hudson

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Author name code: hudson
ADS astronomy entries on 2022-09-14
author:"Hudson, Hugh S."

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Title: Observations of Thomson Scattering from a Loop-prominence
System
Authors: Martínez Oliveros, Juan Carlos; Guevara Gómez, Juan Camilo;
Saint-Hilaire, Pascal; Hudson, Hugh; Krucker, Säm
2022ApJ...936...56M Altcode: 2022arXiv220806007M
We describe observations of the white-light structures in the low
corona following the X8.2 flare SOL 2017-09-10, as observed in full
Stokes parameters by the Helioseismic and Magnetic Imager (HMI) of
the Solar Dynamics Observatory. These data show both bright loops
and a diffuse emission region above them. We interpret the loops as
the white-light counterpart of a classical loop-prominence system,
intermediate between the hot X-ray loops and coronal rain. The diffuse
emission external to the loops is linearly polarized and has a natural
interpretation in terms of Thomson scattering from the hot plasma seen
prior to its cooling and recombination. The polarimetric data from
HMI enable us to distinguish this contribution of scattering from the
HMI pseudocontinuum measurement, and to make a direct estimation of
the coronal mass in the polarized source. For a snapshot at 16:19 UT,
we estimate a mass 8 × 1014 g. We further conclude that
the volumetric filling factor of this source is near unity.

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Title: Fast prograde coronal flows in solar active regions
Authors: Hudson, Hugh S.; Mulay, Sargam M.; Fletcher, Lyndsay;
Docherty, Jennifer; Fitzpatrick, Jimmy; Pike, Eleanor; Strong, Morven;
Chamberlin, Phillip C.; Woods, Thomas N.
2022MNRAS.515L..84H Altcode: 2022MNRAS.tmpL..75H; 2022arXiv220713461H
We report the discovery and characterization of high-speed (&gt;100 km
s-1) horizontal flows in solar active regions, making use
of the Sun-as-a-star spectroscopy in the range 5-105 nm provided by
the EVE (Extreme Ultraviolet Variability Experiment) spectrometers on
the Solar Dynamics Observatory. These apparent flows are persistent on
time-scales of days, and are well observed in lines of Mg X, Si XII,
and Fe XVI for example. They are prograde, as evidenced directly
by blueshifts/redshifts peaking at the east/west limb passages of
isolated active regions. The high-speed flow behaviour does not depend
upon active-region latitude or solar cycle, with similar behaviour in
Cycles 24 and 25.

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Title: The Eclipse Megamovie Project (2017)
Authors: Hudson, Hugh S.; Peticolas, Laura; Johnson, Calvin; White,
Vivian; Bender, Mark; Pasachoff, Jay M.; Martínez Oliveros, Juan
Carlos; Collier, Braxton; Filippenko, Alexei V.; Filippenko, Noelle;
Fraknoi, Andrew; Guevara Gómez, Juan Camilo; Koh, Justin; Konerding,
David; Krista, Larisza; Kruse, Brian; McIntosh, Scott; Mendez, Brian;
Ruderman, Igor; Yan, Darlene; Zevin, Dan
2022arXiv220713704H Altcode:
The total solar eclipse of August 21, 2017, crossed the whole width
of North America, the first occasion for this during the modern age
of consumer electronics. Accordingly, it became a great opportunity
to engage the public and to enlist volunteer observers with relatively
high-level equipment; our program ("Eclipse Megamovie") took advantage
of this as a means of creating a first-ever public database of such
eclipse photography. This resulted in a large outreach program,
involving many hundreds of individuals, supported almost entirely
on a volunteer basis and with the institutional help of Google, the
Astronomical Society of the Pacific, and the University of California,
Berkeley. The project home page is \url{http://eclipsemegamovie.org},
which contains the movie itself. We hope that our comments here will
help with planning for similar activities in the total eclipse of
April 8, 2024.

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Title: NuSTAR observations of a quiet Sun minifilament eruption
Authors: Hannah, Iain; Sterling, Alphonse; Grefenstette, Brian;
Glesener, Lindsay; White, Stephen; Smith, David; Cooper, Kristopher;
Krucker, Sam; Paterson, Sarah; Hudson, Hugh
2022cosp...44.2538H Altcode:
We present a unique set of observations of a confined minifilament
eruption from the quiet-Sun during solar minimum. The Nuclear
Spectroscopic Telescope Array (NuSTAR) spotted a tiny, compact hard
X-ray (HXR) flare on 2019 April 26, peaking about 02:06UT for a few
minutes, finding brief emission &gt;5MK. Observations with SDO/AIA
and Hinode/XRT show this HXR emission was due to a tiny flare arcade
underneath a confined minifilament eruption - behaviour similar to those
seen in both major active-region filament eruptions and minifilament
eruptions that lead to coronal jets. Line-of-sight magnetograms from
SDO/HMI show that this eruption is due to opposite polarity flux
moving together and cancelling and not due to flux emergence. This
eruption occurred near disk-centre, so the Earth orbiting observatories
provide a top-down view of the event, but fortuitously a side-on view
is obtained from STEREO-A/SECCHI, giving a clearer sense of eruption
geometry. We also explore the possibility of non-thermal emission
due to accelerated electrons in the NuSTAR HXR observations of this
small-scale phenomena in the quiet Sun.

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Title: Detecting stellar CMEs through post-flare coronal dimmings
Authors: Veronig, Astrid; Hudson, Hugh S.; Odert, Petra; Leitzinger,
Martin; Dissauer, Karin; Fleck, Nikolaus
2022cosp...44.1379V Altcode:
Coronal dimmings are sudden decreases of the solar EUV and X-ray
emission caused by coronal mass ejections (CMEs). Dimming regions map to
the bipolar ends of closed magnetic field lines that become stretched
or temporarily opened during an eruption, and are a result of the
depletion of coronal plasma caused by the expansion and mass loss due
to the CME. Recently available multi-point imagery from satellites at
different locations in the heliosphere provided us with unprecedented
observations of the three-dimensional evolution of solar CMEs and their
coronal dimmings. These studies showed distinct correlations between CME
mass and speed with key parameters of the associated coronal dimmings
such as their spatial extent and intensity drop. While CMEs from our
Sun are regularly imaged by white-light coronagraphs, and their speeds
and masses are derived from these observations, for stars such direct
imaging is not possible. Here, we present a new approach to detect
stellar mass ejections through post-flare coronal dimmings. To this
aim, we study Sun-as-a-star broad-band EUV light curves derived from
SDO's Extreme ultraviolet Variability Experiment (EVE) as a testbed
to investigate whether coronal dimmings can be also observed on stars
and used for stellar CME detection. We demonstrate that large eruptive
flares are with a high probability associated with a post-flare coronal
dimming, with intensity drops in the 15-25 nm full-Sun light curves
up to 5%. Searching for similar patterns of post-flare dimmings in
the X-ray and EUV light curves of solar-like and late-type stars,
we identify 21 stellar CME candidates. The derived intensity drops
are an order of magnitude larger than for the Sun, suggesting that a
substantial part of the stellar corona gets ejected by the CME. This
study is published in: A.M. Veronig, P. Odert, M. Leitzinger,
K. Dissauer, N. Fleck, H.S. Hudson, Indications of stellar coronal
mass ejections through coronal dimmings, Nature Astronomy 5, 697-706
(2021). https://www.nature.com/articles/s41550-021-01345-9

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Title: NuSTAR observations of small-scale phenomena in the quiet Sun
Authors: Paterson, Sarah; Krucker, Samuel; Grefenstette, Brian;
Glesener, Lindsay; Hannah, Iain; Smith, David; Hudson, Hugh
2022cosp...44.2561P Altcode:
NuSTAR is a sensitive hard X-ray (HXR) focusing telescope capable of
observing the faint emission from small-scale phenomena in the quiet
Sun. During the recent solar minimum, NuSTAR was used several times
to observe the quiet Sun, providing the unique opportunity to perform
imaging spectroscopy on very faint solar HXR sources. We present
analysis on several small features from the NuSTAR 28 September 2018
full disk solar mosaics, including X-ray/coronal bright points, a jet,
and an emerging flux region that later went on to become an active
region. This is the first time these features have been observed with
an HXR imaging spectrometer. To investigate the contribution of these
quiet Sun features to heating the solar atmosphere, we determine their
thermal properties from their X-ray spectra. We combine the X-ray
data from NuSTAR with EUV data from SDO/AIA and soft X-ray data from
Hinode/XRT by reconstructing their differential emission measures in
order to investigate the multithermal temperature evolution of these
small-scale phenomena.

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Title: Variability of Lyman-alpha Emission During Solar Flares and
Implications for Planetary Atmospheres
Authors: Milligan, Ryan; Hudson, Hugh S.; Chamberlin, Phillip;
Hayes, Laura
2022cosp...44..833M Altcode:
Despite decades of observations of the Sun in the Lyman-alpha
(Ly$\alpha$; 1216\AA) line of neutral hydrogen - the strongest emission
line in the solar spectrum - few instruments have had the sensitivity,
cadence, or duty cycle to measure changes in the solar irradiance
at this wavelength on the timescales of solar flares. The few flare
observations that previously existed were often contradictory, and the
ionospheric impacts of enhanced Ly$\alpha$ irradiance were often deemed
to be negligible in comparison with the corresponding increase in soft
X-rays. With the availability of spatially and spectrally integrated
Ly$\alpha$ flare observations from GOES/EUVS, SDO/EVE, MAVEN/EUM,
and PROBA2/LYRA during Solar Cycle 24, there has been a resurgence of
interest in this fundamental chromospheric emission line. Statistical
studies by Milligan et al. (2020) and Milligan (2021) have shown that,
despite increases in irradiance of only a few percent, Ly$\alpha$
emission may be responsible for inducing currents in the E-layer of the
ionosphere (the so-called magnetic crochet, or solar flare effect, Sfe)
due the ionoisation of NO, while weaker flares were found to produce
greater enhancements of Ly$\alpha$ emission than some larger events
when associated with failed filament eruptions, pointing to a possible
coronal origin. This presentation will include an overview of Ly$\alpha$
flare observations to date, the outstanding questions, and how these may
be resolved with the influx of Ly$\alpha$ flare observations anticipated
from GOES/EXIS, Solar Orbiter, Solar-C, and ASO-S during Solar Cycle 25.

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Title: Solar Guidance for Stellar CMEs
Authors: Hudson, Hugh
2022cosp...44.1356H Altcode:
Solar influences on the Earth involve flares, CMEs, and SEPs
in particular, but the latter involve coronagraphic and in-situ
observational tools that we don't have in the stellar case. SEP ions
("solar cosmic rays") in particular have no remote-sensing astronomical
signatures, since gamma-ray detection is so difficult. This also applies
to extreme events analogous to those detectable in the terrestrial
radioisotope history. One ray of light is that Sun-as-a-star EUV
observations from the SDO/EVE instrument provide an excellent basis for
assessing stellar CME occurrence via the dimming signature. I describe
the solar observations and place them within the context of other solar
(hence often spatially resolved) signatures, as discussed by Hudson
&amp; Cliver (2001JGR...10625199H) in terms of "calibrating” the
relationships of such proxies with direct CME observations. Recently
dimming signatures of stellar CMEs by this method have been confirmed
by Veronig et al. (2021NatAs...5..697V).

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Title: Revisting the Orrall-Zirker Effect: Identifying the
suprathermal proton distribution during solar flares from Lyman
line emission
Authors: Kerr, Graham; Allred, Joel; Milligan, Ryan; Kowalski, Adam;
Hudson, Hugh
2021AGUFMSH23B..04K Altcode:
It is likely that ions are accelerated during solar flares. However,
due in large part to a lack of observational constraints on the
suprathermal ion population in flares, they are not usually considered
in energy transport models, with the focus being on flare accelerated
electrons. Gamma-ray observations are required to constrain the high
energy (MeV) protons, but lower energy (deka-keV to 1 MeV) protons
can potentially be detected through the Orrall-Zirker effect (Orrall
&amp; Zirker, 1976). Suprathermal protons undergo charge exchange
with ambient neutral hydrogen, creating a population of suprathermal
neutral hydrogen. These energetic neutrals can subsequently emit
extremely Doppler shifted photons. The appearance of a very broad
redshifted feature in the far red wings of certain spectral lines
can indicate the presence of suprathermal ions, and the properties of
the feature has diagnostic potential of the distribution of those ions
(e.g. Brosius &amp; Woodgate 1999). This effect is revisited here using
modern state-of-the-art flare simulations that track the ionisation
stratification and suprathermal proton distribution as a function of
time in proton beam driven flares (RADYN+FP, Allred et al 2020), and
using up-to-date charge exchange cross sections. We have developed a
post-processing radiation transfer code (OrrallZirkerPy) that takes
those flare atmospheres as input and makes time-dependent predictions
of red-shifted features. The characteristics of non-thermal emission
of Lyman alpha and Lyman beta, and their potential as diagnostics of
flare accelerated protons, are presented. These predictions are of
particular interest now that we have current and planned missions that
can observe these lines during solar flares (e.g. SolO/SPICE, SDO/EVE,
EUVST, SNIFS).

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Title: Sun-as-a-star Spectral Irradiance Observations of Transiting
Active Regions: a Milestone for Characterization of Stellar Active
Regions
Authors: Toriumi, Shin; Airapetian, Vladimir; Hudson, Hugh; Schrijver,
Karel; Cheung, Chun Ming Mark; DeRosa, Marc
2021AGUFM.U43B..05T Altcode:
Recent observations have revealed that solar-type stars can produce
massive "superflares". The strongest flares on the Sun are almost
always associated with large, complex, rapidly-evolving active regions
(ARs) including sunspots. Therefore, to understand why and how stellar
flares and coronal eruptions occur, which may directly determine
the circumstances of exoplanets, it is critically important to gain
information on stellar ARs. One possible way to do so is to monitor the
star in multiple wavelengths. In this study, we perform multi-wavelength
irradiance monitoring of transiting solar ARs by using full-disk
observational (i.e. Sun-as-a-star) data from four satellites. We find
that the near UV light curves show strong correlations with photospheric
total magnetic flux and that there are time lags between the coronal
and photospheric light curves when ARs are close to the limb. Such time
lags result from high-arching, bright coronal loops above stellar ARs
being visible even when the AR is behind the limb. It is also found
that the EUV light curves sensitive to transition-region temperatures
are sometimes dimmed because of a reduction in the emission measure of
0.60.8 MK due to the plasma being heated to higher temperatures over a
wide area around the AR. These results indicate that, by measuring the
stellar light curves in multiple wavelengths, we may obtain information
on the structures and evolution of stellar ARs.

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Title: NuSTAR Observations of a Repeatedly Microflaring Active Region
Authors: Cooper, Kristopher; Hannah, Iain; Grefenstette, Brian;
Glesener, Lindsay; Krucker, Sam; Hudson, Hugh; White, Stephen; Smith,
David; Duncan, Jessie
2021AGUFMSH22B..03C Altcode:
We present observations of microflares from Sep 9-10 2018 in X-rays
with NuSTAR, EUV with SDO/AIA, and photosphere magnetic field with
SDO/HMI. We investigate how the energy released in these small flares
contributes to the heating of the solar atmosphere and the role
flux cancellation/emergence plays during onset and occurrence of the
microflares. 10 microflares were studied from AR12721, all equivalent
to GOES &lt;A1 Class, and we find that many contain plasma heated
to 5-10 MK with energies of 10261028 erg. One particularly small
microflare, equivalent to GOES Class A0.005, demonstrated emission
from 6.7 MK plasma with a thermal energy of 1.11026 erg. Another
microflare, equivalent to GOES Class A0.1, showed clear non-thermal
emission in the X-ray spectra, with non-thermal energy of 1.31027
erg, making it one of the faintest non-thermal X-ray microflares on
record. For 8 of the 10 microflares we can identify areas of magnetic
flux cancellation/emergence at the footpoints, indicating the role
these play in this active region repeatedly producing microflares.

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Title: The eclipse Megamovie Project (2017)
Authors: Hudson, Hugh; Peticolas, Laura; Johnson, Calvin; White,
Vivian; Bender, Mark; Pasachoff, Jay M.; Oliveros, Juan Carlos
Martínez; Filippenko, Alexei V.; et al.
2021JAHH...24.1080H Altcode:
The total solar eclipse of August 21, 2017, crossed the whole width
of North America, the first occasion for this during the modern age
of consumer electronics. Accordingly, it became a great opportunity
to engage the public and to enlist volunteer observers with relatively
high-level equipment; our program ('Eclipse Megamovie') took advantage
of this as a means of creating a first-ever public database of
such eclipse photography. This resulted in a large outreach program,
involving more than one thousand individuals, supported almost entirely
on a volunteer basis and with the institutional help of Google, the
Astronomical Society of the Pacific, and the University of California,
Berkeley. The project home page at eclipsemegamovie.org contains
the finished movie itself. We hope that our comments here will help
with planning for similar activities during the total eclipse of 8
April 2024.

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Title: Thomson Scattering in the Lower Corona in the Presence
of Sunspots
Authors: Saint-Hilaire, Pascal; Martínez Oliveros, Juan Carlos;
Hudson, Hugh S.
2021ApJ...923..276S Altcode:
Polarized scattered light from low (few tens of megameter altitudes)
coronal transients has been recently reported in Solar Dynamics
Observatory/Helioseismic and Magnetic Image (HMI) observations. In
a classic paper, Minnaert (1930) provided an analytic theory of
polarization via electron scattering in the corona. His work assumed
axisymmetric input from the photosphere with a single-parameter
limb-darkening function. This diagnostic has recently been used to
estimate the free-electron number and mass of HMI transients near
the solar limb, but it applies equally well to any coronal material,
at any height. Here we extend his work numerically to incorporate
sunspots, which can strongly effect the polarization properties of the
scattered light in the low corona. Sunspot effects are explored first
for axisymmetric model cases, and then applied to the full description
of two sunspot groups as observed by HMI. We find that (1) as previously
reported by Minnaert, limb darkening has a strong influence, usually
increasing the level of linear polarization tangential to the limb; (2)
unsurprisingly, the effects of the sunspot generally increase at the
lower scatterer altitudes, and increase the larger the sunspot is and
the closer to their center the scatterer subpoint is; (3) assuming the
Stokes Q &gt; 0 basis to be tangential to the limb, sunspots typically
decrease the Stokes Q/I polarization and the perceived electron
densities below the spotless case, sometimes dramatically; and (4)
typically, a sizeable non-zero Stokes U/I polarization component will
appear when a sunspot's influence becomes non-negligible. However,
that is not true in rare cases of extreme symmetry (e.g., scattering
mass at the center of an axisymmetric sunspot). The tools developed
here are generally applicable to an arbitrary image input.

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Title: Hot onsets of solar flares
Authors: Hudson, Hugh; Fletcher, Lyndsay; Hannah, Iain; Hayes, Laura;
Simoes, Paulo
2021AGUFMSH22B..02H Altcode:
The GOES/XRS data show low-level soft X-ray emissions prior to
a flare in a "hot onset" precursor of the main flare development
(2021MNRAS.501.1273H). This phenomenon provides clear evidence for
energy release not identifiable with the impulsive phase, and apparently
not mediated by non-thermal particles. The hot onset phase may last for
tens of seconds to minutes, characterized by gradual and roughly linear
growth of emission measure at characteristic isothermal temperatures
of 10-15 MK and no clear pattern of temperature increase in the sense
of dT/dt &gt; 0. The figure shows an example of this behavior, from
SOL2004-02-26 (X1.1). The newer GOES-R data also show this effect,
providing higher time resolution (1 s) and better noise properties,
although with higher background levels. Hot onsets occur in most if
not all flares, but their properties do not appear to provide any
guidance towards the magnitude of the flare that follows.

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Title: Finding Fast Gamma-ray Variability in Solar Flares
Authors: Zeitohn, Hind; Briggs, Michael; Veres, Peter; Hudson, Hugh;
Lesage, Stephen
2021AGUFMSH35E2122Z Altcode:
Fast Radio Bursts (FRBs) are fast, bright, extragalactic transients,
typically associated with magnetars. In early 2021, a very sensitive
radio array observed a unique FRB-like event which localized to
the Sun. This solar-FRB (sFRB) lasted for only a few milliseconds
at 1.4 GHz and had a flux density of 9.1 Mega-Janskys (910 solar
flux). This millisecond long sFRB was seen as a "spike" in the radio
data, which raises the question, can sFRBs be detected in other
wavelengths? Fermi-GBM has a 4 pi steradian field of view of the sky
and measures gamma-rays down to 2 microseconds temporal resolution,
which makes it the perfect instrument for detecting sFRBs. If such
variability were to be found in the Fermi-GBM data, it would provide
evidence that this unique energetic process spans multiple 9 orders of
magnitude in energy. We performed a blind search of Solar Flares in
the Fermi-GBM time-tagged event (TTE) data, binned at 10, 20, and 50
milliseconds in the 5 keV to 20 keV range starting from November 27th,
2012, and ending on April 23rd, 2021. Of the ~3500 solar flares that
were examined, no statistically significant gamma-ray emission were
detected. We derive upper limit density flux range for the gamma-ray
emission to be 0.2 to 0.9 Janskys.

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Title: Detection of stellar CMEs through post-flare coronal dimmings
Authors: Veronig, Astrid; Odert, Petra; Leitzinger, Martin; Dissauer,
Karin; Fleck, Nikolaus; Hudson, Hugh
2021AGUFM.U43B..07V Altcode:
Coronal mass ejections (CMEs) from our Sun are regularly imaged by
white-light coronagraphs, and their speeds and masses are derived
from these observations. However, for stars such direct imaging is
not possible, and so far, only a few candidates for stellar CMEs
have been reported. Here, we present a new approach to detect stellar
mass ejections through post-flare coronal dimmings. Coronal dimmings
are sudden decreases of the solar EUV and X-ray emission caused by
CMEs. Dimming regions map to the bipolar ends of closed magnetic field
lines that become stretched or temporarily opened during an eruption,
and are a result of the depletion of coronal plasma caused by the
expansion and mass loss due to the CME. Recently available multi-point
imagery from satellites at different locations in the heliosphere
provided us with unprecedented observations of the three-dimensional
evolution of solar CMEs and their coronal dimmings. These studies showed
distinct correlations between CME mass and speed with key parameters
of the associated coronal dimmings such as their spatial extent
and intensity drop. In this contribution, we present Sun-as-a-star
broad-band EUV light curves derived from SDOs Extreme ultraviolet
Variability Experiment (EVE) as a testbed to study whether coronal
dimmings can be also observed on stars and used for stellar CME
detection. We demonstrate that large flares associated with CMEs have
a high probability to show a post-flare coronal dimming, with intensity
drops in the 15-25 nm full-Sun light curves of up to 5%. Searching for
similar patterns of post-flare dimmings in the X-ray and EUV light
curves of solar-like and late-type stars, we identify 21 stellar
CME candidates, which is more than all previous reports of stellar
CMEs. The derived intensity drops are an order of magnitude larger than
for the Sun, suggesting that a substantial part of the stellar corona
gets ejected by the CME. This study paves the way for comprehensive
detections and characterizations of CMEs on stars, which are important
factors in planetary habitability and stellar evolution. This
study is published in: A.M. Veronig, P. Odert, M. Leitzinger,
K. Dissauer, N. Fleck, H.S. Hudson, Indications of stellar coronal
mass ejections through coronal dimmings, Nature Astronomy 5, 697-706
(2021). https://www.nature.com/articles/s41550-021-01345-9

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Title: NuSTAR observations of a repeatedly microflaring active region
Authors: Cooper, Kristopher; Hannah, Iain G.; Grefenstette, Brian W.;
Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; White, Stephen M.;
Smith, David M.; Duncan, Jessie
2021MNRAS.507.3936C Altcode: 2021arXiv210900263C; 2021MNRAS.tmp.2159C
We investigate the spatial, temporal, and spectral properties of 10
microflares from AR12721 on 2018 September 9 and 10 observed in X-rays
using the Nuclear Spectroscopic Telescope ARray and the Solar Dynamic
Observatory's Atmospheric Imaging Assembly and Helioseismic and Magnetic
Imager. We find GOES sub-A class equivalent microflare energies of
1026-1028 erg reaching temperatures up to 10
MK with consistent quiescent or hot active region (AR) core plasma
temperatures of 3-4 MK. One microflare (SOL2018-09-09T10:33), with
an equivalent GOES class of A0.1, has non-thermal hard X-ray emission
during its impulsive phase (of non-thermal power ~7 × 1024
erg s-1) making it one of the faintest X-ray microflares
to have direct evidence for accelerated electrons. In 4 of the 10
microflares, we find that the X-ray time profile matches fainter and
more transient sources in the extreme-ultraviolet, highlighting the need
for observations sensitive to only the hottest material that reaches
temperatures higher than those of the AR core (&gt;5 MK). Evidence
for corresponding photospheric magnetic flux cancellation/emergence
present at the footpoints of eight microflares is also observed.

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Title: Carrington Events
Authors: Hudson, Hugh S.
2021ARA&A..59..445H Altcode:
The Carrington event in 1859, a solar flare with an associated
geomagnetic storm, has served as a prototype of possible superflare
occurrence on the Sun. Recent geophysical (14C signatures
in tree rings) and precise time-series photometry [the bolometric
total solar irradiance (TSI) for the Sun, and the broadband photometry
from Kepler and Transiting Exoplanet Survey Satellite, for the stars]
have broadened our perspective on extreme events and the threats that
they pose for Earth and for Earth-like exoplanets. This review assesses
the mutual solar and/or stellar lessons learned and the status of our
theoretical understanding of the new data, both stellar and solar,
as they relate to the physics of the Carrington event. The discussion
includes the event's implied coronal mass ejection, its potential "solar
cosmic ray" production, and the observed geomagnetic disturbances
based on the multimessenger information already available in that
era. Taking the Carrington event as an exemplar of the most extreme
solar event, and in the context of our rich modern knowledge of solar
flare and/or coronal mass ejection events, we discuss the aspects of
these processes that might be relevant to activity on solar-type stars,
and in particular their superflares. The Carrington flare of 1859,
though powerful, did not significantly exceed the magnitudes of the
greatest events observed in the modern era. Stellar "superflare" events
on solar-type stars may share common paradigms, and also suggest the
possibility of a more extreme solar event at some time in the future. We
benefit from comparing the better-known microphysics of solar flares and
coronal mass ejections with the diversity of related stellar phenomena.

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Title: Characterizing a "Solar FRB"
Authors: Hudson, H.; Briggs, M.; Chitta, L.; Fletcher, L.; Gary, D.;
Monstein, C.; Nimmo, K.; Saint-Hilaire, P.; White, S.
2021AAS...23812716H Altcode:
A remarkable solar microwave (1.4 GHz) burst,
SOL2019-05-06T17:47:35.385, has been reported by the STARE2 fast cosmic
transient survey (Bochenek et al., 2020). Its behavior resembles
that of the Fast Radio Burst (FRB) extragalactic events in having a
relatively broad spectral bandwidth and brief (19-msec) duration. It
also had no measureable dispersion. The associated flare, GOES class
C1, had a relatively hard X-ray spectrum as observed by Fermi/GBM,
but no temporal association at the msec time scale suggested by the
microwaves. Although msec variability in the microwave domain has
been known to solar radio astronomy since the 1970s, the brightness
and isolation of this event (both spatial and temporal) suggests
novelty. Accordingly we survey the available correlative data from
many sources and discuss possible interpretations in terms of type
III-like behavior, electron cyclotron masering, and gyrosynchrotron
emission. We note that the radio data (e-Callisto and EOVSA) revealed
abundant type III activity in the vicinity, and the related flares
as observed by GOES had exceptionally short time scales, suggesting
burst origins in the lower solar atmosphere.

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Title: Sun-as-a-star Spectral Irradiance Observations: Milestone
For Characterizing The Stellar Active Regions
Authors: Toriumi, S.; Airapetian, V.; Hudson, H.; Schrijver, C.;
Cheung, M.; DeRosa, M.
2021AAS...23820503T Altcode:
For understanding the physical mechanism behind the solar flares, it
is crucial to measure the magnetic fields of active regions (ARs) from
the photosphere to the corona and investigate their scale, complexity,
and evolution. This is true for the stellar flares. However, it is
still difficult to spatially resolve the starspots, and one possible
way to probe their evolution and structure is to monitor the star in
multiple wavelengths. To test this possibility with the solar data,
we perform multi-wavelength irradiance monitoring of transiting solar
ARs by using full-disk observation data from SDO, Hinode, GOES, and
SORCE. As a result, we find, for instance, that the near UV light
curves show strong correlations with photospheric total magnetic flux
and that there are time lags between the coronal and photospheric light
curves when ARs are close to the limb, which together may enable one
to discern how high bright coronal loops extend above stellar ARs. It
is also revealed that the sub-MK (i.e. transition-region temperature)
EUV light curves are sometimes dimmed because the emission measure
is reduced owing to the heating over a wide area around the AR. These
results indicate that, by measuring the stellar light curves in multiple
wavelengths, we may obtain information on the structure and evolution
of stellar ARs.

---------------------------------------------------------
Title: Carrington's lost photograph
Authors: Cliver, E. W.; Fletcher, Lyndsay; Hudson, H. S.
2021A&G....62.2.40C Altcode:
Ed Cliver, Lyndsay Fletcher and Hugh Hudson are looking for a photograph
of Richard Carrington. Can you help?

---------------------------------------------------------
Title: Carrington Events
Authors: Hudson, Hugh
2021csss.confE.162H Altcode:
Poster

---------------------------------------------------------
Title: Sun-as-a-star Multi-wavelength Observations: A Milestone for
Characterization of Stellar Active Regions
Authors: Toriumi, Shin; Airapetian, Vladimir S.; Hudson, Hugh S.;
Schrijver, Carolus J.; Cheung, Mark C. M.; DeRosa, Marc L.
2021csss.confE..46T Altcode:
It has been revealed that "superflares" can occur on solar-type
stars. The magnetic energy of the flares is likely to be stored in
active-region atmospheres. Therefore, to explain the energy storage and
occurrence of the flares, it is important to monitor the evolutions of
the active regions, not only in visible light but also in ultraviolet
(UV) and X-rays. To demonstrate this, we perform multi-wavelength
irradiance monitoring of transiting solar active regions by using
full-disk observation data. As a result of this sun-as-a-star spectral
irradiance analysis, we confirm that the visible continuum that
corresponds to the photosphere becomes darkened when the spot is at the
central meridian, whereas most of the UV, EUV and X-rays, which are
sensitive to chromospheric to coronal temperatures, are brightened,
reflecting the bright magnetic features above the starspots. The
time lags between the coronal and photospheric light curves have
the potential to probe the extent of coronal magnetic fields above
the starspots. These results indicate that, by measuring the stellar
light curves in multiple wavelengths, we may obtain information on
the structures and evolution of stellar active regions.

---------------------------------------------------------
Title: Carrington Events
Authors: Hudson, Hugh
2021csss.confE.149H Altcode:
Haiku

---------------------------------------------------------
Title: Hot X-ray onsets of solar flares
Authors: Hudson, Hugh S.; Simões, Paulo J. A.; Fletcher, Lyndsay;
Hayes, Laura A.; Hannah, Iain G.
2021MNRAS.501.1273H Altcode: 2020MNRAS.tmp.3462H; 2020arXiv200705310H
The study of the localized plasma conditions before the impulsive phase
of a solar flare can help us understand the physical processes that
occur leading up to the main flare energy release. Here, we present
evidence of a hot X-ray 'onset' interval of enhanced isothermal plasma
temperatures in the range of 10-15 MK over a period of time prior to
the flare's impulsive phase. This 'hot onset' interval occurs during
the initial soft X-ray increase and definitely before any detectable
hard X-ray emission. The isothermal temperatures, estimated by the
Geostationary Operational Environmental Satellite X-ray sensor,
and confirmed with data from the Reuven Ramaty High Energy Solar
Spectroscopic Imager, show no signs of gradual increase, and the
'hot onset' phenomenon occurs regardless of flare classification or
configuration. In a small sample of four representative flare events, we
tentatively identify this early hot onset soft X-ray emission to occur
within footpoint and low-lying loop regions, rather than in coronal
structures, based on images from the Atmospheric Imaging Assembly. We
confirm this via limb occultation of a flaring region. These hot
X-ray onsets appear before there is evidence of collisional heating
by non-thermal electrons, and hence challenge the standard modelling
techniques.

---------------------------------------------------------
Title: NuSTAR Observation of Energy Release in 11 Solar Microflares
Authors: Duncan, Jessie; Glesener, Lindsay; Grefenstette, Brian W.;
Vievering, Juliana; Hannah, Iain G.; Smith, David M.; Krucker, Säm;
White, Stephen M.; Hudson, Hugh
2021ApJ...908...29D Altcode: 2020arXiv201106651D
Solar flares are explosive releases of magnetic energy. Hard X-ray
(HXR) flare emission originates from both hot (millions of Kelvin)
plasma and nonthermal accelerated particles, giving insight into flare
energy release. The Nuclear Spectroscopic Telescope ARray (NuSTAR)
utilizes direct-focusing optics to attain much higher sensitivity
in the HXR range than that of previous indirect imagers. This paper
presents 11 NuSTAR microflares from two active regions (AR 12671 on
2017 August 21 and AR 12712 on 2018 May 29). The temporal, spatial, and
energetic properties of each are discussed in context with previously
published HXR brightenings. They are seen to display several "large
flare" properties, such as impulsive time profiles and earlier peak
times in higher-energy HXRs. For two events where the active region
background could be removed, microflare emission did not display
spatial complexity; differing NuSTAR energy ranges had equivalent
emission centroids. Finally, spectral fitting showed a high-energy
excess over a single thermal model in all events. This excess was
consistent with additional higher-temperature plasma volumes in 10/11
microflares and only with an accelerated particle distribution in the
last. Previous NuSTAR studies focused on one or a few microflares at a
time, making this the first to collectively examine a sizable number of
events. Additionally, this paper introduces an observed variation in
the NuSTAR gain unique to the extremely low livetime (&lt;1%) regime
and establishes a correction method to be used in future NuSTAR solar
spectral analysis.

---------------------------------------------------------
Title: Indications of stellar coronal mass ejections through coronal
dimmings
Authors: Veronig, Astrid M.; Odert, Petra; Leitzinger, Martin;
Dissauer, Karin; Fleck, Nikolaus C.; Hudson, Hugh S.
2021NatAs...5..697V Altcode: 2021arXiv211012029V; 2021NatAs.tmp...72V
Coronal mass ejections (CMEs) are huge expulsions of magnetized matter
from the Sun and stars, traversing space with speeds of millions
of kilometres per hour. Solar CMEs can cause severe space weather
disturbances and consumer power outages on Earth, whereas stellar CMEs
may even pose a hazard to the habitability of exoplanets. Although CMEs
ejected by our Sun can be directly imaged by white-light coronagraphs,
for stars this is not possible. So far, only a few candidates for
stellar CME detections have been reported. Here we demonstrate a
different approach that is based on sudden dimmings in the extreme
ultraviolet and X-ray emission caused by the CME mass loss. We report
dimming detections associated with flares on cool stars, indicative
of stellar CMEs, and which are benchmarked by Sun-as-a-star extreme
ultraviolet measurements. This study paves the way for comprehensive
detections and characterizations of CMEs on stars, which are important
factors in planetary habitability and stellar evolution.

---------------------------------------------------------
Title: NuSTAR Observation of Eleven Solar Microflares
Authors: Duncan, J. M.; Glesener, L.; Grefenstette, B.; Vievering,
J. T.; Hannah, I. G.; Smith, D. M.; Krucker, S.; White, S. M.; Hudson,
H. S.
2020AGUFMSH045..07D Altcode:
This work presents eleven microflares observed by the Nuclear
Spectroscopic Telescope ARray (NuSTAR), representing the first
time that a sizable number of these events have been examined
collectively. NuSTAR's direct focusing optics give it a dramatic
increase in sensitivity over indirect imagers in the hard X-ray
(HXR) range. HXR emission in solar flares originates from both hot
(millions of Kelvin) plasma and nonthermal accelerated particles,
both of which are diagnostic of flare energy release. Therefore,
NuSTAR solar observation campaigns can give unique insight into the
energetics of faint microflares, including those that were unobservable
with previous-generation HXR instruments. We discuss the temporal,
spatial, and energetic properties of all eleven microflares in context
with previously published HXR brightenings. They are seen to display
several `large-flare' properties, such as impulsive time profiles
and earlier peaktimes in higher energy HXRs. For two events where
active region background could be removed, microflare emission did
not display spatial complexity: differing NuSTAR energy ranges had
equivalent emission centroids. Finally, spectral fitting showed a high
energy excess over a single thermal model in all events. This excess
was found to most likely originate from additional higher-temperature
plasma volumes in 10/11 microflares, and from an accelerated particle
distribution in the last. Finally, we introduce an observed variation
in the NuSTAR gain unique to the extremely low-livetime (&lt; 1%)
regime, and establish a correction method to be used in future NuSTAR
solar spectral analysis.

---------------------------------------------------------
Title: Hot Onsets of Solar Flares
Authors: Hudson, H. S.; Simoes, P. J. D. A.; Fletcher, L.; Hayes,
L.; Hannah, I. G.
2020AGUFMSH0500003H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Sun-as-a-star Spectral Irradiance Observations of Transiting
Active Regions
Authors: Toriumi, Shin; Airapetian, Vladimir S.; Hudson, Hugh S.;
Schrijver, Carolus J.; Cheung, Mark C. M.; DeRosa, Marc L.
2020ApJ...902...36T Altcode: 2020arXiv200804319T
Major solar flares are prone to occur in active-region (AR) atmospheres
associated with large, complex, dynamically evolving sunspots. This
points to the importance of monitoring the evolution of starspots,
not only in visible but also in ultraviolet (UV) and X-rays, in
understanding the origin and occurrence of stellar flares. To this end,
we perform spectral irradiance analysis on different types of transiting
solar ARs by using a variety of full-disk synoptic observations. The
target events are an isolated sunspot, spotless plage, and emerging flux
in prolonged quiet-Sun conditions selected from the past decade. We find
that the visible continuum and total solar irradiance become darkened
when the spot is at the central meridian, whereas it is bright near
the solar limb; UV bands sensitive to the chromosphere correlate well
with the variation of total unsigned magnetic flux in the photosphere;
amplitudes of extreme ultraviolet (EUV) and soft X-ray increase with
the characteristic temperature, whose light curves are flat-topped
due to their sensitivity to the optically thin corona; the transiting
spotless plage does not show the darkening in the visible irradiance,
while the emerging flux produces an asymmetry in all light curves about
the central meridian. The multiwavelength Sun-as-a-star study described
here indicates that the time lags between the coronal and photospheric
light curves have the potential to probe the extent of coronal magnetic
fields above the starspots. In addition, EUV wavelengths that are
sensitive to temperatures just below 1 MK sometimes show antiphased
variations, which may be used for diagnosing plasmas around starspots.

---------------------------------------------------------
Title: Solar Flare Build-Up and Release
Authors: Hudson, Hugh S.
2020SoPh..295..132H Altcode:
Flares and coronal mass ejections should follow a pattern of build-up
and release, with the build-up phase understood as the gradual
addition of stress to the coronal magnetic field. Recently Hudson
(Mon. Not. Roy. Astron. Soc.491, 4435, 2020) presented observational
evidence for this pattern in two isolated active regions from 1997 and
2006, finding a correlation between the waiting time after the event,
and the event magnitude. In this article we systematically search for
related evidence in the largest 14 active regions of Solar Cycle 24,
chosen as those with peak sunspot area exceeding 1000 millionths of
the solar hemisphere (MSH). The smallest of these regions, NOAA 12673,
produced the exceptional flares SOL2017-09-06 and SOL2017-09-10. None
of these regions showed significant correlations of waiting times
and flare magnitudes, although two hinted at such an interval-size
relationship. Correlations thus appear to be non-existent or
intermittent, depending on presently unknown conditions.

---------------------------------------------------------
Title: Solar Flare Energy Partitioning and Transport -- the Impulsive
Phase (a Heliophysics 2050 White Paper)
Authors: Kerr, Graham S.; Alaoui, Meriem; Allred, Joel C.; Bian,
Nicholas H.; Dennis, Brian R.; Emslie, A. Gordon; Fletcher, Lyndsay;
Guidoni, Silvina; Hayes, Laura A.; Holman, Gordon D.; Hudson, Hugh
S.; Karpen, Judith T.; Kowalski, Adam F.; Milligan, Ryan O.; Polito,
Vanessa; Qiu, Jiong; Ryan, Daniel F.
2020arXiv200908400K Altcode:
Solar flares are a fundamental component of solar eruptive events (SEEs;
along with solar energetic particles, SEPs, and coronal mass ejections,
CMEs). Flares are the first component of the SEE to impact our
atmosphere, which can set the stage for the arrival of the associated
SEPs and CME. Magnetic reconnection drives SEEs by restructuring the
solar coronal magnetic field, liberating a tremendous amount of energy
which is partitioned into various physical manifestations: particle
acceleration, mass and magnetic-field eruption, atmospheric heating,
and the subsequent emission of radiation as solar flares. To explain
and ultimately predict these geoeffective events, the heliophysics
community requires a comprehensive understanding of the processes that
transform and distribute stored magnetic energy into other forms,
including the broadband radiative enhancement that characterises
flares. This white paper, submitted to the Heliophysics 2050 Workshop,
discusses the flare impulsive phase part of SEEs, setting out the
questions that need addressing via a combination of theoretical,
modelling, and observational research. In short, by 2050 we must
determine the mechanisms of particle acceleration and propagation,
and must push beyond the paradigm of energy transport via nonthermal
electron beams, to also account for accelerated protons &amp; ions
and downward directed Alfven waves.

---------------------------------------------------------
Title: Solar Flare Energy Partitioning and Transport -- the Gradual
Phase (a Heliophysics 2050 White Paper)
Authors: Kerr, Graham S.; Alaoui, Meriem; Allred, Joel C.; Bian,
Nicholas H.; Dennis, Brian R.; Emslie, A. Gordon; Fletcher, Lyndsay;
Guidoni, Silvina; Hayes, Laura A.; Holman, Gordon D.; Hudson, Hugh
S.; Karpen, Judith T.; Kowalski, Adam F.; Milligan, Ryan O.; Polito,
Vanessa; Qiu, Jiong; Ryan, Daniel F.
2020arXiv200908407K Altcode:
Solar flares are a fundamental component of solar eruptive events
(SEEs; along with solar energetic particles, SEPs, and coronal
mass ejections, CMEs). Flares are the first component of the SEE
to impact our atmosphere, which can set the stage for the arrival
of the associated SEPs and CME. Magnetic reconnection drives SEEs
by restructuring the solar coronal magnetic field, liberating a
tremendous amount of energy which is partitioned into various physical
manifestations: particle acceleration, mass and magnetic-field eruption,
atmospheric heating, and the subsequent emission of radiation as solar
flares. To explain and ultimately predict these geoeffective events,
the heliophysics community requires a comprehensive understanding of
the processes that transform and distribute stored magnetic energy
into other forms, including the broadband radiative enhancement that
characterises flares. This white paper, submitted to the Heliophysics
2050 Workshop, discusses the flare gradual phase part of SEEs, setting
out the questions that need addressing via a combination of theoretical,
modelling, and observational research. In short, the flare gradual phase
persists much longer than predicted so, by 2050, we must identify the
characteristics of the significant energy deposition sustaining the
gradual phase, and address the fundamental processes of turbulence
and non-local heat flux.

---------------------------------------------------------
Title: Active Region Irradiance during Quiescent Periods: New Insights
from Sun-as-a-star Spectra
Authors: Kazachenko, Maria D.; Hudson, Hugh S.
2020ApJ...901...64K Altcode: 2020arXiv200802702K
How much energy do solar active regions (ARs) typically radiate during
quiescent periods? This is a fundamental question for storage and
release models of flares and ARs, yet it is presently poorly answered
by observations. Here we use the "Sun-as-a-point-source" spectra from
the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory
to provide a novel estimate of radiative energy losses of an evolving
AR. Although EVE provides excellent spectral (5-105 nm) and temperature
(2-25 MK) coverage for AR analysis, to our knowledge, these data have
not been used for this purpose due to the lack of spatial resolution
and the likelihood of source confusion. Here we present a way around
this problem. We analyze EVE data time series, when only one large AR
11520 was present on the disk. By subtracting the quiet-Sun background,
we estimate the radiative contribution in EUV from the AR alone. We
estimate the mean AR irradiance and cumulative AR radiative energy
losses in the 1-300 Å and astronomical standard ROSAT-PSPC, 3-124
Å, passbands and compare these to the magnetic energy injection
rate through the photosphere, and to variations of the solar cycle
luminosity. We find that while AR radiative energy losses are ∼100
times smaller than typical magnetic energy injection rates at the
photosphere, they are an order of magnitude larger or similar to the
bolometric radiated energies associated with large flares. This study
is the first detailed analysis of AR thermal properties using EVE
Sun-as-a-star observations, opening doors to AR studies on other stars.

---------------------------------------------------------
Title: NuSTAR X-ray Observations of a Minuscule Microflare from a
Repeatedly Microflaring Active Region
Authors: Cooper, K.; Hannah, I.; Grefenstette, B.; Glesener, L.;
Krucker, S.; Hudson, H.
2020SPD....5121101C Altcode:
Highly frequent, small flares are thought to contribute to heating
the Sun's atmosphere, particularly in active regions. This impulsive
energy release would heat plasma to at least 10 MK and accelerate
electrons, producing weak thermal and non-thermal signatures that
could be observed by a very sensitive X-ray telescope. No such solar
telescope exists (yet) so we use Nuclear Spectroscopic Telescope Array
(NuSTAR), an astrophysical X-ray telescope, with focusing optics imaging
spectroscopy providing a unique sensitivity for observing the Sun
above 2.5 keV. We present an overview of several microflares from the
recently emerged active region AR12721 on 2018 September 9-10. Using
NuSTAR's imaging spectroscopy and the Solar Dynamics Observatory's
Atmospheric Imaging Assembly's (SDO/AIA) EUV imaging capabilities we
can analyse the temporal, spatial, and spectral evolution of these
microflares, determining the energy release and associated heating of
the solar atmosphere. All microflares from AR12721 were below GOES A1
equivalent level and the heated coronal loops were all visible in an
Fe XVIII proxy channel derived from SDO/AIA channels. In particular,
we present our recently published analysis of the weakest microflare
from AR12721 (Cooper et al. 2020 ApJL 893 2) finding it to be one
of the smallest active region X-ray flares on record, with material
heated up to 7MK and a thermal energy of just 1e26 erg.

---------------------------------------------------------
Title: Hot X-ray Onsets of Solar Flares
Authors: Hayes, L.; Hudson, H.; Simoes, P.; Fletcher, L.; Hannah, I.
2020SPD....5121113H Altcode:
The study of the localized plasma conditions before the impulsive phase
of a solar flare can help us understand the physical processes that
occur leading up to the main flare energy release. Here, we present
evidence of a hot X-ray 'onset' interval of enhanced isothermal plasma
temperatures in the range of 10-15 MK up to tens of seconds prior
to the flare's impulsive phase. This 'hot onset' interval occurs
during the pre-flare time during which elevated GOES soft X-ray
flux is detected, but prior to detectable hard X-ray emission. The
isothermal temperatures, estimated by the Geostationary Operational
Environmental Satellite (GOES) X-ray sensor, and confirmed with data
from RHESSI, show no signs of gradual increase, and occurs regardless
of flare classification or configuration. In a small sample of four
representative flare events we identify this early hot onset soft
X-ray emission mainly within footpoint and low-lying loops, rather
than with coronal structures, based on images from the Atmospheric
Imaging Assembly (AIA) and the use of limb occultation. These hot
X-ray onsets appear before there is evidence of collisional heating by
non-thermal electrons, and hence challenges the standard flare heating
modeling techniques.

---------------------------------------------------------
Title: NuSTAR Observation of Quiet Sun X-ray Bright Points
Authors: Paterson, S.; Hannah, I.; Grefenstette, B.; Hudson, H.;
Krucker, S.; Glesener, L.
2020SPD....5121013P Altcode:
NuSTAR is a focusing hard X-ray telescope designed for observing
astrophysical sources, but it is also capable of being pointed at
the Sun. NuSTAR's much greater sensitivity compared to RHESSI and the
current minimum of the solar activity cycle provide a unique opportunity
to investigate quiet Sun features that it has previously not been
possible to with X-ray imaging spectroscopy. We present analysis from
NuSTAR quiet Sun full disk mosaics from April 2019. With the absence
of very bright sources, these mosaics show very small and faint
X-ray bright points. We investigate the contribution of these small
events to heating the solar atmosphere. The X-ray spectra of these
features have been fit, allowing for estimates of their temperatures
and emission measures to be obtained. The temperatures were found to
lie in the range 1.2-3.5 MK. The temperatures and emission measures
predicted by the spectral fits can be tested through comparison to
SDO/AIA observations. Using the characteristics of the bright points
found from their spectra, heating processes occurring in these features
will be investigated.

---------------------------------------------------------
Title: Lyman-alpha Variability During Solar Flares Over Solar Cycle
24 Using GOES-15/EUVS-E
Authors: Milligan, Ryan O.; Hudson, Hugh S.; Chamberlin, Phillip C.;
Hannah, Iain G.; Hayes, Laura A.
2020SpWea..1802331M Altcode: 2019arXiv191001364M
The chromospheric Lyman-alpha line of neutral hydrogen (Lyα; 1216 Å)
is the strongest emission line in the solar spectrum. Fluctuations in
Lyα are known to drive changes in planetary atmospheres, although few
instruments have had the ability to capture rapid Lyα enhancements
during solar flares. In this paper, we describe flare-associated
emissions via a statistical study of 477 M- and X-class flares
as observed by the Extreme UltraViolet Sensor on board the 15th
Geostationary Operational Environmental Satellite, which has been
monitoring the full-disk solar Lyα irradiance on 10-s timescales over
the course of Solar Cycle 24. The vast majority (95%) of these flares
produced Lyα enhancements of 10% or less above background levels,
with a maximum increase of ∼30%. The irradiance in Lyα was found to
exceed that of the 1-8 Å X-ray irradiance by as much as two orders
of magnitude in some cases, although flares that occurred closer to
the solar limb were found to exhibit less of a Lyα enhancement. This
center-to-limb variation was verified through a joint, stereoscopic
observation of an X-class flare that appeared near the limb as viewed
from Earth, but close to disk center as viewed by the MAVEN spacecraft
in orbit around Mars. The frequency distribution of peak Lyα was found
to have a power-law slope of 2.8±0.27. We also show that increased Lyα
flux is closely correlated with induced currents in the ionospheric
E-layer through the detection of the solar flare effect as observed
by the Kakioka magnetometer.

---------------------------------------------------------
Title: NuSTAR Observation of a Minuscule Microflare in a Solar
Active Region
Authors: Cooper, Kristopher; Hannah, Iain G.; Grefenstette, Brian W.;
Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; White, Stephen M.;
Smith, David M.
2020ApJ...893L..40C Altcode: 2020arXiv200411176C
We present X-ray imaging spectroscopy of one of the weakest active
region (AR) microflares ever studied. The microflare occurred
at ∼11:04 UT on 2018 September 9 and we studied it using the
Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic
Observatory's Atmospheric Imaging Assembly (SDO/AIA). The microflare
is observed clearly in 2.5-7 keV with NuSTAR and in Fe XVIII emission
derived from the hotter component of the 94 Å SDO/AIA channel. We
estimate the event to be three orders of magnitude lower than a GOES
A class microflare with an energy of 1.1 × 1026 erg. It
reaches temperatures of 6.7 MK with an emission measure of 8.0 ×
1043 cm-3. Non-thermal emission is not detected
but we instead determine upper limits to such emission. We present the
lowest thermal energy estimate for an AR microflare in literature, which
is at the lower limits of what is still considered an X-ray microflare.

---------------------------------------------------------
Title: Accelerated Electrons Observed Down to &lt;7 keV in a NuSTAR
Solar Microflare
Authors: Glesener, Lindsay; Krucker, Säm; Duncan, Jessie; Hannah,
Iain G.; Grefenstette, Brian W.; Chen, Bin; Smith, David M.; White,
Stephen M.; Hudson, Hugh
2020ApJ...891L..34G Altcode: 2020arXiv200312864G
We report the detection of emission from a nonthermal electron
distribution in a small solar microflare (GOES class A5.7) observed
by the Nuclear Spectroscopic Telescope Array, with supporting
observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI). The flaring plasma is well accounted for by a thick-target
model of accelerated electrons collisionally thermalizing within the
loop, akin to the "coronal thick-target" behavior occasionally observed
in larger flares. This is the first positive detection of nonthermal
hard X-rays from the Sun using a direct imager (as opposed to indirectly
imaging instruments). The accelerated electron distribution has a
spectral index of 6.3 ± 0.7, extends down to at least 6.5 keV, and
deposits energy at a rate of ∼2 × 1027 erg s-1,
heating the flare loop to at least 10 MK. The existence of dominant
nonthermal emission in X-rays down to &lt;5 keV means that RHESSI
emission is almost entirely nonthermal, contrary to what is usually
assumed in RHESSI spectroscopy. The ratio of nonthermal to thermal
energies is similar to that of large flares, in contrast to what has
been found in previous studies of small RHESSI flares. We suggest
that a coronal thick target may be a common property of many small
microflares based on the average electron energy and collisional mean
free path. Future observations of this kind will enable understanding
of how flare particle acceleration changes across energy scales,
and will aid the push toward the observational regime of nanoflares,
which are a possible source of significant coronal heating.

---------------------------------------------------------
Title: Cosmic ray interactions in the solar atmosphere
Authors: Hudson, Hugh S.; MacKinnon, Alec; Szydlarski, Mikolaj;
Carlsson, Mats
2020MNRAS.491.4852H Altcode: 2019arXiv191001186H; 2019MNRAS.tmp.3116H
High-energy particles enter the solar atmosphere from Galactic or
solar coronal sources, and produce 'albedo' emission from the quiet
Sun that is now observable across a wide range of photon energies. The
interaction of high-energy particles in a stellar atmosphere depends
essentially upon the joint variation of the magnetic field and plasma
density, which heretofore has been characterized parametrically as
P ∝ Bα with P the gas pressure and B the magnitude of
the magnetic field. We re-examine that parametrization by using a
self-consistent 3D MHD model (Bifrost) and show that this relationship
tends to P ∝ B3.5 ± 0.1 based on the visible portions
of the sample of open-field flux tubes in such a model, but with large
variations from point to point. This scatter corresponds to the strong
meandering of the open-field flux tubes in the lower atmosphere, which
will have a strong effect on the prediction of the emission anisotropy
(limb brightening). The simulations show that much of the open flux
in coronal holes originates in weak-field regions within the granular
pattern of the convective motions seen in the simulations.

---------------------------------------------------------
Title: SmallSat Solar Axion and Activity X-ray Imager (SSAXI)
Authors: Hong, J.; Romaine, S.; Kenter, A.; Moore, C.; Reeves, K.;
Ramsey, B.; Kilaru, K.; Vogel, J.; Ruz Armendariz, J.; Hudson, H.;
Perez, K.
2020AAS...23527101H Altcode:
The axion is a promising dark matter candidate as well as a solution
to the strong charge-parity (CP) problem in quantum chromodynamics
(QCD). We describe a new concept for SmallSat Solar Axion and Activity
X-ray Telescope (SSAXI) to search for solar axions or axion-like
particles (ALPs) and to monitor solar activity over a wide dynamic
range. SSAXI aims to unambiguously identify X-rays converted from
axions in the solar magnetic field along the line of sight to the
solar core, effectively imaging the solar core. SSAXI employs Miniature
lightweight Wolter-I focusing X-ray optics (MiXO) and monolithic CMOS
X-ray sensors in a compact package. The wide energy range (0.5 - 5 keV)
of SSAXI can easily distinguish spectra of axion-converted X-rays
from solar X-ray spectra, while encompassing the prime energy band
(3 - 4.5 keV) of axion-converted X-rays. The high angular resolution
(30 arcsec) and large field of view (40 arcmin) in SSAXI will easily
resolve the enhanced X-ray flux over the 3 arcmin wide solar core
while fully covering the X-ray activity over the entire solar disc. The
fast readout in the inherently radiation tolerant CMOS X-ray sensors
enables high resolution spectroscopy over a wide dynamic range with a
broad range of operational temperatures. We present multiple mission
implementation options for SSAXI under ESPA class. SSAXI will operate
in a Sun-synchronous orbit for 1 yr preferably near a solar minimum
to accumulate sufficient X-ray photon statistics.

---------------------------------------------------------
Title: A correlation in the waiting-time distributions of solar flares
Authors: Hudson, Hugh S.
2020MNRAS.491.4435H Altcode: 2019arXiv190808749H; 2019MNRAS.tmp.2713H
In a limited sample of isolated solar active regions, we find that the
waiting times between flares may correlate well with flare magnitudes
as determined by the Geostationary Operational Environmental Satellites
(GOES) soft X-ray fluxes. A 'build-up and release' (BUR) scenario for
magnetic energy storage in the solar corona suggests the existence
of such a relationship, relating the slowly varying subphotospheric
energy sources to the sudden coronal energy releases of flares and
coronal mass ejections (CMEs). Substantial amounts of research effort
had not previously found any obvious observational evidence for such
a BUR process. This has posed a puzzle since coronal magnetic energy
storage represents the consensus view of the basic flare mechanism. We
have revisited the GOES soft X-ray flare statistics for any evidence
of correlations, using two isolated active regions, and have found
significant evidence for a 'saturation' correlation. Rather than a
'reset' form of this relaxation, in which the time before a flare
correlates with its magnitude, the 'saturation' relationship results
in the time after the flare showing the correlation. The observed
correlation competes with the 'obscuration' effect of reduced GOES
sensitivity following a strong event, by which weaker events can
be under-reported systematically. This complicates the observed
correlation, and we discuss several approaches to remedy this.

---------------------------------------------------------
Title: NuSTAR observations of the quietest Sun
Authors: Hannah, I. G.; Cooper, K.; Grefenstette, B.; Glesener, L.;
Krucker, S.; Smith, D. M.; Hudson, H. S.; White, S. M.; Kuhar, M.
2019AGUFMSH41F3335H Altcode:
Observing X-rays (above a few keV) from the Sun provides a
direct insight into energy release (heating and/or particle
acceleration) in the solar atmosphere. Targeting the faintest
X-ray emission allows the study of the smallest flares and
eruption, and crucially their contribution to heating the solar
atmosphere. NuSTAR is an astrophysics telescope that uses directly
focusing X-rays optics to detect weak X-rays from the Sun. We have
observed the Sun many times since the start of solar pointings
in Sep 2014 through to our latest observations in 2019. See <A
href="http://ianan.github.io/nsovr/">http://ianan.github.io/nsovr/</A>
for an overview. During the current solar minimum, when the Sun is
devoid of active regions and presenting the very quietest levels of
activity, NuSTAR has targeted the Sun several times. We have detected
X-rays from a variety of sources: large diffuse sources, steady compact
sources, brief flares/brightenings and small eruptions. The NuSTAR
X-ray images of these weak sources are related to features seen at
other wavelengths, such as in softer X-rays with Hinode/XRT and EUV
with SDO/AIA. Crucially, NuSTAR's imaging spectroscopy allows us to
obtain and fit the X-ray spectrum from these small events determining
their thermal (and potentially non-thermal) properties. We will present
some of the latest solar observations with NuSTAR as we go through
the current solar minimum.

---------------------------------------------------------
Title: Lyman-alpha Variability During Solar Flares Over Solar Cycle
24 Using GOES-15/EUVS-E
Authors: Milligan, R. O.; Hudson, H. S.; Chamberlin, P.; Hannah, I. G.
2019AGUFMSA11B3221M Altcode:
The chromospheric Lyman-alpha line of hydrogen (Lyα; 1216Å) is
the strongest emission line in the solar spectrum. Fluctuations in
Lyα are known to drive changes in the dynamics and composition of
planetary atmospheres, and many space weather monitoring platforms
have included Lyα photometers to measure such changes. However, few
instruments have had the capacity to capture Lyα enhancements during
solar flares. The EUV Sensor (EUVS) on board the 15th Geostationary
Operational Environmental Satellite (GOES-15) has been monitoring the
full-disk solar Lyα irradiance on 10s timescales over the course of
Solar Cycle 24. These data, thus far little used in the astrophysics
community, show variability on all time scales, and here we describe
flare-associated emissions via a statistical study of 477 M- and X-class
flares observed in Lyα emission by GOES-15/EUVS-E. We find a strong
impulsive-phase component of the flare emission, but without as much of
the rapid variability characteristic of the hard X-ray bremsstrahlung
or microwave gyrosynchrotron emissions defining this phase. The vast
majority (98%) of these flares produced Lyα enhancements of 10% or
less above background levels, with a maximum increase of ~30%. However,
the energy contained in this flare excess averages about 10 times
more than that of the GOES soft X-ray energy (1-8Å; also a driver
of atmospheric fluctuations), with as much as a factor of 100 in some
events. Conversely, flares that occurred closer to the solar limb were
found to exhibit less of a Lyα enhancement due to either absorption
by the solar atmosphere along the line of sight, foreshortening of the
flare ribbons, or occultation by the solar disk. This center-to-limb
variation was verified through a joint observation of an X-class
flare that appeared on the limb as viewed by GOES, but was closer to
disk center as viewed by the MAVEN spacecraft in orbit around Mars,
which also carries a Lyα photometer. The MAVEN data showed a ~60%
higher flux increase relative to the GOES data.

---------------------------------------------------------
Title: Cosmic Rays Across the Rainbow Bridge: Particle Interactions
in a Magnetized Plasma Atmosphere
Authors: Hudson, H. S.; MacKinnon, A.; Szydlarski, M.; Carlsson, M.
2019AGUFMSH21B..02H Altcode:
High-energy particles enter the solar atmosphere from Galactic or solar
sources, producing gamma-ray "albedo” radiations. These emissions come
from the quiet Sun and from the large-scale corona, as well as from
the vicinity of flares, and have been observed across a wide range of
photon energies (MeV to GeV). The interaction of high-energy particles
in the solar atmosphere depends essentially upon the joint variation of
the magnetic field and the gas, and this has heretofore generally been
characterized parametrically as P ~ Balpha (Zweibel &amp;
Haber 1983), with P the gas pressure and B the magnitude of the magnetic
field. We have checked this relationship with a Bifrost 3D MHD model,
approximating the particle transport as a guiding-center motion along
"open” (large-scale) fieldlines. We find alpha ~ 2.2 in the strongest
(kG) fields in the simulation, but individual field structures have
widely disparate relationships. The scatter corresponds to the strong
meandering of the open-field flux tubes in the lower atmosphere and
to their incessant dynamics. We discuss this and other aspects of
the interactions of high-energy particles with the Sun (the "solar
Størmer problem”).

---------------------------------------------------------
Title: Principles Of Heliophysics: a textbook on the universal
processes behind planetary habitability
Authors: Schrijver, Karel; Bagenal, Fran; Bastian, Tim; Beer,
Juerg; Bisi, Mario; Bogdan, Tom; Bougher, Steve; Boteler, David;
Brain, Dave; Brasseur, Guy; Brownlee, Don; Charbonneau, Paul; Cohen,
Ofer; Christensen, Uli; Crowley, Tom; Fischer, Debrah; Forbes, Terry;
Fuller-Rowell, Tim; Galand, Marina; Giacalone, Joe; Gloeckler, George;
Gosling, Jack; Green, Janet; Guetersloh, Steve; Hansteen, Viggo;
Hartmann, Lee; Horanyi, Mihaly; Hudson, Hugh; Jakowski, Norbert;
Jokipii, Randy; Kivelson, Margaret; Krauss-Varban, Dietmar; Krupp,
Norbert; Lean, Judith; Linsky, Jeff; Longcope, Dana; Marsh, Daniel;
Miesch, Mark; Moldwin, Mark; Moore, Luke; Odenwald, Sten; Opher, Merav;
Osten, Rachel; Rempel, Matthias; Schmidt, Hauke; Siscoe, George;
Siskind, Dave; Smith, Chuck; Solomon, Stan; Stallard, Tom; Stanley,
Sabine; Sojka, Jan; Tobiska, Kent; Toffoletto, Frank; Tribble, Alan;
Vasyliunas, Vytenis; Walterscheid, Richard; Wang, Ji; Wood, Brian;
Woods, Tom; Zapp, Neal
2019arXiv191014022S Altcode:
This textbook gives a perspective of heliophysics in a way that
emphasizes universal processes from a perspective that draws attention
to what provides Earth (and similar (exo-)planets) with a relatively
stable setting in which life as we know it can thrive. The book is
intended for students in physical sciences in later years of their
university training and for beginning graduate students in fields of
solar, stellar, (exo-)planetary, and planetary-system sciences.

---------------------------------------------------------
Title: Joint X-Ray, EUV, and UV Observations of a Small Microflare
Authors: Hannah, Iain G.; Kleint, Lucia; Krucker, Säm; Grefenstette,
Brian W.; Glesener, Lindsay; Hudson, Hugh S.; White, Stephen M.;
Smith, David M.
2019ApJ...881..109H Altcode: 2018arXiv181209214H
We present the first joint observation of a small microflare in X-rays
with the Nuclear Spectroscopic Telescope ARray (NuSTAR), in UV with the
Interface Region Imaging Spectrograph (IRIS), and in EUV with the Solar
Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). These
combined observations allow us to study the hot coronal and cooler
chromospheric/transition region emission from the microflare. This small
microflare peaks from 2016 July 26 23:35 to 23:36 UT, in both NuSTAR,
SDO/AIA, and IRIS. Spatially, this corresponds to a small loop visible
in the SDO/AIA Fe XVIII emission, which matches a similar structure
lower in the solar atmosphere seen by IRIS in SJI1330 and 1400 Å. The
NuSTAR emission in both 2.5-4 and 4-6 keV is located in a source at
this loop location. The IRIS slit was over the microflaring loop,
and fits show little change in Mg II but do show intensity increases,
slight width enhancements, and redshifts in Si IV and O IV, indicating
that this microflare had most significance in and above the upper
chromosphere. The NuSTAR microflare spectrum is well fitted by a
thermal component of 5.1 MK and 6.2 × 1044 cm-3,
which corresponds to a thermal energy of 1.5 × 1026 erg,
making it considerably smaller than previously studied active region
microflares. No non-thermal emission was detected but this could be
due to the limited effective exposure time of the observation. This
observation shows that even ordinary features seen in UV can remarkably
have a higher-energy component that is clear in X-rays.

---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener,
Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Gburek, Szymon;
Steslicki, Marek; Allred, Joel C.; Battaglia, Marina; Baumgartner,
Wayne H.; Drake, James; Goetz, Keith; Grefenstette, Brian; Hannah,
Iain; Holman, Gordon D.; Inglis, Andrew; Ireland, Jack; Klimchuk,
James A.; Ishikawa, Shin-Nosuke; Kontar, Eduard; Massone, Anna-maria;
Piana, Michele; Ramsey, Brian; Schwartz, Richard A.; Woods, Thomas N.;
Chen, Bin; Gary, Dale E.; Hudson, Hugh S.; Kowalski, Adam; Warmuth,
Alexander; White, Stephen M.; Veronig, Astrid; Vilmer, Nicole
2019AAS...23422501C Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI), a SMEX mission concept
in Phase A, is the first-ever solar-dedicated, direct-imaging, hard
X-ray telescope. FOXSI provides a revolutionary new approach to
viewing explosive magnetic-energy release on the Sun by detecting
signatures of accelerated electrons and hot plasma directly in
and near the energy-release sites of solar eruptive events (e.g.,
solar flares). FOXSI's primary science objective is to understand the
mystery of how impulsive energy release leads to solar eruptions, the
primary drivers of space weather at Earth, and how those eruptions are
energized and evolve. FOXSI addresses three important science questions:
(1) How are particles accelerated at the Sun? (2) How do solar plasmas
get heated to high temperatures? (3) How does magnetic energy released
on the Sun lead to flares and eruptions? These fundamental physics
questions are key to our understanding of phenomena throughout
the Universe from planetary magnetospheres to black hole accretion
disks. FOXSI measures the energy distributions and spatial structure of
accelerated electrons throughout solar eruptive events for the first
time by directly focusing hard X-rays from the Sun. This naturally
enables high imaging dynamic range, while previous instruments have
typically been blinded by bright emission. FOXSI provides 20-100 times
more sensitivity as well as 20 times faster imaging spectroscopy
than previously available, probing physically relevant timescales
(&lt;1 second) never before accessible. FOXSI's launch in July 2022
is aligned with the peak of the 11-year solar cycle, enabling FOXSI
to observe the many large solar eruptions that are expected to take
place throughout its two-year mission.

---------------------------------------------------------
Title: Hard X-ray Spectroscopy of Six NuSTAR Microflares
Authors: Duncan, Jessie McBrayer; Glesener, Lindsay; Hannah, Iain;
Smith, David M.; Krucker, Sam; Hudson, Hugh S.; Grefenstette, Brian
2019AAS...23420404D Altcode:
Hard X-ray (HXR) emission in solar flares can originate from regions
of high temperature plasma, as well as from non-thermal particle
populations. Both of these sources of HXR radiation make solar
observation in this band important for study of flare energetics. NuSTAR
is the first HXR telescope with direct focusing optics, giving it
a dramatic increase in sensitivity over previous indirect imaging
methods. Here we present NuSTAR observation of six microflares from
one solar active region during a period of several hours on May 29th,
2018. Spectral fitting of emission at each flare time shows excess
high energy emission over an isothermal spectral component in all
six flares. The most likely origin of this excess could be either
additional volumes of high-temperature plasma, or non-thermally
accelerated particles. For each event, characterization of this excess
is presented, including determination of upper limits on the non-thermal
emission possible in events where it is not directly observed.

---------------------------------------------------------
Title: Eclipse Megamovie 2017 Successes and Potential For Future Work
Authors: Peticolas, L.; Hudson, H.; Johnson, C.; Zevin, D.; White,
V.; Oliveros, J. C. M.; Ruderman, I.; Koh, J.; Konerding, D.; Bender,
M.; Cable, C.; Kruse, B.; Yan, D.; Krista, L.; Collier, B.; Fraknoi,
A.; Pasachoff, J. M.; Filippenko, A. V.; Mendez, B.; McIntosh, S. W.;
Filippenko, N. L.
2019ASPC..516..337P Altcode:
In 2011, an "Eclipse Megamovie" was envisioned for the 2017 total
solar eclipse that would be created using the public's photographs of
the Sun's corona as frames in a movie illuminating dynamic changes in
the chromosphere and corona. On August 21, 2017, our team collected
photographs of the total solar eclipse from thousands of volunteers
with telescopes, DSLR (Digital Single-Lens Reflex) cameras, and mobile
device cameras setup across the path of totality. Our efforts resulted
in 1,190 photographers contributing 50,016 DSLR photographs in a final
open-source, public archive that is 766 GB in size. All photographs
in this archive are Creative Commons zero (CC0), making them freely
available for public use. From mobile devices, we obtained an archive
of 60,000 images, 211 GB in size. The first Eclipse Megamovie video
was compiled and made available to the public a few hours after the
Moon's shadow left the U.S. East Coast. For two weeks, additional
images were added to this video, as volunteers uploaded them to the
project server. The project also resulted in a comprehensive website
with 12,749 users sufficiently interested in the project to each create
a user profile on the website, several short documentaries, 190 articles
and press releases, open-source code for use in future related efforts,
and hundreds of public presentations across the country prior to the
eclipse. Information on how to access these resources is included in
this paper.

---------------------------------------------------------
Title: First detection of non-thermal emission in a NuSTAR solar
microflare
Authors: Glesener, Lindsay; Krucker, Sam; Duncan, Jessie McBrayer;
Hannah, Iain; Grefenstette, Brian; Chen, Bin; Smith, David M.; White,
Stephen M.; Hudson, Hugh S.
2019AAS...23422503G Altcode:
We report the detection of emission from a non-thermal electron
distribution in a small solar microflare observed by the Nuclear
Spectroscopic Telescope Array (NuSTAR). On 2017 August 21, NuSTAR
observed a solar active region for approximately an hour before the
region was eclipsed by the Moon. The active region emitted several
small microflares of GOES class A and smaller. In this work, we present
spectroscopy demonstrating evidence of electron acceleration in one of
these microflares (GOES class A5.7) and we compare energetic aspects
of the accelerated distribution to commonly studied larger flares. The
flaring plasma observed by NuSTAR, with supporting observation by
the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), is
well accounted for by a thick-target model of accelerated electrons
collisionally thermalizing within the loop, akin to the "coronal
thick target" behavior occasionally observed in larger flares. Future
observations of this kind will enable understanding of how flare
particle acceleration changes across energy scales, and will aid
the push toward the observational regime of nanoflares, which are a
possible source of significant coronal heating.

---------------------------------------------------------
Title: Chapter 9 - High-Energy Solar Physics
Authors: Hudson, H. S.; MacKinnon, A. L.
2019sgsp.book..301H Altcode:
This chapter deals generally with the high-energy astrophysics of the
Sun, specifically with solar flares and coronal mass ejections (CMEs),
but it also touches on the whole range of nonthermality or departures
from Maxwellian distributions in solar plasmas. Radio, x-ray, and
Î³-ray observations provide primary remote-sensing observations
of these departures, but such signatures can be hidden by brighter
thermal emissions that may not be as fundamental in physics events. The
solar paradigm for flare/CME development appears to match many of the
new stellar observations of similar phenomena, but the limitations of
observational sensitivity mean that we have few direct observations of
the expected hard x-rays and none at all of the Î³-rays that could
confirm this.

---------------------------------------------------------
Title: First high-resolution look at the quiet Sun with ALMA at 3mm
Authors: Nindos, A.; Alissandrakis, C. E.; Bastian, T. S.; Patsourakos,
S.; De Pontieu, B.; Warren, H.; Ayres, T.; Hudson, H. S.; Shimizu,
T.; Vial, J. -C.; Wedemeyer, S.; Yurchyshyn, V.
2018A&A...619L...6N Altcode: 2018arXiv181005223N
We present an overview of high-resolution quiet Sun observations,
from disk center to the limb, obtained with the Atacama Large
millimeter and sub-millimeter Array (ALMA) at 3 mm. Seven quiet-Sun
regions were observed at a resolution of up to 2.5″ by 4.5″. We
produced both average and snapshot images by self-calibrating the ALMA
visibilities and combining the interferometric images with full-disk
solar images. The images show well the chromospheric network, which,
based on the unique segregation method we used, is brighter than the
average over the fields of view of the observed regions by ∼305
K while the intranetwork is less bright by ∼280 K, with a slight
decrease of the network/intranetwork contrast toward the limb. At 3
mm the network is very similar to the 1600 Å images, with somewhat
larger size. We detect, for the first time, spicular structures,
rising up to 15″ above the limb with a width down to the image
resolution and brightness temperature of ∼1800 K above the local
background. No trace of spicules, either in emission or absorption,
is found on the disk. Our results highlight the potential of ALMA for
the study of the quiet chromosphere.

---------------------------------------------------------
Title: NuSTAR's observations of tiny flares and big eruptions
Authors: Hannah, Iain; Grefenstette, Brian; Glesener, Lindsey; Krucker,
Sam; Smith, David; Hudson, Hugh; White, Stephen; Kuhar, Matej
2018csc..confE.118H Altcode:
NuSTAR is an astrophysics X-ray telescope, with direct imaging
spectroscopy providing a unique sensitivity for observing the Sun above
2.5keV. Targeting the faintest X-ray emission from the solar atmosphere
allows the study of the smallest flares, and their contribution to
heating the corona. However, it can also be used to observe weak
high-coronal sources that are associated with the energy release in
large, but occulted, eruptions. NuSTAR has observed the Sun over a
dozen times since Sep 2014, through to our latest observations in
2018: see http://ianan.github.io/nsovr/ for a quicklook overview of
NuSTAR's solar observations. We will present some of the latest solar
observations with NuSTAR and compare them to the emission seen at
lower energy wavelengths, particularly in EUV with SDO/AIA and also
the derived Fe18 emission.

---------------------------------------------------------
Title: The association of the Hale sector boundary with RHESSI solar
flares and active longitudes
Authors: Loumou, K.; Hannah, I. G.; Hudson, H. S.
2018A&A...618A...9L Altcode: 2018arXiv180805866L
Context. The heliospheric magnetic field (HMF) is structured into large
sectors of positive and negative polarity. The parts of the boundary
between these sectors where the change in polarity matches that of
the leading-to-following sunspot polarity in that solar hemisphere,
are called Hale sector boundaries (HSB). Aims: We investigate
the flare occurrence rate near HSBs and the association between HSBs
and active longitudes. Methods: Previous work determined the
times HSBs were at solar central meridian, using the detection of
the HMF sector boundary crossing at the Earth. In addition to this,
we use a new approach which finds the HSB locations at all times
by determining them from potential field source surface (PFSS)
extrapolations of photospheric magnetograms. We use the RHESSI
X-ray flare list for comparison to the HSB as it provides accurate
flare locations over 14 years, from February 2002 to February 2016,
covering both Cycles 23 and 24. For the active longitude positions we
use previously published work based on sunspot observations. 
Results: We find that the two methods of determining the HSB generally
agree and that 41% (Cycle 23) and 47% (Cycle 24) of RHESSI flares occur
within 30° of the PFSS determined-HSB. The behaviour of the HSBs varies
over the two Cycles studied, and as expected they swap in hemisphere
as the Cycles change. The HSBs and active longitudes do overlap but
not consistently. They often move at different rates relative to each
other (and the Carrington solar rotation rate) and these vary over
each Cycle. The HSBs provide a useful additional activity indicator,
particularly during periods when active longitudes are difficult
to determine.

---------------------------------------------------------
Title: The Relationship between Long-Duration Gamma-Ray Flares and
Solar Cosmic Rays
Authors: Hudson, Hugh S.
2018IAUS..335...49H Altcode: 2017arXiv171105583H
A characteristic pattern of solar hard X-ray emission, first identified
in SOL1969-03-30 by Frost &amp; Dennis (1971), turns out to have a close
association with the prolonged high-energy gamma-ray emission originally
observed by Forrest et al. (1985). This identification has become clear
via the observations of long-duration γ-ray flares by the Fermi/LAT
experiment, for example in the event SOL2014-09-01. The distinctive
features of these events include flat hard X-ray spectra extending
well above 100 keV, a characteristic pattern of time development,
low-frequency gyrosynchrotron peaks, CME association, and gamma-rays
identifiable with pion decay originating in GeV ions. The identification
of these events with otherwise known solar structures nevertheless
remains elusive, in spite of the wealth of EUV imagery available from
SDO/AIA. The quandary is that these events have a clear association with
SEPs in the high corona, and yet the gamma-ray production implicates
the photosphere itself, despite the strong mirror force that should
focus the particles away from the Sun We discuss the morphology of
these phenomena and propose a solution to this problem.

---------------------------------------------------------
Title: On the observation of a classical loop-prominence system
during the 2017 September 10 flare
Authors: Martinez Oliveros, Juan Carlos; Hudson, Hugh; Krucker, Säm;
Guevara Gomez, Juan Camilo
2018shin.confE.258M Altcode:
We report observations of white-light ejecta in the low corona after
the 2017 September 10 flare, using data from the Helioseismic and
Magnetic Imager (HMI) of the Solar Dynamics Observatory. We report the
observation of a classical loop-prominence system, but are brighter
than expected and possibly seen here in the continuum rather than
line emission. We studied the spatial and temporal relation between
RHESSI X-ray and the white-light emissions. We also studied the HMI
spectroscopic data to determine the most probable emission mechanism
that can explain the observation of the loop-prominence system.

---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener,
Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel C.; Chen,
Bin; Battaglia, Marina; Drake, James Frederick; Gary, Dale E.; Goetz,
Keith; Gburek, Szymon; Grefenstette, Brian; Hannah, Iain G.; Holman,
Gordon; Hudson, Hugh S.; Inglis, Andrew R.; Ireland, Jack; Ishikawa,
Shin-nosuke; Klimchuk, James A.; Kontar, Eduard; Kowalski, Adam F.;
Massone, Anna Maria; Piana, Michele; Ramsey, Brian; Schwartz, Richard;
Steslicki, Marek; Ryan, Daniel; Warmuth, Alexander; Veronig, Astrid;
Vilmer, Nicole; White, Stephen M.; Woods, Thomas N.
2018tess.conf40444C Altcode:
We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer
(SMEX) Heliophysics mission that is currently undergoing a Phase A
concept study. FOXSI will provide a revolutionary new perspective on
energy release and particle acceleration on the Sun. FOXSI's primary
instrument, the Direct Spectroscopic Imager (DSI), is a direct imaging
X-ray spectrometer with higher dynamic range and better than 10x the
sensitivity of previous instruments. Flown on a 3-axis-stabilized
spacecraft in low-Earth orbit, DSI uses high-angular-resolution
grazing-incidence focusing optics combined with state-of-the-art
pixelated solid-state detectors to provide direct imaging of solar hard
X-rays for the first time. DSI is composed of a pair of X-ray telescopes
with a 14-meter focal length enabled by a deployable boom. DSI has a
field of view of 9 arcminutes and an angular resolution of better than 8
arcsec FWHM; it will cover the energy range from 3 up to 50-70 keV with
a spectral resolution of better than 1 keV. DSI will measure each photon
individually and will be able to create useful images at a sub-second
temporal resolution. FOXSI will also measure soft x-ray emission down
to 0.8 keV with a 0.25 keV resolution with its secondary instrument,
the Spectrometer for Temperature and Composition (STC) provided by
the Polish Academy of Sciences. Making use of an attenuator-wheel and
high-rate-capable detectors, FOXSI will be able to observe the largest
flares without saturation while still maintaining the sensitivity to
detect X-ray emission from weak flares, escaping electrons, and hot
active regions. This presentation will cover the data products and
software that can be expected from FOXSI and how they could be used
by the community.

---------------------------------------------------------
Title: GeV Particles in the Inner Heliosphere
Authors: Hudson, Hugh S.; MacKinnon, Alec
2018tess.conf22301H Altcode:
The Fermi/LAT observations of GeV solar gamma rays, both from flares
and from two distingishable quiet-Sun sources, make the cosmic-ray
environment of the inner heliosphere topically interesting. The
Parker Solar Probe and Solar Orbiter will explore this territory in
new ways. We discuss recent developments related both to flare/CME
processes and to the quiet Sun, noting that the solar equivalent of
the Stoermer theory remains to be worked out. We describe extensive
shower products from the GeV particles in the solar atmosphere (and
in the atmosphere of Venus) as another item of interest. ~

---------------------------------------------------------
Title: Do "Last Best" Flares Conclude Solar Cycles?
Authors: Hudson, Hugh S.
2018tess.conf31906H Altcode:
The Sun has just produced two major X-class flares in the waning phase
of Solar Cycle 24, to be thoroughly discussed at TESS 2018. This has
happened in previous cycles: Cycle 21: SOL1984-04-24 (X10; SMM
on furlough), Cycle 22: SOL1996-07-09 (X2.2 or maybe X2.6; first
sunquake), Cycle 23: SOL2006-12-13 (X3.4; Hinode), Cycle 24:
SOL2017-09-10 (X8.2; Fermi sustained gamma-rays). These could be
termed "last best" events, each one quite memorable and each one at
the very end of its 11-year cycle. Above a certain magnitude, flare
occurrence approaches a Poisson distribution and one can ignore the
solar cycle. These considerations don't reflect any particular physics,
and yet...

---------------------------------------------------------
Title: NuSTAR Detection of X-Ray Heating Events in the Quiet Sun
Authors: Kuhar, Matej; Krucker, Säm; Glesener, Lindsay; Hannah,
Iain G.; Grefenstette, Brian W.; Smith, David M.; Hudson, Hugh S.;
White, Stephen M.
2018ApJ...856L..32K Altcode: 2018arXiv180308365K
The explanation of the coronal heating problem potentially lies in
the existence of nanoflares, numerous small-scale heating events
occurring across the whole solar disk. In this Letter, we present
the first imaging spectroscopy X-ray observations of three quiet
Sun flares during the Nuclear Spectroscopic Telescope ARray (NuSTAR)
solar campaigns on 2016 July 26 and 2017 March 21, concurrent with
the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA)
observations. Two of the three events showed time lags of a few minutes
between peak X-ray and extreme ultraviolet emissions. Isothermal fits
with rather low temperatures in the range 3.2-4.1 MK and emission
measures of (0.6-15) × 1044 cm-3 describe
their spectra well, resulting in thermal energies in the range (2-6)
× 1026 erg. NuSTAR spectra did not show any signs of a
nonthermal or higher temperature component. However, as the estimated
upper limits of (hidden) nonthermal energy are comparable to the thermal
energy estimates, the lack of a nonthermal component in the observed
spectra is not a constraining result. The estimated Geostationary
Operational Environmental Satellite (GOES) classes from the fitted
values of temperature and emission measure fall between 1/1000 and
1/100 A class level, making them eight orders of magnitude fainter in
soft X-ray flux than the largest solar flares.

---------------------------------------------------------
Title: Exploring the Sun with ALMA
Authors: Bastian, T. S.; Bárta, M.; Brajša, R.; Chen, B.; Pontieu,
B. D.; Gary, D. E.; Fleishman, G. D.; Hales, A. S.; Iwai, K.; Hudson,
H.; Kim, S.; Kobelski, A.; Loukitcheva, M.; Shimojo, M.; Skokić,
I.; Wedemeyer, S.; White, S. M.; Yan, Y.
2018Msngr.171...25B Altcode:
The Atacama Large Millimeter/submillimeter Array (ALMA) Observatory
opens a new window onto the Universe. The ability to perform continuum
imaging and spectroscopy of astrophysical phenomena at millimetre and
submillimetre wavelengths with unprecedented sensitivity opens up new
avenues for the study of cosmology and the evolution of galaxies, the
formation of stars and planets, and astrochemistry. ALMA also allows
fundamentally new observations to be made of objects much closer
to home, including the Sun. The Sun has long served as a touchstone
for our understanding of astrophysical processes, from the nature of
stellar interiors, to magnetic dynamos, non-radiative heating, stellar
mass loss, and energetic phenomena such as solar flares. ALMA offers
new insights into all of these processes.

---------------------------------------------------------
Title: Eclipse Megamovie 2017: How did we do?
Authors: Hudson, Hugh; Bender, Mark; Collier, Braxton; Johnson,
Calvin; Koh, Justin; Konerding, David; Martinez Oliveros, Juan Carlos;
Peticolas, Laura; White, Vivian; Zevin, Dan
2018AAS...23122003H Altcode:
The Eclipse Megamovie program, as set up for the Great American Eclipse
of 21 August 2017, achived a massive volunteer participation, making
maximal use existing equipment but with coordinated training. Everything
worked fine, and the archive entered the public domain on Friday,
October 6. It comprises about 800 GB of data from DSLR cameras and
telescopes. An additional 200 GB of data were obtained by smartphone
cameras operating a dedicated free app. The massive oversampling made
possible by the many (about 2500) volunteer observers has opened
new parameter space for tracking coronal and chromospheric time
development. Fortuitously some solar activity appeared during the
90-minute period of totality, including a C-class flare and an ongoing
CME. At the smartphone level, with the advantage of precise GPS timing,
we have data on solar structure via the timing of Baily's Beads at the
2nd and 3rd contacts. The Megamovie archive is an historical first,
and we hope that it has already been a springboard for citizen-science
projects. We discuss the execution of the program, presenting some of
the 2017 science plans and results. We expect that the eclipse of 2024
will be better still.

---------------------------------------------------------
Title: Detection of 3-Minute Oscillations in Full-Disk Lyman-alpha
Emission During A Solar Flare
Authors: Milligan, R. O.; Ireland, J.; Fleck, B.; Hudson, H. S.;
Fletcher, L.; Dennis, B. R.
2017AGUFMSH41A2739M Altcode:
We report the detection of chromospheric 3-minute oscillations in
disk-integrated EUV irradiance observations during a solar flare. A
wavelet analysis of detrended Lyman-alpha (from GOES/EUVS) and
Lyman continuum (from SDO/EVE) emission from the 2011 February 15
X-class flare revealed a 3-minute period present during the flare's
main phase. The formation temperature of this emission locates this
radiation to the flare's chromospheric footpoints, and similar behaviour
is found in the SDO/AIA 1600A and 1700A channels, which are dominated
by chromospheric continuum. The implication is that the chromosphere
responds dynamically at its acoustic cutoff frequency to an impulsive
injection of energy. Since the 3-minute period was not found at hard
X-ray energies (50-100 keV) in RHESSI data we can state that this
3-minute oscillation does not depend on the rate of energization of, or
energy deposition by, non-thermal electrons. However, a second period of
120 s found in both hard X-ray and chromospheric emission is consistent
with episodic electron energization on 2-minute timescales. Our
finding on the 3-minute oscillation suggests that chromospheric
mechanical energy should be included in the flare energy budget, and
the fluctuations in the Lyman-alpha line may influence the composition
and dynamics of planetary atmospheres during periods of high activity.

---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission
Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.;
Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.;
Drake, J. F.; Gary, D. E.; Goetz, K.; Gburek, S.; Grefenstette, B.;
Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland,
J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.;
Massone, A. M.; Piana, M.; Ramsey, B.; Schwartz, R.; Steslicki, M.;
Turin, P.; Ryan, D.; Warmuth, A.; Veronig, A.; Vilmer, N.; White,
S. M.; Woods, T. N.
2017AGUFMSH44A..07C Altcode:
We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer
(SMEX) Heliophysics mission that is currently undergoing a Phase A
concept study. FOXSI will provide a revolutionary new perspective
on energy release and particle acceleration on the Sun. FOXSI is
a direct imaging X-ray spectrometer with higher dynamic range and
better than 10x the sensitivity of previous instruments. Flown
on a 3-axis-stabilized spacecraft in low-Earth orbit, FOXSI uses
high-angular-resolution grazing-incidence focusing optics combined
with state-of-the-art pixelated solid-state detectors to provide direct
imaging of solar hard X-rays for the first time. FOXSI is composed of
a pair of x-ray telescopes with a 14-meter focal length enabled by a
deployable boom. Making use of a filter-wheel and high-rate-capable
solid-state detectors, FOXSI will be able to observe the largest flares
without saturation while still maintaining the sensitivity to detect
x-ray emission from weak flares, escaping electrons, and hot active
regions. This mission concept is made possible by past experience with
similar instruments on two FOXSI sounding rocket flights, in 2012 and
2014, and on the HEROES balloon flight in 2013. FOXSI's hard X-ray
imager has a field of view of 9 arcminutes and an angular resolution
of better than 8 arcsec; it will cover the energy range from 3 up to
50-70 keV with a spectral resolution of better than 1 keV; and it will
have sub-second temporal resolution.

---------------------------------------------------------
Title: Anticipated Results from the FOXSI SMEX Mission
Authors: Shih, A. Y.; Christe, S.; Krucker, S.; Glesener, L.;
Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.;
Drake, J. F.; Gary, D. E.; Gburek, S.; Goetz, K.; Grefenstette, B.;
Gubarev, M.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.;
Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski,
A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Ryan, D.; Schwartz,
R.; Steslicki, M.; Turin, P.; Veronig, A.; Vilmer, N.; Warmuth, A.;
White, S. M.; Woods, T. N.
2017AGUFMSH43C..03S Altcode:
While there have been significant advances in our understanding
of impulsive energy release at the Sun since the advent of RHESSI
observations, there is a clear need for new X-ray observations that
can capture the full range of emission in flares (e.g., faint coronal
sources near bright chromospheric sources), follow the intricate
evolution of energy release and changes in morphology, and search
for the signatures of impulsive energy release in even the quiescent
Sun. The FOXSI Small Explorer (SMEX) mission, currently undergoing a
Phase A concept study, combines state-of-the-art grazing-incidence
focusing optics with pixelated solid-state detectors to provide
direct imaging of hard X-rays for the first time on a solar
observatory. FOXSI's X-ray observations will provide quantitative
information on (1) the non-thermal populations of accelerated electrons
and (2) the thermal plasma distributions at the high temperatures
inaccessible through other wavelengths. FOXSI's major science questions
include: Where are electrons accelerated and on what time scales? Where
do escaping flare-accelerated electrons originate? What is the energy
input of accelerated electrons into the chromosphere and corona? How
much do flare-like processes heat the corona above active regions? Here
we present examples with simulated observations to show how FOXSI's
capabilities will address and resolve these and other questions.

---------------------------------------------------------
Title: Solar Coronal Events with Extended Hard X-ray and Gamma-ray
Emission
Authors: Hudson, H. S.
2017AGUFMSH44B..02H Altcode:
A characteristic pattern of solar hard X-ray emission, first identified
in SOL1969-03-31 by Frost &amp; Dennis (1971) now has been linked to
prolonged high-energy gamma-ray emission detected by the Fermi/LAT
experiment, for example in SOL2014-09-01. The distinctive features
of these events include flat hard X-ray spectra extending well above
100 keV, a characteristic pattern of time development, low-frequency
gyrosynchrotron peaks, CME association, and gamma-rays identifiable
with pion decay originating in GeV ions. The identification of these
events with otherwise known solar structures nevertheless remains
elusive, in spite of the wealth of imagery available from AIA. The
quandary is that these events have a clear association with CMEs
in the high corona, and yet the gamma-ray production implicates the
photosphere itself. The vanishingly small loss cone in the nominal
acceleration region makes this extremely difficult. I propose direct
inward advection of a part of the SEP particle population, as created
on closed field structures, as a possible resolution of this puzzle,
and note that this requires retracting magnetic structures on long
time scales following the flare itself.

---------------------------------------------------------
Title: The First ALMA Observation of a Solar Plasmoid Ejection from
an X-Ray Bright Point
Authors: Shimojo, M.; Hudson, H. S.; White, S. M.; Bastian, T.;
Iwai, K.
2017AGUFMSH41A2754S Altcode:
Eruptive phenomena are important features of energy releases events,
such solar flares, and have the potential to improve our understanding
of the dynamics of the solar atmosphere. The 304 A EUV line of helium,
formed at around 10^5 K, is found to be a reliable tracer of such
phenomena, but the determination of physical parameters from such
observations is not straightforward. We have observed a plasmoid
ejection from an X-ray bright point simultaneously with ALMA, SDO/AIA,
and Hinode/XRT. This paper reports the physical parameters of the
plasmoid obtained by combining the radio, EUV, and X-ray data. As
a result, we conclude that the plasmoid can consist either of
(approximately) isothermal ∼10^5 K plasma that is optically thin
at 100 GHz, or a ∼10^4 K core with a hot envelope. The analysis
demonstrates the value of the additional temperature and density
constraints that ALMA provides, and future science observations with
ALMA will be able to match the spatial resolution of space-borne and
other high-resolution telescopes.

---------------------------------------------------------
Title: High-Energy Aspects of Small-Scale Energy Release at the Sun
Authors: Glesener, L.; Vievering, J. T.; Wright, P. J.; Hannah,
I. G.; Panchapakesan, S. A.; Ryan, D.; Krucker, S.; Hudson, H. S.;
Grefenstette, B.; White, S. M.; Smith, D. M.; Marsh, A.; Kuhar, M.;
Christe, S.; Buitrago-Casas, J. C.; Musset, S.; Inglis, A. R.
2017AGUFMSM33E..04G Altcode:
Large, powerful solar flares have been investigated in detail for
decades, but it is only recently that high-energy aspects of small
flares could be measured. These small-scale energy releases offer
the opportunity to examine how particle acceleration characteristics
scale down, which is critical for constraining energy transfer theories
such as magnetic reconnection. Probing to minuscule flare sizes also
brings us closer to envisioning the characteristics of the small
"nanoflares" that may be responsible for heating the corona. A new
window on small-scale flaring activity is now opening with the use of
focusing hard X-ray instruments to observe the Sun. Hard X-rays are
emitted by flare-accelerated electrons and strongly heated plasma,
providing a relatively direct method of measuring energy release
and particle acceleration properties. This work will show the first
observations of sub-A class microflares using the FOXSI sounding
rocket and the NuSTAR astrophysics spacecraft, both of which directly
focus hard X-rays but have limited observing time on the Sun. These
instruments serve as precursors to a spacecraft version of FOXSI, which
will explore energy release across the entire range of flaring activity.

---------------------------------------------------------
Title: First NuSTAR Limits on Quiet Sun Hard X-Ray Transient Events
Authors: Marsh, Andrew J.; Smith, David M.; Glesener, Lindsay;
Hannah, Iain G.; Grefenstette, Brian W.; Caspi, Amir; Krucker, Säm;
Hudson, Hugh S.; Madsen, Kristin K.; White, Stephen M.; Kuhar, Matej;
Wright, Paul J.; Boggs, Steven E.; Christensen, Finn E.; Craig,
William W.; Hailey, Charles J.; Harrison, Fiona A.; Stern, Daniel;
Zhang, William W.
2017ApJ...849..131M Altcode: 2017arXiv171105385M
We present the first results of a search for transient hard X-ray (HXR)
emission in the quiet solar corona with the Nuclear Spectroscopic
Telescope Array (NuSTAR) satellite. While NuSTAR was designed as
an astrophysics mission, it can observe the Sun above 2 keV with
unprecedented sensitivity due to its pioneering use of focusing
optics. NuSTAR first observed quiet-Sun regions on 2014 November 1,
although out-of-view active regions contributed a notable amount
of background in the form of single-bounce (unfocused) X-rays. We
conducted a search for quiet-Sun transient brightenings on timescales
of 100 s and set upper limits on emission in two energy bands. We set
2.5-4 keV limits on brightenings with timescales of 100 s, expressed
as the temperature T and emission measure EM of a thermal plasma. We
also set 10-20 keV limits on brightenings with timescales of 30, 60,
and 100 s, expressed as model-independent photon fluxes. The limits in
both bands are well below previous HXR microflare detections, though
not low enough to detect events of equivalent T and EM as quiet-Sun
brightenings seen in soft X-ray observations. We expect future
observations during solar minimum to increase the NuSTAR sensitivity
by over two orders of magnitude due to higher instrument livetime and
reduced solar background.

---------------------------------------------------------
Title: Formation of the thermal infrared continuum in solar flares
Authors: Simões, Paulo J. A.; Kerr, Graham S.; Fletcher, Lyndsay;
Hudson, Hugh S.; Giménez de Castro, C. Guillermo; Penn, Matt
2017A&A...605A.125S Altcode: 2017arXiv170609867S
 Aims: Observations of the Sun with the Atacama Large Millimeter
Array have now started, and the thermal infrared will regularly be
accessible from the NSF's Daniel K. Inouye Solar Telescope. Motivated by
the prospect of these new data, and by recent flare observations in the
mid infrared, we set out here to model and understand the source of the
infrared continuum in flares, and to explore its diagnostic capability
for the physical conditions in the flare atmosphere. Methods:
We use the one-dimensional (1D) radiation hydrodynamics code RADYN
to calculate mid-infrared continuum emission from model atmospheres
undergoing sudden deposition of energy by non-thermal electrons. Results: We identify and characterise the main continuum thermal
emission processes relevant to flare intensity enhancement in the
mid- to far-infrared (2-200 μm) spectral range as free-free emission
on neutrals and ions. We find that the infrared intensity evolution
tracks the energy input to within a second, albeit with a lingering
intensity enhancement, and provides a very direct indication of the
evolution of the atmospheric ionisation. The prediction of highly
impulsive emission means that, on these timescales, the atmospheric
hydrodynamics need not be considered in analysing the mid-IR signatures.

---------------------------------------------------------
Title: Microflare Heating of an Active Region Observed with NuSTAR,
Hinode/XRT, and SDO/AIA
Authors: Wright, Paul James; Hannah, Iain; Grefenstette, Brian;
Glesener, Lindsay; Krucker, Sam; Hudson, Hugh S.; Smith, David M.;
Marsh, Andrew; White, Stephen M.; Kuhar, Matej
2017SPD....4810802W Altcode:
We present the first joint observation of a GOES equivalent A0.2
microflare that occurred on the 29 Apr 2015 with Hinode/XRT and
NuSTAR. During the three hours of combined observation we observe
distinctive loop heating in the soft X-rays from Hinode/XRT, and
the hottest channels from SDO/AIA. Crucially the impulsive phase of
this microflare was also observed by NuSTAR, a highly sensitive hard
X-ray (2.5-80 keV; Harrison et al. 2013) focussing optics imaging
spectrometer. The NuSTAR spectrum before and after the microflare
is well-fitted by a single thermal model of about 3.3 - 3.5 MK, but
at the impulsive phase shows additional material up to 10 MK. This
higher temperature emission is confirmed when we produce the DEM
using a combination of SDO/AIA, Hinode/XRT, and NuSTAR data. During
the impulsive phase of the microflare we determine the heating rate to
be about 3 x 1025 erg s-1. Although non-thermal
emission is not detected we find upper-limits that are consistent with
the required heating rate.

---------------------------------------------------------
Title: The thermal infrared continuum in solar flares
Authors: Fletcher, Lyndsay; Simoes, Paulo; Kerr, Graham Stewart;
Hudson, Hugh S.; Gimenez de Castro, C. Guillermo; Penn, Matthew J.
2017SPD....4810821F Altcode:
Observations of the Sun with the Atacama Large Millimeter Array have
now started, and the thermal infrared will regularly be accessible
from the NSF’s Daniel K. Inouye Solar Telescope. Motivated by the
prospect of these new observations, and by recent flare detections in
the mid infrared, we set out here to model and understand the source
of the infrared continuum in flares, and to explore its diagnostic
capability for the physical conditions in the flare atmosphere. We use
the 1D radiation hydrodynamics code RADYN to calculate mid-infrared
continuum emission from model atmospheres undergoing sudden deposition
of energy by non-thermal electrons. We identify and characterise the
main continuum thermal emission processes relevant to flare intensity
enhancement in the mid- to far-infrared (2-200 micron) spectral range
as free-free emission on neutrals and ions. We find that the infrared
intensity evolution tracks the energy input to within a second,
albeit with a lingering intensity enhancement, and provides a very
direct indication of the evolution of the atmospheric ionization. The
prediction of highly impulsive emission means that, on these timescales,
the atmospheric hydrodynamics need not be considered in analysing the
mid-IR signatures.

---------------------------------------------------------
Title: Solar Commissioning Observations of the Sun with ALMA
Authors: White, Stephen M.; Shimojo, Masumi; Bastian, Timothy S.;
Iwai, Kazumasa; Hales, Antonio; Brajsa, Roman; Skokic, Ivica; Kim,
Sujin; Hudson, Hugh S.; Loukitcheva, Maria; Wedemeyer, Sven
2017SPD....4820402W Altcode:
PI-led science observations have commenced with the Atacama
Large Millimeter-submillimeter Array (ALMA) following an extensive
commissioning effort. This talk will summarize that effort and discuss
some of the scientific results derived from the commissioning data. As
the solar cycle declines, ALMA observations will mainly address
chromospheric science topics. Examples of data obtained during
commissioning, both from the interferometer and from single-dish
observations, will be presented. The temperatures of the layers that
ALMA is most sensitive to have been determined for the two frequency
bands currently used for solar observations. Curious behavior in a
sunspot umbra and an observations of a small chromospheric ejection
will be discussed.

---------------------------------------------------------
Title: Results from NuSTAR: Dynamics and time evolution in a sub-A
class hard X-ray flare
Authors: Glesener, Lindsay; Krucker, Sam; Hannah, Iain; Hudson,
Hugh S.; Grefenstette, Brian; White, Stephen M.; Smith, David M.;
Marsh, Andrew
2017SPD....4810803G Altcode:
We report a NuSTAR observation of a solar microflare,
SOL2015-09-01T04. Although it was too faint to be observed by the
GOES X-ray Sensor, we estimate the flare to be an A0.2 class flare in
brightness. This flare, with only ∼5 counts s-1 detector-1 observed
by RHESSI, is fainter than any hard X-ray (HXR) flare in the existing
literature. The flare occurred during a solar pointing by the highly
sensitive NuSTAR astrophysical observatory, which used its direct
focusing optics to produce detailed HXR flare spectra and images. The
flare exhibits HXR properties commonly observed in larger flares,
including a fast rise and more gradual decay, earlier peaking time with
higher energy, similar spatial dimensions to the RHESSI microflares,
and a high-energy excess beyond an isothermal spectral component
during the impulsive phase. The flare is small in emission measure,
temperature, and energy, though not in physical size; observations
are consistent with its arising via the interaction of at least two
magnetic loops. We estimate the increase in thermal energy at the time
of the flare to be 1.8×1027 ergs. The observation suggests that flares
do indeed scale down to extremely small energies and retain what we
customarily think of as “flarelike” properties.

---------------------------------------------------------
Title: Magnetic Properties of Solar Active Regions that Govern Large
Solar Flares and Eruptions
Authors: Toriumi, Shin; Schrijver, Carolus J.; Harra, Louise; Hudson,
Hugh S.; Nagashima, Kaori
2017SPD....4820001T Altcode:
Strong flares and CMEs are often produced from active regions (ARs). In
order to better understand the magnetic properties and evolutions of
such ARs, we conducted statistical investigations on the SDO/HMI and
AIA data of all flare events with GOES levels &gt;M5.0 within 45 deg
from the disk center for 6 years from May 2010 (from the beginning to
the declining phase of solar cycle 24). Out of the total of 51 flares
from 29 ARs, more than 80% have delta-sunspots and about 15% violate
Hale’s polarity rule. We obtained several key findings including
(1) the flare duration is linearly proportional to the separation
of the flare ribbons (i.e., scale of reconnecting magnetic fields)
and (2) CME-eruptive events have smaller sunspot areas. Depending on
the magnetic properties, flaring ARs can be categorized into several
groups, such as spot-spot, in which a highly-sheared polarity inversion
line is formed between two large sunspots, and spot-satellite, where a
newly-emerging flux next to a mature sunspot triggers a compact flare
event. These results point to the possibility that magnetic structures
of the ARs determine the characteristics of flares and CMEs. In the
presentation, we will also show new results from the systematic flux
emergence simulations of delta-sunspot formation and discuss the
evolution processes of flaring ARs.

---------------------------------------------------------
Title: NuSTAR Hard X-Ray Observation of a Sub-A Class Solar Flare
Authors: Glesener, Lindsay; Krucker, Säm; Hannah, Iain G.; Hudson,
Hugh; Grefenstette, Brian W.; White, Stephen M.; Smith, David M.;
Marsh, Andrew J.
2017ApJ...845..122G Altcode: 2017arXiv170704770G
We report a Nuclear Spectroscopic Telescope Array (NuSTAR) observation
of a solar microflare, SOL2015-09-01T04. Although it was too faint
to be observed by the GOES X-ray Sensor, we estimate the event to be
an A0.1 class flare in brightness. This microflare, with only ∼5
counts s-1 detector-1 observed by the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI), is fainter
than any hard X-ray (HXR) flare in the existing literature. The
microflare occurred during a solar pointing by the highly sensitive
NuSTAR astrophysical observatory, which used its direct focusing optics
to produce detailed HXR microflare spectra and images. The microflare
exhibits HXR properties commonly observed in larger flares, including a
fast rise and more gradual decay, earlier peak time with higher energy,
spatial dimensions similar to the RHESSI microflares, and a high-energy
excess beyond an isothermal spectral component during the impulsive
phase. The microflare is small in emission measure, temperature,
and energy, though not in physical size; observations are consistent
with an origin via the interaction of at least two magnetic loops. We
estimate the increase in thermal energy at the time of the microflare
to be 2.4 × 1027 erg. The observation suggests that flares
do indeed scale down to extremely small energies and retain what we
customarily think of as “flare-like” properties.

---------------------------------------------------------
Title: The Eclipse Megamovie Project
Authors: Hudson, Hugh; Bender, Mark
2017S&T...134b..20H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Microflare Heating of a Solar Active Region Observed with
NuSTAR, Hinode/XRT, and SDO/AIA
Authors: Wright, Paul J.; Hannah, Iain G.; Grefenstette, Brian W.;
Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; Smith, David M.;
Marsh, Andrew J.; White, Stephen M.; Kuhar, Matej
2017ApJ...844..132W Altcode: 2017arXiv170606108W
NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has
observed several small microflares in its initial solar pointings. In
this paper, we present the first joint observation of a microflare
with NuSTAR and Hinode/XRT on 2015 April 29 at ∼11:29 UT. This
microflare shows the heating of material to several million Kelvin,
observed in soft X-rays with Hinode/XRT, and was faintly visible in
the extreme ultraviolet with SDO/AIA. For three of the four NuSTAR
observations of this region (pre-flare, decay, and post-flare phases),
the spectrum is well fitted by a single thermal model of 3.2-3.5 MK,
but the spectrum during the impulsive phase shows additional emission
up to 10 MK, emission equivalent to the A0.1 GOES class. We recover
the differential emission measure (DEM) using SDO/AIA, Hinode/XRT,
and NuSTAR, giving unprecedented coverage in temperature. We find that
the pre-flare DEM peaks at ∼3 MK and falls off sharply by 5 MK;
but during the microflare’s impulsive phase, the emission above 3
MK is brighter and extends to 10 MK, giving a heating rate of about
2.5× {10}25 erg s-1. As the NuSTAR spectrum is
purely thermal, we determined upper limits on the possible non-thermal
bremsstrahlung emission. We find that for the accelerated electrons to
be the source of heating, a power-law spectrum of δ ≥slant 7 with a
low-energy cutoff {E}c≲ 7 keV is required. In summary, this
first NuSTAR microflare strongly resembles much more powerful flares.

---------------------------------------------------------
Title: Observing the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA): High-Resolution Interferometric
Imaging
Authors: Shimojo, M.; Bastian, T. S.; Hales, A. S.; White, S. M.;
Iwai, K.; Hills, R. E.; Hirota, A.; Phillips, N. M.; Sawada, T.;
Yagoubov, P.; Siringo, G.; Asayama, S.; Sugimoto, M.; Brajša, R.;
Skokić, I.; Bárta, M.; Kim, S.; de Gregorio-Monsalvo, I.; Corder,
S. A.; Hudson, H. S.; Wedemeyer, S.; Gary, D. E.; De Pontieu, B.;
Loukitcheva, M.; Fleishman, G. D.; Chen, B.; Kobelski, A.; Yan, Y.
2017SoPh..292...87S Altcode: 2017arXiv170403236S
Observations of the Sun at millimeter and submillimeter wavelengths
offer a unique probe into the structure, dynamics, and heating of the
chromosphere; the structure of sunspots; the formation and eruption
of prominences and filaments; and energetic phenomena such as jets
and flares. High-resolution observations of the Sun at millimeter and
submillimeter wavelengths are challenging due to the intense, extended,
low-contrast, and dynamic nature of emission from the quiet Sun,
and the extremely intense and variable nature of emissions associated
with energetic phenomena. The Atacama Large Millimeter/submillimeter
Array (ALMA) was designed with solar observations in mind. The
requirements for solar observations are significantly different from
observations of sidereal sources and special measures are necessary
to successfully carry out this type of observations. We describe the
commissioning efforts that enable the use of two frequency bands,
the 3-mm band (Band 3) and the 1.25-mm band (Band 6), for continuum
interferometric-imaging observations of the Sun with ALMA. Examples of
high-resolution synthesized images obtained using the newly commissioned
modes during the solar-commissioning campaign held in December 2015
are presented. Although only 30 of the eventual 66 ALMA antennas
were used for the campaign, the solar images synthesized from the
ALMA commissioning data reveal new features of the solar atmosphere
that demonstrate the potential power of ALMA solar observations. The
ongoing expansion of ALMA and solar-commissioning efforts will continue
to enable new and unique solar observing capabilities.

