explanation blue bibcodes open ADS page with paths to full text
Author name code: alexander
ADS astronomy entries on 2022-09-14
author:"Alexander, David" AND (aff:"Rice" OR aff:"Lockheed")
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Title: Effects of Region 2 Field-Aligned Currents on Ionospheric
Saturation and Resulting Impact on Auroral Radio Emission
Authors: Tracy, Preston; Sciola, Anthony; Toffoletto, Frank; Alexander,
David; Merkin, Viacheslav; Sorathia, Kareem
2021AGUFMSM25D2036T Altcode:
We compare the effects of ionospheric saturation on auroral radio
emission when driven by the differing sources of an increased
ionospheric conductance and a decreased solar wind conductance
through the use of the Multiscale Atmosphere Geospace Environment
(MAGE) developed by the Center for Geospace Storms (CGS), which
includes a coupled global magnetohydrodynamic (MHD) and inner
magnetosphere model. This work is of particular interest to the
detection of exoplanets, as the current method for estimating radio
emission for magnetized Solar System planets, the empirically derived
Radiometric Bodes Law (RBL), would predict exoplanets that produce
radio emission capable of being detected with current observational
methods. Because there have been no confirmed detections of exoplanetary
radio emission yet, RBL is expected to be inaccurate for these extreme
environments. Ionospheric saturation is a compelling explanation for
the reduced radio emission of these exoplanets. While the mechanism by
which ionospheric saturation occurs is still unresolved, the conditions
for a saturated system are agreed to be met for a planetary system
in which the ionospheric (Pedersen) conductance dominates the solar
wind (Alfven) conductance. Current analytic models for estimating
the effects of ionospheric saturation on auroral radio emission
give a proportional relationship between the radio emission and the
ratio of Alfven and Pedersen conductances. We explore the effect on
radio emission via the two avenues of saturation: increasing Bz (the
main mechanism of saturation during geomagnetic storms at Earth),
and increasing the ionospheric conductance (the likely cause of
saturation at exoplanets). We examine this relation using the MAGE
model including the Magnetosphere-Ionosphere Coupler (ReMIX), which
is used to compute radio emission from region 1 (high-latitude) field
aligned currents (FACs) and the Rice Convection Model (RCM) which
is used to compute the radio emission from region 2 (low-latitude)
FACs. This work offers a unique insight into testing how region 2
currents scale with the degree of saturation. This work was supported
by the Rice Space Institute and the Chancey and Evelyn Juday Endowment.
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Title: Simulating the X-ray and EUV Emission of Cool Exoplanet
Host Stars
Authors: Farrish, Alison; Barnes, Will; Alexander, David; Garcia-Sage,
Katherine
2021AGUFM.P55D1954F Altcode:
By analogy with the Earth-Sun system, the search for potentially
habitable exoplanets focuses mainly on terrestrial exoplanets orbiting
cool stars. Cool stars include the range of partially-convective stars
of late-F, G, K, and early-M types, in addition to fully-convective
late-M stars. In previous work we have employed a surface flux transport
(SFT) model (Schrijver 2001, Schrijver et al. 2003) to examine the
emergence and dynamics of magnetic flux on the surfaces of cool stars
like the Sun and other exoplanet host stars of interest. Terrestrial
exoplanets orbiting cool stars are influenced by their host stars
in a variety of ways, including via interaction with the stellar
magnetic field (Garaffo et al. 2016, Farrish et al. 2019) and by
stellar coronal X-ray emission (e.g., Farrish et al. 2021) which may
ionize planetary atmospheric gases. Exoplanet atmospheres are also
influenced by stellar emission in the extreme ultraviolet (EUV)
wavelength regime (~100-900 Å) through photochemical reactions
and escape processes. An understanding of the high-energy emission
of the central host star through its X-ray and EUV (collectively,
XUV) output is therefore integral to the study of atmospheric and
ionospheric evolution at the associated exoplanets. However, stellar
EUV observations are historically extremely sparse (Youngblood et
al. 2019). Thus, detailed modeling of the dependence of host star
XUV emission on stellar magnetic activity can fill many gaps in our
current understanding of exoplanet atmospheric processes. We present
a study integrating our previous simulations of exoplanet host star
magnetic activity with models of coronal heating and the associated
XUV emission for a range of cool stars. Particular attention is paid
to the relevance of this high-energy emission to atmospheric processes
at the associated planets.
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Title: Modeling Stellar Activity-rotation Relations in Unsaturated
Cool Stars
Authors: Farrish, Alison O.; Alexander, David; Johns-Krull, Christopher
M.; Li, Minjing
2021ApJ...916...99F Altcode:
We apply a surface flux transport model developed for the
Sun to reconstruct the stellar activity-rotation relationship,
L<SUB>X</SUB>/L<SUB>bol</SUB> versus Ro, observed for unsaturated
cool stars (Rossby numbers Ro ≳ 0.1). This empirical flux transport
model incorporates modulations of magnetic flux strength consistent
with observed solar activity cycles, as well as surface flux dynamics
consistent with observed and modeled stellar relationships. We find
that for stellar flux models corresponding to a range of 0.1 ≲
(Ro/Ro<SUB>Sun</SUB>) ≲ 1.2, the L<SUB>X</SUB>/L<SUB>bol</SUB>
versus Ro relation matches well the power-law behavior observed in
the unsaturated regime of cool stars. Additionally, the magnetic
activity cycles captured by the stellar simulations produce a spread
about the power-law relation consistent with that observed in cool
star populations, indicating that the observed spread may be caused
by intrinsic variations resulting from cyclic stellar behavior. The
success of our flux transport modeling in reproducing the observed
activity relationship across a wide range of late-F, G, K, and M stars
suggests that the photospheric magnetic fields of all unsaturated cool
stars exhibit similar flux emergence and surface dynamic behavior,
and may hint at possible similarities in stellar dynamo action across
a broad range of stellar types.
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Title: Incorporating Inner Magnetosphere Current-driven Electron
Acceleration in Numerical Simulations of Exoplanet Radio Emission
Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David;
Sorathia, Kareem; Merkin, Viacheslav; Farrish, Alison
2021ApJ...914...60S Altcode:
We present calculations of auroral radio emission for an Earth-like
planet produced by field-aligned current (FAC) driven electron
acceleration using a coupled global magnetohydrodynamic (MHD) and
inner magnetosphere model, extending the capabilities of previous
works which focus solely on the direct transmission of magnetic energy
between the stellar wind and ionosphere. Magnetized exoplanets are
expected to produce radio emission via interaction between the host
star's stellar wind and planetary magnetosphere-ionosphere system. The
empirically derived Radiometric Bode's Law (RBL) is a linear relation
between the magnetic solar wind power and total emitted radio power
from magnetized Solar System planets, and is often extrapolated to
extreme exoplanet systems. It has been shown that the magnitudes
of the FACs coupling the stellar wind to planetary ionospheres are
likely to be significantly limited (often referred to as ionospheric
saturation), resulting in an estimated radio power up to several
orders of magnitude less than that predicted by RBL. In this paper,
we demonstrate the significance of intense, sporadic FACs, driven by
nightside magnetic reconnection and inner magnetosphere plasma flow,
to the total radio power produced by wind-ionosphere interaction in
terrestrial planets. During periods of strong stellar wind variability,
the contribution from these secondary currents can be over an order
of magnitude greater than the primary current systems that previous
models describe. The results highlight the role of the variability
of the stellar wind on the magnitude and location of the resulting
emission, subsequently affecting the conditions for detectability.
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Title: Using the Sun-Earth Interaction to Explore Exoplanetary Systems
Authors: Alexander, D.; Toffoletto, F.; Farrish, A.; Sciola, A.
2021BAAS...53c1025A Altcode:
A crucial component in assessing the potential habitability of
an exoplanet is an understanding of its interaction with the host
star. As more terrestrial "habitable zone" exoplanets are discovered,
the detailed characterization of the space environment of these planets
raises new challenges, both from a physical and an observational
perspective. The "space weather" environment of the planet is primarily
governed by the level of magnetic activity of the star (XUV flux,
stellar wind and high energy transients), the orbital distance of the
planet, the nature and strength of the exoplanet's magnetic field and
the magnetic and electromagnetic interactions of this coupled system. To
address this, we take advantage of the wealth of knowledge gained about
the sole existing habitable system of which we are sure, namely, the
Sun-Earth system. We approach this by modeling the stellar activity,
which governs much of the expected star-planet interaction and so has
an important role to play on potential habitability, and the planetary
response, which enables us to place constraints on the expected emission
signatures of the star-planet interaction. <P />On the stellar side,
we employ a magnetic flux transport model (SFT), devised from a full
22-yr solar magnetic cycle, to characterize the asterospheric magnetic
field in systems with stars of varying levels of activity, up to 10x
that of the Sun. This empirical flux transport model incorporates
modulations of magnetic flux strength consistent with observed solar
activity cycles, as well as surface flux dynamics consistent with
observed stellar relationships. We verify the viability of the SFT
model for application to stars other than the Sun by reproducing the
observed stellar activity-rotation relationship across a wide range of
stellar types. We find that the simulations match the activity-rotation
relationship in the unsaturated regime of cool stars extremely well
and that the observed spread in the observations can be reasonably
explained as a result of cycle variability. From our modeling of
the asterospheric field at the various levels of activity consider,
we are able to detail the star-exoplanet interaction through several
quantitative measures such as the ratio of open to total stellar
magnetic flux and its variation with stellar latitude, the location
and variability of the mean stellar Alfven surface, and the strength
of interplanetary magnetic field polarity inversions, all of which have
the potential to influence the magnetic environment of the exoplanet. <P
/>On the planetary side, we explore the coupling of the stellar activity
to the planetary magnetic environment and determine whether or not such
interactions produce potentially observable signatures. In this work,
we focus on the expected signatures of auroral radio emission for
Earth-like planets orbiting active stars. Magnetized exoplanets are
expected to produce radio emission via interaction between the host
star's stellar wind and planetary magnetosphere-ionosphere system both
of which can be significantly enhanced for very active stars. Auroral
radio emission is produced by field-aligned current (FAC) driven
electron acceleration and this is calculated using a coupled global
magnetohydrodynamic (MHD) and inner magnetosphere model, extending the
capabilities of previous work. We find that intense, sporadic FACs,
driven by night-side magnetic reconnection and inner magnetosphere
plasma flow, contribute significantly to the total radio power produced
by wind-ionosphere interaction in terrestrial planets. During periods
of strong stellar wind variability, the contribution from these
secondary currents can be up to several orders of magnitude greater
than the primary current systems which previous models describe. This
may be even more pronounced for systems in which the primary current
system is strongly limited (e.g. ionospheric saturation). The results
suggest that magnetized exoplanets may temporarily produce greater
radio power than previously estimated increasing their likelihood
of producing a detectable signature. Additionally, due to the strong
beaming of the emission, the ideal observing angle is dependent on the
intensity of the interaction between the stellar wind and exoplanetary
magnetosphere. Such observations could provide direct information on
the strength of the planetary magnetic field and consequently knowledge
about planetary dynamos, planetary evolution, atmospheric escape,
and the offset of magnetic and rotation axes.
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Title: Acceleration of Non-Maxwellian Electron Distributions and
Estimates of Radio Emission Observables
Authors: Sciola, A.; Toffoletto, F.; Lin, D.; Alexander, D.
2020AGUFMSA0270002S Altcode:
Mono-energetic electron precipitation is driven by strong
field-aligned currents (FACs) which require the acceleration of
otherwise assumed to be Maxwellian electron distributions. In many MHD
magnetosphere-ionosphere models the associated field-aligned potential
drop is calculated using the MHD plasma temperature, which is also
assumed to be Maxwellian. In this work, we utilize the discrete electron
energy channels used by the Rice Convection Model (RCM) coupled to the
3D MHD model GAMERA, developed as part of the NASA DRIVE Science Center
for Geospace Storms (CGS), to test the validity of the assumption of a
Maxwellian electron distribution, and the effects on the field-aligned
potential drop when the distribution deviates from the Maxwellian
model. Additionally we estimate the radio emission produced by the
accelerated electrons via the Electron Cyclotron Maser Instability
(ECMI) as would be observable by satellite.
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Title: Anomalous Active Region Dynamics and Influence on Activity
Cycles
Authors: Farrish, A.; Alexander, D.
2020AGUFMSH006..05F Altcode:
We present a study of the influence of anomalous active regions on
solar variability, in particular on the suppression of solar activity
cycles in grand minima. The study involves the application of a Solar
Flux Transport (SFT) model [Schrijver 2001] which has, in this work,
been modified to allow for the emergence and evolution of anomalously
oriented (anti-Hale and/or anti-Joy) active regions. Such anomalous
active regions have been shown preliminarily to `shut down' dynamo
action in dynamo modeling efforts, potentially explaining the appearance
of Grand Minima in the Sun's cycle behavior [Nagy et al. 2017]. <P
/>Following on from these efforts, we test the behavior of anomalous
active regions with the computationally lightweight SFT model, which
tracks the emergence, evolution, and decay of photospheric magnetic
flux concentrations without complex feedback to interior dynamo
models. Previous work [Farrish et al. 2019, Farrish et al. 2020 (in
prep.)] that the SFT model can be extended to represent the behavior
of other cool stars. We aim to study the behavior and influence of
anomalous active regions on magnetic activity cycles for the Sun and
other stars, with an eye toward the impact of magnetic cycle variability
on associated exoplanets.
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Title: Modeling Stellar Activity-Rotation Relations in Unsaturated
Cool Stars
Authors: Farrish, A.; Alexander, D.; Johns-Krull, C.; Li, M.
2020SPD....5120702F Altcode:
We apply a surface flux transport (SFT) model developed for the
Sun to reconstruct the stellar activity-rotation relationship,
L<SUB>X</SUB>/L<SUB>bol</SUB> vs. Ro, observed for "unsaturated" cool
stars (Rossby numbers Ro > about 0.1). This empirical flux transport
model incorporates modulations of magnetic flux strength consistent
with observed stellar activity cycles, as well as surface flux dynamics
consistent with observed stellar relationships. We find that for stellar
flux models corresponding to a range ~0.1 < (Ro/Ro<SUB>Sun</SUB>)
< 2, the L<SUB>X</SUB>/L<SUB>bol</SUB> vs. Ro relation matches the
power-law behavior observed in the unsaturated regime of cool stars
extremely well. Additionally, the magnetic activity cycles captured by
the stellar simulations produce a spread about the power-law relation
consistent with that observed in cool star populations, indicating that
the observed spread may be caused by intrinsic variations resulting from
cyclic stellar behavior. The success of our flux transport modeling
in reproducing the observed activity relationship across a wide range
of late-F, G, K, and M stars suggests that the photospheric magnetic
fields of all unsaturated cool stars exhibit similar flux emergence
and surface dynamic behavior, and may hint at possible similarities
in stellar dynamo action across a broad range of stellar types.
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Title: Modeling Luminosity-Activity Relations in Unsaturated Cool
Stars
Authors: Farrish, A.; Alexander, D.; Johns-Krull, C.
2020AAS...23620706F Altcode:
We investigate whether cyclic variations in stellar activity can
account for the observed spread in fractional X-ray luminosity,
L<SUB>X</SUB>/L<SUB>bol</SUB>, for cool stars in the unsaturated range
of Rossby number, Ro ≳ 0.1. To address this question, we employ an
empirical flux transport model of the stellar surface, incorporating
modulations of magnetic flux strength consistent with observed stellar
activity cycles. We find that for stellar flux models corresponding
to a range ~0.1 ≲ Ro ≲ 2, the L<SUB>X</SUB>/L<SUB>bol</SUB>
vs. Ro relation matches well the power-law behavior observed in
the "unsaturated" regime of cool stars. Additionally, the magnetic
activity cycles incorporated into the stellar simulations produce
a spread about the power-law relation consistent with the observed
spread in unsaturated cool star populations. We find, therefore,
that the solar-based flux transport approach employed in this work
can reproduce the X-ray luminosity-magnetic activity relation observed
across the range of unsaturated late-F, G, K, and M stars, providing
support for the hypothesis of a universal dynamo mechanism operating
in all unsaturated cool stars. We further conclude that the spread in
fractional X-ray luminosity, L<SUB>X</SUB>/L<SUB>bol</SUB>, across
the unsaturated range of stellar activity corresponding to ~0.1 ≲
Ro ≲ 2 can be explained by the intrinsic variation due to stellar
activity cycles.
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Title: Characterizing the Magnetic Environment of Exoplanet Stellar
Systems
Authors: Farrish, Alison O.; Alexander, David; Maruo, Mei; DeRosa,
Marc; Toffoletto, Frank; Sciola, Anthony M.
2019ApJ...885...51F Altcode:
We employ a flux transport model incorporating observed stellar
activity relations to characterize stellar interplanetary fields
on cycle timescales for a range of stellar activity defined by the
Rossby number. This framework allows us to examine the asterospheric
environments of exoplanetary systems and yields references against
which exoplanetary observations can be compared. We examine several
quantitative measures of star-exoplanet interaction: the ratio of open
to total stellar magnetic flux, the location of the stellar Alfvén
surface, and the strength of interplanetary magnetic field polarity
inversions, all of which influence planetary magnetic environments. For
simulations in the range of Rossby numbers considered (0.1-5
Ro<SUB>Sun</SUB>), we find that (1) the fraction of open magnetic flux
available to interplanetary space increases with Rossby number, with a
maximum of around 40% at stellar minimum for low-activity stars, while
the open flux for very active stars (Ro ∼ 0.1-0.25 Ro<SUB>Sun</SUB>)
is ∼1-5% (2) the mean Alfvén surface radius, R <SUB>A</SUB>,
varies between 0.7 and 1.3 R <SUB>A,Sun</SUB> and is larger for
lower stellar activity; and (3) at high activity, the asterospheric
current sheet becomes more complex with stronger inversions, possibly
resulting in more frequent reconnection events (e.g., magnetic storms)
at the planetary magnetosphere. The simulations presented here serve to
bound a range of asterospheric magnetic environments within which we
can characterize the conditions impacting any exoplanets present. We
relate these results to several known exoplanets and discuss how they
might be affected by changes in asterospheric magnetic field topologies.
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Title: Planetary Magnetic Response to Young Star Stellar Wind
Environment
Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David
2019ESS.....432208S Altcode:
Young stars are known to exhibit strong magnetic activity in the form
of both steady and eruptive outflow. This, combined with the majority
of known Earth-like planets orbiting their stars closer than Mercury
orbits the Sun, results in an extreme stellar wind environment at the
planet. We employ a 3D coupled MHD model to simulate the planetary
response to this environment with an emphasis on the exchange of plasma
between the planetary magnetosphere, ionosphere and stellar wind. We
will present the results of two cases: the case of extreme, steady
stellar wind and the case of successive Sun-like Coronal Mass Ejection
(CME) impacts over short timescales. These results demonstrate how
exoplanetary environments differ from Earth's and provide insight into
how early stage star-planet interactions may impact future evolution
of the planetary environment.
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Title: Simulating the Inner Asterospheric Magnetic Fields of Exoplanet
Host Stars
Authors: Farrish, Alison; Alexander, David; Maruo, Mei; Sciola,
Anthony; Toffoletto, Frank; DeRosa, Marc L.
2019AAS...23430305F Altcode:
We study magnetic and energetic activity across a range of stellar
behavior via the application of an observationally-based heliophysics
modeling framework. We simulate the inner asterospheric magnetic fields
of host stars with the aim of better understanding and constraining the
space weather environments of exoplanets, and improving our knowledge of
the solar-stellar connection. As astronomy instrumentation has improved,
Earth-like exoplanets are increasingly being found orbiting in the
habitable zones of a variety of stars, ranging from the smallest
and coolest M dwarfs to larger and more solar-like stars. We are
therefore interested in characterizing a broad range of stellar magnetic
activity and the resulting impacts on asterospheric environments. We
will present our work simulating stellar magnetic activity on cycle
timescales via the integration of modeled magnetic flux emergence,
coronal field structure and related plasma emission, and stellar
winds. We use this self-consistent framework of heliophysics-based
models to simulate stellar and asterospheric evolution, in order
to better understand the dynamic connections between host stars and
potential impacts on planetary space weather and habitability. We also
remark on the comparative heliophysics approach which we plan to extend
to star-planet interactions via coupling to models of magnetospheric
activity and dynamo-driven stellar flux emergence.
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Title: Magnetic Properties of Asterospheres of Exoplanet Systems
Authors: Farrish, Alison; Alexander, David; Maruo, Mei; Sciola,
Anthony; Toffoletto, Frank; DeRosa, Marc
2019shin.confE.152F Altcode:
We study magnetic and energetic activity across a range of stellar
behavior via the application of an observationally-based heliophysics
modeling framework. We simulate the inner asterospheric magnetic fields
of host stars with the aim of better understanding and constraining the
space weather environments of exoplanets, and improving our knowledge of
the solar-stellar connection. As astronomy instrumentation has improved,
Earth-like exoplanets are increasingly being found orbiting in the
habitable zones of a variety of stars, ranging from the smallest
and coolest M dwarfs to larger and more solar-like stars. We are
therefore interested in characterizing a broad range of stellar magnetic
activity and the resulting impacts on asterospheric environments. We
will present our work simulating stellar magnetic activity on cycle
timescales via the integration of modeled magnetic flux emergence,
coronal field structure and related plasma emission, and stellar
winds. We use this self-consistent framework of heliophysics-based
models to simulate stellar and asterospheric evolution, in order
to better understand the dynamic connections between host stars and
potential impacts on planetary space weather and habitability. We also
remark on the comparative heliophysics approach which we plan to extend
to star-planet interactions via coupling to models of magnetospheric
activity and dynamo-driven stellar flux emergence.
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Title: Reconstructing Extreme Space Weather From Planet Hosting Stars
Authors: Airapetian, Vladimir; Adibekyan, V.; Ansdell, M.; Alexander,
D.; Barklay, T.; Bastian, T.; Boro Saikia, S.; Cohen, O.; Cuntz,
M.; Danchi, W.; Davenport, J.; DeNolfo, G.; DeVore, R.; Dong, C. F.;
Drake, J. J.; France, K.; Fraschetti, F.; Herbst, K.; Garcia-Sage,
K.; Gillon, M.; Glocer, A.; Grenfell, J. L.; Gronoff, G.; Gopalswamy,
N.; Guedel, M.; Hartnett, H.; Harutyunyan, H.; Hinkel, N. R.; Jensen,
A. G.; Jin, M.; Johnstone, C.; Kahler, S.; Kalas, P.; Kane, S. R.;
Kay, C.; Kitiashvili, I. N.; Kochukhov, O.; Kondrashov, D.; Lazio, J.;
Leake, J.; Li, G.; Linsky, J.; Lueftinger, T.; Lynch, B.; Lyra, W.;
Mandell, A. M.; Mandt, K. E.; Maehara, H.; Miesch, M. S.; Mickaelian,
A. M.; Mouschou, S.; Notsu, Y.; Ofman, L.; Oman, L. D.; Osten, R. A.;
Oran, R.; Petre, R.; Ramirez, R. M.; Rau, G.; Redfield, S.; Réville,
V.; Rugheimer, S.; Scheucher, M.; Schlieder, J. E.; Shibata, K.;
Schnittman, J. D.; Soderblom, David; Strugarek, A.; Turner, J. D.;
Usmanov, A.; Van Der Holst, B.; Vidotto, A.; Vourlidas, A.; Way, M. J.;
Wolk, Scott J.; Zank, G. P.; Zarka, P.; Kopparapu, R.; Babakhanova,
S.; Pevtsov, A. A.; Lee, Y.; Henning, W.; Colón, K. D.; Wolf, E. T.
2019BAAS...51c.564A Altcode: 2019astro2020T.564A; 2019arXiv190306853A
The goal of this white paper is to identify and describe promising key
research goals to aid the theoretical characterization and observational
detection of ionizing radiation from quiescent and flaring upper
atmospheres of planet hosts as well as properties of stellar coronal
mass ejections (CMEs) and stellar energetic particle (SEP) events.
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Title: Earth-like Exoplanet Response to Extreme Northward IMF
Stellar Wind
Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David
2019shin.confE.147S Altcode:
Periods of solar wind where the interplanetary magnetic field
(IMF) is pointing Northward are traditionally considered not to
be geo-effective, or having little interaction with the Earth's
magnetosphere and ionosphere. We simulate and present an example of
extreme Northward IMF stellar wind impacting an Earth-like, closely
orbiting exoplanet. In this regime magnetic reconnection occurs above
the magnetic poles, allowing for stellar wind to penetrate and convect
through the magnetosphere. This results in the loss of planetary plasma
via reconnected field lines and charge exchange with the captured
stellar wind population, which may have long-term implications on how
the planetary environment evolves. We compare this scenario to that
where the same stellar wind event is incident on the Earth-like planet
at 1AU.
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Title: Active Star Space Weather and its Planetary Consequences
Authors: Toffoletto, F.; Alexander, D.; Farrish, A.; Sciola, A.;
Barnes, W.
2018AGUFM.P43H3847T Altcode:
Of particular importance to understanding the space weather effects on
exo-planetary systems is an understanding of the quiescent state of such
systems, frequently observed around very active M-dwarf stars. We apply
an empirical photospheric magnetic flux transport model, derived from
solar behavior, and a magnetically-driven stellar atmosphere model to
explore the range of stellar effects on the habitability of Earth-size
exoplanets around range of active stars, delineated by their rotation
rates. We discuss how the stellar activity influences the asterospheric
magnetic field and its consequent impact on any planets present. We also
apply a hydrostatic coronal loop heating model to estimate the expected
EUV and X-ray emission associated with the magnetic fields of these
stars. Finally, we consider the response of the planetary magnetosphere
and ionosphere to the stellar activity, in the more active stars,
and, in particular, the role of accurately incorporating the inner
magnetosphere and enhanced ionospheric currents resulting from the
significantly larger EUV and X-ray fluxes. These all play an important
role in understanding the potential and rate of atmospheric loss.
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Title: Simulation of Exoplanet Host Star Magnetic Activity on Stellar
Cycle Timescales
Authors: Farrish, Alison; Maruo, M.; Barnes, W.; Alexander, D.;
Bradshaw, S.; DeRosa, M.
