Author name code: warren
ADS astronomy entries on 2022-09-14
author:"Warren, Harry P."
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Title: A Publicly Available Multiobservatory Data Set of an Enhanced
Network Patch from the Photosphere to the Corona
Authors: Kobelski, Adam R.; Tarr, Lucas A.; Jaeggli, Sarah A.; Luber,
Nicholas; Warren, Harry P.; Savage, Sabrina
Bibcode: 2022ApJS..261...15K
Altcode: 2022arXiv220501766K
New instruments sensitive to chromospheric radiation at X-ray, UV,
visible, IR, and submillimeter wavelengths have become available that
significantly enhance our ability to understand the bidirectional
flow of energy through the chromosphere. We describe the calibration,
coalignment, initial results, and public release of a new data set
combining a large number of these instruments to obtain multiwavelength
photospheric, chromospheric, and coronal observations capable of
improving our understanding of the connectivity between the photosphere
and the corona via transient brightenings and wave signatures. The
observations center on a bipolar region of enhanced-network
magnetic flux near disk center on SOL2017-03-17T14:00-17:00. The
comprehensive data set provides one of the most complete views to
date of chromospheric activity related to small-scale brightenings
in the corona and chromosphere. Our initial analysis shows a strong
spatial correspondence between the areas of broadest width of the
hydrogen-α spectral line and the hottest temperatures observed in
Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 radio
data, with a linear coefficient of 6.12 × 10-5Å/K. The
correspondence persists for the duration of cotemporal observations
(≍60 m). Numerous transient brightenings were observed in multiple
data series. We highlight a single, well-observed transient brightening
in a set of thin filamentary features with a duration of 20 minutes. The
timing of the peak intensity transitions from the cooler (ALMA, 7000 K)
to the hotter (XRT, 3 MK) data series.
Title: Parallel Plasma Loops and the Energization of the Solar Corona
Authors: Peter, Hardi; Chitta, Lakshmi Pradeep; Chen, Feng; Pontin,
David I.; Winebarger, Amy R.; Golub, Leon; Savage, Sabrina L.;
Rachmeler, Laurel A.; Kobayashi, Ken; Brooks, David H.; Cirtain,
Jonathan W.; De Pontieu, Bart; McKenzie, David E.; Morton, Richard J.;
Testa, Paola; Tiwari, Sanjiv K.; Walsh, Robert W.; Warren, Harry P.
Bibcode: 2022ApJ...933..153P
Altcode: 2022arXiv220515919P
The outer atmosphere of the Sun is composed of plasma heated to
temperatures well in excess of the visible surface. We investigate
short cool and warm (<1 MK) loops seen in the core of an active
region to address the role of field-line braiding in energizing these
structures. We report observations from the High-resolution Coronal
imager (Hi-C) that have been acquired in a coordinated campaign with
the Interface Region Imaging Spectrograph (IRIS). In the core of the
active region, the 172 Å band of Hi-C and the 1400 Å channel of IRIS
show plasma loops at different temperatures that run in parallel. There
is a small but detectable spatial offset of less than 1″ between
the loops seen in the two bands. Most importantly, we do not see
observational signatures that these loops might be twisted around each
other. Considering the scenario of magnetic braiding, our observations
of parallel loops imply that the stresses put into the magnetic field
have to relax while the braiding is applied: the magnetic field never
reaches a highly braided state on these length scales comparable to
the separation of the loops. This supports recent numerical 3D models
of loop braiding in which the effective dissipation is sufficiently
large that it keeps the magnetic field from getting highly twisted
within a loop.
Title: Geometric Assumptions in Hydrodynamic Modeling of Coronal
and Flaring Loops
Authors: Reep, Jeffrey W.; Ugarte-Urra, Ignacio; Warren, Harry P.;
Barnes, Will T.
Bibcode: 2022ApJ...933..106R
Altcode: 2022arXiv220304385R
In coronal loop modeling, it is commonly assumed that the loops
are semicircular with a uniform cross-sectional area. However,
observed loops are rarely semicircular, and extrapolations of the
magnetic field show that the field strength decreases with height,
implying that the cross-sectional area expands with height. We examine
these two assumptions directly, to understand how they affect the
hydrodynamic and radiative response of short, hot loops to strong,
impulsive electron beam heating events. Both the magnitude and rate
of area expansion impact the dynamics directly, and an expanding cross
section significantly lengthens the time for a loop to cool and drain,
increases upflow durations, and suppresses sound waves. The standard
T ~ n 2 relation for radiative cooling does not hold
with expanding loops, which cool with relatively little draining. An
increase in the eccentricity of loops, on the other hand, only increases
the draining timescale, and is a minor effect in general. Spectral
line intensities are also strongly impacted by the variation in the
cross-sectional area because they depend on both the volume of the
emitting region as well as the density and ionization state. With a
larger expansion, the density is reduced, so the lines at all heights
are relatively reduced in intensity, and because of the increase of
cooling times, the hottest lines remain bright for significantly
longer. Area expansion is critical to accurate modeling of the
hydrodynamics and radiation, and observations are needed to constrain
the magnitude, rate, and location of the expansion-or lack thereof.
Title: Constraining Global Coronal Models with Multiple Independent
Observables
Authors: Badman, Samuel T.; Brooks, David H.; Poirier, Nicolas;
Warren, Harry P.; Petrie, Gordon; Rouillard, Alexis P.; Nick Arge,
C.; Bale, Stuart D.; de Pablos Agüero, Diego; Harra, Louise; Jones,
Shaela I.; Kouloumvakos, Athanasios; Riley, Pete; Panasenco, Olga;
Velli, Marco; Wallace, Samantha
Bibcode: 2022ApJ...932..135B
Altcode: 2022arXiv220111818B
Global coronal models seek to produce an accurate physical
representation of the Sun's atmosphere that can be used, for example, to
drive space-weather models. Assessing their accuracy is a complex task,
and there are multiple observational pathways to provide constraints
and tune model parameters. Here, we combine several such independent
constraints, defining a model-agnostic framework for standardized
comparison. We require models to predict the distribution of coronal
holes at the photosphere, and neutral line topology at the model's outer
boundary. We compare these predictions to extreme-ultraviolet (EUV)
observations of coronal hole locations, white-light Carrington maps of
the streamer belt, and the magnetic sector structure measured in situ
by Parker Solar Probe and 1 au spacecraft. We study these metrics for
potential field source surface (PFSS) models as a function of source
surface height and magnetogram choice, as well as comparing to the more
physical Wang-Sheeley-Arge (WSA) and the Magnetohydrodynamic Algorithm
outside a Sphere (MAS) models. We find that simultaneous optimization
of PFSS models to all three metrics is not currently possible, implying
a trade-off between the quality of representation of coronal holes
and streamer belt topology. WSA and MAS results show the additional
physics that they include address this by flattening the streamer belt
while maintaining coronal hole sizes, with MAS also improving coronal
hole representation relative to WSA. We conclude that this framework
is highly useful for inter- and intra-model comparisons. Integral to
the framework is the standardization of observables required of each
model, evaluating different model aspects.
Title: Detection of Stellar-like Abundance Anomalies in the Slow
Solar Wind
Authors: Brooks, David H.; Baker, Deborah; van Driel-Gesztelyi, Lidia;
Warren, Harry P.; Yardley, Stephanie L.
Bibcode: 2022ApJ...930L..10B
Altcode: 2022arXiv220409332B
The elemental composition of the Sun's hot atmosphere, the corona,
shows a distinctive pattern that is different from the underlying
surface or photosphere. Elements that are easy to ionize in the
chromosphere are enhanced in abundance in the corona compared to
their photospheric values. A similar pattern of behavior is often
observed in the slow-speed (<500 km s-1) solar wind
and in solar-like stellar coronae, while a reversed effect is seen
in M dwarfs. Studies of the inverse effect have been hampered in the
past because only unresolved (point-source) spectroscopic data were
available for these stellar targets. Here we report the discovery of
several inverse events observed in situ in the slow solar wind using
particle-counting techniques. These very rare events all occur during
periods of high solar activity that mimic conditions more widespread
on M dwarfs. The detections allow a new way of connecting the slow
wind to its solar source and are broadly consistent with theoretical
models of abundance variations due to chromospheric fast-mode waves
with amplitudes of 8-10 km s-1, sufficient to accelerate
the solar wind. The results imply that M-dwarf winds are dominated
by plasma depleted in easily ionized elements and lend credence to
previous spectroscopic measurements.
Title: Solar Flare Irradiance: Observations and Physical Modeling
Authors: Reep, Jeffrey W.; Siskind, David E.; Warren, Harry P.
Bibcode: 2022ApJ...927..103R
Altcode: 2021arXiv211006310R
We examine Solar Dynamics Observatory (SDO)/EUV Variability Experiment
(EVE) data to better understand solar flare irradiance, and how that
irradiance may vary for large events. We measure scaling laws relating
Geostationary Orbital Environmental Satellites (GOES) flare classes
to irradiance in 21 lines measured with SDO/EVE, formed across a
wide range of temperatures, and find that this scaling depends on the
line-formation temperature. We extrapolate these irradiance values to
large events, exceeding X10. In order to create full spectra, however,
we need a physical model of the irradiance. We present the first
results of a new physical model of solar flare irradiance, NRLFLARE,
that sums together a series of flare loops to calculate the spectral
irradiance ranging from the X-rays through the far-UV (≍0 to 1250
Å), constrained only by GOES/X-ray Sensors observations. We test
this model against SDO/EVE data. The model spectra and time evolution
compares well in high-temperature emission, but cooler lines show
large discrepancies. We speculate that the discrepancies are likely
due to both a nonuniform cross-section of the flaring loops as well as
opacity effects. We then show that allowing the cross-sectional area to
vary with height significantly improves agreement with observations,
and is therefore a crucial parameter needed to accurately model the
intensity of spectral lines, particularly in the transition region
from $4.7\lesssim \mathrm{log}T\lesssim 6$ .
Title: The EUV High-Throughput Spectroscopic Telescope (EUVST)
Authors: Warren, Harry
Bibcode: 2021AGUFMSH51A..07W
Altcode:
The EUV High-Throughput Spectroscopic Telescope (Solar-C/EUVST) is an
international mission to understand the origins of solar activity by
observing fundamental physical processes in the solar atmosphere. EUVST
is a next generation spectrometer and slit-jaw imaging system that will
provide seamless spectroscopic coverage of the chromosphere, transition
region, corona, and flare plasma at very high temporal resolution
and high spatial resolution (0.4 arcsec or 300 km). This project is
led by Japan with contributions from the United States and European
partners. Launch is currently scheduled for late 2026. Understanding
the release of energy during solar flares is one of the EUVST science
objectives. EUVST observations of flare ribbons will achieve cadences
below 500 ms. We use hydrodynamic simulations to show that observations
of chromospheric and transition region emission lines at these time
scales will be able to differentiate among different energy transport
mechanisms.
Title: NRLFLARE: A physical model of solar flare irradiance
Authors: Reep, Jeffrey; Siskind, David; Warren, Harry
Bibcode: 2021AGUFMSH43A..07R
Altcode:
We introduce the NRLFLARE model, a physically-derived model of solar
flare irradiance. The model constrains energy release and volume of
a flare using soft X-ray observations, with which it drives a series
of hydrodynamic simulations to construct a flare arcade. From these
simulations, we have synthesized the irradiance from the chromosphere
through the corona, from the X-rays through near ultraviolet, at high
cadence and spectral resolution. To test the model, we compare to
irradiance observations of X-, M-, and C-class flares measured with
the Extreme Ultraviolet Variability Experiment (EVE) onboard the
Solar Dynamics Observatory (SDO), as well as to the predictions of
the empirical FISM2 model. We find good agreement in spectral lines
formed at high temperatures (> few x 106 K), but find the model
significantly over-estimates intensities of transition region lines
(105 K < T < 106 K). Finally, we scale the heating rates and
volumes up to extrapolate to flares exceeding X50 in class to predict
spectra for such unobserved flare scales.
Title: Observations and Modeling of Long, Cool, and Overdense Loops
in Active Region 11575
Authors: Barnes, Will; Warren, Harry
Bibcode: 2021AGUFMSH15E2066B
Altcode:
Long coronal loops at the periphery of active regions have been
observed to be steady over intervals greater than a radiative
cooling time, overdense, near-isothermal at approximately 1.5 MK,
and have flat filter ratios. These relatively steady, high-density
structures cannot be explained by either hydrostatic equilibrium or
simple post-nanoflare radiative cooling and thus pose a challenge
to current models of quiescent active region heating. To address
these ambiguities, we analyze observations of active region 11575 as
observed on 29 September 2012 by the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO) and the Extreme
Ultraviolet Imaging Spectrometer (EIS) onboard the Hinode spacecraft. We
manually isolate a single long loop near the periphery of the active
region in SDO/AIA 171 A and calculate the density, emission measure
distribution, and filter ratio. Additionally, we analyze the time
variability of this structure in the EUV channels of AIA over a 12
h interval and compute cross-correlations between these channels. We
then model the hydrodynamic evolution of this loop structure using the
field-aligned Hydrodynamics and Radiation (HYDRAD) model for several
different heating scenarios, including steady uniform heating as well as
steady and time-dependent stratified footpoint heating. From our model
results, we derive density and temperature diagnostics, emission measure
distributions, and cross-correlations between synthetic SDO/AIA light
curves in order to compare with our observations and thus constrain
the parameter space of feasible heating models. While stratified,
fully-asymmetric footpoint heating greatly increases the density of
a 1.5 MK loop over hydrostatic equilibrium, we find that the modeled
densities for all heating scenarios are significantly lower than those
we derive from the EIS observations. Furthermore, we find that impulsive
heating as well as thermal non-equilibrium, as induced by symmetric
stratified footpoint heating, lead to emission measure distributions
that are much broader than the observed distributions.
Title: Preliminary Results from the Marshall Grazing Incidence X-ray
Spectrometer (MaGIXS)
Authors: Winebarger, Amy; Savage, Sabrina; Kobayashi, Ken; Champey,
Patrick; Golub, Leon; Walsh, Robert; Athiray, P. S.; Bradshaw, Stephen;
Cheimets, Peter; Cirtain, Jonathan; DeLuca, Edward; Del Zanna, Giulio;
Mason, Helen; McKenzie, David; Ramsey, Brian; Reeves, Katharine;
Testa, Paola; Vigil, Genevieve; Warren, Harry
Bibcode: 2021AGUFMSH51A..06W
Altcode:
Coronal heating mechanisms are notoriously difficult to constrain with
current observations. We present new observations from an instrument
designed to measure a critical diagnostic of the frequency heating
events in active regions. The Marshall Grazing Incidence X-ray
Spectrometer (MaGIXS) is a sounding rocket mission that aims to
observe the soft x-ray solar spectrum (0.6 2.5 nm) with both spatial
and spectral resolution. This wavelength range has several high
temperature and abundance diagnostics that can be used to infer the
coronal heating frequency. MaGIXS will observe the Sun through a 12
x 33 slot, producing ``overlappograms, where the spatial and spectral
information are overlapped and must be unfolded. In this presentation,
I will report on the MaGIXS launch and data collection and provide
preliminary analysis of MaGIXS observations.
Title: A Multicomponent Magnetic Proxy for Solar Activity
Authors: Warren, Harry P.; Floyd, Linton E.; Upton, Lisa A.
Bibcode: 2021SpWea..1902860W
Altcode:
We present a new, multicomponent magnetic proxy for solar
activity derived from full disk magnetograms that can be used in
the specification and forecasting of the Sun's radiative output. To
compute this proxy we project Carrington maps, such as the synchronic
Carrington maps computed with the Advective Flux Transport (AFT)
surface flux transport model, to heliographic cartesian coordinates and
determine the total unsigned flux as a function of absolute magnetic
flux density. Performing this calculation for each day produces
an array of time series, one for each flux density interval. Since
many of these time series are strongly correlated, we use principal
component analysis to reduce them to a smaller number of uncorrelated
time series. We show that the first few principal components accurately
reproduce widely used proxies for solar activity, such the the 10.7
cm radio flux and the Mg core-to-wing ratio. This suggests that these
magnetic time series can be used as a proxy for irradiance variability
for emission formed over a wide range of temperatures.
Title: The Formation and Lifetime of Outflows in a Solar Active Region
Authors: Brooks, David H.; Harra, Louise; Bale, Stuart D.; Barczynski,
Krzysztof; Mandrini, Cristina; Polito, Vanessa; Warren, Harry P.
Bibcode: 2021ApJ...917...25B
Altcode: 2021arXiv210603318B
Active regions are thought to be one contributor to the slow solar
wind. Upflows in EUV coronal spectral lines are routinely observed at
their boundaries, and provide the most direct way for upflowing material
to escape into the heliosphere. The mechanisms that form and drive these
upflows, however, remain to be fully characterized. It is unclear how
quickly they form, or how long they exist during their lifetimes. They
could be initiated low in the atmosphere during magnetic flux emergence,
or as a response to processes occurring high in the corona when the
active region is fully developed. On 2019 March 31 a simple bipolar
active region (AR 12737) emerged and upflows developed on each side. We
used observations from Hinode, SDO, IRIS, and Parker Solar Probe (PSP)
to investigate the formation and development of the upflows from the
eastern side. We used the spectroscopic data to detect the upflow,
and then used the imaging data to try to trace its signature back to
earlier in the active region emergence phase. We find that the upflow
forms quickly, low down in the atmosphere, and that its initiation
appears associated with a small field-opening eruption and the onset
of a radio noise storm detected by PSP. We also confirmed that the
upflows existed for the vast majority of the time the active region
was observed. These results suggest that the contribution to the solar
wind occurs even when the region is small, and continues for most of
its lifetime.
Title: Measurements of Coronal Magnetic Field Strengths in Solar
Active Region Loops
Authors: Brooks, David H.; Warren, Harry P.; Landi, Enrico
Bibcode: 2021ApJ...915L..24B
Altcode: 2021arXiv210610884B
The characteristic electron densities, temperatures, and thermal
distributions of 1 MK active region loops are now fairly well
established, but their coronal magnetic field strengths remain
undetermined. Here we present measurements from a sample of coronal
loops observed by the Extreme-ultraviolet Imaging Spectrometer on
Hinode. We use a recently developed diagnostic technique that involves
atomic radiation modeling of the contribution of a magnetically
induced transition to the Fe X 257.262 Å spectral line intensity. We
find coronal magnetic field strengths in the range of 60-150 G. We
discuss some aspects of these new results in the context of previous
measurements using different spectropolarimetric techniques, and their
influence on the derived Alfvén speeds and plasma β in coronal loops.
Title: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS)
Authors: Caspi, A.; Shih, A. Y.; Panchapakesan, S.; Warren, H. P.;
Woods, T. N.; Cheung, M.; DeForest, C. E.; Klimchuk, J. A.; Laurent,
G. T.; Mason, J. P.; Palo, S. E.; Seaton, D. B.; Steslicki, M.;
Gburek, S.; Sylwester, J.; Mrozek, T.; Kowaliński, M.; Schattenburg,
M.; The CubIXSS Team
Bibcode: 2021AAS...23821609C
Altcode:
The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) is a 6U
CubeSat proposed to NASA H-FORT. CubIXSS is motivated by a compelling
overarching science question: what are the origins of hot plasma in
solar flares and active regions? Elemental abundances are a unique
diagnostic of how mass and energy flow into and within the corona,
and CubIXSS addresses its science question through sensitive, precise
measurements of abundances of key trace ion species, whose spectral
signatures reveal the chromospheric or coronal origins of heated plasma
across the entire temperature range from ~1 to >30 MK. CubIXSS
measurements of the coronal temperature distribution and elemental
abundances directly address longstanding inconsistencies from prior
studies using instruments with limited, differing temperature and
composition sensitivities. CubIXSS comprises two co-optimized
and cross-calibrated instruments that fill a critical observational
gap: MOXSI, a novel diffractive spectral imager using a pinhole
camera and X-ray transmission diffraction grating for spectroscopy of
flares and active regions from 1 to 55 Å, with spectral and spatial
resolutions of 0.28-0.37 Å and 29-39 arcsec FWHM, respectively;
and SASS, a suite of four spatially-integrated off-the-shelf
spectrometers for high-cadence, high-sensitivity X-ray spectra from
0.5 to 50 keV, with spectral resolution of 0.06-0.5 keV FWHM across
that range. If selected for implementation, CubIXSS will launch
in late 2023 to mid-2024 to observe intense solar flares and active
regions during the rising phase and peak of the solar cycle. Its 1-year
prime mission is well timed with perihelia of Parker Solar Probe and
Solar Orbiter, and with the launches of complementary missions such
as the PUNCH Small Explorer. CubIXSS is a pathfinder for the next
generation of Explorer-class missions with improved capabilities for
SXR imaging spectroscopy. We present the CubIXSS motivating science
background, its suite of instruments and expected performances, and
other highlights from the completed Concept Study Report, including
novel analysis techniques to fully exploit the rich data set of CubIXSS
spectral observations.
Title: Understanding Heating Properties in Hot and Warm Active Region
Loops through Hydrodynamics and Forward Modeling
Authors: Barnes, W.; Warren, H.; Reep, J. W.
Bibcode: 2020AGUFMSH0370003B
Altcode:
While it is generally agreed that the energy to power the
multi-million-degree corona is contained in the complex solar
magnetic field, the processes behind how this energy is transferred
from the stressed magnetic field to the coronal plasma remain
poorly understood. Active region observations from a number of solar
observatories have shown that short, compact loops near the center of
the active region are "hot," sometimes exceeding temperatures of 4 MK,
and are consistent with steady heating, while long loops closer to the
periphery of the active region are significantly cooler (around 1 MK)
and may be powered by more intermittent heating. In this poster, we
use a field-aligned hydrodynamic model, combined with loop properties
constrained from observations and forward modeling, to better understand
the heating properties across the active region. Specifically,
we use the HYDRAD code to survey an array of heating parameters,
from impulsive heating to thermal non-equilibrium induced by
highly-stratified, localized foot point heating for a selection of loop
geometries derived from field extrapolations. We then forward model
spectroscopic observations from the EUV Imaging Spectrometer instrument
onboard Hinode as well as narrow-band imaging observations from the
Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory,
in order to make comparisons between observed loops and our models and
thus constrain the parameter space of heating scenarios. In doing so,
we gain insight into both how different types of loops are heated and
how heating properties vary across the active region.
Title: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS)
Authors: Caspi, A.; Shih, A. Y.; Warren, H.; Winebarger, A. R.; Woods,
T. N.; Cheung, C. M. M.; DeForest, C.; Klimchuk, J. A.; Laurent,
G. T.; Mason, J. P.; Palo, S. E.; Schwartz, R.; Seaton, D. B.;
Steslicki, M.; Gburek, S.; Sylwester, J.; Mrozek, T.; Kowaliński,
M.; Schattenburg, M.
Bibcode: 2020AGUFMSH0480007C
Altcode:
The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) is a 6U
CubeSat currently in a formulation phase under the 2019 NASA H-FORT
program. CubIXSS is motivated by a compelling overarching science
question: what are the origins of hot plasma in solar flares and active
regions? Elemental abundances are a unique diagnostic of how mass
and energy flow into and within the corona, and CubIXSS addresses
its science question through sensitive, precise measurements of
abundances of key trace ion species, whose spectral signatures reveal
the chromospheric or coronal origins of heated plasma across the
entire range of coronal temperatures, from ~1 to >30 MK. CubIXSS
measurements of the coronal temperature distribution and elemental
abundances directly address longstanding inconsistencies from prior
studies using instruments with limited, differing temperature and
composition sensitivities. CubIXSS comprises two co-optimized and
cross-calibrated instruments that fill a critical observational gap:
MOXSI, a novel diffractive spectral imager using a pinhole camera
and X-ray transmission diffraction grating to achieve spectroscopy of
flares and active regions from 1 to 55 Å, with spectral resolution of
0.24 Å FWHM and a spatial resolution of 25 arcsec FWHM; and SASS,
a suite of four spatially-integrated off-the-shelf spectrometers for
high-cadence, high-sensitivity measurements of soft and hard X-rays,
from 0.5 to 50 keV, with spectral resolution from 0.06 to 0.5 keV
FWHM. If selected for implementation, CubIXSS will launch in
mid-2023 to observe intense solar flares and active regions during
the rising phase of the solar cycle. Its nominal 1-year mission is
well timed with perihelia of Parker Solar Probe and Solar Orbiter,
and with the launches of complementary missions such as the PUNCH
Small Explorer. CubIXSS is also a pathfinder for the next generation
of Explorer-class missions with improved capabilities for SXR imaging
spectroscopy. We present the CubIXSS motivating science background, its
suite of instruments and expected performances, and other highlights
from the completed Concept Study Report, including novel analysis
techniques to fully exploit the rich data set of CubIXSS spectral
observations.
Title: Constraining Global Coronal Models with Multiple Independent
Observables
Authors: Badman, S. T.; Brooks, D.; Petrie, G. J. D.; Poirier, N.;
Warren, H.; Bale, S. D.; de Pablos, D.; Harra, L.; Rouillard, A. P.;
Panasenco, O.; Velli, M. C. M.
Bibcode: 2020AGUFMSH032..08B
Altcode:
Global coronal models seek to produce an accurate physical
representation of the Sun's atmosphere which can be used to probe
the dominant plasma physics processes, to connect remote and in situ
observations and operationally to predict space weather events which
can impact the Earth. Assessing their accuracy and usefulness is a
complex task and there are multiple observational pathways to provide
constraints on such models and tune their input parameters. In this
work, we aim to combine several such independent constraints in
a systematic fashion on coronal models. We study the intervals of
Parker Solar Probe's early solar encounters to leverage the unique in
situ observations taken close to the Sun, and the wealth of supporting
observations and prior work analyzing these time intervals. We require
our coronal models to predict the distribution of coronal holes on
the solar surface, and the neutral line topology. We compare these
predictions to (1) direct Extreme Ultraviolet (EUV) observations
of coronal hole locations, (2) white light Carrington maps of the
probable neutral line location at a few solar radii, (3) the magnetic
sector structure measured in situ by Parker Solar Probe as well as
1AU assets. For each of these constraints we compute a simple metric
to evaluate model agreement and compare and contrast these metrics
to evaluate and rank the overall accuracy of the models over a range
of input parameters. Initial results using the coronal hole metric
to analyze Potential Field Source Surface (PFSS) models indicate the
optimum source surface height (Rss) parameter varied from encounter to
encounter. Rss = 1.5 - 2.0 R_sun is shown to work best for Encounters
1 and 3, but higher (2.0-2.5 R_sun) for encounter 2, in agreement with
the magnetic sector structure metric and previous work (e.g. Panasenco
et al. 2020). We discuss the extension of these results to all three
metrics, assess differences in model accuracy among input photospheric
boundary conditions and investigate models with more physics than PFSS.
Title: The Solar-C (EUVST) mission: the latest status
Authors: Shimizu, Toshifumi; Imada, Shinsuke; Kawate, Tomoko; Suematsu,
Yoshinori; Hara, Hirohisa; Tsuzuki, Toshihiro; Katsukawa, Yukio; Kubo,
Masahito; Ishikawa, Ryoko; Watanabe, Tetsuya; Toriumi, Shin; Ichimoto,
Kiyoshi; Nagata, Shin'ichi; Hasegawa, Takahiro; Yokoyama, Takaaki;
Watanabe, Kyoko; Tsuno, Katsuhiko; Korendyke, Clarence M.; Warren,
Harry; De Pontieu, Bart; Boerner, Paul; Solanki, Sami K.; Teriaca,
Luca; Schuehle, Udo; Matthews, Sarah; Long, David; Thomas, William;
Hancock, Barry; Reid, Hamish; Fludra, Andrzej; Auchère, Frederic;
Andretta, Vincenzo; Naletto, Giampiero; Poletto, Luca; Harra, Louise
Bibcode: 2020SPIE11444E..0NS
Altcode:
Solar-C (EUVST) is the next Japanese solar physics mission to
be developed with significant contributions from US and European
countries. The mission carries an EUV imaging spectrometer with
slit-jaw imaging system called EUVST (EUV High-Throughput Spectroscopic
Telescope) as the mission payload, to take a fundamental step towards
answering how the plasma universe is created and evolves and how the
Sun influences the Earth and other planets in our solar system. In
April 2020, ISAS (Institute of Space and Astronautical Science) of JAXA
(Japan Aerospace Exploration Agency) has made the final down-selection
for this mission as the 4th in the series of competitively chosen
M-class mission to be launched with an Epsilon launch vehicle in mid
2020s. NASA (National Aeronautics and Space Administration) has selected
this mission concept for Phase A concept study in September 2019 and
is in the process leading to final selection. For European countries,
the team has (or is in the process of confirming) confirmed endorsement
for hardware contributions to the EUVST from the national agencies. A
recent update to the mission instrumentation is to add a UV spectral
irradiance monitor capability for EUVST calibration and scientific
purpose. This presentation provides the latest status of the mission
with an overall description of the mission concept emphasizing on key
roles of the mission in heliophysics research from mid 2020s.
Title: Current Status of the Solar-C_EUVST Mission
Authors: Imada, S.; Shimizu, T.; Kawate, T.; Toriumi, S.; Katsukawa,
Y.; Kubo, M.; Hara, H.; Suematsu, Y.; Ichimoto, K.; Watanabe, T.;
Watanabe, K.; Yokoyama, T.; Warren, H.; Long, D.; Harra, L. K.;
Teriaca, L.
Bibcode: 2020AGUFMSH056..05I
Altcode:
Solar-C_EUVST (EUV High-Throughput Spectroscopic Telescope) is designed
to comprehensively understand the energy and mass transfer from the
solar surface to the solar corona and interplanetary space, and to
investigate the elementary processes that take place universally
in cosmic plasmas. As a fundamental step towards answering how the
plasma universe is created and evolves, and how the Sun influences
the Earth and other planets in our solar system, the proposed mission
is designed to comprehensively understand how mass and energy are
transferred throughout the solar atmosphere. Understanding the solar
atmosphere, which connects to the heliosphere via radiation, the solar
wind and coronal mass ejections, and energetic particles is pivotal
for establishing the conditions for life and habitability in the solar
system. The two primary science objectives for Solar-C_EUVST are :
I) Understand how fundamental processes lead to the formation of the
solar atmosphere and the solar wind, II) Understand how the solar
atmosphere becomes unstable, releasing the energy that drives solar
flares and eruptions. Solar-C_EUVST will, A) seamlessly observe all
the temperature regimes of the solar atmosphere from the chromosphere
to the corona at the same time, B) resolve elemental structures of the
solar atmosphere with high spatial resolution and cadence to track their
evolution, and C) obtain spectroscopic information on the dynamics of
elementary processes taking place in the solar atmosphere. In this
talk, we will first discuss the science target of the Solar-C_EUVST,
and then discuss the current status of the Solar-C_EUVST mission.
Title: A sensitivity analysis of the updated optical design for
EUVST on the Solar-C mission
Authors: Kawate, Tomoko; Tsuzuki, Toshihiro; Shimizu, Toshifumi;
Imada, Shinsuke; Katsukawa, Yukio; Hara, Hirohisa; Suematsu, Yoshinori;
Ichimoto, Kiyoshi; Hattori, Tomoya; Narasaki, Shota; Warren, Harry P.;
Teriaca, Luca; Korendyke, Clarence M.; Brown, Charles M.; Auchere,
Frederic
Bibcode: 2020SPIE11444E..3JK
Altcode:
The EUV high-throughput spectroscopic telescope (EUVST) onboard the
Solar-C mission has the high spatial (0.4'') resolution over a wide
wavelength range in the vacuum ultraviolet. To achieve high spatial
resolution under a design constraint given by the JAXA Epsilon launch
vehicle, we further update the optical design to secure margins
needed to realize 0.4'' spatial resolution over a field of view of
100''×100''. To estimate the error budgets of spatial and spectral
resolutions due to installation and fabrication errors, we perform a
sensitivity analysis for the position and orientation of each optical
element and for the grating parameters by ray tracing with the Zemax
software. We obtain point spread functions (PSF) for rays from 9
fields and at 9 wavelengths on each detector by changing each parameter
slightly. A full width at half maximum (FWHM) of the PSF is derived at
each field and wavelength position as a function of the perturbation
of each optical parameter. Assuming a mount system of each optical
element and an error of each optical parameter, we estimate spatial
and spectral resolutions by taking installation and fabrication errors
into account. The results of the sensitivity analysis suggest that
budgets of the total of optical design and the assembly errors account
for 15% and 5.8% of our budgets of the spatial resolution in the long
wavelength and short wavelength bands, respectively. On the other hand,
the grating fabrication errors give a large degradation of spatial and
spectral resolutions, and investigations of compensators are needed
to relax the fabrication tolerance of the grating surface parameters.
Title: MinXSS-2 CubeSat mission overview: Improvements from the
successful MinXSS-1 mission
Authors: Mason, James Paul; Woods, Thomas N.; Chamberlin, Phillip
C.; Jones, Andrew; Kohnert, Rick; Schwab, Bennet; Sewell, Robert;
Caspi, Amir; Moore, Christopher S.; Palo, Scott; Solomon, Stanley C.;
Warren, Harry
Bibcode: 2020AdSpR..66....3M
Altcode: 2019arXiv190501345M
The second Miniature X-ray Solar Spectrometer (MinXSS-2) CubeSat, which
begins its flight in late 2018, builds on the success of MinXSS-1,
which flew from 2016-05-16 to 2017-05-06. The science instrument is
more advanced - now capable of greater dynamic range with higher
energy resolution. More data will be captured on the ground than
was possible with MinXSS-1 thanks to a sun-synchronous, polar orbit
and technical improvements to both the spacecraft and the ground
network. Additionally, a new open-source beacon decoder for amateur
radio operators is available that can automatically forward any captured
MinXSS data to the operations and science team. While MinXSS-1 was only
able to downlink about 1 MB of data per day corresponding to a data
capture rate of about 1%, MinXSS-2 will increase that by at least a
factor of 6. This increase of data capture rate in combination with
the mission's longer orbital lifetime will be used to address new
science questions focused on how coronal soft X-rays vary over solar
cycle timescales and what impact those variations have on the earth's
upper atmosphere.
Title: Observation and Modeling of High-temperature Solar Active
Region Emission during the High-resolution Coronal Imager Flight of
2018 May 29
Authors: Warren, Harry P.; Reep, Jeffrey W.; Crump, Nicholas A.;
Ugarte-Urra, Ignacio; Brooks, David H.; Winebarger, Amy R.; Savage,
Sabrina; De Pontieu, Bart; Peter, Hardi; Cirtain, Jonathan W.; Golub,
Leon; Kobayashi, Ken; McKenzie, David; Morton, Richard; Rachmeler,
Laurel; Testa, Paola; Tiwari, Sanjiv; Walsh, Robert
Bibcode: 2020ApJ...896...51W
Altcode:
Excellent coordinated observations of NOAA active region 12712 were
obtained during the flight of the High-resolution Coronal Imager (Hi-C)
sounding rocket on 2018 May 29. This region displayed a typical active
region core structure with relatively short, high-temperature loops
crossing the polarity inversion line and bright "moss" located at the
footpoints of these loops. The differential emission measure (DEM) in
the active region core is very sharply peaked at about 4 MK. Further,
there is little evidence for impulsive heating events in the moss, even
at the high spatial resolution and cadence of Hi-C. This suggests that
active region core heating is occurring at a high frequency and keeping
the loops close to equilibrium. To create a time-dependent simulation of
the active region core, we combine nonlinear force-free extrapolations
of the measured magnetic field with a heating rate that is dependent
on the field strength and loop length and has a Poisson waiting time
distribution. We use the approximate solutions to the hydrodynamic
loop equations to simulate the full ensemble of active region core
loops for a range of heating parameters. In all cases, we find that
high-frequency heating provides the best match to the observed DEM. For
selected field lines, we solve the full hydrodynamic loop equations,
including radiative transfer in the chromosphere, to simulate transition
region and chromospheric emission. We find that for heating scenarios
consistent with the DEM, classical signatures of energy release,
such as transition region brightenings and chromospheric evaporation,
are weak, suggesting that they would be difficult to detect.
Title: The Drivers of Active Region Outflows into the Slow Solar Wind
Authors: Brooks, David H.; Winebarger, Amy R.; Savage, Sabrina; Warren,
Harry P.; De Pontieu, Bart; Peter, Hardi; Cirtain, Jonathan W.; Golub,
Leon; Kobayashi, Ken; McIntosh, Scott W.; McKenzie, David; Morton,
Richard; Rachmeler, Laurel; Testa, Paola; Tiwari, Sanjiv; Walsh, Robert
Bibcode: 2020ApJ...894..144B
Altcode: 2020arXiv200407461B
Plasma outflows from the edges of active regions have been suggested as
a possible source of the slow solar wind. Spectroscopic measurements
show that these outflows have an enhanced elemental composition,
which is a distinct signature of the slow wind. Current spectroscopic
observations, however, do not have sufficient spatial resolution to
distinguish what structures are being measured or determine the driver
of the outflows. The High-resolution Coronal Imager (Hi-C) flew on a
sounding rocket in 2018 May and observed areas of active region outflow
at the highest spatial resolution ever achieved (250 km). Here we use
the Hi-C data to disentangle the outflow composition signatures observed
with the Hinode satellite during the flight. We show that there are
two components to the outflow emission: a substantial contribution
from expanded plasma that appears to have been expelled from closed
loops in the active region core and a second contribution from dynamic
activity in active region plage, with a composition signature that
reflects solar photospheric abundances. The two competing drivers of the
outflows may explain the variable composition of the slow solar wind.
Title: Simulating Solar Flare Irradiance with Multithreaded Models
of Flare Arcades
Authors: Reep, Jeffrey W.; Warren, Harry P.; Moore, Christopher S.;
Suarez, Crisel; Hayes, Laura A.
Bibcode: 2020ApJ...895...30R
Altcode: 2020arXiv200310505R
Understanding how energy is released in flares is one of the
central problems of solar and stellar astrophysics. Observations of
high-temperature flare plasma hold many potential clues as to the
nature of this energy release. It is clear, however, that flares are
not composed of a few impulsively heated loops, but are the result
of heating on many small-scale threads that are energized over time,
making it difficult to compare observations and numerical simulations
in detail. Several previous studies have shown that it is possible
to reproduce some aspects of the observed emission by considering the
flare as a sequence of independently heated loops, but these studies
generally focus on small-scale features while ignoring the global
features of the flare. In this paper, we develop a multithreaded model
that encompasses the time-varying geometry and heating rate for a
series of successively heated loops composing an arcade. To validate,
we compare with spectral observations of five flares made with the
MinXSS CubeSat, as well as light curves measured with GOES/XRS and
SDO/AIA. We show that this model can successfully reproduce the light
curves and quasi-periodic pulsations in GOES/XRS, the soft X-ray spectra
seen with MinXSS, and the light curves in various AIA passbands. The
AIA light curves are most consistent with long-duration heating, but
elemental abundances cannot be constrained with the model. Finally, we
show how this model can be used to extrapolate to spectra of extreme
events that can predict irradiance across a wide wavelength range,
including unobserved wavelengths.
Title: Is the High-Resolution Coronal Imager Resolving Coronal
Strands? Results from AR 12712
Authors: Williams, Thomas; Walsh, Robert W.; Winebarger, Amy R.;
Brooks, David H.; Cirtain, Jonathan W.; De Pontieu, Bart; Golub,
Leon; Kobayashi, Ken; McKenzie, David E.; Morton, Richard J.; Peter,
Hardi; Rachmeler, Laurel A.; Savage, Sabrina L.; Testa, Paola; Tiwari,
Sanjiv K.; Warren, Harry P.; Watkinson, Benjamin J.
Bibcode: 2020ApJ...892..134W
Altcode: 2020arXiv200111254W
Following the success of the first mission, the High-Resolution
Coronal Imager (Hi-C) was launched for a third time (Hi-C 2.1)
on 2018 May 29 from the White Sands Missile Range, NM, USA. On this
occasion, 329 s of 17.2 nm data of target active region AR 12712 were
captured with a cadence of ≈4 s, and a plate scale of 0.129 arcsec
pixel-1. Using data captured by Hi-C 2.1 and co-aligned
observations from SDO/AIA 17.1 nm, we investigate the widths of 49
coronal strands. We search for evidence of substructure within the
strands that is not detected by AIA, and further consider whether these
strands are fully resolved by Hi-C 2.1. With the aid of multi-scale
Gaussian normalization, strands from a region of low emission that can
only be visualized against the contrast of the darker, underlying moss
are studied. A comparison is made between these low-emission strands
and those from regions of higher emission within the target active
region. It is found that Hi-C 2.1 can resolve individual strands as
small as ≈202 km, though the more typical strand widths seen are
≈513 km. For coronal strands within the region of low emission, the
most likely width is significantly narrower than the high-emission
strands at ≈388 km. This places the low-emission coronal strands
beneath the resolving capabilities of SDO/AIA, highlighting the need
for a permanent solar observatory with the resolving power of Hi-C.
Title: Solar physics in the 2020s: DKIST, parker solar probe, and
solar orbiter as a multi-messenger constellation
Authors: Martinez Pillet, V.; Tritschler, A.; Harra, L.; Andretta, V.;
Vourlidas, A.; Raouafi, N.; Alterman, B. L.; Bellot Rubio, L.; Cauzzi,
G.; Cranmer, S. R.; Gibson, S.; Habbal, S.; Ko, Y. K.; Lepri, S. T.;
Linker, J.; Malaspina, D. M.; Matthews, S.; Parenti, S.; Petrie, G.;
Spadaro, D.; Ugarte-Urra, I.; Warren, H.; Winslow, R.
Bibcode: 2020arXiv200408632M
Altcode:
The National Science Foundation (NSF) Daniel K. Inouye Solar Telescope
(DKIST) is about to start operations at the summit of Haleakala
(Hawaii). DKIST will join the early science phases of the NASA
and ESA Parker Solar Probe and Solar Orbiter encounter missions. By
combining in-situ measurements of the near-sun plasma environment and
detail remote observations of multiple layers of the Sun, the three
observatories form an unprecedented multi-messenger constellation to
study the magnetic connectivity inside the solar system. This white
paper outlines the synergistic science that this multi-messenger
suite enables.
Title: The Solar Wind Speed Expansion Factor [v -fs]
Relationship at the Inner Boundary (18 R⊙) of the
Heliosphere
Authors: Wu, Chin-Chun; Liou, Kan; Warren, Harry
Bibcode: 2020SoPh..295...25W
Altcode:
The accuracy of data-driven magnetohydrodynamics (MHD) models
depends on accurate boundary conditions specified at the inner
heliosphere. However, not all of the MHD parameters [B ,v ,ρ ,T ]
are measurable close to the Sun at the present time, except the vector
magnetic field [B ] at the photosphere. The solar wind speed [v ], which
is probably most relevant to space-weather forecasting, is often modeled
by the standard Wang-Sheeley (WS) formula, which is based on an inverse
relationship between the solar wind speed [v ] at 1 AU and the expansion
factor [fs] estimated at 2.5 solar radii [R⊙], with the
following generic form: v =v1+v2fs−α
(where v is the solar wind speed at 18 R⊙, fs is the
magnetic-field expansion factor, and v1, v2, and
α are three free parameters to be determined). While the WS formula
uses "source projection" to determine the solar wind source, it does
not treat the solar wind as plasma because it uses the solar wind
speed observed at 1 AU to derive the empirical relationship. Thus, the
resulting formula ignores the transport and acceleration of the solar
wind as it propagates out into the heliosphere. The purpose of this
study is to rectify this omission by using a numerical MHD simulation
to find the optimal set of free parameters that relate the magnetic
properties at the source surface to the plasma parameters at 1 AU. In
addition to the expansion factor, conservation of mass [ρ v ], magnetic
flux [r2B ], and total pressure along the stream line are
assumed to obtain the solar wind mass density, magnetic field, and
temperature at 18 R⊙. These parameters are used as the inner boundary
conditions of our global three-dimensional MHD (G3DMHD) code to simulate
solar wind plasma and field parameters out to ≈1 AU. The simulation
results are compared with the in-situ data from Wind to assess the
accuracy. Such a procedure is repeated (880 times) to cover the three
parameter regimes (100 <v1<350kms−1; 250
<v2<700 kms−1; and 0.2 <α <0.9 )
to find the optimal set. The simulation is performed for the period of
CR2082 [30 March 2019 to 27 April 2009]. It is found that v =189 +679
fs−0.7 is the best formula to relate the solar wind speed
at 18 R⊙ to the expansion factor. Strictly speaking, this
formula is most applicable for solar equatorial regions and near the
times of solar minimum when there are few coronal mass ejection events.
Title: RHESSI - GOES Comparisons of Soft X-ray Emission from Solar
Flares, 2002 - 2017
Authors: McTiernan, J. M.; Caspi, A.; Warren, H.
Bibcode: 2019AGUFMSH13D3427M
Altcode:
This work is a comparison of the low energy (3 to 20 keV) response
of the 9 detectors on-board the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) with the X-Ray sensors on-board the
series of Geostationary Operational Environmental Satellites (GOES),
for the duration of the RHESSI mission. The purpose is to estimate
the loss of sensitivity for each RHESSI detector during the mission,
relative to GOES detectors which are expected to be more consistent over
time. Comparisons are made during the decay phase of large solar flares,
where non-thermal emission from accelerated electrons is expected
to be unimportant; these large (GOES class M and X) solar flares are
present in the RHESSI database from February 2002 through September
2017. Calculations are done for each of the (3) different RHESSI
attenuator states. The possibility for detection of an energy dependent
time variation in the RHESSI detectors will also be investigated.
Title: Comparing Coronal Hole Wave Properties and Density Profiles
Derived from Indirect and Direct Observations
Authors: Weberg, M. J.; Ko, Y. K.; Laming, J. M.; Warren, H.
Bibcode: 2019AGUFMSH53B3368W
Altcode:
Transverse (or "Alfvénic") waves are commonly invoked by theories and
models to explain coronal heating and solar wind acceleration. However,
direct measurements are sparse and most of what we know about wave
activity in the corona is derived from indirect proxies. Furthermore,
previous studies using direct observations have revealed systematic
discrepancies between the wave properties and energy values reported
by indirect and direct methods. In this study we examine the root
causes and contributing factors of this discrepancy in wave properties
by analyzing the same coronal hole using both indirect and direct
methodologies. In the former case, we apply standard, spectrographic
methods to data from Hinode / EIS to obtain an electron density profile
(using line intensity ratios) and average wave velocity amplitudes
(using measurements of non-thermal line widths). Direct measurements
are made by identifying and tracking transverse motions in SDO / AIA
images using the Northumbria University Wave Tracking (NUWT) code, which
provides more detailed wave parameters as well as a relative density
profile. While the two methodologies produce results with similar
trends, we find that part of the discrepancy stems from the fact
that the two methods measure complementary, rather than identical,
structures and wave motions. We also investigate the latitudinal
variation of wave parameters within a coronal hole and consider the
total wave energy flux using a wider spectrum of wave frequencies than
previously used. This study helps improve our understanding of existing
analysis methods and builds confidence in promising new techniques.
Title: Hi-C 2.1 Observations of Jetlet-like Events at Edges of Solar
Magnetic Network Lanes
Authors: Panesar, Navdeep K.; Sterling, Alphonse C.; Moore, Ronald L.;
Winebarger, Amy R.; Tiwari, Sanjiv K.; Savage, Sabrina L.; Golub, Leon
E.; Rachmeler, Laurel A.; Kobayashi, Ken; Brooks, David H.; Cirtain,
Jonathan W.; De Pontieu, Bart; McKenzie, David E.; Morton, Richard J.;
Peter, Hardi; Testa, Paola; Walsh, Robert W.; Warren, Harry P.
Bibcode: 2019ApJ...887L...8P
Altcode: 2019arXiv191102331P
We present high-resolution, high-cadence observations of six,
fine-scale, on-disk jet-like events observed by the High-resolution
Coronal Imager 2.1 (Hi-C 2.1) during its sounding-rocket flight. We
combine the Hi-C 2.1 images with images from the Solar Dynamics
Observatory (SDO)/Atmospheric Imaging Assembly (AIA) and the Interface
Region Imaging Spectrograph (IRIS), and investigate each event’s
magnetic setting with co-aligned line-of-sight magnetograms from the
SDO/Helioseismic and Magnetic Imager (HMI). We find that (i) all six
events are jetlet-like (having apparent properties of jetlets), (ii)
all six are rooted at edges of magnetic network lanes, (iii) four of
the jetlet-like events stem from sites of flux cancelation between
majority-polarity network flux and merging minority-polarity flux, and
(iv) four of the jetlet-like events show brightenings at their bases
reminiscent of the base brightenings in coronal jets. The average
spire length of the six jetlet-like events (9000 ± 3000 km) is three
times shorter than that for IRIS jetlets (27,000 ± 8000 km). While
not ruling out other generation mechanisms, the observations suggest
that at least four of these events may be miniature versions of both
larger-scale coronal jets that are driven by minifilament eruptions
and still-larger-scale solar eruptions that are driven by filament
eruptions. Therefore, we propose that our Hi-C events are driven by
the eruption of a tiny sheared-field flux rope, and that the flux rope
field is built and triggered to erupt by flux cancelation.
Title: The solar wind speed - expansion factor (v - fs)
relationship at the inner boundary (18 R⊙) of the
heliosphere
Authors: Liou, K.; Wu, C. C.; Warren, H.
Bibcode: 2019AGUFMSH41F3329L
Altcode:
The accuracy of data-driven magnetohydrodynamics (MHD) models
depends on accurate boundary conditions specified at the inner
heliosphere. However, all of the MHD parameters (B, v, ρ, T)
close to the Sun are not measurable at the present time,except the
total magnetic field (|B|) at the photosphere. The solar wind speed
(v), which is probably most relevant to space weather forecasting,
is often modeled by the standard Wang-Sheely-Arge (WSA) empirical
formula. The WSA formula is based on an inverse relationship
between the solar wind speed measured at 1 AU and the magnetic field
expansion factor estimated at 2.5 solar radii (R⊙ ),
with the following generic form: v = v1 +v2
fs -α (where v is the solar wind
speed at 18 R⊙ , fs is the magnetic field
expansion factor, and v1, v2, and α are three
free parameters to be determined). Because it uses the solar wind
speed at 1 AU, the formula ignores the transport of solar wind in the
heliosphere. While the WSA formula uses "source projection" to account
for the transport of the solar wind, it does not treat the solar wind
as plasma. The purpose of this study is to rectify this omission by
using numerical MHD simulations to find the optimal set of the free
parameters that relate the magnetic properties at the source surface
to the plasma parameters at 1 AU. In addition to the expansion factor,
conservation of mass (ρv), magnetic flux (r2B), and total
pressure along the stream line are assumed to obtain a complete set
of MHD parameters at 18 R⊙ . These parameters are used
as the inner boundary conditions of our global three-dimensional MHD
(G3DMHD) code to simulate solar wind plasma and field parameters out
to ~1 AU. The simulation results are compared with the in situ data
from Wind to assess the accuracy. Such a procedure is repeated (880
times) to cover the three parameter regimes (100 < v1
< 350 km/s; 250 < v2 < 700 km/s; and 0.2 < α
< 0.9) to find the optimal set. The simulation is performed for
the period of CR2082. It is found that v = 189 + 679 fs
-0.7 is the best formula to relate the solar wind speed
at 18 R⊙ to the expansion factor. Strictly speaking,
this formula applies only to periods around solar minimum. *
Work of CCW was partially supported by the Chief of Naval Research.
Title: Fine-scale Explosive Energy Release at Sites of Prospective
Magnetic Flux Cancellation in the Core of the Solar Active Region
Observed by Hi-C 2.1, IRIS, and SDO
Authors: Tiwari, Sanjiv K.; Panesar, Navdeep K.; Moore, Ronald L.;
De Pontieu, Bart; Winebarger, Amy R.; Golub, Leon; Savage, Sabrina L.;
Rachmeler, Laurel A.; Kobayashi, Ken; Testa, Paola; Warren, Harry P.;
Brooks, David H.; Cirtain, Jonathan W.; McKenzie, David E.; Morton,
Richard J.; Peter, Hardi; Walsh, Robert W.
Bibcode: 2019ApJ...887...56T
Altcode: 2019arXiv191101424T
The second Hi-C flight (Hi-C 2.1) provided unprecedentedly high spatial
and temporal resolution (∼250 km, 4.4 s) coronal EUV images of Fe IX/X
emission at 172 Å of AR 12712 on 2018 May 29, during 18:56:21-19:01:56
UT. Three morphologically different types (I: dot-like; II: loop-like;
III: surge/jet-like) of fine-scale sudden-brightening events (tiny
microflares) are seen within and at the ends of an arch filament system
in the core of the AR. Although type Is (not reported before) resemble
IRIS bombs (in size, and brightness with respect to surroundings),
our dot-like events are apparently much hotter and shorter in span
(70 s). We complement the 5 minute duration Hi-C 2.1 data with SDO/HMI
magnetograms, SDO/AIA EUV images, and IRIS UV spectra and slit-jaw
images to examine, at the sites of these events, brightenings and
flows in the transition region and corona and evolution of magnetic
flux in the photosphere. Most, if not all, of the events are seated
at sites of opposite-polarity magnetic flux convergence (sometimes
driven by adjacent flux emergence), implying likely flux cancellation
at the microflare’s polarity inversion line. In the IRIS spectra
and images, we find confirming evidence of field-aligned outflow from
brightenings at the ends of loops of the arch filament system. In types
I and II the explosion is confined, while in type III the explosion
is ejective and drives jet-like outflow. The light curves from Hi-C,
AIA, and IRIS peak nearly simultaneously for many of these events,
and none of the events display a systematic cooling sequence as seen in
typical coronal flares, suggesting that these tiny brightening events
have chromospheric/transition region origin.
Title: The High-Resolution Coronal Imager, Flight 2.1
Authors: Rachmeler, Laurel A.; Winebarger, Amy R.; Savage, Sabrina L.;
Golub, Leon; Kobayashi, Ken; Vigil, Genevieve D.; Brooks, David H.;
Cirtain, Jonathan W.; De Pontieu, Bart; McKenzie, David E.; Morton,
Richard J.; Peter, Hardi; Testa, Paola; Tiwari, Sanjiv K.; Walsh,
Robert W.; Warren, Harry P.; Alexander, Caroline; Ansell, Darren;
Beabout, Brent L.; Beabout, Dyana L.; Bethge, Christian W.; Champey,
Patrick R.; Cheimets, Peter N.; Cooper, Mark A.; Creel, Helen K.;
Gates, Richard; Gomez, Carlos; Guillory, Anthony; Haight, Harlan;
Hogue, William D.; Holloway, Todd; Hyde, David W.; Kenyon, Richard;
Marshall, Joseph N.; McCracken, Jeff E.; McCracken, Kenneth; Mitchell,
Karen O.; Ordway, Mark; Owen, Tim; Ranganathan, Jagan; Robertson,
Bryan A.; Payne, M. Janie; Podgorski, William; Pryor, Jonathan; Samra,
Jenna; Sloan, Mark D.; Soohoo, Howard A.; Steele, D. Brandon; Thompson,
Furman V.; Thornton, Gary S.; Watkinson, Benjamin; Windt, David
Bibcode: 2019SoPh..294..174R
Altcode: 2019arXiv190905942R
The third flight of the High-Resolution Coronal Imager (Hi-C 2.1)
occurred on May 29, 2018; the Sounding Rocket was launched from White
Sands Missile Range in New Mexico. The instrument has been modified
from its original configuration (Hi-C 1) to observe the solar corona
in a passband that peaks near 172 Å, and uses a new, custom-built
low-noise camera. The instrument targeted Active Region 12712, and
captured 78 images at a cadence of 4.4 s (18:56:22 - 19:01:57 UT; 5
min and 35 s observing time). The image spatial resolution varies due
to quasi-periodic motion blur from the rocket; sharp images contain
resolved features of at least 0.47 arcsec. There are coordinated
observations from multiple ground- and space-based telescopes providing
an unprecedented opportunity to observe the mass and energy coupling
between the chromosphere and the corona. Details of the instrument
and the data set are presented in this paper.
Title: Model studies of photoionization and photoelectron production
in response to solar flares
Authors: Samaddar, S.; Siskind, D. E.; Bailey, S. M.; Reep, J. W.;
Warren, H.
Bibcode: 2019AGUFMSA11B3222S
Altcode:
The solar flux, shortward of 102.6 nm deposits energy into
the Earth's thermosphere and initiates chemical processes that
affect the composition and structure of the ionospheric D and E
regions. One of the primary processes is the photoionization of
the major neutral constituents N2, O2 and
O. The photoionization of the major species leads to the formation
of energetic photoelectrons. These primary photoelectrons create
secondary electrons that can cause further ionization, dissociation,
and excitation of particles. We use a photoelectron model to study
the effects of variability of the solar flux in the production of the
primary and secondary photoelectrons in the D and E regions of the
thermosphere. Using a detailed hydrodynamic model of a solar flare
arcade, we have synthesized the spectral irradiance from a large solar
flare, extending to energies in the hard X-rays. We use this synthetic
spectrum to study the effects of solar flares at altitudes lower than
90 km, i.e. the D region. We have also revised the ionization and
absorption cross-sections of the neutral species, including wavelengths
based on new laboratory data. The new cross-sections are significantly
different in the neighborhood of the Lyman beta emission. In this
presentation, we examine the role of ionization by both photons
and photoelectrons due to Lyman beta in the context of the revised
cross sections. The improved cross-sections and extension of the
input solar flux to higher energies and therefore to lower altitudes,
give us a better understanding of the effects of solar flares on the
Earth's ionosphere.
Title: A Next Generation Spectrometer: The EUV High-Throughput
Spectroscopic Telescope (EUVST)
Authors: Warren, H. P.
Bibcode: 2019AGUFMSH31C3319W
Altcode:
An advanced spectrometer and slit-jaw imaging system has been proposed
by an international team to JAXA's competitively selected M-class
missions science program. The main scientific goal of the proposed
instrument, the EUV High-Throughput Spectroscopic Telescope (EUVST),
is to understand the transfer of mass and energy from the solar surface
to the solar corona and interplanetary space by observing fundamental
processes occurring in the solar atmosphere. The mission has two
specific scientific objectives: (I) to understand how fundamental
processes lead to the formation of the solar atmosphere and the solar
wind, and (II) to understand how the solar atmosphere becomes unstable,
releasing the energy that drives solar flares and eruptions. EUVST
will make major advances by combining a seamless temperature coverage
of the solar photosphere, chromosphere, transition region, and corona
with very high spatial resolution (0.4ʺ or 300km) and unprecedented
cadence (as high as 0.1s). This instrument will complement new solar
observatories such as DKIST, the Parker Solar Probe, and Solar Orbiter
that will be operational during the proposed mission.
Title: Global Energetics of Solar Flares and Coronal Mass Ejections
Authors: Aschwanden, Markus J.; Caspi, Amir; Cohen, Christina M. S.;
Holman, Gordon; Jing, Ju; Kretzschmar, Matthieu; Kontar, Eduard
P.; McTiernan, James M.; Mewaldt, Richard A.; O'Flannagain, Aidan;
Richardson, Ian G.; Ryan, Daniel; Warren, Harry P.; Xu, Yan
Bibcode: 2019JPhCS1332a2002A
Altcode:
We investigate the global energetics and energy closure of various
physical processes that are energetically important in solar flares
and coronal mass ejections (CMEs), which includes: magnetic energies,
thermal energies, nonthermal energies (particle acceleration),
direct and indirect plasma heating processes, kinetic CME energies,
gravitational CME energies, aerodynamic drag of CMEs, solar
energetic particle events, EUV and soft X-ray radiation, white-light,
and bolometric energies. Statistics on these forms of energies is
obtained from 400 GOES M- and X-class events during the first 3.5
years of the Solar Dynamics Observatory (SDO) mission. A primary
test addressed in this study is the closure of the various energies,
such as the equivalence of the dissipated magnetic energies and the
primary dissipated are energies (accelerated particles, direct heating,
CME acceleration), which faciliate the energy of secondary processes
(plasma heating, shock acceleration) and interactions with the solar
wind (aerodynamic drag). Our study demonstrates energy closure in the
statistical average, while individual events may have considerable
uncertainties, requiring improved nonlinear force-free field models,
and particle acceleration models with observationally constrained
low-energy cutoffs.
Title: Solar Active Region Heating Diagnostics from High-temperature
Emission Using the MaGIXS
Authors: Athiray, P. S.; Winebarger, Amy R.; Barnes, Will T.; Bradshaw,
Stephen J.; Savage, Sabrina; Warren, Harry P.; Kobayashi, Ken; Champey,
Patrick; Golub, Leon; Glesener, Lindsay
Bibcode: 2019ApJ...884...24A
Altcode: 2019arXiv190902541A
The relative amount of high-temperature plasma has been found to be
a useful diagnostic to determine the frequency of coronal heating on
sub-resolution structures. When the loops are infrequently heated,
a broad emission measure (EM) over a wider range of temperatures
is expected. A narrower EM is expected for high-frequency heating
where the loops are closer to equilibrium. The soft X-ray spectrum
contains many spectral lines that provide high-temperature diagnostics,
including lines from Fe XVII-XIX. This region of the solar spectrum
will be observed by the Marshall Grazing Incidence Spectrometer (MaGIXS)
in 2020. In this paper, we derive the expected spectral line intensity
in MaGIXS to varying amounts of high-temperature plasma to demonstrate
that a simple line ratio provides a powerful diagnostic to determine
the heating frequency. Similarly, we examine ratios of AIA channel
intensities, filter ratios from a XRT, and energy bands from the FOXSI
sounding rocket to determine their sensitivity to this parameter. We
find that both FOXSI and MaGIXS provide good diagnostic capabilities
for high-temperature plasma. We then compare the predicted line ratios
to the output of a numerical model and confirm that the MaGIXS ratios
provide an excellent diagnostic for heating frequency.
Title: The Variability of Solar Coronal Abundances in Active Regions
and the Quiet Sun
Authors: Doschek, G. A.; Warren, H. P.
Bibcode: 2019ApJ...884..158D
Altcode:
Measurements of elemental abundances hold important clues to how mass
and energy flow through the solar atmosphere. Variations in abundances
are organized by an element’s first ionization potential (FIP),
and many previous studies have assumed that low FIP (less than 10 eV)
elements are enriched by a factor of 3-4 in the corona. In this paper,
we use spatially resolved observations from the Extreme-ultraviolet
Imaging Telescope on board the Hinode spacecraft to examine the spatial
variability of elemental abundance in and around active regions. We find
substantial variations within some active regions. In general, however,
we find that the enrichment of low FIP elements is limited to bright,
active region structures. In faint active region structures and in the
dark, quiet regions around active regions, the measured abundances are
close to photospheric. These measurements use the ratio of low FIP
Si to high FIP S. Similar conclusions concerning quiet Sun regions
have been reached recently by Del Zanna using full-Sun spectra. He
has found that the coronal quiet Sun (at temperatures greater than
1 MK) has photospheric abundances. Transition region abundances (at
temperatures less than 1 MK in the solar atmosphere) have been found
to be photospheric. These results and results from this paper suggest
that a coronal composition is not a general property of million-degree
plasma, but is limited to bright active region loops, and is variable.
Title: Achievements of Hinode in the first eleven years
Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick;
Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks,
David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio;
Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra,
Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.;
Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao,
Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota,
Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu,
Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin;
Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young,
Peter R.