---------------------------------------------------------
Title: The Solar X-Ray Limb
Authors: Battaglia, Marina; Hudson, Hugh S.; Hurford, Gordon J.;
Krucker, Säm; Schwartz, Richard A.
2017ApJ...843..123B Altcode: 2017arXiv170511044B
We describe a new technique to measure the height of the X-ray limb
with observations from occulted X-ray flare sources as observed
by the RHESSI (the Reuven Ramaty High-Energy Spectroscopic Imager)
satellite. This method has model dependencies different from those
present in traditional observations at optical wavelengths, which depend
upon detailed modeling involving radiative transfer in a medium with
complicated geometry and flows. It thus provides an independent and
more rigorous measurement of the “true” solar radius, which means
that of the mass distribution. RHESSI’s measurement makes use of the
flare X-ray source’s spatial Fourier components (the visibilities),
which are sensitive to the presence of the sharp edge at the lower
boundary of the occulted source. We have found a suitable flare event
for analysis, SOL2011-10-20T03:25 (M1.7), and report a first result
from this novel technique here. Using a four-minute integration
over the 3-25 keV photon energy range, we find {R}{{X} -
{ray}}=960.11+/- 0.15+/- 0.29 arcsec, at 1 au, where the uncertainties
include statistical uncertainties from the method and a systematic
error. The standard VAL-C model predicts a value of 959.94 arcsec,
which is about 1σ below our value.

---------------------------------------------------------
Title: Observing the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
Authors: White, S. M.; Iwai, K.; Phillips, N. M.; Hills, R. E.; Hirota,
A.; Yagoubov, P.; Siringo, G.; Shimojo, M.; Bastian, T. S.; Hales,
A. S.; Sawada, T.; Asayama, S.; Sugimoto, M.; Marson, R. G.; Kawasaki,
W.; Muller, E.; Nakazato, T.; Sugimoto, K.; Brajša, R.; Skokić, I.;
Bárta, M.; Kim, S.; Remijan, A. J.; de Gregorio, I.; Corder, S. A.;
Hudson, H. S.; Loukitcheva, M.; Chen, B.; De Pontieu, B.; Fleishmann,
G. D.; Gary, D. E.; Kobelski, A.; Wedemeyer, S.; Yan, Y.
2017SoPh..292...88W Altcode: 2017arXiv170504766W
The Atacama Large Millimeter/submillimeter Array (ALMA) radio
telescope has commenced science observations of the Sun starting
in late 2016. Since the Sun is much larger than the field of view
of individual ALMA dishes, the ALMA interferometer is unable to
measure the background level of solar emission when observing the
solar disk. The absolute temperature scale is a critical measurement
for much of ALMA solar science, including the understanding of energy
transfer through the solar atmosphere, the properties of prominences,
and the study of shock heating in the chromosphere. In order to provide
an absolute temperature scale, ALMA solar observing will take advantage
of the remarkable fast-scanning capabilities of the ALMA 12 m dishes
to make single-dish maps of the full Sun. This article reports on the
results of an extensive commissioning effort to optimize the mapping
procedure, and it describes the nature of the resulting data. Amplitude
calibration is discussed in detail: a path that uses the two loads in
the ALMA calibration system as well as sky measurements is described
and applied to commissioning data. Inspection of a large number of
single-dish datasets shows significant variation in the resulting
temperatures, and based on the temperature distributions, we derive
quiet-Sun values at disk center of 7300 K at λ =3 mm and 5900 K at
λ =1.3 mm. These values have statistical uncertainties of about 100
K, but systematic uncertainties in the temperature scale that may be
significantly larger. Example images are presented from two periods
with very different levels of solar activity. At a resolution of about
25″, the 1.3 mm wavelength images show temperatures on
the disk that vary over about a 2000 K range. Active regions and plages
are among the hotter features, while a large sunspot umbra shows up as
a depression, and filament channels are relatively cool. Prominences
above the solar limb are a common feature of the single-dish images.

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Title: The First ALMA Observation of a Solar Plasmoid Ejection from
an X-Ray Bright Point
Authors: Shimojo, Masumi; Hudson, Hugh S.; White, Stephen M.; Bastian,
Timothy S.; Iwai, Kazumasa
2017ApJ...841L...5S Altcode: 2017arXiv170404881S
Eruptive phenomena such as plasmoid ejections or jets are important
features of solar activity and have the potential to improve our
understanding of the dynamics of the solar atmosphere. Such ejections
are often thought to be signatures of the outflows expected in
regions of fast magnetic reconnection. The 304 Å EUV line of helium,
formed at around 105 K, is found to be a reliable tracer
of such phenomena, but the determination of physical parameters
from such observations is not straightforward. We have observed
a plasmoid ejection from an X-ray bright point simultaneously at
millimeter wavelengths with ALMA, at EUV wavelengths with SDO/AIA,
and in soft X-rays with Hinode/XRT. This paper reports the physical
parameters of the plasmoid obtained by combining the radio, EUV, and
X-ray data. As a result, we conclude that the plasmoid can consist
either of (approximately) isothermal ∼105 K plasma that
is optically thin at 100 GHz, or a ∼104 K core with a
hot envelope. The analysis demonstrates the value of the additional
temperature and density constraints that ALMA provides, and future
science observations with ALMA will be able to match the spatial
resolution of space-borne and other high-resolution telescopes.

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Title: Science Objective: Understanding Energy Transport by Alfvénic
Waves in Solar Flares
Authors: Reep, Jeffrey W.; Warren, Harry P.; Leake, James E.; Tarr,
Lucas A.; Russell, Alexander J. B.; Kerr, Graham S.; Hudson, Hugh S.
2017arXiv170201667R Altcode:
Solar flares are driven by the release of magnetic energy from
reconnection events in the solar corona, whereafter energy is
transported to the chromosphere, heating the plasma and causing the
characteristic radiative losses. In the collisional thick-target model,
electrons accelerated to energies exceeding 10 keV traverse the corona
and impact the chromosphere, where they deposit their energy through
collisions with the much denser plasma in the lower atmosphere. While
there are undoubtedly high energy non-thermal electrons accelerated
in flares, it is unclear whether these electron beams are the sole
mechanism of energy transport, or whether they only dominate in certain
phases of the flare's evolution. Alfvénic waves are generated during
the post-reconnection relaxation of magnetic field lines, so it is
important to examine their role in energy transport.

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Title: Exploring impulsive solar magnetic energy release and particle
acceleration with focused hard X-ray imaging spectroscopy
Authors: Christe, Steven; Krucker, Samuel; Glesener, Lindsay; Shih,
Albert; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel; Battaglia,
Marina; Chen, Bin; Drake, James; Dennis, Brian; Gary, Dale; Gburek,
Szymon; Goetz, Keith; Grefenstette, Brian; Gubarev, Mikhail; Hannah,
Iain; Holman, Gordon; Hudson, Hugh; Inglis, Andrew; Ireland, Jack;
Ishikawa, Shinosuke; Klimchuk, James; Kontar, Eduard; Kowalski, Adam;
Longcope, Dana; Massone, Anna-Maria; Musset, Sophie; Piana, Michele;
Ramsey, Brian; Ryan, Daniel; Schwartz, Richard; Stęślicki, Marek;
Turin, Paul; Warmuth, Alexander; Wilson-Hodge, Colleen; White, Stephen;
Veronig, Astrid; Vilmer, Nicole; Woods, Tom
2017arXiv170100792C Altcode:
How impulsive magnetic energy release leads to solar eruptions and how
those eruptions are energized and evolve are vital unsolved problems
in Heliophysics. The standard model for solar eruptions summarizes
our current understanding of these events. Magnetic energy in the
corona is released through drastic restructuring of the magnetic
field via reconnection. Electrons and ions are then accelerated by
poorly understood processes. Theories include contracting loops,
merging magnetic islands, stochastic acceleration, and turbulence at
shocks, among others. Although this basic model is well established,
the fundamental physics is poorly understood. HXR observations
using grazing-incidence focusing optics can now probe all of the key
regions of the standard model. These include two above-the-looptop
(ALT) sources which bookend the reconnection region and are likely
the sites of particle acceleration and direct heating. The science
achievable by a direct HXR imaging instrument can be summarized by the
following science questions and objectives which are some of the most
outstanding issues in solar physics (1) How are particles accelerated
at the Sun? (1a) Where are electrons accelerated and on what time
scales? (1b) What fraction of electrons is accelerated out of the
ambient medium? (2) How does magnetic energy release on the Sun lead
to flares and eruptions? A Focusing Optics X-ray Solar Imager (FOXSI)
instrument, which can be built now using proven technology and at modest
cost, would enable revolutionary advancements in our understanding of
impulsive magnetic energy release and particle acceleration, a process
which is known to occur at the Sun but also throughout the Universe.

---------------------------------------------------------
Title: Magnetic Properties of Solar Active Regions That Govern Large
Solar Flares and Eruptions
Authors: Toriumi, Shin; Schrijver, Carolus J.; Harra, Louise K.;
Hudson, Hugh; Nagashima, Kaori
2017ApJ...834...56T Altcode: 2016arXiv161105047T
Solar flares and coronal mass ejections (CMEs), especially the larger
ones, emanate from active regions (ARs). With the aim of understanding
the magnetic properties that govern such flares and eruptions, we
systematically survey all flare events with Geostationary Orbiting
Environmental Satellite levels of ≥M5.0 within 45° from disk center
between 2010 May and 2016 April. These criteria lead to a total of 51
flares from 29 ARs, for which we analyze the observational data obtained
by the Solar Dynamics Observatory. More than 80% of the 29 ARs are found
to exhibit δ-sunspots, and at least three ARs violate Hale’s polarity
rule. The flare durations are approximately proportional to the distance
between the two flare ribbons, to the total magnetic flux inside the
ribbons, and to the ribbon area. From our study, one of the parameters
that clearly determine whether a given flare event is CME-eruptive
or not is the ribbon area normalized by the sunspot area, which may
indicate that the structural relationship between the flaring region
and the entire AR controls CME productivity. AR characterization shows
that even X-class events do not require δ-sunspots or strong-field,
high-gradient polarity inversion lines. An investigation of historical
observational data suggests the possibility that the largest solar
ARs, with magnetic flux of 2 × 1023 Mx, might be able to
produce “superflares” with energies of the order of 1034
erg. The proportionality between the flare durations and magnetic
energies is consistent with stellar flare observations, suggesting a
common physical background for solar and stellar flares.

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Title: Evidence of Significant Energy Input in the Late Phase of a
Solar Flare from NuSTAR X-Ray Observations
Authors: Kuhar, Matej; Krucker, Säm; Hannah, Iain G.; Glesener,
Lindsay; Saint-Hilaire, Pascal; Grefenstette, Brian W.; Hudson, Hugh
S.; White, Stephen M.; Smith, David M.; Marsh, Andrew J.; Wright, Paul
J.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey,
Charles J.; Harrison, Fiona A.; Stern, Daniel; Zhang, William W.
2017ApJ...835....6K Altcode: 2017arXiv170107759K
We present observations of the occulted active region AR 12222
during the third Nuclear Spectroscopic Telescope ARray (NuSTAR) solar
campaign on 2014 December 11, with concurrent Solar Dynamics Observatory
(SDO)/AIA and FOXSI-2 sounding rocket observations. The active region
produced a medium-size solar flare 1 day before the observations, at
∼18 UT on 2014 December 10, with the post-flare loops still visible
at the time of NuSTAR observations. The time evolution of the source
emission in the SDO/AIA 335 Å channel reveals the characteristics
of an extreme-ultraviolet late-phase event, caused by the continuous
formation of new post-flare loops that arch higher and higher in
the solar corona. The spectral fitting of NuSTAR observations yields
an isothermal source, with temperature 3.8-4.6 MK, emission measure
(0.3-1.8) × 1046 cm-3, and density estimated at
(2.5-6.0) × 108 cm-3. The observed AIA fluxes
are consistent with the derived NuSTAR temperature range, favoring
temperature values in the range of 4.0-4.3 MK. By examining the
post-flare loops’ cooling times and energy content, we estimate that
at least 12 sets of post-flare loops were formed and subsequently cooled
between the onset of the flare and NuSTAR observations, with their total
thermal energy content an order of magnitude larger than the energy
content at flare peak time. This indicates that the standard approach
of using only the flare peak time to derive the total thermal energy
content of a flare can lead to a large underestimation of its value.

---------------------------------------------------------
Title: NuSTAR's X-ray search for high energy emission from weakly
flaring active regions
Authors: Hannah, I. G.; Grefenstette, B.; Glesener, L.; Krucker, S.;
Hudson, H. S.; Smith, D. M.; White, S.; Marsh, A.; Wright, P. J.;
Kuhar, M.
2016AGUFMSH11D..07H Altcode:
The NuSTAR X-ray focusing optics telescope, provides highly sensitivity
imaging spectroscopy over 2-78 keV. Most of NuSTAR's time is spent on
targets outside of the solar system but some is devoted to the Sun;
solar observations began in late 2014 (Grefenstette et al. 2016
ApJ). Although not optimized for solar observations it is highly
capable of searching for the weak X-ray emission from high temperature
(&gt; 5MK) or non-thermal components from the weakly or non-flaring
Sun. Such emission provides strong constraints on the nature of
energy release during these more quiescent times. NuSTAR has observed
quiescent/non-flaring active regions from its first observations late in
2014, finding sources between 3.1-4.4 MK. These data placed strict upper
limits on higher temperature emission (Hannah et al. 2016 ApJL). These
observations had limited spectral dynamic range due to short effective
exposures (duration and high deadtime), restricting our ability to
detect higher temperature or non-thermal emission. With weakening
solar activity since then we present further observations during 2015
and 2016 of microflares and non-flaring active regions with longer
exposures (in part due to the decreasing detector deadtime). We also
present robust multi-thermal emission measure distributions obtained
by combining our NuSTAR observations with EUV data from SDO/AIA and
softer X-rays from Hinode/XRT.

---------------------------------------------------------
Title: Focusing Solar Hard X-rays: Expected Results from a FOXSI
Spacecraft
Authors: Glesener, L.; Christe, S.; Shih, A. Y.; Dennis, B. R.;
Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Ryan, D.; Inglis,
A. R.; Hannah, I. G.; Caspi, A.; Klimchuk, J. A.; Drake, J. F.;
Kontar, E.; Holman, G.; White, S. M.; Alaoui, M.; Battaglia, M.;
Vilmer, N.; Allred, J. C.; Longcope, D. W.; Gary, D. E.; Jeffrey,
N. L. S.; Musset, S.; Swisdak, M.
2016AGUFMSH13A2282G Altcode:
Over the course of two solar cycles, RHESSI has examined high-energy
processes in flares via high-resolution spectroscopy and imaging of
soft and hard X-rays (HXRs). The detected X-rays are the thermal
and nonthermal bremsstrahlung from heated coronal plasma and from
accelerated electrons, respectively, making them uniquely suited to
explore the highest-energy processes that occur in the corona. RHESSI
produces images using an indirect, Fourier-based method and has made
giant strides in our understanding of these processes, but it has also
uncovered intriguing new mysteries regarding energy release location,
acceleration mechanisms, and energy propagation in flares. Focusing
optics are now available for the HXR regime and stand poised to perform
another revolution in the field of high-energy solar physics. With
two successful sounding rocket flights completed, the Focusing Optics
X-ray Solar Imager (FOXSI) program has demonstrated the feasibility and
power of direct solar HXR imaging with its vastly superior sensitivity
and dynamic range. Placing this mature technology aboard a spacecraft
will offer a systematic way to explore high-energy aspects of the
solar corona and to address scientific questions left unanswered by
RHESSI. Here we present examples of such questions and show simulations
of expected results from a FOXSI spaceborne instrument to demonstrate
how these questions can be addressed with the focusing of hard X-rays.

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Title: Arcade Implosion Caused by a Filament Eruption in a Flare
Authors: Wang, Juntao; Simões, P. J. A.; Fletcher, L.; Thalmann,
J. K.; Hudson, H. S.; Hannah, I. G.
2016ApJ...833..221W Altcode: 2016arXiv161005931W
Coronal implosions—the convergence motion of plasmas and entrained
magnetic field in the corona due to a reduction in magnetic
pressure—can help to locate and track sites of magnetic energy
release or redistribution during solar flares and eruptions. We report
here on the analysis of a well-observed implosion in the form of an
arcade contraction associated with a filament eruption, during the
C3.5 flare SOL2013-06-19T07:29. A sequence of events including the
magnetic flux-rope instability and distortion, followed by a filament
eruption and arcade implosion, lead us to conclude that the implosion
arises from the transfer of magnetic energy from beneath the arcade
as part of the global magnetic instability, rather than due to local
magnetic energy dissipation in the flare. The observed net contraction
of the imploding loops, which is found also in nonlinear force-free
field extrapolations, reflects a permanent reduction of magnetic
energy underneath the arcade. This event shows that, in addition to
resulting in the expansion or eruption of an overlying field, flux-rope
instability can also simultaneously implode an unopened field due to
magnetic energy transfer. It demonstrates the “partial opening of
the field” scenario, which is one of the ways in 3D to produce a
magnetic eruption without violating the Aly-Sturrock hypothesis. In
the framework of this observation, we also propose a unification of
three main concepts for active region magnetic evolution, namely the
metastable eruption model, the implosion conjecture, and the standard
“CSHKP” flare model.

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Title: RHESSI/SAS Observations of the Optical Solar Limb Over More
Than 15 Years
Authors: Fivian, M. D.; Hudson, H. S.; Krucker, S.
2016AGUFMSH42B..08F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb with a cadence typically set at 100 samples/s.RHESSI
has observed the Sun continuously since its launch in early 2002, and we
have acquired a unique data set ranging over more than a full 11-year
solar cycle and consisting of about 4x10^10 single data points.The
optics has a point spread of about 4.5 arcsec FWHM imaging the red
continuum onto three linear CCD sensors with a pixel resolution of 1.7
arcsec.However, careful study of systematics, masking of contaminated
data, and accumulation of data over appropriate time intervals has
led to measurementswith sub-milli arcsec accuracy.Analyzing data for
an initial period in 2004, these measurements have led to the most
accurate oblateness measurement to date, 8.01+-0.14 milli arcsec
(Fivian et al., 2008), a value consistent with models predicting
an oblateness from surface rotation.An excess oblateness term can
be attributed to magnetic elements possibly located in the enhanced
network.We also study photometric properties of our data. Previous
observations of latitude-dependent brightness variations at the limb
had suggested the presence of a polar temperature excess as large
as 1.5 K.The RHESSI observations, made with a rotating telescope in
space, have great advantages in the rejection of systematic errors
in the very precise photometry required for such an observation.Our
measurements of latitude-dependent brightness variations at the limb
lead to a quadrupolar term (a pole-to-equator temperature variation)
of the order of 0.1 K, an order of magnitude smaller than previously
reported.We present the analysis of these unique data, an overview of
some results and we report on our progress as we apply our developed
analysis method to the whole 15 years of data.

---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission
Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.;
Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen,
B.; Drake, J. F.; Gary, D. E.; Goetz, K.; Grefenstette, B.; Hannah,
I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa,
S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.;
Piana, M.; Ramsey, B.; Gubarev, M.; Schwartz, R. A.; Steslicki, M.;
Ryan, D.; Turin, P.; Warmuth, A.; White, S. M.; Veronig, A.; Vilmer,
N.; Dennis, B. R.
2016AGUFMSH13A2281C Altcode:
We present FOXSI (Focusing Optics X-ray Solar Imager), a recently
proposed Small Explorer (SMEX) mission that will provide a revolutionary
new perspective on energy release and particle acceleration on the
Sun. FOXSI is a direct imaging X-ray spectrometer with higher dynamic
range and better than 10x the sensitivity of previous instruments. Flown
on a 3-axis stabilized spacecraft in low-Earth orbit, FOXSI uses
high-angular-resolution grazing-incidence focusing optics combined
with state-of-the-art pixelated solid-state detectors to provide direct
imaging of solar hard X-rays for the first time. FOXSI is composed of
two individual x-ray telescopes with a 14-meter focal length enabled by
a deployable boom. Making use of a filter-wheel and high-rate-capable
solid-state detectors, FOXSI will be able to observe the largest flares
without saturation while still maintaining the sensitivity to detect
x-ray emission from weak flares, escaping electrons, and hot active
regions. This SMEX mission is made possible by past experience with
similar instruments on two sounding rocket flights, in 2012 and 2014,
and on the HEROES balloon flight in 2013. FOXSI will image the Sun
with a field of view of 9 arcminutes and an angular resolution of
better than 8 arcsec; it will cover the energy range from 3 to 100
keV with a spectral resolution of better than 1 keV; and it will have
sub-second temporal resolution.

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Title: EVE Doppler Signatures in X-class Flares
Authors: Hudson, H. S.; Chamberlin, P. C.; MacKinnon, A.; Woods, T. N.
2016AGUFMSH13A2291H Altcode:
The MEGS-A spectroscopy of the Sun as a star has produced a greatdeal
of interesting solar physics, but without much exploitationyet
of its Doppler capabilities (see Chamberlin, 2016 for details).In
this presentation we examine the He II 304 line specifically andin
specifically on the database of X-class flares from MEGS-A'sactive
lifetime (34 events). The gradual-phase emission allows usto obtain
a local reference for the rest wavelength to good accuracy,and the
impulsive-phase centroid wavelengths typically shift to thered with
values consistent with those of Milligan and Dennis (2009),but with
smaller uncertainties. The exception events include SOL2014-02-25
(X4.9), which we find to have a blueshifted He IIline at an apparent
105 +- 5 km/s relative to its rest value. Thisevent also includes an
unusual coronal hard X-ray source and aFermi/LAT high-energy gamma-ray
emission. The EVE Doppler shiftanalysis can also be done at many other
emission lines, includingthose covered by MEGS-B in the 33-106 nm range.

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Title: Sunquake Generation by Coronal Magnetic Restructuring
Authors: Russell, A. J. B.; Mooney, M. K.; Leake, J. E.; Hudson, H. S.
2016ApJ...831...42R Altcode: 2016arXiv160208245R
Sunquakes are the surface signatures of acoustic waves in the Sun’s
interior that are produced by some but not all flares and coronal
mass ejections (CMEs). This paper explores a mechanism for sunquake
generation by the changes in magnetic field that occur during flares
and CMEs, using MHD simulations with a semiempirical FAL-C atmosphere to
demonstrate the generation of acoustic waves in the interior in response
to changing magnetic tilt in the corona. We find that Alfvén-sound
resonance combined with the ponderomotive force produces acoustic waves
in the interior with sufficient energy to match sunquake observations
when the magnetic field angle changes of the order of 10° in a region
where the coronal field strength is a few hundred gauss or more. The
most energetic sunquakes are produced when the coronal field is strong,
while the variation of magnetic field strength with height and the
timescale of the change in tilt are of secondary importance.

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Title: Flare differentially rotates sunspot on Sun's surface
Authors: Liu, Chang; Xu, Yan; Cao, Wenda; Deng, Na; Lee, Jeongwoo;
Hudson, Hugh S.; Gary, Dale E.; Wang, Jiasheng; Jing, Ju; Wang, Haimin
2016NatCo...713104L Altcode: 2016arXiv161002969L
Sunspots are concentrations of magnetic field visible on the solar
surface (photosphere). It was considered implausible that solar flares,
as resulted from magnetic reconnection in the tenuous corona, would
cause a direct perturbation of the dense photosphere involving bulk
motion. Here we report the sudden flare-induced rotation of a sunspot
using the unprecedented spatiotemporal resolution of the 1.6 m New
Solar Telescope, supplemented by magnetic data from the Solar Dynamics
Observatory. It is clearly observed that the rotation is non-uniform
over the sunspot: as the flare ribbon sweeps across, its different
portions accelerate (up to ~50° h-1) at different times
corresponding to peaks of flare hard X-ray emission. The rotation may be
driven by the surface Lorentz-force change due to the back reaction of
coronal magnetic restructuring and is accompanied by a downward Poynting
flux. These results have direct consequences for our understanding of
energy and momentum transportation in the flare-related phenomena.

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Title: Properties and Developments of Flaring Active Regions
Authors: Toriumi, Shin; Schrijver, Carolus J.; Harra, Louise K.;
Hudson, Hugh; Nagashima, Kaori
2016usc..confE..15T Altcode:
Larger flares and CMEs are often produced from active regions (ARs). In
order to better understand the magnetic properties and evolutions
of such ARs, we picked up all flare events with GOES levels &gt;M5.0
with heliocentric angles of &lt;45 deg in the period of May 2010 to
April 2016, which led to a total of 29 ARs with 51 flares. We analyzed
the observational data obtained by SDO and found that more than 80%
of the 29 ARs have delta-sunspots. Most of them can be classified
depending on their magnetic structures into (1) spot-spot, where a long
sheared polarity inversion line (PIL: characterized by flare ribbons)
is formed between two major sunspots, and (2) spot-satellite, where
a newly-emerging minor bipole next to a pre-existing spot creates a
compact PIL. The remaining minor groups are (3) quadrupole, where two
emerging bipoles produce a PIL in between, and (4) inter-AR, which
produces flares not from delta-spots but from between two separated
ARs. From statistical investigations we found for example that the
spot-spot group generally shows long-duration events due to large
coronal structures, while the spot-satellite has impulsive events
because of their compact magnetic nature. We will also present flux
emergence simulations and discuss their formation processes.

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Title: The Characteristics of Solar X-Class Flares and CMEs: A
Paradigm for Stellar Superflares and Eruptions?
Authors: Harra, Louise K.; Schrijver, Carolus J.; Janvier, Miho;
Toriumi, Shin; Hudson, Hugh; Matthews, Sarah; Woods, Magnus M.; Hara,
Hirohisa; Guedel, Manuel; Kowalski, Adam; Osten, Rachel; Kusano,
Kanya; Lueftinger, Theresa
2016SoPh..291.1761H Altcode: 2016SoPh..tmp..111H
This paper explores the characteristics of 42 solar X-class flares that
were observed between February 2011 and November 2014, with data from
the Solar Dynamics Observatory (SDO) and other sources. This flare
list includes nine X-class flares that had no associated CMEs. In
particular our aim was to determine whether a clear signature could
be identified to differentiate powerful flares that have coronal
mass ejections (CMEs) from those that do not. Part of the motivation
for this study is the characterization of the solar paradigm for
flare/CME occurrence as a possible guide to the stellar observations;
hence we emphasize spectroscopic signatures. To do this we ask the
following questions: Do all eruptive flares have long durations? Do
CME-related flares stand out in terms of active-region size vs. flare
duration? Do flare magnitudes correlate with sunspot areas, and, if so,
are eruptive events distinguished? Is the occurrence of CMEs related to
the fraction of the active-region area involved? Do X-class flares with
no eruptions have weaker non-thermal signatures? Is the temperature
dependence of evaporation different in eruptive and non-eruptive
flares? Is EUV dimming only seen in eruptive flares? We find only one
feature consistently associated with CME-related flares specifically:
coronal dimming in lines characteristic of the quiet-Sun corona,
i.e. 1 - 2 MK. We do not find a correlation between flare magnitude
and sunspot areas. Although challenging, it will be of importance to
model dimming for stellar cases and make suitable future plans for
observations in the appropriate wavelength range in order to identify
stellar CMEs consistently.

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Title: The First Focused Hard X-ray Images of the Sun with NuSTAR
Authors: Grefenstette, Brian W.; Glesener, Lindsay; Krucker, Säm;
Hudson, Hugh; Hannah, Iain G.; Smith, David M.; Vogel, Julia K.; White,
Stephen M.; Madsen, Kristin K.; Marsh, Andrew J.; Caspi, Amir; Chen,
Bin; Shih, Albert; Kuhar, Matej; Boggs, Steven E.; Christensen, Finn
E.; Craig, William W.; Forster, Karl; Hailey, Charles J.; Harrison,
Fiona A.; Miyasaka, Hiromasa; Stern, Daniel; Zhang, William W.
2016ApJ...826...20G Altcode: 2016arXiv160509738G
We present results from the the first campaign of dedicated solar
observations undertaken by the Nuclear Spectroscopic Telescope ARray
(NuSTAR) hard X-ray (HXR) telescope. Designed as an astrophysics
mission, NuSTAR nonetheless has the capability of directly imaging the
Sun at HXR energies (&gt;3 keV) with an increase in sensitivity of at
least two magnitude compared to current non-focusing telescopes. In
this paper we describe the scientific areas where NuSTAR will make
major improvements on existing solar measurements. We report on the
techniques used to observe the Sun with NuSTAR, their limitations
and complications, and the procedures developed to optimize solar
data quality derived from our experience with the initial solar
observations. These first observations are briefly described, including
the measurement of the Fe K-shell lines in a decaying X-class flare,
HXR emission from high in the solar corona, and full-disk HXR images
of the Sun.

---------------------------------------------------------
Title: Implications Of The Mid-IR For ALMA Flare Observations
Authors: Hudson, Hugh; Krucker, Sam; Penn, Matt; Simoes, Paulo
2016csss.confE..49H Altcode:
Poster presented at the Cool Stars 19 meeting in Uppsala, Sweden,
June 2016

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Title: The smallest hard X-ray flare?
Authors: Glesener, Lindsay; Krucker, Sam; Hannah, Iain; Smith, David
M.; Grefenstette, Brian; Marsh, Andrew; Hudson, Hugh S.; White,
Stephen M.; Chen, Bin
2016SPD....4740302G Altcode:
We report a NuSTAR observation of a small solar flare on 2015
September 1, estimated to be on the order of a GOES class A.05 flare
in brightness. This flare is fainter than any hard X-ray (HXR) flares
in the existing literature, and with a peak rate of only ∼5 counts
s-1 detector-1 observed by RHESSI, is effectively
the smallest that can just barely be detected by the current standard
(indirectly imaging) solar HXR instrumentation, though we expect
that smaller flares will continue to be discovered as instrumental
and observational techniques progress. The flare occurred during a
solar observation by the highly sensitive NuSTAR astrophysical HXR
spacecraft, which used its direct focusing optics to produce detailed
flare spectra and images. The flare exhibits properties commonly
observed in larger flares, including a fast rise and more gradual
decay, and similar spatial dimensions to the RHESSI microflares. We
will discuss the presence of non-thermal (flare-accelerated) electrons
during the impulsive phase. The flare is small in emission measure,
temperature, and energy, though not in physical dimensions. Its presence
is an indication that flares do indeed scale down to smaller energies
and retain what we customarily think of as “flarelike” properties.

---------------------------------------------------------
Title: BBSO/NST Observations of the Sudden Differential Rotation of
a Sunspot Caused by a Major Flare
Authors: Liu, Chang; Xu, Yan; Deng, Na; Cao, Wenda; Lee, Jeongwoo;
Hudson, Hugh S.; Gary, Dale E.; Wang, Jiasheng; Jing, Ju; Wang, Haimin
2016SPD....47.0615L Altcode:
Sunspots are concentrations of magnetic field visible on the solar
surface (photosphere), from which the field extends high into the
corona. Complex plasma motions that drag field in the photosphere can
build up free energy in the corona that powers solar eruptions. It
is known that solar flares and the often associated coronal ejections
(CMEs) can produce various radiations in the low atmosphere. However,
it was considered implausible that disturbances created in the tenuous
corona would cause a direct perturbation of the dense photosphere
involving bulk motion. Here we report the sudden rotational motion of
a sunspot clearly induced by a major solar flare (SOL2015-06-22T18:23
M6.6), using the unprecedented spatiotemporal resolution of the 1.6 m
New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO). It is
particularly striking that the rotation is not uniform over the sunspot:
as the flare ribbon sweeps across, its different portions accelerate
(up to ~50 degree per hour) at different times corresponding to peaks
of flare hard X-ray emission. The intensity and magnetic field of
the sunspot also change significantly associated with the flare. Our
results reveal an intrinsic relationship between the photospheric
plasma bulk motions and coronal energy release, with direct consequences
for our understanding of energy and momentum balance in the flare/CME
phenomenon. This work is mainly supported by NASA grants NNX13AF76G
and NNX13AG13G (LWS), and NNX16AF72G, and NSF grants AGS 1250818
and 1408703.

---------------------------------------------------------
Title: Science Objectives of the FOXSI Small Explorer Mission Concept
Authors: Shih, Albert Y.; Christe, Steven; Alaoui, Meriem; Allred,
Joel C.; Antiochos, Spiro K.; Battaglia, Marina; Buitrago-Casas,
Juan Camilo; Caspi, Amir; Dennis, Brian R.; Drake, James; Fleishman,
Gregory D.; Gary, Dale E.; Glesener, Lindsay; Grefenstette, Brian;
Hannah, Iain; Holman, Gordon D.; Hudson, Hugh S.; Inglis, Andrew R.;
Ireland, Jack; Ishikawa, Shin-Nosuke; Jeffrey, Natasha; Klimchuk, James
A.; Kontar, Eduard; Krucker, Sam; Longcope, Dana; Musset, Sophie; Nita,
Gelu M.; Ramsey, Brian; Ryan, Daniel; Saint-Hilaire, Pascal; Schwartz,
Richard A.; Vilmer, Nicole; White, Stephen M.; Wilson-Hodge, Colleen
2016SPD....47.0814S Altcode:
Impulsive particle acceleration and plasma heating at the Sun, from the
largest solar eruptive events to the smallest flares, are related to
fundamental processes throughout the Universe. While there have been
significant advances in our understanding of impulsive energy release
since the advent of RHESSI observations, there is a clear need for
new X-ray observations that can capture the full range of emission
in flares (e.g., faint coronal sources near bright chromospheric
sources), follow the intricate evolution of energy release and changes
in morphology, and search for the signatures of impulsive energy
release in even the quiescent Sun. The FOXSI Small Explorer (SMEX)
mission concept combines state-of-the-art grazing-incidence focusing
optics with pixelated solid-state detectors to provide direct imaging
of hard X-rays for the first time on a solar observatory. We present
the science objectives of FOXSI and how its capabilities will address
and resolve open questions regarding impulsive energy release at the
Sun. These questions include: What are the time scales of the processes
that accelerate electrons? How do flare-accelerated electrons escape
into the heliosphere? What is the energy input of accelerated electrons
into the chromosphere, and how is super-heated coronal plasma produced?

---------------------------------------------------------
Title: Chasing White-Light Flares
Authors: Hudson, H. S.
2016SoPh..291.1273H Altcode: 2016SoPh..tmp...77H
In this memoir I describe my life in research, mostly in the area
of solar physics. The recurring theme is "white-light flares,"
and several sections of this paper deal with this and related
phenomena; I wind up describing how I see the state of the art in
this still-interesting and crucially important (as it has been since
1859) area of flare research. I also describe my participation in two
long-lived satellite programs dedicated to solar observations (Yohkoh
and RHESSI) and elaborate on their discoveries. These have both helped
with white-light flares both directly and also with closely related
X-ray and γ -ray emissions), with the result that this article leans
heavily in that direction.

---------------------------------------------------------
Title: RHESSI/SAS Observations of the Optical Solar Limb Over More
Than 14 Years
Authors: Fivian, Martin; Hudson, Hugh S.; Krucker, Sam
2016SPD....47.1204F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb with a cadence typically set at 100 samples/s.RHESSI
has observed the Sun continuously since its launch in early 2002, and we
have acquired a unique data set ranging over more than a full 11-year
solar cycle and consisting of about 4x10^10 single data points.The
optics has a point spread of about 4.5 arcsec FWHM imaging the red
continuum onto three linear CCD sensors with a pixel resolution of 1.7
arcsec.However, careful study of systematics, masking of contaminated
data, and accumulation of data over appropriate time intervals has
led to measurementswith sub-milli arcsec accuracy.Analyzing data for
an initial period in 2004, these measurements have led to the most
accurate oblateness measurement to date, 8.01+-0.14 milli arcsec
(Fivian et al., 2008), a value consistent with models predicting
an oblateness from surface rotation.An excess oblateness term can
be attributed to magnetic elements possibly located in the enhanced
network.We also study photometric properties of our data. Previous
observations of latitude-dependent brightness variations at the limb
had suggested the presence of a polar temperature excess as large
as 1.5 K.The RHESSI observations, made with a rotating telescope in
space, have great advantages in the rejection of systematic errors
in the very precise photometry required for such an observation.Our
measurements of latitude-dependent brightness variations at the limb
lead to a quadrupolar term (a pole-to-equator temperature variation)
of the order of 0.1 K, an order of magnitude smaller than previously
reported.We present the analysis of these unique data, an overview of
some results and we report on our progress as we apply our developed
analysis method to the whole 14 years of data.

---------------------------------------------------------
Title: White-light flares, Hard X-Rays, and Heights
Authors: Martinez Oliveros, Juan Carlos; Hudson, Hugh S.; Krucker, Sam
2016SPD....47.0617M Altcode:
The white-light continuum of a solar flare was the first manifestation
of a solar flare ever detected. Nevertheless, its mechanisms remain
unknown, even today. Improved observations confirm the identification
of white-light continuum emission and hard X-rays during the impulsive
phase of a solar flare, both in space and in time, to within the
observational limits. Two events observed near the limb, but not
occulted by it (SOL2011-02-24 and SOL2012-02-18), show that these
emissions appear to have physical heights lower than predicted by models
by hundreds of kms, referring height to the location of optical-depth
unity at disk center in the 500 nm continuum. We describe these results
and place them in the context of the three extreme-limb events (within
about 1o) reported by Krucker et al. (2015). The electrons
responsible for hard X-ray bremsstrahlung coincide with the most intense
flare energy release, but we do not presently understand the physics
of energy transport nor the nature of particle acceleration apparently
taking place at heights below the preflare temperature minimum.

---------------------------------------------------------
Title: Solar Observations with the Atacama Large
Millimeter/submillimeter Array (ALMA)
Authors: Kobelski, A.; Bastian, T. S.; Bárta, M.; Brajša, R.; Chen,
B.; De Pontieu, B.; Fleishman, G.; Gary, D.; Hales, A.; Hills, R.;
Hudson, H.; Hurford, G.; Loukitcheva, M.; Iwai, K.; Krucker, S.;
Shimojo, M.; Skokić, I.; Wedemeyer, S.; White, S.; Yan, Y.; ALMA
Solar Development Team
2016ASPC..504..327K Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a
joint North American, European, and East Asian project that opens
the mm-sub mm wavelength part of the electromagnetic spectrum for
general astrophysical exploration, providing high-resolution imaging
in frequency bands currently ranging from 84 GHz to 950 GHz (300
microns to 3 mm). It is located in the Atacama desert in northern
Chile at an elevation of 5000 m. Despite being a general purpose
instrument, provisions have been made to enable solar observations
with ALMA. Radiation emitted at ALMA wavelengths originates mostly
from the chromosphere, which plays an important role in the transport
of matter and energy, and the in heating the outer layers of the solar
atmosphere. Despite decades of research, the solar chromosphere remains
a significant challenge: both to observe, owing to the complicated
formation mechanisms of currently available diagnostics; and to
understand, as a result of the complex nature of the structure and
dynamics of the chromosphere. ALMA has the potential to change the
scene substantially as it serves as a nearly linear thermometer at
high spatial and temporal resolution, enabling us to study the complex
interaction of magnetic fields and shock waves and yet-to-be-discovered
dynamical processes. Moreover, ALMA will play an important role in
the study of energetic emissions associated with solar flares at
sub-THz frequencies.

---------------------------------------------------------
Title: Solar Science with the Atacama Large Millimeter/Submillimeter
Array—A New View of Our Sun
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.;
Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu,
B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin,
P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz,
A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.;
Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary,
D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van
der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.;
Selhorst, C. L.; Barta, M.
2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new
powerful tool for observing the Sun at high spatial, temporal, and
spectral resolution. These capabilities can address a broad range
of fundamental scientific questions in solar physics. The radiation
observed by ALMA originates mostly from the chromosphere—a complex
and dynamic region between the photosphere and corona, which plays a
crucial role in the transport of energy and matter and, ultimately,
the heating of the outer layers of the solar atmosphere. Based on
first solar test observations, strategies for regular solar campaigns
are currently being developed. State-of-the-art numerical simulations
of the solar atmosphere and modeling of instrumental effects can help
constrain and optimize future observing modes for ALMA. Here we present
a short technical description of ALMA and an overview of past efforts
and future possibilities for solar observations at submillimeter and
millimeter wavelengths. In addition, selected numerical simulations
and observations at other wavelengths demonstrate ALMA's scientific
potential for studying the Sun for a large range of science cases.

---------------------------------------------------------
Title: Spectral and Imaging Observations of a White-light Solar
Flare in the Mid-infrared
Authors: Penn, Matt; Krucker, Säm; Hudson, Hugh; Jhabvala, Murzy;
Jennings, Don; Lunsford, Allen; Kaufmann, Pierre
2016ApJ...819L..30P Altcode: 2015arXiv151204449P
We report high-resolution observations at mid-infrared wavelengths of
a minor solar flare, SOL2014-09-24T17:50 (C7.0), using Quantum Well
Infrared Photodetector cameras at an auxiliary of the McMath-Pierce
telescope. The flare emissions, the first simultaneous observations
in two mid-infrared bands at 5.2 and 8.2 μ {{m}} with white-light and
hard X-ray coverage, revealed impulsive time variability with increases
on timescales of ∼4 s followed by exponential decay at ∼10 s in
two bright regions separated by about 13\prime\prime . The
brightest source is compact, unresolved spatially at the diffraction
limit (1\_\_AMP\_\_farcs;72 at 5.2 μ {{m}}). We identify the IR
sources as flare ribbons also seen in white-light emission at 6173 Å
observed by SDO/HMI, with twin hard X-ray sources observed by Reuven
Ramaty High Energy Solar Spectroscopic Imager, and with EUV sources
(e.g., 94 Å) observed by SDO/AIA. The two infrared points have
nearly the same flux density (fν, W m-2 Hz)
and extrapolate to a level of about an order of magnitude below that
observed in the visible band by HMI, but with a flux of more than two
orders of magnitude above the free-free continuum from the hot (∼15
MK) coronal flare loop observed in the X-ray range. The observations
suggest that the IR emission is optically thin; this constraint and
others suggest major contributions from a density less than about
4× {10}13 cm-3. We tentatively interpret this
emission mechanism as predominantly free-free emission in a highly
ionized but cool and rather dense chromospheric region.

---------------------------------------------------------
Title: The First X-Ray Imaging Spectroscopy of Quiescent Solar Active
Regions with NuSTAR
Authors: Hannah, Iain G.; Grefenstette, Brian W.; Smith, David M.;
Glesener, Lindsay; Krucker, Säm; Hudson, Hugh S.; Madsen, Kristin
K.; Marsh, Andrew; White, Stephen M.; Caspi, Amir; Shih, Albert Y.;
Harrison, Fiona A.; Stern, Daniel; Boggs, Steven E.; Christensen,
Finn E.; Craig, William W.; Hailey, Charles J.; Zhang, William W.
2016ApJ...820L..14H Altcode: 2016arXiv160301069H
We present the first observations of quiescent active regions (ARs)
using the Nuclear Spectroscopic Telescope Array (NuSTAR), a focusing
hard X-ray telescope capable of studying faint solar emission from
high-temperature and non-thermal sources. We analyze the first directly
imaged and spectrally resolved X-rays above 2 keV from non-flaring ARs,
observed near the west limb on 2014 November 1. The NuSTAR X-ray images
match bright features seen in extreme ultraviolet and soft X-rays. The
NuSTAR imaging spectroscopy is consistent with isothermal emission of
temperatures 3.1-4.4 MK and emission measures 1-8 × 1046
cm-3. We do not observe emission above 5 MK, but our short
effective exposure times restrict the spectral dynamic range. With
few counts above 6 keV, we can place constraints on the presence of an
additional hotter component between 5 and 12 MK of ∼ {10}46
cm-3 and ∼ {10}43 cm-3, respectively,
at least an order of magnitude stricter than previous limits. With
longer duration observations and a weakening solar cycle (resulting
in an increased livetime), future NuSTAR observations will have
sensitivity to a wider range of temperatures as well as possible
non-thermal emission.

---------------------------------------------------------
Title: Correlation of Hard X-Ray and White Light Emission in Solar
Flares
Authors: Kuhar, Matej; Krucker, Säm; Martínez Oliveros, Juan Carlos;
Battaglia, Marina; Kleint, Lucia; Casadei, Diego; Hudson, Hugh S.
2016ApJ...816....6K Altcode: 2015arXiv151107757K
A statistical study of the correlation between hard X-ray and white
light emission in solar flares is performed in order to search for a
link between flare-accelerated electrons and white light formation. We
analyze 43 flares spanning GOES classes M and X using observations
from the Reuven Ramaty High Energy Solar Spectroscopic Imager and
Helioseismic and Magnetic Imager. We calculate X-ray fluxes at 30
keV and white light fluxes at 6173 Å summed over the hard X-ray
flare ribbons with an integration time of 45 s around the peak hard-X
ray time. We find a good correlation between hard X-ray fluxes and
excess white light fluxes, with a highest correlation coefficient
of 0.68 for photons with energy of 30 keV. Assuming the thick target
model, a similar correlation is found between the deposited power by
flare-accelerated electrons and the white light fluxes. The correlation
coefficient is found to be largest for energy deposition by electrons
above ∼50 keV. At higher electron energies the correlation decreases
gradually while a rapid decrease is seen if the energy provided by
low-energy electrons is added. This suggests that flare-accelerated
electrons of energy ∼50 keV are the main source for white light
production.

---------------------------------------------------------
Title: ALMA Observations of the Sun in Cycle 4 and Beyond
Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.;
Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.;
Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson,
M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.;
Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov,
V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões,
P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig,
A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
2016arXiv160100587W Altcode:
This document was created by the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) in preparation of
the first regular observations of the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA), which are anticipated to start
in ALMA Cycle 4 in October 2016. The science cases presented here
demonstrate that a large number of scientifically highly interesting
observations could be made already with the still limited solar
observing modes foreseen for Cycle 4 and that ALMA has the potential
to make important contributions to answering long-standing scientific
questions in solar physics. With the proposal deadline for ALMA Cycle
4 in April 2016 and the Commissioning and Science Verification campaign
in December 2015 in sight, several of the SSALMON Expert Teams composed
strategic documents in which they outlined potential solar observations
that could be feasible given the anticipated technical capabilities
in Cycle 4. These documents have been combined and supplemented
with an analysis, resulting in recommendations for solar observing
with ALMA in Cycle 4. In addition, the detailed science cases also
demonstrate the scientific priorities of the solar physics community
and which capabilities are wanted for the next observing cycles. The
work on this White Paper effort was coordinated in close cooperation
with the two international solar ALMA development studies led by
T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be
further updated until the beginning of Cycle 4 in October 2016. In
particular, we plan to adjust the technical capabilities of the solar
observing modes once finally decided and to further demonstrate the
feasibility and scientific potential of the included science cases by
means of numerical simulations of the solar atmosphere and corresponding
simulated ALMA observations.

---------------------------------------------------------
Title: EUV &amp; X-ray observations of microflare heating of AR12333
Authors: Hannah, I. G.; Wright, P. J.; Grefenstette, B.; Glesener,
L.; Hudson, H. S.; Smith, D. M.; Krucker, S.; Marsh, A.; White, S. M.
2015AGUFMSH13B2442H Altcode:
We present a study of the heating in AR12333 due to small microflares
between 10:30 and 13:30UT on 29 April 2015. This region is well observed
in EUV by the Solar Dynamics Observatory/Atmospheric Imaging Assembly
(SDO/AIA) as well as Hinode's X-ray Telescope (XRT) operating in
a higher cadence mode, switching through the five thicker filters
(sensitive to the higher temperature range). The Hinode observations
were a coordinated campaign with the NuSTAR hard X-ray focusing
optics telescope (Harrison et al. 2013). NuSTAR was conducting a
full disk mosaic observation of the Sun and caught AR12333 several
times, providing imaging spectroscopy &gt;2 keV. We investigate the
heating in the active region due to several small microflares (about
A1-Class). These were visible with the thicker XRT filters and only
clear in EUV once the FeXVIII component was extracted from SDO/AIA 94Å,
indicating heating primarily &gt;3MK. Using the regularized inversion
method of Hannah &amp; Kontar 2012, we recover the DEM from the SDO/AIA
and Hinode/XRT data and compare this to the thermal characteristics
derived from NuSTAR.

---------------------------------------------------------
Title: NuSTAR X-ray observations of small flares and non-flaring
active regions
Authors: Hannah, I. G.; Grefenstette, B.; Smith, D. M.; Marsh, A.;
Glesener, L.; Krucker, S.; Hudson, H. S.; White, S.; Madsen, K.;
Caspi, A.; Vogel, J.; Shih, A.
2015AGUFMSH31D..03H Altcode:
We present imaging spectroscopy of the Sun with the NuSTAR hard X-ray
(HXR) telescope, an astrophysics mission that uses focusing optics to
directly image X-rays between ~2-80 keV. Although not optimized for
solar observations, NuSTAR's high sensitivity can probe previously
inaccessible X-ray emission from the Sun - crucial for searching for
high temperature and non-thermal emission from "non-flaring" active
regions. We present analysis of the first NuSTAR solar observations,
that began in late 2014 and continued through 2015. These include
using its imaging spectroscopy capabilities to derive the thermal
characteristics of several "non-flaring" active regions, providing
limits to the high temperature emission. We also show NuSTAR
observations of several small microflares that were also observed
by Hinode/XRT (in multiple thicker filters sensitive to higher
temperatures) and RHESSI. This combination of three separate X-ray
telescopes provides a broad observational characterization of active
region heating by these very small microflares.

---------------------------------------------------------
Title: SSALMON - The Solar Simulations for the Atacama Large
Millimeter Observatory Network
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson,
H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu,
B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.;
Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.;
Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier,
A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M.
2015AdSpR..56.2679W Altcode: 2015arXiv150205601W
The Solar Simulations for the Atacama Large Millimeter Observatory
Network (SSALMON) was initiated in 2014 in connection with two ALMA
development studies. The Atacama Large Millimeter/submillimeter Array
(ALMA) is a powerful new tool, which can also observe the Sun at
high spatial, temporal, and spectral resolution. The international
SSALMONetwork aims at co-ordinating the further development of solar
observing modes for ALMA and at promoting scientific opportunities
for solar physics with particular focus on numerical simulations,
which can provide important constraints for the observing modes and
can aid the interpretation of future observations. The radiation
detected by ALMA originates mostly in the solar chromosphere - a
complex and dynamic layer between the photosphere and corona, which
plays an important role in the transport of energy and matter and the
heating of the outer layers of the solar atmosphere. Potential targets
include active regions, prominences, quiet Sun regions, flares. Here,
we give a brief overview over the network and potential science cases
for future solar observations with ALMA.

---------------------------------------------------------
Title: Fast Single-Dish Scans of the Sun Using ALMA
Authors: Phillips, N.; Hills, R.; Bastian, T.; Hudson, H.; Marson,
R.; Wedemeyer, S.
2015ASPC..499..347P Altcode: 2015arXiv150206122P
We have implemented control and data-taking software that makes it
possible to scan the beams of individual ALMA antennas to perform
quite complex patterns while recording the signals at high rates. We
conducted test observations of the Sun in September and December,
2014. The data returned have excellent quality; in particular they
allow us to characterize the noise and signal fluctuations present
in this kind of observation. The fast-scan experiments included both
Lissajous patterns covering rectangular areas, and “double-circle”
patterns of the whole disk of the Sun and smaller repeated maps of
specific disk-shaped targets. With the latter we find that we can
achieve roughly Nyquist sampling of the Band 6 (230 GHz) beam in 60
s over a region 300” in diameter. These maps show a peak-to-peak
brightness-temperature range of up to 1000 K, while the time-series
variability at any given point appears to be of order 0.5% RMS over
times of a few minutes. We thus expect to be able to separate the
noise contributions due to transparency fluctuations from variations in
the Sun itself. Such timeseries have many advantages, in spite of the
non-interferometric observations. In particular such data should make
it possible to observe microflares in active regions and nanoflares
in any part of the solar disk and low corona.

---------------------------------------------------------
Title: Solar ALMA Observations - A New View of Our Host Star
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Shimojo,
M.; Hales, A.; Yagoubov, P.; Hudson, H.
2015ASPC..499..345W Altcode: 2015arXiv150206397W
ALMA provides the necessary spatial, temporal and spectral resolution to
explore central questions in contemporary solar physics with potentially
far-reaching implications for stellar atmospheres and plasma physics. It
can uniquely constraint the thermal and magnetic field structure in
the solar chromosphere with measurements that are highly complementary
to simultaneous observations with other ground-based and space-borne
instruments. Here, we highlight selected science cases.

---------------------------------------------------------
Title: Capabilities of a FOXSI Small Explorer
Authors: Inglis, A. R.; Christe, S.; Glesener, L.; Krucker, S.; Dennis,
B. R.; Shih, A.; Wilson-Hodge, C.; Gubarev, M.; Hudson, H. S.; Kontar,
E.; Buitrago Casas, J. C.; Drake, J. F.; Caspi, A.; Holman, G.; Allred,
J. C.; Ryan, D.; Alaoui, M.; White, S. M.; Saint-Hilaire, P.; Klimchuk,
J. A.; Hannah, I. G.; Antiochos, S. K.; Grefenstette, B.; Ramsey,
B.; Jeffrey, N. L. S.; Reep, J. W.; Schwartz, R. A.; Ireland, J.
2015AGUFMSH43B2456I Altcode:
We present the FOXSI (Focusing Optics X-ray Solar Imager) small explorer
(SMEX) concept, a mission dedicated to studying particle acceleration
and energy release on the Sun. FOXSI is designed as a 3-axis stabilized
spacecraft in low-Earth orbit making use of state-of-the-art grazing
incidence focusing optics, allowing for direct imaging of solar
X-rays. The current design being studied features three telescope
modules deployed in a low-inclination low-earth orbit (LEO). With a 15
meter focal length enabled by a deployable boom, FOXSI will observe
the Sun in the 3-50 keV energe range. The FOXSI imaging concept has
already been tested on two sounding rocket flights, in 2012 and 2014
and on the HEROES balloon payload flight in 2013. FOXSI will image
the Sun with an angular resolution of 5”, a spectral resolution of
0.5 keV, and sub-second temporal resolution using CdTe detectors. In
this presentation we investigate the science objectives and targets
which can be accessed from this mission. Because of the defining
characteristic of FOXSI is true imaging spectroscopy with high dynamic
range and sensitivity, a brand-new perspective on energy release on the
Sun is possible. Some of the science targets discussed here include;
flare particle acceleration processes, electron beams, return currents,
sources of solar energetic particles (SEPs), as well as understanding
X-ray emission from active region structures and the quiescent corona.

---------------------------------------------------------
Title: Švestka's Research Then and Now. Invited Review
Authors: Hudson, H. S.
2015SoPh..290.3383H Altcode: 2015SoPh..tmp...36H
Zdeněk Švestka's research work influenced many fields of solar
physics, especially in the area of flare research. In this article
I take five of the areas that particularly interested him and assess
them in a "then and now" style. His insights in each case were quite
sound, although of course in the modern era we have learned things
that he could not readily have envisioned. His own views about his
research life have been published recently in this journal, to which he
contributed so much, and his memoir contains much additional scientific
and personal information (Švestka in Solar Phys.267, 235, 2010).

---------------------------------------------------------
Title: SDO/HMI -- RHESSI White-Light Flare Catalog: Matsushita
Analysis
Authors: Martinez Oliveros, J. C.; Hudson, H. S.; Krucker, S.
2015AGUFMSH52A..05M Altcode:
In recent years several observation of white-light flare features in
the low corona using data from the Helioseismic and Magnetic Imager
onboard the Solar Dynamics Observatory have been reported. We present
the first results of a white-light flare catalog based on SDO/HMI
6173A Intensity observations and the RHESSI flare catalog. We selected
flares during 2011 and 2013 with GOES classification above M1.0 that
were fully or partially observed by RHESSI as reported in the RHESSI
flare catalog. We found that at least one third of the flares present
white-light enhancement in the 6173A line and at least one fifth of the
events show above the limb white-light sources. We will also discuss the
results of a comparative analysis between the Hard X-ray and white-light
positions. This analysis show the statistical average-height variation
using the analysis technique described by Matsushita et al. 1992.

---------------------------------------------------------
Title: Sunquake Generation by Coronal Magnetic Restructuring
Authors: Russell, A. J. B.; Mooney, M.; Leake, J. E.; Hudson, H. S.
2015AGUFMSH22A..05R Altcode:
Solar flares and coronal mass ejections are powered by major
restructurings of the coronal magnetic field, which appear to strongly
perturb the magnetic field in the photosphere as well. Could the
associated Lorentz forces generate sunquakes, as suggested by Hudson
et al. 2008? Here, we present the first MHD simulations of sunquake
generation by magnetic field perturbations, and explore the details of
this mechanism. The downgoing magnetic field change is modelled as an
Alfven wave, which propagates into the lower atmosphere. When it reaches
the vicinity of the beta=1 layer (where the Alfven and sound speeds are
equal), non-linear coupling excites a downgoing acoustic wave, which we
interpret as a sunquake. The amplitude of the acoustic wave increases
nonlinearly with the amplitude of the magnetic perturbation, reaching
a limit where around 35% of the injected Poynting flux is transferred
to the seismic wave - enough energy to match sunquake observations.

---------------------------------------------------------
Title: The NuSTAR Sensitivity to Quiet-Sun Transient Events
Authors: Marsh, A.; Smith, D. M.; Glesener, L.; Hannah, I. G.; Krucker,
S.; Hudson, H. S.; Grefenstette, B.; Madsen, K.; Caspi, A.
2015AGUFMSH13B2441M Altcode:
We present the NuSTAR sensitivity to quiet Sun (QS) transient events,
which have been seen in wavelengths from UV to soft X-rays. Although
not optimized for solar observations, NuSTAR can observe X-ray
emission from the Sun with unprecedented sensitivity in the hard X-ray
range; this is crucial for detecting individual events in the quiet
corona. While NuSTAR has not yet detected any such events, we use QS
data from the 01-November-2014 observations (at a GOES-level ~B4) to
determine what types of events we could have detected. In particular,
we place lower limits on the detectable flare emission measure for
isothermal temperatures between 2 - 10 MK. While our sensitivity to
date has been limited by noise due to active regions outside the field
of view, these limits are still &gt;2 orders of magnitude below the
RHESSI detection limits at corresponding temperatures. We expect to
increase our sensitivity by at least an order of magnitude with future
observations at lower solar flux levels and with fewer active regions
on the disk.

---------------------------------------------------------
Title: Soft X-Ray Pulsations in Solar Flares
Authors: Simões, P. J. A.; Hudson, H. S.; Fletcher, L.
2015SoPh..290.3625S Altcode: 2014arXiv1412.3045S; 2015SoPh..tmp...50S
The soft X-ray emissions (hν&gt;1.5 keV) of solar flares mainly come
from the bright coronal loops at the highest temperatures normally
achieved in the flare process. Their ubiquity has led to their use
as a standard measure of flare occurrence and energy, although the
overwhelming bulk of the total flare energy goes elsewhere. Recently
Dolla et al. (Astrophys. J. Lett.749, L16, 2012) noted quasi-periodic
pulsations (QPP) in the soft X-ray signature of the X-class flare
SOL2011-02-15, as observed by the standard photometric data from the
GOES (Geostationary Operational Environmental Satellite) spacecraft. In
this article we analyse the suitability of the GOES data for this type
of analysis and find them to be generally valuable after September,
2010 (GOES-15). We then extend the result of Dolla et al. to a complete
list of X-class flares from Cycle 24 and show that most of them (80 %)
display QPPs in the impulsive phase. The pulsations show up cleanly in
both channels of the GOES data, making use of time-series of irradiance
differences (the digital time derivative on the 2-s sampling). We deploy
different techniques to characterise the periodicity of GOES pulsations,
considering the red-noise properties of the flare signals, finding
a range of characteristic time scales of the QPPs for each event,
but usually with no strong signature of a single period dominating
in the power spectrum. The QPP may also appear on somewhat longer
time scales during the later gradual phase, possibly with a greater
tendency towards coherence, but the sampling noise in GOES difference
data for high irradiance values (X-class flares) makes these more
uncertain. We show that there is minimal phase difference between
the differenced GOES energy channels, or between them and the hard
X-ray variations on short time scales. During the impulsive phase,
the footpoints of the newly forming flare loops may also contribute
to the observed soft X-ray variations.

---------------------------------------------------------
Title: The Atacama Large Millimeter/Submillimeter Array: a New Asset
for Solar and Heliospheric Physics
Authors: Bastian, Timothy S.; Barta, Miroslav; Brajsa, Roman; Chen,
Bin; De Pontieu, Bart; Fleishman, Gregory; Gary, Dale; Hales, Antonio;
Hills, Richard; Hudson, Hugh; Iwai, Kazamasu; Shimojo, Masumi; White,
Stephen; Wedemeyer, Sven; Yan, Yihua
2015IAUGA..2257295B Altcode:
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint
North American, European, and East Asian interferometric array that
opens the mm-submm wavelength part of the electromagnetic spectrum
for general astrophysical exploration, providing high-resolution
imaging in frequency bands ranging from 86 to 950 GHz. Despite being
a general purpose instrument, provisions have been made to enable
solar observations with ALMA. Radiation emitted at ALMA wavelengths
originates mostly from the chromosphere, which plays an important
role in the transport of energy and matter and the heating of the
outer layers of the solar atmosphere. In this paper we describe
recent efforts to ensure that ALMA can be usefully exploited by
the scientific community to address outstanding questions in solar
physics. We summarize activities under North American and European
ALMA development studies, including instrument testing, calibration
and imaging strategies, a science simulations. With the support of
solar observations, ALMA joins next-generation groundbased instruments
that can be used alone or in combination with other ground-based and
space-based instruments to address outstanding questions in solar
and heliospheric physics. Opportunities for the wider community to
contribute to these efforts will be highlighted.

---------------------------------------------------------
Title: Solar ALMA observations - A revolutionizing new view at our
host star
Authors: Wedemeyer, Sven; Brajsa, Roman; Bastian, Timothy S.; Barta,
Miroslav; Hales, Antonio; Yagoubov, Pavel; Hudson, Hugh; Loukitcheva,
Maria; Fleishman, Gregory
2015IAUGA..2256732W Altcode:
Observations of the Sun with the Atacama Large Millimeter/submillimeter
Array (ALMA) have a large potential for revolutionizing our
understanding of our host star with far reaching implications
for stars in general. The radiation emitted at ALMA wavelengths
originates mostly from the chromosphere - a complex and dynamic layer
between the photosphere and the corona, which plays an important
role in the transport of energy and matter and the heating of the
outer layers of the solar atmosphere.Despite decades of intensive
research, the chromosphere is still elusive and challenging to
observe owing to the complicated formation mechanisms of currently
available diagnostics. ALMA will change the scene substantially as
it serves as a nearly linear thermometer at high spatial, temporal,
and spectral resolution, enabling us to study the complex interaction
of magnetic fields and shock waves and yet-to-be-discovered dynamical
processes. Furthermore, radio recombination and molecular lines
may have great diagnostic potential but need to be investigated
first. These unprecedented capabilities promise important new findings
for a large range of topics in solar physics including the structure,
dynamics and energy balance of quiet Sun regions, active regions and
sunspots, flares and prominences. As a part of ongoing development
studies, an international network has been initiated, which aims at
defining and preparing key solar science with ALMA through simulation
studies: SSALMON -- Solar Simulations for the Atacama Large Millimeter
Observatory Network (http://ssalmon.uio.no). Here, we give an overview
of potential science cases.

---------------------------------------------------------
Title: Solar extreme events
Authors: Hudson, Hugh S.
2015JPhCS.632a2058H Altcode: 2015arXiv150404755H
Solar flares and CMEs have a broad range of magnitudes. This review
discusses the possibility of “extreme events,” defined as those
with magnitudes greater than have been seen in the existing historical
record. For most quantitative measures, this direct information does
not extend more than a century and a half into the recent past. The
magnitude distributions (occurrence frequencies) of solar events
(flares/CMEs) typically decrease with the parameter measured or inferred
(peak flux, mass, energy etc. Flare radiation fluxes tend to follow a
power law slightly flatter than S-2, where S represents a
peak flux; solar particle events (SPEs) follow a still flatter power law
up to a limiting magnitude, and then appear to roll over to a steeper
distribution, which may take an exponential form or follow a broken
power law. This inference comes from the terrestrial 14C
record and from the depth dependence of various radioisotope
proxies in the lunar regolith and in meteorites. Recently major new
observational results have impacted our use of the relatively limited
historical record in new ways: the detection of actual events in the
14C tree-ring records, and the systematic observations of
flares and “superflares” by the Kepler spacecraft. I discuss how
these new findings may affect our understanding of the distribution
function expected for extreme solar events.

---------------------------------------------------------
Title: Division II: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Scrijver, Karel J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
2015IAUTB..28..106V Altcode:
The Business Meeting of Commission 10 was held as part of the Business
Meeting of Division II (Sun and Heliosphere), chaired by Valentin
Martínez-Pillet, the President of the Division. The President of
Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took
the chair for the business meeting of C10. She summarised the activities
of C10 over the triennium and the election of the incoming OC.

---------------------------------------------------------
Title: High-sensitivity search for transient solar X-ray emission
with NuSTAR
Authors: Marsh, Andrew; Hannah, Iain; Glesener, Lindsay; Smith, David
M.; Grefenstette, Brian; Madsen, Kristin; Krucker, Sam; Hudson, Hugh;
White, Stephen; Caspi, Amir; Christe, Steven; Shih, Albert; Mewaldt,
Richard; Pivovaroff, Michael; Vogel, Julia
2015TESS....121302M Altcode:
We present the first results of a search for transient X-ray emission in
quiet solar regions with the NuSTAR astrophysics satellite. Transient
brightenings of 1024-1027 ergs, or "nanoflares,"
have been observed as thermal emission in EUV and soft X-rays, but
never in hard X-rays (HXRs) due to lack of sensitivity. Frequent
nanoflares could account for a significant fraction of the energy
release needed to heat the corona to &gt;1 MK. NuSTAR directly images
X-rays from ~2-80 keV, with much higher sensitivity than dedicated
solar HXR instruments. More importantly it can point at the Sun without
suffering damage, a rare capability for an astrophysics instrument. We
have developed an algorithm to search the NuSTAR data in space and
time for transient events, while taking into account instrumental
and systematic effects. Preliminary analysis yields a sensitivity to
events ~0.001 times as bright as an “typical” RHESSI microflare
(Hannah et al. 2008), for linear scaling and event duration of 10
seconds. Future observations at full-Sun flux levels below GOES ~B5
will increase our sensitivity by an order of magnitude or more.

---------------------------------------------------------
Title: The solar magnetic activity band interaction and instabilities
that shape quasi-periodic variability
Authors: McIntosh, Scott W.; Leamon, Robert J.; Krista, Larisza D.;
Title, Alan M.; Hudson, Hugh S.; Riley, Pete; Harder, Jerald W.; Kopp,
Greg; Snow, Martin; Woods, Thomas N.; Kasper, Justin C.; Stevens,
Michael L.; Ulrich, Roger K.
2015NatCo...6.6491M Altcode: 2015NatCo...6E6491M
Solar magnetism displays a host of variational timescales of which
the enigmatic 11-year sunspot cycle is most prominent. Recent work
has demonstrated that the sunspot cycle can be explained in terms of
the intra- and extra-hemispheric interaction between the overlapping
activity bands of the 22-year magnetic polarity cycle. Those
activity bands appear to be driven by the rotation of the Sun's
deep interior. Here we deduce that activity band interaction can
qualitatively explain the `Gnevyshev Gap'--a well-established feature
of flare and sunspot occurrence. Strong quasi-annual variability in the
number of flares, coronal mass ejections, the radiative and particulate
environment of the heliosphere is also observed. We infer that this
secondary variability is driven by surges of magnetism from the activity
bands. Understanding the formation, interaction and instability of
these activity bands will considerably improve forecast capability in
space weather and solar activity over a range of timescales.

---------------------------------------------------------
Title: The Radiated Energy Budget Of Chromospheric Plasma In A Major
Solar Flare Deduced From Multi-Wavelength Observations
Authors: Milligan, Ryan; Kerr, Graham Stewart; Dennis, Brian; Hudson,
Hugh; Fletcher, Lyndsay; Allred, Joel; Chamberlin, Phillip; Ireland,
Jack; Mathioudakis, Mihalis; Keenan, Francis
2015TESS....130209M Altcode:
The response of the lower solar atmosphere is an important diagnostic
tool for understanding energy transport during solar flares. The 15
February 2011 X-class flare was fortuitously observed by a host of
space-based instruments that sampled the chromospheric response over
a range of lines and continua at &lt;20s cadence. These include the
free-bound EUV continua of H I (Lyman), He I, and He II, plus the
emission lines of He II at 304Å and H I (Lyα) at 1216Å by SDO/EVE,
the UV continua at 1600Å and 1700Å by SDO/AIA, and the white light
continuum at 4504Å, 5550Å, and 6684Å, along with the Ca II H line
at 3968Å using Hinode/SOT. RHESSI also observed the entire event at
energies up to ~100keV, making it possible to determine the properties
of the nonthermal electrons deemed to be responsible for driving the
enhanced chromospheric emission under the assumption of thick-target
collisions. Integrating over the duration of the impulsive phase,
the total energy contained in the nonthermal electrons was found to be
&gt;2×1031 erg. By comparison, the summed energy detected by
instruments onboard SDO and Hinode amounted to ~3×1030 erg;
about 15% of the total nonthermal energy. The Lyα line was found to
dominate the measured radiative losses in contrast to the predictions
of numerical simulations. Parameters of both the driving electron
distribution and the resulting chromospheric response are presented
in detail to encourage the numerical modeling of flare heating for
this event to determine the depth of the solar atmosphere at which
these line and continuum processes originate, and the mechanism(s)
responsible for their generation.

---------------------------------------------------------
Title: Hard X-ray imaging spectroscopy of hot coronal sources and
active regions with NuSTAR
Authors: Hannah, Iain; Marsh, Andrew; Glesener, Lindsay; Smith,
David; Grefenstette, Brian; Madsen, Kristin; Krucker, Sam; Hudson,
Hugh; White, Stephen; Shih, Albert Y.
2015TESS....120402H Altcode:
We present imaging spectroscopy of the Sun with the NuSTAR hard X-ray
(HXR) telescope, searching for high temperature and non-thermal emission
in the “non-flaring” Sun. Launched in 2012, NASA's astrophysics
mission NuSTAR uses focusing optics to directly image X-rays between
~2-80 keV. In the band below ~50 keV the field of view is 12'x12'
and the instrument has an energy resolution of ~0.4 keV. Although not
optimized for solar observations, NuSTAR’s high sensitivity can probe
previously inaccessible X-ray emission from the Sun. NuSTAR observed the
Sun three times during late 2014 and we present these first directly
imaged hard X-rays from non-flaring active regions. Using NuSTAR’s
imaging spectroscopy capabilities we are able to derive the active
region’s multi-thermal characteristics. We will also discuss a hot
(&gt;3MK) source that appears to linger high in the corona and could
be associated with the occulted active region AR12192.

---------------------------------------------------------
Title: Sub-Milli Arcsecond Resolution Observations of the Optical
Solar Limb with RHESSI/SAS
Authors: Fivian, Martin D.; Hudson, Hugh; Krucker, Sam
2015TESS....120325F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb with a cadence typically set at 100 samples/s. RHESSI
has observed the Sun continuously since its launch in early 2002, and we
have acquired a unique data set ranging over more than a full 11-year
solar cycle and consisting of about 4x10^10 single data points. The
optics has a point spread of about 4.5 arcsec FWHM imaging the red
continuum onto three linear CCD sensors with a pixel resolution of 1.7
arcsec. However, careful study of systematics, masking of contaminated
data, and accumulation of data over appropriate time intervals has led
to measurements with sub-milli arcsec accuracy. Analyzing data for
an initial period in 2004, these measurements have led to the most
accurate oblateness measurement to date, 8.01+-0.14 milli arcsec
(Fivian et al., 2008), a value consistent with models predicting
an oblateness from surface rotation. An excess oblateness term can
be attributed to magnetic elements possibly located in the enhanced
network. We also study photometric properties of our data. Previous
observations of latitude-dependent brightness variations at the limb
had suggested the presence of a polar temperature excess as large as
1.5 K. The RHESSI observations, made with a rotating telescope in
space, have great advantages in the rejection of systematic errors
in the very precise photometry required for such an observation. Our
measurements of latitude-dependent brightness variations at the limb
lead to a quadrupolar term (a pole-to-equator temperature variation)
of the order of 0.1 K, an order of magnitude smaller than previously
reported. We present the analysis of these unique data, an overview of
some results and we report on our progress as we apply our developed
analysis method to the whole 13 years of data.

---------------------------------------------------------
Title: SDO/HMI - RHESSI White-Light Flare Catalog: First Results
Authors: Martinez Oliveros, Juan Carlos; Hudson, Hugh; Saint Hilaire,
Pascal
2015TESS....130904M Altcode:
In recent years several observation of white-light flare features in
the low corona using data from the Helioseismic and Magnetic Imager
onboard the Solar Dynamics Observatory have been reported. We present
the first results of a white-light flare catalog based on SDO/HMI
6173A Intensity observations and the RHESSI flare catalog. We selected
flares during 2011 and 2012 with GOES classification above M1.0 that
were fully or partially observed by RHESSI as reported in the RHESSI
flare catalog. We found that at least one third of the flares present
white-light enhancement in the 6173A line and at least one fifth of the
events show above the limb white-light sources. We will also discuss
the physical implications of these observations.

---------------------------------------------------------
Title: Radio Observations of the CME-poor region AR2192: a type II
burst with no CME driver
Authors: Hudson, Hugh; Vilmer, Nicole; Wakeford, Peter
2015TESS....140803H Altcode:
The remarkable sunspot group NOAA AR 2192 (October 2014) produced
X-class flares without CMEs, and in general was large and powerful but
with little heliospheric interaction. We discuss radio perspectives
on the development of this region. In particular there were
decametric type II bursts observed in association with jet-like
flares SOL2014-10-21T12:28 (C4.4) and SOL2014-10-21T13:38 (M1.2),
as first noted in the Glasgow Callisto observatory and confirmed
via the Meudon decametric array. In cases such as this, the global
coronal wave responsible for the type II emission seems to originate
from an ejection of material flowing along a previously established
field structure, rather than perpendicular to it as in a CME.

---------------------------------------------------------
Title: Co-Spatial White Light and Hard X-Ray Flare Footpoints Seen
Above the Solar Limb
Authors: Krucker, Säm; Saint-Hilaire, Pascal; Hudson, Hugh S.;
Haberreiter, Margit; Martinez-Oliveros, Juan Carlos; Fivian, Martin
D.; Hurford, Gordon; Kleint, Lucia; Battaglia, Marina; Kuhar, Matej;
Arnold, Nicolas G.
2015ApJ...802...19K Altcode:
We report analysis of three solar flares that occur within 1° of limb
passage, with the goal to investigate the source height of chromospheric
footpoints in white light (WL) and hard X-rays (HXR). We find the
WL and HXR (≥30 keV) centroids to be largely co-spatial and from
similar heights for all events, with altitudes around 800 km above
the photosphere or 300-450 km above the limb height. Because of the
extreme limb location of the events we study, emissions from such
low altitudes are influenced by the opacity of the atmosphere and
projection effects. STEREO images reveal that for SOL2012-11-20T12:36
the projection effects are smallest, giving upper limits of the absolute
source height above the nominal photosphere for both wavelengths of
∼1000 km. To be compatible with the standard thick target model,
these rather low altitudes require very low ambient densities within
the flare footpoints, in particular if the HXR-producing electrons
are only weakly beamed. That the WL and HXR emissions are co-spatial
suggests that the observed WL emission mechanism is directly linked
to the energy deposition by flare accelerated electrons. If the WL
emission is from low-temperature (≤slant {{10}4} K) plasma
as currently thought, the energy deposition by HXR-producing electrons
above ∼30 keV seems only to heat chromospheric plasma to such low
temperatures. This implies that the energy in flare-accelerated
electrons above ∼30 keV is not responsible for chromospheric
evaporation of hot (\gt {{10}6} K) plasma, but that their
energy is lost through radiation in the optical range.

---------------------------------------------------------
Title: Svestka's Research: Then and Now
Authors: Hudson, Hugh S.
2015arXiv150304452H Altcode:
Zdenek Svestka's research work influenced many fields of solar physics,
especially in the area of flare research. In this article I take five of
the areas that particularly interested him and assess them in a "then
and now" style. His insights in each case were quite sound, although
of course in the modern era we have learned things that he could not
readily have envisioned. His own views about his research life have
been published recently in this journal, to which he contributed so
much, and his memoir contains much additional scientific and personal
information (Svestka, 2010).

---------------------------------------------------------
Title: Electron Energy Partition in the Above-the-looptop Solar Hard
X-Ray Sources
Authors: Oka, Mitsuo; Krucker, Säm; Hudson, Hugh S.; Saint-Hilaire,
Pascal
2015ApJ...799..129O Altcode:
Solar flares produce non-thermal electrons with energies up to tens
of MeVs. To understand the origin of energetic electrons, coronal
hard X-ray (HXR) sources, in particular above-the-looptop sources,
have been studied extensively. However, it still remains unclear how
energies are partitioned between thermal and non-thermal electrons
within the above-the-looptop source. Here we show that the kappa
distribution, when compared to conventional spectral models, can
better characterize the above-the-looptop HXRs (gsim15 keV) observed
in four different cases. The widely used conventional model (i.e., the
combined thermal plus power-law distribution) can also fit the data,
but it returns unreasonable parameter values due to a non-physical sharp
lower-energy cutoff Ec. In two cases, extreme-ultraviolet
data were available from SDO/AIA and the kappa distribution was still
consistent with the analysis of differential emission measure. Based
on the kappa distribution model, we found that the 2012 July 19 flare
showed the largest non-thermal fraction of electron energies about
50%, suggesting equipartition of energies. Considering the results of
particle-in-cell simulations, as well as density estimates of the four
cases studied, we propose a scenario in which electron acceleration is
achieved primarily by collisionless magnetic reconnection, but the
electron energy partition in the above-the-looptop source depends
on the source density. In low-density above-the-looptop regions
(few times 109 cm-3), the enhanced non-thermal
tail can remain and a prominent HXR source is created, whereas in
higher-densities (&gt;1010 cm-3), the non-thermal
tail is suppressed or thermalized by Coulomb collisions.

---------------------------------------------------------
Title: The Solar Activity Cycle
Authors: Balogh, André; Hudson, Hugh; Petrovay, Kristóf; von
Steiger, Rudolf
2015sac..book.....B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Sector Structure
Authors: Hudson, Hugh S.; Svalgaard, Leif; Hannah, Iain G.
2015sac..book...17H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Introduction to the Solar Activity Cycle: Overview of Causes
and Consequences
Authors: Balogh, A.; Hudson, H. S.; Petrovay, K.; von Steiger, R.
2014SSRv..186....1B Altcode: 2014SSRv..tmp...60B
The 11-year activity cycle is a dominant characteristic of the Sun. It
is the result of the evolution in time the solar dynamo that generates
the solar magnetic field. The nearly periodic variation in the sunspot
number has been known since the mid-1800s; as the observations of
the Sun broadened to cover an increasing number of phenomena, the
same 11-year periodicity was noted in most of them. The discovery of
solar magnetic fields introduced a 22-year periodicity, as the magnetic
polarities of the polar regions change sign every 11 years. Correlations
have been identified and quantified among all the measured parameters,
but in most cases such correlations remain empirical rather than
grounded in physical processes. This introductory paper and the reviews
in the volume describe and discuss the current state of understanding
of the causal chains that lead from the variable nature of the solar
magnetic fields to the variability of solar phenomena. The solar
activity cycle is poorly understood: predictions made for the current
Cycle 24 have proved to be generally wrong. However, the re-evaluation
of the relationships in the light of unexpected shortcomings is likely
to lead to a better physical understanding of solar physics. This
will help in the systematic reassessment of solar activity indices and
their usefulness in describing and predicting the solar activity cycle.

---------------------------------------------------------
Title: Hmi and Rhessi Measurements of the Radial Location of Solar
Flare Footpoints to Subarcsecond Accuracy
Authors: Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Haberreiter,
M.; Kleint, L.; Hurford, G. J.; Fivian, M. D.; Battaglia, M.; Martinez
Oliveros, J. C.
2014AGUFMSH31C..05K Altcode:
We report analysis of three solar flares that occur within one degree
of limb passage, with the goal to investigate the source height of
chromospheric footpoints in white light (WL) and hard X-rays (HXR). The
optical observations are from the Helioseismic and Magnetic Imager
(HMI) around 617.3 nm, providing high precision observations with an
absolute positional accuracy in the radial direction below 0.1 arcsec
(~70 km), as referred to the adjacent limb. The Reuven Ramaty Higher
Energy Solar Spectroscopic Imager (RHESSI) gives HXR source centroids to
a similar accuracy depending on counting statistics. The observed height
of the emissions at either wavelength is influenced by the opacity of
the atmosphere at that wavelength and the height must correspond to a
radial distance from Sun center that is greater than the solar limb at
that wavelength (~350 km for WL and ~450 km for HXR). We find the WL
and HXR (~30 keV) centroids to be largely co-spatial and from similar
heights for all events, with altitudes around 800 km above the height of
the photosphere. The observed altitudes are limited by the uncertainty
of the precise heliographic locations near the limb and the resulting
projection effects. STEREO images reveal that for SOL2012-11-20T12:36
the projection effects are smallest, giving upper limits of the absolute
source height of 979+-70 km for the WL emission and 926+-51 km for HXR
source. Hence, the peak of the WL and HXR must be below 1000 km. To
be compatible with the standard thick target model, these rather low
altitudes require low ambient densities within the flare footpoints, in
particular if the HXR-producing electrons are only weakly beamed. That
the WL and HXR emissions are co-spatial suggests that the observed WL
emission mechanism is directly linked to the energy deposition by flare
accelerated electrons with energies above ~30 keV. If the WL emission
is from low-temperature (~10 000 K) plasma as currently thought, the
energy deposition by HXR-producing electrons above ~30 keV seems only
to heat chromospheric plasma to such low temperatures. This implies
that the energy in flare-accelerated electrons above ~30 keV is lost
through radiation in the optical range rather than heating chromospheric
plasma to coronal (&gt; MK) temperatures.

---------------------------------------------------------
Title: Electron Energy Partition in the Above-the-looptop Solar Hard
X-ray Sources
Authors: Oka, M.; Krucker, S.; Hudson, H. S.; Saint-Hilaire, P.
2014AGUFMSH23A4147O Altcode:
Solar flares produce non-thermal electrons with energies up to tens of
MeVs. To understand the origin of energetic electrons, coronal hard
X-ray sources in particular 'above-the-looptop' sources have been
studied extensively. However, it still remains unclear how energies
are partitioned between thermal and non-thermal electrons within the
above-the-looptop source. Here we show that the kappa distribution,
when compared to conventional spectral models, can better characterize
the above-the- looptop hard X-rays (&gt;~15 keV) observed in four
different cases. The combined thermal plus power-law distribution can
also fit the data, but it returns unreasonable parameter values due to
the artifact of its sharp, lower-energy cutoff Ec. In two cases with
extreme-ultraviolet data from SDO/AIA, the analysis of differential
emission measure (DEM) did not rule out the kappa distribution
model. Based on the kappa distribution model, we found that the 2012
July 19 flare showed the largest non-thermal fraction of electron
energies about 50%, suggesting equipartition. Considering results of
particle-in-cell (PIC) simulations as well as density estimates of the
four cases studied, we propose a scenario in which electron energization
is achieved primarily by collisionless magnetic reconnection but the
non-thermal tail can be suppressed or thermalized by Coulomb collisions.

---------------------------------------------------------
Title: NuSTAR's First Solar Observations: Search for Transient
Brightenings / Nanoflares
Authors: Marsh, A.; Hannah, I. G.; Glesener, L.; Smith, D. M.;
Grefenstette, B.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White,
S.; Caspi, A.; Christe, S.; Shih, A.; Mewaldt, R. A.; Pivovaroff,
M.; Vogel, J.
2014AGUFMSH13C4129M Altcode:
We present a timing analysis of the Sun with the NuSTAR hard X-ray
(HXR) telescope, searching for transient brightenings / nanoflares in
the quiet Sun and active regions. A substantial amount of flare energy
goes into accelerating electrons. HXR observations are a crucial tool
for understanding this non-thermal emission and the energy release
in flares. RHESSI is able to study this emission over many orders of
magnitude (active region flares from X-class to A-class microflares),
but it cannot detect the emission from smaller events. Such "nanoflares"
have been postulated as a possible source of coronal heating and their
existence and relationship to larger flares is still uncertain. In
order to detect these events in HXRs, instruments more sensitive
than RHESSI are required. Launched in 2012, the astrophysics mission
NuSTAR uses focusing optics to directly image X-rays between ~2-80
keV. Although not optimized for solar observations, NuSTAR's highly
sensitive imaging spectroscopy will be used to search for the faintest
X-ray emission from the Sun. These solar observations will begin in
September 2014; here we present the first results of our search for
transient brightenings that could relate to nanoflares.

---------------------------------------------------------
Title: Wavelengths for EVE coronal dimming signatures
Authors: Hudson, H. S.; Simoes, P. J. D. A.; Kukstas, E.
2014AGUFMSH13B4111H Altcode:
The EVE instrument on SDO detects post-flare dimmings, mainly in
the spectral regions of Fe IX-XII in its MEGS-A range. We have shown
that dimmings occurred in most of the 31 X-class flares that occurred
between SDO launch and the end of April 2014. Based upon earlier X-ray
observations, we interpret these dimmings as the result of CME mass
ejections from the low corona. We can estimate the masses involved in
these dimmings by deriving a best pre-event temperature and emission
measure in the dimmed region from EVE, and a source volume from AIA
images. The dimming for SOL2011-02-15, the first of these events,
"peaked" at -3.4% in Fe IX in terms of the pre-event emission from the
whole Sun, with smaller relative depletions in higher ionization states
of Fe. Because of its high photon throughput, EVE data determine line
centroids with precisions of a few km/s equivalent. In the present study
we analyze the wavelengths of the dimmed regions, characterizing their
displacements from the mean wavelengths as functions of heliographic
position, time, event magnitude, and excitation state of Fe.

---------------------------------------------------------
Title: Statistical Study of Coronal Hard X-ray Source Heights
and Fluxes
Authors: Glesener, L.; Oka, M.; Krucker, S.; Hudson, H. S.
2014AGUFMSH11D..03G Altcode:
Hard X-ray observation of partly occulted flares has proven useful
for studying flare-accelerated electrons in the solar corona. These
nonthermal electrons emit bremsstrahlung hard X-rays (HXRs), but
are difficult to observe for on-disk flares because the much brighter
chromospheric footpoints tend to dominate HXR images. Previous research
using the RHESSI spacecraft has performed individual and statistical
study of HXR sources in partly occulted flares to investigate,
for example, spectral characteristics and the relationship between
nonthermal coronal sources and thermal loops. Source heights are not
usually measured in these cases because of the difficulty in determining
heliographic locations of flares beyond the limb. Occasionally,
multi-spacecraft observation will identify a source location and thus
calculate an absolute HXR source height. Microflare source heights have
also been studied statistically by fitting distribution functions to the
observed projected locations. But so far, a statistical study of coronal
HXR sources in which source heights are determined individually for each
flare has not been performed. In this work, we study flares jointly
observed by RHESSI and STEREO/EUVI over a 2+ year time range. From
RHESSI data, we obtain coronal HXR source positions and fluxes. The
alternate viewing angle offered by STEREO provides flare locations,
enabling RHESSI source positions to be translated into absolute heights
above the photosphere. We will present the distribution of coronal HXR
source heights and will discuss their possible corresponding features
in the standard flare model, i.e. thermal loops, nonthermal looptop
sources, above-the-looptop sources, and ejecta.

---------------------------------------------------------
Title: RHESSI/SAS Observations of the Optical Solar Limb Over a Full
Solar Cycle
Authors: Fivian, M. D.; Hudson, H. S.
2014AGUFMSH41C4156F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb in the red continuum with a cadence typically set at
16 samples/s in each of three linear CCD sensors. RHESSI has observed
the Sun continuously since its launch in early 2002, and we have
acquired a unique data set ranging over a full 11-year solar cycle
and consisting of about 3x10^10 single data points. Analyzing data
for an initial period in 2004, these measurements have led to the most
accurate oblateness measurement to date, 8.01+-0.14 milli arcsec (Fivian
et al., 2008), a value consistent with models predicting an oblateness
from surface rotation. An excess oblateness term can be attributed to
magnetic elements possibly located in the enhanced network. We have
started to also study photometric properties of our data. Previous
observations of latitude-dependent brightness variations at the limb
had suggested the presence of a polar temperature excess as large
as 1.5 K. The RHESSI observations, made with a rotating telescope in
space, have great advantages in the rejection of systematic errors in
the very precise photometry required for such an observation. Our new
measurements of latitude-dependent brightness variations at the limb
lead to a quadrupolar term (a pole-to-equator temperature variation)
of the order of 0.1 K, an order of magnitude smaller than previously
reported. We present the analysis of these unique data, an overview of
some results and we report on our progress as we apply our developed
analysis method to the whole 12 years of data.

---------------------------------------------------------
Title: NuSTAR's first solar observations: Search for a high energy
X-ray component to the "non-flaring" Sun
Authors: Marsh, A.; Hannah, I. G.; Glesener, L.; Smith, D. M.;
Grefenstette, B.; Krucker, S.; Hudson, H. S.; Hurford, G. J.; White,
S.; Caspi, A.; Christe, S.; Shih, A.; Mewaldt, R. A.; Pivovaroff,
M.; Vogel, J.
2014AGUFMSH12A..04M Altcode:
We present spectroscopy of the Sun with the NuSTAR hard X-ray (HXR)
telescope, searching for high temperature and non-thermal emission
in the "non-flaring" Sun. A substantial amount of flare energy goes
into accelerating electrons. HXR observations are a crucial tool
for understanding this non-thermal emission and the energy release
in flares. RHESSI is able to study this emission over many orders of
magnitude (active region flares from X-class to A-class microflares),
but it cannot detect the emission from smaller events. Such "nanoflares"
have been postulated as a possible source of coronal heating and their
existence and relationship to larger flares is still uncertain. In
order to detect these events in HXRs, instruments more sensitive
than RHESSI are required. Launched in 2012, the astrophysics mission
NuSTAR uses focusing optics to directly image X-rays between ~2-80
keV. Although not optimized for solar observations, NuSTAR's highly
sensitive imaging spectroscopy will be used to search for the faintest
X-ray emission from the Sun. These solar observations will begin in
September 2014. Here we present the first results of our search for
transient brightenings in active and quiet Sun regions with NuSTAR.

---------------------------------------------------------
Title: Solar Sector Structure
Authors: Hudson, Hugh S.; Svalgaard, Leif; Hannah, Iain G.
2014SSRv..186...17H Altcode: 2014SSRv..tmp...56H; 2015arXiv150304477H
The interplanetary magnetic field near 1 AU has a characteristic
"sector" structure that reflects its polarity relative to the solar
direction. Typically we observe large-scale coherence in these
directions, with two or four "away" or "towards" sectors per solar
rotation, from any platform in deep space and near the ecliptic
plane. In a simple picture, this morphology simply reflects the idea
that the sources of the interplanetary field lie mainly in or near the
Sun, and that the solar-wind flow enforces a radial component in this
field. The sector boundaries are sharply defined in the interplanetary
field near one AU, but have more complicated sources within the Sun
itself. Recent evidence confirms that the origins of this pattern also
appear statistically at the level of the photosphere, with signatures
found in the highly concentrated fields of sunspots and even solar
flares. This complements the associations already known between the
interplanetary sectors and large-scale coronal structures (i.e., the
streamers). This association with small-scale fields strengthens at
the Hale sector boundary, defining the Hale boundary as the one for
which the polarity switch matches that of the leading-to-following
polarity alternation in the sunspots of a given hemisphere. Surface
features that appear 4.5 days prior to the sector crossings observed
at 1 AU correlate with this sense of polarity reversal.

---------------------------------------------------------
Title: The Radiated Energy Budget of Chromospheric Plasma in a Major
Solar Flare Deduced from Multi-wavelength Observations
Authors: Milligan, Ryan O.; Kerr, Graham S.; Dennis, Brian R.; Hudson,
Hugh S.; Fletcher, Lyndsay; Allred, Joel C.; Chamberlin, Phillip C.;
Ireland, Jack; Mathioudakis, Mihalis; Keenan, Francis P.
2014ApJ...793...70M Altcode: 2014arXiv1406.7657M
This paper presents measurements of the energy radiated by the lower
solar atmosphere, at optical, UV, and EUV wavelengths, during an
X-class solar flare (SOL2011-02-15T01:56) in response to an injection
of energy assumed to be in the form of nonthermal electrons. Hard
X-ray observations from RHESSI were used to track the evolution of
the parameters of the nonthermal electron distribution to reveal the
total power contained in flare accelerated electrons. By integrating
over the duration of the impulsive phase, the total energy contained
in the nonthermal electrons was found to be &gt;2 × 1031
erg. The response of the lower solar atmosphere was measured in
the free-bound EUV continua of H I (Lyman), He I, and He II, plus
the emission lines of He II at 304 Å and H I (Lyα) at 1216 Å by
SDO/EVE, the UV continua at 1600 Å and 1700 Å by SDO/AIA, and the
white light continuum at 4504 Å, 5550 Å, and 6684 Å, along with the
Ca II H line at 3968 Å using Hinode/SOT. The summed energy detected
by these instruments amounted to ~3 × 1030 erg about 15%
of the total nonthermal energy. The Lyα line was found to dominate
the measured radiative losses. Parameters of both the driving electron
distribution and the resulting chromospheric response are presented
in detail to encourage the numerical modeling of flare heating for
this event, to determine the depth of the solar atmosphere at which
these line and continuum processes originate, and the mechanism(s)
responsible for their generation.

---------------------------------------------------------
Title: Relationship between the photospheric Poynting flux and the
active region luminosity
Authors: Kazachenko, Maria D.; Canfield, Richard C.; Fisher, George
H.; Hudson, Hugh S.; Welsch, Brian
2014AAS...22412349K Altcode:
How does energy radiated by active regions compare with magnetic energy
that propagates lower across the photosphere? This is a fundamental
question for energy storage and release in active regions, yet it is
presently poorly understood. In this work we quantify and compare
both energy terms using SDO observations of the active region (AR)
11520. To quantify the magnetic energy crossing the photosphere, or
the Poynting flux, we need to know both the magnetic field vector B and
electric field vector E as well. Our current electric field inversion
technique, PDFI, combines the Poloidal-Toroidal-Decomposition method
with information from Doppler measurements, Fourier local correlation
tracking (FLCT) results, and the ideal MHD constraint, to determine
the electric field from vector magnetic field and Doppler data. We
apply the PDFI method to a sequence of Helioseismic and Magnetic
Imager (HMI/SDO) vector magnetogram data, to find the electric-field
and hence the Poynting-flux evolution in AR 11520. We find that most
of the magnetic energy in this AR is injected in the range of $10^7$
to $10^8$ $ergs/{cm^2 s}$, with the largest fluxes reaching $10^{10}$
$ergs/{cm^2 s}$. Integrating over the active region this yields a
total energy of order $10^{28}$ ergs/s. To quantify the active region
luminosity, we use EUV Variability Experiment (EVE) and Atmospheric
Imaging Assembly (AIA) spectrally resolved observations. We find the
active region luminosity of order $10^{28}$ ergs/s. We compare derived
magnetic and radiated energy fluxes on different temporal and spatial
scales and estimate their uncertainties. We also discuss the roles
that potential/non-potential and emerging/shearing terms play in the
total magnetic energy budget.

---------------------------------------------------------
Title: Current and future solar observation using focusing hard
X-ray imagers
Authors: Glesener, Lindsay; Caspi, Amir; Christe, Steven; Hannah, Iain;
Hudson, Hugh S.; Hurford, Gordon J.; Grefenstette, Brian; Krucker,
Sam; Marsh, Andrew; Mewaldt, Richard A.; Pivovaroff, Michael; Shih,
Albert Y.; Smith, David M.; Vogel, Julia; White, Stephen M.
2014AAS...22412364G Altcode:
The efficient processes that accelerate particles in solar flares
are not currently understood. Hard X-rays (HXRs) are one of the
best diagnostics of flare-accelerated electrons, and therefore of
acceleration processes. Past and current solar HXR observers rely on
indirect Fourier imaging and thus lack the necessary sensitivity and
imaging dynamic range to make detailed studies of faint HXR sources in
the solar corona (where particle acceleration is thought to occur). A
future generation of solar HXR observers will instead likely rely on
direct HXR focusing, which can provide far superior sensitivity and
imaging dynamic range.The first wave of focused solar HXR studies
is already underway, including sounding rocket and high-altitude
balloon payloads, and, in the near future, solar observation by the
NuSTAR astrophysics observatory. This poster will (1) summarize the
capabilities of current solar HXR instruments, comparing the science
that can be done from each platform, and (2) discuss the scientific
power of a future, dedicated, spaceborne observatory optimized to
observe HXRs from the Sun.

---------------------------------------------------------
Title: MESSENGER soft X-ray observations of the quiet solar corona
Authors: Schwartz, Richard A.; Hudson, Hugh S.; Tolbert, Anne K;
Dennis, Brian R.
2014AAS...22432349S Altcode:
In a remarkable result from their "SphinX" experiment, Sylwester
et al. (2012) found a non-varying base level of soft X-ray emission
at the quietest times in 2009. We describe comparable data from the
soft X-ray monitor on board MESSENGER (en route to Mercury) which had
excellent coverage both in 2009 and during the true solar minimum of
2008. These observations overlap SphinX's and also are often exactly at
Sun-MESSENGER-Earth conjunctions. During solar minimum the Sun-MESSENGER
distance varied substantially, allowing us to use the inverse-square law
to help distinguish the aperture flux (ie, solar X-rays) from that due
to sources of background in the 2-5 keV range. The MESSENGER data show
a non-varying background level for many months in 2008 when no active
regions were present. We compare these data in detail with those from
SphinX. Both sets of data reveal a different behavior when magnetic
active regions are present on the Sun, and when they are not.Reference:
Sylwester et al., ApJ 751, 111 (2012)

---------------------------------------------------------
Title: RHESSI/SAS Observations of the Optical Solar Limb Over a Full
Solar Cycle
Authors: Fivian, Martin; Hudson, Hugh S.
2014AAS...22421827F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb in the red continuum with a cadence typically set at
16 samples/s in each of three linear CCD sensors. RHESSI has observed
the Sun continuously since its launch in early 2002, and we have
acquired a unique data set ranging over a full 11-year solar cycle
and consisting of about 3x10^10 single data points. Analyzing data
for an initial period in 2004, these measurements have led to the most
accurate oblateness measurement to date, 8.01+-0.14 milli arcsec (Fivian
et al., 2008), a value consistent with models predicting an oblateness
from surface rotation. An excess oblateness term can be attributed to
magnetic elements possibly located in the enhanced network. We have
started to also study photometric properties of our data. Previous
observations of latitude-dependent brightness variations at the limb
had suggested the presence of a polar temperature excess as large
as 1.5 K. The RHESSI observations, made with a rotating telescope in
space, have great advantages in the rejection of systematic errors in
the very precise photometry required for such an observation. Our new
measurements of latitude-dependent brightness variations at the limb
lead to a quadrupolar term (a pole-to-equator temperature variation)
of the order of 0.1 K, an order of magnitude smaller than previously
reported. We present the analysis of these unique data, an overview of
some results and we report on our progress as we apply our developed
analysis method to the whole 12 years of data.

---------------------------------------------------------
Title: Chromospheric and Coronal HMI Flare Sources
Authors: Martinez Oliveros, Juan Carlos; Saint-Hilaire, Pascal;
Couvidat, Sebastien; Hudson, Hugh S.; Krucker, Sam
2014AAS...22412331M Altcode:
We present observations of white-light features in the low corona, for
three flares SOL20110308T1935, SOL20110308T0230 and SOL2013-05-13T16:01,
using data from the Helioseismic and Magnetic Imager (HMI) of the
Solar Dynamics Observatory. At least two distinct kinds of sources
appear (chromospheric and coronal) in the early and later phases of
flare development, in addition to the white-light footpoint sources
commonly observed in the lower atmosphere. The gradual emissions have
a clear identification with the classical loop-prominence system,
with emission contributions from electron scattering and from the
free-free continuum (as seen in soft X-rays). These sources may also
contain other continuum and/or line emissions and lead clearly to
coronal rain in some cases observed

---------------------------------------------------------
Title: CME Mass Estimates via EVE Coronal Dimmings for X-class Flares
Authors: Hudson, Hugh S.; Hannah, Iain; Schrijver, Karel
2014AAS...22421810H Altcode:
The EVE instrument on SDO detects post-flare dimmings, mainly in the
spectral regions of Fe IX-XII in its MEGS-A range, which is available
for most of the 29 X-class flares that have occurred between SDO launch
and the end of April 2014. Based upon earlier X-ray observations
we interpret these dimmings as the result of CME mass ejection from
the low corona. We estimate the masses involved in these dimmings by
deriving a best pre-event temperature and emission measure in the dimmed
region from EVE, and a source volume from AIA images. The dimming for
SOL2011-02-15, the first of these events, "peaked"at -3.4% in Fe IX
in terms of the pre-event emission from the whole Sun, with smaller
relative depletions in higher ionization states of Fe. The "maximum"
occurred more than one hour after GOES peak. The dimming signature is
generally cleanly measurable in the EVE/MEGS-A spectral samples at10
s cadence, with the dominant source of uncertainty stemming from the
"sun-as-a-star" integrations; for example flare-related excess emission
at a given wavelength tends to compensate for the dimming,and in this
sense the mass estimate must be considered a lower limit. We address
these uncertainties for the solar case by appealing to the AIA images,
but for analogous processes in stellar flares one would not have
this luxury.

---------------------------------------------------------
Title: Helioseismic and Magnetic Imager Observations of Linear
Polarization from a Loop Prominence System
Authors: Saint-Hilaire, Pascal; Schou, Jesper; Martinez Oliveros, Juan
Carlos; Hudson, Hugh S.; Krucker, Sam; Bain, Hazel; Couvidat, Sebastien
2014AAS...22412311S Altcode:
White-light observations by the Solar Dynamics Observatory's
Helioseismic and Magnetic Imager of a loop-prominence system occurring
in the aftermath of an X-class flare on 2013 May 13 near the eastern
solar limb show a linearly polarized component, reaching up to 20%
at an altitude of 33 Mm, about the maximal amount expected if the
emission were due solely to Thomson scattering of photospheric light
by the coronal material. The mass associated with the polarized
component was 8.2x10^14 g. At 15 Mm altitude, the brightest part of
the loop was 3(+/-0.5)% linearly polarized, only about 20% of that
expected from pure Thomson scattering, indicating the presence of an
additional unpolarized component at wavelengths near Fe I (617.33 nm),
probably thermal emission. We estimated the free electron density of
the white-light loop system to possibly be as high as 1.8x10^12 cm^-3.

---------------------------------------------------------
Title: Observations of Linear Polarization in a Solar Coronal Loop
Prominence System Observed near 6173 Å
Authors: Saint-Hilaire, Pascal; Schou, Jesper; Martínez Oliveros,
Juan-Carlos; Hudson, Hugh S.; Krucker, Säm; Bain, Hazel; Couvidat,
Sébastien
2014ApJ...786L..19S Altcode: 2014arXiv1402.7016S
White-light observations by the Solar Dynamics Observatory's
Helioseismic and Magnetic Imager of a loop-prominence system occurring
in the aftermath of an X-class flare on 2013 May 13 near the eastern
solar limb show a linearly polarized component, reaching up to ~20%
at an altitude of ~33 Mm, about the maximum amount expected if the
emission were due solely to Thomson scattering of photospheric light by
the coronal material. The mass associated with the polarized component
was 8.2 × 1014 g. At 15 Mm altitude, the brightest part
of the loop was 3(±0.5)% linearly polarized, only about 20% of that
expected from pure Thomson scattering, indicating the presence of an
additional unpolarized component at wavelengths near Fe I (617.33
nm). We estimate the free electron density of the white-light loop
system to possibly be as high as 1.8 × 1012 cm-3.

---------------------------------------------------------
Title: The Role of Magnetic Fields in Transient Seismic Emission
Driven by Atmospheric Heating in Flares
Authors: Lindsey, C.; Donea, A. -C.; Martínez Oliveros, J. C.;
Hudson, H. S.
2014SoPh..289.1457L Altcode: 2013arXiv1303.3299L; 2014SoPh..tmp....9L
Transient seismic emission in flares remains largely mysterious. Its
discoverers proposed that seismic transients are driven by impulsive
heating of the flaring chromosphere. Simulations of such heating
show strong shocks, but these are damped by heavy radiative losses
as they proceed downward. Because compression of the gas the shock
enters both heats it and increases its density, the radiative losses
increase radically with the strength of the shock, leaving doubt
that sufficient energy can penetrate into the solar interior to
explain helioseismic signatures. We note that simulations to date
have no account for strong, inclined magnetic fields characteristic
of transient-seismic-source environments. A strong horizontal magnetic
field, for example, greatly increases the compressional modulus of the
chromospheric medium, greatly reducing compression of the gas, hence
radiative losses. Inclined magnetic fields, then, must be fundamental
to the role of impulsive heating in transient seismic emission.

---------------------------------------------------------
Title: Cycle 23 Variation in Solar Flare Productivity
Authors: Hudson, Hugh; Fletcher, Lyndsay; McTiernan, Jim
2014SoPh..289.1341H Altcode: 2014arXiv1401.6474H
The NOAA listings of solar flares in cycles 21 - 24, including the GOES
soft X-ray magnitudes, enable a simple determination of the number of
flares each flaring active region produces over its lifetime. We have
studied this measure of flare productivity over the interval 1975 -
2012. The annual averages of flare productivity remained approximately
constant during cycles 21 and 22, at about two reported M- or X-flares
per region, but then increased significantly in the declining phase
of cycle 23 (the years 2004 - 2005). We have confirmed this by using
the independent RHESSI flare catalog to check the NOAA events listings
where possible. We note that this measure of solar activity does not
correlate with the solar cycle. The anomalous peak in flare productivity
immediately preceded the long solar minimum between cycles 23 and 24.

---------------------------------------------------------
Title: Transient Artifacts in a Flare Observed by the Helioseismic
and Magnetic Imager on the Solar Dynamics Observatory
Authors: Martínez Oliveros, J. C.; Lindsey, C.; Hudson, H. S.;
Buitrago Casas, J. C.
2014SoPh..289..809M Altcode: 2013arXiv1307.5097M
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) provides a new tool for the systematic observation
of white-light flares, including Doppler and magnetic information as
well as continuum. In our initial analysis of the highly impulsive -ray
flare SOL2010-06-12T00:57 (Martínez Oliveros et al., Solar Phys.269,
269, 2011), we reported the signature of a strong blueshift in the
two footpoint sources. Concerned that this might be an artifact due
to aliasing peculiar to the HMI instrument, we undertook a comparative
analysis of Global Oscillation Network Group (GONG++) observations of
the same flare, using the PArametric Smearing Correction ALgorithm
(PASCAL) algorithm to correct for artifacts caused by variations in
atmospheric smearing. This analysis confirms the artifactual nature
of the apparent blueshift in the HMI observations, finding weak
redshifts at the footpoints instead. We describe the use of PASCAL
with GONG++ observations as a complement to the SDO observations
and discuss constraints imposed by the use of HMI far from its design
conditions. With proper precautions, these data provide rich information
on flares and transients.

---------------------------------------------------------
Title: Chromospheric and Coronal Observations of Solar Flares with
the Helioseismic and Magnetic Imager
Authors: Martínez Oliveros, Juan-Carlos; Krucker, Säm; Hudson, Hugh
S.; Saint-Hilaire, Pascal; Bain, Hazel; Lindsey, Charles; Bogart,
Rick; Couvidat, Sebastien; Scherrer, Phil; Schou, Jesper
2014ApJ...780L..28M Altcode: 2013arXiv1311.7412M
We report observations of white-light ejecta in the low corona, for
two X-class flares on 2013 May 13, using data from the Helioseismic
and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least
two distinct kinds of sources appeared (chromospheric and coronal),
in the early and later phases of flare development, in addition to
the white-light footpoint sources commonly observed in the lower
atmosphere. The gradual emissions have a clear identification
with the classical loop-prominence system, but are brighter than
expected and possibly seen here in the continuum rather than line
emission. We find the HMI flux exceeds the radio/X-ray interpolation
of the bremsstrahlung produced in the flare soft X-ray sources by at
least one order of magnitude. This implies the participation of cooler
sources that can produce free-bound continua and possibly line emission
detectable by HMI. One of the early sources dynamically resembles
"coronal rain", appearing at a maximum apparent height and moving
toward the photosphere at an apparent constant projected speed of 134
± 8 km s-1. Not much literature exists on the detection of
optical continuum sources above the limb of the Sun by non-coronagraphic
instruments and these observations have potential implications for our
basic understanding of flare development, since visible observations
can in principle provide high spatial and temporal resolution.

---------------------------------------------------------
Title: Prominences in SDO/EVE spectra: contributions from large
solar structures
Authors: Labrosse, Nicolas; Hudson, Hugh; Kazachenko, Maria
2014IAUS..300..439L Altcode:
The EVE instrument on SDO is making accurate measurements of the
solar spectral irradiance in the EUV between 30 and 1069 Å, with 1
Å spectral resolution and 10 s sampling rate. These data define solar
variability in the “Sun-as-a-star” mode and reveal many interesting
kinds of variation. Its high sensitivity also makes it suitable for
spectroscopic diagnostics of solar features such as flares. Here we
present EVE's potential contribution to the diagnostics of large-scale,
slowly evolving features such as prominences and active regions,
and what we can learn from this.