2018shin.confE...4F Altcode:
We apply an empirical photospheric magnetic flux transport
model, derived from solar behavior, and a magnetically-driven
stellar atmosphere model to explore the range of stellar effects on
habitability of Earth-size exoplanets around M dwarf stars. We create
detailed, dynamic simulations of stellar activity and its variability
over stellar cycle timescales. In particular, we examine how the
asterospheric magnetic field and related extreme ultraviolet (EUV)
and X-ray emission vary in time and consider the potential impact on
exoplanet habitability.
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Title: Predicting Radio Emission of Exoplanets in Enhanced Stellar
Wind Environments
Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David
2018shin.confE...5S Altcode:
The Radiometric Bode's Law is a scaling law that predicts the power of
a planet's radio emission based on the incident stellar wind power,
which works for the magnetized solar system planets. This scaling
law has since been used to predict the radio emission from exoplanets
such as Proxima Centauri b without addressing several space weather
factors that greatly alter the assumptions used in the calculation,
which may result in an incorrect prediction of the actual emission
power. Utilizing 3D MHD modeling coupled with inner magnetosphere
and ionosphere, we provide a means of more accurately estimating the
efficiency at which the incident stellar wind power penetrates into
the ionosphere and is available for the production of planetary radio
emission. This more representative efficiency can be taken into account
when using the Radiometric Bode's Law for exoplanetary systems whose
space weather environment strongly differ from that of our solar system.
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Title: Modeling the Space Weather environment of Proxima-b.
Authors: Toffoletto, Frank; Alexander, David; Sciola, Anthony; Farrish,
Alison; Sazykin, Stanislav Y.
2018tess.conf11901T Altcode:
There has been a tremendous increase in interest in exoplanet research
in recent years with it an increased emphasis on discovering potentially
habitable planets around other stars. Based on our understanding of our
own solar system, habitability is most feasible around a magnetically
active star that has the protection provided by a substantial planetary
magnetosphere. The recent discovery of a planetary system around
Proxima Centauri has been particularly exciting because of it being
in the closest to our solar system, which increases the possibility
of detection of the magnetic interaction as well as the exploration
of potential habitability. This presentation will summarize recent
work that has focused on the space plasma conditions of this system
via 2 related avenues: one is to get a better understanding of the
environment of the host star using an empirical solar magnetic flux
transport model, and the other is to a 3D coupled magnetosphere
model, that was originally developed for the Earth, that has been
adapted for the more extreme environments thought to represent the
around Proxima-b. We also investigate the question of the how large
a ring current can a planetary magnetosphere contain. In the future,
we expect use the empirical flux transport model to better constrain
the input stellar wind conditions for the planetary magnetosphere.
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Title: Simulated Coronal EUV Emission from Exoplanet Host Stars
Authors: Farrish, Alison; Barnes, Will; Alexander, David; Bradshaw,
Stephen J.; DeRosa, Mark L.
2018tess.conf40649F Altcode:
We apply a modified solar flux transport (SFT) model, developed
by Schrijver and colleagues, to emulate the magnetic activity of
the host stars of recently discovered habitable-zone planets. The
magnetic flux distributions produced by the SFT simulations serve
as a first-order proxy for the expected magnetic behavior of an
exoplanet host star. We couple the simulated magnetic structure
to a coronal heating model and simulate the expected EUV and X-ray
emission of the target star. The EUV emission is a key energetic input
to the exoplanetary atmosphere, governing ionospheric conductance and
therefore influencing the magnetospheric response to stellar activity -
a key factor in determining the overall atmospheric loss and ultimately
the potential of the planet for habitation. In addition, the simulated
stellar coronal emission may provide signatures for comparison with
astronomical observations. We consider the simulated activity over a
number of stellar cycles to explore the long-term impact of the star
on the exo-planetary environment.
---------------------------------------------------------
Title: Coupled MHD and Inner Magnetosphere Modeling of Proxima-b
Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David; Sazykin,
Stanislav Y.
2018tess.conf40650S Altcode:
<span color="#000000" Being our closest stellar neighbor, much
effort has gone into modeling the Proxima system in order to predict
the likelihood of habitabilty. In order to better understand Proxima
b's magnetic response to the enhanced stellar wind of its active M
dwarf host, we employ the 3D MHD model BATSRUS coupled with the Rice
Convection Model (RCM) driven by an enhanced stellar wind assumed
to simulate the properties of the host star. The RCM's modeling of
the inner magnetosphere is important as it can be a source region of
enhanced plasma population and pressure that can influence the global
response of the planet's magnetosphere. It can also lead to enhanced
region-2 field aligned currents in the polar caps. This detailed
modeling of the magnetosphere enables us to more accurately estimate
the parameters that govern habitability, such as rate of thermal ion
loss, as well as potential detectability of auroral radio emission.
---------------------------------------------------------
Title: The Application of Solar Flux Transport Modeling to Exoplanet
Systems
Authors: Alexander, David; Farrish, Alison; Maruo, Mei; De Rosa,
Marc L.
2018tess.conf40648A Altcode:
Earth-size exoplanets are preferentially detected close-in around small
stars; Proxima Centauri b, Ross 128 b, and the TRAPPIST-1 planets are
newly discovered exoplanets in this class. The effects of magnetic
interactions between the host star and such close-in exoplanets are
still not well-constrained. We utilize an empirical solar magnetic flux
transport model, first developed by Schrijver and colleagues, to explore
the possible relationships between stellar properties (e.g. rotation
period, radius, flux emergence rate, and meridional and differential
flow rates) and the expected surface magnetic flux distributions,
along with their evolution over the stellar cycle. The surface field
then provides key information about the interplanetary magnetic fields,
stellar winds, dynamic activity, and coronal emission, all of which
influence the star-planet interactions. We present simulated magnetic
flux patterns representing a range of possible exoplanet host stars,
including potential observables such as starspots, active regions,
and stellar cycle variations. The magnetic and energetic environment at
an exoplanet and its impact on the magnetospheric-atmospheric coupling
are important components of a planet's habitability that we expect to
constrain further with this application of simulated stellar magnetic
activity.
---------------------------------------------------------
Title: Planetary Magnetospheric Response to Increased Stellar Activity
Authors: Sciola, Anthony; Toffoletto, Frank; Alexander, David
2017shin.confE..97S Altcode:
As exemplified by the Proxima Centauri and TRAPPIST-1 systems,
transit-detected Earth-like exoplanets are likely to be closely orbiting
M dwarfs. The lower temperature of these stars cause their Habitable
Zones to be closer in towards the star, which in turn results in the
planet experiencing much stronger stellar wind conditions in the form
of heightened dynamic and magnetic pressures. With the high activity
and expectedly stronger stellar wind of Proxima Centauri as motivation,
I present the response to these conditions of an Earth-like planet's
magnetosphere using the Space Weather Modeling Framework computational
model. An emphasis is placed on the ring current's role in both helping
buff the magnetopause standoff distance and in storing pressure in the
inner magnetosphere between storm events. This understanding of the
magnetic interaction between star and planet can help determine other
characteristics such as atmospheric loss rate and auroral emissions.
---------------------------------------------------------
Title: Hard X-Ray Asymmetry Limits in Solar Flare Conjugate Footpoints
Authors: Daou, Antoun G.; Alexander, David
2016ApJ...832...63D Altcode:
The transport of energetic electrons in a solar flare is modeled
using a time-dependent one-dimensional Fokker-Planck code that
incorporates asymmetric magnetic convergence. We derive the
temporal and spectral evolution of the resulting hard X-ray (HXR)
emission in the conjugate chromospheric footpoints, assuming thick
target photon production, and characterize the time evolution of the
numerically simulated footpoint asymmetry and its relationship to the
photospheric magnetic configuration. The thick target HXR asymmetry
in the conjugate footpoints is found to increase with magnetic field
ratio as expected. However, we find that the footpoint HXR asymmetry
saturates for conjugate footpoint magnetic field ratios ≥4. This
result is borne out in a direct comparison with observations of 44
double-footpoint flares. The presence of such a limit has not been
reported before, and may serve as both a theoretical and observational
benchmark for testing a range of particle transport and flare morphology
constraints, particularly as a means to differentiate between isotropic
and anisotropic particle injection.
---------------------------------------------------------
Title: Observation of the Evolution of a Current Sheet in a Solar
Flare
Authors: Zhu, Chunming; Liu, Rui; Alexander, David; McAteer, James
2016SPD....4730205Z Altcode:
We report multi-wavelength and multi-viewpoint observations of a solar
eruptive event which involves loop-loop interactions. During a C2.0
flare, motions associated with inflowing and outflowing plasma provide
evidence for ongoing magnetic reconnection. The flare loop top and a
rising "concave-up" feature are connected by a current-sheet-like
structure (CSLS). The physical properties (thickness, length,
temperature, and density) of the CSLS are evaluated. In regions adjacent
to the CSLS, the EUV emission (characteristic temperature at 1.6 MK)
begins to increase more than ten minutes prior to the onset of the
flare, and steeply decreases during the decay phase. The reduction
of the emission resembles that expected from coronal dimming. The
dynamics of this event imply a magnetic reconnection rate in the
range 0.01--0.05.
---------------------------------------------------------
Title: Observation of the Evolution of a Current Sheet in a Solar
Flare
Authors: Zhu, Chunming; Liu, Rui; Alexander, David; McAteer,
R. T. James
2016ApJ...821L..29Z Altcode: 2016arXiv160307062Z
We report multi-wavelength and multi-viewpoint observations of a solar
eruptive event that involves loop-loop interactions. During a C2.0
flare, motions associated with inflowing and outflowing plasma provide
evidence for ongoing magnetic reconnection. The flare loop top and a
rising “concave-up” feature are connected by a current-sheet-like
structure (CSLS). The physical properties (thickness, length,
temperature, and density) of the CSLS are evaluated. In regions adjacent
to the CSLS, the EUV emission (characteristic temperature at 1.6 MK)
begins to increase more than 10 minutes prior to the onset of the flare,
and steeply decreases during the decay phase. The reduction of the
emission resembles that expected from coronal dimming. The dynamics
of this event imply a magnetic reconnection rate in the range 0.01-0.05.
---------------------------------------------------------
Title: Complex Flare Dynamics Initiated by a Filament-Filament
Interaction
Authors: Zhu, Chunming; Liu, Rui; Alexander, David; Sun, Xudong;
McAteer, R. T. James
2015ApJ...813...60Z Altcode: 2015arXiv150705889Z
We report on an eruption involving a relatively rare filament-filament
interaction on 2013 June 21, observed by SDO and STEREO-B. The
two filaments were separated in height with a “double-decker”
configuration. The eruption of the lower filament began simultaneously
with a descent of the upper filament, resulting in a convergence
and direct interaction of the two filaments. The interaction was
accompanied by the heating of surrounding plasma and an apparent
crossing of a loop-like structure through the upper filament. The
subsequent coalescence of the filaments drove a bright front ahead
of the erupting structures. The whole process was associated with a
C3.0 flare followed immediately by an M2.9 flare. Shrinking loops and
descending dark voids were observed during the M2.9 flare at different
locations above a C-shaped flare arcade as part of the energy release,
giving us unique insight into the flare dynamics.
---------------------------------------------------------
Title: Optimal Electron Energies for Driving Chromospheric Evaporation
in Solar Flares
Authors: Reep, J. W.; Bradshaw, S. J.; Alexander, D.
2015ApJ...808..177R Altcode: 2015arXiv150608115R
In the standard model of solar flares, energy deposition by a beam
of electrons drives strong chromospheric evaporation leading to a
significantly denser corona and much brighter emission across the
spectrum. Chromospheric evaporation was examined in great detail by
Fisher et al., who described a distinction between two different
regimes, termed explosive and gentle evaporation. In this work,
we examine the importance of electron energy and stopping depths on
the two regimes and on the atmospheric response. We find that with
explosive evaporation, the atmospheric response does not depend strongly
on electron energy. In the case of gentle evaporation, lower energy
electrons are significantly more efficient at heating the atmosphere and
driving up-flows sooner than higher energy electrons. We also find that
the threshold between explosive and gentle evaporation is not fixed at
a given beam energy flux, but also depends strongly on the electron
energy and duration of heating. Further, at low electron energies,
a much weaker beam flux is required to drive explosive evaporation.
---------------------------------------------------------
Title: Complex Flare Dynamics Initiated by a Filament-Filament
Interaction
Authors: Zhu, Chunming; Liu, Rui; Alexander, David; Sun, Xudong;
McAteer, James
2015TESS....120317Z Altcode:
We report on a filament eruption that led to a relatively rare
filament-filament interaction event. The filaments were located
at different heights above the same segment of a circular polarity
inversion line (PIL) around a condensed leading sunspot. The onset of
the eruption of the lower of the two filaments was accompanied by a
simultaneous descent of the upper filament resulting in a convergence
and direct interaction of the two filaments. The interaction led to
the subsequent merger of the filaments into a single magnetically
complex structure that erupted to create a large solar flare and an
array of complex dynamical activity. A hard X-ray coronal source and
an associated enhancement of hot plasma are observed at the interface
between the two interacting filaments. These phenomena are related to
the production of a small C flare and the subsequent development of a
much stronger M flare. Magnetic loop shrinkage and descending dark voids
were observed at different locations as part of the large flare energy
release giving us a unique insight into these dynamic flare phenomena.
---------------------------------------------------------
Title: The Role of Interchange Reconnection in Facilitating a
Filament Eruption
Authors: Zhu, C.; Alexander, D.; Sun, X.; Daou, A.
2014SoPh..289.4533Z Altcode: 2014SoPh..tmp..123Z
We study the interaction between an erupting solar filament and a
nearby coronal hole, based on multi-viewpoint observations from the
Solar Dynamics Observatory and STEREO. During the early evolution of
the filament eruption, it exhibits a clockwise rotation that brings
its easternmost leg in contact with the oppositely aligned field at the
coronal hole boundary. The interaction between the two magnetic-field
systems is manifested as the development of a narrow contact layer
in which we see enhanced EUV brightening and bi-directional flows,
suggesting that the contact layer is a region of strong and ongoing
magnetic reconnection. The coronal mass ejection (CME) resulting from
this eruption is highly asymmetric, with its southern portion opening
up to the upper corona, while the northern portion remains closed
and connected to the Sun. We suggest that the erupting flux rope
that made up the filament reconnected with both the open and closed
fields at the coronal hole boundary via interchange reconnection and
closed-field disconnection, respectively, which led to the observed
CME configuration.
---------------------------------------------------------
Title: Eruption of a Bifurcated Solar Filament
Authors: Zhu, Chunming; Alexander, David
2014SoPh..289..279Z Altcode:
We study the partial eruption of a solar filament observed by the
Solar Dynamics Observatory (SDO) and the Solar TErrestrial RElations
Observatory-Ahead (STEREO-A) spacecraft on 9 May 2012. This filament
was located in Active Region NOAA 11475 and consisted of two distinct
branches, separated in height above the active region's primary
polarity-inversion line. For two days prior to the filament eruption,
several threads of filament material were observed to connect the lower
branch to the upper branch with evidence of a transfer of mass along
them. The eruption commenced as a slow rise of the upper branch that
began at 9 May 2012 23:40 UT, with the main eruption occurring half
an hour later, producing a coronal mass ejection (CME). During the
eruption, the upper branch was observed to rotate approximately 120
degrees in a counter-clockwise direction. We suggest that the mass
transfer events also comprised a transfer of magnetic flux that led
the upper branch of the filament to lose equilibrium as a result of
a helical kink instability or torus instability.
---------------------------------------------------------
Title: Eruption of a Bifurcated Solar Filament
Authors: Zhu, Chunming; Alexander, D.
2013SPD....44...23Z Altcode:
We study the partial eruption of a solar filament observed by the SDO
and the STEREO-A spacecraft on 9 May 2012. This filament was located
in active region NOAA 11475, and consisted of two distinct branches,
separated in height above the active region's primary polarity-inversion
line. For two days prior to the filament eruption, several threads
of filament material were observed to connect the lower branch to
the upper branch with evidence of a transfer of mass along them. The
eruption commenced as a slow rise of the upper branch that began at
9 May 2012 23:40 UT, with the main eruption occurring half an hour
later, producing a coronal mass ejection (CME). During the eruption,
the upper branch was observed to rotate approximately 120 degrees
in a counter-clockwise direction. We suggest that the mass transfer
events also comprised a transfer of magnetic flux that led the upper
branch of the filament to lose equilibrium as a result of a helical
kink instability or torus instability.
---------------------------------------------------------
Title: Eruption of a Bifurcated Solar Filament
Authors: Zhu, Chunming; Alexander, David
2013shin.confE.100Z Altcode:
We study the partial eruption of a solar filament observed by the SDO
and the STEREO-A spacecraft on 9 May 2012. This filament was located
in active region NOAA 11475, and consisted of two distinct branches,
separated in height above the active region's primary polarity-inversion
line. For two days prior to the filament eruption, several threads
of filament material were observed to connect the lower branch to
the upper branch with evidence of a transfer of mass along them. The
eruption commenced as a slow rise of the upper branch that began at
9 May 2012 23:40 UT, with the main eruption occurring half an hour
later, producing a coronal mass ejection (CME). During the eruption,
the upper branch was observed to rotate approximately 120 degrees
in a counter-clockwise direction. We suggest that the mass transfer
events also comprised a transfer of magnetic flux that led the upper
branch of the filament to lose equilibrium as a result of a helical
kink instability or torus instability.
---------------------------------------------------------
Title: Velocity Characteristics of Rotating Sunspots
Authors: Zhu, C.; Alexander, D.; Tian, L.
2012SoPh..278..121Z Altcode:
A statistical study is carried out to investigate the detailed
relationship between rotating sunspots and the emergence of magnetic
flux tubes. This paper presents the velocity characteristics of 132
sunspots in 95 solar active regions. The rotational characteristics of
the sunspots are calculated from successive SOHO/MDI magnetograms by
applying the Differential Affine Velocity Estimator (DAVE) technique
(Schuck, 2006, Astrophys. J.646, 1358). Among 82 sunspots in active
regions exhibiting strong flux emergence, 63 showed rotation with
rotational angular velocity larger than 0.4° h<SUP>−1</SUP>. Among
50 sunspots in active regions without well-defined flux emergence, 14
showed rotation, and the rotation velocities tend to be slower, compared
to those in emerging regions. In addition, we investigated 11 rotating
sunspot groups in which both polarities show evidence for co-temporary
rotation. In seven of these cases the two polarities co-rotate, while
the other four are found to be counter-rotating. Plausible reasons
for the observed characteristics of the rotating sunspots are discussed.
---------------------------------------------------------
Title: Multi-wavelength Observations Of The Evolution Of A
Multi-filament Complex
Authors: Alexander, David; Zhu, C.
2012AAS...22020107A Altcode:
The strong and clear association between eruptive filaments and the
production of solar flares and CMEs provides a natural starting point
from which to explore the connections between solar conditions and
eruptive events. The development of the filament in the build up to
any eruption is critical to understanding how eruptions occur. We
use high cadence ground-based observations from the Mauna Loa Solar
Observatory (MLSO), in conjunction with complementary data from SDO,
and STEREO to quantify the development of a multipolar magnetic
complex comprising several distinct filaments, that formed in a
decaying active region group over the course of November/December
2011. Two neighboring filaments in this complex erupt within 6 hours
of each other on 2011 December, 25. In this paper we describe the
evolution of this complex over the course of its development across
the solar disk, as viewed from STEREO A, B and SDO, and discuss the
role played by the field development, filament dynamics and twisting
motions in forming the filament complex, initiating the eruptions, and
controlling the subsequent reformation of the filament channels. The
MLSO CHIP instrument, in particular, allows for some measure of Doppler
velocity discrimination in the He I line at 10830Å providing a unique
diagnostic capability for determining the strength and location of the
filament dynamics, particularly the twisting and writhing motions in
the erupting filaments. These data are combined with a 3D reconstruction
using STEREO EUVI and SDO AIA data to better determine the 3D velocity
evolution and spatial distribution of the twist.
---------------------------------------------------------
Title: Spectral Characteristics of Hard X-Ray Emission in Double
Footpoint Flares
Authors: Alexander, David; Lastufka, Erica; Daou, Antoun
2011shin.confE..90A Altcode:
Utilizing RHESSI observations we examine the evolution of footpoint
asymmetries in the hard X-ray emission over energy and time for
53 double-footpoint flares. Most of the flares observed displayed
a low asymmetry and very little change in asymmetry with energy,
as expected for comparable footpoint field strengths and isotropic
particle injection. However, a number of events exhibited clear
dependences of the footpoint hard X-ray asymmetry on photon energy
confirming the results of a previous study by Alexander and Metcalf
(2002). The presence of an energy dependence in the footpoint emission
cannot be accounted for by simple particle transport with scattering due
to Coulomb collisions and a converging field geometry. McClements &
Alexander (2005) used a Fokker-Planck model of particle transport with
an anisotropic injection of particles to reproduce the observed energy
dependence in the footpoint asymmetry. The need for an anisotropic
distribution of accelerated particle places constraints on the particle
acceleration processes occurring in solar flares. Furthermore, the
observed time development of the observed asymmetry provides further
constraints on the nature of the accelerated particles injected into
the flaring structure and provides insight into the 3D nature of the
flaring process.
---------------------------------------------------------
Title: Velocity characteristics of rotating sunspots
Authors: Zhu, Chunming; Alexander, David; Tian, Lirong
2011shin.confE..56Z Altcode:
A statistical study is carried out to investigate the detailed
relationship between rotating sunspots and the emergence of magnetic
flux tubes. This study presents the velocity characteristics of 132
sunspots. The rotational characteristics of the sunspots are calculated
from successive SOHO/MDI magnetograms by applying the Differential
Affine Velocity Estimator (DAVE) technique (Schuck, 2006). Among 82
sunspots in active regions exhibiting strong flux emergence, which
are related to flux tube emergence, or footpoint separation, 63 are
rotating with rotational angular velocity larger than 0.4 deg/hr. Among
50 sunspots in active regions without well-defined flux emergence, 14
are rotating, and the rotation velocities tend to be slower, compared
to those in emerging regions. In addition, we investigated 11 rotating
sunspot groups in which both polarities show evidence for co-temporary
rotation. In seven of these cases the two polarities co-rotate while
the other four are found to be counter-rotating. Plausible reasons
for the observed characteristics of the rotating sunspots are discussed.
---------------------------------------------------------
Title: Spatial and Temporal Development of Fiducial Current Systems
in Solar Flares
Authors: Daou, Antoun Georges; Alexander, David
2011shin.confE..87D Altcode:
We use the unprecedented spatial and spectral resolution of the
RHESSI telescope to investigate the temporal evolution of thermal and
non-thermal hard X-ray emission in spatially independent substructures
in 2 major flares: the 2006 December 6 X6.5 and the 2003 November
2 X8.3, one of the large Halloween 2003 flares. The hard X-ray
emission in these flares is concentrated in a number of distinct
sources displaying independent temporal and spectral evolution. We
find that while the total flare energy content rises during the
impulsive phase, the non-thermal hard X-ray fluxes in many of the
identified substructures saturates after an initial rise phase. In a
previous study, we investigated this non-thermal flux saturation at
the peak of the hard X-ray emission in 10 flares spanning a factor of
200 (M1 to X17) in the GOES 1-8A peak flux. The indication was that
the saturation was due to a transition in the dominant energy loss
mechanism from Coulomb collisions to return current Ohmic dissipation
as the flare peak magnitude increased. In this study, we analyze the
temporal variation of the hard X-ray fluxes within two large flares,
and infer the spectral properties of the emitting electron population,
as the flare transits from one energy loss regime to the other. We
find that the increase in the non-thermal hard X-ray flux in the
substructures towards saturation is mainly due to a hardening of the
spectrum, with more higher energy electrons accelerated, while the
total flux of lower energy electrons remaining roughly the same.
---------------------------------------------------------
Title: On Asymmetry of Magnetic Helicity in Emerging Active Regions:
High Resolution Observations
Authors: Alexander, David; Tian, Lirong; Demoulin, Pascal; Zhu,
Chunming
2011shin.confE..11A Altcode:
We employ the DAVE (differential affine velocity estimator, Schuck
2005; 2006) tracking technique on a time series of MDI/1m high spatial
resolution lineof- sight magnetograms to measure the photospheric
flow velocity for three newly emerging bipolar active regions. We
separately calculate the magnetic helicity injection rate of the
leading and following polarities to confirm or refute the magnetic
helicity asymmetry, found by Tian & Alexander (2009) using MDI/96m
low spatial resolution magnetograms. Our results demonstrate that the
magnetic helicity asymmetry is robust being present in the three active
regions studied, two of which have an observed balance of the magnetic
flux. The magnetic helicity injection rate measured is found to depend
little on the window size selected, but does depend on the time interval
used between the two successive magnetograms tracked. It is found
that the measurement of the magnetic helicity injection rate performs
well for a window size between 12-10 and 18-15 pixels, and at a time
interval ∼t=10 minutes. Moreover, the short-lived magnetic structures,
10-60 minutes, are found to contribute 30-50% of the magnetic helicity
injection rate. Comparing with the results calculated by MDI/96m data,
we find that the MDI/96m data, in general, can outline the main trend
of the magnetic properties, but they significantly underestimate
the magnetic flux in strong field region and are not appropriate for
quantitative tracking studies, so provide a poor estimate of the amount
of magnetic helicity injected into the corona.
---------------------------------------------------------
Title: Velocity Characteristics Of Rotating Sunspots
Authors: Zhu, Chunming; Alexander, D.; Tian, L.
2011SPD....42.1709Z Altcode: 2011BAAS..43S.1709Z
A statistical study is carried out to investigate the detailed
relationship between the rotating sunspots and the emergence of magnetic
flux tubes. This paper presents the velocity characteristics of 132
sunspots in 95 solar active regions. The rotational characteristics of
the sunspots are calculated from successive SOHO/MDI magnetograms by
applying the Differential Affine Veloicty Estimator (DAVE) technique
(Schuck, Astrophys. J. 646, 1358, 2006). Among 82 sunspots in active
regions exhibiting strong flux emergence, 77% are rotating sunspots with
rotational angular velocity larger than 0.4 deg/hr. Among 50 sunspots in
active regions without well-defined flux emergence, 28% are rotating,
and the sunspot rotation tend to be slower, even ceased, compared to
those during the emergence. In addition, we investigated 11 rotating
sunspot groups in which both polarities show evidence for contemporary
rotation. In seven of these cases the two polarities co-rotate while
the other four are found to be counter-rotating. Plausible reasons
for the observed characteristics of the rotating sunspots are discussed.