Bibcode: 2019PASJ...71R...1H
Altcode:
Hinode is Japan's third solar mission following Hinotori (1981-1982)
and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in
operation currently. Hinode carries three instruments: the Solar Optical
Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These
instruments were built under international collaboration with the
National Aeronautics and Space Administration and the UK Science and
Technology Facilities Council, and its operation has been contributed
to by the European Space Agency and the Norwegian Space Center. After
describing the satellite operations and giving a performance evaluation
of the three instruments, reviews are presented on major scientific
discoveries by Hinode in the first eleven years (one solar cycle long)
of its operation. This review article concludes with future prospects
for solar physics research based on the achievements of Hinode.
Title: Concept study of Solar-C_EUVST optical design
Authors: Kawate, Tomoko; Shimizu, Toshifumi; Imada, Shinsuke; Tsuzuki,
Toshihiro; Katsukawa, Yukio; Hara, Hirohisa; Suematsu, Yoshinori;
Ichimoto, Kiyoshi; Warren, Harry; Teriaca, Luca; Korendyke, Clarence
M.; Brown, Charles
Bibcode: 2019SPIE11118E..1NK
Altcode:
The main characteristics of Solar-C_EUVST are the high temporal and
high spatial resolutions over a wide temperature coverage. In order
to realize the instrument for meeting these scientific requirements
under size constraints given by the JAXA Epsilon vehicle, we examined
four-dimensional optical parameter space of possible solutions of
geometrical optical parameters such as mirror diameter, focal length,
grating magnification, and so on. As a result, we have identified
the solution space that meets the EUVST science objectives and rocket
envelope requirements. A single solution was selected and used to define
the initial optical parameters for the concept study of the baseline
architecture for defining the mission concept. For this solution, we
optimized the grating and geometrical parameters by ray tracing of the
Zemax software. Consequently, we found an optics system that fulfills
the requirement for a 0.4" angular resolution over a field of view of
100" (including margins) covering spectral ranges of 170-215, 463-542,
557-637, 690-850, 925-1085, and 1115-1275 A. This design achieves an
effective area 10 times larger than the Extreme-ultraviolet Imaging
Spectrometer onboard the Hinode satellite, and will provide seamless
observations of 4.2-7.2 log(K) plasmas for the first time. Tolerance
analyses were performed based on the optical design, and the moving
range and step resolution of focus mechanisms were identified. In
the presentation, we describe the derivation of the solution space,
optimization of the optical parameters, and show the results of ray
tracing and tolerance analyses.
Title: The Solar-C_EUVST mission
Authors: Shimizu, Toshifumi; Imada, Shinsuke; Kawate, Tomoko;
Ichimoto, Kiyoshi; Suematsu, Yoshinori; Hara, Hirohisa; Katsukawa,
Yukio; Kubo, Masahito; Toriumi, Shin; Watanabe, Tetsuya; Yokoyama,
Takaaki; Korendyke, Clarence M.; Warren, Harry P.; Tarbell, Ted; De
Pontieu, Bart; Teriaca, Luca; Schühle, Udo H.; Solanki, Sami; Harra,
Louise K.; Matthews, Sarah; Fludra, A.; Auchère, F.; Andretta, V.;
Naletto, G.; Zhukov, A.
Bibcode: 2019SPIE11118E..07S
Altcode:
Solar-C EUVST (EUV High-Throughput Spectroscopic Telescope) is a
solar physics mission concept that was selected as a candidate for
JAXA competitive M-class missions in July 2018. The onboard science
instrument, EUVST, is an EUV spectrometer with slit-jaw imaging
system that will simultaneously observe the solar atmosphere from the
photosphere/chromosphere up to the corona with seamless temperature
coverage, high spatial resolution, and high throughput for the first
time. The mission is designed to provide a conclusive answer to the
most fundamental questions in solar physics: how fundamental processes
lead to the formation of the solar atmosphere and the solar wind, and
how the solar atmosphere becomes unstable, releasing the energy that
drives solar flares and eruptions. The entire instrument structure
and the primary mirror assembly with scanning and tip-tilt fine
pointing capability for the EUVST are being developed in Japan, with
spectrograph and slit-jaw imaging hardware and science contributions
from US and European countries. The mission will be launched and
installed in a sun-synchronous polar orbit by a JAXA Epsilon vehicle in
2025. ISAS/JAXA coordinates the conceptual study activities during the
current mission definition phase in collaboration with NAOJ and other
universities. The team is currently working towards the JAXA final
down-selection expected at the end of 2019, with strong support from
US and European colleagues. The paper provides an overall description
of the mission concept, key technologies, and the latest status.
Title: The Multi-instrument (EVE-RHESSI) DEM for Solar Flares,
and Implications for Nonthermal Emission
Authors: McTiernan, James M.; Caspi, Amir; Warren, Harry P.
Bibcode: 2019ApJ...881..161M
Altcode: 2018arXiv180512285M
Solar flare X-ray spectra are typically dominated by thermal
bremsstrahlung emission in the soft X-ray (≲10 keV) energy range;
for hard X-ray energies (≳30 keV), emission is typically nonthermal
from beams of electrons. The low-energy extent of nonthermal emission
has only been loosely quantified. It has been difficult to obtain
a lower limit for a possible nonthermal cutoff energy due to the
significantly dominant thermal emission. Here we use solar flare data
from the extreme ultraviolet Variability Experiment on board the Solar
Dynamics Observatory and X-ray data from the Reuven Ramaty High Energy
Spectroscopic Imager to calculate the Differential Emission Measure
(DEM). This improvement over the isothermal approximation and any
single-instrument DEM helps to resolve ambiguities in the range where
thermal and nonthermal emission overlap, and to provide constraints
on the low-energy cutoff. In the model, thermal emission is from a
DEM that is parameterized as multiple Gaussians in Log(T). Nonthermal
emission results from a photon spectrum obtained using a thick-target
emission model. Spectra for both instruments are fit simultaneously
in a self-consistent manner. Our results have been obtained using a
sample of 52 large (Geostationary Operational Environmental Satellite
X- and M-class) solar flares observed between 2011 and 2013. It turns
out that it is often possible to determine low-energy cutoffs early
(in the first two minutes) during large flares. Cutoff energies at
these times are typically low, less than 10 keV, when assuming coronal
abundances. With photospheric abundances, cutoff energies are typically
∼10 keV higher, in the ∼17-25 keV range.
Title: The Magnetic Properties of Heating Events on High-temperature
Active-region Loops
Authors: Ugarte-Urra, Ignacio; Crump, Nicholas A.; Warren, Harry P.;
Wiegelmann, Thomas
Bibcode: 2019ApJ...877..129U
Altcode: 2019arXiv190411976U
Understanding the relationship between the magnetic field and coronal
heating is one of the central problems of solar physics. However,
studies of the magnetic properties of impulsively heated loops have
been rare. We present results from a study of 34 evolving coronal loops
observed in the Fe XVIII line component of 94 Å filter images obtained
by the Atmospheric Imaging Assembly (AIA)/Solar Dynamics Observatory
(SDO) from three active regions with different magnetic conditions. We
show that the peak intensity per unit cross section of the loops depends
on their individual magnetic and geometric properties. The intensity
scales proportionally to the average field strength along the loop (B
avg) and inversely with the loop length (L) for a combined
dependence of {({B}avg}/L)}0.52+/- 0.13. These
loop properties are inferred from magnetic extrapolations of the
photospheric Helioseismic and Magnetic Imager (HMI)/SDO line-of-sight
and vector magnetic field in three approximations: potential and two
nonlinear force-free (NLFF) methods. Through hydrodynamic modeling
(enthalpy-based thermal evolution loop (EBTEL) model) we show that
this behavior is compatible with impulsively heated loops with a
volumetric heating rate that scales as {ɛ }{{H}}∼
{B}avg}0.3+/- 0.2/{L}0.2{+/-
0.10.2}.
Title: Advancing the Advective Flux Transport Model
Authors: Upton, Lisa; Ugarte-Urra, Ignacio; Warren, Harry
Bibcode: 2019AAS...23411802U
Altcode:
The Advective Flux Transport (AFT) model has proven to be a reliable
surface flux transport model for describing the evolution of the global
magnetic field, accurately reproducing the evolution of the polar
field. AFT has also been shown to accurately (within a factor of 2)
reproduce the evolution of the total unsigned flux of simple active
regions over the course of their lifetimes. Here we will discuss the
work being done to validate and advance the AFT model. We will discuss
the ability of AFT to reproduce other active region properties, such
as tilt angles, polarity separation, area expansion and magnetic
elements size distribution, for simple and more complex active
regions. Currently, AFT uses data assimilation to incorporate the
magnetic field from magnetograms from the Earth's vantage point. We will
also discuss the work that is being done to develop an automated process
for adding in far-side active regions observed by STEREO in 304 Å.
Title: Comprehensive Determination of the Hinode/EIS Roll Angle
Authors: Pelouze, Gabriel; Auchère, Frédéric; Bocchialini, Karine;
Harra, Louise; Baker, Deborah; Warren, Harry P.; Brooks, David H.;
Mariska, John T.
Bibcode: 2019SoPh..294...59P
Altcode: 2019arXiv190311923P
We present a new coalignment method for the EUV Imaging Spectrometer
(EIS) on board the Hinode spacecraft. In addition to the pointing
offset and spacecraft jitter, this method determines the roll angle
of the instrument, which has never been systematically measured, and
which is therefore usually not corrected. The optimal pointing for EIS
is computed by maximizing the cross-correlations of the Fe XII 195.119
Å line with images from the 193 Å band of the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO). By
coaligning 3336 rasters with high signal-to-noise ratio, we estimate
the rotation angle between EIS and AIA and explore the distribution
of its values. We report an average value of (−0.387±0.007 )
∘. We also provide a software implementation of this method
that can be used to coalign any EIS raster.
Title: Efficient Calculation of Non-local Thermodynamic Equilibrium
Effects in Multithreaded Hydrodynamic Simulations of Solar Flares
Authors: Reep, Jeffrey W.; Bradshaw, Stephen J.; Crump, Nicholas A.;
Warren, Harry P.
Bibcode: 2019ApJ...871...18R
Altcode: 2018arXiv180609574R
Understanding the dynamics of the chromosphere is crucial to
understanding energy transport across the solar atmosphere. The
chromosphere is optically thick at many wavelengths and described by
non-local thermodynamic equilibrium (NLTE), making it difficult to
interpret observations. Furthermore, there is considerable evidence
that the atmosphere is filamented, and that current instruments do not
resolve small-scale features. In flares, it is likely that multithreaded
models are required to describe the heating. The combination of NLTE
effects and multithreaded modeling requires computationally demanding
calculations, which has motivated the development of a model that can
efficiently treat both. We describe the implementation of a solver in a
hydrodynamic code for the hydrogen level populations that approximates
the NLTE solutions. We derive an accurate electron density across the
atmosphere that includes the effects of nonequilibrium ionization for
helium and metals. We show the effects on hydrodynamic simulations,
which are used to synthesize light curves using a postprocessing
radiative transfer code. We demonstrate the utility of this model on
IRIS observations of a small flare. We show that the Doppler shifts
in Mg II, C II, and O I can be explained with a multithreaded model
of loops subjected to electron beam heating, so long as NLTE effects
are treated. The intensities, however, do not match the observed values
very well, which is due to assumptions about the initial atmosphere. We
briefly show how altering the initial atmosphere can drastically alter
line profiles and derived quantities and suggest that it should be
tuned to preflare observations.
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.
Bibcode: 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: Incorporating Uncertainties in Atomic Data into the Analysis
of Solar and Stellar Observations: A Case Study in Fe XIII
Authors: Yu, Xixi; Del Zanna, Giulio; Stenning, David C.;
Cisewski-Kehe, Jessi; Kashyap, Vinay L.; Stein, Nathan; van Dyk,
David A.; Warren, Harry P.; Weber, Mark A.
Bibcode: 2018ApJ...866..146Y
Altcode: 2018arXiv180906173Y
Information about the physical properties of astrophysical objects
cannot be measured directly but is inferred by interpreting
spectroscopic observations in the context of atomic physics
calculations. Ratios of emission lines, for example, can be used
to infer the electron density of the emitting plasma. Similarly,
the relative intensities of emission lines formed over a wide range
of temperatures yield information on the temperature structure. A
critical component of this analysis is understanding how uncertainties
in the underlying atomic physics propagate to the uncertainties in
the inferred plasma parameters. At present, however, atomic physics
databases do not include uncertainties on the atomic parameters and
there is no established methodology for using them even if they
did. In this paper we develop simple models for uncertainties in
the collision strengths and decay rates for Fe XIII and apply them
to the interpretation of density-sensitive lines observed with the
EUV (extreme ultraviolet) Imagining spectrometer (EIS) on Hinode. We
incorporate these uncertainties in a Bayesian framework. We consider
both a pragmatic Bayesian method where the atomic physics information is
unaffected by the observed data, and a fully Bayesian method where the
data can be used to probe the physics. The former generally increases
the uncertainty in the inferred density by about a factor of 5 compared
with models that incorporate only statistical uncertainties. The latter
reduces the uncertainties on the inferred densities, but identifies
areas of possible systematic problems with either the atomic physics
or the observed intensities.
Title: Probing the evolution of a coronal cavity within a solar
coronal mass ejection.
Authors: Harra, Louise K.; Doschek, G. A.; Hara, Hirohisa; Long,
David; Warren, Harry; Matthews, Sarah; Lee, Kyoung-Sun; Jenkins, Jack
Bibcode: 2018cosp...42E1381H
Altcode:
On the 10 September 2017, an X-class solar flare erupted at the solar
limb. The associated coronal mass ejection (CME) had the classic three
part structure with a bright core surrounded by a dark cavity. This
event was captured perfectly by the Hinode EUV imaging spectrometer
(EIS). The EIS instrument captured spectroscopically the flaring
loops, the current sheet and the cavity for the first time. In the
'standard flare model', magnetic reconnection of coronal loops occurs
following the eruption of a magnetic flux rope. The flux rope is a key
element of the flare process and eruption but is inherently difficult
to observe. Dark cavities observed within a CME are assumed to be
flux ropes. The observations we describe here, provide an insight
into the characteristics of a cavity, and how the rapid injection of
energy from the flare underneath forces the rapid expansion of the
flux rope resulting in the eruption. Doppler shifts of over 200 km/s
are measured at either end of the cavity. There is mixed temperature
plasma - cool material in the centre that also has strong flows, and
hot FeXXIV emission being observed. SDO Atmospheric Imaging Assembly
(AIA) data shows that the cavity erupts rapidly, and is being driven
by the non-thermal energy input from the flare below as measured from
Fermi data.
Title: A Chandra/LETGS Survey of Main-sequence Stars
Authors: Wood, Brian E.; Laming, J. Martin; Warren, Harry P.;
Poppenhaeger, Katja
Bibcode: 2018ApJ...862...66W
Altcode: 2018arXiv180605111W
We analyze the X-ray spectra of 19 main-sequence stars observed
by Chandra using its LETGS configuration. Emission measure (EM)
distributions are computed based on emission line measurements, an
analysis that also yields evaluations of coronal abundances. The
use of newer atomic physics data results in significant changes
compared to past published analyses. The stellar EM distributions
correlate with surface X-ray flux (F X) in a predictable
way, regardless of spectral type. Thus, we provide EM distributions
as a function of F X, which can be used to estimate the
EM distribution of any main-sequence star with a measured broadband
X-ray luminosity. Comparisons are made with solar EM distributions,
both full-disk distributions and spatially resolved ones from active
regions (ARs), flares, and the quiet Sun. For moderately active stars,
the slopes and magnitudes of the EM distributions are in excellent
agreement with those of solar ARs for {log}T< 6.6, suggesting that
such stars have surfaces completely filled with solar-like ARs. A
stellar surface covered with solar X-class flares yields a reasonable
approximation for the EM distributions of the most active stars. Unlike
the EM distributions, coronal abundances are strongly dependent on
spectral type, and we provide relations with surface temperature
for both relative and absolute abundances. Finally, the coronal
abundances of the exoplanet host star τ Boo A (F7 V) are anomalous,
and we propose that this is due to the presence of the exoplanet.
Title: New solar diagnostics enabled by novel soft x-ray imaging
spectroscopy, and future missions
Authors: Caspi, Amir; Sylwester, Janusz; Gburek, Szymon; Crowley,
Geoff; Woods, Thomas; Shih, Albert Y.; DeForest, Craig; Steslicki,
Marek; Warren, Harry; Mason, James
Bibcode: 2018cosp...42E.525C
Altcode:
Solar soft X-ray (SXR) observations provide unique diagnostics of
plasma heating, during solar flares and quiescent times. Spectrally-
and temporally-resolved measurements are crucial for understanding the
dynamics and evolution of these energetic processes; spatially-resolved
measurements are essential for understanding energy transport. A
critical observational gap exists from ∼0.2 to ∼3 keV (∼4-60
Å), where spectrally-resolved stellar observations are plentiful
but have not been routinely made for the Sun in many decades. This
energy range includes spectral lines from highly-ionized atoms with
both low and high first ionization potential (FIP), as well as thermal
free-free (bremsstrahlung) and free-bound (radiative recombination)
continua. These SXR emissions provide crucial diagnostics of plasma
temperature distributions, as well as elemental abundances that
probe plasma origins over a wide range of temperatures, that are
not available from observations at other wavelengths. A better
understanding of thermal plasma also informs our interpretation of
hard X-ray (HXR) observations of nonthermal particles, improving our
understanding of the relationships between particle acceleration,
plasma heating, and the underlying release of magnetic energy during
reconnection.We discuss a proposed small satellite pathfinder mission,
the CubeSat Imaging X-ray Solar Spectrometer (CubIXSS), to measure
spectrally- and spatially-resolved SXRs from the quiescent and
flaring Sun from a 6U CubeSat platform in low-Earth orbit during
a nominal 1-year mission. CubIXSS includes the Amptek X123-FastSDD
silicon drift detector, a low-noise, commercial off-the-shelf (COTS)
instrument enabling full-Sun SXR spectroscopy from ∼0.5 to ∼20
keV with ∼0.15 keV FWHM spectral resolution with low power, mass,
and volume requirements. Multiple detectors and tailored apertures
provide sensitivity to SXR emission from deep solar minimum to >X5
flares. An X123-CdTe cadmium-telluride detector is also included for
∼5-50 keV HXR spectroscopy with ∼0.5 keV FWHM resolution. The
precise spectra from these instruments will provide detailed
measurements of the coronal temperature distribution and elemental
abundances during flares and quiescent times, and, for large flares,
context information of flare-accelerated electrons.CubIXSS also
includes a novel spectro-spatial imager - the first ever solar imager
on a CubeSat - utilizing a custom pinhole camera and Chandra-heritage
X-ray transmission diffraction grating to provide spatially- resolved,
full-Sun imaging spectroscopy from ∼0.2 to ∼10 keV (∼1-60
Å), with ∼25 arcsec and ∼0.25 Å FWHM spatial and spectral
resolutions, respectively. Additional pinholes with tailored filters
provide non-dispersed images with coarse spectral information to seed
analysis of the dispersed spectro-spatial images and for improved
sensitivity to quiescent conditions. MOXSI's unique capabilities
enable SXR spectroscopy and corresponding temperature and elemental
abundance diagnostics of individual flares and active regions over a
spectral range never before accessed by any prior solar mission.CubIXSS
is a pathfinder for larger satellites with improved resolution and
sensitivity. Through these groundbreaking new measurements, CubIXSS
and future missions will improve our physical understanding of thermal
plasma processes and impulsive energy release in the solar corona,
from quiet Sun to solar flares.
Title: Solar Cycle Observations of the Neon Abundance in the
Sun-as-a-star
Authors: Brooks, David H.; Baker, Deborah; van Driel-Gesztelyi, Lidia;
Warren, Harry P.
Bibcode: 2018ApJ...861...42B
Altcode: 2018arXiv180507032B
Properties of the Sun’s interior can be determined accurately
from helioseismological measurements of solar oscillations. These
measurements, however, are in conflict with photospheric elemental
abundances derived using 3D hydrodynamic models of the solar
atmosphere. This divergence of theory and helioseismology is known as
the “solar modeling problem.” One possible solution is that the
photospheric neon abundance, which is deduced indirectly by combining
the coronal Ne/O ratio with the photospheric O abundance, is larger
than generally accepted. There is some support for this idea from
observations of cool stars. The Ne/O abundance ratio has also been
found to vary with the solar cycle in the slowest solar wind streams
and coronal streamers, and the variation from solar maximum to minimum
in streamers (∼0.1-0.25) is large enough to potentially bring some
of the solar models into agreement with the seismic data. Here we use
daily sampled observations from the EUV Variability Experiment on the
Solar Dynamics Observatory taken in 2010-2014, to investigate whether
the coronal Ne/O abundance ratio shows a variation with the solar cycle
when the Sun is viewed as a star. We find only a weak dependence on,
and moderate anti-correlation with, the solar cycle with the ratio
measured around 0.2-0.3 MK falling from 0.17 at solar minimum to
0.11 at solar maximum. The effect is amplified at higher temperatures
(0.3-0.6 MK) with a stronger anti-correlation and the ratio falling
from 0.16 at solar minimum to 0.08 at solar maximum. The values we
find at solar minimum are too low to solve the solar modeling problem.
Title: On the Synthesis of GOES Light Curves from Numerical Models
Authors: Reep, Jeffrey W.; Warren, Harry P.
Bibcode: 2018RNAAS...2...48R
Altcode: 2018RNAAS...2b..48R
No abstract at ADS
Title: Toward a Quantitative Comparison of Magnetic Field
Extrapolations and Observed Coronal Loops
Authors: Warren, Harry P.; Crump, Nicholas A.; Ugarte-Urra, Ignacio;
Sun, Xudong; Aschwanden, Markus J.; Wiegelmann, Thomas
Bibcode: 2018ApJ...860...46W
Altcode: 2018arXiv180500281W
It is widely believed that loops observed in the solar atmosphere
trace out magnetic field lines. However, the degree to which magnetic
field extrapolations yield field lines that actually do follow loops
has yet to be studied systematically. In this paper, we apply three
different extrapolation techniques—a simple potential model, a
nonlinear force-free (NLFF) model based on photospheric vector data,
and an NLFF model based on forward fitting magnetic sources with
vertical currents—to 15 active regions that span a wide range of
magnetic conditions. We use a distance metric to assess how well each
of these models is able to match field lines to the 12202 loops traced
in coronal images. These distances are typically 1″-2″. We also
compute the misalignment angle between each traced loop and the local
magnetic field vector, and find values of 5°-12°. We find that the
NLFF models generally outperform the potential extrapolation on these
metrics, although the differences between the different extrapolations
are relatively small. The methodology that we employ for this study
suggests a number of ways that both the extrapolations and loop
identification can be improved.
Title: A Next Generation Spectrometer: The EUV High-Throughput
Spectroscopic Telescope (EUVST)
Authors: Warren, Harry
Bibcode: 2018tess.conf41003W
Altcode:
<span class="s1" In response to the Next Generation Solar Physics
Mission report, an advanced spectrometer has been proposed to JAXA's
competitively selected M-class missions science program. The main
scientific goal of the proposed instrument, the EUV High-Throughput
Spectroscopic Telescope (EUVST), is to understand the transfer
of mass and energy from the solar surface to the solar corona and
interplanetary space by observing fundamental processes occurring in the
solar atmosphere. The mission has two specific scientific objectives:
(I) to understand how fundamental processes lead to the formation
of the solar atmosphere and the solar wind, and (II) to understand
how the solar atmosphere becomes unstable, releasing the energy that
drives solar flares and eruptions. EUVST will make major advances by
combining a seamless temperature coverage of the solar chromosphere,
transition region, and corona with very high spatial resolution
(0.4ʺ or 300km) and unprecedented cadence (as high as 0.2s). This
instrument will complement new solar observatories such as DKIST,
the Parker Solar Probe, and Solar Orbiter that will be operational
during the proposed mission.
Title: The Magnetic Properties of High-Temperature Active Region Loops
Authors: Ugarte-Urra, Ignacio; Crump, Nicholas A.; Warren, Harry
Bibcode: 2018tess.conf22206U
Altcode:
Understanding the relationship between the magnetic field and coronal
heating is one of the central problems of solar physics. However,
studies of the magnetic properties of impulsively heated loops have
been rare. We present results from a study of 34 coronal loops observed
in the in the Fe XVIII line component of AIA/SDO 94 Å filter images
from three active regions with different magnetic conditions. We
show that the peak radiance per unit volume of the Fe XVIII loops is
correlated to their individual magnetic and geometric properties, namely
field strength (B) and length (L). These are inferred from magnetic
extrapolations of the photospheric field, in three approximations
(potential and two NLFF methods), thus providing an uncertainty in
our estimate of those quantities. Our results provide support, for
the first time at the scale of individual loops, to the B/L scaling
in the heating that has been successful in modeling full active regions.
Title: Spectroscopic Observations of Current Sheet Formation and
Evolution
Authors: Warren, Harry; Brooks, David; Ugarte-Urra, Ignacio; Crump,
Nicholas A.; Doschek, George A.; Stenborg, Guillermo; Reep, Jeffrey W.
Bibcode: 2018tess.conf31904W
Altcode:
<span class="s1" We report on the structure and evolution of
a current sheet that formed in the wake of an eruptive X8.3 flare
observed at the west limb of the Sun on September 10, 2017. Using
observations from the Hinode/EIS and SDO/AIA, we find that plasma
in the current sheet reaches temperatures of about 20MK and that the
range of temperatures is relatively narrow. The highest temperatures
occur at the base of the current sheet, in the region near the top
of the post-flare loop arcade. The broadest high temperature line
profiles, in contrast, occur at the largest observed heights. Further,
line broadening is strong very early in the flare and diminishes over
time. The current sheet can be observed in the AIA 211 and 171 channels,
which have a considerable contribution from thermal bremsstrahlung
at flare temperatures. Comparisons of the emission measure in these
channels with other EIS wavelengths and AIA channels dominated by
Fe line emission indicate a coronal composition and suggest that
the current sheet is formed by the heating of plasma already in the
corona. Finally, we also investigate the structure in the current sheet
as imaged by AIA and find clear evidence for collapsing loops. Taken
together, these observations suggest that some flare heating occurs
in the current sheet while additional energy is released as newly
reconnected field lines relax and become more dipolar.
Title: A Novel Soft X-ray Slitless Imaging Spectrograph for Unique
Diagnostics of Hot Coronal Plasma
Authors: Caspi, Amir; Shh, Albert Y.; Warren, Harry; Woods, Thomas
N.; Mason, James Paul; Steslicki, MArek; Gburek, Szymon; Sylwester,
Janusz; DeForest, Craig; Schwartz, Richard; Crowley, Geoff
Bibcode: 2018tess.conf41006C
Altcode:
Solar soft X-ray (SXR) observations from ∼0.2 to ∼3 keV
(∼4-60 Å), during both solar flares and quiescent times, provide
crucial diagnostics that are not available from observations at other
wavelengths. Specifically, SXRs reveal plasma temperature distributions,
as well as elemental abundances that probe plasma origins over a wide
range of temperatures. Spectrally- and temporally-resolved measurements
are essential for understanding the dynamics and evolution of these
energetic processes; spatially-resolved measurements are essential
for understanding energy transport. The NGSPM study calls out an X-ray
spectroscopic imager (T-10) as a high-priority instrument, in particular
with a spectral resolution of better than 100 eV for SXR emission
lines. We describe a novel approach for a spectro-spatial imager
- combining a pinhole camera with a X-ray transmission diffraction
grating - that can achieve the required combination of spectral and
angular resolutions at SXR energies. Such an instrument has already
been demonstrated as a protoype on a sounding-rocket flight and can be
proven thoroughly on a small satellite, specifically as part of the
instrument complement of the proposed CubeSat Imaging X-ray Solar
Spectrometer (CubIXSS) mission. CubIXSS will measure spectrally-
and spatially-resolved SXRs from ~1 to 60 Å (~0.2-10 keV) with ~0.25
Å and ~25 arcsec FWHM resolutions, respectively, from the quiescent
and flaring Sun from a 6U CubeSat platform in low-Earth orbit during
a nominal 1-year mission. Accordingly, CubIXSS is a pathfinder for
larger satellites with improved resolution (<0.1 Å, ~few arcsec)
and sensitivity, that could be integrated with focusing optics if
desired. Through these groundbreaking new measurements, CubIXSS and
future missions will improve our physical understanding of thermal
plasma processes and impulsive energy release in the solar corona,
from quiet Sun to solar flares.
Title: The Duration of Energy Deposition on Unresolved Flaring Loops
in the Solar Corona
Authors: Reep, Jeffrey W.; Polito, Vanessa; Warren, Harry P.; Crump,
Nicholas A.
Bibcode: 2018ApJ...856..149R
Altcode: 2018arXiv180208884R
Solar flares form and release energy across a large number of magnetic
loops. The global parameters of flares, such as the total energy
released, duration, physical size, etc., are routinely measured, and the
hydrodynamics of a coronal loop subjected to intense heating have been
extensively studied. It is not clear, however, how many loops comprise
a flare, nor how the total energy is partitioned between them. In
this work, we employ a hydrodynamic model to better understand the
energy partition by synthesizing Si IV and Fe XXI line emission and
comparing to observations of these lines with the Interface Region
Imaging Spectrograph (IRIS). We find that the observed temporal
evolution of the Doppler shifts holds important information on the
heating duration. To demonstrate this, we first examine a single loop
model, and find that the properties of chromospheric evaporation seen
in Fe XXI can be reproduced by loops heated for long durations, while
persistent redshifts seen in Si IV cannot be reproduced by any single
loop model. We then examine a multithreaded model, assuming both a
fixed heating duration on all loops and a distribution of heating
durations. For a fixed heating duration, we find that durations of
100-200 s do a fair job of reproducing both the red- and blueshifts,
while a distribution of durations, with a median of about 50-100 s,
does a better job. Finally, we compare our simulations directly to
observations of an M-class flare seen by IRIS, and find good agreement
between the modeled and observed values given these constraints.
Title: Plasma Evolution within an Erupting Coronal Cavity
Authors: Long, David M.; Harra, Louise K.; Matthews, Sarah A.; Warren,
Harry P.; Lee, Kyoung-Sun; Doschek, George A.; Hara, Hirohisa; Jenkins,
Jack M.
Bibcode: 2018ApJ...855...74L
Altcode: 2018arXiv180201391L
Coronal cavities have previously been observed to be associated
with long-lived quiescent filaments and are thought to correspond
to the associated magnetic flux rope. Although the standard flare
model predicts a coronal cavity corresponding to the erupting flux
rope, these have only been observed using broadband imaging data,
restricting an analysis to the plane-of-sky. We present a unique set of
spectroscopic observations of an active region filament seen erupting
at the solar limb in the extreme ultraviolet. The cavity erupted and
expanded rapidly, with the change in rise phase contemporaneous with an
increase in nonthermal electron energy flux of the associated flare. Hot
and cool filamentary material was observed to rise with the erupting
flux rope, disappearing suddenly as the cavity appeared. Although
strongly blueshifted plasma continued to be observed flowing from
the apex of the erupting flux rope, this outflow soon ceased. These
results indicate that the sudden injection of energy from the flare
beneath forced the rapid eruption and expansion of the flux rope,
driving strong plasma flows, which resulted in the eruption of an
under-dense filamentary flux rope.
Title: Coronal Elemental Abundances in Solar Emerging Flux Regions
Authors: Baker, Deborah; Brooks, David H.; van Driel-Gesztelyi,
Lidia; James, Alexander W.; Démoulin, Pascal; Long, David M.; Warren,
Harry P.; Williams, David R.
Bibcode: 2018ApJ...856...71B
Altcode: 2018arXiv180108424B
The chemical composition of solar and stellar atmospheres differs from
the composition of their photospheres. Abundances of elements with low
first ionization potential (FIP) are enhanced in the corona relative
to high-FIP elements with respect to the photosphere. This is known as
the FIP effect and it is important for understanding the flow of mass
and energy through solar and stellar atmospheres. We used spectroscopic
observations from the Extreme-ultraviolet Imaging Spectrometer on board
the Hinode observatory to investigate the spatial distribution and
temporal evolution of coronal plasma composition within solar emerging
flux regions inside a coronal hole. Plasma evolved to values exceeding
those of the quiet-Sun corona during the emergence/early-decay phase
at a similar rate for two orders of magnitude in magnetic flux, a rate
comparable to that observed in large active regions (ARs) containing
an order of magnitude more flux. During the late-decay phase, the rate
of change was significantly faster than what is observed in large,
decaying ARs. Our results suggest that the rate of increase during the
emergence/early-decay phase is linked to the fractionation mechanism
that leads to the FIP effect, whereas the rate of decrease during
the later decay phase depends on the rate of reconnection with the
surrounding magnetic field and its plasma composition.
Title: Photospheric and Coronal Abundances in an X8.3 Class Limb Flare
Authors: Doschek, G. A.; Warren, H. P.; Harra, L. K.; Culhane, J. L.;
Watanabe, T.; Hara, H.
Bibcode: 2018ApJ...853..178D
Altcode:
We analyze solar elemental abundances in coronal post-flare
loops of an X8.3 flare (SOL2017-09-10T16:06) observed on the west
limb on 2017 September 10 near 18 UT using spectra recorded by
the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode
spacecraft. The abundances in the corona can differ from photospheric
abundances due to the first ionization potential (FIP) effect. In
some loops of this flare, we find that the abundances appear to be
coronal at the loop apices or cusps, but steadily transform from
coronal to photospheric as the loop footpoint is approached. This
result is found from the intensity ratio of a low-FIP ion spectral
line (Ca XIV) to a high-FIP ion spectral line (Ar XIV) formed at
about the same temperature (4-5 MK). Both lines are observed close in
wavelength. Temperature, which could alter the interpretation, does
not appear to be a factor based on intensity ratios of Ca XV lines
to a Ca XIV line. We discuss the abundance result in terms of the
Laming model of the FIP effect, which is explained by the action of
the ponderomotive force in magnetohydrodynamic (MHD) waves in coronal
loops and in the underlying chromosphere.
Title: Spectroscopic Observations of Current Sheet Formation and
Evolution
Authors: Warren, Harry P.; Brooks, David H.; Ugarte-Urra, Ignacio;
Reep, Jeffrey W.; Crump, Nicholas A.; Doschek, George A.
Bibcode: 2018ApJ...854..122W
Altcode: 2017arXiv171110826W
We report on the structure and evolution of a current sheet that formed
in the wake of an eruptive X8.3 flare observed at the west limb of
the Sun on 2017 September 10. Using observations from the EUV Imaging
Spectrometer (EIS) on Hinode and the Atmospheric Imaging Assembly
(AIA) on the Solar Dynamics Observatory, we find that plasma in the
current sheet reaches temperatures of about 20 MK and that the range
of temperatures is relatively narrow. The highest temperatures occur
at the base of the current sheet, in the region near the top of the
post-flare loop arcade. The broadest high temperature line profiles,
in contrast, occur at the largest observed heights. Furthermore,
line broadening is strong very early in the flare and diminishes over
time. The current sheet can be observed in the AIA 211 and 171 channels,
which have a considerable contribution from thermal bremsstrahlung
at flare temperatures. Comparisons of the emission measure in these
channels with other EIS wavelengths and AIA channels dominated by
Fe line emission indicate a coronal composition and suggest that
the current sheet is formed by the heating of plasma already in the
corona. Taken together, these observations suggest that some flare
heating occurs in the current sheet, while additional energy is released
as newly reconnected field lines relax and become more dipolar.
Title: The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS)
Authors: Winebarger, A. R.; Savage, S. L.; Kobayashi, K.; Champey,
P. R.; McKenzie, D. E.; Golub, L.; Testa, P.; Reeves, K.; Cheimets,
P.; Cirtain, J. W.; Walsh, R. W.; Bradshaw, S. J.; Warren, H.; Mason,
H. E.; Del Zanna, G.
Bibcode: 2017AGUFMSH44A..06W
Altcode:
For over four decades, X-ray, EUV, and UV spectral observations have
been used to measure physical properties of the solar atmosphere. At
wavelengths below 10 nm, however, observations of the solar corona
with simultaneous spatial and spectral resolution are limited,
and not since the late 1970's have spatially resolved solar X-ray
spectra been measured. Because the soft X-ray regime is dominated
by emission lines formed at high temperatures, X-ray spectroscopic
techniques yield insights to fundamental physical processes that are
not accessible by any other means. Using a novel implementation of
corrective optics, the Marshall Grazing Incidence X-ray Spectrometer
(MaGIXS) will measure, for the first time, the solar spectrum from 0.6-
2.4 nm with a 6 arcsec resolution over an 8 arcmin slit. The MaGIXS
mission will address on of the fundamental problems of coronal physics:
the nature of coronal heating. There are several observables in the
MaGIXS wavelength range that will constrain the heating frequency and
hence discriminate between competing coronal heating theories. In this
presentation, we will present the MaGIXS scientific motivation and
provide an update on instrument development. MaGIXS will be launched
from White Sands Missile Range in the summer of 2019.
Title: Modeling Coronal Response in Decaying Active Regions with
Magnetic Flux Transport and Steady Heating
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.; Upton, Lisa A.;
Young, Peter R.
Bibcode: 2017ApJ...846..165U
Altcode: 2017arXiv170804324U
We present new measurements of the dependence of the extreme ultraviolet
(EUV) radiance on the total magnetic flux in active regions as obtained
from the Atmospheric Imaging Assembly (AIA) and the Helioseismic
and Magnetic Imager on board the Solar Dynamics Observatory. Using
observations of nine active regions tracked along different stages of
evolution, we extend the known radiance—magnetic flux power-law
relationship (I\propto {{{Φ }}}α ) to the AIA 335
Å passband, and the Fe xviii 93.93 Å spectral line in the 94 Å
passband. We find that the total unsigned magnetic flux divided by the
polarity separation ({{Φ }}/D) is a better indicator of radiance for
the Fe xviii line with a slope of α =3.22+/- 0.03. We then use these
results to test our current understanding of magnetic flux evolution
and coronal heating. We use magnetograms from the simulated decay of
these active regions produced by the Advective Flux Transport model
as boundary conditions for potential extrapolations of the magnetic
field in the corona. We then model the hydrodynamics of each individual
field line with the Enthalpy-based Thermal Evolution of Loops model with
steady heating scaled as the ratio of the average field strength and the
length (\bar{B}/L) and render the Fe xviii and 335 Å emission. We find
that steady heating is able to partially reproduce the magnitudes and
slopes of the EUV radiance—magnetic flux relationships and discuss
how impulsive heating can help reconcile the discrepancies. This
study demonstrates that combined models of magnetic flux transport,
magnetic topology, and heating can yield realistic estimates for the
decay of active region radiances with time.
Title: Sunspots, Starspots, and Elemental Abundances
Authors: Doschek, George A.; Warren, Harry P.
Bibcode: 2017SPD....4810601D
Altcode:
The composition of plasma in solar and stellar atmospheres is not fixed,
but varies from feature to feature. These variations are organized by
the First Ionization Potential (FIP) of the element. Solar measurements
often indicate that low FIP elements (< 10eV, such as Fe, Si, Mg)
are enriched by factors of 3-4 in the corona relative to high FIP
elements (>10 eV, such as C, N, O, Ar, He) compared to abundances
in the photosphere. Stellar observations have also shown similar
enrichments. An inverse FIP effect, where the low FIP elements are
depleted, has been observed in stellar coronae of stars believed
to have large starspots in their photospheres. The abundances are
important for determining radiative loss rates in models, tracing the
origin of the slow solar wind, and for understanding wave propagation
in the chromosphere and corona. Recently, inverse FIP effects have
been discovered in the Sun (Doschek, Warren, & Feldman 2015, ApJ,
808, L7) from spectra obtained by the Extreme-ultraviolet Imaging
Spectrometer (EIS) on the Hinode spacecraft. The inverse FIP regions
seem always to be near sunspots and cover only a very small area
(characteristic length = a few arcseconds). However, in pursuing the
search for inverse FIP regions, we have found that in some sunspot
groups the coronal abundance at a temperature of 3-4 MK can be near
photospheric over much larger areas of the sun near the sunspots (e.g.,
6,000 arcsec2). Also, sometimes the abundances at 3-4 MK
are in between coronal and photospheric values. This can occur in small
areas of an active region. It is predicted (Laming 2015, Sol. Phys., 12,
2) that the FIP effect should be highly variable in the corona. Several
examples of coronal abundance variations are presented. Our work
indicates that a comprehensive re-investigation of solar abundances
is highly desirable. This work is supported by a NASA Hinode grant.
Title: A Solar cycle correlation of coronal element abundances in
Sun-as-a-star observations
Authors: Brooks, David H.; Baker, Deborah; van Driel-Gesztelyi, Lidia;
Warren, Harry P.
Bibcode: 2017NatCo...8..183B
Altcode: 2018arXiv180200563B
The elemental composition in the coronae of low-activity solar-like
stars appears to be related to fundamental stellar properties such as
rotation, surface gravity, and spectral type. Here we use full-Sun
observations from the Solar Dynamics Observatory, to show that when
the Sun is observed as a star, the variation of coronal composition
is highly correlated with a proxy for solar activity, the F10.7 cm
radio flux, and therefore with the solar cycle phase. Similar cyclic
variations should therefore be detectable spectroscopically in X-ray
observations of solar analogs. The plasma composition in full-disk
observations of the Sun is related to the evolution of coronal magnetic
field activity. Our observations therefore introduce an uncertainty
into the nature of any relationship between coronal composition and
fixed stellar properties. The results highlight the importance of
systematic full-cycle observations for understanding the elemental
composition of solar-like stellar coronae.
Title: Modeling Active Region Evolution - at the Sun’s Surface
and into the Corona
Authors: Upton, Lisa; Ugarte-Urra, Ignacio; Warren, Harry; Young,
Peter R.
Bibcode: 2017SPD....4840502U
Altcode:
The STEREO mission provides the first opportunity to track the long-term
evolution of Active Regions over multiple rotations. The Advective Flux
Transport (AFT) model is a state of the art Surface Flux Transport
model, which simulates the observed near-surface flows to model
the transport of magnetic flux over the entire Sun. Combining STEREO
observations with AFT has allowed us to characterize the flux-luminosity
relationship for He 304 Å and to validate the far-side evolution of
individual active regions produced with AFT. Here, we present recent
results in which we extend this radiance - magnetic flux power-law
relationship to the AIA 335 Å passband, and the Fe XVIII 93.93 Å
spectral line in the 94 Å passband. We use these results to test
our current understanding of magnetic flux evolution and coronal
heating by modeling the hydrodynamics of individual field lines with
the Enthalpy-based Thermal Evolution of Loops (EBTEL) model including
steady heating scaled as the ratio of the average field strength and
the length (B/L). We find that steady heating is able to partially
reproduce the EUV radiance - magnetic flux relationships and their
observed temporal evolution. We also discuss how time-dependent
heating may be able to explain the remaining discrepancies. This
study demonstrates that combined models of magnetic flux transport,
magnetic topology and heating can yield realistic estimates for the
decay of active region radiances with time.
Title: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS)
Mission Concept
Authors: Caspi, Amir; Shih, Albert Y.; Warren, Harry; DeForest,
Craig; Laurent, Glenn Thomas; Schwartz, Richard A.; Woods, Thomas
N.; Mason, James; Palo, Scott; Steslicki, Marek; Sylwester, Janusz;
Gburek, Szymon; Mrozek, Tomasz; Kowalinski, Miroslaw; Torre, Gabriele;
Crowley, Geoffrey; Schattenburg, Mark
Bibcode: 2017SPD....4830503C
Altcode:
Solar soft X-ray (SXR) observations provide important diagnostics of
plasma heating, during solar flares and quiescent times. Spectrally-
and temporally-resolved measurements are crucial for understanding
the dynamics, origins, and evolution of these energetic processes,
providing probes both into the temperature distributions and elemental
compositions of hot plasmas; spatially-resolved measurements are
critical for understanding energy transport and mass flow. A better
understanding of the thermal plasma improves our understanding of the
relationships between particle acceleration, plasma heating, and the
underlying release of magnetic energy during reconnection. We introduce
a new proposed small satellite mission, the CubeSat Imaging X-ray Solar
Spectrometer (CubIXSS), to measure spectrally- and spatially-resolved
SXRs from the quiescent and flaring Sun from a 6U CubeSat platform in
low-Earth orbit during a nominal 1-year mission. CubIXSS includes the
Amptek X123-FastSDD silicon drift detector, a low-noise, commercial
off-the-shelf (COTS) instrument enabling solar SXR spectroscopy from
~0.5 to ~30 keV with ~0.15 keV FWHM spectral resolution with low
power, mass, and volume requirements. Multiple detectors and tailored
apertures provide sensitivity to a wide range of solar conditions,
optimized for a launch during solar minimum. The precise spectra
from these instruments will provide detailed measurements of the
coronal temperature distribution and elemental abundances from the
quiet Sun to active regions and flares. CubIXSS also includes a
novel spectro-spatial imager -- the first ever solar imager on a
CubeSat -- utilizing a custom pinhole camera and Chandra-heritage
X-ray transmission diffraction grating to provide spatially- resolved,
full-Sun imaging spectroscopy from ~0.1 to ~10 keV, with ~25 arcsec and
~0.1 Å FWHM spatial and spectral resolutions, respectively. MOXSI’s
unique capabilities enable SXR spectroscopy and temperature diagnostics
of individual active regions and flares. Through its groundbreaking
new measurements, CubIXSS will improve our physical understanding of
thermal plasma processes and impulsive energy release in the solar
corona, from quiet Sun to solar flares.
Title: Sunspots, Starspots, and Elemental Abundances
Authors: Doschek, G. A.; Warren, H. P.
Bibcode: 2017ApJ...844...52D
Altcode:
Element abundances in the solar photosphere, chromosphere, transition
region, and corona are key parameters for investigating sources of
the solar wind and for estimating radiative losses in the quiet corona
and in dynamical events such as solar flares. Abundances in the solar
corona and photosphere differ from each other depending on the first
ionization potential (FIP) of the element. Normally, abundances with
FIP values less than about 10 eV are about 3-4 times more abundant in
the corona than in the photosphere. However, recently, an inverse FIP
effect was found in small regions near sunspots where elements with
FIP less than 10 eV are less abundant relative to high FIP elements
(≥slant 10 eV) than they are in the photosphere. This is similar to
fully convective stars with large starspots. The inverse FIP effect
is predicted to occur in the vicinity of sunspots/starspots. Up to
now, the solar anomalous abundances have only been found in very
spatially small areas. In this paper, we show that in the vicinity of
sunspots there can be substantially larger areas with abundances that
are between coronal and photospheric abundances and sometimes just
photospheric abundances. In some cases, the FIP effect tends to shut
down near sunspots. We examine several active regions with relatively
large sunspots that were observed with the Extreme-ultraviolet Imaging
Spectrometer on the Hinode spacecraft in cycle 24.
Title: Measuring Velocities in the Early Stage of an Eruption:
Using “Overlappogram” Data from Hinode EIS
Authors: Harra, Louise K.; Hara, Hirohisa; Doschek, George A.;
Matthews, Sarah; Warren, Harry; Culhane, J. Leonard; Woods, Magnus M.
Bibcode: 2017ApJ...842...58H
Altcode:
In order to understand the onset phase of a solar eruption, plasma
parameter measurements in the early phases are key to constraining
models. There are two current instrument types that allow us to make
such measurements: narrow-band imagers and spectrometers. In the
former case, even narrow-band filters contain multiple emission lines,
creating some temperature confusion. With imagers, however, rapid
cadences are achievable and the field of view can be large. Velocities
of the erupting structures can be measured by feature tracking. In the
spectrometer case, slit spectrometers can provide spectrally pure images
by “rastering” the slit to build up an image. This method provides
limited temporal resolution, but the plasma parameters can be accurately
measured, including velocities along the line of sight. Both methods
have benefits and are often used in tandem. In this paper we demonstrate
for the first time that data from the wide slot on the Hinode EUV
Imaging Spectrometer, along with imaging data from AIA, can be used to
deconvolve velocity information at the start of an eruption, providing
line-of-sight velocities across an extended field of view. Using He
II 256 Å slot data at flare onset, we observe broadening or shift(s)
of the emission line of up to ±280 km s-1. These are seen at
different locations—the redshifted plasma is seen where the hard X-ray
source is later seen (energy deposition site). In addition, blueshifted
plasma shows the very early onset of the fast rise of the filament.
Title: Tracking the Magnetic Flux in and around Sunspots
Authors: Sheeley, N. R., Jr.; Stauffer, J. R.; Thomassie, J. C.;
Warren, H. P.
Bibcode: 2017ApJ...836..144S
Altcode:
We have developed a procedure for tracking sunspots observed by the
Helioseismic and Magnetic Imager on the Solar Dynamics Observatory
and for making curvature-corrected space/time maps of the associated
line-of-sight magnetic field and continuum intensity. We apply
this procedure to 36 sunspots, each observed continuously for nine
days around its central meridian passage time, and find that the
proper motions separate into two distinct components depending on
their speeds. Fast (∼3-5 km s-1) motions, comparable
to Evershed flows, are produced by weak vertical fluctuations of the
horizontal canopy field and recur on a timescale of 12-20 min. Slow
(∼0.3-0.5 km s-1) motions diverge from a sunspot-centered
ring whose location depends on the size of the sunspot, occurring in the
mid-penumbra for large sunspots and at the outer edge of the penumbra
for small sunspots. The slow ingoing features are contracting spokes of
a quasi-vertical field of umbral polarity. These inflows disappear when
the sunspot loses its penumbra, and may be related to inward-moving
penumbral grain. The slow outgoing features may have either polarity
depending on whether they originate from quasi-vertical fields of umbral
polarity or from the outer edge of the canopy. When a sunspot decays,
the penumbra and canopy disappear, and the moat becomes filled with
slow outflows of umbral polarity. We apply our procedure to decaying
sunspots, to long-lived sunspots, and to numerical simulations of a
long-lived sunspot by Rempel.
Title: Sparse Bayesian Inference and the Temperature Structure of
the Solar Corona
Authors: Warren, Harry P.; Byers, Jeff M.; Crump, Nicholas A.
Bibcode: 2017ApJ...836..215W
Altcode: 2016arXiv161005972W
Measuring the temperature structure of the solar atmosphere is critical
to understanding how it is heated to high temperatures. Unfortunately,
the temperature of the upper atmosphere cannot be observed directly,
but must be inferred from spectrally resolved observations of
individual emission lines that span a wide range of temperatures. Such
observations are “inverted” to determine the distribution of plasma
temperatures along the line of sight. This inversion is ill posed and,
in the absence of regularization, tends to produce wildly oscillatory
solutions. We introduce the application of sparse Bayesian inference
to the problem of inferring the temperature structure of the solar
corona. Within a Bayesian framework a preference for solutions that
utilize a minimum number of basis functions can be encoded into the
prior and many ad hoc assumptions can be avoided. We demonstrate the
efficacy of the Bayesian approach by considering a test library of 40
assumed temperature distributions.
Title: Global Energetics of Solar Flares. V. Energy Closure in Flares
and Coronal Mass Ejections
Authors: Aschwanden, Markus J.; Caspi, Amir; Cohen, Christina M. S.;
Holman, Gordon; Jing, Ju; Kretzschmar, Matthieu; Kontar, Eduard
P.; McTiernan, James M.; Mewaldt, Richard A.; O'Flannagain, Aidan;
Richardson, Ian G.; Ryan, Daniel; Warren, Harry P.; Xu, Yan
Bibcode: 2017ApJ...836...17A
Altcode: 2017arXiv170101176A
In this study we synthesize the results of four previous studies
on the global energetics of solar flares and associated coronal
mass ejections (CMEs), which include magnetic, thermal, nonthermal,
and CME energies in 399 solar M- and X-class flare events observed
during the first 3.5 yr of the Solar Dynamics Observatory (SDO)
mission. Our findings are as follows. (1) The sum of the mean
nonthermal energy of flare-accelerated particles ({E}{nt}),
the energy of direct heating ({E}{dir}), and the
energy in CMEs ({E}{CME}), which are the primary
energy dissipation processes in a flare, is found to have a ratio of
({E}{nt}+{E}{dir}+{E}{CME})/{E}{mag}=0.87+/-
0.18, compared with the dissipated magnetic free energy
{E}{mag}, which confirms energy closure within the
measurement uncertainties and corroborates the magnetic origin of
flares and CMEs. (2) The energy partition of the dissipated magnetic
free energy is: 0.51 ± 0.17 in nonthermal energy of ≥slant 6 {keV}
electrons, 0.17 ± 0.17 in nonthermal ≥slant 1 {MeV} ions, 0.07 ±
0.14 in CMEs, and 0.07 ± 0.17 in direct heating. (3) The thermal
energy is almost always less than the nonthermal energy, which is
consistent with the thick-target model. (4) The bolometric luminosity
in white-light flares is comparable to the thermal energy in soft
X-rays (SXR). (5) Solar energetic particle events carry a fraction
≈ 0.03 of the CME energy, which is consistent with CME-driven shock
acceleration. (6) The warm-target model predicts a lower limit of the
low-energy cutoff at {e}c≈ 6 {keV}, based on the mean peak
temperature of the differential emission measure of T e =
8.6 MK during flares. This work represents the first statistical study
that establishes energy closure in solar flare/CME events.
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.
Bibcode: 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.
Title: Diagnosing Coronal Heating Processes with Spectrally Resolved
Soft X-ray Measurements
Authors: Caspi, Amir; Shih, Albert Y.; Warren, Harry P.; Stęślicki,
Marek; Sylwester, Janusz
Bibcode: 2017arXiv170100619C
Altcode:
Decades of astrophysical observations have convincingly shown that
soft X-ray (SXR; ~0.1--10 keV) emission provides unique diagnostics
for the high temperature plasmas observed in solar flares and active
regions. SXR observations critical for constraining models of energy
release in these phenomena can be provided using instruments that
have already been flown on sounding rockets and CubeSats, including
miniaturized high-resolution photon-counting spectrometers and a
novel diffractive spectral imager. These instruments have relatively
low cost and high TRL, and would complement a wide range of mission
concepts. In this white paper, we detail the scientific background and
open questions motivating these instruments, the measurements required,
and the instruments themselves that will make groundbreaking progress
in answering these questions.
Title: Propagation of atmospheric density errors to satellite orbits
Authors: Emmert, J. T.; Warren, H. P.; Segerman, A. M.; Byers, J. M.;
Picone, J. M.
Bibcode: 2017AdSpR..59..147E
Altcode:
We develop and test approximate analytic expressions relating
time-dependent atmospheric density errors to errors in the mean motion
and mean anomaly orbital elements. The mean motion and mean anomaly
errors are proportional to the first and second integrals, respectively,
of the density error. This means that the mean anomaly (and hence the
in-track position) error variance grows with time as t3
for a white noise density error process and as t5 for a
Brownian motion density error process. Our approximate expressions
are accurate over a wide range of orbital configurations, provided
the perigee altitude change is less than ∼0.2 atmospheric scale
heights. For orbit prediction, density forecasts are driven in large
part by forecasts of solar extreme ultraviolet (EUV) irradiance; we
show that errors in EUV ten-day forecasts (and consequently in the
density forecasts) approximately follow a Brownian motion process.
Title: Advancing our Understanding of Active Region Evolution and
Surface Flux Transport Using Far Side Imaging from STEREO 304
Authors: Upton, L.; Ugarte-Urra, I.; Warren, H. P.; Hathaway, D. H.
Bibcode: 2016AGUFMSH42B..02U
Altcode:
The STEREO mission, combined with SDO, provides a unique opportunity
to view the solar surface continuously. These continuous observations
provide the first opportunity to track the long-term evolution of Active
Regions over multiple rotations. We present recent results in which we
illustrate how He 304 Å images can be used as a proxies for magnetic
flux measurements. We will present the long-term evolution of select
isolated Active Regions as seen in He 304 Å. These data are then
used to validate the far-side evolution of individual active regions
produced with our Advective Flux Transport model - AFT. The AFT model
is a state of the art Surface Flux Transport model, which simulates
the observed near-surface flows (including an evolving convective flow
velocity field) to model the transport of magnetic flux over the entire
Sun. Finally, we will show that when new flux emergence occurs on the
far-side of the Sun, 304 Å images can provide sufficient information
about the active region to predict its evolution. These far-side Active
Regions have a substantial impact on the coronal and interplanetary
field configuration used for space weather predictions.
Title: Combining MinXSS and RHESSI X-ray Spectra for a Comprehensive
View of the Temperature Distribution in Solar Flares
Authors: Caspi, A.; McTiernan, J. M.; Mason, J. P.; Moore, C. S.;
Shih, A. Y.; Warren, H.; Woods, T. N.
Bibcode: 2016AGUFMSH13A2288C
Altcode:
Solar flares explosively release large amounts of magnetic energy,
a significant fraction of which goes into transient heating of coronal
plasma to temperatures up to tens of MK. Decades of observations have
shown that flares are multi-thermal, exhibiting broad temperature
distributions or "differential emission measures" (DEMs). Recent
studies suggest that the hottest parts of the DEM evolve differently
from, and are heated by a different physical mechanism than, the
DEM bulk. For example, the peak temperature of the hot, likely
in-situ-heated plasma observed by RHESSI correlates significantly
differently with flare intensity (GOES class) than does the cooler,
likely chromospherically evaporated plasma observed by GOES XRS
and/or Yohkoh BCS. These studies, however, used discrete (iso-/bi-)
thermal approximations, in part because temperature determinations
by the ratio of 2-channel GOES photometer data or selected BCS lines
necessitated such methods. Consequently, the exact DEM profile, its
evolution, and how these correlate with other flare parameters, remain
poorly known. The MinXSS CubeSat deployed from the ISS in May 2016,
and since June has observed (at least) 7 M-class and over 40 C-class
flares. MinXSS's X123 spectrometer measures solar soft X-rays (SXRs)
from 0.5 to 30 keV with 0.15 keV FWHM resolution; this energy range
entirely covers both GOES XRS passbands, and overlaps with and extends
the RHESSI observing range with 5x better resolution. It includes the
thermal continuum emission from plasmas with temperatures down to 2 MK,
as well as a number of mid- and high-temperature spectral lines from
various low- and high-FIP ion species, providing critical temperature
diagnostics for studying flare DEMs with far greater fidelity than is
possible with GOES, or using RHESSI alone. We present spectral analyses
of several flares observed simultaneously by MinXSS and RHESSI. We
compare and contrast the observations of each instrument separately, and
present the results of a joint-instrument DEM analysis that forward-fits
a parametrized DEM model - including variable elemental abundances -
to the combined spectra of both instruments simultaneously. We discuss
the DEM evolution and its correlation with other flare parameters,
and discuss the implications for plasma heating in solar flares.
Title: Observational Signatures of Coronal Heating
Authors: Dahlburg, R. B.; Einaudi, G.; Ugarte-Urra, I.; Warren, H. P.;
Rappazzo, A. F.; Velli, M.; Taylor, B.
Bibcode: 2016AGUFMSH42A..06D
Altcode:
Recent research on observational signatures of turbulent heating of
a coronal loop will be discussed. The evolution of the loop is is
studied by means of numericalsimulations of the fully compressible
three-dimensionalmagnetohydrodynamic equations using the HYPERION
code. HYPERION calculates the full energy cycle involving footpoint
convection, magnetic reconnection,nonlinear thermal conduction and
optically thin radiation.The footpoints of the loop magnetic field
are convected by random photospheric motions. As a consequence
the magnetic field in the loop is energized and develops turbulent
nonlinear dynamics characterized by the continuous formation and
dissipation of field-aligned current sheets: energy is deposited
at small scales where heating occurs. Dissipation is non-uniformly
distributed so that only a fraction of thecoronal mass and volume gets
heated at any time. Temperature and density are highly structured at
scales which, in the solar corona, remain observationally unresolved:
the plasma of the simulated loop is multi-thermal, where highly
dynamical hotter and cooler plasma strands arescattered throughout
the loop at sub-observational scales. Typical simulated coronal loops
are 50000 km length and have axial magnetic field intensities ranging
from 0.01 to 0.04 Tesla.To connect these simulations to observations
the computed numberdensities and temperatures are used to synthesize
the intensities expected inemission lines typically observed with
the Extreme ultraviolet Imaging Spectrometer(EIS) on Hinode. These
intensities are then employed to compute differentialemission measure
distributions, which are found to be very similar to those derivedfrom
observations of solar active regions.
Title: Linear and Non-Linear Forecasts of Solar Activity
Authors: Warren, H.
Bibcode: 2016AGUFMSH11C2237W
Altcode:
Variations in thermospheric density play a major role in perturbing the
orbits of objects in low Earth orbit. These variations are strongly
influenced by changes in the solar irradiance at extreme ultraviolet
(EUV) wavelengths that are ultimately driven by changing levels of
solar magnetic activity. Thus predicting the conjunction of operational
satellites with orbital debris requires accurate forecasts of solar
activity. Current operational models rely on forecasts of proxies
for solar activity based on simple linear extrapolation methods. In
this poster we present a systematic study of these methods applied
to the 10.7 cm solar radio flux, a composite Mg core-to-wing ratio,
the total unsigned solar magnetic flux, and the He II 304 irradiance
observed by the EVE instrument on the Solar Dynamics Observatory. We
find that although RMS errors in these forecasts appear to be small,
the corresponding errors in very simple models, such as the persistence
of the last measurement, are also small, and the formal skill scores
are relatively modest. The use of these proxies and measurements in
non-linear methods, such Gaussian process regression and recurrent
neural networks, will also be discussed.
Title: The importance of high-resolution observations of the solar
corona
Authors: Winebarger, A. R.; Cirtain, J. W.; Golub, L.; Walsh, R. W.;
De Pontieu, B.; Savage, S. L.; Rachmeler, L.; Kobayashi, K.; Testa,
P.; Brooks, D.; Warren, H.; Mcintosh, S. W.; Peter, H.; Morton, R. J.;
Alexander, C. E.; Tiwari, S. K.
Bibcode: 2016AGUFMSH31B2577W
Altcode:
The spatial and temporal resolutions of the available coronal
observatories are inadequate to resolve the signatures of coronal
heating. High-resolution and high-cadence observations available with
the Interface Region Imaging Spectrograph (IRIS) and the High-resolution
Coronal Imager (Hi-C) instrument hint that 0.3 arcsec resolution images
and < 10 s cadence provide the necessary resolution to detect
heating events. Hi-C was launched from White Sands Missile Range on
July 11, 2012 (before the launch with IRIS) and obtained images of
a solar active region in the 19.3 nm passband. In this presentation,
I will discuss the potential of combining a flight in Hi-C with a 17.1
nm passband, in conjunction with IRIS. This combination will provide,
for the first time, a definitive method of tracing the energy flow
between the chromosphere and corona and vice versa.
Title: Solar Soft X-ray Spectral Measurements and the Temperature
Structure of Active Regions and Flares
Authors: Warren, H.