---------------------------------------------------------
Title: Solar Eruptive Events (SEE) 2020 Mission Concept
Authors: Lin, R. P.; Caspi, A.; Krucker, S.; Hudson, H.; Hurford,
G.; Bandler, S.; Christe, S.; Davila, J.; Dennis, B.; Holman, G.;
Milligan, R.; Shih, A. Y.; Kahler, S.; Kontar, E.; Wiedenbeck, M.;
Cirtain, J.; Doschek, G.; Share, G. H.; Vourlidas, A.; Raymond, J.;
Smith, D. M.; McConnell, M.; Emslie, G.
2013arXiv1311.5243L Altcode:
Major solar eruptive events (SEEs), consisting of both a large flare and
a near simultaneous large fast coronal mass ejection (CME), are the most
powerful explosions and also the most powerful and energetic particle
accelerators in the solar system, producing solar energetic particles
(SEPs) up to tens of GeV for ions and hundreds of MeV for electrons. The
intense fluxes of escaping SEPs are a major hazard for humans in space
and for spacecraft. Furthermore, the solar plasma ejected at high speed
in the fast CME completely restructures the interplanetary medium
(IPM) - major SEEs therefore produce the most extreme space weather
in geospace, the interplanetary medium, and at other planets. Thus,
understanding the flare/CME energy release process(es) and the related
particle acceleration processes are major goals in Heliophysics. To
make the next major breakthroughs, we propose a new mission concept,
SEE 2020, a single spacecraft with a complement of advanced new
instruments that focus directly on the coronal energy release and
particle acceleration sites, and provide the detailed diagnostics of
the magnetic fields, plasmas, mass motions, and energetic particles
required to understand the fundamental physical processes involved.

---------------------------------------------------------
Title: Implosion of Coronal Loops during the Impulsive Phase of a
Solar Flare
Authors: Simões, P. J. A.; Fletcher, L.; Hudson, H. S.; Russell,
A. J. B.
2013ApJ...777..152S Altcode: 2013arXiv1309.7090S
We study the relationship between implosive motions in a solar
flare, and the energy redistribution in the form of oscillatory
structures and particle acceleration. The flare SOL2012-03-09T03:53
(M6.4) shows clear evidence for an irreversible (stepwise) coronal
implosion. Extreme-ultraviolet (EUV) images show at least four
groups of coronal loops at different heights overlying the flaring
core undergoing fast contraction during the impulsive phase of the
flare. These contractions start around a minute after the flare onset,
and the rate of contraction is closely associated with the intensity
of the hard X-ray and microwave emissions. They also seem to have
a close relationship with the dimming associated with the formation
of the coronal mass ejection and a global EUV wave. Several studies
now have detected contracting motions in the corona during solar
flares that can be interpreted as the implosion necessary to release
energy. Our results confirm this, and tighten the association with
the flare impulsive phase. We add to the phenomenology by noting
the presence of oscillatory variations revealed by Geostationary
Operational Environmental Satellite soft X-rays (SXR) and spatially
integrated EUV emission at 94 and 335 Å. We identify pulsations
of ≈60 s in SXR and EUV data, which we interpret as persistent,
semi-regular compressions of the flaring core region which modulate
the plasma temperature and emission measure. The loop oscillations,
observed over a large region, also allow us to provide rough estimates
of the energy temporarily stored in the eigenmodes of the active-region
structure as it approaches its new equilibrium.

---------------------------------------------------------
Title: Estimating active region luminosity using EVE/SDO observations
Authors: Kazachenko, Maria D.; Hudson, H. S.; Fisher, G. H.; Canfield,
R. C.
2013SPD....44...44K Altcode:
Do solar active regions typically radiate more coronal energy during
flares than the quiescent periods between them? This is a fundamental
question for storage and release models of flares and active regions,
yet it is presently poorly answered by observations. The EUV Variability
Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides
spectrally resolved observations of the Sun in the "Sun-as-a-point
source" mode. It covers a wide range of temperatures and thus allows
a detailed study of thermal emissions. Here we present two approaches
for computing the active region luminosity, using EVE observations of
fourteen Fe lines (FeIX-FeXXIV). In the first approach, we analyze EVE
data in a time-series sense, when only one active region is present on
the disk; this allows us to subtract the background due to the quiet
sun and get the contribution from the active region alone. In the
second approach, we analyze correlations of the radiative signatures
with proxy indices (total solar magnetic and Poynting fluxes) during
several months of data, when multiple active regions are present
on the solar disk. We discuss capabilities of the two approaches,
and what we can learn from them.Abstract (2,250 Maximum Characters):
Do solar active regions typically radiate more coronal energy during
flares than the quiescent periods between them? This is a fundamental
question for storage and release models of flares and active regions,
yet it is presently poorly answered by observations. The EUV Variability
Experiment (EVE) on the Solar Dynamics Observatory (SDO) provides
spectrally resolved observations of the Sun in the "Sun-as-a-point
source" mode. It covers a wide range of temperatures and thus allows
a detailed study of thermal emissions. Here we present two approaches
for computing the active region luminosity, using EVE observations of
fourteen Fe lines (FeIX-FeXXIV). In the first approach, we analyze EVE
data in a time-series sense, when only one active region is present on
the disk; this allows us to subtract the background due to the quiet
sun and get the contribution from the active region alone. In the
second approach, we analyze correlations of the radiative signatures
with proxy indices (total solar magnetic and Poynting fluxes) during
several months of data, when multiple active regions are present on
the solar disk. We discuss capabilities of the two approaches, and
what we can learn from them.

---------------------------------------------------------
Title: Using X-ray absorption to measure the height of the solar
atmosphere
Authors: Hudson, Hugh S.; Battaglia, M.; Hurford, G. J.; Krucker,
S.; Schwartz, R. A.
2013SPD....44..117H Altcode:
The X-ray image of a partially-occulted solar flare, one occurring
just behind the limb of the Sun, can have a sharply defined X-ray edge
resulting from attenuation in the atmosphere of the quiet Sun in the
foreground. Our analysis makes use of RHESSI's direct measurement of
image Fourier visibilities, and we estimate that the ultimate precision
of the limb height will on the order of the photospheric scale height in
the region of dominant absorption. This occurs at an altitude depending
on the X-ray photon energies used for the measurement, but generally in
the upper photosphere and chromosphere. We give a preliminary report on
analysis of one suitable event, the flare SOL2002-04-04T15:32 (M6.1),
where we find a clean signature of this attenuation up to the RHESSI
hard X-ray range 12-25 keV. At this energy Compton scattering begins
to dominate the attenuation, greatly reducing the model-dependence of
the result; at lower photon energies photoelectric absorption becomes
more important. These data determine the physical altitude of the
mean atmospheric density, with minimal model dependence, and therefore
provide an independent calibration of the atmospheric height scale.

---------------------------------------------------------
Title: White-light and Hard X-ray source heights of the
SOL2011-01-28T00:24 solar flare
Authors: Martinez Oliveros, Juan Carlos; Glesener, L.; Hudson, H. S.;
Krucker, S.; Hurford, G. J.
2013SPD....44...86M Altcode:
White-light continuum and hard X-ray emission in flares have strong
correlations in time, but at present we do not have a clear idea about
their height structures. Recently, several studies of the relative
positions of the white-light and hard X-ray sources have been made using
observations of flares near the solar limb. However, these results are
still inconclusive due to the small number of flares observed. On 28
January 2011 a white-light flare (SOL2011-01-28T07:35) was observed
on the western limb, observed simultaneously by the Helioseismic
Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the
Solar TErrestrial RElations Observatory (STEREO). This observation
provides the heights of these emissions directly, limited only by
the limb references for the two spacecraft, with almost no projection
uncertainty. We report the results of this analysis and discuss our
findings in terms of present models of particle acceleration and energy
transport in the impulsive phase.Abstract (2,250 Maximum Characters):
White-light continuum and hard X-ray emission in flares have strong
correlations in time, but at present we do not have a clear idea about
their height structures. Recently, several studies of the relative
positions of the white-light and hard X-ray sources have been made using
observations of flares near the solar limb. However, these results are
still inconclusive due to the small number of flares observed. On 28
January 2011 a white-light flare (SOL2011-01-28T07:35) was observed
on the western limb, observed simultaneously by the Helioseismic
Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO), the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the
Solar TErrestrial RElations Observatory (STEREO). This observation
provides the heights of these emissions directly, limited only by
the limb references for the two spacecraft, with almost no projection
uncertainty. We report the results of this analysis and discuss our
findings in terms of present models of particle acceleration and energy
transport in the impulsive phase.

---------------------------------------------------------
Title: Photospheric Temperature Variations near the Solar Limb II
Authors: Fivian, Martin; Hudson, H. S.
2013SPD....44..121F Altcode:
We use observations from the Solar Aspect Sensor (SAS) of RHESSI
to characterize the latitude dependence of the temperature of the
photosphere near the solar limb. Previous observations had suggested
the presence of a polar temperature excess as large as 1.5 K. The
RHESSI observations, made with a rotating telescope in space, have
great advantages in the rejection of systematic errors in the very
precise photometry required for such an observation. This photometry
is differential, i.e. relative to a mean limb-darkening function. The
data base consists of about 1,000 images per day from linear CCDs
with 1.73 arc sec square pixels, observing a narrow band (12nm FWHM)
at 670 nm. Each image shows a chord crossing the disk at a different
location as the spacecraft rotates and precesses around its average
solar pointing. We fit an average limb-darkening function and reassemble
the residuals into synoptic maps of differential intensity variations as
a function of position angle. We further mask these images against EUV
images (SOHO/EIT 284A for older data and SDO/AIA for more recent data)
in order to eliminate magnetic regions. We present results from our new
analysis which shows significantly larger signals of latitude-dependent
temperature variations than what has been presented earlier in our
preliminary analysis and interpretation.Abstract (2,250 Maximum
Characters): We use observations from the Solar Aspect Sensor (SAS)
of RHESSI to characterize the latitude dependence of the temperature
of the photosphere near the solar limb. Previous observations had
suggested the presence of a polar temperature excess as large as 1.5
K. The RHESSI observations, made with a rotating telescope in space,
have great advantages in the rejection of systematic errors in the very
precise photometry required for such an observation. This photometry
is differential, i.e. relative to a mean limb-darkening function. The
data base consists of about 1,000 images per day from linear CCDs
with 1.73 arc sec square pixels, observing a narrow band (12nm FWHM)
at 670 nm. Each image shows a chord crossing the disk at a different
location as the spacecraft rotates and precesses around its average
solar pointing. We fit an average limb-darkening function and reassemble
the residuals into synoptic maps of differential intensity variations as
a function of position angle. We further mask these images against EUV
images (SOHO/EIT 284A for older data and SDO/AIA for more recent data)
in order to eliminate magnetic regions. We present results from our new
analysis which shows significantly larger signals of latitude-dependent
temperature variations than what has been presented earlier in our
preliminary analysis and interpretation.

---------------------------------------------------------
Title: Flare Ribbon Energetics in the Early Phase of an SDO Flare
Authors: Fletcher, L.; Hannah, I. G.; Hudson, H. S.; Innes, D. E.
2013ApJ...771..104F Altcode: 2014arXiv1401.6538F
The sites of chromospheric excitation during solar flares are
marked by extended extreme ultraviolet ribbons and hard X-ray
(HXR) footpoints. The standard interpretation is that these are
the result of heating and bremsstrahlung emission from non-thermal
electrons precipitating from the corona. We examine this picture using
multi-wavelength observations of the early phase of an M-class flare
SOL2010-08-07T18:24. We aim to determine the properties of the heated
plasma in the flare ribbons, and to understand the partition of the
power input into radiative and conductive losses. Using GOES, SDO/EVE,
SDO/AIA, and RHESSI, we measure the temperature, emission measure (EM),
and differential emission measure of the flare ribbons, and deduce
approximate density values. The non-thermal EM, and the collisional
thick target energy input to the ribbons are obtained from RHESSI using
standard methods. We deduce the existence of a substantial amount
of plasma at 10 MK in the flare ribbons, during the pre-impulsive
and early-impulsive phase of the flare. The average column EM of
this hot component is a few times 1028 cm-5,
and we can calculate that its predicted conductive losses dominate
its measured radiative losses. If the power input to the hot ribbon
plasma is due to collisional energy deposition by an electron beam
from the corona then a low-energy cutoff of ~5 keV is necessary to
balance the conductive losses, implying a very large electron energy
content. Independent of the standard collisional thick-target electron
beam interpretation, the observed non-thermal X-rays can be provided
if one electron in 103-104 in the 10 MK (1 keV)
ribbon plasma has an energy above 10 keV. We speculate that this could
arise if a non-thermal tail is generated in the ribbon plasma which
is being heated by other means, for example, by waves or turbulence.

---------------------------------------------------------
Title: The properties of flare kernels observed by the Dunn Solar
Telescope
Authors: Fletcher, Lyndsay; Kowalski, A.; Cauzzi, G.; Hawley, S. L.;
Hudson, H. S.
2013SPD....44...67F Altcode:
We report on a campaign at the Dunn Solar Telescope which resulted in
successful imaging and spectroscopic observations of a C1.1 solar flare
on 18th August 2011. This flare exhibited ribbons with complicated
fine structure at the resolution of the DST/IBIS instrument, and a
number of bright kernels with sizes comparable to the smallest scales
sampled by IBIS, around 2-4 pixels (0."3-0."6) FWHM. We focus on these
bright kernels, describing their spatial characteristics in the core
and wing of H alpha and Ca II 8542, and in the UV and EUV with SDO. We
also show preliminary broad-band spectroscopy of the kernels which may
demonstrate the presence of an optical continuum in this small flare.

---------------------------------------------------------
Title: Remote sensing of low-energy SEPs via charge exchange
Authors: Hudson, H. S.; MacKinnon, A. L.; Badnell, N. R.
2013AIPC.1539...19H Altcode:
Charge-exchange reactions at high energies provide new channels for
the remote sensing of solar high-energy particles, as demonstrated by
the recent detection of 1.8-5 MeV hydrogen atoms from a solar flare
[1]. Orrall and Zirker [2] had earlier proposed the detection of
low-energy protons via charge-exchange atomic reactions in the solar
atmosphere, leading in the simplest case to extended red-wing emission
in the Lyman-alpha line. We discuss the analogous process for the He
II 304 A˚ line (for alpha particles) and also assess the feasibility
of the analogous process in the solar wind, whereby ambient He and
(C, N, O) ions allow low-energy alpha particles to undergo resonant
charge exchange in the ambient corona and thereby produce 304 A˚ wing
emission close to the acceleration region.

---------------------------------------------------------
Title: Solar flares at submillimeter wavelengths
Authors: Krucker, Säm; Giménez de Castro, C. G.; Hudson, H. S.;
Trottet, G.; Bastian, T. S.; Hales, A. S.; Kašparová, J.; Klein,
K. -L.; Kretzschmar, M.; Lüthi, T.; Mackinnon, A.; Pohjolainen, S.;
White, S. M.
2013A&ARv..21...58K Altcode:
We discuss the implications of the first systematic observations of
solar flares at submillimeter wavelengths, defined here as observing
wavelengths shorter than 3 mm (frequencies higher than 0.1 THz). The
events observed thus far show that this wave band requires a new
understanding of high-energy processes in solar flares. Several events,
including observations from two different observatories, show during
the impulsive phase of the flare a spectral component with a positive
(increasing) slope at the highest observable frequencies (up to 405
GHz). To emphasize the increasing spectra and the possibility that
these events could be even more prominent in the THz range, we term
this spectral feature a "THz component". Here we review the data and
methods, and critically assess the observational evidence for such
distinct component(s). This evidence is convincing. We also review the
several proposed explanations for these feature(s), which have been
reported in three distinct flare phases. These data contain important
clues to flare development and particle acceleration as a whole, but
many of the theoretical issues remain open. We generally have lacked
systematic observations in the millimeter-wave to far-infrared range
that are needed to complete our picture of these events, and encourage
observations with new facilities.

---------------------------------------------------------
Title: Coronal Post-Flare Dynamics for 14 August 2010 Late Phase of
the Coronal Dimming Event
Authors: Didkovsky, Leonid; Judge, Darrell; Wieman, Seth; Woods, Tom;
Hock, Rachel; Chamberlin, Phillip; Tobiska, Kent; Hudson, Hugh
2013enss.confE..22D Altcode:
Coronal irradiance dynamics related to the 14 August 2010 C4.4
post-flare event was analyzed using EUV spectral emission lines in the
range of logT from 5.8 to 6.4 (0.7 to 2.6 MK). Temporal changes of
high-resolution spectral irradiance for different thermal layers of
the Corona from the Extreme ultraviolet Variability Experiment (EVE)
onboard the Solar Dynamics Observatory (SDO) were compared to study both
a decrease and an increase of the spectral irradiance during the late
phase of the coronal dimming. We found an irradiance decrease which
propagated from the low-temperature layer to the higher temperature
layers with much lower speed than the speed of the `horizontal'
dimming wave inside the low-temperature layer. This upward decrease
of the irradiance in the coronal layers may represent a `vertical'
spatial perturbation of the plasma within these layers in response to
the plasma density and temperature decreases of the dimming wave. The
perturbation may cause a redistribution of the coronal loops and
trigger the late phase of the flare. The increase of the irradiance
detected in the high-temperature coronal layers where the late phase
of the flare occurred after 12 UT was propagating in part downward,
restoring the pre-dimming plasma conditions in these Coronal layers.

---------------------------------------------------------
Title: Non-thermal processes in coronae and beyond
Authors: Poppenhaeger, K.; Günther, H. M.; Beiersdorfer, P.;
Brickhouse, N. S.; Carter, J. A.; Hudson, H. S.; Kowalski, A.; Lalitha,
S.; Miceli, M.; Wolk, S. J.
2013AN....334..101P Altcode: 2013csss...17..101P; 2012arXiv1210.2960P
This contribution summarizes the splinter session “Non-thermal
processes in coronae and beyond” held at the Cool Stars 17 workshop
in Barcelona in 2012. It covers new developments in high energy
non-thermal effects in the Earth's exosphere, solar and stellar flares,
the diffuse emission in star forming regions and reviews the state
and the challenges of the underlying atomic databases.

---------------------------------------------------------
Title: GRB 130427A: RHESSI observations.
Authors: Smith, D. M.; Csillaghy, A.; Hurley, K.; Hudson, H.; Boggs,
S.; Inglis, A.
2013GCN.14590....1S Altcode: 2013GCN..14590...1S
No abstract at ADS

---------------------------------------------------------
Title: Opportunities for Solar Science with NuSTAR
Authors: Glesener, Lindsay; Boggs, S. E.; Christensen, F.; Craig,
W. W.; Hailey, C. J.; Grefenstette, B.; Harrison, F.; Hudson, H. S.;
Hurford, G. J.; Krucker, S.; Marsh, A.; Mewaldt, R. A.; Pivovaroff,
M.; Smith, D. M.; Stern, D.; Vogel, J.; White, S. M.; Zhang, W.;
NuSTAR Team
2013AAS...22124423G Altcode:
While NuSTAR was designed to observe faint cosmic sources in hard
X-rays (HXR), its unprecedented sensitivity can also be used to address
several outstanding questions in high energy solar physics. Medium- and
large-sized solar flares have been well -studied in HXR by the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI), launched in
2002. These flares are always found in active regions and usually
emit nonthermal HXR from accelerated electrons, along with thermal
bremsstrahlung as those electrons lose their energy and heat the
ambient plasma. To date, no HXR flares outside active regions have been
observed, though thermal brightenings in soft X-rays and EUV suggest
that small "nanoflares" may occur frequently across the entire solar
disk, even at quiet times when no active regions are present. Even a
few minutes of NuSTAR solar observations will allow a search for HXR
from quiet-Sun nanoflares with better sensitivity than any previous
study. These observations will have important implications for the
role of flares in supplying the corona with its surprisingly hot
temperature (1--2 MK, as compared with the photospheric temperature
of 5800 K). NuSTAR will also make the first observations of escaping
flare electrons associated with Type III radio emission, can image
faint coronal sources in partially occulted flares that are below
RHESSI's sensitivity, and, combined with RHESSI data, could study the
faint, earliest phase of flares, where direct signatures of particle
acceleration are most likely to be observed.

---------------------------------------------------------
Title: A Coral Sea Rehearsal for the Eclipse Megamovie
Authors: Hudson, H. S.; Davey, A. R.; Ireland, J.; Jones, L.; Mcintosh,
S. W.; Paglierani, R.; Pasachoff, J. M.; Peticolas, L. M.; Russell,
R. M.; Suarez Sola, F. I.; Sutherland, L.; Thompson, M. J.
2012AGUFMSH11C..06H Altcode:
The "Eclipse on the Coral Sea" - 13/14 November 2012 (GMT/Australia)
- will have happened already. Our intention is to have used this
opportunity as a trial run for the eclipse in 2017, which features
1.5 hours of totality across the whole width of the continental
US. Conceived first and foremost as an education and public outreach
activity, the plan is to engage the public in solar science and
technology by providing a way for them to include images they have taken
of the solar eclipse, into a movie representation of coronal evolution
in time. This project will assimilate as much eclipse photography as
possible from the public. The resulting movie(s) will cover all ranges
of expertise, and at the basic smartphone or hand-held digital camera
level, we expect to have obtained a huge number of images in the case
of good weather conditions. The capability of modern digital technology
to handle such a data flow is new. The basic purpose of this and the
2017 Megamovie observations is to explore this capability and its
ability to engage people from many different communities in the solar
science, astronomy, mathematics, and technology. The movie in 2017,
especially, may also have important science impact because of the
uniqueness of the corona as seen under eclipse conditions. In this
presentation we will describe our smartphone application development
(see the "Transit of Venus" app for a role model here). We will also
summarize data acquisition via both the app and more traditional web
interfaces. Although for the Coral Sea eclipse event we don't expect to
have a movie product by the time of the AGU, for the 2017 event we do
intend to assemble the heterogenous data into beautiful movies within a
short space of time after the eclipse. These movies may have relatively
low resolution but would extend to the base of the corona. We encourage
participation in the 2012 observations, noting that no total eclipse,
prior to 2017, will occur in a region with good infrastructure for
extended observations. The National Center for Atmospheric Research is
sponsored by the National Science Foundation. The Megamovie project
is supported by NSF grant AGS-1247226, and JMP's eclipse work about
the eclipses of 2012 is supported by NSF grant AGS-1047726.

---------------------------------------------------------
Title: New Coronal Developments in Flare Research
Authors: Hudson, H. S.
2012AGUFMSH42A..04H Altcode:
The past decade has produced spectacular new observations that have
affected our views of the relationships between flares and coronal
phenomena. We have substantial new clues about the nature of the energy
release that results from the restructuring of the coronal field. The
energy partitioning in a flare/CME event strongly suggests a close
relationship between the microphysics of particle acceleration and
the global scales of the necessary coronal restructuring. Among the
sources of information about the accelerated particles we can now
list hard X-rays, gamma-rays, SEPs, and ENAs, as well as the radio
spectrum. Unexpectedly, the EUV spectroscopy from SDO/EVE shows that
the Orrall-Zirker mechanism (Doppler-shifted He II 304 A, resulting
from charge-exchange reactions) cannot easily be used to link these
scales. In this presentation I synthesize the available data and
suggest some desirable future developments of theory and observation.

---------------------------------------------------------
Title: Magnetism of Solar Flares and Prominences
Authors: Heinzel, P.; Hudson, H. S.
2012ASPC..463..121H Altcode:
We give an overview of magnetic fields in solar flares and
prominences. Magnetic fields related to flares play a crucial role in
the process of energy release and transport to the lower atmosphere,
and thus magnetometry under the coronal and chromospheric conditions is
extremely challenging. Magnetic fields in prominences are supposed to
keep the prominence plasma at coronal heights against the gravity. Their
measurements have been numerous, but high-resolution mapping is still
missing. We discuss various flare and prominence models in connection
to current and future high-resolution observations.

---------------------------------------------------------
Title: The Solar Oblateness at Solar Minimum as Observed by
RHESSI/SAS II
Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.
2012AGUFMSH13C2264F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb in the red continuum with a cadence typically set at
32 samples/s in each of three linear CCD sensors. RHESSI has observed
the Sun continuously now for more than 10 years, and we have acquired a
unique data set ranging almost over a full solar cycle and consisting
of about 25x10^9 single data points. For a three month period during
the active phase of the last solar cycle in 2004, the shape of the
solar disk has been measured discovering an apparent excess oblateness
which we attributed to the enhanced network. These measurements have led
to the most accurate oblateness measurement to date, 8.01+-0.14 milli
arcsec (Fivian et al., 2008), a value consistent with models predicting
an oblateness from surface rotation. In order to avoid confusion between
magnetic activity and a correlated brightness enhancement in the SAS
signal, the SAS data has been masked using the SOHO/EIT284A data, and
SDO/AIA for more recent data. The measured oblateness as function of
the masking level is then extrapolated for a value of the underlaying,
presumably non-magnetic sun. A recent and significantly improved
calibration of the SAS data have allowed a new access to a measurement
of the solar oblateness during the last, extended solar minimum. Here,
we present the analysis of the RHESSI/SAS data during the solar minimum
with the inferred interpretation for the oblateness signal.

---------------------------------------------------------
Title: Magneto-Acoustic Energetics Study of the Seismically Active
Flare of 15 February 2011
Authors: Alvarado-Gómez, J. D.; Buitrago-Casas, J. C.;
Martínez-Oliveros, J. C.; Lindsey, C.; Hudson, H.; Calvo-Mozo, B.
2012SoPh..280..335A Altcode: 2012arXiv1203.3907A; 2012SoPh..tmp..131A
Multi-wavelength studies of energetic solar flares with seismic
emissions have revealed interesting common features between
them. We studied the first GOES X-class flare of Solar Cycle 24,
as detected by the Solar Dynamics Observatory (SDO). For context,
seismic activity from this flare (SOL2011-02-15T01:55-X2.2, in NOAA
AR 11158) has been reported by Kosovichev (Astrophys. J. Lett.,
734, L15, 2011) and Zharkov et al. (Astrophys. J. Lett., 741, L35,
2011). Based on Dopplergram data from the Helioseismic and Magnetic
Imager (HMI), we applied standard methods of local helioseismology
in order to identify the seismic sources in this event. RHESSI hard
X-ray data are used to check the correlation between the location of
the seismic sources and the particle-precipitation sites in during
the flare. Using HMI magnetogram data, the temporal profile of
fluctuations in the photospheric line-of-sight magnetic field is used
to estimate the magnetic-field change in the region where the seismic
signal was observed. This leads to an estimate of the work done by the
Lorentz-force transient on the photosphere of the source region. In this
instance, this is found to be a significant fraction of the acoustic
energy in the attendant seismic emission, suggesting that Lorentz forces
can contribute significantly to the generation of sunquakes. However,
there are regions in which the signature of the Lorentz force is much
stronger, but from which no significant acoustic emission emanates.

---------------------------------------------------------
Title: Estimating the frequency of extremely energetic solar events,
based on solar, stellar, lunar, and terrestrial records
Authors: Schrijver, C. J.; Beer, J.; Baltensperger, U.; Cliver,
E. W.; Güdel, M.; Hudson, H. S.; McCracken, K. G.; Osten, R. A.;
Peter, T.; Soderblom, D. R.; Usoskin, I. G.; Wolff, E. W.
2012JGRA..117.8103S Altcode: 2012arXiv1206.4889S; 2012JGRA..11708103S
The most powerful explosions on the Sun - in the form of bright
flares, intense storms of solar energetic particles (SEPs), and fast
coronal mass ejections (CMEs) - drive the most severe space-weather
storms. Proxy records of flare energies based on SEPs in principle
may offer the longest time base to study infrequent large events. We
conclude that one suggested proxy, nitrate concentrations in polar
ice cores, does not map reliably to SEP events. Concentrations of
select radionuclides measured in natural archives may prove useful in
extending the time interval of direct observations up to ten millennia,
but as their calibration to solar flare fluences depends on multiple
poorly known properties and processes, these proxies cannot presently be
used to help determine the flare energy frequency distribution. Being
thus limited to the use of direct flare observations, we evaluate the
probabilities of large-energy solar events by combining solar flare
observations with an ensemble of stellar flare observations. We conclude
that solar flare energies form a relatively smooth distribution from
small events to large flares, while flares on magnetically active,
young Sun-like stars have energies and frequencies markedly in excess
of strong solar flares, even after an empirical scaling with the mean
coronal activity level of these stars. In order to empirically quantify
the frequency of uncommonly large solar flares extensive surveys
of stars of near-solar age need to be obtained, such as is feasible
with the Kepler satellite. Because the likelihood of flares larger
than approximately X30 remains empirically unconstrained, we present
indirect arguments, based on records of sunspots and on statistical
arguments, that solar flares in the past four centuries have likely
not substantially exceeded the level of the largest flares observed
in the space era, and that there is at most about a 10% chance of a
flare larger than about X30 in the next 30 years.

---------------------------------------------------------
Title: SDO/EVE spectra of solar flares
Authors: Heinzel, Petr; Avrett, Eugene; Dzifcakova, Elena; Hudson,
Hugh S.
2012cosp...39..743H Altcode: 2012cosp.meet..743H
For selected flare events we present the SDO/EVE spectra of hydrogen
and helium resonance continua and compare them with the results of
non-LTE transfer computations based on various flare models. We
discuss the formation of these continua and their diagnostic
potential for determination of the temperature structure of the
flaring atmosphere. Under the optically-thin conditions, we synthesize
these resonance continua using the CHIANTI database where we added
the relevant non-thermal atomic rates. This shows how the electron
or proton beams may affect the continua during an impulsive phase of
solar flares. As a benchmark we present our comparison of the quiet-Sun
EVE spectra with the flux synthesized from the model C6 of Avrett and
Loeser (2008). Our study has a more general relevance to the physics
of stellar flares.

---------------------------------------------------------
Title: The Height of a White-light Flare and Its Hard X-Ray Sources
Authors: Martínez Oliveros, Juan-Carlos; Hudson, Hugh S.; Hurford,
Gordon J.; Krucker, Säm; Lin, R. P.; Lindsey, Charles; Couvidat,
Sebastien; Schou, Jesper; Thompson, W. T.
2012ApJ...753L..26M Altcode: 2012arXiv1206.0497M
We describe observations of a white-light (WL) flare
(SOL2011-02-24T07:35:00, M3.5) close to the limb of the Sun, from which
we obtain estimates of the heights of the optical continuum sources and
those of the associated hard X-ray (HXR) sources. For this purpose, we
use HXR images from the Reuven Ramaty High Energy Spectroscopic Imager
and optical images at 6173 Å from the Solar Dynamics Observatory. We
find that the centroids of the impulsive-phase emissions in WL and HXRs
(30-80 keV) match closely in central distance (angular displacement
from Sun center), within uncertainties of order 0farcs2. This directly
implies a common source height for these radiations, strengthening the
connection between visible flare continuum formation and the accelerated
electrons. We also estimate the absolute heights of these emissions
as vertical distances from Sun center. Such a direct estimation has
not been done previously, to our knowledge. Using a simultaneous 195
Å image from the Solar-Terrestrial RElations Observatory spacecraft
to identify the heliographic coordinates of the flare footpoints,
we determine mean heights above the photosphere (as normally defined;
τ = 1 at 5000 Å) of 305 ± 170 km and 195 ± 70 km, respectively, for
the centroids of the HXR and WL footpoint sources of the flare. These
heights are unexpectedly low in the atmosphere, and are consistent
with the expected locations of τ = 1 for the 6173 Å and the ~40 keV
photons observed, respectively.

---------------------------------------------------------
Title: Charge-exchange Limits on Low-energy α-particle Fluxes in
Solar Flares
Authors: Hudson, H. S.; Fletcher, L.; MacKinnon, A. L.; Woods, T. N.
2012ApJ...752...84H Altcode: 2014arXiv1401.6477H
This paper reports on a search for flare emission via charge-exchange
radiation in the wings of the Lyα line of He II at 304 Å, as
originally suggested for hydrogen by Orrall &amp; Zirker. Via this
mechanism a primary α particle that penetrates into the neutral
chromosphere can pick up an atomic electron and emit in the He
II bound-bound spectrum before it stops. The Extreme-ultraviolet
Variability Experiment on board the Solar Dynamics Observatory gives
us our first chance to search for this effect systematically. The
Orrall-Zirker mechanism has great importance for flare physics because
of the essential roles that particle acceleration plays; this mechanism
is one of the few proposed that would allow remote sensing of primary
accelerated particles below a few MeV nucleon-1. We study
10 events in total, including the γ-ray events SOL2010-06-12 (M2.0)
and SOL2011-02-24 (M3.5) (the latter a limb flare), seven X-class
flares, and one prominent M-class event that produced solar energetic
particles. The absence of charge-exchange line wings may point to a
need for more complete theoretical work. Some of the events do have
broadband signatures, which could correspond to continua from other
origins, but these do not have the spectral signatures expected from
the Orrall-Zirker mechanism.

---------------------------------------------------------
Title: Flare Ribbons In The Early Phase Of An SDO Flare: Emission
Measure And Energetics
Authors: Fletcher, Lyndsay; Hannah, I. G.; Hudson, H. S.; Innes, D. E.
2012AAS...22050902F Altcode:
We report on the M1.0 flare of 7th August 2010, which displayed
extended early phase chromospheric ribbons, well observed by SDO/AIA
and RHESSI. Most large flares saturate rapidly in the high-temperature
AIA channels, however this event could be followed in unsaturated AIA
images for ten minutes in the build-up to and first few minutes of the
impulsive phase. Analysis of GOES, RHESSI and SDO/AIA demonstrates
the presence of high temperature ( 10MK), compact plasma volumes in
the chromospheric flare ribbons, with a column emission measure of
on average 3-7 x 1028 cm-5. We construct a
time-resolved energy budget for the ribbon plasma, including also
SDO/EVE data, and discuss the implications of the observed ribbon
properties for flare energisation. This work was supported by
the UK’s Science and Technology Facilities Council (ST/1001801),
and by the European Commission through the FP7 HESPE project
(FP7-2010-SPACE-263086).

---------------------------------------------------------
Title: The Journey of Sungrazing Comet Lovejoy
Authors: Bryans, Paul; A'Hearn, M.; Battams, K.; Biesecker, D.;
Bodewits, D.; Boice, D.; Brown, J.; Caspi, A.; Chodas, P.; Hudson,
H.; Jia, Y.; Jones, G.; Keller, H. U.; Knight, M.; Linker, J.; Lisse,
C.; Liu, W.; McIntosh, S.; Pesnell, W. D.; Raymond, J.; Saar, S.;
Saint-Hilaire, P.; Schrijver, C.; Snow, M.; Tarbell, T.; Thompson,
W.; Weissman, P.; Comet Lovejoy Collaboration Team
2012AAS...22052507B Altcode:
Comet Lovejoy (C/2011 W3) was the first sungrazing comet, observed
by space-based instruments, to survive perihelion passage. First
observed by ground-based telescopes several weeks prior to perihelion,
its journey towards the Sun was subsequently recorded by several solar
observatories, before being observed in the weeks after perihelion by
a further array of space- and ground-based instruments. Such a surfeit
of wide-ranging observations provides an unprecedented insight into
both sungrazing comets themselves, and the solar atmosphere through
which they pass. This paper will summarize what we have learnt from the
observations thus far and offer some thoughts on what future sungrazing
comets may reveal about comets, the Sun, and their interaction.

---------------------------------------------------------
Title: Solar Flare Observations of the EUV Continua
Authors: Milligan, Ryan O.; Chamberlin, P.; Hudson, H.; Woods, T.;
Mathioudakis, M.; Fletcher, L.; Kowalski, A.; Keenan, F.
2012AAS...22052105M Altcode:
Recent solar flare simulations suggest that the energy deposited in the
chromosphere by nonthermal electrons during a flare's impulsive phase
is re-emitted in the form of recombination (free-bound) continua, in
particular, the Lyman, Balmer, and Paschen continua of hydrogen, and
the He I and He II continua (Allred et al. 2005). However, definitive
observations of free-bound emission during solar flares have been scarce
in recent years as many modern, space-based instruments do not have
the required sensitivity, wavelength coverage, or duty cycle. With
the launch of SDO, these observations are now routinely available
thanks to the EUV Variability Experiment (EVE) instrument. Here we
present unambiguous, spectrally and temporally resolved detections of
enhanced free-free and free-bound continua during the first X-class
solar flare of Solar Cycle 24. While we find that the flare energy
in the EVE spectral range amounts to at most a few percent of the
total flare energy, these findings highlight the capability of EVE
in giving us the first comprehensive look at these diagnostically
important continuum components.

---------------------------------------------------------
Title: Global Forces in Eruptive Solar Flares: The Lorentz Force
Acting on the Solar Atmosphere and the Solar Interior
Authors: Fisher, George H.; Bercik, D. J.; Welsch, B. T.; Hudson, H. S.
2012AAS...22020440F Altcode:
We compute the change in the Lorentz force integrated over the outer
solar atmosphere implied by observed changes in vector magnetograms
that occur during large, eruptive solar flares. This force perturbation
should be balanced by an equal and opposite force perturbation acting
on the solar photosphere and solar interior. The resulting expression
for the estimated force change in the solar interior generalizes the
earlier expression presented by Hudson, Fisher, and Welsch, providing
horizontal as well as vertical force components, and provides a more
accurate result for the vertical component of the perturbed force. We
show that magnetic eruptions should result in the magnetic field at
the photosphere becoming more horizontal, and hence should result
in a downward (toward the solar interior) force change acting on the
photosphere and solar interior, as recently argued from an analysis
of magnetogram data by Wang and Liu. We suggest the existence of an
observational relationship between the force change computed from
changes in the vector magnetograms, the outward momentum carried by
the ejecta from the flare, and the properties of the helioseismic
disturbance driven by the downward force change. We use the impulse
driven by the Lorentz-force change in the outer solar atmosphere to
derive an upper limit to the mass of erupting plasma that can escape
from the Sun. Finally, we compare the expected Lorentz-force change
at the photosphere with simple estimates from flare-driven gasdynamic
disturbances and from an estimate of the perturbed pressure from
radiative backwarming of the photosphere in flaring conditions.

---------------------------------------------------------
Title: A RHESSI and SDO Campaign Measuring Latitude-dependent Limb
Profiles and Oblateness of the Optical Solar Disk II
Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P.; Bush, R. I.;
Emilio, M.; Kuhn, J. R.; Scholl, I. F.
2012AAS...22020511F Altcode:
The SDO spacecraft conducts special roll maneuvers every 6 months. These
SDO maneuvers enable its HMI instrument to obtain precise observations
of the global structure of the limb. During the SDO roll on 2011
April 6 05:50-12:30 UT, we also successfully obtained RHESSI optical
observations at very high cadence, 128 samples per sec for each
of the three linear CCDs. A second coordinated observation with
optimized parameter settings for RHESSI is planned for the time of
the SDO roll maneuver in April 2012. The data from the two instruments
(RHESSI/SAS and SDO/HMI), give different means for the investigation of
the variation of the solar limb properties as a function of position
angle (latitude). At the normal RHESSI cadence very long integrations
(of order 3 months) are needed to obtain precise limb measurements, but
in this case we are able to report results within the exact time frame
of the SDO roll maneuver. The special RHESSI data rate was about 10,000
times larger than the standard rate and will achieve high precision in
a relatively short time. We will compare these results with our earlier
RHESSI observations (Fivian et al., 2008) and those obtained by Kuhn et
al. (1998) and Emilio et al. (2007) with the earlier MDI roll maneuvers,
and as well as with the most recent analysis of HMI data.

---------------------------------------------------------
Title: On The Energetics Of Seismic Excitation Mechanisms
Authors: Martinez Oliveros, Juan Carlos; Bain, H.; Krucker, S.; Donea,
A.; Hudson, H.; Lin, R. P.; Lindsey, C.
2012AAS...22020503M Altcode:
Some solar flares emit strong acoustic transients into the solar
interior during their impulsive phases (Kosovichev and Zharkova,
1998). These transients penetrate thousands of kilometers beneath the
active region photosphere and refract back to the surface, where they
produce a characteristic helioseismic signature tens of thousands
of kilometers from their origin over the succeeding hour. Several
mechanisms of seismic excitation have been proposed, ranging from
hydrodynamic shocks to Lorentz force perturbations. However, regardless
of the mechanism of generation, it is clear that not all flares induce
an acoustic response in the interior of the Sun. A concrete hypothesis
or theory about the nature of this is still a topic of ongoing
investigations. For some particular flares, we present a comparative
study between the energy deposited by the proposed mechanisms of seismic
excitation and the acoustic energy deduced using holographic techniques.

---------------------------------------------------------
Title: Solar Hard X-ray Observations with NuSTAR
Authors: Marsh, Andrew; Smith, D. M.; Krucker, S.; Hudson, H. S.;
Hurford, G. J.; White, S. M.; Mewaldt, R. A.; Harrison, F. A.;
Grefenstette, B. W.; Stern, D.
2012AAS...22052112M Altcode:
High-sensitivity imaging of coronal hard X-rays allows detection
of freshly accelerated nonthermal electrons at the acceleration
site. A few such observations have been made with Yohkoh and RHESSI,
but a leap in sensitivity could help pin down the time, place, and
manner of reconnection. Around the time of this meeting, the Nuclear
Spectroscopic Telescope ARray (NuSTAR), a NASA Small Explorer for high
energy astrophysics that uses grazing-incidence optics to focus X-rays
up to 80 keV, will be launched. Three weeks will be dedicated to solar
observing during the baseline two-year mission. NuSTAR will be 200
times more sensitive than RHESSI in the hard X-ray band. This will allow
the following new observations, among others: 1) Extrapolation of the
micro/nanoflare distribution by two orders of magnitude down in flux;
2) Search for hard X-rays from network nanoflares (soft X-ray bright
points) and evaluation of their role in coronal heating; 3) Discovery
of hard X-ray bremsstrahlung from the electron beams driving type III
radio bursts, and measurement of their electron spectrum; 4) Hard X-ray
studies of polar soft X-ray jets and impulsive solar energetic particle
events at the edge of coronal holes; 5) Study of coronal bremsstrahlung
from particles accelerated by coronal mass ejections as they are
first launched; 6) Study of particles at the coronal reconnection
site when flare footpoints and loops are occulted; 7) Search for weak
high-temperature coronal plasmas in active regions that are not flaring;
and 8) Search for hypothetical axion particles created in the solar
core via the hard X-ray signal from their conversion to X-rays in the
coronal magnetic field. NuSTAR will also serve as a pathfinder for a
future dedicated space mission with enhanced capabilities, such as a
satellite version of the FOXSI sounding rocket.

---------------------------------------------------------
Title: Energetic Neutral Atoms (ENAs): A new window on Solar Energetic
Particle (SEP) acceleration
Authors: Lin, Robert P.; Wang, L.; Hudson, H.; Hurford, G.; Duncan,
N.; Li, G.; Shih, A. Y.; Mewaldt, R. A.
2012AAS...22042406L Altcode:
Large solar eruptive events accelerate ions up to GeV energies in both
flares and fast coronal mass ejections (CMEs). Measurements of gamma-ray
line emission show that up to 10-50% of the total energy released in
the flare is contained in the few to 100 MeV ions accelerated in the
flare. Observations in the interplanetary medium near 1 AU indicate that
the number of SEP (Solar Energetic Particles) ions can be comparable
or even up to 1-2 orders of magnitude larger than in flares. These
SEPs are believed to be accelerated by shocks driven by fast CMEs at
altitudes of from 1.5 to tens of solar radii, with the total energy
in the SEPs of order 10% of the total kinetic energy of the CME. The
ground-breaking discovery of 1.6-15 MeV energetic neutral atoms (ENAs,
produced by charge exchange of SEPs with the ambient corona) from
the Sun just prior to an SEP event (Mewaldt et al 2009) suggest the
possibility that, for the first time, ions being accelerated by CME
shocks close to the Sun can be remotely sensed and imaged, providing
a completely new window on SEPs. We present model simulations of
the SEP ENA production, and possible techniques for remote sensing
and mapping SEP ions from seed particle energies ( 5 keV) to 10s of
MeV. R. A. Mewaldt, et al, Astrophys. J., 693: L11-15, 2009.

---------------------------------------------------------
Title: Results and Analysis of 10 Years of RHESSI/SAS Observations
of the Optical Solar Limb
Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P.
2012AAS...22042306F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb in the red continuum with a cadence typically set at
16 samples/s in each of three linear CCD sensors. RHESSI has observed
the Sun continuously now for more than 10 years, and we have acquired
a unique data set ranging almost over a full solar cycle and consisting
of about 25x10^9 single data points. These measurements have led to the
most accurate oblateness measurement to date, 8.01+-0.14 milli arcsec
(Fivian et al., 2008), a value consistent with models predicting
an oblateness from surface rotation. An excess oblateness term can
be attributed to magnetic elements possibly located in the enhanced
network. New measurements of latitude-dependent brightness variations
at the limb lead to a quadrupolar term (a pole-to-equator temperature
variation) of 0.04+-0.02 K. We present the analysis of these unique
data, an overview of some results and we report on our progress as we
apply our developed analysis method to the whole 10 years of data.

---------------------------------------------------------
Title: X-Ray Searches for Solar Axions
Authors: Hudson, H. S.; Acton, L. W.; DeLuca, E. E.; Hannah, I. G.;
Reardon, K.; Van Bibber, K.
2012ASPC..455...25H Altcode: 2012arXiv1201.4607H
Axions generated thermally in the solar core can convert nearly directly
to X-rays as they pass through the solar atmosphere via interaction with
the magnetic field. The result of this conversion process would be a
diffuse centrally-concentrated source of few-keV X-rays at disk center;
it would have a known dimension, of order 10% of the solar diameter, and
a spectral distribution resembling the blackbody spectrum of the solar
core. Its spatial structure in detail would depend on the distribution
of mass and field in the solar atmosphere. The brightness of the source
depends upon these factors as well as the unknown coupling constant
and the unknown mass of the axion; this particle is hypothetical and
no firm evidence for its existence has been found yet. We describe the
solar magnetic environment as an axion/photon converter and discuss
the upper limits obtained by existing and dedicated observations from
three solar X-ray observatories: Yohkoh, RHESSI, and Hinode.

---------------------------------------------------------
Title: Direct Measurement Of The Height Of A White-light Flare
Authors: Hudson, Hugh S.; Martinez-Oliveros, J.; Krucker, S.; Hurford,
G.; Thompson, W.; Schou, J.; Couvidat, S.; Lindsey, C.
2012AAS...22020441H Altcode:
We have used RHESSI and HMI observations to observe hard X-ray and
white-light continuum sources of the limb flare SOL2011-02-24, and
find the source centroids to coincide within errors of about 0.2 arc s,
with the conclusion that the emissions form at the same height in the
atmosphere. This greatly strengthens the known association between
non-thermal electrons and white-light continuum formation. We also
use STEREO observations to find the heliographic coordinates of the
flare. This determines the projected height of the photosphere directly
below the flare emissions. With this information, the RHESSI metrology
determines the absolute height of the sources to be remarkably low
in the solar atmosphere: the two footpoints have comparable heights,
which we estimate at about 290 +- 138 km above the photosphere. This
location lies significantly below the visible-light limb height,
estimated at 500 km by Brown &amp; Christensen-Dalsgaard (1998), and
the height of optical depth unity to Thomson scattering, estimated
at a higher altitude. The results are not consistent with any current
models of these processes.

---------------------------------------------------------
Title: Generation of electric currents via neutral-ion drag in the
chromosphere and ionosphere
Authors: Krasnoselskikh, V.; Abbett, W. P.; Hudson, H.; Vekstein,
G.; Bale, S. D.
2012AIPC.1439...42K Altcode:
We consider the generation of electric currents in the solar
chromosphere. The ionization level in this region is generally supposed
to be low. We show that the ambient electrons are magnetized even
for weak magnetic fields (30 G), i.e. their gyrofrequency is larger
than the collision frequency; ion motions continue to be dominated by
ion-neutral collisions in this region. Under such conditions the ions
are dragged by neutrals. As a result, the dynamics of magnetic field
resembles frozen-in motion of the field with the neutral gas. On the
other hand magnetized electrons drift under the action of the electric
and magnetic fields induced in the reference frame of ions moving with
the neutral gas. This relative motion of electrons and ions results in
the generation of quite intense electric currents. The dissipation of
these currents leads to the resistive electron heating and efficient
gas ionization. Ionization by electron-neutral impact does not alter
the dynamics of the heavy particles; thus the gas turbulent motions
persist even when the plasma becomes fully ionized and the resistive
current dissipation continues to heat electrons and ions. This heating
process is so efficient that it can result in typical temperature
increases with altitude as large as 0.1-0.3 eV/km. We conclude that this
process can play a major role in the heating of the chromosphere and
corona. We show that the physical conditions in the solar chromosphere,
in particular the neutral and ion density dependencies upon altitude,
are very similar to those in the lower ionosphere of the Earth. A
very similar process of current generation occurs in the ionosphere
after strong earthquakes, resulting in the generation of strong
perturbations in the ionosphere. We then present well-known results of
the observations of such perturbations, which allow an evaluation of the
increment of the growth of the perturbations with altitude, making use
of ionospheric sounding. These results are in perfect agreement with
estimates obtained making use a model similar to ours. We consider
that these observations clearly show the efficiency of the physical
mechanisms discussed, and thus provide strong support for our ideas.

---------------------------------------------------------
Title: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
2012IAUTA..28...69V Altcode:
Commission 10 of the International Astronomical Union has more than
650 members who study a wide range of activity phenomena produced by
our nearest star, the Sun. Solar activity is intrinsically related
to solar magnetic fields and encompasses events from the smallest
energy releases (nano- or even picoflares) to the largest eruptions
in the Solar System, coronal mass ejections (CMEs), which propagate
into the Heliosphere reaching the Earth and beyond. Solar activity is
manifested in the appearance of sunspot groups or active regions, which
are the principal sources of activity phenomena from the emergence of
their magnetic flux through their dispersion and decay. The period
2008-2009 saw an unanticipated extended solar cycle minimum and
unprecedentedly weak polar-cap and heliospheric field. Associated with
that was the 2009 historical maximum in galactic cosmic rays flux since
measurements begun in the middle of the 20th Century. Since then Cycle
24 has re-started solar activity producing some spectacular eruptions
observed with a fleet of spacecraft and ground-based facilities. In
the last triennium major advances in our knowledge and understanding
of solar activity were due to continuing success of space missions as
SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched
by the breathtaking images of the solar atmosphere produced by the
Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the
framework of NASA's Living with a Star program. In August 2012, at the
time of the IAU General Assembly in Beijing when the mandate of this
Commission ends, we will be in the unique position to have for the
first time a full 3-D view of the Sun and solar activity phenomena
provided by the twin STEREO missions about 120 degrees behind and
ahead of Earth and other spacecraft around the Earth and ground-based
observatories. These new observational insights are continuously
posing new questions, inspiring and advancing theoretical analysis
and modelling, improving our understanding of the physics underlying
magnetic activity phenomena. Commission 10 reports on a vigorously
evolving field of research produced by a large community. The number
of refereed publications containing `Sun', `heliosphere', or a synonym
in their abstracts continued the steady growth seen over the preceding
decades, reaching about 2000 in the years 2008-2010, with a total of
close to 4000 unique authors. This report, however, has its limitations
and it is inherently incomplete, as it was prepared jointly by the
members of the Organising Committee of Commission 10 (see the names
of the primary contributors to the sections indicated in parentheses)
reflecting their fields of expertise and interest. Nevertheless, we
believe that it is a representative sample of significant new results
obtained during the last triennium in the field of solar activity.

---------------------------------------------------------
Title: Observations of Enhanced Extreme Ultraviolet Continua during
an X-Class Solar Flare Using SDO/EVE
Authors: Milligan, Ryan O.; Chamberlin, Phillip C.; Hudson, Hugh S.;
Woods, Thomas N.; Mathioudakis, Mihalis; Fletcher, Lyndsay; Kowalski,
Adam F.; Keenan, Francis P.
2012ApJ...748L..14M Altcode: 2012arXiv1202.1731M
Observations of extreme ultraviolet (EUV) emission from an X-class solar
flare that occurred on 2011 February 15 at 01:44 UT are presented,
obtained using the EUV Variability Experiment (EVE) on board the
Solar Dynamics Observatory. The complete EVE spectral range covers
the free-bound continua of H I (Lyman continuum), He I, and He II,
with recombination edges at 91.2, 50.4, and 22.8 nm, respectively. By
fitting the wavelength ranges blueward of each recombination edge
with an exponential function, light curves of each of the integrated
continua were generated over the course of the flare, as was emission
from the free-free continuum (6.5-37 nm). The He II 30.4 nm and Lyα
121.6 nm lines, and soft X-ray (SXR; 0.1-0.8 nm) emission from GOES are
also included for comparison. Each free-bound continuum was found to
have a rapid rise phase at the flare onset similar to that seen in the
25-50 keV light curves from RHESSI, suggesting that they were formed
by recombination with free electrons in the chromosphere. However,
the free-free emission exhibited a slower rise phase seen also in the
SXR emission from GOES, implying a predominantly coronal origin. By
integrating over the entire flare the total energy emitted via
each process was determined. We find that the flare energy in the
EVE spectral range amounts to at most a few percent of the total
flare energy, but EVE gives us a first comprehensive look at these
diagnostically important continuum components.

---------------------------------------------------------
Title: Global Forces in Eruptive Solar Flares: The Lorentz Force
Acting on the Solar Atmosphere and the Solar Interior
Authors: Fisher, G. H.; Bercik, D. J.; Welsch, B. T.; Hudson, H. S.
2012SoPh..277...59F Altcode: 2010arXiv1006.5247F; 2011SoPh..tmp..419F; 2011SoPh..tmp..415F
We compute the change in the Lorentz force integrated over the
outer solar atmosphere implied by observed changes in vector
magnetograms that occur during large, eruptive solar flares. This
force perturbation should be balanced by an equal and opposite force
perturbation acting on the solar photosphere and solar interior. The
resulting expression for the estimated force change in the solar
interior generalizes the earlier expression presented by Hudson,
Fisher, and Welsch (Astron. Soc. Pac. CS-383, 221, 2008), providing
horizontal as well as vertical force components, and provides a more
accurate result for the vertical component of the perturbed force. We
show that magnetic eruptions should result in the magnetic field at
the photosphere becoming more horizontal, and hence should result
in a downward (toward the solar interior) force change acting on the
photosphere and solar interior, as recently argued from an analysis
of magnetogram data by Wang and Liu (Astrophys. J. Lett. 716, L195,
2010). We suggest the existence of an observational relationship between
the force change computed from changes in the vector magnetograms,
the outward momentum carried by the ejecta from the flare, and the
properties of the helioseismic disturbance driven by the downward
force change. We use the impulse driven by the Lorentz-force change
in the outer solar atmosphere to derive an upper limit to the mass of
erupting plasma that can escape from the Sun. Finally, we compare the
expected Lorentz-force change at the photosphere with simple estimates
from flare-driven gasdynamic disturbances and from an estimate of the
perturbed pressure from radiative backwarming of the photosphere in
flaring conditions.

---------------------------------------------------------
Title: Momentum Distribution in Solar Flare Processes
Authors: Hudson, H. S.; Fletcher, L.; Fisher, G. H.; Abbett, W. P.;
Russell, A.
2012SoPh..277...77H Altcode:
We discuss the consequences of momentum conservation in processes
related to solar flares and coronal mass ejections (CMEs), in particular
describing the relative importance of vertical impulses that could
contribute to the excitation of seismic waves ("sunquakes"). The
initial impulse associated with the primary flare energy transport
in the impulsive phase contains sufficient momentum, as do the
impulses associated with the acceleration of the evaporation flow (the
chromospheric shock) or the CME itself. We note that the deceleration
of the evaporative flow, as coronal closed fields arrest it, will tend
to produce an opposite impulse, reducing the energy coupling into
the interior. The actual mechanism of the coupling remains unclear
at present.

---------------------------------------------------------
Title: What is there before a flare?
Authors: Hudson, Hugh S.
2012decs.confE.119H Altcode:
The physical parameters in a region about to flare (or to
make a micro-event of any sort) should be of interest to many
people, and should be a suitable topic for (serendipitous) IRIS
observations. Flaring is associated generally with magnetic fields,
but apparently only infrequently with pre-existing coronal structures
at high temperatures. This poster reviews what is known and aims at
eliciting discussion of what could be observed with IRIS.

---------------------------------------------------------
Title: Thermal Properties of a Solar Coronal Cavity Observed with
the X-Ray Telescope on Hinode
Authors: Reeves, Katharine K.; Gibson, Sarah E.; Kucera, Therese A.;
Hudson, Hugh S.; Kano, Ryouhei
2012ApJ...746..146R Altcode:
Coronal cavities are voids in coronal emission often observed above
high latitude filament channels. Sometimes, these cavities have areas of
bright X-ray emission in their centers. In this study, we use data from
the X-ray Telescope (XRT) on the Hinode satellite to examine the thermal
emission properties of a cavity observed during 2008 July that contains
bright X-ray emission in its center. Using ratios of XRT filters, we
find evidence for elevated temperatures in the cavity center. The area
of elevated temperature evolves from a ring-shaped structure at the
beginning of the observation, to an elongated structure two days later,
finally appearing as a compact round source four days after the initial
observation. We use a morphological model to fit the cavity emission,
and find that a uniform structure running through the cavity does not
fit the observations well. Instead, the observations are reproduced
by modeling several short cylindrical cavity "cores" with different
parameters on different days. These changing core parameters may be
due to some observed activity heating different parts of the cavity
core at different times. We find that core temperatures of 1.75 MK,
1.7 MK, and 2.0 MK (for July 19, July 21, and July 23, respectively)
in the model lead to structures that are consistent with the data,
and that line-of-sight effects serve to lower the effective temperature
derived from the filter ratio.

---------------------------------------------------------
Title: Destruction of Sun-Grazing Comet C/2011 N3 (SOHO) Within the
Low Solar Corona
Authors: Schrijver, C. J.; Brown, J. C.; Battams, K.; Saint-Hilaire,
P.; Liu, W.; Hudson, H.; Pesnell, W. D.
2012Sci...335..324S Altcode:
Observations of comets in Sun-grazing orbits that survive solar
insolation long enough to penetrate into the Sun's inner corona provide
information on the solar atmosphere and magnetic field as well as on
the makeup of the comet. On 6 July 2011, the Solar Dynamics Observatory
(SDO) observed the demise of comet C/2011 N3 (SOHO) within the low solar
corona in five wavelength bands in the extreme ultraviolet (EUV). The
comet penetrated to within 0.146 solar radius (~100,000 kilometers)
of the solar surface before its EUV signal disappeared. Before that,
material released into the coma - at first seen in absorption - formed
a variable EUV-bright tail. During the final 10 minutes of observation
by SDO's Atmospheric Imaging Assembly, ~6 × 10^8 to 6 × 10^10 grams
of total mass was lost (corresponding to an effective nucleus diameter
of ~10 to 50 meters), as estimated from the tail's deceleration due to
interaction with the surrounding coronal material; the EUV absorption
by the comet and the brightness of the tail suggest that the mass was
at the high end of this range. These observations provide evidence
that the nucleus had broken up into a family of fragments, resulting
in accelerated sublimation in the Sun's intense radiation field.

---------------------------------------------------------
Title: Suzaku/WAM and RHESSI observation of non-thermal electrons
in solar microflares
Authors: Ishikawa, S.; Krucker, S.; Ohno, M.; Hudson, H. S.; Christe,
S.; Lin, R. P.
2011AGUFMSH41A1908I Altcode:
We report on hard X-ray spectroscopy of solar microflares observed by
the Wide-band All-sky Monitor (WAM), onboard the Suzaku satellite,
and by the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI). WAM transient data provide wide energy band (50 keV - 5 MeV)
spectrum over a large field of view (~2π str) with a time resolution
of 1 s. While WAM was mainly designed for gamma-ray bursts and other
bursts of hard X-ray and gamma-ray from transient non-solar objects,
it is also attractive as a hard X-ray solar flare monitor thanks to
its large effective area (~800 cm2 at 100 keV, ~13 times
larger than that of RHESSI). Hard X-ray (&gt;50 keV) emissions from
17 GOES B-class flares were detected by WAM by Febrary 2010, and
7 of them were also observed by RHESSI. The GOES classes of these
events range from B1.3 to B9.5, and the RHESSI non-thermal spectra
are well-fit by power-laws with photon spectral indices between 3 and
5. The durations of both the WAM and RHESSI non-thermal emissions are
~1 minute, and the detected WAM fluxes are more than ~20 times smaller
than RHESSI backgrounds at energies above ~100 keV. The WAM spectra show
the high-energy extension of the non-thermal power-law distribution
seen by RHESSI, showing that microflares, similar to regular flares,
accelerate electrons to energies above 50 keV. We discuss high-energy
(&gt;50 keV) particle acceleration in solar microflares and its relation
to large flares.

---------------------------------------------------------
Title: The EUV Emission in Comet-Solar Corona Interactions
Authors: Bryans, P.; Pesnell, W. D.; Schrijver, C. J.; Brown, J. C.;
Battams, K.; Saint-Hilaire, P.; Liu, W.; Hudson, H. S.
2011AGUFMSH34B..05B Altcode:
The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory
(SDO) viewed a comet as it passed through the solar corona on 2011 July
5. This was the first sighting of a comet by a EUV telescope. For 20
minutes, enhanced emission in several of the AIA wavelength bands marked
the path of the comet. We explain this EUV emission by considering
the evolution of the cometary atmosphere as it interacts with the
ambient solar atmosphere. Water ice in the comet rapidly sublimates
as it approaches the Sun. This water vapor is then photodissociated,
primarily by Ly-α, by the solar radiation field to create atomic H and
O. Other molecules present in the comet also evaporate and dissociate
to give atomic Fe and other metals. Subsequent ionization of these
atoms can be achieved by a number of means, including photoionization,
electron impact, and charge exchange with coronal protons and other
highly-charged species. Finally, particles from the cometary atmosphere
are thermalized to the background temperature of the corona. Each step
could cause emission in the AIA bandpasses. We will report here on
their relative contribution to the emission seen in the AIA telescopes.

---------------------------------------------------------
Title: Non-Observation of the He II 304 A Charge-Exchange Continuum
in Major Solar Flares
Authors: Hudson, H. S.; Fletcher, L.; MacKinnon, A.; Woods, T. N.
2011AGUFMSH41A1909H Altcode:
We report on a search for flare emission via charge-exchange continuum
radiation in the wings of the Lyman-alpha line of He ii at 304 A, as
originally suggested for hydrogen by Orrall and Zirker (1976). Via this
mechanism a primary alpha particle that penetrates into the neutral
chromosphere can pick up an atomic electron and radiate recombination
continuum before it stops. The Extreme-ultraviolet Variability
Experiment (EVE) on board the Solar Dynamics Observatory (SDO) gives
us our first chance to search for this effect systematically. The
Orrall-Zirker charge-exchange mechanism has great importance for flare
physics because of the essential roles that particle acceleration plays;
this mechanism is one of the few proposed that would allow remote
sensing of primary accelerated particles below a few MeV/nucleon. We
study four EVE events: the gamma-ray events SOL2010-06-12 (M2.0) and
SOL20 11-02-24 (M3.5), the latter a limb flare, and the X-class flares
SOL2010-02-15 (X2.2) and SOL2011-03-09 (X1.2). No clear signature of
the charge-exchange continuum appears, but SOL2010-02-15 (X2.2) does
reveal a gradual broad-band signature that we tentatively interpret
as due to unresolved emission lines or instrumental scattering.

---------------------------------------------------------
Title: Splinter Session "Solar and Stellar Flares"
Authors: Fletcher, L.; Hudson, H.; Cauzzi, G.; Getman, K. V.; Giampapa,
M.; Hawley, S. L.; Heinzel, P.; Johnstone, C.; Kowalski, A. F.; Osten,
R. A.; Pye, J.
2011ASPC..448..441F Altcode: 2011csss...16..441F; 2012arXiv1206.3997F
This summary reports on papers presented at the Cool Stars-16 meeting in
the splinter session "Solar and Stellar flares." Although many topics
were discussed, the main themes were the commonality of interests,
and of physics, between the solar and stellar flare communities,
and the opportunities for important new observations in the near future.

---------------------------------------------------------
Title: Using the EUV to Weigh a Sun-grazing Comet as it Disappears
in the Solar Corona
Authors: Pesnell, W. D.; Schrijver, C. J.; Brown, J. C.; Battams,
K.; Saint-Hilaire, P.; Hudson, H. S.; Lui, W.
2011AGUFMSH33A2040P Altcode:
On July 6, 2011, the Atmospheric Imaging Assembly (AIA) on the
Solar Dynamics Observatory (SDO) observed a comet in most of its
EUV passbands. The comet disappeared while moving through the solar
corona. The comet penetrated to 0.146 solar radii ( ∼~100,000
km) above the photosphere before its EUV faded. Before then, the
comet's coma and a tail were observed in absorption and emission,
respectively. The material in the variable tail quickly fell behind
the nucleus. An estimate of the comet's mass based on this effect,
one derived from insolation, and one using the tail's EUV brightness,
all yield ∼ 50 giga-grams some 10 minutes prior to the end of
its visibility. These unique first observations herald a new era in
the study of Sun-grazing comets close to their perihelia and of the
conditions in the solar corona and solar wind. We will discuss the
observations and interpretation of the comet by SDO as well as the
coronagraph observations from SOHO and STEREO. A search of the SOHO
comet archive for other comets that could be observed in the SDO/AIA
EUV channels will be described.

---------------------------------------------------------
Title: Results and Analysis of the RHESSI/SAS Observations of the
Optical Solar Limb
Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.
2011AGUFMSH34B..03F Altcode:
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb in the red continuum with a cadence typically set at
16 samples/s in each of three linear CCD sensors. RHESSI has observed
the Sun continuously now for more than 9 years, and we have acquired a
unique data set ranging almost over a full solar cycle and consisting of
about 25x10^9single data points. These measurements have led to the most
accurate oblateness measurement to date, 8.01±0.14 milliarcsec(Fivian
et al., 2008), a value consistent with models predicting an oblateness
from surface rotation. An excess oblateness term can be attributed to
the enhanced network. New measurements of latitude-dependent brightness
variations at the limb lead to a quadrupolarterm (a pole-to-equator
temperature variation) of 0.04±0.02 K. We present the analysis of
these unique data and an overview of some results.

---------------------------------------------------------
Title: The EVE Doppler Sensitivity and Flare Observations
Authors: Hudson, H. S.; Woods, T. N.; Chamberlin, P. C.; Fletcher,
L.; Del Zanna, G.; Didkovsky, L.; Labrosse, N.; Graham, D.
2011SoPh..273...69H Altcode: 2011SoPh..tmp..362H
The Extreme-ultraviolet Variability Experiment (EVE; see Woods et al.,
2009) obtains continuous EUV spectra of the Sun viewed as a star. Its
primary objective is the characterization of solar spectral irradiance,
but its sensitivity and stability make it extremely interesting for
observations of variability on time scales down to the limit imposed
by its basic 10 s sample interval. In this paper we characterize the
Doppler sensitivity of the EVE data. We find that the 30.4 nm line of
He II has a random Doppler error below 0.001 nm (1 pm, better than 10
km s−1 as a redshift), with ample stability to detect
the orbital motion of its satellite, the Solar Dynamics Observatory
(SDO). Solar flares also displace the spectrum, both because of Doppler
shifts and because of EVE's optical layout, which (as with a slitless
spectrograph) confuses position and wavelength. As a flare develops,
the centroid of the line displays variations that reflect Doppler shifts
and therefore flare dynamics. For the impulsive phase of the flare
SOL2010-06-12, we find the line centroid to have a redshift of 16.8 ±
5.9 km s−1 relative to that of the flare gradual phase
(statistical errors only). We find also that high-temperature lines,
such as Fe XXIV 19.2 nm, have well-determined Doppler components for
major flares, with decreasing apparent blueshifts as expected from
chromospheric evaporation flows.

---------------------------------------------------------
Title: Momentum Balance in Eruptive Solar Flares: The Lorentz Force
Acting on the Solar Atmosphere and the Solar Interior
Authors: Fisher, George H.; Bercik, David J.; Welsch, Brian T.;
Hudson, Hugh S.
2011sdmi.confE...9F Altcode:
We compute the change in the Lorentz force integrated over the outer
solar atmosphere implied by observed changes in vector magnetograms
that occur during large, eruptive solar flares. This force perturbation
should be balanced by an equal and opposite force perturbation acting on
the solar photosphere and solar interior. The resulting expression for
the estimated force change in the solar interior generalizes the earlier
expression presented by Hudson, Fisher &amp; Welsch (2008), providing
horizontal as well as vertical force components, and provides a more
accurate result for the vertical component of the perturbed force. We
show that magnetic eruptions should result in the magnetic field at
the photosphere becoming more horizontal, and hence should result in
a downward (towards the solar interior) force change acting on the
photosphere and solar interior, as recently argued from an analysis
of magnetogram data by Wang &amp; Liu. We suggest that there should
be an observational relationship between the force change computed
from changes in the vector magnetograms, the outward momentum carried
by the ejecta from the flare, and the amplitude of the helioseismic
disturbance driven by the downward force change. We use the impulse
driven by the Lorentz force change in the outer solar atmosphere to
derive an upper limit to the mass of erupting plasma that can escape
from the Sun. Finally, we compare the expected Lorentz force change
at the photosphere with simple estimates from flare-driven gasdynamic
disturbances and from an estimate of the perturbed pressure from
radiative backwarming of the photosphere in flaring conditions.

---------------------------------------------------------
Title: High-resolution Imaging of Solar Flare Ribbons and Its
Implication on the Thick-target Beam Model
Authors: Krucker, Säm; Hudson, H. S.; Jeffrey, N. L. S.; Battaglia,
M.; Kontar, E. P.; Benz, A. O.; Csillaghy, A.; Lin, R. P.
2011ApJ...739...96K Altcode:
We report on high-resolution optical and hard X-ray observations of
solar flare ribbons seen during the GOES X6.5 class white-light flare
of 2006 December 6. The data consist of imaging observations at 430 nm
(the Fraunhofer G band) taken by the Hinode Solar Optical Telescope
with the hard X-rays observed by the Reuven Ramaty High Energy Solar
Spectroscopic Imager. The two sets of data show closely similar ribbon
structures, strongly suggesting that the flare emissions in white light
and in hard X-rays have physically linked emission mechanisms. While
the source structure along the ribbons is resolved at both wavelengths
(length ~ 30”), only the G-band observations resolve the width of the
ribbon, with values between ~0farcs5 and ~1farcs8. The unresolved
hard X-ray observations reveal an even narrower ribbon in hard
X-rays (the main footpoint has a width perpendicular to the ribbon
of &lt;1farcs1 compared to the G-band width of ~1farcs8) suggesting
that the hard X-ray emission comes from the sharp leading edge of
the G-band ribbon. Applying the thick-target beam model, the derived
energy deposition rate is &gt;5 × 1012 erg s-1
cm-2 provided by an electron flux of 1 × 1020
electrons s-1 cm-2 above 18 keV. This requires
that the beam density of electrons above 18 keV be at least 1 ×
1010 cm-3. Even if field lines converge toward
the chromospheric footpoints, the required beam in the corona has too
high a density to be described as a dilute tail population on top of
a Maxwellian core. We discuss this issue and others associated with
this extreme event, which poses serious questions to the standard
thick target beam interpretation of solar flares.

---------------------------------------------------------
Title: Flare Seismology from SDO Observations
Authors: Lindsey, Charles; Martinez Oliveros, Juan Carlos; Hudson, Hugh
2011sdmi.confE..10L Altcode:
Some flares release intense seismic transients into the solar
interior. These transients are the sole instance we know of in which
the Sun's corona exerts a conspicuous influence on the solar interior
through flares. The desire to understand this phenomenon has led to
ambitious efforts to model the mechanisms by which energy stored in
coronal magnetic fields drives acoustic waves that penetrate deep
into the Sun's interior. These mechanisms potentially involve the
hydrodynamic response of the chromosphere to thick-target heating
by high-energy particles, radiative exchange in the chromosphere
and photosphere, and Lorentz-force transients to account for
acoustic energies estimated up to at 5X10^27 erg and momenta of
order 6X10^19 dyne sec. An understanding of these components of
flare mechanics promises more than a powerful diagnostic for local
helioseismology. It could give us fundamental new insight into
flare mechanics themselves. The key is appropriate observations
to match the models. Helioseismic observations have identified the
compact sources of transient seismic emission at the foot points of
flares. The Solar Dynamics Observatory is now giving us high quality
continuum-brightness and Doppler observations of acoustically active
flares from HMI concurrent with high-resolution EUV observations from
AIA. Supported by HXR observations from RHESSI and a broad variety
of other observational resources, the SDO promises a leading role in
flare research in solar cycle 24.

---------------------------------------------------------
Title: An Observational Overview of Solar Flares
Authors: Fletcher, L.; Dennis, B. R.; Hudson, H. S.; Krucker, S.;
Phillips, K.; Veronig, A.; Battaglia, M.; Bone, L.; Caspi, A.; Chen,
Q.; Gallagher, P.; Grigis, P. T.; Ji, H.; Liu, W.; Milligan, R. O.;
Temmer, M.
2011SSRv..159...19F Altcode: 2011SSRv..tmp..261F; 2011arXiv1109.5932F
We present an overview of solar flares and associated phenomena,
drawing upon a wide range of observational data primarily from the
RHESSI era. Following an introductory discussion and overview of
the status of observational capabilities, the article is split into
topical sections which deal with different areas of flare phenomena
(footpoints and ribbons, coronal sources, relationship to coronal mass
ejections) and their interconnections. We also discuss flare soft X-ray
spectroscopy and the energetics of the process. The emphasis is to
describe the observations from multiple points of view, while bearing
in mind the models that link them to each other and to theory. The
present theoretical and observational understanding of solar flares is
far from complete, so we conclude with a brief discussion of models,
and a list of missing but important observations.

---------------------------------------------------------
Title: Microflares and the Statistics of X-ray Flares
Authors: Hannah, I. G.; Hudson, H. S.; Battaglia, M.; Christe, S.;
Kašparová, J.; Krucker, S.; Kundu, M. R.; Veronig, A.
2011SSRv..159..263H Altcode: 2011SSRv..tmp..262H; 2011SSRv..tmp...87H; 2011arXiv1108.6203H;
2011SSRv..tmp..243H; 2011SSRv..tmp..163H
This review surveys the statistics of solar X-ray flares, emphasising
the new views that RHESSI has given us of the weaker events (the
microflares). The new data reveal that these microflares strongly
resemble more energetic events in most respects; they occur solely
within active regions and exhibit high-temperature/nonthermal emissions
in approximately the same proportion as major events. We discuss the
distributions of flare parameters (e.g., peak flux) and how these
parameters correlate, for instance via the Neupert effect. We also
highlight the systematic biases involved in intercomparing data
representing many decades of event magnitude. The intermittency of
the flare/microflare occurrence, both in space and in time, argues
that these discrete events do not explain general coronal heating,
either in active regions or in the quiet Sun.

---------------------------------------------------------
Title: Overview of the Volume
Authors: Dennis, B. R.; Emslie, A. G.; Hudson, H. S.
2011SSRv..159....3D Altcode: 2011SSRv..tmp..231D; 2011arXiv1109.5831D; 2011SSRv..tmp..277D;
2011SSRv..tmp..248D; 2011SSRv..tmp..155D
In this introductory chapter, we provide a brief summary of the
successes and remaining challenges in understanding the solar flare
phenomenon and its attendant implications for particle acceleration
mechanisms in astrophysical plasmas. We also provide a brief overview
of the contents of the other chapters in this volume, with particular
reference to the well-observed flare of 2002 July 23.

---------------------------------------------------------
Title: Preface
Authors: Emslie, A. Gordon; Dennis, Brian R.; Hudson, Hugh; Lin,
Robert P.
2011SSRv..159....1E Altcode: 2011SSRv..tmp..161E; 2011SSRv..tmp..288E; 2011SSRv..tmp..257E;
2011SSRv..tmp..239E
No abstract at ADS

---------------------------------------------------------
Title: Solar Hard X-ray Observations with NuSTAR
Authors: Smith, David M.; Krucker, S.; Hurford, G.; Hudson, H.; White,
S.; Mewaldt, R.; Grefenstette, B.; Harrison, F.; NuSTAR Science Team
2011HEAD...12.4309S Altcode:
High-sensitivity imaging of solar hard X-rays allows detection of
freshly accelerated nonthermal electrons at the acceleration site. A
few such observations have been made with Yohkoh and RHESSI, but a
leap in sensitivity could help pin down the time, place, and manner
of reconnection. The Nuclear Spectroscopic Telescope Array (NuSTAR)
Small Explorer will be capable of solar pointing, and three weeks
will be dedicated to solar observing during the baseline two-year
mission. NuSTAR will be 200 times more sensitive than RHESSI in the
hard X-ray band. This will allow the following new solar observations,
among others: 1) Extrapolation of the micro/nanoflare distribution
by two orders of magnitude down in flux 2) Search for hard X-rays
from network nanoflares (soft X-ray bright points) and evaluation
of their role in coronal heating 3) Discovery of hard X-ray
bremsstrahlung from the electron beams driving type III radio bursts,
and measurement of their electron spectrum 4) Hard X-ray studies
of polar soft X-ray jets and impulsive solar energetic particle events
at the edge of coronal holes, and comparison of these events with
observations of 3He and other particles in interplanetary space 5)
Study of coronal bremsstrahlung from particles accelerated by coronal
mass ejections as they are first launched 6) Study of particles
at the coronal reconnection site when flare footpoints are occulted;
and 7) Search for hypothetical axion particles created in the
solar core via the hard X-ray signal from their conversion to X-rays
in the coronal magnetic field.

---------------------------------------------------------
Title: The U.S. Eclipse Megamovie in 2017: a white paper on a unique
outreach event
Authors: Hudson, Hugh S.; McIntosh, Scott W.; Habbal, Shadia R.;
Pasachoff, Jay M.; Peticolas, Laura
2011arXiv1108.3486H Altcode:
Totality during the solar eclipse of 2017 traverses the entire breadth
of the continental United States, from Oregon to South Carolina. It thus
provides the opportunity to assemble a very large number of images,
obtained by amateur observers all along the path, into a continuous
record of coronal evolution in time; totality lasts for an hour and
a half over the continental U.S. While we describe this event here as
an opportunity for public education and outreach, such a movie -with
very high time resolution and extending to the chromosphere - will also
contain unprecedented information about the physics of the solar corona.

---------------------------------------------------------
Title: Estimates of Densities and Filling Factors from a Cooling
Time Analysis of Solar Microflares Observed with RHESSI
Authors: Baylor, R. N.; Cassak, P. A.; Christe, S.; Hannah, I. G.;
Krucker, Säm; Mullan, D. J.; Shay, M. A.; Hudson, H. S.; Lin, R. P.
2011ApJ...736...75B Altcode: 2011arXiv1107.3997B
We use more than 4500 microflares from the RHESSI microflare data set to
estimate electron densities and volumetric filling factors of microflare
loops using a cooling time analysis. We show that if the filling factor
is assumed to be unity, the calculated conductive cooling times are
much shorter than the observed flare decay times, which in turn are
much shorter than the calculated radiative cooling times. This is likely
unphysical, but the contradiction can be resolved by assuming that the
radiative and conductive cooling times are comparable, which is valid
when the flare loop temperature is a maximum and when external heating
can be ignored. We find that resultant radiative and conductive cooling
times are comparable to observed decay times, which has been used as an
assumption in some previous studies. The inferred electron densities
have a mean value of 1011.6 cm-3 and filling
factors have a mean of 10-3.7. The filling factors are lower
and densities are higher than previous estimates for large flares,
but are similar to those found for two microflares by Moore et al.

---------------------------------------------------------
Title: Temperature and Density Estimates of Extreme-ultraviolet
Flare Ribbons Derived from TRACE Diffraction Patterns
Authors: Krucker, Säm; Raftery, Claire L.; Hudson, Hugh S.
2011ApJ...734...34K Altcode:
We report on Transition Region And Coronal Explorer 171 Å observations
of the GOES X20 class flare on 2001 April 2 that shows EUV flare ribbons
with intense diffraction patterns. Between the 11th to 14th order, the
diffraction patterns of the compact flare ribbon are dispersed into two
sources. The two sources are identified as emission from the Fe IX line
at 171.1 Å and the combined emission from Fe X lines at 174.5, 175.3,
and 177.2 Å. The prominent emission of the Fe IX line indicates that
the EUV-emitting ribbon has a strong temperature component near the
lower end of the 171 Å temperature response (~0.6-1.5 MK). Fitting
the observation with an isothermal model, the derived temperature is
around 0.65 MK. However, the low sensitivity of the 171 Å filter to
high-temperature plasma does not provide estimates of the emission
measure for temperatures above ~1.5 MK. Using the derived temperature
of 0.65 MK, the observed 171 Å flux gives a density of the EUV ribbon
of 3 × 1011 cm-3. This density is much lower
than the density of the hard X-ray producing region (~1013
to 1014 cm-3) suggesting that the EUV sources,
though closely related spatially, lie at higher altitudes.

---------------------------------------------------------
Title: Transient Artifacts in SDO/HMI Flare Observations
Authors: Martinez Oliveros, Juan Carlos; Lindsey, C.; Hudson, H.;
Schou, J.; Couvidat, S.
2011SPD....42.2123M Altcode: 2011BAAS..43S.2123M
The Helioseismic and Magnetic Imager (HMI) on SDO provides a new
tool for the systematic observation of white-light flares, including
Doppler and magnetic information as well as continuum. In our initial
analysis of the highly impulsive gamma-ray flare SOL2010-06-12T00:57
(Martinez-Oliveros et al. 2011), we detected an apparently artifactual
blue shift in the two footpoint sources. We have now deployed the PASCAL
algorithm for the same flare as viewed in GONG++ data. This algorithm
makes it possible to obtain much better photometry (plus Doppler and
magnetic measurements) from the ground-based data. Using GONG++ we
have demonstrated the artifactual nature of the apparent blueshift,
finding instead weak redshifts at the foopoints. We discuss the flare
physics associated with these observations and describe the use of
PASCAL (with GONG++ or other ground-based data) as a complement to
the systematic SDO data.

---------------------------------------------------------
Title: Solar Hard X-ray Observations with NuSTAR
Authors: Smith, David M.; Krucker, S.; Hudson, H. S.; Hurford, G. J.;
White, S. M.; Mewaldt, R. A.; Stern, D.; Grefenstette, B. W.; Harrison,
F. A.
2011SPD....42.1501S Altcode: 2011BAAS..43S.1501S
High-sensitivity imaging of coronal hard X-rays allows detection of
freshly accelerated nonthermal electrons at the acceleration site. A
few such observations have been made with Yohkoh and RHESSI, but a
leap in sensitivity could help pin down the time, place, and manner
of reconnection. In 2012, the Nuclear Spectroscopic Telescope
Array (NuSTAR), a NASA Small Explorer for high energy astrophysics
that uses grazing-incidence optics to focus X-rays up to 80 keV, will
be launched. NuSTAR is capable of solar pointing, and three weeks
will be dedicated to solar observing during the baseline two-year
mission. NuSTAR will be 200 times more sensitive than RHESSI in the
hard X-ray band. This will allow the following new observations, among
others: 1) Extrapolation of the micro/nanoflare distribution by two
orders of magnitude down in flux 2) Search for hard X-rays from
network nanoflares (soft X-ray bright points) and evaluation of their
role in coronal heating 3) Discovery of hard X-ray bremsstrahlung
from the electron beams driving type III radio bursts, and measurement
of their electron spectrum 4) Hard X-ray studies of polar soft
X-ray jets and impulsive solar energetic particle events at the edge
of coronal holes, and comparison of these events with observations
of 3He and other particles in interplanetary space 5) Study of
coronal bremsstrahlung from particles accelerated by coronal mass
ejections as they are first launched 6) Study of particles at
the coronal reconnection site when flare footpoints are occulted; and
7) Search for hypothetical axion particles created in the solar
core via the hard X-ray signal from their conversion to X-rays in the
coronal magnetic field. NuSTAR will also serve as a pathfinder
for a future dedicated space mission with enhanced capabilities,
such as a satellite version of the FOXSI sounding rocket.

---------------------------------------------------------
Title: Flaring Solar Hale Sector Boundaries
Authors: Svalgaard, L.; Hannah, I. G.; Hudson, H. S.
2011ApJ...733...49S Altcode: 2010arXiv1010.2710S
The sector structure that organizes the magnetic field of the solar
wind into large-scale domains has a clear pattern in the photospheric
magnetic field as well. The rotation rate, 27-28.5 days, implies an
effectively rigid rotation originating deeper in the solar interior
than the sunspots. The photospheric magnetic field is known to be
concentrated near that portion (the Hale boundary) in each solar
hemisphere, where the change in magnetic sector polarity matches
that between the leading and following sunspot polarities in active
regions in the respective hemispheres. We report here that flares and
microflares also concentrate at the Hale boundaries, implying that
flux emergence and the creation of free magnetic energy in the corona
also have a direct cause in the deep interior.

---------------------------------------------------------
Title: The Height of White-light Flare Continuum Formation
Authors: Martinez Oliveros, J.; Hudson, Hugh; Krucker, S.; Schou,
J.; Couvidat, S.
2011SPD....42.2211M Altcode: 2011BAAS..43S.2211M
White-light continuum and hard X-ray emission in flares have strong
correlations in time and in horizontal position, but at present we do
not have a clear idea about their height structures. On 24 February
2011 a white-light flare (SOL2011-02-24T07:35) was observed on the
east limb, simultaneously by the Helioseismic Magnetic Imager (HMI)
on the Solar Dynamics Observatory (SDO), and by the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI). This observation
gives us the opportunity to determine the heights of these emissions
directly, limited only by the limb references for the two spacecraft,
with almost no projection undertainty. HMI obtained clear images in
the pseudo-continuum around 6173A, and RHESSI obtained hard X-ray
images. For both data sets, the precision of centroid determination
is of order 0.1 arc s. We believe that the position of the white-light
limb, as a local reference, can also be understood at a corresponding
level of accuracy for the two data sets. We report the results of
this analysis and discuss our findings in terms of present models of
particle acceleration and energy transport in the impulsive phase.

---------------------------------------------------------
Title: UV Diagnostics of Stellar and Solar Flares
Authors: Kowalski, Adam; Hawley, S. L.; Hudson, H. S.
2011AAS...21821303K Altcode: 2011BAAS..43G21303K
The UV spectral regime provides a comprehensive view of the plasma
dynamics and atmospheric temperature structure during stellar flares. We
review the major developments in UV spectroscopy of flares on low mass
stars that shape our understanding of the flare process and challenge
the predictions of current radiative hydrodynamic models. We put the
ultraviolet properties in context with the radiation in the neighboring
X-ray and visible wavelength regimes. We also show how SDO/EVE data
of several Cycle 24 solar flares allow for new comparisons to be made
between solar and stellar flares.

---------------------------------------------------------
Title: A RHESSI And SDO Campaign Measuring Latitude-dependent Limb
ProfilesAnd Oblateness Of The Optical Solar Disk
Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P.; Bush, R. I.;
Emilio, M.; Kuhn, J. R.; Scholl, I. F.
2011SPD....42.1706F Altcode: 2011BAAS..43S.1706F
The SDO spacecraft conducted a special roll maneuver, 2011 April
6 05:50-12:30 UT, to enable its HMI instrument to obtain precise
observations of the global structure of the limb. Similar maneuvers
had been carried out with SOHO for MDI in the past. On this occasion
we also successfully obtained RHESSI optical observations at very
high cadence, 128 samples per sec for each of the three linear
CCDs. The data from the two instrument (RHESSI/SAS and SDO/HMI),
give different means for the investigation of the variation of the
solar limb properties as a function of position angle (latitude). At
the normal RHESSI cadence very long integrations (of order 3 months)
are needed to obtain precise limb measurements, but in this case we
expect to be able to report results within the exact time frame of
the SDO roll maneuver. The special RHESSI data rate was about 10,000
times larger than the standard rate and will achieve high precision in
a relatively short time. We will compare these results with our earlier
RHESSI observations (Fivian et al., 2008) and those obtained by Kuhn et
al. (1998) and Emilio et al. (2007) with the earlier MDI roll maneuvers.

---------------------------------------------------------
Title: Doppler Signatures In EVE Spectra
Authors: Hudson, Hugh S.; Chamberlin, P.; Woods, T.; Fletcher, L.;
Graham, D.
2011SPD....42.2124H Altcode: 2011BAAS..43S.2124H
The Extreme-ultraviolet Variability Experiment (EVE) on SDO is providing
a comprehensive set of EUV spectra of the Sun as a star. The routine
sampling is with 10 s integrations at a resolution of 0.1 nm. Although
this resolution corresponds to only some 1000 km/s in velocity space,
we demonstrate that the instrument is stable enough to detect the SDO
orbital motion of a few km/s readily in the bright He II line at 30.4
nm. We find the random error in the centroid location of this line to be
less than one pm (less than 1 km/s) per 10 s integration. We also note
systematic effects from a variety of causes. For flare observations,
the line centroid position depends on the flare position. We discuss the
calibration of this effect and show that EVE can nonetheless provide
clear Doppler signatures that may be interpreted in terms of flare
dynamics. This information has some value in and of itself, because of
EVE's sensitivity, but we feel that it will be of greatest importance
when combined with imagery (e.g., via AIA) a modeling. We discuss flare
signatures in several events, e.g. the gamma-ray flare SOL2010-06-12
and SOL2011-02-16T:07:44, taking advantage of AIA image comparisons.

---------------------------------------------------------
Title: Results and Analysis of the RHESSI/SAS Observations of the
Optical Solar Limb
Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P.
2011SPD....42.1724F Altcode: 2011BAAS..43S.1724F
The Solar Aspect System (SAS) of the RHESSI satellite measures the
optical solar limb in the red continuum with a cadence typically
set at 16 samples/s in each of three linear CCD sensors. RHESSI
has observed the Sun continuously now for more than 9 years,
and we have acquired a unique data set ranging almost over a full
solar cycle and consisting of about 25x109 single data
points. These measurements have led to the most accurate oblateness
measurement to date, 8.01+-0.14 milli arcsec (Fivian et al., 2008),
a value consistent with models predicting an oblateness from surface
rotation. An excess oblateness term can be attributed to the enhanced
network. New measurements of latitude-dependent brightness variations
at the limb lead to a quadrupolar term (a pole-to-equator temperature
variation) of 0.04+-0.02 K. We present the analysis of these unique
data and an overview of some results.

---------------------------------------------------------
Title: On the Magnetic Field Variations and HXR Emission of the
First X-class Flare in the 24th Solar Cycle
Authors: Martinez Oliveros, Juan Carlos; Alvarado Gomez, J.; Buitrago
Casas, J.; Lindsey, C.; Hudson, H.; Calvo-Mozo, B.
2011SPD....42.2225M Altcode: 2011BAAS..43S.2225M
Multi-wavelength studies of energetic solar flares with seismic
emissions have revealed interesting common features between them. We
studied the first seismically active flare of the 24th solar cycle (SOL2011-02-15T01:52 X2.2) detected by HMI/SDO (Kosovichev, 2011)
using a pixel-by-pixel light-curve characterization of the fluctuations
of the photospheric longitudinal magnetic field based on HMI data. For
context we used HXR RHESSI data to find a correlation between these
sources and the spatial location of the transient longitudinal magnetic
field changes in the photospheric region where this flare took place.

---------------------------------------------------------
Title: Magnetic Oscillations Mark Sites of Magnetic Transients in
an Acoustically Active Flare
Authors: Lindsey, Charles A.; Donea, A.; Hudson, H. S.; Martinez
Oliveros, J.; Hanson, C.
2011SPD....42.2207L Altcode: 2011BAAS..43S.2207L
The flare of 2011 February 15, in NOAA AR11158, was the first
acoustically active flare of solar cycle 24, and the first observed by
the Solar Dynamics Observatory (SDO). It was exceptional in a number
of respects (Kosovichev 2011a,b). Sharp ribbon-like transient Doppler,
and magnetic signatures swept over parts of the active region during
the impulsive phase of the flare. We apply seismic holography to a 2-hr
time series of HMI observations encompassing the flare. The acoustic
source distribution appears to have been strongly concentrated in a
single highly compact penumbral region in which the continuum-intensity
signature was unusually weak. The line-of-sight magnetic transient
was strong in parts of the active region, but relatively weak in
the seismic-source region. On the other hand, the neighbourhoods of
the regions visited by the strongest magnetic transients maintained
conspicuous 5-minutes-period variations in the line of sight magnetic
signature for the full 2-hr duration of the time series, before
the flare as well as after. We apply standard helioseismic control
diagnostics for clues as to the physics underlying 5-minute magnetic
oscillations in regions conducive to magnetic transients during a
flare and consider the prospective development of this property as
an indicator of flare potentiality on some time scale. We make use of
high-resolution data from AIA, using diffracted images where necessary
to obtain good photometry where the image is otherwise saturated. This
is relevant to seismic emission driven by thick-target heating in the
absence of back-warming. We also use RHESSI imaging spectroscopy to
compare the source distributions of HXR and seismic emission.

---------------------------------------------------------
Title: Solar and Stellar Flares
Authors: Hudson, Hugh
2011APS..APR.L3001H Altcode:
Flares in the atmosphere of the Sun and of many other stars appear
to result from the sudden conversion of electromagnetic field
energy into a wide array of observable forms. Of these products
the definitive modern observations are the X-rays and γ-rays,
signifying the common occurrence of particle acceleration to mildly
relativistic or higher energies. Abundant direct (the radiation)
and indirect information confirms that this particle acceleration is
energetically significant, as well as common. We are thus led to the
physics of particle distribution functions that may deviate radically
from Maxwellian distributions. Stellar observations allow us to study
these phenomena across a wide variety of environments, whereas solar
and planetary observations allow us to do imaging spectroscopy and
thereby get a better understanding of the global structures of the
processes. In particular we have spectacular new data from satellite
solar observatories such as RHESSI (hard X-rays and γ-rays) and
others, most recently the Solar Dynamics Observatory. Of particular
interest from the point of view of plasma physics is the flare
environment: a low-beta corona linked to a massive body through an
intermediate weakly-ionized layer (the chromosphere). The chromosphere
is extraordinarily complicated; its behavior is coming again to be
recognized as fundamental to the overall flare process, and in this
presentation I will attempt to clarify its role.

---------------------------------------------------------
Title: Imaging Spectroscopy of a White-Light Solar Flare
Authors: Martínez Oliveros, J. C.; Couvidat, S.; Schou, J.; Krucker,
S.; Lindsey, C.; Hudson, H. S.; Scherrer, P.
2011SoPh..269..269M Altcode: 2011SoPh..tmp....7M; 2010arXiv1012.0344M
We report observations of a white-light solar flare
(SOL2010-06-12T00:57, M2.0) observed by the Helioseismic Magnetic
Imager (HMI) on the Solar Dynamics Observatory (SDO) and the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The HMI data
give us the first space-based high-resolution imaging spectroscopy
of a white-light flare, including continuum, Doppler, and magnetic
signatures for the photospheric Fe I line at 6173.34 Å and its
neighboring continuum. In the impulsive phase of the flare, a bright
white-light kernel appears in each of the two magnetic footpoints. When
the flare occurred, the spectral coverage of the HMI filtergrams (six
equidistant samples spanning ±172 mÅ around nominal line center)
encompassed the line core and the blue continuum sufficiently far from
the core to eliminate significant Doppler crosstalk in the latter, which
is otherwise a possibility for the extreme conditions in a white-light
flare. RHESSI obtained complete hard X-ray and γ-ray spectra (this
was the first γ-ray flare of Cycle 24). The Fe I line appears to be
shifted to the blue during the flare but does not go into emission; the
contrast is nearly constant across the line profile. We did not detect
a seismic wave from this event. The HMI data suggest stepwise changes
of the line-of-sight magnetic field in the white-light footpoints.

---------------------------------------------------------
Title: Global Properties of Solar Flares
Authors: Hudson, Hugh S.
2011SSRv..158....5H Altcode: 2011SSRv..tmp....7H; 2011arXiv1108.3490H
This article broadly reviews our knowledge of solar flares. There
is a particular focus on their global properties, as opposed to
the microphysics such as that needed for magnetic reconnection or
particle acceleration as such. Indeed solar flares will always remain
in the domain of remote sensing, so we cannot observe the microscales
directly and must understand the basic physics entirely via the
global properties plus theoretical inference. The global observables
include the general energetics—radiation in flares and mass loss in
coronal mass ejections (CMEs)—and the formation of different kinds of
ejection and global wave disturbance: the type II radio-burst exciter,
the Moreton wave, the EIT "wave", and the "sunquake" acoustic waves
in the solar interior. Flare radiation and CME kinetic energy can
have comparable magnitudes, of order 1032 erg each for an
X-class event, with the bulk of the radiant energy in the visible-UV
continuum. We argue that the impulsive phase of the flare dominates
the energetics of all of these manifestations, and also point out that
energy and momentum in this phase largely reside in the electromagnetic
field, not in the observable plasma.

---------------------------------------------------------
Title: Three-dimensional morphology of a coronal prominence cavity
Authors: Gibson, S. E.; Kucera, T. A.; Rastawicki, D.; Dove, J.; de
Toma, G.; Hao, J.; Hill, S. M.; Hudson, H. S.; Marque, C.; McIntosh,
P. S.; Rachmeler, L.; Reeves, K. K.; Schmieder, B.; Schmit, D. J.;
Sterling, A.; Tripathi, D.; Williams, D. R.; Zhang, M.
2010AGUFMSH51A1667G Altcode:
We present a three-dimensional density model of coronal prominence
cavities, and a morphological fit that has been tightly constrained
by a uniquely well-observed cavity. Observations were obtained as part
of an International Heliophysical Year campaign by instruments from a
variety of space- and ground-based observatories, spanning wavelengths
from radio to soft-X-ray to integrated white light. From these data
it is clear that the prominence cavity is the limb manifestation of
a longitudinally-extended polar-crown filament channel, and that
the cavity is a region of low density relative to the surrounding
corona. As a first step towards quantifying density and temperature
from campaign spectroscopic data, we establish the three-dimensional
morphology of the cavity. This is critical for taking line-of-sight
projection effects into account, since cavities are not localized in the
plane of the sky and the corona is optically thin. We have augmented
a global coronal streamer model to include a tunnel-like cavity with
elliptical cross-section and a Gaussian variation of height along
the tunnel length. We have developed a semi-automated routine that
fits ellipses to cross-sections of the cavity as it rotates past the
solar limb, and have applied it to Extreme Ultraviolet Imager (EUVI)
observations from the two Solar Terrestrial Relations Observatory
(STEREO) spacecraft. This defines the morphological parameters of our
model, from which we reproduce forward-modeled cavity observables. We
find that cavity morphology and orientation, in combination with the
viewpoints of the observing spacecraft, explains the observed variation
in cavity visibility for the east vs. west limbs.

---------------------------------------------------------
Title: Three-dimensional Morphology of a Coronal Prominence Cavity
Authors: Gibson, S. E.; Kucera, T. A.; Rastawicki, D.; Dove, J.; de
Toma, G.; Hao, J.; Hill, S.; Hudson, H. S.; Marqué, C.; McIntosh,
P. S.; Rachmeler, L.; Reeves, K. K.; Schmieder, B.; Schmit, D. J.;
Seaton, D. B.; Sterling, A. C.; Tripathi, D.; Williams, D. R.;
Zhang, M.
2010ApJ...724.1133G Altcode:
We present a three-dimensional density model of coronal prominence
cavities, and a morphological fit that has been tightly constrained
by a uniquely well-observed cavity. Observations were obtained as part
of an International Heliophysical Year campaign by instruments from a
variety of space- and ground-based observatories, spanning wavelengths
from radio to soft X-ray to integrated white light. From these data
it is clear that the prominence cavity is the limb manifestation of
a longitudinally extended polar-crown filament channel, and that the
cavity is a region of low density relative to the surrounding corona. As
a first step toward quantifying density and temperature from campaign
spectroscopic data, we establish the three-dimensional morphology
of the cavity. This is critical for taking line-of-sight projection
effects into account, since cavities are not localized in the plane of
the sky and the corona is optically thin. We have augmented a global
coronal streamer model to include a tunnel-like cavity with elliptical
cross-section and a Gaussian variation of height along the tunnel
length. We have developed a semi-automated routine that fits ellipses
to cross-sections of the cavity as it rotates past the solar limb, and
have applied it to Extreme Ultraviolet Imager observations from the
two Solar Terrestrial Relations Observatory spacecraft. This defines
the morphological parameters of our model, from which we reproduce
forward-modeled cavity observables. We find that cavity morphology
and orientation, in combination with the viewpoints of the observing
spacecraft, explain the observed variation in cavity visibility for
the east versus west limbs.

---------------------------------------------------------
Title: Generation of Electric Currents in the Chromosphere via
Neutral-Ion Drag
Authors: Krasnoselskikh, V.; Vekstein, G.; Hudson, H. S.; Bale, S. D.;
Abbett, W. P.
2010ApJ...724.1542K Altcode: 2010arXiv1011.5834K
We consider the generation of electric currents in the solar
chromosphere where the ionization level is typically low. We show that
ambient electrons become magnetized even for weak magnetic fields (30
G); that is, their gyrofrequency becomes larger than the collision
frequency while ion motions continue to be dominated by ion-neutral
collisions. Under such conditions, ions are dragged by neutrals,
and the magnetic field acts as if it is frozen-in to the dynamics of
the neutral gas. However, magnetized electrons drift under the action
of the electric and magnetic fields induced in the reference frame of
ions moving with the neutral gas. We find that this relative motion of
electrons and ions results in the generation of quite intense electric
currents. The dissipation of these currents leads to resistive electron
heating and efficient gas ionization. Ionization by electron-neutral
impact does not alter the dynamics of the heavy particles; thus, the
gas turbulent motions continue even when the plasma becomes fully
ionized, and resistive dissipation continues to heat electrons and
ions. This heating process is so efficient that it can result in
typical temperature increases with altitude as large as 0.1-0.3 eV
km-1. We conclude that this process can play a major role
in the heating of the chromosphere and corona.

---------------------------------------------------------
Title: Morphology of a hot coronal cavity core as observed by
Hinode/XRT
Authors: Reeves, K. K.; Gibson, S. E.; Kucera, T. A.; Hudson, H. S.
2010AGUFMSH51A1669R Altcode:
We follow a coronal cavity that was observed by Hinode/XRT during the
summer of 2008. This cavity has a persistent area of relatively bright
X-ray emission in its center. We use multifilter data from XRT to
study the thermal emission from this cavity, and find that the bright
center is hotter than the surrounding cavity plasma with temperatures
of about 1.6 MK. We follow the morphology of this hot feature as the
cavity structure rotates over the limb during the several days between
July 19 - 23 2008. We find that the hot structure at first looks fairly
circular, then appears to expand and elongate, and then shrinks again
to a compact circular shape. We interpret this apparent change in shape
as being due to the morphology of the filament channel associated with
the cavity, and the change in viewing angle as the structure rotates
over the limb of the Sun.

---------------------------------------------------------
Title: A "black light flare" observed by HMI?
Authors: Martinez Oliveros, J.; Hudson, H. S.; Krucker, S.
2010AGUFMSH23A1827M Altcode:
We report the observation of a negative precursor to the white-light
flare SOL2010-06-12T00:57 (M2.0), as observed via SDO/HMI in narrow
bands near 6173.3 A. RHESSI and Fermi hard X-ray and gamma-ray
observations for this flare are also available and reveal an unusually
hard spectrum in the hard X-ray range. The "black light flare" locations
are in both of the two white-light flare emission regionsand precede
them by about one HMI time step of 45 s. The timing thus resembles
that predicted by Henoux et al. (1990) based on the interplay between
ionization (leading to opacity) and heating (leading to emission). We
discuss this remarkable event in its full observational scope.

---------------------------------------------------------
Title: The Solar Oblateness at Solar Minimum as Observed by RHESSI/SAS
Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.
2010AGUFMSH53B..05F Altcode:
The RHESSI solar aspect sensor (SAS) has provided oblateness
measurements of the optical solar disk with unprecedented precision. SAS
measures the optical solar profile at 670 nm in the red continuum. SAS
consists of three spacially uniformly distributed linear CCDs mounted
on a rotating spacecraft; a crucial ingredient in access of calibration
parameters. From the SAS data, differential radius measurements can
be derived at about 100 Hz including about 1000 full CCD readouts
per day for calibration purposes. For a three month period during the
active phase of the solar cycle in 2004, the shape of the solar disk
has been measured discovering an apparent excess oblateness which
we attributed to the enhanced network. In order to avoid confusion
between magnetic activity and a correlated brightness enhancement
in the SAS signal at 670 nm, the SAS data has been masked using the
SOHO/EIT284A data. The measured oblateness as function of the masking
level is then extrapolated for a value of the underlaying, presumably
non-magnetic sun. Here, we present the analysis of the RHESSI/SAS
data during the solar minimum with the inferred interpretation for
the oblateness signal.

---------------------------------------------------------
Title: Generation of electric currents in the chromosphere via
neutral-ion drag
Authors: Krasnoselskikh, V.; Vekstein, G.; Hudson, H. S.; Bale, S.;
Abbett, W. P.
2010AGUFMSH31C1810K Altcode:
We consider the generation of electric currents in the solar
chromosphere. The ionization level in this region is generally supposed
to be low. We show that the ambient electrons become magnetized even for
weak magnetic fields (30 G), i.e. their gyrofrequency becomes larger
than the collision frequency; ion motions continue to be dominated by
ion-neutral collisions in this region. Under such conditions the ions
are dragged by neutrals and magnetic field dynamics resembles frozen-in
motion of the field with the neutral gas. On the other hand magnetized
electrons drift under the action of the electric and magnetic fields
induced in the reference frame of ions moving with the neutral gas. This
relative motion of electrons and ions results in the generation of quite
intense electric currents. The dissipation of these currents leads to
the resistive electron heating and efficient gas ionization. Ionization
by electron-neutral impact does not alter the dynamics of the heavy
particles; thus the gas turbulent motions continue even when the plasma
becomes fully ionized and the resistive current dissipation continues
to heat electrons and ions. This heating process is so efficient that
it can result in typical temperature increases with altitude as large
as 0.1-0.3 eV/km. We conclude that this process can play a major role
in the heating of the chromosphere and corona.

---------------------------------------------------------
Title: Uncovering Mechanisms of Coronal Magnetism via Advanced 3D
Modeling of Flares and Active Regions
Authors: Fleishman, Gregory; Gary, Dale; Nita, Gelu; Alexander,
David; Aschwanden, Markus; Bastian, Tim; Hudson, Hugh; Hurford,
Gordon; Kontar, Eduard; Longcope, Dana; Mikic, Zoran; DeRosa, Marc;
Ryan, James; White, Stephen
2010arXiv1011.2800F Altcode:
The coming decade will see the routine use of solar data of
unprecedented spatial and spectral resolution, time cadence, and
completeness. To capitalize on the new (or soon to be available)
facilities such as SDO, ATST and FASR, and the challenges they present
in the visualization and synthesis of multi-wavelength datasets,
we propose that realistic, sophisticated, 3D active region and flare
modeling is timely and critical, and will be a forefront of coronal
studies over the coming decade. To make such modeling a reality, a
broad, concerted effort is needed to capture the wealth of information
resulting from the data, develop a synergistic modeling effort, and
generate the necessary visualization, interpretation and model-data
comparison tools to accurately extract the key physics.

---------------------------------------------------------
Title: Constraining the Hard X-ray Properties of the Quiet Sun with
New RHESSI Observations
Authors: Hannah, I. G.; Hudson, H. S.; Hurford, G. J.; Lin, R. P.
2010ApJ...724..487H Altcode: 2010arXiv1009.2918H
We present new RHESSI upper limits in the 3-200 keV energy range for
solar hard X-ray emission in the absence of flares and active regions,
i.e., the quiet Sun, using data obtained between 2005 July and 2009
April. These new limits, substantially deeper than any previous ones,
constrain several physical processes that could produce hard X-ray
emission. These include cosmic-ray effects and the generation of
axions within the solar core. The data also limit the properties of
"nanoflares," a leading candidate to explain coronal heating. We find
it unlikely for nanoflares involving nonthermal effects to heat the
corona because such events would require a steep electron spectrum
E -δ with index δ&gt;5 extending to very low energies
(&lt;1 keV), into the thermal energy range. We also use the limits
to constrain the parameter space of an isothermal model and coronal
thin-target emission models (power-law and kappa distributions).

---------------------------------------------------------
Title: On the Origin of the Solar Moreton Wave of 2006 December 6
Authors: Balasubramaniam, K. S.; Cliver, E. W.; Pevtsov, A.; Temmer,
M.; Henry, T. W.; Hudson, H. S.; Imada, S.; Ling, A. G.; Moore, R. L.;
Muhr, N.; Neidig, D. F.; Petrie, G. J. D.; Veronig, A. M.; Vršnak,
B.; White, S. M.
2010ApJ...723..587B Altcode:
We analyzed ground- and space-based observations of the eruptive flare
(3B/X6.5) and associated Moreton wave (~850 km s-1 ~270°
azimuthal span) of 2006 December 6 to determine the wave driver—either
flare pressure pulse (blast) or coronal mass ejection (CME). Kinematic
analysis favors a CME driver of the wave, despite key gaps in coronal
data. The CME scenario has a less constrained/smoother velocity versus
time profile than is the case for the flare hypothesis and requires an
acceleration rate more in accord with observations. The CME picture is
based, in part, on the assumption that a strong and impulsive magnetic
field change observed by a GONG magnetograph during the rapid rise phase
of the flare corresponds to the main acceleration phase of the CME. The
Moreton wave evolution tracks the inferred eruption of an extended
coronal arcade, overlying a region of weak magnetic field to the west
of the principal flare in NOAA active region 10930. Observations of
Hα foot point brightenings, disturbance contours in off-band Hα
images, and He I 10830 Å flare ribbons trace the eruption from 18:42
to 18:44 UT as it progressed southwest along the arcade. Hinode EIS
observations show strong blueshifts at foot points of this arcade
during the post-eruption phase, indicating mass outflow. At 18:45
UT, the Moreton wave exhibited two separate arcs (one off each flank
of the tip of the arcade) that merged and coalesced by 18:47 UT to
form a single smooth wave front, having its maximum amplitude in
the southwest direction. We suggest that the erupting arcade (i.e.,
CME) expanded laterally to drive a coronal shock responsible for the
Moreton wave. We attribute a darkening in Hα from a region underlying
the arcade to absorption by faint unresolved post-eruption loops.