---------------------------------------------------------
Title: On Asymmetry of Magnetic Helicity in Emerging Active Regions:
High-resolution Observations
Authors: Tian, Lirong; Démoulin, Pascal; Alexander, David; Zhu,
Chunming
2011ApJ...727...28T Altcode: 2011arXiv1101.1068T
We employ the DAVE (differential affine velocity estimator) tracking
technique on a time series of Michelson Doppler Imager (MDI)/1 minute
high spatial resolution line-of-sight magnetograms to measure the
photospheric flow velocity for three newly emerging bipolar active
regions (ARs). We separately calculate the magnetic helicity injection
rate of the leading and following polarities to confirm or refute the
magnetic helicity asymmetry, found by Tian & Alexander using MDI/96
minute low spatial resolution magnetograms. Our results demonstrate
that the magnetic helicity asymmetry is robust, being present in
the three ARs studied, two of which have an observed balance of the
magnetic flux. The magnetic helicity injection rate measured is found to
depend little on the window size selected, but does depend on the time
interval used between the two successive magnetograms being tracked. It
is found that the measurement of the magnetic helicity injection rate
performs well for a window size between 12 × 10 and 18 × 15 pixels
and at a time interval Δt = 10 minutes. Moreover, the short-lived
magnetic structures, 10-60 minutes, are found to contribute 30%-50%
of the magnetic helicity injection rate. Comparing with the results
calculated by MDI/96 minute data, we find that the MDI/96 minute data,
in general, can outline the main trend of the magnetic properties,
but they significantly underestimate the magnetic flux in strong field
regions and are not appropriate for quantitative tracking studies,
so provide a poor estimate of the amount of magnetic helicity injected
into the corona.
---------------------------------------------------------
Title: Comparing Spatial Distributions of Solar Prominence Mass
Derived from Coronal Absorption
Authors: Gilbert, Holly; Kilper, Gary; Alexander, David; Kucera,
Therese
2011ApJ...727...25G Altcode:
In a previous study, Gilbert et al. derived the column density and total
mass of solar prominences using a new technique, which measures how much
coronal radiation in the Fe XII (195 Å) spectral band is absorbed by
prominence material, while considering the effects of both foreground
and background radiation. In the present work, we apply this method
to a sample of prominence observations in three different wavelength
regimes: one in which only H<SUP>0</SUP> is ionized (504 Å < λ
< 911 Å), a second where both H<SUP>0</SUP> and He<SUP>0</SUP> are
ionized (228 Å < λ < 504 Å), and finally at wavelengths where
H<SUP>0</SUP>, He<SUP>0</SUP>, and He<SUP>+</SUP> are all ionized (λ
< 228 Å). This approach, first suggested by Kucera et al., permits
the separation of the contributions of neutral hydrogen and helium to
the total column density in prominences. Additionally, an enhancement
of the technique allowed the calculation of the two-dimensional (2D)
spatial distribution of the column density from the continuum absorption
in each extreme-ultraviolet observation. We find the total prominence
mass is consistently lower in the 625 Å observations compared to lines
in the other wavelength regimes. There is a significant difference in
total mass between the 625 Å and 195 Å lines, indicating the much
higher opacity at 625 Å is causing a saturation of the continuum
absorption and thus, a potentially large underestimation of mass.
---------------------------------------------------------
Title: Magnetic helicity injection by rotating sunspots
Authors: Zhu, Chunming; Alexander, David; Tian, Lirong
2010shin.confE.100Z Altcode:
We present the observations by SOHO/MDI on four active regions with
rotating sunspots. The magnetic helicity and the angular speed of
sunspot rotating are calculated and the relations with the emergence
of these acitvie regions are discussed. The asymmetry of the magnetic
helcity should be responsible for the asymmetry of the magnetic
morphology, as shown by Tian & Alexander (2009). The observations
also show that CMEs could result from the accumulation of the magnetic
helicity injection.
---------------------------------------------------------
Title: Checking Asymmetry of Magnetic Helicity Using Magnetograms
with High Spatial and Temporal Resolution
Authors: Tian, Lirong; Zhu, C.; Alexander, D.
2010AAS...21640114T Altcode: 2010BAAS...41..859T
In order to check if the helicity imbalance is robust between the
leading and following polarities, found by Tian & Alexander, we
use an improved technique, differential affline velocity estimator
(DAVE), on series of MDI 1m and 96m line-of-sight magnetograms with
spatial resolution of 0.6 and 2 arcsecs. respectively, to measure
photospheric flow motions of an emerging active region: NOAA 10365
(S08). A better parameter of helicity density (G<SUB>θ</SUB>) than
G<SUB>A</SUB> is employed to calculate helicity injection rate of
leading and following polarities. <P />Our results display that the
helicity injection rate of using MDI/1m data is 2 times larger than
that of using MDI/ 96m data. The helicity injection rate is little
affected by the size of apodizing window selected and the noise level
(20 Gauss). However, it is improved so much due to decreasing time
difference (up to Δt=10 mines) of two images tracked. The helicity
injection rate of two polarities of the active region developed as
roughly same step with flux emergence,and maintain its imbalance with
more amount in the negative (leading) polarity over tracking period of
three days, which is a similar development tendency no matter using
MDI/1m data or MDI/96m data. <P />These results reflect that the
time difference of two tracking images is the most important factor
affecting amount of helicity injection rate, while there is little
relation with spatial resolution of data, the size of apodizing window,
and the noise level. Therefore, it should be reliable to study the
development of helicity injection rate and imbalanced relationship of
two polarities when using MDI/96m data, though the amount calculated
is two times smaller. <P />Further test for MDI/96m data of ARs 8214
and 0656 confirm that the helicity imbalance indeed exists between
the leading and following polarities.
---------------------------------------------------------
Title: On the Origin of the Asymmetric Helicity Injection in Emerging
Active Regions
Authors: Fan, Y.; Alexander, D.; Tian, L.
2009ApJ...707..604F Altcode:
To explore the possible causes of the observed asymmetric helicity flux
in emerging active regions between the leading and following polarities
reported in a recent study by Tian & Alexander, we examine the
subsurface evolution of buoyantly rising Ω-shaped flux tubes using
three-dimensional, spherical-shell anelastic MHD simulations. We find
that due to the asymmetric stretching of the Ω-shaped tube by the
Coriolis force, the leading side of the emerging tube has a greater
field strength, is more buoyant, and remains more cohesive compared
to the following side. As a result, the magnetic field lines in the
leading leg show more coherent values of local twist α ≡ (∇ × B)
· B/B <SUP>2</SUP>, whereas the values in the following leg show large
fluctuations and are of mixed sign. On average, however, the field
lines in the leading leg do not show a systematically greater mean
twist compared to the following leg. Due to the higher rise velocity
of the leading leg, the upward helicity flux through a horizontal
cross section at each depth in the upper half of the convection zone
is significantly greater in the leading polarity region than that in
the following leg. This may contribute to the observed asymmetric
helicity flux in emerging active regions. Furthermore, based on a
simplified model of active region flux emergence into the corona by
Longcope & Welsch, we show that a stronger field strength in the
leading tube can result in a faster rotation of the leading polarity
sunspot driven by torsional Alfvén waves during flux emergence into
the corona, contributing to a greater helicity injection rate in the
leading polarity of an emerging active region.
---------------------------------------------------------
Title: Sunspot activity in the build-up to the extended solar minimum
Authors: Alexander, D.; Martus, C.
2009AGUFMSH11A1497A Altcode:
Comparison between the recent, extended, solar minimum (between Cycles
23 and 24) with the previous one (between Cycles 22 and 23) shows that
one of the key distinctive features is the high degree of low latitude
structure present in most, if not all, measures of solar activity. In
particular, the surface magnetic configuration is significantly more
complex in the more recent minimum than in the previous one and this has
an impact throughout the heliosphere, leading to many of the observed
discriminators used in the comparison of the minima (e.g. low latitude
coronal holes, recurring fast solar wind streams). In this paper we
categorize the emergence, magnetic structure, evolution and dispersion
of sunspots in the years leading up to the two minima and investigate
the role that these have in determining the nature of the subsequent
solar minimum.
---------------------------------------------------------
Title: The Evolution of Prominence Mass and its Relation to CMEs
Authors: Gilbert, H. R.; Xie, H.; St. Cyr, C.; Alexander, D.
2009AGUFMSH41B1672G Altcode:
Prominences have long been associated with coronal mass ejections (CMEs)
and provide an important component in our understanding of the Sun-Earth
system as a whole. Although some prominences are long-lived and others
erupt or disappear on short time scales, all types of prominences are
dynamical and many show mass loss in observations through draining. Such
mass loss often occurs without an eruption, while sometimes occurring
prior to and/or during an eruption. We explore the relationship between
the nature of a prominence eruption (i.e., full, partial, failed)
and the associated CME properties (e.g., speed, mass, and energy).
---------------------------------------------------------
Title: Implications of Temporal Development of Localized Ultraviolet
and Hard X-ray Emission for Large Solar Flares
Authors: Coyner, Aaron J.; Alexander, David
2009ApJ...705..554C Altcode:
Ultraviolet and hard X-ray (HXR) emissions in solar flares provide
observational signatures of the interaction of flare-accelerated
particles with the chromospheric plasma. Earlier studies have shown
clear evidence of temporal and spatial relationships between UV
and HXR emission, suggesting a common physical origin. However,
more recent spatially resolved case studies suggest significant
variations in the spatial distributions of the emission signatures,
implying that the large-scale magnetic topology of the flaring region
must play a crucial role in the spatial and temporal development of
the localized UV and HXR emission. We present here an analysis of
spatially resolved HXR emission and localized UV emission sources from
11 M and X class flares with observations from RHESSI and high-cadence
1600 Å observations from TRACE. Within each flare we address the
overall temporal development of individual UV sources and relate them
to associated impulsive bursts within the HXR flare profile. We find,
for these large flares, that in the initial impulsive bursts of flare
activity, the majority of the temporally correlated emission evolves
in a series of localized co-spatial sources along the UV ribbons
consistent with previous two-dimensional reconnection pictures. However,
observations of impulses late in flares, show significant departures
from the traditional co-spatial/co-temporal picture. The new results
include extended UV ribbons with no corresponding HXR emission and
marked spatial separations between temporally correlated sources of UV
and HXR emission. In seven of the multi-burst events, we observe the
development of independent sets of UV and HXR sources corresponding
to the individual impulses seen in the temporal profile. The observed
separations and the spatial development of co-temporal emission in
multi-burst events emphasize the importance of a complex and time
varying magnetic topology in shaping the observed emission distributions
in both wavelengths. In four of the events, we observe late developing
UV sources which show no relationship with the HXR emission. In these
events, the emission sources show a strong relationship with lower
energy, more spatially extended X-ray emission believed to be of thermal
origin. This suggests that, in the later phase of these complex flares,
emission from thermal processes comes to dominate non-thermal processes
in the production of the UV emission.
---------------------------------------------------------
Title: Comparison of STEREO/EUVI Loops with Potential Magnetic
Field Models
Authors: Sandman, A. W.; Aschwanden, M. J.; DeRosa, M. L.; Wülser,
J. P.; Alexander, D.
2009SoPh..259....1S Altcode:
The Solar Terrestrial Relations Observatory (STEREO) provides the
first opportunity to triangulate the three-dimensional coordinates of
active region loops simultaneously from two different vantage points in
space. Three-dimensional coordinates of the coronal magnetic field have
been calculated with theoretical magnetic field models for decades,
but it is only with the recent availability of STEREO data that a
rigorous, quantitative comparison between observed loop geometries and
theoretical magnetic field models can be performed. Such a comparison
provides a valuable opportunity to assess the validity of theoretical
magnetic field models. Here we measure the misalignment angles between
model magnetic fields and observed coronal loops in three active
regions, as observed with the Extreme Ultraviolet Imager (EUVI) on
STEREO on 30 April, 9 May, and 19 May 2007. We perform stereoscopic
triangulation of some 100 - 200 EUVI loops in each active region and
compute extrapolated magnetic field lines using magnetogram information
from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric
Observatory (SOHO). We examine two different magnetic extrapolation
methods: (1) a potential field and (2) a radially stretched potential
field that conserves the magnetic divergence. We find considerable
disagreement between each theoretical model and the observed loop
geometries, with an average misalignment angle on the order of 20°
- 40°. We conclude that there is a need for either more suitable
(coronal rather than photospheric) magnetic field measurements or more
realistic field extrapolation models.
---------------------------------------------------------
Title: Mass Composition in Pre-eruption Quiet Sun Filaments
Authors: Kilper, Gary; Gilbert, Holly; Alexander, David
2009ApJ...704..522K Altcode:
Filament eruptions are extremely important phenomena due to their
association with coronal mass ejections and their effects on space
weather. Little is known about the filament mass and composition in the
eruption process, since most of the related research has concentrated
on the evolution and disruption of the magnetic field. Following up
on our previous work, we present here an analysis of nineteen quiet
Sun filament eruptions observed by Mauna Loa Solar Observatory in Hα
and He I 10830 Å that has identified a compositional precursor common
to all of these eruptions. There is a combined trend of an apparent
increase in the homogenization of the filament mass composition, with
concurrent increases in absorption in Hα and He I and in the level of
activity, all starting at least one day prior to eruption. This finding
suggests that a prolonged period of mass motions, compositional mixing,
and possibly even extensive mass loading is occurring during the build
up of these eruptions.
---------------------------------------------------------
Title: Solar Cycle Variations of the Multipolar Field and White
Light Corona
Authors: Young, Daniel Edward; Alexander, David; St. Cyr, O. C.
2009shin.confE.186Y Altcode:
We measure clear cyclic variances in the solar magnetic field and white
light corona that we can therefore use to predict upcoming iterations
of the solar cycle.
---------------------------------------------------------
Title: Can We Use STEREO/EUVI to Improve Boundary Conditions for
Magnetic Modeling?
Authors: Sandman, Anne; Aschwanden, Markus J.; Alexander, David
2009shin.confE..19S Altcode:
The STEREO mission provides a unique opportunity to observe
active region loops simultaneously from two different vantage
points in space. Using the stereoscopic data, we can triangulate
3D coordinates for active region loops and use them to perform
quantitative comparisons between observed loop geometries and
theoretical magnetic field models. These comparisons provide some
insight into the validity of magnetic field models, but recent work
(DeRosa et al. 2009, Sandman et al. 2009) has shown that potential and
non-potential models yield equally poor agreement with observed loop
structures. In order to improve the results of magnetic modeling we
must resolve the discrepancy between the typically force-free domain
(the corona) and non-force-free boundary condition (the photosphere
or chromosphere). We seek to address this discrepancy by 'correcting'
the boundary condition using observational constraints. The magnetic
field in the low corona cannot currently be mapped directly, but using
STEREO 3D loop coordinates we can constrain the orientation of the
magnetic field in the low corona. In a previous study we measured the
misalignment angles between model magnetic fields and observed coronal
loops in three active regions, as observed with STEREO/EUVI on April 30,
May 9, and May 19, 2007. We now attempt to use these misalignment angles
to modify the magnetogram input to the magnetic field model such that
the modified model has minimal misalignment with the observed coronal
loops near the base of the corona. We present the results of a test
case exploring the potential and limitations of this technique.
---------------------------------------------------------
Title: Hard X-ray Emission in Kinking Filaments
Authors: Liu, Rui; Alexander, David
2009ApJ...697..999L Altcode:
We present an observational study on the impact of the dynamic evolution
of kinking filaments on the production of hard X-ray (HXR) emission. The
investigation of two kinking-filament events in this paper, occurring
on 2003 June 12 and 2004 November 10, respectively, combined with our
earlier study on the failed filament eruption of 2002 May 27, suggests
that two distinct processes take place during the kink evolution,
leading to HXR emission with different morphological connections to the
overall magnetic configuration. The first phase of the evolution (Phase
I) is characterized by compact HXR footpoint sources at the endpoints of
the filament, and the second phase (Phase II) by a ribbon-like footpoint
emission extending along the endpoints of the filament. The HXR emission
in both the 2002 May 27 and 2004 November 10 events shows a transition
from Phase I to Phase II. In the 2002 May 27 event, coronal emission
was observed to be associated with EUV brightening sheaths aligned
along two filament legs in Phase I, while in Phase II, it was located
near the projected crossing point of the kink. The coronal emission
in the 2004 November 10 event does not exhibit a clear morphological
transition as in the 2002 May 27 event, probably due to the filament's
relatively small size. The 2003 June 12 event mostly features a Phase
I emission, with a compact footpoint emission located at one end
of the filament, and an elongated coronal source oriented along the
same filament leg. We propose the following scenarios to explain the
different flare morphology: magnetic reconnection in Phase I occurs
as a result of the interactions of the two writhing filament legs;
reconnection in Phase II occurs at an X-type magnetic topology beneath
the filament arch when the filament ascends and expands.
---------------------------------------------------------
Title: Implosion in a Coronal Eruption
Authors: Liu, Rui; Wang, Haimin; Alexander, David
2009ApJ...696..121L Altcode:
We present the observations of the contraction of the
extreme-ultraviolet coronal loops overlying the flaring region during
the preheating as well as the early impulsive phase of a GOES class
C8.9 flare. During the relatively long, 6 minutes, preheating phase,
hard X-ray (HXR) count rates at lower energies (below 25 keV) as well as
soft X-ray fluxes increase gradually and the flare emission is dominated
by a thermal looptop source with the temperature of 20-30 MK. After
the onset of impulsive HXR bursts, the flare spectrum is composed of a
thermal component of 17-20 MK, corresponding to the looptop emission,
and a nonthermal component with the spectral index γ = 3.5-4.5,
corresponding to a pair of conjugate footpoints. The contraction of
the overlying coronal loops is associated with the converging motion
of the conjugate footpoints and the downward motion of the looptop
source. The expansion of the coronal loops following the contraction
is associated with the enhancement in Hα emission in the flaring
region, and the heating of an eruptive filament whose northern end is
located close to the flaring region. The expansion eventually leads to
the eruption of the whole magnetic structure and a fast coronal mass
ejection. It is the first time that such a large scale contraction of
the coronal loops overlying the flaring region has been documented,
which is sustained for about 10 minutes at an average speed of ~5 km
s<SUP>-1</SUP>. Assuming that explosive chromospheric evaporation plays
a significant role in compensating for the reduction of the magnetic
pressure in the flaring region, we suggest that a prolonged preheating
phase dominated by coronal thermal emission is a necessary condition
for the observation of coronal implosion. The dense plasma accumulated
in the corona during the preheating phase may effectively suppress
explosive chromospheric evaporation, which explains the continuation
of the observed implosion up to ~7 minutes into the impulsive phase.
---------------------------------------------------------
Title: Using Prominence Mass Inferences in Different Coronal Lines
to Obtain the He/H Abundance
Authors: Gilbert, Holly; Kilper, G.; Alexander, D.; Kucera, T.
2009SPD....40.1011G Altcode:
In a previous study we developed a new technique for deriving prominence
mass by observing how much coronal radiation in the Fe XII (λ195)
spectral line is absorbed by prominence material. In the present work
we apply this method, which allows us to consider the effects of both
foreground and background radiation in our calculations, to a sample
of prominences absorbing in a coronal line that ionizes both H and He
(λ < 504 Å), and a line that ionizes only H (504 Å < λ <
911 Å). This approach, first suggested by Kucera et al. (1998),
permits the determination of the abundance ratio of neutral helium
and hydrogen in the prominence. This ratio should depend on how the
prominence is formed, on its current thermodynamic state, and on its
dynamical evolution. Thus, it may provide useful insights into the
formation and evolution of prominences.
---------------------------------------------------------
Title: Mass Composition in Pre-Eruption Quiet Sun Filaments
Authors: Kilper, Gary; Gilbert, H.; Alexander, D.
2009SPD....40.1012K Altcode:
Filament eruptions are extremely important phenomena due to their
association with coronal mass ejections and their effects on space
weather. Little is known about the filament mass and composition in the
eruption process, since most of the related research has concentrated
on the evolution and disruption of the magnetic field. Following up
on our previous work, we are presenting an analysis of nineteen quiet
Sun filament eruptions observed by MLSO in H-alpha and He I 10830 A
that has identified a compositional precursor common to all of these
eruptions. A general trend of increasing homogenization of the filament
mass composition with concurrent increases in absorption in H-alpha
and He I and the level of activity, all starting at least one day prior
to eruption, suggests the possibility that an extended period of mass
loading is occurring during the build-up to these eruptions. Funding
was provided by a NASA GSRP training grant from GSFC.
---------------------------------------------------------
Title: Implosion in a Coronal Eruption
Authors: Liu, Rui; Wang, H.; Alexander, D.
2009SPD....40.1918L Altcode:
We present the observations of the contraction of the EUV coronal
loops overlying the flaring region during the pre-heating as well as
the early impulsive phase of a C8.9 flare. During the relatively long,
6 minutes, pre-heating phase, the flare emission is dominated by a
thermal looptop source with the temperature of 20 - 30 MK. After the
onset of impulsive hard X-ray bursts, the flare spectrum is composed
of a thermal component of 17 - 20 MK, corresponding to the looptop
emission, and a nonthermal component with the spectral index (3.5 -
4.5), corresponding to a pair of conjugate footpoints. The contraction
of the overlying coronal loops is associated with the converging
motion of the conjugate footpoints and the downward motion of the
looptop source. The expansion of the coronal loops following the
contraction is associated with the enhancement in H-alpha emission
in the flaring region, and the heating of an eruptive filament whose
northern end is located close to the flaring region. The expansion
eventually leads to the eruption of the whole magnetic structure and
a fast coronal mass ejection. It is the first time that such a large
scale contraction of the coronal loops overlying the flaring region
has been documented, which is sustained for about 10 min at an average
speed of 5 km/s. Assuming that explosive chromospheric evaporation plays
a significant role in compensating for the reduction of the magnetic
pressure in the flaring region, we suggest that a prolonged pre-heating
phase dominated by coronal thermal emission is a necessary condition
for the observation of coronal implosion. The dense plasma accumulated
in the corona during the pre-heating phase may effectively suppress
explosive chromospheric evaporation, which explains the continuation
of the observed implosion up to 7 minutes into the impulsive phase.
---------------------------------------------------------
Title: Can We Use STEREO/EUVI to Improve Boundary Conditions for
Magnetic Modeling?
Authors: Sandman, Anne; Aschwanden, M. J.; Alexander, D.
2009SPD....40.1220S Altcode:
The STEREO mission provides a unique opportunity to observe
active region loops simultaneously from two different vantage
points in space. Using the stereoscopic data, we can triangulate
3D coordinates for active region loops and use them to perform
quantitative comparisons between observed loop geometries and
theoretical magnetic field models. These comparisons provide some
insight into the validity of magnetic field models, but recent work
(DeRosa et al. 2009, Sandman et al. 2009) has shown that potential and
non-potential models yield equally poor agreement with observed loop
structures. In order to improve the results of magnetic modeling we
must resolve the discrepancy between the typically force-free domain
(the corona) and non-force-free boundary condition (the photosphere or
chromosphere). We seek to address this discrepancy by "correcting”
the boundary condition using observational constraints. The magnetic
field in the low corona cannot currently be mapped directly, but using
STEREO 3D loop coordinates we can constrain the orientation of the
magnetic field in the low corona. In a previous study we measured the
misalignment angles between model magnetic fields and observed coronal
loops in three active regions, as observed with STEREO/EUVI on April 30,
May 9, and May 19, 2007. We now use these misalignment angles to modify
the magnetogram input to the magnetic field model. The resulting model
field has minimal misalignment with the observed coronal loops near the
base of the corona, allowing for a more realistic field extrapolation.
---------------------------------------------------------
Title: Effect of Magnetic Flux Imbalance on the Magnetic Helicity
Imbalance in Emerging Active Regions
Authors: Zhu, Chunming; Tian, L.; Alexander, D.
2009SPD....40.0911Z Altcode:
Tian & Alexander (2009) studied 15 emerging active regions and found
that magnetic helicity flux injected into the corona by the leading
polarity is generally several times larger than that injected by the
following polarity. They thought that the asymmetry of the magnetic
helicity should be responsible for the asymmetry of the magnetic
morphology, i.e, the leading magnetic field of bipolar active regions
is often spatially more compact, while more dispersed and fragmented in
following polarity. Though they take into account and "remove" possible
effect from magnetic flux imbalance, however, we still don't know how
much it affects. In this paper, we will calculate the magnetic flux
and helicity flux of two emerging active regions observed by SOHO/MDI
and Hinode, find their difference, and study possible origins resulting
in the helicity flux imbalance.
---------------------------------------------------------
Title: Asymmetry of Helicity Injection Flux in Emerging Active Regions
Authors: Tian, Lirong; Alexander, David
2009ApJ...695.1012T Altcode:
Observational and modeling results indicate that typically the leading
magnetic field of bipolar active regions (ARs) is often spatially more
compact, while more dispersed and fragmented in following polarity. In
this paper, we address the origin of this morphological asymmetry,
which is not well understood. Although it may be assumed that, in
an emerging Ω-shaped flux tube, those portions of the flux tube
in which the magnetic field has a higher twist may maintain its
coherence more readily, this has not been tested observationally. To
assess this possibility, it is important to characterize the nature
of the fragmentation and asymmetry in solar ARs and this provides the
motivation for this paper. We separately calculate the distribution
of the helicity flux injected in the leading and following polarities
of 15 emerging bipolar ARs, using the Michelson Doppler Image 96
minute line-of-sight magnetograms and a local correlation tracking
technique. We find from this statistical study that the leading
(compact) polarity injects several times more helicity flux than the
following (fragmented) one (typically 3-10 times). This result suggests
that the leading polarity of the Ω-shaped flux tube possesses a much
larger amount of twist than the following field prior to emergence. We
argue that the helicity asymmetry between the leading and following
magnetic field for the ARs studied here results in the observed
magnetic field asymmetry of the two polarities due to an imbalance
in the magnetic tension of the emerging flux tube. We suggest that
the observed imbalance in the helicity distribution results from a
difference in the speed of emergence between the leading and following
legs of an inclined Ω-shaped flux tube. In addition, there is also
the effect of magnetic flux imbalance between the two polarities with
the fragmented following polarity displaying spatial fluctuation in
both the magnitude and sign of helicity measured.