Bibcode: 2016AGUFMSH11D..01W
Altcode: 2016AGUFMSH11D..01C
How solar and stellar atmospheres are heated to millions of degrees
is a fundamental problem in astrophysics. The Parker nanoflare model,
in which the topological complexity created by turbulent photospheric
motions is dissipated by magnetic reconnection, is perhaps the most
widely studied theory of coronal heating. Although this model is
conceptually similar to our understanding of how large flares work,
recent results suggest that they may be fundamentally different. Large
flares, for example, have a peak in the emission measure distribution
near 10 MK, while active regions appear to have relatively little
plasma at that temperature. For large flares, several studies have
indicated a composition close to that of the photosphere, while
active region structures show a clear enhancement in elements with
low first ionization potential. These results rely on observations
at extreme ultraviolet wavelengths, which do not provide the rich
array of temperature and abundance diagnostics that are available
at soft X-ray wavelengths. In this talk we will review these recent
results and explore the potential for observations from new soft X-ray
instrumentation such as MinXSS to advance our understanding of coronal
heating mechanisms.
Title: The EVE plus RHESSI DEM for Solar Flares, and Implications
for Residual Non-Thermal Soft X-Ray Emission
Authors: McTiernan, J. M.; Caspi, A.; Warren, H.
Bibcode: 2016AGUFMSH13A2289M
Altcode:
We combine observations of solar flares from the EUV Variability
Experiment (EVE) on-board the Solar Dynamics Observatory (SDO) with
X-ray data from the Reuven Ramaty High Energy Spectroscopic Imager
(RHESSI) to calculate the Differential Emission Measure (DEM). This
improvement over the isothermal approximation is intended to help
to resolve ambiguities in the range where thermal and non-thermal
emission overlap. For this current project we are interested in
constraining cutoffs in the "residual" non-thermal spectrum; i.e.,
the RHESSI spectrum that is left over after the DEM has accounted
for the bulk of the soft X-ray emission. (Previous work by Caspi
et.al. 2014ApJ...788L..31C concentrated on obtaining DEM models that
fit both instruments' observations well). Solar flare spectra are
typically dominated by thermal bremsstrahlung emission in the soft
X-ray (< 10 keV) energy range; at higher hard X-ray energies (>
30 keV) the emission is non-thermal from beams of electrons. The low
energy extent of non-thermal emission can typically only be loosely
quantified. In particular, it is difficult to obtain a lower limit
for any possible non-thermal cutoff energy due to the larger amount of
thermal emission. In this model, thermal emission is due to a DEM that
is parametrized as multiple gaussians in Log(T). Non-thermal emission
is modeled as a photon spectrum obtained using thin and thick-target
emission models. Spectra for both instruments are fit simultaneously
in a self-consistent manner. Preliminary results have been obtained
using a sample of 102 large (GOES X and M class) solar flares observed
between February 2011 and February 2013. These results show that it
is possible to determine low energy cutoffs and breaks early during
large flares, and to get good values for the low energy limit to the
non-thermal cutoff.
Title: The Electron Density in Explosive Transition Region Events
Observed by IRIS
Authors: Doschek, G. A.; Warren, H. P.; Young, P. R.
Bibcode: 2016ApJ...832...77D
Altcode:
We discuss the intensity ratio of the O IV line at 1401.16 Å to the
Si IV line at 1402.77 Å in Interface Region Imaging Spectrograph
(IRIS) spectra. This intensity ratio is important if it can be used
to measure high electron densities that cannot be measured using line
intensity ratios of two different O IV lines from the multiplet within
the IRIS wavelength range. Our discussion is in terms of considerably
earlier observations made from the Skylab manned space station and
other spectrometers on orbiting spacecraft. The earlier data on the O
IV and Si IV ratio and other intersystem line ratios not available to
IRIS are complementary to IRIS data. In this paper, we adopt a simple
interpretation based on electron density. We adopt a set of assumptions
and calculate the electron density as a function of velocity in the Si
IV line profiles of two explosive events. At zero velocity the densities
are about 2-3 × 1011 cm-3, and near 200 km
s-1 outflow speed the densities are about 1012
cm-3. The densities increase with outflow speed up to
about 150 km s-1 after which they level off. Because of the
difference in the temperature of formation of the two lines and other
possible effects such as non-ionization equilibrium, these density
measurements do not have the precision that would be available if
there were some additional lines near the formation temperature of O IV.
Title: Transition Region and Chromospheric Signatures of Impulsive
Heating Events. I. Observations
Authors: Warren, Harry P.; Reep, Jeffrey W.; Crump, Nicholas A.;
Simões, Paulo J. A.
Bibcode: 2016ApJ...829...35W
Altcode: 2016arXiv160609045W
We exploit the high spatial resolution and high cadence of the Interface
Region Imaging Spectrograph (IRIS) to investigate the response of
the transition region and chromosphere to energy deposition during
a small flare. Simultaneous observations from the Reuven Ramaty
High Energy Solar Spectroscopic Imager provide constraints on the
energetic electrons precipitating into the flare footpoints, while
observations of the X-Ray Telescope, Atmospheric Imaging Assembly, and
Extreme Ultraviolet Imaging Spectrometer (EIS) allow us to measure
the temperatures and emission measures from the resulting flare
loops. We find clear evidence for heating over an extended period on
the spatial scale of a single IRIS pixel. During the impulsive phase
of this event, the intensities in each pixel for the Si IV 1402.770 Å,
C II 1334.535 Å, Mg II 2796.354 Å, and O I 1355.598 Å emission lines
are characterized by numerous small-scale bursts typically lasting 60
s or less. Redshifts are observed in Si IV, C II, and Mg II during the
impulsive phase. Mg II shows redshifts during the bursts and stationary
emission at other times. The Si IV and C II profiles, in contrast, are
observed to be redshifted at all times during the impulsive phase. These
persistent redshifts are a challenge for one-dimensional hydrodynamic
models, which predict only short-duration downflows in response to
impulsive heating. We conjecture that energy is being released on many
small-scale filaments with a power-law distribution of heating rates.
Title: Correlation of Coronal Plasma Properties and Solar Magnetic
Field in a Decaying Active Region
Authors: Ko, Yuan-Kuen; Young, Peter R.; Muglach, Karin; Warren,
Harry P.; Ugarte-Urra, Ignacio
Bibcode: 2016ApJ...826..126K
Altcode:
We present the analysis of a decaying active region observed by
the EUV Imaging Spectrometer on Hinode during 2009 December 7-11. We
investigated the temporal evolution of its structure exhibited by plasma
at temperatures from 300,000 to 2.8 million degrees, and derived the
electron density, differential emission measure, effective electron
temperature, and elemental abundance ratios of Si/S and Fe/S (as a
measure of the First Ionization Potential (FIP) Effect). We compared
these coronal properties to the temporal evolution of the photospheric
magnetic field strength obtained from the Solar and Heliospheric
Observatory Michelson Doppler Imager magnetograms. We find that, while
these coronal properties all decreased with time during this decay
phase, the largest change was at plasma above 1.5 million degrees. The
photospheric magnetic field strength also decreased with time but
mainly for field strengths lower than about 70 Gauss. The effective
electron temperature and the FIP bias seem to reach a “basal” state
(at 1.5 × 106 K and 1.5, respectively) into the quiet Sun
when the mean photospheric magnetic field (excluding all areas <10 G)
weakened to below 35 G, while the electron density continued to decrease
with the weakening field. These physical properties are all positively
correlated with each other and the correlation is the strongest in
the high-temperature plasma. Such correlation properties should be
considered in the quest for our understanding of how the corona is
heated. The variations in the elemental abundance should especially
be considered together with the electron temperature and density.
Title: Transition Region and Chromospheric Signatures of Impulsive
Heating Events. II. Modeling
Authors: Reep, Jeffrey W.; Warren, Harry P.; Crump, Nicholas A.;
Simões, Paulo J. A.
Bibcode: 2016ApJ...827..145R
Altcode: 2016arXiv160706684R
Results from the Solar Maximum Mission showed a close connection
between the hard X-ray (HXR) and transition region (TR) emission in
solar flares. Analogously, the modern combination of RHESSI and IRIS
data can inform the details of heating processes in ways that were
never before possible. We study a small event that was observed with
RHESSI, IRIS, SDO, and Hinode, allowing us to strongly constrain the
heating and hydrodynamical properties of the flare, with detailed
observations presented in a previous paper. Long duration redshifts
of TR lines observed in this event, as well as many other events,
are fundamentally incompatible with chromospheric condensation
on a single loop. We combine RHESSI and IRIS data to measure the
energy partition among the many magnetic strands that comprise the
flare. Using that observationally determined energy partition, we show
that a proper multithreaded model can reproduce these redshifts in
magnitude, duration, and line intensity, while simultaneously being
well constrained by the observed density, temperature, and emission
measure. We comment on the implications for both RHESSI and IRIS
observations of flares in general, namely that: (1) a single loop model
is inconsistent with long duration redshifts, among other observables;
(2) the average time between energization of strands is less than 10
s, which implies that for a HXR burst lasting 10 minutes, there were
at least 60 strands within a single IRIS pixel located on the flare
ribbon; (3) the majority of these strands were explosively heated with
an energy distribution well described by a power law of slope ≈ -1.6;
(4) the multi-stranded model reproduces the observed line profiles,
peak temperatures, differential emission measure distributions,
and densities.
Title: Properties and Modeling of Unresolved Fine Structure Loops
Observed in the Solar Transition Region by IRIS
Authors: Brooks, David H.; Reep, Jeffrey W.; Warren, Harry P.
Bibcode: 2016ApJ...826L..18B
Altcode: 2016arXiv160605440B
Recent observations from the Interface Region Imaging Spectrograph
(IRIS) have discovered a new class of numerous low-lying dynamic loop
structures, and it has been argued that they are the long-postulated
unresolved fine structures (UFSs) that dominate the emission of the
solar transition region. In this letter, we combine IRIS measurements
of the properties of a sample of 108 UFSs (intensities, lengths, widths,
lifetimes) with one-dimensional non-equilibrium ionization simulations,
using the HYDRAD hydrodynamic model to examine whether the UFSs are now
truly spatially resolved in the sense of being individual structures
rather than being composed of multiple magnetic threads. We find that
a simulation of an impulsively heated single strand can reproduce most
of the observed properties, suggesting that the UFSs may be resolved,
and the distribution of UFS widths implies that they are structured on
a spatial scale of 133 km on average. Spatial scales of a few hundred
kilometers appear to be typical for a range of chromospheric and
coronal structures, and we conjecture that this could be an important
clue for understanding the coronal heating process.
Title: The Mysterious Case of the Solar Argon Abundance near Sunspots
in Flares
Authors: Doschek, G. A.; Warren, H. P.
Bibcode: 2016ApJ...825...36D
Altcode:
Recently we discussed an enhancement of the abundance of Ar xiv relative
to Ca xiv near a sunspot during a flare, observed in spectra recorded
by the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode
spacecraft. The observed Ar xiv/Ca xiv ratio yields an argon/calcium
abundance ratio seven times greater than expected from the photospheric
abundance. Such a large abundance anomaly is unprecedented in the solar
atmosphere. We interpreted this result as being due to an inverse first
ionization potential (FIP) effect. In the published work, two lines of
Ar xiv were observed, and one line was tentatively identified as an Ar
xi line. In this paper, we report observing a similar enhancement in a
full-CCD EIS flare spectrum in 13 argon lines that lie within the EIS
wavelength ranges. The observed lines include two Ar xi lines, four
Ar xiii lines, six Ar xiv lines, and one Ar xv line. The enhancement
is far less than reported in Doschek et al. but exhibits similar
morphology. The argon abundance is close to a photospheric abundance
in the enhanced area, and the abundance could be photospheric. This
enhancement occurs in association with a sunspot in a small area only
a few arcseconds (1″ = about 700 km) in size. There is no enhancement
effect observed in the normally high-FIP sulfur and oxygen line ratios
relative to lines of low-FIP elements available to EIS. Calculations of
path lengths in the strongest enhanced area in Doschek et al. indicate
a depletion of low-FIP elements.
Title: Transition Region Abundance Measurements During Impulsive
Heating Events
Authors: Warren, Harry P.; Brooks, David H.; Doschek, George A.;
Feldman, Uri
Bibcode: 2016ApJ...824...56W
Altcode: 2015arXiv151204447W
It is well established that elemental abundances vary in the solar
atmosphere and that this variation is organized by first ionization
potential (FIP). Previous studies have shown that in the solar corona,
low-FIP elements such as Fe, Si, Mg, and Ca, are generally enriched
relative to high-FIP elements such as C, N, O, Ar, and Ne. In this paper
we report on measurements of plasma composition made during impulsive
heating events observed at transition region temperatures with the
Extreme Ultraviolet Imaging Spectrometer (EIS) on Hinode. During these
events the intensities of O IV, v, and VI emission lines are enhanced
relative to emission lines from Mg v, VI, and vii and Si VI and vii,
and indicate a composition close to that of the photosphere. Long-lived
coronal fan structures, in contrast, show an enrichment of low-FIP
elements. We conjecture that the plasma composition is an important
signature of the coronal heating process, with impulsive heating leading
to the evaporation of unfractionated material from the lower layers of
the solar atmosphere and higher-frequency heating leading to long-lived
structures and the accumulation of low-FIP elements in the corona.
Title: The Light at the End of the Tunnel: Uncertainties in Atomic
Physics, Bayesian Inference, and the Analysis of Solar and Stellar
Observations
Authors: Warren, Harry
Bibcode: 2016SPD....4720801W
Altcode:
We report on the efforts of a multidisciplinary International Space
Science Institute team that is investigating the limits of our ability
to infer the physical properties of solar and stellar atmospheres from
remote sensing observations. As part of this project we have estimated
the uncertainties in the collisional cross sections and radiative
decay rates for Fe XIII and O VII and created 1000 realizations of the
CHIANTI atomic database. These perturbed atomic data are then used
to analyze solar observations from the EIS spectrometer on Hinode
and stellar observations from the LETG on Chandra within a Bayesian
framework. For the solar case we find that the systematic errors
from the atomic physics dominate the statistical uncertainties from
the observations. For many cases the uncertainties are about 10 times
larger when variations in the atomic data are included. This indicates
the need for very accurate atomic physics. Comparisons among recent
Fe XIII calculations suggest that for some transitions the collision
rates are currently known well enough to measure the electron density
and emission measure to about 15%.
Title: Numerical Simulation of DC Coronal Heating
Authors: Dahlburg, Russell B.; Einaudi, G.; Taylor, Brian D.;
Ugarte-Urra, Ignacio; Warren, Harry; Rappazzo, A. F.; Velli, Marco
Bibcode: 2016SPD....47.0305D
Altcode:
Recent research on observational signatures of turbulent heating of
a coronal loop will be discussed. The evolution of the loop is is
studied by means of numerical simulations of the fully compressible
three-dimensional magnetohydrodynamic equations using the HYPERION
code. HYPERION calculates the full energy cycle involving footpoint
convection, magnetic reconnection, nonlinear thermal conduction
and optically thin radiation. The footpoints of the loop magnetic
field are convected by random photospheric motions. As a consequence
the magnetic field in the loop is energized and develops turbulent
nonlinear dynamics characterized by the continuous formation and
dissipation of field-aligned current sheets: energy is deposited
at small scales where heating occurs. Dissipation is non-uniformly
distributed so that only a fraction of thecoronal mass and volume gets
heated at any time. Temperature and density are highly structured at
scales which, in the solar corona, remain observationally unresolved:
the plasma of the simulated loop is multi thermal, where highly
dynamical hotter and cooler plasma strands are scattered throughout
the loop at sub-observational scales. Typical simulated coronal loops
are 50000 km length and have axial magnetic field intensities ranging
from 0.01 to 0.04 Tesla. To connect these simulations to observations
the computed number densities and temperatures are used to synthesize
the intensities expected in emission lines typically observed with
the Extreme ultraviolet Imaging Spectrometer (EIS) on Hinode. These
intensities are then employed to compute differential emission measure
distributions, which are found to be very similar to those derived
from observations of solar active regions.
Title: The Mysterious Case of the Solar Argon Abundance Near Sunspots
in Flares
Authors: Doschek, George A.; Warren, Harry
Bibcode: 2016SPD....4730207D
Altcode:
Recently Doschek et al. (2015, ApJL, 808, L7) reported on an observation
of an enhancement of the abundance of Ar XIV relative to Ca XIV
of about a factor of 30 near a sunspot during a flare, observed in
spectra recorded by the Extreme-ultraviolet Imaging Spectrometer (EIS)
on the Hinode spacecraft. This enhancement yields an argon/calcium
abundance ratio 7 times greater than expected from the photospheric
abundances. Such a large abundance anomaly is unprecedented in the
solar atmosphere. We interpreted this result as due to an inverse
First Ionization Potential (FIP) effect. Argon is a high-FIP element
and calcium is a low-FIP element. In the published work two lines of
Ar XIV were observed and one line was tentatively identified as an Ar
XI line. The number of argon lines was limited by the limitations of
the flare study that was executed. In this paper we report observing
a similar enhancement in a full-CCD EIS flare spectrum in argon lines
with reasonable statistics and lack of blending that lie within the
EIS wavelength ranges. The observed lines include two Ar XI lines,
four Ar XIII lines, six Ar XIV lines, and one Ar XV line. The
enhancement is far less than reported in Doschek et al. (2015)
but exhibits similar morphology. The argon abundance is close to a
photospheric abundance in the enhanced area, and is only marginally
an inverse FIP effect. However, as for the published cases, this newly
discovered enhancement occurs in association with a sunspot in a small
area only a few arcsec in size and therefore we feel it is produced
by the same physics that produced the strong inverse FIP case. There
is no enhancement effect observed in the normally high-FIP sulfur and
oxygen line ratios relative to lines of low-FIP elements available
to EIS. Calculations of path lengths in the strongest enhanced area
in Doschek et al. (2015) indicate that the argon/calcium enhancement
is due to a depletion of low-FIP elements. This work is supported by
a NASA Hinode grant.
Title: Transition Region and Chromospheric Signatures of Impulsive
Heating Events
Authors: Warren, Harry; Reep, Jeffrey; Crump, Nicholas
Bibcode: 2016SPD....4740303W
Altcode:
We exploit the high spatial resolution and high cadence of the Interface
Region Imaging Spectrograph (IRIS) to investigate the response of
the transition region and chromosphere to energy deposition during
several small flares. We find that during the impulsive phase of
these events the intensities of the C II 1334.535 and Si IV 1402.770
A emission lines are characterized by numerous, small-scale impulsive
bursts typically lasting 60 s or less followed by a slower decay
over several minutes. These variations in intensity are usually
accompanied by impulsive redshifts of 20-40 km/s, although some
blueshifted profiles are also observed. For one particularly well
observed event we combine the IRIS observations with co-temporal
measurements of hard X-ray emission from RHESSSI, transition region
density from EIS, and high-temperature coronal loops with XRT and AIA
to constrain 1D hydrodynamic models of loop evolution. Many aspects
of the observations can be explained with simple heating scenarios,
but some cannot. The simulated Doppler shifts, for example, show very
short-duration redshifts during the initial phase of the heating while
the observed redshifts persist over several minutes.
Title: The EVE plus RHESSI DEM for Solar Flares, and Implications
for Residual Non-Thermal X-Ray Emission
Authors: McTiernan, James; Caspi, Amir; Warren, Harry
Bibcode: 2016SPD....47.0618M
Altcode:
Solar flare spectra are typically dominated by thermal emission in the
soft X-ray energy range. The low energy extent of non-thermal emission
can only be loosely quantified using currently available X-ray data. To
address this issue, we combine observations from the EUV Variability
Experiment (EVE) on-board the Solar Dynamics Observatory (SDO) with
X-ray data from the Reuven Ramaty High Energy Spectroscopic Imager
(RHESSI) to calculate the Differential Emission Measure (DEM) for solar
flares. This improvement over the isothermal approximation helps to
resolve the ambiguity in the range where the thermal and non-thermal
components may have similar photon fluxes. This "crossover" range can
extend up to 30 keV.Previous work (Caspi et.al. 2014ApJ...788L..31C)
concentrated on obtaining DEM models that fit both instruments'
observations well. For this current project we are interested in breaks
and cutoffs in the "residual" non-thermal spectrum; i.e., the RHESSI
spectrum that is left over after the DEM has accounted for the bulk
of the soft X-ray emission. As in our earlier work, thermal emission
is modeled using a DEM that is parametrized as multiple gaussians
in temperature. Non-thermal emission is modeled as a photon spectrum
obtained using a thin-target emission model ('thin2' from the SolarSoft
Xray IDL package). Spectra for both instruments are fit simultaneously
in a self-consistent manner.For this study, we have examined the DEM
and non-thermal resuidual emission for a sample of relatively large
(GOES M class and above) solar flares observed from 2011 to 2014. The
results for the DEM and non-thermal parameters found using the combined
EVE-RHESSI data are compared with those found using only RHESSI data.
Title: Science Goals and First Light Analysis from the Miniature
X-ray Solar Spectrometer (MinXSS) CubeSat
Authors: Caspi, Amir; Woods, Thomas N.; Warren, Harry; Chamberlin,
Phillip C.; Jones, Andrew; Mason, James; McTiernan, James; Moore,
Christopher; Palo, Scott; Solomon, Stanley
Bibcode: 2016SPD....47.0306C
Altcode:
The Miniature X-ray Solar Spectrometer (MinXSS) is a 3U CubeSat with
deployment from the ISS planned in Q2 2016. Its goal is to measure the
solar soft X-ray (SXR) spectral irradiance, an observational signature
of hot plasma in the solar corona. Over the last few decades, there
have been very few spectrally resolved observations from ~0.2 to ~4 keV
(~0.3-6 nm). This range is sensitive to high-temperature plasma and
contains many spectral lines (e.g., Mg, Si, Fe, S, Ar), the abundances
of which probe plasma transport and provide valuable constraints on
plasma heating mechanisms during both flares and quiescence. This
solar SXR emission is primarily absorbed in the E-region of Earth's
ionosphere, and the subsequently driven dynamical processes are still
poorly understood, in large part because the energy distribution of the
incident SXRs is not yet well characterized.MinXSS flies a miniature
commercial off-the-shelf soft X-ray (SXR) spectrometer, the Amptek
X123-SDD. The silicon drift detector has 0.5 mm fully depleted thickness
and a 25 mm^2 physical area, with a ~16 micron Be entrance window;
with on-board thermoelectric cooling and pulse pile-up rejection,
it is sensitive to solar SXRs from ~0.5 to 30 keV with ~0.15 keV FWHM
resolution. MinXSS also includes a broadband SXR photometer, providing
an integrated intensity over a similar energy range for comparison,
cross-calibration, and additional data, especially useful during more
intense flares at the upper end of the X123 dynamic range.We present
the MinXSS science goals for studying hot plasma in the solar corona,
including impulsive flare heating and quiescent coronal heating,
and the impact of the resultant SXR emission on Earth's ionosphere,
thermosphere, and mesosphere. We present analysis of MinXSS first
light results (depending on deployment date from the ISS), as well as
modeling and predictions of future observations over the MinXSS 6-12
month mission lifetime.
Title: The Rapid Acquisition Imaging Spectrograph Experiment (RAISE)
Sounding Rocket Investigation
Authors: Laurent, Glenn T.; Hassler, Donald M.; Deforest, Craig;
Slater, David D.; Thomas, Roger J.; Ayres, Thomas; Davis, Michael; de
Pontieu, Bart; Diller, Jed; Graham, Roy; Michaelis, Harald; Schuele,
Udo; Warren, Harry
Bibcode: 2016JAI.....540006L
Altcode:
We present a summary of the solar observing Rapid Acquisition
Imaging Spectrograph Experiment (RAISE) sounding rocket program
including an overview of the design and calibration of the instrument,
flight performance, and preliminary chromospheric results from the
successful November 2014 launch of the RAISE instrument. The RAISE
sounding rocket payload is the fastest scanning-slit solar ultraviolet
imaging spectrograph flown to date. RAISE is designed to observe the
dynamics and heating of the solar chromosphere and corona on time
scales as short as 100-200ms, with arcsecond spatial resolution and
a velocity sensitivity of 1-2km/s. Two full spectral passbands over
the same one-dimensional spatial field are recorded simultaneously
with no scanning of the detectors or grating. The two different
spectral bands (first-order 1205-1251Å and 1524-1569Å) are imaged
onto two intensified Active Pixel Sensor (APS) detectors whose focal
planes are individually adjusted for optimized performance. RAISE
reads out the full field of both detectors at 5-10Hz, recording up
to 1800 complete spectra (per detector) in a single 6-min rocket
flight. This opens up a new domain of high time resolution spectral
imaging and spectroscopy. RAISE is designed to observe small-scale
multithermal dynamics in Active Region (AR) and quiet Sun loops,
identify the strength, spectrum and location of high frequency waves
in the solar atmosphere, and determine the nature of energy release
in the chromospheric network.
Title: Measurements of Non-thermal Line Widths in Solar Active Regions
Authors: Brooks, David H.; Warren, Harry P.
Bibcode: 2016ApJ...820...63B
Altcode: 2015arXiv151102313B
Spectral line widths are often observed to be larger than can be
accounted for by thermal and instrumental broadening alone. This excess
broadening is a key observational constraint for both nanoflare and
wave dissipation models of coronal heating. Here we present a survey
of non-thermal velocities measured in the high temperature loops (1-4
MK) often found in the cores of solar active regions. This survey of
Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) observations
covers 15 non-flaring active regions that span a wide range of solar
conditions. We find relatively small non-thermal velocities, with a
mean value of 17.6 ± 5.3 km s-1, and no significant trend
with temperature or active region magnetic flux. These measurements
appear to be inconsistent with those expected from reconnection jets in
the corona, chromospheric evaporation induced by coronal nanoflares,
and Alfvén wave turbulence models. Furthermore, because the observed
non-thermal widths are generally small, such measurements are difficult
and susceptible to systematic effects.
Title: Converging Supergranular Flows and the Formation of Coronal
Plumes
Authors: Wang, Y. -M.; Warren, H. P.; Muglach, K.
Bibcode: 2016ApJ...818..203W
Altcode:
Earlier studies have suggested that coronal plumes are energized
by magnetic reconnection between unipolar flux concentrations and
nearby bipoles, even though magnetograms sometimes show very little
minority-polarity flux near the footpoints of plumes. Here we use
high-resolution extreme-ultraviolet (EUV) images and magnetograms
from the Solar Dynamics Observatory (SDO) to clarify the relationship
between plume emission and the underlying photospheric field. We
find that plumes form where unipolar network elements inside coronal
holes converge to form dense clumps, and fade as the clumps disperse
again. The converging flows also carry internetwork fields of both
polarities. Although the minority-polarity flux is sometimes barely
visible in the magnetograms, the corresponding EUV images almost
invariably show loop-like features in the core of the plumes, with the
fine structure changing on timescales of minutes or less. We conclude
that the SDO observations are consistent with a model in which plume
emission originates from interchange reconnection in converging flows,
with the plume lifetime being determined by the ∼1 day evolutionary
timescale of the supergranular network. Furthermore, the presence of
large EUV bright points and/or ephemeral regions is not a necessary
precondition for the formation of plumes, which can be energized
even by the weak, mixed-polarity internetwork fields swept up by
converging flows.
Title: Observational Signatures of Coronal Loop Heating and Cooling
Driven by Footpoint Shuffling
Authors: Dahlburg, R. B.; Einaudi, G.; Taylor, B. D.; Ugarte-Urra,
I.; Warren, H. P.; Rappazzo, A. F.; Velli, M.
Bibcode: 2016ApJ...817...47D
Altcode: 2015arXiv151203079D
The evolution of a coronal loop is studied by means of
numerical simulations of the fully compressible three-dimensional
magnetohydrodynamic equations using the HYPERION code. The footpoints
of the loop magnetic field are advected by random motions. As a
consequence, the magnetic field in the loop is energized and develops
turbulent nonlinear dynamics characterized by the continuous formation
and dissipation of field-aligned current sheets: energy is deposited
at small scales where heating occurs. Dissipation is nonuniformly
distributed so that only a fraction of the coronal mass and volume gets
heated at any time. Temperature and density are highly structured at
scales that, in the solar corona, remain observationally unresolved:
the plasma of our simulated loop is multithermal, where highly dynamical
hotter and cooler plasma strands are scattered throughout the loop at
sub-observational scales. Numerical simulations of coronal loops of
50,000 km length and axial magnetic field intensities ranging from 0.01
to 0.04 T are presented. To connect these simulations to observations,
we use the computed number densities and temperatures to synthesize
the intensities expected in emission lines typically observed with the
Extreme Ultraviolet Imaging Spectrometer on Hinode. These intensities
are used to compute differential emission measure distributions using
the Monte Carlo Markov Chain code, which are very similar to those
derived from observations of solar active regions. We conclude that
coronal heating is found to be strongly intermittent in space and time,
with only small portions of the coronal loop being heated: in fact,
at any given time, most of the corona is cooling down.
Title: New Instruments for Spectrally-Resolved Solar Soft X-ray
Observations from CubeSats, and Larger Missions
Authors: Caspi, A.; Shih, A.; Warren, H. P.; DeForest, C. E.; Woods,
T. N.
Bibcode: 2015AGUFMSH13B2444C
Altcode: 2015AGUFMSH13B2444D
Solar soft X-ray (SXR) observations provide important diagnostics of
plasma heating, during solar flares and quiescent times. Spectrally-
and temporally-resolved measurements are crucial for understanding the
dynamics and evolution of these energetic processes; spatially-resolved
measurements are critical for understanding energy transport. A better
understanding of the thermal plasma informs our interpretation of
hard X-ray (HXR) observations of nonthermal particles, improving our
understanding of the relationships between particle acceleration,
plasma heating, and the underlying release of magnetic energy during
reconnection. We introduce a new proposed mission, the CubeSat
Imaging X-ray Solar Spectrometer (CubIXSS), to measure spectrally-
and spatially-resolved SXRs from the quiescent and flaring Sun from
a 6U CubeSat platform in low-Earth orbit during a nominal 1-year
mission. CubIXSS includes the Amptek X123-SDD silicon drift detector,
a low-noise, commercial off-the-shelf (COTS) instrument enabling
solar SXR spectroscopy from ~0.5 to ~30 keV with ~0.15 keV FWHM
spectral resolution with low power, mass, and volume requirements. An
X123-CdTe cadmium-telluride detector is also included for ~5-100
keV HXR spectroscopy with ~0.5-1 keV FWHM resolution. CubIXSS also
includes a novel spectro-spatial imager -- the first ever solar imager
on a CubeSat -- utilizing a pinhole aperture and X-ray transmission
diffraction grating to provide full-Sun imaging from ~0.1 to ~10 keV,
with ~25 arcsec and ~0.1 Å FWHM spatial and spectral resolutions,
respectively. We discuss scaled versions of these instruments, with
greater sensitivity and dynamic range, and significantly improved
spectral and spatial resolutions for the imager, for deployment on
larger platforms such as Small Explorer missions.
Title: Energy Release and Transport in Super-Hot Solar Flares
Authors: Caspi, A.; McTiernan, J. M.; Shih, A.; Martinez Oliveros,
J. C.; Allred, J. C.; Warren, H. P.
Bibcode: 2015AGUFMSH22A..08C
Altcode: 2015AGUFMSH22A..08M
Solar flares efficiently convert the magnetic energy stored in the
Sun's complex coronal magnetic field into the kinetic energies of hot
plasma, accelerated particles, and bulk flows. In intense flares, up to
10^32-33 ergs can go into heating plasma to tens of MK, accelerating
electrons to hundreds of MeV and ions to tens of GeV, and ejecting
10^9-10 kg of coronal material into the heliosphere at thousands of
km/s. However, the exact physical mechanisms behind these phenomena are
poorly understood. For example, while "super-hot" (T > 30 MK) plasma
temperatures appear to be common in the most intense, X-class flares,
how that plasma is so efficiently heated remains unknown. Current
studies favor an in situ heating process for super-hot plasma, versus
chromospheric evaporation for cooler plasma, although the specific
mechanism is under debate. X-class flares are also often associated
with enhanced photospheric/chromospheric white light emission, which
is itself poorly understood, and with fast (>1000 km/s) CMEs;
super-hot flares are more commonly observed in eruptive two-ribbon
arcade flares than in highly-confined events. These phenomena may well
have common underlying drivers. We discuss the current understanding of
super-hot plasma in solar flares, its formation, and evolution, based on
observations from RHESSI, SDO/EVE, SDO/AIA, and other instruments. We
discuss the energetics of these events and their relationship to white
light enhancement and fast CMEs. We explore the possibility of energy
deposition by accelerated ions as a common driver for super-hot plasmas
and white light enhancement, and discuss future instrumentation -- both
for CubeSats and Explorers -- that will provide a deeper understanding
of these phenomena and their interrelationships.
Title: Modeling Chromospheric Nanoflares with HYDRAD
Authors: Reep, J. W.; Warren, H. P.
Bibcode: 2015AGUFMSH31D..02R
Altcode:
Observational advances with IRIS have given the ability to observe
details of the coronal transition region (TR) with extremely high
spatial resolution. Spectral lines formed in the TR, in particular,
illuminate the dynamics of mass and energy flow between the chromosphere
and corona. Using a sophisticated hydrodynamic model, we simulate
nanoflares driven by different heating mechanisms - electron beams, in
situ thermal heating, and Alfvenic waves. By examining the atmospheric
response and by forward modeling of spectral lines, we can directly
compare with observations of the TR in order to differentiate potential
heating mechanisms. We thus present the results of a large, systematic
investigation of the parameter space of chromospheric nanoflares. We
discuss similarities and differences predicted by the different heating
mechanisms, all within the context of observed quantities.
Title: Active Region Soft X-Ray Spectra as Observed Using Sounding
Rocket Measurements from the Solar Aspect Monitor (SAM), - a Modified
SDO/EVE Instrument
Authors: Wieman, S. R.; Didkovsky, L. V.; Woods, T. N.; Jones, A. R.;
Caspi, A.; Warren, H. P.
Bibcode: 2015AGUFMSH23B2446W
Altcode:
Observations of solar active regions (ARs) in the soft x-ray spectral
range (0.5 to 3.0 nm) were made on sounding rocket flight NASA 36.290
using a modified Solar Aspect Monitor (SAM), a pinhole camera on the
EUV Variability Experiment (EVE) sounding rocket instrument. The suite
of EVE rocket instruments is designed for under-flight calibrations
of the orbital EVE on SDO. While the sounding rocket EVE instrument
is for the most part a duplicate of the EVE on SDO, the SAM channel
on the rocket version was modified in 2012 to include a free-standing
transmission grating so that it could provide spectrally resolved
images of the solar disk with the best signal to noise ratio for
the brightest features on it, such as ARs. Calibrations of the EVE
sounding rocket instrument at the National Institute of Standards and
Technology Synchrotron Ultraviolet Radiation Facility (NIST SURF) have
provided a measurement of the SAM absolute spectral response function
and a mapping of wavelength separation in the grating diffraction
pattern. For solar observations, this spectral separation is on a
similar scale to the spatial size of the AR on the CCD, so dispersed
AR images associated with emission lines of similar wavelength tend
to overlap. Furthermore, SAM shares a CCD detector with MEGS-A, a
separate EVE spectrometer channel, and artifacts of the MEGS-A signal
(a set of bright spectral lines) appear in the SAM images. For these
reasons some processing and analysis of the solar images obtained by
SAM must be performed in order to determine spectra of the observed
ARs. We present a method for determining AR spectra from the SAM
rocket images and report initial soft X-ray spectra for two of the
major active regions (AR11877 and AR11875) observed on flight 36.290
on 21 October 2013 at about 18:30 UT. We also compare our results with
concurrent measurements from other solar soft x-ray instrumentation.
Title: Magnetic Flux Transport and the Long-term Evolution of Solar
Active Regions
Authors: Ugarte-Urra, Ignacio; Upton, Lisa; Warren, Harry P.; Hathaway,
David H.
Bibcode: 2015ApJ...815...90U
Altcode: 2015arXiv151104030U
With multiple vantage points around the Sun, Solar Terrestrial Relations
Observatory (STEREO) and Solar Dynamics Observatory imaging observations
provide a unique opportunity to view the solar surface continuously. We
use He ii 304 Å data from these observatories to isolate and track
ten active regions and study their long-term evolution. We find
that active regions typically follow a standard pattern of emergence
over several days followed by a slower decay that is proportional in
time to the peak intensity in the region. Since STEREO does not make
direct observations of the magnetic field, we employ a flux-luminosity
relationship to infer the total unsigned magnetic flux evolution. To
investigate this magnetic flux decay over several rotations we use
a surface flux transport model, the Advective Flux Transport model,
that simulates convective flows using a time-varying velocity field
and find that the model provides realistic predictions when information
about the active region's magnetic field strength and distribution at
peak flux is available. Finally, we illustrate how 304 Å images can
be used as a proxy for magnetic flux measurements when magnetic field
data is not accessible.
Title: The Missing Solar Irradiance Spectrum: 1 to 7 nm
Authors: Sojka, J. J.; Lewis, M.; David, M.; Schunk, R. W.; Woods,
T. N.; Eparvier, F. G.; Warren, H. P.
Bibcode: 2015AGUFMSH32A..02S
Altcode:
During large X-class flares the Earth's upper atmospheric
E-region responds immediately to solar photons in the 1 to 7 nm
range. The response can change the E-region density by factors
approaching 10, create large changes in conductivity, and plague HF
communications. GOES-XRS provide 0.1 to 0.8 nm and a 0.05 to 0.4 nm
integral channels; SOHO-SEM provided a 0 to 50 nm irradiance; TIMED and
SORCE-XPS diode measurements also integrated down to 0.1 nm; and most
recently SDO-EVE provided a 0.1 to 7 nm irradiance. For atmospheric
response to solar flares the cadence is also crucial. Both GOES and SDO
provided integral measurements at 10 seconds or better. Unfortunately
these measurements have failed to capture the 1 to 7 nm spectral
changes that occur during flares. It is these spectral changes that
create the major impact since the ionization cross-section of the
dominant atmospheric species, N2 and O2, both contain step function
changes in the cross-sections. Models of the solar irradiance over
this critical wavelength regime have suffered from the need to model
the spectral variability based on incomplete measurements. The most
sophisticated empirical model FISM [Chamberlin et al., 2008] used 1
nm spectral binning and various implementations of the above integral
measurements to describe the 1 to 7 nm irradiance. Since excellent solar
observations exist at other wavelengths it is possible to construct an
empirical model of the solar atmosphere and then use this model to infer
the spectral distribution at wavelengths below 5 nm. This differential
emission measure approach has been used successfully in other contexts
[e.g., Warren, 2005, Chamberlin et al., 2009]. This paper contrasts
the broadband versus spectrally resolved descriptions of the incoming
irradiance that affects the upper atmospheric E-layer. The results
provide a prescription of what wavelength resolution would be needed to
adequately measure the incoming solar irradiance in the 1 to 7 nm range.
Title: Flare Footpoint Regions and a Surge Observed by Hinode/EIS,
RHESSI, and SDO/AIA
Authors: Doschek, G. A.; Warren, H. P.; Dennis, B. R.; Reep, J. W.;
Caspi, A.
Bibcode: 2015ApJ...813...32D
Altcode: 2015arXiv151007088D
The Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode
spacecraft observed flare footpoint regions coincident with a surge
for an M3.7 flare observed on 2011 September 25 at N12 E33 in active
region 11302. The flare was observed in spectral lines of O vi, Fe x,
Fe xii, Fe xiv, Fe xv, Fe xvi, Fe xvii, Fe xxiii, and Fe xxiv. The
EIS observations were made coincident with hard X-ray bursts observed
by RHESSI. Overlays of the RHESSI images on the EIS raster images
at different wavelengths show a spatial coincidence of features in
the RHESSI images with the EIS upflow and downflow regions, as well
as loop-top or near-loop-top regions. A complex array of phenomena
were observed, including multiple evaporation regions and the surge,
which was also observed by the Solar Dynamics Observatory/Atmospheric
Imaging Assembly telescopes. The slit of the EIS spectrometer covered
several flare footpoint regions from which evaporative upflows in Fe
xxiii and Fe xxiv lines were observed with Doppler speeds greater than
500 km s-1. For ions such as Fe xv both evaporative outflows
(∼200 km s-1) and downflows (∼30-50 km s-1)
were observed. Nonthermal motions from 120 to 300 km s-1
were measured in flare lines. In the surge, Doppler speeds are
found from about 0 to over 250 km s-1 in lines from ions
such as Fe xiv. The nonthermal motions could be due to multiple
sources slightly Doppler-shifted from each other or turbulence in
the evaporating plasma. We estimate the energetics of the hard X-ray
burst and obtain a total flare energy in accelerated electrons of ≥7
× 1028 erg. This is a lower limit because only an upper
limit can be determined for the low-energy cutoff to the electron
spectrum. We find that detailed modeling of this event would require
a multithreaded model owing to its complexity.
Title: Benchmark Test of Differential Emission Measure Codes and
Multi-thermal Energies in Solar Active Regions
Authors: Aschwanden, Markus J.; Boerner, Paul; Caspi, Amir; McTiernan,
James M.; Ryan, Daniel; Warren, Harry
Bibcode: 2015SoPh..290.2733A
Altcode: 2015arXiv150907546A; 2015SoPh..tmp..146A
We compare the ability of 11 differential emission measure (DEM)
forward-fitting and inversion methods to constrain the properties
of active regions and solar flares by simulating synthetic data
using the instrumental response functions of the Solar Dynamics
Observatory/Atmospheric Imaging Assembly (SDO/AIA) and EUV Variability
Experiment (SDO/EVE), the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI), and the Geostationary Operational Environmental
Satellite/X-ray Sensor (GOES/XRS). The codes include the single-Gaussian
DEM, a bi-Gaussian DEM, a fixed-Gaussian DEM, a linear spline DEM,
the spatial-synthesis DEM, the Monte-Carlo Markov Chain DEM, the
regularized DEM inversion, the Hinode/X-Ray Telescope (XRT) method, a
polynomial spline DEM, an EVE+GOES, and an EVE+RHESSI method. Averaging
the results from all 11 DEM methods, we find the following accuracies
in the inversion of physical parameters: the EM-weighted temperature
Twfit/Twsim=0.9
±0.1 , the peak emission measure
EMpfit/EMpsim=0.6
±0.2 , the total emission measure
EMtfit/EMtsim=0.8
±0.3 , and the multi-thermal energies
Ethfit/EMthapprox=1.2
±0.4 . We find that the AIA spatial-synthesis, the EVE+GOES, and the
EVE+RHESSI method yield the most accurate results.
Title: VizieR Online Data Catalog: Global energetics of solar
flares. II. (Aschwanden+, 2015)
Authors: Aschwanden, M. J.; Boerner, P.; Ryan, D.; Caspi, A.;
McTiernan, J. M.; Warren, H. P.
Bibcode: 2015yCat..18020053A
Altcode:
The dataset we are analyzing for this project on the global energetics
of flares includes all M- and X-class flares observed with the Solar
Dynamics Observatory (SDO) during the first 3.5yr of the mission
(2010 June 1 to 2014 January 31), which amounts to 399 flare events,
as described in Paper I (Aschwanden et al. 2014, J/ApJ/797/50). We
attempt to calculate the thermal energies in all 399 cataloged
events, but we encountered eight events with incomplete or corrupted
Atmospheric Imaging Assembly (AIA) data, so that we are left with
391 events suitable for thermal data analysis. AIA provides EUV
images corresponding to an effective spatial resolution of ~1.6". (1 data file).
Title: Anomalous Relative Ar/Ca Coronal Abundances Observed by the
Hinode/EUV Imaging Spectrometer Near Sunspots
Authors: Doschek, G. A.; Warren, H. P.; Feldman, U.
Bibcode: 2015ApJ...808L...7D
Altcode:
In determining the element abundance of argon (a high first ionization
potential; FIP element) relative to calcium (a low FIP element) in
flares, unexpectedly high intensities of two Ar xiv lines (194.40,
187.96 Å) relative to a Ca xiv line (193.87 Å) intensity were
found in small (a few arcseconds) regions near sunspots in flare
spectra recorded by the Extreme-ultraviolet Imaging Spectrometer
on the Hinode spacecraft. In the most extreme case the Ar xiv line
intensity relative to the Ca xiv intensity was 7 times the value
expected from the photospheric abundance ratio, which is about 30
times the abundance of argon relative to calcium in active regions,
i.e., the measured Ar/Ca abundance ratio is about 10 instead of 0.37
as in active regions. The Ar xiv and Ca xiv lines are formed near 3.4
MK and have very similar contribution functions. This is the first
observation of the inverse FIP effect in the Sun. Other regions show
increases of 2-3 over photospheric abundances, or just photospheric
abundances. This phenomenon appears to occur rarely and only over
small areas of flares away from the regions containing multi-million
degree plasma, but more work is needed to quantify the occurrences
and their locations. In the bright hot regions of flares the Ar/Ca
abundance ratio is coronal, i.e., the same as in active regions. In
this Letter we show three examples of the inverse FIP effect.
Title: Multi-thermal Energies of Solar Flares
Authors: Ryan, Daniel; Aschwanden, Markus; Boerner, Paul; Caspi,
Amir; McTiernan, James; Warren, Harry
Bibcode: 2015TESS....130215R
Altcode:
Measuring energy partition in solar eruptions is key to understanding
how different processes affect their evolution. In order to improve
our knowledge on this topic, we are participating in a multi-study
project to measure the energy partition of 400 M- and X-class flares
and associated coronal mass ejections (CMEs). In this study we focus
on the flare thermal energies of 391 of these events. We improve upon
previous studies in the following ways: 1) We determine thermal energy
using spatially resolved multi-thermal differential emission measures
(DEMs) determined from AIA (Atmospheric Imaging Assembly) rather than
relying on the isothermal assumption; 2) We determine flare volumes
by thresholding these DEM maps rather than relying on single passband
observations which may not show the full flare volume; 3) We analyze
a greater number of events than previous similar studies to increase
the statistical reliability of our results. We find that the thermal
energies of these flares lie in the range 10^26.8—10^32 erg. These
results are compared to those of Aschwanden et al. (2014) who examined
a subset of these events. They determined the dissipated non-potential
magnetic energy which is thought to be the total energy available
to drive solar eruptions. For the 171 events common to both studies,
we find that the ratio of flare thermal energy to dissipated magnetic
energy ranges from 2%—40%. This is an order of magnitude higher than
previously found by Emslie et al. (2012). This may be because Emslie et
al. (2012) had to assume the amount of non-potential magnetic energy,
or that they relied on the isothermal assumption to determine flare
thermal energies. The improved results found here will help us better
understand the role played by flare thermal processes in dissipating
the overall energy of solar eruptions.
Title: On the long-term evolution of solar active regions from full
Sun observations, magnetic flux transport and hydrodynamic modeling
Authors: Ugarte-Urra, Ignacio; Upton, Lisa; Warren, Harry; Hathaway,
David H.
Bibcode: 2015TESS....120104U
Altcode:
With their multiple vantage points around the Sun, STEREO and SDO
observations provide a unique opportunity to view the solar surface
continuously. We use data from these observatories to study the
long-term evolution of solar active regions in He II 304 A. We
show that active regions follow a universal pattern of emergence
over several days followed by a decay that is proportional to the
peak intensity in the region. We find that magnetic surface flux
transport simulations are able to reproduce this evolution. Since
STEREO does not make direct observations of the magnetic field, we use
the flux-luminosity relationship to infer the total unsigned magnetic
flux from the He 304 A images. We also illustrate the use of far-side
imaging to introduce solar active regions into magnetic surface flux
transport simulations. We finally show how these models can be used to
determine the long-term coronal emission evolution in active regions
by coupling extrapolations of the magnetic flux transport simulations
field with EBTEL solutions to the hydrodynamic loop equations.
Title: Spectrally-resolved Soft X-ray Observations and the Temperature
Structure of the Solar Corona
Authors: Caspi, Amir; Warren, Harry; McTiernan, James; Woods, Thomas N.
Bibcode: 2015TESS....120403C
Altcode:
Solar X-ray observations provide important diagnostics of plasma
heating and particle acceleration, during solar flares and quiescent
periods. How the corona is heated to its ~1-3 MK nominal temperature
remains one of the fundamental unanswered questions of solar physics;
heating of plasma to tens of MK during solar flares -- particularly
to the hottest observed temperatures of up to ~50 MK -- is also still
poorly understood. Soft X-ray emission (~0.1-10 keV; or ~0.1-10 nm)
is particularly sensitive to hot coronal plasma and serves as a probe
of the thermal processes driving coronal plasma heating. Spectrally-
and temporally-resolved measurements are crucial for understanding
these energetic processes, but there have historically been very
few such observations. We present new solar soft X-ray spectra from
the Amptek X123-SDD, measuring quiescent solar X-ray emission from
~0.5 to ~30 keV with ~0.15 keV FWHM resolution from two SDO/EVE
calibration sounding rocket underflights in 2012 and 2013. Combined
with observations from RHESSI, GOES/XRS, SDO/EVE, and SDO/AIA, the
temperature distribution derived from these data suggest significant hot
(5-10 MK) emission from active regions, and the 2013 spectra suggest a
low-FIP enhancement of only ~1.6 relative to the photosphere, 40% of the
usually-observed value from quiescent coronal plasma. We explore the
implications of these findings on coronal heating. We discuss future
missions for spectrally-resolved soft X-ray observations using the
X123-SDD, including the upcoming MinXSS 3U CubeSat using the X123-SDD
and scheduled for deployment in mid-2015, and the CubIXSS 6U CubeSat
mission concept.
Title: Modelling nanoflares in active regions and implications for
coronal heating mechanisms
Authors: Cargill, P. J.; Warren, H. P.; Bradshaw, S. J.
Bibcode: 2015RSPTA.37340260C
Altcode:
Recent observations from the Hinode and Solar Dynamics Observatory
spacecraft have provided major advances in understanding the heating of
solar active regions (ARs). For ARs comprising many magnetic strands
or sub-loops heated by small, impulsive events (nanoflares), it is
suggested that (i) the time between individual nanoflares in a magnetic
strand is 500-2000 s, (ii) a weak 'hot' component (more than 106.6 K)
is present, and (iii) nanoflare energies may be as low as a few 1023
ergs. These imply small heating events in a stressed coronal magnetic
field, where the time between individual nanoflares on a strand is of
order the cooling time. Modelling suggests that the observed properties
are incompatible with nanoflare models that require long energy build-up
(over 10 s of thousands of seconds) and with steady heating.
Title: RAISE (Rapid Acquisition Imaging Spectrograph Experiment):
Results and Instrument Status
Authors: Laurent, Glenn T.; Hassler, Donald; DeForest, Craig; Ayres,
Tom; Davis, Michael; DePontieu, Bart; Diller, Jed; Graham, Roy;
Schule, Udo; Warren, Harry
Bibcode: 2015TESS....140203L
Altcode:
We present initial results from the successful November 2014 launch of
the RAISE (Rapid Acquisition Imaging Spectrograph Experiment) sounding
rocket program, including intensity maps, high-speed spectroheliograms
and dopplergrams, as well as an update on instrument status. The
RAISE sounding rocket payload is the fastest high-speed scanning-slit
imaging spectrograph flown to date and is designed to observe the
dynamics and heating of the solar chromosphere and corona on time
scales as short as 100-200ms, with arcsecond spatial resolution and a
velocity sensitivity of 1-2 km/s. The instrument is based on a class of
UV/EUV imaging spectrometers that use only two reflections to provide
quasi-stigmatic performance simultaneously over multiple wavelengths
and spatial fields. The design uses an off-axis parabolic telescope
mirror to form a real image of the sun on the spectrometer entrance
aperture. A slit then selects a portion of the solar image, passing its
light onto a near-normal incidence toroidal grating, which re-images
the spectrally dispersed radiation onto two array detectors. Two
full spectral passbands over the same one-dimensional spatial field
are recorded simultaneously with no scanning of the detectors or
grating. The two different spectral bands (1st-order 1205-1243Å and
1526-1564Å) are imaged onto two intensified Active Pixel Sensor (APS)
detectors whose focal planes are individually adjusted for optimized
performance. RAISE reads out the full field of both detectors at 5-10
Hz, allowing us to record over 1,500 complete spectral observations in
a single 5-minute rocket flight, opening up a new domain of high time
resolution spectral imaging and spectroscopy. RAISE is designed to
study small-scale multithermal dynamics in active region (AR) loops,
explore the strength, spectrum and location of high frequency waves
in the solar atmosphere, and investigate the nature of transient
brightenings in the chromospheric network.
Title: Measuring Elemental Abundances in Impulsive Heating Events
with EIS
Authors: Warren, Harry; Doschek, George A.; Young, Peter
Bibcode: 2015TESS....121306W
Altcode:
It is well established that elemental abundances vary in the solar
atmosphere and that this variation is organized by first ionization
potential (FIP). Previous studies have indicated that in the solar
corona low FIP elements, such as Fe, Si, and Mg, are enriched relative
to high FIP elements, such as H, He, C, N, and O. In this paper we
report on measurements of plasma composition made during transient
heating events observed at transition region temperatures with the
Extreme Ultraviolet Imaging Spectrometer (EIS) on Hinode. During these
events the intensities of O IV, V, and VI emission lines are enhanced
relative to emission lines from Mg V, VI, and VII and indicate a
composition close to that of the photosphere. Differential emission
measure calculations show a broad distribution of temperatures in
these events. Long-lived coronal structures, in contrast, show an
enrichment of low FIP elements and relatively narrow temperature
distributions. We conjecture that plasma composition is an important
signature of the coronal heating process, with impulsive heating
leading to the evaporation of unfractionated material from the lower
layers of the solar atmosphere and higher frequency heating leading
to the accumulation of low-FIP elements in the corona.
Title: Magnetic and Hydrodynamic Energy Scaling Laws in Solar Flares
Authors: Aschwanden, Markus; Boerner, Paul; Xu, Yan; Ju, Jing; Ryan,
Dan; Caspi, Amir; McTiernan, James; Warren, Harry
Bibcode: 2015TESS....140603A
Altcode:
We determine the dissipated non-potential magnetic energy and measure
the multi-thermal energy in a sample of about 400 M and X-class
flares observed with AIA and HMI during the first 4 years of the SDO
mission. The free energy is determined with two nonlinear force-free
field (NLFFF) models, one is based on the 3D vectorphotospheric magnetic
field and the other uses forward-fitting of a vertical-current model to
automatically traced coronal loops.The multi-thermal energy is measured
with a spatial-synthesis differential emission measure (DEM) code,
which yields a more comprehensive multi-thermal energy (being larger
by an averagefactor of 14) than iso-thermal estimates. We show how the
correlations and powerlaw-like size distributions of energies and other
geometrical and physical parameters reveal magnetic and hydrodynamic
scaling lawsthat are in agreement with recent statistical models of
nonlinear dissipative systems governed by self-organized criticality.
Title: The Multi-Instrument (EVE-RHESSI) DEM for Solar Flares,
and Implications for Residual Non-Thermal Soft X-Ray Emission
Authors: McTiernan, James M.; Caspi, Amir; Warren, Harry
Bibcode: 2015TESS....130210M
Altcode:
In the soft X-ray energy range, solar flare spectra are typically
dominated by thermal emission. The low energy extent of non-thermal
emission can only be loosely quantified using currently available
X-ray data. To address this issue, we combine observations from the EUV
Variability Experiment (EVE) on-board the Solar Dynamics Observatory
(SDO) with X-ray data from the Reuven Ramaty High Energy Spectroscopic
Imager (RHESSI). The improvement over the isothermal approximation is
intended to resolve the ambiguity in the range where the thermal and
non-thermal components may have similar photon fluxes. This "crossover"
range can extend up to 30 keV for medium to large solar flares.Previous
work (Caspi et.al. 2014ApJ...788L..31C) has concentrated on obtaining
DEM models that fit both instruments' observations well. Now we are
interested in any breaks and cutoffs in the "residual" non-thermal
spectrum; i.e., the RHESSI spectrum that is left over after the
DEM has accounted for the bulk of the soft X-ray emission. Thermal
emission is again modeled using a DEM that is parametrized as multiple
gaussians in temperature; the non-thermal emission is modeled as a
photon spectrum obtained using a thin-target emission model ('thin2'
from the SolarSoft Xray IDL package). Spectra for both instruments
are fit simultaneously in a self-consistent manner. The results for
non-thermal parameters then are compared with those found using RHESSI
data alone, with isothermal and double-thermal models.
Title: Global Energetics of Solar Flares: II. Thermal Energies
Authors: Aschwanden, Markus J.; Boerner, Paul; Ryan, Daniel; Caspi,
Amir; McTiernan, James M.; Warren, Harry P.
Bibcode: 2015ApJ...802...53A
Altcode: 2015arXiv150205941A
We present the second part of a project on the global energetics of
solar flares and coronal mass ejections that includes about 400 M-
and X-class flares observed with the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO) during the
first 3.5 yr of its mission. In this Paper II we compute the
differential emission measure (DEM) distribution functions and
associated multithermal energies, using a spatially-synthesized
Gaussian DEM forward-fitting method. The multithermal DEM function
yields a significantly higher (by an average factor of ≈14),
but more comprehensive (multi-)thermal energy than an isothermal
energy estimate from the same AIA data. We find a statistical
energy ratio of {{E}th}/{{E}diss} ≈ 2-40%
between the multithermal energy Eth and the magnetically
dissipated energy Ediss, which is an order of magnitude
higher than the estimates of Emslie et al. 2012. For the analyzed
set of M- and X-class flares we find the following physical
parameter ranges: L={{10}8.2}{{-10}9.7}
cm for the length scale of the flare areas,
{{T}p}={{10}5.7}{{-10}7.4}
K for the DEM peak temperature,
{{T}w}={{10}6.8}{{-10}7.6}
K for the emission measure-weighted temperature,
{{n}p}={{10}10.3}-{{10}11.8}
cm-3 for the average electron density,
E{{M}p}={{10}47.3}-{{10}50.3}
cm-3 for the DEM peak emission measure, and
{{E}th}={{10}26.8}-{{10}32.0} erg
for the multithermal energies. The deduced multithermal energies
are consistent with the RTV scaling law {{E}th,RTV}=7.3×
{{10}-10} Tp3Lp2,
which predicts extremal values of {{E}th,max }≈ 1.5×
{{10}33} erg for the largest flare and {{E}th,min
}≈ 1× {{10}24} erg for the smallest coronal
nanoflare. The size distributions of the spatial parameters exhibit
powerlaw tails that are consistent with the predictions of the
fractal-diffusive self-organized criticality model combined with the
RTV scaling law.
Title: New Observations of the Solar 0.5-5 keV Soft X-Ray Spectrum
Authors: Caspi, Amir; Woods, Thomas N.; Warren, Harry P.
Bibcode: 2015ApJ...802L...2C
Altcode: 2015arXiv150201725C
The solar corona is orders of magnitude hotter than the underlying
photosphere, but how the corona attains such high temperatures is
still not understood. Soft X-ray (SXR) emission provides important
diagnostics for thermal processes in the high-temperature corona, and
is also an important driver of ionospheric dynamics at Earth. There is
a crucial observational gap between ∼0.2 and ∼4 keV, outside the
ranges of existing spectrometers. We present observations from a new
SXR spectrometer, the Amptek X123-SDD, which measured the spatially
integrated solar spectral irradiance from ∼0.5 to ∼5 keV, with
∼0.15 keV FWHM resolution, during sounding rocket flights on 2012
June 23 and 2013 October 21. These measurements show that the highly
variable SXR emission is orders of magnitude greater than that during
the deep minimum of 2009, even with only weak activity. The observed
spectra show significant high-temperature (5-10 MK) emission and are
well fit by simple power-law temperature distributions with indices
of ∼6, close to the predictions of nanoflare models of coronal
heating. Observations during the more active 2013 flight indicate
an enrichment of low first-ionization potential elements of only
∼1.6, below the usually observed value of ∼4, suggesting that
abundance variations may be related to coronal heating processes. The
XUV Photometer System Level 4 data product, a spectral irradiance
model derived from integrated broadband measurements, significantly
overestimates the spectra from both flights, suggesting a need for
revision of its non-flare reference spectra, with important implications
for studies of Earth ionospheric dynamics driven by solar SXRs.
Title: Full-Sun observations for identifying the source of the slow
solar wind
Authors: Brooks, David H.; Ugarte-Urra, Ignacio; Warren, Harry P.
Bibcode: 2015NatCo...6.5947B
Altcode: 2016arXiv160509514B; 2015NatCo...6E5947B
Fast (>700 km s-1) and slow
(~400 km s-1) winds stream from the Sun, permeate
the heliosphere and influence the near-Earth environment. While the
fast wind is known to emanate primarily from polar coronal holes,
the source of the slow wind remains unknown. Here we identify possible
sites of origin using a slow solar wind source map of the entire Sun,
which we construct from specially designed, full-disk observations
from the Hinode satellite, and a magnetic field model. Our map
provides a full-Sun observation that combines three key ingredients
for identifying the sources: velocity, plasma composition and magnetic
topology and shows them as solar wind composition plasma outflowing on
open magnetic field lines. The area coverage of the identified sources
is large enough that the sum of their mass contributions can explain
a significant fraction of the mass loss rate of the solar wind.
Title: The VAULT2.0 Observing Campaign: A Comprehensive Investigation
of the Chromosphere-Corona Interface at Sub-arcsecond scales
Authors: Vourlidas, A.; Korendyke, C.; Tun-Beltran, S. D.; Ugarte-Urra,
I.; Morrill, J. S.; Warren, H. P.; Young, P.; De Pontieu, B.; Gauzzi,
G.; Reardon, K.
Bibcode: 2014AGUFMSH41C4155V
Altcode:
We report the first results from an observing campaign in support of
the VAULT2.0 sounding rocket launch on September 30, 2014. VAULT2.0
is a Lya (1216Å) spectroheliograph capable of 0.3" (~250 km) spatial
resolution. The objective of the VAULT2.0 project is the study of
the chromosphere-corona interface. This interface has acquired renewed
emphasis over the last few years, thanks to high-resolution observations
from Hinode/SOT and EIS instruments and the Lya imaging from the two
VAULT flights. The observations have shown that the upper chromosphere
may play a more important role in heating the corona and in affecting
EUV observations that previously thought: (1) by supplying the mass
via Type-II spicules and, (2) by absorbing coronal emission. Many of
the required clues for further progress are located in sub-arcsecond
structures with temperatures between 10000 and 50000 K, a regime not
accessible by Hinode or SDO. Lyman-alpha observations are, therefore,
ideal, for filling in this gap. The observing campaign in support of
the VAULT2.0 is closely coordinated with the Hinode and IRIS missions
to study the mass/energy flow from the chromosphere to the corona with
joint observations of type-II spicules, and the magnetic connectivity
of coronal loops using the full imaging and spectral capabilities of
IRIS, Hinode and SDO. Several ground-based observatories also provide
important observations (IBIS, BBSO, SOLIS). The VAULT2.0 project is
funded by the NASA LCAS program.
Title: New Solar Soft X-ray Observations from the X123 Spectrometer
Authors: Caspi, A.; McTiernan, J. M.; Warren, H. P.; Woods, T. N.