---------------------------------------------------------
Title: Solar Flares and the Chromosphere
Authors: Fletcher, L.; Turkmani, R.; Hudson, H. S.; Hawley, S. L.;
Kowalski, A.; Berlicki, A.; Heinzel, P.
2010arXiv1011.4650F Altcode:
A white paper prepared for the Space Studies Board, National Academy
of Sciences (USA), for its Decadal Survey of Solar and Space Physics
(Heliophysics), reviewing and encouraging studies of flare physics in
the chromosphere.

---------------------------------------------------------
Title: The Optical Depth of White-light Flare Continuum
Authors: Potts, Hugh; Hudson, Hugh; Fletcher, Lyndsay; Diver, Declan
2010ApJ...722.1514P Altcode: 2010arXiv1004.1039P
The white-light continuum emission of a solar flare remains a puzzle as
regards its height of formation and its emission mechanism(s). This
continuum and its extension into the near-UV contain the bulk of
the energy radiated by a flare, and so its explanation is a high
priority. We describe a method to determine the optical depth of the
emitting layer and apply it to the well-studied flare of 2002 July
15, making use of MDI pseudo-continuum intensity images. We find the
optical depth of the visible continuum in all flare images, including
an impulsive ribbon to be small, consistent with the observation of
Balmer and Paschen edges in other events.

---------------------------------------------------------
Title: Solar Flares and the Chromosphere: A white paper for the
Decadal Survey
Authors: Hudson, H. S.; Fletcher, L.; Turkmani, R.; Hawley, S. L.;
Kowalski, A. F.; Berlicki, A.; Heinzel, P.
2010helio2010....1H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar physics: Solar flares add up
Authors: Hudson, Hugh S.
2010NatPh...6..637H Altcode:
Solar flares are the most energetic events in our Solar System,
but relatively little is known about their contribution to the total
energy the Earth receives from the Sun. The detection of a moderate
solar flare in the total solar irradiance suggests their impact on
the variability of the Sun's output could be larger than expected.

---------------------------------------------------------
Title: Flare and CME Properties and Rates at Sunspot Minimum
Authors: Hudson, H. S.; Li, Y.
2010ASPC..428..153H Altcode: 2010arXiv1003.4289H
The corona at solar minimum generally differs greatly from that
during active times. We discuss the current Cycle 23/24 minimum from
the point of view of the occurrence of flares and CMEs (coronal mass
ejections). By comparison with the previous minimum, the flare/CME
ratio diminished by almost an order of magnitude. This suggests that
the environmental effect in flare/CME association differed in the sense
that the Cycle 23/24 minimum corona was relatively easy to disrupt.

---------------------------------------------------------
Title: The Solar Microwave Flux and the Sunspot Number
Authors: Svalgaard, L.; Hudson, H. S.
2010ASPC..428..325S Altcode: 2010arXiv1003.4281S
The solar F10.7 index is has been a reliable and sensitive activity
index since 1947. As with other indices, it has been showing unusual
behavior in the Cycle 23/24 minimum. The origins of the solar microwave
flux lie in a variety of features, and in two main emission mechanisms:
free-free and gyroresonance. In past solar cycles, F10.7 has correlated
well with the sunspot number SSN. We find that this correlation has
broken down in Cycle 23, confirming this with Japanese fixed-frequency
radiometric microwave data.

---------------------------------------------------------
Title: The Signature of Flares in VIRGO Total Solar Irradiance
Measurements
Authors: Quesnel, A.; Dennis, B. R.; Fleck, B.; Fröhlich, C.; Hudson,
H. S.; Tolbert, A. K.
2010ASPC..428..133Q Altcode: 2010arXiv1003.4194Q
We use Total Solar Irradiance (TSI) measurements from the VIRGO
(Variability of solar Irradiance and Gravity Oscillations) instrument
on board SOHO to obtain preliminary estimates of the mean total
radiative energy emitted by X-class solar flares. The basic tool is
that of summed-epoch analysis, which has also enabled us to detect and
partially characterize systematic errors present in the basic data. We
describe these errors, which significantly degrade the photometry at
high frequencies. We find the ratio of GOES 1-8 Å luminosity to total
bolometric luminosity to be of order 0.01.

---------------------------------------------------------
Title: Thermal Properties of Coronal Cavities as Observed by the
X-Ray Telescope on Hinode
Authors: Reeves, Kathy; Gibson, S. E.; Kucera, T. A.; Hudson, H. S.;
Tripathi, D.
2010AAS...21640511R Altcode: 2010BAAS...41..891R
Coronal cavities are voids in coronal emission often observed above
high latitude filament channels. Sometimes, these cavities have
areas of bright X-ray emission in their centers (i.e. Hudson et al
1999). In this study, we use data from the X-ray Telescope (XRT)
on Hinode to examine the thermal emission properties of two kinds of
coronal cavities, those with and without enhanced emission at their
centers. For cavities with bright X-ray emission in their centers,
we find evidence for elevated temperatures in the cavity center. We
find no obvious correlation between the presence of bright cavity
cores and filament presence or eruption. This work is part of the
effort of the International Space Science Institute International Team
on Prominence Cavities

---------------------------------------------------------
Title: Observations of solar and stellar eruptions, flares, and jets
Authors: Hudson, Hugh
2010hssr.book..123H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Flare Global Waves Of Three Kinds
Authors: Hudson, Hugh S.; Martinez-Oliveros, J. C.
2010AAS...21640429H Altcode: 2010BAAS...41..904H
Flares produce at least three kinds of global waves: Moreton, metric
type II, and seismic. In addition, EIT waves, coronal dimmings, and
CMES may also have wave-like properties. Each of these three global
waves arises in the impulsive phase of a flare. This is also the time
of sudden stepwise changes in the photospheric line-of-sight field,
and the time of the CME acceleration phase. We review the observational
material, starting with the published seismic events, and ask whether
or not a common origin is consistent with the physical parameters in
the likely region of origin.

---------------------------------------------------------
Title: Measurements of the Coronal Acceleration Region of a Solar
Flare
Authors: Krucker, Sam; Hudson, H. S.; White, S. M.; Masuda, S.; Lin,
R. P.
2010AAS...21630603K Altcode:
The most discussed coronal hard X-ray source has been the
above-the-loop-top source observed in the Masuda flare. We present new
RHESSI hard X-ray observations of a similar event with simultaneous
microwave observations by NoRH. These observations clearly establish
the non-thermal nature of the above-the-loop-top source and triggered
a new interpretation. To account for the extremely bright hard X-ray
source in a rather low ambient density plasma, all electrons in the
above-the-loop-top source seem to be accelerated, suggesting that the
above-the-loop-top source is itself the electron acceleration region.

---------------------------------------------------------
Title: G-band and Hard X-ray Emissions of the 2006 December 14 Flare
Observed by Hinode/SOT and Rhessi
Authors: Watanabe, Kyoko; Krucker, Säm; Hudson, Hugh; Shimizu,
Toshifumi; Masuda, Satoshi; Ichimoto, Kiyoshi
2010ApJ...715..651W Altcode: 2010arXiv1004.4259W
We report on G-band emission observed by the Solar Optical Telescope on
board the Hinode satellite in association with the X1.5-class flare on
2006 December 14. The G-band enhancements originate from the footpoints
of flaring coronal magnetic loops, coinciding with nonthermal hard
X-ray bremsstrahlung sources observed by the Reuven Ramaty High Energy
Solar Spectroscopic Imager. At the available 2 minute cadence, the
G-band and hard X-ray intensities are furthermore well correlated in
time. Assuming that the G-band enhancements are continuum emission from
a blackbody, we derived the total radiative losses of the white-light
flare (white-light power). If the G-band enhancements additionally have
a contribution from lines, the derived values are overestimates. We
compare the white-light power with the power in hard X-ray producing
electrons using the thick-target assumption. Independent of the cutoff
energy of the accelerated electron spectrum, the white-light power and
the power of accelerated electrons are roughly proportional. Using
the observed upper limit of ~30 keV for the cutoff energy, the hard
X-ray producing electrons provide at least a factor of 2 more power
than needed to produce the white-light emission. For electrons above
40 keV, the powers roughly match for all four of the time intervals
available during the impulsive phase. Hence, the flare-accelerated
electrons contain enough energy to produce the white-light flare
emissions. The observed correlation in time, space, and power strongly
suggests that electron acceleration and white-light production in solar
flares are closely related. However, the results also call attention
to the inconsistency in apparent source heights of the hard X-ray
(chromosphere) and white-light (upper photosphere) sources.

---------------------------------------------------------
Title: Geometric Model of a Coronal Cavity
Authors: Kucera, Therese A.; Gibson, S. E.; Rastawicki, D.; Dove, J.;
de Toma, G.; Hao, J.; Hudson, H. S.; Marque, C.; McIntosh, P. S.;
Reeves, K. K.; Schmidt, D. J.; Sterling, A. C.; Tripathi, D. K.;
Williams, D. R.; Zhang, M.
2010AAS...21640510K Altcode: 2010BAAS...41..890K
We observed a coronal cavity from August 8-18 2007 during a
multi-instrument observing campaign organized under the auspices of
the International Heliophysical Year (IHY). Here we present initial
efforts to model the cavity with a geometrical streamer-cavity
model. The model is based the white-light streamer model of Gibson et
al. (2003), which has been enhanced by the addition of a cavity and
the capability to model EUV and X-ray emission. The cavity is modeled
with an elliptical cross-section and Gaussian fall-off in length and
width inside the streamer. Density and temperature can be varied in the
streamer and cavity and constrained via comparison with data. Although
this model is purely morphological, it allows for three-dimensional,
multi-temperature analysis and characterization of the data, which
can then provide constraints for future physical modeling. Initial
comparisons to STEREO/EUVI images of the cavity and streamer show that
the model can provide a good fit to the data. This work is part of the
effort of the International Space Science Institute International Team
on Prominence Cavities.

---------------------------------------------------------
Title: Photospheric Temperature Variations near the Solar Limb
Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P.; Zahid, H. J.
2010AAS...21631305F Altcode: 2010BAAS...41R.888F
We use observations from the solar aspect sensor of RHESSI to
characterize the latitude dependence of the temperature of the
photosphere near the solar limb. Previous observations had suggested
the presence of a polar temperature excess as large as 1.5 K. The
RHESSI observations, made with a rotating telescope in space,
have great advantages in the rejection of systematic errors in
the very precise photometry required for such an observation. This
photometry is differential, i.e. relative to a mean limb-darkening
function. The data base consists of about 1,000 images per day from
linear CCDs with 1.73 arc sec square pixels, observing a narrow band
(12nm FWHM) at 670 nm. Each image shows a chord crossing the disk at a
different location as the spacecraft rotates and precesses around its
average solar pointing. We fit an average limb-darkening function and
reassemble the residuals into synoptic maps of differential intensity
variations as a function of position angle. We further mask these
images against SOHO/EIT 284A images in order to eliminate magnetic
regions. The analysis establishes a limit on the quadrupole dependence
of temperature (brightness) on position angle of order 0.04 K, with
a comparable uncertainty.

---------------------------------------------------------
Title: Measurements of the Coronal Acceleration Region of a Solar
Flare
Authors: Krucker, Säm; Hudson, H. S.; Glesener, L.; White, S. M.;
Masuda, S.; Wuelser, J. -P.; Lin, R. P.
2010ApJ...714.1108K Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and
the Nobeyama Radioheliograph (NoRH) are used to investigate coronal
hard X-ray and microwave emissions in the partially disk-occulted solar
flare of 2007 December 31. The STEREO mission provides EUV images of the
flare site at different viewing angles, establishing a two-ribbon flare
geometry and occultation heights of the RHESSI and NoRH observations
of ~16 Mm and ~25 Mm, respectively. Despite the occultation, intense
hard X-ray emission up to ~80 keV occurs during the impulsive phase
from a coronal source that is also seen in microwaves. The hard X-ray
and microwave source during the impulsive phase is located ~6 Mm above
thermal flare loops seen later at the soft X-ray peak time, similar in
location to the above-the-loop-top source in the Masuda flare. A single
non-thermal electron population with a power-law distribution (with
spectral index of ~3.7 from ~16 keV up to the MeV range) radiating
in both bremsstrahlung and gyrosynchrotron emission can explain
the observed hard X-ray and microwave spectrum, respectively. This
clearly establishes the non-thermal nature of the above-the-loop-top
source. The large hard X-ray intensity requires a very large number
(&gt;5 × 1035 above 16 keV for the derived upper limit
of the ambient density of ~8 × 109 cm-3)
of suprathermal electrons to be present in this above-the-loop-top
source. This is of the same order of magnitude as the number of ambient
thermal electrons. We show that collisional losses of these accelerated
electrons would heat all ambient electrons to superhot temperatures
(tens of keV) within seconds. Hence, the standard scenario, with hard
X-rays produced by a beam comprising the tail of a dominant thermal core
plasma, does not work. Instead, all electrons in the above-the-loop-top
source seem to be accelerated, suggesting that the above-the-loop-top
source is itself the electron acceleration region.

---------------------------------------------------------
Title: On The Brightness and Waiting-Time Distributions of a Type
III Radio Storm Observed By Stereo/Waves
Authors: Eastwood, J. P.; Wheatland, M. S.; Hudson, H. S.; Krucker,
S.; Bale, S. D.; Maksimovic, M.; Goetz, K.; Bougeret, J. -L.
2010ApJ...708L..95E Altcode: 2009arXiv0911.4131E
Type III solar radio storms, observed at frequencies below ~16 MHz
by space-borne radio experiments, correspond to the quasi-continuous,
bursty emission of electron beams onto open field lines above active
regions. The mechanisms by which a storm can persist in some cases
for more than a solar rotation whilst exhibiting considerable radio
activity are poorly understood. To address this issue, the statistical
properties of a type III storm observed by the STEREO/WAVES radio
experiment are presented, examining both the brightness distribution
and (for the first time) the waiting-time distribution (WTD). Single
power-law behavior is observed in the number distribution as a function
of brightness; the power-law index is ~2.1 and is largely independent of
frequency. The WTD is found to be consistent with a piecewise-constant
Poisson process. This indicates that during the storm individual type
III bursts occur independently and suggests that the storm dynamics are
consistent with avalanche-type behavior in the underlying active region.

---------------------------------------------------------
Title: The white-light continuum in the impulsive phase of a solar
flare.
Authors: Hudson, H. S.; Fletcher, L.; Krucker, S.
2010MmSAI..81..637H Altcode: 2010arXiv1001.1005H
We discuss the IR/visible/VUV continuum emission of the impulsive
phase of a solar flare, using TRACE UV and EUV images to characterize
the spectral energy distribution. This continuum has been poorly
observed but energetically dominates the radiant energy output
. Recent bolometric observations of solar flares furthermore point to
the impulsive phase as the source of a major fraction of the radiant
energy. This component appears to exhibit a Balmer jump and thus must
originate in an optically thin region above the quiet photosphere,
with an elevated temperature and strong ionization.

---------------------------------------------------------
Title: The RHESSI Microflare Height Distribution
Authors: Christe, Steven; Krucker, Samuel; Hudson, Hugh
2010cosp...38.2963C Altcode: 2010cosp.meet.2963C
We present the first in-depth statistical survey of flare source
heights observed by RHESSI between March 2002 and March 2007,
a total of 25,705 events. These flares were found using a new
flare-finding algorithm designed to search the 6-12 keV count-rate
when RHESSI's full sensitivity was available in order to find the
smallest events. Thermal (4-10 keV) and nonther-mal (15-25 keV)
images were made for all microflares and source centroid locations
were found for each event. In order to extract the height information
from source positions, a Monte-Carlo model was developed with an
assumed source height distribution where height is measured from the
photosphere. We find that the best source height model is given by an
exponential distri-bution with a scale height of 2.1 (0.3) Mm and a
minimum height of 3.1 (0.3) Mm. Comparing with previously published
loop length measurements, we find that the average loop tilt is 44
degrees as measured from the vertical.

---------------------------------------------------------
Title: The McClymont Jerk: A driver of solar seismicity
Authors: Martinez Oliveros, J. C.; Hudson, H. S.; Krucker, S.
2009AGUFMSH13A1508M Altcode:
Our Sun is a vibrating and amazing celestial object, full of
little-understood phenomena. One such phenomenon is the so-called
Sunquake, originally observed by Kosovichev and Zharkova (1998). Studies
by Donea and Lindsey (2005) and Besliu et al. (2008) now show that
the sunquake is not a rare phenomenon. They are powerful events,
hardly visible on the solar surface, and result directly from energy
release. They can be detected using acoustic techniques as high
frequencies oscillations in the sun. The first models proposed to
explain sunquakes involved pressure pulses, perhaps associated with
heating revealed by white-light flare emission. Hudson, Fisher and Welsh
(2008) proposed a mechanism to generate seismic waves based on the
dynamical behavior of the solar magnetic field during flares. In this
poster we study the variations of the magnetic field as an alternative
mechanism for the generation of seismic waves.

---------------------------------------------------------
Title: Ulysses/GRB Measurements of Hard X-Ray Flares on the Far Side
of the Sun
Authors: Tranquille, C.; Hurley, K.; Hudson, H. S.
2009AGUFMSH21C..03T Altcode:
The Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) on Ulysses
provided continuous measurements of energetic (25-150 keV) solar
X-ray activity from launch in October 1990 to November 2003, when
it was temporarily switched off due to instrument power sharing
requirements. The unique high latitude orbit of the spacecraft made it
possible to monitor solar X-ray flare activity on the far side of the
Sun during extended periods of the mission. We correlate all X-class
flares measured by the GOES fleet of satellites listed in the NOAA
reports, with count rate increases measured by GRB to confirm that
energetic flares have hard non-thermal components which extend into the
energy range above 25 keV. By comparing peak GRB count rates with the
GOES flare magnitudes, we establish a scaling law between the two sets
of measurements, characterized by a power-law fit. Having accounted
for all the X-class flares seen simultaneously by both GRB and GOES,
we are able to identify signatures of intense X-ray activity in the
GRB data set which must originate from flares on the hidden face of
the Sun. In total, we list 82 such events during the 13 years of GRB
operation. We provide timing information for each event and also
coarse flare site locations based on the geometry of the Ulysses
orbit. Estimates of the flare intensity are made using the scaling
law derived from the correlated GRB and GOES measurements. Global
monitoring of flare activity on the complete surface of the Sun can
provide useful information to validate and refine models and numerical
simulations of heliospheric and space weather processes.

---------------------------------------------------------
Title: High Resolution Imaging of Solar Flare Footpoints in White
Light and Hard X-rays
Authors: Krucker, S.; Hudson, H. S.; Lin, R. P.
2009AGUFMSH21C..01K Altcode:
We test the standard thick target beam scenario of solar flares
using high resolution G-band (430 nm) observations (~0.2 arcsec)
taken by HINODE/SOT and hard X-ray observations (2.3 arcsec) from
the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
of the December 6, 2006 GOES X9 flare. At both wavelengths, several
co-spatial footpoint sources are seen on the flare ribbons that show
similar relative intensities. This excellent correlation suggests that
the suprathermal electrons producing the hard X-ray emission are also
the source of energy for the white light emission, excluding energetic
protons as a possible source. If both emissions indeed come from the
same location, the higher resolution G-band observations suggest that
the individual hard X-ray sources are unresolved. Using the footpoint
area from the G-band images, the energy deposition rate by the hard
X-ray producing electron beam in cold thick target approximation become
enormous with values of 2x10 12 erg/s/cm2 for 25 keV (9x1012 erg/s/cm2
for 10 keV). This corresponds to a giant electron beam density within
the hard X-ray source of 0.3x1010 cm-3 above 25 keV (5x1010 cm-3 above
10 keV). These estimates pose serious questions for the thick-target
beam interpretation. We will discuss alternative scenarios, including
the idea of a purely non-thermal electron distribution as the source
of the hard X-ray emission.

---------------------------------------------------------
Title: The Solar Radio Microwave Flux and the Sunspot Number
Authors: Svalgaard, L.; Hudson, H. S.
2009AGUFMSH13C..03S Altcode:
Since 1947 the flux of microwaves from the Sun at wavelengths between
3 and 30 cm [frequencies between 10 and 1 GHz] has been routinely
measured. This emission comes from both thromosphere and the corona and
has two main sources: thermal bremsstrahlung (free-free emission) and
thermal gyroradiation. These mechanisms give rise to enhanced radiation
when the density and magnetic field increase, so the microwave radiation
is a good measure of general solar activity. Strong magnetic fields
occur in the network and can persist for weeks or longer; hence there is
a strong rotational signal in the emission superposed on a solar cycle
variation of the background coronal signal. The radio flux measurements
can be calibrated absolutely and are not very sensitive to observing
conditions, and in principle have no personal equation. They may thus be
the most objective measure of solar activity, and our many decades-long
flux record could throw light on the important issue of the long-term
variation of solar activity. The longest series of observations F10.7,
begun by Covington in Ottawa, Canada in April 1947 and maintained to
this day. Other observatories also have long and continuing series of
measurements of the microwave flux. One can now ask how this measure
of solar activity compares to other measures, in particular the sunspot
number. We correlate the sunspot number against the F10.7 flux for the
interval 1951-1988, and obtain a good polynomial fit (R^2 = 0.977)
up until ~1989.0 after which time the observed sunspot number falls
progressively below the fitted number. Three obvious hypotheses present
themselves: 1) The sunspot counting procedure or observers have changed,
with resulting artificial changes of the sunspot number as they have
in the past. 2) Physical changes in the corona or chromosphere have
occurred. 3) Livingston &amp; Penn’s observations that the sunspots
are getting warmer during the last decade, leading to a decreased
contrast with the surrounding photosphere and hence lessened visibility,
possibly resulting in an undercount of sunspots. The near constancy of
the flux at minima since 1954 argues against a change of the physical
conditions at the source locations, leaving the exciting possibility
that Livingston &amp; Penn may be correct.

---------------------------------------------------------
Title: Latitude-Dependent Temperature Variations at the Solar Limb
Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.; Zahid, H. J.
2009AGUFMSH23B1549F Altcode:
We use observations from the solar aspect sensor of RHESSI to
characterize the latitude dependence of the temperature of the
photosphere at the solar limb. Previous observations have suggested
the presence of a polar temperature excess as large as 1.5 K. The
RHESSI observations, made with a rotating telescope in space,
have great advantages in the rejection of systematic errors in
the very precise photometry required for such an observation. This
photometry is differential, i.e. relative to a mean limb-darkening
function. The data base consists of about 1,000 images per day from
linear CCDs with 1.73 arc sec square pixels, observing a narrow band
(12nm FWHM) at 670 nm. Each image shows a chord crossing the disk at a
different location as the spacecraft rotates and precesses around its
average solar pointing. We fit an average limb-darkening function and
reassemble the residuals into synoptic maps of differential intensity
variations as function of position angle. We further mask these images
against SOHO/EIT 284A images in order to eliminate magnetic regions. The
analysis establishes a limit on the quadrupole dependence of temperature
(brightness) on position angle of 0.04 +/- 0.02 K. This results in a
possible correction of our precise measurement of the solar oblateness
which is smaller than its rms error of 0.14 mas.

---------------------------------------------------------
Title: The Ulysses Catalog of Solar Hard X-Ray Flares
Authors: Tranquille, C.; Hurley, K.; Hudson, H. S.
2009SoPh..258..141T Altcode:
Ulysses was launched in October 1990, and its Solar X-ray/Cosmic
Gamma-Ray Burst Experiment (GRB) has provided more than 13 years
of uninterrupted observations of solar X-ray flare activity. Due to
the large variation of the relative solar latitude and longitude of
the spacecraft orbit with respect to the Earth, the perspective of
the GRB instrument often differed significantly from that of X-ray
instruments on Earth-orbiting satellites. During extended periods the
GRB experiment made direct observations of flares on the hidden face
of the Sun, providing a unique record of events not visible to other
instruments. The small detector area of GRB and its optimization for
very high counting rates minimized the effects of pulse pile-up. We
interpret the spectra, time histories, and occurrence distribution
patterns of GRB data in terms of "thermal feed-through", the confusion
of thermal soft X-rays and non-thermal hard X-rays. This effect is a
systematic problem for scintillation-counter spectrometers observing
the solar hard X-ray spectrum. This paper provides a definitive catalog
of the Ulysses X-ray flare observations and discusses various features
of this unique database. For the equivalent GOES range X2 - X25, we
find a power-law fit for the (differential) occurrence frequency at
&gt;25 keV with slope −1.61±0.04, with no evidence for a downturn
at the highest event magnitudes (for the relatively small sample of
such events available in this study). If the nine most intense events
are excluded because of concerns about the effects of pulse pile-up,
the slope steepens to −1.75±0.08.

---------------------------------------------------------
Title: X-ray Behavior of the CMEless X-class flares
Authors: Hudson, Hugh S.
2009shin.confE.179H Altcode:
The RHESSI data reveal a wealth of coronal hard X-ray sources, both
in the impulsive phase and in the extended nonthermal phase. The
latter are strongly associated with CME occurrence and the related
global shock wave. Major flare events without CMEs have significantly
different morphology: no soft-hard-harder evolution in the hard X-ray
spectrum and weak soft X-ray precursors. This presentation reviews the
data for the CMEless X-class flare list of Gopalswamy et al. (2009) and
discusses interpretations of these and of the phenomena associated with
CME flares. Time permitting we will also discuss the impulsive-phase
coronal hard X-ray sources (e.g. Masuda).

---------------------------------------------------------
Title: Coronal Radiation Belts
Authors: Hudson, H. S.; MacKinnon, A. L.; De Rosa, M. L.; Frewen,
S. F. N.
2009ApJ...698L..86H Altcode: 2009arXiv0905.3824H
The magnetic field of the solar corona has a large-scale dipole
character, which maps into the bipolar field in the solar wind. Using
standard representations of the coronal field, we show that high-energy
ions can be trapped stably in these large-scale closed fields. The
drift shells that describe the conservation of the third adiabatic
invariant may have complicated geometries. Particles trapped in these
zones would resemble the Van Allen belts and could have detectable
consequences. We discuss potential sources of trapped particles.

---------------------------------------------------------
Title: Flares and the chromosphere
Authors: Hudson, Hugh S.; Fletcher, Lyndsay
2009EP&S...61..577H Altcode: 2009EP&S...61L.577H
The chromosphere (the link between the photosphere and the corona)
plays a crucial role in flare and CME development. In analogies between
flares and magnetic substorms, it is normally identified with the
ionosphere, but we argue that the correspondence is not exact. Much
of the important physics of this interesting region remains to be
explored. We discuss chromospheric flares in the context of recent
observations of white-light flares and hard X-rays as observed by
TRACE and RHESSI, respectively. We interpret key features of these
observations as results of the stepwise changes a flare produces in
the photospheric magnetic field.

---------------------------------------------------------
Title: Gamma Rays and Energetic Neutral Atoms from Solar Flares
Authors: Hudson, Hugh S.; Lin, R. P.; MacKinnon, A. N.; Raymond,
J. C.; Shih, A. Y.; Wang, L.
2009SPD....40.3604H Altcode:
The recent discovery of energetic (1.6-5.0 MeV) neutral hydrogen atoms
(ENAs) from an X9 solar flare on 2006 Dec. 5 (Mewaldt et al. 2009)
raises the exciting possibility that they could represent a lower-energy
range of the same population of accelerated ions responsible for flare
gamma-ray emission. If so the neutralization would take place at low
altitudes in the flaring loop, and the neutral atoms might need to
escape through multiple epochs of re-ionization and neutralization. The
probability for eventual escape is high because of the large mirror
ratios of the coronal fields and because of small energy losses of
the ENAs and the ions they become. Using a standard density model,
we have shown that the collisional lifetime of re-ionized particles
exceeds the observed injection time. We use Monte Carlo simulations
of particle trajectories, including both neutral and ionized states,
to estimate escape probabilities and directivity more generally within
the context of a standard PFSS (potential-field source surface) model
for the field above the Mewaldt et al. flare, and compare the results
with RHESSI gamma-ray observations of this flare.

---------------------------------------------------------
Title: High Resolution Imaging of Solar Flare Footpoints in White
Light and Hard X-rays
Authors: Krucker, Sam; Hudson, H. S.; Lin, R. P.
2009SPD....40.1919K Altcode:
High resolution G-band (430 nm) observations ( 0.2 arcsec) taken by
HINODE/SOT and hard X-ray observations (2.3 arcsec) from the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are used to study
footpoint sources of the December 6, 2006 GOES X9 class solar flare. At
both wavelengths, several co-spatial footpoint sources are seen on the
flare ribbons that show similar relative intensities. This excellent
correlation suggests that the energetic electrons producing the hard
X-ray emission are also the source of energy for the white light
emission, excluding energetic protons as a possible source. If both
emissions indeed come from the same location, the higher resolution
G-band observation suggest that the individual hard X-ray sources
are unresolved. Using the area from the G-band images, the cold
thick target model gives the enormous energy deposition rate of 2e12
erg/s/cm2 for 25 keV (9e12 erg/s/cm2 for 10 keV) and huge densities of
beam electrons within the hard X-ray source of 0.3e10 cm-3 above 25 keV
(5d10 cm-3 above 10 keV). These estimates pose serious questions for
the thick-target interpretation. We will discuss alternative scenarios,
including the idea of a purely non-thermal electron distribution as
the source of the hard X-ray emission.

---------------------------------------------------------
Title: Latitude-Dependent Temperature Variations at the Solar Limb
Authors: Fivian, Martin; Hudson, H. S.; Lin, R. P.; Zahid, H. J.
2009SPD....40.0925F Altcode:
We use observations from the solar aspect sensor of RHESSI to
characterize the latitude dependence of the temperature of the
photosphere at the solar limb. Previous observations have suggested
the presence of a polar temperature excess as large as 1.5 K. The
RHESSI observations, made with a rotating telescope in space, have
great advantages in the rejection of systematic errors in the very
precise photometry required for such an observation. This photometry
is differential, i.e. relative to a mean limb-darkening function. The
data base consists of about 1,000 images per day from linear CCDs
with 1.73 arc sec square pixels, observing a narrow band (12nm FWHM)
at 670 nm. Each image shows a chord crossing the disk at a different
location as the spacecraft rotates and precesses around its average
solar pointing. We fit an average limb-darkening function and reassemble
the residuals into synoptic maps of differential intensity variations
as function of position angle. We further mask these images against
SOHO/EIT 284A images in order to eliminate magnetic regions. The
analysis establishes limits on the quadrupole dependence of brightness
(temperature) on position angle, a crucial unknown in our precise
measurement of the solar oblateness.

---------------------------------------------------------
Title: Direct Observations of the Coronal Acceleration Region of a
Solar Flare
Authors: Krucker, Sam; Hudson, H. S.; White, S. M.; Lin, R. P.
2009SPD....40.3601K Altcode:
Solar flares essentially convert the intrinsic energy of coronal
magnetic field into the kinetic energy of accelerated particles. Hard
X-ray emission from flare-accelerated electrons produced by the
bremsstrahlung mechanism provides the most direct diagnostics of
electron acceleration. The most discussed coronal hard X-ray source has
been the above-the-loop-top source observed in the Masuda flare. The
poor spectral resolution of these observations, however, made an
interpretation ambiguous, and the exact location of the acceleration
remained elusive. We present high spatial and spectral resolution
RHESSI hard X-ray observations of an above-the-loop-top source with
simultaneous microwave observations from NoRH. These observations
provide a unambiguous interpretation of above-the-loop-top sources:
The above-the-loop-top source itself is the acceleration region, where
all electrons within an extended volume (1e27 cm3) are accelerated. The
distribution of the accelerated electrons is definitely non-thermal,
with a power law distribution extending from 10 keV up to the
relativistic range (few MeV). The plasma beta in the acceleration region
changes from the pre-flare value of 0.01 to 1, indicating that roughly
half of the magnetic energy has been transformed into kinetic energy.

---------------------------------------------------------
Title: Solar Trapped Particles
Authors: Hudson, Hugh S.; MacKinnon, A. L.; Frewen, S. F.
2009SPD....40.1205H Altcode:
High-energy particles (&gt;10 MeV protons) can be trapped in large-scale
coronal magnetic fields for periods of days to weeks. We model this
trapping by following the adiabatic motions of particles in test fields,
including the Schrijver-DeRosa PFSS models. These are available in a
SolarSoft interface for the entire duration of the SOHO mission thus
far. In spite of the complexity of the field, we find drift shells
in which particles can circulate completely around the Sun, and thus
conserve the third adiabatic invariant of motion well. In this work
we study the morphology of the these drift shells, including their
appearance as a function of phase in the solar cycle.

---------------------------------------------------------
Title: The RHESSI Microflare Height Distribution
Authors: Christe, S.; Krucker, Sam; Hudson, H.; Lin, R.
2009SPD....40.1903C Altcode:
We present the first in-depth statistical survey of flare source
heights observed by RHESSI between March 2002 and March 2007, a total
of 25,705 events. These flares were found using a new flare-finding
algorithm designed to search the 6-12 keV count-rate when RHESSI's
full sensitivity was available in order to find the smallest
events. Thermal (4-10 keV) and nonthermal (15-25 keV) images were
made for all microflares and source centroid locations were found for
each event. In order to extract the height information from source
positions, a Monte-Carlo model was developed with an assumed source
height distribution where height is measured from the photosphere. We
find that the best source height model is given by an exponential
distribution with a scale height of 2.1 (0.3) Mm and a minimum height
of 3.1 (0.3) Mm. Comparing with previously published loop length
measurements, we find that the average loop tilt is 44 degrees as
measured from the vertical.

---------------------------------------------------------
Title: Particle Acceleration and Transport on the Sun
Authors: Bastian, T. S.; Emslie, G.; Fleishman, G.; Gary, D. E.;
Holman, G.; Hudson, H.; Hurford, G.; Krucker, S.; Lee, J.; Miller,
J.; White, S.
2009astro2010S..13B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: RHESSI Observations of Facular Limb Darkening at 670 nm
Authors: Zahid, H. J.; Chapman, G.; Fivian, M. D.; Hudson, H.
2008AGUFMSH23A1623Z Altcode:
We use observations from the solar aspect sensor of RHESSI to
characterize the facular limb brightening function. The RHESSI
observations, made with a rotating telescope in space, have great
advantages in the rejection of systematic errors in the very precise
photometry required for such an observation. The facular photometry is
differential relative to a mean background limb-darkening function. The
data base consists of about 1,000 images per day from linear CCDs with
1.73 arc sec square pixels, observing a narrow band at 670 nm. Each
image shows a chord crossing the disk at a different location as the
spacecraft rotates and precesses around its nominal solar pointing,
with amplitude of a few arc sec. We reassemble these line images into
synoptic images with a relatively low time cadence but an almost full
coverage of more than six years. We further mask these images against
SOHO/EIT 284A images in order to select magnetic regions. The resulting
mean limb-darkening function is clearly resolved in radius and has a
maximum at mu = 0.24 and approaches zero at the limb, consistent with
Spruit's "hot wall" model. The contrast is positive at disk center,
and we discuss explanations for this.

---------------------------------------------------------
Title: Spatial and Temporal Relationships Between WL/UV Continuum
and hard X-ray Footpoints in Solar Flares
Authors: Hudson, H. S.; Fletcher, L.; McTiernan, J.
2008AGUFMSH41A1605H Altcode:
Hard X-rays show the presence of energetic electrons in the
impulsive phase of a solar flare. According to standard models, these
electrons contain a large fraction of the total flare energy. We
show that comparable amounts of energy are present in the compact,
rapidly variable WL and UV bright points that constitute white-light
flares. This suggests that these structures can be identified with
each other, and indeed the image centroids and time variations match
well. There are image differences that we believe mainly to be due to
the different resolving powers of Hinode and TRACE WL/UV imaging on the
one hand, and RHESSI hard X-rays on the other. We therefore also use
RHESSI modeling software to simulate hard X-ray images using TRACE and
Hinode data as templates to understand this relationship more precisely.

---------------------------------------------------------
Title: The Global Photospheric Temperature Field
Authors: Fivian, M. D.; Zahid, H. J.; Hudson, H. S.; Lin, R. P.
2008AGUFMSH23A1624F Altcode:
We use observations from the solar aspect sensor of RHESSI to
characterize the large-scale temperature variation across the solar
disk. Previous observations have suggested the presence of a polar
temperature excess as large as 1.5 K. The RHESSI observations, made with
a rotating telescope in space, have great advantages in the rejection
of systematic errors in the very precise photometry required for such
an observation. This photometry is differential relative to a mean
limb-darkening function. The data base consists of about 1,000 images
per day from linear CCDs with 1.73 arc sec square pixels, observing a
narrow band at 670 nm. Each image shows a chord crossing the disk at a
different location as the spacecraft rotates and precesses around its
nominal solar pointing. We reassemble these line images into synoptic
images with a relatively low time cadence but an almost full coverage of
more than six years. We further mask these images against SOHO/EIT 284A
images in order to eliminate magnetic regions. The analysis establishes
limits on the quadrupole dependence of brightness (temperature) on
position angle, a crucial unknown in our precise measurement of the
solar oblateness.

---------------------------------------------------------
Title: Modeling of Solar Radiation Belts
Authors: Frewen, S. S.; De Rosa, M.; Hudson, H.; MacKinnon, A.
2008AGUFMSH13B1526F Altcode:
Stable particle trapping in the complicated magnetic field of the solar
corona -- "solar radiation belts" -- at first seems unlikely in the face
of the Sun's complex, variable magnetic field. By integrating particle
orbit equations in the guiding-center approximation, we investigate
the fates of energetic ions in model coronal magnetic fields. We use
both PFSS (Potential Field Source Surface) and simple analytic field
models. Contrary to naive expectation, we find that significant numbers
of particles remain trapped more than long enough to circumnavigate
the Sun, neither precipitating to the surface nor attaining open field
lines. The drift "shells" corresponding to conservation of the third
adiabatic invariant may be complicated in form. A close look at the
dependence of the cross-field drift speed on magnetic field strength
and topology accounts for this finding.

---------------------------------------------------------
Title: A Large Excess in Apparent Solar Oblateness Due to Surface
Magnetism
Authors: Fivian, Martin D.; Hudson, Hugh S.; Lin, Robert P.; Zahid,
H. Jabran
2008Sci...322..560F Altcode:
The shape of the Sun subtly reflects its rotation and internal
flows. The surface rotation rate, ~2 kilometers per second at the
equator, predicts an oblateness (equator-pole radius difference) of
7.8 milli arc seconds, or ~0.001%. Observations from the Reuven Ramaty
High-Energy Solar Spectroscopic Imager satellite show unexpectedly large
flattening, relative to the expectation from surface rotation. This
excess is dominated by the quadrupole term and gives a total oblateness
of 10.77 ± 0.44 milli arc seconds. The position of the limb correlates
with a sensitive extreme ultraviolet proxy, the 284 angstrom limb
brightness. We relate the larger radius values to magnetic elements
in the enhanced network and use the correlation to correct for it as
a systematic error term in the oblateness measurement. The corrected
oblateness of the nonmagnetic Sun is 8.01 ± 0.14 milli arc seconds,
which is near the value expected from rotation.

---------------------------------------------------------
Title: Hard X-ray emission from the solar corona
Authors: Krucker, S.; Battaglia, M.; Cargill, P. J.; Fletcher, L.;
Hudson, H. S.; MacKinnon, A. L.; Masuda, S.; Sui, L.; Tomczak, M.;
Veronig, A. L.; Vlahos, L.; White, S. M.
2008A&ARv..16..155K Altcode: 2008A&ARv.tmp....8K
This review surveys hard X-ray emissions of non-thermal electrons in the
solar corona. These electrons originate in flares and flare-related
processes. Hard X-ray emission is the most direct diagnostic of
electron presence in the corona, and such observations provide
quantitative determinations of the total energy in the non-thermal
electrons. The most intense flare emissions are generally observed
from the chromosphere at footpoints of magnetic loops. Over the years,
however, many observations of hard X-ray and even γ-ray emission
directly from the corona have also been reported. These coronal sources
are of particular interest as they occur closest to where the electron
acceleration is thought to occur. Prior to the actual direct imaging
observations, disk occultation was usually required to study coronal
sources, resulting in limited physical information. Now RHESSI has
given us a systematic view of coronal sources that combines high
spatial and spectral resolution with broad energy coverage and high
sensitivity. Despite the low density and hence low bremsstrahlung
efficiency of the corona, we now detect coronal hard X-ray emissions
from sources in all phases of solar flares. Because the physical
conditions in such sources may differ substantially from those of
the usual “footpoint” emission regions, we take the opportunity
to revisit the physics of hard X-radiation and relevant theories of
particle acceleration.

---------------------------------------------------------
Title: Impulsive Flare Energy Transport by Large-Scale Alfven Waves,
and Flare Electron Acceleration
Authors: Fletcher, L.; Hudson, H. S.
2008ESPM...12.3.62F Altcode:
The impulsive phase of a solar flare marks the epoch of rapid conversion
of energy stored in the pre-flare coronal magnetic field. Hard X-ray
observations imply that a substantial fraction of flare energy released
during the impulsive phase is converted to the kinetic energy of mildly
relativistic electrons (10-100 keV). The liberation of the magnetic free
energy can occur as the coronal magnetic field reconfigures and relaxes
following reconnection. Motivated by observations pointing to a high
local Alfven speed in parts of the corona, and by considerations from
magnetospheric physics, we investigate a scenario in which products of
the reconfiguration - large-scale Alfven wave pulses - transport the
energy and magnetic-field changes rapidly through the corona to the
lower atmosphere. We investigate the opportunities that such a scenario
offers for heating of the chromospheric plasma in flare footpoints, and
for electron acceleration, and confront our findings with observational
constraints, including energetics, HXR timing, and radio signatures.

---------------------------------------------------------
Title: Constraining the Properties of Hard X-ray Nanoflares with
RHESSI Observations of the Quiet Sun
Authors: Hannah, I. G.; Hudson, H. S.; Hurford, G. J.; Lin, R. P.
2008ESPM...12.2.83H Altcode:
We present new results from the Reuven Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI) of solar X-ray emission not associated
with active regions, sunspots, or flares (the quiet Sun). RHESSI has
greater sensitivity in the 5-25 keV range than previous missions,
but since the quiet Sun sources may be well-dispersed spatially across
the disk, RHESSI's normal imaging technique is not well suited to the
task. Instead, we obtain observations in a special mode ("fan-beam
modulation," Hannah et al. RSI 78, 024501, 2007) to "chop" the quiet
solar signal. This technique has been used fourteen times between
between June 2005 and June 2008, obtaining limits to the emission
between 3-200 keV. These limits improve on those previously reported
(Hannah et al. ApJ 659L, 77, 2007). They are both lower and also
extend the energy range covered by the pre-RHESSI results. We use the
new limits to constrain the possible properties of the thermal and
non-thermal emission of the quiet Sun. In particular we discuss the
possible properties of hard X-ray nanoflares and the implications for
nanoflare coronal heating models.

---------------------------------------------------------
Title: Facular Contrast at 670 nm
Authors: Hudson, H. S.; Zahid, H. J.; Fivian, M. D.
2008ESPM...12.2.59H Altcode:
The Solar Aspect System (SAS) on the RHESSI satellite consists of three
simple lenses with linear CCD detectors, whose data we use to synthesize
images of solar photospheric brightness. The three independent detectors
record about 1,000 line images per day each, with chord locations at
roughly random positions on the disk. The synthesized images allow us
to study the mean distribution of faculae and enhanced network. RHESSI
has been observing since launch in 2002. Here we report an analysis of
three years (2003-2005) of the these data. We find strong correlations
between the facular excess signals in these data, and EUV brightness
as derived from SOHO/EIT. The data interior to mu of about 0.24 fit a
generalization of the "hot wall" model, but this fails closer to the
limb. We discuss the fit and its model discrepancy in terms of the
geometry of the observation, including the possibility of a "cloud"
model, ie one involving emitting material above the height of the
photosphere.

---------------------------------------------------------
Title: Microflares with RHESSI and Hinode/XRT
Authors: Hannah, I. G.; Krucker, S.; Christe, S.; Hudson, H. S.; Lin,
R. P.
2008ASPC..397..169H Altcode:
In this article we discuss the opportunities for analyzing microflares
with RHESSI and Hinode/XRT. We present analysis of one microflare,
using the RHESSI to obtain the thermal and non-thermal spectral
parameters and compare the RHESSI images of the thermal (4-8 keV) and
non-thermal (12-50 keV) emission with the Hinode/XRT images. The RHESSI
non-thermal emission in this event matches spatial and temporally the
initial brightest emission from XRT.

---------------------------------------------------------
Title: Physical Conditions in Coronal Structures About to Flare
Authors: Hudson, H. S.; Hannah, I. G.; Deluca, E. E.; Weber, M.
2008ASPC..397..130H Altcode:
We use Hinode observations to study coronal structures about to flare,
based on their apparent footpoints as a guide to identification. The
high resolution and excellent stability of the Hinode observations
makes the identifications much more precise than those done with the
soft X-ray telescope (SXT) on board Yohkoh. The physical conditions in
the coronal structure about to flare are important in understanding
the nature of the plasma processes leading to the eruption. We find
examples of soft X-ray microflares that agree with the SXT conclusions:
the structure is essentially invisible prior to the flare in most
cases. We present an estimation of preflare temperature and density
and find that in these cases, the flare appears to happen in flux tubes
with undetectably low electron density, less than ∼10^{8} cm^{-3}. A
similar program with the full instrument set of Hinode would be
extremely powerful, owing to the broad temperature coverage available.

---------------------------------------------------------
Title: How Solar Flares Work
Authors: Hudson, H. S.
2008AGUSM.U22A..01H Altcode:
Geophysics, radio astronomy, Japan, the ionosphere, X and gamma rays:
all have contributed to my view of how solar flares and their partner
coronal mass ejections (CMEs) work. A solar flare (and a CME, it
turns out) has an "impulsive phase" in which catastrophic and dominant
energy release from magnetic storage takes place, resulting in particle
acceleration. The impulsive restructuring of the coronal currents and
fields leads directly (if still mysteriously) to the many observable
phenomena, which can reach the surface of the Earth. In particular
we now recognize that the term "impulsive phase" correctly captures
the basic morphology of the process: it is highly intermittent in
both space and time, even though it underlies large-scale phenomena
such as CME eruptions. We still cannot resolve the scales of the flare
intermittency, but we can use in-situ observations of possibly analogous
processes in the solar wind and magnetosphere for guidance.

---------------------------------------------------------
Title: The role of large-scale Alfvén waves in solar flare energy
release and particle acceleration
Authors: Fletcher, L.; Hudson, H. S.
2008AGUSMSH51C..06F Altcode:
The impulsive phase of a solar flare marks the epoch of rapid conversion
of energy stored in the pre-flare coronal magnetic field. Hard X-ray
observations imply that a substantial fraction of flare energy
released during the impulsive phase is converted to the kinetic
energy of mildly relativistic electrons (10-100 keV). The liberation
of the magnetic free energy can occur as the coronal magnetic field
reconfigures and relaxes following reconnection. We investigate
a scenario, inspired in part by magnetospheric physics, in which
products of the reconfiguration - large-scale Alfvén wave pulses
- transport the energy and magnetic-field changes rapidly through
the corona to the lower atmosphere. This offers two possibilities
for electron acceleration. Firstly, in a coronal plasma with E &lt;
me/mp, the waves propagate as inertial Alfvén waves. In the presence
of strong spatial gradients, these generate field-aligned electric
fields that can accelerate electrons to energies on the order of 10
keV and above, including by repeated interactions between electrons
and wavefronts. Secondly, when they reflect in the chromosphere,
a cascade to high wave numbers may develop. This will also accelerate
electrons by turbulence, in a medium with a locally high electron number
density. This concept, which bridges MHD-based and particle- based
views of a flare, provides an interpretation of the recently-observed
rapid variations of the line-of-sight component of the photospheric
magnetic field across the flare impulsive phase, and offers solutions
to some perplexing flare problems, such as the flare "number problem" of
finding and resupplying sufficient electrons to explain impulsive-phase
hard X-ray emission.

---------------------------------------------------------
Title: Limb Darkening at 670 nm Measured from RHESSI
Authors: Zahid, H. J.; Fivian, M. D.; Hudson, H. S.
2008AGUSMSP21B..04Z Altcode:
The Solar Aspect System (SAS) on RHESSI uses linear CCD detectors
to make chord images across the solar disk. The three independent
detectors record about 1,000 images per day each, and RHESSI has been
observing since launch in 2002. Here we report on the these data,
including models of the limb-darkening and point spread function,
our methods for calibration, and a survey of the observations. We
will present results on possible time and latitude dependences of the
limb-darkening coefficients, and discuss sources of contamination to a
'quiet-sun' limb-darkening model.

---------------------------------------------------------
Title: Masking RHESSI radius measures against EUV brightness
Authors: Hudson, H. S.; Zahid, H. J.; Fivian, M. D.
2008AGUSMSP21B..05H Altcode:
The RHESSI radius observations, made with its Solar Aspect Sensor (SAS),
are providing our best characterization of the shape of the Sun. In
spite of the differential nature of our measurement, which somewhat
resembles that of Dicke's original (ground-based) Solar Oblateness
Telescope, we still have a sensitivity in the radius measure to the
presence of faculae and other small-scale magnetic features in the
solar atmosphere. We find an clear positive correlation between radius
and EUV brightness, as obtained from SOHO/EIT images; in addition
we clearly see the Wilson Depression as a negative correlation. The
facular correlation has been successfully used to screen RHESSI data
from our initial study interval in 2004. We describe the nature of
the correlation and discuss its interpretation.

---------------------------------------------------------
Title: RHESSI Observations of a Large Excess Solar Oblateness and
its Identification as Magnetic in Nature
Authors: Fivian, M. D.; Hudson, H. S.; Zahid, H. J.; Lin, R. P.
2008AGUSMSP21B..06F Altcode:
The RHESSI solar aspect sensors (SAS) serendipitously provide precise
measures of the shape of the Sun at 670 nm. The data rate is high
(more than 106 points since launch in 2002), and each point has a
statistical precision of the order of 10 mas. We present reduced
data from a three-month interval in 2004. The full data gives
an oblateness (axisymmetric quadrupole shape term, expressed as
the difference between equatorial and polar radii) of 10.74 ± 0.44
mas. For comparison, Dicke's 1970 estimate, based on uniform rotation,
predicted 8.10 mas. The apparent radius strongly correlates with
the EUV limb brightness. Accordingly, by restricting the data base
to avoid faculae, including a component outside the active regions,
we obtain a lower value for the oblateness. We find a value of 7.98 ±
0.14 mas. Based on a comparison of our results with previous balloon
and satellite observations, the apparent excess oblateness may have
a positive correlation with the solar cycle.

---------------------------------------------------------
Title: How Solar Flares Work
Authors: Hudson, Hugh S.
2008AAS...212.6501H Altcode: 2008BAAS...40..256H
Geophysics, radio astronomy, Japan, the ionosphere, X and gamma rays:
all have contributed to my view of how solar flares and their partner
coronal mass ejections (CMEs) work. A solar flare (and a CME, it turns
out) has an "impulsive phase” in which catastrophic and dominant
energy release from magnetic storage takes place, resulting in particle
acceleration. The impulsive restructuring of the coronal currents and
fields leads directly (if still mysteriously) to the many observable
phenomena, which canreach the surface of the Earth. In particular we
now recognize that the term "impulsive phase” correctly captures
the basic morphology of the process: it is highly intermittent in
both space and time, even though it underlies large-scale phenomena
such as CME eruptions. We still cannot resolve the scales of the flare
intermittency, but we can use in-situ observations of possibly analogous
processes in the solar wind and magnetosphere for guidance.

---------------------------------------------------------
Title: Searching the X-ray Sun For Solar Axions
Authors: Hudson, Hugh S.; Acton, L. W.; DeLuca, E. E.; Hannah, I. G.;
Hurford, G. J.; Lin, R. P.; Reardon, K. P.; van Bibber, K.
2008AAS...212.0402H Altcode: 2008BAAS...40..193H
The axion is a hypothetical weakly-interacting elementary particle. The
solar core may produce a copious axion flux via the Primakoff
effect. This same process can also convert a tiny fraction of the
axions back into photons via interaction with the magnetic field
threading the solar atmosphere. The spectral signature of the emitted
X-rays is determined mainly by the temperature of the solar core, and
the spatial distribution also depends strongly on the solar magnetic
field in the back-conversion process. The X-ray intensity thus varies as
Gaγγ(&amp;#8747BperpdL)2,
where Bperp is the perpendicular component of the
chromospheric and coronal magnetic field in the appropriate zone
for interaction and photon escape, and Gaγγ
is the (unknown) coupling constant, dependent on the (unknown) mass
of the axion. We describe observational tests suitable for solar
X-ray imagers and discuss projects now under way with Yohkoh/SXT,
RHESSI, and Hinode/XRT. The successful detection of axions would have
implications for basic physics and for cosmological dark matter. It
would also help us to characterize the ill-understood extension of the
solar magnetic field into the chromosphere and corona. We specifically
focus on applying the existing understanding of solar (and stellar)
magnetism to this problem.

---------------------------------------------------------
Title: An intriguing solar microflare observed with RHESSI, Hinode,
and TRACE
Authors: Hannah, I. G.; Krucker, S.; Hudson, H. S.; Christe, S.; Lin,
R. P.
2008A&A...481L..45H Altcode: 2007arXiv0712.0369H
Aims:We investigate particle acceleration and heating in a solar
microflare. Methods: In a microflare with non-thermal emission
to remarkably high energies (&gt;50 keV), we investigate the hard
X-rays with RHESSI imaging and spectroscopy and the resulting thermal
emission seen in soft X-rays with Hinode/XRT and in EUV with TRACE. Results: The non-thermal footpoints observed with RHESSI spatially and
temporally match bright footpoint emission in soft X-rays and EUV. There
is the possibility that the non-thermal spectrum extends down to 4
keV. The hard X-ray burst clearly does not follow the expected Neupert
effect, with the time integrated hard X-rays not matching the soft
X-ray time profile. So, although this is a simple microflare with good
X-ray observation coverage it does not fit the standard flare model.

---------------------------------------------------------
Title: RHESSI Microflare Statistics. II. X-Ray Imaging, Spectroscopy,
and Energy Distributions
Authors: Hannah, I. G.; Christe, S.; Krucker, S.; Hurford, G. J.;
Hudson, H. S.; Lin, R. P.
2008ApJ...677..704H Altcode: 2007arXiv0712.2544H
We present the first statistical analysis of the thermal and nonthermal
X-ray emission of all 25,705 microflares (RHESSI) observed between
2002 March and 2007 March. These events were found by searching
the 6-12 keV energy range (see Paper I) and are small active region
flares, from low (GOES) C class to below A class. Each microflare is
automatically analyzed at the peak time of the 6-12 keV emission:
the thermal source size is found by forward-fitting the complex
visibilities for 4-8 keV, and the spectral parameters (temperature,
emission measure, power-law index) are found by forward-fitting a
thermal plus nonthermal model. The resulting wealth of information we
determine about the events allows a range of the thermal and nonthermal
properties to be investigated. In particular, we find that there is
no correlation between the thermal loop size and the flare magnitude,
indicating that microflares are not necessarily spatially small. We
present the first thermal energy distribution of RHESSI flares and
compare it to previous thermal energy distributions of transient
events. We also present the first nonthermal power distribution of
RHESSI flares and find that a few microflares have unexpectedly large
nonthermal powers up to 1028 erg s-1. The total
microflare nonthermal energy, however, is still small compared to
that of large flares as it occurs for shorter durations. These large
energies and difficulties in analyzing the steep nonthermal spectra
suggest that a sharp broken power law and thick-target bremsstrahlung
model may not be appropriate for microflares.

---------------------------------------------------------
Title: Impulsive Phase Flare Energy Transport by Large-Scale Alfvén
Waves and the Electron Acceleration Problem
Authors: Fletcher, L.; Hudson, H. S.
2008ApJ...675.1645F Altcode: 2007arXiv0712.3452F
The impulsive phase of a solar flare marks the epoch of rapid conversion
of energy stored in the preflare coronal magnetic field. Hard X-ray
observations imply that a substantial fraction of flare energy released
during the impulsive phase is converted to the kinetic energy of mildly
relativistic electrons (10-100 keV). The liberation of the magnetic free
energy can occur as the coronal magnetic field reconfigures and relaxes
following reconnection. We investigate a scenario in which products
of the reconfiguration—large-scale Alfvén wave pulses—transport
the energy and the magnetic field changes rapidly through the
corona to the lower atmosphere. This offers two possibilities
for electron acceleration. First, in a coronal plasma with β &lt;
me/mp, the waves propagate as inertial Alfvén
waves. In the presence of strong spatial gradients, these generate
field-aligned electric fields that can accelerate electrons to energies
on the order of 10 keV and above, including by repeated interactions
between electrons and wave fronts. Second, when they reflect and
mode-convert in the chromosphere, a cascade to high wavenumbers
may develop. This will also accelerate electrons by turbulence, in
a medium with a locally high electron number density. This concept,
which bridges MHD-based and particle-based views of a flare, provides
an interpretation of the recently observed rapid variations of the
line-of-sight component of the photospheric magnetic field across the
flare impulsive phase, and offers solutions to some perplexing flare
problems, such as the flare "number problem" of finding and resupplying
sufficient electrons to explain the impulsive-phase hard X-ray emission.

---------------------------------------------------------
Title: An universal flaring mechanism from the Sun to the stars? .
Authors: Isola, C.; Favata, F.; Micela, G.; Hudson, H.
2008MmSAI..79..255I Altcode:
We present an analysis of solar flares and its extension to active
stars with the goal to investigate the observability of non-thermal
components of stellar flares with future instrumentation, in particular
with Simbol-X. We derive a scaling law for the relationship between soft
(thermal) and hard (non-thermal) peak X-ray fluxes which we extrapolate
from the solar case to energetic stellar flares.

---------------------------------------------------------
Title: Flare Energy and Magnetic Field Variations
Authors: Hudson, H. S.; Fisher, G. H.; Welsch, B. T.
2008ASPC..383..221H Altcode:
We describe ways in which the photospheric vector magnetic field might
vary across the duration of a solar flare or CME. We also quantitatively
assess the back reaction on the photosphere and solar interior by the
coronal field evolution required to release flare energy. Our estimates
suggest that the work done by Lorentz forces in this back reaction
could supply enough energy to explain observations of flare-driven
seismic waves.

---------------------------------------------------------
Title: Energetic Particles in the Quiet Corona
Authors: MacKinnon, A. L.; Hudson, H. S.
2007AGUFMSH41A0303M Altcode:
We address the fates of energetic particles in typical solar minimum
coronal magnetic fields. A baseline density of such particles will
be set by cosmic ray albedo neutron decay. Other, electromagnetic
mechanisms may augment this. PFSS extrapolation is combined
with synoptic magnetogram data to constrain the coronal fields,
and particle trajectories are determined in the guiding center
approximation. We discuss the extent to which solar analogues of
terrestrial magnetospheric phenomena (radiation belts, ring current)
may exist near solar minimum.

---------------------------------------------------------
Title: X-ray Microflares with Hinode and RHESSI.
Authors: Hannah, I. G.; Christe, S.; Krucker, S.; Hudson, H.; Lin,
R. P.; Deluca, E.
2007AGUFMSH52C..07H Altcode:
We present analysis of microflares (small active region associated
flares below GOES C class) using RHESSI and Hinode/XRT. RHESSI has
observed well over 1,000 microflares since Hinode launched late in 2006
and of these over 150 have good Hinode/XRT coverage. We use RHESSI to
obtain the temperature, emission measure and non-thermal power-law
parameters from spectral fitting. We compare RHESSI and Hinode/XRT
images to locate the thermal and non-thermal emissions. Taking advantage
of the sensitive high-resolution capability of XRT for the softer
X-rays, we investigate the resulting heating and evaporation from the
accelerated electrons observed via the non-thermal emission by RHESSI.

---------------------------------------------------------
Title: Solar Shape Measurements from RHESSI: A Large Excess Oblateness
Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.; Zahid, H. J.
2007AGUFMSH53A1076F Altcode:
The Solar Aspect System of the RHESSI spacecraft scans the limb at the
~4 sec rotation period of the spacecraft, producing a large quantity
of precise differential measurements of the solar radius at optical
wavelengths (monochromatic at 670 nm). These data provide the most
precise determinations of the oblateness prior in particular to the
launch of the Picard mission in 2008. The observation of standing
waves in the body of the Sun (helioseismology) provided the first
direct way to study the interior of a star. The astrometric shape of
the solar limb gives independent constraints on interior structures
and flows; the surface rotation predicts an oblate ellipsoid with
an equator-pole radius difference of some 8 mas (~0.001%). Here we
report the most accurate observations to date of the solar shape,
which show a much larger apparent oblateness with an equator-pole
radius difference of 13.72± 0.44 mas. This new component can easily
be distinguished spatially from the effects of faculae in the active
latitude zones. Comparison with earlier observations suggests that
this excess oblateness results from solar magnetic activity, as do
the frequency variations of the helioseismic modes.

---------------------------------------------------------
Title: Impulsive Flare Energy Transport by Large-Scale Alfven Waves
and the Electron Acceleration Problem
Authors: Fletcher, L.; Hudson, H. S.
2007AGUFMSM53B1278F Altcode:
In this poster, we investigate an alternative scenario for solar
flare energy transport. Usually, the energy stored in the solar
corona and released during a flare is thought to be transported to
the lower atmosphere by electron beams. However, based on microwave
observations, magnetic fields of the order of a few 100 Gauss to
a kilogauss are measured in the corona above the core of an active
region. This implies an Alfvén speed on the order of 0.1c, meaning
that Alfvén wave pulses become plausible agents for transporting
the stored energy of the flare. We investigate this scenario, and the
opportunities it presents for both heating the lower chromosphere and
accelerating electrons to HXR-emitting energies.

---------------------------------------------------------
Title: Investigation of Ion Acceleration in Small RHESSI Flares
Authors: Shih, A. Y.; Hudson, H. S.; Smith, D. M.; Lin, R. P.
2007AGUFMSH44C..01S Altcode:
Solar flares accelerate both ions and electrons, and the energy
content in accelerated ions may be comparable to that in accelerated
electrons. The signature of ion acceleration is the emission of
gamma-ray lines that result from nuclear interactions in the ambient
solar atmosphere, and the gamma-ray line most easily observed by the
high-resolution detectors on the Reuven Ramaty High-Energy Solar
Spectroscopic Imager ( RHESSI) is the 2.223~MeV neutron-capture
line. Unfortunately, gamma-ray lines are often too weak to observe
in all but the largest flares, but it is of interest to be able to
put constraints on ion acceleration in the more frequently occurring
smaller flares. We combine the spectra of many small flares to obtain
limits on the average neutron-capture line flux for small flares and
discuss the implications of these limits relative to previous studies
of larger flares. The work at the University of California, Berkeley,
was supported by NASA contract NAS 5-98033.

---------------------------------------------------------
Title: Synoptic Views of the Solar Limb: RHESSI Radius and SOHO Images
Authors: Zahid, H. J.; Fivian, M. D.; Hudson, H. S.
2007AGUFMSH32A0792Z Altcode:
The RHESSI mission includes precise astrometric measurements
of the solar limb shape at optical wavelengths as a part of its
aspect-determination system. These data have precisions below 1 mas
and extend over the full lifetime of the mission (from February,
2002). Synoptic maps of the limb shape reveal facular regions as
increases, and sunspots as decreases, in the apparent radius. We compare
these signatures with synoptic SOHO images for a 3-month period in
2004. The patterns are strongly similar, but the EUV synoptic maps have
contributions from features not at the exact limb, which dominates the
RHESSI data. This study anticipates making use of such high-contrast
coronal or chromospheric measurements to provide a masking function
to screen against these features in determinations of the true solar
oblateness and higher-order permanent shape features. We also explore
the possibility of cross-correlating RHESSI sunspot images against those
of other optical telescopes, such as MDI, as a means of calibrating
the roll coordinate of the telescope pointing.

---------------------------------------------------------
Title: What is There Before a Flare?
Authors: Hudson, H. S.; Hannah, I. G.; Krucker, S.; Watanabe, K.
2007AGUFMSH53A1055H Altcode:
The physical parameters in a coronal volume prior to the occurrence
of a flare are generally unknown, but may play an important role
in identifying the processes involved in flaring or eruption. Now
we have observations from Hinode at very high resolution that can
provide the best possible values for preflare temperature and density,
for example. We make use of Hinode XRT observations of the preflare
magnetic structure with the same footpoint locations as a flaring
loop, as identified in RHESSI images. We additionally introduce a
method based on conductive equilibrium (RTV scaling) to reduce the
uncertainty on estimates of physical parameters due to lack of knowledge
of the detailed geometry. Preliminary results are consistent with the
finding at lower resolution from Yohkoh: in a majority of cases, the
preflare conditions are not observable in soft X-rays. We discuss the
upper limits that result, which point to low temperatures, densities,
and plasma beta values, but high Alfven speeds. We hope also to be
able to extend this conclusion with EIS observations.

---------------------------------------------------------
Title: The correlation between soft and hard X-rays component in
flares: from the Sun to the stars
Authors: Isola, C.; Favata, F.; Micela, G.; Hudson, H. S.
2007A&A...472..261I Altcode: 2007arXiv0707.2322I
Aims:We study the correlation between the soft (1.6-12.4 keV, mostly
thermal) and the hard (20-40 and 60-80 keV, mostly non-thermal) X-ray
emission in solar flares up to the most energetic events, spanning about
4 orders of magnitude in peak flux, establishing a general scaling law
and extending it to the most intense stellar flaring events observed
to date. Methods: We used the data from the Reuven Ramaty
High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, a NASA
Small Explorer launched in February 2002. RHESSI has good spectral
resolution (≃1 keV in the X-ray range) and broad energy coverage
(3 keV-20 MeV), which makes it well suited to distinguish the thermal
from non-thermal emission in solar flares. Our study is based on the
detailed analysis of 45 flares ranging from the GOES C-class, to the
strongest X-class events, using the peak photon fluxes in the GOES
1.6-12.4 keV and in two bands selected from RHESSI data, i.e. 20-40
keV and 60-80 keV. Results: We find a significant correlation
between the soft and hard peak X-ray fluxes spanning the complete
sample studied. The resulting scaling law has been extrapolated to the
case of the most intense stellar flares observed, comparing it with
the stellar observations. Conclusions: Our results show that an
extrapolation of the scaling law derived for solar flares to the most
active stellar events is compatible with the available observations
of intense stellar flares in hard X-rays.

---------------------------------------------------------
Title: The Unpredictability of the Most Energetic Solar Events
Authors: Hudson, Hugh S.
2007ApJ...663L..45H Altcode: 2007arXiv0707.1118H
Observations over the past two solar cycles show a highly irregular
pattern of occurrence for major solar flares, γ-ray events,
and solar energetic particle (SEP) fluences. Such phenomena do not
appear to follow the direct indices of solar magnetic activity, such
as the sunspot number. I show that this results from the non-Poisson
occurrence for the most energetic events. This Letter also points out
a particularly striking example of this irregularity in a comparison
between the declining phases of the recent two solar cycles (1993-1995
and 2004-2006, respectively) and traces it through the radiated energies
of the flares, the associated SEP fluences, and the sunspot areas. These
factors suggest that processes in the solar interior involved with
the supply of magnetic flux up to the surface of the Sun have strong
correlations in space and time, leading to a complex occurrence pattern
that is presently unpredictable on timescales longer than active region
lifetimes (weeks) and not correlated well with the solar cycle itself.

---------------------------------------------------------
Title: Solar Magnetic Activity Observed as Distortions of the
Apparent Radius
Authors: Zahid, H. J.; Fivian, M. D.; Hudson, H. S.
2007AAS...210.2223Z Altcode: 2007BAAS...39..323Z
The RHESSI spacecraft carries a high-resolution X-ray and gamma-ray
telescope. To create images and to align them requires a high-resolution
limb aspect sensor with high time resolution. This system has a random
error of less than 20 mas per point, and accumulates about 100 samples
per second in normal operation. The data clearly show faculae and
spots (including the Wilson depression) as they cross the limb. We
characterize these signatures and discuss their significance.

---------------------------------------------------------
Title: Energy Deposition in White Light Flares with TRACE and RHESSI
Authors: Fletcher, L.; Hannah, I. G.; Hudson, H. S.; Metcalf, T. R.
2007ASPC..368..423F Altcode:
In Fletcher et al. (2007) we investigated the white light (WL) continuum
during solar flares and its relationship to energy deposition by
electron beams. In 9 flare events, spanning GOES classifications from
C4.8 to M9.1, we have high cadence TRACE WL and RHESSI hard X-ray
observations, and compare the WL radiative power output with that
provided by flare electrons. Under the thick--target model assumptions,
we find that the electron beam must extend down to 15--20 keV, and the
energy input to the chromosphere should occur within the collisional
stopping depth of these electrons - approximately 2× 10-4
g cm-2. In this short paper, we discuss some ideas on flare
WL emission, summarise the results of the Fletcher et al. (2007)
study and discuss their implications for chromospheric heating and
white light flare emission.

---------------------------------------------------------
Title: Chromospheric Flares
Authors: Hudson, H. S.
2007ASPC..368..365H Altcode: 2007arXiv0704.0823H
In this topical review I revisit the “chromospheric flare”. This
should currently be an outdated concept, because modern data seem
to rule out the possiblity of a major flare happening independently
in the chromosphere alone, but the chromosphere still plays a major
observational role in many ways. It is the source of the bulk of
a flare's radiant energy -- in particular the visible/UV continuum
radiation. It also provides tracers that guide us to the coronal source
of the energy, even though we do not yet understand the propagation
of the energy from its storage in the corona to its release in the
chromosphere. The formation of chromospheric radiations during a flare
presents several difficult and interesting physical problems.

---------------------------------------------------------
Title: Electron Acceleration By Inertial Alfven Waves In The Impulsive
Phase Of A Solar Flare
Authors: Hudson, Hugh S.; Fletcher, L.
2007AAS...210.9303H Altcode: 2007BAAS...39R.211H
The impulsive phase of a solar flare marks the epoch of rapid conversion
of energy stored in the pre-flare coronal magnetic field. Hard X-ray
observations imply that a substantial fraction of flare energy released
during the impulsive phase is converted to the kinetic energy of mildly
relativistic electrons (10-100 keV). The liberation of the magnetic
free energy occurs as the coronal magnetic field reconfigures and
relaxes following reconnection. Therefore, we investigate a scenario
in which the electron acceleration is caused by the agents of this
reconfiguration -- the large-scale Alfven waves which propagate the
field changes throughout the atmosphere. In a plasma with β &lt;
me/mp, these waves propagate as inertial
Alfven waves which, in the presence of strong spatial gradients,
generate field-aligned electric fields that can accelerate electrons
to energies on the order of 10 keV and above. This novel view also
provides an interpretation of the recently-observed rapid variations of
the line-of-sight component of the photospheric magnetic field during
the flare impulsive phase, and offers solutions to some perplexing flare
problems, such as the flare “number problem” of finding sufficient
and resupplying sufficient electrons to explain the impulsive-phase
hard X-ray emission.

---------------------------------------------------------
Title: Variations Of The Optical Solar Limb As Observed By RHESSI
Authors: Fivian, Martin; Hudson, H. S.; Zahid, H. J.
2007AAS...210.2222F Altcode: 2007BAAS...39..128F
The Solar Aspect System of the RHESSI spacecraft scans the limb at the
4 sec rotation period of the spacecraft, producing a large quantity
of precise differential measurements of the solar radius at optical
wavelengths (monochromatic at 670 nm). These data provide the most
precise determinations of the oblateness prior in particular to the
launch of the Picard mission in 2008. The statistical error of an
individual data point is now below 20 mas (milli arc sec) and we hope
to improve on this still further. The database of about 100 samples per
second now exceeds five years, following RHESSI's launch in February
2002. We present results on solar shape and its time variations,
including several signatures of solar magnetic activity (spots and
faculae). We report new improvements made possible by an analysis of
subtle systematic errors due to temperature variations in the spacecraft
and telescope and extensive modeling of the solar limb shape.

---------------------------------------------------------
Title: First Limits on the 3-200 keV X-Ray Spectrum of the Quiet
Sun Using RHESSI
Authors: Hannah, I. G.; Hurford, G. J.; Hudson, H. S.; Lin, R. P.;
van Bibber, K.
2007ApJ...659L..77H Altcode: 2007astro.ph..2726H
We present the first results using the Reuven Ramaty High-Energy
Solar Spectroscopic Imager (RHESSI) to observe solar X-ray emission
not associated with active regions, sunspots, or flares (the quiet
Sun). Using a newly developed chopping technique (fan-beam modulation)
during seven periods of offpointing between 2005 June and 2006 October,
we obtained upper limits over 3-200 keV for the quietest times when the
GOES 12 1-8 Å flux fell below 10-8 W m-2. These
values are smaller than previous limits in the 17-120 keV range and
extend them to both lower and higher energies. The limit in 3-6 keV is
consistent with a coronal temperature &lt;=6 MK. For quiet-Sun periods
when the GOES 12 1-8 Å background flux was between 10-8 and
10-7 W m-2, the RHESSI 3-6 keV flux correlates
to this as a power law, with an index of 1.08+/-0.13. The power-law
correlation for microflares has a steeper index of 1.29+/-0.06. We
also discuss the possibility of observing quiet-Sun X-rays due to
solar axions and use the RHESSI quiet-Sun limits to estimate the
axion-to-photon coupling constant for two different axion emission
scenarios.

---------------------------------------------------------
Title: A TRACE White Light and RHESSI Hard X-Ray Study of Flare
Energetics
Authors: Fletcher, L.; Hannah, I. G.; Hudson, H. S.; Metcalf, T. R.
2007ApJ...656.1187F Altcode:
In this paper we investigate the formation of the white-light (WL)
continuum during solar flares and its relationship to energy deposition
by electron beams inferred from hard X-ray emission. We analyze nine
flares spanning GOES classifications from C4.8 to M9.1, seven of which
show clear cospatial RHESSI hard X-ray and TRACE WL footpoints. We
characterize the TRACE WL/UV continuum energy under two simplifying
assumptions: (1) a blackbody function, or (2) a Paschen-Balmer
continuum model. These set limits on the energy in the continuum,
which we compare with that provided by flare electrons under the
usual collisional thick-target assumptions. We find that the power
required by the white-light luminosity enhancement is comparable to
the electron beam power required to produce the HXR emission only if
the low-energy cutoff to the spectrum is less than 25 keV. The bulk
of the energy required to power the white-light flare (WLF) therefore
resides at these low energies. Since such low-energy electrons cannot
penetrate deep into a collisional thick target, this implies that the
continuum enhancement is due to processes occurring at moderate depths
in the chromosphere.

---------------------------------------------------------
Title: A new method of observing weak extended x-ray sources with
the Reuven Ramaty High-Energy Solar Spectroscopic Imager
Authors: Hannah, Iain G.; Hurford, Gordon J.; Hudson, Hugh S.; Lin,
Robert P.
2007RScI...78b4501H Altcode: 2007astro.ph..2217H
We present a new method, fan-beam modulation, for observing weak
extended x-ray sources with the Reuven Ramaty High-Energy Solar
Spectroscopic Imager (RHESSI). This space-based solar x-ray and γ-ray
telescope has much greater sensitivity than previous experiments in the
3-25 keV range, but is normally not well suited to detecting extended
sources since their signal is not modulated by RHESSI's rotating
grids. When the spacecraft is offpointed from the target source,
however, the fan-beam modulation time-modulates the transmission
by shadowing resulting from exploiting the finite thickness of the
grids. In this article we detail how the technique is implemented and
verify its consistency with sources with clear known signals that
have occurred during RHESSI offpointing: microflares and the Crab
Nebula. In both cases the results are consistent with previous and
complementary measurements. Preliminary work indicates that this new
technique allows RHESSI to observe the integrated hard x-ray spectrum
of weak extended sources on the quiet Sun.

---------------------------------------------------------
Title: Small Scale Energy Release and the Acceleration and Transport
of Energetic Particles
Authors: Hudson, Hugh; Vilmer, Nicole
2007LNP...725...81H Altcode:
We report on results presented at the sessions of Working Group~1
at CESRA 2004, which covered the topic area of the title of this
paper. The working-group participants are listed in the Appendix, and
the topics discussed have been brought together in several general areas
of focus. The emphasis on the discussion is from the point of view of
radiophysics. We organize the material by presenting new constraints
imposed by the recent high-energy and radio observations. We note though
that multi-wavelength knowledge is generally vital in understanding
all of the phenomena involved. The new constraints include exciting
new millimeter-wave discoveries, among others. We then place these
observations into the framework of our knowledge of the acceleration
and propagation of high-energy particles, and of their radio emission
mechanisms. The RHESSI1 results are the most distinctive
in this time frame, and they have made possible several new advances.

---------------------------------------------------------
Title: Review of Selected RHESSI Solar Results
Authors: Dennis, Brian R.; Hudson, Hugh S.; Krucker, Säm
2007LNP...725...33D Altcode:
We review selected science results from RHESSI solar observations made
since launch on 5 February 2002. A brief summary of the instrumentation
is given followed by a sampling of the major science results obtained
from the soft X-ray, hard X-ray, and gamma-ray energy domains. The
thermal continuum measurements and detection of Fe-line features are
discussed as they relate to parameters of the thermal flare plasma for
several events, including microflares. Observations of X-ray looptop,
and rising above-the-loop sources are discussed as they relate to
standard models of eruptive events and the existence of a current
sheet between the two. Hard X-ray spectra and images of footpoints and
coronal sources are presented, showing how they can be used to separate
thermal and nonthermal sources and determine the magnetic reconnection
rate. Gamma-ray line images and spectra are presented as they relate
to determining the location, spectra, and angular distribution of
the accelerated ions and the temperature of the chromospheric target
material. Finally, we discuss the overall energy budget for two of
the larger events seen with RHESSI.

---------------------------------------------------------
Title: RHESSI observations of the solar radius
Authors: Fivian, M. D.; Hudson, H. S.; Zahid, H. J.
2006AGUFMSH23B0368F Altcode:
The Solar Aspect System of the RHESSI spacecraft scans the limb at the
~4 sec rotation period of the spacecraft, producing a large quantity
of precise differential measurements of the solar radius at optical
wavelengths (monochromatic at 670~nm). These data provide the most
precise determinations of the oblateness prior in particular to the
launch of the Picard mission in 2008. The statistical error of an
individual data point is now below 20 mas (milli arc sec) and we hope
to improve on this still further. Data accumulate at the rate of about
100 points per sec, beginning at launch in February 2002. We present
results on solar shape and its time variations, including several
signatures of solar magnetic activity (spots and faculae). We report
new improvements made possible by an analysis of subtle systematic
errors due to temperature variations in the spacecraft and telescope.

---------------------------------------------------------
Title: High Energy Solar Physics from Lunar-Based Observatories
Authors: Emslie, G.; Dennis, B. R.; Holman, G. D.; Hudson, H. S.;
Krucker, S.; Lin, R. P.; Murphy, R.; Ryan, J. M.; Share, G. H.
2006AGUFMSM52A..05E Altcode:
High-energy and optical solar observations from large telescopes will
greatly improve our understanding of the physical processes responsible
for particle acceleration in the Sun and in other astrophysical
sources. In addition, such observations will allow us to identify
the conditions preceding solar eruptive events that are potentially
hazardous to astronauts and equipment on the Moon and on interplanetary
flights, and to power transmission and communications on the Earth. The
moon provides an exceptionally large and stable platform on which to
position instruments that can be used to observe the Sun (and other
astrophysical sources) at photon energies from &lt; 1 keV to &gt;
100 MeV. Uninterrupted measurements over extended periods of time
(1/2 lunar day or 14 days, which is also the duration of the East-West
passage of an active region on the Sun) would be possible, and the
gradual rotation rate of the Moon also allows horizon occultation
measurements (at a drift rate ~ 0.5 arc seconds/second) to be made.

---------------------------------------------------------
Title: Coronal particle trapping revisited
Authors: Hudson, H. S.; MacKinnon, A.; De Rosa, M.
2006AGUFMSH54A..07H Altcode:
We re-examine the idea of long-term particle storage in the solar
corona in the context of modern PFSS (potential-field source surface)
magnetic models. As pointed out by H. Elliot in 1964 and others since
then, such particles could be energetically important, at the level
of some large fraction of the magnetic energy density B2/8π. We
estimate the distribution and time scales of particle trapping by
using representative PFSS coronal models from the Schrijver-De Rosa
SolarSoft code. As the coronal field simplifies during solar minimum, it
approaches axisymmetry and thus contains volumes inaccessible to charged
particles under the guiding-center approximation. We conclude that time
scales can be sufficiently long, so long in fact that the azimuthal
drift time scale (third adiabatic invariant of guiding-center motion),
for the large-scale dipolar configuration characteristic of solar
minimum, can exceed one solar cycle. We discuss the possible sources
of trapped particles, starting with the basic CRAND (cosmic-ray albedo
neutron decay) mechanism, and relate their X-ray and γ-ray signatures
to future observational capabilities including the Sentinels spacecraft.

---------------------------------------------------------
Title: Occulted Hard X-ray Flare Observations with Sentinels
Authors: Krucker, S.; Hannah, I.; Hudson, H. S.; Hurford, G.; Lin,
R. P.
2006AGUFMSH53C..07K Altcode:
Multi-spacecraft hard X-ray observations provided by the Sentinel
mission will give different view angles of solar flares, including the
possibility of partially occulted observations. Occultation of the main
flare emission by the solar limb allows us to study fainter coronal
emission that otherwise would be hidden by the limited dynamic range
of the observations. Furthermore, it will allow to measure directivity
and reconstruct the 3 dimensional geometry of X-ray sources. We use
statistical results from RHESSI observations to derive the probability
of occulted flare observations with Sentinels. A statistical study of
19 giant flares with fast CMES (v&gt;1500 km/s) occurring behind the
solar limb is presented. We find that all events occurring 45 degrees
or less behing the limb (corresponding to an occultation height h &lt;
0.4~Rsun) show X-ray enhancements, while the three events occurring
more than 50 degrees (h&gt;1~Rsun) behind the limb do not. This result
indicates that on the average, two Sentinels will see the total flare,
while one spacecraft will see the flare partially occulted. The most
prominent event in our RHESSI survey will be discussed in detail. It
has an occultation high of 0.3 solar radius, but still shows emission
up to 80 keV. The observed hard X-ray source is extended, with a size
of about 200", and moves upwards with a velocity of ~1000 km/s.

---------------------------------------------------------
Title: Gamma Rays and the Evolving, Compact Structures of the 2003
October 28 X17 Flare
Authors: Schrijver, C. J.; Hudson, H. S.; Murphy, R. J.; Share, G. H.;
Tarbell, T. D.
2006ApJ...650.1184S Altcode:
The X17 flare on 2003 October 28 was observed by high-resolution
imaging or spectroscopic instruments on CORONAS, GOES, INTEGRAL,
RHESSI, SOHO, and TRACE. These spacecraft observed the temporal
evolution of the γ-ray positron-annihilation and nuclear de-excitation
line spectra, imaged the hard X-ray bremsstrahlung and EUV and UV
emission, and measured the surface magnetic field and subphotospheric
pressure perturbations. In the usual pattern, the onset of the flare
is dominated by particle acceleration and interaction, and by the
filling of coronal magnetic structures with hot plasma. The associated
positron-annihilation signatures early in the impulsive phase from
11:06 to 11:16 UT have a line-broadening temperature characteristic of
a few hundred thousand kelvins. The most intense precipitation sites
within the extended flare ribbons are very compact, with diameters
of less than 1400 km, and a 195 Å TRACE intensity that can exceed
7500 times the quiescent active-region value. These regions appear to
move at speeds of up to 60 km s-1. The associated rapidly
evolving, compact perturbations of the photosphere below these sites
excite acoustic pulses that propagate into the solar interior. Less
intense precipitation sites typically persist for several minutes
behind the advancing flare ribbons. After ~1 ks, the flare enters
a second phase, dominated by coronal plasma cooling and downflows
and by annihilation-line radiation characteristic of a photospheric
environment. We point out (1) that these detailed observations
underscore that flare models need to explicitly incorporate the
multitude of successively excited environments whose evolving signals
differ at least in their temporal offsets and energy budgets, if not
also in the exciting particle populations and penetration depths, and
(2) that the spectral signatures of the positron annihilation do not
fit conventional model assumptions.

---------------------------------------------------------
Title: The Optical Solar Limb As Observed By RHESSI
Authors: Fivian, Martin D.; Hudson, H. S.; Zahid, H. J.
2006SPD....37.3005F Altcode: 2006BAAS...38Q.257F
The RHESSI limb sensors routinely make precise differentialobservations
of the solar limb shape and its time variations. Therandom error
for a single point is less than 40 milli-arcsecondsand in regular
operation we obtain about 100 samples per second.The database now
exceeds four years, following RHESSI's launch inFebruary 2002. Because
of RHESSI's rotation (nominally at 4 secperiod) the basic measurement
resembles that pioneered by Dicke's"Oblateness Telescope". The data
clearly show several solar features,including the oblateness as well
as sunspots, faculae, and p-modes.We give a general description and
report preliminary values for theoblateness. We find the oblateness
measurement to depend sensitivelyon the level of magnetic activity,
and give a representative value of9.72 +/- 0.19 MAS (statistical error
only) for a 60-hour intervalstarting 10:15 UT 14 August 2004.

---------------------------------------------------------
Title: Limb Distortions Related To Solar Magnetic Activity
Authors: Hudson, Hugh S.; Fivian, M. D.; Wilson, B. M.; Zahid, H. J.
2006SPD....37.0714H Altcode: 2006BAAS...38..230H
We use the RHESSI aspect-sensor observations of the solar limb
tocharacterize the effects of solar magnetic activity on the
apparentradius as a function of position angle. Sunspots at the
exact limbresult in a depression (the Wilson effect), which we
observe directly.The results are interpreted as lower limits
of a few hundred km,as limited by the umbral diameter, based on
simple geometricalmodels. Faculae produce clear signatures with a
presently ambiguousinterpretation because of lack of knowledge of the
limb-darkeningfunction and fine structure. We display the limb data
as stackplotsof radius as a function of position angle and compare
these with othermeasures of solar activity.

---------------------------------------------------------
Title: Searching For Quiet Sun X-ray Emission With RHESSI
Authors: Hannah, Iain G.; Hurford, G. J.; Hudson, H. S.; Lin, R. P.
2006SPD....37.1402H Altcode: 2006BAAS...38R.243H
In this work we are searching for X-ray emission from the Sun that is
not associated with active regions. RHESSI has greater sensitivity
in the 5-25 keV range than previous missions, but since the quiet
Sun sources may be well-dispersed spatially across the disk, RHESSI's
normal imaging technique is not well suited to the task. An alternate
observational technique involves pointing RHESSI slightly away from the
Sun. The resulting time-dependent transmission of the signal through
RHESSI's rotating modulation collimators allows the quiet Sun signal
to be distinguishable from the background. However, a large sample
of data is required in this mode so that we can determine whether
the observations are noise limited or quiet Sun signal. This novel
offpointing technique has been used 4 times since July 2005, during
which the GOES background level dropped below A1. From the data acquired
we have been able to obtain limits to the quiet Sun X-ray flux between
3-100 keV. At the lowest energies, below 7 keV, the limits correlate
with the observed GOES flux, suggesting a signal. We will discuss the
consequences of interpreting this emission as originating from either
a thermal or non-thermal electron population, and the relevance to
coronal heating.

---------------------------------------------------------
Title: Coronal Mass Ejections: Overview of Observations
Authors: Hudson, H. S.; Bougeret, J. -L.; Burkepile, J.
2006SSRv..123...13H Altcode: 2006SSRv..tmp...63H
We survey the subject of Coronal Mass Ejections (CMEs), emphasizing
knowledge available prior to about 2003, as a synopsis of the
phenomenology and its interpretation.

---------------------------------------------------------
Title: Coronal Observations of CMEs. Report of Working Group A
Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.;
Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.;
Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.;
Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J.
2006SSRv..123..127S Altcode: 2006SSRv..tmp...58S
CMEs have been observed for over 30 years with a wide variety of
instruments. It is now possible to derive detailed and quantitative
information on CME morphology, velocity, acceleration and mass. Flares
associated with CMEs are observed in X-rays, and several different
radio signatures are also seen. Optical and UV spectra of CMEs both on
the disk and at the limb provide velocities along the line of sight
and diagnostics for temperature, density and composition. From the
vast quantity of data we attempt to synthesize the current state of
knowledge of the properties of CMEs, along with some specific observed
characteristics that illuminate the physical processes occurring during
CME eruption. These include the common three-part structures of CMEs,
which is generally attributed to compressed material at the leading
edge, a low-density magnetic bubble and dense prominence gas. Signatures
of shock waves are seen, but the location of these shocks relative
to the other structures and the occurrence rate at the heights where
Solar Energetic Particles are produced remains controversial. The
relationships among CMEs, Moreton waves, EIT waves, and EUV dimming
are also cloudy. The close connection between CMEs and flares suggests
that magnetic reconnection plays an important role in CME eruption
and evolution. We discuss the evidence for reconnection in current
sheets from white-light, X-ray, radio and UV observations. Finally, we
summarize the requirements for future instrumentation that might answer
the outstanding questions and the opportunities that new space-based
and ground-based observatories will provide in the future.

---------------------------------------------------------
Title: Multi-Wavelength Observations of CMEs and Associated Phenomena.
Report of Working Group F
Authors: Pick, M.; Forbes, T. G.; Mann, G.; Cane, H. V.; Chen, J.;
Ciaravella, A.; Cremades, H.; Howard, R. A.; Hudson, H. S.; Klassen,
A.; Klein, K. L.; Lee, M. A.; Linker, J. A.; Maia, D.; Mikic,
Z.; Raymond, J. C.; Reiner, M. J.; Simnett, G. M.; Srivastava, N.;
Tripathi, D.; Vainio, R.; Vourlidas, A.; Zhang, J.; Zurbuchen, T. H.;
Sheeley, N. R.; Marqué, C.
2006SSRv..123..341P Altcode: 2006SSRv..tmp...60P
This chapter reviews how our knowledge of CMEs and CME-associated
phenomena has been improved, since the launch of the SOHO mission,
thanks to multi-wavelength analysis. The combination of data obtained
from space-based experiments and ground based instruments allows us
to follow the space-time development of an event from the bottom of
the corona to large distances in the interplanetary medium. Since CMEs
originate in the low solar corona, understanding the physical processes
that generate them is strongly dependant on coordinated multi-wavelength
observations. CMEs display a large diversity in morphology and kinematic
properties, but there is presently no statistical evidence that those
properties may serve to group them into different classes. When a CME
takes place, the coronal magnetic field undergoes restructuring. Much
of the current research is focused on understanding how the corona
sustains the stresses that allow the magnetic energy to build up and
how, later on, this magnetic energy is released during eruptive flares
and CMEs. Multi-wavelength observations have confirmed that reconnection
plays a key role during the development of CMEs. Frequently, CMEs
display a rather simple shape, exhibiting a well known three-part
structure (bright leading edge, dark cavity and bright knot). These
types of events have led to the proposal of the ‘`standard model’'
of the development of a CME, a model which predicts the formation
of current sheets. A few recent coronal observations provide some
evidence for such sheets. Other more complex events correspond to
multiple eruptions taking place on a time scale much shorter than the
cadence of coronagraph instruments. They are often associated with
large-scale dimming and coronal waves. The exact nature of these waves
and the physical link between these different manifestations are not
yet elucidated. We also discuss what kind of shocks are produced during
a flare or a CME. Several questions remain unanswered. What is the
nature of the shocks in the corona (blast-wave or piston-driven?) How
they are related to Moreton waves seen in Hα? How they are related
to interplanetary shocks? The last section discusses the origin of
energetic electrons detected in the corona and in the interplanetary
medium. “Complex type III-like events,”which are detected at
hectometric wavelengths, high in the corona, and are associated with
CMEs, appear to originate from electrons that have been accelerated
lower in the corona and not at the bow shock of CMEs. Similarly,
impulsive energetic electrons observed in the interplanetary medium
are not the exclusive result of electron acceleration at the bow shocks
of CMEs; rather they have a coronal origin.

---------------------------------------------------------
Title: White-Light Flares: A TRACE/RHESSI Overview
Authors: Hudson, H. S.; Wolfson, C. J.; Metcalf, T. R.
2006SoPh..234...79H Altcode:
The Transition Region and Coronal Explorer (TRACE) instrument includes
a "white light" imaging capability with novel characteristics. Many
flares with such white-light emission have been detected, and this paper
provides an introductory overview of these data. These observations
have 0.5″ pixel size and use the full broad-band response of the
CCD sensor; the images are not compromised by ground-based seeing and
have excellent pointing stability as well as high time resolution. The
spectral response of the TRACE white-light passband extends into the
UV, so these data capture, for the first time in images, the main
radiative energy of a flare. This initial survey is based on a sample
of flares observed at high time resolution for which the Reuven Ramaty
High-Energy Solar Spectroscopic Imager (RHESSI) had complete data
coverage, a total of 11 events up to the end of 2004. We characterize
these events in terms of source morphology and contrast against the
photosphere. We confirm the strong association of the TRACE white-light
emissions - which include UV as well as visual wavelengths - with hard
X-ray sources observed by RHESSI. The images show fine structure at
the TRACE resolution limit, and often show this fine structure to
be extended over large areas rather than just in simple footpoint
sources. The white-light emission shows strong intermittency both in
space and in time and commonly contains features unresolved at the
TRACE resolution. We detect white-light continuum emission in flares
as weak as GOES C1.6. limited by photon statistics and background
solar fluctuations, and support the conclusion of Neidig (1989) that
white-light continuum occurs in essentially all flares.

---------------------------------------------------------
Title: Preflare Nonthermal Emission Observed in Microwaves and
Hard X-Rays
Authors: Asai, Ayumi; Nakajima, Hiroshi; Shimojo, Masumi; White,
Stephen M.; Hudson, Hugh S.; Lin, Robert P.
2006PASJ...58L...1A Altcode:
We present a detailed examination on nonthermal emissions during
the preflare phase of the X4.8 flare that occurred on 2002 July
23. The microwave (17GHz and 34GHz) data obtained with the Nobeyama
Radioheliograph at Nobeyama Solar Radio Observatory and the hard X-ray
data taken with the Reuven Ramaty High Energy Solar Spectroscopic Imager
obviously showed nonthermal features in the preflare phase. We also
found a faint ejection associated with the flare in the EUV images
taken with the Transition Region and Coronal Explorer. We discuss
the temporal and spatial features of the nonthermal emissions in the
preflare phase, and their relation with the ejection.

---------------------------------------------------------
Title: Multi-Wavelength Observations of CMEs and Associated Phenomena
Authors: Pick, M.; Forbes, T. G.; Mann, G.; Cane, H. V.; Chen, J.;
Ciaravella, A.; Cremades, H.; Howard, R. A.; Hudson, H. S.; Klassen,
A.; Klein, K. L.; Lee, M. A.; Linker, J. A.; Maia, D.; Mikic,
Z.; Raymond, J. C.; Reiner, M. J.; Simnett, G. M.; Srivastava, N.;
Tripathi, D.; Vainio, R.; Vourlidas, A.; Zhang, J.; Zurbuchen, T. H.;
Sheeley, N. R.; Marqué, C.
2006cme..book..341P Altcode:
This chapter reviews how our knowledge of CMEs and CME-associated
phenomena has been improved, since the launch of the SOHO mission,
thanks to multi-wavelength analysis. The combination of data obtained
from space-based experiments and ground based instruments allows us
to follow the space-time development of an event from the bottom of
the corona to large distances in the interplanetary medium. Since
CMEs originate in the low solar corona, understanding the physical
processes that generate them is strongly dependant on coordinated
multi-wavelength observations. CMEs display a large diversity in
morphology and kinematic properties, but there is presently no
statistical evidence that those properties may serve to group them
into different classes. When a CME takes place, the coronal magnetic
field undergoes restructuring. Much of the current research is focused
on understanding how the corona sustains the stresses that allow the
magnetic energy to build up and how, later on, this magnetic energy is
released during eruptive flares and CMEs. Multiwavelength observations
have confirmed that reconnection plays a key role during the development
of CMEs. Frequently, CMEs display a rather simple shape, exhibiting a
well known three-part structure (bright leading edge, dark cavity and
bright knot). These types of events have led to the proposal of the
"standard model" of the development of a CME, a model which predicts
the formation current sheets. A few recent coronal observations provide
some evidence for such sheets. Other more complex events correspond
to multiple eruptions taking place on a time scale much shorter than
the cadence of coronagraph instruments. They are often associated with
large-scale dimming and coronal waves. The exact nature of these waves
and the physical link between these different manifestations are not
yet elucidated. We also discuss what kind of shocks are produced during
a flare or a CME. Several questions remain unanswered. What is the
nature of the shocks in the corona (blast-wave or piston-driven?) How
they are related to Moreton waves seen in Hα? How they are related
to interplanetary shocks? The last section discusses the origin of
energetic electrons detected in the corona and in the interplanetary
medium. "Complex type III-like events," which are detected at
hectometric wavelengths, high in the corona, and are associated with
CMEs, appear to originate from electrons that have been accelerated
lower in the corona and not at the bow shock of CMEs. Similarly,
impulsive energetic electrons observed in the interplanetary medium
are not the exclusive result of electron acceleration at the bow shocks
of CMEs; rather they have a coronal origin.

---------------------------------------------------------
Title: Coronal Observations of CMEs
Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.;
Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.;
Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.;
Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J.
2006cme..book..127S Altcode:
CMEs have been observed for over 30 years with a wide variety of
instruments. It is now possible to derive detailed and quantitative
information on CME morphology, velocity, acceleration and mass. Flares
associated with CMEs are observed in X-rays, and several different
radio signatures are also seen. Optical and UV spectra of CMEs both on
the disk and at the limb provide velocities along the line of sight
and diagnostics for temperature, density and composition. From the
vast quantity of data we attempt to synthesize the current state of
knowledge of the properties of CMEs, along with some specific observed
characteristics that illuminate the physical processes occurring during
CME eruption. These include the common three-part structures of CMEs,
which is generally attributed to compressed material at the leading
edge, a low-density magnetic bubble and dense prominence gas. Signatures
of shock waves are seen, but the location of these shocks relative
to the other structures and the occurrence rate at the heights where
Solar Energetic Particles are produced remains controversial. The
relationships among CMEs, Moreton waves, EIT waves, and EUV dimming
are also cloudy. The close connection between CMEs and flares suggests
that magnetic reconnection plays an important role in CME eruption
and evolution. We discuss the evidence for reconnection in current
sheets from white-light, X-ray, radio and UV observations. Finally, we
summarize the requirements for future instrumentation that might answer
the outstanding questions and the opportunities that new space-based
and ground-based observatories will provide in the future.

---------------------------------------------------------
Title: Preflare Nonthermal Emission Observed in Microwaves and
Hard X-Rays
Authors: Asau, A.; Nakajima, H.; Shimojo, M.; White, S. M.; Hudson,
H. S.
2006apri.meet...46A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Coronal Mass Ejections: Overview of Observations
Authors: Hudson, H. S.; Bougeret, J. -L.; Burkepile, J.
2006cme..book...13H Altcode:
We survey the subject of Coronal Mass Ejections (CMEs), emphasizing
knowledge available prior to about 2003, as a synopsis of the
phenomenology and its interpretation.

---------------------------------------------------------
Title: Variations of Solar Radius: Observations with Rhessi
Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.
2005ESASP.600E...4F Altcode: 2005dysu.confE...4F; 2005ESPM...11....4F
No abstract at ADS

---------------------------------------------------------
Title: Late-phase hard X-ray emission from flares
Authors: Hudson, H. S.; Krucker, S.
2005AGUFMSH13A0290H Altcode:
In a few major flare events observed by RHESSI, we see hard X-ray
signatures long after (tens of minutes to more than one hour) the
impulsive-phase onset. A prototype for such phenomena was the flare
of March 31, 1969 (Frost and Dennis, 1971). Recent RHESSI examples
include January 19, 2005, a GOES X1.5 event with gradual variations, and
September 7, 2005 an X17 event with more impulsive variability. RHESSI
images show footpoint emissions in both cases; the spectra are hard and
become harder with time as in the Frost-Dennis event. The existence
of impulsive variability and footpoint emission allows us to discuss
trapping and injection. We discuss the morphology of these events,
including other examples such as April 21, 2002 (X1.5).

---------------------------------------------------------
Title: RHESSI observations of solar radius
Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.
2005AGUFMSH11A0241F Altcode:
The Solar Aspect System (SAS) of the rotating (at 15 rpm) RHESSI
spacecraft has three simple solar optical telescopes. Each of these
measures the position of the limb by sampling the solar chord profile
with a linear CCD using a narrow bandwidth filter at 670 nm. With
a resolution of each CCD of 1.7 arcsec/pixel, the precision of each
of the 6 limb positions is observed to be better than 50 mas using
4 pixels at each limb. Since the launch of RHESSI early 2002, data
have been obtained at a rate of at least 100 samples/sec. This has
provided a database of currently 7~×~109 individual measurements. The
main function of SAS is to determine the RHESSI pointing relative to
Sun center. Using these data we observe the solar oblateness and see
signatures of magnetic activity (spots and faculae).

---------------------------------------------------------
Title: A Search for Hard X-Ray Emission from the Quiet Spotless Sun
Authors: Hannah, I. G.; Hurford, G. J.; Hudson, H. S.
2005AGUFMSH11A0242H Altcode:
RHESSI observations often show hard x-ray emission from non-flaring
active regions. The objective of this work is to search for hard
x-ray emission that is not associated with active regions. There are
a number of potential sources for such emission, some of which are
relevant to coronal heating. With its shutters open RHESSI has greater
sensitivity in the 5-25 keV range than previous missions. It uses a
set of 9 rotating modulation collimators to image sources with size
scales from 2 arcseconds to 3 arcminutes. However, since the quiet
Sun sources may be well-dispersed spatially across the disk, RHESSI's
normal imaging technique is not well suited to the task. An alternate
observational technique involves pointing RHESSI slightly away from
the Sun. Then the time-dependent transmission of the thick individual
grids (whose field of view is 1 degree) chops the integrated solar
signal at ~0.5 Hz to provide effective background suppression. This
paper describes the technique and initial results from a period of
offpointing observations (19-July-05 to 25-July-05) acquired when the
GOES background level dropped to below A2 and there were no active
regions visible on the disk.

---------------------------------------------------------
Title: TRACE white light and RHESSI hard X-rays
Authors: Fletcher, L.; Allred, J.; Hannah, I.; Hudson, H.; Metcalf, T.
2005AGUFMSH13A0286F Altcode:
We study the energetics of RHESSI and TRACE observations for a
sample of 11 solar flares well observed in the TRACE “white light"
channel. In general, the data show excellent correlations between hard
X-rays and white light. We discuss the energetics of this relationship
based upon simple models for the visible/UV continuum, namely (i)
a non-thermal approximation (Balmer and Paschen continuum), (ii)
a thermal approximation (blackbody), and (iii) model spectra derived
from radiation-hydrodynamic modeling. We relate the white-light energy
and the low-energy cutoff energy of the primary electron spectrum
required for energetic equivalence. This comparison will be made in
the context of the flare WL morphology and in comparison with RHESSI
hard X-ray images.

---------------------------------------------------------
Title: Refinements to flare energy estimates: A followup to “Energy
partition in two solar flare/CME events” by A. G. Emslie et al.
Authors: Emslie, A. G.; Dennis, B. R.; Holman, G. D.; Hudson, H. S.
2005JGRA..11011103E Altcode:
Emslie et al. (2004) reported estimates of the energy in the different
flare and coronal mass ejection (CME) components of two major solar
events with unprecedented observational coverage, one on 21 April
2002 and the other on 23 July 2002. On the basis of these estimates,
it appeared that the summed energy content of the different flare
components was significantly lower than the total energy of the CME,
leading them to reach the "cautious" conclusion that "in both events
the coronal mass ejection has the dominant component of the released
energy," amounting to approximately 30% of the available magnetic
energy. In this note we present revised estimates of the flare thermal
energies in the two events and also add a consideration of the total
radiant energy of the events obtained by scaling the measured soft X-ray
luminosity based on Solar Radiation and Climate Experiment (SORCE) total
solar irradiance measurements for the 28 October 2003 event. Recognizing
that many of these energetic components are interrelated, we also take
care to distinguish between "primary" components of energy (e.g.,
the magnetic field), "intermediate" components (e.g., accelerated
particles and thermal plasma), and "final" components (e.g., kinetic
energy of ejecta, radiant energy in various wave bands). We note that
since the values of these components are not all independent, careful
tallying is necessary to arrive at an overall energy budget for the
event. The best estimates for the energies of the various components
still show that the CME contains the greatest fraction of the released
energy in both events. However, given the large uncertainties in the
energies of the different flare components and the higher estimates of
radiant energy obtained by scaling from the SORCE measurements, the
results are also consistent with the flare and CME energies in both
events being comparable, with a common value of ∼1032 ergs.

---------------------------------------------------------
Title: Coronal shock waves observed in images
Authors: Hudson, H. S.
2005AIPC..781..336H Altcode:
The large-scale coronal shock waves observed from radio type II bursts
and from Moreton waves have proven surprisingly difficult to detect
in coronal images. I review the evidence for such waves in radio,
optical, EUV, and soft X-ray images. The data generally support the
conclusion that the metric type II bursts can be identified with weak
fast-mode shock waves launched at the impulsive phase of the associated
flares. Other coronal waves, well seen by EIT, are more closely related
to CMEs.

---------------------------------------------------------
Title: Outburst Evolution and Pulse Period of 1A 0535+262
Authors: Smith, D. M.; Hazelton, B.; Coburn, W.; Boggs, S. E.; Fivian,
M.; Hurford, G. J.; Hudson, H. S.; Grefenstette, B.; Gilmore, R.
2005ATel..557....1S Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
spacecraft was pointed to the accreting pulsar 1A 0535+262 after the
discovery by Tueller et al. (ATEL #504) that it was in a major outburst
for the first time since 1994. Operations to point away from the Sun
began on 3 June and the pulsar gradually moved into our field of view
over the next week (RHESSI repointing is very slow since the spacecraft
spins near 15 rpm and has only magnetic torquers to repoint it).

---------------------------------------------------------
Title: Energetics of RHESSI X-Class Flares
Authors: Dennis, B. R.; Holman, G. D.; Haga, L.; Hudson, H. S.
2005AGUSMSP21A..01D Altcode:
The thermal and nonthermal energies of several X-class flares seen
with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
will be presented. The same techniques described by Emslie et al. (JGR,
109, A10104, 2004) are used to take the RHESSI imaging spectroscopic
observations and compute the energies in the thermal plasma and
in the nonthermal electrons as a function of time throughout the
flares. Radiative and conductive cooling rates are estimated and total
thermal and nonthermal energies are computed for each flare. Typically,
the energy in nonthermal electrons integrated up to the time of peak
soft X-ray emission is equal to or exceeds the energy in the thermal
plasma at that time. This suggests that energy must have been converted
into a form not visible with RHESSI and that the total energy released
by the flares may be significantly greater than the sum of energies
calculated from the RHESSI observations alone. This conclusion is
supported by the high radiative energy seen with SORCE during the
impulsive phase of the 28 October 2003 flare. The peak increase in
total solar irradiance of 270 mW m-2 measured with SORCE was over two
orders of magnitude higher than the peak soft X-ray flux seen with
GOES or RHESSI. The implications of this new observation as compared
to the energetics derived from the X-ray observations of that flare
will be discussed along with the energetics analysis of most of the
other X-class flares in October/November 2003.

---------------------------------------------------------
Title: RHESSI Observations of Relativistic Electron Precipitation
at L=1.0-2.6 Following Large Solar Energetic Particle Events
Authors: Goodhue, A. C.; Lin, R. P.; Smith, D. M.; Hudson, H. S.
2005AGUSMSM43B..06G Altcode:
The past eighteen months have produced the largest solar energetic
particle events in the current solar cycle. We present a preliminary
investigation of the effects of large solar storms on trapped electron
populations in the magnetosphere using observations from the Reuven
Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) following the
28 October 2003, 27 July 2004, and 10 November 2004 storms. Although
RHESSI's germanium detectors were designed to measure solar hard
x-rays and gamma rays, they are large enough (7.1 cm diameter x 8.5
cm) to stop penetrating electrons and measure their energy up to 17
MeV, as well as detect bremsstrahlung radiation from the precipitating
electrons. We first obtain raw counts spectra, which are a superposition
of bremsstrahlung and direct electron detection signatures. We then
apply a GEANT simulation model of the spacecraft and instrument in order
to infer the original spectra of incoming electrons from the measured
spectra. After the October 2003 storm, RHESSI detected electrons up
to ~5 MeV that were transported into the slot region (below L=2.6),
and subsequently began to diffuse away. In the month following the
storm, the spectrum became softer as high-energy electrons receded from
the L=1.0-2.6 region. By the end of November 2003, the highest-energy
electrons detected dropped to ~2 MeV. At the beginning of June 2004, six
months after the original storm, the relativistic electrons disappeared
completely. A similar spectral analysis is done for the July 2004 and
November 2004 storms.

---------------------------------------------------------
Title: RHESSI soft X-ray imaging spectroscopy of a flare
Authors: Hudson, H.; Caspi, A.; Dennis, B.; Phillips, K.
2005AGUSMSH31A..03H Altcode:
We apply RHESSI imaging spectroscopy to a well-observed solar flare on
26~April~2003. This GOES M2-class flare (N20W69) exercised all three of
the RHESSI shutter states, and was simultaneously observed by the RESIK
high-resolution X-ray spectrometer on board the CORONAS-F spacecraft
(Dennis et al., 2005). It thus provides an excellent opportunity to
study the behavior of the high-throughput RHESSI observations of the
Fe~emission-line complex at ~6.7~keV. The equivalent width of this
feature has a unique dependence on the assumed isothermal temperature
of the source (Phillips, 2004). Comparing this feature-derived
temperature between the onset and decay phases of the event, we note
a discrepancy between it and the temperature derived directly from
the continuum. We analyze the causes of this discrepancy in terms
of image morphology, non-isothermality, non-equilibrium excitation,
and instrument properties.