---------------------------------------------------------
Title: POLAR investigation of the Sun—POLARIS
Authors: Appourchaux, T.; Liewer, P.; Watt, M.; Alexander, D.;
Andretta, V.; Auchère, F.; D'Arrigo, P.; Ayon, J.; Corbard, T.;
Fineschi, S.; Finsterle, W.; Floyd, L.; Garbe, G.; Gizon, L.; Hassler,
D.; Harra, L.; Kosovichev, A.; Leibacher, J.; Leipold, M.; Murphy,
N.; Maksimovic, M.; Martinez-Pillet, V.; Matthews, B. S. A.; Mewaldt,
R.; Moses, D.; Newmark, J.; Régnier, S.; Schmutz, W.; Socker, D.;
Spadaro, D.; Stuttard, M.; Trosseille, C.; Ulrich, R.; Velli, M.;
Vourlidas, A.; Wimmer-Schweingruber, C. R.; Zurbuchen, T.
2009ExA....23.1079A Altcode: 2008ExA...tmp...40A; 2008arXiv0805.4389A
The POLAR Investigation of the Sun (POLARIS) mission uses a combination
of a gravity assist and solar sail propulsion to place a spacecraft
in a 0.48 AU circular orbit around the Sun with an inclination of 75°
with respect to solar equator. This challenging orbit is made possible
by the challenging development of solar sail propulsion. This first
extended view of the high-latitude regions of the Sun will enable
crucial observations not possible from the ecliptic viewpoint or from
Solar Orbiter. While Solar Orbiter would give the first glimpse of
the high latitude magnetic field and flows to probe the solar dynamo,
it does not have sufficient viewing of the polar regions to achieve
POLARIS’s primary objective: determining the relation between the
magnetism and dynamics of the Sun’s polar regions and the solar cycle.
---------------------------------------------------------
Title: Asymmetric Eruptive Filaments
Authors: Liu, Rui; Alexander, David; Gilbert, Holly R.
2009ApJ...691.1079L Altcode:
Filaments are often observed to erupt asymmetrically, during which
one leg is fixed to the photosphere (referred to as the anchored leg)
while the other undertakes most of the dynamic motions (referred
to as the active leg) during the eruptive process. In this paper,
we present observations of a group of asymmetric eruptive filaments,
in which two types of eruptions are identified: whipping-like, where
the active leg whips upward, and hard X-ray sources shift toward the
end of the anchored leg; and zipping-like, where the visible end of the
active leg moves along the neutral line like the unfastening of a zipper
as the filament arch rises and expands. During a zipping-like eruption,
hard X-ray sources shift away from where the eruption initiates toward
where the visible end of the active leg eventually stops moving. Both
types of asymmetric eruptions can be understood in terms of how the
highly sheared filament channel field, traced by filament material,
responds to an external asymmetric magnetic confinement, where force
imbalance occurs in the neighborhood of the visible end of the active
leg. The dynamic motions of the active leg have a distinct impact on
how hard X-ray sources shift, as observed by RHESSI.
---------------------------------------------------------
Title: Using STEREO/EUVI to Study Active Region Magnetic Fields
Authors: Sandman, A.; Aschwanden, M.; Wuelser, J.; De Rosa, M.;
Alexander, D.
2008AGUFMSH13B1523S Altcode:
We examine the effect of linear transformations on the misalignment
between model magnetic fields and coronal loops in active regions,
as observed with STEREO/EUVI on three separate occasions between
April 30 and May 19, 2007. We perform stereoscopic triangulation of
some 100 EUVI loops in each active region, and identify the tangent
vectors along every loop. Using magnetogram information from SOHO/MDI
we compute a 3D potential field and interpolate the magnetic field
vector at every position along the EUVI loops. The angle between the
loop tangent vector and the magnetic field vector provides a measure
of the misalignment angle between the observed field configuration
and the model. We then transform the field in a way that preserves
the divergence-free condition while injecting electric currents into
the system. With this modified field we repeat our calculation of the
misalignment angles between the magnetic field vectors and the EUV
loop tangent vectors, quantifying the improvement of the transformed
magnetic field model. Results of this type of magnetic modeling are
presented for three active regions.
---------------------------------------------------------
Title: Temporal and Spatial Evolution of Localized UV and Hard X-ray
Emission in Solar Flares
Authors: Coyner, A. J.; Alexander, D.
2008AGUFMSH13A1512C Altcode:
Localized ultraviolet and hard X-ray emission sources provide
observational diagnostics of flare-accelerated particles and energy
deposition within the chromosphere. The evolution of these emission
sources in space and intensity provide insight into the evolving
magnetic structure and energy release dynamics of the flaring
region. Initial studies of UV and hard X-ray emission indicated a
strong co-temporal relationship between the two emission suggesting
a common energetic origin often attributed to accelerated particle
production resulting from magnetic reconnection in the corona. While
confirming the overall temporal correlation, recent spatially-resolved
observations have determined that localized correlated UV and HXR
sources can occur in spatially distinct locations emphasizing the
importance of a complex evolving magnetic topology in governing the
spatial emission distributions observed. We present here an analysis of
M and X class flares observed in TRACE 1600 Å images and RHESSI 25-100
keV X-rays at high cadence. While observations of the initial impulses
show strong co-spatial emission in both wavelengths; however, as the
flare evolves, we find significant spatial separations between the
temporally correlated emission sources requiring that a time varying
complex magnetic structure must be present to transport particles
and produce the varied spatial distributions. In addition, we find,
for 6 events showing multiple impulses in their X-ray time profiles,
that each X-ray impulse corresponds to a distinct distribution of X-ray
emission emission sources suggesting that each impulse may correspond to
additional magnetic reconnection events is distinct locations within the
corona. Finally, we find UV sources which deviate from the expected hard
X-ray temporal correlation and instead appear more strongly associated
with lower energy thermal X-ray emission indicating thermal processes
or the thermal particle acceleration must contribute significantly in
the flare energy release process.
---------------------------------------------------------
Title: Using Prominence Mass Inferences in Different Coronal Lines
to Obtain the He/H Abundance
Authors: Gilbert, H.; Kilper, G.; Alexander, D.; Kucera, T.
2008AGUFMSH13A1509G Altcode:
In a previous study we developed a new technique for deriving prominence
mass by observing how much coronal radiation in the Fe XII (19.5 nm)
spectral line is absorbed by prominence material. In the present work
we apply this method, which allows us to consider the effects of both
foreground and background radiation in our calculations, to a sample
of prominences absorbing in a coronal line that ionizes both H and He
(lambda < 50.4 nm), and a line that ionizes only H (50.4 nm <
lambda < 91.1 nm). This approach, first suggested by Kucera et
al. (1998), permits the determination of the abundance ratio of neutral
helium and hydrogen in the prominence. This ratio should depend on
how the prominence is formed, on its current thermodynamic state,
and on its dynamical evolution. Thus, it may provide useful insights
into the formation and evolution of prominences.
---------------------------------------------------------
Title: Origins of Coronal Energy and Helicity in NOAA 10030
Authors: Tian, Lirong; Alexander, David; Nightingale, Richard
2008ApJ...684..747T Altcode:
Exploring the origins of coronal helicity and energy, as well as
determining the mechanisms that lead to coronal energy release, is a
fundamental topic in solar physics. Using MDI 96 minute line-of-sight
and HSOS vector magnetograms in conjunction with TRACE white-light
and UV (1600 Å) images and BBSO Hα and SOHO EIT (195 Å) images,
we find in active region NOAA 10030 that a large positive polarity
sunspot, located in the center of the region, exhibited significant
counterclockwise rotation, which continued for 6 days during the period
2002 July 12-18. This rotating sunspot was related to the formation of
inverse-\textsf{S}-shaped filaments, left-handed twist of the vector
magnetic fields, and the production of strong negative vertical current,
but exhibited little emergence of magnetic flux. In all, five M-class
and two X-class flares were produced around this rotating sunspot over
the 6 day period. The observed characteristics of the strongly rotating
sunspot suggest that sunspots can undergo strong intrinsic rotation,
the source of which may originate below the photosphere and can play
a significant role in helicity production and injection and energy
buildup in the corona. A sunspot with negative magnetic polarity showed
fast and significant emergence in the eastern portion of the active
region, and moved northeastward over several days, but exhibited
little rotation. The moving sunspot also exhibited the formation
of inverse-\textsf{S}-shaped filaments, left-handed twist of vector
magnetic fields and coronal structure, and the production of stronger
positive current. The observed characteristics of the emerging sunspot
suggest that significant emergence of twisted magnetic fields may not
always result in the rotation of the associated sunspots, but they do
play a very important role in the coronal helicity accumulation and
free-energy build-up.
---------------------------------------------------------
Title: The Filament-Moreton Wave Interaction of 2006 December 6
Authors: Gilbert, Holly R.; Daou, Antoun G.; Young, Daniel; Tripathi,
Durgesh; Alexander, David
2008ApJ...685..629G Altcode:
We utilize chromospheric observations obtained at MLSO of the 2006
December 6 Moreton wave, which exhibits two distinct fronts, and
subsequent filament activation to conduct a comprehensive analysis of
the wave-filament interaction. By determining the period, amplitude, and
evolution of the oscillations in the activated filament, we make certain
inferences regarding the physical properties of both the wave and the
filament. The large-amplitude oscillations induced in the filament by
the wave passage last on the order of 180 minutes and demonstrate a
complicated mixture of transverse and perpendicular motion with respect
to the filament spine. These oscillations are predominantly along the
filament axis, with a period of ~29 minutes and maximum line-of-sight
velocity amplitude of ~41 km s<SUP>-1</SUP>. A careful examination of
the complex oscillatory response of the filament elucidates some of the
fundamental characteristics of the related Moreton wave. Specifically,
we infer the maximum total kinetic energy involved in the interaction,
the structure and topology of the passing wave, and discuss implications
for the topology of the responding magnetic structure supporting the
filament. The results of this observational study equip us with a
better understanding of how filaments become activated and the nature
of their responses to large propagating disturbances.
---------------------------------------------------------
Title: The Effect of Magnetic Reconnection and Writhing in a Partial
Filament Eruption
Authors: Liu, Rui; Gilbert, Holly R.; Alexander, David; Su, Yingna
2008ApJ...680.1508L Altcode:
We present observations from 2007 March 2 of a partial filament
eruption characterized by two distinct phases of writhing motions: a
quasi-static, slowly evolving phase followed by a rapid kinking phase
showing a bifurcation of the filament. The quasi-static kinking motions
are observed before there is any heating or flaring evident in EUV or
soft X-ray (SXR) observations. As the writhe of the filament develops,
a sigmoid becomes sharply defined in the SXR. Prior to eruption onset,
the sigmoid in the EUV appears to be composed of two separate looplike
structures, which are discontinuous at the projected location where the
sigmoid crosses the filament. Coincident with the onset of the eruption
and the production of a GOES class B2 flare, the original "two-loop"
EUV sigmoid is now observed as a single continuous structure lying above
the filament, signifying the presence of magnetic reconnection and the
associated dissipative heating of field lines above the filament. During
the eruption, the escaping portion of the filament rotates quickly
and erupts together with the expanding arched sigmoid. The portion of
the filament that is left behind develops into an inverse S-shaped
configuration. The separation of the filament, the EUV brightening
at the separation location, and the surviving sigmoidal structure are
all signatures of magnetic reconnection occurring within the body of
the original filament. Other features of the same event reported by
Sterling and coworkers, e.g., the flux cancellation at the polarity
inversion line prior to the eruption and the SXR compact loop formed
underneath the erupting sigmoid during the eruption, indicate that
magnetic reconnection also occurred in the sheared core field beneath
the filament. These results suggest that a combination of the kinking
motions and internal tether-cutting are responsible for the initiation
of the eruption.
---------------------------------------------------------
Title: Temporal Development of Localized UV and Hard X-ray Emissions
in Large Solar Flares
Authors: Coyner, A. J.; Alexander, D.
2008AGUSMSP51C..10C Altcode:
Ultraviolet and hard X-ray (HXR) emissions in solar flares provide
signatures of flare-accelerated particles with the chromosphere. Initial
studies of flare emission at these wavelengths have indicated a strong
co-temporal relationship during the impulsive phase of solar flares,
suggesting they arise from a common origin, often attributed to the
accelerated particle production via coronal magnetic reconnection. While
spatially-unresolved studies have shown a strong temporal correlation,
recent spatially-resolved observations indicate that co- temporal UV
and HXR emission can originate from distinct locations, raising the
question as to how the global magnetic topology and energy release
mechanisms relate to the spatial and temporal development of the
localized UV and HXR emission. The spatial resolution provided by TRACE
and RHESSI imaging presents an opportunity to address these global
relationships on the level of individually distinguishable sources. We
present an analysis and comparison of spatially-resolved hard X-ray
and localized UV emission sources for a number of M and X class flares
with observations from RHESSI and high-cadence 1600 Å observations
from TRACE. Within each flare we investigate the overall temporal
development of individual UV sources and relate them to individual
impulsive hard X-ray bursts. We find that for these large flares,
the bulk of the temporally correlated UV and HXR emission evolves in a
series of localized co-spatial sources along the UV ribbons consistent
with previous 2-D reconnection pictures, however, extended UV ribbon
emission, the evolution of multiple burst events, and the existence
of UV sources developing late in the flare evolution indicate the
need for a 3-D magnetic topology scenario along with the suggestion
that multiple physical mechanisms appear necessary to reproduce the
emission in these flares.
---------------------------------------------------------
Title: Association of magnetic topology and radiative emissions in
solar flares
Authors: Alexander, D.; Tian, L.; Coyner, A.
2008AGUSMSP51C..14A Altcode:
We relate the energetic process in flares to the detailed topology,
dynamics and evolution of magnetic field for NOAA active region 10720,
which proved to be the most flare productive region of Cycle 23. We
obtain the locations of quasi-separatrix layers (QSLs) from photospheric
vector magnetic field observations based on the squashing degree, Q of
Titov et al. (2002)and compare these to the distribution of vertical
current, magnetic field evolution, and the chromospheric sites of UV
and hard X-ray flare emissions. We find a close relationship between
the distribution of the QSLs and the flare emission sources. Moreover,
opposite vertical current is found to concentrate near the QSLs
and that several large flares occur in the vicinity of the current
concentrations and QSLs. We argue that the magnetic emergence and
fast evolution of the sunspot magnetic fields alter the topology of
the field in the corona resulting in the production of large solar
flares and the release of the free energy associated with the currents.
---------------------------------------------------------
Title: Using Prominence Mass Inferences in Different Coronal Lines
to Obtain the He/H Abundance
Authors: Gilbert, H. R.; Kilper, G.; Kucera, T.; Alexander, D.
2008AGUSMSP43B..05G Altcode:
In a previous study we developed a new technique for deriving prominence
mass by observing how much coronal radiation in the Fe XII (λ195)
spectral line is absorbed by prominence material. In the present work
we apply this method, which allows us to consider the effects of both
foreground and background radiation in our calculations, to a sample
of prominences absorbing in a coronal line that ionizes both H and He
(λ < 504 Å), and a line that ionizes only H (504 Å < λ <
911 Å). This approach, first suggested by Kucera et al. (1998),
permits the determination of the abundance ratio of neutral helium
and hydrogen in the prominence. This ratio should depend on how the
prominence is formed, on its current thermodynamic state, and on its
dynamical evolution. Thus, it may provide useful insights into the
formation and evolution of prominences.
---------------------------------------------------------
Title: Spatial and Temporal Development of Fiducial Current Systems
in Solar Flares
Authors: Daou, A. G.; Alexander, D.
2008AGUSMSP51C..16D Altcode:
We use the unprecedented spatial and spectral resolution of the
RHESSI telescope to investigate the temporal evolution of thermal and
non-thermal hard X-ray emission in spatially independent substructures
in 2 major flares: the 2006 December 6 X6.5 and the 2003 November 2 X
8.3, one of the large Halloween '03 flares. The hard X-ray emission in
these flares is concentrated in a number of distinct sources displaying
independent temporal and spectral evolution. We find that while the
total flare energy content rises during the impulsive phase, the
non-thermal hard X-ray fluxes in many of the identified substructures
saturates after an initial rise phase. In a previous study, we have
investigated this non-thermal flux saturation at the peak of the hard
X-ray emission in 10 flares spanning a factor of ~200 (M1 to X17) in
the GOES 1-8Å~peak flux. The indication was that the saturation was
due to a transition in the dominant energy loss mechanism from Coulomb
collisions to return current ohmic dissipation as the flare magnitude
increased. In this study, we analyze the temporal variation of the hard
X-ray fluxes within two large flares, and infer the spectral properties
of the emitting electron population, as the flare transits from from
one energy loss regime to the other. We find that the increase in the
non-thermal hard X-ray flux in the substructures towards saturation
is mainly due to a hardening of the spectrum rather than an increase
in the normalized flux in the electron beams.
---------------------------------------------------------
Title: Theoretical modeling for the stereo mission
Authors: Aschwanden, Markus J.; Burlaga, L. F.; Kaiser, M. L.; Ng,
C. K.; Reames, D. V.; Reiner, M. J.; Gombosi, T. I.; Lugaz, N.;
Manchester, W.; Roussev, I. I.; Zurbuchen, T. H.; Farrugia, C. J.;
Galvin, A. B.; Lee, M. A.; Linker, J. A.; Mikić, Z.; Riley, P.;
Alexander, D.; Sandman, A. W.; Cook, J. W.; Howard, R. A.; Odstrčil,
D.; Pizzo, V. J.; Kóta, J.; Liewer, P. C.; Luhmann, J. G.; Inhester,
B.; Schwenn, R. W.; Solanki, S. K.; Vasyliunas, V. M.; Wiegelmann, T.;
Blush, L.; Bochsler, P.; Cairns, I. H.; Robinson, P. A.; Bothmer,
V.; Kecskemety, K.; Llebaria, A.; Maksimovic, M.; Scholer, M.;
Wimmer-Schweingruber, R. F.
2008SSRv..136..565A Altcode: 2006SSRv..tmp...75A
We summarize the theory and modeling efforts for the STEREO mission,
which will be used to interpret the data of both the remote-sensing
(SECCHI, SWAVES) and in-situ instruments (IMPACT, PLASTIC). The
modeling includes the coronal plasma, in both open and closed magnetic
structures, and the solar wind and its expansion outwards from the Sun,
which defines the heliosphere. Particular emphasis is given to modeling
of dynamic phenomena associated with the initiation and propagation
of coronal mass ejections (CMEs). The modeling of the CME initiation
includes magnetic shearing, kink instability, filament eruption, and
magnetic reconnection in the flaring lower corona. The modeling of CME
propagation entails interplanetary shocks, interplanetary particle
beams, solar energetic particles (SEPs), geoeffective connections,
and space weather. This review describes mostly existing models of
groups that have committed their work to the STEREO mission, but is by
no means exhaustive or comprehensive regarding alternative theoretical
approaches.
---------------------------------------------------------
Title: On the Origin of Magnetic Helicity in the Solar Corona
Authors: Tian, Lirong; Alexander, David
2008ApJ...673..532T Altcode:
Twenty-three active regions associated with pronounced sigmoidal
structure in Yohkoh soft X-ray observations are selected to investigate
the origin of magnetic helicity in the solar corona. We calculate
the radial magnetic flux of each polarity, the rate of magnetic
helicity injection, and total flux of the helicity injection (Δ
H<SUB>LCT</SUB>) over 4-5 days using MDI 96 minute line-of-sight
magnetograms and a local correlation tracking technique. We also
estimate the contribution from differential rotation to the overall
helicity budget (Δ H<SUB>rot</SUB>). It is found that of the
seven active regions for which the flux emergence exceeds 1.0 ×
10<SUP>22</SUP> Mx, six exhibited a helicity flux injection exceeding
1.0 × 10<SUP>43</SUP> Mx<SUP>2</SUP> (i.e., Δ H = Δ H<SUB>LCT</SUB>
- Δ H<SUB>rot</SUB>). Moreover, the rate of helicity injection and
the total helicity flux are larger (smaller) during periods of more
(less) increase of magnetic flux. Of the remaining 16 active regions,
with flux emergence less than 10<SUP>22</SUP> Mx, only 4 had significant
injection of helicity, exceeding 10<SUP>43</SUP> Mx<SUP>2</SUP>. Typical
contributions from differential rotation over the same period were 2-3
times smaller than that of the strong magnetic field emergence. These
statistical results signify that the strong emergence of magnetic
field is the most important origin of the coronal helicity, while
horizontal motions and differential rotation are insufficient to
explain the measured helicity injection flux. Furthermore, the study
of the helicity injection in nineteen newly emerging active regions
confirms the result on the important role played by strong magnetic
flux emergence in controlling the injection of magnetic helicity into
the solar corona.
---------------------------------------------------------
Title: Observational Evidence Supporting Cross-field Diffusion of
Neutral Material in Solar Filaments
Authors: Gilbert, Holly; Kilper, Gary; Alexander, David
2007ApJ...671..978G Altcode:
We investigate the temporal and spatial variation of the relative
abundance of He to H in a sample of solar filaments by comparing
cotemporal observations of Hα and He I λ10830 obtained at
MLSO. Motivated by indications that cross-field diffusion of neutral
filament material is an important mechanism in mass loss, the present
study offers results that provide a convincing test of the mechanisms
proposed in Gilbert and coworkers. Specifically, when observed across
an entire disk passage, we find a majority of stable, quiescent
filaments show a relative helium deficit in the upper portions of
their structure coupled with a relative helium surplus in the lower
regions, a consequence of the large loss timescale for neutral helium
compared to neutral hydrogen. Moreover, we find that the variation of
the relative He/H ratio is uniform across filament barbs and footpoints
on both short and long timescales.
---------------------------------------------------------
Title: 3D Magnetic Modeling of Active Regions Using STEREO/EUVI
Authors: Sandman, A.; Aschwanden, M. J.; Alexander, D.; Wuelser, J.
2007AGUFMSH32A0770S Altcode:
With the recent availability of stereoscopic data from the Extreme
Ultraviolet Imager (EUVI) on the Solar Terrestrial Relations Observatory
(STEREO) we have an unprecedented opportunity to investigate the
accuracy of 3D magnetic field models. These data will be put to
best use by modeling techniques that make no assumptions about the
nature of the field (that it is potential, force-free, etc.). The
Gary-Alexander radial stretching method use a series of transformations
to map a simple potential field to a more complicated target field, and
compare the transformed field lines with observed coronal structures
in the EUV. Unlike many other simulation techniques, this approach
requires only that the field remain divergence-free and continuous
at the photosphere. Here we apply this transformation method to
STEREO/EUVI data. We obtain a 3D potential field extrapolation using
an MDI magnetogram, and utilize stereoscopy to derive the 3D field
line coordinates from pairs of EUV images at 171Å. By comparing the
3D coordinates of the transformed model field lines with those of
the real field lines as seen by EUVI, we can place constraints on the
distribution of magnetic field and current in an active region.
---------------------------------------------------------
Title: Filament Kinking and Its Implications for Eruption and
Re-formation
Authors: Gilbert, Holly R.; Alexander, David; Liu, Rui
2007SoPh..245..287G Altcode:
Solar filaments exhibit a range of eruptive-like dynamic activity from
the full, or partial, eruption of the filament mass and surrounding
magnetic structure, as a CME, to a fully confined dynamic evolution or
"failed" eruption, sometimes producing a flare but no CME. Additionally,
observations of erupting filaments often show a clear helical
structure, indicating the presence of a magnetic flux rope. Dynamic
helical structures, in addition to being twisted, frequently show
evidence of being kinked, with the axis of the flux rope exhibiting
a large-scale writhe. Motivated by the fact that kinking motions
are also detected in filaments that fail to erupt, we investigate
the possible relationship between the kinking of a filament and its
success or failure to erupt. We present an analysis of kinking in
filaments and its implications for other filament phenomena such as
the nature of the eruption, eruptive acceleration, and post-eruptive
re-formation. We elucidate the relationship between kinking and the
various filament phenomena via a simple physical picture of the forces
involved in kinking together with specific definitions of the types of
filament eruption. The present study offers results directly applicable
to observations, allowing a thorough exploration of the implications of
the observational relationship between kinking and filament phenomena
and provides new insight for modelers of CME initiation.
---------------------------------------------------------
Title: Saturation of Nonthermal Hard X-Ray Emission in Solar Flares
Authors: Alexander, David; Daou, Antoun G.
2007ApJ...666.1268A Altcode:
We consider a number of flares spanning a range of magnitudes over two
decades in GOES soft X-ray luminosity and demonstrate an observational
confirmation of the saturation of the nonthermal hard X-ray photon flux
in large solar flares. Using the unprecedented spectral and spatial
resolution of the RHESSI telescope, we find that the integrated photon
flux above 20 keV asymptotically approaches a limiting value with
increasing flare intensity, suggesting a saturation of the nonthermal
photon production for high electron-beam fluxes in flares. This
result is strengthened by considering each of the large flares as an
aggregation of multiple fiducial energy release events identified using
the high spatial resolution of RHESSI. We argue that the saturation
of the photon production sets in as a result of the transition from
Coulomb collisions to return current ohmic dissipation dominating the
energy losses as the flare magnitudes increase.
---------------------------------------------------------
Title: Kink-induced Catastrophe in a Coronal Eruption
Authors: Liu, Rui; Alexander, David; Gilbert, Holly R.