Bibcode: 2014AGUFMSH53B4220C
Altcode:
The Amptek X123 is a new soft X-ray photon-counting spectrometer, based
on a silicon drift detector with integrated thermoelectric cooler,
vacuum housing, and multi-channel analyzer (including pulse pile-up
rejection), capable of measuring solar line and continuum emission from
~0.5 to ~30 keV with ~0.15 keV FWHM resolution. It was flown on two
recent SDO/EVE sounding rocket calibration underflights, is the primary
science instrument on the upcoming Miniature X-ray Solar Spectrometer
(MinXSS) NASA CubeSat, and is part of the proposed instrument payload
for the CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) mission
concept. With the best resolution yet obtained from a broadband X-ray
spectrometer, the X123 will enable new studies of plasma heating and
particle acceleration, during flares and quiescent periods, and help to
fill a crucial observational gap from ~0.2 to ~1.2 keV, not currently
measured by existing instruments but critical for understanding
solar-driven dynamics in Earth's upper atmosphere (ionosphere,
thermosphere, mesosphere). We present results from a new analysis of
X123 data obtained from the SDO/EVE rocket flights. In preparation
for future MinXSS and CubIXSS data, we adapt a recently-developed
technique combining EUV and X-ray spectra from SDO/EVE and RHESSI,
respectively, to obtain a self-consistent differential emission measure
(DEM) over the full range of coronal temperatures, ~2-50 MK. Including
the X123 rocket X-ray spectra, we apply the adapted technique to examine
both the coronal DEM and composition during quiescent (non-flaring)
times with varying activity levels, obtaining constraints on the
high-temperature extent of the quiescent DEM, the elemental abundances,
and any potential non-thermal emission, and use the observations to
extrapolate the spectrum to the poorly-observed ~0.2-1.2 keV band. We
compare these results with those from a parallel technique using SDO/AIA
imaging data. We discuss the implications for coronal plasma heating
and the expectations for future observations from MinXSS and CubIXSS.
Title: Computing Solar EUV Irradiance Variability
Authors: Warren, H. P.
Bibcode: 2014AGUFMSH21C4130W
Altcode:
The solar EUV irradiance plays a central role in determining the state
of the Earth's upper atmosphere. The EUV irradiance at the shortest
wavelengths, which is highly variable over time scales from seconds
to decades, is particularly important for many aspects of space
weather. Systematic spectrally resolved observations at the shortest
EUV wavelengths, however, have been rare and there is a need to develop
a methodology for estimating and forecasting the solar irradiance
at all EUV wavelengths from sparse data sets. In this presentation
we report on our efforts to use AIA DEM calculations to estimate the
solar EUV irradiance at wavelength below 450 Å, where the emission
is predominately optically thin. To validate our AIA DEM calculations
we have performed extensive comparisons with simultaneous observations
from the EVE instrument on SDO and the EIS instrument on Hinode and find
that with the proper constraints we can generally reproduce the results
obtained with detailed spectroscopic observations. Using a proxy for
solar activity derived from photospheric magnetic field measurements
we extend our model calculations to previous solar cycles and discuss
how the model can be used to forecast EUV irradiance variability over
short time scales. Finally, we speculate on what is needed to further
develop semi-empirical and physical models for use in understanding
the solar spectral irradiance at these wavelengths.
Title: Using Running Difference Images to Track Proper Motions of
XUV Coronal Intensity on the Sun
Authors: Sheeley, N. R., Jr.; Warren, H. P.; Lee, J.; Chung, S.;
Katz, J.; Namkung, M.
Bibcode: 2014ApJ...797..131S
Altcode:
We have developed a procedure for observing and tracking proper
motions of faint XUV coronal intensity on the Sun and have applied
this procedure to study the collective motions of cellular plumes and
the shorter-period waves in sunspots. Our space/time maps of cellular
plumes show a series of tracks with the same 5-8 minute repetition
times and ~100 km s-1 sky-plane speeds found previously
in active-region fans and in coronal hole plumes. By synchronizing
movies and space/time maps, we find that the tracks are produced by
elongated ejections from the unipolar flux concentrations at the bases
of the cellular plumes and that the phases of these ejections are
uncorrelated from cell to cell. Thus, the large-scale motion is not a
continuous flow, but is more like a system of independent conveyor belts
all moving in the same direction along the magnetic field. In contrast,
the proper motions in sunspots are clearly waves resulting from periodic
disturbances in the sunspot umbras. The periods are ~2.6 minutes, but
the sky-plane speeds and wavelengths depend on the heights of the waves
above the sunspot. In the chromosphere, the waves decelerate from 35-45
km s-1 in the umbra to 7-8 km s-1 toward the outer
edge of the penumbra, but in the corona, the waves accelerate to ~60-100
km s-1. Because chromospheric and coronal tracks originate
from the same space/time locations, the coronal waves must emerge from
the same umbral flashes that produce the chromospheric waves.
Title: Propagation of Forecast Errors from the Sun to LEO
Trajectories: How Does Drag Uncertainty Affect Conjunction Frequency?
Authors: Emmert, J.; Byers, J.; Warren, H.; Segerman, A.
Bibcode: 2014amos.confE..48E
Altcode:
Atmospheric drag is the largest source of error in the prediction
of trajectories of most objects in low-Earth orbit, and solar
variability is the largest source of error in upper atmospheric density
forecasts. There is thus a need to accurately propagate solar forecast
uncertainty to atmospheric density uncertainty and thence to satellite
position uncertainty. Furthermore, the collective position uncertainty
of the LEO population determines the frequency of conjunctions that must
be assessed in order to avoid collisions. To maintain Space Situational
Awareness of the growing LEO population, the number of conjunctions
must be kept at a manageable level to avoid being overwhelmed by false
alarms. This criterion can be used to define solar and atmospheric
forecast accuracy requirements. In this paper, we examine how solar
forecast errors grow with increasing forecast time, and how this
uncertainty maps to atmospheric density uncertainty as a function of
altitude. We then develop analytical approximations of the mapping from
density uncertainty to in-track position uncertainty, as a function of
perigee height, orbital eccentricity, ballistic coefficient, background
atmospheric conditions, and forecast time. Finally, we estimate the
conjunction frequency between operational LEO satellites and the entire
LEO population (separately considering objects larger than 10 cm and
objects larger than 1 cm), based on the statistical distributions of
the key orbital parameters (perigee height, eccentricity, inclination
and ballistic coefficient) and assumed solar and density forecast
uncertainties.
Title: Exploiting the Magnetic Origin of Solar Activity in Forecasting
Thermospheric Density Variations
Authors: Warren, H.; Emmert, J.
Bibcode: 2014amos.confE.111W
Altcode:
A detailed understanding of solar irradiance and its variability
at extreme ultraviolet (EUV) wavelengths is required to model
thermospheric density and to specify and forecast satellite
drag. Current operational models rely on forecasts of proxies for
solar activity based on autoregression. The forecasts from these
models generally degrade to climatology after only a few days. Solar
magnetic fields are ultimately responsible for variations in the EUV
irradiance. The evolution of solar magnetic fields is well understood
and results from a combination of solar rotation, diffusion, meridional
flow, and magnetic flux emergence. In this presentation we review the
current state of autoregressive proxy models and compare their forecast
skill against new activity models based on magnetic flux transport.
Title: VizieR Online Data Catalog: UV spectrum of the quiet Sun
above the limb (Warren+, 2014)
Authors: Warren, H. P.; Ugarte-Urra, I.; Landi, E.
Bibcode: 2014yCat..22130011W
Altcode:
First, we compare full-disk mosaics constructed by scanning the EIS slot
over the Sun with irradiance observations made by the EUV Variability
Experiment (EVE; Woods et al. 2012SoPh..275..115W) on the Solar
Dynamics Observatory (SDO) mission. These comparisons provide a means
of establishing the absolute calibration for EIS. Second, we combine
extended EIS observations from above the limb in the quiet Sun with a
simple temperature model to simultaneously determine the differential
emission measure (DEM) distribution and the time-dependent changes
to the effective areas that best fit all of the available spectral
lines. In Figure 2 we show the average spectrum from an observation
of seven consecutive runs of ELFULLCCDWSUMER. The
observations began on 2007 November 4 19:12 and ended on the same date
at 23:51 UT. The EIS field of view was centered at (990", -50") about
22" above the limb of the Sun. The central 129 pixels along the slit
have been averaged over 38 exposures (11 exposures were corrupted in
transmission to the ground) for a total of 4902 intensity measurements
at each wavelength. Since each exposure is 300s, the spectrum represents
1470600 pixels of effective exposure time and allows weak lines at
the ends of the detector to be measured. (1 data file).
Title: The Absolute Calibration of the EUV Imaging Spectrometer
on Hinode
Authors: Warren, Harry P.; Ugarte-Urra, Ignacio; Landi, Enrico
Bibcode: 2014ApJS..213...11W
Altcode: 2013arXiv1310.5324W
We investigate the absolute calibration of the EUV Imaging Spectrometer
(EIS) on Hinode by comparing EIS full-disk mosaics with irradiance
observations from the EUV Variability Experiment on the Solar Dynamics
Observatory. We also use extended observations of the quiet corona above
the limb combined with a simple differential emission measure model
to establish new effective area curves that incorporate information
from the most recent atomic physics calculations. We find that changes
to the EIS instrument sensitivity are a complex function of both time
and wavelength. We find that the sensitivity is decaying exponentially
with time and that the decay constants vary with wavelength. The EIS
short wavelength channel shows significantly longer decay times than
the long wavelength channel.
Title: Constraining Solar Flare Differential Emission Measures with
EVE and RHESSI
Authors: Caspi, Amir; McTiernan, James M.; Warren, Harry P.
Bibcode: 2014ApJ...788L..31C
Altcode: 2014arXiv1405.7068C
Deriving a well-constrained differential emission measure (DEM)
distribution for solar flares has historically been difficult,
primarily because no single instrument is sensitive to the full range
of coronal temperatures observed in flares, from lsim2 to gsim50
MK. We present a new technique, combining extreme ultraviolet (EUV)
spectra from the EUV Variability Experiment (EVE) onboard the Solar
Dynamics Observatory with X-ray spectra from the Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI), to derive, for the first
time, a self-consistent, well-constrained DEM for jointly observed
solar flares. EVE is sensitive to ~2-25 MK thermal plasma emission,
and RHESSI to gsim10 MK together, the two instruments cover the full
range of flare coronal plasma temperatures. We have validated the
new technique on artificial test data, and apply it to two X-class
flares from solar cycle 24 to determine the flare DEM and its temporal
evolution; the constraints on the thermal emission derived from the EVE
data also constrain the low energy cutoff of the non-thermal electrons,
a crucial parameter for flare energetics. The DEM analysis can also
be used to predict the soft X-ray flux in the poorly observed ~0.4-5
nm range, with important applications for geospace science.
Title: Absolute Abundance Measurements in Solar Flares
Authors: Warren, Harry
Bibcode: 2014AAS...22412301W
Altcode:
We present measurements of elemental abundances in solar flares with
EVE/SDO and EIS/Hinode. EVE observes both high temperature Fe emission
lines Fe XV-XXIV and continuum emission from thermal bremsstrahlung
that is proportional to the abundance of H. By comparing the relative
intensities of line and continuum emission it is possible to determine
the enrichment of the flare plasma relative to the composition of
the photosphere. This is the first ionization potential or FIP bias
(F). Since thermal bremsstrahlung at EUV wavelengths is relatively
insensitive to the electron temperature it is important to account for
the distribution of electron temperatures in the emitting plasma. We
accomplish this by using the observed spectra to infer the differential
emission measure distribution and FIP bias simultaneously. In
each of the 21 flares that we analyze we find that the observed
composition is close to photospheric. The mean FIP bias in our sample
is F=1.17+-0.22. Furthermore, we have compared the EVE measurements
with corresponding flare observations of intermediate temperature S,
Ar, Ca, and Fe emission lines taken with EIS. Our initial calculations
also indicate a photospheric composition for these observations. This
analysis suggests that the bulk of the plasma evaporated during a flare
comes from deep in the chromosphere, below the region where elemental
fractionation in the non-flaring corona occurs.
Title: Photometric and Thermal Cross-calibration of Solar EUV
Instruments
Authors: Boerner, P. F.; Testa, P.; Warren, H.; Weber, M. A.;
Schrijver, C. J.
Bibcode: 2014SoPh..289.2377B
Altcode: 2013arXiv1307.8045B
We present an assessment of the accuracy of the calibration measurements
and atomic physics models that go into calculating the SDO/AIA response
as a function of wavelength and temperature. The wavelength response
is tested by convolving SDO/EVE and Hinode/EIS spectral data with the
AIA effective area functions and by comparing the predictions with
AIA observations. For most channels, the AIA intensities summed over
the disk agree with the corresponding measurements derived from the
current version (V2) of the EVE data to within the estimated 25 %
calibration error. This agreement indicates that the AIA effective
areas are generally stable in time. The AIA 304 Å channel, however,
does show degradation by a factor of almost 3 from May 2010 through
September 2011, when the throughput apparently reached a minimum. We
also found some inconsistencies in the 335 Å passband, possibly due to
higher-order contamination of the EVE data. The intensities in the AIA
193 Å channel agree to within the uncertainties with the corresponding
measurements from EIS full CCD observations. Analysis of high-resolution
X-ray spectra of the solar-like corona of Procyon and of EVE spectra
allowed us to investigate the accuracy and completeness of the CHIANTI
database in the AIA shorter wavelength passbands. We found that in
the 94 Å channel, the spectral model significantly underestimates the
plasma emission owing to a multitude of missing lines. We derived an
empirical correction for the AIA temperature responses by performing
differential emission measure (DEM) inversion on a broad set of EVE
spectra and adjusting the AIA response functions so that the count
rates predicted by the full-disk DEMs match the observations.
Title: The Multi-Instrument, Comprehensive Differential Emission
Measure (DEM) of the Solar Corona During Flares and Quiescent Periods
Authors: Caspi, Amir; McTiernan, James; Warren, Harry; Woods, Thomas N.
Bibcode: 2014AAS...22412307C
Altcode:
Thermal plasma in the solar corona, while often modeled as isothermal
for ease of analysis, is in fact decidedly multi-thermal, ranging
from ~1-2 MK in the quiescent corona to ~30-50 MK in intensely
flaring loops. It has proven difficult to obtain a well-constrained
differential emission measure (DEM) from a single instrument, as
the wavelength ranges of individual instruments, even those with
broadband coverage, provide sensitivity to only a limited range of
plasma temperatures. Recently, we developed a new technique using
combined extreme ultraviolet (EUV) and soft and hard X-ray (SXR, HXR)
data from the EUV Variability Experiment (EVE) onboard the Solar
Dynamics Observatory (SDO) and the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI), respectively, to obtain a self-consistent
DEM that is strongly constrained across the full range of coronal plasma
temperatures (<2 to >50 MK). An accurate, precise determination
of the plasma temperature distribution enables not only studies of
plasma heating and thermal plasma evolution, but can also provide
strong constraints on the non-thermal accelerated electron population,
including the low-energy cutoff which is typically determined only as
a loose upper limit.We present EVE+RHESSI DEM results from selected
intense (X-class) flares from solar cycle 24, including determining
the non-thermal low-energy cutoff and examining how this evolves with
the temperature distribution. We also apply this technique to combine
EUV data from EVE with SXR data from the GOES X-ray Sensor (XRS)
and the X123, a new SXR spectrometer flown on two recent SDO/EVE
calibration sounding rockets, to examine the DEM during quiescent
(non-flaring) times with varying activity levels; the X-ray data
provide crucial constraints on the high-temperate extent of the DEM
and any potential non-thermal emission. We compare these results with
those from a parallel technique to derive DEMs from imaging data from
the Atmospheric Imaging Assembly (AIA) onboard SDO, and we discuss the
implications for plasma heating, both during flares and in the quiescent
corona. This research is supported by NASA contracts NAS5-98033 and
NAS5-02140, and NASA Heliophysics Guest Investigator Grant NNX12AH48G.
Title: Plasma Dynamics Above Solar Flare Soft X-Ray Loop Tops
Authors: Doschek, G. A.; McKenzie, D. E.; Warren, H. P.
Bibcode: 2014ApJ...788...26D
Altcode:
We measure non-thermal motions in flare loop tops and above the loop
tops using profiles of highly ionized spectral lines of Fe XXIV and Fe
XXIII formed at multimillion-degree temperatures. Non-thermal motions
that may be due to turbulence or multiple flow regions along the line of
sight are extracted from the line profiles. The non-thermal motions are
measured for four flares seen at or close to the solar limb. The profile
data are obtained using the Extreme-ultraviolet Imaging Spectrometer on
the Hinode spacecraft. The multimillion-degree non-thermal motions are
between 20 and 60 km s-1 and appear to increase with height
above the loop tops. Motions determined from coronal lines (i.e., lines
formed at about 1.5 MK) tend to be smaller. The multimillion-degree
temperatures in the loop tops and above range from about 11 MK to 15 MK
and also tend to increase with height above the bright X-ray-emitting
loop tops. The non-thermal motions measured along the line of sight,
as well as their apparent increase with height, are supported by
Solar Dynamics Observatory Atmospheric Imaging Assembly measurements
of turbulent velocities in the plane of the sky.
Title: The Hydrodynamics of High Temperature Plasma: Reproducing
the Properties of High Temperature Emission in Solar Active Regions
Authors: Ugarte-Urra, Ignacio; Warren, Harry
Bibcode: 2014AAS...22431205U
Altcode:
The launch of Hinode and SDO have revolutionized our ability to
measure the plasma properties of the solar corona. Many studies have
documented both the temperature structure of the corona as well as
its temporal variability. Of particular interest is the behavior
of high temperature loops that are typically found in the core of
an active region. Temperature distributions in these regions are
often sharply peaked near 4 MK but rapidly evolving loops are also
observed. In this talk we will present results from our effort to
perform hydrodynamic simulations of 15 solar active regions that cover
a wide range of solar conditions and to reconcile these simulations
with observations. In this work we have coupled non-linear force
free extrapolations with solutions to the hydrodynamic loop equations
approximated by EBTEL. Using relatively simple heating scenarios we are
able to reproduce three important properties of the observations: the
dependance of the observed intensity on magnetic flux, the sharply
peaked emission measure distributions for large regions, and the
general frequency distribution of the observed events. Our current
simulations, however, suggest much stronger 1MK emission near the
neutral line than is observed, indicating the heating of small loops
is not well understood. We also do not properly reproduce the relative
distribution of large and small events in these active regions.
Title: Non-thermal Motions in and Above Flare Loop Tops Measured by
the Extreme-ultraviolet Imaging Spectrometer (EIS) on Hinode
Authors: Doschek, George A.; McKenzie, David Eugene; Warren, Harry P
Bibcode: 2014AAS...22411105D
Altcode:
The plasma volume above the soft X-ray emitting loop tops is of
particular interest for studying the formation of flare loops. We
present EIS observations of non-thermal motions (turbulence) determined
from spectral line profiles of Fe XXIII and Fe XXIV ions for three
well-observed flares near the solar limb. We compare the non-thermal
motions at temperatures near 10 MK with the motions along the same
lines-of-sight determined from lines of coronal ions such as Fe XII, Fe
XIV, and Fe XV formed at 1-2 MK. The take-away is that the non-thermal
motions obtained from Fe XXIII and Fe XXIV lines increase with height
towards the reconnection region, up to speeds of about 50-60 km/s
for the largest heights that we can observe. The implication is that
considerable plasma heating occurs outside the reconnection region. In
addition, we discuss the implications of results obtained for flares
from earlier X-ray Yohkoh observations of line profiles of Fe XXV and
Ca XIX on the current results from EIS and AIA. Fe XXV is formed at
significantly higher temperatures than any strong flare EUV spectral
line observed by EIS or by imaging telescopes such as AIA or TRACE. This
work is supported by NASA grants.
Title: Measurements of Absolute Abundances in Solar Flares
Authors: Warren, Harry P.
Bibcode: 2014ApJ...786L...2W
Altcode: 2013arXiv1310.4765W
We present measurements of elemental abundances in solar flares
with the EUV Variability Experiment (EVE) on the Solar Dynamics
Observatory. EVE observes both high temperature Fe emission lines
(Fe XV-Fe XXIV) and continuum emission from thermal bremsstrahlung
that is proportional to the abundance of H. By comparing the relative
intensities of line and continuum emission it is possible to determine
the enrichment of the flare plasma relative to the composition of
the photosphere. This is the first ionization potential or FIP bias
(f). Since thermal bremsstrahlung at EUV wavelengths is relatively
insensitive to the electron temperature, it is important to account for
the distribution of electron temperatures in the emitting plasma. We
accomplish this by using the observed spectra to infer the differential
emission measure distribution and FIP bias simultaneously. In each of
the 21 flares that we analyze we find that the observed composition is
close to photospheric. The mean FIP bias in our sample is f = 1.17 ±
0.22. This analysis suggests that the bulk of the plasma evaporated
during a flare comes from deep in the chromosphere, below the region
where elemental fractionation occurs.
Title: Determining Heating Timescales in Solar Active Region Cores
from AIA/SDO Fe XVIII Images
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.
Bibcode: 2014ApJ...783...12U
Altcode: 2013arXiv1311.6346U
We present a study of the frequency of transient brightenings in the
core of solar active regions as observed in the Fe XVIII line component
of AIA/SDO 94 Å filter images. The Fe XVIII emission is isolated using
an empirical correction to remove the contribution of "warm" emission to
this channel. Comparing with simultaneous observations from EIS/Hinode,
we find that the variability observed in Fe XVIII is strongly correlated
with the emission from lines formed at similar temperatures. We examine
the evolution of loops in the cores of active regions at various stages
of evolution. Using a newly developed event detection algorithm,
we characterize the distribution of event frequency, duration, and
magnitude in these active regions. These distributions are similar for
regions of similar age and show a consistent pattern as the regions
age. This suggests that these characteristics are important constraints
for models of solar active regions. We find that the typical frequency
of the intensity fluctuations is about 1400 s for any given line of
sight, i.e., about two to three events per hour. Using the EBTEL 0D
hydrodynamic model, however, we show that this only sets a lower limit
on the heating frequency along that line of sight.
Title: Using Coronal Cells to Infer the Magnetic Field Structure
and Chirality of Filament Channels
Authors: Sheeley, N. R., Jr.; Martin, S. F.; Panasenco, O.; Warren,
H. P.
Bibcode: 2013ApJ...772...88S
Altcode: 2013arXiv1306.2273S
Coronal cells are visible at temperatures of ~1.2 MK in Fe XII
coronal images obtained from the Solar Dynamics Observatory and Solar
Terrestrial Relations Observatory spacecraft. We show that near a
filament channel, the plumelike tails of these cells bend horizontally
in opposite directions on the two sides of the channel like fibrils
in the chromosphere. Because the cells are rooted in magnetic flux
concentrations of majority polarity, these observations can be used
with photospheric magnetograms to infer the direction of the horizontal
field in filament channels and the chirality of the associated magnetic
field. This method is similar to the procedure for inferring the
direction of the magnetic field and the chirality of the fibril pattern
in filament channels from Hα observations. However, the coronal cell
observations are easier to use and provide clear inferences of the
horizontal field direction for heights up to ~50 Mm into the corona.
Title: High Spatial Resolution Observations of Loops in the Solar
Corona
Authors: Brooks, David H.; Warren, Harry P.; Ugarte-Urra, Ignacio;
Winebarger, Amy R.
Bibcode: 2013ApJ...772L..19B
Altcode: 2013arXiv1305.2246B
Understanding how the solar corona is structured is of fundamental
importance to determine how the Sun's upper atmosphere is heated to
high temperatures. Recent spectroscopic studies have suggested that an
instrument with a spatial resolution of 200 km or better is necessary
to resolve coronal loops. The High Resolution Coronal Imager (Hi-C)
achieved this performance on a rocket flight in 2012 July. We use Hi-C
data to measure the Gaussian widths of 91 loops observed in the solar
corona and find a distribution that peaks at about 270 km. We also
use Atmospheric Imaging Assembly data for a subset of these loops and
find temperature distributions that are generally very narrow. These
observations provide further evidence that loops in the solar corona
are often structured at a scale of several hundred kilometers, well
above the spatial scale of many proposed physical mechanisms.
Title: Turbulence in the Flare Reconnection Region
Authors: Doschek, George A.; McKenzie, D. E.; Warren, H.
Bibcode: 2013SPD....4430401D
Altcode:
The physical conditions such as temperature, density, and dynamical
properties in the flare reconnection region, located above the bright
soft X-ray loops, are basically not known although there have been
measurements of non-thermal hard X-ray emission properties by RHESSI
and earlier by HXT on Yohkoh. The advent of Hinode and the Solar
Dynamics Observatory (SDO) spatially resolved observations, however,
has changed this and it is now possible to measure in more detail some
of the properties of the reconnection region. AIA imagery on SDO and
the Extreme-ultraviolet Imaging Spectrometer (EIS) and X-ray Telescope
(XRT) on Hinode allow values of non-thermal motions or turbulence in
the reconnection region to be determined. Turbulence is predicted by
theoretical models of magnetic reconnection in flares (e.g., see Liu et
al. 2008, ApJ, 676, 704) and has long been inferred spectroscopically
from non-thermal broadening of flare emission lines. Studies with
Hinode/XRT and SDO/AIA demonstrate that two-dimensional investigations
of flare velocity fields can be made, by imaging the plasma sheets
above post-CME flare arcades. These measurements are made possible
through the use of local correlation tracking (LCT), as shown by
McKenzie (2013), ApJ, 766, 39, and reveal signatures of turbulence,
including temporally and spatially varying vorticity. For some flares
the AIA and XRT results can be combined with Doppler measurements of
turbulence obtained with EIS. EIS data consist of raster scans that
include the reconnection region for flares on the limb or near the
limb. A set of spectral lines are observed that cover temperatures
from 0.25 MK up to ~20 MK. A temperature in the reconnection region
is calculated from the Fe XXIII/Fe XXIV line ratio and the thermal
Doppler and instrumental widths are subtracted from the total line
widths. The remainder is non-thermal motions or turbulence. We will
present coordinated analyses of EIS and AIA observations of plasma
sheets in post-CME flares, and demonstrate that the turbulent speeds
found by LCT are about the same magnitude as those derived from EIS
spectral line profiles obtained in the same or nearby locations.
Title: EVE-RHESSI Observations of Thermal and Nonthermal Solar
Flare Emission
Authors: McTiernan, James; Caspi, A.; Warren, H.
Bibcode: 2013SPD....44...55M
Altcode:
Solar flares accelerate electrons up to hundreds of MeV and heat plasma
to tens of MK. In large (GOES M- and X-class) flares, in addition
to the 10-25 MK plasma thought to be the result of chromospheric
evaporation, even hotter plasma (up to 50 MK) may be directly heated
in the corona. While observations of hard X-ray bremmstrahlung directly
probe the nonthermal electron population, for large flares the spectra
below 20-30 keV are typically dominated by thermal emission. The low
energy extent of the nonthermal spectrum can be only loosely quantified
by hard X-ray spectrometers, resulting in significant implications
for calculating flare energy budgets and for constraining possible
acceleration mechanisms. A precise characterization of the thermal
emission is imperative. Extreme ultraviolet observations from the EUV
Variability Experiment (EVE) on-board the Solar Dynamics Observatory
(SDO), combined with X-ray data from the Reuven Ramaty High Energy
Spectroscopic Imager (RHESSI), currently offer the most comprehensive
view of the flare temperature distribution. EVE observes EUV emission
lines with peak formation temperatures of 2-20 MK, while RHESSI observes
the X-ray bremsstrahlung of hot, 10-50 MK plasma; combined, the two
instruments cover the full range of flare plasma temperatures. In this
work, we handle the EVE-RHESSI data for a few large flares in three
steps; first we calculate differential emission measures (DEMs) using
EVE and RHESSI independently for purposes of cross-calibration. Second,
we create combined EVE-RHESSI DEMs, fixing the nonthermal spectral
parameters to those found using a RHESSI-only spectral fit. The final
step is to unconstrain the nonthermal parameters (in particular,
the low-energy cutoff of the spectrum) and let them be fit in the same
process as the EVE-RHESSI DEM, to obtain a fully self-consistent thermal
plus nonthermal model. This research is supported by NASA Heliophysics
Guest Investigator Grant NNX12AH48G.
Title: Progress toward high resolution EUV spectroscopy
Authors: Korendyke, C.; Doschek, G. A.; Warren, H.; Young, P. R.;
Chua, D.; Hassler, D. M.; Landi, E.; Davila, J. M.; Klimchuck, J.;
Tun, S.; DeForest, C.; Mariska, J. T.; Solar C Spectroscopy Working
Group; LEMUR; EUVST Development Team
Bibcode: 2013SPD....44..143K
Altcode:
HIgh resolution EUV spectroscopy is a critical instrumental technique
to understand fundamental physical processes in the high temperature
solar atmosphere. Spectroscopic observations are used to measure
differential emission measure, line of sight and turbulent flows,
plasma densities and emission measures. Spatially resolved, spectra of
these emission lines with adequate cadence will provide the necessary
clues linking small scale structures with large scale, energetic
solar phenomena. The necessary observations to determine underlying
physical processes and to provide comprehensive temperature coverage
of the solar atmosphere above the chromosphere will be obtained by the
proposed EUVST instrument for Solar C. This instrument and its design
will be discussed in this paper. Progress on the VEry high Resolution
Imaging Spectrograph (VERIS) sounding rocket instrument presently under
development at the Naval Research Laboratory will also be discussed.
Title: Heating Frequency in the core of Active Regions
Authors: Ugarte-Urra, Ignacio; Warren, H.
Bibcode: 2013SPD....4430502U
Altcode:
We present a study of the frequency and duration of brightenings in the
core of solar active regions as observed in the Fe XVIII line component
of AIA/SDO 94 A filter images. The Fe XVIII emission was isolated by
removing the "warm" emission contribution using as proxy the emission
from the AIA 193 and 171 channels. We examined the evolution of loop
in cores of several active regions that span a wide range of total
magnetic field strengths and at various stages of evolution. Using
a newly developed event detector algorithm we find that the typical
frequency of occurrence of detectable brightness enhancements is in
the order of 20 minutes. Using EBTEL, a 0D hydrodynamical model, we
show that a single loop heated a that frequency would be experiencing
effectively steady heating. Then we evaluate different heating scenarios
with multiple loops along the line-of-sight. Finally, we report on our
preliminary efforts to reproduce those characteristic timescales on
full active region models where field lines from a non-linear force
free extrapolation are populated with EBTEL solutions.
Title: Status of RAISE, the Rapid Acquisition Imaging Spectrograph
Experiment
Authors: Laurent, Glenn T.; Hassler, D. M.; DeForest, C.; Ayres,
T. R.; Davis, M.; De Pontieu, B.; Schuehle, U.; Warren, H.
Bibcode: 2013SPD....44..145L
Altcode:
The Rapid Acquisition Imaging Spectrograph Experiment (RAISE) sounding
rocket payload is a high speed scanning-slit imaging spectrograph
designed to observe the dynamics and heating of the solar chromosphere
and corona on time scales as short as 100 ms, with 1 arcsec spatial
resolution and a velocity sensitivity of 1-2 km/s. The instrument is
based on a new class of UV/EUV imaging spectrometers that use only
two reflections to provide quasi-stigmatic performance simultaneously
over multiple wavelengths and spatial fields. The design uses an
off-axis parabolic telescope mirror to form a real image of the sun
on the spectrometer entrance aperture. A slit then selects a portion
of the solar image, passing its light onto a near-normal incidence
toroidal grating, which re-images the spectrally dispersed radiation
onto two array detectors. Two full spectral passbands over the same
one-dimensional spatial field are recorded simultaneously with no
scanning of the detectors or grating. The two different spectral
bands (1st-order 1205-1243Å and 1526-1564Å) are imaged onto two
intensified Active Pixel Sensor (APS) detectors whose focal planes are
individually adjusted for optimized performance. The telescope and
grating are coated with B4C to enhance short wavelength (2nd order)
reflectance, enabling the instrument to record the brightest lines
between 602-622Å and 761-780Å at the same time. RAISE reads out the
full field of both detectors at 5-10 Hz, allowing us to record over
1,500 complete spectral observations in a single 5-minute rocket flight,
opening up a new domain of high time resolution spectral imaging and
spectroscopy. We present an overview of the project, a summary of the
maiden flight results, and an update on instrument status.Abstract
(2,250 Maximum Characters): The Rapid Acquisition Imaging Spectrograph
Experiment (RAISE) sounding rocket payload is a high speed scanning-slit
imaging spectrograph designed to observe the dynamics and heating of the
solar chromosphere and corona on time scales as short as 100 ms, with 1
arcsec spatial resolution and a velocity sensitivity of 1-2 km/s. The
instrument is based on a new class of UV/EUV imaging spectrometers
that use only two reflections to provide quasi-stigmatic performance
simultaneously over multiple wavelengths and spatial fields. The design
uses an off-axis parabolic telescope mirror to form a real image of
the sun on the spectrometer entrance aperture. A slit then selects
a portion of the solar image, passing its light onto a near-normal
incidence toroidal grating, which re-images the spectrally dispersed
radiation onto two array detectors. Two full spectral passbands over
the same one-dimensional spatial field are recorded simultaneously with
no scanning of the detectors or grating. The two different spectral
bands (1st-order 1205-1243Å and 1526-1564Å) are imaged onto two
intensified Active Pixel Sensor (APS) detectors whose focal planes are
individually adjusted for optimized performance. The telescope and
grating are coated with B4C to enhance short wavelength (2nd order)
reflectance, enabling the instrument to record the brightest lines
between 602-622Å and 761-780Å at the same time. RAISE reads out the
full field of both detectors at 5-10 Hz, allowing us to record over
1,500 complete spectral observations in a single 5-minute rocket flight,
opening up a new domain of high time resolution spectral imaging and
spectroscopy. We present an overview of the project, a summary of the
maiden flight results, and an update on instrument status.
Title: Observations of Thermal Flare Plasma with the EUV Variability
Experiment
Authors: Warren, Harry P.; Mariska, John T.; Doschek, George A.
Bibcode: 2013ApJ...770..116W
Altcode: 2012arXiv1211.1875W
One of the defining characteristics of a solar flare is the impulsive
formation of very high temperature plasma. The properties of the
thermal emission are not well understood, however, and the analysis of
solar flare observations is often predicated on the assumption that the
flare plasma is isothermal. The EUV Variability Experiment (EVE) on the
Solar Dynamics Observatory provides spectrally resolved observations of
emission lines that span a wide range of temperatures (e.g., Fe XV-Fe
XXIV) and allow for thermal flare plasma to be studied in detail. In
this paper we describe a method for computing the differential emission
measure distribution in a flare using EVE observations and apply it to
several representative events. We find that in all phases of the flare
the differential emission measure distribution is broad. Comparisons
of EVE spectra with calculations based on parameters derived from
the Geostationary Operational Environmental Satellites soft X-ray
fluxes indicate that the isothermal approximation is generally a poor
representation of the thermal structure of a flare.
Title: Properties of a Solar Flare Kernel Observed by Hinode and SDO
Authors: Young, P. R.; Doschek, G. A.; Warren, H. P.; Hara, H.
Bibcode: 2013ApJ...766..127Y
Altcode: 2012arXiv1212.4388Y
Flare kernels are compact features located in the solar chromosphere
that are the sites of rapid heating and plasma upflow during the rise
phase of flares. An example is presented from a M1.1 class flare in
active region AR 11158 observed on 2011 February 16 07:44 UT for which
the location of the upflow region seen by EUV Imaging Spectrometer (EIS)
can be precisely aligned to high spatial resolution images obtained by
the Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic
Imager (HMI) on board the Solar Dynamics Observatory (SDO). A string
of bright flare kernels is found to be aligned with a ridge of strong
magnetic field, and one kernel site is highlighted for which an upflow
speed of ≈400 km s-1 is measured in lines formed at 10-30
MK. The line-of-sight magnetic field strength at this location is
≈1000 G. Emission over a continuous range of temperatures down to
the chromosphere is found, and the kernels have a similar morphology
at all temperatures and are spatially coincident with sizes at the
resolution limit of the AIA instrument (lsim400 km). For temperatures
of 0.3-3.0 MK the EIS emission lines show multiple velocity components,
with the dominant component becoming more blueshifted with temperature
from a redshift of 35 km s-1 at 0.3 MK to a blueshift of
60 km s-1 at 3.0 MK. Emission lines from 1.5-3.0 MK show a
weak redshifted component at around 60-70 km s-1 implying
multi-directional flows at the kernel site. Significant non-thermal
broadening corresponding to velocities of ≈120 km s-1 is
found at 10-30 MK, and the electron density in the kernel, measured
at 2 MK, is 3.4 × 1010 cm-3. Finally, the Fe
XXIV λ192.03/λ255.11 ratio suggests that the EIS calibration has
changed since launch, with the long wavelength channel less sensitive
than the short wavelength channel by around a factor two.
Title: Chromospheric Evaporation in an M1.8 Flare Observed by the
Extreme-ultraviolet Imaging Spectrometer on Hinode
Authors: Doschek, G. A.; Warren, H. P.; Young, P. R.
Bibcode: 2013ApJ...767...55D
Altcode: 2012arXiv1212.4027D
We discuss observations of chromospheric evaporation for a complex
flare that occurred on 2012 March 9 near 03:30 UT obtained from the
Extreme-ultraviolet Imaging Spectrometer (EIS) on board the Hinode
spacecraft. This was a multiple event with a strong energy input
that reached the M1.8 class when observed by EIS. EIS was in raster
mode and fortunately the slit was almost at the exact location of
a significant energy input. Also, EIS obtained a full-CCD spectrum
of the flare, i.e., the entire CCD was readout so that data were
obtained for about the 500 lines identified in the EIS wavelength
ranges. Chromospheric evaporation characterized by 150-200 km
s-1 upflows was observed in multiple locations in
multi-million degree spectral lines of flare ions such as Fe XXII,
Fe XXIII, and Fe XXIV, with simultaneous 20-60 km s-1
upflows in million degree coronal lines from ions such as Fe XII-Fe
XVI. The behavior of cooler, transition region ions such as O VI, Fe
VIII, He II, and Fe X is more complex, but upflows were also observed
in Fe VIII and Fe X lines. At a point close to strong energy input in
space and time, the flare ions Fe XXII, Fe XXIII, and Fe XXIV reveal
an isothermal source with a temperature close to 14 MK and no strong
blueshifted components. At this location there is a strong downflow
in cooler active region lines from ions such as Fe XIII and Fe XIV,
on the order of 200 km s-1. We speculate that this downflow
may be evidence of the downward shock produced by reconnection in the
current sheet seen in MHD simulations. A sunquake also occurred near
this location. Electron densities were obtained from density sensitive
lines ratios from Fe XIII and Fe XIV. Atmospheric Imaging Assembly
(AIA) observations from the Solar Dynamics Observatory are used with
JHelioviewer to obtain a qualitative overview of the flare. However,
AIA data are not presented in this paper. In summary, spectroscopic data
from EIS are presented that can be used for predictive tests of models
of chromospheric evaporation as envisaged in the Standard Flare Model.
Title: Flare Footpoint Regions Observed by the Extreme-ultraviolet
Imaging Spectrometer (EIS) on Hinode
Authors: Doschek, G. A.; Warren, H. P.; Young, P. R.; Caspi, A.
Bibcode: 2013enss.confE..74D
Altcode:
The Extreme-ultraviolet Imaging Spectrometer (EIS) on Hinode has
observed flare footpoints using a variety of studies designed to
observe various aspects of the solar flare Standard Model. Some of
these observations are accompanied by imaging data from RHESSI. We
present observations of upflows in flare footpoint regions obtained
from picket-fence raster observations of two flares that occurred
on 24 and 25 September 2011. The observations consist of upflow and
non-thermal speeds at various temperatures (from about 1 MK to 15
MK) at footpoint regions as well as a limited differential emission
measure. RHESSI observations provide constraints on the energetic
electron precipitation into the footpoints. Electron densities are
available from an Fe XIV ratio, and SDO AIA data are also investigated
for context. The RHESSI energy input will be assessed in terms of 1D
models of the footpoint regions.
Title: Heating frequency in active region cores as observed in AIA
Fe XVIII images
Authors: Ugarte-Urra, I.; Warren, H. P.
Bibcode: 2013enss.confE..85U
Altcode:
We present a study of the frequency and duration of brightenings in the
core of solar active regions as observed in the Fe XVIII line component
of AIA/SDO 94 A filter images. The Fe XVIII emission was isolated by
removing the "warm" emission contribution using as proxy the emission
from the AIA 193 and 171 channels. We examined the evolution of loop
in cores of several active regions that span a wide range of total
magnetic field strengths and at various stages of evolution. Using
a newly developed event detector algorithm we find that the typical
frequency of occurrence of brightness enhancements is in the order
of tens of minutes. We then use those values to evaluate different
scenarios of heating frequency using 1D hydrodynamical models of loops.
Title: Computing the Solar EUV Irradiance at Wavelengths Below 450 Å
Authors: Warren, Harry
Bibcode: 2013enss.confE..47W
Altcode:
The solar EUV irradiance plays a central role in determining the state
of the Earth's upper atmosphere. The EUV irradiance at the shortest
wavelengths, which is highly variable over time scales from seconds
to decades, is particularly important for many aspects of space
weather. Systematic spectrally resolved observations at the shortest
EUV wavelengths, however, have been rare and there is a need to develop
a methodology for estimating and forecasting the solar irradiance at
all EUV wavelengths from sparse data sets. The AIA on SDO provides full
Sun solar images in 7 narrow EUV wavelength ranges. These channels were
selected to provide complete thermal coverage of the corona and allow
for calculation of the differential emission measure distribution. In
this presentation we report on our efforts to use AIA DEM calculations
to estimate the solar EUV irradiance at wavelength below 450 Å, where
the emission is predominately optically thin. To validate our AIA DEM
calculations we have performed extensive comparisons with simultaneous
observations from the EIS instrument on Hinode and find that with the
proper constraints we can generally reproduce the results obtained
with detailed spectroscopic observations using AIA. We also present
comparisons with existing time series of QEUV, the integrated solar
irradiance at wavelengths below 450 Å.
Title: SDO and Hinode observations of coronal heating at a flare
kernel site
Authors: Young, P. R.; Doschek, G. A.; Warren, H. P.; Hara, H.
Bibcode: 2013enss.confE..36Y
Altcode:
Flare kernels are compact features located in the chromosphere that
are the sites of rapid heating and plasma upflow during the rise phase
of flares. They provide an excellent opportunity for testing models of
energy transport and dissipation in the solar atmosphere as they are
very bright and emit over a wide temperature range. A M1.1 class flare
that peaked at 07:44 UT on 2011 February 16 was observed simultaneously
by SDO and Hinode, and one flare kernel observed prior to the flare
peak is highlighted. It is found to emit at all temperatures from
the chromosphere through to 30 MK, with all AIA channels brightening
simultaneously and rise times of only 1 minute. The kernel is located on
a ridge of strong magnetic field close to a neutral line in the active
region. The kernel is at the resolution limit of AIA, suggesting a size
of < 0.6 arcsec. Hinode/EIS allows velocity patterns in the kernel
to be tracked over a wide temperature range and reveals a dominant high
speed upflow of 400 km/s at temperatures of 10-30 MK, with both down
and upflows measured at cooler temperatures of 1.5-3.0 MK, suggesting
unresolved structures. All emission lines show evidence of significant
non-thermal broadening, and the electron density of the plasma is 3.4
x 10^10 cm-3. The observations are compared to models of chromospheric
evaporation and similarities and differences are highlighted.
Title: Exploring Thermal and Non-Thermal Flare Emission with EVE
and RHESSI
Authors: Caspi, Amir; McTiernan, James M.; Warren, Harry P.
Bibcode: 2013enss.confE.121C
Altcode:
Solar flares accelerate electrons up to hundreds of MeV and heat plasma
to tens of MK, but the physical processes behind these phenomena
remain poorly understood. In intense (GOES M- and X-class) flares,
in addition to the common 10-25 MK plasma thought to be the result
of chromospheric evaporation, even hotter plasma (up to 50 MK) may
be directly heated in the corona. While observations of hard X-ray
bremmstrahlung directly probe the non-thermal electron population, for
large flares, the spectra below 20-30 keV are typically dominated by
this strong thermal emission. The low-energy extent of the non-thermal
spectrum can be only loosely quantified, resulting in significant
implications for calculating flare energy budgets and for constraining
possible acceleration mechanisms. A precise characterization of the
thermal electron population is imperative, and this requires an equally
precise characterization of the thermal emission. Extreme ultraviolet
observations from the EUV Variability Experiment (EVE) on-board the
Solar Dynamics Observatory (SDO), combined with X-ray data from the
Reuven Ramaty High Energy Spectroscopic Imager (RHESSI), currently offer
the most comprehensive view of the flare temperature distribution. EVE
observes EUV emission lines with peak formation temperatures of 2-20
MK, while RHESSI observes the X-ray bremsstrahlung of hot, 10-50 MK
plasma; combined, the two instruments cover the full range of flare
plasma temperatures. Previously, we have calculated differential
emission measures (DEMs) using EVE and RHESSI independently, for a
small number of flares, and showed that they tend to agree well in the
10-20 MK region, where their responses overlap, but that, as expected,
they disagree significantly outside this range, where the DEM is
poorly constrained by one instrument or the other, exemplifying the
need for a unified solution. Recently, we have developed a technique
for determining flare DEMs using both EVE and RHESSI simultaneously,
with each instrument constraining the other. We apply this technique
to a number of synthetic test cases to show that it robustly recovers
the input test DEMs, and then show results of analyzing real data from
two intense, X-class flares. Through this technique, for the first
time, we can determine self-consistent DEMs over the complete flare
temperature range of 3-50 MK, and this precise determination of the
thermal emission will later enable detailed studies of the non-thermal
electron populations, as well.
Title: Is Active Region Core Variability Age Dependent?
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.
Bibcode: 2012ApJ...761...21U
Altcode:
The presence of both steady and transient loops in active region cores
has been reported from soft X-ray and extreme-ultraviolet observations
of the solar corona. The relationship between the different loop
populations, however, remains an open question. We present an
investigation of the short-term variability of loops in the core of
two active regions in the context of their long-term evolution. We
take advantage of the nearly full Sun observations of STEREO and Solar
Dynamics Observatory spacecraft to track these active regions as they
rotate around the Sun multiple times. We then diagnose the variability
of the active region cores at several instances of their lifetime
using EIS/Hinode spectral capabilities. We inspect a broad range of
temperatures, including for the first time spatially and temporally
resolved images of Ca XIV and Ca XV lines. We find that the active
region cores become fainter and steadier with time. The significant
emission measure at high temperatures that is not correlated with a
comparable increase at low temperatures suggests that high-frequency
heating is viable. The presence, however, during the early stages,
of an enhanced emission measure in the "hot" (3.0-4.5 MK) and "cool"
(0.6-0.9 MK) components suggests that low-frequency heating also plays
a significant role. Our results explain why there have been recent
studies supporting both heating scenarios.
Title: Exploring Thermal and Non-Thermal Flare Emission with EVE
and RHESSI
Authors: McTiernan, J. M.; Warren, H. P.; Caspi, A.
Bibcode: 2012AGUFMSH52B..02M
Altcode:
Solar flares accelerate electrons up to hundreds of MeV and heat plasma
to tens of MK, but the physical processes behind these phenomena remain
poorly understood. In large (GOES M- and X-class) flares, in addition
to the 10-25 MK plasma thought to be the result of chromospheric
evaporation, even hotter plasma (up to 50 MK) may be directly heated
in the corona. While observations of hard X-ray bremmstrahlung
directly probe the non-thermal electron population, for large flares
the spectra below 20-30 keV are typically dominated by this strong
thermal emission. The low energy extent of the non-thermal spectrum
can be only loosely quantified, resulting in significant implications
for calculating flare energy budgets and for constraining possible
acceleration mechanisms. A precise characterization of the non-thermal
electron population requires an equally precise characterization of
the thermal emission. Extreme ultraviolet observations from the EUV
Variability Experiment (EVE) on-board the Solar Dynamics Observatory
(SDO), combined with X-ray data from the Reuven Ramaty High Energy
Spectroscopic Imager (RHESSI), currently offer the most comprehensive
view of the flare temperature distribution. EVE observes EUV emission
lines with peak formation temperatures of 2-20 MK, while RHESSI observes
the X-ray bremsstrahlung of hot, 10-50 MK plasma; combined, the two
instruments cover the full range of flare plasma temperatures. We
have calculated differential emission measures (DEMs), using EVE and
RHESSI independently, for a small number of flares. In this work we
concentrate on comparing the observed DEM functions from EVE and RHESSI
with each other, during different phases of flares, for the purpose
of cross-calibration of the two instruments. When cross-calibration
is successful, we will combine the data from the two instruments to
create a DEM function for the temperature range up to 50 MK.
Title: Chromospheric Evaporation in an M1.8 Flare Observed by the
Extreme-ultraviolet Imaging Spectrometer (EIS) on Hinode
Authors: Doschek, G. A.; Warren, H. P.
Bibcode: 2012AGUFMSH52B..04D
Altcode:
We discuss observations of chromospheric evaporation for a flare
that occurred on 9 March 2012 near 03:30 UT obtained from the
Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode
spacecraft. This was a multiple event with a strong energy input
that reached the M1.8 class when observed by EIS. EIS was in raster
mode and fortunately the slit reached almost the exact location of a
significant energy input. Also, fortunately EIS obtained a full-CCD
spectrum of the flare, i.e., the entire CCD was readout so that data
were obtained for about the 500 lines identified in the EIS wavelength
ranges. Chromospheric evaporation characterized by 150-200 km/s upflows
was observed in several locations in multi-million degree spectral lines
of flare ions such as Fe XXII, Fe XXIII, Fe XXIV, with simultaneous 20 -
60 km/s upflows in a host of million degree coronal lines from ions such
as Fe XI - Fe XVI. The behavior of cooler, transition region ions such
as O VI, Fe VIII, He II, and Fe X is more complex. At a point close
to strong energy input, the flare ions reveal an isothermal source
with a temperature close to 14 MK. At this point there is a strong
downflow in cooler active region lines from ions such as Fe XIII
and Fe XIV. Electron densities were obtained from density sensitive
lines ratios from Fe XIII and Fe XIV. The results to be presented
are refined from the preliminary data given above and combined with
context AIA observations for a comparison with predictions of models
of chromospheric evaporation as envisaged in the Standard Flare Model.
Title: A Systematic Survey of High Temperature Emission in Solar
Active Regions
Authors: Warren, H. P.
Bibcode: 2012AGUFMSH31B..01W
Altcode:
The temperature structure of the solar corona holds many important
clues as to how the solar atmosphere is heated. Recent observations
with EIS/Hinode and AIA/SDO have shown that well constrained temperature
measurements can be made over a wide range of solar conditions. In this
talk I will present results from a systematic study of the differential
emission measure distribution in 15 active region cores. We focus on
measurements in the "inter-moss" region, that is, the region between
the loop footpoints, where the observations are easier to interpret. To
reduce the uncertainties at the highest temperatures we present a
new method for isolating the Fe XVIII emission in the AIA/SDO 94
channel. The resulting differential emission measure distributions
show that the temperature distribution in an active region core is
often strongly peaked near 4MK. We will compare these results to
the analysis of evolving million degree loops, which show a similar,
sharply peaked temperature distribution. This work was sponsored by
the Office of Naval Research and by NASA
Title: The Fundamental Structure of Coronal Loops
Authors: Winebarger, A. R.; Warren, H. P.; Cirtain, J. W.; Kobayashi,
K.; Korreck, K. E.; Golub, L.; Kuzin, S.; Walsh, R. W.; DeForest,
C.; De Pontieu, B.; Title, A. M.; Weber, M.
Bibcode: 2012AGUFMSH31B..06W
Altcode:
During the past ten years, solar physicists have attempted to infer the
coronal heating mechanism by comparing observations of coronal loops
with hydrodynamic model predictions. These comparisons often used
the addition of sub-resolution strands to explain the observed loop
properties. On July 11, 2012, the High Resolution Coronal Imager (Hi-C)
was launched on a sounding rocket. This instrument obtained images of
the solar corona was 0.2-0.3'' resolution in a narrowband EUV filter
centered around 193 Angstroms. In this talk, we will compare these
high resolution images to simultaneous density measurements obtained
with the Extreme Ultraviolet Imaging Spectrograph (EIS) on Hinode to
determine whether the structures observed with Hi-C are resolved.
Title: Computing the Solar EUV Irradiance at Wavelengths Below 450 Å
Authors: Warren, H. P.
Bibcode: 2012AGUFMSH13C2270W
Altcode:
The solar EUV irradiance plays a central role in determining the state
of the Earth's upper atmosphere. The EUV irradiance at the shortest
wavelengths, which is highly variable over time scales from seconds
to decades, is particularly important for many aspects of space
weather. Systematic spectrally resolved observations at the shortest
EUV wavelengths, however, have been rare and there is a need to develop
a methodology for estimating and forecasting the solar irradiance at
all EUV wavelengths from sparse data sets. The AIA on SDO provides full
Sun solar images in 7 narrow EUV wavelength ranges. These channels were
selected to provide complete thermal coverage of the corona and allow
for calculation of the differential emission measure distribution. In
this presentation we report on our efforts to use AIA DEM calculations
to estimate the solar EUV irradiance at wavelength below 450 Å,
where the emission is predominately optically thin. To validate our
AIA DEM calculations we have performed extensive comparisons with
simultaneous observations from the EIS instrument on Hinode and find
that with the proper constraints we can generally reproduce the results
obtained with detailed spectroscopic observations using AIA. We also
anticipate presenting comparisons with existing time series of QEUV,
the integrated solar irradiance at wavelengths below 450 Å.
Title: Hinode/EIS measurements of Abundances in Solar Active Region
Outflows
Authors: Brooks, D.; Warren, H. P.
Bibcode: 2012AGUFMSH52A..04B
Altcode:
Peripheral outflows appear to be a common feature of active regions,
and may be a significant source of the slow speed solar wind. Spectral
line profiles from the Hinode EUV Imaging Spectrometer (EIS) show that
the bulk outflows reach speeds of ~50km/s with a much faster component
reaching hundreds of km/s. I will review recent measurements of the
elemental composition of the outflows obtained by EIS, with particular
attention paid to AR 10978 that was observed as it crossed the solar
disk in December 2007. EIS measurements show that the temperature
distribution of the outflows is dominated by coronal emission, and
that plasma with a slow wind-like composition flowed from the edge of
AR 10978 for at least five days. Furthermore, when the outflow from
the Western side was favorably oriented in the Earth direction, the
composition was found to match the value measured a few days later by
ACE/SWICS. The composition of the high speed component of the outflows
was also found to be similar to that of the slow speed wind, implying
that it may also be a contributor. Observations and models indicate
that it takes time for plasma to evolve to the enhanced composition
typical of the slow wind, suggesting that the material in the outflows
is trapped on closed loops before escaping, perhaps by interchange
reconnection. The results, therefore, also identify the high speed
component of the plasma as having a coronal origin. A significant
constraint on the mechanisms that drive the outflows.
Title: A Systematic Survey of High-temperature Emission in Solar
Active Regions
Authors: Warren, Harry P.; Winebarger, Amy R.; Brooks, David H.
Bibcode: 2012ApJ...759..141W
Altcode: 2012arXiv1204.3220W
The recent analysis of observations taken with the EUV Imaging
Spectrometer and X-Ray Telescope instruments on Hinode suggests that
well-constrained measurements of the temperature distribution in solar
active regions can finally be made. Such measurements are critical
for constraining theories of coronal heating. Past analysis, however,
has suffered from limited sample sizes and large uncertainties at
temperatures between 5 and 10 MK. Here we present a systematic study
of the differential emission measure distribution in 15 active region
cores. We focus on measurements in the "inter-moss" region, that is, the
region between the loop footpoints, where the observations are easier
to interpret. To reduce the uncertainties at the highest temperatures
we present a new method for isolating the Fe XVIII emission in the
AIA/SDO 94 Å channel. The resulting differential emission measure
distributions confirm our previous analysis showing that the temperature
distribution in an active region core is often strongly peaked near 4
MK. We characterize the properties of the emission distribution as a
function of the total unsigned magnetic flux. We find that the amount
of high-temperature emission in the active region core is correlated
with the total unsigned magnetic flux, while the emission at lower
temperatures, in contrast, is inversely related. These results provide
compelling evidence that high-temperature active region emission is
often close to equilibrium, although weaker active regions may be
dominated by evolving million degree loops in the core.
Title: The Coronal Source of Extreme-ultraviolet Line Profile
Asymmetries in Solar Active Region Outflows
Authors: Brooks, David H.; Warren, Harry P.
Bibcode: 2012ApJ...760L...5B
Altcode: 2012arXiv1210.1274B
High-resolution spectra from the Hinode EUV Imaging Spectrometer have
revealed that coronal spectral line profiles are sometimes asymmetric,
with a faint enhancement in the blue wing on the order of 100 km
s-1. These asymmetries could be important since they may
be subtle yet diagnostically useful signatures of coronal heating or
solar wind acceleration processes. It has also been suggested that
they are signatures of chromospheric jets supplying mass and energy
to the corona. Until now, however, there have been no studies of the
physical properties of the plasma producing the asymmetries. Here we
identify regions of asymmetric profiles in the outflows of AR 10978
using an asymmetric Gaussian function and extract the intensities
of the faint component using multiple Gaussian fits. We then derive
the temperature structure and chemical composition of the plasma
producing the asymmetries. We find that the asymmetries are dependent
on temperature, and are clearer and stronger in coronal lines. The
temperature distribution peaks around 1.4-1.8 MK with an emission
measure at least an order of magnitude larger than that at 0.6
MK. The first ionization potential bias is found to be 3-5, implying
that the high-speed component of the outflows may also contribute to
the slow-speed wind. Observations and models indicate that it takes
time for plasma to evolve to a coronal composition, suggesting that
the material is trapped on closed loops before escaping, perhaps by
interchange reconnection. The results, therefore, identify the plasma
producing the asymmetries as having a coronal origin.
Title: LEMUR: Large European module for solar Ultraviolet
Research. European contribution to JAXA's Solar-C mission
Authors: Teriaca, Luca; Andretta, Vincenzo; Auchère, Frédéric;
Brown, Charles M.; Buchlin, Eric; Cauzzi, Gianna; Culhane, J. Len;
Curdt, Werner; Davila, Joseph M.; Del Zanna, Giulio; Doschek, George
A.; Fineschi, Silvano; Fludra, Andrzej; Gallagher, Peter T.; Green,
Lucie; Harra, Louise K.; Imada, Shinsuke; Innes, Davina; Kliem,
Bernhard; Korendyke, Clarence; Mariska, John T.; Martínez-Pillet,
Valentin; Parenti, Susanna; Patsourakos, Spiros; Peter, Hardi; Poletto,
Luca; Rutten, Robert J.; Schühle, Udo; Siemer, Martin; Shimizu,
Toshifumi; Socas-Navarro, Hector; Solanki, Sami K.; Spadaro, Daniele;
Trujillo-Bueno, Javier; Tsuneta, Saku; Dominguez, Santiago Vargas;
Vial, Jean-Claude; Walsh, Robert; Warren, Harry P.; Wiegelmann,
Thomas; Winter, Berend; Young, Peter
Bibcode: 2012ExA....34..273T
Altcode: 2011ExA...tmp..135T; 2011arXiv1109.4301T
The solar outer atmosphere is an extremely dynamic environment
characterized by the continuous interplay between the plasma and the
magnetic field that generates and permeates it. Such interactions play a
fundamental role in hugely diverse astrophysical systems, but occur at
scales that cannot be studied outside the solar system. Understanding
this complex system requires concerted, simultaneous solar observations
from the visible to the vacuum ultraviolet (VUV) and soft X-rays, at
high spatial resolution (between 0.1'' and 0.3''), at high temporal
resolution (on the order of 10 s, i.e., the time scale of chromospheric
dynamics), with a wide temperature coverage (0.01 MK to 20 MK,
from the chromosphere to the flaring corona), and the capability of
measuring magnetic fields through spectropolarimetry at visible and
near-infrared wavelengths. Simultaneous spectroscopic measurements
sampling the entire temperature range are particularly important. These
requirements are fulfilled by the Japanese Solar-C mission (Plan B),
composed of a spacecraft in a geosynchronous orbit with a payload
providing a significant improvement of imaging and spectropolarimetric
capabilities in the UV, visible, and near-infrared with respect to
what is available today and foreseen in the near future. The Large
European Module for solar Ultraviolet Research (LEMUR), described
in this paper, is a large VUV telescope feeding a scientific payload
of high-resolution imaging spectrographs and cameras. LEMUR consists
of two major components: a VUV solar telescope with a 30 cm diameter
mirror and a focal length of 3.6 m, and a focal-plane package composed
of VUV spectrometers covering six carefully chosen wavelength ranges
between 170 Å and 1270 Å. The LEMUR slit covers 280'' on the Sun with
0.14'' per pixel sampling. In addition, LEMUR is capable of measuring
mass flows velocities (line shifts) down to 2 km s - 1 or
better. LEMUR has been proposed to ESA as the European contribution
to the Solar C mission.
Title: Constraints on the Heating Time Scale in Active Regions
Authors: Brooks, D. H.; Warren, H. P.
Bibcode: 2012ASPC..454..189B
Altcode:
Understanding the heating time scale is important for constraining
models of active region emission. Hinode observations of moss at the
bases of high temperature active region core loops are allowing us to
study this problem in unprecedented detail. Here we discuss some of
our recent results studying the variability of moss properties such as
intensity, magnetic flux, Doppler and non-thermal velocity. We find that
most of these quantities are relatively constant. One interpretation is
that the heating is therefore effectively steady , i.e., heating events
occur with a rapid repetition rate. Alternatively, the heating could be
low frequency, but only if it occurs on sub-resolution spatial scales.
Title: Spectroscopic Observations of Fe XVIII in Solar Active Regions
Authors: Teriaca, Luca; Warren, Harry P.; Curdt, Werner
Bibcode: 2012ApJ...754L..40T
Altcode: 2012arXiv1206.4228T
The large uncertainties associated with measuring the amount of
high temperature emission in solar active regions (ARs) represents
a significant impediment to making progress on the coronal heating
problem. Most current observations at temperatures of 3 MK and above
are taken with broadband soft X-ray instruments. Such measurements
have proven difficult to interpret unambiguously. Here, we present the
first spectroscopic observations of the Fe XVIII 974.86 Å emission
line in an on-disk AR taken with the SUMER instrument on SOHO. Fe
XVIII has a peak formation temperature of 7.1 MK and provides important
constraints on the amount of impulsive heating in the corona. Detailed
evaluation of the spectra and comparison of the SUMER data with soft
X-ray images from the X-Ray Telescope on Hinode confirm that this line
is unblended. We also compare the spectroscopic data with observations
from the Atmospheric Imaging Assembly (AIA) 94 Å channel on the Solar
Dynamics Observatory. The AIA 94 Å channel also contains Fe XVIII, but
is blended with emission formed at lower temperatures. We find that it
is possible to remove the contaminating blends and form relatively pure
Fe XVIII images that are consistent with the spectroscopic observations
from SUMER. The observed spectra also contain the Ca XIV 943.63 Å
line that, although a factor 2-6 weaker than the Fe XVIII 974.86 Å
line, allows us to probe the plasma around 3.5 MK. The observed ratio
between the two lines indicates (isothermal approximation) that most
of the plasma in the brighter Fe XVIII AR loops is at temperatures
between 3.5 and 4 MK.