---------------------------------------------------------
Title: Initial localization and kinematic characteristics of the
structural components of a coronal mass ejection
Authors: Uralov, A. M.; Grechnev, V. V.; Hudson, H. S.
2005JGRA..110.5104U Altcode: 2005JGRA..11005104U
The leading component of a coronal mass ejection (CME), its observed
frontal structure (FS), has been detected close to the solar surface
in a few near-the-limb events only. Thus far, no manifestations of
such a frontal structure have been reported in reasonable proximity
to a preeruptive filament located away from the solar limb. Thus
the identification of the FS with preevent coronal structures
remains unclear. We propose a method to estimate the parameters of
the initial volume of a CME, using comparative measurements of the
spatial locations of the erupting filament and FS with a self-similar
solution of the magnetohydrodynamic equations describing the expansion
of the CME. We develop this method by analyzing observations of a large
eruptive filament on the solar disk on 4 September 2000, using data
acquired with the Solar Heliospheric Observatory (SOHO), Large-Angle
Spectroscopic Coronagraph (LASCO), and EUV Imaging Telescope (EIT)
instruments and the Siberian Solar Radio Telescope. We show that if
a magnetic structure corresponding to the FS prior to the filament
eruption does exist, then it is localized at a relatively low height
(here, about 100-150 Mm above the filament). At the initial stage
of the motion, the shape of the hot FS approximately reproduces the
configuration of the cool eruptive filament. In addition, we conclude
that the coronal dimming observed in this event could be also due to
CME-caused suppression of the heating and/or mass supply of the dimmed
structures rather than due to their opening only. We also obtain in a
simple way an exact self-similar solution of MHD equations in a form
suitable for analyses of experimental data.

---------------------------------------------------------
Title: An exceptionally bright flare from SGR 1806-20 and the origins
of short-duration γ-ray bursts
Authors: Hurley, K.; Boggs, S. E.; Smith, D. M.; Duncan, R. C.; Lin,
R.; Zoglauer, A.; Krucker, S.; Hurford, G.; Hudson, H.; Wigger, C.;
Hajdas, W.; Thompson, C.; Mitrofanov, I.; Sanin, A.; Boynton, W.;
Fellows, C.; von Kienlin, A.; Lichti, G.; Rau, A.; Cline, T.
2005Natur.434.1098H Altcode: 2005astro.ph..2329H
Soft-γ-ray repeaters (SGRs) are galactic X-ray stars that emit
numerous short-duration (about 0.1s) bursts of hard X-rays during
sporadic active periods. They are thought to be magnetars: strongly
magnetized neutron stars with emissions powered by the dissipation
of magnetic energy. Here we report the detection of a long (380s)
giant flare from SGR 1806-20, which was much more luminous than any
previous transient event observed in our Galaxy. (In the first 0.2s,
the flare released as much energy as the Sun radiates in a quarter
of a million years.) Its power can be explained by a catastrophic
instability involving global crust failure and magnetic reconnection
on a magnetar, with possible large-scale untwisting of magnetic field
lines outside the star. From a great distance this event would appear
to be a short-duration, hard-spectrum cosmic γ-ray burst. At least
a significant fraction of the mysterious short-duration γ-ray bursts
may therefore come from extragalactic magnetars.

---------------------------------------------------------
Title: Rhessi Microflare Statistics
Authors: Hannah, I. G.; Christe, S.; Krucker, S.; Hudson, H. S.;
Fletcher, L.; Hendry, M. A.
2004ESASP.575..259H Altcode: 2004soho...15..259H
No abstract at ADS

---------------------------------------------------------
Title: RHESSI: First Results
Authors: Hudson, H. S.
2004ASPC..325..335H Altcode:
The RHESSI observations consist of imaging spectroscopy in the γ-ray,
hard X-ray, and “firm X-ray” (3--20 keV) bands. These data are
now the most extensive and capable solar high-energy observations
at high spectral and spatial resolution. The low-energy hard X-ray
spectrum bridges the thermal and non-thermal ranges of solar electron
distributions in flares systematically for the first time. In
this presentation I survey some results from the first 18 months
of observation, including findings on image and spectral morphology
(both hard X-ray and γ-ray), and the behavior of microflares in the
3--20 keV band.

---------------------------------------------------------
Title: Overview of Early Rhessi Results
Authors: Krucker, S.; Hudson, H. S.
2004ESASP.575..247K Altcode: 2004soho...15..247K
No abstract at ADS

---------------------------------------------------------
Title: The hard X-ray spectral structure of flare ribbons
Authors: Hudson, H.; Fletcher, L.; Krucker, S.; Pollock, J.
2004AGUFMSH24A..02H Altcode:
We examine the spatial distribution of hard X-ray spectral parameters
in flares exhibiting the classic two-ribbon structure using RHESSI
observations. The flares studied include July~15 and July~17, 2002,
and October~29, 2003. We confirm the existence of a tendency for
the localization of the hard X-ray sources into dominant bright
“footpoint” regions which do not show ribbon structure as extensive
as that seen in Hα or UV~images. As a part of the study we characterize
the ribbons photometrically in the EUV as observed by TRACE, confirming
earlier results that find complicated relationships between EUV and
hard X-rays. We seek an empirical explanation for the restricted
hard X-ray footpoints in terms of a spatial analog of the well-known
“soft-hard-soft” morphology: the regions of weaker hard X-ray emission
correspond to steeper X-ray energy spectra and hence to softer electron
precipitation spectra. This relationship may be as predicted by the
1D radiation hydrodynamics models of flaring loops.

---------------------------------------------------------
Title: The Halloween 2003 Storm's Effect on Trapped Electron
Populations
Authors: Goodhue, A. C.; Hudson, M. K.; Hudson, H. S.; McNab, M. C.
2004AGUFMSM41A1110G Altcode:
We have investigated the effects of the October and November 2003 solar
energetic particle (SEP) events on trapped electron populations using
data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI) in conjunction with the findings from the Solar, Anomalous,
Magnetospheric Particle Explorer (SAMPEX). Immediately after the
Halloween storm, RHESSI, saw an order of magnitude increase in the
electron population in its low particle energy bin (less than 600 KeV);
the population lingered for more than thirty days at L=1.9-2.2. The
increase is caused by an inward transportation of trapped magnetospheric
electrons from higher L values, and the decay is produced by pitch
angle diffusion. Similar effects were seen by SAMPEX. However, the
appearance of SEP's, whose energies are much greater and range up to
20 MeV, does not occur until months later, as these electrons are not
pitch angle diffused as quickly as the lower energy population.

---------------------------------------------------------
Title: Coronal Loop Oscillations and Flare Shock Waves
Authors: Hudson, H. S.; Warmuth, A.
2004ApJ...614L..85H Altcode:
A statistical analysis of coronal loop oscillations observed by the
Transition Region and Coronal Explorer (TRACE) shows that 12 of 28 cases
were associated with metric type II bursts. The timing is consistent
with the idea that in many cases the loop oscillations result from the
passage of a large-scale wave disturbance originating in a flare in
the nearby active region. The GOES classifications for these flares
range from C4.2 to X20. Typically, the oscillating structures are
not disrupted, implying that the disturbance has passed through the
medium, which has returned to an equilibrium near that seen prior to
the event. This is consistent with the Uchida interpretation of the
disturbance as a weak fast-mode blast wave (i.e., a simple wave at
a low Alfvénic Mach number) propagating in the ambient corona. We
note that all 12 of the associated events were also associated with
coronal mass ejections (CMEs) and conclude that the CME eruptions in
these cases corresponded to only partial openings of the active-region
magnetic fields.

---------------------------------------------------------
Title: Overview of Solar Flares
Authors: Hudson, Hugh; Fletcher, Lyndsay; Khan, Josef I.; Kosugi, Takeo
2004ASSL..314..153H Altcode:
This chapter reviews the physics of solar flares, with special emphasis
on the past decade. During this decade first Yohkoh and then TRACE
have drastically improved our observational capabilities for flares,
with contributions also from the essentially non-flare instrumentation
on SOHO and of course the ground-based observatories. In this review
we assess how these new observations have changed our understanding
of the basic physics of flares and consider the implications of these
results for future observations with FASR. The discussion emphasizes
flaring loops, flare ejecta, particle acceleration, and microflares.

---------------------------------------------------------
Title: Analysis of the impulsive phase of a solar flare at
submillimeter wavelengths
Authors: Raulin, Jean Pierre; Makhmutov, Vladimir S.; Kaufmann, Pierre;
Pacini, Alessandra Abe; Lüthi, Thomas; Hudson, Hugh S.; Gary, Dale E.
2004SoPh..223..181R Altcode:
We present a report on the strong X5.3 solar flare which occurred
on 25 August 2001, producing high-level γ-ray activity, nuclear
lines and a dramatic long-duration white-light continuum. The bulk
of millimeter radio fluxes reached a peak of ∼100 000 solar flux
units at 89.4 GHz, and a few thousands of solar flux units were
detected in the submillimeter range during the impulsive phase. In
this paper we focus on and discuss (i) the implications inferred
from high frequency radio observations during the impulsive phase;
(ii) the dynamics of the low corona active region during the impulsive
phase. In particular we found that 4-5 × 1036 accelerated
(&gt;20 keV) electrons s−1 radiating in a 1000-1100 G
region, are needed to explain the millimeter to submillimeter-wave
emissions. We present evidence that the magnetic field in the active
region was very dynamic, and that strong non-thermal processes were
triggered by the appearance of new, compact, low-lying (few thousand
kilometers) loop systems, suggesting the acceleration site(s) were
also located in the low solar atmosphere.

---------------------------------------------------------
Title: Total Solar Irradiance Variation During Rapid Sunspot Growth
Authors: Zahid, H. Jabran; Hudson, Hugh S.; Fröhlich, Claus
2004SoPh..222....1Z Altcode:
Large sunspot areas correspond to dips in the total solar irradiance
(TSI), a phenomenon associated with the local suppression of
convective energy transport in the spot region. This results in a
strong correlation between sunspot area and TSI. During the growth
phase of a sunspot other physics may affect this correlation; if the
physical growth of the sunspot resulted in surface flows affecting
the temperature, for example, we might expect to see an anomalous
variation in TSI. In this paper we study NOAA active region 8179,
in which large sunspots suddenly appeared near disk center, at a time
(March 1998) when few competing sunspots or plage regions were present
on the visible hemisphere. We find that the area/TSI correlation does
not significantly differ from the expected pattern of correlation,
a result consistent with a large thermal conductivity in solar
convection zone. In addition we have searched for a smaller-scale
effect by analyzing white-light images from MDI (the Michelson Doppler
Imager) on SOHO. A representative upper-limit energy consistent with
the images is on the order of 3×1031 ergs, assuming the
time scale of the actual spot area growth. This is of the same order
of magnitude as the buoyant energy of the spot emergence even if it
is shallow. We suggest that detailed image analyses of sunspot growth
may therefore show `transient bright rings' at a detectable level.

---------------------------------------------------------
Title: The hard X-ray spectral structure of flare ribbons
Authors: Fletcher, L.; Hudson, H. S.; Krucker, S.; Pollock, J. A.
2004AAS...204.5403F Altcode: 2004BAAS...36..758F
We examine the spatial distribution of hard X-ray spectral parameters
in flares exhibiting the classic two-ribbon structure using RHESSI
observations. The flares studied include July 15 and July 17, 2002,
and October 29, 2003. We confirm the existence of a tendency for
the localization of the hard X-ray sources into dominant bright
“footpoint” regions which do not show ribbon structure as extensive
as that seen in Hα or UV images. As a part of the study we characterize
the ribbons photometrically in the EUV as observed by TRACE, confirming
earlier results that find complicated relationships between EUV and
hard X-rays. We seek an empirical explanation for the restricted
hard X-ray footpoints in terms of a spatial analog of the well-known
“soft-hard-soft” morphology: the regions of weaker hard X-ray emission
correspond to steeper X-ray energy spectra and hence to softer electron
precipitation spectra. This relationship may be as predicted by the
1D radiation hydrodynamics models of flaring loops.

---------------------------------------------------------
Title: Remarkable Low Temperature Emission of the 4 November 2003
Limb Flare
Authors: Leibacher, J. W.; Harvey, J. W.; Kopp, G.; Hudson, H.;
GONG Team
2004AAS...204.0213L Altcode: 2004BAAS...36..669L
Strong (&gt; 1.5 times normal intensity) continuum and photospheric line
emission of the 4 November 2003 X28 flare was recorded simultaneously
by three widely separated GONG instruments. Emission was seen from
on the disk to &gt; 20" above the limb for nearly one hour, likely
making this event the longest duration white light flare observed
to date. GONG observations are one-minute duration integrations of
intensity averaged across a Lyot filter bandpass of about 90 pm FWHM
centered on the Ni I line at 676.8 nm with 2.5" instrument pixel
size. Spatial resolution is limited by diffraction and seeing to
greater than 5". Additional measurements include the Doppler shift and
strength of the spectrum line. These latter measurements indicate that
continuum and line emission contributed about equally to the observed
intensity signal. Light curves and images of the flare show a notable
two-kernel disk event starting at about 19:33 UTC followed by a much
stronger event that peaked at about 19:44. Rare, white-light prominences
were visible above the limb after 19:34. Comparison of total solar
irradiance measurements from the TIM instrument on board the SORCE
spacecraft with full-disk integrated GONG intensities shows the global
five-minute oscillation and the white light flare. The latter is much
weaker in the GONG data, suggesting that most of the TIM flare signal
arises from other, most likely shorter, wavelengths. This work
utilizes data obtained by the Global Oscillation Network Group (GONG)
Program, managed by the National Solar Observatory, which is operated
by AURA, Inc. under a cooperative agreement with the National Science
Foundation. SORCE is supported by NASA NAS5-97045

---------------------------------------------------------
Title: Nonlinear Force Free Field Models of AR 0486
Authors: McTiernan, J. M.; Hudson, H. S.; Metcalf, T. R.
2004AAS...204.0204M Altcode: 2004BAAS...36Q.668M
In this work we present nonlinear force free field (NLFFF)
extrapolations from vector magnetograms of AR0486 taken before, during
and after the X11 flare of 29 October 2003. The extrapolations will be
used to interpret the different source patterns and motions seen in
RHESSI and TRACE images of the flare. In particular we will examine
the footpoint motions observed in the flare by RHESSI, and also the
expansion and other changes in the high temperature thermal sources
visible in low energy (less than 20 keV) RHESSI images and TRACE 195
A images. This work is funded by NASA contract NAS 5-98033.

---------------------------------------------------------
Title: Variations of Solar Radius: Observations with RHESSI
Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.
2004AAS...204.3719F Altcode: 2004BAAS...36..712F
The Solar Aspect System (SAS) of the rotating (at 15 rpm) RHESSI
spacecraft has three subsystems. Each of these measures the position
of the limb by sampling the full solar chord profile with a linear
CCD using a narrow-band filter at 670 nm. With a CCD pixel size of 1.7
arcsec, the accuracy of each of the six limb positions is theoretically
better than 50 mas using four pixels at each limb. Since the launch
of RHESSI early 2002, solar limbs have been sampled with at least 100
Hz. That provides database currently containin 4 × 109
single radius measurements. The main function of SAS is to determine
the RHESSI pointing relative to Sun center. The observed precision of
this determination has a typical instantaneous (16 Hz) value of the
order of 50 mas (rms). We present initial RHESSI observations of the
radius, including signatures of oblateness and of magnetic activity
(spots and faculae).

---------------------------------------------------------
Title: Coronal Loop Oscillations and Flare Shock Waves
Authors: Hudson, H. S.; Warmuth, A.
2004AAS...204.5411H Altcode: 2004BAAS...36..759H
A preliminary statistical analysis of coronal loop oscillations
observed by TRACE shows that 12/31 cases were associated with metric
type II bursts. The timing is consistent with the idea that the loop
oscillations represent the passage of a disturbance launched by an
eruption in an active region; the GOES classifications for these
flares range from C4.2 to X20. Typically the oscillating structures
are not disrupted, implying that the disturbance has passed through
the medium. This is consistent with the Uchida interpretation of the
disturbance as a weak fast-mode blast wave propagating in the ambient
corona. We note that all twelve of the events were also associated
with CMEs.

---------------------------------------------------------
Title: Photospheric field variations during the Oct. 28 and 29
solar events
Authors: Li, Y.; Welsch, B.; Fisher, G.; Luhmann, J.; Hudson, H.
2004AGUSMSH51A..01L Altcode:
Solar photospheric magnetic field variations around the Oct. 28th
and 29th, 2003 large flares and CMEs are investigated. The essential
data for the study are the high cadence MDI full disk line of sight
magnetograms. Abrupt and permanent changes of the field strength
occur at the times of both X-flares (Oct. 28, ~10:30UT and Oct. 29,
20:45UT). GONG+ magnetograms are used to provide a possible confirmation
of the observed field changes. Velocity fields in the CME related
active region and their evolution around the time of the events are
obtained using the Local Correlation Tracking (LCT) technique on the
MDI magnetograms. Some Mees/IVM vector magnetic field data analysis
results may also likely be available.This level of observation of major
active region fields spawning superstorm conditions is unprecedented.

---------------------------------------------------------
Title: Soft X-ray analysis of a loop flare on the Sun
Authors: Khan, J. I.; Hudson, H. S.; Mouradian, Z.
2004A&A...416..323K Altcode:
We present the results of an analysis of soft X-ray images for a
solar flare which occurred on 1992 July 11. This flare, as seen
in Yohkoh Soft X-ray Telescope (SXT) images was of comparatively
simple geometry, consisting of two bright footpoints early in the
flare with a bright loop seen later in the flare. We examine how
closely this flare compares with the supposed paradigm of a confined
simple-loop flare. Closer examination of the SXT images reveals
that the flare structure consisted of at least two adjacent loops,
one much fainter than the other. We examine the brighter of the two
soft X-ray loops. The SXT images reveal an apparent slow, northward
motion of this loop (roughly transverse to its major axis). Examination
of derived emission measure and temperature images also indicate an
apparent northward motion. In addition, we find an increase in the
cross-sectional width at the top of the loop with time. Emission
measure maps derived from the SXT images also indicates an apparent
broadening of the loop-top region. We infer that the apparent northward
motion and the apparent broadening of the soft X-ray emission can be
explained in a reconnection scenario where successive magnetic field
structures do not lie in a plane but are tilted to the south of the
line of sight but with successively brightening loops oriented at less
tilted angles. Hα images for this flare reveal an evolution from a
few brilliant points to a short two-ribbon-like appearance. Comparison
of the SXT images with the Hα images shows that the Hα patches are
aligned with the footpoints of the soft X-ray loops, suggesting the
presence of a small arcade structure. There is no clear evidence for
an eruptive signature in our observations nor in reports from other
observations. The lack of an eruptive signature could suggest that
the flare may have been a confined simple-loop flare, but this is not
compelling due to a gap in the coronal observations prior to and early
in the event. Analysis of our observations indicate that the flare
exhibited characteristics suggesting that it may be better understood
as a mini-arcade flare. These results casts doubt on the validity of
the supposed paradigm of a confined simple-loop flare, at least for
this flare. They indicate that even an apparently simple-loop flare may
be considered to be a variety of arcade flare. We also find an effect
which, to our knowledge, has not been reported before: the hot flaring
regions later become cooler than the surrounding quiescent corona. That
is, the flare loops do not evolve into bright active region loops, but
into cooler loops. This may indicate an increase in the efficiency of
the cooling mechanism or a transformed equilibrium state within the
flaring loops.

---------------------------------------------------------
Title: Homologous large-scale activity in solar eruptive events of
24-26 November 2000
Authors: Chertok, I. M.; Grechnev, V. V.; Hudson, H. S.; Nitta, N. V.
2004JGRA..109.2112C Altcode:
We study large-scale activity on the solar disk associated with a
24-26 November 2000 series of six recurrent major flares and "halo"
coronal mass ejections (CMEs). The analysis is based mainly on the
SOHO/EIT data, particularly by using properly rotated difference
full-disk images with 12-min intervals at 195 Å as well as with
6-hour intervals at 171, 195, 284, and 304 Å. We demonstrate that
these eruptive events were homologous not only by their flare and
CME characteristics, as [2001] showed, but also in terms of their
large-scale CME-associated manifestations in the EUV corona. These
include long and narrow channeled dimmings, some transequatorial;
anisotropic coronal waves, propagating in a restricted angular sector;
and additional quasi-stationary emitting fronts. As a whole, in all of
these six events, the homologous CME-associated disturbances covered a
considerable portion of the solar disk. The homology tendency appears
to be due to significant disturbance, partial eruption, and relatively
fast restoration of the same large-scale structures involved in the
repeating CME events. We briefly discuss the implications of the
analysis in connection with the nature of coronal equilibrium.

---------------------------------------------------------
Title: Solar Energy Flux Variations
Authors: Hudson, Hugh S.
2004GMS...141...85H Altcode:
The chapters in this section of the monograph deal with the basic raw
material of solar variability, namely the measurements themselves. With
complete characterization of the spectral components of the irradiance,
one might imagine an easy task in putting them all together to determine
the energy flux from the Sun. As the length and depth of these chapters
shows, however, the simple characterization of a spectral irradiance at
the accuracies permitted by the available technology already becomes
a sophisticated business. The scope of the problem becomes apparent
when one realizes that the observed bolometric variability of the total
irradiance does not exceed fractions of a percent, and has well-measured
components with amplitudes below one part per million (Fröhlich)!

---------------------------------------------------------
Title: Total solar irradiance variation during rapid sunspot growth
Authors: Zahid, H. J.; Hudson, H. S.; Frohlich, C.
2004cosp...35.2769Z Altcode: 2004cosp.meet.2769Z
Large sunspot areas correspond to dips in the total solar irradiance
(TSI), a phenomenon associated with the local suppression of convective
energy transport in the spot region. During the growth of a sunspot
other physics might conceivably affect the resulting correlation
between sunspot area and TSI. We study NOAA active region 8179, in
which large sunspots suddenly appeared near disk center, at a time
(March 1998) when few competing sunspots or plage regions were present
on the visible hemisphere. We find that the area/TSI correlation does
not significantly differ from the expected pattern of correlation,
a result consistent with the expected thermal conductivity of the
solar convection zone. In addition we have searched for a smaller-scale
effect by analyzing white-light images from MDI (the Michelson Doppler
Imager) on SOHO. A representative upper-limit energy consistent with
the images is on the order of 3 X 1031 ergs, assuming the time scale
of the actual spot area growth. This is the same order of magnitude
as the buoyant energy of the spot emergence even if it is shallow. We
suggest that detailed image analyses of sunspot growth may therefore
show "transient bright rings" at a detectable level.

---------------------------------------------------------
Title: Solar Variability and its Effects on Climate. Geophysical
Monograph 141
Authors: Pap, Judit M.; Fox, Peter; Frohlich, Claus; Hudson, Hugh S.;
Kuhn, Jeffrey; McCormack, John; North, Gerald; Sprigg, William; Wu,
S. T.
2004GMS...141.....P Altcode:
This monograph presents a state-of-the-art description of the most
recent results on solar variability and its possible influence on the
Earth's climate and atmosphere. Our primary goal in doing so is to
review solar energy flux variations (both electromagnetic and particle)
and understand their relations to solar magnetic field changes and
global effects, their impact on different atmospheric layers, and—as
a collaboration of scientists working on solar-terrestrial physics—to
note unresolved questions on an important interdisciplinary area. One of the highest-level questions facing science today is whether
the Earth's atmosphere and climate system changes in a way that we
can understand and predict. The Earth's climate is the result of
a complex and incompletely understood system of external inputs and
interacting parts. Climate change can occur on various time scales as a
consequence of natural variability—including solar variability—or
anthropogenic causes, or both. The Sun's variability in the form
of sunspots and related magnetic activity has been the subject of
careful study ever since the earliest telescopic observations. High
precision photometric observations of solar-type stars clearly show that
year-to-year brightness variations connected with magnetic activity
are a widespread phenomenon among such stars. As our nearest star,
the Sun is the only star where we can observe and identify a variety of
structures and processes which lead to variations in the solar energy
output, in both radiative and particle fluxes. Studying event tiny
changes in solar energy flux variations may teach us about internal
processes taking place in the Sun's convective zone and below.

---------------------------------------------------------
Title: Preface
Authors: Pap, Judit M.; Fox, Peter; Fröhlich, Claus; Hudson, Hugh S.;
Kuhn, Jeffrey; McCormack, John; North, Gerald; Sprigg, William; Wu,
S. T.
2004GMS...141D...7P Altcode:
This monograph presents a state-of-the-art description of the most
recent results on solar variability and its possible influence on the
Earth's climate and atmosphere. Our primary goal in doing so is to
review solar energy flux variations (both electromagnetic and particle)
and understand their relations to solar magnetic field changes and
global effects, their impact on different atmospheric layers, and—as
a collaboration of scientists working on solar-terrestrial physics—to
note unresolved questions on an important interdisciplinary area. One of the highest-level questions facing science today is whether
the Earth's atmosphere and climate system changes in a way that we
can understand and predict. The Earth's climate is the result of
a complex and incompletely understood system of external inputs and
interacting parts. Climate change can occur on various time scales as a
consequence of natural variability—including solar variability—or
anthropogenic causes, or both. The Sun's variability in the form
of sunspots and related magnetic activity has been the subject of
careful study ever since the earliest telescopic observations. High
precision photometric observations of solar-type stars clearly show that
year-to-year brightness variations connected with magnetic activity
are a widespread phenomenon among such stars. As our nearest star,
the Sun is the only star where we can observe and identify a variety of
structures and processes which lead to variations in the solar energy
output, in both radiative and particle fluxes. Studying even tiny
changes in solar energy flux variations may teach us about internal
processes taking place in the Sun's convective zone and below.

---------------------------------------------------------
Title: A scenario for three "homologous" CMEs from AR 8038 in May 1997
Authors: Li, Y.; Luhmann, J. G.; Hudson, H.
2003AGUFMSH42B0522L Altcode:
The halo CME event of May 12, 1997 has been exceptionally
well-documented, in part because of the Sun-to-Earth coverage
afforded by the combination of comprehensive SOHO imaging, ground-based
observations, and WIND spacecraft in-situ measurements, but also because
it exhibited what some regard as a classic set of event signatures with
exceptional clarity: C-class flare, Halo CME, coronal double dimming,
EIT wave, type II radio burst, energetic particles, magnetic cloud, and
moderate geomagnetic storm. One problem with interpreting halo CMEs is
the head-on view. While the STEREO twin-spacecraft mission is aimed at
resolving the question of what a halo event looks like from the side,
and visualizations based on numerical simulations suggest a number of
possible interpretations, nature has provided an opportunity in the
form of nearly homologous events initiated at the same active region
AR 8038. One is at the east limb on May 5, another near the west limb
on May 16. Being shortly after the solar minimum, the Sun is very quiet
and the coronal magnetic field is simple. During the entire disk passage
of AR8038, it is the only active region present. We take advantage of
this period to make a first comparison of the three similar events at
different view angles. We also model the geometry of the CME in 3D,
and discuss improved physical parameters such as the speed.

---------------------------------------------------------
Title: Variations of Solar Radius Observed with RHESSI
Authors: Fivian, M. D.; Hudson, H. S.; Lin, R. P.
2003AGUFMSH32A1103F Altcode:
The Solar Aspect System (SAS) of the rotating (at 15 rpm) RHESSI
spacecraft has three subsystems. Each of these measures the position
of the limb by sampling the full solar chord profile with a linear
CCD using a narrow bandwidth filter at 670 nm. With a resolution of
each CCD of 1.7 arcsec/pixel, the accuracy of each of the 6 limb
positions is theoretically better than 50 mas using 4 pixels at
each limb. Since the launch of RHESSI early 2002, solar limbs are
sampled with at least 100 Hz. That provides a database of currently
4 x 109 single radius measurements. The main function of
SAS is to determine the RHESSI pointing relative to Sun center. The
observed precision of this determination has a typical instantaneous
(16 Hz) value of about 200 mas (rms). We show and discuss first results
of variations of solar radius observed with RHESSI.

---------------------------------------------------------
Title: Radiative Hydrodynamic Models of Solar White Light Flares
Authors: Allred, J. C.; Hawley, S. L.; Abbett, W. P.; Fisher, G. H.;
Hudson, H. S.; Metcalf, T. R.
2003AGUFMSH22A0175A Altcode:
We report on theoretical radiative hydrodynamic simulations of solar
white light flares. The solar atmosphere is modeled in detail from
the transition region to the photosphere. The coronal pressure and
X-ray backheating are included self-consistently. Flare heating is
assumed to be from an electron beam which is modeled for several
white light flares using data from RHESSI, TRACE and Yohkoh. We also
investigate the possibility that the 511 keV line width is produced
from a significant column depth of atmosphere at transition region
temperatures. We compare our new solar flare models to previous results,
and to models of M dwarf stellar flares.

---------------------------------------------------------
Title: Solar irradiance variation during rapid sunspot growth
Authors: Zahid, H.; Frohlich, C.; Hudson, H.
2003AGUFMSH32A1102Z Altcode:
Large sunspot areas correspond to dips in the total solar irradiance
(TSI), a phenomenon associated with the local suppression of
convective energy transport in the spot region. Observations show
a strong correlation between spot area and TSI. During the growth
phase of a sunspot, though, other physics might conceivably affect
this correlation. In this study we analyze the growth phase of NOAA
active region 8179, in which large sunspots suddenly appeared near
disk center at a time when few competing sunspots or plage regions were
present on the visible hemisphere. The spot area of AR 8179 increased
by a factor of two in less than 12 hours on March 13, 1998. The study
makes use of the continuum images from SOHO/MDI and total-irradiance
data from SOHO/VIRGO. We find that the area/TSI correlation does
not significantly differ from the expected pattern of correlation,
a result consistent with high thermal conductivity even at small
depths below the photosphere. We have also searched for localized
irradiance variations (transient bright rings) associated with the
period of rapid growth. We discuss the significance of this lack of
irradiance signatures of spot formation.

---------------------------------------------------------
Title: Gamma-ray flare occurrence patterns
Authors: Hudson, H. S.; Lin, R. P.; Smith, D. M.
2003AGUFMSH22A0168H Altcode:
As of 2003 September 4, RHESSI (the Reuven Ramaty High-Energy
Spectroscopic Imager) had obtained coverage for the entire GOES duration
(&gt;95%) for 98 M- and 6 X-class flares, and for each of these we
estimate the ratio of the 2.223 MeV line fluence to the GOES soft
X-ray fluence. All are upper limits except for one M-class event and
one X-class event. The GOES fluence is known to scale well with total
flare energy. The statistics of these observations, considering as
well the solar gamma-ray line observations from other spacecraft plus
the statistics of proton events in the heliosphere, are not consistent
with the hypothesis that ion acceleration scales proportionally with
total flare energy.

---------------------------------------------------------
Title: Homologous large-scale activity in solar eruptive events of
November 24-26, 2000
Authors: Chertok, I. M.; Grechnev, V. V.; Hudson, H. S.; Nitta, N. V.
2003AGUFMSH22A0180C Altcode:
We study large-scale activity on the solar disk associated with a
November 24-26, 2000 series of six recurrent major flares and “halo”
coronal mass ejections (CMEs). The analysis is based mainly on the
SOHO/EIT data, particularly by using properly rotated difference
full-disk images with 12-min intervals at 195Å~ as well as with
6-hour intervals at 171, 195, 284, and 304Å. We demonstrate that
these eruptive events were homologous not only by their flare and CME
characteristics, as Nitta and Hudson [2001] showed, but also in terms of
their large-scale CME-associated manifestations in the EUV corona. These
include long and narrow channeled dimmings, some transequatorial;
anisotropic coronal waves, propagating in a restricted angular sector;
and additional quasi-stationary emitting fronts. As a whole, in all of
these six events, the homologous CME-associated disturbances covered a
considerable portion of the solar disk. The homology tendency appears
to be due to significant disturbance, partial eruption and relatively
fast restoration of the same large-scale structures involved in the
repeating CME events. We briefly discuss the implications of the
analysis in connection with the coronal equilibrium as indicated by
recent TRACE observations of oscillating loop systems.

---------------------------------------------------------
Title: A catalogue of white-light flares observed by Yohkoh
Authors: Matthews, S. A.; van Driel-Gesztelyi, L.; Hudson, H. S.;
Nitta, N. V.
2003A&A...409.1107M Altcode:
The aspect camera of the Soft X-ray Telescope (SXT) on Yohkoh provided
the first systematic survey of white-light flares from an observatory in
space. The observations were made in the Fraunhofer g-band at a pixel
size of 2.46 arcsec and a typical sample interval on the order of ten
seconds. A total of 28 flares with clear white-light signatures were
detected, corresponding to GOES events down to the C7.8 level in one
case. Above the X-class threshold, all 5 events observed by SXT were
observed in white light, and the maximum average contrast observed
was 30% relative to the pre-flare continuum brightness of the flare
location. We have made comprehensive comparisons of Yohkoh soft X-ray
and hard X-ray data for this list of flares. In addition we compare
the properties of the WLF sample to a sample of 31 flares that showed
no white-light emission. These comparisons show that while white-light
continuum emission has a strong association with hard X-ray emission
it is also strongly related to coronal overpressure, as determined
from the soft X-ray spectrum, indicating a component with a thermal,
rather than non-thermal origin. Appendices are only available in
electronic form at http://www.edpsciences.org

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Title: High-Resolution Observation of the Solar Positron-Electron
Annihilation Line
Authors: Share, Gerald H.; Murphy, Ronald J.; Skibo, Jeffrey G.;
Smith, David M.; Hudson, Hugh S.; Lin, Robert P.; Shih, Albert Y.;
Dennis, Brian R.; Schwartz, Richard A.; Kozlovsky, Benzion
2003ApJ...595L..85S Altcode:
The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has
observed the positron-electron annihilation line at 511 keV produced
during the 2002 July 23 solar flare. The shape of the line is consistent
with annihilation in two vastly different solar environments. It can
be produced by formation of positronium by charge exchange in flight
with hydrogen in a quiet solar atmosphere at a temperature of ~6000
K. However, the measured upper limit to the 3γ/2γ ratio (ratio
of annihilation photons in the positronium continuum to the number
in the line) is only marginally consistent with what is calculated
for this environment. The annihilation line can also be fitted by a
thermal Gaussian having a width of 8.1+/-1.1 keV (FWHM), indicating
temperatures of ~(4-7)×105 K. The measured 3γ/2γ ratio
does not constrain the density when the annihilation takes place in such
an ionized medium, although the density must be high enough to slow
down the positrons. This would require the formation of a substantial
mass of atmosphere at transition-region temperatures during the flare.

---------------------------------------------------------
Title: RHESSI Observations of Particle Acceleration and Energy
Release in an Intense Solar Gamma-Ray Line Flare
Authors: Lin, R. P.; Krucker, S.; Hurford, G. J.; Smith, D. M.; Hudson,
H. S.; Holman, G. D.; Schwartz, R. A.; Dennis, B. R.; Share, G. H.;
Murphy, R. J.; Emslie, A. G.; Johns-Krull, C.; Vilmer, N.
2003ApJ...595L..69L Altcode:
We summarize Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) hard X-ray (HXR) and γ-ray imaging and spectroscopy
observations of the intense (X4.8) γ-ray line flare of 2002 July 23. In
the initial rise, a new type of coronal HXR source dominates that has
a steep double-power-law X-ray spectrum and no evidence of thermal
emission above 10 keV, indicating substantial electron acceleration
to tens of keV early in the flare. In the subsequent impulsive phase,
three footpoint sources with much flatter double-power-law HXR spectra
appear, together with a coronal superhot (T~40 MK) thermal source. The
north footpoint and the coronal source both move systematically to the
north-northeast at speeds up to ~50 km s-1. This footpoint's
HXR flux varies approximately with its speed, consistent with magnetic
reconnection models, provided the rate of electron acceleration
varies with the reconnection rate. The other footpoints show similar
temporal variations but do not move systematically, contrary to simple
reconnection models. The γ-ray line and continuum emissions show that
ions and electrons are accelerated to tens of MeV during the impulsive
phase. The prompt de-excitation γ-ray lines of Fe, Mg, Si, Ne, C,
and O-resolved here for the first time-show mass-dependent redshifts
of 0.1%-0.8%, implying a downward motion of accelerated protons and
α-particles along magnetic field lines that are tilted toward the
Earth by ~40°. For the first time, the positron annihilation line is
resolved, and the detailed high-resolution measurements are obtained
for the neutron-capture line. The first ever solar γ-ray line and
continuum imaging shows that the source locations for the relativistic
electron bremsstrahlung overlap the 50-100 keV HXR sources, implying
that electrons of all energies are accelerated in the same region. The
centroid of the ion-produced 2.223 MeV neutron-capture line emission,
however, is located ~20”+/-6” away, implying
that the acceleration and/or propagation of the ions must differ from
that of the electrons. Assuming that Coulomb collisions dominate the
energetic electron and ion energy losses (thick target), we estimate
that a minimum of ~2×1031 ergs is released in accelerated
&gt;~20 keV electrons during the rise phase, with ~1031
ergs in ions above 2.5 MeV nucleon-1 and about the same
in electrons above 30 keV released in the impulsive phase. Much more
energy could be in accelerated particles if their spectra extend to
lower energies.

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Title: TRACE and Yohkoh Observations of a White-Light Flare
Authors: Metcalf, Thomas R.; Alexander, David; Hudson, Hugh S.;
Longcope, Dana W.
2003ApJ...595..483M Altcode:
We present observations of a large solar white-light flare observed
on 2001 August 25, using data from the Transition Region and Coronal
Explorer (TRACE) white-light channel and Yohkoh/HXT. These emissions are
consistent with the classic type I white-light flare mechanism, and we
find that the enhanced white-light emission observed by TRACE originates
in the chromosphere and temperature minimum regions via nonequilibrium
hydrogen ionization induced by direct collisions with the electron beam
and by back-warming of the lower atmosphere. The three flare kernels
observed in hard X-rays and white light are spatially associated with
magnetic separatrices, and one of the kernels is observed to move along
a magnetic separatrix at 400 km s-1. This is evidence in
favor of particle acceleration models, which energize the electrons
via magnetic reconnection at magnetic separators.

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Title: Narrow coronal holes in Yohkoh soft X-ray images and the slow
solar wind
Authors: Arge, C. N.; Harvey, K. L.; Hudson, H. S.; Kahler, S. W.
2003AIPC..679..202A Altcode:
Soft X-ray images of the solar corona sometimes show narrow dark
features not obviously present in HE I 10830Å images. We term
these “narrow coronal holes” (NCHs). A prototype for this type of
structure crossed solar central meridian on October 29, 2001. Standard
source-surface models showed open magnetic field lines in this
feature, tending to confirm its identification as a coronal hole. The
magnetic field in this example is relatively strong (above 100 G in
the low-resolution Kitt Peak magnetograms), and the boundaries of the
open-field domain fall within the unipolar area as expected. We have
surveyed the Yohkoh SXT data for other examples of this phenomenon,
and have found several candidates. From observations of the associated
solar wind, and from modeling, we find these regions to be sources of
slow solar wind.

---------------------------------------------------------
Title: Yutaka Uchida
Authors: Hudson, Hugh S.; Kosugi, Takeo
2003PhT....56i..78H Altcode:
No abstract at ADS

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Title: Detecção da fase impulsiva de uma explosão solar gigante
até 405 GHz
Authors: Raulin, J. -P.; Makhmutov, V.; Kaufmann, P.; Pacini, A. A.;
Luethi, T.; Hudson, H. S.; Gary, D. E.; Yoshimori, M.
2003BASBr..23..179R Altcode:
A explosão ocorrida no dia 25/08/2001 foi uma das mais
intensas do presente ciclo solar em ondas de rádio de altas
frequências. Foram medidas em ondas milimétricas e submilimétricas,
aproximadamente, 105 e vários milhares de unidades de fluxo solar,
respectivamente. Apresentamos um estudo deste evento em múltiplas
frequências, desde microondas (1GHz), até ondas submilimétricas
(405 GHz) detectadas pelo Telescópio Solar para ondas Submilimétricas
(SST). Esta base de dados foi complementada utilizando-se o experimento
Yohkoh, incluindo a emissão em raios-X duros e raios-g (até 100 MeV),
e imagens em raios-X moles da região ativa envolvida. Enfocamos e
discutimos principalmente os seguintes aspectos da fase impulsiva do
evento: (i) as implicações deduzidas do espectro eletromagnético,
obtido pela primeira vez até 405 GHz; (ii) a dinâmica da região
ativa. Os resultados mostram que para explicar o espectro rádio
observado, são necessários entre 3.5×1037 e 1.5×1039 elétrons
acelerados acima de 20 keV em uma região de campo magnético entre
300 e 800 Gauss. A estimativa do fluxo de fótons que seria produzido
por estes elétrons, mostra que grande parte deles não precipitou
na baixa atmosfera. A evolução temporal da emissão em raios-X
moles revela que a configuração magnética da região ativa foi
muito dinâmica durante a fase impulsiva da explosão. Em particular,
mostramos que a produção dos elétrons altamente energéticos foi
iniciada junto com a aparição, na baixa coroa solar, de um novo
sistema compacto de estruturas magnéticas. Este fato sugere que os
locais de aceleração estão localizados na baixa atmosfera do Sol,
como resultado da interação entre o novo sistema compacto e o campo
magnético ambiente da região ativa.

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Title: RHESSI Observation of the Solar Annihilation Line
Authors: Share, Gerald H.; Murphy, R. J.; Skibo, Jeffrey G.; Smith,
David M.; Hudson, Hugh S.; Lin, Robert P.; Shih, Albert Y.; Dennis,
Brian R.; Schwartz, Richard A.
2003ICRC....6.3199S Altcode: 2003ICRC...28.3199S; 2003ICRC....6.3199M
RHESSI has observed the positron-electron annihilation line at 511 keV
produced during the 2002 July 23 solar flare. The shape of the line is
consistent with formation of positronium by charge exchange in flight
with hydrogen in a quiet solar atmosphere at a temperature of ∼ 6000
K. However, the measured upper limit to the 3γ /2γ ratio (ratio of
annihilation photons in the positronium continuum to the number in the
line) is only marginally consistent with what is calculated for this
environment. The annihilation line can also be fit by a thermal Gaussian
having a width of 8.1 ± 1.1 keV (FWHM), indicating temperatures of
∼ 4 - 7 × 105 K. This would require the formation of a substantial
mass of atmosphere at transition-region temperatures during the flare.

---------------------------------------------------------
Title: A Study of Extremely Low Density Solar Wind Detected on Three
Successive Solar Rotations
Authors: Wang, L. -H.; Lin, R. P.; Larson, D. E.; Hudson, H. S.
2003SPD....34.0611W Altcode: 2003BAAS...35..819W
Three periods of extremely low density solar wind (n &lt; 1 cm-3)
were detected by the WIND 3D plasma and Energetic Particle (3DP)
instrument on successive solar rotations, July 4, July 31 and Aug
28,1999. The measurements show that the halo electron pitch angle
distributions were far narrower than those in normal solar wind. With
the increasing distance from the Sun, adiabactic focusing narrows
the pitch angle distribution, while Coulomb collisions, which are
proportional to the electron number density, widens the pitch angle
distribution.In the low density events, fewer Coulomb collisions lead
to a more anisotropic halo electron distribution. The observations are
compared to numerical results from a 1-dimensional simulation based
on the Fokker-Planck equation. We also examine solar observations from
SOHO and YOHKOH to search for the source of the low density solar wind.

---------------------------------------------------------
Title: High-Resolution Observation of the Solar Positron-Electron
Annihilation Line
Authors: Share, G. H.; Murphy, R. J.; Skibo, J. G.; Smith, D. M.;
Hudson, H. S.; Lin, R. P.; Shih, A. Y.; Schwartz, R. A.; Kozlovsky, B.
2003SPD....34.1408S Altcode: 2003BAAS...35R.830S
RHESSI has observed the positron-electron annihilation line at 511
keV produced during the 2002 July 23 solar flare. The shape of the
line is consistent with annihilation in two vastly different solar
environments. It can be produced by formation of positronium by
charge exchange in flight with hydrogen in a quiet solar atmosphere
at a temperature of ∼ 6000 K. However, the measured upper limit to
the 3γ /2γ ratio (ratio of annihilation photons in the positronium
continuum to the number in the line) is only marginally consistent with
what is calculated for this environment. The annihilation line can also
be fit by a thermal Gaussian having a width of 8.1 +/- 1.1 keV (FWHM),
indicating temperatures of ∼ 4 - 7 x 105 K. The measured
3γ /2γ ratio does not constrain the density when the annihilation
takes place in such an ionized medium, however the density must be high
enough to slow down the positrons. This would require the formation
of a substantial mass of atmosphere at transition-region temperatures
during flares.

---------------------------------------------------------
Title: Coronal scattering as a source of flare-associated polarized
hard X-rays
Authors: Hudson, H. S.; Hurford, G. J.; Brown, J. C.
2003SoPh..214..171H Altcode:
We consider the scattering of flare-associated X-rays above 1
keV at coronal heights, particularly from regions of enhanced
density. This includes a discussion of the polarization of the
scattered X-rays. Although the scattered radiation would not be
bright by comparison with the total hard X-ray flux from a flare,
its detectability would be enhanced for events located a few degrees
behind the limb for which the dominant `footpoint' hard X-ray sources
are occulted. Thus we predict that major flares occurring beyond the
solar limb may be detectable via scattering in density enhancements
that happen to be visible above the limb, and that such sources
may be strongly polarized. Since thin-target bremsstrahlung will
generally greatly exceed the scattered thick-target flux in flare loops
themselves, these considerations apply only to coronal structures that
do not contain significant populations of non-thermal electrons.

---------------------------------------------------------
Title: The nature of impulsive solar energetic particle events
Authors: Nitta, N. V.; Hudson, H. S.; De Rosa, M. L.
2003SPD....34.1606N Altcode: 2003BAAS...35..833N
Impulsive solar energetic particle (SEP) events, as opposed to
gradual SEP events, are usually thought to originate from flares in
the well-connected regions. In order to test this idea, we compute
the solar locations of the field lines that cross the spacecraft
encountering particles from SEP events, and compare them with
the flare locations. We combine two frequently used techniques,
i.e., the ballistic approximation for the interplanetary magnetic
field and the potential-field source-surface model for the coronal
field. Such comparisons are made for selected impulsive SEP events
during 1995-2001. We check the validity of the techniques using
Yohkoh SXT and SOHO EIT images, which often show coronal holes on
the disk. Furthermore, we study the properties of the flares that are
identified with impulsive SEP events, and compare them with those of
other flares in the well-connected areas but without impulsive SEP
events. This will give us a clue as to the importance of the flare
processes relative to the magnetic field connectivity on the detection
of impulsive SEP events.

---------------------------------------------------------
Title: Thermal and Nonthermal Contributions to the Flare X-ray Flux
Authors: Dennis, B. R.; Phillips, K. J. H.; Schwartz, R. A.; Tolbert,
A. K.; Hudson, H. S.; RHESSI Team
2003SPD....34.1809D Altcode: 2003BAAS...35..841D
The relative thermal and nonthermal contributions to the total
flare energy budget are being determined through analysis of RHESSI
imaging and spectroscopy X-ray observations in the critical energy
range from a few keV to a few tens of keV. The classic ways of
differentiating between the thermal and nonthermal components -
exponential vs. power-law spectra, impulsive vs. gradually varying
flux, compact vs. extended sources - can now be combined together for
individual flares. In addition, RHESSI's sensitivity and fine energy
resolution of 1 keV FWHM allow the intensity and mean energy of the
iron-line complex between 6 and 7 keV to be measured as a function
of time. Thus, the temperature and emission measure of the thermal
plasma can be determined in a consistent manner assuming only the
abundance of iron and the thermal origin of the emission. This then
defines the thermal continuum spectrum and allows it to be cleanly
separated from any nonthermal component that may be required to fit
the measured spectrum at higher energies. Examples of such separations
of the thermal and nonthermal components will be presented for several
flares. The intensity of the iron line complex is consistent with an
iron abundance equal to the solar coronal abundance as given by Feldman
(1992), i.e., three times higher than the photospheric abundance

---------------------------------------------------------
Title: Characterization of the RHESSI 3-10 keV spectrum
Authors: Hudson, H. S.; Dennis, B. R.; Phillips, K. J. H.; Schwartz,
R. A.; Smith, D. M.
2003SPD....34.1806H Altcode: 2003BAAS...35..840H
We describe an empirical characterization of the RHESSI X-ray
spectrum in the 3-10 keV range, using a sample of flares for which
the attenuating shutters remained open. The main spectral features
comprise the continuum, which we study in two broad-band ranges (F3:
2.83-5.5 keV, and F8: 7.5-10.17 keV). The lower-energy band F3 normally
shows time profiles resembling those of the GOES hard channel, and the
F8/F3 ratio exhibits a heating/cooling variation similar to that of the
two GOES channels; in addition an impulsive component appears in the F8
channel. In between these bands lies the prominent Fe line complex. We
fit the Fe excess emission to a Gaussian, using 2-sec integrations, and
find values of sigma on the order of 0.3-0.4 keV for most of the RHESSI
detectors. This emission feature embraces the 2-1 transitions for all
of the Fe ionization states. The centroid of the Fe line feature varies
significantly in energy as the flare evolves; the time variability
resembles the F8 continuum more than the F3 continuum. We present
statistical results for these properties of the 3-10 keV spectrum,
including the systematic variation of Fe feature equivalent width.

---------------------------------------------------------
Title: Observations of CMEs associated with trans-equatorial loop
systems
Authors: Khan, J. I.; Hudson, H. S.
2003SPD....34.1004K Altcode: 2003BAAS...35Q.825K
Soft X-ray observations frequently show the existence of
trans-equatorial loop systems, often rooted near an active region. The
visibility in soft X-rays implies relatively high temperature,
a fact that has no ready explanation. The Yohkoh SXT observations
from April-May 1998 showed the persistent formation and re-formation
of trans-equatorial loops associated with NOAA AR 8210. At least four
such events happened: May 2, 6, 8, and 9. In these events the erupting
loop systems become parts of the associated CMEs. These events were
also associated with major flares and with global waves visible in
soft X-rays and type II bursts. We suggest that this morphology, even
in cases in which the trans-equatorial structure was not hot enough to
be visible in soft X-rays, could explain the occurrence of the larger
angular scales of some CMEs as opposed to their associated flares.

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Title: RHESSI and non-thermal solar physics in the IHY
Authors: Hudson, H.; Davila, J.; Dennis, B.; Emslie, G.; Lin, R.;
Ryan, J.; Share, G.
2003EAEJA.....7939H Altcode:
The signatures of non-thermal activity on the Sun - X-rays, gamma-rays,
and high-energy particles - present us with the closest possible view
of the essential physics underlying solar activity and its heliospheric
consequences. During the International Heliophysical Year (2007) we
will have a rich harvest of measurements from current sunspot maximum
from an unprecedented array of observations from space. This poster
will present the most recent observations from the newest spacecraft,
RHESSI (the Reuven Ramaty High-Energy Solar Spectroscopic Imager) in
the context of the IHY and possible future programs. The RHESSI data
discussed emphasize the gamma-ray line flare of July 23, 2002 as well
as discoveries made in the April 21, 2002 and other events.

---------------------------------------------------------
Title: X-ray and radio observations of the activation stages of an
X-class solar flare
Authors: Fárník, F.; Hudson, H. S.; Karlický, M.; Kosugi, T.
2003A&A...399.1159F Altcode:
We report interesting developments prior to the impulsive phase
of an X-class solar flare that occurred on September 24, 2001. Our
multiwavelength study makes use of X-ray data from the Yohkoh satellite,
the Ondřejov radio spectral observations in the decimetric band,
and the new Hard X-Ray Spectrometer instrument (HXRS) on board the MTI
satellite. The GOES time history of this event showed a “precursor”
phase starting as early as two hours prior to the impulsive phase,
and we have used various data sets to identify what parts of this
development could be associated with the flare itself. The most
interesting time interval was identified roughly one hour before
the main peak when an unusual drifting radio continuum was observed
together with two radio sources (at 327 and 164 MHz) in positions
corresponding to expanding loops seen in Yohkoh/SXT and SOHO/EIT images,
accompanied by a filament disappearence during the same period. Hard
X-ray observations revealed a soft spectrum that we interpret as
non-thermal, located within loop structures observed in soft X-rays
along the magnetic neutral line. The hard X-ray emission continued
for more than one hour, as observed in turn by the two spacecraft. In
the initial phase of the flare itself, the hard X-ray emission arose
in structures closely identifiable with the early soft X-ray loops,
which appeared to evolve smoothly into the post-flare loop system of
the flare maximum. The decimeter spectra showed loosely-correlated
spiky emission at frequencies consistent with the densities inferred
from soft X-rays, but with rapid drifts implying motions along field
lines. From all these data we infer that the initiation of the flare
involved non-thermal processes extending along the neutral line in
the photosphere, systematically including open magnetic field lines
as shown by the occurrence of interplanetary Type III bursts observed
by the WAVES spectrometer on board the WIND spacecraft.

---------------------------------------------------------
Title: Soft X-ray observation of a large-scale coronal wave and
its exciter
Authors: Hudson, Hugh S.; Khan, Josef I.; Lemen, James R.; Nitta,
Nariaki V.; Uchida, Yutaka
2003SoPh..212..121H Altcode:
Recent extreme ultraviolet (EUV) observations from SOHO have shown
the common occurrence of flare-associated global coronal waves
strongly correlated with metric type II bursts, and in some cases
with chromospheric Moreton waves. Until now, however, few direct soft
X-ray detections of related global coronal waves have been reported. We
have studied Yohkoh Soft X-ray Telescope (SXT) imaging observations to
understand this apparent discrepancy, and describe the problems in this
paper. We have found good X-ray evidence for a large-scale coronal wave
associated with a major flare on 6 May 1998. The earliest direct trace
of the wave motion on 6 May consisted of an expanding volume within
20 Mm (projected) of the flare-core loops, as established by loop
motions and a dimming signature. Wavefront analyses of the soft X-ray
observations point to this region as the source of the wave, which began
at the time of an early hard X-ray spike in the impulsive phase of the
flare. The emission can be seen out to a large radial distance (some
220 Mm from the flare core) by SXT, and a similar structure at a still
greater distance by EIT (the Extreme Ultraviolet Imaging Telescope) on
SOHO. The radio dynamic spectra confirm that an associated disturbance
started at a relatively high density, consistent with the X-ray
observations, prior to the metric type II burst emission onset. The
wavefront tilted away from the vertical as expected from refraction if
the Alfvén speed increases with height in the corona. From the X-ray
observations we estimate that the electron temperature in the wave,
at a distance of 120 Mm from the flare core, was on the order of 2-4
MK, consistent with a Mach number in the range 1.1-1.3.

---------------------------------------------------------
Title: Modeling large solar flares
Authors: Somov, B. V.; Kosugi, T.; Hudson, H. S.; Sakao, T.; Masuda, S.
2003AdSpR..32.2439S Altcode:
The basic ideas to model the large solar flares are reviewed and
illustrated. Some fundamental properties of potential and non-potential
fields in the solar atmosphere are recalled. In particular, we consider
a classification of the non-potential fields or, more exactly, related
electric currents, including reconnecting current layers. The so-called
'rainbow reconnection' model is presented with its properties and
predictions. This model allows us to understand main features of large
flares in terms of reconnection. We assume that in the two-ribbon
flares, like the Bastille-day flare, the magnetic separatrices are
involved in a large-scale shear photospheric flow in the presence of
reconnecting current layers generated by a converging flow.

---------------------------------------------------------
Title: Unusual large-scale flaring structure
Authors: Chertok, I. M.; Hudson, H. S.; Kahler, S. W.
2002ESASP.506..569C Altcode: 2002ESPM...10..569C; 2002svco.conf..569C
The Yohkoh/SXT data of August 21, 1999 displayed a spectacular
transient brightening of a large-scale double whip-like structure
extending through the corona to a length greater than 950 Mm. The
transient originated at a relatively small middle-latitude soft X-ray
bright point (XBP), which was also visible in the EUV range and had a
small underlying Hα plage with a bipolar magnetic configuration. The
structure developed in the high-temperature (&gt;2.5 MK) soft X-ray
emission to both sides of the XBP during a few tens of minutes and could
be recognized for about two hours. The observations suggest sudden
energy release during the interaction of the magnetic structures of
the XBP and the filament channel.

---------------------------------------------------------
Title: Two wave morphologies in SOHO/EIT - EIT waves and Moreton waves
Authors: Biesecker, D. A.; Thompson, B. J.; Hudson, H. S.; Warmuth,
A.; White, S.
2002AGUFMSH52A0450B Altcode:
EIT waves are global waves observed to propagate across quiet
coronal field regions in the SOHO/EIT data. The waves are initiated in
association with other transient activity. The literature now contains
many references to EIT waves and observers have published observations
of associated waves at a variety of wavelengths. We show in this
poster that there is confusion in the literature as to what an EIT
wave is. We use YOHKOH SXT, Nobeyama Radioheliograph and He I 10830A
observations to show that the EIT instrument observes waves with two
distinct properties and morphologies. These two morphologies correspond
to the classical Moreton wave and to what we call the EIT wave. The
Moreton waves in EIT appear as a sharp, bright feature, travelling at
super-Alfvenic velocities. The EIT waves instead appear as a diffuse,
faint feature, moving at relatively slower velocities. The EIT waves
appear much more frequently than the Moreton waves and Moreton waves
are usually seen in tandem with EIT waves. Both types of waves have
been modelled by various researchers as MHD waves.

---------------------------------------------------------
Title: Spectral variations of flare hard X-rays
Authors: Hudson, H. S.; Fárník, F.
2002ESASP.506..261H Altcode: 2002ESPM...10..261H; 2002svco.conf..261H
We report on a set of eight solar hard X-ray bursts corresponding
to M-class flares and well-observed with the RHESSI and HXRS
instruments. We find, as previously reported, that the impulsive
phases of these events invariably show the soft-hard-soft spectral
pattern (SHS) in which the power-law spectral index anticorrelates
with the flux. The RHESSI data have higher spectral resolution but
give the same result. The SHS pattern occurs in the spectral domain
above the typical break energy of double power-law fits as well as
below. Some of the events also show other patterns, including the
soft-hard-harder evolutions seen in eruptive events. We discuss the
physical interpretation of the spectral behavior.

---------------------------------------------------------
Title: Mapping RHESSI footpoints with potential-field models
Authors: Fletcher, L.; Hudson, H. S.; Metcalf, T. R.
2002AGUFMSH52A0452F Altcode:
RHESSI hard X-ray observations help us to identify the locations of
magnetically conjugate footpoints, and to study their apparent motions
during the evolution of the impulsive phase of a flare. We put this
information into the context of an M-class flare that occurred 2002
March 14 01:50 UT (GOES peak time) at S12, E23 (NOAA region 9866) by
making potential-field mappings of the coronal magnetic structure. In
principle the hard X-ray sources (plus the mapping) constrain the
site of magnetic energy release, and the maps reveal the location
of the stored energy. The RHESSI source centroids can be determined
to better than 1” (rms) for an M-class flare. This analysis is an
exploration of the feasibility of such an approach, since full success
would require understanding the magnetic restructuring in detail. If
suitable X-class RHESSI flares occur we will be able to present data
with better precision.

---------------------------------------------------------
Title: Intense Flares Without Solar Energetic Particle Events
Authors: Nitta, N. V.; Cliver, E. W.; Hudson, H. S.
2002AGUFMSH61A0437N Altcode:
We study favorably located (western hemisphere) X-class flares that were
not associated with solar energetic particle (SEP) events. Three of the
four such flares that occurred during the present cycle lacked coronal
mass ejections (CMEs), consistent with the current paradigm. Soft X-ray
data for these three events show either outward moving loops above
the main flare loop or a much slower rise of the flare loop itself,
as predicted in bipolar reconnection models. However, unlike fully
eruptive events, the outward moving loops appear to stop at a certain
distance. We speculate that they are held back by overlying magnetic
field, as implied in soft X-ray images. The flare that was associated
with a CME (but not an SEP event) produced metric and decametric type
III bursts, but those without CMEs did not. Other characteristics for
the flares not associated with SEP events include relatively short
decay times of hard X-ray emission at 30--50 keV. We are extending
our study to include additional (somewhat smaller) events to try to
identify key parameters that keep intense flares from erupting and
accompanying SEP events.

---------------------------------------------------------
Title: Five years of Yohkoh science nuggets
Authors: Hudson, H. S.; McKenzie, D. E.; Nitta, N. V.
2002AGUFMSH52A0453H Altcode:
The Yohkoh "science nuggets", weekly Web-based reports emphasizing
current Yohkoh observations, began October 24, 1997. Many writers
(the SXT "chief observers" in particular) contributed, and over the
years a characteristic style developed: these were educational pieces
rather than public-relations puffs, and they each attempted to describe
a particular item for a technically savvy non-specialist. In this poster
we summarize the nugget philosophy and point out some of our favorites,
such as the remarkable "triple jet." This and many other interesting
observations have not yet otherwise been published. Since December 2001
we have gradually broadened our science basis to include SOHO, TRACE,
and now RHESSI input. The topic index lists more than 60 categories,
and there is a general search facility. We present this poster partly
to encourage discussion of the future development of the series.

---------------------------------------------------------
Title: Coronal blast waves detected in soft X-rays
Authors: Khan, J.; Hudson, H. S.; Nitta, N. V.
2002AGUFMSH52A0487K Altcode:
Several examples of rather clear X-ray detection of coronal blast
waves have now been reported (e.g., Khan and Aurass, A&amp;A 383,
1018, 2002). Interestingly, most of them have been reported from
two particular active regions -- NOAA 8100 (November 1997) and 8210
(April-May 1998). The soft X-ray observations in some cases have
high time resolution and the ability to look close to the core of the
flare responsible for the wave. We summarize the observations to date,
emphasizing the distinction between ejecta (magnetic loops) and freely
running waves. Related observations now exist at metric and centimetric
wavelengths, in the EUV, and in chromospheric lines (H-alpha and He
10830), and we describe the observational relationships among these
different observations.

---------------------------------------------------------
Title: Spectral and Spatial Variations of Flare Hard X-ray Footpoints
Authors: Fletcher, L.; Hudson, H. S.
2002SoPh..210..307F Altcode:
In a sample of strong RHESSI M-class flares we have made a study of
the relationship between the `hardness' of the HXR spectrum and the
intensity in the 30-50 keV energy range. In all events we find clear
evidence for a `soft-hard-soft' pattern of correlation between hardness
and flux, on time scales as short as 10 s. We investigate whether or
not this pattern is intrinsic to the acceleration mechanism. The RHESSI
images in this energy range are dominated by footpoint brightenings,
and we have searched for a correlation between footpoint separation
velocity and spectral hardness, to be compared qualitatively with
theoretical flare models. We find quite systematic footpoint motions,
and also note that episodes in which footpoint separation varies
rapidly often correspond with episodes of significant change in the
flare spectral index, though not as the simplest flare models would
predict. We report also on one of our events, on 14 March 2002, which
exhibits highly sheared HXR footpoint ribbons extending over a scale
of 100 arc sec. For this flare we find a correlation between footpoint
motion and hard X-ray flux.

---------------------------------------------------------
Title: Magnetic Reconnection Scenario of the Bastille Day 2000 Flare
Authors: Somov, Boris V.; Kosugi, Takeo; Hudson, Hugh S.; Sakao,
Taro; Masuda, Satoshi
2002ApJ...579..863S Altcode:
On the basis of Yohkoh Hard X-Ray Telescope data and the magnetograms
taken by the SOHO Michelson Doppler Imager and the Solar Magnetic
Field Telescope at Huairou Solar Observing Station, we suggest an
interpretation of the well-observed “Bastille Day 2000” flare. The
large-scale structure and dynamics of the flare, as seen in hard X-rays,
can be explained in terms of the three-dimensional reconnection at
a separator in the corona. More specifically, we suggest that before
occurrence of two-ribbon flares with significant decrease of a distance
between the hard X-ray (HXR) footpoints, like the Bastille Day flare,
the bases of magnetic field separatrices are moved by the large-scale
photospheric flows of two types. First, the shear flows, which are
parallel to the photospheric neutral line, increase the length of field
lines in the corona and produce an excess of magnetic energy. Second,
the converging flows, i.e., the flows directed to the neutral line,
create the preflare current layers in the corona and provide an excess
of energy sufficient to produce a large flare. During the flare, both
excesses of magnetic energy are released completely or partially. In
the Bastille Day flare, the model describes two kinds of apparent
motions of the HXR kernels. One is an increase of a distance between
the flare ribbons in which the HXR kernels appear. The effect results
from fast reconnection in a coronal current layer. The second effect is
a decrease of the distance between the kernels moving to each other as a
result of relaxation of magnetic tensions generated by the photospheric
shear flows.

---------------------------------------------------------
Title: The Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI)
Authors: Lin, R. P.; Dennis, B. R.; Hurford, G. J.; Smith, D. M.;
Zehnder, A.; Harvey, P. R.; Curtis, D. W.; Pankow, D.; Turin, P.;
Bester, M.; Csillaghy, A.; Lewis, M.; Madden, N.; van Beek, H. F.;
Appleby, M.; Raudorf, T.; McTiernan, J.; Ramaty, R.; Schmahl, E.;
Schwartz, R.; Krucker, S.; Abiad, R.; Quinn, T.; Berg, P.; Hashii,
M.; Sterling, R.; Jackson, R.; Pratt, R.; Campbell, R. D.; Malone,
D.; Landis, D.; Barrington-Leigh, C. P.; Slassi-Sennou, S.; Cork, C.;
Clark, D.; Amato, D.; Orwig, L.; Boyle, R.; Banks, I. S.; Shirey,
K.; Tolbert, A. K.; Zarro, D.; Snow, F.; Thomsen, K.; Henneck,
R.; Mchedlishvili, A.; Ming, P.; Fivian, M.; Jordan, John; Wanner,
Richard; Crubb, Jerry; Preble, J.; Matranga, M.; Benz, A.; Hudson,
H.; Canfield, R. C.; Holman, G. D.; Crannell, C.; Kosugi, T.; Emslie,
A. G.; Vilmer, N.; Brown, J. C.; Johns-Krull, C.; Aschwanden, M.;
Metcalf, T.; Conway, A.
2002SoPh..210....3L Altcode:
RHESSI is the sixth in the NASA line of Small Explorer (SMEX)
missions and the first managed in the Principal Investigator mode,
where the PI is responsible for all aspects of the mission except
the launch vehicle. RHESSI is designed to investigate particle
acceleration and energy release in solar flares, through imaging and
spectroscopy of hard X-ray/gamma-ray continua emitted by energetic
electrons, and of gamma-ray lines produced by energetic ions. The
single instrument consists of an imager, made up of nine bi-grid
rotating modulation collimators (RMCs), in front of a spectrometer
with nine cryogenically-cooled germanium detectors (GeDs), one behind
each RMC. It provides the first high-resolution hard X-ray imaging
spectroscopy, the first high-resolution gamma-ray line spectroscopy,
and the first imaging above 100 keV including the first imaging of
gamma-ray lines. The spatial resolution is as fine as ∼ 2.3 arc sec
with a full-Sun (≳ 1°) field of view, and the spectral resolution
is ∼ 1-10 keV FWHM over the energy range from soft X-rays (3 keV)
to gamma-rays (17 MeV). An automated shutter system allows a wide
dynamic range (&gt;107) of flare intensities to be handled
without instrument saturation. Data for every photon is stored in a
solid-state memory and telemetered to the ground, thus allowing for
versatile data analysis keyed to specific science objectives. The
spin-stabilized (∼ 15 rpm) spacecraft is Sun-pointing to within ∼
0.2° and operates autonomously. RHESSI was launched on 5 February
2002, into a nearly circular, 38° inclination, 600-km altitude orbit
and began observations a week later. The mission is operated from
Berkeley using a dedicated 11-m antenna for telemetry reception and
command uplinks. All data and analysis software are made freely and
immediately available to the scientific community.

---------------------------------------------------------
Title: Solar Submillimeter and Gamma-Ray Burst Emission
Authors: Kaufmann, P.; Raulin, J. -P.; Melo, A. M.; Correia, E.; Costa,
J. E. R.; de Castro, C. G. Giménez; Silva, A. V. R.; Yoshimori, M.;
Hudson, H. S.; Gan, W. Q.; Gary, D. E.; Gallagher, P. T.; Levato,
H.; Marun, A.; Rovira, M.
2002ApJ...574.1059K Altcode:
Solar flare emission was measured at 212 GHz in the submillimeter
range by the Submillimeter Solar Telescope in the 1.2-18 GHz microwave
range by the Owens Valley Solar Array and in the gamma-ray energy
range (continuum) by experiments on board the Yohkoh (&gt;1.2 MeV)
and Shenzhou 2 (&gt;0.2 MeV) satellites. At the burst onset, the
submillimeter and microwave time profiles were well correlated with
gamma rays to the limit of the temporal resolution (&lt;=10 s). At 212
GHz, fast pulses (&lt;1 s), defined as time structures in excess of the
bulk emission, were identified as the flux increased. Their spatial
positions were scattered by tens of arcseconds with respect to the
main burst emission position. Correlation of submillimeter emission
with gamma-ray fast time structures shorter than 500 ms is suggested
at the gamma-ray maximum. The time variation of the rate of occurrence
of the submillimeter rapid pulses was remarkably well correlated with
gamma-ray intensities in the energy range (&gt;1.2 MeV), attaining
nearly 50 pulses per minute at the maximum. These results suggest that
gamma rays might be the response to multiple rapid pulses at 212 GHz
and might be produced at different sites within the flaring region.

---------------------------------------------------------
Title: Boundary Structures and Changes in Long-lived Coronal Holes
Authors: Kahler, S. W.; Hudson, H. S.
2002ApJ...574..467K Altcode:
We report a first systematic morphological study of the boundaries
of coronal holes (CHs) as viewed in soft X-ray images from the
Yohkoh Soft X-Ray Telescope. The special emphasis is on long-lived
(several rotations) CHs that extend from the solar polar regions to
midlatitudes. As shown earlier, such equatorward extensions tend to
show rigid, rather than differential, rotation. Magnetic reconnection
must occur at the “closing” boundary, in such a case, to maintain the
CH integrity. We find three kinds of CH boundaries in the soft X-ray
observations. The majority are generally ragged and not sharply defined;
we also find smooth boundaries to occur near a matching-polarity active
region (AR), and loopy boundaries to occur near an opposite-polarity
AR. In this latter case the loops clearly do not extend far enough to
reach another CH but instead end in normal corona. The CH boundaries
evolve slowly, and neither large-scale transient X-ray events nor
coronal bright points appeared significant factors in long-term CH
boundary development. No direct evidence for magnetic reconnection is
seen. We compare these results with those expected from current models,
derived largely from considerations of heliospheric conditions rather
than the detailed appearance of CHs in the low corona.

---------------------------------------------------------
Title: X-ray and radio observations in the initial development of
an X-class solar flare
Authors: Fárnik, F.; Karlický, M.; Hudson, H.; Kosugi, T.
2002ESASP.508..441F Altcode: 2002soho...11..441F
The EIT/SOHO and SXT/YOHKOH plasma ejecta accompanied by an unusual
drifting radio continuum and early hard X-ray emission observed
prior to the impulsive phase of the September 24, 2001, X-class
flare are analyzed. The paper presents some of the first reported
observations from the new Hard X-ray Spectrometer instrument (HXRS),
as well as imaging data from YOHKOH plus radio spectral observations
in the decimetric band. The early hard X-ray observations revealed a
soft spectrum that we interpret as non-thermal, located within loop
structures observed in soft X-rays along the magnetic neutral line. The
hard X-ray emission continued for more than one hour. In the initial
phase of the flare, the hard X-ray emission arose in structures closely
identifiable with the early soft X-ray loops, which appeared to evolve
smoothly into the post-flare loop system of the flare maximum. At this
time the decimeter spectra showed loosely-correlated spiky emission at
frequencies consistent with the densities inferred from soft X-rays,
but with rapid drifts implying motions along magnetic field lines.

---------------------------------------------------------
Title: Coronal holes as seen in soft X-rays by Yohkoh
Authors: Hudson, H. S.
2002ESASP.508..341H Altcode: 2002soho...11..341H
This paper reviews literature on the 10-plus years of Yohkoh soft
X-ray observations of coronal holes (CHs), which span almost half
a Hale cycle. They thus extend and generalize the results of Skylab,
providing better sampling and duration. These modern X-ray data confirm
the tendency towards rigid rotation of equatorward extensions of polar
holes, but show no clear sign of the mechanism of magnetic reconnection
that makes this possible. Coronal-hole boundary evolution does not
seem to depend upon transient magnetic activity such as X-ray bright
points or large-scale arcade events associated with transient coronal
holes. The morphology of the coronal holes as seen in X-rays differs
from that seen in He I λ10830: X-rays generally do not do so well with
the polar holes or other holes near the limb, because of foreground
confusion, but they show narrow features better.

---------------------------------------------------------
Title: Simultaneous Observation of a Moreton Wave on 1997 November
3 in Hα and Soft X-Rays
Authors: Narukage, N.; Hudson, H. S.; Morimoto, T.; Akiyama, S.;
Kitai, R.; Kurokawa, H.; Shibata, K.
2002ApJ...572L.109N Altcode:
We report the observation of a Moreton wave in Hα (line center and
+/-0.8 Å) with the Flare Monitoring Telescope at the Hida Observatory
of Kyoto University at 4:36-4:41 UT on 1997 November 3. The same
region (NOAA Active Region 8100) was simultaneously observed in soft
X-rays with the soft X-ray telescope on board Yohkoh, and a wavelike
disturbance (“X-ray wave”) was also found. The position of the wave
front as well as the direction of propagation of the X-ray wave roughly
agree with those of the Moreton wave. The propagation speeds of the
Moreton wave and the X-ray wave are about 490+/-40 and 630+/-100 km
s-1, respectively. Assuming that the X-ray wave is an MHD
fast-mode shock, we can estimate the propagation speed of the shock, on
the basis of MHD shock theory and the observed soft X-ray intensities
ahead of and behind the X-ray wave front. The estimated fast shock
speed is 400-760 km s-1, which is in rough agreement with
the observed propagation speed of the X-ray wave. The fast-mode Mach
number of the X-ray wave is also estimated to be about 1.15-1.25. These
results suggest that the X-ray wave is a weak MHD fast-mode shock
propagating through the corona and hence is the coronal counterpart
of the Moreton wave.

---------------------------------------------------------
Title: Imaging Spectroscopy of the February 20, 2002 flare: RHESSI
Observations
Authors: Krucker, S.; Lin, R. P.; Caspi, A.; Hudson, H.; Schwartz,
R. S.; Johns-Krull, C. M.; RHESSI Team
2002AAS...200.7607K Altcode: 2002BAAS...34..776K
First RHESSI imaging spectroscopy results of X-ray bursts from solar
flares are presented. In a first step, different spatial features
such as footpoints, loops in-between footpoints, etc. are identified
in the X-ray images. By reconstructing images at different energies
(with a spectral resolution down to 1 keV) the X-ray photon spectrum
for each of the spatial features can be extracted independently of each
other. Here, images at high cadence (4 seconds) are analyzed allowing
to follow the temporal evolution of the spectra of different spatial
features. The results from imaging spectroscopy of individual spatial
features are compared with the total spectrum and the differences are
discussed. For the event of February 20, 11:05 UT, in-situ observations
at 1 AU of 1-300 keV electron taken by the 3DP instrument on the WIND
spacecraft show an impulsive electron event of solar origin. The
electron onset times at different energies reveal that the solar
release time of these escaping electrons coincides with the HXR burst
seen with RHESSI. Therefore, a direct comparison of the different X-ray
photon spectra and the electron spectrum observed at 1 AU is possible.

---------------------------------------------------------
Title: Investigation of the Sources of Irradiance Variation on the
Sun (ISIS)
Authors: LaBonte, B. J.; Bernasconi, P. N.; Rust, D.; Foukal, P.;
Hudson, H.; Spruit, H.
2002AAS...200.5608L Altcode: 2002BAAS...34..736L
There is a persistent correlation of the longterm climate change and
solar irradiance. ISIS is designed to understand the physical basis of
this correlation. ISIS combines an innovative bolometric imager and a
multiband CCD imager. The bolometric imager has uniform response from
200 nm to 3000 nm, spatial resolution &lt; 5 arcseconds, and precision
of &lt; 0.1% in a one minute integration. The multiband imager records
ultraviolet irradiance variation in the band from 200 to 350 nm,
measures photospheric temperature structure, and provides chromospheric
structure in Ca II K and H-alpha, with spatial resolution &lt;1.0
arcsecond. Designed for flight on the Solar Dynamics Observatory,
ISIS will provide the comprehensive photometric measurements needed
to characterize the irradiance variation from identifiable structures
and challenge theoretical models of convection and the solar dynamo.

---------------------------------------------------------
Title: Coronal Dimming Associated with Coronal Mass Ejections
Authors: Nitta, N. V.; Hudson, H. S.; Newmark, J. S.
2002AAS...200.2905N Altcode: 2002BAAS...34..681N
Observations from Yohkoh SXT have identified different patterns
of coronal dimming around the times of coronal mass ejections
(CMEs), but the mapping between the dimming and the CME is still
not well understood. There is always a possibility that the observed
dimming may simply reflect cooling rather than mass depletion due to
ejection. Empirically, dimming seen in SOHO EIT images appears to be
more intimately associated with CMEs, although EIT images with narrower
sensitivities should be more susceptible to changes in temperature. In
this work, we compare SXT and EIT images systematically for various
patterns of dimming, in order to understand possibly different origins
of CMEs. We use new calibrations for both sets of images that allow us
to more accurately estimate the changes of temperature/density that
account for the observed level of dimming. We concentrate on events
with good coverage to study the time dependence of coronal dimming at
different temperatures.

---------------------------------------------------------
Title: Soft-hard-soft spectral evolution observed at HESSI resolution
Authors: Hudson, H. S.; Dennis, B. R.; Krucker, S.; Lin, R. P.; Sato,
J.; Schwartz, R. A.; Smith, D. M.
2002AAS...200.6905H Altcode: 2002BAAS...34..758H
Hard X-ray bursts from solar flares show a strong correlation between
spectral index and flux level in the 20-100 keV hard X-ray range. Prior
to HESSI this pattern could mainly be studied only with scintillation
counters, at relatively low resolution. HESSI has observed more than
ten M-class flares, and we will report how this morphology appears
at ~1-keV resolution for these events. Specifically, we address the
question of whether the time evolution of the break energy in a double
power-law fit can play a role in defining the soft-hard-soft morphology.

---------------------------------------------------------
Title: Hard X-ray and White Light Observations of the August 25,
2001 X Flare
Authors: Metcalf, T. R.; Alexander, D.; Hudson, H. S.; Longcope, D.;
Myers, D.
2002AAS...200.6803M Altcode: 2002BAAS...34..755M
An X5.3 flare occurred about 16:31 UT on 2001 August 25 and was well
observed by the Yohkoh and TRACE spacecraft. The flare showed gamma-ray
emission, nuclear lines, and was a dramatic white light flare seen in
TRACE data. A preliminary analysis of the hard X-ray images from the
Yohkoh/HXT instrument shows two clear footpoints and a moving HXR
source in this very energetic flare. The moving hard X-ray source
appears to move along a magnetic separatrix at 400 km/sec. We will
discuss the hard X-ray and white light structure of this flare and
discuss the energetics and possible mechanisms for the formation of
the TRACE white light emission.

---------------------------------------------------------
Title: Solar Disappearing Filament Inside a Coronal Hole
Authors: Chertok, I. M.; Obridko, E. I.; Mogilevsky, V. N.; Shilova,
N. S.; Hudson, H. S.
2002ApJ...567.1225C Altcode:
Based on Yohkoh/SXT, SOHO/EIT images and movies, as well as on Hα,
He I 10830 Å heliograms and other relevant data, we analyze an event
of 1999 December 28, which is interesting in at least two aspects. (1)
A major horseshoe-shaped Hα filament appeared to be located within a
large transequatorial coronal hole (CH) in the eastern hemisphere. (2)
This filament subsequently disappeared, with its eruption combined
with a number of dynamic phenomena, including large-scale ones. The
probable location of the filament inside the CH was confirmed in detail
by calculations of the open field regions and, for the first time,
the quasi-separatrix layers in the global solar magnetic field. The
filament eruption was accompanied by significant evolution of the soft
X-ray and EUV-emitting structures inside the CH as well as by a coronal
mass ejection. The analysis indicates that CHs need not have the simple
and uniform structure normally assumed and can sometimes contain local
areas with low-altitude closed magnetic fields. It demonstrates also
that the erupting filament inside the CH was a part of a much more
global, evolving magnetic structure associated with activity extending
through at least the entire eastern half of the disk.

---------------------------------------------------------
Title: Energetic particles HXR emission as a diagnostic for energy
release
Authors: Hudson, Hugh
2002ocnd.confE..15H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: RHESSI and TRACE Observations of an X-class Flare
Authors: Hudson, H.; Dennis, B.; Gallagher, P.; Krucker, S.; Reeves,
K.; Warren, H.
2002cosp...34E3101H Altcode: 2002cosp.meetE3101H
RHESSI and TRACE both obtained excellent observations of an X1.5 flare
on April 21, 2002. In this paper we provide an overview of the flare
and discuss the high- energy imaging and spectra in detail. The TRACE
images in the 195A passband (Fe XII and FeXXIV) reveal this flare to
have a spiky arcade with post-flare flow field in the "supra-arcade
downflow" pattern discovered by Yohkoh. Below the spikes, but above
the FeXII loops, TRACE observes a region with complex motions and fine
structure. We confirm with RHESSI that this region has an elevated
temperature and discuss the transition between thermal and non-thermal
sources. RHESSI also observes footpoint emission distributed along
the flare ribbons.

---------------------------------------------------------
Title: Multi-Wavelength Observation of A Moreton Wave on November
3, 1997
Authors: Narukage, N.; Shibata, K.; Hudson, H. S.; Eto, S.; Isobe,
H.; Asai, A.; Morimoto, T.; Kozu, H.; Ishii, T. T.; Akiyama, S.;
Kitai, R.; Kurokawa, H.
2002mwoc.conf..295N Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Multi-Wavelength Observations of Yohkoh White-Light Flares
Authors: Matthews, S. A.; van Driel-Gesztelyi, L.; Hudson, H. S.;
Nitta, N. V.
2002mwoc.conf..289M Altcode:
The problem of accounting for the continuum emission that is observed
in solar flares is still one which is largely unresolved. These
white-light flares place severe constraints on the energy requirements
and transport mechanisms operating in the flare, raising the question
of whether partial or total in-situ heating is required to account for
this deep atmospheric heating. Since it is widely believed that the
energy release in solar flares occurs in the corona and that energy is
then transported to the low chromosphere where the optical emission is
produced, most attempts to explain the origin of white-light flares
have centred on canonical mechanisms. However, it has become clear
that the spatial and temporal correspondence between white-light and
HXR is not one-to one. In order to further our understanding of these
events we study the temporal and spatial relationships between emission
in the visible, SXR and HXR regimes in all of the white-light flares
observed by Yohkoh prior to the failure of the Soft X-ray Telescope
(SXT) Aspect Camera in 1992; a total of approximately 30 events.

---------------------------------------------------------
Title: Simultaneous observations of Moreton waves in Hα and Soft
X-ray
Authors: Narukage, N.; Hudson, H.; Morimoto, T.; Kitai, R.; Kurokawa,
H.; Shibata, K.
2002cosp...34E1337N Altcode: 2002cosp.meetE1337N
Moreton waves are flare-associated waves observed to propagate
across the solar disk in H (Moreton, 1960). Such waves have been
identified as the intersections of a coronal fast-mode shock fronts
and the chromosphere (Uchida, 1967). We report the two observations of
Moreton waves in H (line center and +/- 0.8 A) with the Flare Monitoring
Telescope (FMT) at the Hida Observatory of Kyoto University. The both
events were simultaneously observed in soft X-rays with the Soft X-ray
Telescope (SXT) on board Yohkoh, and wave-like disturbances ("X-ray
wave") were also found. One event occurred in solar-disk on November 3,
1997, the other near solar limb on March 3, 2000. Assuming that the
X-ray waves are the MHD fast shocks, we can estimate the propagation
speeds of the shocks, based on the MHD shock theory and the observed
soft X-ray intensities ahead and behind the X-ray wave fronts. It is
found that the estimated fast shock speeds are in rough agreement with
the observed propagation speeds of the X-ray waves. The fast mode Mach
numbers of the X-ray waves are also estimated. These results suggest
that the X-ray waves are MHD fast shocks propagating through the corona
and hence are the coronal counterparts of the Moreton waves.

---------------------------------------------------------
Title: Observations of Moreton Waves and EIT Waves
Authors: Shibata, K.; Eto, S.; Narukage, N.; Isobe, H.; Morimoto,
T.; Kozu, H.; Asai, A.; Ishii, T.; Akiyama, S.; Ueno, S.; Kitai, R.;
Kurokawa, H.; Yashiro, S.; Thompson, B. J.; Wang, T.; Hudson, H. S.
2002mwoc.conf..279S Altcode:
The Moreton wave is a flare-associated wave observed in H alpha, and
is now established to be a fast mode MHD shock emitted from the flare,
but the physical mechanism to create the wave is still puzzling. On
the other hand, the EIT wave is a newly discovered flare-associated
wave observed in EUV with the Extreme ultraviolet Imaging Telescope
(EIT) aboard SOHO, and in this case, not only its origin but also
its physical property are both puzzling. We study the relationship
of these two flare-associated waves, Moreton waves and EIT waves, by
analyzing 4 events observed on Nov. 3 and 4, 1997, Aug. 8, 1998, and
Mar. 3, 2000 (Narukage et al. 2001). The Moreton waves were observed
in Ha, Ha+0.8A and Ha-0.8A with the Flare Monitoring Telescope (FMT)
at the Hida Observatory of Kyoto University, while the EIT waves were
observed with SOHO/EIT. In the typical case associated with an X-class
flare in AR 8100 on 4 November 1997 (Eto et al. 2001) the propagation
speeds of the Moreton wave and the EIT wave were approximately 780
km/s and 200 km/s respectively. The data on speed and location show
clearly that the Moreton wave differs physically from the EIT wave in
this case. The detailed analyses of the other events (Nov. 3, 1997,
Aug. 8, 1998, and Mar. 3, 2000) will also be presented, with Yohkoh/SXT
data in the lucky case.

---------------------------------------------------------
Title: Modeling big flares: Principles and practice
Authors: Somov, B.; Kosugi, T.; Hudson, H.; Sakao, T.; Masuda, S.
2002cosp...34E..43S Altcode: 2002cosp.meetE..43S
The basic ideas, which are under current use to model the large-scale
solar flares, are reviewed and illustrated. The so-called `rainbow
reconnection' model is applied to the observations of flares with
the HXT on board Yohkoh, the MDI instrument on the SOHO, the TRACE
satellite, and the Solar Magnetic Field Telescope (SMFT) of the
Beijing Astronomical Observatory. This allows us to improve a theory of
large solar flare (Somov, B., Cosmic Plasma Physics, 2000, Dordrect:
Kluwer). In particular, the famous Bastille day 2000 flare is studied
and interpreted. It is shown that the main large-scale structure and
dynamics of this flare can be explained in terms of the collisionless
3D reconnection.

---------------------------------------------------------
Title: CMEs: How do the puzzle pieces fit together?
Authors: Cliver, E. W.; Hudson, H. S.
2002JASTP..64..231C Altcode: 2002JATP...64..231C
This review consists of questions to participants in the S-RAMP
Symposium (S3) on CMEs and Coronal Holes, as well as to a few others,
and their responses in a “town meeting” format (originally conducted
on Hugh Hudson's website). Here we deal only with CMEs. The questions we
ask aim at probing the weaknesses of existing models and highlighting
controversies, thereby providing guidance toward a more complete view
of solar eruptions. Topics covered include: the “solar flare myth”,
flux ropes, new phenomena (EIT waves, dimmings, global brightenings),
helicity and sigmoids, and transequatorial loops (as sources of
CMEs). Although this is a review, we're more concerned here with
what is not known than what is already agreed upon. We asked people
to speculate freely in advance of the observational, analytical, and
theoretical work that will provide definitive answers-this is not the
standard Scientific Method at work!

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Title: A Rapidly Moving Hard X-Ray Source in a CME
Authors: Hudson, H. S.
2002mwoc.conf..379H Altcode:
We have observed a high-speed coronal ejection in hard X-rays,
detectable to an altitude of some 2 times 105 km in the
Yohkoh 33-53 keV energy bands. Simultaneous imaging at 17 and 34 GHz
from the Nobeyama Radio Observatory shows complex moving features
simultaneous with the ejection, including a compact source that
we identify with the rapid X-ray source motion. The hard X-ray and
microwave observations agree upon ejection velocities in the vicinity
of 1000 km s-1. The hard X-ray sources also corresponded
in position angle to a bright coronal mass ejection detected about 15
minutes later, and to both fast-drift and slow-drift radio bursts in the
decimeter-meter bands. Other components of coronal hard X-ray emission
were also detected, including an extended long-duration event with a
hard spectrum. We suggest that a major eruptive flare occurred in NOAA
region 9415, approximately 26circ beyond the W limb at the
time of the event. Estimating a source density of 4 times 109
cm-3 from the compact source observed at 17 GHz, we find
a total electron number (&gt; 20 keV) of approximately N20
~2.5 times 1035 for the compact part of the source.

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Title: Origin and development of transient coronal holes
Authors: Kahler, S. W.; Hudson, H. S.
2001JGR...10629239K Altcode:
Solar transient coronal holes (TCHs) are short-lived (&lt;=2 days)
regions of dimmed X-ray intensity sometimes observed in association
with coronal mass ejections. These features, first discovered from
Skylab observations, can occur in magnetic unipolar regions on either
side of the X-ray posteruptive arcades. They have been suggested as
the magnetically open footpoints of associated transient flux ropes
observed at 1 AU. We have used images from the Yohkoh Soft X-ray
Telescope (SXT) to study the development of 19 TCH events obtained in a
survey of 9 years of Yohkoh observations. We find that the boundaries
of the TCHs are never static. The boundaries closer to the magnetic
neutral line generally move away from it as the closed-loop X-ray
arcades expand. In addition, previously closed coronal loops at the
ends of the arcades often continue to expand and open on the outer
boundaries of the TCHs. These processes typically last for hours. The
arcade brightenings do not extend into the full areas of the TCHs. The
TCHs tend to disappear only by a net contraction of the boundaries,
rather than by brightening within their boundaries. The location of a
TCH appears to coincide with a large-scale curvature of the magnetic
neutral line or the occurrence of a nearby active region at one end of
the coronal eruption. This distinguishes the formation of TCHs from the
arcade development, suggesting that there is no requirement for a pair
of TCHs or even any TCH to be formed in an eruptive event. The moving
magnetic boundaries, uniformly dark interiors, and short lifetimes of
TCHs pose significant problems for the interpretation that TCHs are
footpoints of interplanetary magnetic flux ropes.

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Title: High-Energy Aspects of CMEs Associated with X-Class Flares
in the Present Cycle
Authors: Nitta, N. V.; Hudson, H. S.
2001AGUFMSH32B..04N Altcode:
We review the properties of X-class flares as observed by Yohkoh in
the present solar cycle. More than a half of about four dozen X-class
flares were caught by Yohkoh from an early phase. Most of these flares
have durations not as long as long decay events (LDEs), and yet their
association with coronal mass ejections (CMEs) is very high. They often
correspond to extended or halo events. The flares not associated with
CMEs do not show ejecta in soft X-rays. The flare core usually shows
compact morphology. Within the sensitivity of the Yohkoh Hard X-ray
Spectrometer, the hard X-ray spectra extend to the MeV range only
in 20% of these flares, and their temporal variations are typically
soft-hard-soft. Concerning their association with interplanetary proton
events at 20 MeV, not all the proton-associated flares are associated
with CMEs or located close to the well-connected longitudes. We plan
to incorporate analysis of additional data such as metric/kilometric
radio spectra to study when and where the shocks form.

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Title: A Hard X-ray Two-Ribbon Flare Observed with Yohkoh/HXT
Authors: Masuda, S.; Kosugi, T.; Hudson, H. S.
2001SoPh..204...55M Altcode:
The Yohkoh hard X-ray telescope (HXT) observed hard X-rays from the
impulsive phase of a long-duration event (LDE) occurring on 14 July
2000. The Yohkoh soft X-ray telescope (SXT) and other instruments
observed a large arcade, with width and length ∼30 000 km and
∼120 000 km, respectively. In hard X-rays, for the first time,
a two-ribbon structure was clearly observed in the energy range above
30 keV. This result suggests that electrons are in fact accelerated in
the whole system of this arcade, not merely in a particular dominant
loop. We analyzed the motions of bright kernels in the two hard X-ray
ribbons in detail. Assuming these bright kernels to be footpoints of
newly reconnected loops, we infer from their motions that the loops
reconnecting early are highly sheared, while the loops reconnecting
later are less sheared. We have also analyzed the hard X-ray spectra
of the two ribbons independently. At the outer edge of a ribbon,
the spectrum tends to be harder than that in the inner edge. This
suggests that higher-energy electrons precipitate at the footpoints
of outer loops and lower ones do at those of inner loops. We discuss
what kind of model can support this tendency.

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Title: The Magnetic Structure and Generation of EUV Flare Ribbons
Authors: Fletcher, L.; Hudson, H.
2001SoPh..204...69F Altcode:
The `ribbons' of two-ribbon flares show complicated patterns reflecting
the linkages of coronal magnetic field lines through the lower solar
atmosphere. We describe the morphology of the EUV ribbons of the July
14, 2000 flare, as seen in SOHO, TRACE, and Yohkoh data, from this
point of view. A successful co-alignment of the TRACE, SOHO/MDI and
Yohkoh/HXT data has allowed us to locate the EUV ribbon positions on
the underlying field to within ∼ 2”, and thus to investigate the
relationship between the ribbons and the field, and also the sites
of electron precipitation. We have also made a determination of the
longitudinal magnetic flux involved in the flare reconnection event,
an important parameter in flare energetic considerations. There are
several respects in which the observations differ from what would be
expected in the commonly-adopted models for flares. Firstly, the flare
ribbons differ in fine structure from the (line-of-sight) magnetic
field patterns underlying them, apparently propagating through regions
of very weak and probably mixed polarity. Secondly, the ribbons split
or bifurcate. Thirdly, the amount of line-of-sight flux passed over by
the ribbons in the negative and positive fields is not equal. Fourthly,
the strongest hard X-ray sources are observed to originate in stronger
field regions. Based on a comparison between HXT and EUV time-profiles
we suggest that emission in the EUV ribbons is caused by electron
bombardment of the lower atmosphere, supporting the hypothesis that
flare ribbons map out the chromospheric footpoints of magnetic field
lines newly linked by reconnection. We describe the interpretation of
our observations within the standard model, and the implications for
the distribution of magnetic fields in this active region.

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Title: Numerical Modelling of Trapped Electrons in an Expanding
Solar Loop
Authors: Fletcher, L.; Hudson, H.
2001AGUFMSH42A0777F Altcode:
Recent observations with the Yohkoh Hard X-ray telescope and the
Nobeyama Radioheliograph [Hudson et al., 2001] have shown a moving
hard X-ray coronal source, associated in space and time with moving
microwave and decimeter sources. Because of the electron energies
involved in producing these emissions, we hypothesize that the
radiation indicates the presence of a coronal population of high
energy electrons, trapped in a magnetic loop which is expanding
outward. The expansion will lead to a `betatron' deceleration effect,
which, combined with the normal Coulomb scattering and energy losses,
will lead to an evolution of the particle energy density, spectrum and
pitch angle distribution. We model this process using a stochastic
test particle simulation. &gt;http://isass1.solar.isas.ac.jp/
~hudson/drafts/apr18.pdf&lt;/a&gt;

---------------------------------------------------------
Title: Hard X-rays Associated with CMEs
Authors: Hudson, H. S.
2001AGUFMSH32B..03H Altcode:
In principle we can use hard X-ray bremsstrahlung (&gt; few keV)
to study the non-thermal tail of the electron distribution function
in the solar corona. This approach is less sensitive than the
techniques of radio astronomy, because of the low emissivity and low
detection sensitivity for hard X-rays, but observations of major
events are now possible; HESSI should greatly improve the breadth
of such observations. These highly energetic flare events commonly
are associated with CMEs. The view offered by such observations
complements the radio techniques in interesting ways. This talk
reviews data from several coronal hard X-ray events observed by the
Yohkoh Hard X-ray Telescope and other instruments in the context of
the radio observations.

---------------------------------------------------------
Title: Hard X-Radiation from a Fast Coronal Ejection
Authors: Hudson, H. S.; Kosugi, T.; Nitta, N. V.; Shimojo, M.
2001ApJ...561L.211H Altcode:
We have observed a high-speed coronal ejection in hard X-rays,
detectable to an altitude of some 2×105 km in the Yohkoh
23-53 keV energy bands. Simultaneous imaging at 17 and 34 GHz from the
Nobeyama radioheliograph shows complex moving features simultaneous with
the ejection, including a compact source that we identify with the rapid
X-ray source motion. The hard X-ray and microwave observations agree on
ejection velocities in the vicinity of 1000 km s-1. The hard
X-ray sources also corresponded in position angle to a bright coronal
mass ejection (CME) detected about 15 minutes later and temporally to
both fast-drift and slow-drift radio bursts in the decimeter-meter
bands. Other components of coronal hard X-ray emission were also
detected, including an extended long-duration event with a nonthermal
spectrum. We suggest that a major eruptive flare occurred in NOAA Active
Region 9415, approximately 26° beyond the west limb at the time of the
event. Estimating a source density of 4×109 cm-3
from the compact source observed at 17 GHz, we find a total electron
number (&gt;20 keV) of approximately N20~1.3×1036
for the compact part of the source. We infer that these electrons
were trapped in expanding loops forming a part of the CME and may have
contributed substantial pressure within these loops.

---------------------------------------------------------
Title: Observing coronal mass ejections without coronagraphs
Authors: Hudson, H. S.; Cliver, E. W.
2001JGR...10625199H Altcode:
A coronal mass ejection (CME), strictly speaking, is a phenomenon
observed via a white-light coronal imager. In addition to
coronagraphs, a wide variety of other instruments provide independent
observations of CMEs, in regimes ranging from the chromosphere to
interplanetary space. In this paper we list the most important of these
noncoronagraphic signatures, many of which had been known even before
CMEs were first identified in coronagraph observations about 30 years
ago. We summarize the new aspects of CMEs discovered in the past several
years, primarily with instruments on the Yohkoh and SOHO satellites. We
emphasize the need for detailed statistically based comparisons
between SOHO CMEs and their noncoronagraphic manifestations. We discuss
how the various aspects of CMEs fit into the current standard model
(sigmoids, flux rope, double dimming, arcade). While a class of CMEs
follows this pattern, it does not appear to work for all events. In
particular, some CMEs involve extended dimming regions and erupting
transequatorial X-ray loops, indicating a more complex geometry than
a simple bipolar magnetic configuration.

---------------------------------------------------------
Title: Recurrent flare/CME events from an emerging flux region
Authors: Nitta, Nariaki V.; Hudson, Hugh S.
2001GeoRL..28.3801N Altcode:
We report on six recurrent ‘halo’ coronal mass ejections (CMEs) that
occurred (in November 2000) during a 60-hour period in clear association
with major flares in an active region on the solar disk. The region was
undergoing dynamic restructuring due to flux emergence. The flares were
not long-decay events (LDEs) in terms of soft X-ray light curves and
morphologies, although, in the impulsive phase, they produced ejections
in soft X-rays that are characteristic of CMEs. We do not detect global
changes in EUV and X-ray full-disk images prior to these flares. We
suggest that emerging magnetic flux in the core of an active region
may be responsible for the occurrence of such repeated flare/CME events.

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Title: Chromospheric Damping of Alfvén Waves
Authors: De Pontieu, B.; Martens, P. C. H.; Hudson, H. S.
2001ApJ...558..859D Altcode:
We analytically study the damping of Alfvén mode oscillations in the
chromosphere and in coronal loops. In the partially ionized chromosphere
the dominant damping process of Alfvén waves is due to collisions
between ions and neutrals. We calculate the damping time for Alfvén
waves of a given frequency, propagating through model chromospheres
of various solar structures such as active region plage, quiet sun,
and the penumbra and umbra of sunspots. For a given wave frequency,
the maximum damping always occurs at temperature minimum heights and
in the coldest structure(s), i.e., the umbra of sunspots. Energy
dissipation due to ion-neutral damping of Alfvén waves with an
energy flux of 107 ergs cm-3 s- 1 can
play a considerable role in the energy balance of umbrae, quiet sun,
and plage for Alfvén wave periods of the order, respectively, 50,
5, and 0.5 s. We also consider Alfvén waves in coronal loops and the
leakage of wave energy through the footpoints. We assume a three-layer
model of coronal loops with constant Alfvén speed vA
(and no damping) in the corona, vA varying exponentially
with height in the dissipative chromosphere, and vA again
constant in the photosphere at the end of the loop. We find an exact
analytical solution in the chromospheric part. Using these solutions, we
estimate the leakage of wave energy from the coronal volume through the
footpoint regions of the loop and find that the presence of a moderate
amount of chromospheric damping can enhance the footpoint leakage. We
apply this result to determine the damping time of standing waves in
coronal loops. The enhanced footpoint leakage also has implications
for theories of coronal heating based on resonant absorption. Finally,
we find exact expressions for the damping of Alfvén waves launched
in the photosphere and upward propagating through the chromosphere
and into the corona. The partially ionized chromosphere presents an
effective barrier for upward propagating Alfvén waves with periods
less than a few seconds.

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Title: Hard X-rays from Slow Flares
Authors: Hudson, H. S.; McKenzie, D. E.
2001EP&S...53..581H Altcode:
"Slow LDEs" are those for which the rise phase is slow, as well as
the decay phase. Such flares follow the Neupert effect, which implies
that the non-thermal energy release has a similar relationship to
heating as in a normal impulsive flare. Based on a sample of 53
slow LDEs during the first nine years of Yohkoh observations, we
find 19 for which substantial overlap occurs with BATSE hard X-ray
observations. These events tend strongly to have extended hard X-ray
emission even though their hard X-ray emission does not tend to be
"impulsive" in the sense of rapid variation. The hard X-ray fluences
for these 19 events correlate with the soft X-ray peak fluxes, implying
strong non-thermal particle acceleration even for these relatively slow
energy-release rates. These events often correspond to the occurrence of
"supra-arcade downflows," a phenomenon consistent with the classical
reconnection model for gradual-phase flare energy release. This
correspondence suggests a close relationship, not depending strongly
upon time scale, between large-scale reconnection and the acceleration
of non-thermal electrons.

---------------------------------------------------------
Title: Energetics of an Active Region Observed from Helium-Like
Sulphur Lines
Authors: Watanabe, Tetsuya; Sterling, Alphonse C.; Hudson, Hugh S.;
Harra, Louise K.
2001SoPh..201...71W Altcode:
We report temperature diagnostics derived from helium-like ions of
sulphur for an active region NOAA 7978 obtained with Bragg Crystal
Spectrometer (BCS) on board the Yohkoh satellite. For the same
region we estimate conductive flux downward to the chromosphere by
the Coronal Diagnostic Spectrometer (CDS) on board the Solar and
Heliospheric Observatory (SOHO) satellite. This region appeared as a
region of soft X-ray enhancement in May 1996, underwent a period of
enhanced activity coinciding with flux emergence between 6 July and
12 July, and then continued to exist in a nearly flareless state for
several solar rotations until November 1996. Energy balance of the
non-flaring active region is basically consistent with a model of
an arcade of coronal loops having an average loop-top temperature of
4×106 K. Energy from flare activity during a period of flux
emergence is comparable to the energy requirements of the non-flaring
active region. However, the non-flaring energy is roughly constant
for the subsequent solar rotations following the birth of the active
region even after the flare activity essentially subsided. Energy
partition between flare activity and steady active-region heating
thus varies significantly over the lifetime of the active region,
and active-region emission cannot always be identified with flaring.

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Title: Downflows and structure above LDE arcades: Possible signatures
of reconnection?
Authors: McKenzie, D. E.; Hudson, H. S.
2001EP&S...53..577M Altcode:
We will introduce a set of observations made during the decay phase of
long duration event (LDE) flares on the Sun. In a number of events,
the soft X-ray images from Yohkoh SXT indicate a downward directed
flow field in the region immediately above the flare arcade. These
are tentatively identified as evidence of reconnection outflows. The
prototypical event is the M5 flare of 20 January 1999, presented by
McKenzie and Hudson (1999); since the time that paper was completed,
sixteen more events have been found in the interval April 1998 to
February 2000. As in the 20 January event, the speeds of downflow are
35-500 km s-1, lower than both the freefall speed and the
typically assumed Alfvén speed. The motion is evidenced by both dark
and bright (i.e., X-ray emitting) features, some of which may have a
looplike morphology; no cool counterparts have been detected in Hα
or EUV observations. Movies depicting some of the LDE downflows were
presented at this meeting; these also appear in the journal Solar
Physics (McKenzie, 2000), since they cannot be presented in these
Proceedings.

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Title: Onset of the Magnetic Explosion in Solar Flares and Coronal
Mass Ejections
Authors: Moore, Ronald L.; Sterling, Alphonse C.; Hudson, Hugh S.;
Lemen, James R.
2001ApJ...552..833M Altcode:
We present observations of the magnetic field configuration and its
transformation in six solar eruptive events that show good agreement
with the standard bipolar model for eruptive flares. The observations
are X-ray images from the Yohkoh soft X-ray telescope (SXT) and
magnetograms from Kitt Peak National Solar Observatory, interpreted
together with the 1-8 Å X-ray flux observed by GOES. The observations
yield the following interpretation. (1) Each event is a magnetic
explosion that occurs in an initially closed single bipole in which the
core field is sheared and twisted in the shape of a sigmoid, having an
oppositely curved elbow on each end. The arms of the opposite elbows are
sheared past each other so that they overlap and are crossed low above
the neutral line in the middle of the bipole. The elbows and arms seen
in the SXT images are illuminated strands of the sigmoidal core field,
which is a continuum of sheared/twisted field that fills these strands
as well as the space between and around them. (2) Although four of
the explosions are ejective (appearing to blow open the bipole) and
two are confined (appearing to be arrested within the closed bipole),
all six begin the same way. In the SXT images, the explosion begins
with brightening and expansion of the two elbows together with the
appearance of short bright sheared loops low over the neutral line
under the crossed arms and, rising up from the crossed arms, long
strands connecting the far ends of the elbows. (3) All six events are
single-bipole events in that during the onset and early development
of the explosion they show no evidence for reconnection between the
exploding bipole and any surrounding magnetic fields. We conclude that
in each of our events the magnetic explosion was unleashed by runaway
tether-cutting via implosive/explosive reconnection in the middle of the
sigmoid, as in the standard model. The similarity of the onsets of the
two confined explosions to the onsets of the four ejective explosions
and their agreement with the model indicate that runaway reconnection
inside a sheared core field can begin whether or not a separate system
of overlying fields, or the structure of the bipole itself, allows the
explosion to be ejective. Because this internal reconnection apparently
begins at the very start of the sigmoid eruption and grows in step
with the explosion, we infer that this reconnection is essential for
the onset and growth of the magnetic explosion in eruptive flares and
coronal mass ejections.

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Title: The Physical Nature of the Loop-Top X-Ray Sources in the
Gradual Phase of Solar Flares
Authors: Nitta, Nariaki V.; Sato, Jun; Hudson, Hugh S.
2001ApJ...552..821N Altcode:
We have analyzed Yohkoh soft and hard X-ray images of 36 flares,
primarily to study the loop-top source that often prevails in these
wavelengths during and following the impulsive phase. There are
typically two patterns for the location of the low-energy (15-30 keV)
hard X-ray (HXR) source with respect to the soft X-ray (SXR) loop. In
a quarter of the flares, the HXR source lies in an extended structure
separate from the brightest SXR loop. In other flares, the HXR source
appears to be part of the same bipolar structure as the SXR loop,
but its centroid is often displaced from the SXR loop-top source. The
fact that the HXR source is not cospatial with the SXR source may
reflect the presence of a distinct hotter structure. According to
Yohkoh X-ray emission-line spectroscopy, the ~20 MK plasma accounts
for only a fraction of the HXR counts. The temperature maps obtained
from the SXR broadband photometry occasionally reveal high-temperature
areas outside the bright loop, but they also tend to be displaced from
the HXR source, indicating that they do not represent the superhot
(&gt;~30 MK) plasma. We discuss possible distributions of plasma of
different temperatures that could be consistent with the data.

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Title: Ribbons and field at high resolution
Authors: Fletcher, L.; Hudson, H. S.
2001AGUSM..SH31D10F Altcode:
The TRACE data let us define the geometry of flare ribbons with great
precision, as for example in the Bastille Day 2000 flare. We study the
UV and EUV ribbon motions for this event in comparison with the loops
seen by SXT and in the TRACE high-temperature response, and compare
these also with the photospheric magnetic field as observed by MDI. In
this work we assume the standard model for large-scale coronal magnetic
reconnection as a source of flare energy, and search for a correlation
between footpoint locations, magnetic field strength and flare energy
release as measured by hard X-ray emission.

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Title: A Search for Discrete Changes in Coronal Hole Boundaries
Authors: Kahler, S. W.; Hudson, H. S.
2001AGUSM..SH51C05K Altcode:
Coronal hole boundaries separate the large-scale open and closed
magnetic fields of the solar corona. Mid-latitude coronal holes
often last for several rotations, maintain their structure and
rotate more slowly than photospheric magnetic structures such as
active regions. Thus it is assumed that magnetic reconnection between
open and closed field lines must occur at the boundaries of holes to
maintain their shapes and rotation rates. We use Yohkoh SXT difference
images to study the changes in coronal hole boundaries as long and
short-lived holes cross the disk near central meridian. Specifically we
are interested in learning whether the reconnection proceeds smoothly or
in a stepwise fashion. We focus on the equatorial extensions of polar
coronal holes observed in 1992 and in 2000. We use similar techniques
to study the evolution of the boundaries of transient coronal holes.