2007ApJ...661.1260L Altcode:
We investigate the kinking motion and its role in the eruption of a
filament/cavity system that occurred on 2002 October 31. The evolution
of the eruptive filament consists of four distinct phases. After
an initial slow upward acceleration, the filament experiences
a quasi-static phase exhibiting kinking motions of the filament
axis. The kinking phase is followed by a sudden jump, coincident with
the onset of the unkinking of the filament. The loss of equilibrium
initiates a gradual relaxation phase at the end of which the filament
reattains a similar unkinked configuration as its initial state. The
filament/cavity structure, evident in the white-light observations,
interacts with a large-scale coronal helmet streamer to the north,
and material is observed to eject outward, aligned with a preexisting,
low-density, dark channel that originally separated the northern
helmet streamer from the southern streamer, where the dark cavity
resides. The bulk of the filament, however, remains confined in the
lower corona throughout the eruption along the channel. This suggests
a partial eruption of the filament/cavity structure. The observations
presented here manifest a catastrophic loss of equilibrium in response
to the evolution of kinking motions in the filament activation.
---------------------------------------------------------
Title: Kinking and Solar Filament Eruptions
Authors: Liu, Rui; Alexander, D.; Gilbert, H. R.
2007AAS...210.9502L Altcode: 2007BAAS...39..223L
We conduct observational studies to explore the role of kinking in
coronal eruptions. Our recent work has shown evidence supporting the
role of writhe of the filament axis as a possible driver and regulator
of filament eruptions. We identify coronal X-ray source emission
occurring beneath the apex of a writhed filament with reconnection
driven by kinking motions. We indicate that quasi-static kinking
motions induce a catastrophe in a filament/cavity eruption. We find
brightening soft X-ray and EUV features propagating along filament
legs down to the solar surface during filament eruptions, suggestive
of heating in the current sheet formed at the interface of writhed
filaments and ambient field. We propose that weakening of magnetic
tethers that anchor either leg of a twisted and/or writhed flux rope
which supports the filament result in whipping and/or rotating motions
observed in “zipper-like” filament eruptions.
---------------------------------------------------------
Title: Observational Evidence Supporting Cross-field Diffusion of
Neutral Prominence Material
Authors: Gilbert, Holly; Kilper, G.; Alexander, D.
2007AAS...210.9505G Altcode: 2007BAAS...39..224G
Prominences over all lifetime scales are dynamical, and many exhibit
mass loss in observations through draining. Draining occurs along
magnetic field lines but it can also occur across field lines since
prominences are partially ionized. In a careful analysis of cross-field
diffusion of neutral material in a partially ionized hydrogen-helium
plasma, Gilbert et al. (2002) found the loss time scales of hydrogen to
be much longer than that of helium. One way to evaluate the importance
of this mechanism is to compare observations of filaments in Hα
and He I (1083 nm) to see if there is a relative helium deficit. We
present the results of such an analysis on a large sample of filaments,
which show an obvious relative helium deficit in the edges of filaments
residing higher in the solar atmosphere than the lower portions.
---------------------------------------------------------
Title: Relationship Of Sunspot Rotation And Emergence Of Twisted
Magnetic Fields?
Authors: Tian, Lirong; Alexander, D.; Nightingale, R.
2007AAS...210.2403T Altcode: 2007BAAS...39..128T
Is it the sunspot rotation that twists the magnetic fields? Or does
the emergence of twisted magnetic fields create the sunspot rotation
viewed in the photosphere due to a geometric effect resulting from
the emergence of twisted magnetic fields? Using TRACE white light and
1600A images, BBSO/H_a and SOHO/EIT/195A observations with MDI/96m
line-of-sight and HSOS vector magnetograms, we find in active region
NOAA 10030 that a large positive polarity sunspot, located in the
center of the region, exhibited significant counter-clockwise rotation,
which continued for six days during July 12-18, 2002. This rotating
sunspot was related to the formation of inverse-S-shaped filaments,
left-handed twist of the vector magnetic fields, and the production of
strong negative vertical current, but exhibiting little emergence of
magnetic flux. All five M-class and two X-class flares were produced
around this rotating sunspot over the six days period. A sunspot with
negative magnetic polarity showed fast and significant emergence in
the eastern portion of the active region, and moved north-eastward
over several days, but exhibited no rotation. The moving sunspot also
exhibited the formation of inverse-S-shaped filaments, left-handed twist
of vector magnetic fields and coronal structure, and the production
of stronger positive current. The observed characteristics of the
emerging sunspot suggest that the significant emergence of twisted
magnetic fields play an important role in coronal helicity injection
and free-energy buildup, but does not have to result in a sunspot
rotation viewed in the photosphere, due to a geometric effect resulting
from the emergence of twisted magnetic fields. While from the observed
characteristics of the strong rotating sunspot, we suggest that physical
rotation of sunspots originates from the solar interior, and plays
a remarkable role in coronal helicity and free-energy accumulation,
but does not have to result from the geometric effect of the emergence
of twisted magnetic fields.
---------------------------------------------------------
Title: Laboratory Exploration of Solar Energetic Phenomena
Authors: Alexander, David
2007Ap&SS.307..197A Altcode: 2006Ap&SS.tmp..535A
The solar atmosphere displays a wide variety of dynamic phenomena
driven by the interaction of magnetic fields and plasma. In particular,
plasma jets in the solar chromosphere and corona, coronal heating,
solar flares and coronal mass ejections all point to the presence
of magnetic phenomena such as reconnection, flux cancellation, the
formation of magnetic islands, and plasmoids. While we can observe
the signatures and gross features of such phenomena we cannot probe
the essential physics driving them, given the spatial resolution
of current instrumentation. Flexible and well-controlled laboratory
experiments, scaled to solar parameters, open unique opportunities
to reproduce the relevant unsteady phenomena under various simulated
solar conditions. The ability to carefully control these parameters
in the laboratory allows one to diagnose the dynamical processes which
occur and to apply the knowledge gained to the understanding of similar
processes on the Sun, in addition directing future solar observations
and models. This talk introduces the solar phenomena and reviews the
contributions made by laboratory experimentation.
---------------------------------------------------------
Title: Hard X-Ray Production in a Failed Filament Eruption
Authors: Alexander, David; Liu, Rui; Gilbert, Holly R.
2006ApJ...653..719A Altcode:
We revisit the “failed” filament eruption of 2002 May 27, first
studied in detail by Ji et al. We investigate the temporal and spatial
relationship between the filament dynamics and the production of
hard X-ray emission using spatially resolved high-cadence data from
TRACE and RHESSI. We confirm the presence of a hard X-ray source in
the corona above the filament prior to the main activation phase and
identify a second coronal hard X-ray source, not considered by earlier
studies, that occurs under the apex of the filament during the erupting
phase when the filament is clearly strongly kinked. We argue that this
second source of coronal hard X-ray emission implies ongoing magnetic
reconnection in a current sheet formed via a kink instability resulting
from the interaction of the two adjacent legs underneath the writhing
filament, in agreement with simulation results. The presence of this
second energy release site has important implications for models of
solar eruptions.
---------------------------------------------------------
Title: Getting the Word Out: Undergradute Space Physics at Rice
University
Authors: Reiff, P. H.; Alexander, D.
2006AGUFMED52B..04R Altcode:
At Rice University we emphasize space physics in our non-major Physics
and Astronomy undergraduate classes in addition to our graduate and
majors program. In "ASTR 202" (solar system exploration for non-
majors), we typically use a textbook which includes magnetospheric
and auroral topics in it (many do not). In recent years, we have
also created two new courses for undergraduates which highlight
space physics. In spring 2005 we began PHYS 401, The Physics of Ham
Radio, which includes a significant portion on the Sun, ionosphere,
radio propagation, and space storms. It is a fun hands-on way to
learn about circuits, electrical theory, antennas, and the effects
of space weather, while creating a new hobby at the same time. The
students are required to attempt the FCC "Technician" exam as their
midterm exam, and all of the class members passed. This course is
taken both by undergraduates and by local teachers in the Master of
Science Teaching program (the teacher tuition is partially supported
by CISM), and is offered every other year (it will be offered again
in Spring 2007). In fall 2005 one of us (Alexander) started a new
course, ASTR 243 "Exploring the Sun-Earth Connection", which focuses
entirely on solar and space weather topics. It required the students
to perform several projects over the course of the semester, and used
many online resources. The feedback from the first session was very
favorable, so it also will likely be offered every other year. Two
of the students extended their experience by participating in summer
research, one at an REU at the National Solar Observatory working on
helioseismology data, and one at an international summer school in
the U.K. where she focused on coronal heating. Thus with two courses
in an every-other-year rotation, each academic year one undergraduate
course in space physics is available at Rice. Furthermore, all senior
majors are required to perform research, and each year several students
choose a solar or space physics topic for their senior research, and
often go on to graduate study at schools around the nation. Sun-Earth
course page: http://www.owlnet.rice.edu/~astr243/ Ham radio course
page: http://space.rice.edu/PHYS401/
---------------------------------------------------------
Title: Investigating the Temporal Behavior of Localized UV and Hard
X-ray Emission in Large Solar Flares
Authors: Coyner, Aaron J.; Alexander, D.
2006SPD....37.1302C Altcode: 2006BAAS...38..241C
While many studies have related the timing of localized UV emission
tospatially unresolved hard X-ray lightcurves, few have compared the
timeprofiles of spatially localized UV and hard X-ray sources. In
this workwe present a study of large two ribbon flares observed with
RHESSI andTRACE in which multiple hard X-ray sources develop along
the ribbonsduring the course of the flare. Our preliminary findings
indicate thatlate in these multi-burst events, the UV emission onset
precedes theX-ray emission with delays on scales of seconds. These
observations havepotential implications for particle production and
transport mechanismswithin complex flares.
---------------------------------------------------------
Title: A Brief History of CME Science
Authors: Alexander, David; Richardson, Ian G.; Zurbuchen, Thomas H.
2006SSRv..123....3A Altcode: 2006SSRv..tmp...54A
We present here a brief summary of the rich heritage of observational
and theoretical research leading to the development of our current
understanding of the initiation, structure, and evolution of Coronal
Mass Ejections.
---------------------------------------------------------
Title: The Pre-CME Sun
Authors: Gopalswamy, N.; Mikić, Z.; Maia, D.; Alexander, D.; Cremades,
H.; Kaufmann, P.; Tripathi, D.; Wang, Y. -M.
2006SSRv..123..303G Altcode: 2006SSRv..tmp...77G
The coronal mass ejection (CME) phenomenon occurs in closed magnetic
field regions on the Sun such as active regions, filament regions,
transequatorial interconnection regions, and complexes involving a
combination of these. This chapter describes the current knowledge
on these closed field structures and how they lead to CMEs. After
describing the specific magnetic structures observed in the CME source
region, we compare the substructures of CMEs to what is observed before
eruption. Evolution of the closed magnetic structures in response to
various photospheric motions over different time scales (convection,
differential rotation, meridional circulation) somehow leads to the
eruption. We describe this pre-eruption evolution and attempt to link
them to the observed features of CMEs. Small-scale energetic signatures
in the form of electron acceleration (signified by nonthermal radio
bursts at metric wavelengths) and plasma heating (observed as compact
soft X-ray brightening) may be indicative of impending CMEs. We survey
these pre-eruptive energy releases using observations taken before
and during the eruption of several CMEs. Finally, we discuss how the
observations can be converted into useful inputs to numerical models
that can describe the CME initiation.
---------------------------------------------------------
Title: Coronal Observations of CMEs. Report of Working Group A
Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.;
Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.;
Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.;
Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J.
2006SSRv..123..127S Altcode: 2006SSRv..tmp...58S
CMEs have been observed for over 30 years with a wide variety of
instruments. It is now possible to derive detailed and quantitative
information on CME morphology, velocity, acceleration and mass. Flares
associated with CMEs are observed in X-rays, and several different
radio signatures are also seen. Optical and UV spectra of CMEs both on
the disk and at the limb provide velocities along the line of sight
and diagnostics for temperature, density and composition. From the
vast quantity of data we attempt to synthesize the current state of
knowledge of the properties of CMEs, along with some specific observed
characteristics that illuminate the physical processes occurring during
CME eruption. These include the common three-part structures of CMEs,
which is generally attributed to compressed material at the leading
edge, a low-density magnetic bubble and dense prominence gas. Signatures
of shock waves are seen, but the location of these shocks relative
to the other structures and the occurrence rate at the heights where
Solar Energetic Particles are produced remains controversial. The
relationships among CMEs, Moreton waves, EIT waves, and EUV dimming
are also cloudy. The close connection between CMEs and flares suggests
that magnetic reconnection plays an important role in CME eruption
and evolution. We discuss the evidence for reconnection in current
sheets from white-light, X-ray, radio and UV observations. Finally, we
summarize the requirements for future instrumentation that might answer
the outstanding questions and the opportunities that new space-based
and ground-based observatories will provide in the future.
---------------------------------------------------------
Title: Temporal and Spatial Relationships between Ultraviolet and
Hard X-Ray Emission in Solar Flares
Authors: Alexander, David; Coyner, Aaron J.
2006ApJ...640..505A Altcode:
Hard X-ray and ultraviolet emissions in solar flares provide evidence
for the interaction of particles accelerated in the solar corona
with the ambient plasma in the chromosphere. Previous studies have
shown that these emissions are temporally correlated indicating a
common source, presumably magnetic reconnection, governing the flare
energy release. We use the high spatial resolution of the RHESSI and
TRACE telescopes to explore the temporal connections of spatially
resolved hard X-ray and UV sources in a flare occurring on 2002
July 16. While exhibiting a strong temporal correlation, there are
distinct differences in the spatial distribution of the UV and hard
X-ray emissions. The UV emission is found to be distributed in two
well-defined ribbons spanning 50"-100" and separated by some 50"-70",
while the hard X-ray emission is localized to a compact distribution
at one end of the southernmost ribbon. We argue that the temporally
correlated, but spatially well-separated, emissions result from the
interaction of an evolving multipolar magnetic flux system with the
flare energy release predominantly occurring along a separator marking
the intersection of separate topological domains defined by an emerging
bipolar system and the preexisting active region magnetic field.
---------------------------------------------------------
Title: An Introduction to the Pre-CME Corona
Authors: Alexander, David
2006SSRv..123...81A Altcode: 2006SSRv..tmp....4A
Coronal mass ejections provide a gateway to understanding the physics
of energy release and conversion in the solar corona. While it is
generally accepted that the energy required to power a CME is contained
in the pre-eruption coronal magnetic field, the pre-CME state of that
field and the conditions leading up to the release of the magnetic
energy are still not entirely clear. Recent studies point to various
phenomena which are common to many, if not all, CME events, suggesting
that there may be identifiable characteristics of the pre-CME corona
which signal the impending eruption. However, determining whether these
phenomena are necessary or even sufficient has yet to be achieved. In
this paper we attempt to summarize the state of the solar corona and
its evolution in the build up to a CME.
---------------------------------------------------------
Title: A Brief History of CME Science
Authors: Alexander, David; Richardson, Ian G.; Zurbuchen, Thomas H.
2006cme..book....3A Altcode:
We present here a brief summary of the rich heritage of observational
and theoretical research leading to the development of our current
understanding of the initiation, structure, and evolution of Coronal
Mass Ejections.
---------------------------------------------------------
Title: An Introduction to the pre-CME Corona
Authors: Alexander, David
2006cme..book...81A Altcode:
Coronal mass ejections provide a gateway to understanding the physics
of energy release and conversion in the solar corona. While it is
generally accepted that the energy required to power a CME is contained
in the pre-eruption coronal magnetic field, the pre-CME state of that
field and the conditions leading up to the release of the magnetic
energy are still not entirely clear. Recent studies point to various
phenomena which are common to many, if not all, CME events, suggesting
that there may be identifiable characteristics of the pre-CME corona
which signal the impending eruption. However, determining whether these
phenomena are necessary or even sufficient has yet to be achieved. In
this paper we attempt to summarize the state of the solar corona and
its evolution in the build up to a CME.
---------------------------------------------------------
Title: Role of Sunspot and Sunspot-Group Rotation in Driving Sigmoidal
Active Region Eruptions
Authors: Tian, Lirong; Alexander, David
2006SoPh..233...29T Altcode:
We study active region NOAA 9684 (N06L285) which produced an X1.0/3B
flare on November 4, 2001 associated with a fast CME (1810 km
s<SUP>−1</SUP>) and the largest proton event (31 700 pfu) in cycle
23. SOHO/MDI continuum image data show that a large leading sunspot
rotated counter-clockwise around its umbral center for at least 4
days prior to the flare. Moreover, it is found from SOHO/MDI 96 m
line-of-sight magnetograms that the systematic tilt angle of the
bipolar active region, a proxy for writhe of magnetic fluxtubes,
changed from a positive value to a negative one. This signifies a
counter-clockwise rotation of the spot-group as a whole. Using vector
magnetograms from Huairou Solar Observing Station (HSOS), we find that
the twist of the active region magnetic fields is dominantly left handed
(α<SUB>best</SUB> = −0.03), and that the vertical current and current
helicity are predominantly negative, and mostly distributed within the
positive rotating sunspot. The active region exhibits a narrow inverse
S-shaped H<SUB>α</SUB> filament and soft X-ray sigmoid distributed
along the magnetic neutral line. The portion of the filament which
is most closely associated with the rotating sunspot disappeared on
November 4, and the corresponding portion of the sigmoid was observed
to erupt, producing the flare and initiating the fast CME and proton
event. These results imply that the sunspot rotation is a primary
driver of helicity production and injection into the corona. We suggest
that the observed active region dynamics and subsequent filament and
sigmoid eruption are driven by a kink instability which occurred due
to a large amount of the helicity injection.
---------------------------------------------------------
Title: Coronal Observations of CMEs
Authors: Schwenn, R.; Raymond, J. C.; Alexander, D.; Ciaravella, A.;
Gopalswamy, N.; Howard, R.; Hudson, H.; Kaufmann, P.; Klassen, A.;
Maia, D.; Munoz-Martinez, G.; Pick, M.; Reiner, M.; Srivastava, N.;
Tripathi, D.; Vourlidas, A.; Wang, Y. -M.; Zhang, J.
2006cme..book..127S Altcode:
CMEs have been observed for over 30 years with a wide variety of
instruments. It is now possible to derive detailed and quantitative
information on CME morphology, velocity, acceleration and mass. Flares
associated with CMEs are observed in X-rays, and several different
radio signatures are also seen. Optical and UV spectra of CMEs both on
the disk and at the limb provide velocities along the line of sight
and diagnostics for temperature, density and composition. From the
vast quantity of data we attempt to synthesize the current state of
knowledge of the properties of CMEs, along with some specific observed
characteristics that illuminate the physical processes occurring during
CME eruption. These include the common three-part structures of CMEs,
which is generally attributed to compressed material at the leading
edge, a low-density magnetic bubble and dense prominence gas. Signatures
of shock waves are seen, but the location of these shocks relative
to the other structures and the occurrence rate at the heights where
Solar Energetic Particles are produced remains controversial. The
relationships among CMEs, Moreton waves, EIT waves, and EUV dimming
are also cloudy. The close connection between CMEs and flares suggests
that magnetic reconnection plays an important role in CME eruption
and evolution. We discuss the evidence for reconnection in current
sheets from white-light, X-ray, radio and UV observations. Finally, we
summarize the requirements for future instrumentation that might answer
the outstanding questions and the opportunities that new space-based
and ground-based observatories will provide in the future.
---------------------------------------------------------
Title: The Pre-CME Sun
Authors: Gopalswamy, N.; Mikić, Z.; Maia, D.; Alexander, D.; Cremades,
H.; Kaufmann, P.; Tripathi, D.; Wang, Y. -M.
2006cme..book..303G Altcode:
The coronal mass ejection (CME) phenomenon occurs in closed magnetic
field regions on the Sun such as active regions, filament regions,
transequatorial interconnection regions, and complexes involving a
combination of these. This chapter describes the current knowledge
on these closed field structures and how they lead to CMEs. After
describing the specific magnetic structures observed in the CME source
region, we compare the substructures of CMEs to what is observed before
eruption. Evolution of the closed magnetic structures in response to
various photospheric motions over different time scales (convection,
differential rotation, meridional circulation) somehow leads to the
eruption. We describe this pre-eruption evolution and attempt to link
them to the observed features of CMEs. Small-scale energetic signatures
in the form of electron acceleration (signified by nonthermal radio
bursts at metric wavelengths) and plasma heating (observed as compact
soft X-ray brightening) may be indicative of impending CMEs. We survey
these pre-eruptive energy releases using observations taken before
and during the eruption of several CMEs. Finally, we discuss how the
observations can be converted into useful inputs to numerical models
that can describe the CME initiation.
---------------------------------------------------------
Title: High Spectral Resolution Cryogenic Imaging Detectors for
Solar Physics
Authors: Stern, R.; Martinez-Galarce, D.; Rausch, A.; Shing, L.;
Deiker, S.; Boerner, P.; Metcalf, T.; Cabrera, B.; Leman, S.; Brink,
P.; Irwin, K.; Ullom, J.; Alexander, D.
2005AGUFMSH41B1130S Altcode:
X-ray microcalorimeters have the potential to substantially
improve our understanding of magnetic reconnection in the early
phases of flares or during microflare events. Operating in the
sub-Kelvin temperature range, they provide high non-dispersive
energy resolution at ~ keV energies (e.g, E/Δ E ~ 1500 at 6 keV),
coupled with high (msec or better) time resolution. Coupled with
grazing-incidence X-ray optics, microcalorimeter arrays or position
sensitive readout schemes will form the basis for a new generation of
solar imaging spectrometers. An Explorer-class solar mission within
the next 5-10 years, based upon these detectors, is rapidly becoming
technically feasible. LMSAL currently has both internally and NASA
funded laboratory research programs to investigate TES (Transition
Edge Sensor) microcalorimeters. In this presentation, we will discuss
the current status of these programs and their applicability to future
Explorer missions and Roadmap missions such as RAM.
---------------------------------------------------------
Title: MTRAP: the magnetic transition region probe
Authors: Davis, J. M.; West, E. A.; Moore, R. L.; Gary, G. A.;
Kobayashi, K.; Oberright, J. E.; Evans, D. C.; Wood, H. J.; Saba,
J. L. R.; Alexander, D.
2005SPIE.5901..273D Altcode:
The Magnetic Transition Region Probe is a space telescope designed to
measure the magnetic field at several heights and temperatures in the
solar atmosphere, providing observations spanning the chromospheric
region where the field is expected to become force free. The primary
goal is to provide an early warning system (hours to days) for solar
energetic particle events that pose a serious hazard to astronauts in
deep space and to understand the source regions of these particles. The
required magnetic field data consist of simultaneous circular and linear
polarization measurements in several spectral lines over the wavelength
range from 150 to 855 nm. Because the observations are photon limited
an optical telescope with a large (>18m<SUP>2</SUP>) collecting area
is required. To keep the heat dissipation problem manageable we have
chosen to implement MTRAP with six separate Gregorian telescopes, each
with ~ 3 m<SUP>2</SUP> collecting area, that are brought to a common
focus. The necessary large field of view (5 × 5 arcmin<SUP>2</SUP>)
and high angular resolution (0.025 arcsec pixels) require large
detector arrays and, because of the requirements on signal to noise
(10<SUP>3</SUP>), pixels with large full well depths to reduce the
readout time and improve the temporal resolution. The optical and
engineering considerations that have gone into the development of a
concept that meets MTRAP's requirements are described.
---------------------------------------------------------
Title: The Role of the Kink Instability of a Long-Lived Active Region
AR 9604
Authors: Tian, Lirong; Liu, Yang; Yang, Jing; Alexander, David
2005SoPh..229..237T Altcode:
We have traced the long-term evolution of a non-Hale active region
composed of NOAA 9604-9632-9672-9704-9738, which displayed strong
transient activity with associated geomagnetic effects from September
to December, 2001. By studying the development of spot-group
and line-of-sight magnetic field together with the evolution of
H<SUB>α</SUB> filaments, the EUV and X-ray corona (TRACE 171 Å,
Yohkoh/SXT), we have found that the magnetic structure of the active
region exhibited a continuous clockwise rotation throughout its entire
life. Vector magnetic data obtained from Huairou Solar Observing Station
(HSOS) and full-disk line-of-sight magnetograms from SOHO/MDI allowed
the determination of the best-fit force-free parameter (proxy of twist),
α<SUB>best</SUB>, and the systematic tilt angle (proxy of writhe) which
were both found to take positive values. Soft X-ray coronal loops from
Yohkoh/SXT displayed a pronounced forward-sigmoid structure in period of
NOAA 9704. These observations imply that the magnetic flux tube (loops)
with the same handedness (right) of the writhe and the twist rotated
clockwise in the solar atmosphere for a long time. We argue that the
continuous clockwise rotation of the long-lived active region may be
a manifestation that a highly right-hand twisted and kinked flux tube
was emerging through the photosphere and chromosphere into the corona.
---------------------------------------------------------
Title: Magnetic Twist and Writhe of δ Active Regions
Authors: Tian, Lirong; Alexander, David; Liu, Yang; Yang, Jing
2005SoPh..229...63T Altcode:
We have selected 104 active regions with a δ magnetic configuration
from 1996 to 2002 to study how important a role the kink instability
plays in such active regions. In this study, we employ the systematic
tilt angle of each active region as a proxy for the writhe of a flux
tube and the force-free parameter, α<SUB>best</SUB>, as a proxy for
the magnetic field twist in the flux tube. It is found that 65-67%
of the active regions have the same sign of twist and writhe. About
34% (32%) of the active regions violate (follow) the Hale-Nicholson
and Joy's Laws (HNJL) but follow (violate) the hemispheric helicity
rule (HHR). Sixty-one (61) of the 104 active regions studied each
produced more than five large flares. Active regions violating HNJL,
but following HHR, have a much stronger tendency to produce X-class
flares and/or strong proton events. Comparing with previous studies
for active regions with well-defined (simpler) bipolar magnetic
configuration, it is found that the numbers following both HNJL and HHR
are significantly lower in the δ-configuration case, while numbers
violating one of the laws and the rule significantly increase with
the increase of the magnetic complexity of the active regions. These
results support the prediction for the presence of a kink instability,
that the twist and writhe of the magnetic fields exhibit the same sign
for δ active regions (Linton et al., Astrophys. J.507, 40, 1998,
Astrophys. J.522, 1205, 1999; Fan et al., Astrophys. J.521, 460,
1999). Finally, we analyze possible origins of the twist and writhe
of the magnetic fields for the active regions studied.