Title: Solar Coronal Loops Resolved by Hinode and the Solar Dynamics
Observatory
Authors: Brooks, David H.; Warren, Harry P.; Ugarte-Urra, Ignacio
Bibcode: 2012ApJ...755L..33B
Altcode:
Despite decades of studying the Sun, the coronal heating problem remains
unsolved. One fundamental issue is that we do not know the spatial scale
of the coronal heating mechanism. At a spatial resolution of 1000 km or
more, it is likely that most observations represent superpositions of
multiple unresolved structures. In this Letter, we use a combination
of spectroscopic data from the Hinode EUV Imaging Spectrometer and
high-resolution images from the Atmospheric Imaging Assembly on the
Solar Dynamics Observatory to determine the spatial scales of coronal
loops. We use density measurements to construct multi-thread models of
the observed loops and confirm these models using the higher spatial
resolution imaging data. The results allow us to set constraints on the
number of threads needed to reproduce a particular loop structure. We
demonstrate that in several cases million degree loops are revealed to
be single monolithic structures that are fully spatially resolved by
current instruments. The majority of loops, however, must be composed
of a number of finer, unresolved threads, but the models suggest that
even for these loops the number of threads could be small, implying
that they are also close to being resolved. These results challenge
heating models of loops based on the reconnection of braided magnetic
fields in the corona.
Title: Solar EUV and XUV energy input to thermosphere on solar
rotation time scales derived from photoelectron observations.
Authors: Peterson, W. K. Bill; Solomon, Stanley; Warren, Harry;
Fontenla, Juan; Woods, Thomas; Richards, Phil; Chamberlin, Phillip;
Tobiska, W. Kent
Bibcode: 2012cosp...39.1489P
Altcode: 2012cosp.meet.1489P
Solar radiation below ~100 nm produces photoelectrons, a substantial
portion of the F region ionization, most of the E region ionization,
and drives chemical reactions in the thermosphere. Unquantified
uncertainties in thermospheric models exist because of uncertainties
in solar irradiance models used to fill spectral and temporal gaps in
solar irradiance observations. We investigate uncertainties in solar
energy input to the thermosphere on solar rotation time scales using
photoelectron observations from the FAST satellite. We compare observed
and modeled photoelectron energy spectra using two photoelectron
production codes driven by five different solar irradiance models. We
observe about 1.7 per cent of the ionizing solar irradiance power in the
escaping photoelectron flux. Most of the code/model pairs used reproduce
the average escaping photoelectron flux over a 109-day interval in
late 2006. The code/model pairs we used do not completely reproduce
the observed spectral and solar cycle variations in photoelectron power
density. For the interval examined, 30 per cent of the variability in
photoelectron power density with equivalent wavelengths between 18
and 45 nm was not captured in the code/model pairs. For equivalent
wavelengths below ~ 16 nm, most of the variability was missed. This
result implies that thermospheric model runs based on the solar
irradiance models we tested systematically underestimate the energy
input from ionizing radiation on solar rotation time scales.
Title: A Comprehensive View of the Temperature Distribution in Solar
Flares from EVE and RHESSI
Authors: Caspi, Amir; McTiernan, J. M.; Warren, H. P.
Bibcode: 2012AAS...22020411C
Altcode:
Solar flares accelerate electrons up to hundreds of MeV and heat
plasma up to tens of MK, but the physical processes behind these
phenomena remain poorly understood. While the ubiquitous 10-25 MK
plasma is commonly accepted to result from chromospheric evaporation,
evidence suggests that in intense (GOES M- and X-class) flares, the
hottest, 20-50 MK plasma is directly heated in the corona, although
the heating mechanism and its connection to the flare-accelerated
non-thermal electrons is not yet understood. While observations of
hard X-ray bremmstrahlung directly probe the non-thermal electron
population, the spectra below 20-30 keV are typically dominated by
strong thermal emission. The low-energy extent of the non-thermal
spectrum can thus be only loosely quantified, which has significant
implications for calculating flare energy budgets and for constraining
possible acceleration mechanisms. Hence, a precise characterization
of the thermal electron population is imperative. New extreme
ultraviolet observations from the EUV Variability Experiment (EVE)
on-board the Solar Dynamics Observatory (SDO), combined with X-ray data
from the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI), offer
the most comprehensive view into the flare temperature distribution to
date. EVE observes a wealth of EUV emission lines with peak formation
temperatures of 2-20 MK, while RHESSI observes the X-ray bremsstrahlung
of hot, 10-50 MK plasmas; combined, the two instruments have excellent
temperature sampling and coverage over the full range of flare plasma
temperatures. We have calculated differential emission measures
(DEMs) using EVE and RHESSI independently, for separately observed
events. We present a novel method of combining simultaneous EVE and
RHESSI observations to determine the flare DEM, and its evolution,
over the full 1-100 MK range during intense M/X flares. We present
preliminary results from the 2011-Feb-15 X2.2 flare, and compare with
the RHESSI non-thermal emission to discuss the implications for flare
plasma heating.
Title: Evidence of a Connection Between Active Region Outflows and
the Solar Wind
Authors: Brooks, D. H.; Warren, H. P.
Bibcode: 2012ASPC..455..327B
Altcode:
We present new evidence of a connection between active region (AR)
outflows and the slow speed solar wind from chemical composition
measurements made by the EUV Imaging Spectrometer (EIS) on Hinode. By
combining the differential emission measure (DEM) distribution derived
using low First Ionization Potential (FIP) elements (Fe and Si) with
the modeling of the high FIP element S, we are able to measure the
degree of FIP bias in an observed region. We have applied this analysis
to the outflow areas of AR 10978 observed in December 2007. Since the
results of our study have already been presented by Brooks & Warren
(2011), we use this short conference proceeding to show one illustrative
example and the methodology in detail. We focus on the western outflow
from AR 10978 observed on December 11 and show that the FIP bias of
3.4 matches the value of 3.5 measured in situ three days later at
Earth by the Solar Wind Ion Composition Spectrometer (SWICS) on the
ACE spacecraft. We consider this to be compelling evidence that the
plasma in the outflow region really travels to the slow wind at Earth.
Title: Evidence for Two Separate But Interlaced Components of the
Chromospheric Magnetic Field
Authors: Muglach, Karin; Reardon, K.; Wang, Y.; Warren, H.
Bibcode: 2012AAS...22012403M
Altcode:
Chromospheric fibrils are generally thought to trace out horizontal
magnetic fields that fan out from flux concentrations in the
photosphere. A high-resolution (0.2") image taken in the core of the
Ca II 854.2 nm line shows the dark fibrils within an active region
remnant as fine, looplike features that are aligned parallel to each
other and have lengths on the order of a supergranular diameter ( 30
Mm). Comparison with a line-of-sight magnetogram confirms that the
fibrils are centered above intranetwork areas, with one end rooted
just inside the neighboring plage or strong unipolar network
but the other endpoint less clearly defined. Focusing on a particular
arcade-like structure lying entirely on one side of a filament channel
(large-scale polarity inversion), we find that the total amount of
positive-polarity flux underlying this ``fibril arcade'' is 50 times
greater than the total amount of negative-polarity flux. Thus, if the
fibrils represent closed loops, they must consist of very weak fields
(in terms of flux density), which are interpenetrated by a more vertical
field that contains most of the flux. This surprising result suggests
that the fibrils in unipolar regions connect the network to the nearby
intranetwork flux, while the bulk of the network flux is diverted
upward into the corona and connects to remote regions of the opposite
polarity. We conclude that the chromospheric field near the edge of
the network has an interlaced structure resembling that in sunspot
penumbrae, with the fibrils representing the low-lying horizontal flux
that remains trapped within the highly nonpotential chromospheric layer.
Title: Solar Coronal Loops Resolved by Hinode and SDO
Authors: Brooks, David H.; Warren, Harry P.; Ugarte-Urra, Ignacio
Bibcode: 2012arXiv1205.5814B
Altcode:
Despite decades of studying the Sun, the coronal heating problem remains
unsolved. One fundamental issue is that we do not know the spatial scale
of the coronal heating mechanism. At a spatial resolution of 1000 km or
more it is likely that most observations represent superpositions of
multiple unresolved structures. In this letter, we use a combination
of spectroscopic data from the Hinode EUV Imaging Spectrometer (EIS)
and high resolution images from the Atmospheric Imaging Assembly
(AIA) on the Solar Dynamics Observatory to determine the spatial
scales of coronal loops. We use density measurements to construct
multi-thread models of the observed loops and confirm these models
using the higher spatial resolution imaging data. The results allow
us to set constraints on the number of threads needed to reproduce
a particular loop structure. We demonstrate that in several cases
million degree loops are revealed to be single monolithic structures
that are fully spatially resolved by current instruments. The majority
of loops, however, must be composed of a number of finer, unresolved
threads; but the models suggest that even for these loops the number
of threads could be small, implying that they are also close to being
resolved. These results challenge heating models of loops based on
the reconnection of braided magnetic fields in the corona.
Title: Solar EUV and XUV energy input to thermosphere on solar
rotation time scales derived from photoelectron observations
Authors: Peterson, W. K.; Woods, T. N.; Fontenla, J. M.; Richards,
P. G.; Chamberlin, P. C.; Solomon, S. C.; Tobiska, W. K.; Warren, H. P.
Bibcode: 2012JGRA..117.5320P
Altcode: 2012JGRA..11705320P
Solar radiation below ∼100 nm produces photoelectrons, a substantial
portion of the F region ionization, most of the E region ionization,
and drives chemical reactions in the thermosphere. Unquantified
uncertainties in thermospheric models exist because of uncertainties
in solar irradiance models used to fill spectral and temporal gaps
in solar irradiance observations. We investigate uncertainties
in solar energy input to the thermosphere on solar rotation time
scales using photoelectron observations from the FAST satellite. We
compare observed and modeled photoelectron energy spectra using two
photoelectron production codes driven by five different solar irradiance
models. We observe about 1.7% of the ionizing solar irradiance power
in the escaping photoelectron flux. Most of the code/model pairs
used reproduce the average escaping photoelectron flux over a 109-day
interval in late 2006. The code/model pairs we used do not completely
reproduce the observed spectral and solar rotation variations in
photoelectron power density. For the interval examined, 30% of the
variability in photoelectron power density with equivalent wavelengths
between 18 and 45 nm was not captured in the code/model pairs. For
equivalent wavelengths below ∼16 nm, most of the variability was
missed. This result implies that thermospheric model runs based on
the solar irradiance models we tested systematically underestimate
the energy input from ionizing radiation on solar rotation time scales.
Title: Hinode/EIS Flare Spectra During RHESSI Hard X-ray Bursts
Authors: Young, Peter R.; Warren, H.; Doschek, G.
Bibcode: 2012AAS...22020442Y
Altcode:
The standard flare model requires a beam of non-thermal electrons
- generated at the coronal flare site - to hit the chromosphere
and trigger heating and chromospheric evaporation. Ultraviolet
spectrometers allow the heated, evaporating plasma to be observed
and its properties measured. Observations of a M3 flare observed in
2011 September with Hinode/EIS, RHESSI and SDO/AIA will be presented,
revealing the physical conditions in the flare ribbons at the time of
the hard X-ray bursts. At the hottest temperatures (20 MK) upflowing
plasma with speeds up to 500 km/s are found co-spatial with stationary
plasma, while at cooler temperatures (0.5-2 MK) small downflows and
large non-thermal broadening are found. These observations will be
compared with predictions from multi-strand hydrodynamic simulations
that take the RHESSI-derived electron beam spectrum as input.
Title: Can We Resolve Coronal Loops with Hinode and SDO?
Authors: Ugarte-Urra, Ignacio; Brooks, D. H.; Warren, H. P.
Bibcode: 2012AAS...22030903U
Altcode:
A combination of spectral data from the Hinode EUV Imaging Spectrometer
(EIS) and high resolution imaging from the Solar Dynamics Observatory
(SDO) Atmospheric Imaging Assembly (AIA) are used to investigate
the fundamental spatial scales of coronal loops. We construct
multi-isothermal thread models and find that we are able to successfully
reproduce the cross-loop intensity profiles observed by EIS and AIA. The
models allow us to set constraints on the number of threads needed to
reproduce a particular loop structure, and the results suggest that
although most coronal loops remain unresolved, current instruments
are close to resolving them. We discuss implications for future high
resolution EUV spectral imaging instruments.
Title: Coronal Cells
Authors: Sheeley, N. R., Jr.; Warren, H. P.
Bibcode: 2012ApJ...749...40S
Altcode:
We have recently noticed cellular features in Fe XII 193 Å images of
the 1.2 MK corona. They occur in regions bounded by a coronal hole
and a filament channel, and are centered on flux elements of the
photospheric magnetic network. Like their neighboring coronal holes,
these regions have minority-polarity flux that is ~0.1-0.3 times
their flux of majority polarity. Consequently, the minority-polarity
flux is "grabbed" by the majority-polarity flux to form low-lying
loops, and the remainder of the network flux escapes to connect with
its opposite-polarity counterpart in distant active regions of the
Sun. As these regions are carried toward the limb by solar rotation,
the cells disappear and are replaced by linear plumes projecting toward
the limb. In simultaneous views from the Solar Terrestrial Relations
Observatory and Solar Dynamics Observatory spacecraft, these plumes
project in opposite directions, extending away from the coronal hole in
one view and toward the hole in the other view, suggesting that they are
sky-plane projections of the same radial structures. We conclude that
these regions are composed of closely spaced radial plumes, extending
upward like candles on a birthday cake and visible as cells when seen
from above. We suppose that a coronal hole has this same discrete,
cellular magnetic structure, but that it is not seen until the
encroachment of opposite-polarity flux closes part or all of the hole.
Title: Defining the "Blind Spot" of Hinode EIS and XRT Temperature
Measurements
Authors: Winebarger, Amy R.; Warren, Harry P.; Schmelz, Joan T.;
Cirtain, Jonathan; Mulu-Moore, Fana; Golub, Leon; Kobayashi, Ken
Bibcode: 2012ApJ...746L..17W
Altcode:
Observing high-temperature, low emission measure plasma is key to
unlocking the coronal heating problem. With current instrumentation,
a combination of EUV spectral data from Hinode Extreme-ultraviolet
Imaging Spectrometer (EIS; sensitive to temperatures up to 4 MK)
and broadband filter data from Hinode X-ray Telescope (XRT; sensitive
to higher temperatures) is typically used to diagnose the temperature
structure of the observed plasma. In this Letter, we demonstrate that a
"blind spot" exists in temperature-emission measure space for combined
Hinode EIS and XRT observations. For a typical active region core with
significant emission at 3-4 MK, Hinode EIS and XRT are insensitive
to plasma with temperatures greater than ~6 MK and emission measures
less than ~1027 cm-5. We then demonstrate that
the temperature and emission measure limits of this blind spot depend
upon the temperature distribution of the plasma along the line of sight
by considering a hypothetical emission measure distribution sharply
peaked at 1 MK. For this emission measure distribution, we find that
EIS and XRT are insensitive to plasma with emission measures less
than ~1026 cm-5. We suggest that a spatially and
spectrally resolved 6-24 Å spectrum would improve the sensitivity to
these high-temperature, low emission measure plasma.
Title: Extreme Ultraviolet Variability Experiment (EVE) on the
Solar Dynamics Observatory (SDO): Overview of Science Objectives,
Instrument Design, Data Products, and Model Developments
Authors: Woods, T. N.; Eparvier, F. G.; Hock, R.; Jones, A. R.;
Woodraska, D.; Judge, D.; Didkovsky, L.; Lean, J.; Mariska, J.;
Warren, H.; McMullin, D.; Chamberlin, P.; Berthiaume, G.; Bailey,
S.; Fuller-Rowell, T.; Sojka, J.; Tobiska, W. K.; Viereck, R.
Bibcode: 2012SoPh..275..115W
Altcode:
The highly variable solar extreme ultraviolet (EUV) radiation is
the major energy input to the Earth's upper atmosphere, strongly
impacting the geospace environment, affecting satellite operations,
communications, and navigation. The Extreme ultraviolet Variability
Experiment (EVE) onboard the NASA Solar Dynamics Observatory (SDO) will
measure the solar EUV irradiance from 0.1 to 105 nm with unprecedented
spectral resolution (0.1 nm), temporal cadence (ten seconds), and
accuracy (20%). EVE includes several irradiance instruments: The
Multiple EUV Grating Spectrographs (MEGS)-A is a grazing-incidence
spectrograph that measures the solar EUV irradiance in the 5 to 37 nm
range with 0.1-nm resolution, and the MEGS-B is a normal-incidence,
dual-pass spectrograph that measures the solar EUV irradiance in the
35 to 105 nm range with 0.1-nm resolution. To provide MEGS in-flight
calibration, the EUV SpectroPhotometer (ESP) measures the solar EUV
irradiance in broadbands between 0.1 and 39 nm, and a MEGS-Photometer
measures the Sun's bright hydrogen emission at 121.6 nm. The EVE data
products include a near real-time space-weather product (Level 0C),
which provides the solar EUV irradiance in specific bands and also
spectra in 0.1-nm intervals with a cadence of one minute and with a
time delay of less than 15 minutes. The EVE higher-level products
are Level 2 with the solar EUV irradiance at higher time cadence
(0.25 seconds for photometers and ten seconds for spectrographs) and
Level 3 with averages of the solar irradiance over a day and over each
one-hour period. The EVE team also plans to advance existing models of
solar EUV irradiance and to operationally use the EVE measurements in
models of Earth's ionosphere and thermosphere. Improved understanding
of the evolution of solar flares and extending the various models to
incorporate solar flare events are high priorities for the EVE team.
Title: Plasma Diagnostics of an EIT Wave Observed by Hinode/EIS
and SDO/AIA
Authors: Veronig, A. M.; Gömöry, P.; Kienreich, I. W.; Muhr, N.;
Vršnak, B.; Temmer, M.; Warren, H. P.
Bibcode: 2011ApJ...743L..10V
Altcode: 2011arXiv1111.3505V
We present plasma diagnostics of an Extreme-Ultraviolet
Imaging Telescope (EIT) wave observed with high cadence in
Hinode/Extreme-Ultraviolet Imaging Spectrometer (EIS) sit-and-stare
spectroscopy and Solar Dynamics Observatory/Atmospheric Imaging
Assembly imagery obtained during the HOP-180 observing campaign on 2011
February 16. At the propagating EIT wave front, we observe downward
plasma flows in the EIS Fe XII, Fe XIII, and Fe XVI spectral lines
(log T ≈ 6.1-6.4) with line-of-sight (LOS) velocities up to 20
km s-1. These redshifts are followed by blueshifts with
upward velocities up to -5 km s-1 indicating relaxation
of the plasma behind the wave front. During the wave evolution, the
downward velocity pulse steepens from a few km s-1 up to 20
km s-1 and subsequently decays, correlated with the relative
changes of the line intensities. The expected increase of the plasma
densities at the EIT wave front estimated from the observed intensity
increase lies within the noise level of our density diagnostics from
EIS Fe XIII 202/203 Å line ratios. No significant LOS plasma motions
are observed in the He II line, suggesting that the wave pulse was not
strong enough to perturb the underlying chromosphere. This is consistent
with the finding that no Hα Moreton wave was associated with the
event. The EIT wave propagating along the EIS slit reveals a strong
deceleration of a ≈ -540 m s-2 and a start velocity of v
0 ≈ 590 km s-1. These findings are consistent
with the passage of a coronal fast-mode MHD wave, pushing the plasma
downward and compressing it at the coronal base.
Title: Observations of Reconnecting Flare Loops with the Atmospheric
Imaging Assembly
Authors: Warren, Harry P.; O'Brien, Casey M.; Sheeley, Neil R., Jr.
Bibcode: 2011ApJ...742...92W
Altcode: 2011arXiv1109.2474W
Perhaps the most compelling evidence for the role of magnetic
reconnection in solar flares comes from the supra-arcade downflows that
have been observed above many post-flare loop arcades. These downflows
are thought to be related to highly non-potential field lines that have
reconnected and are propagating away from the current sheet. We present
new observations of supra-arcade downflows taken with the Atmospheric
Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO). The
morphology and dynamics of the downflows observed with AIA provide new
evidence for the role of magnetic reconnection in solar flares. With
these new observations we are able to measure downflows originating
at larger heights than in previous studies. We find, however,
that the initial velocities measured here (~144 km s-1)
are well below the Alfvén speed expected in the lower corona, and
consistent with previous results. We also find no evidence that the
downflows brighten with time, as would be expected from chromospheric
evaporation. These observations suggest that simple two-dimensional
models cannot explain the detailed observations of solar flares.
Title: Evidence for Two Separate but Interlaced Components of the
Chromospheric Magnetic Field
Authors: Reardon, K. P.; Wang, Y. -M.; Muglach, K.; Warren, H. P.
Bibcode: 2011ApJ...742..119R
Altcode:
Chromospheric fibrils are generally thought to trace out low-lying,
mainly horizontal magnetic fields that fan out from flux concentrations
in the photosphere. A high-resolution (~0farcs1 pixel-1)
image, taken in the core of the Ca II 854.2 nm line and covering
an unusually large area, shows the dark fibrils within an active
region remnant as fine, looplike features that are aligned parallel
to each other and have lengths comparable to a supergranular
diameter. Comparison with simultaneous line-of-sight magnetograms
confirms that the fibrils are centered above intranetwork areas
(supergranular cell interiors), with one end rooted just inside the
neighboring plage or strong unipolar network but the other endpoint
less clearly defined. Focusing on a particular arcade-like structure
lying entirely on one side of a filament channel (large-scale polarity
inversion), we find that the total amount of positive-polarity flux
underlying this "fibril arcade" is ~50 times greater than the total
amount of negative-polarity flux. Thus, if the fibrils represent closed
loops, they must consist of very weak fields (in terms of total magnetic
flux), which are interpenetrated by a more vertical field that contains
most of the flux. This surprising result suggests that the fibrils in
unipolar regions connect the network to the nearby intranetwork flux,
while the bulk of the network flux links to remote regions of the
opposite polarity, forming a second, higher canopy above the fibril
canopy. The chromospheric field near the edge of the network thus has
an interlaced structure resembling that in sunspot penumbrae.
Title: Cross-Calibration and Thermal Analysis with SDO/AIA
Authors: Boerner, P.; Warren, H. P.; Testa, P.; Weber, M.; Schrijver,
C. J.
Bibcode: 2011AGUFMSH13B1955B
Altcode:
The measured intensity in each pixel of the images from SDO/AIA and
similar narrowband EUV imagers can be used to perform quantitative
analysis of the temperature and density of the plasma along the
line of sight. This type of analysis depends very sensitively on the
accuracy of the instrument calibration and the atomic physics models
used to estimate the plasma emissivity. Unfortunately, it is difficult
to assess the accuracy of these parameters, other than by verifying
the consistency of datasets from different instruments and analysis
techniques. Here we use differential emission measure models of the
plasma temperature structure constrained by spectroscopic observations
from SDO/EVE and Hinode/EIS to assess the AIA temperature response
functions. The response functions generated using the CHIANTI database
underestimate the emission from the non-flaring corona in the 94 and 131
Å channels. We find empirical corrections to the temperature response
functions for these channels that are internally consistent and provide
good agreement with DEMs obtained from other instruments. We present an
assessment of the reliability of thermal analysis using AIA data based
on the current state of the instrument calibration and spectral models.
Title: Can a Long Nanoflare Storm Explain the Observed Emission
Measure Distributions in Active Region Cores?
Authors: Mulu-Moore, Fana M.; Winebarger, Amy R.; Warren, Harry P.
Bibcode: 2011ApJ...742L...6M
Altcode: 2012arXiv1205.5486M
All theories that attempt to explain the heating of the high-temperature
plasma observed in the solar corona are based on short bursts of
energy. The intensities and velocities measured in the cores of
quiescent active regions, however, can be steady over many hours of
observation. One heating scenario that has been proposed to reconcile
such observations with impulsive heating models is the "long nanoflare
storm," where short-duration heating events occur infrequently on many
sub-resolution strands; the emission of the strands is then averaged
together to explain the observed steady structures. In this Letter,
we examine the emission measure distribution predicted for such a long
nanoflare storm by modeling an arcade of strands in an active region
core. Comparisons of the computed emission measure distributions with
recent observations indicate that the long nanoflare storm scenario
implies greater than five times more 1 MK emission than is actually
observed for all plausible combinations of loop lengths, heating rates,
and abundances. We conjecture that if the plasma had "super coronal"
abundances, the model may be able to match the observations at low
temperatures.
Title: Using a Differential Emission Measure and Density Measurements
in an Active Region Core to Test a Steady Heating Model
Authors: Winebarger, Amy R.; Schmelz, Joan T.; Warren, Harry P.;
Saar, Steve H.; Kashyap, Vinay L.
Bibcode: 2011ApJ...740....2W
Altcode: 2011arXiv1106.5057W
The frequency of heating events in the corona is an important
constraint on the coronal heating mechanisms. Observations indicate
that the intensities and velocities measured in active region cores are
effectively steady, suggesting that heating events occur rapidly enough
to keep high-temperature active region loops close to equilibrium. In
this paper, we couple observations of active region (AR) 10955 made
with the X-Ray Telescope and the EUV Imaging Spectrometer on board
Hinode to test a simple steady heating model. First we calculate the
differential emission measure (DEM) of the apex region of the loops in
the active region core. We find the DEM to be broad and peaked around
3 MK. We then determine the densities in the corresponding footpoint
regions. Using potential field extrapolations to approximate the loop
lengths and the density-sensitive line ratios to infer the magnitude
of the heating, we build a steady heating model for the active region
core and find that we can match the general properties of the observed
DEM for the temperature range of 6.3 < log T < 6.7. This model,
for the first time, accounts for the base pressure, loop length,
and distribution of apex temperatures of the core loops. We find that
the density-sensitive spectral line intensities and the bulk of the
hot emission in the active region core are consistent with steady
heating. We also find, however, that the steady heating model cannot
address the emission observed at lower temperatures. This emission may
be due to foreground or background structures, or may indicate that the
heating in the core is more complicated. Different heating scenarios
must be tested to determine if they have the same level of agreement.
Title: New Solar Extreme-ultraviolet Irradiance Observations during
Flares
Authors: Woods, Thomas N.; Hock, Rachel; Eparvier, Frank; Jones,
Andrew R.; Chamberlin, Phillip C.; Klimchuk, James A.; Didkovsky,
Leonid; Judge, Darrell; Mariska, John; Warren, Harry; Schrijver,
Carolus J.; Webb, David F.; Bailey, Scott; Tobiska, W. Kent
Bibcode: 2011ApJ...739...59W
Altcode:
New solar extreme-ultraviolet (EUV) irradiance observations from the
NASA Solar Dynamics Observatory (SDO) EUV Variability Experiment provide
full coverage in the EUV range from 0.1 to 106 nm and continuously at
a cadence of 10 s for spectra at 0.1 nm resolution and even faster,
0.25 s, for six EUV bands. These observations can be decomposed into
four distinct characteristics during flares. First, the emissions
that dominate during the flare's impulsive phase are the transition
region emissions, such as the He II 30.4 nm. Second, the hot coronal
emissions above 5 MK dominate during the gradual phase and are highly
correlated with the GOES X-ray. A third flare characteristic in the
EUV is coronal dimming, seen best in the cool corona, such as the
Fe IX 17.1 nm. As the post-flare loops reconnect and cool, many of
the EUV coronal emissions peak a few minutes after the GOES X-ray
peak. One interesting variation of the post-eruptive loop reconnection
is that warm coronal emissions (e.g., Fe XVI 33.5 nm) sometimes exhibit
a second large peak separated from the primary flare event by many
minutes to hours, with EUV emission originating not from the original
flare site and its immediate vicinity, but rather from a volume of
higher loops. We refer to this second peak as the EUV late phase. The
characterization of many flares during the SDO mission is provided,
including quantification of the spectral irradiance from the EUV late
phase that cannot be inferred from GOES X-ray diagnostics.
Title: Constraints on the Heating of High-temperature Active Region
Loops: Observations from Hinode and the Solar Dynamics Observatory
Authors: Warren, Harry P.; Brooks, David H.; Winebarger, Amy R.
Bibcode: 2011ApJ...734...90W
Altcode: 2010arXiv1009.5976W
We present observations of high-temperature emission in the core
of a solar active region using instruments on Hinode and the Solar
Dynamics Observatory (SDO). These multi-instrument observations allow
us to determine the distribution of plasma temperatures and follow the
evolution of emission at different temperatures. We find that at the
apex of the high-temperature loops the emission measure distribution
is strongly peaked near 4 MK and falls off sharply at both higher and
lower temperatures. Perhaps most significantly, the emission measure at
0.5 MK is reduced by more than two orders of magnitude from the peak at
4 MK. We also find that the temporal evolution in broadband soft X-ray
images is relatively constant over about 6 hr of observing. Observations
in the cooler SDO/Atmospheric Imaging Assembly (AIA) bandpasses
generally do not show cooling loops in the core of the active region,
consistent with the steady emission observed at high temperatures. These
observations suggest that the high-temperature loops observed in the
core of an active region are close to equilibrium. We find that it is
possible to reproduce the relative intensities of high-temperature
emission lines with a simple, high-frequency heating scenario where
heating events occur on timescales much less than a characteristic
cooling time. In contrast, low-frequency heating scenarios, which are
commonly invoked to describe nanoflare models of coronal heating, do
not reproduce the relative intensities of high-temperature emission
lines and predict low-temperature emission that is approximately an
order of magnitude too large. We also present an initial look at images
from the SDO/AIA 94 Å channel, which is sensitive to Fe XVIII.
Title: Flares Observed By Hinode During 14-18 February 2011
Authors: Young, Peter R.; Doschek, G. A.; Warren, H. P.
Bibcode: 2011SPD....42.2213Y
Altcode: 2011BAAS..43S.2213Y
Active region AR 11158 produced an X1 flare and several M flares during
2011 February 14-18, and yielded the best set of flare observations
captured by the Hinode satellite in four years. Finding the mechanisms
responsible for flares was one of the major science goals of the Hinode
mission, and data from AR 11158 will be presented to demonstrate how
this goal is being achieved with Hinode data. A particular focus will
be on relating plasma flows and temperature and density changes measured
with the EIS instrument to the magnetic field evolution observed by SOT,
and the coronal evolution observed with SDO/AIA.
Title: Determining the Structure of Solar Coronal Loops Using Their
Evolution
Authors: Mulu-Moore, Fana M.; Winebarger, Amy R.; Warren, Harry P.;
Aschwanden, Markus J.
Bibcode: 2011ApJ...733...59M
Altcode:
Despite significant progress in understanding the dynamics of the
corona, there remain several unanswered questions about the basic
physical properties of coronal loops. Recent observations from
different instruments have yielded contradictory results about some
characteristics of coronal loops, specifically as to whether the
observed loops are spatially resolved. In this paper, we examine the
evolution of coronal loops through two extreme-ultraviolet filters
and determine if they evolve as a single cooling strand. We measure
the temporal evolution of eight active region loops previously
studied and found to be isothermal and resolved by Aschwanden &
Nightingale. All eight loops appear in "hotter" TRACE filter images
(Fe XII 195 Å) before appearing in the "cooler" (Fe IX/Fe X 171 Å)
TRACE filter images. We use the measured delay between the two filters
to calculate a cooling time and then determine if that cooling time is
consistent with the observed lifetime of the loop. We do this twice:
once when the loop appears (rise phase) and once when it disappears
(decay phase). We find that only one loop appears consistent with a
single cooling strand and hence could be considered to be resolved by
TRACE. For the remaining seven loops, their observed lifetimes are
longer than expected for a single cooling strand. We suggest that
these loops could be formed of multiple cooling strands, each at a
different temperature. These findings indicate that the majority of
loops observed by TRACE are unresolved.
Title: EUV Spectral Line Formation and the Temperature Structure of
Active Region Fan Loops: Observations with Hinode/EIS and SDO/AIA
Authors: Brooks, David H.; Warren, Harry P.; Young, Peter R.
Bibcode: 2011ApJ...730...85B
Altcode: 2011arXiv1101.5240B
With the aim of studying active region fan loops using observations
from the Hinode EUV Imaging Spectrometer (EIS) and Solar Dynamics
Observatory Atmospheric Imaging Assembly (AIA), we investigate a number
of inconsistencies in modeling the absolute intensities of Fe VIII
and Si VII lines, and address why spectroheliograms formed from these
lines look very similar despite the fact that ionization equilibrium
calculations suggest that they have significantly different formation
temperatures: log(Te /K) = 5.6 and 5.8, respectively. It is
important to resolve these issues because confidence has been undermined
in their use for differential emission measure (DEM) analysis, and
Fe VIII is the main contributor to the AIA 131 Å channel at low
temperatures. Furthermore, the strong Fe VIII 185.213 Å and Si VII
275.368 Å lines are the best EIS lines to use for velocity studies
in the transition region, and for assigning the correct temperature
to velocity measurements in the fans. We find that the Fe VIII 185.213
Å line is particularly sensitive to the slope of the DEM, leading to
disproportionate changes in its effective formation temperature. If
the DEM has a steep gradient in the log(Te /K) = 5.6-5.8
temperature range, or is strongly peaked, Fe VIII 185.213 Å and Si VII
275.368 Å will be formed at the same temperature. We show that this
effect explains the similarity of these images in the fans. Furthermore,
we show that the most recent ionization balance compilations resolve the
discrepancies in absolute intensities. With these difficulties overcome,
we combine EIS and AIA data to determine the temperature structure of
a number of fan loops and find that they have peak temperatures of
0.8-1.2 MK. The EIS data indicate that the temperature distribution
has a finite (but narrow) width < log (σ_{T_e}/K) = 5.5 which,
in one detailed case, is found to broaden substantially toward the
loop base. AIA and EIS yield similar results on the temperature,
emission measure magnitude, and thermal distribution in the fans,
though sometimes the AIA data suggest a relatively larger thermal
width. The result is that both the Fe VIII 185.213 Å and Si VII
275.368 Å lines are formed at log(Te /K)~ 5.9 in the fans,
and the AIA 131 Å response also shifts to this temperature.
Title: Temporal Variability of Active Region Outflows
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.
Bibcode: 2011ApJ...730...37U
Altcode: 2010arXiv1008.4730U
Recent observations from the Extreme-ultraviolet Imaging Spectrometer
(EIS) on board Hinode have shown that low-density areas on the
periphery of active regions are characterized by strong blueshifts in
the emission of spectral lines formed at 1 MK. These Doppler shifts have
been associated with outward propagating disturbances observed with
extreme-ultraviolet and soft X-ray imagers. Since these instruments
can have broad temperature responses, we investigate these intensity
fluctuations using the monochromatic imaging capabilities of the EIS
wide slit (slot) and confirm their 1 MK nature. We also look into
their spectral temporal variability using narrow slit observations and
present the first Doppler movies of the outflow regions. We find that
the Fe XII 195.119 Å blueshifted spectral profiles at their footpoints
exhibit transient blue wing enhancements on timescales as short as the 5
minute cadence. We have also looked at the fan peripheral loops observed
at 0.6 MK in Si VII 275.368 Å in those regions and find no sign of
the recurrent outward propagating disturbances with velocities of
40-130 km s-1 seen in Fe XII. We do observe downward trends
(15-20 km s-1) consistent with the characteristic redshifts
measured at their footpoints. We, therefore, find no evidence that the
structures at these two temperatures and the intensity fluctuations
they exhibit are related to one another.
Title: The Temperature Dependence of Solar Active Region Outflows
Authors: Warren, Harry P.; Ugarte-Urra, Ignacio; Young, Peter R.;
Stenborg, Guillermo
Bibcode: 2011ApJ...727...58W
Altcode: 2010arXiv1008.2696W
Spectroscopic observations with the EUV Imaging Spectrometer (EIS)
on Hinode have revealed large areas of high-speed outflows at the
periphery of many solar active regions. These outflows are of interest
because they may connect to the heliosphere and contribute to the
solar wind. In this paper, we use slit rasters from EIS in combination
with narrowband slot imaging to study the temperature dependence and
morphology of an outflow region and show that it is more complicated
than previously thought. Outflows are observed primarily in emission
lines from Fe XI to Fe XV. Observations at lower temperatures (Si VII),
in contrast, show bright fan-like structures that are dominated by
inflows. These data also indicate that the morphology of the outflows
and the fans is different, outflows are observed in regions where
there is no emission in Si VII. This suggests that the fans, which
are often associated with outflows in studies involving imaging data,
are not directly related to the active region outflows.
Title: Establishing a Connection Between Active Region Outflows and
the Solar Wind: Abundance Measurements with EIS/Hinode
Authors: Brooks, David H.; Warren, Harry P.
Bibcode: 2011ApJ...727L..13B
Altcode: 2010arXiv1009.4291B
One of the most interesting discoveries from Hinode is the presence
of persistent high-temperature high-speed outflows from the edges
of active regions (ARs). EUV imaging spectrometer (EIS) measurements
indicate that the outflows reach velocities of 50 km s-1
with spectral line asymmetries approaching 200 km s-1. It
has been suggested that these outflows may lie on open field lines
that connect to the heliosphere, and that they could potentially
be a significant source of the slow speed solar wind. A direct link
has been difficult to establish, however. We use EIS measurements of
spectral line intensities that are sensitive to changes in the relative
abundance of Si and S as a result of the first ionization potential
(FIP) effect, to measure the chemical composition in the outflow
regions of AR 10978 over a 5 day period in 2007 December. We find that
Si is always enhanced over S by a factor of 3-4. This is generally
consistent with the enhancement factor of low FIP elements measured
in situ in the slow solar wind by non-spectroscopic methods. Plasma
with a slow wind-like composition was therefore flowing from the edge
of the AR for at least 5 days. Furthermore, on December 10 and 11,
when the outflow from the western side was favorably oriented in the
Earth direction, the Si/S ratio was found to match the value measured
a few days later by the Advanced Composition Explorer/Solar Wind Ion
Composition Spectrometer. These results provide strong observational
evidence for a direct connection between the solar wind, and the
coronal plasma in the outflow regions.
Title: Photoelectrons as a tool to evaluate spectral and temporal
variations of solar EUV and XUV irradiance models over solar rotation
and solar cycle time scales
Authors: Peterson, W. K.; Woods, T. N.; Fontenla, J. M.; Richards,
P. G.; Tobiska, W.; Solomon, S. C.; Warren, H. P.
Bibcode: 2010AGUFMSA33B1766P
Altcode:
Solar radiation below 50 nm produces a substantial portion of the F
region ionization and most of the E region ionization that drives
chemical reactions in the thermosphere. Because of a lack of high
temporal and spectral resolution Solar EUV and XUV observations,
particularly below 27 nm, various solar irradiance models have
been developed. We have developed a technique to use observations of
escaping photoelectron fluxes from the FAST satellite and two different
photoelectron production codes driven by model solar irradiance values
to systematically examine differences between observed and calculated
escaping photoelectron fluxes. We have compared modeled and observed
photoelectron fluxes from the start of TIMED/SEE data availability
(2002) to the end of FAST photoelectron observations (2009). Solar
irradiance inputs included TIMED/SEE data, which is derived from a model
below 27 nm, and the FISM Version 1, the SRPM predictive model based on
solar observation, HEUVAC, S2000, and NRL, solar irradiance models. We
used the GLOW and FLIP photoelectron production codes. We find that
model photoelectron spectra generated using the HEUVAC solar irradiance
model have the best overall agreement with observations. Photoelectron
spectra generated with the the TIMED/SEE based FISM model best agree
with the observations on solar cycle time scales. Below ~27 nm all but
the HEUVAC solar irradiance model produces photoelectron fluxes that are
systematically below observations. We also noted systematic differences
in the photoelectron energy spectra below 25 eV produced by the GLOW
and FLIP photoelectron production codes for all solar irradiance inputs.
Title: Solar flare impulsive phase observations from SDO and other
observatories
Authors: Chamberlin, P. C.; Woods, T. N.; Schrijver, C. J.; Warren,
H. P.; Milligan, R. O.; Christe, S.; Brosius, J. W.
Bibcode: 2010AGUFMSH23A1832C
Altcode:
With the start of normal operations of the Solar Dynamics Observatory
in May 2010, the Extreme ultraviolet Variability Experiment (EVE)
and the Atmospheric Imaging Assembly (AIA) have been returning the
most accurate solar XUV and EUV measurements every 10 and 12 seconds,
respectively, at almost 100% duty cycle. The focus of the presentation
will be the solar flare impulsive phase observations provided by EVE
and AIA and what these observations can tell us about the evolution
of the initial phase of solar flares. Also emphasized throughout
is how simultaneous observations with other instruments, such as
RHESSI, SOHO-CDS, and HINODE-EIS, will help provide a more complete
characterization of the solar flares and the evolution and energetics
during the impulsive phase. These co-temporal observations from the
other solar instruments can provide information such as extending
the high temperature range spectra and images beyond that provided
by the EUV and XUV wavelengths, provide electron density input into
the lower atmosphere at the footpoints, and provide plasma flows of
chromospheric evaporation, among other characteristics.
Title: Physical Properties of Solar Flares: New Results from EVE/SDO
Authors: Warren, H. P.; Mariska, J. T.; Doschek, G. A.; Eve Team
Bibcode: 2010AGUFMSH13A..06W
Altcode:
Much of our current understanding of the temperature and density
structure of solar flares has been derived from broad band X-ray
instruments, such as RHESSI, GOES, and SXT/Yohkoh, or the observation
of isolated emission lines, such as from BCS/Yohkoh. This has lead
to uncertainties in determining the distribution of temperatures and
densities in a flare. The EUV Variability Experiment (EVE) on the Solar
Dynamics Observatory (SDO) provides an unprecedented opportunity to
observe a very wide range of high-temperature emission lines at high
cadence (10 s) and relatively high spectral resolution (1 A). The
spectral range between 90 and 200 Angstroms is particularly rich in
emission lines from Fe that are formed at temperatures above 7 MK
(Fe XVIII - Fe XXIV). This range also includes one of the few density
diagnostics (Fe XXI 145.66/128.75) that is useful in solar flare
observations. Our initial calculations suggest very broad differential
emission measure distributions and indicate high densities (Log Ne as
high as 11.7), which implies a very rapid cooling of flare plasma. These
observations are broadly consistent with flare models that allow for
the release of energy on many independent threads and we will discuss
these results in the context of current theories of solar flares.
Title: Science Objectives for an X-Ray Microcalorimeter Observing
the Sun
Authors: Laming, J. Martin; Adams, J.; Alexander, D.; Aschwanden, M;
Bailey, C.; Bandler, S.; Bookbinder, J.; Bradshaw, S.; Brickhouse,
N.; Chervenak, J.; Christe, S.; Cirtain, J.; Cranmer, S.; Deiker, S.;
DeLuca, E.; Del Zanna, G.; Dennis, B.; Doschek, G.; Eckart, M.; Fludra,
A.; Finkbeiner, F.; Grigis, P.; Harrison, R.; Ji, L.; Kankelborg,
C.; Kashyap, V.; Kelly, D.; Kelley, R.; Kilbourne, C.; Klimchuk, J.;
Ko, Y. -K.; Landi, E.; Linton, M.; Longcope, D.; Lukin, V.; Mariska,
J.; Martinez-Galarce, D.; Mason, H.; McKenzie, D.; Osten, R.; Peres,
G.; Pevtsov, A.; Porter, K. Phillips F. S.; Rabin, D.; Rakowski, C.;
Raymond, J.; Reale, F.; Reeves, K.; Sadleir, J.; Savin, D.; Schmelz,
J.; Smith, R. K.; Smith, S.; Stern, R.; Sylwester, J.; Tripathi, D.;
Ugarte-Urra, I.; Young, P.; Warren, H.; Wood, B.
Bibcode: 2010arXiv1011.4052L
Altcode:
We present the science case for a broadband X-ray imager with
high-resolution spectroscopy, including simulations of X-ray spectral
diagnostics of both active regions and solar flares. This is part of
a trilogy of white papers discussing science, instrument (Bandler et
al. 2010), and missions (Bookbinder et al. 2010) to exploit major
advances recently made in transition-edge sensor (TES) detector
technology that enable resolution better than 2 eV in an array that
can handle high count rates. Combined with a modest X-ray mirror, this
instrument would combine arcsecondscale imaging with high-resolution
spectra over a field of view sufficiently large for the study of
active regions and flares, enabling a wide range of studies such as
the detection of microheating in active regions, ion-resolved velocity
flows, and the presence of non-thermal electrons in hot plasmas. It
would also enable more direct comparisons between solar and stellar
soft X-ray spectra, a waveband in which (unusually) we currently have
much better stellar data than we do of the Sun.
Title: Radiative and magnetic properties of solar active
regions. II. Spatially resolved analysis of O V 62.97 nm transition
region emission
Authors: Fludra, A.; Warren, H.
Bibcode: 2010A&A...523A..47F
Altcode:
Context. Global relationships between the photospheric magnetic flux
and the extreme ultraviolet emission integrated over active region
area have been studied in a previous paper by Fludra & Ireland
(2008, A&A, 483, 609). Spatially integrated EUV line intensities
are tightly correlated with the total unsigned magnetic flux, and
yet these global power laws have been shown to be insufficient for
accurately determining the coronal heating mechanism owing to the
mathematical ill-conditioning of the inverse problem.
Aims:
Our aim is to establish a relationship between the EUV line intensities
and the photospheric magnetic flux density on small spatial scales in
active regions and investigate whether it provides a way of identifying
the process that heats the coronal loops.
Methods: We compare
spatially resolved EUV transition region emission and the photospheric
magnetic flux density. This analysis is based on the O V 62.97 nm line
recorded by the SOHO Coronal Diagnostic Spectrometer (CDS) and SOHO MDI
magnetograms for six solar active regions. The magnetic flux density
ϕ is converted to a simulated O V intensity using a model relationship
I(ϕ, L) = Cϕδ Lλ, where the loop length L is
obtained from potential magnetic field extrapolations. This simulated
spatial distribution of O V intensities is convolved with the CDS
instrument's point spread function and compared pixel by pixel with the
observed O V line intensity. Parameters δ and λ are derived to give
the best fit for the observed and simulated intensities.
Results:
Spatially-resolved analysis of the transition region emission reveals
the complex nature of the heating processes in active regions. In some
active regions, particularly large, local intensity enhancements up to
a factor of five are present. When areas with O V intensities above
3000 erg cm-2 s-1 sr-1 are ignored,
a power law has been fitted to the relationship between the local O
V line intensity and the photospheric magnetic flux density in each
active region. The average power index δ from all regions is 0.4±0.1
and λ = -0.15±0.07. However, the scatter of intensities in all
regions is significantly greater than ±3σ from the fitted model. We
therefore determine for the first time an empirical lower boundary for
the IOV-ϕ relationship that is the same for five active
regions. We postulate that it represents a basal heating. Because this
boundary is present in the spatially-resolved data, this is compelling
proof that the magnetic field is one of the major factors contributing
to the basal component of the heating of the coronal plasma. We discuss
the implications for the diagnostics of the coronal heating mechanism.
Title: Characteristics and Evolution of the Magnetic Field and
Chromospheric Emission in an Active Region Core Observed by Hinode
Authors: Brooks, David H.; Warren, Harry P.; Winebarger, Amy R.
Bibcode: 2010ApJ...720.1380B
Altcode: 2010arXiv1006.5776B
We describe the characteristics and evolution of the magnetic field and
chromospheric emission in an active region core observed by the Solar
Optical Telescope (SOT) on Hinode. Consistent with previous studies,
we find that the moss is unipolar, the spatial distribution of magnetic
flux evolves slowly, and that the magnetic field is only moderately
inclined. We also show that the field-line inclination and horizontal
component are coherent, and that the magnetic field is mostly sheared
in the inter-moss regions where the highest magnetic flux variability
is seen. Using extrapolations from spectropolarimeter magnetograms,
we show that the magnetic connectivity in the moss is different from
that in the quiet Sun because most of the magnetic field extends to
significant coronal heights. The magnetic flux, field vector, and
chromospheric emission in the moss also appear highly dynamic but
actually show only small-scale variations in magnitude on timescales
longer than the cooling times for hydrodynamic loops computed from
our extrapolations, suggesting high-frequency (continuous) heating
events. Some evidence is found for flux (Ca II intensity) changes on
the order of 100-200 G (DN) on timescales of 20-30 minutes that could
be taken as indicative of low-frequency heating. We find, however,
that only a small fraction (10%) of our simulated loops would be
expected to cool on these timescales, and we do not find clear evidence
that the flux changes consistently produce intensity changes in the
chromosphere. Using observations from the EUV Imaging Spectrometer
(EIS), we also determine that the filling factor in the moss is ~16%,
consistent with previous studies and larger than the size of an SOT
pixel. The magnetic flux and chromospheric intensity in most individual
SOT pixels in the moss vary by less than ~20% and ~10%, respectively,
on loop cooling timescales. In view of the high energy requirements of
the chromosphere, we suggest that these variations could be sufficient
for the heating of "warm" EUV loops, but that the high basal levels
may be more important for powering the hot core loops rooted in the
moss. The magnetic field and chromospheric emission appear to evolve
gradually on spatial scales comparable to the cross-field scale of
the fundamental coronal structures inferred from EIS measurements.
Title: An Argument for Two Coronal Heating Models
Authors: Winebarger, Amy R.; Warren, H. P.; Brooks, D. H.; Ugarte-Urra,
I.
Bibcode: 2010AAS...21630002W
Altcode:
There are several different models for the time and spatial scale
of the energy release in the corona. Over the past several years,
several different attempts at reconciling model predictions with
observations have been made with varying degrees of success. In this
talk, we review three popular heating models and discuss the expected
observables associated with each model. We then show observations
and compare them to the predictions. We find that observations of
loops with a peak temperature near 1 MK are consistent with impulsive
heating while observations of high temperature loops rooted in the
moss are consistent with a steady heating model. This indicates the
time scale of the heating, and potentially the heating mechanism,
are markedly different in the two structures.
Title: Determining the Temperature Structure of Solar Coronal Loops
using their Temporal Evolution
Authors: Mulu, Fana; Winebarger, A. R.; Warren, H. P.; Aschwanden,
M. J.; Klimchuk, J. A.
Bibcode: 2010AAS...21630001M
Altcode:
Despite much progress toward understanding the dynamics of the
corona, the physical properties of coronal loops are not yet fully
understood. Recent investigations and observations from different
instruments have yielded contradictory results about the true physical
properties of coronal loops, specifically as to whether the observed
loops are isothermal structures or the convolution of several
multi-thermal strands. In this talk, we introduce a new technique
to determine if an observed loop is isothermal or multi-thermal. We
determine the evolution of ten selected loops in multiple filter
images from the Transition Region and Coronal Explorer (TRACE). Our
new technique calculates the delay, calculates a cooling time,
and determines if that cooling time is consistent with the observed
lifetime. If the observational lifetime of the loop agrees with the
calculated lifetime, then we can conclude that the loop is a single
"monolithic” structure that heats and cools as a homogeneous unit,
with isothermal temperature over the cross-section. If not, the
loop must be a bundle of multiple multi-thermal strands, all being
heated and cooling independently. In the second part of the talk,
we utilize the concept of nanoflare storms to understand the reason
behind the extended lifetimes. By simulating the observed light curves
of the loops using EBTEL (Enthalpy-Based Thermal Evolution of Loops),
we find that the longer observed lifetimes can be reproduced by using
a set of small-scale impulsively heated strands.
Title: Steady Heating Model of an Active Region Core
Authors: Winebarger, Amy R.; Schmelz, J. T.; Saar, S. H.; Kashyap,
V. L.; Warren, H. P.
Bibcode: 2010AAS...21640711W
Altcode: 2010BAAS...41R.861W
If the heating in an active region core is steady, the base pressure of
loop as well as its loop length determines exactly the apex temperature,
density and required heating rate. In this research, we analyze data
of an active region core that is observed with both Hinode XRT and
EIS instruments. We use the density sensitve Fe XII line ratios to
determine the base pressure of the loops and geometrical constraints
to determine the loop lengths. We use the hotter spectral lines coupled
with the XRT filter intensities to determine the differential emission
measure (DEM) of the core plasma. Using the base pressures and loop
lengths, we populate loops in a model active region to determine a
model DEM. We then compare this emission measure distribution to the
observed distribution.
Title: Variability of Hot Plasma in Solar Active Regions.
Authors: Ugarte-Urra, Ignacio; Warren, H. P.
Bibcode: 2010AAS...21640712U
Altcode: 2010BAAS...41..861U
The core of a solar active region is generally dominated by hot,
high density, slowly evolving loops that appear to be consistent with
steady heating. However, these loops are generally studied using
instruments with a broad temperature response, which may mask some
of the variability. Here we investigate the evolution of coronal
loops emitting at temperatures above 3MK in fast scans taken with
the Extreme-ultraviolet Imaging Spectrometer on board Hinode. We show
evidence for short term variability at these temperatures and present
the differential emission measure changes associated with them.
Title: SDO Extreme Ultraviolet Variability Experiment (EVE):
Instrument and First Light
Authors: Woods, Thomas N.; Eparvier, F.; Hock, R.; Jones, A.;
Didkovsky, L.; Judge, D.; Chamberlin, P.; Lean, J.; Warren, H.;
Mariska, J.
Bibcode: 2010AAS...21630802W
Altcode:
The Extreme ultraviolet Variability Experiment (EVE) aboard the
NASA Solar Dynamics Observatory (SDO) was launched on 11 February
2010. The EVE instruments measure the solar extreme ultraviolet
(EUV) irradiance from 0.1 to 105 nm with unprecedented spectral
resolution (0.1 nm), temporal cadence (10 sec minimum), and accuracy
(20% or better). The highly variable solar EUV irradiance is a key
measurement for the NASA Living With the Star (LWS) program as it is
the major energy input into the Earth's upper atmosphere and thus
impacts the geospace environment that affects satellite operations
and communication and navigation systems. The EVE measurements, along
with additional solar measurements from SDO and other satellite and
ground-based instruments, will be used to advance our understanding of
the solar EUV irradiance variability. For short time scales, EVE will
make detailed observations on the evolution of flare events that are
an important interest for space weather research and operations. For
longer time scales, EVE measurements will be compared to other solar
EUV irradiance measurements to help establish a composite time series
of the solar EUV irradiance. The EVE instrument will be described, and
first light results from EVE during the rise of solar cycle 24 will be
presented. The EVE instrument team is supported by the NASA SDO Project.
Title: The Observation and Modeling of High Temperature Active Region
Emission with Hinode
Authors: Warren, Harry; Brooks, D. H.; Winebarger, A. R.
Bibcode: 2010AAS...21640709W
Altcode: 2010BAAS...41Q.861W
The high temperature emission that is present in the core of an active
region is generally unresolved by current solar instrumentation. This
makes it difficult to isolate and study individual loops. One way
to circumvent this problem is to study the moss, which is the bright
emission from the footpoints of hot active region loops seen in many
solar EUV images. Moss observations are particularly useful because
they provide boundary conditions for physical models of coronal loops
without the need to know the loop geometry. With their high spatial and
temporal resolution and broad array of diagnostics, the instruments
on Hinode have provided many new insights into the properties of
the moss. These observations appear to be generally consistent with
steady heating in high temperature active region loops. Fast scans
with EIS, for example, show that the moss intensities, Doppler shifts,
and nonthermal velocities are constant over many hours. Our initial
analysis of SOT Ca data from the moss also shows remarkably constant
emission even at this very high spatial resolution. Steady heating
models are consistent with the intensities observed with EIS. In this
talk we review Hinode observations and modeling of the moss emission
and extend these models to simulations of entire active regions.
Title: Modeling Evolving Coronal Loops with Observations from Stereo,
Hinode, and Trace
Authors: Warren, Harry P.; Kim, David M.; DeGiorgi, Amanda M.;
Ugarte-Urra, Ignacio
Bibcode: 2010ApJ...713.1095W
Altcode: 2009arXiv0904.3920W
The high densities, long lifetimes, and narrow emission measure
distributions observed in coronal loops with apex temperatures
near 1 MK are difficult to reconcile with physical models of the
solar atmosphere. It has been proposed that the observed loops are
actually composed of sub-resolution "threads" that have been heated
impulsively and are cooling. We apply this heating scenario to nearly
simultaneous observations of an evolving post-flare loop arcade
observed with EUVI/STEREO, EIS/Hinode, XRT/Hinode, and TRACE. We
find that it is possible to reproduce the extended loop lifetime,
high electron density, and the narrow differential emission measure
with a multi-thread hydrodynamic model provided that the timescale
for the energy release is sufficiently short. The model, however,
does not reproduce the evolution of the very high temperature emission
observed with XRT. In XRT the emission appears diffuse and it may be
that this discrepancy is simply due to the difficulty of isolating
individual loops at these temperatures. This discrepancy may also
reflect fundamental problems with our understanding of post-reconnection
dynamics during the conductive cooling phase of loop evolution.
Title: Evidence for Steady Heating: Observations of an Active Region
Core with Hinode and TRACE
Authors: Warren, Harry P.; Winebarger, Amy R.; Brooks, David H.
Bibcode: 2010ApJ...711..228W
Altcode: 2009arXiv0910.0458W
The timescale for energy release is an important parameter for
constraining the coronal heating mechanism. Observations of "warm"
coronal loops (~1 MK) have indicated that the heating is impulsive and
that coronal plasma is far from equilibrium. In contrast, observations
at higher temperatures (~3 MK) have generally been consistent with
steady heating models. Previous observations, however, have not been
able to exclude the possibility that the high temperature loops are
actually composed of many small-scale threads that are in various stages
of heating and cooling and only appear to be in equilibrium. With new
observations from the EUV Imaging Spectrometer and X-ray Telescope
(XRT) on Hinode we have the ability to investigate the properties of
high temperature coronal plasma in extraordinary detail. We examine
the emission in the core of an active region and find three independent
lines of evidence for steady heating. We find that the emission observed
in XRT is generally steady for hours, with a fluctuation level of
approximately 15% in an individual pixel. Short-lived impulsive heating
events are observed, but they appear to be unrelated to the steady
emission that dominates the active region. Furthermore, we find no
evidence for warm emission that is spatially correlated with the hot
emission, as would be expected if the high temperature loops are the
result of impulsive heating. Finally, we also find that intensities in
the "moss," the footpoints of high temperature loops, are consistent
with steady heating models provided that we account for the local
expansion of the loop from the base of the transition region to the
corona. In combination, these results provide strong evidence that
the heating in the core of an active region is effectively steady,
that is, the time between heating events is short relative to the
relevant radiative and conductive cooling times.
Title: Bright Points and Jets in Polar Coronal Holes Observed by
the Extreme-Ultraviolet Imaging Spectrometer on Hinode
Authors: Doschek, G. A.; Landi, E.; Warren, H. P.; Harra, L. K.
Bibcode: 2010ApJ...710.1806D
Altcode:
We present observations of polar coronal hole bright points (BPs)
made with the Extreme-ultraviolet Imaging Spectrometer (EIS) on the
Hinode spacecraft. The data consist of raster images of BPs in multiple
spectral lines from mostly coronal ions, e.g., Fe X-Fe XV. The BPs
are observed for short intervals and thus the data are snapshots of
the BPs obtained during their evolution. The images reveal a complex
unresolved temperature structure (EIS resolution is about 2''), with the
highest temperature being about 2 × 106 K. Some BPs appear
as small loops with temperatures that are highest near the top. But
others are more point-like with surrounding structures. However, the
thermal time evolution of the BPs is an important factor in their
appearance. A BP may appear quite different at different times. We
discuss one BP with an associated jet that is bright enough to allow
statistically meaningful measurements. The jet Doppler speed along the
line of sight is about 15-20 km s-1. Electron densities of
the BPs and the jet are typically near 109 cm-3,
which implies path lengths along the line of sight on the order of a
few arcsec. We also construct differential emission measure curves for
two of the best observed BPs. High spatial resolution (significantly
better than 1'') is required to fully resolve the BP structures.
Title: Nitric oxide density enhancements due to solar flares
Authors: Rodgers, E. M.; Bailey, S. M.; Warren, H. P.; Woods, T. N.;
Eparvier, F. G.
Bibcode: 2010AdSpR..45...28R
Altcode:
A differential emission measure technique is used to determine flare
spectra using solar observations from the soft X-ray instruments aboard
the Thermosphere Ionosphere Mesosphere Energetics Dynamics and Solar
Radiation and Climate Experiment satellites. We examine the effect
of the solar flare soft X-ray energy input on the nitric oxide (NO)
density in the lower thermosphere. The retrieved spectrum of the 28
October 2003 X18 flare is input to a photochemical thermospheric NO
model to calculate the predicted flare NO enhancements. Model results
are compared to Student Nitric Oxide Explorer Ultraviolet Spectrometer
observations of this flare. We present results of this comparison and
show that the model and data are in agreement. In addition, the NO
density enhancements due to several flares are studied. We present
results that show large solar flares can deposit the same amount of
0.1-2 and 0.1-7 nm energy to the thermosphere during a relatively
short time as the Sun normally deposits in one day. The NO column
density nearly doubles when the daily integrated energy above 5 J
m-2 is doubled.
Title: The Temperature Structure of Active Region Loops
Authors: Warren, H. P.; Ugarte-Urra, I.; Degiorgi, A.
Bibcode: 2009ASPC..415..303W
Altcode:
Previous solar observations have shown that coronal loops near 1 MK
are difficult to reconcile with simple heating models. These loops
have lifetimes that are long relative to a radiative cooling time and
densities that are large relative to thermodynamic equilibrium. Models
proposed to explain these properties generally rely on the existence of
small scale filaments that are in various stages of heating and cooling,
suggesting a distribution of temperatures within a loop. We present
the observation of an evolving coronal loop observed with EIS/Hinode
and TRACE. This loop has a high density, a narrow distribution of
temperatures, and a lifetime that is long relative to a radiative
cooling time. These properties will be difficult to reconcile with
physical models of coronal loops.
Title: Hinode Coronal Loop Observations
Authors: Ugarte-Urra, I.; Warren, H. P.; Brooks, D. H.
Bibcode: 2009ASPC..415..241U
Altcode:
Coronal loops are the building blocks of the solar
atmosphere. Understanding their mechanism of formation means
understanding the mechanism responsible for heating the solar
corona. The properties of coronal loops are only partially
established. Several issues such as the relationship among structures
seen at different temperatures, the temperature distribution of the
emission, or the degree of filamentation within a volume remain under
discussion. Hinode, as the new generation solar observatory, provides
the best suited diagnostics to address some of these issues. We
demonstrate that this is the case using a case study, AR 10978,
and encourage systematic studies of larger samples as solar activity
increases. Results for AR 10978 are consistent with loops rooted in
highly dynamic unipolar magnetic field areas made of multiple strands
that get heated to at least 2.5 MK, and cool down rather coherently
to transition region temperatures.
Title: Ultra-Hot Plasma in Active Regions Observed by the
Extreme-Ultraviolet Imaging Spectrometer on Hinode
Authors: Ko, Y. -K.; Doschek, G. A.; Warren, H. P.; Yount, P. R.
Bibcode: 2009ASPC..415..275K
Altcode:
We present a specific algorithm to extract the Ca XVII λ192.858
line from the blending with two Fe XI and six O V lines in the
Hinode/EIS data. This is a review of the work to be published by Ko et
al. (2009). We demonstrate that the Ca XVII line can be satisfactorily
extracted from the blend if the Ca XVII emission contributes to at
least 10% of the blend. This Ca XVII line, with formation temperature
at 6 million degrees, is thus a viable line in the EIS data to probe the
thermal structure in non-flaring active regions at its high temperature
end and provides valuable constraints for coronal heating models.