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Title: Automated Search for Limb-Occulted Flares
Authors: Freeland, S.; Hudson, H.
2001AGUSM..SP51A07F Altcode:
Limb-occulted flares give instruments with limited spatial resolution
or limited dynamic range an opportunity to study coronal processes
without the competition of bright sources in the lower atmosphere,
for example at the footpoints of coronal loops. We have created an
automated search procedure for Yohkoh soft X-ray and hard X-ray data
and have tested it for the 1999 data, finding a total of 59 candidate
events that occurred quite near the limb. The procedure produces a Web
page for each candidate event, including images and lightcurves plus a
set of parameters intended to guide the distinction between front-side
and back-side events. The initial application of the search will be
to develop a comprehensive list of coronal hard X-ray events from the
first 11 years of Yohkoh observations.

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Title: Coronal Structure from the Yohkoh Perspective
Authors: Hudson, H.
2000SPD....31.1302H Altcode: 2000BAAS...32..845H
The soft X-ray observations from Yohkoh delineate the higher-temperature
regions of the solar corona. In general the large-scale corona generally
appears diffuse outside of coronal holes and active regions. However
trans-equatorial loop structures often appear, and appear to be
especially prominent during the current rise to maximum.

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Title: Electron Trapping and Precipitation in Asymmetric Solar
Flare Loops
Authors: Aschwanden, M. J.; Fletcher, L.; Sakao, T.; Kosugi, T.;
Hudson, H.
2000IAUS..195..375A Altcode:
Acceleration, propagation, and energy loss of particles energized in
solar flares cannot be studied separately because their radiative
signatures observed in the form of hard X-ray bremsstrahlung or
radio gyrosynchrotron emission represent a convolution of all these
processes. We analyze hard X-ray emission from solar flares using
a kinematic model that includes free-streaming electrons (having an
energy-dependent time-of-flight delay) as well as temporarily trapped
electrons (which are pitch-angle scattered by Coulomb collisional
scattering) to determine various physical parameters (trapping times,
flux asymmetry, loss-cone angles, magnetic mirror ratios) in flare
loops with asymmetric magnetic fields.

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Title: Chromospheric Damping of Alfvén Waves
Authors: De Pontieu, B.; Martens, P. C. H.; Hudson, H. S.
2000SPD....31.0131D Altcode: 2000BAAS...32..806D
We study the damping of Alfvén mode oscillations on coronal loops and
in the chromosphere. First we consider damping of standing waves on
coronal loops, such as those observed in the aftermath of a flare with
the Transition Region and Coronal Explorer (TRACE). We calculate the
leakage of wave energy from the coronal volume through the footpoints
of a coronal loop, assuming constant Alfvén speed vA in
the corona and vA varying exponentially with height in the
photosphere/chromosphere at both ends of the loop. We study analytically
the influence of chromospheric damping of standing waves on a coronal
loop and find that, for a moderate amount of chromospheric damping,
the footpoint leakage can be enhanced. The damping in the partially
ionized chromosphere is mostly due to collisions between ions and
neutrals. In a second part we calculate the damping time for Alfvén
waves of a given frequency, propagating through (model) chromospheres
of various solar structures such as active region plage, quiet sun
and the penumbra and umbra of sunspots. For a given wave frequency,
the maximum damping always occurs at temperature minimum heights
and in the coldest structure(s), i.e. the umbra of a sunspot. Energy
dissipation due to ion-neutral damping of Alfvén waves could play a
considerable role in the energy balance of umbrae, quiet sun and plage
for wave periods of the order, respectively, 100, 10 and 1 s.

---------------------------------------------------------
Title: Max Millennium/Whole Sun Month Observations of a Sigmoid Region
(AR 8668)
Authors: Zarro, D. M.; Canfield, R. C.; Nitta, N.; Myers, D. C.;
Gregory, S. E.; Qiu, J.; Alexander, D.; Hudson, H. S.; Thompson,
B. J.; LaBonte, B. J.
2000SPD....31.0236Z Altcode: 2000BAAS...32..817Z
We report on observations of a sigmoidal region AR 8668 obtained
during the Whole Sun Month #3 campaign and Max Millennium Coordinated
Observing Program #2. The observations pertain to the period 1999
August 16-17 during which several GOES B and C class flares occurred
in AR 8668. Near simultaneous observations were obtained by SOHO (EIT
195 Angstroms/ and MDI full-disk magnetograms), TRACE 171 Angstroms/,
Yohkoh SXT, Big Bear (Hα ), and Mees (IVM vector magnetograms). The
multi-wavelength nature of these data, combined with their overlapping
spatial and temporal coverages, provide a unique opportunity to study
the magnetic topology and flaring evolution of twisted flux structures
associated with sigmoids. An objective of this study is to co-align
images and magnetograms obtained before and during the observed flares,
and compare the results with inferences from the topological model of
Titov and Demoulin, A&amp;A 351, 707 (1999). We will present examples
of these coalignments and identify sites of magnetic energy release
that are associated with topological features (e.g. separatrices)
predicted by this model.

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Title: Observations of the 24 September 1997 Coronal Flare Waves
Authors: Thompson, B. J.; Reynolds, B.; Aurass, H.; Gopalswamy, N.;
Gurman, J. B.; Hudson, H. S.; Martin, S. F.; St. Cyr, O. C.
2000SoPh..193..161T Altcode:
We report coincident observations of coronal and chromospheric `flare
wave' transients in association with a flare, large-scale coronal
dimming, metric radio activity and a coronal mass ejection. The two
separate eruptions occurring on 24 September 1997 originate in the
same active region and display similar morphological features. The
first wave transient was observed in EUV and Hα data, corresponding
to a wave disturbance in both the chromosphere and the solar corona,
ranging from 250 to approaching 1000 km s−1 at different
times and locations along the wavefront. The sharp wavefront had a
similar extent and location in both the EUV and Hα data. The data did
not show clear evidence of a driver, however. Both events display a
coronal EUV dimming which is typically used as an indicator of a coronal
mass ejection in the inner corona. White-light coronagraph observations
indicate that the first event was accompanied by an observable coronal
mass ejection while the second event did not have clear evidence of a
CME. Both eruptions were accompanied by metric type II radio bursts
propagating at speeds in the range of 500-750 km s−1,
and neither had accompanying interplanetary type II activity. The
timing and location of the flare waves appear to indicate an origin
with the flaring region, but several signatures associated with coronal
mass ejections indicate that the development of the CME may occur in
concert with the development of the flare wave.

---------------------------------------------------------
Title: Homologous sudden disappearances of transequatorial
interconnecting loops in the solar corona
Authors: Khan, Josef I.; Hudson, Hugh S.
2000GeoRL..27.1083K Altcode:
We have found a remarkable sequence of homologous disappearances of
transequatorial X-ray loops linking active regions. Each disappearance
was closely associated with a major flare and coronal mass ejection
(CME). In each case the flarings precede the disappearances and
the CMEs. Mass estimates for the X-ray loops are similar to CME
masses. This, the timing of the disappearances, their morphology,
and the homology of the events in the sequence, provide direct
evidence for a new class of CME origins in the low corona. We also
briefly report observations of features which we infer to be the soft
X-ray counterparts of shock waves emanating from the flare region. The
inferred shocks appeared to play a vital role in the disappearances. Our
results suggest that flare-generated shock waves may destabilize large
transequatorial loops, causing them to erupt.

---------------------------------------------------------
Title: Implosions in Coronal Transients
Authors: Hudson, H. S.
2000ApJ...531L..75H Altcode:
Coronal events such as flares or coronal mass ejections derive their
energy from the energy stored locally in the magnetic field. This leads
to the conjecture that a magnetic implosion must occur simultaneously
with the energy release. The site of the implosion would show the
location of preflare energy storage, and its detection should have
a high priority. The Transition Region and Coronal Explorer EUV
observations, for example, have sufficient resolution to show the
geometry of a flare implosion by following the motions of tracers in
the images.

---------------------------------------------------------
Title: Yohkoh SXT and SOHO EIT Observations of Sigmoid-to-Arcade
Evolution of Structures Associated with Halo Coronal Mass Ejections
Authors: Sterling, Alphonse C.; Hudson, Hugh S.; Thompson, Barbara J.;
Zarro, Dominic M.
2000ApJ...532..628S Altcode:
A subset of the solar-disk counterparts to halo coronal mass ejections
(CMEs) displays an evolution in soft X-rays (SXR) that is characterized
by a preflare S-shaped structure, dubbed a “sigmoid,” which evolves
into a postflare cusp or arcade. We examine the morphological properties
of the evolution of sigmoids into cusps and arcades for four such
regions associated with SXR flares, using the Soft X-Ray Telescope
(SXT) on the Yohkoh satellite and the EUV Imaging Telescope (EIT) on
the Solar and Heliospheric Observatory (SOHO) satellite. Most of our
EIT observations are with the 1.5 MK 195 Å Fe XII channel. At most,
there is only a weak counterpart to the SXR sigmoid in the preflare 195
Å EUV images, indicating that the preflare sigmoid has a temperature
greater than 1.5 MK. While more identifiable than in the 195 Å channel,
a clear preflare sigmoid is also not observed in the 2.0 MK EIT 284 Å
Fe XV channel. During the time of the flare, however, an EUV sigmoid
brightens near the location of the SXR preflare sigmoid. Initially
the SXR sigmoid lies along a magnetic neutral line. As the SXR flare
progresses, new field lines appear with orientation normal to the
neutral line and with footpoints rooted in regions of opposite polarity;
these footpoints are different from those of the preflare sigmoid. The
cusp structures in SXRs develop from these newly ignited field lines. In
EIT images, the EUV sigmoid broadens as the flare progresses, forming
an arcade beneath the SXR cusp. Our findings are consistent with a
standard picture in which the origin of the flare and CME is caused by
the eruption of a filament-like feature, with the stretching of field
lines producing a cusp. We infer that the cusp-producing fields may
be overlying the sigmoid fields in the preflare phase, but we do not
directly observe such preflare overlying fields.

---------------------------------------------------------
Title: Solar Activity and the Formation of Coronal Holes
Authors: Harvey, Karen L.; Hudson, Hugh S.
2000AdSpR..25.1735H Altcode:
We describe the evolution of a complex of active regions belonging
to cycle 22 and how its interaction with two new-cycle (23) regions
resulted in the formation of several isolated coronal holes, in the
development of the large coronal hole extending from the north polar
hole observed in late August 1996, and in significant changes in both
polar coronal holes

---------------------------------------------------------
Title: Hard X-rays from "Slow LDEs"
Authors: Hudson, H. S.; McKenzie, D. E.
2000ASPC..206..221H Altcode: 2000hesp.conf..221H
No abstract at ADS

---------------------------------------------------------
Title: Yohkoh Observations of White-Light Flares
Authors: Matthews, S.; van Driel-Gesztelyi, L.; Hudson, H.; Nitta, N.
2000ASPC..206..239M Altcode: 2000hesp.conf..239M
No abstract at ADS

---------------------------------------------------------
Title: The Sun as an X-Ray Star. II. Using theYohkoh/Soft X-Ray
Telescope-derived Solar Emission Measure versus Temperature to
Interpret Stellar X-Ray Observations
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H.
2000ApJ...528..537P Altcode:
This paper is the second of a project dedicated to using solar
Yohkoh/SXT data as a guide and a template to interpret data on
stellar coronae. In the light of the large differences in scope
and approach between solar and stellar studies, we have developed a
method to translate Yohkoh/SXT data of the whole solar corona into
stellar-like data, i.e., to put them in the same format and context
as the stellar ones. First from the Yohkoh/SXT images we derive
the whole-Sun X-ray emission measure versus temperature [EM(T)],
in the range 105.5-108 K, during the specific
observation. Then, we synthesize the solar X-ray spectrum; finally,
we fold the spectrum through the instrumental response of nonsolar
X-ray observatories, for instance, ROSAT/PSPC and ASCA/SIS. Finally,
we analyze such solar coronal data in the same band and with the
same methods used for stellar observations, allowing a direct
and homogeneous comparison with them. In this paper we present in
detail our method and, as an example of results, we show and discuss
EM(T) and stellar-like spectra for three phases of the solar cycle:
maximum, intermediate phase, and minimum. The total amount and the
distribution of the emission measure change dramatically during the
cycle, in particular at temperatures above 106 K. We also
show the EM(T) of the whole solar corona during a large flare. The
ROSAT/PSPC- and ASCA/SIS-like X-ray spectra of the Sun as a star
that we obtain are discussed in the context of stellar coronal
physics. The Sun's coronal total luminosity in the ROSAT/PSPC band
ranges from ~2.7×1026 ergs s-1 (at minimum)
to ~4.7×1027 ergs s-1 (at maximum). We discuss
future developments and possible applications of our method.

---------------------------------------------------------
Title: Hot Cores in Coronal Filament Cavities
Authors: Hudson, H.; Schwenn, R.
2000AdSpR..25.1859H Altcode:
Filaments represent cold intrusions in the corona, embedded in magnetic
configurations termed “filament cavities.” Such cavities may occur
without actually containing prominence material. A cavity then may
erupt, leading to a coronal mass ejection (CME). Studies of Yohkoh
soft X-ray images have previously shown that such eruptions may contain
elongated high-temperature regions closely aligned with the Hα filament
material. We report in this paper multi-wavelength observations of an
extremely stable filament cavity, observed by Yohkoh and SOHO during
July-September 1997. Hot multi-thermal structures persistently occupied
the core of this large-scale polar-crown cavity

---------------------------------------------------------
Title: Global Coronal Waves: Implications for HESSI
Authors: Hudson, H. S.; Karlický, M.
2000ASPC..206..268H Altcode: 2000hesp.conf..268H
No abstract at ADS

---------------------------------------------------------
Title: X-ray spectra of the Sun as a star: how different coronal
regions contribute to the observed X-ray spectrum
Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Hudson, H.
2000ASPC..198..479O Altcode: 2000scac.conf..479O
No abstract at ADS

---------------------------------------------------------
Title: Structure and Dynamics of the Corona Surrounding an Eruptive
Prominence
Authors: Gopalswamy, N.; Hanaoka, Y.; Hudson, H. S.
2000AdSpR..25.1851G Altcode:
We report on the 1997 December 14 prominence eruption event that was
accompanied by eruptive signatures in X-rays, EUV and white light:
coronal dimming, X-ray arcade formation , X-ray brightenings, EUV
eruption, and a white light CME. The data used were obtained by
the Nobeyama Radioheliograph, Yohkoh Soft X-ray Telescope (SXT) and
SOHO/LASCO and EIT. We identified various substructures of the eruption
and their inter-relationships. We found that the pre-disruption swelling
of the equatorial streamer was caused by the outward displacement of
the coronal material around the prominence location. The dynamical
behaviors of the CME and the accompanying eruptive prominence seem to
be very different

---------------------------------------------------------
Title: TRACE and Yohkoh Observations of High-Temperature Plasma in
a Two-Ribbon Limb Flare
Authors: Warren, H. P.; Bookbinder, J. A.; Forbes, T. G.; Golub, L.;
Hudson, H. S.; Reeves, K.; Warshall, A.
1999ApJ...527L.121W Altcode:
The ability of the Transition Region and Coronal Explorer
(TRACE) to image solar plasma over a wide range of temperatures
(Te~104-107 K) at high spatial
resolution (0.5" pixels) makes it a unique instrument for observing
solar flares. We present TRACE and Yohkoh observations of an M2.4
two-ribbon flare that began on 1999 July 25 at about 13:08 UT. We
observe impulsive footpoint brightenings that are followed by the
formation of high-temperature plasma (Te&gt;~10 MK)
in the corona. After an interval of about 1300 s, cooler loops
(Te&lt;2 MK) form below the hot plasma. Thus, the
evolution of the event supports the qualitative aspects of the standard
reconnection model of solar flares. The TRACE and Yohkoh data show that
the bulk of the flare emission is at or below 10 MK. The TRACE data
are also consistent with the Yohkoh observations of hotter plasma
(Te~15-20 MK) existing at the top of the arcade. The
cooling time inferred from these observations is consistent with a
hybrid cooling time based on thermal conduction and radiative cooling.

---------------------------------------------------------
Title: The Global Dynamics of the High-Temperature Corona
Authors: Hudson, Hugh
1999SoPh..190...91H Altcode:
This paper surveys coronal motions detected by the Yohkoh soft
X-ray telescope SXT, emphasizing `global restructuring'. Large-scale
structures in the solar corona can persist for time scales much longer
than those of the supergranulation, and may have larger spatial
scales. Flares and coronal mass ejections (CMEs) can disrupt these
structures. A grazing-incidence telescope such as SXT provides a view
of the corona biased in temperature towards the hotter components,
but in a thick `observing slice' (spatial contribution function). This
helps in seeing excitation (energy release) but may conceal some of
the structural changes. The observations of restructuring largely
appear to consist of expanding or outwards magnetic motions, which
are endoergic. This suggests a conjecture regarding the existence
of magnetic implosions on scales not yet detected, as a source of
free energy.

---------------------------------------------------------
Title: Multi-Wavelength Observations of a Large-Scale Jet and an
Eruptive- Prominence on 28 August 1992
Authors: Watanabe, Ta.; Ashizawa, K.; Nakagawa, Y.; Miyazaki, H.;
Irie, M.; Ichimoto, K.; Kurokawa, H.; Hudson, H.; Yatagai, H.
1999spro.proc..171W Altcode:
An eruption of a large (15o) north-south aligned quiescent
prominence and associated coronal disturbance, which took place above
the eastern solar limb near the equator on 28 August 1992, were observed
at a wide range of wavelengths ranging from soft X-rays (Yohkoh) to
microwaves (Nobeyama). The eruption was preceded by the formation of
a large-scale jet which was apparently ejected near the root of the
southern leg of the prominence. The characteristic outward speed of the
jet was 450 km s-1. A potential-field presentation of the
coronal magnetic field suggests that the jet was formed along the open
field which was located immediately to the west of the magnetic arcade,
originally surrounding the eruptive prominence. The temperature of the
jet is suggested to be comparable to that of the nearby quiet corona
(2 × 106 K). In the course of the prominence eruption,
helically twisted loops surrounding the prominence were observed. This
suggests that magnetic reconnection of the sheared arcade took place
underneath the erupting prominence.

---------------------------------------------------------
Title: Coronal Mass Ejections at High Temperatures
Authors: Hudson, H.
1999spro.proc..159H Altcode:
We now have extensive X-ray (Yohkoh) and EUV observations of the
behavior of coronal mass ejections (CMEs) at high temperatures in the
lower corona. We also now have coronagraph observations from space
with which to make identifications of the related phenomena. This
paper reviews theories and observations of CMEs in this new context.

---------------------------------------------------------
Title: Reply
Authors: Kahler, S. W.; Cane, H. V.; Hudson, H. S.; Kurt, V. G.;
Gotselyuk, Y. V.; MacDowall, R. J.; Bothmer, V.
1999JGR...10422411K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Transequatorial Interconnecting Loops and Increase of Their
Length in the New Cycle
Authors: Fárnik, F.; Ŝvestka, Z.; Karlický, M.; Hudson, H. S.
1999ESASP.446..305F Altcode: 1999soho....8..305F
Using Yohkoh SXT data, we show that active regions of the new solar
cycle, appearing at high latitudes, can be connected across the
solar equator by loops as long as 60 heliographic degrees (730000
km). This length greatly exceeds the limit of 37 degrees found for
transequatorial interconnecting loops on Skylab and implies that the
Skylab limit was simply due to the low latitudes of active regions
during the period of Skylab observations. By modelling the loops in
force-free approximation using Kitt Peak magnetograms, we find strong
support for the interpretation that these long interconnecting loops
originate through reconnection of magnetic field lines which extend from
the two active regions towards and beyond the equator, and confirm the
earlier finding by Canfield, Pevtsov, and McClymont that a favourable
condition for the reconnection is the same chirality (i.e., same sign
of the force-free parameter alpha and same direction of current flow)
in the two active regions. As we were unable to find any longitudinal
(i.e., east-west) loops of a comparable length, we suggest that an
important component of the driving force for the reconnection of
transequatorial interconnecting loops may be the differential solar
rotation. It can help to drive the reconnection of loops extending in
the north- south direction, whereas it does not help in the case of
longitudinal loops. These conclusions are based on loop observations
in December 1997 and May 1998, when only short-lived transequatorial
loops connected the high-latitude active regions on the northern
and southern hemispheres. However, in February 1999 a rich system
of transequatorial loops as long as 550 000 km could be observed
during its whole transit across the visible solar disk. We are now
studying this system (not all supporting data have been available yet
when writing this Abstract), comparing it with a similar system of
much shorter transequatorial loops which crossed the solar disk in
March/April 1992 during the preceeding cycle. We again try to model
these long-lived loop systems in current-free approximation, using
both Kitt Peak and Big Bear magnetograms and, for the February 1999
transit, also the more frequent, high-resolution full-disk magnetic
maps from SOHO. This modelling verifies the location of the footpoints
of interconnecting loops in low magnetic fields at peripheries of the
interconnected active regions, indicates how some of the loops have
been formed, and provides information about some of the reasons which
lead to the shape and brightness variations in the loops system.

---------------------------------------------------------
Title: SOHO EIT Observations of Extreme-Ultraviolet “Dimming”
Associated with a Halo Coronal Mass Ejection
Authors: Zarro, Dominic M.; Sterling, Alphonse C.; Thompson, Barbara
J.; Hudson, Hugh S.; Nitta, Nariaki
1999ApJ...520L.139Z Altcode:
A solar flare was observed on 1997 April 7 with the Soft X-ray Telescope
(SXT) on Yohkoh. The flare was associated with a “halo” coronal
mass ejection (CME). The flaring region showed areas of reduced soft
X-ray (SXR) brightness--“dimmings”--that developed prior to the CME
observed in white light and persisted for several hours following the
CME. The most prominent dimming regions were located near the ends of
a preflare SXR S-shaped (sigmoid) feature that disappeared during the
event, leaving behind a postflare SXR arcade and cusp structure. Based
upon these and similar soft X-ray observations, it has been postulated
that SXR dimming regions are the coronal signatures (i.e., remnants)
of magnetic flux ropes ejected during CMEs. This Letter reports
new observations of coronal dimming at extreme-ultraviolet (EUV)
wavelengths obtained with the Extreme-ultraviolet Imaging Telescope
(EIT) on the Solar and Heliospheric Observatory (SOHO). A series of
EIT observations in the 195 Å Fe XII wavelength band were obtained
simultaneously with SXT during the 1997 April 7 flare/CME. The EIT
observations show that regions of reduced EUV intensity developed at
the same locations and at the same time as SXR dimming features. The
decrease in EUV intensity (averaged over each dimming region) occurred
simultaneously with an increase in EUV emission from flaring loops in
the active region. We interpret these joint observations within the
framework of flux-rope eruption as the cause of EUV and SXR coronal
dimmings, and as the source of at least part of the CME.

---------------------------------------------------------
Title: How the Sun's Corona Gets Hot
Authors: Hudson, H. S.; Kosugi, T.
1999Sci...285..849H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: X-Ray Observations of Motions and Structure above a Solar
Flare Arcade
Authors: McKenzie, D. E.; Hudson, H. S.
1999ApJ...519L..93M Altcode:
In this Letter, we describe a solar flare that was observed by Yohkoh
in 1999 January 20. This long-duration event is notable because the
Yohkoh images show not only the formation of the arcade associated
with the coronal mass ejection but also a considerable amount of motion
above the arcade in the region normally identified with a large-scale
current sheet or the outflow from magnetic reconnection in the current
sheet. A number of arcade events of this morphological type (i.e.,
a fan of spikelike “rays” above the posteruption loops) have been
seen by Yohkoh, but in this case we have a much clearer view of mass
motions in the region above the arcade. The motions indicate field-line
retraction without the formation of long-lasting cusps during the rise
phase of the flare, and a downward flow above the arcade during the
decay phase. The late-phase downward motion is in the form of X-ray
dark voids moving at 100-200 km s-1, i.e., at velocities
much smaller than the free-fall speed or the assumed Alfvén speed. We
interpret the voids as cross sections of evacuated flux tubes resulting
from intermittent reconnection following the associated coronal mass
ejection. We believe these data represent the first direct evidence
of high-speed flows in the region immediately above the flare loops.

---------------------------------------------------------
Title: Deconvolution of Directly Precipitating and Trap-precipitating
Electrons in Solar Flare Hard X-Rays. III.Yohkoh Hard X-Ray Telescope
Data Analysis
Authors: Aschwanden, Markus J.; Fletcher, Lyndsay; Sakao, Taro;
Kosugi, Takeo; Hudson, Hugh
1999ApJ...517..977A Altcode:
We analyze the footpoint separation d and flux asymmetry A of
magnetically conjugate double footpoint sources in hard X-ray images
from the Yohkoh Hard X-Ray Telescope (HXT). The data set of 54 solar
flares includes all events simultaneously observed with the Compton
Gamma Ray Observatory (CGRO) in high time resolution mode. From the CGRO
data we deconvolved the direct-precipitation and trap-precipitation
components previously (in Paper II). Using the combined measurements
from CGRO and HXT, we develop an asymmetric trap model that allows
us to quantify the relative fractions of four different electron
components, i.e., the ratios of direct-precipitating (qP1,
qP2) and trap-precipitating electrons (qT1,
qT2) at both magnetically conjugate footpoints. We find mean
ratios of qP1=0.14+/-0.06, qP2=0.26+/-0.10,
and qT=qT1+qT2=0.60+/-0.13. We
assume an isotropic pitch-angle distribution at the
acceleration site and double-sided trap precipitation
(qT2/qT1=qP2/qP1)
to determine the conjugate loss-cone angles
(α1=42deg+/-11deg and
α2=52deg+/-10deg) and magnetic
mirror ratiosat both footpoints (R1=1.6,...,4.0 and
R2=1.3,...,2.5). From the relative displacement of
footpoint sources we also measure altitude differences of hard
X-ray emission at different energies, which are found to decrease
systematically with higher energies, with a statistical height
difference of hLo-hM1=980+/-250 km and
hM1-hM2=310+/-300 km between the three lower
HXT energy channels (Lo, M1, M2).

---------------------------------------------------------
Title: Quiet solar wind signatures above active regions observed
in X-rays
Authors: Hick, P.; Svestka, Z.; Jackson, B. V.; Farnik, F.; Hudson, H.
1999AIPC..471..231H Altcode: 1999sowi.conf..231H
X-ray images from the Yohkoh satellite, obtained following occurrences
of limb flares sometimes show coronal fan-like structures extending
above a growing post-flare loop system. We show one such event
observed in AR 7270 on the east limb of the Sun on 28/29 August
1992. We suggest that these rays are `ministreamers,' formed as a
result of the re-structuring of the corona following the occurrence
of a flare-associated CME. Synoptic maps of the solar wind density,
constructed from a tomographic analysis of interplanetary scintillation
(IPS) measurements, show enhanced scintillation matching the position
of AR 7270 if we assume a radial outflow at a reasonable slow solar
wind speed of 400 km s-1. From this agreement we argue that outflow
of mass occurs from the active region into interplanetary space.

---------------------------------------------------------
Title: Modeling CMEs in three dimensions using an analytic MHD model
Authors: Gibson, Sarah E.; Alexander, David; Biesecker, Doug; Fisher,
Richard; Guhathakurta, Madhulika; Hudson, Hugh; Thompson, B. J.
1999AIPC..471..645G Altcode: 1999sowi.conf..645G
Because coronal mass ejections (CMEs) are viewed in projection,
it is difficult to determine their three-dimensional nature. We use
an analytic model of CMEs as an example of a fully three-dimensional
magnetic field structure in MHD force balance with an emerging CME. We
present the CME magnetic field and its associated density structure,
seen projected at the limb from two viewing angles perpendicular
to the plane of the sky, and emerging from disk center representing
“earth-directed” CME events. The range of CME structures thus produced
compares well to existing CME white-light coronagraph and full disk
EUV and X-ray observations. In particular, we find that both 3-part
“front-cavity-core” and “U-shaped” white light CMEs, as well as the
twin dimmings (also referred to as transient coronal holes) observed in
X-ray and EUV, can successfully be reproduced by the CME model. All of
these structures are a direct consequence of a single three-dimensional
magnetic field topology, viewed from different directions.

---------------------------------------------------------
Title: Soft X-ray Observation of a Flare-Associated Coronal Wave
Authors: Hudson, H. S.; Lemen, J. R.; Thompson, B.; Uchida, Y.
1999AAS...194.2205H Altcode: 1999BAAS...31..860H
Recent EUV observations from the EIT instrument of SOHO have shown
the common occurrence of flare-associated global coronal waves, allies
of Type II burst exciters and chromospheric Moreton waves. Until now,
however, no direct soft X-ray detections have been reported. We have
studied Yohkoh SXT observations to understand this apparent discrepancy
between EUV and soft X-ray observations,and have now found good X-ray
evidence for a large-scale coronal wave launched during an X-class
flare of May 6, 1998. During the impulsive phase of this flare, a rapid
( ~ 10(3) km s(-1) ) bright front appeared to the north of the flare
core; later a more normal loop-like ejection emerged to the west at a
lower projected speed. We identify the rapid front with enhanced X-ray
emission from a global coronal wave. Wave signatures also appear in
SOHO EIT images and in the meter-wave dynamic spectrum from Hiraiso,
and SOHO LASCO detected a coronal mass ejection. NASA supported this
work under contract NAS 8-37334.

---------------------------------------------------------
Title: YOHKOH SXT and SOHO EIT Observations of “Sigmoid-to-Arcade”
Evolution of Structures Associated with Halo CMEs
Authors: Sterling, A. C.; Hudson, H. S.; Thompson, B. J.; Zarro, D. M.
1999AAS...19410107S Altcode: 1999BAAS...31..999S
A subset of the solar-disk counterparts to halo coronal mass ejections
(CMEs) display an evolution in soft X-rays (SXR) characterized by a
preflare “S”-shaped structure, dubbed a “sigmoid,” evolving into
a postflare cusp or arcade. We examine the morphological properties
of the evolution of sigmoids into cusps and arcades for four such
regions associated with SXR flares, using the Soft X-ray Telescope
(SXT) on Yohkoh and the 195 Angstroms Fe xii\ channel of the EUV Imaging
Telescope (EIT) on SOHO. There is, at most, only a weak counterpart to
the SXR sigmoid in the preflare EUV images, indicating that the preflare
sigmoid has a temperature &gt;1.5 MK\@. During the time of the flare
itself, however, an EUV sigmoid brightens near the location of the
SXR preflare sigmoid. Initially the SXR sigmoid lies along a magnetic
neutral line. As the SXR flare progresses new field lines appear with
orientation normal to the neutral line and with footpoints rooted in
opposite polarity regions; these footpoints are different from those of
the preflare sigmoid. The cusp structures in SXRs develop from these
newly-ignited field lines. In EIT images the EUV sigmoid broadens out
as the flare progresses, forming an arcade which resides beneath the
SXR cusp. In many respects, our findings are consistent with a standard
picture where the origin of the flare and CME is due to the eruption
of a filament-like feature, and the stretching of overlying preflare
fields produces the cusp. We do not, however, observe these preflare
overlying fields prior to flare onset. This work was supported by the
NRL Naval basic research program and NASA.

---------------------------------------------------------
Title: Arcade Structure and Dynamics in the 20-Jan-99 M5 Solar Flare
Authors: McKenzie, D. E.; Hudson, H. S.
1999AAS...19410108M Altcode: 1999BAAS...31..999M
We present observations of a remarkable solar flare which was observed
by Yohkoh on 20-Jan-99. This long duration event is notable because the
Yohkoh images show not only the formation of the arcade associated with
the coronal mass ejection, but also a considerable amount of structure
and motion in the current sheet above the arcade. Though not unique, the
structure of this arcade is different from the majority of flare arcades
observed by Yohkoh. The motions indicate field line shrinkage during the
rise phase of the flare, and downward flow above the arcade during the
decay phase. The late-phase downward motion suggests the possibility
either of a high-Beta regime in the current sheet, or of shrinkage of
evacuated flux tubes downward through the current sheet. This research
is supported by NASA under MSFC contract NAS8-40801.

---------------------------------------------------------
Title: A Stable Filament Cavity with a Hot Core
Authors: Hudson, H. S.; Acton, L. W.; Harvey, K. L.; McKenzie, D. E.
1999ApJ...513L..83H Altcode:
We present observations of a long-lived solar filament cavity with
soft X-ray sources along its axis. This structure appeared above the
southern polar crown polarity-inversion line for approximately three
rotations during 1997 June-August, centered at a west-limb passage on
approximately July 3. At the limb, the Yohkoh soft X-ray data showed
a bright region situated above and around the projected filament
location but near the axis of the cavity. We describe measurements
of the geometry of the cavity, which we interpret as a flux rope that
is partially embedded in the photosphere, and use the Yohkoh data to
describe the physical parameters of the structure. We find that the
core consists of an unresolved mass of filamentary substructures, with a
volume filling factor significantly less than unity for the soft X-ray
telescope (SXT) resolution. The core has a higher temperature than the
cavity surrounding it, ruling out explanations in terms of a transition
region supported by thermal conduction. Transient activity occurred in
the polar crown region, but no detectable destabilization or eruption
of the cavity structure resulted from it. We suggest that the bright
structure at the core of the cavity corresponds to higher altitude
coronal segments of the field lines that support the filament material.

---------------------------------------------------------
Title: Sigmoidal morphology and eruptive solar activity
Authors: Canfield, Richard C.; Hudson, Hugh S.; McKenzie, David E.
1999GeoRL..26..627C Altcode:
Soft X-ray images of solar active regions frequently show S- or
inverse-S (sigmoidal) morphology. We have studied the Yohkoh Soft
X-Ray Telescope video movie for 1993 and 1997. We have classified
active regions according to morphology (sigmoidal or non-sigmoidal)
and nature of activity (eruptive or non-eruptive). As well, we have
used NOAA sunspot areas for each region as a measure of size. We find
that regions are significantly more likely to be eruptive if they are
either sigmoidal or large.

---------------------------------------------------------
Title: The Solar-Stellar Connection in X-rays: How to Take Advantage
of the YOHKOH data
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H.
1999ASPC..158..391P Altcode: 1999ssa..conf..391P
No abstract at ADS

---------------------------------------------------------
Title: Observational Tests of a Double Loop Model for Solar Flares
Authors: Hardy, S. J.; Melrose, D. B.; Hudson, H. S.
1998PASA...15..318H Altcode: 1998PASA...15..317H
A model for the energetics of solar flares, developed by Melrose
(1997), is based on magnetic reconnection between two current-carrying
magnetic loops. A detailed numerical investigation of the model has
been made to identify those configurations that lead to energy release
in a flare. Our results predict a strong relation between the ratio of
currents in the interacting loops for a favoured flare configuration,
and provide further support for a proposed method of generating long
loops connecting different active regions. Both of these predictions
are amenable to observational verification.

---------------------------------------------------------
Title: High-Energy Solar Spectroscopic Imager (HESSI) Small Explorer
mission for the next (2000) solar maximum
Authors: Lin, Robert P.; Hurford, Gordon J.; Madden, Norman W.;
Dennis, Brian R.; Crannell, Carol J.; Holman, Gordon D.; Ramaty,
Reuven; von Rosenvinge, Tycho T.; Zehnder, Alex; van Beek, H. Frank;
Bornmann, Patricia L.; Canfield, Richard C.; Emslie, A. Gordon; Hudson,
Hugh S.; Benz, Arnold; Brown, John C.; Enome, Shinzo; Kosugi, Takeo;
Vilmer, Nicole; Smith, David M.; McTiernan, Jim; Hawkins, Isabel;
Slassi-Sennou, Said; Csillaghy, Andre; Fisher, George; Johns-Krull,
Chris; Schwartz, Richard; Orwig, Larry E.; Zarro, Dominic; Schmahl,
Ed; Aschwanden, Markus; Harvey, Peter; Curtis, Dave; Pankow, Dave;
Clark, Dave; Boyle, Robert F.; Henneck, Reinhold; Michedlishvili,
Akilo; Thomsen, K.; Preble, Jeff; Snow, Frank
1998SPIE.3442....2L Altcode:
The primary scientific objective of the High Energy Solar Spectroscopic
Imager (HESSI) Small Explorer mission selected by NASA is to investigate
the physics of particle acceleration and energy release in solar
flares. Observations will be made of x-rays and (gamma) rays from
approximately 3 keV to approximately 20 MeV with an unprecedented
combination of high resolution imaging and spectroscopy. The HESSI
instrument utilizes Fourier- transform imaging with 9 bi-grid rotating
modulation collimators and cooled germanium detectors. The instrument
is mounted on a Sun-pointed spin-stabilized spacecraft and placed
into a 600 km-altitude, 38 degrees inclination orbit.It will provide
the first imaging spectroscopy in hard x-rays, with approximately
2 arcsecond angular resolution, time resolution down to tens of ms,
and approximately 1 keV energy resolution; the first solar (gamma)
ray line spectroscopy with approximately 1-5 keV energy resolution;
and the first solar (gamma) -ray line and continuum imaging,with
approximately 36-arcsecond angular resolution. HESSI is planned for
launch in July 2000, in time to detect the thousands of flares expected
during the next solar maximum.

---------------------------------------------------------
Title: Large-Scale Active Coronal Phenomena in Yohkoh SXT Images
IV. Solar Wind Streams from Flaring Active Regions
Authors: Švestka, Zdeněk; Fárník, František; Hudson, Hugh S.;
Hick, Paul
1998SoPh..182..179S Altcode:
We demonstrate limb events on the Sun in which growing flare loop
systems are embedded in hot coronal structures looking in soft X-rays
like fans of coronal rays. These structures are formed during the flare
and extend high into the corona. We analyze one of these events, on
28-29 August 1992, which occurred in AR 7270 on the eastern limb, and
interpret these fans of rays either as temporary multiple ministreamers
or plume-like structures formed as a result of restructuring due to
a CME. We suggest that this configuration reflects mass flow from the
active region into interplanetary space. This suggestion is supported
by synoptic maps of solar wind sources constructed from scintillation
measurements which show a source of enhanced solar wind density at
the position of AR 7270, which disappears when 5 days following the
event are removed from the synoptic map data. We also check synoptic
maps for two other active regions in which existence of these fan-like
structures was indicated when the active regions crossed both the east
and west limbs of the Sun, and both these regions appear to be sources
of a density enhancement in the solar wind.

---------------------------------------------------------
Title: On the relationship between coronal mass ejections and
magnetic clouds
Authors: Gopalswamy, N.; Hanaoka, Y.; Kosugi, T.; Lepping, R. P.;
Steinberg, J. T.; Plunkett, S.; Howard, R. A.; Thompson, B. J.;
Gurman, J.; Ho, G.; Nitta, N.; Hudson, H. S.
1998GeoRL..25.2485G Altcode:
We compare the substructures of the 1997 February 07 coronal mass
ejection (CME) observed near the Sun with a corresponding event in
the interplanetary medium to determine the origin of magnetic clouds
(MCs). We find that the eruptive prominence core of the CME observed
near the Sun may not directly become a magnetic cloud as suggested by
some authors and that it might instead become the ”pressure pulse”
following the magnetic cloud. We substantiate our conclusions using time
of arrival, size and composition estimates of the CME-MC substructures
obtained from ground based, SOHO and WIND observations.

---------------------------------------------------------
Title: X-ray coronal changes during Halo CMEs
Authors: Hudson, H. S.; Lemen, J. R.; St. Cyr, O. C.; Sterling, A. C.;
Webb, D. F.
1998GeoRL..25.2481H Altcode:
Using the Yohkoh soft X-ray images, we examine the coronal structures
associated with “halo” coronal mass ejections (CMEs). These may
correspond to events near solar disk center. Starting with a list
of eleven confirmed halo CMEs over the time range from December 1996
through May 1997, we find seven with surface features identifiable in
soft X-rays, with GOES classifications ranging from A1 to M1.3. These
have a characteristic pattern of sigmoid → arcade development. In each
of these events, the pre-flare structure disrupted during the flare,
leaving the appearance of compact transient coronal holes. The four
remaining events had weak or indistinguishable signatures in the X-ray
images. For the events for which we could see well-defined coronal
changes, we confirm our previous result that the estimated mass loss
inferred from the soft X-ray dimming is a small fraction of typical
CME masses [Sterling &amp; Hudson 1997].

---------------------------------------------------------
Title: The solar origin of the January 1997 coronal mass ejection,
magnetic cloud and geomagnetic storm
Authors: Webb, D. F.; Cliver, E. W.; Gopalswamy, N.; Hudson, H. S.;
St. Cyr, O. C.
1998GeoRL..25.2469W Altcode:
The magnetic cloud and geomagnetic storm on January 10-11, 1997 were
associated with a halo-type Coronal Mass Ejection (CME) observed by
the SOHO/LASCO coronagraphs near the sun on January 6. We summarize
the solar activity related to this CME and the subsequent storm at
Earth. This solar activity was remarkably weak and unimpressive. If
the wide CME had not been observed, the storm would not have been
forecast. Thus this case represents an extreme example of so-called
“problem” magnetic storms that lack obvious surface signatures of
eruptive solar activity. It supports the view that CMEs involve the
destabilization of large-scale coronal structures which may or may
not have associated surface activity, and that CMEs, not the surface
activity, are the key causal link between solar eruptions and space
weather at Earth.

---------------------------------------------------------
Title: The solar energetic particle event of April 14, 1994, as a
probe of shock formation and particle acceleration
Authors: Kahler, S. W.; Cane, H. V.; Hudson, H. S.; Kurt, V. G.;
Gotselyuk, Y. V.; MacDowall, R. J.; Bothmer, V.
1998JGR...10312069K Altcode:
Gradual solar energetic particle (SEP) events observed at 1 AU are
associated with coronal mass ejections (CME) that drive shocks which
are presumed to accelerate the ions and electrons to suprathermal
energies. However, high-energy (&gt;30 MeV) proton and (&gt;1 MeV)
electron events are nearly always associated with both CMEs and flares,
suggesting that the acceleration of those particles, particularly
the electrons, could be attributed to the associated flares. Only one
clear example of a high-energy SEP event without an active region flare
association has been reported previously. We discuss a second such SEP
event, on April 14, 1994, associated with a well-observed solar X ray
arcade structure spanning ~150° of solar longitude. The SEP event,
observed by detectors on the IMP 8 and Koronas I spacecraft, began
~10 hrs after the beginning of the X ray event and was temporally
and spatially associated with the last of three weak interplanetary
type III radio bursts observed by the Ulysses low-frequency radio
experiment. The delayed onset and rapid rise of the SEP intensities
preclude a recent interpretation in which SEPs were presumed to be
accelerated by a shock driven by a CME which erupted at the onset of
the X ray event. Yohkoh soft X ray subtracted images show a large-scale
arcade brightening west of ~E10° beginning about 8 hours after the
initial brightening near the east limb. We suggest that the April 14
SEP event at Earth was produced by a shock driven by a CME associated
with the later brightening near central meridian. The initial X ray
brightening may also have been associated with an earlier CME.

---------------------------------------------------------
Title: Precise Determination of the Coordinate Systems for the YOHKOH
Telescopes and the Application of a Transit of Mercury
Authors: Wülser, J. -P.; Hudson, H. S.; Nishio, M.; Kosugi, T.;
Masuda, S.; Morrison, M.
1998SoPh..180..131W Altcode:
The Yohkoh solar X-ray observatory carries two telescopes that
require coalignment at a level better than the minimum pixel size
of 2_45″. This coalignment is needed both internally within
Yohkoh and for many scientific applications involving data from
ground-based radio and optical observatories. We describe the methods
successfully developed for this purpose and now incorporated in the
Yohkoh software. Soft X-ray observations of the 1993 transit of Mercury
across the solar disk provided key information for the calibration of
the coalignment procedures.

---------------------------------------------------------
Title: Fan-Like coronal X-ray Structures as Sources of Solar Wind
Authors: Hick, P.; Svestka, Z.; Farnik, F.; Hudson, H. S.; Jackson,
B. V.
1998AAS...192.1503H Altcode: 1998BAAS...30..840H
We show coronal soft X-ray images from the Yohkoh satellite, obtained
following occurrences of limb flares. These images show rising
post-flare loops, which are embedded in hot coronal structures looking
like fans of coronal rays. We analyze the event on 28/29 August 1992,
which occurred in AR 7270 on the east limb of the Sun. We suggest that
these rays are multiple 'ministreamers', which apparently are formed as
a result of the restructuring of the corona following the occurrence
of a flare-associated CME. We argue that this configuration allows
outflow of mass from the active region into interplanetary space. This
is supported by synoptic maps of solar wind sources constructed from
scintillation measurements showing a source of enhanced scintillation
at the position of AR 7270.

---------------------------------------------------------
Title: 3-Dimensional Models of Active Region Loops
Authors: Aschwanden, M. J.; Neupert, W. M.; Newmark, J.; Thompson,
B. J.; Brosius, J. W.; Holman, G. D.; Harrison, R. A.; Bastian, T. S.;
Nitta, N.; Hudson, H. S.; Zucker, A.
1998ASPC..155..145A Altcode: 1998sasp.conf..145A
No abstract at ADS

---------------------------------------------------------
Title: An Interdisciplinary Study of the Eruptive Prominence of 28
August 1992
Authors: Watanabe, Ta.; Yamamoto, M.; Hudson, H.; Irie, M.; Ichimoto,
K.; Kurokawa, H.; Yatagai, H.
1998ASSL..229..101W Altcode: 1998opaf.conf..101W
No abstract at ADS

---------------------------------------------------------
Title: Solar-Stellar Connection: Relevance of YOHKOH Data
Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Hudson, H.
1998ASPC..154.1130O Altcode: 1998csss...10.1130O
The similarity of late-type stars to the Sun is often assumed
when studying the physical conditions in their coronae. In order
to explore better such a “solar-stellar” connection we use the
Yohkoh/SXT X-ray images to generate the distribution of the emission
measure vs. temperature of the Sun and, from that, the expected
emission, as it would be observed by non-solar X-ray telescopes
such as ROSAT/PSPC and ASCA/SIS. We discuss the role of the various
solar structures in determining the total distribution of the emission
measure vs. temperature and in determining the stellar-like synthesized
X-ray spectra.

---------------------------------------------------------
Title: Birth Place of the 1998 January 21 CME
Authors: Gopalswamy, N.; Hanaoka, Y.; Kaiser, M.; Gurman, J.; Hudson,
H.; Howard, R. A.
1998cee..workE..40G Altcode:
The 1998 January 21 halo coronal mass ejection was launched
from high southern latitudes in association with a filament
disappearance observed by the Nobeyama Radioheliograph. Signatures
of the initial destabilization of the filament were observed by
the Extreme-ultraviolaet Imaging Telescope (EIT) on board the SOHO
spacecraft and by the Soft X-ray Telescope (SXT) on board Yohkoh. The
Wind/WAVES experiment observed a type II burst in the 600-300 kHz
range. The data coverage for this event is unusually high and we make
use of it to understand the origin and evolution of the eruption. We
address several issues based on these data: (i) relation between
filament eruption and arcade formation beneath the filament, (ii)
comparison between the hot arcade formation in X-rays and EUV, (iii)
relation between the filament eruption and the white light CME, (iv)
relation between the CME and the interplanetary shock inferred from the
WAVES data. A summary of near-surface activities associated with the
eruption can be seen in the Figure 1. Figure 1. SOHO/MDI longitudinal
magnetogram, with radio filament (white contours) and X-ray emission
(enclosed by dark lines) are overlaid. The thick white line from E to
W is the neutral line over which the eruption took place. North is to
the top and east is to the left. F is the location where the filament
split at the time of eruption.

---------------------------------------------------------
Title: The Formation and Evolution of the Coronal Holes Associated
with NOAA Region 7978
Authors: Harvey, K. L.; Hudson, H. S.
1998ASSL..229..315H Altcode: 1998opaf.conf..315H
No abstract at ADS

---------------------------------------------------------
Title: NOAA 7978: the Last best Old-Cycle Region
Authors: Hudson, H. S.; Labonte, B. J.; Sterling, A. C.; Watanabe, Te.
1998ASSL..229..237H Altcode: 1998opaf.conf..237H
No abstract at ADS

---------------------------------------------------------
Title: Soft X-ray Observations of Eruptive Prominences
Authors: Watanabe, T.; Yamamoto, M.; Hudson, H.; Irie, M.; Ichimoto,
K.; Kurokawa, H.; Yatagai, H.
1998ASPC..150..376W Altcode: 1998IAUCo.167..376W; 1998npsp.conf..376W
No abstract at ADS

---------------------------------------------------------
Title: The Sun as an X-Ray Star: Overview of the Method
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H.
1998ASSL..229...29P Altcode: 1998opaf.conf...29P
No abstract at ADS

---------------------------------------------------------
Title: Coronal and Interplanetary Disturbances Associated with an
Eruptive Prominence of 28 August 1992
Authors: Watanabe, T.; Yamamoto, M.; Hudson, H.; Irie, M.; Ichimoto,
K.; Kurokawa, H.; Yatagai, H.
1998asct.conf..313W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Evolution of a Rapidly-Expanding Active Region Loop into
a Trans-Equatorial Coronal Mass Ejection
Authors: Simnett, G. M.; Hudson, H. S.
1997ESASP.415..437S Altcode: 1997cpsh.conf..437S
No abstract at ADS

---------------------------------------------------------
Title: Large-Scale Active Coronal Phenomena in YOHKOH SXT
Images. III. Enhanced Post-Flare Streamer
Authors: Švestka, Zdeněk; Fárník, František; Hick, Paul; Hudson,
Hugh S.; Uchida, Yutaka
1997SoPh..176..355S Altcode:
We demonstrate several events where an eruptive flare close to the
limb gave rise to a transient coronal streamer visible in X-rays in
Yohkoh SXT images, and analyze one of these events, on 28-29 October
1992, in detail. A coronal helmet streamer began to appear 2 hours
after the flare, high above rising post-flare loops; the streamer
became progressively narrower, reaching its minimum width 7-12 hours
after the flare, and widened again thereafter, until it eventually
disappeared. Several other events behaved in a similar way. We suggest
that the minimum width indicates the time when the streamer became
fully developed. All the time the temperature in the helmet streamer
structure was decreasing, which can explain the subsequent fictitious
widening of the X-ray streamer. It is suggested that we may see here
two systems of reconnection on widely different altitudes, one giving
rise to the post-flare loops while the other creates (or re-forms)
the coronal helmet streamer. A similar interpretation was suggested in
1990 by Kopp and Polettofor post-flare giant arches observed on board
the SMM; indeed, there are some similarities between these post-flare
helmet streamers and giant arches and, with the low spatial resolution
of SMM instruments, it is possible that some helmet streamers could
have been considered to be a kind of a giant arch.

---------------------------------------------------------
Title: The High Energy Solar Spectroscopic Imager (HESSI): A Small
Explorer for the Start of the New Millennium
Authors: Holman, G. D.; Lin, R. P.; Dennis, B. R.; Crannell, C. J.;
Ramaty, R. R.; Rosenvinge, T. T.; Canfield, R. C.; Emslie, A. G.;
Hudson, H. S.; Hurford, G. J.; Madden, N. W.; van Beek, H. F.; Benz,
A.; Bornmann, P. L.; Brown, J. C.; Enome, S.; Kosugi, T.; Vilmer,
N.; Zehnder, A.
1997AAS...191.7416H Altcode: 1997BAAS...29R1326H
The High Energy Solar Spectroscopic Imager (HESSI) has been selected
for launch in mid 2000, at the peak of the solar activity cycle. The
primary scientific objective of HESSI is to understand particle
acceleration and explosive energy release in the magnetized plasma at
the Sun. HESSI will provide the first high-spectral-resolution x-ray and
gamma -ray images of the Sun. It will obtain the first imaging above
100 keV, the first imaging of solar gamma -ray lines, and the first
high-resolution spectroscopy of solar gamma -ray lines, including the
first determination of line shapes. In two years HESSI is expected to
obtain observations of tens of thousands of microflares, thousands of
hard x-ray flares, and of order a hundred gamma -ray line flares. HESSI
will also monitor and provide high-spectral-resolution observations of
cosmic and terrestrial hard x-ray and gamma -ray transients, as well
as imaging of the Crab Nebula. HESSI's high spectral, spatial, and
temporal resolution and dynamic range will allow the first detailed
studies of the evolution of both accelerated particles and hot,
thermal plasma in solar flares.

---------------------------------------------------------
Title: Solar Energetic Particle Events and Coronal Mass Ejections:
New Insights from SOHO
Authors: Bothmer, V.; Posner, A.; Kunow, H.; Müller-Mellin, R.;
Herber, B.; Pick, M.; Thompson, B. J.; Delaboudinière, J. -P.;
Brueckner, G. E.; Howard, R. A.; Michels, D. J.; Cyr, C. St.; Szabo,
A.; Hudson, H. S.; Mann, G.; Classen, H. -T.; McKenna-Lawlor, S.
1997ESASP.415..207B Altcode: 1997cpsh.conf..207B
No abstract at ADS

---------------------------------------------------------
Title: Yohkoh SXT Observations of X-Ray “Dimming” Associated with
a Halo Coronal Mass Ejection
Authors: Sterling, Alphonse C.; Hudson, Hugh S.
1997ApJ...491L..55S Altcode:
A sudden depletion or intensity “dimming” of the X-ray corona
sometimes accompanies a solar eruptive flare or coronal mass ejection
(CME). We have identified a dimming that occurred just prior to a
“halo” CME, observed on 1997 April 7 using the Soft X-ray Telescope
(SXT) on Yohkoh. Halo CMEs are prime candidates for “space weather”
effects. The dimming occurred in compact regions near a flare of
14 UT on April 7, over a projected area of about 1020
cm-2, and indicate that a mass of a few times 1014
g was ejected. This is a lower limit imposed by the obscuration
of the dimming volume by the brightness of the accompanying flare
and other factors. Most of the mass deficit comes from two regions
close to the ends of a preflare S-shaped active-region structure,
and the resulting dimmings in these regions persisted for more than
three days following the flare. A cusp-shaped loop--not apparent
prior to the flare--dominates the emission in the flare decay phase,
and has a mass comparable to that lost in the dimming regions. Our
findings are consistent with the source of the CME being a flux rope
that erupted, leaving behind the dimming regions. The cusp-shaped loop
probably represents magnetic fields reconfigured or reconnected by the
eruption. We do not see an X-ray analog of the wavelike disturbance
evident in SOHO EUV images.

---------------------------------------------------------
Title: An Overview of IACG Campaign 4: Solar Sources of Heliospheric
Structure Observed Out of the Ecliptic
Authors: Galvin, A. B.; Hudson, H. S.
1997ESASP.415...39G Altcode: 1997cpsh.conf...39G
No abstract at ADS

---------------------------------------------------------
Title: Post-Flare Loops Embedded in a Hot Coronal Fan-Like Structure
Authors: Švestka, Z.; Fárnik; Hudson, H. S.; Hick, P.
1997ESASP.415..139S Altcode: 1997cpsh.conf..139S
No abstract at ADS

---------------------------------------------------------
Title: The Solar Energetic Particle Event of 14 April 1994 as a
Probe of Shock Formation and Particle Acceleration
Authors: Kahler, S. W.; Cane, H. V.; Hudson, H. S.; Kurt, V. G.;
MacDowall, R. J.; Bothmer, V.
1997AAS...191.7412K Altcode: 1997BAAS...29.1326K
Gradual solar energetic particle (SEP) events observed at 1 AU are
associated with coronal mass ejections (CMEs) that drive shocks which
accelerate the ions and electrons to suprathermal energies. However,
high energy (&gt; 30 MeV) proton and (&gt; 1 MeV) electron events are
nearly always associated with both CMEs and flares, suggesting that
the acceleration of those particles, particularly the electrons, could
be attributed to the associated flares. Only one clear example of a
high energy SEP event without a flare association has been reported
previously. We discuss a second such SEP event, on 14 April 1994,
associated with a well observed solar X-ray arcade structure spanning ~
150deg of solar longitude. The SEP event, observed by detectors on the
IMP-8 and Koronas-I space craft, began about 10 hrs after the beginning
of the X-ray event and was temporally and spatially associated with
the last of three weak interplanetary type III radio bursts observed
by the Ulysses low frequency radio experiment. The delayed onset and
rapid rise of the SEP intensities preclude a recent interpretation in
which SEPs were accelerated by a shock driven by a CME which erupted
at the onset of the X-ray event. Yohkoh soft X-ray subtracted images
show a large-scale arcade brightening west of ~ E10deg beginning about 8
hours after the initial brightening near the east limb. We suggest that
the 14 April event consisted of at least two CMEs with progressively
westward source regions and that the SEP event was produced in a second
shock driven by a later CME.

---------------------------------------------------------
Title: Correlated Studies at Activity Maximum: the Sun and the
Solar Wind
Authors: Hudson, H. S.; Galvin, A. B.
1997ESASP.415..275H Altcode: 1997cpsh.conf..275H
No abstract at ADS

---------------------------------------------------------
Title: Electron Trapping Times and Trap Densities in Solar Flare
Loops Measured with COMPTON and YOHKOH
Authors: Aschwanden, Markus J.; Bynum, Robert M.; Kosugi, Takeo;
Hudson, Hugh S.; Schwartz, Richard A.
1997ApJ...487..936A Altcode:
We measure energy-dependent time delays of ~20-200 keV hard X-ray
(HXR) emission from 78 flares observed simultaneously with the Compton
Gamma Ray Observatory and Yohkoh. Fast time structures (&lt;~1 s)
are filtered out, because their time delays have been identified
in terms of electron time-of-flight (TOF) differences from directly
precipitating electrons (Aschwanden et al.). For the smooth HXR flux,
we find systematic time delays in the range of τS = t50
keV-t200 keV ~ -(1 ... 10) s, with a sign opposite
to TOF delays, i.e., the high-energy HXRs lag the low-energy HXRs. We interpret these time delays of the smooth HXR flux in terms of
electron trapping, and we fitted a model of the collisional deflection
time tDefl(E)~E3/2n-1e
to the observed HXR delays in order to infer electron densities
nTrape in the trap. Independently,
we determine the electron density nSXRe
in flare loops from soft X-ray (SXR) peak emission measures EM=
n2edh, using loop width (w) measurements to
estimate the column depth dh ~ w. Comparing the two independent
density measurements in HXR and SXR, we find a mean ratio of
qe=nTrape/nSXRe~1,
with a relatively small scatter by a factor of ~2. Generally, it is
likely that the SXR-bright flare loops have a higher density than
the trapping regions (when qe &lt; 1), but they also are
subject to filling factors less than unity (when qe &gt;
1). Our measurements provide comprehensive evidence that electron
trapping in solar flares is governed in the weak-diffusion limit, i.e.,
that the trapping time corresponds to the collisional deflection time,
while pitch-angle scattering by resonant waves seems not to be dominant
in the 20-200 keV energy range. The measurements do not support a
second-step acceleration scenario for energies &lt;=200 keV.

---------------------------------------------------------
Title: The IACG Campaign IV: solar sources of heliospheric structure
observed out of the ecliptic
Authors: Galvin, A. B.; Hudson, H. S.
1997AdSpR..20..631G Altcode:
The Inter-Agency Consultative Group, representing the four space
agencies with contributing missions (the European Space Agency, the
Japanese Institute of Space and Astronautical Science, the Russian
Space Agency, and the United States National Aeronautics and Space
Administration), has initiated a data analysis campaign “Solar
Sources of Heliospheric Structure Observed out of the Ecliptic”
(IACG Campaign IV) based on the unique capabilities and opportunities
afforded by the core campaign missions, Ulysses and Yohkoh. The campaign
is strongly benefited by data sets from solar ground observatories and
other spacecraft missions (SPARTAN 201, IMP, WIND, SoHO). The campaign
themes emphasize the joint analysis of these remote-sensing and in-situ
data sets for features such as coronal holes, coronal mass ejections,
and other solar sources of heliospheric structure.

---------------------------------------------------------
Title: (Erratum) YOHKOH observations of flares with flat hard X-ray
spectra.
Authors: Farnik, F.; Hudson, H.; Watanabe, T.
1997A&A...324..433F Altcode:
Erratum to Astron. Astrophys. 320, 620 (1997)

---------------------------------------------------------
Title: Statistics of Fluctuations in the Solar Soft X-Ray Emission
Authors: UeNo, S.; Mineshige, S.; Negoro, H.; Shibata, K.; Hudson,
H. S.
1997ApJ...484..920U Altcode:
X-ray emission from the Sun fluctuates as a result of occasional flare
events. We have calculated the power spectra of the solar soft X-ray
variations using the photometric data of the GOES 6 satellite. The data
cover the period 1991 September to 1994 April, about 32 months in total,
and we have worked with 10 minute averages. We find that the total power
spectral densities (PSDs) have three distinct components; a flat or
slightly decreasing low-frequency section, a 1/f-like moderate decline
medium-frequency section, and steep decline high-frequency parts. The
break frequencies separating three parts are fbreak ~=
10-4.7 Hz and 10-3.8 Hz , respectively. Such
downward breaks are expected from the shot-noise (flarelike)
character of solar X-ray emission, and we interpret the lower break
frequency as indicating an upper limit on flare timescales. These
break frequencies do not vary appreciably with activity level. This
suggests the existence of a universal mechanism for triggering flares
in the solar corona. Moreover, the power-law index (where we assume
PSD ~ f-β, f being frequency) of each part does not vary
appreciably with the level of activity either; its average is β ~=
0.45, 0.95, and 1.5, respectively. The overall shape of the PSD is
quite similar to those of other astrophysical objects such as black
hole candidate stars and active galactic nuclei, albeit on a vastly
different scale.

---------------------------------------------------------
Title: The Solar-B Mission
Authors: Antiochos, Spiro; Acton, Loren; Canfield, Richard; Davila,
Joseph; Davis, John; Dere, Kenneth; Doschek, George; Golub, Leon;
Harvey, John; Hathaway, David; Hudson, Hugh; Moore, Ronald; Lites,
Bruce; Rust, David; Strong, Keith; Title, Alan
1997STIN...9721329A Altcode:
Solar-B, the next ISAS mission (with major NASA participation), is
designed to address the fundamental question of how magnetic fields
interact with plasma to produce solar variability. The mission has
a number of unique capabilities that will enable it to answer the
outstanding questions of solar magnetism. First, by escaping atmospheric
seeing, it will deliver continuous observations of the solar surface
with unprecedented spatial resolution. Second, Solar-B will deliver the
first accurate measurements of all three components of the photospheric
magnetic field. Solar-B will measure both the magnetic energy driving
the photosphere and simultaneously its effects in the corona. Solar-B
offers unique programmatic opportunities to NASA. It will continue an
effective collaboration with our most reliable international partner. It
will deliver images and data that will have strong public outreach
potential. Finally, the science of Solar-B is clearly related to the
themes of origins and plasma astrophysics, and contributes directly
to the national space weather and global change programs.

---------------------------------------------------------
Title: The Sun as AN X-Ray Star: Overview of the Method
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H.
1997SoPh..172..239P Altcode: 1997ESPM....8..239P
We present a method to study the solar-stellar connection, i.e., the
close similarity of the physical phenomena occurring on the Sun and
on late-type active stars, by taking advantage of Yohkoh/SXT X-ray
images. From such images, we first generate distribution functions
of the whole disk differential emission measure, and then synthesize
from these spectra analogous to those collected by X-ray telescope
instruments aimed at stars other than the Sun. Here we illustrate the
application of this method to the ROSAT/PSPC and ASCA/SIS, and discuss
test cases as well as future applications.

---------------------------------------------------------
Title: Tracking a CME from Cradle to Grave: A Multi-wavelength
Analysis of the February 6-7, 1997 Event
Authors: Gopalswamy, N.; Kundu, M. R.; Hanaoka, Y.; Kosugi, T.; Hudson,
H.; Nitta, N.; Thompson, B.; Gurman, J.; Plunkett, S.; Howard, R.;
Burkepile, J.
1997SPD....28.0501G Altcode: 1997BAAS...29..908G
The partially earth-directed coronal mass ejection (CME) event of 1997
February 6-7 originated from the southwest quadrant of the sun. The
CME accelerated from 170 km/s to about 830 km/s when it reached a
distance of 25 solar radii. The CME was an arcade eruption followed
by bright prominence core structures. The prominence core was tracked
continuously from the solar surface to the interplanetary medium by
combining data from the Nobeyama radioheliograph (microwaves), Mauna Loa
Solar Observatory (He 10830 { Angstroms}), SOHO/EIT (EUV) and SOHO/LASCO
(white light). The CME was accompanied by an arcade formation, fully
observed by the YOHKOH/SXT (soft X-rays) and SOHO/EIT (EUV). The X-ray
and EUV observations suggest that the reconnection proceeded from
the northwest end to the southeast end of a filament channel. In the
SOHO/EIT images, the the feet of the soft X-ray arcade were observed
as EUV ribbons. The CME event also caused a medium sized geomagnetic
storm: The hourly equatorial Dst values attained storm level during
18:00-19:00 UT on February 09. This means the disturbance took about
2.25 days to reach the Earth. The first signatures of an IP shock was
a pressure jump in the WIND data around 13:00 UT on Feb 09, 1997 which
lasted for about 14 hours, followed by flux rope signatures. This CME
event confirms a number of ideas about CMEs: The three part structure
(frontal bright arcade, dark cavity and prominence core), disappearing
filament, elongated arcade formation, and terrestrial effects. We make
use of the excellent data coverage from the solar surface to the Earth
to address a number of issues regarding the origin and propagation of
the geoeffective solar disturbances. We benefited from discussions at
the first SOHO-Yohkoh Coordinated Data Analysis Workshop, held March
3-7, 1997, at Goddard Space Flight Center.

---------------------------------------------------------
Title: The Solar Source of the January 1997 CME/Magnetic Cloud;
Recurrent Activity on a Polar Crown Filament Channel
Authors: Webb, David; Cliver, E.; McIntosh, P.; Gopalswamy, N.;
Hudson, H.
1997SPD....28.1501W Altcode: 1997BAAS...29..918W
The magnetic cloud and geomagnetic storm on 10-11 January 1997 was
associated with a halo-type CME observed by LASCO near the sun on 6
January. As part of the ISTP and SHINE collaboration on this event we
are studying the solar source region of the CME and its propagation
through the interplanetary medium to Earth. We summarize the rather
weak solar activity that apparently was associated with this CME, and
contrast it with the strong surface and coronal activity associated
with the 9-11 February 1997 magnetic cloud/storm. We present evidence
that the solar sources of both of these events occurred over an area
where the southern polar crown filament channel diverted sharply to
the north and in the decaying remnants of the first large-scale active
region to form during the new solar cycle. Our results also suggest
that this region was the site of earlier CMEs; i.e., it was a key site
of recurrent activity during 1996-97 which, when aimed toward Earth,
produced recurrent magnetic clouds and storms.

---------------------------------------------------------
Title: Observations of Superhot Plasma in Solar Flares
Authors: Nitta, N.; Hudson, H. S.
1997SPD....28.0162N Altcode: 1997BAAS...29..890N
We have shown in an M1-class flare that a structure away from
the main bright loop filled with superhot ( ~ 30 MK) plasma. The
analysis incorporated Yohkoh SXT measurements in three filters and
BCS Fe XXV and Ca XIX spectra. We made use of the fact that even the
thickest filter of SXT is sensitive to plasma of ~ 10 MK, biasing
the temperature determination towards lower values. This resulted in
an estimated temperature of the imaged superhot plasma consistent
with that derived from hard X-rays (the Yohkoh HXT M1/L channel
ratio). We now apply the same technique to several other flares to
study the existence of superhot plasma, its timing and location with
respective to the impulsive component and its range of parameters. We
identify flares with superhot plasma irrespective of the results from
the BCS Fe XXVI diagnostic (which has large uncertainty), suggesting
that the latter should not be the only method to determine whether a
flare has superhot plasma. We also discuss the data in terms of the
true differential emission measure (a local property of the plasma)
rather than the spatial-composite emission measure (the average along
the line of sight).

---------------------------------------------------------
Title: Electron Temperatures of the Corona Above a Solar Active
Region Determined from S XV Spectra
Authors: Sterling, Alphonse C.; Hudson, Hugh S.; Watanabe, Tetsuya
1997SPD....28.0136S Altcode: 1997BAAS...29..885S
We present high resolution soft X-ray spectral observations of the
corona above an active region, using data from the Bragg crystal
spectrometer (BCS) on board the Yohkoh satellite. We observed NOAA AR
7978 as it rotated beyond the solar limb so that the lower portions of
the region were occulted. Long integrations from times after the region
had totally disappeared some days later show a substantial background
in S xv. Since the background spectrum is featureless, spectral lines
obtained from the time of occultation must originate from the upper
corona of the active region. Our results support previous findings that
the corona consists of two components: a cooler, steady component with
T_e ~ 3 MK, and a hotter, transient component in excess of 5 MK. This
hotter component is due to microflares; outside the time of microflares
there is relatively little or no active region upper coronal plasma
with T_e gtrsim 3.5 MK. There is evidence for a decrease in T_e with
height for the cool component.

---------------------------------------------------------
Title: Interacting X-ray Loops in a Coronal Mass Ejection
Authors: Hudson, Hugh S.
1997SPD....28.0137H Altcode: 1997BAAS...29..886H
The Yohkoh soft X-ray telescope observed a part of a large coronal
mass ejection on 5 October 1996. The event originated beyond the west
limb, probably by at least two days' rotation to be consistent with
the location of the successor of AR 7978. Coronal soft X-ray dimming
accompanied the ejection, suggesting that the X-ray brightness of the
source region extended to high altitude, again consistent with the
identification with the successor of AR 7978. Following the dimming, a
network of filamentary structures rose outward, giving the appearance
of an opening flux rope that twisted as it rose. The south end of
the flux rope appeared to be confined by an arcade of loops. There
was no evidence for loop-loop reconnection between the two sets of
filamentary structures.

---------------------------------------------------------
Title: Temporal Variations of Solar Flare Spectral Properties: Hard
X-Ray Fluxes and Fe XXV, Ca XIX, and Wide-Band Soft X-Ray Fluxes,
Temperatures, and Emission Measures
Authors: Sterling, Alphonse C.; Hudson, Hugh S.; Lemen, James R.;
Zarro, Dominic A.
1997ApJS..110..115S Altcode:
We present fluxes, temperatures, and emission measures for nine
solar flares, using data from both the Fe XXV and Ca XIX channels
of the Bragg Crystal Spectrometer (BCS) experiment on the Yohkoh
satellite and from the wide-band soft X-ray spectrometers on the GOES
spacecraft. We also present hard X-ray fluxes from the Hard X-ray
Telescope (HXT) on Yohkoh and the BATSE spectrometer on the Compton
Gamma-Ray Observatory (CGRO). All events occurred during 1992 and
ranged in size from GOES class C5 to M2. Three of the events occurred
near the solar limb. For each flare we give two sets of plots. The
first set shows flux, electron temperature, and emission measures
for Fe XXV, Ca XIX, and GOES as functions of time. The second set of
plots gives log electron temperature as functions of log (emission
measure)1/2 for these three wavelength ranges; we refer to
these plots as E1/2-T diagrams. Hard X-ray flux information
is included in both sets of plots. Our observations indicate that (1)
cooler plasmas are located along the legs of, or are evenly distributed
along, the flaring loops, while hotter plasmas are concentrated near
the loop tops, (2) peaks in temperature in each of the wavelength
bands are closely associated with hard X-ray enhancements, and (3)
the emission from both relatively hot and relatively cool flaring
plasmas emanates from the same loop or from closely related loops.

---------------------------------------------------------
Title: YOHKOH observations of flares with flat hard X-ray spectra.
Authors: Farnik, F.; Hudson, H.; Watanabe, T.
1997A&A...320..620F Altcode:
A series of flares with exceptionally hard spectral indices in the hard
X-ray band occurred on 3 October 1993. The non-thermal bremsstrahlung
spectra may extend to a few keV in these events, one of which was
detectable in the Yohkoh Bragg Crystal Spectrometer at 7keV as well as
by the hard X-ray instruments at higher energies. We present Yohkoh
soft and hard X-ray imaging, spectroscopy and energetics analysis
of these events, with the idea that flares with such flat spectra
(power-law as hard as 1.98 below 33keV) might differ appreciably from
ordinary flares. The series of events is strongly homologous, with no
systematic variations in structure over a period of 3.5-hours except
for jet-like ejecta accompanying Type III/V bursts. Unlike other hard
events, these flares are large (footpoint separation about 3x10^4^km)
and therefore well resolved by the Yohkoh imaging instruments. The time
variations match the Neupert effect. The hard and soft X-ray images
also show footpoint brightening and loop filling. The spikes with
the flattest spectra have the weakest Neupert-effect signature, but no
perceptible time delays between the hard X-ray time profile and the soft
X-ray time derivative. These events do not produce superhot emission
but are probably microwave-rich. We find no evidence for large-scale
magnetic reconnection in the development of these flares. We note
two discrepancies between the observations and the existing numerical
hydrodynamic models of flare energetics, and suggest that rapid spike
events of this type provide good tests of such models.

---------------------------------------------------------
Title: Electron Temperatures of the Corona Above a Solar Active
Region Determined from S XV Spectra
Authors: Sterling, Alphonse C.; Hudson, Hugh S.; Watanabe, Tetsuya
1997ApJ...479L.149S Altcode:
We present the first high-resolution soft X-ray spectral observations of
the corona above an active region, using the Bragg crystal spectrometer
(BCS) on board the Yohkoh satellite. We observed NOAA AR 7978 as it
rotated beyond the solar limb so that the lower portions of the region
were occulted. Long integrations from times after the region had totally
disappeared some days later show a substantial, variable background
in S XV. Since the background spectrum is featureless, spectral lines
obtained from the time of occultation must originate from the upper
corona of the active region. Our results support previous findings that
the active region corona consists of two components: a cooler, steady
component with Te ~ 3 MK and a hotter, transient component
in excess of 5 MK. This hotter component is due to microflares; outside
the time of microflares there is relatively little or no active region
upper coronal plasma with Te &gt;~ 3.5 MK. There is evidence
for a decrease in Te with height for the cool component.

---------------------------------------------------------
Title: Isolating the Footpoint Characteristics of a Solar Flare Loop
Authors: Harra-Murnion, L. K.; Culhane, J. L.; Hudson, H. S.; Fujiwara,
T.; Kato, T.; Sterling*, A. C.
1997SoPh..171..103H Altcode:
We analyse the physical characteristics of a C5.7 class flare which
was observed on 27 September, 1993 using data from the soft X-ray
telescope (SXT), the Bragg crystal spectrometer (BCS), and the hard
X-ray telescope (HXT) on Yohkoh. The flare takes the form of a simple
loop which is much brighter at one of its footpoints than anywhere
else for a period of 2 min. During this time there is an increase in
the soft X-ray fluxes, and a corresponding peak in hard X-rays. The
parameters derived from the hard X-ray and soft X-ray spectra and images
are assumed to be from the footpoint. This flare showed two peaks in
the non-thermal velocity, the first one simultaneous with the footpoint
brightening. The non-thermal velocity corresponding to these first few
minutes is unusually large - by a factor of 80%, 68%, and 26% relative
to the second peak in the Fexxv, Caxix, and Sxv channels respectively.

---------------------------------------------------------
Title: A Multi-Wavelength Analysis of the February 6/7, 1997 Coronal
Mass Ejection
Authors: Plunkett, S. P.; Gopalswamy, N.; Kundu, M. R.; Howard, R. A.;
Thompson, B. J.; Gurman, J. B.; Lepping, R. P.; Hudson, H. S.; Nitta,
N.; Hansoka, Y.; Kosugi, T.; Burkepile, J. T.
1997ESASP.404..615P Altcode: 1997cswn.conf..615P
No abstract at ADS

---------------------------------------------------------
Title: Active Region Coronal Temperatures from YOHKOH BCS Sxv Spectra
Authors: Sterling, A. C.; Hudson, H. S.; Watanabe, T.
1997IAUJD..19E...9S Altcode:
Using the Bragg crystal spectrometer (BCS) on board the Yohkoh
satellite, we present high resolution soft X-ray spectral observations
of the corona above an active region. We observed NOAA AR 7978 as
it rotated beyond the solar limb so that the lower portions of the
region were occulted. Long integrations from times after the region
had totally disappeared some days later show a substantial background
in Sxv. Since the background spectrum is featureless, spectral lines
obtained from the time of occultation must originate from the upper
corona of the active region. Our results support previous findings
that the active region corona consists of two components: a cooler,
steady component with electron temperature T_e ~3 MK, and a hotter,
transient component in excess of 5 MK @. This hotter component is due
to micro-flares; outside the time of micro-flares there is relatively
little or no active region upper coronal plasma with T_e higher than
about 3.5 MK @. There is evidence for a decrease in T_e with height
for the cool component.

---------------------------------------------------------
Title: Active Region Energetics via Yohkoh/BCS and SOHO/CDS
Authors: Watanabe, T.; Sterling, Alphonse C.; Hudson, Hugh S.;
Harra-Murnion, Louise K.
1997ESASP.404..723W Altcode: 1997cswn.conf..723W
No abstract at ADS

---------------------------------------------------------
Title: Limb Events Observed by YOHKOH and Coronal Mass Ejections:
A Filamentary Soft X-ray Structure on 5 October 1996
Authors: Watari, S.; Watanabe, Takashi; Acton, L. W.; Hudson, H. S.
1997ESASP.404..725W Altcode: 1997cswn.conf..725W
No abstract at ADS

---------------------------------------------------------
Title: The Sun as an X-ray Star: Overview of the Method
Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H.
1997IAUJD..19E..37P Altcode:
We present a method to study the solar-stellar connection, i.e., the
close similarity of the physical phenomena occurring on the Sun and
on late-type active stars, by taking advantage of Yohkoh/SXT X-ray
images. From such images, we first generate distribution functions
of the whole disk differential emission measure, and then synthesize
from these spectra analogous to those collected by X-ray telescope
instruments aimed at stars other than the Sun. Here we illustrate
the application of this method to the ROSAT/PSPC and ASCA/SIS,
and discuss test cases as well as future applications. For a more
detailed discussion, please refer to a paper, by the same authors in
"OBSERVATIONAL PLASMA ASTROPHYSICS: FIVE YEARS OF YOHKOH AND BEYOND",
T. Watanabe, T. Kosugi, and A. C. Sterling, eds., Kluwer Academic
Publishers, the Proceedings of the Yohkoh 5th Anniversary Symposium,
held in November 1996, in Yoyogi, Tokyo, Japan.

---------------------------------------------------------
Title: Comparison between arch filaments and coronal loops.
Authors: Yoshimura, K.; Kurokawa, H.; Sano, S.; Hudson, H.
1997IAUJD..19E..60Y Altcode:
Kawai et al. (1992) presented preliminary results from comparison
between arch filament system (AFS) and soft X-ray (SXR) loops. They
found the AFS was covered with the SXR bright features. Coordinated
observations with YOHKOH enable us to make more detail comparison
between arch filament(AFS) and SXR loops. (With small brightening
points both in SXR and H alpha images, we can co-align those images
accurately enough.) We intended to see how each AFS contributes to
SXR brightenings. We will show examples of AFS which no particular
SXR brightening occurred around. And we will present observations of
SXR major brightenings of loops, which was related to H alpha dark
features. The dark features did not seem to be AFS. They may be small
active region filaments which indicates magnetic sheared structure
(evidence for storages of extra magnetic energy).

---------------------------------------------------------
Title: X-ray Observations of Coronal Mass Ejections
Authors: Hudson, H. S.
1997IAUJD..19E..24H Altcode:
X-ray and XUV telescopes on Yohkoh and SOHO show the full coronal
structure, both over the limbs and in front of the solar disk. A
coronal mass ejection (CME) must result in a perceptible alteration
of the coronal structure, and indeed “depletions” have long been
known from coronagraph and corona-meter data. The X-ray observations
also show depletion effects, which we term “dimming”, including the
transient coronal holes observed by the X-ray telescopes on Skylab. The
new X-ray observations allow us to study the actual outward flow of
the coronal material in some cases; we find both diffuse clouds and
highly filamented loop-like structures in the “dimming” volumes and
can estimate the CME masses. The observations are generally consistent
with the standard model of magnetic reconnection used to explain CME's
and long-duration flares, except that there appears to be little delay
between the eruption and the flaring.

---------------------------------------------------------
Title: The Solar Flare of 1992 August 17 23:58 UT} %
Authors: Takahashi, Masaaki; Watanabe, Tetsuya; Sakai, Jun-Ichi;
Sakao, Taro; Kosugi, Takeo; Sakurai, Takashi; Enome, Shinzo; Hudson,
Hugh S.; Hashimoto, Shizuyo; Nitta, Nariaki
1996PASJ...48..857T Altcode:
A small flare (C4.3 in the GOES X-ray class) was well observed by
all of the instruments on board Yohkoh. The X-ray light curves have
double peaks which are about 5 min apart. Until the first peak from
flare onset, four compact areas brighten up in the soft X-ray region,
which are aligned almost on one straight line. We regard them as being
footpoints of two sets of loops, which are identifiable in soft X-ray
images, since their locations match those of hard X-ray sources. Indeed,
after the second peak, the temporal behavior of the temperature and
emission measure at each point is consistent with the existence of
two such loops. Comparing our results with recent MHD simulations,
we propose a possible scenario for this flare that is based on the
coalescence of two loops.

---------------------------------------------------------
Title: Comprehensive Multiwavelength Observations of the 1992 January
7 Solar Flare
Authors: Silva, Adriana V. R.; White, Stephen M.; Lin, Robert P.;
de Pater, Imke; Gary, Dale E.; McTiernan, James M.; Hudson, Hugh S.;
Doyle, J. Gerry; Hagyard, Mona J.; Kundu, Mukul R.
1996ApJS..106..621S Altcode:
Observations of a solar flare that occurred at 2022 UT on 1992
January 7, during the 1991 December/1992 January Max `91 campaign,
are presented. This flare was observed simultaneously in Hα, radio
(at microwave and millimeter wavelengths), and soft and hard X-rays
(by the Yohkoh spacecraft) with high spatial and moderate spectral
resolution. A comparison of magneto grams before and after the flare
shows evidence of the emergence of new magnetic flux of opposite
polarity at the flare site. Although this flare was only of moderate
size (GOES classification C8.9 and Hα importance SF), it exhibited
several distinct bursts and at least 10 spatially distinct hard/soft
X-ray sources. Cospatial Hα brightenings suggest that most of the
X-ray sources are located at footpoints of magnetic loops. Two of the
hard X-ray sources have no Hα counterparts and are therefore believed
to be located at loop tops. The flare consisted of three bursts
of particle acceleration followed by a purely thermal phase. High
spectral resolution Ca XIX line profiles indicate upflows shortly
after the second acceleration phase. Analysis of the microwave/hard
X-ray/soft X-ray emission from individual sources provides information
on the radio emission mechanisms, the energetic electron population,
the magnetic field strength, and the plasma density. These parameters
were estimated for the two microwave sources observed during the
third acceleration burst; these sources were simultaneously detected
in soft X-rays, and one of the sources is also seen in hard X-ray
maps. Although the microwave emission is consistent with the gyro
synchrotron mechanism, the millimeter emission, which peaks during the
thermal phase when all nonthermal activity has ceased, is likely due
to thermal bremsstrahlung from the soft X-ray emitting hot plasma. The
energy lost to collisions by the energetic (&gt;15 keV) electrons
and the energy contained in the thermal plasma are calculated for
each source. The energy injected by the nonthermal electrons from all
sources is estimated to be 1030 ergs. Only the soft X-ray
sources with gradual time profiles seem to show the Neupert effect.

---------------------------------------------------------
Title: A Long-Duration Solar Flare with Mass Ejection and Global
Consequences
Authors: Hudson, H. S.; Acton, L. W.; Freeland, S. L.
1996ApJ...470..629H Altcode:
We report observations of a long-duration flare with mass ejection
from the corona, using the Yohkoh soft X-ray telescope (SXT). This
flare occurred 1994 November 13 near disk center during quiet solar
conditions, with excellent temporal coverage of both the core activity
in the active region itself and of the global corona. The initial
X-ray images reveal two arcades of cusped magnetic loops, connected
via a series of thin loops. These loops rise rapidly during the
increasing phase of soft X-ray flare brightness. In its final state,
the flare has the configuration of postflare loops with a cusp. Large
regions of the X-ray corona appear to empty during the evolution of the
event. We suggest that this corresponds a coronal mass ejection (CME)
seen in soft X-rays. Its detection in the SXT images is consistent with
the finding that material participating in a CME exists at elevated
coronal temperatures (2.8 x 106 K in this case) before
the ejection. We estimate a mass &gt;4 x 1014 g for the
ejected material. The X-ray morphology of the event has strong points
of similarity with the classical reconnection picture of long-duration
event (LDE) formation, but there are significant discrepancies: there
is no observed inward flow during the rise phase, the expansions are
multiple and appear to be nonradial, and none of the observed motions
suggest a reconnection jet. We note the subsequent occurrence of very
large scale coronal disturbances, including regions near the boundaries
of coronal holes at both poles. We suggest that this global disturbance
implies a perturbation reaching as far outward as the heliospheric
neutral sheet. The exciter would require a horizontal velocity of
approximately 200 km s-1 in such a case, consistent with
the projected velocity of the plasma cloud that we identify with a
CME in the process of launching.

---------------------------------------------------------
Title: The Scaling Law between Electron Time-of-Flight Distances
and Loop Lengths in Solar Flares
Authors: Aschwanden, Markus J.; Kosugi, Takeo; Hudson, Hugh S.; Wills,
Meredith J.; Schwartz, Richard A.
1996ApJ...470.1198A Altcode:
From the complete data set of solar flares simultaneously observed
with the Burst and Transient Source Experiment (BATSE) on board the
Compton Gamma Ray Observatory (CGRO) in the high-time resolution mode
(64 ms) and the Hard X-ray Telescope (HXT) on board Yohkoh, we were
able to determine the electron time-of-flight (TOF) distance l' and
the flare loop geometry in 42 events. The electron TOF distances were
determined from time delays (of ≍ 10-100 ms) of hard X-ray (HXR)
pulses (measured in 16 channel spectra over ≍ 20-200 keV), produced
by the velocity difference of the HXR-producing electrons. The flare
loops were mostly identified from double footpoint sources in ≥30
keV HXT images, with radii in the range r = 3000-25,000 km. We find
a scaling law between the electron TOF distance l' and the flare loop
half-length 5 = r(π/2), having a mean ratio (and standard deviation)
of l'/s = 1.4±0.3. In five flares, we observe coronal ≥ 30 keV HXR
sources of the Masuda type in the cusp region above the flare loop and
find that their heights are consistent with the electron TOF distance
to the footpoints. These results provide strong evidence that particle
acceleration in solar flares occurs in the cusp region above the flare
loop and that the coronal HXR sources discovered by Masuda et al. are
a signature of the acceleration site, probably controlled by a magnetic
reconnection process.

---------------------------------------------------------
Title: Large-scale active coronal phenomena in Yohkoh SXT images
Authors: Fárník, František; Švestka, Zdeněk; Hudson, Hugh S.;
Uchida, Yutaka
1996SoPh..168..331F Altcode: 1996SoPh..168..331U
We discuss Yohkoh SXT observations of stationary giant post-flare
arches which occurred on 3-6 May, 1992 and study in detail the last
arch, associated with the flare at 19:02 UT on 5 May, which extended
above the west limb. The arch was similar to the first giant arch
discovered on board the SMM, on 21-22 May, 1980. We demonstrate that
the long lifetimes of these structures necessarily imply additional
energy input from the underlying active region: otherwise, conduction
would cool these arches in less than one hour and even with the
unlikely assumption of conduction inhibited, pure radiative cooling
would not produce the temperature decrease observed. All arch tops,
although varying in brightness, stayed for several days at a fairly
constant altitude of ∼ 100 000 km, and the arch studied, on 5-6 May,
was just a new brightening of the pre-existing decaying structure. The
brightening was apparently due to inflow of hot plasma from the flare
region. Yohkoh data confirm that these stationary arches are rare
phenomena when compared with the rising arches studied in Paper I and
with Uchida et al.'s expanding active regions.

---------------------------------------------------------
Title: Electron Time-of-Flight Distances and Flare Loop Geometries
Compared from CGRO and YOHKOH Observations
Authors: Aschwanden, Markus J.; Wills, Meredith J.; Hudson, Hugh S.;
Kosugi, Takeo; Schwartz, Richard A.
1996ApJ...468..398A Altcode:
The distance between the coronal acceleration site and the chromospheric
hard X-ray (HXR) emission site can be determined from velocity-dependent
electron time-of-flight (TO F) differences in the framework of the
thick-target model. We determine these electron TOF distances 1 with
relative time delay measurements in the 30-300 keV energy range,
using 16 channel data from BATSE/CGRO for the eight largest flares
simultaneously observed with Yohkoh. We filter the HXR fine structure
from the smoothly varying HXR flux with a Fourier filter in order to
separate competing time delays. In the Yohkoh/HXT images we identify the
corresponding flare loops that show ≥30 keV HXR footpoint emission
and project the electron TOF distances into the loop plane, assuming
a semicircular shape (with radius r). The flare loop radii vary in
the range of r = 5600-17,000 km. In all eight flares we find that the
projected electron TOF distance l' exceeds the loop half-length s =
r(π/2), with a scale-invariant ratio of l'/s = 1.3±0.2. Projecting
the electron TOF distances onto an open field line that extends to the
cusp region above the flare loop, we find an average ratio of h/r =
1.7±0.4 for the height h of the acceleration site. This geometry is
compatible with acceleration mechanisms operating in the cusp region,
perhaps associated with magnetic reconnection processes above the flare
loop. Alternatively, acceleration sites inside the flare loop cannot
be ruled out (since l'/s &lt; 2), but they do not provide a natural
explanation for the observed length ratio l'/s. Large-scale electric
DC field acceleration mechanisms are found to be less suitable to
explain the observed HXR timing and pulse durations.

---------------------------------------------------------
Title: Solar identification of solar-wind disturbances observed
at Ulysses
Authors: Lemen, J. R.; Acton, L. W.; Alexander, D.; Galvin, A. B.;
Harvey, K. L.; Hoeksema, J. T.; Zhao, X.; Hudson, H. S.
1996AIPC..382...92L Altcode:
The Ulysses polar passages are producing a unique set of observations of
solar-wind disturbances at high heliographic latitudes. In this paper
we use the Yohkoh soft X-ray telescope (SXT) to locate some of these
events, as defined by the Ulysses/SWICS data, in the solar corona. Of 8
events, we identify two with flares, three with front-side large arcade
events, two with far-side events, and one was not seen in the Ulysses
data. The arcade events generally resemble long-duration flares seen
in active regions, but are larger, slower, and cooler. We present
Yohkoh images of each of these events. In the large arcade events
(see Alexander et al., 1996, for a detailed look at one of them) the
magnetic morphology at the location of the Yohkoh arcade is generally
consistent with the development of a large system of loops. Some of
the identifications are ambiguous, and we summarize the reasons for
this. From the SWICS data we have obtained ionization temperatures for
several events, and find that they have no obvious pattern in relation
to the X-ray temperatures; this may be expected on the basis that the
interplanetary plasma cloud is physically distinct from the plasma
trapped in the corona. Soft X-ray observations of the solar corona
show occasional occurrences of large-scale brightenings in the form
of arcades of loops. Such structures have been known since Skylab
(e.g., Sturrock, 1980), and have a clear relationship with coronal
mass ejections (e.g., Kahler, 1977). We now may study this phenomenon
statistically with the much more comprehensive Yohkoh observations;
with Yohkoh movies we can also begin to extend our knowledge to the
three-dimensional development of the structures. At the same time
Ulysses has sampled the latitude dependence of the interplanetary
effects. With this paper we introduce this subject and provide a
preliminary listing of events from the passage of Ulysses through
high heliographic latitudes. The starting point of the present
survey is a list of interplanetary plasma clouds (IPC's) derived
from Ulysses/SWICS data. These are essentially the same as the events
termed CMEs by Gosling et al. (1994a, 1994b). For this identification
the presence of bidirectional streaming in the suprathermal electron
distribution is one of the main criteria. We note that there are no
direct coronagraph observations, however. The Yohkoh observations
were examined at the apparent time of origin of each Ulysses event,
resulting in some clear and some less-certain identifications. We
also studied the ionization temperatures of the IPC material as a
beginning step to give the identifications a physical basis. There
has been little study thus far of the Yohkoh soft X-ray observations
in relationship to CMEs, which we believe to be closely related to
the interplanetary disturbances. Hiei et al. (1993) reported the only
Yohkoh event yet studied in conjunction with white-light coronagraph
observations. However Klimchuk et al. (1994) showed that X-ray eruptive
phenomena with parameters similar to those of CMEs occur frequently at
the limb, and there have been several studies of individual eruptive
events (e.g., Watanabe et al., 1992). Presently there is no systematic
knowledge of the X-ray coronal counterparts of CMEs, and the survey
represented here is part of the effort to rectify this situation.

---------------------------------------------------------
Title: Energetic particles and coronal mass ejections in the high
latitude heliosphere: Ulysses-LET observations
Authors: Bothmer, V.; Marsden, R. G.; Sanderson, T. R.; Trattner,
K. J.; Wenzel, K. -P.; Balogh, A.; Forsyth, R. J.; Goldstein, B. E.;
Uchida, Y.; Hudson, H. S.
1996AIPC..382..445B Altcode:
We have investigated energetic ions of non-corotating nature in the high
latitude heliosphere. Major particle events were observed by Ulysses up
to latitudes of 60 °S. All were associated with passage of coronal mass
ejections (CMEs) over the spacecraft. The relationship of these events
with solar activity was investigated using Yohkoh soft X-ray images.

---------------------------------------------------------
Title: The large scale eruptive event of 1994 April 14
Authors: Alexander, David; Harvey, K. L.; Hudson, H. S.; Hoeksema,
J. T.; Zhao, X.
1996AIPC..382...80A Altcode:
The polar crown event of 1994 April 14 is one of the largest scale
eruptive events observed by the Yohkoh/SXT. Associated with the
formation of an arcade of soft X-ray loops at the Sun was the
detection of an interplanetary forward/reverse shock event by the
Ulysses spacecraft some 7 days later. The relationship between the
coronal and interplanetary signatures of these events is important if
we are to address fully the initialisation and consequent development of
interplanetary phenomena, such as CMEs and counter-streaming electrons,
originating at the Sun. We investigate the development of the energetics
of the 1994 April 14 event and their relationship to the dynamics of
the eruption are investigated. The arcade formation, together with the
eruption of material into interplanetary space, suggests a large-scale
temporary reconfiguration of the coronal magnetic field. We examine
the effects of the formation of such a coronal arcade structure on
the HCS and discuss the dynamics involved with the passage of a large
scale disturbance through the interplanetary magnetic field.

---------------------------------------------------------
Title: The solar origins of two high-latitude interplanetary
disturbances
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Harvey, K. L.;
Kahler, S. W.; Kurokawa, H.; Lemen, J. R.
1996AIPC..382...84H Altcode:
Two extremely similar interplanetary forward/reverse shock events,
with bidirectional electron streaming, were detected by Ulysses in
1994 [Gosling et al., 1994]. Both events resulted in geomagnetic
storms and presumably were associated with coronal mass ejections. In
this paper we use the Yohkoh soft X-ray observations to characterize
the conditions in the lower corona at the times appropriate for the
launching of these two events. We find two strikingly different solar
events to be the likeliest candidates: an LDE flare on 20 Feb. 1994,
and a extremely large-scale arcade event on 14 April 1994.

---------------------------------------------------------
Title: Pixon-based Multiresolution Image Reconstruction for Yohkoh's
Hard X-Ray Telescope
Authors: Metcalf, Thomas R.; Hudson, Hugh S.; Kosugi, Takeo; Puetter,
R. C.; Pina, R. K.
1996ApJ...466..585M Altcode:
We present results from the application of pixon-based multiresolution
image reconstruction to real and simulated data from Yohkoh's Hard
X-ray Telescope (HXT). The goal of the pixon algorithm is to minimize
the number of degrees of freedom used to describe an image within the
accuracy allowed by the noise. This leads to a reconstruction that is
optimally constrained. We apply the pixon code to two solar flares in
the HXT database and compare the results of the pixon reconstruction to
the results of a direct, linear, smoothed inversion of the HXT Fourier
synthesis data and to a maximum entropy reconstruction. The maximum
entropy reconstruction is vastly better than the direct inversion,
but the pixon reconstruction gives superior noise suppression and
photometry. Further, the pixon reconstruction does not suffer from
overresolution of the images.

---------------------------------------------------------
Title: Electron Time-of-Flight Measurements during the Masuda Flare,
1992 January 13
Authors: Aschwanden, Markus J.; Hudson, Hugh; Kosugi, Takeo; Schwartz,
Richard A.
1996ApJ...464..985A Altcode:
The solar flare of 1992 January 13, 1729 UT, has become famous because
Masuda's discovery of a hard X-ray loop-top source (Masuda 1994). Here
we analyze energy-dependent time delays occurring in 30-120 keV hard
X-ray (HXR) emission during this flare, observed by BATSE on board
the Compton Gamma Ray Observatory with a time resolution of 64 ms. The
purpose of this study is to reconstruct the kinematics of HXR-producing
electrons from energy-dependent HXR delays, and to relate the inferred
time-of-flight distance to the spatial geometry of the flare loop,
as observed by SXT and HXT on board Yohkoh. The findings are the
following: 1. The HXR flux can be decomposed into a sequence of
pulses with ≍2-3 s duration and into a smoothly varying envelope
that accounts for 90% of the ≥ 30 keV. flux. Cross-correlating the
pulses between five different energy channels in the 30-120 keV range,
we find that the HXR pulses are delayed (τP = 40-220 ms)
at the lower energies with respect to the higher energies. For the
HXR envelopes, we find much larger delays (-τE = 2.1-6.6
s) of opposite sign. 2. We fit kinematic models that quantify
electron acceleration and propagation times to the observed HXR timing,
for small-scale and large-scale accelerating fields, in semicircular
and cusplike flare loop geometries. We find that the acceleration
site is most likely located in an altitude of h = 44,000±6000 km,
in the cusp region above the SXR-emitting flare loop (h = 12,500 km),
and also significantly above Masuda's coronal HXR source (h = 22,100
km). This finding offers an interpretation of Masuda's HXR source in
terms of nonthermal bremsstrahlung by electrons partially confined
in the cusp region either by magnetic mirroring or by wave turbulence
in the reconnection outflow. 3.The delay of the smoothly varying
HXR flux is found to be consistent with trapping time differences in
terms of collisional deflection, based on estimates of the electron
density (ne ≤ 2 x 1011 cm-3)
from SXT emission measure maps. This study provides the first
quantitative localization of the electron acceleration site in a
solar flare, and demonstrates that energy-dependent HXR delays offer
a sensitive diagnostic for electron acceleration, propagation, and
trapping in solar flares.

---------------------------------------------------------
Title: YOHKOH Observations of an Over-the-Limb Solar Flare with
Large Spectral Line Shifts
Authors: Sterling, Alphonse C.; Harra-Murnion, Louise K.; Hudson,
Hugh S.; Lemen, James R.
1996ApJ...464..498S Altcode:
We present observations of a solar flare of 1993 April 15 near 9 UT,
using data from the Yohkoh Bragg crystal spectrometer (BC S) and soft
X-ray telescope (SXT). Observations from SXT indicate that the flare
occurred well beyond the solar limb, meaning that our observations
are restricted to the uppermost portions of the flaring structure. BCS
spectra show strong bulk blueshifts of the spectral line profiles for a
short period near the start of the event, followed by an extended period
of strong bulk redshifts of the line profiles. Concurrent with these
bulk line shifts, the spectra show "blue wing" asymmetries. Both bulk
line shifts and blue wings are infrequent characteristics of flares
observed near the solar limb. Our observations are consistent with
strong upward mass motions occurring on a high-altitude flaring loop
oriented edge-on with the Earth. We find nonthermal line broadenings
in the spectra which are qualitatively and quantitatively similar to
line broadenings in spectra of disk flares. Near peak intensity of the
flare, ≤10% of the residual nonthermal broadening can be explained
by the spatial distribution of the soft X-ray flaring structure.

---------------------------------------------------------
Title: Yohkoh observations of flares with superhot properties
Authors: Hudson, H. S.; Nitta, N.
1996AIPC..374..285H Altcode: 1996hesp.conf..285H
Solar flares, almost as their defining property, fill coronal magnetic
flux tubes with hot plasma. When the temperature of a significant
fraction of this plasma exceeds about 30×106 K, we call
the event “superhot”, following the initial observation of the
hard X-ray continuum of such an event by Lin et al. (11). The Yohkoh
observations include many examples of similar events, of which three
have been published thus far. This paper reports a survey of the Yohkoh
observations, based mainly on the hard X-ray spectra obtained by the HXT
instrument. While comprehensive conclusions will not be possible until
the survey includes the Yohkoh imaging observations, we make tentative
suggestions here about the nature of flares with superhot properties.

---------------------------------------------------------
Title: Temporal Variations of Solar Flare Spectral Properties in CA
XIX and GOES
Authors: Sterling, A. C.; Hudson, H. S.; Lemen, J. R.; Zarro, D. A.
1996AAS...188.1905S Altcode: 1996BAAS...28..850S
Since the advent of space borne X-ray observations, there has been
a strong interest in the nature of the X-ray solar flare. Examining
the relationships between radiations produced in different portions
of the X-ray spectrum gives us information on the properties of the
constituent flaring plasmas. We have studied the joint variation of
electron temperatures and emission measures for a number of flares in
two different wavelength ranges, using data from the narrow band Ca xix
channel (near 3.18 Angstroms) of the Bragg crystal spectrometer (BCS)
experiment on board the Yohkoh spacecraft, and data from the wide band
X-ray monitors on the Geostationary Operational Environmental Satellites
(GOES, covering 0.5---8 Angstroms). A power law relationship often
describes the relationship between temperature and emission measure
during the decay phase in both wavelength ranges. According to work of
Sylwester et al. (1993, A&amp;A 267, 586), energy input parameters and
physical properties of the flaring loop(s) determine the slope of this
power law. We find that ratios of Ca xix to GOES slopes generally fall
between .6 and 1.0, when slopes in both channels are measured during the
flare decay in each respective wavelength range. This relatively good
agreement between slopes in the two channels suggests that emissions
in both wavelength ranges originate from either the same flaring loop,
or differing loops with similar global properties.

---------------------------------------------------------
Title: A Signature of CME Onsets in Soft X-rays
Authors: Webb, D.; Hudson, H.; Lemen, J.
1996AAS...188.7008W Altcode: 1996BAAS...28..939W
Some CME models involve the opening of previously closed magnetic
field lines as the plasma erupts through the lower corona. However, the
identification of observational signatures of this process has proven
illusive. We report on Yohkoh/SXT observations of large-scale arcade
events which reveal the dimming, or depletion, of coronal material
above the bright long-duration arcade region. Viewed in video form, the
impression is of an opening up and evacuation of a local portion of the
lower corona. We interpret this as the process of field line opening
in the initial phase of a CME and, therefore, as the first direct
signature of the mass ejection in soft X-rays. The dimming appears as a
reduction of the soft X-ray intensity by about a factor of 2-3. We are
studying several events of this type which reveal different aspects of
the geometry and kinematics of this process. Highly structured loops
can be seen expanding in the dimming regions of some events and can
be used to characterize the velocity field of the initial CME expansion.

---------------------------------------------------------
Title: The Scaling Law between Electron Time-of-Flight Distances
and Loop Lengths in Solar Flares
Authors: Aschwanden, M. J.; Kosugi, T.; Hudson, H. S.; Wills, M. J.;
Schwartz, R. A.
1996AAS...188.2608A Altcode: 1996BAAS...28..858A
;}} ;}} From the complete dataset of solar flares simultaneously
observed with the Burst and Transient Source Experiment (BATSE) onboard
the Compton Gamma Ray Observatory (CGRO) in the high-time resolution
mode (64 ms) and the Hard X-Ray Telescope (HXT) onboard Yohkoh we
were able to determine the electron time-of-flight (TOF) distance
and the flare loop geometry in 42 events. The electron TOF distances
l' were determined from hard X-ray (HXR) time delays (~ 10-100 ms)
occurring in the 16-channel spectra (at ~ 20-200 keV), produced by the
velocity difference of the HXR-producing electrons. The flare loops
were mostly identified from double footpoint sources in &gt;~ 30 keV
HXT images, with radii ranging from r=3000 to r=25,000 km. We find a
scaling law between the electron TOF distance l' and the flare loop
half length s=r(pi /2), having a mean ratio (and standard deviation)
of l'/s=1.41+/- 0.29. In 5 flares we observe coronal &gt;~ 30 keV
HXR sources of the Masuda-type in the cusp region above the flare
loop, and find that their heights are consistent with the electron
TOF distance to the footpoints. These results provide strong evidence
that particle acceleration in solar flares occurs in the cusp region
above the flare loop and that the coronal HXR sources discovered by
Masuda are a signature of the acceleration site, probably controlled
by a magnetic reconnection process.

---------------------------------------------------------
Title: Observations of Coronal Depletion and Ejection
Authors: Lemen, J. R.; Hudson, H.; Webb, D.; Tsuneta, S.
1996AAS...188.7007L Altcode: 1996BAAS...28..939L
We present the analysis of the Yohkoh/SXT observations of a long-decay
event that was observed on 1992 February 21. This event, previously
reported by Tsuneta et al. \ 1992, had a clearly observed compact
ejection at its onset. There is also evidence for a depletion
of material in the corona above the flare site (coronal dimming)
accompanying the onset of flare brightening. We find a lower limit of
3 x 10(14) g for this mass depletion. This event was observed near the
limb as an arcade viewed almost end-on. Its morphology resembles the
classical reconnection model for a solar flare. Here we describe the
early phase of this event when the outward motions as observed in the
SXT images are interpreted as the X-ray signature of a coronal mass
ejection (no white-light images are available). The coronal depletion
marks the beginning of the outward motion and occurs simultaneously
with hard X-ray emission. If this marks the beginning of a CME, then
this event provides support for a close connection between of X-ray
flares and coronal mass ejections.

---------------------------------------------------------
Title: The High Energy Solar Spectroscopic Imager - HESSI
Authors: Dennis, B. R.; Crannell, C. J.; Holman, G. D.; Ramaty,
R.; von Rosenvinge, T. T.; Benz, A.; Bornmann, P. L.; Brown, J. C.;
Canfield, R. C.; Emslie, A. G.; Enome, S.; Kosugi, T.; Hudson, H. S.;
Hurford, G. J.; Lin, R. P.; Ling, J. C.; Madden, N. W.; van Beek,
H. F.; Vilmer, N.
1996AAS...188.7016D Altcode: 1996BAAS...28..941D
HESSI will investigate the physics of particle acceleration and energy
release in solar flares through an unprecedented combination of high
resolution imaging and spectroscopy of X-rays and gamma rays from 2
keV to 20 MeV during the next solar maximum. It uses Fourier-transform
imaging with 12 bi-grid modulation collimators and cooled germanium and
silicon detectors mounted on a Sun-pointed spin-stabilized spacecraft in
a low-altitude equatorial orbit. HESSI will carry out the first imaging
spectroscopy in hard X-rays with 2 arcseconds angular resolution, time
resolution to tens of ms, and ~ 1 keV energy resolution; the first
gamma-ray line spectroscopy from a spacecraft with ~ 1 keV energy
resolution; and the first gamma-ray line and continuum imaging with
20 arcseconds angular resolution.

---------------------------------------------------------
Title: X-ray Images of Two Type II Bursts
Authors: Hudson, Hugh S.
1996AAS...188.1907H Altcode: 1996BAAS...28Q.851H
The Yohkoh soft X-ray telescope has been providing high-resolution
images of the counterparts of various meter-wave phenomena. We report
here on clear examples of Type II emission in flares of 13 and 20
October, 1995, based upon Culgoora spectral observations. The starting
frequencies of the Type II bursts differed greatly, and both showed
accompanying Type III and continuum structures. The Type II traces
were highly structured. The X-ray images show no sign of concentric
wave fronts, as expected from the standard interpretation of the
Type II burst phenomenon. Instead we see a series of loops emerging
from the flare regions. We interpret the fine structure in the X-ray
images as the origin of the patterns in the dynamic radio spectra,
and attempt thereby to compare the densities independently inferred
from the radio and X-ray observations.

---------------------------------------------------------
Title: Spatial Relations between Preflares and Flares
Authors: Fárník, František; Hudson, Hugh; Watanabe, Tetsuya
1996SoPh..165..169F Altcode:
We have conducted an initial search for discrete preflare brightenings
as observed in soft X-radiation by Yohkoh. The Yohkoh images allow us
to identify, to within a few arc seconds, the location of a preflare
event relative to the succeeding flare. Our initial motivation in
this study was to search for early coronal brightenings leading
to flare effects, as had been suggested by earlier studies; thus we
concentrated on Yohkoh limb events. We find no evidence for such early
coronal brightenings. Between 15% and 41% of the 131 suitable events
matched our criteria for preflare brightening: the same active region;
brightening within one hour of the flare peak; preflare brightness
less than 30% of the flare peak. In the great majority of the preflare
cases, we found that physically separate nearby structures brightened
initially. Often these structures appeared to share a common footpoint
location with the flare brightening itself. In a few cases the preflare
could have occurred in exactly the same structure as the flare.

---------------------------------------------------------
Title: Metric Type III bursts associated with soft X-ray jets.
Authors: Raulin, J. P.; Kundu, M. R.; Hudson, H. S.; Nitta, N.;
Raoult, A.
1996A&A...306..299R Altcode:
From soft X-ray and metric radio observations with high temporal and
spatial resolution, we show that electron acceleration in the form of
Type III bursts occurs in association with coronal jets observed by the
Yohkoh soft X-ray telescope. The excellent correspondence between the
positions of the radio sources observed at different frequencies and
the X-ray jets strongly suggests that electron beams propagate along
the relatively dense paths formed by the jets. Assuming a constant
temperature for the jets, one can estimate the electron density from
the soft X-ray measurements. These computed electron densities agree
well with the values derived from Type III bursts produced by the
plasma emission process. The observations are consistent with the idea
that strong particle acceleration accompanies magnetic reconnection
in these events as well as in solar flares.