---------------------------------------------------------
Title: Cryogenic 3-D Detectors for Solar Physics
Authors: Stern, R. A.; Martinez-Galarce, D.; Rausch, A.; Shing, L.;
Deiker, S.; Boerner, P.; Metcalf, T.; Cabrera, B.; Leman, S. W.;
Brink, P.; Irwin, K.; Alexander, D.
2005AGUSMSP12A..02S Altcode:
Cryogenic microcalorimeters operating in the sub-Kelvin temperature
range provide non-dispersive energy resolution at optical through
gamma ray energies (e.g, E/Δ E ~ 1500 at 6 keV). Microcalorimeters
also have high time resolution (msec or better), and can be made into
imaging arrays through SQUID multiplexing of individual pixels or
employing position sensitive detector structures. The application
of such "3-D" detector technology to solar physics will lead to
significant advances in our understanding of magnetic reconnection
in the Sun, including X-ray jet phenomena, and active region heating
and dynamics. An Explorer-class solar mission within the next 5-10
years, based upon these detectors, is rapidly becoming technically
feasible. LMSAL currently has an internally funded laboratory research
program to investigate TES (Transition Edge Sensor) microcalorimeters;
we recently saw our first X-ray photons using TES detectors supplied
by NIST. In addition, we have recently been funded by NASA to begin
work with NIST on position-sensitive X-ray strip detectors for solar
physics applications. Finally, we are collaborating with with Stanford
and NIST on a solar sounding rocket. In this presentation, we will
discuss the current status of these programs and their applicability
to future Explorer missions and Roadmap missions such as RAM.
---------------------------------------------------------
Title: The Characteristics of Hard X-ray Production in Flares Driven
by Filament Eruptions
Authors: Liu, R.; Alexander, D.
2005AGUSMSP23B..04L Altcode:
We investigate the temporal and spatial relationship between filament
eruptions and the production of hard X-ray emission using spatially
resolved high cadence data from TRACE and RHESSI. In particular, we
focus on comparing the characteristics of the hard X-ray production
in `successful' and `failed' filament eruption cases. Our preliminary
findings indicate even failed eruption events can generate significant
energy release and hard X-ray emission with the hard X-ray production
apparently correlated to the rate of expansion of the filament. The
spatial distribution of the hard X-ray emission, while depending upon
the overall strength of the event, also depends on the evolutionary
behavior of the filament as it erupts, e.g. loop-like versus
"zipper"-like.
---------------------------------------------------------
Title: Photon-Flux Saturation in Large Solar Flares
Authors: Daou, A. G.; Alexander, D.
2005AGUSMSP21A..06D Altcode:
We use the spectral and spatial resolution of RHESSI to explore the
behavior of electron fluxes and their associated currents in large
solar flares, including the Halloween events of October-November
2003. The incident electron spectra at the flaring footpoints are
derived from the RHESSI photon spectra. Spectral images are used to
determine an upper limit as well as a spectrally-averaged estimation
for the footpoint area. We find that, over a wide range of flare X-ray
magnitudes, the integrated fluxes above 20 keV asymptotically approach
a limiting value, suggesting some form of saturation in the particle
production in flares. We discuss the implications of these results in
terms of the assumed model of a stable non-thermal electron beam.
---------------------------------------------------------
Title: Magnetic Twist and Writhe of Delta Active Regions
Authors: Tian, L.; Alexander, D.; Liu, Y.; Yang, J.
2005AGUSMSH54B..03T Altcode:
Active regions with a δ magnetic configuration from 1996 to 2002 were
selected to study how important a role the kink instability plays in
such active regions. We employ the systematic tilt angle of each active
region as a proxy for the writhe of a fluxtube and the force-free
parameter, αbest, as a proxy for the magnetic field twist in the
fluxtube. It is found that 65-67% of the 104 active regions have the
same sign of twist and writhe, which violate the Hale-Nicholson and
Joy's Laws (HNJL) or the hemispheric helicity rule (HHR). 68% (46/68)
of these active regions produced more than five large flares. Active
regions violating HNJL, but following HHR, have a much stronger tendency
to produce X-class flares and/or strong proton events. Continuously
clockwise rotation of magnetic configuration of a long-lived active
region (AR 9604-9632-9672-9704-9738) which produced major flares, fast
CMEs and many strong proton events shows that a kink instability would
play very important role in such active regions. These results support
the prediction for the presence of a kink instability, that the twist
and writhe of the magnetic fields exhibit the same sign for δ active
regions (Linton et al, 1998, 1999, and Fan et al., 1999). Finally,
we analyze possible origins of the twist and writhe of the magnetic
fields for the active regions with different relations between the
twist and writhe.
---------------------------------------------------------
Title: Active region evolution in the build-up to large solar
energetic particle events
Authors: Alexander, D.; Coyner, A.; Daou, A.; Liu, R.
2005AGUSMSH53A..02A Altcode:
The SEP signatures of the solar flares occurring on 21 Apr 2002 and
24 Aug 2002 show marked differences in their compositions above 10
MeV/nucleon, yet at the Sun the events themselves display very similar
characteristics in the chromospheric (hard X-ray) emission and their
coronal (X-ray, EUV) signatures. We consider the prior evolution of the
parent active regions to look for possible signatures in the ambient
corona, magnetic connectivities (within the active region, large-scale
closed field, and open field distributions), and flare/activity history
which may differentiate the solar conditions leading to the observed
disparate particle signatures at 1 AU.
---------------------------------------------------------
Title: Temporal Evolution of Hard X-ray and UV Emission Sources in
Solar Flares
Authors: Coyner, A. J.; Alexander, D.
2005AGUSMSP21A..03C Altcode:
In this work, we investigate, for a number of solar flares, the
evolution of both UV hard x-ray emission as observed by TRACE and
RHESSI, respectively. The spatial resolution of these two instruments
provides an unprecedented capability to study the individual source
regions producing the observed time profiles in these events. Confirming
earlier results, it is found that individual bursts in the hard X-ray
time profiles are correlated with the unresolved UV emission. However,
the temporal evolution of individual sources provides a deeper insight
into the flare energization process. We report a number of results: 1)
the UV-to-UV temporal correlation analysis allows us to identify sources
which exhibit similar time profiles and therefore may indicate conjugate
footpoints of a large coronal loop, 2) the UV emission is distributed
over a wider area than the hard X-ray emission, 3) the bulk of the
hard X-ray emission would seem to be related to a different structure
within the flaring volume but sharing a common footpoint with the UV
emission, and 4) the temporal and spatial behavior observed would seem
to indicate an interacting loop scenario for the flares considered.
---------------------------------------------------------
Title: Multialtitude Observations of a Coronal Jet during the Third
Whole Sun Month Campaign
Authors: Ko, Y. -K.; Raymond, J. C.; Gibson, S. E.; Alexander, D.;
Strachan, L.; Holzer, T.; Gilbert, H.; Cyr, O. C. St.; Thompson, B. J.;
Pike, C. D.; Mason, H. E.; Burkepile, J.; Thompson, W.; Fletcher, L.
2005ApJ...623..519K Altcode:
On 1999 August 26, a coronal jet occurred at the northwest limb near
a sigmoid active region (AR 8668) that was the target for a joint
observation plan (SOHO joint observing program 106) during the third
Whole Sun Month Campaign. This jet was observed by several instruments
at the limb (SOHO/CDS, SOHO/EIT, TRACE, and Mauna Loa Solar Observatory
CHIP and PICS) and at 1.64 R<SUB>solar</SUB> (SOHO/UVCS). At the limb,
this jet event displayed both low- and high-temperature components. Both
high- and low-temperature components were evident during the early phase
(first 20 minutes) of the event. However, the low-temperature component
is maintained for ~1 hr after the higher temperature component is
gone. There is a second brightening (a possible second jet) seen by
EIT and TRACE about 50 minutes after the onset of the first jet. The
line-of-sight motion at the limb began with a 300 km s<SUP>-1</SUP>
redshift and evolved to a 200 km s<SUP>-1</SUP> blueshift. At
1.64 R<SUB>solar</SUB>, the intensities of Lyα and Lyβ in the jet
increased by a factor of several hundred compared with the background
corona. The C III λ977 line also brightened significantly. This
indicates low-temperature [~(1-2)×10<SUP>5</SUP> K] emission in the
jet, while the intensities of O VI λ1032 and O VI λ1037 increased
by as much as a factor of 8. The UVCS data show evidence of heating at
the early phase of the event. The Doppler shift in the lines indicates
that the line-of-sight (LOS) velocity in the jet started from ~150
km s<SUP>-1</SUP> in blueshift and ended at ~100 km s<SUP>-1</SUP>
in redshift. This LOS motion seen at 1.64 R<SUB>solar</SUB> was
apparently opposite to what was observed when the jet emerged from the
limb. The Doppler dimming analysis indicates that the radial outflow
speed correlates with the magnitude of the LOS speed. Interestingly,
UVCS observations at 2.33 and 2.66 R<SUB>solar</SUB> show no trace of
the jet and SOHO/LASCO observations also yield no firm detection. We
find that a simple ballistic model can explain most of the dynamical
properties of this jet, while the morphology and the thermal properties
agree well with reconnection-driven X-ray jet models.
---------------------------------------------------------
Title: Fokker-Planck Modeling of Asymmetric Footpoint Hard X-Ray
Emission in Solar Flares
Authors: McClements, K. G.; Alexander, D.
2005ApJ...619.1153M Altcode:
Chromospheric hard X-ray emission in a solar flare generally occurs
in two magnetically connected “footpoint” regions. Recent spatially
resolved hard X-ray observations carried out using the RHESSI spacecraft
have shown that the ratio of total X-ray fluxes from the two footpoints
is time dependent and demonstrates a weak but detectable photon
energy dependence. A Fokker-Planck code is used to identify possible
scenarios that could reproduce the observed dependence of footpoint
asymmetry on time and energy. The code, which is benchmarked against
analytical results in the limit of collisionless precipitation from a
symmetric flaring loop, includes collisional friction and pitch-angle
scattering, asymmetric magnetic mirroring, and a source term that can
be prescribed arbitrarily. This model is used to demonstrate that the
observed dependence of hard X-ray asymmetry on photon energy can be
attributed to an energetic electron source that is isotropic at low
energy (presumed to be due to Coulomb collisions) and at high energy
(presumed to be due to resonant wave-particle scattering) and strongly
anisotropic at intermediate energies.
---------------------------------------------------------
Title: STEREO/SECCHI Simulations of CMEs and Flares using TRACE Images
Authors: Aschwanden, M. J.; Lemen, J.; Nitta, N.; Metcalf, T.; Wuelser,
J.; Alexander, D.
2004AGUFMSH22A..02A Altcode:
We simulate 3D models of EUV images of flare and CME events, using
TRACE EUV movies. TRACE movies show 2D images in projection along a
particular line-of-sight. We simulate 3D models of erupting filaments,
flare loops, and postflare loops using: (1) a “finger printing”
technique to trace linear structures in 2D images; (2) geometric 3D
models based on force-free fields and curvature radius maximization
of flare loop and flux rope structures; (3) conservation of velocity
and acceleration parameters; (4) multi-temperature plasma filling
according to hydrodyamic scaling laws; and (5) 2D projections from
secondary line-of-sights that correspond to viewpoints of the secondary
STEREO spacecraft. From such simulations we envision to illustrat
3D time-dependent models, what would be observed at the two STEREO
spacecraft positions as well as from a near-Earth spacecraft such as
SoHO. These simulations are used to test STEREO analysis software and
to investigate what physical parameters and geometric 3D reconstructions
can be retrieved from STEREO/SECCHI data.
---------------------------------------------------------
Title: “Transition-Edge Sensors for Solar X-ray Spectral Observations
- An Update”
Authors: Martinez-Galarce, D. S.; Stern, R.; Cabrera, B.; Deiker,
S.; Alexander, D.; Metcalf, T.; Irwin, K.; Brink, P.; Boerner,
P. F.; Morse, K.; Leman, S.; Shing, L.; Rausch, A.; Nichols, T. D.;
Chakraborty, S.
2004AGUFMSH13A1147M Altcode:
ABSTRACT: The advent of cryogenic microcalorimeters (operating at
temperatures of ∼ 0.1 K) in ground-based and space-based astronomy
promises a revolution of new discoveries. Particularly, Transition-Edge
Sensors (TES) have demonstrated high-energy resolution measurements of
soft X-rays of up to E/Δ E ∼ 1500 (at 6keV) and with high temporal
resolution of a msec or less in photon pulse detections. Fabricated
into multiplexed arrays of single detectors, or position sensing
macropixels, and placed at the focus of a Wolter optic would further
yield high spatial resolution capability of 2 arcsec or less, thus
producing unprecedented "3-D" solar observations. We report herein,
on progress to date in the development of these detectors and
particularly, with on-going work with the sounding rocket payload,
the Advanced Technology Solar Spectroscopic Imager, which will debut
a TES instrument operating in the 300eV - 1.5keV range to study active
region magnetic reconnection. Furthermore, as part of our larger effort,
we discuss also current technical developments and plans at the Lockheed
Martin Solar & Astrophysics Laboratory to design a TES instrument
(3 - 8keV range) for realization into a dedicated Explorer-class solar
observatory in the next 5 - 10 years.
---------------------------------------------------------
Title: Tomographic 3D-Modeling of the Solar Corona with FASR
Authors: Aschwanden, Markus J.; Alexander, David; de Rosa, Marc L.
2004ASSL..314..243A Altcode: 2003astro.ph..9501A
The Frequency-Agile Solar Radiotelescope (FASR) literally opens up
a new dimension, in addition to the 3D Euclidian geometry—the
frequency dimension. The 3D geometry is degenerated to 2D in all
images from astronomical telescopes, but the additional frequency
dimension allows us to retrieve the missing third dimension by means of
physical modeling. We call this type of 3D reconstruction Frequency
Tomography. In this study we simulate a realistic 3D model of an
active region, composed of 500 coronal loops with the 3D geometry
[x(s), y(s), z(s)] constrained by magnetic field extrapolations and
the physical parameters of the density n<SUB>e</SUB>(s) and temperature
T<SUB>e</SUB>(s) given by hydrostatic solutions. We simulate a series
of 20 radio images in a frequency range of ν=0.1-10 GHz, anticipating
the capabilities of FASR, and investigate what physical information
can be retrieved from such a dataset. We discuss also forward-modeling
of the chromospheric and Quiet Sun density and temperature structure,
another primary goal of future FASR science.
---------------------------------------------------------
Title: Hard X-ray Footpoint Asymmetries in Solar Flares
Authors: Chollet, E.; Alexander, D.; Metcalf, T. R.; Pollack, L.
2004AAS...204.5406C Altcode: 2004BAAS...36R.758C
RHESSI observations of a M4.0 solar flare on 17 March, 2002 are
presented. This flare exhibits footpoint asymmetries in hard X-ray
emission, allowing us to study the energy and time dependence of
particle transport in this flare. The data, originally presented in
Alexander and Metcalf (2002), is reanalyzed with new calibrations,
shorter integration times and better spatial resolution, allowing
for more accurate determination of the time and energy dependence
of the asymmetries. While the previous work suggested that this
was a simple flare with two footpoints, the new analysis suggests a
more complex structure with three or more footpoints, confirmed by
force free magnetic field extrapolations. The time development of
the evolving flare indicates a transition from one flaring structue
to another which significantly complicates the interpretation of the
measured asymmetry. The implications of this new interpretation for the
previously reported energy dependence will be discussed. We gratefully
acknowledge support from NASA (NAS5-02048).
---------------------------------------------------------
Title: Spatial and temporal correlations between UV continuum and
hard x-ray emissions in solar flares
Authors: Coyner, A. J.; Alexander, D.
2004AAS...204.5405C Altcode: 2004BAAS...36Q.758C
Combined UV and hard x-ray observations provide a means to better
understanding the temporal and spatial relationship between the
energization of the solar corona in flares and the response of the
solar chromosphere. In this work, we investigate the evolution of
an X1.1 solar flare occurring on 2003 Oct 19 at both UV and hard
x-ray wavelengths. This is accomplished through analysis of data from
the TRACE and RHESSI spacecraft, respectively. Uncertainties in the
TRACE pointing were corrected by coaligning TRACE images with EIT
images of the same wavelength for the same time. Coincident RHESSI
hard X-ray images were generated in 5keV steps over the range from
10-80 keV. We compare the spatial and temporal evolution of the UV and
hard X-ray emissions, for each hard X-ray energy bin, to determine the
relationship, if any, between the hard x-ray flare footpoints and the
UV continuum emission. We gratefully acknowledge the support of NASA
under contract NAS5-02048.
---------------------------------------------------------
Title: Multi-Altitude Observations of a Coronal Jet
Authors: Ko, Y. -K.; Raymond, J. C.; Gibson, S. E.; Alexander, D.;
Strachan, L.; Holzer, T.; Gilbert, H.; St. Cyr, O. C.; Thompson,
B. J.; Pike, C. D.; Burkepile, J.; Thompson, W.; Fletcher, L.
2004AAS...204.5413K Altcode: 2004BAAS...36..759K
A coronal jet occurred on August 26, 1999 at the NW limb near a sigmoid
active region (AR8668). This jet was observed by several instruments
at the limb (SOHO/CDS, SOHO/EIT, TRACE, MLSO/CHIP, MLSO/PICS) and
at 1.64 Ro (SOHO/UVCS). At the limb, this jet event has both low and
high temperature components. The high temperature component appeared
at the early phase (first 20 minutes) of the event along with the low
temperature component while the latter seems to last long ( ∼ 1 hour)
after the higher temperature component was gone. The line-of-sight
motion at the limb started with red-shifted (by as much as 300 km/s)
and turned blue-shifted (by as much as 200 km/s). At 1.64 Ro, the
intensities of Lyα , Lyβ in the jet increased by a factor of several
hundreds compared with the background corona. C III λ 977 line also
brightened significantly. This indicates low temperature ( ∼ 1-2×
10<SUP>5</SUP> K) emission in the jet, while the intensities of O VI
λ 1032 and O VI λ 1037 increased by a factor of as large as 8. Both
UVCS and CDS data show evidence of heating at the early phase of the
event. The line-of-sight velocity seen at 1.64 Ro started with ∼ 150
km/sec in blue shift and ended at ∼ 100 km/sec in red shift. This is
apparently opposite to what were observed when the jet emerged from
the limb. The Doppler dimming analysis indicates that the radial
outflow speed correlates with the magnitude of the line-of-sight
speed. Interestingly, UVCS observations at 2.33 and 2.66 Ro show no
trace of the jet and LASCO observations also yield no firm sight of the
jet. In this paper, we present the observations by these instruments
and discuss the dynamical structure and physical properties of this
jet. Y.-K. Ko acknowledges the support by NASA grant NAG5-12865.
---------------------------------------------------------
Title: Non-Relativistic Electron Beam Stability in Solar Flares
Authors: Daou, A. G.; Alexander, D.; Metcalf, T. R.
2004AAS...204.0203D Altcode: 2004BAAS...36..667D
The thick-target electron beam model has been used for decades
as a viable description for the production of solar flare hard
X-ray emission. The required very rapid transport of energy to the
footpoints during solar flares is achieved in this model by fast
electrons traversing the loop to deposit their energy in the dense
chromosphere. For some of the largest flares the currents (up to
10<SUP>17</SUP> Amps or 10<SUP>36</SUP> electrons/sec) inferred
can significantly exceed the Alfven-Lawson limit suggesting that
the assumed electron beam is inherently unstable. In this paper, we
use the spectral and spatial resolution of RHESSI to explore whether
the conditions for a stable non-thermal electron beam exist in large
solar flares. The incident electron spectra at flaring footpoints are
derived from the RHESSI photon spectra while an upper limit to the
footpoint area is detremined form the hard X-ray images. We determine
the electron beam density needed to produce the hard X-ray emission for
two large flares, July 17, 2002 (M8.5) and October 28, 2003 (X17.2)
and determine whether such beams are viable in these cases. <P />We
gratefully acknowledge support from NASA (NAS5-02048).
---------------------------------------------------------
Title: Fokker Planck Modelling of Asymmetric Footpoint Hard X-ray
Emission in Solar Flares
Authors: Alexander, D.; McClements, K. G.
2004AAS...204.5404A Altcode: 2004BAAS...36..758A
Chromospheric hard X-ray emission in a solar flare generally
occurs in two magnetically connected 'footpoint' regions. Recent
spatially-resolved hard X-ray observations carried out using the RHESSI
spacecraft have shown that the ratio of total X-ray fluxes from two such
footpoints is time-dependent and demonstrates a weak but detectable
energy-dependence. We use a Fokker-Planck code to identify possible
scenarios that could reproduce the observed dependence of footpoint
asymmetry on time and energy. The code, which we have benchmarked
against analytical results in the limit of collisionless precipitation
from a symmetric flaring loop, includes collisional friction and pitch
angle scattering, asymmetric magnetic mirroring, and a source term
that can be prescribed arbitrarily. Using this model, we examine in
particular whether the observed imbalance in hard X-ray emission from
the two footpoints in a flare can be attributed primarily to asymmetry
in the loop magnetic structure or asymmetry in the acceleration
process. <P />This work was funded partly by NASA (NAS5-02048) and by
the United Kingdom Engineering and Physical Sciences Research Council.
---------------------------------------------------------
Title: Solar Week: Learning from Experience
Authors: Alexander, D.; Hauck, K.
2003AGUFMED51C1209A Altcode:
Solar Week is a week-long set of games and activities allowing students
to interact directly with solar science and solar scientists. Solar
Week was developed as a spin-off of the highly successful Yohkoh
Public Outreach Project (YPOP). While YPOP provided access to solar
images, movies and activities, the main goal of Solar Week was to
enhance the participation of women, who are under-represented in the
physical sciences. Solar Week achieves this by providing young women,
primarily in grades 6-8, with access to role models in the sciences. The
scientists participating in Solar Week are women from a variety of
backgrounds and with a variety of scientific expertise. In this paper,
our aim is to provide some insight into developing activity-based space
science for the web and to discuss the lessons-learned from tailoring
to a specific group of participants.
---------------------------------------------------------
Title: Cryogenic 3-D Detectors for Solar Physics Using Position
Sensitive Transition-Edge Sensors
Authors: Stern, R. A.; Martinez-Galarce, D.; Metcalf, T.; Lemen, J.;
Cabrera, B.; Brink, P.; Leman, S.; Deiker, S.; Irwin, K.; Alexander, D.
2003AAS...203.1805S Altcode: 2003BAAS...35Q1237S
Space and ground-based astronomy is currently undergoing a revolution
in detector technology with the advent of cryogenic sensors operating in
the sub-Kelvin temperature range. These detectors provide non-dispersive
energy resolution at optical through gamma ray energies (e.g, E/Δ E ∼
1500 at 6 keV), high time resolution (msec or better), and can be made
into imaging arrays through SQUID multiplexing of individual pixels or
employing macropixels with position sensing capability. The application
of such “3-D” detector technology to solar physics will lead to
significant advances in our understanding of magnetic reconnection
phenomena in the Sun, including flares and microflares, X-ray jets,
and active region dynamics. An Explorer-class solar mission based upon
these detectors is a distinct possibility within the next 5-10 years. In
this poster, we will describe some of the recent advances in cryogenic
detector technology with particular applicability to solar physics,
and future technical developments required to make such a mission a
reality. <P />This work was supported in part by the Lockheed Martin
Independent Research Program.
---------------------------------------------------------
Title: TRACE and Yohkoh Observations of a White-Light Flare
Authors: Metcalf, Thomas R.; Alexander, David; Hudson, Hugh S.;
Longcope, Dana W.
2003ApJ...595..483M Altcode:
We present observations of a large solar white-light flare observed
on 2001 August 25, using data from the Transition Region and Coronal
Explorer (TRACE) white-light channel and Yohkoh/HXT. These emissions are
consistent with the classic type I white-light flare mechanism, and we
find that the enhanced white-light emission observed by TRACE originates
in the chromosphere and temperature minimum regions via nonequilibrium
hydrogen ionization induced by direct collisions with the electron beam
and by back-warming of the lower atmosphere. The three flare kernels
observed in hard X-rays and white light are spatially associated with
magnetic separatrices, and one of the kernels is observed to move along
a magnetic separatrix at 400 km s<SUP>-1</SUP>. This is evidence in
favor of particle acceleration models, which energize the electrons
via magnetic reconnection at magnetic separators.
---------------------------------------------------------
Title: Observations of Rotating Sunspots from TRACE
Authors: Brown, D. S.; Nightingale, R. W.; Alexander, D.; Schrijver,
C. J.; Metcalf, T. R.; Shine, R. A.; Title, A. M.; Wolfson, C. J.
2003SoPh..216...79B Altcode:
Recent observations from TRACE in the photospheric white-light channel
have shown sunspots that rotate up to 200° about their umbral centre
over a period of 3-5 days. The corresponding loops in the coronal fan
are often seen to twist and can erupt as flares. In an ongoing study,
seven cases of rotating sunspots have been identified, two of which
can be associated with sigmoid structures appearing in Yohkoh/SXT and
six with events seen by GOES. This paper analyzes the rotation rates
of the sunspots using TRACE white-light data. Observations from AR
9114 are presented in detail in the main text and a summary of the
results for the remaining six sunspots is presented in Appendixes
A-F. Discussion of the key results, particularly common features,
are presented, as well as possible mechanisms for sunspot rotation.