Title: Hinode/Extreme-Ultraviolet Imaging Spectrometer Observations
of the Temperature Structure of the Quiet Corona
Authors: Brooks, David H.; Warren, Harry P.; Williams, David R.;
Watanabe, Tetsuya
Bibcode: 2009ApJ...705.1522B
Altcode: 2009arXiv0905.3603B
We present a differential emission measure (DEM) analysis of the quiet
solar corona on disk using data obtained by the Extreme-ultraviolet
Imaging Spectrometer (EIS) on Hinode. We show that the expected
quiet-Sun DEM distribution can be recovered from judiciously selected
lines, and that their average intensities can be reproduced to
within 30%. We present a subset of these selected lines spanning the
temperature range log T = 5.6-6.4 K that can be used to derive the DEM
distribution reliably, including a subset of iron lines that can be used
to derive the DEM distribution free of the possibility of uncertainties
in the elemental abundances. The subset can be used without the need for
extensive measurements, and the observed intensities can be reproduced
to within the estimated uncertainty in the pre-launch calibration
of EIS. Furthermore, using this subset, we also demonstrate that
the quiet coronal DEM distribution can be recovered on size scales
down to the spatial resolution of the instrument (1'' pixels). The
subset will therefore be useful for studies of small-scale spatial
inhomogeneities in the coronal temperature structure, for example,
in addition to studies requiring multiple DEM derivations in space or
time. We apply the subset to 45 quiet-Sun data sets taken in the period
2007 January to April, and show that although the absolute magnitude
of the coronal DEM may scale with the amount of released energy, the
shape of the distribution is very similar up to at least log T ~ 6.2 K
in all cases. This result is consistent with the view that the shape of
the quiet-Sun DEM is mainly a function of the radiating and conducting
properties of the plasma and is fairly insensitive to the location and
rate of energy deposition. This universal DEM may be sensitive to other
factors such as loop geometry, flows, and the heating mechanism, but
if so they cannot vary significantly from quiet-Sun region to region.
Title: Flows and Motions in Moss in the Core of a Flaring Active
Region: Evidence for Steady Heating
Authors: Brooks, David H.; Warren, Harry P.
Bibcode: 2009ApJ...703L..10B
Altcode: 2009arXiv0905.3462B
We present new measurements of the time variability of intensity,
Doppler, and nonthermal velocities in moss in an active region core
observed by the EUV Imaging Spectrometer on Hinode in 2007 June. The
measurements are derived from spectral profiles of the Fe XII 195
Å line. Using the 2'' slit, we repeatedly scanned 150'' by 150''
in a few minutes. This is the first time it has been possible to
make such velocity measurements in the moss, and the data presented
are the highest cadence spatially resolved maps of moss Doppler and
nonthermal velocities ever obtained in the corona. The observed region
produced numerous C- and M-class flares with several occurring in
the core close to the moss. The magnetic field was therefore clearly
changing in the active region core, so we ought to be able to detect
dynamic signatures in the moss if they exist. Our measurements of
moss intensities agree with previous studies in that a less than 15%
variability is seen over a period of 16 hr. Our new measurements of
Doppler and nonthermal velocities reveal no strong flows or motions
in the moss, nor any significant variability in these quantities. The
results confirm that moss at the bases of high temperature coronal loops
is heated quasi-steadily. They also show that quasi-steady heating
can contribute significantly even in the core of a flare productive
active region. Such heating may be impulsive at high frequency, but
if so it does not give rise to large flows or motions.
Title: The Temperature and Density Structure of the Solar
Corona. I. Observations of the Quiet Sun with the EUV Imaging
Spectrometer on Hinode
Authors: Warren, Harry P.; Brooks, David H.
Bibcode: 2009ApJ...700..762W
Altcode: 2009arXiv0901.1621W
Measurements of the temperature and density structure of the
solar corona provide critical constraints on theories of coronal
heating. Unfortunately, the complexity of the solar atmosphere,
observational uncertainties, and the limitations of current atomic
calculations, particularly those for Fe, all conspire to make this
task very difficult. A critical assessment of plasma diagnostics in
the corona is essential to making progress on the coronal heating
problem. In this paper, we present an analysis of temperature and
density measurements above the limb in the quiet corona using new
observations from the EUV Imaging Spectrometer (EIS) on Hinode. By
comparing the Si and Fe emission observed with EIS we are able to
identify emission lines that yield consistent emission measure
distributions. With these data we find that the distribution of
temperatures in the quiet corona above the limb is strongly peaked
near 1 MK, consistent with previous studies. We also find, however,
that there is a tail in the emission measure distribution that extends
to higher temperatures. EIS density measurements from several density
sensitive line ratios are found to be generally consistent with
each other and with previous measurements in the quiet corona. Our
analysis, however, also indicates that a significant fraction of the
weaker emission lines observed in the EIS wavelength ranges cannot be
understood with current atomic data.
Title: Hot Plasma in Nonflaring Active Regions Observed by the
Extreme-Ultraviolet Imaging Spectrometer on Hinode
Authors: Ko, Yuan-Kuen; Doschek, George A.; Warren, Harry P.; Young,
Peter R.
Bibcode: 2009ApJ...697.1956K
Altcode: 2009arXiv0903.3029K
The Extreme-Ultraviolet Imaging Spectrometer (EIS) on the Hinode
spacecraft obtains high-resolution spectra of the solar atmosphere
in two wavelength ranges: 170-210 and 250-290 Å. These wavelength
regions contain a wealth of emission lines covering temperature regions
from the chromosphere/transition region (e.g., He II, Si VII) up to
flare temperatures (Fe XXIII, Fe XXIV). Of particular interest for
understanding coronal heating is a line of Ca XVII at 192.858 Å,
formed near a temperature of 6 × 106 K. However, this
line is blended with two Fe XI and six O V lines. In this paper we
discuss a specific procedure to extract the Ca XVII line from the
blend. We have performed this procedure on the raster data of five
active regions (ARs) and a limb flare, and demonstrated that the Ca
XVII line can be satisfactorily extracted from the blend if the Ca XVII
flux contributes to at least ~10% of the blend. We show examples of the
high-temperature corona depicted by the Ca XVII emission and find that
the Ca XVII emission has three morphological features in these ARs: (1)
"fat" medium-sized loops confined in a smaller space than the 1 million
degree corona, (2) weaker, diffuse emission surrounding these loops that
spread over the core of the AR, and (3) the locations of the strong Ca
XVII loops are often weak in line emission formed from the 1 million
degree plasma. We find that the emission measure ratio of the 6 million
degree plasma relative to the cooler 1 million degree plasma in the
core of the ARs, using the Ca XVII to Fe XI line intensity ratio as a
proxy, can be as high as 10. Outside of the AR core where the 1 million
degree loops are abundant, the ratio has an upper limit of about 0.5.
Title: Diagnostics of High Temperature Active Region Plasma with
EIS/Hinode
Authors: Warren, Harry; Ko, Y.; Doschek, G.
Bibcode: 2009SPD....40.1212W
Altcode:
Solar active region loops at different temperatures appear to have very
different properties. Coronal loops with apex temperatures near 1 MK
have high densities and lifetimes long relative to a characteristic
cooling time. The properties of these loops are generally consistent
with impulsive heating models. Higher temperature coronal emission,
in contrast, is generally consistent with steady heating models. This
conclusion is largely based on the analysis of broad-band observations
from the SXT on Yohkoh, which had modest spatial resolution and
temperature discrimination. Because of these diagnostic limitations the
time scale for the heating in the core of an active region is still
an open question. The high resolution EIS spectrometer on Hinode
provides new spectroscopic diagnostics of high temperature plasma
in active region cores through the observation of Ca XIV, XV, XVI,
and XVII emission lines. An initial survey of active regions cores
indicates that the emission measure at high temperatures ( 3 MK) is
uncorrelated with the emission measure at lower temperatures ( 1 MK),
suggesting that these loops are not fully cooling. These results will
be discussed in the context of hydrodynamic models.
Title: The Relationship Between Cooling Time and Equilibrium Density
and Temperature
Authors: Winebarger, Amy R.; Warren, H. P.; Mulu-Moore, F. M.
Bibcode: 2009SPD....40.1210W
Altcode:
There now exists significant evidence that the overdense, 1-2 MK
active regions loops observed at EUV wavelengths are cooling. One
heating model for these loops is that they are impulsively heated and
then cool with no additional energy input. At some point in the loop's
evolution before it reaches 1-2 MK, the density and temperature at the
apex of the loop will resemble the expected density and temperature for
a steady, uniformly heated loop; we term this the loop's equilibrium
time. In our previous work, we have determined that the equilibrium
conditions of a loop are related to the total amount of energy released
in the loop. In this poster, we investigate the relationship between
the cooling time measured at 1.5 MK and the equilibrium density and
temperature in hopes to relate the observed cooling time with the
total energy released in the loop. To do this, we calculate a grid of
solutions to the one-dimensional hydrodynamic equations and develop
an empirical relationship between the cooling time and equilibrium
density and temperature. We then discuss the ability to further relate
these values to the total energy released in the loop.
Title: Exploiting EIS/Hinode Imaging Diagnostic Capabilities
Authors: Ugarte-Urra, Ignacio; Warren, H. P.
Bibcode: 2009SPD....40.1219U
Altcode:
Using a wide slit, also called slot, the Extreme-ultraviolet Imaging
Spectrometer, on-board Hinode, is capable of obtaining relatively
fast (1-3 min) simultaneous monochromatic images of various spectral
lines with different formation temperatures ranging 0.4-3 MK. This
mode allows us to study morphology and dynamics of solar coronal
and transition region structures across the temperature spectrum in
a similar way to an EUV imager. This is achieved at the expense of
spectral resolution. In this paper we investigate the plasma diagnostic
capabilities of these spectrally pure images.Wide slit images can be
interpreted as a superposition of simultaneous narrow slit spectra
from adjacent solar positions. From the comparison of consecutive
narrow slit rasters and wide slit images, we demonstrate that by
making simple assumptions it is possible to extract the narrow slit
spectra out of the slot images. This encouraging result opens up the
door for plasma diagnostics, like electron density from spectral line
ratios and differential emission measure analysis, for solar dynamic
events. Various examples, as well as the limitations and validity of
the assumptions, are discussed.
Title: Classifying Coronal Loops as Isothermal or Multi-thermal
Using the Loops' Evolution
Authors: Mulu, Fana; Winebarger, A. R.; Warren, H. P.; Aschwanden,
M. J.
Bibcode: 2009SPD....40.1218M
Altcode:
Despite much progress toward understanding the dynamics of the
corona, the physical properties of coronal loops are not yet fully
understood. Recent investigations and observations from different
instruments have yielded contradictory results about the true
physical properties of coronal loops, specifically as to whether the
observed loops are isothermal structures or the convolution of several
multi-thermal strands. In this poster, we introduce a new technique
to determine if an observed loop is isothermal or multi-thermal. We
will determine the evolution of loops in multiple filter images from
the Transition Region and Coronal Explorer (TRACE). Our new technique
will calculate the delay of the loop between different filter images,
calculate a cooling time, and determine if that cooling time is
consistent with the observed lifetime. We will present preliminary
results that show if the temperature structure across coronal loops
is isothermal or multi-thermal.
Title: Active Region Transition Region Loop Populations and Their
Relationship to the Corona
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.; Brooks, David H.
Bibcode: 2009ApJ...695..642U
Altcode: 2009arXiv0901.1075U
The relationships among coronal loop structures at different
temperatures are not settled. Previous studies have suggested that
coronal loops in the core of an active region (AR) are not seen cooling
through lower temperatures and therefore are steadily heated. If loops
were cooling, the transition region would be an ideal temperature regime
to look for a signature of their evolution. The Extreme-ultraviolet
Imaging Spectrometer on Hinode provides monochromatic images of the
solar transition region and corona at an unprecedented cadence and
spatial resolution, making it an ideal instrument to shed light on
this issue. Analysis of observations of AR 10978 taken in 2007 December
8-19 indicates that there are two dominant loop populations in the AR:
(1) core multitemperature loops that undergo a continuous process of
heating and cooling in the full observed temperature range 0.4-2.5
MK and even higher as shown by the X-Ray Telescope and (2) peripheral
loops which evolve mostly in the temperature range 0.4-1.3 MK. Loops
at transition region temperatures can reach heights of 150 Mm in the
corona above the limb and develop downflows with velocities in the
range of 39-105 km s-1.
Title: Can the Composition of the Solar Corona Be Derived from
Hinode/Extreme-Ultraviolet Imaging Spectrometer Spectra?
Authors: Feldman, U.; Warren, H. P.; Brown, C. M.; Doschek, G. A.
Bibcode: 2009ApJ...695...36F
Altcode:
Elemental abundances appear to be the same everywhere in the
photosphere, but in the solar corona they vary in different
regions. Abundances in quiet Sun (closed) flux tubes are different from
those in coronal hole (CH, open) magnetic field regions, and therefore
abundance variations might possibly be used to determine locations of
slow and fast solar wind in the corona. In active regions, abundances
can change from region to region and can vary with the age of the
region. In the present paper, we evaluate the feasibility of determining
relative elemental abundances in the corona using spectra acquired by
the Extreme-ultraviolet Imaging Spectrometer (EIS) on Hinode. As test
cases, we attempt to evaluate the coronal composition above the limb
in an equatorial quiet region and in a polar CH. We also determine
the elemental composition of coronal regions with moderate activity on
the disk and at the limb. To estimate the accuracy of the instrumental
calibration and the atomic physics used in the calculations, we compare
the derived composition with earlier derivations from spectra recorded
by the Solar Ultraviolet Measurements of Emitted Radiation spectrometer
in similar regions. We find that EIS can be used to determine relative
abundance variations in the inner solar corona. The determination of
absolute abundances can also be attempted after additional calibrations
in space are accomplished.
Title: The Role of Transient Brightenings in Heating the Solar Corona
Authors: Brooks, David H.; Ugarte-Urra, Ignacio; Warren, Harry P.
Bibcode: 2008ApJ...689L..77B
Altcode:
Nanoflare reconnection events have been proposed as a mechanism for
heating the corona. Parker's original suggestion was that frequent
reconnection events occur in coronal loops due to the braiding of the
magnetic field. Many observational studies, however, have focused on the
properties of isolated transient brightenings unassociated with loops,
but their cause, role, and relevance for coronal heating have not
yet been established. Using Hinode SOT magnetograms and high-cadence
EIS spectral data we study the relationship between chromospheric,
transition region, and coronal emission and the evolution of the
magnetic field. We find that hot, relatively steadily emitting coronal
loops and isolated transient brightenings are both associated with
magnetic flux regions that are highly dynamic. An essential difference,
however, is that brightenings are typically found in regions of flux
collision and cancellation whereas coronal loops are generally rooted
in magnetic field regions that are locally unipolar with unmixed
flux. This suggests that the type of heating (transient vs. steady) is
related to the structure of the magnetic field, and that the heating
in transient events may be fundamentally different than in coronal
loops. This implies that they do not play an important role in heating
the "quiescent" corona.
Title: Modeling of the Extreme-Ultraviolet and Soft X-Ray Emission
in a Solar Coronal Bright Point
Authors: Brooks, David H.; Warren, Harry P.
Bibcode: 2008ApJ...687.1363B
Altcode:
Previous studies have been able to reproduce both the observed
intensities and the morphology of high-temperature solar plasma
using steady state heating models. These models, however, have
been unable to reproduce the lower temperature emission observed in
active regions. Here we present results from numerical simulations
of a coronal bright point. We use potential field extrapolations of a
Kitt Peak magnetogram to compute the coronal field lines and populate
them with solutions to the hydrostatic loop equations based on a
volumetric heating function that scales as bar B/L, where bar B is the
magnetic field strength averaged along a field line and L is the loop
length. We consider the effects of altering the magnitude and scale
height of the energy deposition and the effect of allowing the loop
cross sections to expand proportionally to 1/bar B. We then use the
computed densities and temperatures to calculate average intensities
and simulated EUV and soft X-ray images and compared them to Yohkoh
and SOHO observations. We find that our best-case model (apex heating
of expanding loops) can reproduce the high-temperature emission, the
general morphology of the lower temperature emission, and the majority
of the average intensities of reliable lines over a wide range of
temperatures to within ~20%. The morphology in the EUV visualizations,
however, shows some differences from the observations. These results
suggest the role of nonpotential or evolving magnetic fields, or
dynamic processes, but indicate that departures from the potential
field hydrostatic case may not be too large.
Title: Observations of Active Region Loops with the EUV Imaging
Spectrometer on Hinode
Authors: Warren, Harry P.; Ugarte-Urra, Ignacio; Doschek, George A.;
Brooks, David H.; Williams, David R.
Bibcode: 2008ApJ...686L.131W
Altcode: 2008arXiv0808.3227W
Previous solar observations have shown that coronal loops near 1 MK
are difficult to reconcile with simple heating models. These loops have
lifetimes that are long relative to a radiative cooling time, suggesting
quasi-steady heating. The electron densities in these loops, however,
are too high to be consistent with thermodynamic equilibrium. Models
proposed to explain these properties generally rely on the existence
of smaller scale filaments within the loop that are in various stages
of heating and cooling. Such a framework implies that there should be
a distribution of temperatures within a coronal loop. In this paper
we analyze new observations from the EUV Imaging Spectrometer (EIS)
on Hinode. EIS is capable of observing active regions over a wide range
of temperatures (Fe VIII-Fe XVII) at relatively high spatial resolution
(1''). We find that most isolated coronal loops that are bright in Fe
XII generally have very narrow temperature distributions (σT
lesssim 3 × 105 K), but are not isothermal. We also derive
volumetric filling factors in these loops of approximately 10%. Both
results lend support to the filament models.
Title: Solar Observations of High-Temperature Emission with the
Extreme-Ultraviolet Imaging Spectrometer on Hinode
Authors: Warren, Harry P.; Feldman, Uri; Brown, Charles M.
Bibcode: 2008ApJ...685.1277W
Altcode:
We present an analysis of solar coronal emission lines formed above 2.5
MK observed with the Extreme-Ultraviolet Imaging Spectrometer on Hinode
during a small flare. Our main purpose is to evaluate the internal
consistency of the atomic data available for the observed emission. We
find that the observed emission from high-temperature Ca lines (Ca XIV,
XV, XVI, and XVII) is generally consistent with the available atomic
data. The observed Fe XVII emission at these wavelengths, in contrast,
is more difficult to reconcile with current atomic calculations. The
energy levels tabulated in the CHIANTI atomic physics database generally
do not correspond to the observed wavelengths. After associating the
calculated emissivities with the observed emission by hand, we find
that the observed intensities are roughly consistent with what is
predicted. However, the intensity of the strongest unblended line, Fe
XVII 254.87 Å, is not consistent with the intensities of the other Fe
XVII lines at these wavelengths. Several of the Ca XV emission lines,
which are formed at about 4 MK, form density-sensitive line ratios in
the range log ne = 9-11 cm-3. Density measurements
at these temperatures are potentially important for understanding
the coronal heating mechanism. Our initial analysis suggests that
high-temperature active region plasma is underdense relative to the
predictions of steady heating models.
Title: Flows and Nonthermal Velocities in Solar Active Regions
Observed with the EUV Imaging Spectrometer on Hinode: A Tracer of
Active Region Sources of Heliospheric Magnetic Fields?
Authors: Doschek, G. A.; Warren, H. P.; Mariska, J. T.; Muglach, K.;
Culhane, J. L.; Hara, H.; Watanabe, T.
Bibcode: 2008ApJ...686.1362D
Altcode: 2008arXiv0807.2860D
From Doppler velocity maps of active regions constructed from spectra
obtained by the EUV Imaging Spectrometer (EIS) on the Hinode spacecraft
we observe large areas of outflow (20-50 km s-1) that can
persist for at least a day. These outflows occur in areas of active
regions that are faint in coronal spectral lines formed at typical
quiet-Sun and active region temperatures. The outflows are positively
correlated with nonthermal velocities in coronal plasmas. The bulk
mass motions and nonthermal velocities are derived from spectral line
centroids and line widths, mostly from a strong line of Fe XII at
195.12 Å. The electron temperature of the outflow regions estimated
from an Fe XIII to Fe XII line intensity ratio is about (1.2-1.4) ×
106 K. The electron density of the outflow regions derived
from a density-sensitive intensity ratio of Fe XII lines is rather low
for an active region. Most regions average around 7 × 108
cm-3, but there are variations on pixel spatial scales
of about a factor of 4. We discuss results in detail for two active
regions observed by EIS. Images of active regions in line intensity,
line width, and line centroid are obtained by rastering the regions. We
also discuss data from the active regions obtained from other orbiting
spacecraft that support the conclusions obtained from analysis of the
EIS spectra. The locations of the flows in the active regions with
respect to the longitudinal photospheric magnetic fields suggest that
these regions might be tracers of long loops and/or open magnetic
fields that extend into the heliosphere, and thus the flows could
possibly contribute significantly to the solar wind.
Title: Observations of Doppler Shift Oscillations with the EUV
Imaging Spectrometer on Hinode
Authors: Mariska, John T.; Warren, Harry P.; Williams, David R.;
Watanabe, Tetsuya
Bibcode: 2008ApJ...681L..41M
Altcode: 2008arXiv0806.0265M
Damped Doppler shift oscillations have been observed in emission lines
from ions formed at flare temperatures with the Solar Ultraviolet
Measurements of Emitted Radiation spectrometer on the Solar and
Heliospheric Observatory and with the Bragg Crystal Spectrometer
on Yohkoh. This Letter reports the detection of low-amplitude damped
oscillations in coronal emission lines formed at much lower temperatures
observed with the EUV Imaging Spectrometer on the Hinode satellite. The
oscillations have an amplitude of about 2 km s-1 and a
period of around 35 minutes. The decay times show some evidence for
a temperature dependence with the lowest temperature of formation
emission line (Fe XII 195.12 Å) exhibiting a decay time of about 43
minutes, while the highest temperature of formation emission line (Fe XV
284.16 Å) shows no evidence for decay over more than two periods of the
oscillation. The data appear to be consistent with slow magnetoacoustic
standing waves, but may be inconsistent with conductive damping.
Title: Observations of Doppler Shift Oscillations With the EUV
Imaging Spectrometer on Hinode
Authors: Mariska, J. T.; Warren, H. P.; Williams, D. R.; Watanabe, T.
Bibcode: 2008AGUSMSP31A..04M
Altcode:
Damped Doppler shift oscillations have been observed in emission lines
from ions formed at flare temperatures with the Solar Ultraviolet
Measurements of Emitted Radiation spectrometer on SOHO and the Bragg
Crystal Spectrometer on Yohkoh. We report the detection of similar
oscillations in coronal emission lines observed with the EUV Imaging
Spectrometer on the Hinode satellite. The oscillations, which are
present in emission lines formed at lower temperatures than seen with
the instruments listed above, have an amplitude of about 2 km s- 1,
and a period of around 35 min. The decay times show some evidence
for a temperature dependence with the lowest temperature of formation
emission line (Fe~XII 195.12 Å) exhibiting a decay time of about 43
min, while the highest temperature of formation emission line (Fe~XV
284.16 Å) shows no evidence for decay over more than two periods of the
oscillation. The data appear to be consistent with slow magnetoacoustic
standing waves.
Title: Comparison of One-dimensional Hydrodynamic Codes and Analytical
Models for Time- Dependent Heating
Authors: Mulu-Moore, F.; Winebarger, A. R.; Warren, H. P.
Bibcode: 2008AGUSMSP31C..07M
Altcode:
Numerous coronal heating theories have suggested that the heating in
the corona is highly dynamic and strongly time-dependent. One method of
investigating the coronal heating problem is to compare observations
to solutions of the one-dimensional hydrodynamic equations. Because
the numerical simulations are computationally intensive, there are
also analytical models that describe the evolution of the plasma that
has been heated dynamically. In this poster, we present a comparison
of numerical simulations and analytical models for a limited number
of dynamic heating functions.
Title: The Role of Isolated EUV Brightenings in Heating the Corona
Authors: Brooks, D. H.; Warren, H. P.; Ugarte-Urra, I.
Bibcode: 2008AGUSMSP43C..04B
Altcode:
Nanoflare reconnection events have been proposed as a mechanism for
heating the solar corona. Parker's original suggestion was that frequent
reconnection events occur in coronal loops due to the twisting and
braiding of the magnetic field. Many observational studies, however,
have focused on the radiating properties of isolated brightening
events, but their cause, role, and relevance for coronal heating
has not yet been established. Using Hinode Solar Optical Telescope
(SOT) magnetograms and high cadence EUV Imaging Spectrometer (EIS)
slot rasters we study the relationship between transition region and
coronal emission and the evolution of the magnetic field. We find that
hot, relatively steadily emitting coronal loops are generally rooted in
magnetic field regions that are locally unipolar yet highly dynamic,
whereas detailed analysis shows that ubiquitous EUV brightenings are
found in regions of magnetic flux cancellation in the photosphere. This
suggests that the heating in transient events may be fundamentally
different than the heating in coronal loops and that they play no
direct role in the heating of the quiescent corona.
Title: Electron Densities in Active Region Loops Observed with
Hinode/EIS
Authors: Warren, H. P.; Winebarger, A. R.; Brooks, D. H.
Bibcode: 2008AGUSMSP41C..02W
Altcode:
Active region observations with the Transition Region and Coronal
Explorer (TRACE) showed that loops near 1 MK appear to have high
densities relative to the predictions of scaling laws based on steady
heating. These loops also persist much longer than a radiative cooling
time. This lead to the formation of models based on the impulsive
heating of small scale filaments. With the launch of the EUV Imaging
Spectrometer (EIS) on Hinode we now have a much more detailed view of
coronal loops at these temperatures. We find that the temperatures,
densities, and filling factors inferred from the new spectroscopic
data are largely consistent with our interpretation of the earlier
TRACE observations. The impulsive heating models also predict low
densities relative to the steady heating models at high temperatures,
and we will discuss the EIS evidence for hot, underdense loops in
solar active regions.
Title: EIS: a new view of active region transition region loops
Authors: Ugarte-Urra, I.; Warren, H. P.; Brooks, D. H.
Bibcode: 2008AGUSMSP41C..03U
Altcode:
The EUV Imaging Spectrometer (EIS) on board Hinode is providing
unprecedented diagnostics of solar coronal plasmas. One of its less
exploited capabilities is the ability to make instantaneous spectrally
pure images with the 40'' slot. Simultaneous transition region (Mg
VI, Mg VII, Si VII) and coronal (Fe XI - Fe XVI) images allow us
to observe active region loops as we have not been able to before,
given the spatial resolution (1arcsec pixels), cadence (70s) and,
most importantly, the broad temperature coverage. Under this scrutiny
two distinct populations of active region transition region loops can
be differentiated: core loops that result from the cooling of several
million degree plasma; and fan structures with their main contribution
in the 0.6-1 MK temperature range. These results suggest that the cores
of active regions are not as steady as commonly assumed and reinforce
the idea of coexistance of differentiated loop populations within the
active region topology. We present the properties of the loops and we
discuss the implications that these new observations have for current
transition region and coronal models.
Title: Ultra-Hot Plasma in Active Regions Observed by the
Extreme-ultraviolet Imaging Spectrometer on Hinode
Authors: Doschek, G. A.; Warren, H. P.; Feldman, U.
Bibcode: 2008AGUSMSP43C..01D
Altcode:
The Extreme-ultraviolet Imaging Spectrometer (EIS) on the Hinode
spacecraft obtains high resolution spectra of the solar atmosphere in
two wavelength ranges: 170 - 210 and 250 — 290 Angstroms. These
wavelength regions contain a wealth of emission lines covering
temperature regions from the chromosphere/transition region (e.g.,
He II, Si VII) up to soft X-ray flare temperatures (Fe XXIII,
Fe XXIV). EIS can obtain line profiles and intensities for the
spectral lines in these wavelength regions. Of particular interest
for understanding coronal heating is a line of Ca XVII, formed near
a temperature of 6 MK. This line is blended with lines of Fe XI and O
V. However, by using unblended lines of these ions, the Ca XVII line
can be deconvolved from the blended emission. EIS has obtained many
raster observations of active regions by stepping the slit in small
increments across the active region, producing monochromatic images
of the active region. The Ca XVII blend has been included in many of
these rasters. In this paper we discuss the appearance and frequency
of 6 MK plasma in active regions in the absence of strong flaring
activity. This temperature region is not well-observed by normal
incidence imaging spectrometers and therefore the EIS data shed light
on higher temperature areas of active regions than normally available
from imaging instruments alone. We discuss how to deconvolve the blend
and show examples of 6 MK plasma emission in several active regions.
Title: Observation and Modeling of Coronal "Moss" With the EUV
Imaging Spectrometer on Hinode
Authors: Warren, Harry P.; Winebarger, Amy R.; Mariska, John T.;
Doschek, George A.; Hara, Hirohisa
Bibcode: 2008ApJ...677.1395W
Altcode: 2007arXiv0709.0396W
Observations of transition region emission in solar active regions
represent a powerful tool for determining the properties of hot coronal
loops. We present the analysis of new observations of active region
moss taken with the Extreme Ultraviolet Imaging Spectrometer (EIS)
on the Hinode satellite. EIS observations of a density sensitive Fe
XII line ratio suggest moss densities of approximately 1010
cm-3 and pressures of 3 × 1016 cm-3
K. We find that the moss intensities predicted by steady, uniformly
heated loop models are too intense relative to the observations,
consistent with previous work. To bring the steady heating model into
agreement with the observations a filling factor is required. Our
analysis indicates that the filling factor in the moss is nonuniform
and varies inversely with the loop pressure. The intensities predicted
by steady uniform heating are generally consistent with the EIS moss
observations. There are, however, significant discrepancies for the
coolest emission line available in the data we analyze.
Title: Modeling X-Ray Loops and EUV "Moss" in an Active Region Core
Authors: Winebarger, Amy R.; Warren, Harry P.; Falconer, David A.
Bibcode: 2008ApJ...676..672W
Altcode: 2007arXiv0712.0756W
The soft X-ray intensity of loops in active region cores and the
corresponding footpoint, or moss, intensity observed in the EUV remain
steady for several hours of observation. The steadiness of the emission
has prompted many to suggest that the heating in these loops must also
be steady, although no direct comparison between the observed X-ray and
EUV intensities and the steady heating solutions of the hydrodynamic
equations has yet been made. In this paper we perform these simulations
and simultaneously model the X-ray and EUV moss intensities in one
active region core with steady uniform heating. To perform this task,
we introduce a new technique to constrain the model parameters using
the measured EUV footpoint intensity to infer a heating rate. Using an
ensemble of loop structures derived from magnetic field extrapolation
of photospheric field, we associate each field line with an EUV moss
intensity, then determine the steady uniform heating rate on that
field line that reproduces the observed EUV intensity within 5% for
a specific cross-sectional area, or filling factor. We then calculate
the total X-ray filter intensities from all loops in the ensemble and
compare this to the observed X-ray intensities. We complete this task
iteratively to determine the filling factor that returns the best match
to the observed X-ray intensities. We find that a filling factor of 8%
and loops that expand with height provides the best agreement with
the intensity in two X-ray filters, although the simulated SXT Al12
intensity is 147% the observed intensity and the SXT AlMg intensity is
80% the observed intensity. From this solution we determine the required
heating rate scales as bar B0.29L-0.95. Finally,
we discuss the future potential of this type of modeling, such as the
ability to use density measurements to fully constrain filling factor,
and its shortcomings, such as the requirement to use potential field
extrapolations to approximate the coronal field.
Title: Solar Flare Soft X-ray Irradiance and its Impact on the
Earth's Upper Atmosphere
Authors: Rodgers, Erica; Bailey, Scott; Warren, Harry; Woods, Thomas;
Eparvier, Francis
Bibcode: 2008cosp...37.2627R
Altcode: 2008cosp.meet.2627R
Solar flares dramatically enhance the soft X-ray region of the
solar spectrum. A differential emission measure technique is used
to determine flare spectra from Thermosphere Ionosphere Mesosphere
Energetics Dynamics (TIMED) and Solar Radiation and Climate Experiment
(SORCE) solar observations. Results show that flares primarily enhance
the soft X-ray irradiance in the 0.1-2 nm range, and rapidly modify
the energy input to the lower thermosphere. Most of the excess flare
0.1-2 nm irradiance comes from 1-2 nm, thus flares deposit a large
amount of their energy between 100-110 km. One of the key effects of
this energy deposition is to modify nitric oxide (NO), which plays an
important role in the energy balance of the thermosphere as it is a
source of radiative cooling through infrared emissions. The density of
NO is highly variable as a function of time and latitude, and reaches
a maximum in the same altitude region where the flare irradiance
is absorbed. Valid comparisons between Student Nitric Oxide Explorer
(SNOE) satellite NO observations and those predicted by a photochemical
thermospheric model provide a better understanding of low latitude flare
enhanced NO density. Results show that large flares can deposit the
same amount of 0.1-2 nm energy to the thermosphere during a relatively
short time as the Sun normally deposits in one day, thus doubling the
energy, which doubles the NO density.
Title: A Streamer Ejection with Reconnection Close to the Sun
Authors: Sheeley, N. R., Jr.; Warren, H. P.; Wang, Y. -M.
Bibcode: 2007ApJ...671..926S
Altcode:
We previously described coronal events that expand gradually outward
over an interval of 1-2 days and then suddenly tear apart in the
coronagraph's 2-6 Rsolar field of view to form an outgoing
flux rope and an inward system of collapsing loops. Now, we combine
LASCO white-light images of the outer corona with spectrally resolved
EIT images of the inner corona to describe a similar event for which the
separation occurs closer to the Sun. The evolution of this 2006 July
1-2 event had four phases: (1) an expansion phase in which magnetic
loops rise slowly upward and increase the amount of open flux in the
adjacent polar coronal hole and in the low-latitude hole of opposite
polarity; (2) a stretching phase in which the legs of the rising
loops pinch together to form a current sheet; (3) a transition phase
in which field line reconnection produces an outgoing flux rope and a
hot cusp of new loops; and (4) an end phase in which the reconnected
loops become visible at lower temperatures, and the outgoing flux rope
plows through the slow material ahead of it to form a traveling bow
wave. During this time, the photospheric field was relatively weak and
unchanging, as if the eruption had a nonmagnetic origin. We suppose
that coronal heating gradually overpowers magnetic tension and causes
the streamer to separate into a system of collapsing loops and a flux
rope that is carried outward in the solar wind.
Title: Benchmarking 1D Hydrodynamic Codes for Steady State Solutions
Authors: Mulu-Moore, F.; Winebarger, A.; Warren, H.
Bibcode: 2007AGUFMSH33A1097M
Altcode:
Recent studies that investigate the coronal heating problem use
numerical codes to solve the 1D hydrodynamic equations. Many different
numerical solutions are involved for solving the equations, however,
only a few comparison have been made between the different numerical
results. In this study, we begin a benchmarking process by comparing
steady state solutions from the NRL Solar Flux Tube Model(SOLFTM), AAD's
code and Serio's Scaling Law by calculating percentage differences in
their simulated temperatures and densities.
Title: Using Hinode Data to Explicitly Model Active Regions Cores
Authors: Winebarger, A. R.; Warren, H. P.
Bibcode: 2007AGUFMSH52C..01W
Altcode:
There have been many recent studies that have modeled active regions
as an ensemble of steadily heated loops. One limitation of these
studies is that they required a series of assumptions, such as how
the heating rate scales with the magnetic field strength, loop length,
and filling factor. We have developed a new technique that allows us
to infer the heating rate directly from the observations. Our method
uses observations of density sensitive line ratios in the "moss"
(or footpoints) of active regions cores to infer both the heating rate
and the filling factor. In this talk we will present the application
of our modeling technique to new observations from the EIS and XRT
instruments on Hinode.
Title: The Temperature and Density Structure of an Active Region
Observed with the Extreme-Ultraviolet Imaging Spectrometer on Hinode
Authors: Doschek, George A.; Mariska, John T.; Warren, Harry P.;
Culhane, Len; Watanabe, Tetsuya; Young, Peter R.; Mason, Helen E.;
Dere, Kenneth P.
Bibcode: 2007PASJ...59S.707D
Altcode:
The Extreme-Ultraviolet Imaging Spectrometer (EIS) on Hinode
produces high resolution spectra that can be combined via rasters
into monochromatic images of solar structures, such as active
regions. Electron temperature and density maps of the structures can
be obtained by imaging the structures in different spectral lines with
ratios sensitive to either temperature or density. Doppler maps and
ion temperature maps can be made from spectral line wavelengths and
profiles, respectively. In this paper we discuss coronal temperature
and density distributions within an active region, illustrating the
power of EIS for solar plasma diagnostics.
Title: Observations of Transient Active Region Heating with Hinode
Authors: Warren, Harry P.; Ugarte-Urra, Ignacio; Brooks, David H.;
Cirtain, Jonathan W.; Williams, David R.; Hara, Hirohisa
Bibcode: 2007PASJ...59S.675W
Altcode: 2007arXiv0711.0357W
We present observations of transient active region heating events
observed with the Extreme Ultraviolet Imaging Spectrometer (EIS) and
X-ray Telescope (XRT) on Hinode. This initial investigation focuses
on NOAA active region 10940 as observed by Hinode on 2007 February 1
between 12 and 19UT. In these observations we find numerous examples
of transient heating events within the active region. The high spatial
resolution and broad temperature coverage of these instruments allows
us to track the evolution of coronal plasma. The evolution of the
emission observed with XRT and EIS during these events is generally
consistent with loops that have been heated and are cooling. We have
analyzed the most energetic heating event observed during this period,
a small GOES B-class flare, in some detail and present some of the
spectral signatures of the event, such as relative Doppler shifts at
one of the loop footpoints and enhanced line widths during the rise
phase of the event. While the analysis of these transient events has
the potential to yield insights into the coronal heating mechanism,
these observations do not rule out the possibility that there is a
strong steady heating level in the active region. Detailed statistical
analysis will be required to address this question definitively.
Title: On Connecting the Dynamics of the Chromosphere and Transition
Region with Hinode SOT and EIS
Authors: Hansteen, Viggo H.; de Pontieu, Bart; Carlsson, Mats;
McIntosh, Scott; Watanabe, Tetsuya; Warren, Harry P.; Harra, Louise K.;
Hara, Hirohisa; Tarbell, Theodore D.; Shine, Dick; Title, Alan M.;
Schrijver, Carolus J.; Tsuneta, Saku; Katsukawa, Yukio; Ichimoto,
Kiyoshi; Suematsu, Yoshinori; Shimizu, Toshifumi
Bibcode: 2007PASJ...59S.699H
Altcode: 2007arXiv0711.0487H
We use coordinated Hinode SOT/EIS observations that include
high-resolution magnetograms, chromospheric, and transition region
(TR) imaging, and TR/coronal spectra in a first test to study how
the dynamics of the TR are driven by the highly dynamic photospheric
magnetic fields and the ubiquitous chromospheric waves. Initial
analysis shows that these connections are quite subtle and require a
combination of techniques including magnetic field extrapolations,
frequency-filtered time-series, and comparisons with synthetic
chromospheric and TR images from advanced 3D numerical simulations. As a
first result, we find signatures of magnetic flux emergence as well as
3 and 5mHz wave power above regions of enhanced photospheric magnetic
field in both chromospheric, transition region, and coronal emission.
Title: Hinode EUV Imaging Spectrometer Observations of Solar Active
Region Dynamics
Authors: Mariska, John T.; Warren, Harry P.; Ugarte-Urra, Ignacio;
Brooks, David H.; Williams, David R.; Hara, Hirohisa
Bibcode: 2007PASJ...59S.713M
Altcode: 2007arXiv0708.4309M
The EUV Imaging Spectrometer (EIS) on the Hinode satellite is capable of
measuring emission line center positions for Gaussian line profiles to a
fraction of a spectral pixel, resulting in relative solar Doppler-shift
measurements with an accuracy of a less than a km s-1 for
strong lines. We show an example of the application of that capability
to an active region sit-and-stare observation in which the EIS slit
is placed at one location on the Sun and many exposures are taken
while the spacecraft tracking keeps the same solar location within
the slit. For the active region examined (NOAA10930), we find that
significant intensity and Doppler-shift fluctuations as a function of
time are present at a number of locations. These fluctuations appear
to be similar to those observed in high-temperature emission lines
with other space-borne spectroscopic instruments. With its increased
sensitivity over earlier spectrometers and its ability to image many
emission lines simultaneously, EIS should provide significant new
constraints on Doppler-shift oscillations in the corona.
Title: Hinode EUV Imaging Spectrometer Observations of Active Region
Loop Morphology: Implications for Static Heating Models of Coronal
Emission
Authors: Brooks, David H.; Warren, Harry P.; Ugarte-Urra, Ignacio;
Matsuzaki, Keiichi; Williams, David R.
Bibcode: 2007PASJ...59S.691B
Altcode:
Theoretically, magnetic fields are expected to expand as they rise
above the photosphere and into the corona, so the apparent uniform
cross-sections of active region loops are difficult to understand. There
has been some debate as to whether coronal loops really have constant
cross-sections, or are actually unresolved and composed of expanding
threads within the constant cross-section envelopes. Furthermore, loop
expansion is critical to the success or failure of hydrostatic models
in reproducing the intensities and morphology of observed emission. We
analyze Hinode EIS (EUV Imaging Spectrometer) observations of loops
in active region 10953 and detect only moderate apex width expansion
over a broad range of temperatures from log Te / K = 5.6
to 6.25. The expansion is less than required by steady-state heating
models of coronal emission suggesting that such models will have
difficulty reproducing both low and high temperature loop emission
simultaneously. At higher temperatures (> log Te /
K = 6.3) the apex widths increase substantially, but the emission
at these temperatures likely comes from a combination of multiple
loops. These observations demonstrate the advantage of EIS over previous
instruments. For the first time, active region loops can be examined
over a broad temperature range with high temperature fidelity and the
same spatial resolution. The results therefore provide further clues
to the coronal heating timescale and thus have implications for the
direction of future modeling efforts.
Title: Velocity Structure of Jets in a Coronal Hole
Authors: Kamio, Suguru; Hara, Hirohisa; Watanabe, Tetsuya; Matsuzaki,
Keiichi; Shibata, Kazunari; Culhane, Len; Warren, Harry P.
Bibcode: 2007PASJ...59S.757K
Altcode: 2007arXiv0711.2848K
The velocity structures of jets in a coronal hole have been derived
for the first time. Hinode observations revealed the existence
of many bright points in coronal holes. They are loop-shaped and
sometimes associated with coronal jets. Spectra obtained with the
Extreme-ultraviolet Imaging Spectrometer aboard Hinode were analyzed
to infer the Doppler velocity of bright loops and jets in a coronal
hole of the north polar region. Elongated jets above bright loops are
found to be blue-shifted by 30kms-1 at maximum, while foot
points of bright loops are red-shifted. Blue-shifts detected in coronal
jets are interpreted as being upflows produced by magnetic reconnection
between emerging flux and the ambient field in the coronal hole.
Title: Theoretical Predictions of X-Ray and Extreme-UV Flare Emissions
Using a Loss-of-Equilibrium Model of Solar Eruptions
Authors: Reeves, Katharine K.; Warren, Harry P.; Forbes, Terry G.
Bibcode: 2007ApJ...668.1210R
Altcode:
In this paper, we present numerical simulations of solar flares that
couple a loss-of-equilibrium solar eruption model with a one-dimensional
hydrodynamic model. In these calculations, the eruption is initiated by
footpoint motions that disrupt the balance of forces acting on a flux
rope. After the eruption begins, a current sheet forms and an arcade
of flare loops is created by reconnecting magnetic fields. Thermal
energy input into the flare loops is found by assuming the complete
thermalization of the Poynting flux swept into the current sheet. This
thermal energy is input into a one-dimensional hydrodynamic code for
each loop formed in the multithreaded flare arcade. We find that a
density enhancement occurs at the loop top when the two evaporating
plasma fronts in each leg of the loop collide there. Simulated flare
images show that these loop-top density enhancements produce ``bars''
of bright emission similar to those observed in the Transition Region
and Coronal Explorer (TRACE) 195 Å bandpass and loop-top ``knots'' of
bright emission seen in flare observations by the Soft X-Ray Telescope
(SXT) on Yohkoh. We also simulate flare spectra from the Bragg Crystal
Spectrometer (BCS) on Yohkoh. We find that during the early stages of
flare initiation, there are significant blueshifts in the Ca XIX line,
but the intensities are too faint to be observed with BCS. In general,
the results of this model simulate observed flare emissions quite well,
indicating that the reconnection model of solar flares is energetically
consistent with observations.
Title: Nonthermal Velocities in Solar Active Regions Observed with
the Extreme-Ultraviolet Imaging Spectrometer on Hinode
Authors: Doschek, G. A.; Mariska, J. T.; Warren, H. P.; Brown, C. M.;
Culhane, J. L.; Hara, H.; Watanabe, T.; Young, P. R.; Mason, H. E.
Bibcode: 2007ApJ...667L.109D
Altcode:
We discuss nonthermal velocities in an active region as revealed
by the Extreme-Ultraviolet Imaging Spectrometer (EIS) on the Hinode
spacecraft. The velocities are derived from spectral line profiles in
the extreme-ultraviolet (EUV) from a strong line of Fe XII at 195.12 Å
by fitting each line profile to a Gaussian function. We compare maps
of the full width at half-maximum values, the Fe XII spectral line
intensity, the Fe XII Doppler shift, the electron temperature, and
electron density. We find that the largest widths in the active region
do not occur in the most intense regions, but seem to concentrate in
less intense regions, some of which are directly adjacent to coronal
loops, and some of which concentrate in regions which also exhibit
relative Doppler outflows. The increased widths can also occur over
extended parts of the active region.
Title: Static and Dynamic Modeling of a Solar Active Region
Authors: Warren, Harry P.; Winebarger, Amy R.
Bibcode: 2007ApJ...666.1245W
Altcode: 2006astro.ph..9023W
Recent hydrostatic simulations of solar active regions have shown
that it is possible to reproduce both the total intensity and the
general morphology of the high-temperature emission observed at soft
X-ray wavelengths using static heating models. These static models,
however, cannot account for the lower temperature emission. In addition,
there is ample observational evidence that the solar corona is highly
variable, indicating a significant role for dynamical processes
in coronal heating. Because they are computationally demanding,
full hydrodynamic simulations of solar active regions have not been
considered previously. In this paper we make first application of an
impulsive heating model to the simulation of an entire active region,
AR 8156 observed on 1998 February 16. We model this region by coupling
potential field extrapolations to full solutions of the time-dependent
hydrodynamic loop equations. To make the problem more tractable
we begin with a static heating model that reproduces the emission
observed in four different Yohkoh Soft X-Ray Telescope (SXT) filters
and consider impulsive heating scenarios that yield time-averaged SXT
intensities that are consistent with the static case. We find that
it is possible to reproduce the total observed soft X-ray emission in
all of the SXT filters with a dynamical heating model, indicating that
nanoflare heating is consistent with the observational properties of
the high-temperature solar corona. At EUV wavelengths the simulated
emission shows more coronal loops, but the agreement between the
simulation and the observation is still not acceptable.
Title: The Magnetic Topology of Coronal Mass Ejection Sources
Authors: Ugarte-Urra, Ignacio; Warren, Harry P.; Winebarger, Amy R.
Bibcode: 2007ApJ...662.1293U
Altcode: 2007astro.ph..3049U
In an attempt to test current initiation models of coronal mass
ejections (CMEs), with an emphasis on the magnetic breakout model, we
inspect the magnetic topology of the sources of 26 CME events in the
context of their chromospheric and coronal response in an interval
of approximately 9 hr around the eruption onset. First we perform
current-free (potential) extrapolations of photospheric magnetograms
to retrieve the key topological ingredients, such as coronal magnetic
null points. Then we compare the reconnection signatures observed in
the high-cadence and high spatial resolution Transition Region and
Coronal Explorer (TRACE) images with the location of the relevant
topological features. The comparison reveals that only seven events
can be interpreted in terms of the breakout model, which requires
a multipolar topology with preeruption reconnection at a coronal
null. We find, however, that a larger number of events (12) cannot
be interpreted in those terms. No magnetic null is found in six of
them. Seven other cases remain difficult to interpret. We also show
that there are no systematic differences between the CME speed and
flare energies of events under different interpretations.
Title: EIS/Hinode Look At Active Region Dynamics
Authors: Ugarte-Urra, Ignacio; Warren, H. P.; Brooks, D. H.; Williams,
D. R.; Cirtain, J. W.; McKenzie, D. E.; Weber, M.; Hara, H.; Harra,
L. K.
Bibcode: 2007AAS...210.9429U
Altcode: 2007BAAS...39..222U
We present some initial results from the Hinode EUV Imaging Spectrometer
(EIS) on the heating and cooling of active region loops. The events
we observe are part of the short term active region evolution within
the span of 50 hours of sit-and-stare observations. We investigate
the temporal evolution of the intensity and Doppler shift of spectral
lines formed at different temperatures. We then determine time lags
and cooling times at various temperature regimes and discuss the
results in the context of previous results provided by earlier space
missions. This effort is considered a first step into the hydrodynamic
modeling of the loop structures. The morphology of the structures is
obtained from X-Ray Telescope (XRT) and TRACE images.
Title: Impact of Flare Radiation on the Ionosphere
Authors: Slinker, S.; Krall, J.; Huba, J. D.; Warren, H.; Joyce, G.
Bibcode: 2007AGUSMSA33A..03S
Altcode:
We study the impact of the solar flare radiation on the low- to
mid-latitude ionosphere for a number of flares: 28 Oct 2003, 29 Oct
2003, 4 Nov 2004, and 7 Sept 2005. We use the solar EUV spectrum from
the Flare Irradiance Spectral Model (FISM)1; the spectrum considers
10Å bins from 10 -- 1050 Å and a one minute cadence. We use this
spectrum in the NRL three-dimensional ionosphere model SAMI3 to
obtain the global impact of the flare on the mid- to low-latitude
ionosphere. We find that the TEC can increase by ~ 10% which can be
as high as ~ 25 TECU. Finally, we compare our results to GPS data and
discuss improvements to the model. 1 Chamberlin, P.C., T.N. Woods,
and F.G. Eparvier, ILWS Workshop Proc., GOA, 2006. Research supported
by ONR.
Title: Hinode EIS Observations of Solar Active Regions
Authors: Mariska, John T.; Doschek, G. A.; Warren, H. P.; Brooks,
D. H.; Young, P. R.; Watanabe, T.; Culhane, J. L.
Bibcode: 2007AAS...210.7202M
Altcode: 2007BAAS...39R.178M
The EUV Imaging Spectrometer (EIS) on the Hinode satellite provides
high spatial and spectral resolution data along a 512 arcsec slit in
two wavelength ranges, 170--210 Angstroms and 250--290 Angstroms. These
wavelengths mostly contain emission lines from upper transition region
and coronal plasmas. Emission from these wavelengths is routinely
imaged using instruments such as the EIT on SOHO and TRACE, but there
are few high-resolution spectra to aid in more deeply understanding
the physical conditions and dynamics associated with the intensity
variations seen in the images. In this presentation, we show some
initial results from EIS active region studies aimed at mapping the
density, temperature, nonthermal broadening, and Doppler shifts in
active regions. This presentation focusses on spectroheliograms of
active regions in diagnostically interesting spectral lines. These
show the overall active region morphology and the behavior of Doppler
shifts, nonthermal velocities, and densities as a function of position,
but at the expense of high time resolution. Other presentations will
focus on how the observed physical parameters vary with time.
Title: Intercalibration of the X-ray Telescope and the EUV Imaging
Spectrometer on Hinode
Authors: Golub, Leon; Cirtain, J.; DeLuca, E. E.; Hara, H.; Warren,
H.; Weber, M.
Bibcode: 2007AAS...210.9418G
Altcode: 2007BAAS...39..220G
The X-Ray Telescope and the Extreme-Ultra Violet Imaging Spectrometer
on Hinode are designed to measure the emission of excited ions formed
at temperatures ranging from 104-108 K. The
temperature overlap of these two telescope is from 0.7 to 20 MK, and
an on-orbit calibration of the sensitivity of the two instruments
to solar features will provide a basis for future observational
comparisons. Using calibrated samples of data from each instrument,
and relying to a great extent on the CHIANTI spectral code, we have
derived an estimate of the inter-calibration of the two telescope
for a variety of different solar features and conditions. This is a
major step in enhancing our ability to use the instruments together
for providing quantitative diagnostics of the solar plasma.
Title: Determining the Chirality Of Filaments Associated with CMEs
Authors: Mulu, Fana; Winebarger, A.; Ugarte-Urra, I.; Warren, H.
Bibcode: 2007AAS...210.2914M
Altcode: 2007BAAS...39..139M
There is currently much debate over the initiation mechanism of coronal
mass ejections. One distinction in the various models is the degree
of magnetic complexity in the pre-CME active region. For instance,
the magnetic breakout model requires a null point in the coronal
field and for reconnection to occur at that null point. Recently,
Ugarta-Urra et al. (2007) investigated the magnetic topology of the
sources of 26 CME events using extrapolations of photospheric fields to
find the location of coronal magnetic null points, if any, and hence
test the validity of the breakout model. Seven of the 26 CMEs studied
supported the breakout model, 12 did not support the model, and 7 were
difficult to interpret. In this poster, we present preliminary results
from a continuing investigation these 26 events. First we determine
if an Halpha filament was associated with the eruption. If a filament
was present, we find the chirality (handedness) of the filaments using
the method described by Martin (1998). We will categorize the filament
properties in terms of Ugarte-Urra's findings to determine if certain
filament properties are associated with a breakout or non-breakout CMEs.
Title: Static and Dynamic Simulations of a Solar Active Region
Authors: Warren, Harry; Winebarger, A. R.
Bibcode: 2007AAS...210.9104W
Altcode: 2007BAAS...39R.204W
Most previous work on the coronal heating problem has focused on the
observation and simulation of individual loops. The recent proliferation
of high speed, multi-processor computers has made it possible to
simulate large ensembles of loops and consider the emission from entire
active regions using both steady and impulsive heating models. In this
paper we present simulations of AR8156, which was observed by SXT and
EIT in many filters. We model this region by coupling potential field
extrapolations to full solutions of the 1D hydrodynamic loop equations
using both steady and impulsive heating scenarios. The steady heating
model is able to reproduce the total intensity and general morphology
of the soft X-ray emission observed in four SXT filters. The steady
models, however, cannot reproduce the loop emission observed at lower
temperatures. Using the steady heating rate for each field line as the
basis for an impulsive heating scenario we find that the time-dependent
modeling is able to reproduce both the high temperature emission and
many, but not all, features of the emission at lower temperatures. We
will also discuss how new observations from Hinode will greatly improve
the observational constraints on the coronal heating problem.
Title: Post-CME Reconnection and the Generation of Descending Solar
Coronal Voids
Authors: Linton, Mark; Longcope, D.; Warren, H.
Bibcode: 2007AAS...210.2903L
Altcode: 2007BAAS...39R.137L
Observations of solar coronal flares occurring behind coronal mass
ejections (CME's) have shown downflowing voids in the corona, which are
believed to be the signatures of descending magnetic flux tubes. We are
studying the hypothesis that these flux tubes have reconnected in the
current sheet which forms behind the CME in the high corona. We will
present three dimensional MHD simulations of a localized reconnection
event in a Y-type post-CME current sheet. The reconnected field
creates a downflow which rapidly decelerates as it hits the Y-line
and the magnetic loops below it. We will compare this deceleration
with the observed deceleration of coronal voids when they hit coronal
arcades. We will also present studies of the 3D tearing mode in this
current sheet. This tearing generates numerous localized reconnection
patches, and a cascade of reconnected fluxtubes. We will compare this
cascade with the the cascades of descending voids and coronal loops
which are seen following a CME event, providing further evidence that
the observed voids are reconnected flux tubes. This research was
supported by grants from NASA and ONR.
Title: Initial Results On Active Region Loop Morphology From Hinode
EIS And XRT.
Authors: Brooks, David; Warren, H.; Young, P.; Matsuzaki, K.;
Williams, D.
Bibcode: 2007AAS...210.6307B
Altcode: 2007BAAS...39Q.172B
Theoretically, magnetic fields are expected to expand as they rise
above the photosphere and into the corona, so the apparent uniform
cross-sections of active region coronal loops are difficult to
understand. There has been some debate in the community as to whether
coronal loops really have constant cross-sections (as suggested by
TRACE and SXT), or are actually unresolved and composed of expanding
threads within the constant cross-section envelopes. Furthermore,
loop expansion is critical to the success or failure of hydrostatic
models in reproducing the observed intensities and morphology in
active region loops, bright points, and the full Sun. Hinode EIS and
XRT provide unprecedented spatial resolution at high temperatures
that can be used to reexamine the morphology of active region loops
and provide new insights. Here we present initial results from our
study of active region loop widths with EIS and XRT.
Title: Atomic oxygen photoionization rates computed with high
resolution cross sections and solar fluxes
Authors: Meier, R. R.; McLaughlin, Brendan M.; Warren, H. P.;
Bishop, James
Bibcode: 2007GeoRL..34.1104M
Altcode:
Accurate knowledge of photoionization rates is fundamental for the
study and understanding of gases in the solar system. Past calculations
of the photoionization rates of atmospheric gases lack the spectral
resolution to accommodate highly structured autoionization features in
the photoionization cross section. A new theoretical model of the atomic
oxygen photoionization cross section combined with a new solar minimum
spectral irradiance model enables calculations at very high spectral
resolution (0.001 nm). Our analysis of unattenuated photoionization
rates reveals no strong coincidences among myriad bright solar emission
lines and autoionization lines in the cross section. However, deeper
in the terrestrial atmosphere, opacity effects are significant and the
need for high spectral accuracy becomes increasingly important. In
our solar minimum example, factor of 3 errors occur when the lower
thermospheric photoionization rate of atomic oxygen is computed at 1
nm spectral resolution for both the cross section and solar flux.
Title: Soft X-ray irradiances during solar flares observed by
TIMED-SEE
Authors: Rodgers, E. M.; Bailey, S. M.; Warren, H. P.; Woods, T. N.;
Eparvier, F. G.
Bibcode: 2006JGRA..11110S13R
Altcode:
Observations from the Thermosphere Ionosphere Mesosphere Energetics
Dynamics-Solar Extreme Ultraviolet Experiment (TIMED-SEE) are
analyzed to determine the solar soft X-ray energy emission during a
solar flare. The TIMED-SEE X-ray ultraviolet Photometer System (XPS)
detectors utilize broadband photodiodes that observe from 0.1 to 27
nm. The XPS observed 29 flares of various strengths over a 6-month
period in 2002. We describe an emission measure technique to interpret
the broadband observations and determine a best fit flare spectrum. This
technique is applied to the 29 flares observed by the XPS. Our results
show that most of the enhancement in the solar spectrum during a
flare comes from the 0-2 nm wavelength range. We also show that the XPS
calculated 0.1-0.8 nm irradiance for brighter M-class and X-class flares
is in good agreement with the Geostationary Operational Environmental
Satellites (GOES) observed 0.1-0.8 nm irradiance. Also, the XPS 0-7
nm flare irradiance is well correlated with the 0.1-0.8 nm irradiance
observed by GOES. We show that the total 0-7 nm irradiance of smaller
X-class flares (X1-X5) calculated at the time of the XPS observation
provides 2-3 times the energy of the quiet Sun 0-7 nm irradiance.
Title: Using a Loss-of-Equilibrium CME Model to Predict X-Ray and
EUV Emissions Resulting From Solar Flares
Authors: Reeves, K. K.; Warren, H. P.; Forbes, T. G.
Bibcode: 2006IAUJD...3E..68R
Altcode:
In this study, we use a loss-of-equilibrium model for solar eruptions
to calculate the thermal energy input into a system of flare loops. In
this model, the flare consists of a system of reconnecting loops
below a current sheet that connects the flare to an erupting flux
rope. The thermal energy is calculated by assuming that all of the
Poynting flux into the current sheet is thermalized. The density,
temperature and velocity of the plasma in each reconnected loop
are then calculated using a 1D hydrodynamic code. These parameters
are coupled with the instrument response functions of various solar
instruments to calculate flare emissions. We simulate spectra from the
Bragg Crystal Spectrometer (BCS) on Yohkoh, and find that the strong
blueshifts that should be present due to chromospheric evaporation
during flare initiation are difficult to observe with BCS, but may be
better observed with a more sensitive instrument. We also find that a
density enhancement occurs at the top of a loop when evaporating plasma
fronts in each loop leg collide there. This enhancement gives rise to
bright loop-top intensities in simulated Transition Region and Coronal
Explorer (TRACE) and Yohkoh Soft X-ray Telescope (SXT) images. These
loop-top features have been observed in TRACE and SXT images, and are
not explained by single-loop flare models. The Atmospheric Imaging
Assembly (AIA) on the Solar Dynamics Observatory should be able to
observe these features in detail, and we will use this model to help
develop flare observing programs for AIA.
Title: Heating of the Solar Corona: Modeling the EUV/X-ray Emission
Authors: Brooks, D. H.; Warren, H. P.
Bibcode: 2006ESASP.617E..11B
Altcode: 2006soho...17E..11B
No abstract at ADS
Title: Hydrostatic Modeling of the Integrated Soft X-Ray and Extreme
Ultraviolet Emission in Solar Active Regions
Authors: Warren, Harry P.; Winebarger, Amy R.
Bibcode: 2006ApJ...645..711W
Altcode: 2006astro.ph..2052W
Many studies of the solar corona have shown that the observed X-ray
luminosity is well correlated with the total unsigned magnetic flux. In
this paper we present results from the extensive numerical modeling of
active regions observed with the Solar and Heliospheric Observatory
(SOHO) EUV Imaging Telescope (EIT), the Yohkoh Soft X-Ray Telescope
(SXT), and the SOHO Michelson Doppler Imager (MDI). We use potential
field extrapolations to compute magnetic field lines and populate
these field lines with solutions to the hydrostatic loop equations
assuming steady, uniform heating. Our volumetric heating rates are of
the form ɛH~B¯α/Lβ, where B¯ is
the magnetic field strength averaged along a field line and L is the
loop length. Comparisons between the observed and simulated emission
for 26 active regions suggest that coronal heating models that scale
as ɛH~B¯/L are in the closest agreement with the observed
emission at high temperatures. The field-braiding reconnection model
of Parker, for example, is consistent with our results. We find,
however, that the integrated intensities alone are insufficient to
uniquely determine the parameterization of the volumetric heating
rate. Visualizations of the emission are also needed. We also find
that there are significant discrepancies between our simulation results
and the lower temperature emission observed in the EIT channels.
Title: 3D Reconnection Simulations of Descending Coronal Voids
Authors: Linton, Mark; Longcope, D.; Warren, H.; McKenzie, D.
Bibcode: 2006SPD....37.0123L
Altcode: 2006BAAS...38R.219L
We will present simulations of a highly localized, finite durationburst
of 3D reconnection in a post-CME current sheet. Suchreconnection forms
a pair of 3D reconnected flux tubes piercing thecurrent sheet. These
tubes retract from the reconnection region,pushing their way through
the surrounding magnetic field to form apost-reconnection arcade below
the reconnection region. We willdiscuss how the evolution of these
reconnected flux tubes can formthe descending, post-eruption voids
which have been seen in thehigh corona by the Yohkoh, TRACE and LASCO
instruments. Wewill compare the velocities and deceleration profiles of
theobserved voids with those of the simulated reconnected flux tubes.We
will also show how the presence of multiple reconnectionregions in a
single current sheet affects the dynamics of thereconnected tubes.