---------------------------------------------------------
Title: First Images of a Solar Flare at Millimeter Wavelengths
Authors: Silva, Adriana V. R.; White, Stephen M.; Lin, Robert P.;
de Pater, Imke; Shibasaki, K.; Hudson, Hugh S.; Kundu, Mukul R.
1996ApJ...458L..49S Altcode:
We present the first high spatial resolution images of a solar flare
at millimeter wavelengths. On 1994 August 17, a GOES soft X-ray class
M1 flare was observed by the Berkeley-Illinois-Maryland Array at 86
GHz by the Nobeyama 17 GHz array and by the Yohkoh spacecraft. The
flare displayed both a prominent impulsive phase in microwaves and a
gradual phase that lasted over 30 minutes. The millimeter data were
taken only during the gradual phase. The millimeter images show a
source with a size of ~8", a peak brightness temperature of ~106 K,
and maximum optical depth of 0.09. At both X-ray and radio wavelengths,
the emitting region appeared to be compact (&lt;~20"). In soft X-ray,
the images are resolved into two sources: one located at a footpoint
and the other at the top of the flaring loop. The millimeter emission
is consistent with the predicted free-free flux from an isothermal
temperature (~14 MK) loop-top source, a multitemperature footpoint
source with a hot (~22 MK), and a cold (~12 MK) component. Most (80%)
of the millimeter flux density originates from the top of the magnetic
loop, and the footpoint contribution is only 20%.

---------------------------------------------------------
Title: Statistical Study of Solar X-Ray Jets Observed with the YOHKOH
Soft X-Ray Telescope
Authors: Shimojo, Masumi; Hashimoto, Shizuyo; Shibata, Kazunari;
Hirayama, Tadashi; Hudson, Hugh S.; Acton, Loren W.
1996PASJ...48..123S Altcode:
We have found 100 X-ray jets in the database of full Sun images taken
with the Soft X-ray Telescope (SXT) aboard Yohkoh during the period
from 1991 November through 1992 April. A statistical study for these
jets results in the following characteristics: 1)\ Most are associated
with small flares (microflares--subflares) at their footpoints. 2)\ The
lengths lie in the range of a few times 10(4) --4 times 10(5) km. 3)\
The widths are 5 times 10(3) --10(5) km. 4)\ The apparent velocities are
10--1000 km s(-1) with an average velocity of about 200 km s(-1) . 5)\
The lifetime of the jet extends to ~ 10 hours and the distribution
of the observed lifetime is a power law with an index of ~ 1.2. 6)\
76% of the jets show constant or converging shapes; the width of the
jet is constant or decreases with distance from the footpoint. The
converging type tends to be generated with an energetic footpoint
event and the constant type by a wide energy range of the footpoint
event. 7)\ Many jets ( ~ 68%) appear in or near to active regions
(AR). Among the jets ejected from bright-point like features in ARs,
most ( ~ 86%) are observed to the west of the active region. 8)\ 27%
of the jets show a gap ( &gt; 10(4) km) between the exact footpoint of
the jet and the brightest part of the associated flare. 9)\ The X-ray
intensity distribution along an X-ray jet often shows an exponential
decrease with distance from the footpoint. This exponential intensity
distribution holds from the early phase to the decay phase.

---------------------------------------------------------
Title: Comparison and Relation of HeI 1083 NM Two-Ribbon Flares and
Large-Scale Coronal Arcades Observed by YOHKOH
Authors: Harvey, Karen L.; McAllister, Alan; Hudson, Hugh; Alexander,
David; Lemen, James R.; Jones, Harrison P.
1996ASPC...95..100H Altcode: 1996sdit.conf..100H
No abstract at ADS

---------------------------------------------------------
Title: Broadband Imaging Spectroscopy with the Solar Radio Telescope
Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Hudson, H. S.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1996ASPC...93..430B Altcode: 1996ress.conf..430B
No abstract at ADS

---------------------------------------------------------
Title: YOHKOH SXT and BCS Observations of the "Reconnection Region"
of a Solar Flare
Authors: Sterling, Alphonse C.; Hudson, Hugh S.; Lemen, James R.
1996ASPC..111..177S Altcode: 1997ASPC..111..177S
The authors find strong line shifts in Bragg crystal spectrometer
(BCS) spectra of a flare which occurred well beyond the solar limb on
1993 April 15. Since the flare is beyond the limb, only the uppermost
regions of the flare are visible. If reconnection is acting in flares,
than one may expect that the line shifts from this event are due to
reconnection jets emanating from above the region of the main flaring
loops. The authors show, however, that details of the line shifts
are not consistent with this picture. Rather than being a result of
reconnection jets, it is more likely that the line shifts are due
to plasma motions on a flaring loop oriented edge on with respect to
the Earth.

---------------------------------------------------------
Title: Large-Scale Active Coronal Phenomena in YOHKOH SXT Images
Authors: Svestka, Z.; Farnik, F.; Hudson, H. S.; Uchida, Y.; Hick, P.
1996ASPC..111..388S Altcode: 1997ASPC..111..388S
The authors have checked in Yohkoh SXT images the appearance of giant
post-flare arches which were discovered in hard X-ray images from
the HXIS and FCS instruments onboard the SMM. They have verified the
existence of both the rising and stationary arches. In addition to
these two kinds of giant post-flare arches, known before from SMM
observations, Yohkoh also reveals other large post-flare coronal
structures which might have been considered to be giant arches by the
low-resolution SMM instruments. These include coronal helmet streamers
above rising flare loops or fans of hot structures in which the rising
loops are embedded.

---------------------------------------------------------
Title: Magnetodynamic phenomena in the solar atmosphere. Prototypes
of stellar magnetic activity
Authors: Uchida, Yutaka; Kosugi, Takeo; Hudson, Hugh S.
1996mpsa.conf.....U Altcode: 1996IAUCo.153.....U
No abstract at ADS

---------------------------------------------------------
Title: Pixon Reconstruction and the Masuda Event of 1992 January 13
Authors: Alexander, D.; Metcalf, T.; Hudson, H. S.
1996ASPC..111..253A Altcode: 1997ASPC..111..253A
The data set of localised hard X-ray sources observed by the Yohkoh/HXT
has been analysed using maximum entropy methods (MEM) to reconstruct
HXT images. Recently, an alternative method, that of fractal pixon
reconstruction, has been developed for use with the HXT. The authors
have reanalysed the event of 13 January 1992 (the Masuda event),
comparing the MEM and pixon methods. There are distinct differences
in the two sets of results. The pixon method, favoured by the authors,
indicates a less impulsive coronal source than the MEM reconstruction
and also a relatively weaker coronal/footpoint emission ratio.

---------------------------------------------------------
Title: Observational Problems for Flare Models Based on Large-Scale
Magnetic Reconnection (Invited)
Authors: Hudson, Hugh S.; Khan, Josef I.
1996ASPC..111..135H Altcode: 1997ASPC..111..135H
The authors examine the observable consequences of flare models
which describe the energetics in terms of large-scale magnetic
reconnection. The Yohkoh data offer many ways to check whether the
phenomena expected from large-scale reconnection models actually
occur. There are apparent successes, as for example in the morphology
of flares with cusp geometry. Puzzling observational discrepancies
also exist and are discussed: the lack of inward flows toward the
reconnection site; the lack of outward jet flows from the reconnection
site; the implicit existence of open field lines prior to flare onset;
the dominance of apparently single loops; the need for efficient
acceleration of non-thermal particles; the existence of homologous
flares; the existence of intense compact events; the existence of
energetic flares essentially without mass motion; and the rarity
of concave-up field lines in the corona. The authors propose an
observational technique, "conjugate variability", which if successful
could establish reconnection relatively unambiguously.

---------------------------------------------------------
Title: Causal Relation between H alpha Arch Filament Loops and Soft
X-ray Coronal Loops
Authors: Yoshimura, K.; Kurokawa, H.; Sano, S.; Hudson, H.
1996mpsa.conf..457Y Altcode: 1996IAUCo.153..457Y
No abstract at ADS

---------------------------------------------------------
Title: Coronal X-Ray Dimming in Two Limb Flares
Authors: Hudson, Hugh S.; Lemen, James R.; Webb, David F.
1996ASPC..111..379H Altcode: 1997ASPC..111..379H
Yohkoh SXT observations of flares and large-scale arcade events
frequently show coronal dimming accompanying X-ray brightening in
long-duration events. The authors tentatively identify this with the
process of field-line opening in the initial phase of a coronal mass
ejection (CME), although few simultaneous coronagraph and soft X-ray
observations have yet been described. The dimming signature may reduce
the coronal soft X-ray intensity by as much as a factor of 2 - 3, and
thus has a higher contrast than the cavity often seen in white-light
CME observations. In the cases examined thus far, the authors find
a close match between the onsets of X-ray brightening and coronal
dimming, suggesting a close physical relationship. The dimming appears
(in movie representations) to result from outward expansion; highly
structured features (multiple loops) are recognizable in the dimming
regions of some events, suggesting that the soft X-ray data may be
used to characterize the velocity field of the expansion.

---------------------------------------------------------
Title: Nonthermal Radio Emission from Coronal X-ray Jets
Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.;
Raoult, A.
1996ASPC...93..375K Altcode: 1996ress.conf..375K
No abstract at ADS

---------------------------------------------------------
Title: Detection of Nonthermal Radio Emission from Coronal X-ray Jets
Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.;
Raoult, A.; Shibata, K.; Shimojo, M.
1996mpsa.conf..445K Altcode: 1996IAUCo.153..445K
No abstract at ADS

---------------------------------------------------------
Title: YOHKOH Observations of Coronal Mass Ejections
Authors: Hudson, H.
1996mpsa.conf...89H Altcode: 1996IAUCo.153...89H
No abstract at ADS

---------------------------------------------------------
Title: X-ray Observations of an Over-the-Limb Solar Flare with Large
Spectral Line Shifts
Authors: Sterling, A. C.; Harra-Murnion, L. K.; Hudson, H. S.; Lemen,
J. R.; Strong, K. T.
1996mpsa.conf..557S Altcode: 1996IAUCo.153..557S
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamic Modeling of Flares Well-Observed by Yohkoh/SXT
Authors: Reale, F.; Peres, G.; Hudson, H.
1996mpsa.conf..311R Altcode: 1996IAUCo.153..311R
No abstract at ADS

---------------------------------------------------------
Title: Active Region Evolution and Flare Activity
Authors: Nitta, N.; van Driel-Gesztelyi, L.; Leka, K. D.; Hudson, H. S.
1996mpsa.conf..515N Altcode: 1996IAUCo.153..515N
No abstract at ADS

---------------------------------------------------------
Title: Large-Scale Arcade Formation on May 15, 1992 and its
Interplanetary Consequence
Authors: Nakagawa, Y.; Watanabe, T.; Hudson, H.; Kojima, M.
1996mpsa.conf..489N Altcode: 1996IAUCo.153..489N
No abstract at ADS

---------------------------------------------------------
Title: First Images of a Solar Flare at Millimeter Wavelengths
Authors: Silva, Adriana V. R.; White, Stephen M.; Lin, Robert P.;
de Pater, Imke; Shibasaku, K.; Hudson, Hugh S.; Kundu, Mukul R.
1996ADIL...AS...01S Altcode:
We present the first high-spatial-resolution images of a solar flare at
millimeter wavelengths. On 1994 August 17, a GOES soft X--ray class M1
flare was observed by the Berkeley--Illinois--Maryland Array (BIMA) at
86 GHz, by the Nobeyama 17 GHz array, and by the Yohkoh spacecraft. The
flare displayed both a prominent impulsive phase in microwaves and a
gradual phase which lasted over 30 minutes. The millimeter data were
taken only during the gradual phase. The millimeter images show a
source with a size of $\sim$8\arcsec, a peak brightness temperature
of ~ 10^6 K, and maximum optical depth of 0.09. At both X--ray and
radio wavelengths the emitting region appeared to be compact (&lt; 20
arcseconds). In soft X--ray the images are resolved into two sources:
one located at a footpoint and the other at the top of the flaring
loop. The millimeter emission is consistent with the predicted free-free
flux from an isothermal temperature (~ 14 MK) looptop source and a
multi--temperature footpoint source with a hot (~ 22 MK) and a cold (~
12 MK) component. Most (80%) of the millimeter flux density originates
from the top of the magnetic loop, and the footpoint contribution is
only 20%.

---------------------------------------------------------
Title: A Loop Flare Observed by YOHKOH on 1992 July 11
Authors: Khan, Josef I.; Hudson, Hugh S.; Sterling, Alphonse C.;
Lemen, James R.
1996ASPC..111..162K Altcode: 1997ASPC..111..162K
The authors present Yohkoh soft and hard X-ray observations of a
flare. Soft X-ray morphology shows the structure of this flare to
be a relatively simple loop. Nonetheless several interesting points
were found including: (i) bright soft X-ray footpoints persist long
after completion of the impulsive hard X-ray bursts; (ii) both legs
and footpoints of the flare loop appear to move together rather than
apart during the course of the flare; (iii) initially the flare loop
appears to have a fairly uniform thickness but as the flare progresses
the loop-top region becomes broader; (iv) 'low energy' hard X-rays
appear to originate from high in the loop near the loop apex; and (v)
soft X-ray spectra show strong line asymmetries suggesting the presence
of upflowing plasma oriented nearly directly towards the Earth.

---------------------------------------------------------
Title: Large-Scale Active Coronal Phenomena in YOHKOH SXT Images, I
Authors: Švestka, Zdeněk; Fárník, František; Hudson, Hugh S.;
Uchida, Yutaka; Hick, Paul; Lemen, James R.
1995SoPh..161..331S Altcode:
We have found several occurrences of slowly rising giant arches inYohkoh
images. These are similar to the giant post-flare arches previously
discovered by SMM instruments in the 80s. However, we see them now
with 3-5 times better spatial resolution and can recognize well their
loop-like structure. As a rule, these arches followeruptive flares
with gradual soft X-ray bursts, and rise with speeds of 1.1-2.4 km
s−1 which keep constant for &gt;5 to 24 hours, reaching
altitudes up to 250 000 km above the solar limb. These arches differ
from post-flare loop systems by their (much higher) altitudes, (much
longer) lifetimes, and (constant) speed of growth. One event appears
to be a rise of a transequatorial interconnecting loop.

---------------------------------------------------------
Title: Yohkho Soft X-Ray Spectroscopic Observations of the Bright
Loop-Top Kernels of Solar Flares
Authors: Khan, Josef I.; Harra-Murnion, Louise K.; Hudson, Hugh S.;
Lemen, James R.; Sterling, Alphonse C.
1995ApJ...452L.153K Altcode:
Observations of solar flares by the Soft X-ray Telescope (SXT) on
board Yohkoh frequently show strongly enhanced brightenings near the
tops of the magnetic loops containing hot plasma. The Yohkoh Bragg
Crystal Spectrometer (BCS) cannot normally make observations of these
loop-top sources in the absence of contamination by the legs and the
feet of the loops since it has no spatial resolution. We have overcome
this limitation by using the solar limb as an occulting edge in a
sequence of similar flares that occurred over an interval of ~10 hr
near the west limb on 1992 November 24. The progressive occultation by
the limb restricts the line of sight to higher and higher altitudes
during this sequence, with the final event showing only a compact
source of the type often found at loop tops. BCS observations in Fe
XXV, Ca XIX, and S XV show that electron temperatures and nonthermal
velocities in these compact sources are similar to those quantities
determined for disk flares in previous studies. As with disk flares,
the nonthermal line broadening persists late into the decay phase
of the flaring isolated loop tops. Our results favor mechanisms for
nonthermal-velocity generation that are either independent of height
or place the source near the apex of the flaring loop. In addition,
there may be a temporal relationship between the hard X-ray emission
and the nonthermal velocity, which suggests a possible association
between the primary energy release of the flare, the nonthermal-velocity
generation mechanism, and the loop top.

---------------------------------------------------------
Title: Using Fe X 6374 Å and Fe XIV 5303 Å spectral line
intensities to study the effect of line of sight integration on
coronal temperature inferences
Authors: Esser, R.; Brickhouse, N. S.; Habbal, S. R.; Altrock, R. C.;
Hudson, H. S.
1995JGR...10019829E Altcode:
Polar coronal holes are relatively stable structures persisting
over many solar rotations. The appearance of coronal holes in
remote observations, however, can change on a daily basis due to
variations of the denser and hotter plasma surrounding them. We
explore the effect of these denser and hotter surrounding regions on
coronal hole observations, using daily intensity measurements at 1.15
RS of the green Fe XIV 5303 Å and red Fe X 6374 Å spectral
lines. The observations, which were carried out at the National Solar
Observatory a Sacramento Peak, New Mexico, cover at time period of
about four solar rotations. We show that the “coronal hole”
temperatures derived using the line ratio technique, vary by more than
0.8×106 K over the time interval considered here. We also
provide a short discussion of the expected accuracy of the atomic data
for these two iron spectral lines. Using intensity measurements as a
function of distance from the Sun, we briefly discuss how the regions
surrounding the coronal holes might influence the inference of the
temperature gradient in the coronal holes. The line of sight effect
on the temperature gradient should be explored in more detail in the
future using daily observations of the line intensities as a function
of distance. These observations could be provided by ground-based
coronagraphs and by instruments on board SOHO.

---------------------------------------------------------
Title: Detection of Nonthermal Radio Emission from Coronal X-Ray Jets
Authors: Kundu, M. R.; Raulin, J. P.; Nitta, N.; Hudson, H. S.;
Shimojo, M.; Shibata, K.; Raoult, A.
1995ApJ...447L.135K Altcode:
We report the detection of a type III burst in association with a
dynamic X-ray coronal jet observed by Yohkoh/SXT. The type III burst
observed with the Nancay (France) multifrequency radioheliograph is
spatially and temporally coincident with the X-ray jet. The radio
locations at different frequencies (236.6 and 164 MHz) are aligned
along the length of the jet. The observation of the type III burst in
association with the X-ray jet implies the acceleration of electrons
to several tens of keV, along with the heating responsible for the
production of soft X-rays. This association implies the existence of
open field lines in dense coronal structures identified on the Sun's
disk. This is the first observation of dense coronal structures on the
disk, along which type III emitting nonthermal electrons propagate. We
find that this structure begins to form before the type III emission. At
the time of the type III burst we estimate a density of 6--10 x 108
cm-3 for a temperature of ~5--6 MK at an altitude of 20,000 km.

---------------------------------------------------------
Title: Coordinated Observation of the Solar Corona Using the Norikura
Coronagraph and the YOHKOH Soft X-Ray Telescope
Authors: Ichimoto, K.; Hara, H.; Takeda, A.; Kumagai, K.; Sakurai,
T.; Shimizu, T.; Hudson, H. S.
1995ApJ...445..978I Altcode:
Spectroscopic observations of coronal emission lines were carried
out at the Norikura Solar Observatory in cooperation with the soft
X-ray telescope on board the Yohkoh satellite to study the plasma
distributions at different temperatures. Intensity and velocity
distributions in Fe XIV wavelength 5303 (green), Fe X wavelength
6374 (red), and Ca XV wavelength 5694 (yellow) lines are compared
with the soft X-ray images. It is found that the soft X-ray images
closely resemble those of the yellow line that represents a rather
high temperature component of the corona. On the other hand the
low-temperature component seen in the green and the red lines shows
quite a different distribution from that of the high-temperature
component; the low-temperature component consists of many thin loops
or streaks, while the high-temperature component is more diffuse. We
find that the active elements of the cool component, i.e., complex
loop systems, rapid changes of small structures, and localized large
plasma motions, all tend to be cospatial with the hot component.

---------------------------------------------------------
Title: The solar/interplanetary event of 14 April 1994 observed
by Yohkoh/SXT
Authors: Alexander, D.; Harvey, K. L.; Hudson, H. S.; Hoeksema, J. T.;
Zhao, X.
1995sowi.conf...57A Altcode:
The polar crown event of April 14 1994 is one of the largest
scale eruptive events observed by the Yohkoh/SXT. Associated with
the formation of an arcade of soft X-ray loops at the Sun was the
detection of an interplanetary forward/reverse shock event by the
Ulysses spacecraft some 4-7 days later. The relationship between the
coronal and interplanetary signatures of these events is important if we
are to address fully the initialization and consequent acceleration of
interplanetary phenomena, such as CMEs and counter-streaming electrons,
originating at the Sun. From detailed analysis of the energetics
of the arcade formed during the eruption of April 14 1994, we find
peak temperatures and emission measures of approximately 5MK and
approximately 1048cm-3 respectively. The total
thermal content of the arcade loop structure observed in soft X-rays
is calculated to be some 5 x 1029 ergs. The development
of these parameters as the event proceeds and their relationship to
the dynamics of the eruption are investigated. Although spanning a
longitudinal range of some 150 degrees the April 14 event displayed
the typical helmet streamer structure normally associated with coronal
mass ejections These helmet streamers are thought to be related to the
global solar magnetic field through the heliospheric current sheet
(HCS). The arcade formation, together with the eruption of material
into interplanetary space, signifies a large-scale reconfiguration of
the coronal magnetic field. We examine the effects of the formation of
such a coronal arcade structure on the HCS and discuss the dynamics
involved with the passage of a large scale disturbance through the
interplanetary magnetic field.

---------------------------------------------------------
Title: Comparison of YOHKOH x-ray coronal events with ULYSSES
interplanetary events
Authors: Lemen, J. R.; Acton, L. W.; Alexander, D.; Galvin, A. B.;
Harvey, K. L.; Hoecksema, J. T.; Zhao, X.; Hudson, H.
1995sowi.conf...58L Altcode:
The Yohkoh soft X-ray telescope (SXT) has observed several
largescale eruptive events per year for the first three years
of observations (Aug. 1991 - Nov. 1994) Such events are most
prominent at high latitudes, but resemble long-duration flare
events seen in active regions. Some of the high-latitude events
have now been identified in the Ulysses/SWICS data base during the
Ulysses south polar passage. There are puzzling examples of solar
events with no interplanetary counterparts. A comparison of coronal
and interplanetary events can lead to better models for mapping
interplanetary disturbances back to their source location, especially
by combining Yohkoh morphology with three-dimensional representations
of the coronal magnetic field. In this paper we describe the parameters
of the hot plasma seen by SXT. There is clear evidence for non radial
motion in specific events. We present comparisons between the ionization
temperature of the interplanetary plasma with that observed at the
Sun in cases where this is possible.

---------------------------------------------------------
Title: Yohkoh/SXT soft x-ray observations of sudden mass loss from
the solar corona
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Freeland, S. L.;
Lemen, J. R.; Harvey, K. L.
1995sowi.confR..58H Altcode:
Direct X-ray observations allow us to estimate the hot coronal mass
before and after a flare or other disturbance of the type leading to
a coronal mass ejection. The sudden disappearance of a large coronal
structure (scale greater than 105 km) gives evidence that an ejection
has occurred, if the time scales are much shorter than the conductive
or radiative cooling times for such structures. A flare also typically
adds large amounts of new material to the corona via evaporation
resulting from the coronal energy release. This provides a competing
mechanism that makes the estimation of the total mass loss somewhat
difficult. We note that the X-ray observations have the advantage of
covering the entire corona rather than the limb regions unlike the
coronagraph observations. We have identified two examples of coronal
mass disappearances. before and during long duration flare events on
21 Feb. 1992 (on the E limb) and 13 Nov. 1994 (near disk center). In
latter case the total mass amounted to some 4 x 1014 g with a
density of 3 x 108cm-3 and a temperature of 2.8 MK
before its disappearance. This corresponds to a radiative cooling time
of some 104 S. much longer than the observed time of disappearance. We
therefore suggest that these sudden mass disappearances correspond with
coronal mass ejections (CMEs), and suggest that further data analysis
will be able to confirm this by comparison with optical observations
of specific CMEs.

---------------------------------------------------------
Title: The solar origins of two high-latitude interplanetary
disturbances
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Harvey, K. L.;
Kurokawa, H.; Kahler, S.; Lemen, J. R.
1995sowi.confS..58H Altcode:
Two extremely similar interplanetary forward/reverse shock events,
with bidirectional electron streaming were detected by Ulysses in
1994. Ground-based and Yohkoh/SXT observations show two strikingly
different solar events that could be associated with them: an LDE flare
on 20 Feb. 1994, and a extremely large-scale eruptive event on 14 April
1994. Both events resulted in geomagnetic storms and presumably were
associated with coronal mass ejections. The sharply contrasting nature
of these solar events argues against an energetic causal relationship
between them and the bidirectional streaming events observed by Ulysses
during its S polar passage. We suggest instead that for each pair of
events. a common solar trigger may have caused independent instabilities
leading to the solar and interplanetary phenomena.

---------------------------------------------------------
Title: Inference of 3-dimensional structure underlying large-scale
coronal events observed by YOHKOH and ULYSSES
Authors: Slater, G. L.; Freeland, S. L.; Hoeksema, T.; Zhao, X.;
Hudson, H. S.
1995sowi.confQ..63S Altcode:
The Yohkoh/SXT images provide full-disk coverage of the solar corona,
usually extending before and after one of the large-scale eruptive
events that occur in the polar crown These produce large arcades of
X-ray loops, often with a cusp-shaped coronal extension, and are known
to be associated with coronal mass ejections. The Yohkoh prototype of
such events occurred 12 Nov. 1991. This allows us to determine heights
from the apparent rotation rates of these structures. In comparison v
with magnetic-field extrapolations from Wilcox Solar Observatory. use
use this tool to infer the three dimensional structure of the corona
in particular cases: 24 Jan. 1992, 24 Feb. 1993, 14 Apr. 1994, and 13
Nov. 1994. The last event is a long-duration flare event.

---------------------------------------------------------
Title: Comment on “The solar flare myth” by J. T. Gosling
Authors: Hudson, Hugh; Haisch, Bernhard; Strong, Keith T.
1995JGR...100.3473H Altcode:
In a recent paper Gosling (1993) claims that solar flares are relatively
unimportant for understanding the terrestrial consequences of solar
activity, and argues that coronal mass ejections (CMEs) produce the
most powerful terrestrial disturbances. This opinion conflicts with
observation, as it is well known that CMEs and flares are closely
associated, and we disagree with Gosling's insistence on a simplistic
cause-and-effect description of the interrelated phenomena of a solar
flare. In this brief response we present new Yohkoh data and review
older results that demonstrate the close relationships among CMEs,
flares, filament eruptions, and other forms of energy release such as
particle acceleration.

---------------------------------------------------------
Title: A Solar Radio Telescope for the Future: Science Summary from
the SRT Workshop
Authors: Gary, D. E.; Bastian, T. S.; Hudson, H. S.; Hurford, G. J.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1995SPD....26..801G Altcode: 1995BAAS...27..971G
No abstract at ADS

---------------------------------------------------------
Title: Yohkoh Multi-Wavelength Observations of the Bright Loop-Top
Kernels in Solar Flares
Authors: Sterling, A.; Khan, J.; Harra-Murnion, L.; Hudson, H.;
Lemen, J.
1995SPD....26.1211S Altcode: 1995BAAS...27..985S
No abstract at ADS

---------------------------------------------------------
Title: Yohkoh Observations of Impulse-Response Flares
Authors: Hudson, H. S.; Farnik, F.; Watanabe, T.
1995SPD....26.1210H Altcode: 1995BAAS...27..984H
No abstract at ADS

---------------------------------------------------------
Title: Correlated brightness variations in solar radiative output
from the photosphere to the corona
Authors: Lean, J. L.; Mariska, J. T.; Strong, K. T.; Hudson, H. S.;
Acton, L. W.; Rottman, G. J.; Woods, T. N.; Willson, R. C.
1995GeoRL..22..655L Altcode:
Correlated brightness variations are shown to occur in time series of
coronal soft X-rays exclusive of prominent active regions, chromospheric
ultraviolet radiation, and the photospheric total solar irradiance
corrected for sunspot effects. These temporal correlations suggest that
upwardly extending magnetic fields may have a large scale impact on the
solar atmosphere in addition to their demonstrable role of generating
localized active regions. The correlations have implications for
improving and extending solar spectrum variability models.

---------------------------------------------------------
Title: Nouthermal Radio Emission From Coronal X-Ray Jets
Authors: Raulin, J. P.; Kundu, M. R.; Hudson, H. S.; Nitta, N.;
Raoult, A.
1995SPD....26.1318R Altcode: 1995BAAS...27..991R
No abstract at ADS

---------------------------------------------------------
Title: A Solar Radio Telescope for the Future: Strawman Concept from
the SRT Workshop
Authors: Hurford, G. J.; Bastian, T. S.; Gary, D. E.; Hudson, H. S.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1995SPD....26..802H Altcode: 1995BAAS...27..971H
No abstract at ADS

---------------------------------------------------------
Title: Fractal Pixon Image Reconstruction for Yohkoh's Hard X-Ray
Telescope
Authors: Metcalf, T. R.; Hudson, H. S.; Kosugi, T.; Puetter, R. C.;
Piña, R. K.
1995SPD....26.1314M Altcode: 1995BAAS...27..990M
No abstract at ADS

---------------------------------------------------------
Title: The 1991 October 24 Flare: A Challenge for Standard Models
Authors: de La Beaujardiere, J. -F.; Canfield, R. C.; Hudson, H. S.;
Wulser, J. -P.; Acton, L.; Kosugi, T.; Masuda, S.
1995ApJ...440..386D Altcode:
The M9.8 solar flare of 1991 October 24 22:30 UT presents several
interesting characteristics: (1) energy release starts high in
the corona; (2) the primary chromospheric ribbons are initially
well separated and do not move apart at an observable rate; (3) no
evidence is found for an erupting filament or other driver. To explain
this flare, we consider several canonical flare models, including a
filament eruption, a confined filament eruption, current interruption,
and interacting loops. We conclude that none of these scenarios
unequivocally explains this flare. Two possibilities which cannot be
ruled out are (1) the eruption of a filament unobservable in H-alpha
which starts high in the corona and produces no ribbon motions smaller
than our detection threshold and no perceptible expansion of the coronal
X-ray source, and (2) energy release due to spontaneous, propagating
reconnection which allows the system to essentially brighten in place.

---------------------------------------------------------
Title: High-Energy Particles In Solar Flares
Authors: Hudson, H.; Ryan, J.
1995ARA&A..33..239H Altcode:
Accelerated particles appear to co exist inseparably with most forms
of energy release in solar flares and coronal mass ejections. We
identify at least six different populations of high-energy electrons
and ions. High-energy particles, accelerated efficiently in the flare
in great numbers, transport a large fraction of the flare energy to
other sites. This behavior makes them an integral part of the flare
process. Much new data has come from two satellites launched in 1991:
the Compton Gamma-Ray Observatory and Yohkoh. This review concentrates
on particles in flares, mainly using X-ray and gamma-ray data rather
than measurements of "escaping” particles observed in interplanetary
space.

---------------------------------------------------------
Title: Solar flares: No "myth"
Authors: Hudson, H. S.
1995EOSTr..76..405H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Stellar x-ray flares
Authors: Haisch, B.; Uchida, Y.; Kosugi, T.; Hudson, H. S.
1995lock.reptR....H Altcode:
What is the importance of stellar X-ray flares to astrophysics, or
even more, to the world at large? In the case of the Sun, changes in
solar activity at the two temporal extremes can have quite significant
consequences. Longterm changes in solar activity, such as the Maunder
Minimum, can apparently lead to non-negligible alterations of the
earth's climate. The extreme short term changes are solar flares, the
most energetic of which can cause communications disruptions, power
outages and ionizing radiation levels amounting to medical X-ray dosages
on long commercial flights and even potentially lethal exposures for
unshielded astronauts. Why does the Sun exhibit such behaviour? Even
if we had a detailed knowledge of the relevant physical processes on
the Sun - which we may be on the way to having in hand as evidenced
by these Proceedings- our understanding would remain incomplete in
regard to fundamental causation so long as we could not say whether the
Sun is, in this respect, unique among the stars. This current paper
discusses the stellar x-ray flare detections and astronomical models
(quasi-static cooling model and two-ribbon model) that are used to
observe the x-ray emission.

---------------------------------------------------------
Title: Recovering the fine structures in solar images
Authors: Karovska, M.; Habbal, S. R.; Golub, L.; DeLuca, E.; Hudson, H.
1994ESASP.373..183K Altcode: 1994soho....3..183K
No abstract at ADS

---------------------------------------------------------
Title: The large scale coronal eruptive event of April 14 1994
Authors: Alexander, D.; Slater, Greg L.; Hudson, Hugh S.; McAllister,
Alan H.; Harvey, Karen L.
1994ESASP.373..187A Altcode: 1994soho....3..187A
No abstract at ADS

---------------------------------------------------------
Title: The Yohkoh context for high-energy particles in solar flares
Authors: Hudson, Hugh S.
1994AIPC..294..151H Altcode: 1994hesp.conf..151H
Yohkoh, a satellite dedicated to high-energy observations of solar
flares, began observations in September, 1991. It carries (i) a
soft X-ray telescope with arcsecond resolution and excellent temporal
sampling; (ii) a hard X-ray imager making the first images above 30 keV;
(iii) a sensitive Bragg crystal spectrometer for soft X-ray emission
lines; and (iv) a set of proportional and scintillation counters. The
flare observations confirm the central role of impulsive-phase
electron acceleration in causing “evaporation” and white-light
flare emission. SXT has found impulsive soft X-ray time profiles at
the footpoints. It also shows compact bright structures apparently at
the tops of flaring loops during the gradual phase. Large flares may
show cusp-shaped structures that strongly resemble the usual picture of
coronal magnetic reconnection, but otherwise do not match the details
of the classical flare scenario. The data taken as a whole suggest
that large-scale magnetic reconnection in the solar corona does not
drive flare energy release, but rather is driven by the flare; the
reconnection may have an important role in flare triggering.

---------------------------------------------------------
Title: A Gigantic Coronal Jet Ejected from a Compact Active Region
in a Coronal Hole
Authors: Shibata, K.; Nitta, N.; Strong, K. T.; Matsumoto, R.;
Yokoyama, T.; Hirayama, T.; Hudson, H.; Ogawara, Y.
1994ApJ...431L..51S Altcode:
A gigantic coronal jet greater than 3 x 105 km long (nearly
half the solar radius) has been found with the soft X-ray telescope
(SXT) on board the solar X-ray satellite, Yohkoh. The jet was ejected
on 1992 January 11 from an 'anemone-type' active region (AR) appearing
in a coronal hole and is one of the largest coronal X-ray jets observed
so far by SXT. This gigantic jet is the best observed example of many
other smaller X-ray jets, because the spatial structures of both the jet
and the AR located at its base are more easily resolved. The range of
apparent translational velocities of the bulk of the jet was between
90 and 240 km s-1, with the corresponding kinetic energy
estimated to be of order of 1028 ergs. A detailed analysis
reveals that the jet was associated with a loop brightening (a small
flare) that occurred in the active region. Several features of this
observation suggest and are consistent with a magnetic reconnection
mechanism for the production of such a 'jet-loop-brightening' event.

---------------------------------------------------------
Title: Evidence for Both Electron Acceleration and Direct Heating
in Solar Flares
Authors: Dennis, B. R.; Holman, G. D.; Hudson, H. S.; Kosugi, T.;
Strong, K. T.; Zarro, D. M.
1994kofu.symp..217D Altcode:
It is well known that in many impulsive solar flares, the time profile
of the total soft X-ray flux closely matches the time integral of the
hard X-ray profile, the so-called Neupert Effect. We have selected
several flares detected by the X-ray telescopes on Yohkoh that
clearly show this effect and examined the time profiles for different
spatial locations throughout the flaring region. We find that footpoint
locations show coincident impulsive bursts in both soft and hard X-ray
emissions whereas loop-top locations show more gradually varying
soft X-ray emission with weaker hard X-ray emission. We interpret
these observations in terms of an electric field model in which both
Joule heating and electron runaway acceleration take place, with the
ratio of the two dependent on how strong the field is compared to the
local Dreicer field. The pre-impulsive phase emissions and the early
gradually-varying soft X-ray emission can be attributed to the direct
heating by the electri! c field in the coronal part of the loop and
the impulsive footpoint emission can be attributed to both enhanced
Joule heating and electron precipitation.

---------------------------------------------------------
Title: Metric Type III Bursts from Flaring X-ray Bright Points
Authors: Kundu, M. R.; Strong, K. T.; Pick, M.; Harvey, K. T.; Kane,
S. R.; White, S. M.; Hudson, H. S.
1994kofu.symp..343K Altcode:
X-ray bright points (XBP's) are known to show variability on a number
of timescales, including impulsive X-ray brightenings. The relationship
between these XBP “flares” and normal solar flares is poorly known. A
fundamental question is whether nonthermal acceleration of particles
takes place in XBP flares. We address this issue by searching for
nonthermal radio emission at metric wavelengths from flaring XBPs
identified in Yohkoh/SXT data. Unequivocal evidence for type-III-like
radio bursts, usually attributed to beams of nonthermal electrons on
open field lines, is found. This suggests that XBP flares are similar
to normal flares and can indeed accelerate nonthermal populations of
energetic particles.

---------------------------------------------------------
Title: Hard and Soft X-ray Observations of a Super-Hot Thermal Flare
of 6 February, 1992
Authors: Kosugi, T.; Sakao, T.; Masuda, S.; Hara, H.; Shimizu, T.;
Hudson, H. S.
1994kofu.symp..127K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Determining Point Spread Function of Space Observations Using
BID Algorithm
Authors: Karovska, M.; Hudson, H. S.
1994kofu.symp..327K Altcode:
With the advent of improved data analysis tools and the superior
image restoration capabilities of the newly developed Blind Iterative
Deconvolution algorithm, the scientific return from the observations
from space can be significantly improved. We present the results of
the application of this algorithm to the EUV/Skylab images and to a
sample of YOHKOH data.

---------------------------------------------------------
Title: A Search for “Black-Light Flares”
Authors: Driel-Gesztelyi, L. V.; Hudson, H. S.; Anwer, B.; Hiei, E.
1994kofu.symp..375D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Temperature Analysis of the Post-Flare Loops of June 25-26,
1992
Authors: Anwer, B.; Hiei, E.; Hudson, H. S.; Acton, L. W.; Lemen,
J.; Metcalf, T. R.
1994kofu.symp..137A Altcode:
We have performed an analysis of temperatures and emission measures
of thermal plasma on a post--flare loop system following an X3.9
flare of June 25, 1992, at 20:14 UT in NOAA active region 7205 near
the west limb (N09, W67). The filter ratio method was applied to the
data sets taken using the Al 0.1 micron (thin Al) and Al 12 micron
(thick Al) filters of the Yohkoh Soft X-ray Telescope (SXT). We found
that the plasma temperature of the top of loops was in the range 5 -
8 x 10^6 K and log emission measure between 44.6 and 46.7 cm^(-3)
for data sets taken from 22:56:57 UT of June 25 to 09:00 UT of June
26. Furthermore, the occurrence of a C1-class flare at the top of the
flare loops increased the plasma temperature from 5.5 x 10^6 K to 6.6
x 10^6 K at 06:57:11 UT. The loops top was much brighter than the legs
and footpoints, with delta_T was about 0.1 x 10^6 K.

---------------------------------------------------------
Title: Thermal Plasmas in the Solar Corona: the YOHKOH Soft X-Ray
Observations
Authors: Hudson, H. S.
1994kofu.symp....1H Altcode:
The Yohkoh observatory carries two instruments whose main emphasis is
the observation of soft X-radiation, principally emitted by thermal
radiation processes. The other two Yohkoh instruments also observe
some of these thermal sources. The combination of instruments makes
Yohkoh unprecedented in its ability to observe the remarkable array
of hot plasmas in the solar corona, and this has resulted in many new
discoveries as well as in the sharpening of our knowledge of known
(but often unexplained) phenomena. The scope of this review consists
of the Yohkoh observations of thermal plasmas, and it emphasizes the
new discoveries. The SXT data show many phenomena whose geometry and
dynamics strongly suggest magnetic reconnection.

---------------------------------------------------------
Title: Multiwavelength Observations of a Solar Flare
Authors: White, S. M.; Silva, A.; de Pater, I.; Lin, R. P.; Gary,
D. E.; Hudson, H. S.; Doyle, J. G.; Hagyard, M. J.; Kundu, M. R.
1994kofu.symp..203W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Analysis of Three YOHKOH White-Light Flares
Authors: Hudson, H. S.; Driel-Gesztelyi, L. V.; Kosugi, T.
1994kofu.symp..397H Altcode:
Three of the nine white-light flares thus far discovered in the Yohkoh
white-light data were also observed at the Nobeyama Radio Observatory
at its 80 GHz polarimeter. Each of the three flares was relatively
close to the center of the Sun and had an H_alpha importance of 2B-3B,
but the X-ray classes were quite different: the flares of 27 Oct. 1991,
15 Nov. 1991, and 14 Feb. 1992 had GOES X-ray magnitudes of X6.1, X1.5,
and M7.0 respectively. We have analyzed the characteristics of these
flares at white-light, X-ray and radio wavelengths in order to find
clues for the emission mechanisms of the white-light and millimeter-wave
continuum. Each of these three flares had both impulsive and gradual
optical emissions. Using the Yohkoh soft X-ray images we associate the
gradual component of white-light emission in these flares with compact
high-temperature loops. We speculate that the white light itself comes
from fine structures embedded in these loops, at densities greater
than normal photospheric densities.

---------------------------------------------------------
Title: Preface
Authors: Pap, Judit M.; Fröhlich, Claus; Hudson, Hugh S.; Tobiska,
W. Kent
1994SoPh..152D...9P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Improvement of the Photometric Sunspot Index and Changes of
the Disk-Integrated Sunspot Contrast with Time
Authors: Froehlich, Claus; Pap, Judit M.; Hudson, Hugh S.
1994SoPh..152..111F Altcode: 1994IAUCo.143..111F; 1994svs..coll..111F
The photometric sunspot index (PSI) was developed to study the
effects of sunspots on solar irradiance. It is calculated from the
sunspot data published in theSolar-Geophysical Data catalogue. It
has been shown that the formerPSI models overestimate the effect of
dark sunspots on solar irradiance; furthermore results of direct
sunspot photometry indicate that the contrast of spots depends on
their area. An improvedPSI calculation is presented; it takes into
account the area dependence of the contrast and calculates 'true'
daily means for each observation using the differential rotation of
the spots. Moreover, the observations are screened for outliers which
improves the homogeneity of the data set substantially, at least for
the period after December 1981 when NOAA started to report data from
a few instead of one to two stations. A detailed description of the
method is provided. The correlation between the newly calculatedPSI
and total solar irradiance is studied for different phases of the solar
cycles 21 and 22 using bi-variate spectral analysis. The results can be
used as a `calibration' ofPSI in terms of gain, the factor by whichPSI
has to be multiplied to yield the observed irradiance change. This
factor changes with time from about 0.6 in 1980 to 1.1 in 1990. This
unexpected result cannot be interpreted by a change of the contrast
relative to the quiet Sun (as it is normally defined and determined by
direct photometry) but rather as a change of the contrast between the
spots and their surrounding as seen in total irradiance (integrated
over the solar disk). This may partly be explained by a change in the
ratio between the areas of the spots and the surrounding faculae.

---------------------------------------------------------
Title: Astronomical photometry from the moon
Authors: Hudson, Hugh S.
1994AdSpR..14f..99H Altcode: 1994AdSpR..14...99H
The Moon would be an excellent platform for photometric astronomical
observations. This paper discusses such observations, emphasizing
time-series photometry of oscillating stars (asteroseismology), of
faint gravitating bodies (microlensing), and of the interplanetary
medium. To prepare for the deployment of major new telescopes and
instrumentation on the surface of the Moon, I suggest that smaller
“site-survey” instruments be put in place as soon as possible. Each
application suggested can derive great benifits from small site-survey
instruments established relatively soon, and each would ultimately
need extensive arrays of large instruments.

---------------------------------------------------------
Title: Electric Currents and Coronal Heating in NOAA Active Region
6952
Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey,
D. L.; Wulser, J. -P.; Martens, P. C. H.; Tsuneta, S.
1994ApJ...428..860M Altcode:
We examine the spatial and temporal relationship between coronal
structures observed with the soft X-ray telescope (SXT) on board the
Yohkoh spacecraft and the vertical electric current density derived from
photospheric vector magnetograms obtained using the Stokes Polarimeter
at the Mees Solar Observatory. We focus on a single active region:
AR 6952 which we observed on 7 days during 1991 December. For 11
independent maps of the vertical electric current density co-aligned
with non-flaring X-ray images, we search for a morphological
relationship between sites of high vertical current density in the
photosphere and enhanced X-ray emission in the overlying corona. We
find no compelling spatial or temporal correlation between the sites of
vertical current and the bright X-ray structures in this active region.

---------------------------------------------------------
Title: Observations of Jovian Thermal Structure at Mid-Infrared
Wavelengths
Authors: Fisher, B. M.; Jones, B.; Hudson, H. S.
1994DPS....26.1118F Altcode: 1994BAAS...26Q1103F
No abstract at ADS

---------------------------------------------------------
Title: A Yohkoh search for “black-light flares”
Authors: van Driel-Gesztelyi, Lidia; Hudson, Hugh S.; Anwar, Bachtiar;
Hiei, Eijiro
1994SoPh..152..145V Altcode: 1994svs..coll..145V; 1994IAUCo.143..145V
Calculations which predict that a phenomenon analogous to stellar
negative pre-flares could also exist on the Sun were published by
Hénouxet al. (1990), and Aboudarhamet al., (1990), who showed that
at the beginning of a solar white-light flare (WLF) event an electron
beam can cause a transient darkening before the WLF emission starts,
under certain conditions. They named this event a "black light flare"
(BLF). Such a BLF event should appear as diffuse dark patches lasting
for about 20 seconds preceding the WLF emission, which would coincide
with intense and impulsive hard X-ray bursts. The BLF location would
be at (or in the vicinity of) the forthcoming bright patches. Their
predicted contrast depends on the position of the flare on the solar
disc and on the wavelength band of the observation.

---------------------------------------------------------
Title: The sun as a variable star: Solar and stellar irradiance
variations
Authors: Pap, Judit M.; Froehlich, Claus; Hudson, Hugh S.; Tobiska,
W. Kent
1994SoPh..152.....P Altcode: 1994svs..coll.....P; 1994IAUCo.143.....P
Variations in solar and stellar irradiances have long been of
interest. An International Astronomical Union (IAU) colloquium reviewed
such relevant subjects as observations, theoretical interpretations,
and empirical and physical models, with a special emphasis on climatic
impact of solar irradiance variability. Specific topics discussed
included: (1) General Reviews on Observations of Solar and Stellar
Irradiance Variability; (2) Observational Programs for Solar and
Stellar Irradiance Variability; (3) Variability of Solar and Stellar
Irradiance Related to the Network, Active Regions (Sunspots and Plages),
and Large-Scale Magnetic Structures; (4) Empirical Models of Solar Total
and Spectral Irradiance Variability; (5) Solar and Stellar Oscillations,
Irradiance Variations and their Interpretations; and (6) The Response
of the Earth's Atmosphere to Solar Irradiance Variations and Sun-Climate
Connections. For individual titles, see A95-78168 through A95-78218.

---------------------------------------------------------
Title: Nonthermal Processes in Flaring X-Ray--bright Points
Authors: Kundu, M. R.; Strong, K. T.; Pick, M.; White, S. M.; Hudson,
H. S.; Harvey, K. L.; Kane, S. R.
1994ApJ...427L..59K Altcode:
X-ray-bright point (XBPs) are known to show variability on a number of
timescales, including impulsive X-ray brightenings. The relationship
between these XBP 'flares' and normal solar flares is poorly known. A
fundamental question is whether nonthermal acceleration of particles
takes place in XBP flares. We address this issue by searching for
nonthermal radio emission at metric wavelengths from flaring XBPs
identified in Yohkoh soft x-ray telescope (SXT) data. Unequivocal
evidence for type III-like radio bursts, usually attributed to beams
of nonthermal electrons on open field lines, is found. This suggests
that XBP flares are similar to normal flares and can indeed accelerate
nonthermal populations of energetic particles.

---------------------------------------------------------
Title: Impulsive Behavior in Solar Soft X-Radiation
Authors: Hudson, H. S.; Strong, K. T.; Dennis, B. R.; Zarro, D.;
Inda, M.; Kosugi, T.; Sakao, T.
1994ApJ...422L..25H Altcode:
The Yohkoh soft X-ray telescope has observed impulsive, thermal,
soft X-ray emission at the footpoints of magnetic loops during solar
flares. The soft X-ray (thermal) time profiles at the footpoints closely
match the hard X-ray (nonthermal) time profiles, directly demonstrating
the heating of the lower solar atmosphere on short timescales during
the interval of nonthermal energy release. This phenomenon is the rule,
rather than the exception, occurring in the majority of flares that
we have examined with the Yohkoh data. We illustrate the impulsive
behavior with data from the major flare of 1992 January 26. For this
flare, the soft X-ray peak times matched the hard X-ray peak times
within the time resolution of the soft X-ray measurements (about 10 s),
and the soft and hard X-ray locations match within the resolution of
the hard X-ray imager. The impulsive soft X-ray emission clearly has
a thermal spectral signature, but not at the high temperature of a
'superhot' source. We conclude that the impulsive soft X-ray emission
comes from material heated by precipitating electrons at loop footpoints
and evaporating from the deeper atmosphere into the flaring flux tube.

---------------------------------------------------------
Title: YOHKOH Observations of Weak Events Within AR7218
Authors: Linford, G. A.; Hudson, H.; Sterling, A.
1994xspy.conf...49L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: X-ray solar physics from YOHKOH
Authors: Uchida, Yutaka; Watanabe, Tetsuya; Shibata, Kazunari; Hudson,
Hugh S.
1994xspy.conf.....U Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Rapid Sunspot Motion Associated with Large Solar Flares
Authors: Reardon, K. W.; Canfield, R. C.; McClymont, A. N.; Hudson,
H. S.
1994ASPC...68..336R Altcode: 1994sare.conf..336R
No abstract at ADS

---------------------------------------------------------
Title: Eclipses of the solar X-ray corona by Mercury and the Moon.
Authors: Hudson, H.; Freeland, S.; Lemen, J.; Kosugi, T.; Soma, M.;
Watanabe, T.; Hara, H.; Shimizu, T.
1994BAAS...26..795H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Spatial Resolution of Solar Total Irradiance Variability:
The YOHKOH White-Light Observations
Authors: Hudson, H. S.
1994svsp.coll..196H Altcode: 1994IAUCo.143P.196H
No abstract at ADS

---------------------------------------------------------
Title: Two Types of Interaction Between Emerging Flux and Coronal
Magnetic Field
Authors: Shibata, K.; Nitta, N.; Matsumoto, R.; Tajima, T.; Yokoyama,
T.; Hirayama, T.; Hudson, H.
1994xspy.conf...29S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Coronal Eruptions Observed by YOHKOH
Authors: Klimchuk, J. A.; Acton, L. W.; Harvey, K. L.; Hudson, H. S.;
Kluge, K. L.; Sime, D. G.; Strong, K. T.; Watanabe, Ta.
1994xspy.conf..181K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Interplanetary Consequences of Transient Coronal Events
Authors: Watanabe, Ta.; Kojima, M.; Kozuka, Y.; Tsuneta, S.; Lemen,
J. R.; Hudson, H.; Joselyn, J. A.; Klimchuk, J. A.
1994xspy.conf..207W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Poster Proceedings from IAU Colloquium 143: The Sun as a
Variable Star: Solar and Stellar Irradiance Variations
Authors: Pap, J. M.; Frohlich, C.; Hudson, H. S.; Solanki, S. K.
1994svsp.coll.....P Altcode: 1994IAUCo.143P....P
No abstract at ADS

---------------------------------------------------------
Title: Evidence for Impulsive Soft-X Bursts during Flares
Authors: Strong, K.; Hudson, H.; Dennis, B.
1994xspy.conf...65S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Diagnostics of Twisted Flux Emergence (noaa AR7260)
Authors: Leka, K. D.; van Driel-Gesztelyi, L.; Anwar, B.; Canfield,
R. C.; Hudson, H. S.; Metcalf, T. R.; Mickey, D. L.; Nitta, N.;
Kurokawa, H.
1994xspy.conf...25L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Non-Thermal Effects in Slow Solar Flares
Authors: Hudson, H. S.; Acton, L. W.; Sterling, A. C.; Tsuneta, S.;
Fishman, J.; Meegan, C.; Paciesas, W.; Wilson, R.
1994xspy.conf..143H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Loop-Loop Interaction Observed with YOHKOH SXT
Authors: Akioka, M.; Acton, L. W.; Hudson, H. S.
1994xspy.conf..241A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Electric Currents and Coronal Structures in NOAA Active
Region 6952
Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey,
D. L.; Wülser, J. -P.; Tsuneta, S.
1994xspy.conf...51M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Morphological Evolution of the Post-Flare Loops of June
25-26, 1992
Authors: Anwar, B.; Hiei, E.; Hudson, H. S.; Acton, L. W.; Metacalf,
T.; Lemen, J.; Martens, P.
1994xspy.conf..121A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Rapid Sunspot Motion during a Major Solar Flare
Authors: Anwar, B.; Acton, L. W.; Hudson, H. S.; Makita, M.; McClymont,
A. N.; Tsuneta, S.
1993SoPh..147..287A Altcode:
A major solar flare on 15 November, 1991 produced a striking
perturbation in the position and shape of the sunspot related most
closely to the flare. We have studied these perturbations by use of the
aspect-sensor images from the Soft X-ray Telescope on board YOHKOH,
and with ground-based data from the Mees Solar Observatory. The
perturbation occurred during the impulsive phase of the flare, with
a total displacement on the order of 1 arc sec. The apparent velocity
of approximately 2 km s−1 exceeds that typically reported
for sunspot proper motions even in flare events. We estimate that the
magnetic energy involved in displacing the sunspot amounted to less
than 4 × 1030 ergs, comparable to the radiant energy from
the perturbed region. Examination of the Mees Observatory data shows
that the spot continued moving at lower speed for a half-hour after
the impulsive phase. The spot perturbation appears to have been a
result of the coronal restructuring and flare energy release, rather
than its cause.

---------------------------------------------------------
Title: The 1992 January 5 Flare at 13.3 UT: Observations from YOHKOH
Authors: Doschek, G. A.; Strong, K. T.; Bentley, R. D.; Brown, C. M.;
Culhane, J. L.; Fludra, A.; Hiei, E.; Lang, J.; Mariska, J. T.;
Phillips, K. J. H.; Pike, C. D.; Sterling, A. C.; Watanabe, T.; Acton,
L. W.; Bruner, M. E.; Hirayama, T.; Tsuneta, S.; Rolli, E.; Kosugi,
T.; Yoshimori, M.; Hudson, H. S.; Metcalf, T. R.; Wuelser, J. -P.;
Uchida, Y.; Ogawara, Y.
1993ApJ...416..845D Altcode:
We discuss X-ray spectra and soft X-ray images of an M1.9 flare that
occurred on 1992 January 5 near 13.3 UT. These data were obtained
with instrumentation on the Japanese Yohkoh spacecraft. They cover
the entire rise phase of the flare. To supplement these data we have
ground-based magnetograms and Hα spectroheliograms. We calculate
the electron temperature and emission measure of the flare as a
function of time during the early rise phase using X-ray spectral
line intensities and line ratios. Using spectral line widths, line
profile asymmetries, and wavelength shifts due to the Doppler effect,
we calculate the dynamical properties of the flare. The time development
of the morphology of the flare, as revealed by the soft X-ray images
and the Hα spectroheliograms, and the physical quantities inferred
from the X-ray spectra, are compared with chromospheric evaporation
models. There is an enhancement of blueshifted emission that is closely
correlated with the hard X-ray bursts. Heating of one loop in the flare
is consistent with a conduction-evaporation model, but heating is found
in several structures that do not appear to be physically associated
with each other. No standard evaporation model can adequately explain
all of the observations.

---------------------------------------------------------
Title: The Confined Two-Ribbon Flare of 1991 October 24
Authors: de La Beaujardiere, J. F.; Canfield, R. C.; Hudson, H. S.;
Wuelser, J. -P.; Kosugi, T.; Masuda, S.; Acton, L. W.
1993BAAS...25.1178D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Imaging Solar Bolometric and Spectral Intensity Using Thermal
Detector Arrays
Authors: Deming, D.; Glenar, D.; Kostiuk, T.; Bly, V.; Forrest, K.;
Nadler, D.; Hudson, H.; Lindsey, C.; Kopp, G.; Avrett, E.; Terrill,
C. W.
1993BAAS...25R1221D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Electric Currents and Coronal Structures in NOAA Active
Region 6952
Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey,
D. L.; Martens, P. C. H.; Tsuneta, S.
1993BAAS...25.1179M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Survey of Loop-Footpoint Brightenings During the Impulsive
Phase of Flares
Authors: Strong, K.; Dennis, B.; Hudson, H.; Kosugi, T.; Sakao, T.
1993BAAS...25Q1187S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Yohkoh-SXT Observations of AR Brightenings
Authors: Linford, G. A.; Lemen, J. R.; Hudson, H. S.
1993BAAS...25Q1187L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Millimeter, Microwave and X-Ray Morphology and Spectra of
the 07Jan92 Flare
Authors: Silva, A. V.; Lin, R. P.; de Pater, I.; White, S. M.; Kundu,
M. R.; Gary, D. E.; Hudson, H. S.
1993BAAS...25Q1223S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Temperatures in Flares Determined from Fe XXV Spectra,
Resonance Line Ratios, and GOES X-ray Flux
Authors: Sterling, A. C.; Doschek, G. A.; Pike, C. D.; Hudson, H. S.;
Lemen, J. R.; Zarro, D. M.
1993BAAS...25.1178S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Impulsive Soft X-Ray Emission in Solar Flares
Authors: Hudson, H. S.; Strong, K. T.; Dennis, B. R.; Zarro, D.;
Inda, M.; Kosugi, T.; Sakao, T.
1993BAAS...25.1177H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Extended and Diffuse X-Ray Corona Observed by Yohkoh-SXT
Authors: Lemen, J. R.; Slater, G. L.; Hudson, H. S.; Acton, L. W.
1993BAAS...25.1179L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Yohkoh-SXT Observations from the Spartan and Nixt Max91
Campaign
Authors: Morrison, M.; Bruner, M.; Freeland, S.; Lemen, J.; Linford,
G.; Nitta, N.; Slater, G.; Strong, K.; Hara, H.; Kano, R.; Shimizu,
T.; Tsuneta, S.; Hudson, H.; Ogawara, Y.; Kosugi, T.; Sakao, T.;
Watanabe, T.; Takeda, A.; Acton, L.
1993BAAS...25.1213M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Large-scale structure of the solar plasma corona. An analysis
of Yohkoh SXT images.
Authors: Saito, T.; Minami, Shigeyuki; Kozuka, Y.; Takahashi, T.;
Hudson, H.; Tsuneta, Saku; Watanabe, T.
1993ppcn.conf..215S Altcode:
The YOHKOH SXT images are analyzed to find the large-scale structure
of the solar plasma corona. The clarified characteristics are preferred
position of active regions and its possible mechanism, rules governing
the coronal hole channel, a relation between the inner and outer
corona, formation of the 4-sector structure, and the outer corona in
pseudo-aligned phase. These characteristics are in agreement with
both the rotational reversing model and the triple-dipole model on
the structure and dynamics of the heliomagnetosphere.

---------------------------------------------------------
Title: Asteroseismology: the impact of solar space observations.
Authors: Hudson, H. S.
1993ASPC...40...43H Altcode: 1993IAUCo.137...43H; 1993ist..proc...43H
Observations from space relevant to solar global properties
(oscillations, magnetic activity, etc.) are helpful both scientifically
and technically in preparing for stellar observations. This paper
summarizes the results from the main previous experiments (ACRIM,
SOUP, and IPHIR), and also gives an initial technical report from the
SXT instrument on board Yohkoh, launched in August 1991. The solar
observations to date demonstrate the existence of several mechanisms
for low-level variability: spots, faculae, the photospheric network,
granulation, and p-mode oscillations. The observations of oscillations
have been particularly helpful in setting limits on solar interior
rotation. In addition to the solar processes, stars of other types
may have different mechanisms of variability. These may include the
analogues of coronal holes or solar flares, modes of oscillation
not detected in the Sun, collisions with small bodies, duplicity,
and probably mechanisms not invented yet but related in interesting
ways to stellar convection and magnetism.

---------------------------------------------------------
Title: Soft X-ray Imaging of Impulsive Evaporation
Authors: Hudson, H.; Nitta, N.; Strong, K.; Takakura, T.
1992AAS...181.5501H Altcode: 1992BAAS...24.1211H
The time development of a solar flare can often be broken into
two phases, viz. the impulsive phase and the gradual phase. For
many reasons the impulsive phase is known to result from powerful
electron acceleration, to energies of tens of keV (and higher), with
a total energy that is a large fraction of the energy subsequently
radiated in various flare emissions. A grazing-incidence soft X-ray
telescopes on board the Yohkoh spacecraft now enables us to study
the few-keV properties of the impulsive phase for the first time. In
a representative set of impulsive solar flares, we find an excellent
match between the soft X-ray time profiles at the footpoints of coronal
magnetic flux tubes and the hard X-ray impulsive emission. The hard
X-ray images directly show the sites of the particle precipitation. The
impulsive soft X-ray emission could arise directly as non-thermal
bremsstrahlung, extending to the few-keV range; or it could contain
contributions from the impulsively evaporating plasma seen during the
process of the flare explosion from the chromosphere. We discuss these
interpretations and the physics resulting from them.

---------------------------------------------------------
Title: Correlation between X-ray Temporal Variability and Magnetic
Environment in Solar Flares
Authors: Nitta, N.; Harvey, K.; Hudson, H.; Ichimoto, K.; Metcalf,
T.; Mickey, D.; Sakai, J. -I.; Sakao, T.; Sakurai, T.; Takahashi, M.
1992AAS...181.5503N Altcode: 1992BAAS...24.1211N
The X-ray time history of a solar flare can reflect basic processes of
heating and/or acceleration, which in turn may depend on the magnetic
environment of the site. Some flares show a simple rise and fall
temporal behavior, whereas others show more than one peak. Comparisons
of images taken by the Soft X-ray Telescope (SXT) aboard the Yohkoh
spacecraft with ground-based magnetic data (Hawaii, Kitt Peak and
Mitaka) reveal that, at least for a flare-productive active region
(NOAA 7260), flares with double-peaked and single-peaked time profiles
occurred at systematically different locations within the region. We
discuss this result in terms of theoretical models, especially those
of coalescence of two current loops.

---------------------------------------------------------
Title: White-Light Flares Observed by YOHKOH
Authors: Hudson, Hugh S.; Acton, Loren W.; Hirayama, Tadashi; Uchida,
Yutaka
1992PASJ...44L..77H Altcode:
The Yohkoh observatory is producing a first sample of white-light
flares observed from space. We present observations of four of them,
all X-class events. The Yohkoh (SXT) white-light data typically
have a 12-s cadence for images with 2.”46 pixels over a field of
view of 2.'62 in one of two broad-band optical filters, and the 1991
November 15 flare produced a brightness increase of about 38% over the
photospheric brightness in the 30 Angstroms \ passband filter centered
at 4308 Angstroms. The white-light flare morphology in the best-observed
flares displays a double “footpoint” character, establishing a close
relationship with the compact magnetic flux tubes involved with both
hard and soft X-ray emissions. The “footpoint” brightnesses may vary
independently with time. We describe the data in the context of the
soft and hard X-ray observations simultaneously carried out on board
the Yohkoh satellite, emphasizing energetics and timing.

---------------------------------------------------------
Title: The Morphology of 20times 10(6) K Plasma in Large Non-Impulsive
Solar Flares
Authors: Acton, Loren W.; Feldman, Uri; Bruner, Marilyn E.; Doschek,
George A.; Hirayama, Tadashi; Hudson, Hugh S.; Lemen, James R.;
Ogawara, Yoshiaki; Strong, Keith T.; Tsuneta, Saku
1992PASJ...44L..71A Altcode:
We have examined images of 10 flares observed by the Soft X-ray
Telescope on-board the Yohkoh spacecraft. These images show that the
hottest portion of the soft X-ray flare is located in compact regions
that appear to be situated at the tops of loops. These compact regions
form at, or shortly after, flare onset, and persist well into the decay
phase of the flares. In some cases, the compact regions are only a
few thousand kilometers in size and are small compared to the lengths
of flaring loops. This is inconsistent with the smoother intensity
distribution along the loops expected from models of chromospheric
evaporation.

---------------------------------------------------------
Title: Continual Expansion of the Active-Region Corona Observed by
the YOHKOH Soft X-Ray Telescope
Authors: Uchida, Yutaka; McAllister, Alan; Strong, Keith T.; Ogawara,
Yoshiaki; Shimizu, Toshifumi; Matsumoto, Ryoji; Hudson, Hugh S.
1992PASJ...44L.155U Altcode:
We have found from the observations of the Yohkoh Soft X-ray Telescope
(SXT) that the corona above active regions expands occasionally, and
almost continually in the cases of “active" active regions. This
is contrary to the commonly accepted idea of magnetohydrostatic
equilibrium of these regions. The key to this discovery has been a
movie representation of the Yohkoh-SXT data, which, for the first time,
provides adequate sampling and continuity for this purpose. The movies
show ubiquitous expansions above the active regions, with velocities
in the range of a few to a few tens km s(-1) as measured when they are
on the limb. The expansion appears to preserve the overall structure
of the active-region corona. We suggest that the expansion may have a
physical relationship with the transient loop brightenings found within
the active regions. This finding of almost continual expansion of the
active-region corona may affect some of the basic ideas concerning
active regions, as well as those of the mass-loss from the Sun and
Sun-like stars.

---------------------------------------------------------
Title: The X Flare of 1991 November 15: Coordinated Mees/Yohkoh
Observations
Authors: Canfield, Richard C.; Hudson, Hugh S.; Leka, K. D.; Mickey,
Donald L.; Metcalf, Thomas R.; Wuelser, Jean-Pierre; Acton, Loren W.;
Strong, Keith T.; Kosugi, Takeo; Sakao, Taro; Tsuneta, Saku; Culhane,
J. Leonard; Phillips, Andrew; Fludra, Andrzej
1992PASJ...44L.111C Altcode:
This is a preliminary report on two unique new results from coordinated
observations at Mees Solar Observatory and Yohkoh of the X1.5 flare
of 1991 November 15, using vector magnetograms, Hα imaging spectra,
X-ray images, and X-ray spectra. First, we find a close spatial
relationship between Hα redshifts and X-rays from a flare loop and
its footpoints at a time of large X-ray blueshifts. Second, we find
that impulsive-phase hard X-rays originate in regions that are near,
but not coincident with, the peaks of the vertical electrical current
density distribution in AR 6919.

---------------------------------------------------------
Title: The Status of YOHKOH in Orbit: an Introduction to the Initial
Scientific Results
Authors: Ogawara, Yoshiaki; Acton, Loren W.; Bentley, Robert D.;
Bruner, Marilyn E.; Culhane, J. Leonard; Hiei, Eijiro; Hirayama,
Tadashi; Hudson, Hugh S.; Kosugi, Takeo; Lemen, James R.; Strong, Keith
T.; Tsuneta, Saku; Uchida, Yutaka; Watanabe, Tetsuya; Yoshimori, Masato
1992PASJ...44L..41O Altcode:
In this introductory article accompanying the initial scientific
papers from the Yohkoh mission, we briefly summarize the design
and in-orbit function of the spacecraft and its four scientific
instruments. Although these initial results include mainly studies
based upon individual Yohkoh experiments at this early stage, there
are also analyses of combined data sets provided by several on-board
and ground-based instruments in progress. The results presented here,
and anticipated future results, suggest that the Yohkoh observations
with their comprehensive coverage of solar high-energy phenomena will
come to represent a significant milestone in the progress of solar
physics. This will be true not only regarding flares, but also for
fainter coronal structures and even coronal holes.

---------------------------------------------------------
Title: Hard X-Ray Imaging Observations by YOHKOH of the 1991 November
15 Solar Flare
Authors: Sakao, Taro; Kosugi, Takeo; Masuda, Satoshi; Inda, Mika;
Makishima, Kazuo; Canfield, Richard C.; Hudson, Hugh S.; Metcalf,
Thomas R.; Wuelser, Jean-P.; Acton, Loren W.; Ogawara, Yoshiaki
1992PASJ...44L..83S Altcode:
We report on hard X-ray imaging observations of the 1991 November 15
flare with the HXT instrument aboard {Yohkoh}. Distributions of the hard
X-ray sources at various stages of the flare, together with an overlay
of the white-light flare, are presented. Attention is concentrated on
the behavior of hard X-ray sources during the impulsive phase. The
hard X-ray source appeared initially as a single source near the
magnetic neutral line, then evolved into a double-source shape with the
separation increasing with time. We believe that this is evidence for a
multiple loop system flaring successively with a rising energy-release
site. At the minima between the individual spikes of the time profile,
the hard X-rays at 20--30 keV were concentrated near the apex of the
flaring loop, whereas the hard X-rays above 30 keV originated from the
footpoints. These observations are compared with the existing models.

---------------------------------------------------------
Title: Observation of a Solar Flare at the Limb with the YOHKOH Soft
X-Ray Telescope
Authors: Tsuneta, Saku; Hara, Hirohisa; Shimizu, Toshifumi; Acton,
Loren W.; Strong, Keith T.; Hudson, Hugh S.; Ogawara, Yoshiaki
1992PASJ...44L..63T Altcode:
A long-enduring soft X-ray flare at the solar limb was well observed
by the Soft X-ray Telescope aboard the Yohkoh spacecraft from its
pre-flare stage through the post-flare phase. A “helmet streamer"
arch appears several hours prior to the flare, in association with a
continuous expansion and restructuring of the active-region magnetic
structure. This arch then starts to flare, and increases its height and
footpoint separation at v = 10--30 km s(-1) . The arch has a complex
temperature structure in the rising phase, whereas the outer arches
have systematically higher temperatures in the decay phase. Magnetic
reconnection in a neutral sheet at the loop top, created by pre-flare
magnetic restructuring, would explain this type of flare.

---------------------------------------------------------
Title: Estimating Solar Chromospheric UV Fluxes from Sunspot and
Solar Radio Data
Authors: Donnelly, R. F.; Hudson, H.; Pap, J.; Willson, R.
1992sers.conf..275D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Photometric Measurements of Sunspots Deficits and Facular
Excesses
Authors: Chapman, G. A.; Lawrence, J. K.; Hudson, H. S.
1992sers.conf..135C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Precise ground-based solar photometry and variations of
total irradiance
Authors: Chapman, G. A.; Herzog, A. D.; Lawrence, J. K.; Walton,
S. R.; Hudson, H. S.; Fisher, B. M.
1992JGR....97.8211C Altcode:
Variations in the total solar irradiance measured by the active cavity
radiometer irradiance monitor (ACRIM) on SMM have been correlated
with measures of magnetic activity on the solar disk. Quantitative
indices of magnetic activity were derived from ground-based, full-disk,
photometric images of the Sun at red (6723 Å) and violet (3934-Å K
line) wavelengths. The red images have been obtained on a daily basis
at the San Fernando Observatory since 1985, and the K line images
since 1988. Sunspot irradiance deficits are calculated directly
from the red images while proxy measures of facular irradiance
excesses are derived from the K line images. The images analyzed
here were made during 21 days between June 20 and July 14, 1988,
a period centered on the disk passage of a large sunspot group. The
best two-parameter multiple correlation coefficient between the
ACRIM data and the photometric data is R2=0.97 (21 data
points, 18 degrees of freedom). The zero point S0=1367.27
W m-2 agrees well with the solar irradiance
measured by ACRIM/SMM during the 1986 activity minimum: the residual
standard deviation was 0.13 W m-2 (about 100
ppm). The multiple correlations were extended to include measures
of the irradiance contribution of “network” magnetic fields,
unassociated with active regions. NOAA 9 spacecraft observations of UV
MgII lines at 2800 Å gave R2=0.99 (17 degrees of freedom)
with S0=1366.68+0.08 W m-2. The index of 10.7-cm
microwave flux gave R2=0.98, with S0=1366.43+0.11
W m-2. We can thus model short-term irradiance
changes to within 100 ppm relative precision from ground-based data.

---------------------------------------------------------
Title: White--Light Flares Observed by YOHKOH
Authors: Hudson, H. S.; Wulser, J. -P.; Acton, L.; Uchida, Y.
1992AAS...180.2309H Altcode: 1992BAAS...24..761H
The YOHKOH observatory is producing a first sample of white-light
flares observed from space. We present observations of three of
these flares, 1991 Oct. 27, 1991 Nov. 15, and 1991 Dec. 3. Of these,
the 1991 Nov. 15 was also well-observed with H-alpha spectroscopic
imaging observations at Mees Observatory, Haleakala. The YOHKOH (SXT)
white-light data typically have a 12-second cadence for images with
2.46 arc-sec pixels over a field of view of 2.62 arc min in one of two
broad-band optical filters, and the Nov. 15 flare produced a brightness
excess of about 25\ We describe the YOHKOH white-light observations in
terms of morphology and flare energetics in the context of the X-ray
observations. For the Nov. 15 flare, we find that the H-alpha emission
wing spectroheliograms match closely with the continuum images. Such
observations permit tests of models (“electron precipitation”)
in which the non--thermal electrons responsible for the hard X-ray
bremsstrahlung also excite the upper photosphere by direct heating and
ionization. We find by comparison with the hard X-ray data that this
mechanism is viable, but confirm the need for at least one additional
mechanism for the excitation of the continuum.

---------------------------------------------------------
Title: Electric Currents and Coronal Structures in Two Flare-
Productive Active Regions, AR 6850 and AR 6952
Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey,
D. L.; Strong, K. T.; Tsuneta, S.
1992AAS...180.3004M Altcode: 1992BAAS...24R.775M
In this study, we examine the spatial and temporal relationship between
coronal structures observed with the Soft X-Ray Telescope (SXT) on
board the YOHKOH spacecraft and vertical electric currents derived
from vector magnetograms obtained at the Mees Solar Observatory,
Haleakala, Hawaii. We have focused on two active regions, AR 6850
(October 1991) and AR 6952 (December 1991). In both active regions,
we observed significant current structures which persisted over time
scales of days. The SXR emitting coronal structures, however, changed
on much shorter time scales, indicating that there is no compelling,
direct spatial and temporal relationship between the non-flaring SXR
structures and the long-lived electric current systems. We have seen
at least one case (in AR 6952) where a SXR brightening was associated
spatially with a change in the vertical electric current. In this case,
the the vertical current dissipated between December 8, 00:35 UT and
the next observation at 00:48 UT on December 9, leaving a bright SXR
structure which was observed at 24:27 UT on December 8. Hence, although
more data must be analyzed to make a compelling case, it is possible
that the SXR emission is related more closely to changes in the electric
current systems rather than simply to the presence of these currents.

---------------------------------------------------------
Title: SPAM: A Canned Internet-Accessible Database of Interest to
Solar Flare Researchers
Authors: Canfield, R. C.; Hudson, H. S.; Kiernan, E.; Metcalf, T. R.;
Wulser, J. -P.
1992AAS...180.5103C Altcode: 1992BAAS...24..813C
We have established a searchable database, called SPAM (Spectroscopy
and Polarimetry at Mees), which contains logs of observations made at
Mees Solar Observatory (Haleakala, Maui). Of more general interest,
the database also includes the Events List and Region Report from
the Space Environment Laboratory (Boulder). Logs from YOHKOH are
currently being added. Hence, SPAM can be used to determine, for
example, whether Mees has vector magnetograms of a certain NOAA AR
or whether YOHKOH has certain types of observations in specified time
ranges. As well, it can be used to search the SEL database for flares
with selected attributes. Included logs (and searchable attributes, in
addition to date, day of year, and time) are: Mees Solar Observatory Log
(instrument, NOAA AR, data type, observing setup), SEL Event List (NOAA
AR, X-ray Class), SEL Region Report (NOAA AR), YOHKOH Orbit Summary,
YOHKOH SXT Quiet Mode PFI Observations (latitude, longitude, X-ray and
optical image size), YOHKOH Flare Observations (latitude, longitude,
specific channel counts or ratios). SPAM runs on a Sun workstation at
Mees Solar Observatory, and is available over Internet. Simply access
(e.g., telnet) koa.ifa.hawaii.edu (128.171.167.1) from any vt100,
Sun, or xterm emulator. Log on as spam (lower case); there is no
password. New users are asked to read release notes and hints.

---------------------------------------------------------
Title: YOHKOH and Compton Observations of an LDE Event: Reconnection
and the Neupert Effect?
Authors: Hudson, H. S.; Nitta, N.; Paciesas, W.; Fishman, G.; Meegan,
C.; Wilson, R.
1992AAS...180.3409H Altcode: 1992BAAS...24..784H
We present X-ray images and time profiles of a gradual flare (GOES
magnitude M3.2) that occurred on the solar limb at 0430 UT (time of
GOES maximum), Feb. 21, 1992. The soft X-ray structure of this event
showed a remarkably clear loop development with complicated geometry,
including a prominent nearly-vertical bar extending from near the
top of the loop(s). During the presence of this bar, the large-area
BATSE hard X-ray detector on the Compton Observatory showed extended
but highly irregular hard X-ray emission, roughly in the proportion
predicted by the “Neupert Effect” that associates the integral of the
hard X-ray light curve with the instantaneous soft X-ray flux. The peak
hard X-ray flux (about 1 ph/(cm(2) sec) above 25 keV) was at 0321 UT,
and the emission extended throughout the rise phase of the soft X-ray
event. Coronal magnetic reconnection provides a natural interpretation
for this morphology, and the time profiles strongly suggest that we
can identify the hard X-ray emission with bremsstrahlung from electrons
accelerated in the coronal volume of the reconnection region (vertical
bar structure).

---------------------------------------------------------
Title: Electric Currents and Hard X-ray Images in the X Class Flare
of November 15, 1991
Authors: Metcalf, T. R.; Sakao, T.; Acton, L. W.; Canfield, R. C.;
Hudson, H. S.; Inda, M.; Kosugi, T.; Wulser, J. P.
1992AAS...180.3005M Altcode: 1992BAAS...24..776M
We present co-aligned observations of hard x-rays observed with the
Hard X-ray Telescope (HXT) on board the YOHKOH spacecraft and vertical
electric currents derived from a vector magnetogram obtained at the
Mees Solar Observatory, Haleakala, Hawaii. Previous work comparing
the wings of the Hα line to vertical electric currents has suggested
that electron precipitation in flares occurs at the edges of these
currents. The Stark wings of Hα were interpreted as a signature of
non-thermal electrons penetrating the relatively dense chromosphere and
used as a proxy for direct observation of the non-thermal electrons. The
hard X-rays used in this study provide a direct determination of
the locations of the electron energy losses. In the X class flare
of November 15, 1991, we find the same relation between hard X-ray
emission and vertical electric currents as was found between Hα Stark
wing emission and vertical currents: the hard x-ray emission occurs
predominantly at the edges of the vertical current sites, and not
spatially on top of these currents. Canfield, R. C., de La Beaujardiere,
J., and Leka, K. D., in “The Physics of Solar Flares", ed. Culhane and
Jordan, The Royal Society, London, 1991 Canfield, R. C., Leka, K. D.,
and Wulser,J. P., in “Flare Physics in Solar Activity Maximum 22",
ed. Uchida, Canfield, Watanabe, and Hiei, Springer-Verlag, Berlin, 1991

---------------------------------------------------------
Title: Search for evidence of low energy protons in solar flares.
Authors: Metcalf, Thomas R.; Wuelser, Jean-Pierre; Canfield, Richard
C.; Hudson, Hugh S.
1992NASCP3137..536M Altcode: 1992como.work..536M
We searched for linear polarization in the H alpha line using the
Stokes Polarimeter at Mees Solar Observatory and present observations
of a flare from NOAA active region 6659 which began at 01:30 UT on
14 Jun. 1991. Our dataset also includes H alpha spectra from the Mees
charge coupled device (MCCD) imaging spectrograph as well as hard x ray
observations from the Burst and Transient Source Experiment (BATSE)
instrument on board the Gamma Ray Observatory (GRO). The polarimeter
scanned a 40 x 40 inch field of view using 16 raster points in a 4 x
4 grid. Each scan took about 30 seconds with 2 seconds at each raster
point. The polarimeter stopped 8.5 inches between raster points and
each point covered a 6 inch region. This sparse sampling increased
the total field of view without reducing the temporal cadence. At
each raster point, an H alpha spectrum with 20 mA spectral sampling is
obtained covering 2.6 A centered on H alpha line center. The preliminary
conclusions from the research are presented.

---------------------------------------------------------
Title: Gamma-ray and microwave emission from 1991 June events.
Authors: Enome, Shinzo; Nakajima, Hiroshi; Hudson, Hugh S.; Schwartz,
Richard
1992NASCP3137..522E Altcode: 1992como.work..522E
The Sun showed unprecedented microwave activities in Jun. 1991,
which produced four major and numerous weaker bursts and gamma ray
emission measured by the Gamma Ray Observatory. The 4 Jun. 1991
event shows a sharp maximum around 03:41 UT and weak emission a few
minutes before the maximum in the gamma ray record of the Burst and
Transient Source Experiment (BATSE), with a preliminary estimated
energy of 5 MeV. Although the 80-GHz and possible 35-GHz records
show more prominent emission in the pre-maximum stage. This strongly
suggests the first observational evidence for gamma ray and mm-wave
emission from relativistic electrons. Comparisons of the other three
major events on 6 Jun. at 01:00 UT, 9 Jun. at 01:34 UT, and 11 Jun. at
01:51 UT between gamma ray and microwave emission are also in progress.

---------------------------------------------------------
Title: Variability of solar ultraviolet irradiance.
Authors: Pap, J. M.; Donnelly, R. F.; Hudson, H. S.; Rottman, G. J.;
Willson, R. C.
1991JATP...53..999P Altcode:
A model of solar Lyman alpha irradiance developed by multiple linear
regression analysis, including the daily values and 81-day running
means of the full disk equivalent width of the Helium line at 1083
nm, predicts reasonably well both the short- and long-term variations
observed in Lyman alpha. In contrast, Lyman alpha models calculated
from the 10.7 cm radio flux overestimate the observed variations in the
rising portion and maximum period of solar cycle, and underestimates
them during solar minimum. The authors show models of Lyman alpha based
on the He line equivalent width and 10.7 cm radio flux for those time
intervals when no satellite observations exist, namely back to 1974
and after April 1989, when the measurements of the Solar Mesosphere
Satellite were terminated.

---------------------------------------------------------
Title: Using Image Area to Control CCD Systematic Errors in
Spacebourne Photometric and Astrometric Time-Series Measurements
Authors: Buffington, Andrew; Booth, Corwin H.; Hudson, Hugh S.
1991PASP..103..685B Altcode:
The effect of some systematic errors for high-precision time-series
spaceborne photometry and astrometry has been investigated with a CCD
as the detector. The 'pixelization' of the images causes systematic
error in astrometric measurements. It is shown that this pixelization
noise scales as image radius r exp -3/2. Subpixel response gradients,
not correctable by the 'flat field', and in conjunction with telescope
pointing jitter, introduce further photometric and astrometric
errors. Subpixel gradients are modeled using observed properties of
real flat fields. These errors can be controlled by having an image
span enough pixels. Large images are also favored by CCD dynamic
range considerations. However, magnified stellar images can overlap,
thus introducing another source of systematic error. An optimum image
size is therefore a compromise between these competing factors.

---------------------------------------------------------
Title: Book reviews
Authors: Hudson, H. S.; Kleczek, J.; Švestka, Z.
1991SoPh..133..403H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar flares, microflares, nanoflares, and coronal heating
Authors: Hudson, H. S.
1991SoPh..133..357H Altcode:
Solar flare occurrence follows a power-law distribution against total
flare energy W: dN/dW ∼ W−α with an index α ∼ 1.8
as determined by several studies. This implies (a) that microflares
must have a different, steeper distribution if they are energetically
significant, and (b) there must be a high-energy cutoff of the observed
distribution. We identify the distinct `soft' distribution needed
for coronal heating, if such a distribution exists, with Parker's
nanoflares.

---------------------------------------------------------
Title: The Sun's luminosity over a complete solar cycle
Authors: Willson, Richard C.; Hudson, Hugh S.
1991Natur.351...42W Altcode:
THE Active Cavity Radiometer Irradiance Monitor (ACRIM I), an
instrument carried on NASA's Solar Maximum Mission satellite,
measured the Sun's luminosity (total power outflow) from early 1980
to late 19891-5. Here we present the first account of
the complete ACRIM I data set, and give evidence confirming our
previous suggestion that solar luminosity varies with the 11-year
solar cycle6. As previously reported, this slow variation
closely follows statistical measures of the distribution of magnetic and
photospheric features on the Sun's surface4-8. But there was
an exception to this correlation in the form of a remarkable irradiance
excess during 1980, at about the time of the sunspot maximum of solar
cycle 21. The linkage, over a whole cycle, of luminosity variation to
photospheric activity suggests the existence of an unknown physical
mechanism other than the thermal diffusion model that explains
luminosity deficits due to sunspots. Luminosity models connecting
total irradiance to global indicators of solar activity, such as the
equivalent width of the 1,083-nm helium line, are consistent with the
gross features of the variability, but fail to account for the 1980
irradiance excess.

---------------------------------------------------------
Title: Report of the solar physics panel
Authors: Withbroe, George L.; Fisher, Richard R.; Antiochos, Spiro;
Brueckner, Guenter; Hoeksema, J. Todd; Hudson, Hugh; Moore, Ronald;
Radick, Richard R.; Rottman, Gary; Scherrer, Philip
1991spsi....1...67W Altcode:
Recent accomplishments in solar physics can be grouped by the
three regions of the Sun: the solar interior, the surface, and the
exterior. The future scientific problems and areas of interest involve:
generation of magnetic activity cycle, energy storage and release,
solar activity, solar wind and solar interaction. Finally, the report
discusses a number of future space mission concepts including: High
Energy Solar Physics Mission, Global Solar Mission, Space Exploration
Initiative, Solar Probe Mission, Solar Variability Explorer, Janus,
as well as solar physics on Space Station Freedom.

---------------------------------------------------------
Title: Differential Emission-Measure Variations and the "Neupert
Effect"
Authors: Hudson, H. S.
1991BAAS...23R1064H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Total Solar Irradiance Variations Compared with Ground-Based
Photometry at the SFO
Authors: Chapman, G. A.; Lawrence, J. K.; Hudson, H. S.
1991BAAS...23.1067C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Gamma-Ray Spectroscopy with BATSE
Authors: Hudson, H. S.
1991BAAS...23.1074H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Total Irradiance and the Solar Cycle
Authors: Hudson, H. S.; Willson, R. C.
1991BAAS...23.1067H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Time-Series Images of Jupiter at 10 Microns
Authors: Fisher, B.; Hudson, H.; Jones, B.; Piña, R.; Puetter, R.
1991BAAS...23..961F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Ground-Based Modelling of Solar Irradiance Variations
Authors: Lawrence, J. K.; Chapman, G. A.; Herzog, A. D.; Walton,
S. R.; Hudson, H. S.; Fisher, B. M.
1991BAAS...23..960L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Space Parasol as a Countermeasure Against the Greenhouse
Effect
Authors: Hudson, H. S.
1991JBIS...44..139H Altcode:
The greenhouse effect threatens to increase the temperature of the
Earth substantially leading to gross changes in the quality of human
life. This paper suggests a solution to this problem via the deployment
of a "Space Parasol" at the Lagrangian point L1 of the Earth-Sun system,
to intercept some desired fraction of the solar radiant energy.

---------------------------------------------------------
Title: Helioseismology with the ACRIM instrument on the solar
maximum mission
Authors: Hudson, Hugh S.
1991AdSpR..11d..61H Altcode: 1991AdSpR..11R..61H
The Active Cavity Radiometer Irradiance Monitor (ACRIM) instrument on
board SMM pioneered high-precision solar photometry from space, and
provided the first detection of solar p-mode oscillations at low degree
by this technique. The observations extended from February, 1980, until
December, 1989, with a hiatus of low sampling rate in 1981-1984. During
summer 1989, the instrument operated in a “no-shutter” mode with
continuous viewing between the orbital gaps. This resulted in a fourfold
increase of the duty cycle, and an effective increase in the Nyquist
frequency from 3.815 mHz to some tens of mHz. This review discusses
the initial results from this campaign along with a general review of
the analyses to date of the entire ACRIM data set.

---------------------------------------------------------
Title: Capabilities and Limitations of SOLAR-A
Authors: Acton, L. W.; Hudson, H. S.
1991LNP...387...28A Altcode: 1991fpsa.conf...28A
The purpose of this paper is to present an overview useful to
scientists, not familiar with Solar-A, who are interested in the
capabilities of the mission for solar research. In order to keep
the paper to a manageable size it will be assumed that the reader is
generally familiar both with the experimental techniques of Solar-A
and current research in solar high-energy physics. We do not provide
detailed technical descriptions of the Solar-A instruments. We do,
however, briefly discuss the capabilities of Solar-A in the context
of solar activity research in the 1990's.

---------------------------------------------------------
Title: Solar Irradiance Variability from Modern Measurements
Authors: Froehlich, C.; Foukal, P. V.; Hickey, J. R.; Hudson, H. S.;
Willson, R. C.
1991suti.conf...11F Altcode:
Direct measurements from satellites of the solar 'constant' (the total
irradiance at mean sun-earth distance) during more than ten years
show variations over time scales from minutes to years and decades. At
high frequencies, solar oscillations contribute to the variance. The
most important influences are related to solar activity: during the
passage of active regions on the solar disk (sunspots and faculae)
changes of a few 0.1 percent lasting for several days are observed. The
effects of spots can be well reproduced by the projected sunspot index,
whereas the influence of faculae have to be modeled from proxy data
like the Ca-K plage index or the He I index. Long-term trends are
detected which are connected to the 11-yr solar activity cycle.

---------------------------------------------------------
Title: Hard X-ray and gamma-ray imaging spectroscopy for the next
solar maximum.
Authors: Hudson, H. S.; Crannell, C. J.; Dennis, B. R.; Spicer, D. S.;
Davis, J. M.; Hurford, G. J.; Lin, R. P.
1990SPIE.1344..492H Altcode: 1990exrg.conf..492H
A single high-energy instrument based on rotating modulation collimators
with germanium semiconductor spectrometers as the detectors can provide
high angular resolution (&lt;1″, high time resolution (&lt;1 s),
and high spectral resolution (about 1 keV), all in one package. Such
rotating modulation-collimator optics provide excellent spatial coverage
for high-contrast images in the hard X-ray domain, where there will
be a large signal-to-noise ratio during even modest flares. The use of
thick modulation plates will make it possible to image gamma rays with
&lt;5″angular resolution to energies in excess of 10 MeV during the
more energetic flares without compromising the ability of the germanium
detectors to resolve the gamma-ray lines. Energetic neutrons will also
be imaged for the first time with &lt;20″angular resolution. This
combination of imaging and spectroscopy at high resolution will be
a powerful tool for helping to answer central questions of solar
flare physics.

---------------------------------------------------------
Title: Conference on EUV, X-Ray, and Gamma-Ray Instrumentation
for Astronomy
Authors: Siegmund, Oswald H. W.; Hudson, Hugh S.
1990SPIE.1344.....S Altcode:
Various papers on EUV, X-ray, and gamma-ray instrumentation for
astronomy are presented. Individual topics addressed include:
performance of lithium scatterers for X-ray polarimetry, shared
X-ray concentration via crystal diffraction, optimum shields for
spaceborne gamma-ray spectrometers, position-sensitive high-resolution
spectrometer, IPCs for stellar X-ray polarimeter, soft X-ray windows
for position-sensitive proportional counters, EUV imaging telescope
array on the spectrum X-G satellite, European Photon Imaging Camera
for X-ray astronomy, development of a UV auroral imager, background
reduction in microchannel plates, 2D delay-line anode detector for
astronomical imaging, dynamic range considerations for EUV MAMA
detectors, Rosat WFD imaging detectors. Also discussed are: EUV
band-pass filters for the Rosat wide-field camera, calibration of
the Rosat High-Resolution Imager, superconducting tunneling junction
detectors, test results of a prototype dielectric microcalorimeter,
novel high-speed high-resolution position readout SPAN, after emission
in microchannel plate detectors, highly curved microchannel plates,
soft X-ray performance of back-illuminated EEV CCDs, proton damage
effects in EEV CCDs, PN-CCDs for the XMM satellite mission, intensified
CCD detectors using the phosphor TPB, silicon X-ray array detector
concept, multilayer telescope for soft X-ray surveys, hard X-ray and
soft gamma-ray astronomy mission EXOS.

---------------------------------------------------------
Title: The Advanced Solar Observatory
Authors: Walker, Arthur B. C., Jr.; Bailey, Wayne; Chupp, Edward L.;
Hudson, Hugh S.; Moore, Ronald; Roberts, William; Hoover, Richard B.
1990OptEn..29.1306W Altcode:
A conceptual plan for the development of a comprehensive long duration
solar space observatory, The Advanced Solar Observatory (ASO) is
described. The ASO is intended to provide solar astronomers with
the observational power necessary to address fundamental problems
relating to the solar convection zone and activity cycle; the thermal
and nonthermal processes that control the transport of energy, mass, and
magnetic flux in the solar atmosphere; the generation of the solar wind;
and the dynamics of the inner heliosphere. The ASO concept encompasses
three proposed Space Station-based instrument ensembles: (1) the High
Resolution Telescope Cluster, which includes far ultraviolet, extreme
ultraviolet, and X-ray telescopes; (2) the Pinhole/Occulter Facility,
which includes Fourier transform and coded aperture hard X-ray and gamma
ray telescopes and occulted ultraviolet and visible light coronagraphs;
and (3) the High Energy Facility, which contains neutron, gamma ray, and
low frequency radio spectrometers. Two other facilities, the Orbiting
Solar Laboratory, and a package of Global Dynamics Instrumentation,
will, with the Space Station ensembles, form a comprehensive capability
for solar physics. The scientific program of the ASO, current instrument
concepts for the Space Station based ASO instrument ensembles, and
plans for their accommodation on the Space Station are described.

---------------------------------------------------------
Title: Using Image Size to Control CCD Systematic Errors in Spaceborne
Photometric and Astrometric Time-Series Measurements
Authors: Booth, C. H.; Hudson, H. S.; Buffington, A.
1990BAAS...22.1259B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The ACRIM data in the context of stellar variability.
Authors: Hudson, Hugh S.
1990NASCP3086..280H Altcode: 1990cisv.nasa..280H
The ACRIM total-irradiance data from the Solar Maximum Mission
have given us a first comprehensive view of solar variability in the
stellar sense. Five types of solar variability have been identified thus
far. These have small amplitudes, less than a few tenths of one percent,
and are at levels generally not yet detectable on other stars. The
possible stellar analogs are interesting physically, and in particular
may help us to understand solar behavior on longer time scales. This
paper describes the ACRIM data from the stellar point of view and
comments on the present state of stellar time-series photometry.

---------------------------------------------------------
Title: Modeling solar Lyman alpha irradiance.
Authors: Pap, J.; Hudson, H. S.; Rottman, G. J.; Willson, R. C.;
Donnelly, R. F.; London, J.
1990NASCP3086..189P Altcode: 1990cisv.nasa..189P
Solar Lyman alpha irradiance is estimated from various solar indices
using linear regression analyses. Models developed with multiple linear
regression analysis, including daily values and 81-day running means
of solar indices, predict reasonably well both the short-and long-term
variations observed in Lyman alpha. It is shown that the full disk
equivalent width of the He line at 1083 nm offers the best proxy for
Lyman alpha, and that the total irradiance corrected for sunspot effect
also has a high correlation with Lyman alpha.

---------------------------------------------------------
Title: Advanced Solar Observatory
Authors: Walker, Arthur B.; Bailey, Wayne L.; Chupp, Edward L.; Hudson,
Hugh S.; Moore, Ronald L.; Roberts, William T.; Hoover, Richard B.;
Wu, Shi T.
1990SPIE.1235..802W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar physics from the Moon: Summary of panel discussions
Authors: Hudson, Hugh
1990AIPC..207..109H Altcode: 1990am...proc..109H
No abstract at ADS

---------------------------------------------------------
Title: A “solar watch” program for the Human Exploration Initiative
Authors: Hudson, H.; Hildner, E.
1990AIPC..207..584H Altcode: 1990am...proc..584H
Because of the healt hazards of solar activity, human traffic in deep
space will require facilities for monitoring, predicting, and altering
the astronauts to potential danger. This suggests a “solar watch”
program consisting of a network of platforms at one A.U. from the
Sun, capable of monitoring its global behavior. The instrumentation
carried by these network platforms can evolve in sophistication with
successive launches, in order to lead to a deep understanding of the
physical mechanisms of solar activity - the best basis for maximally
reliable activity forecasts. The possibility of steroscopic viewing
of the solar surface and corona will confer unique advantages for the
physical understanding of these physical mechanisms.

---------------------------------------------------------
Title: A Laboratory Measurement of CCD Photometric and Dimensional
Variability
Authors: Buffington, Andrew; Hudson, Hugh S.; Booth, Corwin H.
1990PASP..102..688B Altcode:
The sun exhibits periodic and quasi-periodic variability in its total
luminosity, which provides information about its internal structure
and dynamics. Variability ranges from a few minutes to many-year
time scales, with amplitudes as small as a few ppm in the milliHz
band. Extension of this analysis to a large sample of outer stars would
be interesting: a panoramic detector such as a CCD could record many
stars at once. To meet this objective, a ppm time-series differential
precision is required. Laboratory CCD photometric measurements presented
here are promising for such an instrument. Normalizing the response
from a portion of the CCD area removes most of the individual-frame
variability. When a trend attributed to a thermal transient in the CCD
dewar is removed, the individual-frame photometric precision is about
0.0001, limited by photoelectron counting statistics. The time-series
power spectrum is flat within the desired frequency domain. Analysis
of the dimensional stability of the CCD within the same data set
indicates better than ppm performance, when first-order bulk motion
and magnification changes are removed.

---------------------------------------------------------
Title: Solar Flares and "Microflaring"
Authors: Hudson, H. S.
1990BAAS...22..898H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Luminosity Variability up to 10 mHz
Authors: Fisher, B. M.; Hudson, H. S.; Willson, R. C.
1990BAAS...22..897F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Lunar Based Solar Observatory and the Human Exploration
Initiative
Authors: Davis, J. M.; Hudson, H. S.
1990BAAS...22..880D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Variability Measured by SMM/ACRIM Compared with
Ground-Based Photometry
Authors: Chapman, G. A.; Herzog, A. D.; Lawrence, J. K.; Walton,
S. R.; Hudson, H. S.; Fisher, B.
1990BAAS...22..897C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Space Astrophysics with Large Structures: CASES and P/OF
Authors: Hudson, Hugh S.; Davis, J. M.
1990AIPC..211..134H Altcode: 1990heac.work..134H
Space instruments for remote sensing, of the types used for astrophysics
and solar-terrestrial physics among many disciplines, will grow to
larger physical sizes in the future. The zero-g space environment does
not inherently restrict such growth, because relatively lightweight
structures can be used. Active servo control of the structures can
greatly increase their size for a given mass. The Pinhole/Occulter
Facility, a candidate Space Station attached payload, offers an example:
it will achieve 0.2 arc s resolution by use of a 50-m baseline for
coded-aperture telescopes for hard X-ray and γ-ray imagers. The CASES
experiment (NASA Office of Applications and Space Technology)-deployable
on the Space Shuttle as early as 1994-will provide an engineering and
scientific demonstration of active structural control in this context.

---------------------------------------------------------
Title: Book reviews
Authors: de Graaf, W.; Doom, C.; Wesselius, P. R.; de Jager, Cornelis;
van der Kruit, P. C.; Achterberg, A.; de Waard, H.; van Hugo, Woerden;
Kleczek, J.; Hudson, H. S.; van der Hucht, K. A.
1989SSRv...51..425D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Solar Physics in the 1990'S
Authors: Neidig, D. F.; Hudson, H. S.
1989Sci...246..246N Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Pinhole/Occulter Facility
Authors: Hudson, Hugh S.; Davis, John M.
1989SPIE.1159..318H Altcode:
To image X-radiation efficiently at energies above about 10 keV requires
the use of 'shadow optics' techniques. The Pinhole/Occulter Facility
(P/OF) represents an application of these techniques for observations
in high-energy astrophysics, especially the study of solar coronal
activity in hard X-rays and gamma rays. P/OF will achieve angular
resolutions on the order of 0.2 arcsec for an instrument deployment
length of 50 m. Because of this large structural scale, P/OF has been
proposed as an attached payload for the Space Station. Meanwhile,
several smaller-scale instruments are being developed.

---------------------------------------------------------
Title: Laboratory Analysis of a CCD as a Panoramic Astrometric
Detector
Authors: Buffington, A.; Hudson, H. S.; Booth, C.
1989BAAS...21Q1109B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Imaging Solar Flare Gamma-Rays and Hard X-rays with The Gamma
Ray Imaging Device (GRID) on a Balloon
Authors: Starr, R.; Crannell, C. J.; Orwig, L. E.; Dennis, B. R.;
Davis, J. M.; Johnson, W. N.; Norris, J. P.; Wood, K. S.; Green,
M. E.; Vanbeek, H. F.; Hurford, G. J.; Prince, T. A.; Hudson, H. S.
1989BAAS...21.1151S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Laboratory Analysis of a CCD as a Sensor for Time-Series
Photometry
Authors: Hudson, H. S.; Booth, C.; Buffington, A.
1989BAAS...21R1070H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Report from solar physics
Authors: Walker, A. B. C.; Acton, L.; Brueckner, G.; Chupp, E. L.;
Hudson, H. S.; Roberts, W.
1989stss.work...31W Altcode:
A discussion of the nature of solar physics is followed by a brief
review of recent advances in the field. These advances include: the
first direct experimental confirmation of the central role played
by thermonuclear processes in stars; the discovery that the 5-minute
oscillations of the Sun are a global seismic phenomenon that can be
used as a probe of the structure and dynamical behavior of the solar
interior; the discovery that the solar magnetic field is subdivided into
individual flux tubes with field strength exceeding 1000 gauss. Also
covered was a science strategy for pure solar physics. Brief discussions
are given of solar-terrestrial physics, solar/stellar relationships,
and suggested space missions.

---------------------------------------------------------
Title: Solar Flare Gamma-Ray and Hard X-ray Imaging with the
GRID-on-a-Balloon
Authors: Orwig, L. E.; Crannell, C. J.; Dennis, B. R.; Starr, R.;
Hurford, G. J.; Prince, T. A.; Hudson, H. S.; van Beek, F.; Greene,
M. E.; Johnson, W. N.; Norris, J. P.; Wood, K. S.; Davis, J. M.
1989BAAS...21..861O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The BATSE Experiment for the GRO - Solar Flare Hard X-Ray
and Gamma-Ray Capabilities
Authors: Fishman, G. J.; Meegan, C. A.; Wilson, R. B.; Parnell, T. A.;
Paciesas, W. S.; Pendleton, G. N.; Hudson, H. S.; Matteson, J. L.;
Peterson, L. E.; Cline, T. L.; Teegarden, B. J.; Schaefer, B. E.
1989BAAS...21..860F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Design considerations for a "Solar Mass Ejection Imager"
on a rotating spacecraft.
Authors: Jackson, B. V.; Hudson, H. S.; Nichols, J. D.; Gold, R. E.
1989GMS....54..291J Altcode: 1989opss.conf..291J; 1989sspp.conf..291J
The authors describe an instrument capable of imaging the time-varying
features of the entire outer corona (from near the Sun to beyond
90° elongation) via the Thomson-scattered diffuse solar light. This
"all sky" imager works on a spin-stabilized spacecraft, preferably in
deep space.

---------------------------------------------------------
Title: Solar flare gamma-ray and hard X ray imaging with the
GRID-on-a-balloon
Authors: Orwig, Larry E.; Crannell, C. J.; Dennis, Brian R.; Starr,
R.; Hurford, G. J.; Hudson, H. S.; Vanbeek, F.; Greene, M. E.; Johnson,
W. N.; Norris, J. P.
1989dots.work..143O Altcode:
A primary scientific objective for solar flare research during
the rapidly approaching maximum in solar activity is the imaging of
gamma-ray and hard x ray sources of solar flare emissions. These goals
will be pursued by the Gamma Ray Imaging Device (GRID) instrument, one
of three instruments recently selected for NASA's Max '91 Solar Balloon
Program. The GRID instrument is based on the technique of Fourier
transform imaging and utilizes scanning modulation grid collimator
optics to provide full-Sun imaging with 1.9-arcsecond resolution
over the energy range from 20 to 700 keV at time resolutions from
0.1 to 2 s. The GRID telescope will employ 32 subcollimators, each
composed of a matched pair of high-Z collimator grids separated by
5.2 meters and a phoswich scintillation spectrometer detector having
no spatial resolution. The subcollimators and integrally-mounted
fine aspect system are contained within a telescope canister which
will be pointed to 0.1 degree accuracy and cyclically scanned to
produce source modulation. The 32 subcollimators provide a uniform
distribution of grid slit orientations and a logarithmic distribution
of slit spacings corresponding to angular dimensions of 1.9 arcseconds
to several arcminutes. The instrument is several orders of magnitude
more sensitive than the HXIS instrument on the Solar Maximum Mission
(SMM) and nearly 10 times more sensitive than any similar instrument
scheduled to fly during the next solar maximum. The payload, designed
for long-duration high-altitude balloon capability, is scheduled for
its first science flight (8 to 14 days duration) from the Antarctic
in January of 1992.

---------------------------------------------------------
Title: The BATSE experiment on the Gamma Ray Observatory: Solar
flare hard X ray and gamma-ray capabilities
Authors: Fishman, G. J.; Meegan, C. A.; Wilson, R. B.; Parnell, T. A.;
Paciesas, W. S.; Pendleton, G. N.; Hudson, H. S.; Matteson, J. L.;
Peterson, L. E.; Cline, T. L.
1989dots.work...96F Altcode:
The Burst and Transient Source Experiment (BATSE) for the Gamma Ray
Observatory (GRO) consists of eight detector modules that provide
full-sky coverage for gamma-ray bursts and other transient phenomena
such as solar flares. Each detector module has a thin, large-area
scintillation detector (2025 sq cm) for high time-resolution studies,
and a thicker spectroscopy detector (125 sq cm) to extend the energy
range and provide better spectral resolution. The total energy range of
the system is 15 keV to 100 MeV. These 16 detectors and the associated
onboard data system should provide unprecedented capabilities for
observing rapid spectral changes and gamma-ray lines from solar
flares. The presence of a solar flare can be detected in real-time
by BATSE; a trigger signal is sent to two other experiments on the
GRO. The launch of the GRO is scheduled for June 1990, so that BATSE
can be an important component of the Max '91 campaign.

---------------------------------------------------------
Title: Flare energetics.
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veck, N. J.
1989epos.conf..377W Altcode:
The authors have sought to establish a comprehensive and self-consistent
picture of the sources and transport of energy within a flare. To
achieve this goal, they chose five flares in 1980 that were well
observed with instruments on the Solar Maximum Mission, and with other
space-borne and ground-based instruments. The events were chosen to
represent various types of flares. Details of the observations available
for them and the corresponding physical parameters derived from these
data are presented. The flares were studied from two perspectives,
the impulsive and gradual phases, and then the results were compared
to obtain the overall picture of the energetics of these flares. The
authors also discuss the role that modeling can play in estimating the
total energy of a flare when the observationally determined parameters
are used as the input to a numerical model. Finally, a critique of
our current understanding of flare energetics and the methods used to
determine various energetics terms is outlined, and possible future
directions of research in this area are suggested.

---------------------------------------------------------
Title: Book-Review - Advances in Helio and Asteroseismology
Authors: Christiansen-Dalsgaard, J.; Frandsen, S.; Hudson, H. S.
1989SSRv...51..431C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar response of the BATSE instrument on the gamma-ray
observatory
Authors: Fishman, G. J.; Meegan, C. A.; Parnell, T. A.; Wilson, R. B.;
Paciesas, W.; Cline, T.; Teegarden, B.; Schaefer, B.; Hudson, Hugh;
Matteson, J. L.
1988fnsm.work..204F Altcode:
The Burst and Transient Source Experiment (BATSE) on board the gamma
ray observatory (GRO) aims at comprehensive observations of time
profiles, spectra, and locations of high-energy transient sources. The
mysterious cosmic gamma ray bursts provided the main motivation for
the observations, but BATSE will make excellent observations of many
classes of sources, and in particular solar flares. The solar response
of BATSE, as inferred from its design parameters, is analyzed for two
purposes: the optimization of the solar observations themselves, and the
characterization of the solar effects on ordinary nonsolar observations.

---------------------------------------------------------
Title: Solar luminosity variations in solar cycle 21
Authors: Willson, Richard C.; Hudson, H. S.
1988Natur.332..810W Altcode:
The ACRIM I experiment (Active Cavity Radiometer Irradiance Monitor)
on the solar maximum Mission (SMM) satellite has provided a nearly
continuous record of solar total irradiance variations since early
19801. It has detected variations on time scales ranging
from minutes to SMM's lifetime. The long-term variations have revealed
a downward trend during the declining phase of solar cycle 21 (ref. 2)
of the sunspot cycle, a flat period between mid-1985 and mid-1987,
and an upturn in late 1987 which suggests a direct correlation of
luminosity and solar active region population. If the upturn continues
into the activity maximum of solar cycle 22, a relation between solar
activity and luminosity of possible climatological significance could
have been discovered. The sense of the correlation agrees with what has
been predicted from the coincidence of the 'little ice age' climate
anomaly in the sixteenth and seventeenth centuries and the Maunder
Minimum of solar activity3. The best-fit relationship for
the variation of total irradiance S, with sunspot number Rz,
and 10-cm flux F10, are S=1,366.82+7.7l×10-3
Rz and S=1,366.27+8.98×10-3 F10 (W
m-2). These could be used to approximate total irradiance
variations over the periods for which these indices have been compiled.

---------------------------------------------------------
Title: Solar Capability of the BATSE Experiment on GRO
Authors: Hudson, H.; Matteson, J. L.; Fishman, G. J.; Meegan, C. A.;
Parnell, T. A.; Paciesas, W.; Cline, T. A.; Schaefer, B.; Teegarden, B.
1988BAAS...20..747H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Gamma-Ray and Hard X-Ray Imaging of Solar Flares
Authors: Prince, T. A.; Hurford, G. J.; Hudson, H. S.; Crannell, C. J.
1988SoPh..118..269P Altcode:
We discuss the scientific and technical aspects of high-resolution
γ-ray and X-ray imaging of solar flares. The scientific necessity for
imaging observations of solar flares and the implications of future
observations for the study of solar flare electrons and ions are
considered. Performance parameters for a future hard X-ray and γ-ray
imager are then summarized. We briefly survey techniques for high-energy
photon imaging including direct collimation imaging, coded apertures,
and modulation collimators. We then discuss in detail the technique of
Fourier-transform imaging. The basic formalism is presented, followed by
a discussion of several practical aspects of the technique. We conclude
our discussion of imaging techniques with a description of the options
for detectors and grid fabrication. Several planned future high-energy
imagers are described including the Solar-A hard X-ray imager, the
balloon-borne GRID γ-ray imager, and the Pinhole/Occulter Facility.