---------------------------------------------------------
Title: TRACE, SOHO/EIT, and SOHO/MDI Observations of AR0030, Including
Rotating Sunspots and the July 15, 2002 X3.0 Flare in Ultraviolet
and Extreme Ultraviolet
Authors: Nightingale, R. W.; Shine, R. A.; Alexander, D.; Freeland,
S. L.; Frank, Z. A.; Brown, D. S.
2002AGUFMSH52A0467N Altcode:
On July 15, 2002 TRACE and several SOHO instruments observed an X3.0
flare in AR0030 near 2000 UT. During this period TRACE was primarily
observing in its 1600Å ultraviolet (UV) channel (most sensitive
to temperatures around 100,000 K in the flare). The 195Å extreme
ultraviolet (EUV) channel of SOHO/EIT (which is most sensitive to about
1.6 MK) will be utilized in this poster, in addition to the magnetic
field measurements of SOHO/MDI during this event period. TRACE followed
the active region for over 10 days, starting about 4 days before the
flare. Broadband white light TRACE images of the photosphere indicate
that one or more of the sunspots were rotating, a possible precursor to
the flare. Images and movies of AR0030 in the various wavelengths will
be shown. The flare region was so intense in the TRACE UV that it is
very difficult to show both the quiescent and flaring regions, so the UV
movie will focus on the flaring plasma with its 2 eruptions. In the EIT
EUV, more coronal structure away from the flare can be seen. Analysis
of the rotational rates of the sunspots will be given along with their
possible coupling to the flare. This work was supported by NASA under
contract NAS5-38099.
---------------------------------------------------------
Title: Energy dependence of electron trapping in a solar flare
Authors: Alexander, David; Metcalf, Thomas R.
2002SoPh..210..323A Altcode:
Observations of an energy-dependent asymmetry in footpoint hard X-ray
emission by RHESSI for the M4.0 solar flare of 17 March 2002 allows us
to probe the dynamics of particle transport with energy and time. The
presence of such an asymmetry is most readily explained by the effects
of a converging magnetic field with different rates of convergence at
the different footpoints, as would be expected from realistic surface
field distributions. Such a geometry has been discussed in the context
of a trap-plus-precipitation model where the transport of energetic
particles in the flare is governed by the precipitation out of the
coronal trap via collisions, wave-particle interactions or some other
scattering process, into the high-density chromosphere. Comparison
of RHESSI observations with a trap-plus-precipitation model allows us
to use the energy dependence of the asymmetry and the observed ratio
of footpoint to coronal emission at the different energies to assess
the role of the trapping in the transport of energetic electrons and
to probe the nature of the particle precipitation process inside the
loss cone.
---------------------------------------------------------
Title: Complete Models of Axisymmetric Sunspots: Magnetoconvection
with Coronal Heating
Authors: Hurlburt, Neal E.; Alexander, David; Rucklidge, Alastair M.
2002ApJ...577..993H Altcode:
We present detailed results of numerical experiments into the nature
of complete sunspots. The models remain highly idealized but include
fully nonlinear compressible magnetoconvection in an axisymmetric
layer that drives energy into an overlying, low-β plasma. We survey
a range of parameters in which the resulting magnetoconvection
displays the formation of pore- and sunspot-like behavior and assess
the coronal signatures resulting from the energy generated by the
magnetoconvection. The coronal heating is assumed to be a result of
the dissipation by an unspecified means of a fraction of the Poynting
flux entering the corona. The expected signatures in the EUV and soft
X-ray bandpasses of the Transition Region and Coronal Explorer and
Yohkoh/SXT, respectively, are examined. This ad hoc coupling of the
corona to the subphotospheric region results in a dynamical behavior
that is consistent with recent observational results. This agreement
demonstrates that even simple coupled modeling can lead to diagnostics
for investigations of both subphotospheric sunspot structures and
coronal heating mechanisms.
---------------------------------------------------------
Title: The Structure and Evolution of a Sigmoidal Active Region
Authors: Gibson, S. E.; Fletcher, L.; Del Zanna, G.; Pike, C. D.;
Mason, H. E.; Mandrini, C. H.; Démoulin, P.; Gilbert, H.; Burkepile,
J.; Holzer, T.; Alexander, D.; Liu, Y.; Nitta, N.; Qiu, J.; Schmieder,
B.; Thompson, B. J.
2002ApJ...574.1021G Altcode:
Solar coronal sigmoidal active regions have been shown to be precursors
to some coronal mass ejections. Sigmoids, or S-shaped structures,
may be indicators of twisted or helical magnetic structures, having
an increased likelihood of eruption. We present here an analysis of a
sigmoidal region's three-dimensional structure and how it evolves in
relation to its eruptive dynamics. We use data taken during a recent
study of a sigmoidal active region passing across the solar disk
(an element of the third Whole Sun Month campaign). While S-shaped
structures are generally observed in soft X-ray (SXR) emission, the
observations that we present demonstrate their visibility at a range of
wavelengths including those showing an associated sigmoidal filament. We
examine the relationship between the S-shaped structures seen in SXR
and those seen in cooler lines in order to probe the sigmoidal region's
three-dimensional density and temperature structure. We also consider
magnetic field observations and extrapolations in relation to these
coronal structures. We present an interpretation of the disk passage
of the sigmoidal region, in terms of a twisted magnetic flux rope
that emerges into and equilibrates with overlying coronal magnetic
field structures, which explains many of the key observed aspects of
the region's structure and evolution. In particular, the evolving flux
rope interpretation provides insight into why and how the region moves
between active and quiescent phases, how the region's sigmoidicity is
maintained during its evolution, and under what circumstances sigmoidal
structures are apparent at a range of wavelengths.
---------------------------------------------------------
Title: Numerical Simulations of Supergranular Magnetoconvection
Authors: De Rosa, M. L.; Hurlburt, N. E.; Alexander, D.; Rucklidge,
A. M.
2002AAS...200.0418D Altcode: 2002BAAS...34..646D
The complex interactions between the turbulent fluid motions within
the solar convection zone and the related processes of emergence,
evolution, and cancellation of magnetic field at the photosphere have
received much recent attention. It is likely that such interactions
depend on the relative magnitudes of the field and of the flows,
but the details of this coupling are not well understood. To further
investigate the magnetohydrodynamics within such turbulent convection,
we have constructed several idealized simulations of fully compressible
MHD fluids, each contained within a curved, spherical segment that
approximates a localized volume of subphotospheric convection on the
sun. In some cases, the horizontal extent of the computational volume
spans 30 heliographic degrees in both latitude and longitude, thereby
enabling the dynamics within a large field containing approximately
100 supergranular-sized cells to be studied. By varying the amount of
total (unsigned) flux permeating the domain, we are able to investigate
analogs to patches of subsurface convection that generally resemble
either quiet-sun or active regions when viewed from above. In addition,
simplified potential-field extrapolations into the volume above the
computational domain are used to illustrate how the coronal field
topology might behave in response to the continually evolving magnetic
field within the convecting layers. This work was supported by NASA
through grant NAG 5-3077 to Stanford University and by Lockheed Martin
Independent Research and Development funds.
---------------------------------------------------------
Title: Helicity driven sigmoid evolution
Authors: Alexander, D.; Nightingale, R.; Metcalf, T. R.; Brown, D.
2002AAS...200.3608A Altcode: 2002BAAS...34..694A
Recent observations of rotating sunspots in TRACE white light images
and their apparent association with soft X-ray sigmoids have led to the
intriguing possibility that the sunspot rotation serves as the driver
for both sigmoid formation and their potential eruption. We discuss
the energization of the corona resulting from currents generated by
the vortex motions of the rotating sunspot. We will present data from
events for which we have good white light coverage of the sunspot, an
evolving sigmoid and an associated CME (in those cases where the sigmoid
erupts). We investigate the relationship between the sunspot rotation
and the evolution of the sigmoid structure and attempt to determine
the key physical conditions which result in a sigmoid destabilizing
and ultimately producing a CME.
---------------------------------------------------------
Title: Summer Fun in the Sun
Authors: Alexander, D.; Noldon, D.
2002AAS...200.4803A Altcode: 2002BAAS...34..720A
We report here on the development of a program to incorporate a
math/science component, emphasizing space science and solar physics,
into an existing set of summer activities sponsored by the National
Youth Sports Program (NYSP). NYSP provides summer sports and classroom
training components to youth whose families fall within federal
poverty guidelines. Recently, a partnership between Lockheed Martin
Solar and Astrophysics Lab. and Chabot Community College received NASA
IDEAS funding to develop a summer curriculum in math and science to
augment the already successful program. This provides an opportunity to
significantly enhance the experience of the participating students by
giving them access to the latest in space data and direct interaction
with space scientists. This paper discusses our goals, our approach
and the current status of our curricular materials. We would like to
acknowledge funding by the National Youth Sports Program and NASA IDEAS.
---------------------------------------------------------
Title: GeoSail: A novel solar sail mission concept for geospace
Authors: Alexander, David; McInnes, Colin R.; Angelopoulos, Vassilis;
Sandman, Anne W.; MacDonald, Malcolm
2002AIPC..608..305A Altcode: 2002stai.conf..305A
GeoSail is a mission designed to utilize the versatility of solar sail
propulsion in the exploration of magnetic reconnection and electron
dynamics in the Earth's magnetotail. The GeoSail mission uses a
low-performance solar sail to artificially precess the apse-line of a
long elliptical Earth-centered orbit to match the annual rotation of
the geomagnetic tail. This unique use of a solar sail will enable the
GeoSail science payload to maintain a nearly constant presence in the
geotail providing an exciting opportunity to probe the rapid dynamical
evolution of energetic particle distributions in this critical region
of geospace. The level of solar sail performance required for GeoSail
is typical of that currently being considered for near-term technology
demonstration missions, i.e., a 40m×40m sail with characteristic
acceleration of 0.1-0.15 mm/s<SUP>2</SUP>. GeoSail is therefore capable
of providing both technology validation and a unique science return
from a first solar sail mission. .
---------------------------------------------------------
Title: The Yohkoh Public Outreach Project: A Space Science Resource
for Formal and Informal Education
Authors: Lemen, J. R.; Alexander, D.; Metcalf, T. R.; Freeland, S. L.;
Acton, L. W.; Larson, M.; McKenzie, D.; Slater, T.
2001AGUFMED12A0160L Altcode:
The Yohkoh Public Outreach Project (YPOP) is a NASA-funded web site
maintained by scientists and educators at Lockheed Martin Solar and
Astrophysics Lab. and Montana State University. YPOP includes a range
of activities for youngsters, parents, teachers and anyone interested
in learning more about the Sun. YPOP utilizes a number of approaches
to the dissemination of solar data which incorporates elements of both
formaleducation, via a number of lesson plans and classroom activities,
and informal education, via access to the latest solar images, a
solar tour, and updated movies. This combination has proved extremely
effective in providing quality access to scientific data for a broad
audience with a wide range of interests. The Yohkoh Public Outreach
Project can be found at http://www.LMSAL.com/YPOP.
---------------------------------------------------------
Title: Energization of Rotating Sunspots, Twisted Coronal Fans,
Sigmoid Structures, and Coronal Mass Ejections
Authors: Nightingale, R. W.; Alexander, D.; Brown, D. S.; Metcalf,
T. R.
2001AGUFMSH11C0724N Altcode:
In an on-going study, several sunspots, rotating about their umbral
centers, have been identified in TRACE photospheric white light
(WL) images with accompanying twisting of coronal fans connected to
penumbral magnetic field lines in the corresponding EUV (171, 195
Å) images. These observations can also be temporally and spatially
associated with S or inverse-S shaped regions (sigmoid structures)
appearing in Yohkoh SXT images and with concurrent coronal mass
ejections (CMEs) and/or flares. We have determined the rotational
speed of the sunspot in AR9114 over 8-10 August 2000, established the
inverse-S shape observed in the SXT data, and viewed a rapid flash of
possible reconnection in a TRACE EUV movie. A CME was observed in AR8667
during the 15-18 August 1999 event, which also included an inverse-S
shaped region in the SXT data, and a rotating sunspot and twisting
coronal fans in the TRACE data. The large Bastille Day CME event of 14
July 2000 in AR9077 was accompanied by one or more rotating sunspots
as observed in TRACE WL and by an inverse-S shaped region as seen in
a difference SXT image. We will utilize these data to estimate the
energization of the twisted coronal fans resulting from the sunspot
rotation and compare this with the temporal evolution of the sigmoid
structures. We will investigate the possibility of a direct role of
the observed sunspot rotation and the potential for a sigmoid to erupt
as a CME. This work was supported by NASA under contract NAS5-38099.
---------------------------------------------------------
Title: Coupled modeling of photospheric and coronal dynamics
Authors: Alexander, D.; Hurlburt, N. E.; Rucklidge, A. M.; De Rosa, M.
2001AGUFMSH11C0718A Altcode:
The coupling of the motions within and below te photosphere to the
chromosphere and corona is one of the fundamental issues in solar
physics. We have developed a model coupling the simulated dynamics of
sunspots to the simulated heating of coronal loops. In this paper we
present an extension of our earlier work to the inclusion of (a) fully
three dimensional magnetoconvection, (b) new analytical representations
of hydrostatic loops with spatially-dependent heating rates and (c)
fully time-dependent hydrodynamic coronal modeling. The dynamic loop
model uses the same numerical scheme as the magnetoconvective model
used to simulate the photospheric behavior in this sunspot system,
making it possible to more fully integrate the two regimes. We present
the first results of a hybrid model utilizing a time-dependent coronal
model and a fully three-dimensional magnetoconvective model.
---------------------------------------------------------
Title: Flare Plasma Cooling from 30 MK down to 1 MK modeled from
Yohkoh, GOES, and TRACE Observations during the Bastille-Day Event
(14 July 2000)
Authors: Aschwanden, M. J.; Alexander, D.
2001AGUFMSH32B..05A Altcode:
We present an analysis of the evolution of the thermal flare
plasma during the 2000-Jul-14, 10 UT, Bastille-Day flare event,
using spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and
TRACE. The spatial structure of this double-ribbon flare consists of
a curved arcade with some 100 post-flare loops which brighten up in
a sequential manner from highly-sheared low-lying to less-sheared
higher-lying bipolar loops. We reconstruct an instrument-combined,
average differential emission measure distribution dEM(T)/dT that
ranges from T=1 MK to 40 MK and peaks at T<SUB>0=10.9</SUB> MK. We
find that the time profiles of the different instrument fluxes peak
sequentially over 7 minutes with decreasing temperatures from T≈ 30
MK to 1 MK, indicating the systematic cooling of the flare plasma. From
these temperature-dependent relative peak times t<SUB>peak</SUB>(T)
we reconstruct the average plasma cooling function T(t) for loops
observed near the flare peak time, and find that their temperature
decrease is initially controlled by conductive cooling during the
first 188 s, T(t) ~ [1+(t/τ<SUB>cond</SUB>)]<SUP>-2/7</SUP>,
and then by radiative cooling during the next 592 s, T(t) ~
[1-(t/τ<SUB>rad</SUB>)]<SUP>3/5</SUP>. From the radiative cooling
phase we infer an average electron density of n<SUB>e=4.2x</SUB>
10<SUP>11</SUP> cm<SUP>-3</SUP>, which implies a filling factor
near 100% for the brightest observed 23 loops with diameters
of ≈ 1.8 Mm that appear simultaneously over the flare peak
time and are fully resolved with TRACE. We reproduce the time
delays and fluxes of the observed time profiles near the flare
peak self-consistently with a forward-fitting method of a fully
analytical model. The total integrated thermal energy of this
flare amounts to E<SUB>thermal</SUB>=2.6 x 10<SUP>31</SUP>
erg. >http://www.lmsal.com/~aschwand/publications/publ.html</a>
---------------------------------------------------------
Title: Hard X-ray Observations of the August 25, 2001 X Flare
Authors: Metcalf, T. R.; Alexander, D.; Hudson, H. H.
2001AGUFMSH42A0776M Altcode:
The X5.3 flare which occurred about 16:31 UT on 2001 August 25 was
well observed by the Yohkoh spacecraft. The flare showed gamma-ray
emission, nuclear lines and was a dramatic white light flare. A
preliminary analysis of the hard X-ray images from the Yohkoh/HXT
instrument show two clear footpoints to this very energetic flare,
one nearly stationary and the other apparently moving as the flare
progresses. We will discuss the hard X-ray and soft X-ray structure
of this flare and compare the hard X-ray images to the TRACE and MDI
white light data both spatially and temporally.
---------------------------------------------------------
Title: Flare Plasma Cooling from 30 MK down to 1 MK modeled from
Yohkoh, GOES, and TRACE observations during the Bastille Day Event
(14 July 2000)
Authors: Aschwanden, Markus J.; Alexander, David
2001SoPh..204...91A Altcode:
We present an analysis of the evolution of the thermal flare plasma
during the 14 July 2000, 10 UT, Bastille Day flare event, using
spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and TRACE. The
spatial structure of this double-ribbon flare consists of a
curved arcade with some 100 post-flare loops which brighten up in
a sequential manner from highly-sheared low-lying to less-sheared
higher-lying bipolar loops. We reconstruct an instrument-combined,
average differential emission measure distribution dEM(T)/dT that
ranges from T=1 MK to 40 MK and peaks at T<SUB>0</SUB>=10.9 MK. We
find that the time profiles of the different instrument fluxes peak
sequentially over 7 minutes with decreasing temperatures from T≈30 MK
to 1 MK, indicating the systematic cooling of the flare plasma. From
these temperature-dependent relative peak times t<SUB>peak</SUB>(T)
we reconstruct the average plasma cooling function T(t) for loops
observed near the flare peak time, and find that their temperature
decrease is initially controlled by conductive cooling during the
first 188 s, T(t)∼[1+(t/τ<SUB>cond</SUB>)]<SUP>−2/7</SUP>,
and then by radiative cooling during the next 592 s,
T(t)∼[1−(t/τ<SUB>rad</SUB>)]<SUP>3/5</SUP>. From the
radiative cooling phase we infer an average electron density of
n<SUB>e</SUB>=4.2×10<SUP>11</SUP> cm<SUP>−3</SUP>, which implies
a filling factor near 100% for the brightest observed 23 loops with
diameters of ∼1.8 Mm that appear simultaneously over the flare
peak time and are fully resolved with TRACE. We reproduce the time
delays and fluxes of the observed time profiles near the flare peak
self-consistently with a forward-fitting method of a fully analytical
model. The total integrated thermal energy of this flare amounts to
E<SUB>thermal</SUB>=2.6×10<SUP>31</SUP> erg.
---------------------------------------------------------
Title: Evidence for the Flare Trigger Site and Three-Dimensional
Reconnection in Multiwavelength Observations of a Solar Flare
Authors: Fletcher, L.; Metcalf, T. R.; Alexander, D.; Brown, D. S.;
Ryder, L. A.
2001ApJ...554..451F Altcode:
Based on a multiwavelength data set and a topological model for
the magnetic field, we argue that a M1.9 flare which occurred on
1993 May shows evidence of three-dimensional coronal reconnection
in a spine-fan configuration. Images from the Transition Region and
Coronal Explorer allow the detailed examination of the structures
involved in the flare and preflare in the 171 Å (1 MK) EUV passband
and the Lyα (10,000-20,000 K) passband. Yohkoh Hard X-ray Telescope
maps the position of nonthermal electron precipitation and the
Soft X-ray Telescope reveals preflare and flare heating on large
and small scales. While the flare appears to be driven by changes in
small-scale field close to the photosphere, near the interface between
strong opposite magnetic polarities, the result is the disruption of
large-scale field. We demonstrate how this observed activity on large
and small scales, along with many other aspects of the flare, suggests
a qualitative explanation in the three-dimensional reconfiguration of
coronal magnetic field, following a small-scale flux cancellation at
the photosphere.
---------------------------------------------------------
Title: Chromospheric Heating in the Late Phase of Two-Ribbon Flares
Authors: Czaykowska, A.; Alexander, D.; De Pontieu, B.
2001ApJ...552..849C Altcode:
Fast upflows observed in the late gradual phase of an M6.8 two-ribbon
flare by the Solar and Heliospheric Observatory/Coronal Diagnostic
Spectrometer have provided evidence for the presence of chromospheric
evaporation more than an hour after the impulsive phase of the
flare. The chromospheric heating necessary to generate these upflows
requires the continued injection and deposition of energy, which
we presume to be provided by magnetic reconnection in the flaring
corona. We investigate the nature of the transport of this energy from
the reconnection site to the chromosphere by comparing the observed
upflow velocities with those expected from different chromospheric
heating models. A nonthermal beam of energetic electrons (>~15
keV) that is capable of generating the observed velocities would also
generate significant hard X-ray emission that is not observed at this
stage of the flare. We conclude, therefore, that the most likely energy
transport mechanism is thermal conduction.
---------------------------------------------------------
Title: Observational Diagnostics of Reconnection in Solar Flares
Authors: Alexander, D.
2001AGUSM..SM22A10A Altcode:
Magnetic reconnection is believed to be the prime means of releasing
magnetic energy in a solar flare. The process of reconnection occurs
on spatial scales significantly smaller than the best observationally
resolvable scales.Consequently, we must rely on observational signatures
which, indirectly, imply the presence of magnetic reconfiguration in the
form of reconnection. Recent, high-resolution, multi-wavelength flare
observations are allowing us to use the observed dynamical behavior
to probe the connection between the small-scale localized trigger and
the large-scale coronal field which ultimately participates in the
solar flare. We will discuss some new results which indicate strong
evidence for the presence of 3D coronal reconnection and the role it
plays in the dynamics of the flare plasma.
---------------------------------------------------------
Title: Coronal Heating and the dynamics of subphotospheric magnetic
fields
Authors: Hurlburt, N.; Alexander, D.
2001AGUSM..SH31D05H Altcode:
Simultaneous observations of sunspots in the photosphere and in
the coronal regions above them reveal a close coupling between the
dynamics of the photospheric motion and structure and the heating
of coronal loops. We investigate this relationship through detailed
three-dimensional simulations of dynamic, small-scale structures in
sunspot penumbra and umbra in conjunction with models of coronal
excitation and emission. The numerical models incorporate a fully
three-dimensional magnetoconvection calculation, potential field
extrapolations from the sunspot model boundary conditions, steady-state
and dynamic coronal loops powered by the convective motions at the
surface, EUV and X-ray instrument response functions, and a fieldline
rendering. The result is a simulated dynamical active region in
three dimensions which can be compared directly with observations
and enables us to explore coronal heating and its relationship to the
dynamics of the photosphere and convection zone. We present results
of recent calculations exploring the dynamics of penumbra grains and
their possible influence on the overlying corona.
---------------------------------------------------------
Title: Solar Week: An experiment of the Yohkoh Public Outreach Project
Authors: Alexander, D.
2001AGUSM..ED41A08A Altcode:
Solar Week is a week-long collection of content, games, and
activities centered on the latest data from and discoveries about
the Sun. Targetted at middle-school girls, the key feature of Solar
Week is the "Ask a Scientist" section enabling direct interaction
between participating students and volunteer scientists. All of the
contributing scientists are women, serving as experts in their field and
providing role models to whom the students can relate. Solar Week has
completed two sessions, one in October 2000 and one in April 2001, with
a total of some 80 edcuators and 8,000+ students in over 25 states. A
major success of the Solar Week program has been the ability of the
students to learn more about the scientists as people, through online
biographies, and to discuss a variety of topics ranging from science,
to careers and common hobbies.
---------------------------------------------------------
Title: Analysis of X-ray counterparts for Fast and Slow Halo CMEs
Authors: Alexander, D.; Metcalf, T. R.; Nitta, N.
2001AGUSM..SH22B05A Altcode:
In many cases, coronal mass ejections exhibit a strong counterpart in
the X-ray corona, particularly in flare-related events. Observations
by the Soft X-ray Telescope on Yohkoh have exhibited a number of
manifestations in association with CME eruptions. These include the
well-known dimming events, post-event arcades and expanding loop
systems. We examine the X-ray signatures of a number of fast and
slow halo CME events for evidence of two-classes of CME eruption in
accord with the observed velocity profiles determined from white-light
data. Flare-associated CMEs, which have a tendency to exhibit constant
velocity, necessarily undergo a rapid acceleration in the low corona
and should exhibit enhanced heating of the ambient corona to X-ray
emitting temperatures. Slow CMEs, on the other hand, are expected to
display a more subtle, if any, effect in the hot corona. We examine
the Yohkoh database for evidence of a dichotomy in the X-ray signatures
of halo CMEs.
---------------------------------------------------------
Title: Modeling of Coronal EUV Loops Observed with
TRACE. I. Hydrostatic Solutions with Nonuniform Heating
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Alexander, David
2001ApJ...550.1036A Altcode:
Recent observations of coronal loops in EUV wavelengths with
the Transition Region and Coronal Explorer (TRACE) and the
Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and
Heliospheric Observatory (SOHO) demonstrated three new results that
cannot be explained by most of the existing loop models: (1) EUV loops
are near-isothermal along their coronal segments, (2) they show an
overpressure or overdensity compared with the requirements of steady
state loops with uniform heating, and (3) the brightest EUV loops
exhibit extended scale heights up to 4 times the hydrostatic scale
height. These observations cannot be reconciled with the classical RTV
(Rosner, Tucker, & Vaiana) model, they do not support models with
uniform heating, and they even partially violate the requirements
of hydrostatic equilibrium. In this study we are fitting for the
first time steady state solutions of the hydrodynamic equations to
observed intensity profiles, permitting a detailed consistency test of
the observed temperature T(s) and density profiles n<SUB>e</SUB>(s)
with steady state models, which was not possible in previous studies
based on scaling laws. We calculate some 500 hydrostatic solutions,
which cover a large parameter space of loop lengths (L~4-300 Mm), of
nonuniform heating functions (with heating scale heights in the range
of λ<SUB>H</SUB>~1-300 Mm), approaching also the limit of uniform
heating (λ<SUB>H</SUB>>>L). The parameter space can be subdivided
into three regimes, which contain (1) solutions of stably stratified
loops, (2) solutions of unstably stratified loops (in the case of
short heating scale heights, λ<SUB>H,Mm</SUB>~L<SUB>Mm</SUB>),
and (3) a regime in which we find no numerical solutions (when
λ<SUB>H,Mm</SUB><~L<SUB>Mm</SUB>). Fitting the hydrostatic
solutions to 41 EUV loops observed with TRACE (selected by the
criterion of detectability over their entire length), we find that
only 30% of the loops are consistent with hydrostatic steady state
solutions. None of the observed EUV loops is consistent with a uniform
heating function while in quasi-steady state. Those loops compatible
with a steady state are found to be heated near the footpoints, with a
heating scale height of λ<SUB>H</SUB>=12+/-5 Mm, covering a fraction
λ<SUB>H</SUB>/L=0.2+/-0.1 of the loop length. These results support
coronal heating mechanisms operating in or near the chromosphere and
transition region.