Title: An Investigation into the Variability of Heating in a Solar
Active Region
Authors: Ugarte-Urra, Ignacio; Winebarger, Amy R.; Warren, Harry P.
Bibcode: 2006ApJ...643.1245U
Altcode:
Previous studies have indicated that both steady and impulsive heating
mechanisms play a role in active region heating. In this paper,
we present a study of 20 hours of soft X-ray and EUV observations
of solar active region NOAA AR 8731. We examine the evolution of six
representative loop structures that brighten and fade first from X-ray
images and subsequently from the EUV images. We determine their lifetime
and the delay between their appearance in the different filters. We
find that the lifetime in the EUV filters is much longer than expected
for a single cooling loop. We also notice that the delay in the loops'
appearance in the X-ray and EUV filters is proportional to the loop
length. We model one of the loops using a hydrodynamic model with both
impulsive and quasi-steady heating functions and find that neither of
these simple heating functions can well reproduce the observed loop
characteristics in both the X-ray and EUV images. Hence, although this
active region is dominated by variable emission and the characteristics
of the observed loops are qualitatively consistent with a cooling loop,
the timescale of the heating in this active region remains unknown.
Title: Using X-ray and EUV Active Region Observations to Constrain
the CoronalHeating Rate
Authors: Winebarger, Amy R.; Warren, H. P.
Bibcode: 2006SPD....37.1702W
Altcode: 2006BAAS...38..245W
One constraint on the coronal heating mechanism may come from the
observation that X-ray and EUV intensities of solar active regions
are well correlated with the total unsigned magnetic flux. In this
presentation, we discuss an extensive modeling effort using 26 active
regions observed with EIT, SXT, and MDI. For each active region, we use
potential field extrapolations of the photospheric field to estimate
the coronal field. We then populate the resulting field lines with
solutions to the hydrostatic loop equations assuming steady, uniform
heating rates of the form E B^alpha/L^beta. We compare the resulting
relationship between the simulated X-ray and EUV intensities and total
unsigned flux with the observed relationship. We find that solutions
with E B/L are the most consistent with the X-ray observations as well
as the EUV "moss" intensities, but no heating function well matches
the EUV loop intensities. Because static models are unable to address
the extended emission at lower (1-2 MK) temperatures, we suggest that
dynamical processes play a significant role in active region heating.
Title: Modeling High Resolution Flare Spectra Using Hydrodynamic
Simulations
Authors: Warren, Harry; Doschek, G.
Bibcode: 2006SPD....37.2702W
Altcode: 2006BAAS...38..253W
Understanding the hydrodynamic response of the solar atmosphere to
the release of energy during a flare has been a long standing problem
in solar physics. Early time-dependent hydrodynamic simulations were
able to reproduce the high temperatures and densities observed in solar
flares, but were not able to model the observations in any detail. For
example, these simulations could not account for the relatively slow
decay of the observed emission or the absence of blueshifts in high
spectral resolution line profiles at flare onset. We have found that by
representing the flare as a succession of independently heated filaments
it is possible to reproduce both the evolution of line intensity and
the shape of the line profile using hydrodynamic simulations. Here
we present detailed comparisons between our simulation results and
several flares observed with the Yohkoh Bragg Crystal Spectrometer
(BCS). Comparisons with 3D MHD simulations will also be discussed.
Title: The Magnetic Topology of Coronal Mass Ejection Sources
Authors: Ugarte-Urra, Ignacio; Warren, H. P.
Bibcode: 2006SPD....37.2205U
Altcode: 2006BAAS...38..249U
We present results from the study of the sources of 15 coronal mass
ejections (CME) observed with TRACE. These events are a subset of
the preliminary 48 catalogued CME events with sources that fall
within ±400 arcsec of disk center and were observed with TRACE, in
any of its UV or EUV bandpasses, during the period 1998--2003. The
evolution of the eruptions is analyzed in the context of the magnetic
topology given by a potential field extrapolation of a photospheric
magnetogram. The extrapolations are used to determine the presence of
nulls and quasi-separatrix layers. We discuss the results in the context
of the current CME models and their observational predictions. The
breakout model for CMEs, for example, requires the presence of a null
encompassing the sheared neutral line. Pre-eruption reconnection is
expected to take place at the null. We find magnetic nulls in many
of the events. For most of them, however, we do not see compelling
evidence for pre-eruption reconnection.This research was supported by
the NASA Living With a Star Program.
Title: Modeling of the EUV/Soft X-ray Emission in a Large Coronal
Bright Point
Authors: Brooks, David; Warren, H.
Bibcode: 2006SPD....37.0126B
Altcode: 2006BAAS...38R.219B
We use potential field extrapolations of SOHO/MDI and Kitt Peak Vacuum
Telescope high resolution magnetograms to model the magnetic field line
distribution in a large coronal bright point. We populate the field
lines with solutions to the hydrostatic loop equations and simulate the
emission observed at EUV and soft X-ray wavelengths. The results are
compared in detail to observations obtained by Yohkoh/SXT, SOHO/EIT,
and SOHO/CDS. Previous similar studies of larger active regions have
shown that relatively good agreement is obtained between the simulated
and observed soft X-ray images. The simulations, however, fail to
correctly model the loop emission and integrated intensities at EUV
wavelengths simultaneously. Our results support these conclusions; the
SXT images and integrated intensities agree well with the observations,
but the spatial distributions of the EUV line intensities are incorrect
even when the integrated intensities are correct. We further extend the
analysis to lines formed in the transition region. Here the simulated
intensities are too large by factors of 2--4 and there are differences
in the morphology of the network emission. We explore the consequences
for these comparisons of using loop expansion and different forms of
the heating function in the models. The techniques are also applied
to model the on-disk and off-limb intensities in the quiet corona.This
research was supported by the NASA Guest Investigator Program and the
Office of Naval Research.
Title: The Intercalibration of SOHO EIT, CDS-NIS, and TRACE
Authors: Brooks, David H.; Warren, Harry P.
Bibcode: 2006ApJS..164..202B
Altcode:
Using coordinated observations of a quiet coronal region, we study the
intercalibration of the CDS and EIT instruments on board the Solar and
Heliospheric Observatory (SOHO) and the Transition Region and Coronal
Explorer (TRACE). We derive the differential emission measure (DEM)
distribution from CDS spectral line intensities and convolve it with
EIT and TRACE temperature response functions, calculated with the
latest atomic data from the CHIANTI database, to predict count rates
in their observing channels. We examine different analysis methods and
briefly discuss some more advanced aspects of atomic modeling such as
the density dependence of the ionization fractions. We investigate the
implications for our study using data from the ADAS database. We find
that our CDS DEM can predict the TRACE and EIT 171 and 195 Å channel
count rates to within 25%. However, the accuracy of the predictions
depends on the ionization fractions and elemental abundances used. The
TRACE 284 Å and EIT 284 and 304 Å filter predictions do not agree
well with the observations, even after taking the contribution from
the optically thick He II 304 Å line to the TRACE 284 Å channel into
account. The different CDS DEM solutions we derive using different
ionization fractions produce fairly similar results: the majority of
the CDS line intensities used are reproduced to within 20% with only
around one-fifth reproduced to worse than 50%. However, the comparison
provides us with further clues with which to explain the discrepancies
found for some lines, and highlights the need for accurate equilibrium
ionization balance calculations even at low density.
Title: Photoionization Rate of Atomic Oxygen
Authors: Meier, R. R.; McLaughlin, B. M.; Warren, H. P.; Bishop, J.
Bibcode: 2006AGUSMSA23B..01M
Altcode:
Accurate knowledge of the photoionization rate of atomic oxygen
is important for the study and understanding of the ionospheres
and emission processes of terrestrial, planetary, and cometary
atmospheres. Past calculations of the photoionization rate have
been carried out at various spectral resolutions, but none were at
sufficiently high resolution to accommodate accidental resonances
between solar emission lines and highly structured auto-ionization
features in the photoionization cross section. A new version of the
NRLEUV solar spectral irradiance model (at solar minimum) and a new
model of the O photoionization cross section enable calculations at
very high spectral resolution. We find unattenuated photoionization
rates computed at 0.001 nm resolution are larger than those at moderate
resolution (0.1 nm) by amounts approaching 20%. Allowing for attenuation
in the terrestrial atmosphere, we find differences in photoionization
rates computed at high and moderate resolution to vary with altitude,
especially below 200 km where deviations of plus or minus 20% occur
between the two cases.
Title: Solar Flare Soft X-ray Irradiance and its Impact on the
Earth's Upper Atmosphere
Authors: Rodgers, E. M.; Bailey, S. M.; Warren, H. P.; Woods, T. N.;
Eparvier, F. G.
Bibcode: 2006AGUSMSA23B..03R
Altcode:
Solar flare soft X-ray irradiance provides a highly variable energy
source to the lower thermosphere. Observations from three NASA
satellite missions, the Thermosphere, Ionosphere, Mesosphere,
Energetics and Dynamics - Solar Extreme Ultraviolet Experiment
(TIMED-SEE), the Solar Radiation and Climate Experiment (SORCE) and
the Student Nitric Oxide Explorer (SNOE) are analyzed to determine
how solar soft X-ray irradiance varies during a solar flare and
how this irradiance affects the Earth's lower thermosphere. Solar
soft X-rays are one of the principal energy sources that lead to the
production of thermospheric nitric oxide (NO) through the dissociation
of odd-nitrogen. NO is an important source of radiative cooling in the
thermosphere and therefore performs an important role in the energy
balance. The XUV Photometer System (XPS) aboard TIMED-SEE and the XPS
aboard SORCE both include a suite of photodiode detectors that measure
the solar soft X-ray irradiance in broadband channels from 0.1 to 27
nm. The TIMED-SEE XPS observed 29 flares of various strengths over
a six month period in 2002 and the SORCE XPS observed several large
flares during the fall of 2003. An emission measure technique is used
to interpret the broadband observations and determine a solar flare
spectrum with a model that calculates theoretical spectra for input
differential emission measures (DEM). The DEMs are iterated until
the resulting spectrum reproduces the XPS observations. These solar
flare spectra are used to determine the soft X-ray energy input to
the Earth's lower thermosphere. Most of the solar flare soft X-ray
irradiance comes from the 1 - 2 nm range and is deposited near 106
km. The abundance of NO peaks near 106 km and responds dramatically to
energy deposited in the upper atmosphere. SNOE observed a significant
increase in thermospheric NO following the X17 solar flare on 28 October
2003. Analysis of solar flare NO density enhancements and a comparison
to results from a photochemical model will be presented along with a
description of the solar flare spectral analysis.
Title: Carrington Maps of the Upper Photosphere
Authors: Sheeley, N. R., Jr.; Warren, H. P.
Bibcode: 2006ApJ...641..611S
Altcode:
We have used images of the Sun's disk, obtained in the 6767 Å
continuum with the Michelson Doppler Interferometer (MDI) on the
Solar and Heliospheric Observatory (SOHO), to make Carrington
maps of the upper photosphere during the years 1996-2005. Each map
is constructed from observations near the limb where the continuum
radiation originates relatively high in the photosphere and faculae have
their greatest visibility. Consequently, the Carrington maps resemble
spectroheliograms in temperature-sensitive photospheric lines and show
the global distribution of faculae and all but the smallest sunspots
(which are obscured by overlying faculae). A time-lapse sequence of the
combined east-limb and west-limb maps shows the emergence of active
regions and the evolution of large-scale patterns of faculae with an
average temporal resolution of 14 days during the sunspot cycle. Also,
a longitudinally averaged butterfly diagram of these maps shows that in
each hemisphere there is a facula-free zone separating the old-cycle
polar field from trailing-polarity flux that is migrating poleward
from the sunspot belts. These facula-free zones coincide with the
neutral zones of the axisymmetric component of photospheric magnetic
field and their arrival at the poles in 2001 marks the reversal of the
polar fields. We think that this mapmaking technique can be applied
to white-light images obtained daily at the Mount Wilson Observatory
since 1905 and that the resulting Carrington maps will provide details
about the polar-field reversal process during past sunspot cycles when
high-quality magnetograms were unavailable.
Title: Multithread Hydrodynamic Modeling of a Solar Flare
Authors: Warren, Harry P.
Bibcode: 2006ApJ...637..522W
Altcode: 2005astro.ph..7328W
Past hydrodynamic simulations have been able to reproduce the high
temperatures and densities characteristic of solar flares. These
simulations, however, have not been able to account for the slow
decay of the observed flare emission or the absence of blueshifts
in high spectral resolution line profiles. Recent work has suggested
that modeling a flare as a sequence of independently heated threads
instead of as a single loop may resolve the discrepancies between
the simulations and observations. In this paper, we present a method
for computing multithread, time-dependent hydrodynamic simulations
of solar flares and apply it to observations of the Masuda flare of
1992 January 13. We show that it is possible to reproduce the temporal
evolution of high temperature thermal flare plasma observed with the
instruments on the GOES and Yohkoh satellites. The results from these
simulations suggest that the heating timescale for a individual thread
is on the order of 200 s. Significantly shorter heating timescales
(20 s) lead to very high temperatures and are inconsistent with the
emission observed by Yohkoh.
Title: NRLEUV 2: A new model of solar EUV irradiance variability
Authors: Warren, Harry P.
Bibcode: 2006AdSpR..37..359W
Altcode:
NRLEUV represents an independent approach to modeling the Sun's
EUV irradiance and its variability. Instead of relying on existing
irradiance observations, our model utilizes differential emission
measure distributions derived from spatially and spectrally resolved
solar observations, full-disk solar images, and a database of atomic
physics parameters to calculate the solar EUV irradiance. Recent
updates to the model include the calculation of a new quiet Sun
differential emission measure distribution using data from the CDS and
SUMER spectrometers on SOHO and the use of a more extensive database
of atomic physics parameters. Here, we present comparisons between
the NRLEUV quiet Sun reference spectrum and solar minimum irradiance
observations. Although there are many areas of agreement between
the modeled spectrum and the observations, there are some major
disagreements. The computed spectra cannot reproduce the observed
irradiances at wavelengths below about 160 Å. The observed irradiances
appear to overstate the magnitude of the EUV continua. We also present
some initial comparisons between the NRLEUV irradiance variability model
and TIMED/SEE data. We find that the NRLEUV model tends to overpredict
the absolute magnitude of the irradiance at many wavelengths. The
model also appears to underpredict the magnitude of the solar-cycle
and solar rotational variation in transition region emission lines.
Title: Observing the Solar atmosphere with the Extreme Ultraviolet
Imaging Spectrometer on Solar B
Authors: Korendyke, C. M.; Brown, C.; Dere, K.; Doschek, G.; Klimchuk,
J.; Landi, E.; Mariska, J.; Warren, H.; Lang, J.
Bibcode: 2005AGUFMSH41B1124K
Altcode:
The Extreme Ultraviolet Imaging Spectrometer (EIS) is part of the
instrument complement on the Solar B satellite, scheduled for launch
in the summer of 2006. The instrument has been calibrated and is
presently mounted on the spacecraft. EIS is the most sensitive EUV
solar spectrometer to be flown. The instrument is the first of a new
generation of two optical element, solar spectrographs. Preliminary
results from the laboratory focussing and calibration of the
instrument will be shown. The instrument wavelength coverage includes
reasonably bright spectral lines emitted by plasmas from 0.1 to 20 MK
in temperature. The wavelength range also provides coronal density
diagnostics. Temperature, density and velocity diagnostics will be
discussed. An example observing program for exploring active region
evolution and dynamics will be discussed.
Title: Chromospheric Evaporation in Solar Flares Revisited
Authors: Doschek, G. A.; Warren, H. P.
Bibcode: 2005ApJ...629.1150D
Altcode:
We investigate the initial stage of chromospheric evaporation in flares
using soft X-ray spectra obtained by the Bragg Crystal Spectrometer
(BCS) experiment on Yohkoh. We find that the centroid wavelength
of the Ca XIX line in spectra with the first detectable emission is
within about 8.5×10-4 Å of the rest wavelength, which
corresponds to a Doppler shift of no more than 80 km s-1. We
also determine the minimum detectable soft X-ray flare volume emission
measure from BCS Ca XIX flare spectra. We find that the minimum
detectable emission measured by BCS is produced by an X-ray flux
that is about equal to the peak intensity of a class A6 flare. These
results are difficult to reconcile with one-dimensional hydrodynamic
simulations of an impulsively heated flare loop, which predict large
Doppler shifts during the initial stage of the heating. Furthermore,
inspection of high spatial resolution TRACE images of flare plasma
indicate significant differences between the observed morphology and
the predictions of hydrodynamic models. The evolution of the intensity
and the Doppler shifts are more consistent with models that assume the
sequential heating of small-scale threads rather than the heating of an
individual loop. However, the bright knots of emission and asymmetrical
intensity distributions seen in flare images cannot be explained by
current numerical models of chromospheric evaporation.
Title: Global response of the low-latitude to midlatitude ionosphere
due to the Bastille Day flare
Authors: Huba, J. D.; Warren, H. P.; Joyce, G.; Pi, X.; Iijima, B.;
Coker, C.
Bibcode: 2005GeoRL..3215103H
Altcode:
The first global simulation study and comparison to data of the
ionospheric effects associated with the enhanced EUV irradiance of
the Bastille Day flare are presented. This is done by incorporating a
time-dependent EUV spectrum, based on data and hydrodynamic modeling,
into the NRL ionosphere model SAMI3. The simulation results indicate
that the total electron content (TEC) increases to over 7 TEC units in
the daytime, low-latitude ionosphere. In addition, it is predicted that
the maximum density in the F-layer (NmF2) increases by $\lesssim$20%
and that the height of the maximum electron density (HmF2) decreases by
$\lesssim$20%. These results are explained by the increased ionization
at altitudes <400 km which increases TEC and NmF2 while decreasing
HmF2. The results are in reasonably good agreement with data obtained
from GPS satellites and the TOPEX satellite.
Title: Cooling Active Region Loops Observed with SXT and TRACE
Authors: Winebarger, Amy R.; Warren, Harry P.
Bibcode: 2005ApJ...626..543W
Altcode: 2005astro.ph..2270W
An impulsive heating multiple strand (IHMS) model is able to
reproduce the observational characteristics of EUV (~1 MK) active
region loops. This model implies that some of the loops must reach
temperatures where X-ray filters are sensitive (>2.5 MK) before
they cool to EUV temperatures. Hence, some bright EUV loops must be
preceded by bright X-ray loops. Previous analyses of X-ray and EUV
active region observations, however, have concluded that EUV loops
are not the result of cooling X-ray loops. In this paper, we examine
two active regions observed in both X-ray and EUV filters and analyze
the evolution of five loops over several hours. These loops first
appear bright in the X-ray images and later appear bright in the EUV
images. The delay between the appearance of the loops in the X-ray and
EUV filters is as little as 1 hr and as much as 3 hr. All five loops
appear as single ``monolithic'' structures in the X-ray images but
are resolved into many smaller structures in the (higher resolution)
EUV images. The positions of the loops appear to shift during cooling,
implying that the magnetic field is changing as the loops evolve. There
is no correlation between the brightness of the loop in the X-ray and
EUV filters, meaning that a bright X-ray loop does not necessarily
cool to a bright EUV loop, and vice versa. The progression of the
loops from X-ray images to EUV images and the observed substructure
is qualitatively consistent with the IHMS model.
Title: Reconciling Hydrodynamic Simulations With Yohkoh and RHESSI
Observations of Solar Flares
Authors: Warren, H. P.
Bibcode: 2005AGUSMSP23B..07W
Altcode:
High spatial resolution TRACE observations provide compelling
observational evidence for small-scale filamentation in solar flares. In
this poster we present results from time-dependent hydrodynamic
simulations that treat a flare as a succession of independently
heated filaments. The energy deposited onto each filament and the
volume of each filament are derived from the observed GOES soft X-ray
fluxes. These numerical simulations are able to reproduce both the
evolution of the line intensity and the shape of the line profile
for the Yohkoh BCS Ca XIX and S XV lines. Of particular significance
is the fact that the simulated line profiles are always dominated
by the stationary component, consistent with observations. In this
model the strongly blueshifted emission evident during the initial
heating of a thread is largely masked by emission from threads that
have been heated previously and do not show bulk motions. In addition
to comparisons with Yohkoh we will also present detailed comparisons
between simulation results and RHESSI flare observations.
Title: Chromospheric Evaporation in Solar Flares Revisited
Authors: Doschek, G. A.; Warren, H. P.
Bibcode: 2005AGUSMSP52A..05D
Altcode:
We investigate the initial stage of chromospheric evaporation in
flares using soft X-ray flare spectra obtained by the Bragg Crystal
Spectrometer (BCS) experiment on Yohkoh. We determine the minimum
detectable soft X-ray flare volume emission measure from BCS Ca XIX
flare spectra. We find that the minimum detectable emission measure
by BCS is produced by an X-ray flux that is about equal to the peak
intensity of a class A5 flare. We also find that the centroid wavelength
of the Ca XIX line in spectra with the first detectable emission is
within about 8.5E-4 Angstroms of the rest wavelength, which is 80
km/s in terms of a Doppler shift. We interpret our results assuming
sequential chromospheric evaporation into a multi-threaded flare loop
envelop. Under this assumption, by comparing the BCS results with
images of flares from the Soft X-ray Telescope (SXT) on Yohkoh and
from TRACE, we have determined the minimum energy and electron density
of multi-million degree soft X-ray plasma that can be detected using
presently available spectroscopic X-ray data. In addition we consider
the implications of a multi-thread loop model on TRACE and Yohkoh flare
images, and the differences between the images and the multi-thread
predictions. We find that the multi-million degree flare plasma in TRACE
images frequently exhibits structures that do not resemble the images
of loops expected from the numerical simulations of evaporation. Thus,
while observational signatures of flare dynamics can be consistent
with chromospheric evaporation simulations, problems still remain in
understanding the loop morphology of the multi-million degree plasma.
Title: Are there two coronal heating mechanisms?
Authors: Winebarger, A. R.; Warren, H. P.
Bibcode: 2005AGUSMSP41A..03W
Altcode:
The source of coronal heating remains one of the most significant
unknowns in solar physics. In this poster, we present analysis of two
types of active region structures - relatively long loops that are
bright in EUV images and short, hot loops that are bright in X-ray
images. We compare the temporal evolution of these loops in multiple
filters to the evolution derived from hydrodynamic simulations with
various heating function to determine the most likely heating function
for each structure.
Title: A Solar Minimum Irradiance Spectrum for Wavelengths below
1200 Å
Authors: Warren, Harry P.
Bibcode: 2005ApJS..157..147W
Altcode:
NRLEUV represents an independent approach to modeling the Sun's EUV
irradiance and its variability. Our model utilizes differential emission
measure distributions derived from spatially and spectrally resolved
solar observations, full-disk solar images, and a database of atomic
physics parameters to calculate the solar EUV irradiance. In this paper
we present a new solar minimum irradiance spectrum for wavelengths
below 1200 Å. This spectrum is based on extensive observations of
the quiet Sun taken with the CDS and SUMER spectrometers on the Solar
and Heliospheric Observatory (SOHO) and the most recent version of
the CHIANTI atomic physics database. In general, we find excellent
agreement between this new irradiance spectrum and our previous
quiet-Sun reference spectrum derived primarily from Harvard Skylab
observations. Our analysis does show that the quiet-Sun emission
measure above about 1 MK declines more rapidly than in our earlier
emission measure distribution and that the intensities of the EUV
free-bound continua at some wavelengths are somewhat smaller than
indicated by the Harvard observations. Our new reference spectrum is
also generally consistent with recent irradiance observations taken
near solar minimum. There are, however, two areas of persistent
disagreement. Our solar spectrum indicates that the irradiance
measurements overestimate the contribution of the EUV free-bound
continua at some wavelengths by as much as a factor of 10. Our model
also cannot reproduce the observed irradiances at wavelengths below
about 150 Å. Comparisons with spectrally resolved solar and stellar
observations indicate that only a small fraction of the emission lines
in the 60-120 Å wavelength range are accounted for in CHIANTI.
Title: Reconciling Hydrodynamic Simulations with Spectroscopic
Observations of Solar Flares
Authors: Warren, Harry P.; Doschek, George A.
Bibcode: 2005ApJ...618L.157W
Altcode:
Chromospheric evaporation is a central element of current models
of solar flares. The high-velocity upflows that should accompany
evaporation, however, are rarely observed in high-resolution solar flare
spectra. Thus the absence of blueshifted line profiles represents a
significant discrepancy between the theory and observations of this
phenomenon. In this Letter we present an algorithm for computing
multiple-loop time-dependent hydrodynamic simulations of solar flares
using a minimum of assumptions. We show that these simulations can
accurately reproduce the Ca XIX and S XV line profiles observed with
the Bragg Crystal Spectrometer on Yohkoh during the earliest stages
of a flare. Since our model represents the flare as a succession
of independently heated threads, the strongly blueshifted emission
evident during the initial heating of a thread is largely masked by
emission from threads that have been heated previously and do not show
bulk motions.
Title: Impact of the Bastille Day Solar Flare on the Low- to
Mid-Latitude Ionosphere
Authors: Huba, J.; Warren, H.; Joyce, G.
Bibcode: 2004AGUFMSA21B0357H
Altcode:
We study the impact of the Bastille Day solar flare radiation on the
low- to mid-latitude ionosphere. The methodology is as follows. We
develop an EUV irradiance spectrum based upon observations for the
Bastille Day flare. Since solar irradiance observations typically do not
have the cadence necessary to follow the evolution of a flare, we have
developed techniques for computing flare spectra from the available
solar data. We then use this spectrum in the NRL three-dimensional
ionosphere model SAMI3 to obtain the global impact of the flare on
the mid- to low-latitude ionosphere. We assess the flare's impact
by comparing simulation results with and without the solar flare
enhanced EUV spectrum. A previous study using the NRL two dimensional
ionosphere model SAMI2 and a more simplistic EUV spectrum of the
Bastille Day storm found that flare radiation can increase the F-region
ionosphere density by up to 50% [Meier et al., Geophys. Res. Lett. 29,
10.1029/2001GL013956, 2002]. Research supported by ONR.
Title: The Origin of Postflare Loops
Authors: Sheeley, N. R., Jr.; Warren, H. P.; Wang, Y. -M.
Bibcode: 2004ApJ...616.1224S
Altcode:
We apply a tracking technique, previously developed to study motions
in the outer corona by Sheeley, Walters, Wang, and Howard, to 195 Å
filtergrams obtained with the Transition Region and Coronal Explorer
(TRACE) satellite and obtain height-time maps of the motions in the
hot (10-20 MK) plasma clouds above postflare loop systems. These
maps indicate the following two main characteristics. (1) Within the
plasma cloud, the motions are downward at speeds of approximately 4
km s-1. The cloud itself grows with time, its upper layers
being replenished by the arrival and deceleration of fast inflows and
its lower layers disappearing when they cool to form the tops of new
postflare loops. (2) Early in these events, the inward motions are
turbulent, showing a variety of dark elongated features resembling
``tadpoles'' and some bright features. Later, the inflows are visible
as dark collapsing loops, changing from initially cusp-shaped features
to rounder loops as they move inward. Their speeds initially lie in the
range 100-600 km s-1 but decrease to 4 km s-1
in about 3 minutes, corresponding to an average deceleration ~1500 m
s-2. Combining these observations with similar observations
obtained at reconnection sites in the outer corona by the Large Angle
Spectrometric Coronagraph (LASCO), we conclude that postflare loops are
the end result of the formation, filling, deceleration, and cooling
of magnetic loops produced by the reconnection of field lines blown
open in the flare. The formation of collapsing loops occurs in the
dark tadpoles; the filling of these initially dark loops occurs via
chromospheric evaporation, which also contributes to the deceleration
of the loops; and the radiative cooling ultimately resolves the loops
into sharply defined structures.
Title: Thermal and Nonthermal Emission in Solar Flares
Authors: Warren, Harry P.; Antiochos, Spiro K.
Bibcode: 2004ApJ...611L..49W
Altcode:
The observation that in many flares there is a linear correlation
between the peak soft X-ray emission and the time-integrated nonthermal
emission-the Neupert effect-indicates a strong link between particle
acceleration and chromospheric evaporation. In this Letter we consider
the hydrodynamic response of impulsively heated flare loops. We
find that the peak soft X-ray flux should scale approximately as
E1.75/V0.75L0.25, where E is the total
input energy, V is the flare volume, and L is the loop length. This
scaling is not consistent with the linear relationship implied by the
Neupert effect unless there are additional correlations between the
input energy and the other parameters of the flare.
Title: Can TRACE Extreme-Ultraviolet Observations of Cooling Coronal
Loops Be Used to Determine the Heating Parameters?
Authors: Winebarger, Amy R.; Warren, Harry P.
Bibcode: 2004ApJ...610L.129W
Altcode:
Recent analysis of relatively cool (~1 MK) active region loops observed
with TRACE has suggested that these loops have been heated impulsively
and are cooling through the TRACE bandpasses. In this Letter we
explore the evolution of cooling loops to determine if the TRACE EUV
observations can be used to determine the magnitude, duration, and
location of the energy release. We find that the evolution of the apex
density and temperature in an impulsively heated cooling loop depends
only on the total energy deposited (not the magnitude, duration,
or location of the energy deposition) after the loop cools past an
``equilibrium point,'' where the conductive and radiative cooling
times are comparable. Hence, observations must be made early in the
evolution of a loop to determine the heating parameters. Typical TRACE
observations of cooling loops do not provide adequate information to
discriminate between different heating scenarios.
Title: Evidence for Small-Scale Filamentation and Dynamics in the
Solar Corona
Authors: Warren, H.
Bibcode: 2004AAS...204.8601W
Altcode: 2004BAAS...36..818W
Observations with the Transition Region and Coronal Explorer (TRACE)
have revealed that the solar corona is both highly dynamic and highly
filamented. In this talk I will discuss how dynamics and filamentation
play an important role in explaining some of the observational
properties of the Sun's atmosphere. TRACE observations have shown,
for example, that many relatively cool ( ∼1 MK), long-lived
active region loops have density and temperature profiles that are
difficult to reconcile with static models. By modeling these loops
as a sequence of impulsively heated filaments, in contrast, it is
possible to account for the high densities, flat temperature profiles,
and the temporal evolution of these structures. A similar approach to
modeling the evolution of flare emission yields much better agreement
with observation than treating the flare as a single loop.
Title: Impact of Solar Flare Radiation on the Ionosphere
Authors: Warren, H.; Huba, J. D.; Joyce, G.
Bibcode: 2004AGUSMSA23A..07W
Altcode:
We study the impact of enhanced solar flare radiation on the low-
to mid-latitude ionosphere. The methodology is to develop an EUV
irradiance spectrum based upon observations that can be used in
the NRL ionosphere model SAMI3. Since solar irradiance observations
typically do not have the cadence necessary to follow the evolution
of a flare, we have developed techniques for computing flare spectra
from the available solar data. The initial simulation study will
use a generic flare radiation spectrum to test the technique and
develop a baseline understanding of the impact of flare radiation on
the ionosphere. Subsequent studies will ingest flare spectra based
on actual events and model results will be compared to observations
if available. A previous study of the Bastille Day storm found that
flare radiation can increase the F-region ionosphere density by up
to 50% [Meier et al., Geophys. Res. Lett. 29, 10.1029/2001GL013956,
2002]. Research supported by ONR.
Title: Thermal and Non-Thermal Emission in Two-Ribbon Flares
Authors: Warren, H.
Bibcode: 2004AAS...204.4716W
Altcode: 2004BAAS...36..741W
The observation that in many flares there is a good correlation between
the soft X-ray emission and the time-integrated non-thermal emission
--- the Neupert effect --- indicates a strong link between magnetic
reconnection and particle acceleration. We present hydrodynamic
simulations of flare loops heated by precipitating energetic
electrons. Instead of representing a flare as a single loop, we model
it as a succession of independently heated, small-scale filaments. We
find that to reproduce the observed thermal emission the energy in
the injected electrons must be proportional to the soft X-ray flux,
not the derivative of the soft X-ray flux as suggested by the Neupert
effect. Comparisons between the simulations and GOES and RHESSI
observations indicates that there is not sufficient energy in the
non-thermal electrons to account for the thermal emission observed
in a large, long duration flare. This suggests that there must be in
situ heating of coronal plasma as well as particle acceleration during
magnetic reconnection.
Title: Solar Extreme Ultraviolet and X-ray Irradiance Variations
Authors: Woods, Tom; Acton, Loren W.; Bailey, Scott; Eparvier, Frank;
Garcia, Howard; Judge, Darrell; Lean, Judith; Mariska, John T.;
McMullin, Don; Schmidtke, Gerhard; Solomon, Stanley C.; Tobiska,
W. Kent; Warren, Harry P.; Viereck, Rodney
Bibcode: 2004GMS...141..127W
Altcode:
The solar extreme ultraviolet (EUV) radiation at wavelengths shortward
of 120 nm is a primary energy source for planetary atmospheres and
is also a tool for remote sensing of the planets. For such aeronomic
studies, accurate values of the solar EUV irradiance are needed over
time periods of minutes to decades. There has been a variety of solar
EUV irradiance measurements since the 1960s, but most of the recent
observations have been broadband measurements in the X-ray ultraviolet
(XUV) at wavelengths shortward of 35 nm. A summary of the solar EUV
irradiance measurements and their variability during the last decade is
presented. One of the most significant new solar irradiance results is
the possibility that the irradiance below 20 nm is as much as a factor
of 4 higher than the reference Atmospheric Explorer E (AE-E) spectra
established in the 1970s and 1980s. The primary short-term irradiance
variability is caused by the solar rotation, which has a mean period
of 27 days. The primary long-term variability is related to the solar
dynamo and is known best by the 11-year sunspot cycle. The solar cycle
variability as a function of wavelength can be characterized as 20% to
70% between 120 and 65 nm and as a factor of 1.5 to 10 between 65 and 1
nm. The variability of the total solar EUV irradiance, integrated from
0 to 120 nm, is estimated to be 30-40% for a large 27-day rotational
period and a factor of about 2 for the 11-year solar cycle during the
recent, rather active, solar cycles.
Title: NRLEUV 2: A New Model of Solar EUV Irradiance Variability
Authors: Warren, H.; Mariska, J.
Bibcode: 2004cosp...35.1109W
Altcode: 2004cosp.meet.1109W
NRLEUV represents an independent approach to modeling the Sun's
EUV irradiance and its variability. Our model utilizes differential
emission measure distributions derived from spatially and spectrally
resolved solar observations, full-disk solar images, and an a database
of atomic physics parameters to calculate the solar EUV irradiance. Our
initial version of the model made use of Skylab spectra and a crude
partitioning of solar features into quiet Sun, coronal hole, and active
region components. Despite the simplicity of our initial effort, our
model was able to reproduce the observed EUV irradiance variability
at many wavelengths as well as most existing models based directly on
observations. In this presentation we will discuss a revised version of
the model that is based on extensive observations with the spectrometers
on SOHO, utilizes a continuous distribution of emission measures, and
includes the most comprehensive database of atomic physics parameters
available. Comparisons between our model, other empirical irradiance
models, and recent irradiance observations will also be discussed.
Title: The Magnetic Corona: Magnetic Reconnection in Solar Flares
Authors: Warren, H. P.
Bibcode: 2004IAUS..219...91W
Altcode: 2003IAUS..219E.149W
No abstract at ADS
Title: Density and Temperature Measurements in a Solar Active Region
Authors: Warren, Harry P.; Winebarger, Amy R.
Bibcode: 2003ApJ...596L.113W
Altcode:
We present electron density and temperature measurements from an
active region observed above the limb with the Solar Ultraviolet
Measurements of Emitted Radiation spectrometer on the Solar and
Heliospheric Observatory. Density-sensitive line ratios from Si VIII
and S X indicate densities greater than 108 cm-3
as high as 200" (or 145 Mm) above the limb. At these heights, static,
uniformly heated loop models predict densities close to 107
cm-3. Differential emission measure analysis shows that
the observed plasma is nearly isothermal with a mean temperature of
about 1.5 MK and a dispersion of about 0.2 MK. Both the differential
emission measure and the Si XI/Si VIII line ratios indicate only
small variations in the temperature at the heights observed. These
measurements confirm recent observations from the Transition Region and
Coronal Explorer of ``overdense'' plasma at temperatures near 1 MK in
solar active regions. Time-dependent hydrodynamic simulations suggest
that impulsive heating models can account for the large densities,
but they have a difficult time reproducing the narrow range of observed
temperatures. The observations of overdense, nearly isothermal plasma
in the solar corona provide a significant challenge to theories of
coronal heating.
Title: Evolving Active Region Loops Observed with the Transition
Region and Coronal Explorer. I. Observations
Authors: Winebarger, Amy R.; Warren, Harry P.; Seaton, Daniel B.
Bibcode: 2003ApJ...593.1164W
Altcode:
Observations made with TRACE have detected a class of persistent active
region loops that have flat 195/171 Å filter ratios. The intensity of
these loops implies a density that is as much as 3 orders of magnitude
larger than the densities of static solutions to the hydrodynamic
equations. It has recently been suggested that these loops are
bundles of impulsively heated strands that are cooling through the
TRACE passbands. This scenario implies that the loops would appear in
the hotter (Fe XV 284 Å or Fe XII 195 Å) TRACE filter images before
appearing in the cooler (Fe IX/X 171 Å) TRACE filter images. In this
paper, we test this hypothesis by examining the temporal evolution
of five active region loops in multiple TRACE EUV filter images. We
find that all the loops appear in the hotter filter images before
appearing in cooler filter images. We then use the measured delay
to estimate a cooling time and find that four of the five loops have
lifetimes greater than the expected lifetime of a cooling loop. These
results are consistent with the hypothesis that each apparent loop
is a bundle of sequentially heated strands; other explanations will
also be discussed. To facilitate comparisons between these loops and
hydrodynamic simulations, we use a new technique to estimate the loop
length and geometry.
Title: Evolving Active Region Loops Observed with the Transition
Region and Coronal explorer. II. Time-dependent Hydrodynamic
Simulations
Authors: Warren, Harry P.; Winebarger, Amy R.; Mariska, John T.
Bibcode: 2003ApJ...593.1174W
Altcode:
Observations with the Transition Region and Coronal Explorer (TRACE)
have revealed a new class of active region loops. These loops have
relatively flat filter ratios, suggesting approximately constant
temperatures near 1 MK along much of the loop length. The observed
apex intensities are also higher than static, uniformly heated loop
models predict. These loops appear to persist for much longer than
a characteristic cooling time. Recent analysis has indicated that
these loops first appear in the hotter Fe XV 284 Å or Fe XII 195
Å filters before they appear in the Fe IX/Fe X 171 Å filter. The
delay between the appearance of the loops in the different filters
suggests that the loops are impulsively heated and are cooling when
they are imaged with TRACE. In this paper we present time-dependent
hydrodynamic modeling of an evolving active region loop observed with
TRACE. We find that by modeling the loop as a set of small-scale,
impulsively heated filaments we can generally reproduce the spatial
and temporal properties of the observed loop. These results suggest
that both dynamics and filamentation are crucial to understanding the
observed properties of active region loops observed with TRACE.
Title: Evolving Active Region Loops Observed With TRACE
Authors: Warren, H. P.; Winebarger, A. R.; Mariska, J. T.
Bibcode: 2003SPD....34.1007W
Altcode: 2003BAAS...35Q.826W
Recent observations with TRACE have revealed a new class of active
region loops with very interesting properties. These loops have
relatively flat filter ratios, suggesting approximately constant
temperatures along much of the loop length, and large densities relative
to the predictions of static loop models. Recent analysis has indicated
that these loops first appear in the hotter filters before they appear
in the cooler filters. The delay between the appearance of the loops in
the different filters suggests that the loops are impulsively heated and
are cooling when they are imaged with TRACE. In this paper we present
time-dependent hydrodynamic modeling of evolving active region loops
observed with TRACE. We find that by modeling the loops as small-scale,
impulsively heated filaments we can generally reproduce the spatial
and temporal properties of the observations. These results suggest
that both dynamics and filamentation are crucial to understanding the
observed properties of active region loops observed with TRACE. This research has been funded by the NASA SR&T and Sun-Earth
Connection Guest Investigator programs.
Title: A New Method to Constrain the Iron Abundance from Cooling
Delays in Coronal Loops
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Winebarger,
Amy R.; Warren, Harry P.
Bibcode: 2003ApJ...588L..49A
Altcode: 2003astro.ph..9506A
Recent observations with the Transition Region and Coronal Explorer
reveal that the time delay between the appearance of a cooling loop in
different EUV temperature filters is proportional to the loop length,
Δt12~L. We model this cooling delay in terms of radiative
loss and confirm this linear relationship theoretically. We derive an
expression that can be used to constrain the coronal iron enhancement
αFe=AcorFe/AphFe
relative to the photospheric value as function of the cooling delay
Δt12, flux F2, loop width w, and filling factor
qw<=1. With this relation, we find upper limits on
the iron abundance enhancement of αFe<=4.8+/-1.7 for
10 small-scale nanoflare loops, and αFe<=1.4+/-0.4
for five large-scale loops, in the temperature range of T~1.0-1.4
MK. This result supports the previous finding that low first ionization
potential elements, including Fe, are enhanced in the corona. The
same relation constitutes also a lower limit for the filling factor,
which is qw>=0.2+/-0.1 and qw>=0.8+/-0.2
for the two groups of coronal loops.
Title: Observing the Dynamic Corona: Diagnostics to Determine
Coronal Heating
Authors: Winebarger, A. R.; Warren, H. P.; Mariska, J. T.
Bibcode: 2003SPD....34.1008W
Altcode: 2003BAAS...35R.826W
High resolution observations made with TRACE have uncovered a dynamic
solar corona. Many of these observations indicate that TRACE is
imaging cooling loops (i.e., the loop appears in the TRACE filter
sensitive to hotter plasma before it appears in the cooler TRACE
filters.) Significant information can be garnered from the TRACE
observations. For instance, the delay between the appearance of loop
in different filters provides information on the cooling time of the
plasma and, hence, the total energy deposited in the loop. The cooling
time also indicates a density and hence can be used to determine the
degree of filamentation within a loop. TRACE observations of cooling
loops, however, tell us little about the spatial and temporal scales
for energy deposition. In this talk, we will discuss other diagnostics
necessary to pinpoint the magnitude, duration, and location of the
heating. The purpose of this talk is to establish the criteria of the
necessary spatial, spectral, and temperature resolution necessary to
discriminate between the coronal heating theories.
Title: A New Method to Constrain the Iron Abundance from Cooling
Delays in Coronal Loops
Authors: Aschwanden, M. J.; Schrijver, C. J.; Winebarger, A. R.;
Warren, H. P.
Bibcode: 2003SPD....34.1701A
Altcode: 2003BAAS...35..837A
Recent observations with TRACE reveal that the time delay between
the appearance of a cooling loop in different EUV temperature
filters is proportional to the loop length, dt12
∼ L . We model this cooling delay in terms of radiative loss
and confirm this linear relationship theoretically. We derive an
expression that can be used to constrain the coronal iron enhancement
AFe=AFecor/AFePh
relative to the photospheric value as function of the cooling delay
dt12, flux F2, loop width w, and filling factor
qw < 1. With this relation we find upper limits on
the iron abundance enhancement of AFe < 4.8 +/- 1.7
for 10 small-scale nanoflare loops, and AFe < 1.4 +/-
0.4 for 5 large-scale loops, in the temperature range of T ∼ 1.0-1.4
MK. This result supports the previous finding that low-FIP elements,
including Fe, are enhanced in the corona. The same relation constitutes
also a lower limit for the filling factor, which is qw >
0.2 +/- 0.1 and qw > 0.8 +/- 0.2 for the two groups of
coronal loops.
Title: Magnetic Modulation of Solar 304 Å Irradiance
Authors: Lean, J. L.; Mariska, J. T.; Warren, H. P.; Woods, T. N.;
Eparvier, F. G.; McMullin, D. R.; Judge, D. L.; Newmark, J. S.;
Viereck, R. A.
Bibcode: 2003SPD....34.1902L
Altcode: 2003BAAS...35..842L
Solar 304 Å irradiance is an important source of heating and ionization
in the Earth's upper atmosphere. Because only intermittent observations
exist prior to solar cycle 23, the absolute levels and solar cycle
variability of 304 Å irradiance are uncertain by a factor of two,
based on the range of estimates from four current EUV irradiance
variability models. Large active regions are a significant source of
304 Å radiation but their characteristics are not well specified,
with contrasts reported in the range of two to ten. Statistical
quantification of the role of small scale active regions and network
is also lacking. During solar cycle 23, three different instruments
are observing the Sun's 304 Å radiation concurrently. The EIT on SOHO
records the brightness distribution on the solar disk in a 20 Å band,
SEM on SOHO monitors the disk-integrated emission in an 80 Å band,
and the SEE grating spectrometer on the TIMED spacecraft recently began
observing EUV irradiance spectra with 4 Å resolution. We calculate
daily histograms of the brightness distributions of EIT images after
adjustments for various instrumental effects. Deconstructions of the
histograms permit statistical characterizations of magnetic sources
of 304 Å irradiance variability during solar cycle 23, in terms
of fractional disk areas and contrasts. We also study center-to-limb
variations. The calculations provide independent irradiance variability
estimates for comparison with the SEM and SEE direct irradiance
observations, and the models. We utilize the source characterizations
to revise the NRLEUV model, the present version of which estimates 304
Å emission by assuming that a bright active region has a contrast of
ten, and that source region evolution is temporally similar to the Mg
chromospheric irradiance index. Funded by NASA SEC GI Program.
Title: Transition Region and Coronal Explorer and Soft X-Ray Telescope
Active Region Loop Observations: Comparisons with Static Solutions
of the Hydrodynamic Equations
Authors: Winebarger, Amy R.; Warren, Harry P.; Mariska, John T.
Bibcode: 2003ApJ...587..439W
Altcode:
Active region coronal loop observations with broadband X-ray instruments
have often been found to be consistent with the predictions of static
loop models. Recent observations in the EUV, however, have discovered
a class of active region loops that are difficult to reconcile with
static loop models. In this paper, we take a comprehensive look at
how coronal loops compare with static models. We select 67 loops with
a large range of apex temperatures and half-lengths observed with
either the Transition Region and Coronal Explorer or the Soft X-Ray
Telescope. We compare these observations to static loop models using
both uniform and nonuniform heating. We find that only 2 of the 67
loops are fully consistent with static solutions with uniform heating
and a filling factor of unity. We further find that long, cool (<3
MK) loops are as much as 2500 times ``overdense,'' while short, hot
(>3 MK) loops are as much as 63 times ``underdense'' when compared
to the static solutions with uniform heating. We then consider the
possibility that the disparity in the density could be due to steady,
nonuniform heating along the loop and find that footpoint heating can
increase densities only by a factor of 3 over density solutions with
uniform heating while loop-top heating results in density solutions
that are, at most, a factor of 2.5 smaller than the density solutions
with uniform heating. Only 19 of the 67 loops in this data set could be
fully consistent with hydrodynamic solutions with steady heating. Hence,
we conclude that static loop models are poor representations of most
active region loops.
Title: A new model of solar EUV irradiance variability 2. Comparisons
with empirical models and observations and implications for space
weather
Authors: Lean, J. L.; Warren, H. P.; Mariska, J. T.; Bishop, J.
Bibcode: 2003JGRA..108.1059L
Altcode:
Motivated by the need for reliable specification of the Sun's
electromagnetic radiation in the extreme ultraviolet (EUV)
spectrum, we have developed a new model of solar EUV irradiance
variability at wavelengths from 50 to 1200 Å. Solar images are
used to quantify changes in the sources of EUV irradiance during the
solar cycle. Optically thin EUV emission line fluxes are estimated
from differential emission measures (DEMs) that characterize the
properties of the solar atmosphere in the source regions, while fluxes
for optically thick lines are modeled directly by specifying the source
region contrasts. We compare the new model, NRLEUV, with three different
empirical models of solar EUV irradiance since 1975. For solar cycles
21 and 22, NRLEUV predicts overall lower EUV irradiances and smaller
solar cycle variability than the empirical models. The average total
EUV energy at wavelengths from 50 to 1050 Å is 2.9 mW m-2,
smaller than the HFG, EUVAC, and SOLAR2000 models for which average
energies are 3.7, 4.3, and 5.6 mW m-2, respectively. These
differences have distinct wavelength dependencies. The solar cycle
variation in total EUV energy is 1.9 for NRLEUV compared with 2.7, 2.9,
and 2.3 for HFG, EUVAC, and SOLAR2000. Here, too, the differences are
wavelength dependent. We compare both the NRLEUV and the empirically
modeled EUV irradiances with selected wavelength bands and emission
lines measured during 4 years in cycle 21 by Atmospheric Explorer-E
(AE-E) and two broad bands at 170-200 and 260-340 Å measured in
cycle 23 by the Solar X-Ray Photometer (SXP) on the Student Nitric
Oxide Experiment (SNOE) and the Solar EUV Monitor (SEM) on the Solar
and Heliospheric Observatory (SOHO), respectively. The NRLEUV model
reproduces the variations observed during solar rotation better than, or
as well as, the empirical models. Comparisons of solar cycle variations
are more ambiguous because undetected instrumental drifts can cause
spurious trends in the observations over these longer timescales. Drifts
in the AE-E instruments may explain why the HFG and EUVAC models, which
are based on parameterizations of these data, have larger solar cycle
variations than NRLEUV. We assess the implications for space weather
of the significant differences among the modeled EUV irradiances by
using the Atmospheric Ultraviolet Radiance Integrated Code (AURIC)
to quantify corresponding differences in upper atmosphere energy
deposition and photoionization rates.
Title: The Energy Release Process in Solar Flares; Constraints from
TRACE Observations
Authors: Fletcher, L.; Warren, H. P.
Bibcode: 2003LNP...612...58F
Altcode: 2003ecpa.conf...58F
The Transition Region And Coronal Explorer Satellite, TRACE, launched
in 1998, has proved a valuable tool in the study of solar flares. UV
and EUV observations of the impulsive and gradual phases of many tens
of flares have been made. TRACE's excellent spatial resolution and
image cadence on the order of one second allow the rearrangement of the
magnetic field to be tracked in some detail. The combination of these
observations with data from other instruments, and with magnetic field
reconstructions, have provided strong evidence for (a) UV emission as a
beam proxy in the impulsive phase (b) long duration coronal heating in
the gradual phase (c) very complex and varied magnetic geometries. We
review the observational evidence for the above, discussing implications
for energy release.
Title: Extreme Ultraviolet Variability of the Large Solar Flare on
April 21, 2002 and the Terrestrial Photoelectron Response
Authors: Eparvier, F. G.; Woods, T. N.; Bailey, S. M.; Peterson,
W. K.; Solomon, S. C.; Garcia, H.; Lean, J. L.; Warren, H. P.; Carlson,
C. W.; McFadden, J. P.
Bibcode: 2002AGUFMSA21B0431E
Altcode:
The near-simultaneous observations of the solar extreme ultraviolet
(EUV) irradiance and terrestrial photoelectron distribution during and
after the large solar flare on April 21, 2002 provide for a distinctive
study of the effects that a solar flare can have on Earth's upper
atmosphere. The solar EUV irradiance from 0.1-195 nm was measured by
the Solar EUV Experiment (SEE) aboard the NASA Thermosphere, Ionosphere,
Mesosphere, Energetics, and Dynamics (TIMED) satellite. The terrestrial
photoelectron distribution from 50-1000 eV was measured by the Fast
Auroral Snapshot (FAST) energetic electron sensor. The variations of
the solar EUV irradiance from the X class flare at ~2 UT on April 21,
2002 range from more than a factor of 8 for the X-ray emissions to less
than 10% at longer EUV wavelengths. The spectral shape of this flare
is similar to that predicted for the Bastille Day 2000 flare. Most of
the solar irradiance variation is in the X-ray range and for coronal
emissions. The photoelectron distribution changed by a factor of
about 10 for the high-energy Auger electrons and by very little for the
low-energy thermal electrons. Modeling of the photoelectron distribution
using the measured solar EUV irradiance will also be presented.
Title: Overview of the SDO Extreme ultraviolet Variability Experiment
(EVE)
Authors: Woods, T. N.; Eparvier, F. G.; Rottman, G. J.; Judge,
D. L.; McMullin, D. R.; Lean, J. L.; Mariska, J. T.; Warren, H. P.;
Berthiaume, G. D.; Bailey, S. M.; Viereck, R. A.; Tobiska, W. K.;
Fuller-Rowell, T. J.; Sojka, J. J.
Bibcode: 2002AGUFMSH21C..02W
Altcode:
The NASA Solar Dynamics Observatory (SDO), with its launch in 2007, is
the first mission for the NASA Living With a Star (LWS) program. The
SDO mission will provide measurements and modeling of the solar
radiation and dynamics that can disturb Earth's environment. The
Extreme ultraviolet Variability Experiment (EVE) is one of the
three instrument suites on SDO. The EVE measures the solar extreme
ultraviolet (EUV) irradiance with unprecedented spectral resolution,
temporal cadence, accuracy, and precision. Furthermore, the EVE program
will incorporate physics-based modeling to advance the understanding
of the solar EUV irradiance variations based on the activity of solar
magnetic features. The EVE instrument consists of three subsystems. The
Multiple EUV Grating Spectrograph (MEGS) measures the 4-120 nm spectral
irradiance with 0.1 nm spectral resolution. The Optics Free Spectrometer
(OFS), being ionization cells, provides daily, in-flight calibrations
for the MEGS channels. The EUV Spectrophotometer (ESP) completes the
spectral coverage at 0.1-5 nm and 119-125 nm and provides additional
MEGS calibrations. Collectively, the EVE instrument measures the
solar EUV irradiance from 0.1 to 125 nm with 7% accuracy and 4%
long-term precision.
Title: Hydrodynamic Modeling of Active Region Loops
Authors: Warren, Harry P.; Winebarger, Amy R.; Hamilton, Paul S.
Bibcode: 2002ApJ...579L..41W
Altcode:
Recent observations with the Transition Region and Coronal Explorer
(TRACE) have shown that many apparently cool (Te~1-1.5 MK)
active region loops are much brighter and have flatter temperature
profiles than static loop models predict. Observations also indicate
that these loops can persist much longer than a characteristic cooling
time. Using time-dependent hydrodynamic simulations, we explore the
possibility that these active region loops are actually a collection
of small-scale filaments that have been impulsively heated and
are cooling through the TRACE 171 Å (Fe IX/X) and 195 Å (Fe XII)
bandpasses. We find that an ensemble of independently heated filaments
can be significantly brighter than a static uniformly heated loop and
would have a flat filter ratio temperature when observed with TRACE.
Title: Modeling the Cooling of Postflare Loops
Authors: Reeves, Katharine K.; Warren, Harry P.
Bibcode: 2002ApJ...578..590R
Altcode:
We present a model for the cooling of postflare loops. In our model, we
form an arcade that consists of hundreds of loops with offset formation
times to simulate a rising reconnection site. An initial temperature and
density is assumed in each loop, and then the scaling laws of Cargill,
Mariska, & Antiochos are used to determine the evolution of the
temperature and density in the loop. Once these quantities are found,
they are passed through the instrument response functions for TRACE
and the Yohkoh Soft X-Ray Telescope (SXT) to obtain intensities, which
are integrated over the arcade to give a simulated light curve. This
light curve is then compared to observed light curves from the 2000
July 14 X6 flare. We find that this multiloop, multithermal approach
to simulating the flare cooling fits the observed data much better
than a single-loop model. There are some discrepancies between our
simulations and the observed data in the decay phase of the flare,
however, which may be due to residual late-phase heating. We also
find that the temperatures calculated by using SXT filter ratios
are generally lower than the initial loop temperatures needed in the
simulation to give a good fit to the observed data.
Title: Temperature and Density Measurements in a Quiet Coronal
Streamer
Authors: Warren, Harry P.; Warshall, Andrew D.
Bibcode: 2002ApJ...571..999W
Altcode:
Many previous studies have used emission line or broadband filter
ratios to infer the presence of temperature gradients in the quiet
solar corona. Recently it has been suggested that these temperature
gradients are not real, but result from the superposition of isothermal
loops with different temperatures and density scale heights along
the line of sight. A model describing this hydrostatic weighting
bias has been developed by Aschwanden & Acton. In this paper we
present the application of the Aschwanden & Acton differential
emission measure model to Solar and Heliospheric Observatory Solar
Ultraviolet Measurement of Emitted Radiation (SUMER) observations of
a quiet coronal streamer. Simultaneous Yohkoh soft X-ray telescope
(SXT) observations show increases in the filter ratios with height
above the limb, indicating an increase in temperature. The application
of the Aschwanden & Acton model to these SUMER data, however, show
that the temperature is constant with height and that the distribution
of temperatures in the corona is much too narrow for the hydrostatic
weighting bias to have any effect on the SXT filter ratios. We consider
the possibility that there is a tenuous hot component (~3 MK) that
accounts for the SXT observations. We find that a hot plasma with an
emission measure sufficient to reproduce the observed SXT fluxes would
also produce significant count rates in the high-temperature emission
lines in the SUMER wavelength range. These lines are not observed,
and we conclude that the SUMER spectra are not consistent with the SXT
filter ratio temperatures. Calculations from a hydrodynamic loop model
suggest that nonuniform footpoint heating may be consistent with the
temperatures and densities observed at most heights, consistent with
the recent analysis of relatively cool (~1 MK) active region loops. We
also find, however, that at the lowest heights the observed densities
are smaller than those predicted by uniform or footpoint heating.
Title: TRACE Active Region Loops: Observation and Modeling
Authors: Hamilton, P. S.; Warren, H. P.; DeLuca, E. E.; Boyd, J. F.
Bibcode: 2002AAS...200.0210H
Altcode: 2002BAAS...34..641H
Recent Transition Region and Coronal Explorer (TRACE) observations have
detected active region coronal loops that can not be easily modeled
using hydrostatic models. Analysis of these loops suggests that they
are overdense relative to the predictions of hydrodynamic models with
uniform heating. This modeling, however, assumes that the observed
emission is near 1 MK. Since the TRACE filter ratios are actually
multivalued and high-temperature, uniformly heated models are difficult
to exclude based on the TRACE data alone. Using co-aligned CDS rasters
of overdense TRACE loops we find that these loops contain material at
105.9 K to 106.2 K. From these rasters we perform
a DEM analysis to constrain the input parameters to time-dependent
hydrodynamic models of these loops. TRACE is supported by contract
NAS5-38099 from NASA to LMATC.
Title: Ionospheric and dayglow responses to the radiative phase of
the Bastille Day flare
Authors: Meier, R. R.; Warren, H. P.; Nicholas, A. C.; Bishop, J.;
Huba, J. D.; Drob, D. P.; Lean, J. L.; Picone, J. M.; Mariska, J. T.;
Joyce, G.; Judge, D. L.; Thonnard, S. E.; Dymond, K. F.; Budzien, S. A.
Bibcode: 2002GeoRL..29.1461M
Altcode: 2002GeoRL..29j..99M
The Sun's Bastille Day flare on July 14, 2000 produced a variety of
geoeffective events. This solar eruption consisted of an X-class flare
followed by a coronal mass ejection that produced a major geomagnetic
storm. We have undertaken a study of this event beginning with an
analysis of the effects of the radiative phase of the flare on the
dayglow and the ionosphere. The key new enabling work is a novel
method of evaluating the X-ray and extreme ultraviolet (EUV) solar
spectral irradiance changes associated with the flare. We find that
the solar radiative output enhancements modeled during the flare are
consistent with measurements of both solar EUV irradiance and far UV
Earth thermospheric dayglow. We use the SAMI2 model to predict global
ionospheric changes along a magnetic meridian that show significantly
different northern and southern effects, suggesting that flares can
be used to study ionospheric dynamics.
Title: Temperature and Density Measurements in a Quiet Coronal
Streamer
Authors: Warren, H. P.; Warshall, A. D.
Bibcode: 2002AAS...200.1602W
Altcode: 2002BAAS...34Q.667W
Recent observations with TRACE have revealed a new class of relatively
cool ( ~1 MK) active region loops that have flat temperature
profiles and are much denser than would be expected from hydrostatic
equilibrium. Hydrodynamic loop modeling shows that non-uniform,
footpoint heating is consistent with the observed properties of some
of these loops. In this paper we present temperature and density
measurements derived from SUMER/SoHO observations of a quiet coronal
streamer. The streamer is nearly isothermal at all observed heights
(50--200 Mm above the limb), indicating that the streamer is composed
of loops with very flat temperature profiles. We also find that the
densities measured at most heights are larger than would be expected
from hydrostatic equilibrium. Loop models with non-uniform footpoint
heating can reproduce the observed properties of this streamer at most
heights. This work has been supported by NAG5-11625 and NAG5-10786.
Title: Origins of the Solar Wind
Authors: Warren, Harry
Bibcode: 2002STIN...0251137W
Altcode:
This paper presented the differential emission measure analysis of SUMER
observations of a coroner streamer. We found that: The coroner streamer
is isothermal at all heights. This suggests that the loops comprising
the streamer must have very flat temperature profiles. The coroner
streamer is "overdense" relative to the predictions of hydrostatic
equilibrium at most heights. At the lowest heights the streamer is
actually "underdense". The SUMER temperature measurements are not
consistent with those derived from simultaneous SXT observations. SXT
indicates systematically higher temperatures as well as a strong
temperature gradient. These SUMER measurements yield somewhat lower
temperatures and no gradient in the temperature with height. Previous
work has suggested that there may be a hot component to the streamer
that is preferentially observed with SXT. Our analysis shows that high
temperature emission lines would be observed with SUMER if this were
true and thus discounts this possibility. We suggested that scattered
light in SXT might produce spurious temperature measurements. The
temperature and density structure of this coroner streamer are
very similar to "TRACE" active region loops (flat temperature
profiles, overdense relative to uniform heating, and relatively cool
temperatures).
Title: Hydrodynamic Modeling of Flare Loops
Authors: Reeves, K. K.; Warren, H. P.; DeLuca, E. E.; Boyd, J. F.;
Arber, T. D.
Bibcode: 2002AAS...200.6811R
Altcode: 2002BAAS...34..757R
The study of post-flare loops is instrumental to understanding the
energy deposition in flares. Previously we modeled the evolution of
a flare arcade using a set of scaling laws for the conductive and
radiative cooling of post-flare loops. We found that these simulated
loops decrease in intensity faster than the observed loops. The scaling
laws, however, did not allow for heating during the decay of the flare,
or provide information on variations in temperature and density along
the loop. In the current work, we use a full hydrodynamic simulation
to investigate energy deposition in flaring loops. We will compare our
simulated flare arcades to spatially and temporally resolved TRACE,
SXT and HESSI observations. This work has been supported in part by
the NASA Sun-Earth Connection Guest Investigator Program. TRACE is
supported by Contract NAS5-38099 from NASA to LMATC.
Title: New Determinations of Solar EUV Irradiance Variability for
use in the NRLMSIS Atmospheric Density Specification Model
Authors: Lean, J. L.; Mariska, J. T.; Warren, H. P.; Bishop, J.;
Picone, J. M.