---------------------------------------------------------
Title: An Instrument for High Angular Resolution, Hard X-Ray
Observations of the Galactic Center
Authors: Davis, J. M.; Weisskopf, M. C.; Hudson, H. S.; Hurford, G. J.
1988BAAS...20..677D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Outstanding problems of solar flare research
Authors: Hudson, H. S.
1988AdSpR...8k...7H Altcode: 1988AdSpR...8....7H
This paper describes in very broad terms the current status of problems
in understanding solar flares, from the personal perspective of an
observer. It goes on to summarize new observational directions to
help solve the problems, including support for the suggestion for
stereoscopic imaging of the solar atmosphere.

---------------------------------------------------------
Title: Max '91: Flare research at the next solar maximum
Authors: Dennis, Brian; Canfield, Richard; Bruner, Marilyn; Emslie,
Gordon; Hildner, Ernest; Hudson, Hugh; Hurford, Gordon; Lin, Robert;
Novick, Robert; Tarbell, Ted
1988STIN...8814919D Altcode:
To address the central scientific questions surrounding solar
flares, coordinated observations of electromagnetic radiation and
energetic particles must be made from spacecraft, balloons, rockets,
and ground-based observatories. A program to enhance capabilities
in these areas in preparation for the next solar maximum in 1991 is
recommended. The major scientific issues are described, and required
observations and coordination of observations and analyses are
detailed. A program plan and conceptual budgets are provided.

---------------------------------------------------------
Title: High-energy detector calibration and observation of non-thermal
and “superhot” sources
Authors: Hudson, Hugh S.
1988AdSpR...8k.229H Altcode: 1988AdSpR...8..229H
No abstract at ADS

---------------------------------------------------------
Title: Modelling of total solar irradiance variability: An overview
Authors: Hudson, Hugh S.
1988AdSpR...8g..15H Altcode: 1988AdSpR...8...15H
Several components contribute to the observed variations of the
total solar irradiance. There has been considerable effort expended
on building empirical models for specific components, especially
for sunspots and faculae. These models typically use time series of
ground-based data as a means of representing the total-irradiance
variability. There are several reasons to do this modeling. The
models may help to identify the physical cause of a variation; the
parameters of a model (e.g. the effective temperature of a sunspot) may
be determinable via correlation with the total-irradiance observations;
the models may be used as proxy representations for total-irradiance
variability during periods of no data; finally, the models in principle
can be used as a basis for “correcting” the total-irradiance data,
as a means for better identification of additional components of
variability.

---------------------------------------------------------
Title: The Nature of the Solar-Cycle Variation of Total Irradiance
Authors: Hudson, H.; Willson, R.
1988srov.proc..318H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Observed variability of the solar luminosity.
Authors: Hudson, H. S.
1988ARA&A..26..473H Altcode:
Data on solar luminosity variations are compiled in graphs and
analyzed, with a focus on the SMM satellite bolometric measurements
and other observations made since 1983. Consideration is given to
total irradiance measurements in integrated light; emission-line
variability in the visible, UV, and IR; mechanisms contributing to
the solar variability (sunspots, faculae, rotation, oscillations,
granulation, and active networks); the role of the solar magnetic
cycle; and other observable parameters indicating variability. Also
discussed are the effects of solar variations on the earth climate
and the implications of the observations for theoretical models of
convective-envelope processes (thermal diffusion near the surface and
magnetic pressure in the interior).

---------------------------------------------------------
Title: Solar physics in the 1990s. Proceedings. Workshop XV and
the COSPAR Interdisciplinary Scientific Commission E (Meeting E1)
of the 27. COSPAR Plenary Meeting: Solar physics in the 1990s, Espoo
(Finland), 18 - 29 Jul 1988.
Authors: Neidig, D. F.; Hudson, H. S.
1988AdSpR...8k....N Altcode: 1988AdSpR...8.....N
Papers concerning developments in solar physics are presented, focusing
on scientific planning for the solar maximum and high-energy detector
calibration and observation of nonthermal and superhot sources. Specific
topics include solar radioastronomy, VLA observations of the sun,
coronal loops, solar observation in the Phobos mission, the Solar-A
mission, the Solar and Heliospheric Observatory satellite, the Global
Oscillation Network Group, the relation between convection flows and
magnetic structure and the solar surface, and solar flares research
programs including quasi-dedicated mm-wave imaging, H-alpha, far IR,
X-ray spectroscopy, and optical observations. Additional subjects
include the manifestation of supergranulation structure of active
regions during solar flares, post-flare loops, the relationship of peak
emission measure and temperature to peak flare X-ray flux, turbulent
and directed motions in solar flares, coronal temperature diagnostics
from high-resolution soft W-ray spectra, the study of coronal densities
from X-ray line ratios of Ne IX and Mg XI, electron densities in the
solar atmosphere, the Coronal Magnetic Structures Observing Campaign,
observations of a giant filament, the determination of coronal fieldline
connectivity from photospheric flare observations, MHD simulation of
mass injection, numerical simulation of solar atmospheric dynamics,
intercalibration of hard X-ray spectrometers, the influence of the
energy calibration of broad-band X-ray detectors on the determination
of plasma parameters, and space experiments measuring solar X-rays.

---------------------------------------------------------
Title: Solar variability and oscillations.
Authors: Hudson, Hugh S.
1987RvGeo..25..651H Altcode: 1986RvGSP..25..651H
Within this decade, observations of total solar irradiance have
become good enough to permit us to study directly the solar luminosity
variations on a wide range of time scales, up to several years. At the
same time there has been considerable improvement in understanding
the classical indicators of solar activity, such as UV and visible
chromospheric lines, soft X-rays, and radio fluxes. The observed
variations include the effects of sunspots and plages (which induce
"rotational modulation"), solar-cycle effects, and signatures of
global oscillations and convection. In addition new characteristic
time-scales (154 days and possibly 320 days) have been discovered but
not explained. This review covers these developments and comments
briefly on the subject of helioseismology, the study of the solar
global oscillations.

---------------------------------------------------------
Title: Recent advances in the understanding of solar flares : 2 : 1987
Authors: Hudson, H.; Kai, K.
1987SoPh..113.....H Altcode:
Papers are presented on solar flare discovery, solar flares and magnetic
topology, impulsive and hot thermal solar flares, and plasma dynamics at
the very initial phases of flares. Also considered are a magnetodynamic
mechanism for loop flares, the propagation and confinement of energetic
electrons in solar flares, nuclear processes and accelerated particles
in solar flares, and optical observations of solar flares. Other topics
include delta spots and great flares, chromospheric downflow velocities
as a diagnostic in solar flares, and subphotospheric current systems and
flares. Papers are also presented on a long-duration balloon payload
for hard X-ray and gamma-ray observations of the sun, the derivation
of vector magnetic fields from Stokes profiles, and the Solar-A mission.

---------------------------------------------------------
Title: Subphotospheric current systems and flares
Authors: Hudson, Hugh S.
1987SoPh..113..315H Altcode: 1982SoPh..113..315H
Subphotospheric current systems inferred from recent vector magnetograph
observations (e.g. Gary et al., 1987) imply the existence of electric
currents penetrating the photosphere and thus flowing deep in the
solar convection zone. These currents presumably originate in an
internal dynamo that supplies the observed photospheric magnetic
fields through the buoyant motions of the initially deeply-buried
flux tubes. The coronal fields resulting from this process therefore
must carry slowly-varying currents driven by emf's remote from the
surface. These currents may then drive solar-flare energy release. This
paper discusses the consequences of such a deep origin of the coronal
parallel currents. Simple estimates for a large active region suggest
a mean current-closure depth ≥ 10,000 km, with a subphotospheric
inductance ≥ 100 H and a subphotospheric stored energy ≥
1033 ergs.

---------------------------------------------------------
Title: Solar flare discovery
Authors: Hudson, Hugh S.
1987SoPh..113....1H Altcode: 1982SoPh..113....1H
This paper considers the discoveries that have appreciably changed
our understanding of the physics of solar flares. I identify a
total of 42 discoveries from all disciplines, ranging from Galileo's
initial observation of faculae to the recent discovery of strong limb
brightening in 10-MeV γ-radiation. The rate of discovery increased
dramatically over the past four decades as new observational tools
became available. My assessment of significance suggests that recent
discoveries - though more numerous - are individually less significant;
perhaps this is because the minor early discoveries tend to be taken
for granted. In spite of the many facets of flare physics that have
been explained or at least well-described, many fundamental questions
remain unresolved.

---------------------------------------------------------
Title: Flare energetics
Authors: Wu, S. T.; De Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.;
Bruner, M. E.; Cargill, P. J.
1986epos.conf..5.1W Altcode: 1986epos.confE...1W
In this investigation of flare energetics, researchers sought
to establish a comprehensive and self-consistent picture of the
sources and transport of energy within a flare. To achieve this
goal, they chose five flares in 1980 that were well observed with
instruments on the Solar Maximum Mission, and with other space-borne
and ground-based instruments. The events were chosen to represent
various types of flares. Details of the observations available for
them and the corresponding physical parameters derived from these
data are presented. The flares were studied from two perspectives,
the impulsive and gradual phases, and then the results were compared
to obtain the overall picture of the energics of these flares. The
role that modeling can play in estimating the total energy of a flare
when the observationally determined parameters are used as the input
to a numerical model is discussed. Finally, a critique of the current
understanding of flare energetics and the methods used to determine
various energetics terms is outlined, and possible future directions
of research in this area are suggested.

---------------------------------------------------------
Title: Characterization of the Total Flare Energy
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf.5.41W Altcode: 1986epos.confE..41W
No abstract at ADS

---------------------------------------------------------
Title: Energetics of the Impulsive Phase
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf..5.5W Altcode: 1986epos.confE...5W
No abstract at ADS

---------------------------------------------------------
Title: Energetics of the Gradual Phase
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf.5.20W Altcode: 1986epos.confE..20W
No abstract at ADS

---------------------------------------------------------
Title: Review of Impulsive Phase Phenomena
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf.5.60W Altcode: 1986epos.confE..60W
No abstract at ADS

---------------------------------------------------------
Title: Flares Chosen for Energetics Study
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf.5.47W Altcode: 1986epos.confE..47W
No abstract at ADS

---------------------------------------------------------
Title: Relationships among the Phases
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veek, N. J.
1986epos.conf.5.39W Altcode: 1986epos.confE..39W
No abstract at ADS

---------------------------------------------------------
Title: Long-Term Downward Trend in Total Solar Irradiance
Authors: Willson, R. C.; Hudson, H. S.; Frohlich, C.; Brusa, R. W.
1986Sci...234.1114W Altcode:
The first 5 years (from 1980 to 1985) of total solar irradiance
observations by the first Active Cavity Radiometer Irradiance Monitor
(ACRIM I) experiment on board the Solar Maximum Mission spacecraft show
a clearly defined downward trend of -0.019% per year. The existence
of this trend has been confirmed by the internal self-calibrations of
ACRIM I, by independent measurements from sounding rockets and balloons,
and by observations from the Nimbus-7 spacecraft. The trend appears
to be due to unpredicted variations of solar luminosity on time scales
of years, and it may be related to solar cycle magnetic activity.

---------------------------------------------------------
Title: Components of the Variability of Solar Luminosity
Authors: Hudson, H. S.
1986BAAS...18..981H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Solar-Space Observations and Stellar Prospects
Authors: Harvey, J. W.; Stenflo, J. O.; Hudson, H. S.; Noyes, R. W.;
Kotrc, P.
1986BAICz..37..252H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The concept of the Pinhole/Occulter Facility.
Authors: Hudson, H. S.
1986sfcp.nasa....5H Altcode:
The Pinhole/Occulter Facility is based upon a simple idea for
obtaining high angular resolution in astronomical X-ray observations,
for example for solar flares at energies &gt;10 keV. The scheme
uses a coded aperture device (multiple pinhole camera) with a large
separation between the aperture encoder and the detector. Such an
imaging device can have an angular resolution much better than 1 arc
s if desired. A large structure would also make it possible to have
a large external occulter, which would have powerful applications,
notably for high-sensitivity observations of the corona in EUV and white
light. This capability leads to the definition of the Pinhole/Occulter
Facility, which combines both X-ray and coronal observations. The
present concept is based upon a 35-m deployable boom, erected in the
cargo bay of the Space Shuttle and pointed in the solar direction by
the Instrument Pointing System of Spacelab.

---------------------------------------------------------
Title: Solar and stellar activity. Proceedings of Symposium 11 and
of the Topical Meeting of the COSPAR Interdisciplinary Scientific
Commission E (Meeting E2) of the COSPAR Twenty-sixth Plenary Meeting
held in Toulouse, France, 30th June - 11th July 1986.
Authors: Praderie, Francoise; Hudson, H. S.
1986AdSpR...6h....P Altcode: 1986AdSpR...6.....P
Stellar and solar activity research concerned with the relevance of the
theory of magnetic field production and tracing the manifestations of
the magnetic field in the sun and stars is examined. Topics discussed
include the generation of magnetic flux and energetics; the sun as
a star; coronal activity along the HR diagram; activity and stellar
evolution; and cycles of activity in solar-type stars. Attention is
also given to data obtained from solar investigation from Spacelab 2.

---------------------------------------------------------
Title: Flare energetics.
Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.;
Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner,
M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.;
Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres,
G.; Serio, S.; Sylwester, J.; Veck, N. J.
1986NASCP2439....5W Altcode:
In this investigation of flare energetics, the authors establish a
comprehensive and self-consistent picture of the sources and transport
of energy within a flare. They chose five flares in 1980 that were
well observed with instruments on the SMM, and with other space-borne
and ground-based instruments. Details of the observations available
for them and the corresponding physical parameters derived from these
data are presented. The flares were studied from two perspectives,
the impulsive and gradual phases, and then the results were compared
to obtain the overall picture of the energetics of these flares. The
authors also discuss the role that modeling can play in estimating the
total energy of a flare when the observationally determined parameters
are used as the input to a numerical model.

---------------------------------------------------------
Title: The concept of the Pinhole/Occulter Facility.
Authors: Hudson, H. S.
1986NASCP2421....5H Altcode:
The Pinhole/Occulter Facility is based upon a simple idea for
obtaining high angular resolution in astronomical X-ray observations,
for example for solar flares at energies &gt;10 keV. The scheme
uses a coded aperture device (multiple pinhole camera) with a large
separation between the aperture encoder and the detector. Such an
imaging device can have an angular resolution much better than 1 arc
s if desired. A large structure would also make it possible to have
a large external occulter, which would have powerful applications,
notably for high-sensitivity observations of the corona in EUV and white
light. This capability leads to the definition of the Pinhole/Occulter
Facility, which combines both X-ray and coronal observations. The
present concept is based upon a 35-m deployable boom, erected in the
cargo bay of the Space Shuttle and pointed in the solar direction by
the Instrument Pointing System of Spacelab.

---------------------------------------------------------
Title: Energy balance in solar active regions: The dip of April, 1985
Authors: Hudson, H. S.
1986AdSpR...6h..81H Altcode: 1986AdSpR...6...81H
The presence of a solar active region affects the luminosity of the
sun. Sunspots directly produce “dips” in the total solar irradiance
approximately proportionally to their projected area, while faculae
produce excess energy. These effects were discovered during the solar
maximum period of 1980, and in this paper we examine the sunspot effect
during solar minimum. We examine the “dip” due to an active region
in April, 1985, as observed in the total solar irradiance by the
ACRIM instrument on the Solar Maximum Mission. These data (obtained
after the spacecraft repair in May, 1984) have simple variations,
relative to those observed in 1980, because of the reduced level of
activity approaching solar minimum. We find that the PSI index of
projected sunspot area as defined in 1980 appears to describe this
“dip” satisfactorily.

---------------------------------------------------------
Title: The Explorer program for astronomy and astrophysics
Authors: Savage, Blair D.; Becklin, Eric E.; Cassinelli, Joseph P.;
Dupree, Andrea K.; Elliot, James L.; Hoffmann, William F.; Hudson,
Hugh S.; Jura, Michael; Kurfess, James; Murray, Stephen S.
1986STIN...9014159S Altcode:
This report was prepared to provide NASA with a strategy for proceeding
with Explorer-class programs for research in space astronomy and
astrophysics. The role of Explorers in astronomy and astrophysics and
their past accomplishments are discussed, as are current and future
astronomy and astrophysics Explorers. Specific cost needs for an
effective Explorer program are considered.

---------------------------------------------------------
Title: The over-the-limb hard X-ray events
Authors: Hudson, H. S.
1986AdSpR...6f.199H Altcode: 1986AdSpR...6Q.199H
The over-the-limb hard X-ray events offer us a uniquely direct view
of the hard X-ray emission from the solar corona during a major
flare. Limb occultation at angles greater than about 10 degrees (an
arbitrary definition of this class of events) excludes any confusion
with brighter chromospheric sources. I review the observations of
the group of seven such over-the-limb events now in the literature,
beginning with the prototype 1969 March 30 flare. The hard X-ray spectra
appear to fall into two classes: hard events, with power-law index
of about 2.0; and soft events, with power-law index about 5.4. This
tendency towards bimodality is only significant at the 90% confidence
level due to the smallness of the number of events observed to date. If
borne out by future data, the bimodality would suggest the existence
of two different acceleration mechanisms.

---------------------------------------------------------
Title: Book-Review - Solar / Space Observations and Stellar Prospects
Authors: Harvey, J. W.; Hudson, H. S.; Noyes, R. W.; Zirker, J. B.
1985Sci...230..660H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Energetic ions in solar γ-ray flares
Authors: Hudson, H. S.
1985SoPh..100..515H Altcode:
Solar flares emit line and continuum γ-radiation as well as neutrons
and charged particles. These high-energy emissions require the presence
of energetic ions within the magnetic structures of the flare proper. We
have already learned a great deal about the location and mode of
particle acceleration. The observations have now become extensive
enough so that we can begin to study the dynamics of the energetic
ions within the flare structures themselves. This paper reviews the
γ-ray and neutron observations and the theory of their emission,
and discusses on this basis the presence of energetic ions deep within
the flaring atmosphere.

---------------------------------------------------------
Title: Book-Review - Solar Space Observations and Stellar Prospects
Authors: Harvey, J. W.; Hudson, H. S.; Noyes, R. W.
1985Natur.317...91H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Ion Energy Storage for Post-Flare Loops
Authors: Hudson, H. S.
1985ICRC....4...58H Altcode: 1985ICRC...19d..58H
Low-energy non-thermal protons may have long lifetimes in coronal loops
with low density and high temperature. If energy were stored in such
protons in the initial phases of a solar flare, it could be released
slowly during the later phases. Within the present observational limits
for post-flare loops, this mechanism should be considered in addition
to a field-line reconnection theory of the Kopp and Pneuman type. The
thin-target gamma ray emission from the trapped protons is below
present limits, but more sensitive observations can test the hypothesis.

---------------------------------------------------------
Title: A Extended "superhot" Solar Flare X-Ray Source
Authors: Hudson, H. S.; Ohki, K. I.; Tsuneta, S.
1985ICRC....4...50H Altcode: 1985ICRC...19d..50H
A superhot hard X-ray source in a solar flare occulted by the solar limb
was identified. Its hard X-ray image was found to show great horizontal
extent but little vertical extent. An H alpha brightening at the same
limb position about an hour later suggests a multi-component loop
prominence system, so that it appears that a superhot source can evolve
in the same manner as a normal solar soft X-ray source. The assignment
of plausiable values to physical parameters in the source suggests
(from the simplest form of classical thermal-conduction theory) that
either new physics will be required to suppress conduction, or else
that gradual energy release well after the impulsive phase of the
flare must occur. In this respect too, the superhot source appears
to resemble ordinary soft X-ray sources, except of course that its
temperature is higher.

---------------------------------------------------------
Title: Detectability of extrasolar planetary transits
Authors: Borucki, W. J.; Scargle, J. D.; Hudson, H. S.
1985ApJ...291..852B Altcode:
Precise stellar photometry can be used to detect other planetary
systems. However, the intrinsic variability of stellar luminosity
imposes a fundamental limit on the sensitivity of this method. Based on
recent precise solar observations made from the Solar Maximum Mission
satellite, it appears that the detection of earth-sized planets will
be marginal during periods of high stellar activity. However, with a
suitable photometer larger planets should be readily detectable even in
the presence of stellar activity equal to that of the sun at the peak
of its sunspot cycle. The high precision, multiple-star photometric
system required to detect planets in other stellar systems could be
used to monitor flares, starspots, and global oscillations.

---------------------------------------------------------
Title: Post-flare Loop Heating by Trapped Superthermal Protons
Authors: Hudson, H. S.
1985BAAS...17..628H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Brightness of the Interplanetary Medium in Thomson
Scattering from 0.3 to 1.0 AU: Comparison with a View from Helios B
Authors: Venkataraman, A.; Hudson, H. S.; Jackson, B. V.
1985BAAS...17..638V Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Problems of Energy Transport in Solar Active Regions and Flares
Authors: Hudson, H. S.
1985spit.conf..348H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A survey for photometric variability from space
Authors: Hudson, H. S.
1984NASCP2350...43H Altcode: 1984itp..work...43H
A survey for photometric variability in a wide variety of astronomical
objects would produce much new information about their interiors and
dynamics. Reasons for such a survey are given, as well as an example
of the solar-constant variations that can be used as a guide to what
may be expected from main-sequence stars. A concept for a satellite
dedicated to a survey of photometric variability is proposed.

---------------------------------------------------------
Title: Ionospheric Detection of X-ray Pulsars
Authors: Hudson, H. S.; Te Kolste, R.
1984BAAS...16..982H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Total irradiance observations of low degree p-modes
Authors: Woodard, M.; Hudson, H.
1984MmSAI..55...67W Altcode:
A spectral analysis of 10 months of brightness data from the
Active Cavity Radiometer Irradiance Monitor on the SMM satellite has
revealed new information about the low-degree (l = 0,1, and 2) p-mode
oscillations of the Sun. The mean frequencies, irradiance amplitudes,
and line widths of the largest amplitude modes have been determined
with previously unattained accuracy.

---------------------------------------------------------
Title: The Inconstant Solar Constant
Authors: Willson, R. C.; Hudson, H.; Woodard, M.
1984S&T....67..501W Altcode:
The Active Cavity Radiometer Irradiance Monitor (ACRIM) of the Solar
Maximum Mission satellite measures the radiant power emitted by the sun
in the direction of the earth and has worked flawlessly since 1980. The
main motivation for ACRIM's use to measure the solar constant is the
determination of the extent to which this quantity's variations affect
earth weather and climate. Data from the solar minimum of 1986-1987 is
eagerly anticipated, with a view to the possible presence of a solar
cycle variation in addition to that caused directly by sunspots.

---------------------------------------------------------
Title: Facular limb-darkening functions for irradiance modeling.
Authors: Hirayama, T.; Okamoto, T.; Hudson, H. S.
1984NASCP2310...59H Altcode: 1984siva.work...59H
The limb-darkening function of faculae is an important factor in
estimating facular contributions to solar irradiance variations. The
authors review the existing photometric data and generate a synthetic
limb-darkening function for faculae, which is then compared with the
limb-darkening functions currently in use for irradiance modeling. It is
found that the excess facular flux ranges from 0.017 to 0.0349 of the
solar photospheric flux for the various representations. The present
limitation appears to be the lack of comprehensive photometric data.

---------------------------------------------------------
Title: A Global Irradiance Program
Authors: Hudson, H. S.; Chapman, G. A.; LaBonte, B. J.
1984NASCP2310..313H Altcode: 1984siva.work..311H
Basic requirements for a long-term program of ground-based measurements
of the solar brightness are outlined.

---------------------------------------------------------
Title: Solar Irradiance Variations on Active Region Time Scales
Authors: Labonte, B. J.; Chapman, G. A.; Hudson, H. S.; Willson, R. C.
1984NASCP2310.....L Altcode: 1984QB531.S576.....; 1984siva.work.....L
No abstract at ADS

---------------------------------------------------------
Title: Drift Scan Photometry and Astrometry
Authors: Hudson, H. S.
1984NASCP2310..297H Altcode: 1984siva.work..297H
No abstract at ADS

---------------------------------------------------------
Title: High Energy Aspects of Solar Flares
Authors: Hudson, Hugh S.
1984BAAS...16..480H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hα and Hard X-Ray Development in Two-Ribbon Flares
Authors: Dwivedi, B. N.; Hudson, H. S.; Kane, S. R.; Svestka, Z.
1984SoPh...90..331D Altcode:
Morphological features of two-ribbon flares have been studied, using
simultaneous ISEE-3 hard X-ray records and high-resolution Big Bear
Hα movies for more than 20 events. Long-lasting and complex hard
X-ray bursts are almost invariably found associated with flares of the
two-ribbon type. We find at least three events, namely March 31, 1979,
April 10, 1980, and July 1, 1980, where the occurrence of individual
spikes in hard X-ray radiation coincides with suddenly enhanced Hα
emission covering the sunspot penumbra. There definitely exist important
(≥ 1B) two-ribbon Hα flares without significant hard X-ray emission.

---------------------------------------------------------
Title: Solar irradiance variations on active region time
scales. Proceedingsof a workshop held at the California Institute
of Technology, Pasadena, California, June 20 - 21, 1983.
Authors: Labonte, B. J.; Chapman, G. A.; Hudson, H. S.; Willson,
R. C.; Newkirk, G. A., Jr.; Bruning, D. H.
1984sivo.book.....L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A survey for photometric variability from space
Authors: Hudson, H. S.
1984AdSpR...4h.169H Altcode: 1984AdSpR...4..169H
A survey for photometric variability in a wide variety of astronomical
objects would produce much new information about their interiors and
dynamics. This paper discusses reasons for such a survey, showing the
example of the recent precise SMM observations of total solar irradiance
variations as a guide to what might be expected from main-sequence
stars, and proposes a concept for a satellite dedicated to a survey
of photometric variability.

---------------------------------------------------------
Title: Preface
Authors: Harvey, J. W.; Hudson, H. S.; Noyes, R. W.
1984AdSpR...4d...1H Altcode: 1984AdSpR...4....1H
No abstract at ADS

---------------------------------------------------------
Title: Solar-space observations and stellar prospects. Proceedings
of the Topical Meeting of the COSPAR Interdisciplinary Scientific
Commission E (Meetings E1, E2, and E6) of the COSPAR Twenty-fifth
Plenary Meeting held in Graz, Austria, 25th June - 7th July 1984.
Authors: Harvey, J. W.; Hudson, H. S.; Noyes, R. W.
1984AdSpR...4h....H Altcode: 1984AdSpR...4..180H; 1984AdSpR...4.....H
Selected topics pertaining to solar-space observations and stellar
prospects are discussed. Papers are presented on the fine-scale
structure of solar magnetic fields, increasing solar chromosphere
line intensities with solar activity, and ulraviolet spectroscopy
of the chromosphere and transition zone at high spatial and temporal
resolution. Consideration is also given to solar coronal studies using
normal-incidence X-ray optics, immediate and long-term prospects for
helioseismology, and a compact Dopplergraph/Magnetograph suitable for
space-based measurements of solar oscillations and magnetic fields.

---------------------------------------------------------
Title: Solar total irradiance and sunspot area in 1981
Authors: Hudson, H. S.
1984AdSpR...4h.113H Altcode: 1984AdSpR...4..113H
Precise observations of the total solar irradiance in 1980 from the
Solar Maximum Mission showed a strong correlation with the projected
total sunspot area. This correlation “explained” about half of the
variance (r ~ 0.75) in the total-irradiance data, leaving the other half
for other solar phenomena and errors in the data. We have now begun
the analysis of the intervening three years of “spin-mode” data,
which have reduced coverage. We find that the correlation persists
at about the same qualitative level, but with increased scatter that
can be attributed to the smaller amount of data. The flatness of the
distribution of areas of sunspot groups makes it possible to estimate
PSI approximately from only the large groups.

---------------------------------------------------------
Title: An All-Sky Photometric Explorer
Authors: Hudson, H. S.
1984srps.conf..197H Altcode:
These notes give ideas for an "All-sky Photometric Explorer", which
would survey the sky from space for visual photometric variability
of as many objects as possible. The observations would be as precise
as possible and would have the maximum duty cycle possible for each
object. These conflicting requirements present many difficulties for
optimization, and of course the basic objective - that of studying
variability on scales much finer than those of Earth-based photometry
- also presents many difficulties. Several technical routes may be
available; the author is basing the discussion upon a particular
concept sketched in this paper.

---------------------------------------------------------
Title: Solar Flux Variations and Concepts for a Stellar Photometric
Satellite
Authors: Hudson, Hugh S.
1984LNP...193..173H Altcode: 1984csss....3..173H
No abstract at ADS

---------------------------------------------------------
Title: Sunspot areas and solar irradiance variations during 1980
Authors: Hoyt, D. V.; Eddy, J. A.; Hudson, H. S.
1983ApJ...275..878H Altcode:
The realibility of daily measurements of sunspot areas made at the
Space Environment Laboratories (SEL, Boulder, Colorado) is tested by
comparison with measurements from other observatories, and they are
found to be typically consistent to within 9 percent. The calculated
visibility loss of sunspot due to poor seeing is no more than 13
percent of the projected sunspot areas. The resulting systematic
error in derived solar constant values (S) will be less than 0.007
percent, although random measurement errors for sunspot areas can lead
to corresponding random errors in the solar constant of as much as +
or - 0.05 percent on 5 percnt of the days. The dominant contributor to
short-term variations in S is the direct effect of sunspot blocking. A
Goddard Space Flight Center interpretation of solar constant variations
has used simulated sunspot areas that are systematically higher than
the SEL measurements by 46 percent, which is well outside the expected
range of measurement error or underestimation due to visibility losses.

---------------------------------------------------------
Title: Frequencies, amplitudes and linewidths of solar oscillations
from total irradiance observations
Authors: Woodard, M.; Hudson, H. S.
1983Natur.305..589W Altcode:
Ten months of solar total irradiance data from the Solar Maximum Mission
satellite have generated accurate frequencies, amplitudes and linewidths
for individual ~5-min solar p-mode ocillations of low degree. The modes
can be described as independent and chaotically excited oscillators,
and provide no evidence for the fine structure taken to imply rapid
internal rotation of the Sun.

---------------------------------------------------------
Title: Variations of Total Solar Irradiance During Rapid Sunspot
Growth
Authors: Hudson, H. S.; Jones, H.; McIntosh, P.
1983BAAS...15Q.950H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Upper Limits on the Internal Rotation Rate of the Sun
Authors: Woodard, M. F.; Hudson, H. S.
1983BAAS...15..951W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Observation of AN Absorption Feature in Gamma Ray Burst
Spectrum
Authors: Hueter, G. J.; Hudson, H. S.; Matteson, J. L.; Rothschild,
R. E.; Peterson, L. E.
1983ICRC....1...95H Altcode: 1983ICRC...18a..95H
A gamma ray burst was detected on March 25, 1978 by the High Energy
X-ray and Low Energy Gamma Ray Experiment on HEAO-1. The burst spectrum
shows an absorption feature at 55 + or - 5 keV with an equivalent
width of 13 + or - 3 keV, values commensurate with those of similar
features observed by the KONUS experiment. The burst spectrum also
is characterized by a hard component extending from about 0.25-6
MeV. This component can be interpreted in terms of a fireball model
for gamma ray bursts, which places the distance to the source at 1
kpc. The integrated fluence of the burst between 0.025 and 6 MeV is
1.5 x 10 to the -5th ergs/sq cm. The burst source has been localized
to within a degree of RA = 237.5 deg and Dec = 76.2 deg.

---------------------------------------------------------
Title: Location of X-Ray and Microwave Sources
Authors: Hudson, H.
1983SoPh...86..444H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Open discussion of controversial points
Authors: Hudson, H. S.; Uchida, Y.
1983SoPh...86..435H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Upper Limits on the Total Radiant Energy of Solar Flares
Authors: Hudson, H. S.; Willson, R. C.
1983SoPh...86..123H Altcode:
We establish limits on the total radiant energy of solar flares during
the period 1980 February - November, using the solar-constant monitor
(ACRIM) on board the Solar Maximum Mission. Typical limits amount to
6 × 1029 erg/s for a 32-second integration time, with 5σ
statistical significance, for an impulsive emission; for a gradual
component, about 4 × 1032 ergs total radiant energy. The
limits lie about an order of magnitude higher than the total radiant
energy estimated from the various known emission components, suggesting
that no heretofore unknown dominant component of flare radiation exists.

---------------------------------------------------------
Title: Recent advances in the understanding of solar flares :
proceedings of the U.S.-Japan seminar held at Komaba, Tokyo, 5-8
October 1982
Authors: Kane, S. R.; Uchida, Y.; Tanaka, K.; Hudson, H. S.
1983SoPh...86.....K Altcode:
Papers are presented on recent research concerning the understanding
of solar flares, including the general characteristics of flares;
energy transport, and chromospheric heating and evaporation; emission
processes and source structure; and high energy photons, nuclear
processes, and particle acceleration. Topics addressed include magnetic
theories of solar flares, nonthermal and nonequilibrium effects in
soft X-ray flare spectra, the thermal evolution of flare plasmas,
upper limits on the total radiant energy of solar flares, energetic
electrons as an energy transport mechanism in solar flares, the spatial
characteristics of microwave bursts, and the relation between hard X-ray
spectra and electron energy spectra. Also examined are the pre-flare and
post-flare X-ray variations in active regions, the imaging of impulsive
solar flare phenomena, the vertical structure of hard X-ray flares,
the spatial structure of high energy photon sources, gamma-ray lines
and neutrons from solar flares, and a dynamical interpretation of the
very hot region appearing at the top of a loop. For individual items
see A83-47659 to A83-47701

---------------------------------------------------------
Title: Foreword
Authors: Kane, S. R.; Uchida, Y.; Tanaka, K.; Hudson, H. S.
1983SoPh...86D...9K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Pinhole/Occulter Facility.
Authors: Tandberg-Hanssen, E. A.; Hudson, H. S.; Dabbs, J. R.; Baity,
W. A.
1983pof..rept.....T Altcode:
The Pinhole/Occulter is designed to study the nonthermal phenomena
of plasma dynamics in the solar corona. It consists of a long boom,
erected from the Shuttle bay or other space platform, separating an
X-ray mask/occulting disk from X-ray detectors and coronagraphs. The
long separation permits very high angular resolution.

---------------------------------------------------------
Title: First-order Fermi acceleration in solar flares as a mechanism
for the second-step acceleration of prompt protons and relativistic
electrons
Authors: Bai, T.; Hudson, H. S.; Pelling, R. M.; Lin, R. P.; Schwartz,
R. A.; von Rosenvinge, T. T.
1983ApJ...267..433B Altcode:
Solar flare data from June 27, 1980 balloon-based observations were
studied in terms of the hard X ray component. A temporal delay of 3 sec
was observed for the X ray emissions above 235 keV. The delay occurred
relative to the low-energy electrons and indicated a second acceleration
stage. An estimation of the acceleration rate of the first-order Fermi
process operating in a closed flare loop was found to be in agreement
with the resulting data, including the acceleration of both protons and
relativistic electrons. Additional support for the first-order Fermi
process is noted in the fact that flares generally occur in magnetic
loops, a condition which allows energetic particles to continually
interact with the upward moving shock fronts. A correlation has also
been observed between the delay times and the H-alpha areas, encouraging
the interpretation that the delay times are the shock transit times.

---------------------------------------------------------
Title: Gamma-ray imaging with a rotating modulator
Authors: Durouchoux, P.; Hudson, H.; Matteson, J.; Hurford, G.;
Hurley, K.; Orsal, E.
1983A&A...120..150D Altcode:
A gamma-ray imaging system, called a rotating modulator, is described,
which allows a large area of the sky (radius approximately 27 deg)
to be imaged with a limiting resolution approximately equal to or
less than 1 deg for intense sources. It operates in the 30 keV -10
MeV energy range, and does not rely on position sensitive detection
devices. It possesses a multiplex advantage which allows sky surveys to
be completed in a small fraction of the time needed by a conventional
collimation system. In addition, it is relatively insensitive to
background fluctuations, which makes it ideally suited to satellite
applications. The concept is illustrated by the results from a study
of a gamma-ray spectroscopy experiment.

---------------------------------------------------------
Title: Solar Surface Granulation and Variations of Total Irradiance
Authors: Hudson, H. S.; Woodard, M. F.
1983BAAS...15R.715H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Soft Thermal Component of Flare Hard X-ray Sources
Authors: Hudson, H.; Ohki, K.
1983BAAS...15Q.715H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar coronal non-thermal processes (Solar Maximum Mission)
Authors: Hudson, H. S.
1983ucsd.reptQ....H Altcode:
The Solar Maximum Mission was used to study solar coronal phenomena in
hard X-radiation, since its instrument complement included the first
solar hard X-ray telescope. Phenomena related to those discovered from
OSO-5 and OSO-7 observations were emphasized.

---------------------------------------------------------
Title: Studies with the Pinhole/Occulter Facility
Authors: Tandberg-Hanssen, E.; Dabbs, J.; Hudson, H.; Greene, M.
1983aiaa.meetQ....T Altcode:
The scientific justifications for high-resolution hard X-ray astronomy
are reviewed, and a scheme for making such observations from the Space
Shuttle payload bay is presented. High-resolution X-ray observations
at photon energies above 10 keV are important for the understanding of
the physics of solar flares, coronal disturbances related to flares,
and corona fine structure, as well as nonsolar X-ray sources. In
order to study these phenomena, concepts have been developed for
the Shuttle Pinhole/Occulter Facility (P/OF), an instrument based
on the principles of the pinhole camera which will have an angular
resolution of 0.2 arc sec. The proposed P/OF configuration consists of
four separate telescopes or position-sensitive counters mounted on a
detector plane and looking toward the target through separate portions
of an aperture mask (occulter plane). The two planes are separated by
a self-deployable 50-m boom, which is to be made essentially rigid by
direct control of its lowest normal modes.

---------------------------------------------------------
Title: Variations of the Solar Radiation Input
Authors: Hudson, H. S.
1983wcrs.proc...31H Altcode:
Observations from spacecraft have now provided the first data on the
variations of the total solar irradiance or 'solar constant'. The
active cavity radiometer on the Solar Maximum Mission in particular
produced copious data during its prime interval of about 10 mo during
1980. These data show that the total irradiance may vary by as much as
0.2 percent, with the largest excursions (deficits) due to the presence
of sunspots. Other identified sources of variability include faculae
and probably surface structure such as granulation. The 5-min global
oscillations of the sun produce very-small-amplitude variations in the
solar constant. The slower variations establish that the luminosity of
the sun varies with the growth and decay of active regions; in other
words, that the active regions cause appreciable storage of energy in
the solar convection zone. A study of the Greenwich sunspot data by
Hoyt and Eddy (1982) has shown that this sunspot effect may cause a
modulation of order 0.1 percent on solar-cycle time scales.

---------------------------------------------------------
Title: Solar Oscillations Observed in the Total Irradiance
Authors: Woodard, M.; Hudson, H.
1983SoPh...82...67W Altcode: 1983IAUCo..66...67W
The total solar irradiance measurements obtained by the active-cavity
radiometer on board the Solar Maximum Mission have been analyzed
for evidence of global oscillations. We find that the most energetic
low-degree p-mode oscillations in the five-minute band have amplitudes
of a few parts per million of the total irradiance, and we positively
detect modes with l = 0, 1, and 2. The distribution in l differs from
that of the velocity spectrum, with relatively more power at lower l
values. The individual modes have narrow line widths, corresponding
to values of Q greater than a few thousand, or lifetimes of at least a
week. We do not detect the 160-min oscillation in the power spectrum,
and place an upper limit of 5 parts per million (99.9% confidence)
on its amplitude.

---------------------------------------------------------
Title: Achievements in space astrophysics : proceedings of the topical
meeting of the COSPAR Interdisciplinary Scientific Commission E of
the COSPAR twenty-fourth plenary meeting held in Ottawa, Canada,
16th May-2nd June 1982
Authors: Hudson, Hugh S.; Dupree, Andrea K.; Linsky, Jeffrey L.
1983asap.book.....H Altcode: 1983QB495.A38v2n9..
No abstract at ADS

---------------------------------------------------------
Title: Chromospheric evaporation in a well-observed compact flare
Authors: Acton, L. W.; Leibacher, J. W.; Canfield, R. C.; Gunkler,
T. A.; Hudson, H. S.; Kiplinger, A. L.
1982ApJ...263..409A Altcode:
Hudson and Ohki (1972) pointed out that the increase of the soft
X-ray emission measure during flares might be accounted for in two
different ways, either by 'coronal condensation', or by what they termed
'chromospheric rarefaction', now more commonly called 'chromospheric
evaporation'. They ruled out coronal condensation on the basis of
cornal mass content arguments. Moore et al. (1980) found it highly
probable that the bulk of the mass of the soft X-ray emitting plasma is
supplied during the rise phase by chromospheric evaporation from the
feet of the soft X-ray loops. On the other hand, Cheng et al. (1981)
argued that chromospheric evaporation is not important as a source
of soft X-ray plasma. The present investigation is concerned with
an event in which direct chromospheric observations contradict the
conclusions reached by Cheng et al. Up to now chromospheric evaporation
has always been an inference, without compelling positive evidence. In
the current investigation, observations are considered which constitute
such evidence.

---------------------------------------------------------
Title: The Pinhole/Occulter facility
Authors: Dabbs, J. R.; Tandberg-Hanssen, E. A.; Hudson, H. S.
1982pof..rept.....D Altcode:
The outer solar atmosphere exhibits a great variety of dynamic and
energetic plasma phenomena, from the catastrophic energy release
of solar flares to the steady acceleration of the solar wind. The
Pinhole/Occulter Facility contains the instruments necessary for
broadband X-ray imaging, combined with simultaneous ultraviolet and
white light spectroscopy and imaging.

---------------------------------------------------------
Title: What are Solar Irradiance Observations of Global Oscillations
Telling Us?
Authors: Woodard, M. F.; Hudson, H. S.
1982BAAS...14..864W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Sensitive Upper Limits on the Total Luminosity of Solar Flares
Authors: Hudson, H. S.
1982BAAS...14..899H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar analysis of solar-constant monitoring package (SMM)
Authors: Hudson, H. S.
1982ucsd.rept.....H Altcode:
The activity cavity radiometer irradiance monitor is supplying the first
high precision data on solar total irradiance at the Earth. Thee classes
of variability were discovered: large variations of amplitudes up to
0.2%; small high frequency variations in the form of a continuum in the
periodogram, extending up to the Nyquist frequency; and sharp spikes at
frequencies corresponding to the individual p modes already known from
radial velocity measurements. The observed variations (up to 0.3%, on
time scales of several days) were identified with sunspot darkness. The
data analysis is expected to give information about the solar interior,
as well as about the solar input to the terrestrial climate.

---------------------------------------------------------
Title: Detection of Gamma-Ray Bursts with the Hard X-Ray and Low
Energy Gamma-Ray Experiment on HEAO-1
Authors: Hueter, G. J.; Hudson, H. S.; Matteson, J. L.
1982BAAS...14..619H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Effects of Sunspots on Solar Irradiance
Authors: Hudson, H. S.; Silva, S.; Woodard, M.; Willson, R. C.
1982SoPh...76..211H Altcode:
Sunspots have an obvious direct effect upon the visible radiant
energy falling upon the Earth. We show how to estimate this effect and
compare it quantitatively with recent observations of the solar total
irradiance (Willson et al., 1981). The sunspots explain about half of
the total observed variance of one-day averages. Since the sunspot
effect on irradiance produces an asymmetry of the solar radiation,
rather than (necessarily) a variation of the total luminosity, we have
also estimated the sunspot population on the invisible hemisphere. This
extrapolation allows us to estimate the true luminosity deficit produced
by sunspots, in a manner that tends toward the correct long-term average
value. We find no evidence for instantaneous global re-emission to
compensate for the sunspot flux deficit.

---------------------------------------------------------
Title: Gamma Radiation and Photospheric White-Light Flare Continuum
Authors: Hudson, H. S.; Dwivedi, B. N.
1982SoPh...76...45H Altcode:
Recent gamma-ray observations of solar flares have provided a
better means for estimating the heating of the solar atmosphere by
energetic protons. Such heating has been suggested as the explanation
of the continuum emission of the white-light flare. We have analyzed
the effects on the photosphere of high-energy particles capable of
producing the intense gamma-ray emission observed in the 1978 July 11
flare. Using a simple energy-balance argument and taking into account
hydrogen ionization, we have obtained the following conclusions:
Heating near τ5000 = 1 in the input HSRA model atmosphere
is negligible, even for very high fluxes of energetic particles.

---------------------------------------------------------
Title: Gamma ray transients and related astrophysical phenomena
Authors: Lingenfelter, R. E.; Hudson, H. S.; Worrall, D. M.
1982AIPC...77.....L Altcode: 1982grtr.work.....L
The papers deal with observational and theoretical studies of cosmic
gamma-ray bursts and line transients, X-ray burst sources, and solar
flares, as well as with instrument concepts for X-ray and gamma-ray
burst observations. General characteristics of gamma-ray bursts and
transients are reviewed, along with burst positions and distributions,
energy spectra, emission processes, and source models. HEAO-3
observations of gamma-ray bursts are discussed, as are ISEE-3 searches
for time variations in the 511-keV cosmic gamma-ray line flux, gamma-ray
burst spectra, neutron-star and stellar-flare models of gamma-ray
burst sources, satellite observations and models of X-ray burst
sources, solar energetic transients, and solar-flare energetics. The
instruments described include a burst and transient source detector for
the Gamma-ray Observatory, a modulated multislit camera for improved
localization of gamma-ray bursts, and a space-balloon ion-chamber
gamma-ray burst detector.

---------------------------------------------------------
Title: A third-generation small spectroscopy experiment for hard
transient events
Authors: Klebesadel, R. W.; Evans, W. D.; Laros, J. G.; Nakano, G. H.;
Datlowe, D. W.; Imhof, W. L.; Hudson, H. S.
1982AIPC...77..469K Altcode: 1982grtr.work..469K
We describe an experiment for monitoring high-energy transient events
(X-ray and γ-ray bursts from both solar and stellar sources) as
proposed for the OPEN (Origin of Plasmas in the Earth's Magnetosphere)
mission. This experiment contains Si(Li) detectors, a high-purity
germanium detector, and a bismuth germanate scintillation counter
for high-energy response. Cooling for the solid-state detectors is
provided by a passive radiator. The instrument gives broad spectral
response with high energy resolution; its mass of 15 kg and telemetry
requirement of 200 bps impose only modest demands upon spacecraft
resources. The use of a passive cooler does also place a constraint
upon the orbit of the satellite; deep space would be the preferred
location but other orbits may also be suitable. The ideas embodied
in the design may be of interest in the design of other spectroscopy
measurements from deep space, as for example for participants in a
future triangulation nework for transient events.

---------------------------------------------------------
Title: Second-Stage Acceleration in a Limb-Occulted Flare
Authors: Hudson, H. S.; Lin, R. P.; Stewart, R. T.
1982SoPh...75..245H Altcode:
We analyze hard and soft X-ray, microwave and meter wave radio,
interplanetary particle, and optical data for the complex energetic
solar event of 22 July 1972. The flare responsible for the observed
phenomena most likely occurred ∼20° beyond the NW limb of the Sun,
corresponding to an occultation height of 45 000 km. A group of type
III radio bursts at meter wavelengths appeared to mark the impulsive
phase of the flare, but no impulsive hard X-ray or microwave burst was
observed. These impulsive-phase phenomena were apparently occulted by
the solar disk as was the soft X-ray source that invariably accompanies
an Hα flare. Nevertheless essentially all of the characteristic
phenomena associated with second-stage acceleration in flares - type II
radio burst, gradual second stage hard X-ray burst, meter wave flare
continuum (FC II), extended microwave continuum, energetic electrons
and ions in the interplanetary medium - were observed. The spectrum
of the escaping electrons observed near Earth was approximately the
same as that of the solar population and extended to well above 1 MeV.

---------------------------------------------------------
Title: High-energy observations of stellar flares: comparison with
the sun
Authors: Hudson, H. S.
1982AIPC...77..383H Altcode: 1982grtr.work..383H
This paper reviews recent observations of flaring activity on
flare stars of the UV Ceti type, concentrating on X-ray and radio
data in comparison with possible solar analogs. Although detailed
differences exist, we conclude that similar mechanisms may work for both
cases. Extending the analogy, we estimate the hard X-ray and γ-ray
fluxes expected from typical stellar flares. In the solar case, these
radiations give information about accelerated particles in the flare
region. Hard X-ray observations of stellar flares may be possible,
eventually, but the predicted γ-ray fluxes are prohibitively weak.

---------------------------------------------------------
Title: The pinhole/occulter facility
Authors: Hudson, H. S.
1982AdSpR...2d.307H Altcode: 1982AdSpR...2..307H
The Pinhole/Occulter Facility concept uses a remote occulting mask
to provide high resolution observations of the solar corona and of
astronomical X-ray sources. With coded-aperture and Fourier-transform
techniques, the Pinhole/Occulter makes images at a resolution of
0.2 arc sec for 2 - 120 keV X-rays, using a 50-m boom erected from
the payload bay of the Space Shuttle or mounted on a free-flying
platform. The remote occulter also creates a large shadow area for solar
coronal observations; the Pinhole/Occulter concept includes separate
optical and ultraviolet telescopes with 50-cm apertures. These large
telescopes will provide a new order of resolution and sensitivity for
diagnostic observations of faint structures in the solar corona. The
Pinhole/Occulter is a powerful and versatile tool for general-purpose
X-ray astronomy, with excellent performance in a broad spectral band
complementary to that accessible with AXAF. The large collecting
area of 1.5 m2 results in a 5σ detection threshold of
about 0.02 μJy for the 2 - 10 keV band, or about 10-5
ph(cm2sec keV)-1 at 20 keV.

---------------------------------------------------------
Title: Variability of Solar Total Irradiance
Authors: Woodward, M.; Hudson, H.; Willson, R.
1982pccv.conf..152W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Direct evidence for chromospheric evaporation in a
well-observed compact flare
Authors: Canfield, R. C.; Gunkler, T. A.; Hudson, H. S.; Acton, L. W.;
Leibacher, J. W.; Kiplinger, A. L.
1982AdSpR...2k.145C Altcode: 1982AdSpR...2..145C
Observations of the solar flare of May 7, 1980 using several Solar
Maximum Mission instruments are presented as an investigation of the
phenomenon of chromospheric evaporation. The total amount of plasma
at temperatures greater than 2 x 10 to the 6th K were determined from
the X-ray data, and the amount of plasma that was evaporated from
the chromosphere was determined from the H-alpha data. The H-alpha
profiles indicate that for the flare as a whole, at the time of peak
soft X-ray emission measure, the number of atoms evaporated from the
chromosphere was 7 x 10 to the 37th. The soft X-ray emission measure
of 1 x 10 to the 49th/cu cm, coupled with the flare volume estimate
of 10 to the 26th cu cm, indicates that there were 3 x 10 to the 37th
electrons in the soft X-ray plasma with temperatures greater than 2 x
10 to the 6th K. These results indicate that enough material had been
evaporated from the chromosphere to account for the X-ray plasma. Taken
together, the H-alpha, soft X-ray, and hard X-ray images indicate that
chromospheric evaporation is driven both by flare-accelerated electrons
during the impulsive phase and by conduction during the thermal phase.

---------------------------------------------------------
Title: Achievements in space astrophysics; Proceedings of the Topical
Meeting, Ottawa, Canada, May 16-June 2, 1982
Authors: Hudson, H. S.; Dupree, A. K.; Linsky, J.
1982AdSpR...2i....H Altcode: 1982AdSpR...2.....H
Progress in astrophysics resulting from IUE and Einstein Observatory
studies is reviewed. The topics considered include: the solar corona
as a testing found for plasma astrophysics; seismological studies
of the sun and other stars; magnetic reconnection; IUE and Einstein
observations of cool stars, high-luminosity X-ray binaries, supernovae,
cataclysmic variables, active galactic nuclei, globular clusters and
blue horizontal branch stars, supernova remnants, UV spectra of H
II regions and galaxies, and galactic clusters. Also discussed are:
highly variable X-ray emitting objects in the Rho Oph dark cloud; X-ray
diagnostics of globular clusters; stellar chromospheres and coronae of
solar and late-type dwarfs, active stars and systems, F-, G-, and K-type
stars, hot stars, and cool luminous stars; coronal heating mechanisms;
magnetic flux expulsion as an acceleration mechanism for stellar winds;
and energy balance of the outer atmospheres of solar like stars. For
individual items see A83-33582 to A83-33609

---------------------------------------------------------
Title: The Pinhole/Occulter Facility. Executive summary.
Authors: Dabbs, J. R.; Tandberg-Hanssen, E. A.; Hudson, H. S.
1982NASTP2089.....D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: High-Energy Observations of Stellar Flares - Comparison with
the Sun
Authors: Hudson, H. S.
1982AIPC...77..382H Altcode: 1982grtr.work..382H
No abstract at ADS

---------------------------------------------------------
Title: An Einstein search for X-ray emission from comet Bradfield
(1979l)
Authors: Hudson, H. S.; Ip, W. -H.; Mendis, D. A.
1981P&SS...29.1373H Altcode:
X-radiation may result from active plasma phenomena in the interactions
of comets with the solar wind. We have carried out a limited but
sensitive search for soft X-radiation from Comet Bradfield (1979 l),
on 1980 Feb. 5. No X-radiation was detected at a level (3σ) of 1.7 ×
10 -13 erg(cm 2sec keV) -1 in the 0.2 -
4.0 kev range. This corresponds to a limit on the power dissipated in
the comet by non-thermal electrons of approximately 10 19
ergs sec -1, averaged over the 2568-sec exposure to the
comet. This energy deposition is near the magnitude suggested by simple
theoretical ideas, and further searches of appropriate comets both in
soft X-radiation and at radio wavelengths seem warranted.

---------------------------------------------------------
Title: Interpretation of the 'Second-step' Acceleration in the
Impulsive Phase of a Solar Flare
Authors: Bai, T.; Hudson, H. S.
1981BAAS...13..912B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Pinhole/Occulter Facility
Authors: Hudson, H. S.; Kohl, J. L.; Lin, R. P.; MacQueen, R. M.;
Tandberg-Hanssen, E.; Pabbs, J. R.
1981pof..rept.....H Altcode:
A large occulting system in space can be used for high resolution
X-ray observations and for large aperture coronagraphic observations in
visible and UV light. The X-ray observations can combine high angular
resolution in hand (10 keV) X-radiation with the high sensitivity
of a multiple pinhole camera, and can permit sensitive observations
of bremsstrahlung from nonthermal particles in the corona. The
large aperture coronagraphs have two major advantages: high angular
resolution and good photon collection. This will permit observations
of small scale structures in the corona for the first time and will
give sufficient counting rates above the coronal background rates
for sensitive diagnostic analysis of intensities and line profiles
for coronal structures in the solar wind acceleration region. The
technical basis for performing observations with a large occulting
system in these three wavelength ranges is described as well as a
pinhole/occulter facility presently being considered for Spacelab. Some
indications about future developments are included.

---------------------------------------------------------
Title: Direct Evidence for Chromospheric Evaporation in a
Well-Observed Compact Flare
Authors: Canfield, R. C.; Acton, L. W.; Gunkler, T. A.; Hudson, H. S.;
Kiplinger, A. L.; Leibacher, J. W.
1981BAAS...13R.819C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Effects of Solar Activity on the Total Solar Irradiance
Authors: Hudson, H. S.; Woodward, M.; Willson, R. C.
1981BAAS...13R.877H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Observations of p-mode Oscillations in the Total Solar
Irradiance
Authors: Woodard, M.; Hudson, H.; Willson, R. C.
1981BAAS...13..858W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Variations of solar irradiance
Authors: Willson, R. C.; Hudson, H. S.
1981ApJ...244L.185W Altcode:
The active cavity radiometer experiment on the Solar Maximum Mission
is providing sensitive measurements of time variations of the total
solar irradiance with greater accuracy and precision than previously
achieved. The mean 1 AU irradiance for the first 45 days' operation
is 1368.64 W/sq m with an absolute uncertainty of less than + or
0.5%. Variations about this mean have been observed on time scales
of hours to days with amplitudes up to + or - 0.04%, resolved with a
statistical uncertainty as low as 0.001%. Variations within a single
orbit with amplitudes as large as + or - 0.5% have been resolved with
0.005% or smaller statistical uncertainty. Although these variations do
not display a systematic relationship to conventional solar activity
indices over the period, correlative behavior cannot be ruled out on
the basis of the present limited data set.

---------------------------------------------------------
Title: Observations of Solar Irradiance Variability
Authors: Willson, R. C.; Gulkis, S.; Janssen, M.; Hudson, H. S.;
Chapman, G. A.
1981Sci...211..700W Altcode:
High-precision measurements of total solar irradiance, made by the
active cavity radiometer irradiance monitor on the Solar Maximum
Mission satellite, show the irradiance to have been variable
throughout the first 153 days of observations. The corrected data
resolve orbit-to-orbit variations with uncertainties as small as
0.001 percent. Irradiance fluctuations are typical of a band-limited
noise spectrum with high-frequency cutoff near 0.15 day-1;
their amplitudes about the mean value of 1368.31 watts per square
meter approach ± 0.05 percent. Two large decreases in irradiance of
up to 0.2 percent lasting about 1 week are highly correlated with
the development of sunspot groups. The magnitude and time scale of
the irradiance variability suggest that considerable energy storage
occurs within the convection zone in solar active regions.

---------------------------------------------------------
Title: Solar maximum mission experiment: Initial observations by
the active cavity radiometer
Authors: Willson, R. C.; Hudson, H. S.
1981AdSpR...1m.285W Altcode: 1981AdSpR...1..285W
The Active Cavity Radiometer on board the SMM is providing high-quality
measurements of the solar irradiance. After correction for the
solar distance, the orbital displacement of the satellite, and the
relativistic shift of irradiance due to the satellite motion, the
observed standard deviation is in the range 10-15 parts per million
in a 96-minute integration. Measurable solar variations occur on time
scales of a few minutes to a few days. The total amplitude of the
variations in the daily averages from February 16 to March 31, 1980,
was 0.10% based upon 96-minute averages.

---------------------------------------------------------
Title: Solar Maximum Mission experiment: initial observations by
the Active Cavity Radiometer.
Authors: Willson, R. C.; Hudson, H. S.
1981hea..conf..285W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: X-ray and gamma-ray observations of a white-light flare
Authors: Hudson, H. S.
1981AdSpR...1m.247H Altcode: 1981AdSpR...1..247H
HEAO-1 observed hard radiations (X- and gamma-rays) from a major
solar flare on 11 July 1978. The observations showed gamma-ray line
and continuum emission extending to the highest energy observed. The
lines are identified with the 2.2 MeV line of deuterium formation
and the 4.4 MeV line of inelastic scattering on 12C, both
previously observed in the flares of August 1972 [1]. The 11 July
flare was identified as a white-light flare by observations at Debrecen
[2]. It thus provides the first opportunity for a detailed examination
of white-light flare theories that depend upon proton heating of the
photosphere. The line strength over a four-minute integration at 2.2
MeV was 1.00 +/- 0.29 ph(cm2 sec)-1, and the
gamma-ray emission (excluding the 2.2 MeV line which was appreciably
delayed) lagged by less than 20 sec approximately after the hard
X-ray and microwave fluxes. We conclude that the “second-stage”
acceleration of high-energy solar particles must commence promptly
after the impulsive phase.

---------------------------------------------------------
Title: High-energy astrophysics : proceedings of symposium 5 and
the topical meeting of the COSPAR Interdisciplinary Scientific
Commission E of the COSPAR twenty-third plenary meeting held in
Budapest, Hungary, 2-14 June 1980
Authors: Hudson, H. S.
1981AdSpR...1m....H Altcode: 1981QB495.A38v1n13.; 1981AdSpR...1.....H
Papers are presented on such topics as the X-ray background from
evolving quasars, high-energy neutrinos from powerful radio galaxies,
spectra of accreting X-ray pulsars, cyclotron radiation of magnetized
degenerate dwarfs and neutron stars, and expanding shells of young
pulsars as sources of high-energy neutrinos. Also considered are
low-energy observation of Cygnus X-2 by Ariel VI, evidence from a
cyclotron line at 70 keV from 4U053+21, observations of 1-30 MeV gamma
rays from the galactic center, and results from the SMM experiment.

---------------------------------------------------------
Title: High-energy astrophysics. Proceedings of Symposium 5 and
the Topical Meeting of the COSPAR Interdisciplinary Scientific
Commission E of the COSPAR Twenty-third Plenary Meeting held in
Budapest, Hungary, 2 - 14 June 1980.
Authors: Hudson, H. S.
1981AdSpR...1m...7H Altcode: 1981AdSpR...1....7H
No abstract at ADS

---------------------------------------------------------
Title: Sunspots and solar variability
Authors: Hudson, H. S.; Willson, R. C.
1981phss.conf..434H Altcode:
The analyses of Willson et al. (1981) and Hudson et al. (1981) are
extended in correlating the total solar irradiance monitor (ACRIM)
data with the routine synoptic sunspot data. At the simplest level,
this correlation reveals clearly that the so-called 'missing flux'
is truly missing in the sense that large young active regions do
produce at net diminution of the irradiance when their spots cross the
central meridian. It is pointed out that the irradiance deficit must
of course be made up, either promptly or on intermediate time scales;
this is because the surface effects cannot perturb the energy generation
processes in the interior. In the approach taken here, simple models
of the reemission are constructed, the total reemission is scaled to
the estimated sunspot deficit, and an attempt is made to measure the
parameters of the models by a statistical comparison with ACRIM data.

---------------------------------------------------------
Title: X-ray and gamma-ray observations of a white-light flare.
Authors: Hudson, H. S.
1981hea..conf..247H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Variability of Total Solar Irradiance: the Flux Deficit
of Sunspots
Authors: Hudson, H. S.; Woodard, M.; Willson, R. C.
1980BAAS...12Q.898H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: HEAO 1 observations of gamma-ray lines from a solar flare
Authors: Hudson, H. S.; Bai, T.; Gruber, D. E.; Matteson, J. L.;
Nolan, P. L.; Peterson, L. E.
1980ApJ...236L..91H Altcode:
HEAO 1 observed gamma radiation, including the 2.223 MeV line of
deuterium and the 4.43 MeV line of C-12, from a white-light solar flare
of 1978 July 11. Line strengths over a 4 minute integration were 1.00
+ or - 0.29 and 0.18 + or - 0.07 photons/sq cm s, respectively, and
the continuum in the 1-5 MeV range fitted a spectrum 10 E to the -3rd
photons/sq cm s keV. The 2.2 MeV line lagged 94 + or - 30 s behind the
gamma-ray continuum, which itself was delayed about 20 s from the hard
X-ray (not less than 40 keV) and microwave fluxes. This is the second
flare for which both MeV-range lines and continuum have been observed,
and the first for which simultaneous white-light observations exist. The
prompt gamma-ray line (4.43 MeV) can be directly interpreted as an
energy deposition of not greater than 7 x 10 to the 26th ergs per sec
by energetic protons at photospheric depths. This is insufficient to
maintain the white-light continuum by normal photospheric emission
mechanisms.

---------------------------------------------------------
Title: The Necessary Conditions for White-light Flaring from Proton
Bombardment
Authors: Hudson, H. S.; Dwivedi, B. N.
1980BAAS...12..480H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Impulsive phase of solar flares
Authors: Kane, S. R.; Crannell, C. J.; Datlowe, D.; Feldman, U.;
Gabriel, A.; Hudson, H. S.; Kundu, M. R.; Maetzler, C.; Neidig, D.;
Petrosian, V.
1980sfsl.work..187K Altcode: 1980sofl.symp..187K
The present understanding of the impulsive phase of a solar flare,
characterized by short-duration bursts of impulsive hard X-ray,
EUV, optical and radio emission indicating the release of energetic
electrons is reviewed. Observations of the spectral distribution
of impulsive hard X-ray bursts and of Type III and radio continuum
bursts are presented and interpreted in terms of energetic electron
distributions, and impulsive EUV, XUV, soft X-ray and optical
observations, which provide a lower limit to total energy release
during the impulsive phase, are discussed. The role of energetic
electrons in exciting the hard X-ray, EUV and microwave emissions is
considered, and thin-target, thick-target, partial-precipitation and
thermal models of impulsive phase electron acceleration are evaluated
in light of the observations. It is noted that available data do
not allow discrimination between a thermal or a nonthermal electron
distribution, on which depends the proportion of flare energy supplied
by the energetic electrons, and that data favors models which permit
at least partial electron precipitation. Future observational and
theoretical work is indicated.

---------------------------------------------------------
Title: Solar particle fluxes and the ancient sun
Authors: Lingenfelter, R. E.; Hudson, H. S.
1980asfr.symp...69L Altcode:
The implications of the statistical data on solar flare particle fluxes,
for the present and the ancient sun, considering modern data from the
past two solar cycles, C-14 data from the past 7000 years, and Al-26
and Mn-53 data in lunar samples for the last 10-million years are
reviewed. All of these records suggest that there is a maximum proton
fluence (greater than 10 MeV) from a solar flare on the order of 10 to
the 10th p/sq cm, above which the size-frequency distribution steepens
sharply. From this it is extremely unlikely that energetic particles
from solar flares could have contributed to extinction catastrophes
in the fossil record.

---------------------------------------------------------
Title: Analysis of HEAO-1 Solar Gamma-Ray Spectra
Authors: Bai, T.; Hudson, H. S.; Lingfelter, R. E.
1979BAAS...11R.658B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Ground-based observations of sources in the AFGL infrared
sky survey.
Authors: Gosnell, T. R.; Hudson, H.; Puetter, R. C.
1979AJ.....84..538G Altcode:
A preliminary report is presented on ground-based observations
of sources discovered by the AFGL infrared sky survey. This paper
contains photometric data on 104 sources and spectrophotometry of 14
sources identified in 6.0 x 4.5-arcmin raster scans around the positions
reported by the AFGL. The sources originally identified in the catalog
with known objects are mainly hot bright stars. Two unusual sources
discovered in this program, GL 2636 and GL 4029, appear to be multiple
sources. Both these objects exhibit the 3.3-micron emission feature,
and GL 4029 shows several other emission features previously found in
NGC 7027.

---------------------------------------------------------
Title: Infrared continuum observations of the solar atmosphere
Authors: Hudson, H.; Levan, P.; Lindsey, C.
1979ucsd.rept.....H Altcode:
The far-infrared wavelengths (10 microns to 1 mm) were used to study the
spatial and temporal structure of the solar atmosphere. Observational
results were obtained on flares, faculae, sunspots, and on the
center-to-limb intensity distribution, as well as on time variability
within these regions. A program of precise monitoring of slow variations
in the integrated solar luminosity was shown to be feasible, and
initial steps to implement observations were completed.

---------------------------------------------------------
Title: Observations of particle accelerator in solar flares
Authors: Hudson, H. S.
1979AIPC...56..115H Altcode: 1979pama.work..115H
Solar flares provide several examples of non-thermal particle
acceleration. This paper reviews the information gained about
these processes via X-ray and γ-ray astronomy, which can presently
distinguish among three separate particle-acceleration processes at
the sun: An impulsive acceleration of &gt;~20 keV electrons, a gradual
accelerator of &gt;~20 keV electrons, and a gradual accelerator of
&gt;~10 MeV ions. The acceleration energy efficiency (total particle
energy divided by total flare energy) of any of these mechanisms cannot
be less than about 0.1%, although the gradual acceleration does not
occur in every flare. The observational material suggests that both
the impulsive and gradual accelerations take place preferentially
in closed magnetic-field structures, but that the electrons decay in
these traps before they can escape. The ions escape very efficiently,
on the other hand.

---------------------------------------------------------
Title: Indirect estimation of energy disposition by non-thermal
electrons in solar flares.
Authors: Hudson, H. S.; Canfield, R. C.; Kane, S. R.
1978SoPh...60..137H Altcode:
The broad-band EUV and microwave fluxes correlate strongly with hard
X-ray fluxes in the impulsive phase of a solar flare. This note presents
numerical aids for the estimation of the non-thermal electron fluxes
from these correlations, using the SFD (sudden frequency deviation)
ionospheric data to measure the EUV flux.

---------------------------------------------------------
Title: Hard X-ray imaging from the solar probe
Authors: Hudson, H. S.
1978clus.nasa..132H Altcode:
The solar probe offers a platform with particular advantages for
studying solar nonthermal plasma processes via the observations of
hard X-radiation from energetic electrons in the chromosphere and
corona, these include (1) high sensitivity, (2) a second line of
sign (in addition to the earth's) that can aid in three dimensional
reconstruction of the source distribution, and, (3) the possibility
of correlation with direct measurements of the nonthermal particles
from the probe itself.

---------------------------------------------------------
Title: New perspectives for solar observations
Authors: Hudson, H. S.
1978clus.nasa...81H Altcode:
The merits and demerits of an approach to the Sun (more closely than
about one AU) are examined. High resolution imaging (approximately 0.1
arc sec) to be obtained with the shuttle-borne solar optical telescope,
will permit conclusive observations relating to the structure of the
quiet solar atmosphere, sunspots, spicules, oscillations, and many other
problems of solar astrophysics. Beyond this limit important unresolved
structure will exist, especially in optically thin regions or in
regions with strong magnetic fields. Ambiguity will remain in solar
imagery because a single line of sight cannot suffice completely to
untangle the vertical dimension from the two horizontal dimensions. A
solar probe with a complement of solar telescopes would provide two
lines of sight for solar viewing and increase knowledge of the three
dimensional structure of the solar atmosphere.

---------------------------------------------------------
Title: A purely coronal hard X-ray event.
Authors: Hudson, H. S.
1978ApJ...224..235H Altcode:
OSO 7 observations of a hard X-ray event of coronal origin are
described. This event had a duration of more than 42 min as well as
an abnormally large hard/soft ratio and apparently occurred after the
disappearance of a bright coronal streamer. This gradual hard X-ray
event is tentatively associated with open field lines extending well
above the closed loop structures that participated in the originating
flare. It is noted that a gradual hardening of the hard X-ray spectral
distribution occurred during the event. Physical conditions in the
source are considered, and the results are compared with observations
of other hard X-ray events associated with flares.

---------------------------------------------------------
Title: Large scale telescopes for high resolution X-ray and gamma-ray
astronomy.
Authors: Hudson, H. S.; Lin, R. P.
1978SSI.....4..101H Altcode:
This paper shows that angular-resolution, energy-range, and
structural constraints on image-modulated X-ray telescopes are not
fundamental and that the limits on angular resolution can be overcome
by constructing such telescopes on a very large spatial scale. It is
proposed that widely separated satellites be used for the modulating
mask and detector array. Implementation of this concept is discussed
in terms of a simple system consisting of a pinhole camera (i.e.,
a hole in an opaque mask on one subsatellite and a detector array on
another). Advantages and problems of such systems are briefly discussed,
and a solar X-ray telescope intended for deployment from a Shuttle
orbiter is described. It is noted that such large-scale telescopes can
be constructed to image gamma rays and even energetic neutrons as well.

---------------------------------------------------------
Title: HEAO-1 Response to Solar Gamma Rays.
Authors: Hudson, H. S.; Baity, W. A.; Gruber, D. E.; Knight, F. K.;
Nolan, P. L.; Matteson, J. L.; Peterson, L. E.
1978BAAS...10..516H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Threshold effect in second-stage acceleration.
Authors: Hudson, H. S.
1978SoPh...57..237H Altcode:
Proton fluxes observed at Earth (van Hollebeke et al., 1975) have
a much flatter size distribution than do other parameters of solar
flares more representative of total energy. Peak proton flux varies at
least as rapidly as the fourth power of total flare energy. An absolute
threshold may exist, in view of the flatness of the proton distribution.

---------------------------------------------------------
Title: OSO-7 Observations of Coronal Hard X-Ray Sources.
Authors: Hudson, H. S.
1978BAAS...10..454H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Grid Telescope for Imaging Hard X-Rays
Authors: Hudson, H. S.; Hurford, G. J.; Peterson, L. E.; van Beek,
H. F.
1978nisa.symp..297H Altcode: 1978nisa.conf..297H
No abstract at ADS

---------------------------------------------------------
Title: A study of the planetary nebulae Abell 30 and Abell 78.
Authors: Cohen, M.; Hudson, H. S.; O'Dell, S. L.; Stein, W. A.
1977MNRAS.181..233C Altcode:
The central regions of the planetary nebulae A 30 and A 78
have been studied by UBVRI photometry, optical spectroscopy,
and near-infrared photometry. The spectra contain high-excitation
emission lines and strongly resemble those of Wolf-Rayet stars of
the carbon sequence. Stellar temperatures in excess of 50,000 K
are inferred. The observed 3.5-micron flux of each nebula exceeds
reasonable extrapolations of both the stellar flux and any possible
free-free emission. The color temperature of this excess between
2.28 and 3.5 microns is of the order of 1000 K. For each nebula,
the aperture dependence of the excess emission suggests an extended
(about 10 arcsec radius) region centered on the nucleus. Thermal
radiation from a distribution of dust that is concentrated near the
nuclei seems the most plausible explanation for the excess, but no
theory of dust formation or heating seems totally adequate at present.

---------------------------------------------------------
Title: Do All Flares Have Impulsive Phases?
Authors: Datlowe, D. W.; Elcan, M. J.; Hudson, H. S.; Peterson, L. E.
1977BAAS....9..569D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Effects of Electrons versus Protons in the Solar Atmosphere
(Proceedings of the Meeting `How Can Flares be Understood?', held
during the 16th General Assembly of the IAU in Grenoble, France,
on 27 August, 1976.)
Authors: Hudson, H. S.
1977SoPh...53..295H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Surface photometry of the SB0 galaxy NGC 1023.
Authors: Gallagher, J. S.; Hudson, H. S.
1976PASP...88..824G Altcode:
Beam-switched photoelectric surface photometry has been obtained
for the SB0 galaxy NGC 1023. Initially the disk surface brightness
distribution is approximately exponential with a more rapid fall-off
in surface brightness occurring for R 5 arc minutes ( 15 kpc). No
color gradient is found in (V- i). Key words: galaxies - photometry

---------------------------------------------------------
Title: New Upper Limits on Jovian X Rays
Authors: Peterson, L. E.; Hudson, H. S.; Tsikoudi, V.
1976Icar...29..419P Altcode:
The UCSD X-ray telescope on OSO-3 scanned Jupiter for 33 days during
February and March 1968. We have searched the data for a steady
Jovian flux, and for a burst component at times of decametric radio
bursts. Neither component was detected at a sensitivity of ∼0.1 photon
(cm 2sec) -1 for hv &gt; 7.7 keV. At 4.4AU,
the 3σ upper limits correspond to X-ray luminosities of 7.4 × 10
19 ergs sec -1 for the steady component, and 2 ×
10 20 ergs sec -1 for the burst component. The
observations occurred during a period of high solar activity, during
which three sudden-commencement magnetic storms were observed at
Earth. We compare the upper limits with several different calculations
of the expected flux levels, and conclude that major improvements in
X-ray detection techniques will be required before Jovian X rays can
be detected with near-Earth observations.

---------------------------------------------------------
Title: Hard X-ray imaging facility for space shuttle: A scientific
and conceptual engineering study
Authors: Peterson, L. E.; Hudson, H. S.; Hurford, G.; Schneible, D.
1976STIN...7822016P Altcode:
A shuttle-accommodated instrument for imaging hard X-rays in the study
of nonthermal particles and high temperature particles in various solar
and cosmic phenomena was defined and its feasibility demonstrated. The
imaging system configuration is described as well as the electronics,
aspect systems, mechanical and thermal properties and the ground
support equipment.

---------------------------------------------------------
Title: Surface photometry of the spiral galaxy IC 2233 and the
existence of massive halos.
Authors: Gallagher, J. S.; Hudson, H. S.
1976ApJ...209..389G Altcode:
Using beam-switching to cancel the sky background, deep BVi photometric
measurements have been obtained near the edge-on Scd galaxy IC 2233. No
halo component is detected to a level of 1 percent of the disk surface
brightness. For a concentrated model (half-intensity isophote smaller
than the disk radius), the halo M/L must exceed 100 if it is to meet
minimally the Ostriker and Peebles criterion for stabilizing a cold disk
of stars. Subject headings: galaxies: individual-galaxies: photometry -
galaxies: structure

---------------------------------------------------------
Title: Non-thermal processes in large solar flares.
Authors: Lin, R. P.; Hudson, H. S.
1976SoPh...50..153L Altcode:
We analyze particle acceleration processes in large solar flares, using
observations of the August, 1972, series of large events. The energetic
particle populations are estimated from the hard X-ray and γ-ray
emission, and from direct interplanetary particle observations. The
collisional energy losses of these particles are computed as a function
of height, assuming that the particles are accelerated high in the solar
atmosphere and then precipitate down into denser layers. We compare
the computed energy input with the flare energy output in radiation,
heating, and mass ejection, and find for large proton event flares
that: The ∼10-102 keV electrons accelerated during the
flash phase constitute the bulk of the total flare energy.

---------------------------------------------------------
Title: Far-infrared observations of IRC +10216.
Authors: Campbell, M. F.; Elias, J. H.; Gezari, D. Y.; Harvey, P. M.;
Hoffmann, W. F.; Hudson, H. S.; Neugebauer, G.; Soifer, B. T.; Werner,
M. W.; Westbrook, W. E.
1976ApJ...208..396C Altcode:
Broadband photometric observations of IRC + 10216 in five wavelength
intervals from 50 to 1000 microns are reported. The observed radiation
is interpreted as thermal emission from dust in the extended molecular
cloud heated by the compact 2-20-micron source at the cloud core. The
shape of the 50-1000-micron spectrum suggests that the emissivity of
the dust particles varies approximately as the inverse wavelength over
this spectral interval. The mass of dust inferred from the far-infrared
emission is comparable with the mass of heavy molecules in the cloud.

---------------------------------------------------------
Title: Submillimeter observations of NGC 2024, OMC-2, and Mon R-2.
Authors: Hudson, H. S.; Soifer, B. T.
1976ApJ...206..100H Altcode:
The paper reports submillimeter (effective wavelength about 400 microns)
observations of three dense molecular clouds: NGC 2024 (Orion B),
OMC-2 (in Orion A), and Mon R-2. These objects strongly resemble the
far-infrared source in the Kleinmann-Low nebula in Orion A. An extensive
map of NGC 2024 shows the peak of submillimeter brightness to coincide
with the peak locations of far-infrared continuum and HCN molecular-line
emissions. The submillimeter and far-infrared brightness distributions
differ in spatial detail, suggesting that the submillimeter emission
comes from a cool region with temperature of about 25 K (inferred
from the surface brightness of the optically thick (C-12)(O-16)
line). Arguments based on the column density derived from optically
thin molecular lines yield an effective mass-absorption coefficient
of 17 sq cm/g for the continuum opacity at 400 microns; however, this
estimate is subject to large uncertainties. It is suggested that the
star 2024 No. 2 is imbedded in the molecular cloud and supplies the
energy of the submillimeter emission.

---------------------------------------------------------
Title: Solar limb brightening in submillimeter wavelengths.
Authors: Lindsey, C.; Hudson, H. S.
1976ApJ...203..753L Altcode:
Differential two-beam scans of the sun in submillimeter wavelengths
(350 microns to 1 millimeter) indicate limb brightening approaching 1
percent when the cosine of the angle from the normal equals 0.60. The
observations also show considerable chromospheric structure, both in
active and quiet regions, but with less relative amplitude than at
millimeter and centimeter wavelengths. The limited angular resolution
of the observing system, together with photometric errors due to
fluctuating atmospheric transparency, make the brightness profile of
the extreme limb uncertain. The observed degree of limb brightening is
considerably less than that consistent with spherically symmetric model
atmospheres based on continuum brightness-temperature measurements. The
suppression of limb brightening suggests the existence of irregular
granular structure with both horizontal and vertical characteristic
sizes of the order of 1500 km. High-resolution images in the wings of
the K-line show granular structure of about this horizontal scale.

---------------------------------------------------------
Title: The Solar-Flare Infrared Continuum: Observational Techniques
and Upper Limits
Authors: Hudson, H. S.
1975SoPh...45...69H Altcode:
Exploratory observations at 20 μ and 350 μ have determined detection
thresholds for solar flares in these wavelengths. In the 20 μ range
solar atmospheric fluctuations (the `temperature field') set the basic
limits on flare detectability at ∼5K; at 350 μ the extinction in
the Earth's atmosphere provides the basic limitation of ∼30K. These
thresholds are low enough for the successful detection of several
infrared-emitting components of large flares. Limited observing time
and lack of solar activity have prevented observations of large flares
up to the present, but the techniques promise to be extremely useful
in the future.

---------------------------------------------------------
Title: Upper limits on stellar flare X-ray emission from OSO-3.
Authors: Tsikoudi, V.; Hudson, H.
1975A&A....44..273T Altcode:
The OSO-3 X-ray telescope scanned 82 optically-observed flares on UV
Cet, EV Lac, YZ CMi, and AD Leo as well as four radio flares on UV
Cet. These data are examined individually and by superposed-epoch
analysis. Upper limits of about 1000 and about 10 million are
determined for the X-ray/optical and X-ray/radio luminosity ratios,
respectively. The upper limits conflict with theories of stellar
flaring which directly utilize nonthermal particles to excite the main
part of the flare optical emission, but agree with observations of
solar flares. Moderate improvement of the X-ray observations should
make possible a direct test of the solar-flare analogy in the near
future. The integrated X-ray luminosity of stellar flares may contribute
significantly to the low-energy diffuse X-ray background in the plane
of the Galaxy.

---------------------------------------------------------
Title: The Solar-flare Infrared Continuum
Authors: Ohki, K.; Hudson, H. S.
1975SoPh...43..405O Altcode:
We consider potential sources of infrared (1 μ to 1 mm) continuum in
solar flares. Several mechanisms should produce detectable fluxes: in
the 350 μ window for ground-based observations, impulsive emission will
arise in synchrotron radiation from 1-10 MeV electrons, and possibly
thermal (free-free) continuum from the source of the white-light flare;
the hot flare plasma responsible for soft X-ray emission will also
emit detectable fluxes of free-free continuum in the largest flares. At
shorter wavelengths the dominant infrared emission will come from the
Hα flare itself. Observations in the infrared wavelengths will help
to complete our picture of flare structure in both the impulsive and
gradual phases.

---------------------------------------------------------
Title: Submillimeter Observations of NGC 2024, OMC-2, and Mon R-2
Authors: Hudson, H. S.; Soifer, B. T.
1975BAAS....7..465H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Beam-Switched Surface Photometry of Galaxies
Authors: Gallagher, J. S.; Hudson, H. S.
1975BAAS....7..396G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Hard X-Ray Burst Phenomena.
Authors: Peterson, L. E.; Datlowe, D. W.; Elcan, M. J.; Hudson, H. S.
1975BAAS....7..399P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Center to Limb Variations in Solar Hard X-Ray Spectra
Authors: Datlowe, D. W.; Elcan, M. J.; Hudson, H. S.; Peterson, L. E.
1975BAAS....7..354D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Shape of the Hard Solar X-Ray Spectrum
Authors: Elcan, M. J.; Datlowe, D. W.; Hudson, H. S.; Peterson, L. E.
1975BAAS....7..354E Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Limb Brightening at Submillimeter wavelengths
Authors: Lindsey, C.; Hudson, H. S.
1975BAAS....7..360L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The peculiar object HD 44179 ("The red rectangle").
Authors: Cohen, M.; Anderson, C. M.; Cowley, A.; Coyne, G. V.; Fawley,
W.; Gull, T. R.; Harlan, E. A.; Herbig, G. H.; Holden, F.; Hudson,
H. S.; Jakoubek, R. O.; Johnson, H. M.; Merrill, K. M.; Schiffer,
F. H.; Soifer, B. T.; Zuckerman, B.
1975ApJ...196..179C Altcode:
A strong infrared source detected in the AFCRL sky survey is confirmed,
and is identified with the binary star HD 44179, embedded in a peculiar
nebula. UBVRI and broad-band photometry between 2.2 and 27 microns are
combined with blue, red, and near-infrared spectra, polarimetry and
spectrophotometry of the star, and a range of direct and image-tube
photographs of the nebula, to suggest a composite model of the
system. In this model, the infrared radiation derives from thermal
emission by dust grains contained in a disklike geometry about the
central object, which appears to be of spectral type B9-A0 III and which
may be in pre-main-sequence evolution. Two infrared emission features
are found, peaking at 8.7 and 11.3 microns, the latter corresponding
to the feature seen in the spectrum of the planetary nebula NGC
7027. The complex nebular structure is discussed on the basis of
photographs through narrow-band continuum and emission-line filters. The
polarization data support the suggestion of a disk containing some
large particles. No radio continuum emission is detected.

---------------------------------------------------------
Title: Relationship Between Hard and Soft Solar X-Ray Sources Observed
by OSO 7
Authors: Datlowe, D. W.; Hudson, H. S.
1975IAUS...68..209D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Nonthermal Processes in Large Solar Flares
Authors: Hudson, H. S.; Jones, T. W.; Lin, R. P.
1975IAUS...68..425H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The solar X-ray continuum below 10 keV
Authors: Hudson, H. S.
1975xris.conf..233H Altcode:
Information obtained from soft X-ray (below 10 keV) continuum and
broadband photometric observations of the sun is summarized. The
quasi-thermal origin of the soft X-ray continuum is discussed,
techniques are described for analyzing the spectrum of the continuum
radiation, and OSO-7 data is used to describe the characteristics of
flare-associated soft X-ray bursts. Results are reviewed for broadband
observations of solar flares, thermal emission parameters are determined
for the hot regions in flares, and models are considered for the
temporal evolution of soft X-ray bursts from hot flare regions. It is
shown that these bursts require the injection of additional material
into the corona during the growth phase and that conductive cooling
is the dominant cooling mechanism in hot flare regions. Soft X-ray
emission from active regions is discussed, and radiative losses are
shown to be the dominant cooling mechanism of these regions. It is
concluded that soft solar X-radiation represents the characteristic
emission of hot regions in the solar atmosphere.

---------------------------------------------------------
Title: Relationship between hard and soft X-ray sources observed
by OSO-7.
Authors: Datlowe, D. W.; Hudson, H. S.
1975IAUS...68R.209D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Relationship between hard and soft solar X-ray sources observed
by OSO-7.
Authors: Datlowe, D. W.; Hudson, H. S.
1975IAUS...68Q.209D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Submillimeter Observations of Planets
Authors: Hudson, H. S.; Lindsey, C. A.; Soifer, B. T.
1974Icar...23..374H Altcode:
A new program of ground-based observations at submillimeter (≈400 μm)
wavelengths has yielded observations of Mercury, Venus, Mars, Jupiter,
and Saturn. We report here observations near planetary conjunctions,
which have minimal corrections for atmospheric extinction: Mercury,
361±65 K; Venus, 231±35 K; and Saturn, 205±15 K (based upon the
area of the planetary disk), using Jupiter (150 K) and Mars (220 K) as
photometric standards. The Mercury observations show that the brightness
temperature does not decrease at the submillimeter wavelengths, relative
to observations at 3 mm; for Venus, however, the brightness temperature
appears appreciably lower than at millimeter wavelengths. The results
for Saturn indicate a strong and possibly optically thick contribution
from the rings. We also gave a description of our instrumentation and
observational techniques, with special emphasis upon the effect of
extinction by atmospheric water vapor.

---------------------------------------------------------
Title: OSO-7 observations of solar x-rays in the energy range 10
100 keV
Authors: Datlowe, D. W.; Elcan, M. J.; Hudson, H. S.
1974SoPh...39..155D Altcode:
The solar X-ray experiment on the satellite OSO-7 has provided
extensive observations of hard and soft X-ray bursts. We give a
general description of the hard X-ray data here, in parallel with
the description of the soft X-ray data already published (Datlowe
et al., 1974). The data for this study consist of 123 hard X-ray
bursts which occurred between 10 October 1971 and 6 June 1972. We
examine the behavior of a typical event in terms of its spectral and
flux variations. For the whole data sample, we find that 2/3 of the
soft X-ray bursts have detectable hard X-ray emission. We present the
distributions of frequency of occurrence of peak flux, spectral index,
collisional energy loss, burst duration and the duration at half maximum
of the flux profile. No correlation was found between the flux and the
spectral slope of an individual data sample, nor was there a correlation
between the peak flux and the full width at half maximum of a burst.

---------------------------------------------------------
Title: Submillimeter Observations of the Orion Nebula and NGC 2024
Authors: Soifer, B. T.; Hudson, H. S.
1974ApJ...191L..83S Altcode:
We report here new observations at submillimeter wavelengths ( 4O0 )
of the Orion Nebula and NGC 2024. These observations provide a fairly
detailed (FWHM 1 6) map of the Orion Nebula at these wavelengths. We
attribute the Orion Nebula emission to optically thin thermal radiation
from dust grains in the molecular cloud with peak emission centered on
the KL source. A comparison of our data with those of Harvey et a'. at
longer wavelengths shows that the emissivity must vary as . We find
evidence that the dust temperature varies with position, with the center
having the higher temperature. If optical constants determined from
lunar silicate material are used, then 0.05A).10 g of small particles
would be required to produce the observed submillimeter emission,
but this suggests a far larger mass ( 5 X 102 M0 in the central 1 6)
in the Orion molecular cloud than is normally assumed. Alternatively,
the data require an opacity at 100 greatly exceeding that of the lunar
silicates. Subject headings: infrared - Orion Nebula - photometry

---------------------------------------------------------
Title: Search of OSO-3 data for X-ray emission from stellar flares.
Authors: Tsikoudi, V.; Hudson, H.
1974BAAS....6..264T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Infrared Continuum Observations of 300-sec Oscillations
Authors: Hudson, H. S.
1974BAAS....6R.289H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Non-thermal Processes in Large Solar Flares
Authors: Lin, R. P.; Hudson, H. S.; Jones, T. W.
1974BAAS....6R.290L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Observations of solar X-ray bursts in the energy range 5 15 keV
Authors: Datlowe, D. W.; Hudson, H. S.; Peterson, L. E.
1974SoPh...35..193D Altcode:
Bursts of solar X-rays in the energy range 5-15 keV are associated
with flares and are due to thermal emission from a hot coronal
plasma. In this paper we present the results of the first study
of a large sample of separate bursts, 197 events associated with
subflares and a few importance 1 events. The observations were made
by a proportional counter on the satellite OSO-7 from October 1971
to June 1972. In most cases the temperature characterizing the X-ray
spectrum rises impulsively at the onset of the burst and then declines
slowly throughout the remainder of the burst. The emission measure rises
exponentially with a time scale of 30-100 s and then declines slowly, on
a time scale of the order of 103 s. From these observations
we show that the growth of the thermal energy in the flare plasma
throughout the burst can be due to the heating of new cool material.

---------------------------------------------------------
Title: Observations of Solar X-Ray Bursts in the Energy Range 5-15 keV
Authors: Datlowe, D.; Hudson, H.
1974BAAS....6..285D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Absence of Cosmic Gamma-Ray Bursts in Association with Normal
Stellar Flares
Authors: Hudson, H.; Tsikoudi, V.
1974tcgx.conf...93H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Direct Observation of Temperature Amplitude of Solar 300-SECOND
Oscillations
Authors: Hudson, H. S.; Lindsey, C. A.
1974ApJ...187L..35H Altcode:
The 300-second oscillations form the dominant source of variability
of the solar infrared continuum. We have observed them at 20 with
an amplitude AT, = 3.0 K over an area with an effective diameter of
33". This new mode of observation of the 300-s oscillations should
make possible a fundamental improvement in our knowledge of their
nature and origin. Subject headings: Infrared solar atmospheric motions

---------------------------------------------------------
Title: Absence of Cosmic γ-ray Bursts in Association with Normal
Stellar Flares
Authors: Hudson, H.; Tsikoudi, V.
1973NPhS..245...88H Altcode: 1973Natur.245...88H
The OSO-3 X-ray telescope provided extensive observations of cosmic
X-ray sources between March 1967 and June 1968 (refs 1 and 2). We report
here a search of OSO-3 data during times of known optical stellar
flares for γ-ray bursts similar to those reported by Klebesadel et
al.3 Table 1 shows the number of flares and the range of
observation times for each flare star.

---------------------------------------------------------
Title: Far Infrared Observations of Solar Flares
Authors: Hudson, H. S.
1973BAAS....5..274H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Nonthermal X-rays and Related Processes
Authors: Hudson, H. S.
1973NASSP.342..207H Altcode: 1973heps.conf..207H
No abstract at ADS

---------------------------------------------------------
Title: Search for Weak White-Light Flares by Time-Wise Photographic
Cancellation
Authors: Uchida, Yutaka; Hudson, Hugh
1972SoPh...26..414U Altcode:
This study proposes as a working hypothesis that small white-light
flares accompany all major (proton) flare events and suggests a new
method for systematically finding these `patches' of white-light
emission. The new technique consists of the time-wise application
of the photographic cancellation method to detect small time-varying
features around the time of the impulsive phase of a flare.

---------------------------------------------------------
Title: Infrared Emission from Solar Flares
Authors: Hudson, H. S.; Ohki, K.
1972BAAS....4R.385H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Thick-Target Processes and White-Light Flares
Authors: Hudson, H. S.
1972SoPh...24..414H Altcode:
Observations indicate that fast electrons in solar flares, which cause
the hard X-ray burst and the impulsive microwave burst, lose energy
predominantly by collisional processes. This requires a thick-target
theory of the emission, for which the electron spectrum inferred
from the X-ray spectrum becomes 1.5 powers steeper than in the usual
thin-target theory.

---------------------------------------------------------
Title: Long-Term Temporal Variations of the Hard X-Ray Flux from
the Centaurus Region
Authors: Schwartz, Daniel A.; Peterson, Laurence E.; Hudson, Hugh S.
1972ApJ...174..549S Altcode:
The University of California at San Diego (UCSD) X-ray telescope aboard
the third Orbiting Solar Observatory (OSO-IlI) observed the Centaurus
region daily from 1967 October to 1968 February, and also for five days
in 1968 June. For this period, we derive a stable minimum flux of 0.33
+ 0.03 photons (cm2 s) I between 7.7 and 38 keV from a persistent hard
X-ray source around 1 = 305 . Several single days show enhanced fluxes;
and two extensive flaring episodes, one with a soft and the other with
a very hard spectrum, last at least 10 days.

---------------------------------------------------------
Title: Upper Limits to the X-Ray Luminosities of Five Supernovae
Authors: Ulmer, M.; Grace, V.; Hudson, H. S.; Schwartz, D. A.
1972ApJ...173..205U Altcode:
We have examined data from the 050-Ill X-ray telescope for evidence of
X-ray emission from five optically detected extragalactic supernovae
during the period 1967 March-1968 June. Upper limits to the X-ray
emission in the range 7.7-113 keV near optical maximum (within 30 days)
fall in the range 10- - 10-10 ergo (cm2 s) 2 Reasonable estimates of
the distances to these supernovae lead to upper limits on the total
energies of from 1010 to 1051 ergs.

---------------------------------------------------------
Title: Soft X-Ray and Microwave Observations of Hot Regions in
Solar Flares
Authors: Hudson, H. S.; Ohki, K.
1972SoPh...23..155H Altcode:
Hot regions in solar flares produce X-radiation and microwaves by
thermal processes. Recent X-ray data make it possible to specify the
temperature and emission measure of the soft X-ray source, by using,
for instance, a combination of the 1-8 Å (peak response at about
2 keV) and the 0.5-3 Å (peak response at about 5 keV) broad-band
photometers. The temperatures and emission measures thus derived
satisfactorily explain the radio fluxes, within systematic errors of
about a factor of 3. Comparison of 15 events with differing parameters
shows that a hot solar flare region has an approximately isothermal
temperature distribution. The time evolution of the correlation in a
single event shows that the hot material originates in the chromosphere,
rather than the corona. The density must lie between 1010 and
2 × 1011 cm−3. For an Importance 1 flare, this
implies a stored energy of roughly 2 x 1030-1029
ergs. A refinement of the data will enable us to choose between
conductive and radiative cooling models.

---------------------------------------------------------
Title: Non-thermal electron population in solar flares.
Authors: Hudson, H. S.
1972eucr.conf..127H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Upper limits to the X-ray luminosity from five supernovae.
Authors: Ulmer, M.; Grace, V.; Hudson, H.; Schwartz, D.
1971PASP...83..608U Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Type III Solar Radio Bursts Accompanied by Soft X-Radiation
in the Absence of Hα Flares
Authors: Teske, Richard G.; Soyumer, Tevfik; Hudson, Hugh S.
1971ApJ...165..615T Altcode:
Several Type iii solar radio bursts which occurred in the absence of
reportable flares were observed to be accompanied by weak X-radiation
detectable at photon energies of between 1.0 and 12.5 keV. The X-rays
almost invariably preceded the Type iii burst, an observation which
suggests that a thermal event precedes the Type iii instability.

---------------------------------------------------------
Title: 10 100 keV electron acceleration and emission from solar flares
Authors: Lin, R. P.; Hudson, H. S.
1971SoPh...17..412L Altcode:
We present an analysis of spacecraft observations of non-thermal
X-rays and escaping electrons for 5 selected small solar flares in
1967. OSO-3 multi-channel energetic X-ray measurements during the
non-thermal component of the solar flare X-ray bursts are used to
derive the parent electron spectrum and emission measure. IMP-4 and
Explorer-35 observations of &gt; 22 keV and &gt; 45 keV electrons in the
interplanetary medium after the flares provide a measure of the total
number and spectrum of the escaping particles. The ratio of electron
energy loss due to collisions with the ambient solar flare gas to the
energy loss due to bremsstrahlung is derived. The total energy loss
due to collisions is then computed from the integrated bremsstrahlung
energy loss during the non-thermal X-ray burst. For &gt; 22 keV flare
electrons the total energy loss due to collisions is found to be ∼
104 times greater than the bremsstrahlung energy loss and
∼ 102 times greater than the energy loss due to escaping
electrons. Therefore the escape of electrons into the interplanetary
medium is a negligible energetic electron loss mechanism and cannot
be a substantial factor in the observed decay of the non-thermal X-ray
burst for these solar flares.

---------------------------------------------------------
Title: Observations of Unresolved Galactic X-ray Sources
Authors: Hudson, H. S.; Peterson, L. E.; Schwartz, D. A.
1971Natur.230..177H Altcode:
WE present in this article new data obtained by the OSO III
satellite1 about an apparently diffuse X-ray emission from
the galactic plane. Previous observations2 have established
the existence of this component at a few keV and at about 100 MeV
(ref. 3), but the observations reported here cover the energy range
7.7-115 keV. Theoretical speculation after the first observations has
suggested several emission mechanisms-Compton scattering by cosmic
ray electrons of far-infrared radiation4-7 or of visible
photons8 and the presence of numerous weak discrete sources
(Setti, G., and Woltjer, L., personal communication, and Ryter, C.,
personal communication). Because the new data cover an energy range
between the two earlier sets of observations at a few keV and 100 MeV,
they impose important new constraints on the theories.

---------------------------------------------------------
Title: The Spectrum of Diffuse Cosmic X-Rays 7.7-113 keV
Authors: Schwartz, Daniel A.; Hudson, Hugh S.; Peterson, Laurence E.
1970ApJ...162..431S Altcode:
An X-ray telescope aboard the OSO-III satellite provided data from
which we derive the intensity of diffuse cosmic X-rays in five energy
bands between 77 and 113 keV. A ngle power law cannot represent
our data in this energy range The 1 year of satellite operation has
allowed a determination of systematic effects, so that the four lowest
energy points have an accuracy of about 3 percent They fit a powemlaw
spectrum of index 1.7 + 0.1 which "breaks" at 42 I 4 keV to become 3.0
I 0.3. The normalization constant gives an intensity of 15.2 photons
(cm2 sec sterad keV)-' at 1 keV. A more complicated spectral shape
suggested by Brecher and Morrison fits the present data very well and
provides a better extrapolation to data at keV and a few MeV than does
the power law.

---------------------------------------------------------
Title: Re-Interpretation of OSO-III Scintillation Counter Measurements
of Hard Solar X-Ray Spectra
Authors: Kane, S. R.; Hudson, H. S.
1970SoPh...14..414K Altcode:
Laboratory tests show that pulse pile-up requires a significant
correction to the response of scintillation counters to solar X-ray
bursts with steep energy spectra. The interpretation of the OSO-III
and other satellite data in view of these corrections indicates that
steep solar X-ray spectra with varying emission measure produce an
effect similar to that produced by relatively hard spectra with a
constant emission measure.

---------------------------------------------------------
Title: The Time Variability of X-Ray Emission from Sco X-1
Authors: Hudson, H. S.; Pelling, R. M.; Peterson, L. E.; Schwartz,
D. A.
1970BAAS....2T.200H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Simultaneous X-Ray and Optical Observations of SCO X-1 Flares
Authors: Hudson, Hugh S.; Peterson, Laurence E.; Schwartz, Daniel A.
1970ApJ...159L..51H Altcode:
The X-ray telescope on the Orbiting Solar Observatory III has made
extensive observations of Sco X-1 during 1967 May and June. Two
occasions of X-ray flaring, at o7h26 and O9ht(y U.T. on 1967 June 3,
happened to coindde with optical observations of the same phenomena,
thus proving the connection between optical and X-ray flares on Sco
X-1. The X-ray enhancements amounted to about a factor of 2 over the
quiescent emission in the energy range 7.7-12.S keV.

---------------------------------------------------------
Title: De-Occultation X-Ray Events of 2 December, 1967
Authors: Zirin, Harold; Ingham, William; Hudson, Hugh; McKenzie, David
1969SoPh....9..269Z Altcode:
A flare rising from behind the solar limb was recorded simultaneously
by the UCSD X-ray detector on OSO-III (7.7-200 keV) and the Caltech
photoheliograph on Robinson Laboratory roof (Hα). The de-occultation
gives excellent spatial resolution of the X-ray source. Spectra
suggest that the material was already heated to 27 000 000° and that
the increase in flux was due to the de-occultation. The flux rise to
maximum was proportional to the apparent area. The uniformity of this
rise shows that there was no special kernel of emission. Comparison
of the deduced volume with the bremsstrahlung formula gives a density
of about 1010 for the 27 000 000° component of the flare;
this is confirmed by consideration of the maximum possible coulomb
braking. The actual decay is more likely by escape rather than coulomb
braking.

---------------------------------------------------------
Title: The Hard Solar X-Ray Spectrum Observed from the Third Orbiting
Solar Observatory
Authors: Hudson, Hugh S.; Peterson, Laurence E.; Schwartz, Daniel A.
1969ApJ...157..389H Altcode:
The hard solar X-ray scintillation-counter telescope on the
OSO-Ill satellite covers the energy range 7.7-210 keV with
15-sec time resolution, and six logarithmically spaced energy
channels. Approximately 55 per cent time coverage has been obtained
for the period following the date of launching, March 8, 1967,
until failure of the on-board tape recorder June 28, 1968 This paper
is based mainly on solar X-ray events observed during the first two
weeks of data accumulation. Approximately ten bursts per day were de-
tected during the interval March 9-March 23, 1967, above the threshold
sensitivity of 10-8 erg (cm2 sec)-1 for 7.7 «= liv «= 12.5 keY. About
once per day a burst of peak energy flux greater than 1.6 X 1O~ erg
(cm2 sec)' was observed. Although many variations were observed, the
typical event had an e-folding rise time of 86 sec and a decay time
of 458 sec. The bursts occurred in correlation with almost all listed
flares and subflares (88 per cent), microwave bursts (92 per cent), and
SID's (100 per cent). Numerous bursts were also detected without these
accompanying phenomena. The correlation with type III radio bursts,
although still positive, is not as good (31 per cent), a fact which
suggests that coronal disturbances are not an inevitable consequence
of the process which produces X-ray emission. The X-ray spectrum is
appreciably non-thermal in the initial phase of the burst and thermal
in the decay phase, with an effective temperature often exceeding 50 X
106 °K. The average peak temperature of subfiares exceeds 10 X 106°
K, while that of importance 1 or greater exceeds 14 X 1060 K. The
emission measure flefliV has a constant value of about 1.4 X 1O~ cm3
both during an X-ray burst and from burst to burst. The solar X-ray
bursts therefore differ mainly in the maximum temperature attained. A
less detailed examination of later data shows that the phenomena we
describe are not peculiar characteristics of a single active re- gion

---------------------------------------------------------
Title: De-occultation x-Ray Event of 2 December 1967
Authors: Hudson, Hugh; McKenzie, David; Zirin, Harold; Ingham, William
1969BAAS....1T.280H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Simultaneous x-Ray and Electron Emission from the Sun
Authors: Hudson, H. S.; Lin, R. P.
1969BAAS....1S.280H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar and Cosmic X-Rays above 7.7 keV
Authors: Hudson, Hugh S.; Peterson, Laurence E.; Schwartz, Daniel A.
1969SoPh....6..205H Altcode:
Since its launch on March 8, 1967, the OSO-III has continuously
observed solar and cosmic X-rays over the 7.7-210 keV range. The
sun emits many impulsive X-ray bursts having fluxes several orders
of magnitude above the background level of 8 × 10−9
ergs(cm2-sec)−1 at 7.7 keV and characteristic
times on the order of 5 min. Ninety-five such events having fluxes
&gt;3 × 10−5 ergs(cm2-sec)−1 were
detected in the period from March 8 to June 15, 1967. The cosmic X-ray
source Lupus XR-1 has been observed to have a power law spectral form
and no significant time variations over a 40-day period. Upper limits
have been obtained on the hard X-ray flux of the peculiar galaxy M 87.

---------------------------------------------------------
Title: The time structure of solar X-ray bursts above 7.7 keV
Authors: Hudson, H. S.; Peterson, L. E.; Schwartz, D. A.
1969sfsr.conf..113H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hard Solar X-Ray Bursts Observed by OSO-III.
Authors: Hudson, H. S.; Schwartz, D. A.; Peterson, L. E.
1968AJS....73R..64H Altcode:
OSO-III has observed the solar x-ray spectrum above 7.7 keV since 9
March 1967. Solar x-ray bursts are very frequent (98 were observed
in the first week of operation of the satellite) and are partially
correlated with flares and radio-wave emission. The occurrence of
solar x-ray bursts is well correlated with the occurrence of 2800 MHz
solar radio bursts, but the detailed time structures of the events
are usually different. 65% of the x-ray bursts coincide with flares;
on the other hand, about 85% of flares of all importances produce
detectable x-ray bursts. The peak x-ray flux for a correlated burst
does not depend in a simple way on flare area or brightness. Most solar
x-ray bursts exhibit a simple fast-rise, slow-decay time structure;
the median rise time is about 65 sec. and the median fall time is
about 340 sec. The rise and fall times are widely variable, and
the peak energy flux ranges from 10-8 erg (cm2sec)-1 to more than
10-~ erg (cm2 sec)-1 above 7.7 keV. Assuming the X-ray production
mechanism to be free-free transitions, the OSO data establish that
the onset phase of the x-ray burst is due to a nonthermal electron
distribution. The decaying phase of the burst is essentially thermal in
nature. These conclusions are drawn from the strong dependence of the
x-ray spectrum (or the effective temperature of the emitting region)
on the magnitude of the x-ray flux. The thermal bremsstrahlung model
allows a determination of neN, where N is the number of electrons
radiating and ne the electron density. The distribution of values of
this quantity for different bursts is strongly peaked at about 1.4X
10~~ cm-3. It is a striking fact that this value obtains for the entire
observed range of sizes of solar x-ray bursts.

---------------------------------------------------------
Title: Observations of the Isotropic Component of Cosmic X Rays at
Balloon Altitude
Authors: Hudson, H. S.; Prinbsch, J. H.; Anderson, K. A.
1966JGR....71.5665H Altcode:
The isotropic component of cosmic X-ray flux between 20 and 70 kev
has been determined from measurements made with balloon-borne X-ray
telescopes. In this energy range the spectrum is characterized
by γ = 2.7+1.7-0.6 in the distribution
N(hv) = k(hv)-γ. The measured flux at 40 kev was
0.014-0.003+0.008 photons (cm2
sec ster kev)-1. The observations by Ghielmetti, Becerra,
Godel, Heredia, and Roederer of an X-ray influx over the South Atlantic
geomagnetic anomaly can probably be explained by the cosmic primary
X rays.

---------------------------------------------------------
Title: Energy spectrum of auroral zone electron precipitation
Authors: Hudson, Hugh S.
1966PhDT.......116H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Determinations of the Auroral-Zone X-Ray Spectrum
Authors: Hudson, H. S.; Parks, G. K.; Milton, D. W.; Anderson, K. A.
1965JGR....70.4979H Altcode:
A portion of the bremsstrahlung spectrum of electrons precipitating
into the auroral-zone atmosphere is accessible to precise spectrum
measurements from balloon-borne scintillation detectors. Such
measurements were made near Flin Flon, Manitoba, Canada (geomagnetic
latitude 64°N), during local daytime on a magnetically disturbed day
(September 8, 1964; Ap = 23). The design of the detector embodied
several features that made possible a reliable extrapolation from
the spectrum observed at balloon altitude to the true spectrum at the
stopping level of the electrons: 1. Measurements were made at 3.3 g
cm-2 atmospheric depth, whereas the Compton interaction
length is greater than 5.8 g cm-2. This served to minimize
the contribution of photons that had been Compton-scattered and
hence degraded in energy. That scattered photons did not contribute
appreciably to the counting rates was directly confirmed by directional
measurements made from the same balloon payload [Parks et al.,
1965]. Absorption of the bremsstrahlung through the photoelectric
process increases steeply toward lower energies; this effectively set
the lower limit of the measurements at about 15 kev.

---------------------------------------------------------
Title: Spatial Asymmetry and Periodic Time Variations of X-Ray
Microbursts in the Auroral Zone
Authors: Parks, G. K.; Hudson, H. S.; Milton, D. W.; Anderson, K. A.
1965JGR....70.4976P Altcode:
Describing the properties of short time constant impulsive electron
precipitation in the auroral zone, Anderson and Milton [1964] found
that the time interval between microbursts was nonrandom although power
spectrum analysis revealed only a weak tendency toward periodicity at
0.6 second. It was thought that intrinsically this phenomenon might
exhibit stronger periodicities but that they might not have appeared,
owing to limitations in the experimental methods. A suspected difficulty
was the use of omnidirectional detectors. Thus if several independent
source regions were present differing in periodic behavior the peaks
in the power spectrum could be washed out.