Bibcode: 2002AGUSMSH51B..03L
Altcode:
A variety of space weather models require knowledge of the Sun' s EUV
spectral irradiance because this radiation is the predominant source of
upper atmosphere heating and ionization. Lacking reliable observations
to specify the considerable variability of the EUV spectrum during the
solar activity cycle, space weather research and operational models
continue to use the 10.7 cm radio flux as a proxy for variations in EUV
radiation, even though its shortcomings have been recognized for some
time. For example, the largest source of error in special perturbations
propagation of spacecraft orbits is the parameterization of solar
EUV irradiance in the density models that the codes use to estimate
drag. The strongest lines in the EUV spectrum are formed in the Sun'
s upper chromosphere whereas the 10.7 cm flux is formed primarily in the
hotter corona. We have recently developed a composite chromospheric EUV
irradiance index by combining multiple space and ground-based datasets,
and are reformulating NRL' s upper atmosphere neutral density model
(NRLMSISE-00) to accommodate this new index. In a parallel effort
we have developed a new physics-based irradiance variability model
(NRLEUV) that calculates the EUV spectrum independently of direct
spectral irradiance observations. The model utilizes solar images
to extract information about the fraction of the solar atmosphere
occupied by different active, network and quiet regions, and coronal
holes. Representative differential emission measures are constructed
for each of the features and the EUV disk-integrated spectrum is
calculated by combining the emission measures with theoretically
determined values of plasma emissivity. We use a parameterized
version of the NRLEUV model that includes both chromospheric and
coronal indices to calculate variations during past solar cycles in
the total EUV energy incident at the top of the earth' s atmosphere,
and at selected altitude in the range 100 to 1000 km, for use in future
reformulations of NRLMSIS. We compare the new chromospheric index
and the physics-based EUV irradiance model products with existing EUV
model estimates derived from parameterizations of direct observations,
and describe how NRLMSIS will incorporate these new determinations of
the EUV irradiance to provide an improved density specification for
space weather applications.
Title: Spectral observations of quiescent EUV loops
Authors: Winebarger, A. R.; Mariska, J. T.; Warren, H. P.
Bibcode: 2002AAS...200.1603W
Altcode: 2002BAAS...34R.667W
Recent TRACE observations have detected a class of cool, quiescent
loops that are inconsistent with hydrostatic loop models. These loops
appear static, isothermal (1 MK), and overdense. In this talk, we
present co-aligned TRACE, SUMER and CDS observations of several EUV
loops. Every loop observed shows evidence of significant downflows
along the loop. We confirm that the temperature along some of the
observed loops is sharply peaked around 1 MK. In other loops, however,
we find evidence for emission at hotter (2 MK) and cooler (0.2 MK)
temperatures; this emission may be co-spatial with the loops observed
in TRACE. These observations suggest that non-hydrostatic models are
necessary to describe the loops.
Title: The Development of a New Model of Solar EUV Irradiance
Variability
Authors: Warren, Harry
Bibcode: 2002STIN...0243799W
Altcode:
The goal of this research project is the development of a new model of
solar EUV (Extreme Ultraviolet) irradiance variability. The model is
based on combining differential emission measure distributions derived
from spatially and spectrally resolved observations of active regions,
coronal holes, and the quiet Sun with full-disk solar images. An
initial version of this model was developed with earlier funding from
NASA. The new version of the model developed with this research grant
will incorporate observations from SoHO as well as updated compilations
of atomic data. These improvements will make the model calculations
much more accurate.
Title: Steady Flows Detected in Extreme-Ultraviolet Loops
Authors: Winebarger, Amy R.; Warren, Harry; van Ballegooijen, Adriaan;
DeLuca, Edward E.; Golub, Leon
Bibcode: 2002ApJ...567L..89W
Altcode:
Recent Transition Region and Coronal Explorer (TRACE) observations have
detected a class of active region loops whose physical properties are
inconsistent with previous hydrostatic loop models. In this Letter we
present the first co-aligned TRACE and the Solar Ultraviolet Measurement
of Emitted Radiation (SUMER) observations of these loops. Although these
loops appear static in the TRACE images, SUMER detects line-of-sight
flows along the loops of up to 40 km s-1. The presence
of flows could imply an asymmetric heating function; such a heating
function would be expected for heating that is proportional to
(often asymmetric) footpoint field strength. We compare a steady flow
solution resulting from an asymmetric heating function to a static
solution resulting from a uniform heating function in a hypothetical
coronal loop. We find that the characteristics associated with the
asymmetrically heated loop better compare with the characteristics of
the loops observed in the TRACE data.
Title: Energetics of Explosive Events Observed with SUMER
Authors: Winebarger, Amy R.; Emslie, A. Gordon; Mariska, John T.;
Warren, Harry P.
Bibcode: 2002ApJ...565.1298W
Altcode:
Observations of solar chromosphere-corona transition region plasma show
evidence of small-scale, short-lived dynamic phenomena characterized
by significant nonthermal broadening and asymmetry in the wings
of spectral line profiles. These impulsive mass motions (explosive
events) are thought to be the product of magnetic reconnection and to
be similar in driving mechanism (though larger in size) to nanoflares,
the small-scale events proposed to heat the corona. In this paper,
we present a statistical analysis of the energetics of explosive
events to address the viability of the nanoflare heating theory. We
consider high spectral, spatial, and temporal resolution spectra of the
C III λ977, N IV λ765, O VI λ1032, and Ne VIII λ770 lines observed
with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER)
telescope and spectrometer. Each line profile exhibiting explosive event
characteristics was analyzed using the velocity differential emission
measure (VDEM) technique. A VDEM is a measure of the emitting power
of the plasma as a function of its line-of-sight velocity and hence
provides a method of accurately measuring the energy flux associated
with an explosive event. We find that these events globally release
~4×104 ergs cm-2 s-1 toward both the
corona and chromosphere. This implies that explosive events themselves
are not energetically significant to the solar atmosphere. However, the
distribution of these explosive events as a function of their energy
has a power-law spectral index of α=2.9+/-0.1 for the energy range
1022.7-1025.1 ergs. Since α is greater than 2,
the energy content is dominated by the smallest events. Hence, if this
distribution is representative of the size distribution down to lower
energy ranges (~1022 ergs), such small and (currently)
undetectable events would release enough energy to heat the solar
atmosphere.
Title: RHESSI and TRACE Observations of an X-class Flare
Authors: Hudson, H.; Dennis, B.; Gallagher, P.; Krucker, S.; Reeves,
K.; Warren, H.
Bibcode: 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: Early Results from a Multi-Thermal Model for the Cooling of
Post-Flare Loops
Authors: Reeves, K. K.; Warren, H. P.
Bibcode: 2002mwoc.conf..275R
Altcode:
We have developed a multi-thermal model for the cooling of post-flare
loops. The model consists of an arcade of many nested loops that
reconnect and begin cooling at slightly different times, and have
different cooling profiles because of the different loop lengths across
the arcade. Cooling due to both conductive and radiative processes is
taken into account. The free parameters in the model include initial
temperature and density in the loop, loop width and the initial loop
length. The results from the model are then compared to TRACE and SXT
observations. Our many-loop model does a much better job of predicting
the SXT and TRACE light curves than a similar model with only one loop.
Title: Observations of Preflare Activity with TRACE and Yohkoh
[Invited]
Authors: Warren, H. P.
Bibcode: 2002mwoc.conf..239W
Altcode:
Despite several decades of observational and theoretical effort, a
complete understanding of solar flares remains elusive. It has been
especially difficult to understand how the evolution of the magnetic
field triggers a flare and drives the release of energy. In this talk
I will review TRACE and Yohkoh observations of pre-flare and impulsive
phase dynamics related to nonthermal broadening, flare ribbon evolution,
and breakout reconnection. Studies of these phenomena suggest that
pre-flare activity is a potentially rich source of information on the
mechanisms that power a flare. For example, Yohkoh BCS measurements of
nonthermal broadening have shown that the largest nonthermal velocities
can occur before the onset of significant hard X-ray emission. This
suggests that nonthermal broadening is a signature of a turbulent
phase of the flare, which can begin several minutes before the onset
of the hard x-ray emission. TRACE observations have also yielded
evidence for ribbon brightenings that precede the onset of the hard
X-ray emission. The analysis of very high cadence TRACE data, however,
indicates that energy release during the pre-flare and impulsive phases
of the flare is occurring on different loops. Finally, comparisons of
pre-flare TRACE images with potential field extrapolations have shown
that pre-flare activity associated with a null point in the field is an
essential component of eruptive flares. Understanding the relationships
between these phenomena will require coordinated observations between
many instruments. I will discuss how future observations from Yohkoh,
TRACE, SoHO, HESSI, Stereo, Solar-B, and ground-based observatories
will be used to advance our understanding of flare physics.
Title: Ultraviolet Flare Ribbon Brightenings and the Onset of Hard
X-Ray Emission
Authors: Warren, Harry P.; Warshall, Andrew D.
Bibcode: 2001ApJ...560L..87W
Altcode:
The broadband UV images taken by the Transition Region and Coronal
Explorer (TRACE) provide a unique opportunity to observe transition
region and chromospheric emission from flare footpoints at high spatial
and temporal resolution. In this Letter, we present comparisons of
UV flare footpoint evolution with hard X-ray light curves from the
Yohkoh Hard X-Ray Telescope and the Compton Gamma Ray Observatory Burst
and Transient Source Experiment (BATSE). The nine events analyzed in
this Letter all show evidence for flare footpoint brightenings that
precede the onset of the hard X-ray emission. Regression analysis
between TRACE UV data taken at very high cadence (2-3 s) and hard
X-ray light curves, however, shows that the initial hard X-ray burst
is positively correlated only with footpoints that show no pre-hard
X-ray activity. This indicates that energy release during the preflare
and impulsive phases of the flare is occurring on different loops.
Title: A new model of solar EUV irradiance variability: 1. Model
formulation
Authors: Warren, H. P.; Mariska, J. T.; Lean, J.
Bibcode: 2001JGR...10615745W
Altcode:
We present a new model of solar irradiance variability at extreme
ultraviolet wavelengths (EUV, 50-1200 Å). In this model, quiet Sun,
coronal hole, and active region intensities for optically thin emission
lines are computed from emission measure distributions determined
from spectrally and spatially resolved observations. For optically
thick emission lines and continua, empirical values are used. The
contribution of various solar features to the spectral irradiance
variability is determined from a simple model of limb-brightening
and full-disk solar images taken at the Big Bear Solar Observatory
and by the Soft X-Ray Telescope on Yohkoh. To extend our irradiance
model beyond the time period covered by the available images, we
use correlations with proxies for solar activity. Comparisons with
the available irradiance data from the Atmospheric Explorer E (AE-E)
spacecraft show that our model is capable of reproducing the rotational
modulation of the EUV irradiance near solar maximum. The AE-E data,
however, show systematically more solar cycle variability than our
model estimates.
Title: High Spatial Resolution Observations of a Hot Region in a
Solar Flare with the Transition Region and Coronal Explorer
Authors: Warren, Harry P.; Reeves, Kathy K.
Bibcode: 2001ApJ...554L.103W
Altcode:
The Transition Region and Coronal Explorer (TRACE) provides some
of the highest spatial resolution images ever taken of hot solar
flare plasma. The TRACE 195 Å channel is particularly sensitive to
high-temperature flare plasma because of the presence of the Fe XXIV
λ192 resonance line in this bandpass. The TRACE 171 Å channel observes
emission from thermal bremsstrahlung during a flare. Since this emission
is generally weak, it is usually not possible to derive electron
temperatures for flare plasma from TRACE observations. In this Letter,
we present analysis of the 2000 March 24 X1.8 limb flare that produced
high count rates in both the TRACE 195 and 171 Å channels. We find
evidence for a small, high-temperature region near the top of the flare
arcade. This hot region appears to lie at the base of the cusp-shaped
structure that extends above the arcade. The TRACE observations are
consistent with a strong enrichment of Fe over its photospheric value
in the hot region that suggests in situ heating of this plasma. We
also find that multithermal simulations of flare evolution reproduce
the observations much better than an isothermal model does.
Title: TRACE Observations of Flare Ribbon Evolution
Authors: Warren, H. P.
Bibcode: 2001AGUSM..SP42A12W
Altcode:
We present very high cadence (2--3 s) Transition Region and Coronal
Explorer (TRACE) observations of the M1.1 flare that occurred on 2000
March 17 at 11:10 UT. In this flare we find evidence for flare ribbon
brightenings in TRACE broadband 1600 Å images that precede the onset
of the hard X-ray emission measured with the Burst and Transient Source
Experiment (BATSE) by as much as several minutes. Many of the flare
footpoints that show pre-flare activity also brighten significantly
during impulsive phase of the flare. Regression analysis between the
TRACE and BATSE light curves, however, shows that hard x-ray emission
is more strongly correlated with footpoints that show no pre-flare
activity. This suggests that energy release during the pre-flare and
impulsive phases of the flare may not be directly related. We discuss
these results in the context of recent observations of strong nonthermal
broadening that precedes the onset of hard X-ray emission.
Title: The Temperature and Density Structure of the Quiet Solar Corona
Authors: Winebarger, A. R.; Warren, H. P.
Bibcode: 2001AGUSM..SH31A02W
Altcode:
The temperature and density structure of the quiet solar corona
remains unclear. In this poster, we will present a preliminary
analysis of a quiet solar coronal loop structure observed with SOHO and
TRACE. After determining the magnetic field structure from potential
field extrapolation, we attempt to model this loop using RTV scaling
laws with various heating functions. This work is in preparation for
a full-scale statistical study of SOHO/TRACE data to determine the
structure of the quiet solar corona.
Title: Observation of Large Flares and Their Evolution with the
Transition Region and Coronal Explorer
Authors: Reeves, K. K.; Warren, H. P.
Bibcode: 2001AGUSM..SP51A08R
Altcode:
The Transition Region and Coronal Explorer (TRACE) provides some of
the highest spatial resolution images ever taken of hot solar flare
plasma. The TRACE 195 A channel is particularly sensitive to high
temperature flare plasma because of the presence of the Fe XXIV 192
A resonance line in this bandpass. The TRACE 171 A channel observes
emission from thermal bremsstrahlung during a flare. Since this
emission is generally weak compared to the background corona, it is
difficult to derive electron temperatures for flare plasma from TRACE
filter ratios. In this paper, we examine large flares observed by TRACE
that have significant counts in the 171 A channel when compared to the
background corona. The evolution of the filter ratios of these flares
is examined over time and compared with a simple cooling model. The
effects of non-equilibrium ionization are also examined.
Title: Ionospheric and Thermospheric Effects During the Initial
Radiative Phase of the Bastille Day Event
Authors: Meier, R. R.; Drob, D. P.; Nicholas, A. C.; Bishop, J.;
Picone, J. M.; Thonnard, S. E.; Dymond, K. F.; Budzien, S. A.; Lean,
J.; Mariska, J. T.; Huba, J. D.; Joyce, G.; Warren, H. P.; Judge, D. L.
Bibcode: 2001AGUSM..SA51A08M
Altcode:
Increases in the solar EUV and X-ray irradiance during a solar flare
can produce enhanced ionization and heating in the terrestrial
ionosphere. The resulting energetic photoelectrons in turn
cause increases in the far ultraviolet (FUV) dayglow (100 - 150
nm). Enhancements of some 50 per cent had previously been detected
in OGO-4 nadir-viewing data [C B Opal, Space Research XIII, 797,
1973]. Similar enhancements have now been seen in the FUV limb-viewing
dayglow observations from the ARGOS satellite during the Bastille Day
flare (July 14, 2000). Because extinction of the FUV dayglow by O2
prevents seeing below 140 km tangent altitude on the limb, increases
in the dayglow above that altitude must be caused by the component
of the flare spectral irradiance which is deposited there, namely at
wavelengths greater than 20 nm. This conclusion is corroborated by
the observation of the flare at 30.3 nm made by the SEM instrument on
the SOHO satellite. We study this solar-ionospheric connection using
a modified version of the NRL solar spectrum as input to the SAMI2
ionospheric model, and also calculate thermospheric heating rates for
this event.
Title: Intensity Fluctuations in the Solar Chromosphere
Authors: Kalkofen, W.; Warren, H. P.; Winebarger, A. R.; van
Ballegooijen, A.; Avrett, E. H.
Bibcode: 2001AGUSM..SP41C05K
Altcode:
We examine two very different empirical models of the solar
chromosphere. The first model implies steady heating, is hot at the
top of the chromosphere and has temperature fluctuations of relatively
low amplitude; the second model is heated intermittently, is cold most
of the time and undergoes large temperature variations. Estimates of
intensity fluctuations of chromospheric radiation are very different
for the two models. We compare the model predictions with observations
made by Skylab and by SUMER.
Title: A correlation between erupted lava composition and degree of
subsequent thermal metamorphism for HED-meteoritic basalts.
Authors: Warren, H. P.; Kallemeyn, W. G.
Bibcode: 2001anme...26..154W
Altcode:
No abstract at ADS
Title: Northwest Africa 766: A new ferroan ureilite with variety of
chromium-rich phases and associated Si, Al-rich glasses.
Authors: Sikirdji, M.; Warren, H. P.
Bibcode: 2001anme...26..131S
Altcode:
No abstract at ADS
Title: Time Variability of the ``Quiet'' Sun Observed with
TRACE. II. Physical Parameters, Temperature Evolution, and Energetics
of Extreme-Ultraviolet Nanoflares
Authors: Aschwanden, Markus J.; Tarbell, Ted D.; Nightingale, Richard
W.; Schrijver, Carolus J.; Title, Alan; Kankelborg, Charles C.;
Martens, Piet; Warren, Harry P.
Bibcode: 2000ApJ...535.1047A
Altcode:
We present a detailed analysis of the geometric and physical
parameters of 281 EUV nanoflares, simultaneously detected with the
TRACE telescope in the 171 and 195 Å wavelengths. The detection and
discrimination of these flarelike events is detailed in the first paper
in this series. We determine the loop length l, loop width w, emission
measure EM, the evolution of the electron density ne(t) and
temperature Te(t), the flare decay time τdecay,
and calculate the radiative loss time τloss, the conductive
loss time τcond, and the thermal energy Eth. The
findings are as follows: (1) EUV nanoflares in the energy range of
1024-1026 ergs represent miniature versions
of larger flares observed in soft X-rays (SXR) and hard X-rays
(HXR), scaled to lower temperatures (Te<~2 MK),
lower densities (ne<~109 cm-3),
and somewhat smaller spatial scales (l~2-20 Mm). (2) The cooling
time τdecay is compatible with the radiative cooling
time τrad, but the conductive cooling timescale
τcond is about an order of magnitude shorter, suggesting
repetitive heating cycles in time intervals of a few minutes. (3)
The frequency distribution of thermal energies of EUV nanoflares,
N(E)~10-46(E/1024)-1.8 (s-1
cm-2 ergs-1) matches that of SXR microflares
in the energy range of 1026-1029, and exceeds
that of nonthermal energies of larger flares observed in HXR by a
factor of 3-10 (in the energy range of 1029-1032
ergs). Discrepancies of the power-law slope with other studies, which
report higher values in the range of a=2.0-2.6 (Krucker & Benz;
Parnell & Jupp), are attributed to methodical differences in the
detection and discrimination of EUV microflares, as well as to different
model assumptions in the calculation of the electron density. Besides
the insufficient power of nanoflares to heat the corona, we find also
other physical limits for nanoflares at energies <~1024
ergs, such as the area coverage limit, the heating temperature limit,
the lower coronal density limit, and the chromospheric loop height
limit. Based on these quantitative physical limitations, it appears
that coronal heating requires other energy carriers that are not
luminous in EUV, SXR, and HXR.
Title: Fine Structure in Solar Flares
Authors: Warren, Harry P.
Bibcode: 2000ApJ...536L.105W
Altcode:
We present observations of several large two-ribbon flares observed
with both the Transition Region and Coronal Explorer (TRACE) and the
soft X-ray telescope on Yohkoh. The high spatial resolution TRACE
observations show that solar flare plasma is generally not confined
to a single loop or even a few isolated loops but to a multitude
of fine coronal structures. These observations also suggest that the
high-temperature flare plasma generally appears diffuse while the cooler
(<~2 MK) postflare plasma is looplike. We conjecture that the diffuse
appearance of the high-temperature flare emission seen with TRACE is due
to a combination of the emission measure structure of these flares and
the instrumental temperature response and does not reflect fundamental
differences in plasma morphology at the different temperatures.
Title: The Morphology of the Solar Transition Region and Corona
Authors: Winebarger, A. R.; Warren, H. P.
Bibcode: 2000SPD....31.0204W
Altcode: 2000BAAS...32..811W
The solar transition region is an important boundary condition in
the modeling of the solar corona, yet the basic geometry of the solar
transition region, specifically the magnetic connectivity between the
transition region and corona, has been heavily debated and remains
poorly understood. In this poster, we will present analysis of a SUMER
O VI (105.45 K) spectroheliogram, as well as several TRACE
171 Angstroms (106.0 K) and 195 Angstroms (106.2
K) images. We quantify the structure size at each temperature using a
fast Fourier transform analysis of the images. From this analysis, we
find the typical size of the emitting structures in the O VI image and
the 171 Angstroms images is smaller than that of the structures in the
195 Angstroms images. We have also examined the magnetic connectivity
of the various structures by comparing the images to co-aligned MDI
magnetograms. Most of the structures present in the O VI image do
not connect regions of opposite polarity network fields, while the
structures observed in the 171 Angstroms and 195 Angstroms images do
appear to have footpoints rooted in opposite polarity regions. From
these observations, we conjecture that the geometry of the upper
transition region and lower corona is dominated by spicules at O VI
temperatures, small-scale loops at 171 Angstroms temperatures and
large-scale loops at 195 Angstroms temperatures.
Title: Small Scale Structure in the Solar Transition Region
Authors: Warren, Harry P.; Winebarger, Amy R.
Bibcode: 2000ApJ...535L..63W
Altcode:
We present analysis of a high spatial resolution (~1.5") O VI λ1032
spectroheliogram taken with the Solar Ultraviolet Measurements of
Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric
Observatory. This image shows numerous narrow, looplike structures. The
widths of the narrowest resolved features are at the spatial resolution
of the SUMER spectrometer. In contrast to earlier Skylab observations,
however, comparisons with a Michelson Doppler Imager magnetogram reveals
that the majority of these looplike structures do not connect network
magnetic fields. Instead, they extend from the supergranulation network
into the cell-center regions where no magnetic fields are detected in
these data. We conjecture that these fine structures are related to
spicules and are highly dynamic.
Title: Fine Structure in Solar Flares
Authors: Warren, H. P.
Bibcode: 2000SPD....31.0262W
Altcode: 2000BAAS...32..822W
The high spatial resolution (0.5 arcsec) and broad temperature response
(104-10^7 K) of the Transition Region and Coronal Explorer
(TRACE) make it a unique instrument for observing solar flares. Here
we present analysis of several large two-ribbon flares observed with
both TRACE and Yohkoh. The high spatial resolution TRACE observations
show that solar flare plasma is generally not confined to a single
loop or even a few isolated loops, but a multitude of fine coronal
structures. These observations also suggest that the high temperature
flare plasma generally appears diffuse while the cooler (<2
MK) post-flare plasma is loop-like. We conjecture that the diffuse
appearance of the high temperature flare emission seen with TRACE is due
to a combination of the emission measure structure of these flares and
the instrumental temperature response and does not reflect fundamental
differences in plasma morphology at the different temperatures.
Title: Temperature Profiles of Super-Hot Flare Plasma using the
Transition Region and Coronal Explorer (TRACE)
Authors: Reeves, K. K.; Warren, H. P.
Bibcode: 2000SPD....31.0263R
Altcode: 2000BAAS...32..822R
The Transition Region and Coronal Explorer (TRACE) provides some of
the highest spatial resolution images ever taken of hot solar flare
plasma. The TRACE 195 Angstroms channel is particularly sensitive
to flare plasma because of the presence of the Fe XXIV 192 Angstroms
resonance line in this bandpass. Using TRACE filter ratios to derive
temperatures and emission measures during flares is often difficult,
however, because the 171 Angstroms channel is predominately hydrogen
continuum emission. This emission is generally weak and often hard
to distinguish from the background corona. A very large flare,
such as the X2 flare observed on March 24, 2000, yields high
counts in the 171 Angstroms channel so that background emission
becomes insignificant. Analysis of these data show steep temperature
gradients below the brightest flare plasma in the arcade. These data
also show large 195 Angstroms/171 Angstroms ratios above the arcade,
suggesting localized regions of super-hot plasma. Interpretation of
the filter ratios depends on the absolute abundance of Fe, however,
and we re-examine past measurements of Fe abundances in flares.
Title: Erratum: Morphology of the Quiet Solar Upper Atmosphere in
the 4 W 104 < Te < 1.4 W 106 K Temperature Regime
Authors: Feldman, U.; Widing, K. G.; Warren, H. P.
Bibcode: 2000ApJ...529.1145F
Altcode:
No abstract at ADS
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.
Bibcode: 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>~10 MK)
in the corona. After an interval of about 1300 s, cooler loops
(Te<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: Measuring the Physical Properties of the Solar Corona:
Results from SUMER/SOHO and TRACE
Authors: Warren, H. P.
Bibcode: 1999SoPh..190..363W
Altcode:
Using SUMER observations taken above the limb of a quiet region we
derive electron temperatures, emission measures, and absolute elemental
abundances. This analysis, which uses recently published ionization
balance calculations and the latest solar photospheric abundances,
indicates that the low-FIP elements are enriched by a factor of 2.3±0.7
in the corona, which is smaller than some previous measurements. TRACE
observations of this region yield systematically lower temperatures
and emission measures.
Title: Analyzing the Energetics of Explosive Events Observed by
SUMER on SOHO
Authors: Winebarger, Amy R.; Emslie, A. Gordon; Mariska, John T.;
Warren, Harry P.
Bibcode: 1999ApJ...526..471W
Altcode:
The SUMER spectrometer on SOHO has obtained numerous observations of
optically thin chromosphere-corona transition-region line profiles
with high spatial, spectral, and temporal resolution. Many of these
profiles exhibit asymmetries and broadenings associated with impulsive
mass motions (explosive events) in the solar atmosphere. We present
here a new method of analyzing non-Gaussian line profiles to calculate
the distribution of fluid velocities and hence the associated energy
flux. We illustrate this method through a preliminary analysis
of explosive event line profiles observed by SUMER. We derive the
magnitudes of the energy fluxes directed both toward and away from the
observer, and their (``net flux'') differences. We also identify and
quantify the various components of each (i.e., kinetic, thermal and
nonthermal enthalpy, and the high-energy component associated with
the skewed tail of the distribution). The global energy contribution
of explosive events to the solar atmosphere is then estimated under
two different ``grouping'' assumptions. This preliminary analysis
reveals an average net upward energy flux over the entire Sun of
104-105 ergs cm-2 s-1,
up to an order of magnitude larger than previous estimates based
on characteristic velocities of the fluid. Furthermore, the global
estimate for the separate upward- and downward-directed energy fluxes
is 105-106 ergs cm-2 s-1,
which is comparable to the energy flux required for heating of the quiet
corona and indicates that explosive events may indeed have significant
implications for the energy balance of the chromosphere and corona.
Title: Morphology of the Quiet Solar Upper Atmosphere in the
4×104<Te<1.4×106 K
Temperature Regime
Authors: Feldman, U.; Widing, K. G.; Warren, H. P.
Bibcode: 1999ApJ...522.1133F
Altcode:
Studies on the morphology of the solar upper atmosphere began over
three decades ago. Early models assumed that the temperature structure
of the solar upper atmosphere was continuous with a thin transition
region connecting the chromosphere with the corona. Over the years
it became apparent that the original depiction of the solar upper
atmosphere was too simplistic. In this paper we present a morphological
study of the solar upper atmosphere over a wide range of temperatures
(4×104<=Te<=1.4×106 K)
using high-resolution images (1''-2'')
taken by TRACE, the SUMER spectrometer on SOHO, and the NRL
spectroheliograph on Skylab. The images clearly show that the
4×104<=Te<=1.4×106
K temperature domain of the solar upper atmosphere consists
of a hierarchy of isothermal loop structures. While at the
Te<8×105 K temperature regime the looplike
structures are more abundant along the chromospheric network, at higher
temperatures (Te>9×105 K) no association
between them and the chromospheric network is apparent. The hottest
(Te~1.4×106 K), which are also the longest among
the quiet-Sun loop structures, form a canopy over the lower temperature
loop structures. We discuss in the paper possible relationships between
the morphology of the solar upper atmosphere, its elemental abundance,
and ideas regarding the origin of the slow-speed solar wind.
Title: A new view of the solar outer atmosphere by the Transition
Region and Coronal Explorer
Authors: Schrijver, C. J.; Title, A. M.; Berger, T. E.; Fletcher, L.;
Hurlburt, N. E.; Nightingale, R. W.; Shine, R. A.; Tarbell, T. D.;
Wolfson, J.; Golub, L.; Bookbinder, J. A.; DeLuca, E. E.; McMullen,
R. A.; Warren, H. P.; Kankelborg, C. C.; Handy, B. N.; De Pontieu, B.
Bibcode: 1999SoPh..187..261S
Altcode:
The Transition Region and Coronal Explorer (TRACE) - described in the
companion paper by Handy et al. (1999) - provides an unprecedented
view of the solar outer atmosphere. In this overview, we discuss the
initial impressions gained from, and interpretations of, the first
million images taken with TRACE. We address, among other topics,
the fine structure of the corona, the larger-scale thermal trends,
the evolution of the corona over quiet and active regions, the high
incidence of chromospheric material dynamically embedded in the coronal
environment, the dynamics and structure of the conductively dominated
transition region between chromosphere and corona, loop oscillations
and flows, and sunspot coronal loops. With TRACE we observe a corona
that is extremely dynamic and full of flows and wave phenomena, in
which loops evolve rapidly in temperature, with associated changes in
density. This dynamic nature points to a high degree of spatio-temporal
variability even under conditions that traditionally have been referred
to as quiescent. This variability requires that coronal heating can
turn on and off on a time scale of minutes or less along field-line
bundles with cross sections at or below the instrumental resolution
of 700 km. Loops seen at 171 Å (∼1 MK) appear to meander through
the coronal volume, but it is unclear whether this is caused by the
evolution of the field or by the weaving of the heating through the
coronal volume, shifting around for periods of up to a few tens of
minutes and lighting up subsequent field lines. We discuss evidence
that the heating occurs predominantly within the first 10 to 20 Mm
from the loop footpoints. This causes the inner parts of active-region
coronae to have a higher average temperature than the outer domains.
Title: The transition region and coronal explorer
Authors: Handy, B. N.; Acton, L. W.; Kankelborg, C. C.; Wolfson, C. J.;
Akin, D. J.; Bruner, M. E.; Caravalho, R.; Catura, R. C.; Chevalier,
R.; Duncan, D. W.; Edwards, C. G.; Feinstein, C. N.; Freeland, S. L.;
Friedlaender, F. M.; Hoffmann, C. H.; Hurlburt, N. E.; Jurcevich,
B. K.; Katz, N. L.; Kelly, G. A.; Lemen, J. R.; Levay, M.; Lindgren,
R. W.; Mathur, D. P.; Meyer, S. B.; Morrison, S. J.; Morrison, M. D.;
Nightingale, R. W.; Pope, T. P.; Rehse, R. A.; Schrijver, C. J.;
Shine, R. A.; Shing, L.; Strong, K. T.; Tarbell, T. D.; Title, A. M.;
Torgerson, D. D.; Golub, L.; Bookbinder, J. A.; Caldwell, D.; Cheimets,
P. N.; Davis, W. N.; Deluca, E. E.; McMullen, R. A.; Warren, H. P.;
Amato, D.; Fisher, R.; Maldonado, H.; Parkinson, C.
Bibcode: 1999SoPh..187..229H
Altcode:
The Transition Region and Coronal Explorer (TRACE) satellite, launched
2 April 1998, is a NASA Small Explorer (SMEX) that images the solar
photosphere, transition region and corona with unprecedented spatial
resolution and temporal continuity. To provide continuous coverage
of solar phenomena, TRACE is located in a sun-synchronous polar
orbit. The ∼700 Mbytes of data which are collected daily are made
available for unrestricted use within a few days of observation. The
instrument features a 30-cm Cassegrain telescope with a field of view
of 8.5×.5 arc min and a spatial resolution of 1 arc sec (0.5 arc sec
pixels). TRACE contains multilayer optics and a lumogen-coated CCD
detector to record three EUV wavelengths and several UV wavelengths. It
observes plasmas at selected temperatures from 6000 K to 10 MK with
a typical temporal resolution of less than 1 min.
Title: The density structure of a solar polar coronal hole
Authors: Warren, H. P.; Hassler, D. M.
Bibcode: 1999JGR...104.9781W
Altcode:
Electron densities derived from Si III, Ne VII, Mg VIII, Si VIII, and Mg
IX line ratios observed above the limb of a polar coronal hole with the
Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer
on the Solar and Heliospheric Observatory (SoHO) are presented. The
electron densities are consistent with a constant pressure of
1.6×1014cm-3K (logPe=14.2).
Title: A new view of the solar corona from the transition region
and coronal explorer (TRACE)
Authors: Golub, L.; Bookbinder, J.; Deluca, E.; Karovska, M.; Warren,
H.; Schrijver, C. J.; Shine, R.; Tarbell, T.; Title, A.; Wolfson,
J.; Handy, B.; Kankelborg, C.
Bibcode: 1999PhPl....6.2205G
Altcode:
The TRACE Observatory is the first solar-observing satellite in the
National Aeronautics and Space Administration's (NASA) Small Explorer
series. Launched April 2, 1998, it is providing views of the solar
transition region and low corona with unprecedented spatial and
temporal resolution. The corona is now seen to be highly filamented,
and filled with flows and other dynamic processes. Structure is seen
down to the resolution limit of the instrument, while variability and
motions are observed at all spatial locations in the solar atmosphere,
and on very short time scales. Flares and shock waves are observed,
and the formation of long-lived coronal structures, with consequent
implications for coronal heating models, has been seen. This overview
describes the instrument and presents some preliminary results from
the first six months of operation.
Title: Elemental Abundance Variations and the Structure of the Quiet
Solar Corona and Transition Region
Authors: Warren, H. P.
Bibcode: 1999AAS...19410002W
Altcode: 1999BAAS...31..996W
Recent observations of Doppler shifts with the SUMER spectrometer
on SoHO have shown that there is a transition from net redshifts
(apparent inflows) to blueshifts (outflows) at approximately 630,000
K in solar coronal holes. These outflows appear to be directly related
to the formation of the high speed solar wind. This transition from net
redshifts to blueshifts is significant because it supports the idea that
much of the solar transition region is not connected to the corona,
but is confined on small, closed loops. Measurements of elemental
abundances hold important clues to the structure of solar corona and
the origin of the high speed solar wind. In situ measurements of the
high speed solar wind indicate that low first ionization potential
elements are enriched by about a factor of two or less over their
photospheric values. The slow speed wind, which is formed from quiet
regions, shows much larger enrichements. Skylab-era measurements of
emission formed in the lower transition region suggested no enrichment
of low FIP elements at these temperatures in either the quiet Sun or
coronal holes. In this paper we present a systematic investigation of
abundance variations as a function of temperature in the quiet Sun and
coronal holes using observations from the SUMER and CDS spectrometers
on SoHO. We also discuss the relationship between abundance variations
and the morphology of the solar atmosphere using high resolution images
from TRACE.
Title: On the Ability of an Extreme-Ultraviolet Multilayer
Normal-Incidence Telescope to Provide Temperature Information for
Solar Plasmas
Authors: Feldman, U.; Laming, J. M.; Doschek, G. A.; Warren, H. P.;
Golub, L.
Bibcode: 1999ApJ...511L..61F
Altcode:
In recent years, multilayer-coated optics have been used in solar-soft
X-ray and extreme-ultraviolet telescopes to record high-resolution,
full Sun images. The multilayer coatings reflect efficiently over rather
narrow wavelength bands that are selected to contain spectral emission
lines considered to have plasma diagnostic importance for determining
approximate electron temperatures. The purpose of this Letter is to
discuss the effect of continuum emission on the response of multilayer
passbands and the effect of this response on temperature determinations
in the 4×106-2×107 K range. Significant effects
are largely confined to continuum emission from flare plasma. The
flare free-free continuum in the EUV range is nearly temperature and
wavelength insensitive and dominates the emission in passbands that
are centered on quiet- to active-Sun coronal lines emitted by ions
such as Fe IX-Fe XV.
Title: Differentiation of siderophile elements in the Moon and the
HED parent asteroid.
Authors: Warren, H. P.
Bibcode: 1999anme...24..185W
Altcode:
No abstract at ADS
Title: Observations of High-Temperature Flare Plasma with Transition
Region and Coronal Explorer (TRACE)
Authors: Reeves, K. K.; Golub, L.; Warren, H. P.
Bibcode: 1999agu..meet..234R
Altcode:
The so-called standard model of solar flares makes specific
predictions concerning the amount, location, and timing of both hot
(Te>10 MK) and cool (Te<2 MK) plasma in
solar flares. 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) make it a unique instrument for observing
solar flares and testing the model predictions. We present TRACE and
Yohkoh observations of an M2.4 two-ribbon flare that began on 1999
July 25 at about 13:08 UT. These observations are in qualitative
agreement with the essential elements of the reconnection model. We
observe impulsive footpoint brightenings that are quickly followed
by the formation of high-temperature plasma in the corona. After an
interval of about 1300 s cooler loops form below the hot plasma. The
cooling time inferred from the observations suggests large densities
(ne~1011 cm-3) for the high temperature
plasma so that radiative losses dominate the cooling process. The
TRACE data are consistent with the Yohkoh observations of a ``hot''
(Te~15-20 MK) plasma existing at the top of the arcade.
Title: High-Resolution Observations of the Solar Hydrogen Lyman
Lines in the Quiet Sun with the SUMER Instrument on SOHO
Authors: Warren, H. P.; Mariska, J. T.; Wilhelm, K.
Bibcode: 1998ApJS..119..105W
Altcode:
We present high-resolution observations of the higher H Lyman series
lines taken with the Solar Ultraviolet Measurements of Emitted Radiation
(SUMER) experiment flown on the Solar and Heliospheric Observatory
(SOHO) spacecraft. We have used systematic observations extending
from disk center to the solar limb to compute average profiles for
representative solar features of the quiet Sun, limb-brightening curves,
and full-disk, quiet-Sun profiles for Lyβ through Lyλ(11) and the
Lyman continuum. The effects of radiative transfer are apparent in
all of the line profiles we studied. The average quiet-Sun profiles
for Lyβ through Lyɛ are self-reversed, and the remaining lines are
flat-topped. The characteristics of the line profiles vary markedly
with intensity. We observe strong enhancements in the red wings of
network profiles, while the faint cell-center profiles are nearly
symmetric. We also find that the intensities of the H Lyman lines
increase at the limb, although the limb brightening is weak compared
to optically thin transition region emission lines and largely obscured
by the intensity variations observed in the quiet Sun.
Title: The Electron Pressure in the Solar Lower Transition Region
Determined from O V and Si III Density-sensitive Line Ratios
Authors: Doschek, G. A.; Feldman, U.; Laming, J. M.; Warren, H. P.;
Schüle, U.; Wilhelm, K.
Bibcode: 1998ApJ...507..991D
Altcode:
We determine the electron density at the temperatures of formation
of O+4 and Si+2 ions, which are about 2.5 ×
105 and 3.2 × 104 K in ionization equilibrium,
respectively. These temperatures occur in the lower transition
region of the Sun's atmosphere and allow a test of the often invoked
assumption of constant pressure in quiet-Sun models. The O+4
density is determined from a density-sensitive spectroscopic O V
line ratio involving 2s2p3P-2p23P
transitions that fall near 760 Å. The Si+2 density is
determined from a density-sensitive Si III line ratio within the
3s3p3P-3p23P multiplet near 1300
Å. There are few available line ratio techniques for determining
the density and hence electron pressure in the quiet-Sun and coronal
hole transition regions using lines emitted by the same ion, and
determining these quantities is the principal motivation for this
work. The spectra used in our analysis were obtained from the Solar
Ultraviolet Measurements of Emitted Radiation (SUMER) experiment on the
Solar and Heliospheric Observatory (SOHO). We determine the electron
density and pressure in typical quiet-Sun/coronal hole regions, and
densities in active region brightenings and in an explosive event. Our
O V and Si III results indicate that constant pressure is valid or
nearly valid in quiet-Sun lower transition regions, although there
are complications arising from the weakness of a key Si III line in
the quiet-Sun disk spectra. We also discuss our results in light of
other density measurements and theories regarding the structure and
heating of the transition region.
Title: A new reference spectrum for the EUV irradiance of the quiet
Sun 2. Comparisons with observations and previous models
Authors: Warren, H. P.; Mariska, J. T.; Lean, J.
Bibcode: 1998JGR...10312091W
Altcode:
In a companion paper we presented a new reference spectrum for the EUV
irradiance of the quiet Sun based primarily on intensities calculated
from a quiet Sun emission measure distribution and recent compilations
of atomic data. The contributions of optically thick emission lines
and continua were included empirically. In this paper we present
comparisons between this reference spectrum and independently measured
irradiance observations, previous quiet Sun reference spectra, and
the predictions of empirical EUV irradiance models. These comparisons
indicate relatively good agreement among fluxes of emission lines
formed in the solar chromosphere and transition region. The fluxes for
coronal emission lines in previous quiet Sun reference spectra, however,
do not agree with our calculated fluxes or with a recent irradiance
observation taken at a low level of solar activity. Coronal emission
lines in the Atmospheric Explorer E (AE-E) quiet Sun reference spectrum
SC21REFW have fluxes that are typically smaller than our calculated
fluxes by factors of 2 or more. We also identify inconsistencies in the
earlier reference spectrum of Heroux and Hinteregger [1978] (F74113),
where the fluxes of many coronal emission lines with wavelengths below
250 Å are inconsistent with the fluxes of coronal emission lines at
longer wavelengths. The fluxes of EUV continua in the various reference
spectra and irradiance observations also differ significantly.
Title: A new reference spectrum for the EUV irradiance of the quiet
Sun 1. Emission measure formulation
Authors: Warren, H. P.; Mariska, J. T.; Lean, J.
Bibcode: 1998JGR...10312077W
Altcode:
We present a quiet Sun irradiance spectrum from 50-1200 Å based
primarily on intensities computed from a newly constructed quiet Sun
emission measure distribution. We derive the emission measure from a
spectrum of a portion of the quiet solar disk measured with the Harvard
instrument on Skylab and recent compilations of atomic data. For
some specific emission lines and continua which are not optically
thin and cannot be computed using an emission measure, we either use
intensities from the Harvard spectrum directly or infer them from
other observations. Application of a simple center-to-limb variation
converts the intensities to equivalent full-disk quiet Sun fluxes.
Title: Empirical Models of Temperature, Densities, and Velocities
in the Solar Corona
Authors: Fludra, A.; Strachan, L.; Alexander, D.; Bagenal, F.;
Biesecker, D. A.; Dobrzycka, D.; Galvin, A. B.; Gibson, S.; Hassler,
D.; Yo, Y. -K.; Panasyuk, A. V.; Thompson, B.; Warren, H.; del Zanna,
G.; Zidowitz, S.; Antonucci, E.; Bromage, B. J. I.; Giordano, S.
Bibcode: 1998EOSTr..79..278F
Altcode:
We present empirical results for temperatures, densities, and outflow
velocities of constituents of the solar corona from 1 to 3 Ro in
polar coronal holes and an equatorial streamer. Data were obtained
from a variety of space and ground-based instruments during August
1996 as part of the SOHO Whole Sun Month Campaign. From white light
data obtained with the SOHO/LASCO/C2 and HAO/Mauna Loa coronagraphs,
we determine electron densities and compare them to those determined
from the density-sensitive EUV line ratio of Si IX 350/342 Angstroms
observed by the SOHO/Coronal Diagnostic Spectrometer (CDS). Moreover,
from the white light density profiles we calculate temperature profiles
and compare to temperature diagnostic information from EUV lines and
soft X-ray images from Yohkoh. H I Ly alpha and O VI 1032/1037 Angstrom
intensities from the SOHO Ultraviolet Coronagraph Spectrometer (UVCS)
are used to estimate both the direction and magnitude of plasma outflow
velocities in coronal holes and streamers above 1.5 Ro. The velocities
are derived using densities from white light coronagraph data and
coronal electron temperature estimates derived from Ulysses/SWICS
ion composition data. Near the base of the corona we find the white
light and spectral analysis produce consistent density and temperature
information. In the extended corona we find results consistent with
high outflow velocities and a superradial outflow geometry in polar
coronal holes.
Title: Petrology of unique Fe-Ni metal bearing cumulate eucrite
EET92023.
Authors: Kaneda, K.; Warren, H. P.
Bibcode: 1998anme...23...45K
Altcode:
No abstract at ADS
Title: VDEM Analysis of Transition Region Line Profiles Observed
with the SUMER Instrument on SoHO
Authors: Winebarger, A. R.; Warren, H. P.; Emslie, A. G.; Mariska,
J. T.
Bibcode: 1997AAS...191.7302W
Altcode: 1997BAAS...29.1320W
The SUMER spectrometer has obtained numerous high spatial and spectral
resolution observations of optically-thin transition region line
profiles in various solar features (quiet Sun, active region, coronal
hole, etc.). Frequently, these line profiles show evidence for both
steady and impulsive mass motions, which can have profound implications
for the mass and energy balance of the transition region and
corona. These motions can be quantified using the Velocity Differential
Emission Measure (VDEM) technique to derive the distribution of emission
with respect to the line-of-sight velocity. This VDEM distribution
can then be used to infer mean plasma velocities, momenta, and energy
fluxes. We have found that representations of the energy flux by
either the ``mean flow approximation'' {1 / 2}n m{bar v}(3) or by the
``enthalpy approximation''(whether in a purely thermal form nkT {bar v}
or incorporating nonthermal turbulence {1 / 2}n m /line {v(2}) {bar
v}) all underestimate the true energy flux {1 / 2} n m /line{v(3})
by up to an order of magnitude. In cases where lines formed at
different temperatures have been observed in the SUMER spectral range
simultaneously, we have estimated the divergence of the energy flux
and so the energy deposition rate within the region bounded by the
formation heights of the two spectral lines; implications for coronal
heating are discussed. This work was supported by grants from the SoHO
Guest Investigator Program and by the Office of Naval Research.
Title: Observations of Doppler Shifts in a Solar Polar Coronal Hole
Authors: Warren, H. P.; Mariska, J. T.; Wilhelm, K.
Bibcode: 1997ApJ...490L.187W
Altcode:
Using observations from the Solar Ultraviolet Measurements of Emitted
Radiation experiment flown on the Solar and Heliospheric Observatory
spacecraft, we have measured Doppler wavelength shifts in the north
polar coronal hole in the 1032 and 1038 Å emission lines of O VI and
the 1036 and 1037 Å emission lines of C II relative to chromospheric
emission lines. These observations were obtained on 1996 November 2
when the north polar coronal hole boundary extended southward to about
750'' (cosθ=0.65). Our measurements indicate the presence
of average net redshifts in coronal holes at temperatures of less than
2.9×105 K. Measurements of systematic wavelength shifts
in the Ne VIII resonance lines relative to the quiet Sun suggest a
transition to average net outflows near 6.3×105 K in the
coronal hole.
Title: Doppler Shifts and Nonthermal Broadening in the Quiet Solar
Transition Region: O VI
Authors: Warren, H. P.; Mariska, J. T.; Wilhelm, K.; Lemaire, P.
Bibcode: 1997ApJ...484L..91W
Altcode:
Using observations from the Solar Ultraviolet Measurements of Emitted
Radiation (SUMER) experiment flown on the Solar and Heliospheric
Observatory (SOHO) spacecraft, we have measured Doppler wavelength
shifts and nonthermal broadening in the 1032 and 1038 Å emission lines
of O VI and the 1036 and 1037 Å emission lines of C II. Near Sun center
the C II lines exhibit an average redshift of 2 +/- 3 km s-1,
consistent with earlier observations in this temperature range. The
O VI emission lines exhibit average Doppler velocities of 5 +/- 3
km s-1, suggesting the presence of redshifted material at
2.9 × 105 K. For the O VI lines, the average nonthermal
component of the observed line width is 34 +/- 3 km s-1.
Title: Electron Densities in the Solar Polar Coronal Holes from
Density-Sensitive Line Ratios of Si VIII and S X
Authors: Doschek, G. A.; Warren, H. P.; Laming, J. M.; Mariska, J. T.;
Wilhelm, K.; Lemaire, P.; Schühle, U.; Moran, T. G.
Bibcode: 1997ApJ...482L.109D
Altcode:
We derive electron densities as a function of height in the north and
south polar coronal holes from a forbidden spectral line ratio of Si
VIII. Si VIII is produced at about 8 × 105 K in ionization
equilibrium. We also derive densities from a similar line ratio of S X
(1.3 × 106 K). The spectra were obtained with the Solar
Ultraviolet Measurements of Emitted Radiation spectrometer flown on
the Solar and Heliospheric Observatory spacecraft. In addition to the
primary mechanism of electron impact excitation, the derivation of
theoretical level populations for Si VIII and S X includes both proton
and resonance capture excitation. We compare the coronal hole results
to quiet-Sun coronal measurements obtained outside the east and west
limbs. We find for distances of a few arcseconds outside the solar
limb that the average line-of-sight electron densities in the coronal
holes are about a factor of 2 lower than in quiet-Sun regions. The
decrease of density with height is exponential in the polar holes. We
also confirm the result known from a variety of earlier observations
that the temperature of most of the plasma in coronal holes does not
exceed about 106 K.
Title: The Electron Density, Temperature, and Si/Ne Abundance Ratio
in Polar Coronal Holes from SUMER
Authors: Doschek, G. A.; Laming, J. M.; Warren, H. P.; Lemaire, P.;
Wilhelm, K.
Bibcode: 1997SPD....28.0404D
Altcode: 1997BAAS...29Q.908D
The Solar Ultraviolet Measurements of Emitted Radiation (SUMER)
spectrometer flown on the Solar and Heliospheric Observatory (SOHO)
covers a wavelength range with spectral and spatial resolution and
sensitivity not previously obtained by any other solar spectrometer
experiment. Consequently, new plasma diagnostic techniques have been
used and developed to measure plasma parameters such as electron
density. In this paper we discuss the electron density as a function
of height above the solar surface in the polar coronal holes as
determined from line ratios of Si VIII and S X. The densities vary
between about 3 x 10(6) cm(-3) and 10(8) cm(-3) . We determine the
emission measure at selected temperatures as a function of position
above the polar limbs from lines of Mg VII, Mg VIII, Mg IX, and Mg
X. The electron temperature is lower in the polar holes than in the
quiet Sun. This result is also based on intensities of lines of Fe X,
Fe XI, and Fe XII. We determine the Si/Ne abundance ratio (low/high
first ionization potential elements) using lines of Ne VII, Ne VIII,
Si VII, and Si VIII. The preliminary result is that the Si/Ne abundance
ratio in interplume regions is close to photospheric. We also compare
the coronal hole density and abundance results with similar results
obtained for the quiet Sun in the east and west coronal streamers. We
discuss our conclusions in light of results from previous missions.
Title: Observations of Doppler Shifts and Nonthermal Broadening in
the North Polar Coronal Hole and Adjacent Quiet Sun
Authors: Mariska, J. T.; Warren, H. P.; Wilhelm, K.; Lemaire, P.
Bibcode: 1997SPD....28.0118M
Altcode: 1997BAAS...29..882M
Coronal holes are thought to be the source of high-speed solar
wind streams. It is, however, unclear at what height the outflow
of material first becomes apparent. Using the Solar Ultraviolet
Measurements of Emitted Radiation (SUMER) experiment on the Solar and
Heliospheric Observatory (SOHO), we have obtained observations in the
north polar coronal hole, its boundary region, and the adjacent quiet
Sun at wavelengths that cover emission lines of O VI at 1032 and 1038
Angstroms, Ne VIII at 770 Angstroms, and Mg X at 625 Angstroms. These
lines are formed at temperatures of 0.3, 0.8, and 1.25 MK, respectively,
and should thus help to determine the temperature at which outflows
are first detected. For O VI and Mg X, we will report on Doppler shift
measurements made relative to cooler chromospheric lines. Since there
are no suitable emission lines from ions formed in the chromosphere
for the Ne VIII line, we are only able to measure relative differences
between the coronal hole and the adjacent quiet Sun. We will also
report on the magnitude of the nonthermal broadening in these lines
in the different solar regions. This work was supported by a NASA SOHO
Guest Investigator Program grant.
Title: Doppler Shifts and Nonthermal Broadening in the Quiet Solar
Transition Region: O VI
Authors: Warren, H. P.; Mariska, J. T.; Wilhelm, K.; Lemaire, P.
Bibcode: 1997SPD....28.0117W
Altcode: 1997BAAS...29..882W
Using observations from the Solar Ultraviolet Measurements of Emitted
Radiation (SUMER) experiment flown on the Solar and Heliospheric
Observatory (SOHO ) spacecraft, we have measured Doppler wavelength
shifts and nonthermal broadening in the 1032 and 1038 Angstroms emission
lines of Ovi and the 1036 and 1037 Angstroms emission lines of Cii. Near
Sun center the Cii lines exhibit an average redshift of 2+/-3km s(-1)
, consistent with earlier observations in this temperature range. The
Ovi emission lines exhibit average Doppler velocities of 5+/-3km s(-1)
suggesting the presence of redshifted material at 2.9*E(5) K. For the
Ovi lines, the average nonthermal component of the observed line width
is 34+/-3km s(-1) . These observations indicate a tendency for brighter
regions to have larger average redshifts and line widths than faint
features although the relationship is very weak. Preliminary analysis of
observations at the limb suggests that the Doppler velocities for Ovi do
not approach zero as would be expected for predominately radial motions.
Title: Determination of the Formation Temperature of Si IV in the
Solar Transition Region
Authors: Doschek, G. A.; Mariska, J. T.; Warren, H. P.; Wilhelm, K.;
Lemaire, P.; Kucera, T.; Schühle, U.
Bibcode: 1997ApJ...477L.119D
Altcode:
Using spectra obtained with the Solar Ultraviolet Measurements
of Emitted Radiation (SUMER) spectrometer flown on the Solar and
Heliospheric Observatory spacecraft, we deduce the temperature
of formation of the Si IV ion in the solar transition region
from the Si IV ultraviolet spectral line intensity ratio, 3p
2P3/2-3d 2D3/2,5/2/3s
2S1/2-3p 2P1/2,
and compare the result to the temperature predicted under the
assumption of ionization equilibrium. The wavelengths are as
follows: 2D3/2,5/2, 1128.325, 1128.340 Å
2P1/2, 1402.770 Å. Ratios are derived for
typical features of the quiet Sun, such as cell center and network,
and are systematically higher than those predicted at the 6.3 ×
104 K ionization equilibrium temperature of formation
of Si IV. For most solar features the ratios imply a temperature
of formation of about 8.5 × 104 K. The ratios for the
faintest features imply a temperature of formation of up to 1.6 ×
105 K. It is not clear, however, that all the discrepancies
between the measured and theoretical ratios are due to a temperature
effect. Accurate temperature measurements are important since a large
discrepancy from ionization equilibrium has significant implications
for the physics of the transition region, such as the possible presence
of nonthermal electrons.
Title: Trace element chemistry of volcanic glasses in lunar meteorites
Y 793274 and QUE 94281.
Authors: Arai, T.; Warren, H. P.; Papike, J. J.; Shearer, K. C.;
Takeda, H.
Bibcode: 1997anme...22....3A
Altcode:
No abstract at ADS
Title: The Yamato-793605 martian meteorite consortium.
Authors: Kojima, H.; Miyamoto, M.; Warren, H. P.
Bibcode: 1997anme...22...91K
Altcode:
No abstract at ADS
Title: Yamato-793605 and other presumed martian meteorites:
Composition and petrogenesis.
Authors: Warren, H. P.; Kallemeyn, W. G.
Bibcode: 1997anme...22..200W
Altcode:
No abstract at ADS
Title: Modeling solar extreme ultraviolet irradiance variability
using emission measure distributions
Authors: Warren, H. P.; Mariska, J. T.; Lean, J.; Marquette, W.;
Johannesson, A.
Bibcode: 1996GeoRL..23.2207W
Altcode:
We introduce a new model of solar irradiance variability at extreme
ultraviolet (EUV) wavelengths. The model combines a spectral emission
line database, solar emission measure distributions, and estimates
from ground-based solar images of the fraction of the Sun covered by
the various types of activity to synthesize the irradiance. Using Call
K-line images, the model can be used to estimate the irradiance from
EUV line emission formed in the upper chromosphere and lower transition
region. Comparisons of this new model with existing empirical models
reveal both similarities and disagreements in the absolute magnitude,
the amplitude of the rotational modulation, and the intermediate-term
solar cycle variability of the predicted fluxes.
Title: A New Model of Solar EUV Irradiance Variability
Authors: Warren, H. P.; Mariska, J. T.; Lean, J.
Bibcode: 1996AAS...188.3617W
Altcode: 1996BAAS...28R.875W
Solar soft X-ray (SXR, 1-100 Angstroms) and extreme ultraviolet (EUV,
100--1200 Angstroms) radiation plays a central role in the energetics
and dynamics of the Earth's upper atmosphere. Solar radiation at
these wavelengths is strongly affected by solar magnetic activity
and varies significantly during the solar activity cycle. Empirical
models of solar irradiance variability essentially parameterize
existing full-disk irradiance observations with proxies for solar
activity. However, the limitations of existing EUV observations and
absence of any current irradiance measurements at these wavelengths
limits the utility of empirical irradiance modeling. Motivated by solar
physics experiments on Yohkoh, SOHO, and TRACE we have developed a
new, physics-based approach to modeling solar SXR and EUV irradiance
variability. In this new model, the intensities of optically thin
spectral lines are calculated using theoretically determined values
of plasma emissivity coupled with emission measure distributions for
features of the solar atmosphere: coronal holes, quiet Sun, and active
regions. For emission lines with very complicated formation processes,
such as the Lyman lines of hydrogen and helium, spatially and spectrally
resolved solar observations are used in place of emission measure
calculations. Information about the distribution of emitting regions
on the Sun is inferred from full-disk images of the Sun, such as BBSO
CaII k-line and Yohkoh SXT images, rather than from proxies for solar
activity. Comparison of the model with existing empirical irradiance
models based on F_{10.7} and other proxies for solar activity reveals
disagreements in the absolute magnitude, the amplitude of the rotational
modulation, and the solar cycle variability of the predicted fluxes at
many wavelengths. This research was supported by the NASA SEE program.
Title: VLT-mare glasses of probable pyroclastic origin in lunar
meteorite breccias Yamato 793274 and QUE94281.
Authors: Arai, T.; Warren, H. P.
Bibcode: 1996anme...21....4A
Altcode:
No abstract at ADS
Title: Compositional-petrologic investigation of eucrites and the
QUE94201 Shergottite.
Authors: Warren, H. P.; Kallemeyn, W. G.; Arai, T.; Kaneda, K.
Bibcode: 1996anme...21..195W
Altcode:
No abstract at ADS
Title: Consortium investigation of the Asuka-881371 angrite:
Petrographic, electron microprobe, and ion microprobe observations.
Authors: Warren, H. P.; Davis, M. A.
Bibcode: 1995anme...20..257W
Altcode:
No abstract at ADS
Title: Chemical variations of spinels in Asuka-881757.
Authors: Arai, T.; Takeda, H.; Warren, H. P.
Bibcode: 1995anme...20....4A
Altcode:
No abstract at ADS
Title: Consortium investigation of the Asuka-881371 angrite: Bulk-rock
geochemistry and oxygen isotopes.
Authors: Warren, H. P.; Kallemeyn, W. G.; Mayeda, T.
Bibcode: 1995anme...20..261W
Altcode:
No abstract at ADS
Title: On Arnol'd diffusion in a perturbed magnetic dipole field
Authors: Warren, Harry P.; Bhattacharjee, A.; Mauel, Michael E.
Bibcode: 1992GeoRL..19..941W
Altcode:
For certain initial conditions, the motion of charged particles in a
magnetic dipole field is well described by the hierarchy of adiabatic
invariants: the magnetic moment μ, the longitudinal invariant J, and
the magnetic flux ψ. Electrostatic waves that break the axisymmetry
of the dipole field and resonate with the drift motion can generate
large-scale or so-called thick-layer chaos in ψ. This chaos will
drive Arnol'd diffusion in μ and J, making the motion asymptotically
unstable. Previous studies involving mappings [Tennyson et al., 1980;
Kook and Meiss, 1989] have found the thick-layer Arnol'd diffusion
rate to be proportional to the square of the perturbtion amplitude,
consistent with quasilinear theory. Here we present numerical evidence
that for many cases of physical interest, such as particle motion in
a perturbed dipole field, the thick-layer diffusion rate is greatly
attenuated from the quasilinear result.
Title: Lithophile, siderophile, and volatile geochemistry (consortium
investigations) of two mare-basaltic meteorites.
Authors: Warren, H. P.; Kallemeyn, W. G.
Bibcode: 1992anme...17..113W
Altcode:
No abstract at ADS
Title: Lunar meteorites: A survey of the first eight district moon
rocks from Antarctica.
Authors: Warren, H. P.
Bibcode: 1990anme...15..131W
Altcode:
No abstract at ADS
Title: Geochemistry of lunar meteorite Yamato-86032.
Authors: Warren, H. P.; Kallemeyn, W. G.
Bibcode: 1988anme...13...12W
Altcode:
No abstract at ADS
Title: Antarctic Meteorites XII. Papers presented to the 12th
Symposium on Antarctic Meteorites, NIPR, Tokyo, 8-10 June 1987.
Authors: Jerde, A. E.; Warren, H. P.; Heiken, H. G.; Vaniman, T. D.
Bibcode: 1987anme...12.....J
Altcode:
No abstract at ADS
Title: New data for the bulk compositions of four lunar meteorites,
and for an Fe-rich basaltic clast of probable VLT-mare affinity
from Y-791197.
Authors: Warren, H. P.; Kallemeyn, W. G.
Bibcode: 1987anme...12...22W
Altcode:
No abstract at ADS
Title: Geochemistry of lunar meteorite Yamato-82192: Comparison with
Yamato-791197, ALHA81005 and other lunar samples.
Authors: Warren, H. P.; Kallemeyn, W. G.
Bibcode: 1986anme...11...31W
Altcode:
No abstract at ADS
Title: A potpourri of Apollo regolith breccias: Analogs of lunar
meteorites.
Authors: Jerde, A. E.; Warren, H. P.; Heiken, H. G.; Vaniman, T. D.
Bibcode: 1986anme...11..162J
Altcode:
No abstract at ADS
Title: Geochemistry of lunar meteorites Yamato-971197 and ALHA81005.
Authors: Warren, H. P.; Kallemeyn, W. G.
Bibcode: 1985anme...10...90W
Altcode:
No abstract at ADS
Title: Submicrosecond Comparison of Intercontinental Clock
Synchronization by VLBI and the NTS Satellite
Authors: Hurd, W. J.; Wardrip, S. C.; Bussion, J.; Oaks, J.; McCaskill,
T.; Warren, H.; Whitworth, G.
Bibcode: 1978DSNPR..49...64H
Altcode:
No abstract at ADS