---------------------------------------------------------
Title: Solar Week 2000: Using role models to encourage an interest
in science
Authors: Alexander, D.
2000AAS...19712009A Altcode: 2000BAAS...32.1607A
Solar Week 2000 is a week-long set of games and activities
allowing students to interact directly with solar science and solar
scientists. The main goal of Solar Week was to provide young women,
primarily in grades 6-8, with access to role models in the sciences. The
scientists participating in Solar Week are women from a variety of
backgrounds and with a variety of scientific expertise. An online
bulletin board was used to foster discussion between the students
and the scientists about both science and career issues. In this
presentation I will discuss the successes and failures of the first
run of Solar Week which occurred on 9-13 October 2000. Our aim is to
provide some insight into doing activity-based space science on the
web and to discuss the lessons-learned from tailoring to a specific
group of participants.
---------------------------------------------------------
Title: Evidence for Nonuniform Heating of Coronal Loops Inferred
from Multithread Modeling of TRACE Data
Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Alexander,
David
2000ApJ...541.1059A Altcode:
The temperature T<SUB>e</SUB>(s) and density structure n<SUB>e</SUB>(s)
of active region loops in EUV observed with TRACE is modeled with a
multithread model, synthesized from the summed emission of many loop
threads that have a distribution of maximum temperatures and that
satisfy the steady state Rosner-Tucker-Vaiana (RTV) scaling law,
modified by Serio et al. for gravitational stratification (called
RTVS<SUB>p</SUB> in the following). In a recent Letter, Reale &
Peres demonstrated that this method can explain the almost isothermal
appearance of TRACE loops (observed by Lenz et al.) as derived from the
filter-ratio method. From model-fitting of the 171 and 195 Å fluxes
of 41 loops, which have loop half-lengths in the range of L=4-320
Mm, we find that (1) the EUV loops consist of near-isothermal loop
threads with substantially smaller temperature gradients than are
predicted by the RTVS<SUB>p</SUB> model; (2) the loop base pressure,
p<SUB>0</SUB>~0.3+/-0.1 dynes cm<SUP>-2</SUP>, is independent of
the loop length L, and it agrees with the RTVS<SUB>p</SUB> model
for the shortest loops but exceeds the RTVS<SUB>p</SUB> model up
to a factor of 35 for the largest loops; and (3) the pressure scale
height is consistent with hydrostatic equilibrium for the shortest
loops but exceeds the temperature scale height up to a factor of ~3
for the largest loops. The data indicate that cool EUV loops in the
temperature range of T<SUB>e</SUB>~0.8-1.6 MK cannot be explained with
the static steady state RTVS<SUB>p</SUB> model in terms of uniform
heating but are fully consistent with Serio's model in the case of
nonuniform heating (RTVS<SUB>ph</SUB>), with heating scale heights in
the range of s<SUB>H</SUB>=17+/-6 Mm. This heating function provides
almost uniform heating for small loops (L<~20 Mm), but restricts
heating to the footpoints of large loops (L~50-300 Mm).
---------------------------------------------------------
Title: SOHO/UVCS Observations of a Coronal Jet During the Third
Whole Sun Month Campaign
Authors: Ko, Y. -K.; Raymond, J.; Gibson, S.; Strachan, L.; Alexander,
D.; Fletcher, L.; Holzer, T.; Gilbert, H.; Burkepile, J.; St. Cyr,
C.; Thompson, B.
2000SPD....31.0271K Altcode: 2000BAAS...32R.823K
On August 26 1999, a coronal jet occurred at the north west limb
near a sigmoid active region which has been the target for a joint
observation plan during the third Whole Sun Month Campaign. This jet
was observed by several instruments at the limb (SOHO/CDS, SOHO/EIT,
TRACE, MLSO/CHIP, MLSO/PICS), at 1.7 Ro (SOHO/UVCS), and at the outer
corona (SOHO/LASCO). At 1.7 Ro, the intensities of Lyman alpha, Lyman
beta in the jet increased by as large a factor of 100 compared with the
background corona, while those for O VI 1032 and O VI 1037 increased
by a factor of 2. C III 977 line also brightened significantly. The
line shift in the lines indicates that the line-of-sight velocity in
the jet started from 150 km/sec blue shift and ended at 120 km/sec
red shift. This line-of-sight motion seen at 1.7 Ro apparently was
opposite that observed when the jet emerged from the limb. In this
paper, we present the observation by SOHO/UVCS and discuss the dynamic
structure and physical properties of this jet as it passed through
1.7 Ro. Comparisons will be shown with the observations from other
instruments. This work is supported by NASA Grant number NAG5-7822.
---------------------------------------------------------
Title: Evidence for continued reconnection in solar flares
Authors: Alexander, D.; Czaykowska, A.
2000SPD....31.0270A Altcode: 2000BAAS...32..823A
Fast upflows observed in the late gradual phase of an M6.8 two-ribbon
flare by SOHO/CDS have provided evidence for the presence of
chromospheric evaporation more than an hour after the main impulsive
phase of the flare. The chromospheric heating necessary to generate
these upflows requires the continued injection and deposition of energy,
which we presume to be provided by magnetic reconnection in the flaring
corona. We investigate the nature of the transport of this energy from
the reconnection site to the chromosphere by comparing the observed
velocities with those expected from different chromospheric heating
models. A non-thermal electron-beam model capable of generating the
observed velocities would also generate significant hard X-ray emission
which is not observed at this stage of the flare. We, therefore,
conclude that the most likely energy transport mechanism is thermal
conduction. This work is supported by the Yohkoh/SXT project at LMSAL
(contract NAS8-40801).
---------------------------------------------------------
Title: Solar Sail Mission Concepts
Authors: Winter, H. D.; Alexander, D.; McInnes, C. R.
2000SPD....3102102W Altcode: 2000BAAS...32..829W
Solar sail technology is fast becoming a viable option for spacecraft
propulsion and appears as an enabling technology in many plans for
future space physics missions. We have developed a number of novel
mission concepts which utilize the full potential of solar sail
propulsion. These mission concepts include enhancements to existing
ideas, some of which appear in the most recent SEC Roadmap, in addition
to a number of new mission ideas. Each mission considered incorporates
a range of sail performance levels which allow an examination
of the potential, both near-term and far-term, of attaining the
specified mission goals. Two main issues arose from this work: 1)
High energy orbits can be readily attained from a relatively small
launch vehicle even for low performance sails, significantly reducing
costs; 2) Improving technology can enhance a specific mission by
either decreasing the travel times and increasing maneuverability,
or by increasing the payload mass fraction. This work was supported
by an internal research and development contract from Lockheed Martin
Advanced Technology Center.
---------------------------------------------------------
Title: Structure of a Large low-Latitude Coronal Hole
Authors: Bromage, B. J. J.; Alexander, D.; Breen, A.; Clegg, J. R.;
Del Zanna, G.; DeForest, C.; Dobrzycka, D.; Gopalswamy, N.; Thompson,
B.; Browning, P. K.
2000SoPh..193..181B Altcode:
Coronal holes on the Sun are the source of high-speed solar wind
streams that produce magnetic disturbances at the Earth. A series
of multi-wavelength, multi-instrument observations obtained during
the 1996 `Whole Sun Month' campaign examined a large coronal hole in
greater detail than ever before. It appeared on the Sun in August, and
extended from the north pole to a large active region in the southern
hemisphere. Its physical and magnetic structure and subsequent evolution
are described.
---------------------------------------------------------
Title: Three-dimensional Stereoscopic Analysis of Solar Active Region
Loops. II. SOHO/EIT Observations at Temperatures of 1.5-2.5 MK
Authors: Aschwanden, Markus J.; Alexander, David; Hurlburt, Neal;
Newmark, Jeffrey S.; Neupert, Werner M.; Klimchuk, J. A.; Gary,
G. Allen
2000ApJ...531.1129A Altcode:
In this paper we study the three-dimensional structure of hot
(T<SUB>e</SUB>~1.5-2.5 MK) loops in solar active region NOAA
7986, observed on 1996 August 30 with the Extreme-ultraviolet
Imaging Telescope (EIT) on board the Solar and Heliospheric
Observatory (SOHO). This complements a first study (Paper I) on
cooler (T<SUB>e</SUB>~1.0-1.5 MK) loops of the same active region,
using the same method of Dynamic Stereoscopy to reconstruct the
three-dimensional geometry. We reconstruct the three-dimensional
coordinates x(s), y(s), z(s), the density n<SUB>e</SUB>(s), and
temperature profile T<SUB>e</SUB>(s) of 35 individual loop segments
(as a function of the loop coordinate s) using EIT 195 and 284 Å
images. The major findings are as follows. (1) All loops are found
to be in hydrostatic equilibrium, in the entire temperature regime
of T<SUB>e</SUB>=1.0-2.5 MK. (2) The analyzed loops have a height of
2-3 scale heights, and thus only segments extending over about one
vertical scale height have sufficient emission measure contrast for
detection. (3) The temperature gradient over the lowest scale height
is of order dT/ds~1-10 K km<SUP>-1</SUP>. (4) The radiative loss
rate is found to exceed the conductive loss rate by about two orders
or magnitude in the coronal loop segments, implying that the loops
cannot be in quasi-static equilibrium, since standard steady-state loop
models show that radiative and conductive losses are comparable. (5) A
steady state could only be maintained if the heating rate E<SUB>H</SUB>
matches exactly the radiative loss rate in hydrostatic equilibrium,
requiring a heat deposition length λ<SUB>H</SUB> of the half density
scale height λ. (6) We find a correlation of p~L<SUP>-1</SUP> between
loop base pressure and loop length, which is not consistent with the
scaling law predicted from steady-state models of large-scale loops. All
observational findings indicate consistently that the energy balance
of the observed EUV loops cannot be described by steady-state models.
---------------------------------------------------------
Title: Coronal Trapping of Energetic Flare Particles: Yohkoh/HXT
Observations
Authors: Metcalf, Thomas R.; Alexander, David
2000ASPC..206..233M Altcode: 2000hesp.conf..233M
No abstract at ADS
---------------------------------------------------------
Title: High-resolution Observations of Plasma Jets in the Solar Corona
Authors: Alexander, David; Fletcher, Lyndsay
1999SoPh..190..167A Altcode:
We present recent observations of coronal jets, made by TRACE and
Yohkoh/SXT on 28 May and 19 August 1998. The high spatial resolution
of TRACE enables us to see in detail the process of material ejection;
in the line of Fe ix (one million degrees) we see both bright emitting
material and dark absorbing/scattering material being ejected, i.e.,
both hot and cold material, highly collimated and apparently ejected
along the direction of the overlying field lines. Bright ejecta are
seen simultaneously in Lyman α for one event and Yohkoh/SXT in the
other. The jets on the two days are different in that the 19 August
jet displays the morphology typical of a one-sided anemone jet while
the 28 May jet exhibits a two-sided jet morphology. The 19 August jet
shows evidence for rotation and an interesting bifurcation at large
distances from the energy release site. We study the physical properties
and energetics of these jetting events, and conclude that existing
theoretical models capture the essential physics of the jet phenomena.
---------------------------------------------------------
Title: A Method for Characterizing Rotation Rates in the Soft
X-Ray Corona
Authors: Weber, M. A.; Acton, L. W.; Alexander, D.; Kubo, S.; Hara, H.
1999SoPh..189..271W Altcode:
Differential rotation rates of soft X-ray features in the solar
corona are quantified by a method of harmonic filtering using
the Lomb-Scargle periodogram. This approach leads reasonably to a
quantitative discrimination between uncertainty estimates and spectral
leakage of the fundamental rotation frequency due to the presence
of multiple rotating tracers. Mean rotation rates as a function of
latitude and year are calculated for the years 1992-1997 (roughly
the declining phase of the last solar activity cycle). The corona is
found to have a small but measurable latitudinal gradient in rotation
rate. The presence of multiple features places a lower bound of 1-2%
on the relative uncertainties with which a `mean' rotation rate can
be measured. The results are compared with autocorrelation estimates
and found to agree within 1σ.
---------------------------------------------------------
Title: Heating The Atmosphere Above Sunspots
Authors: Alexander, David; Hurlburt, Neal E.; Rucklidge, Alastair
1999ESASP.446..117A Altcode: 1999soho....8..117A
We present our results of a hybrid model of sunspots and their
overlying corona. The two-layer model considers both the nonlinear,
compressible magnetoconvection beneath the photosphere and potential,
or linear force-free, models of the coronal fields. Heating of the
plasma along the field lines is then consider using quasi-static and
steady-state model with the heating rate being specified by the dynamics
of the magnetoconvection. Two distinct magnetoconvection scenarios are
considered. The first describes magnetoconvection in a 2D axisymmetric
geometry and considers the time development of the overlying coronal
field. The second describes a 3D cylindrical geometry with a static
coronal field configuration. Both scenarios diverge from the standard
practice of assuming constant temperature and vertical magnetic field
conditions at the top surface. Instead a radiative linear force-free
field condition is adopted. Extrapolation of the top surface boundary
conditions results in a coronal field configuration which is assumed to
be filled with plasma heated to coronal temperatures. The heating rate
and thermodynamic behavior of the plasma is related to the sub-surface
model by assuming that individual fluxtubes are heated uniformly
with the necessary energy being generated from the dissipation of the
Poynting flux entering the coronal volume. Radiation and conductive
losses are included. The combination of a sunspot model, whereby the
surface field is completely specified, with a coronal heating model,
in which the plasma parameters are specified for a given energy input
allows us to explore a broad class of heating paradigms.
---------------------------------------------------------
Title: Chromospheric Evaporation In The Gradual Flare Phase
Authors: Czaykowska, A.; de Pontieu, B.; Alexander, D.; Rank, G.
1999ESASP.446..257C Altcode: 1999soho....8..257C
SOHO/CDS observations are used to determine relative line-of-sight
velocities and their temporal evolution in the late gradual flare
phase. The observations started about 1 hour after the M6.8 two-ribbon
flare in NOAA 8210 on April 29, 1998. Velocity maps in O V 629 A (Tmax
= 0.25 MK), Fe XVI 360 A (Tmax = 2 MK), and Fe XIX 592 A (Tmax = 6.3
MK), covering temperatures from the transition region to the corona
show strong gradients at the position of the Hα ribbons. Downflows
are observed in the footpoint regions of the post-flare loops whereas
the velocities observed further away from the magnetic neutral line
are interpreted as upflows due to chromospheric evaporation. Loops are
filled with hot plasma and their footpoints become visible later on at
the former evaporation site. At the same time the Hα ribbon is slowly
moving outward together with the location of the velocity gradient. Our
observations strongly support models in which chromospheric evaporation
driven by magnetic reconnection is responsible for the continuous
formation of loops, which are visible for several hours after the
flare's maximum in EUV and soft X-ray radiation.
---------------------------------------------------------
Title: Coronal Trapping of Energetic Flare Particles:Yohkoh/HXT
Observations
Authors: Metcalf, Thomas R.; Alexander, David
1999ApJ...522.1108M Altcode:
We examine spectroscopic data from the Yohkoh Hard X-Ray Telescope in
a search for spectral evidence of the coronal trapping of energetic
particles during solar flares. Two distinct particle populations with
significantly different spectral properties are found to be present in
three of the six flares studied; the first population is trapped in the
corona, where it encounters a “thick-thin” target, while the second
population precipitates directly to the footpoints. In the remaining
three events, a single population of energetic particles appear to be
responsible for the observed hard X-ray emission, either via thermal
bremsstrahlung (one case) or nonthermal thin-target emission (two
cases). For the three events in which a trapped population is observed,
the spectroscopic observations imply first that there is likely
to be a single acceleration mechanism for both the trapped and the
precipitating populations and second that the magnetic field geometry
in these flares is conducive to trapping in a confined region high in
the corona, above the soft X-ray loops. Both conditions are consistent
with magnetic reconnection models of flares in which energetic particles
are trapped between MHD slow-mode shocks attached to the reconnection
region and a fast-mode shock formed by the reconnection outflow jet.
---------------------------------------------------------
Title: Evidence for Chromospheric Evaporation in the Late Gradual
Flare Phase from SOHO/CDS Observations
Authors: Czaykowska, A.; De Pontieu, B.; Alexander, D.; Rank, G.
1999ApJ...521L..75C Altcode:
Using extreme-ultraviolet (EUV) spectroheliograms from the first
intentional postflare observations with the Coronal Diagnostic
Spectrometer (CDS) on board SOHO, we determine relative line-of-sight
velocities and their temporal evolution during the gradual flare phase
of an M6.8 two-ribbon flare that occurred on 1998 April 29. Dopplergrams
in lines of O V, Fe XVI, and Fe XIX, with formation temperatures
T<SUB>max</SUB> of, respectively, 0.25, 2.0, and 8.0 MK show strong
velocity gradients coincident with the Hα ribbons, visible in Big Bear
Solar Observatory (BBSO) images. These gradients are perpendicular to
and moving with the Hα ribbons. Bright downflowing plasma seems to be
prevalent in the regions, between the ribbons and the magnetic neutral
line, that coincide with the ends of postflare loops seen with the
Extreme-Ultraviolet Imaging Telescope (EIT) on board SOHO. The plasma
on the outer side of the ribbons is less bright in the EUV but shows
strong relative blueshifts. This pattern of upflows and downflows
demonstrates, for the first time in transition region and coronal
lines, the existence of chromospheric evaporation during the late
gradual phase of a flare and provides evidence for ongoing reconnection.
---------------------------------------------------------
Title: Coronal Loop Oscillations Observed with the Transition Region
and Coronal Explorer
Authors: Aschwanden, Markus J.; Fletcher, Lyndsay; Schrijver, Carolus
J.; Alexander, David
1999ApJ...520..880A Altcode:
We report here, for the first time, on spatial oscillations of coronal
loops, which were detected in extreme-ultraviolet wavelengths (171 Å)
with the Transition Region and Coronal Explorer, in the temperature
range of T<SUB>e</SUB>~1.0-1.5 MK. The observed loop oscillations
occurred during a flare that began at 1998 July 14, 12:55 UT and are
most prominent during the first 20 minutes. The oscillating loops
connect the penumbra of the leading sunspot to the flare site in
the trailing portion. We identified five oscillating loops with an
average length of L=130,000+/-30,000 km. The transverse amplitude
of the oscillations is A=4100+/-1300 km, and the mean period
is T=280+/-30 s. The oscillation mode appears to be a standing
wave mode (with fixed nodes at the footpoints). We investigate
different MHD wave modes and find that the fast kink mode with a
period τ=205(L/10<SUP>10</SUP> cm)(n<SUB>e</SUB>/10<SUP>9</SUP>
cm<SUP>-3</SUP>)<SUP>1/2</SUP>(B/10 G)<SUP>-1</SUP> s provides the
best agreement with the observed period. We propose that the onset
of loop oscillations in distant locations is triggered by a signal or
disturbance that propagates from the central flare site with a radial
speed of ~700 km s<SUP>-1</SUP>. Because the observed loop oscillation
periods are comparable to photospheric 5 minute oscillations, a resonant
coupling between the two systems is possible. We further find evidence
for global extreme-UV dimming in the entire active region possibly
associated with a coronal mass ejection.
---------------------------------------------------------
Title: Modeling CMEs in three dimensions using an analytic MHD model
Authors: Gibson, Sarah E.; Alexander, David; Biesecker, Doug; Fisher,
Richard; Guhathakurta, Madhulika; Hudson, Hugh; Thompson, B. J.
1999AIPC..471..645G Altcode: 1999sowi.conf..645G
Because coronal mass ejections (CMEs) are viewed in projection,
it is difficult to determine their three-dimensional nature. We use
an analytic model of CMEs as an example of a fully three-dimensional
magnetic field structure in MHD force balance with an emerging CME. We
present the CME magnetic field and its associated density structure,
seen projected at the limb from two viewing angles perpendicular
to the plane of the sky, and emerging from disk center representing
“earth-directed” CME events. The range of CME structures thus produced
compares well to existing CME white-light coronagraph and full disk
EUV and X-ray observations. In particular, we find that both 3-part
“front-cavity-core” and “U-shaped” white light CMEs, as well as the
twin dimmings (also referred to as transient coronal holes) observed in
X-ray and EUV, can successfully be reproduced by the CME model. All of
these structures are a direct consequence of a single three-dimensional
magnetic field topology, viewed from different directions.
---------------------------------------------------------
Title: Cylindrical Compressible Magnetoconvection and Model Sunspots
Authors: Hurlburt, N.; Alexander, D.; Rucklidge, A.
1999AAS...194.5502H Altcode: 1999BAAS...31..910H
We present results of hybrid models of sunspots and pores which
encompasses both the nonlinear, compressible magnetoconvection beneath
the photosphere, potential models of the coronal fields and includes
quasistatic coronal heating models. We solve the equations that describe
compressible magnetoconvection in 2D axisymmetric and 3D cylindrical
geometries using compact finite difference scheme. The convecting layer
consists of electrically conducting gas which experiences a uniform
gravitational acceleration directed downwards. The gas possesses a
shear viscosity, a thermal conductivity, a magnetic diffusivity, and a
magnetic permeability which are all assumed to be constant. We assume
that the fluid satisfies the equation of state for a perfect monatomic
gas with constant heat capacities. At the bottom of the cylinder,
we impose a constant temperature and vertical magnetic field. On
the top surface apply instead a radiative, and linear force-free
field condition. The outer boundary is insolating and perfectly
conducting. The magnetic fields above the computational domain are
then extrapolated and heated using a quasistatic model. The heating
problem is solved in an empirical way by assuming that individual
fluxtubes are heated in a manner that is proportional to one or more
of the parameters defining the fluxtube, e.g. pressure, length, field
strength, current density etc. The combination of a sunspot model,
whereby the surface field is completely specified, with a coronal
heating model, in which the plasma parameters are specified for a
given energy input allows us to explore a broad class of heating
paradigms. We present result of 2D simulations with no net magnetic
flux which display phenomena similar to that observed in sunspot moats,
and 3D simulations which develop penumbral-like structure. This work
was supported by NASA contract NAG5-7376.
---------------------------------------------------------
Title: The role of the large-scale corona in the production of
solar flares
Authors: Alexander, D.; Simnett, G. M.
1999AAS...194.5503A Altcode: 1999BAAS...31..910A
We investigate the spatial location of large solar flares (>M5)
occurring in the GOES record from 1-JAN-1986 to 1-JAN-1999. It is found,
with very few exceptions, that a single active region dominates the
flare production over any given time. In other words, before a flare
can occur in an active region, flare production must have completely
ceased in any other active regions present. This suggests some sort of
communication between the active regions on the solar disk and that the
large-scale corona plays an important role in the production of these
large solar flares. One intriguing possibility is the treatment of the
global solar corona as a multi-flux system with each part physically
related to all others. In this scenario, the transfer of flux between
active regions, caused by a solar flare in one of them, suppresses
the ability of the other to erupt. This has important implications
for flare studies and flare prediction in general.
---------------------------------------------------------
Title: Making YOHKOH SXT Images Available to the Public: The YOHKOH
Public Outreach Project
Authors: Larson, M. B.; McKenzie, D.; Slater, T.; Acton, L.; Alexander,
D.; Freeland, S.; Lemen, J.; Metcalf, T.
1999AAS...194.7024L Altcode: 1999BAAS...31..941L
The NASA funded Yohkoh Public Outreach Project (YPOP) provides public
access to high quality Yohkoh SXT data via the World Wide Web. The
products of this effort are available to the scientific research
community, K-12 schools, and informal education centers including
planetaria, museums, and libraries. The project utilizes the intrinsic
excitement of the SXT data, and in particular the SXT movies, to develop
science learning tools and classroom activities. The WWW site at URL:
http://solar.physics.montana.edu/YPOP/ uses a movie theater theme to
highlight available Yohkoh movies in a format that is entertaining and
inviting to non-scientists. The site features informational tours of
the Sun as a star, the solar magnetic field, the internal structure
and the Sun's general features. The on-line Solar Classroom has proven
very popular, showcasing hand-on activities about image filtering,
the solar cycle, satellite orbits, image processing, construction of a
model Yohkoh satellite, solar rotation, measuring sunspots and building
a portable sundial. The YPOP Guestbook has been helpful in evaluating
the usefulness of the site with over 300 detailed comments to date.
---------------------------------------------------------
Title: Constructing the Coronal Magnetic Field By Correlating
Parameterized Magnetic Field Lines With Observed Coronal Plasma
Structures
Authors: Gary, G. Allen; Alexander, David
1999SoPh..186..123G Altcode:
A method is presented for constructing the coronal magnetic field
from photospheric magnetograms and observed coronal loops. A set of
magnetic field lines generated from magnetogram data is parameterized
and then deformed by varying the parameterized values. The coronal flux
tubes associated with this field are adjusted until the correlation
between the field lines and the observed coronal loops is maximized. A
mathematical formulation is described which ensures that (i) the
normal component of the photospheric field remains unchanged, (ii)
the field is given in the entire corona over an active region, (iii)
the field remains divergence-free, and (iv) electric currents are
introduced into the field. It is demonstrated that a parameterization
of a potential field, comprising a radial stretching of the field,
can provide a match for a simple bipolar active region, AR 7999,
which crossed the central meridian on 1996 November 26. The result is a
non-force-free magnetic field with the Lorentz force being of the order
of 10<SUP>−5.5</SUP> g cm s<SUP>−2</SUP> resulting from an electric
current density of 0.079 μA m<SUP>−2</SUP>. Calculations show that
the plasma beta becomes larger than unity at a relatively low height of
∼0.25 r<SUB>⊙</SUB> supporting the non-force-free conclusion. The
presence of such strong non-radial currents requires large transverse
pressure gradients to maintain a magnetostatic atmosphere, required
by the relatively persistent nature of the coronal structures observed
in AR 7999. This scheme is an important tool in generating a magnetic
field solution consistent with the coronal flux tube observations and
the observed photospheric magnetic field.
---------------------------------------------------------
Title: Utilizing solar sails for solar physics
Authors: Strong, K. T.; Alexander, D.; McInnes, C. R.; Lemen, J. R.
1999AAS...194.6508S Altcode: 1999BAAS...31..928S
Recently, there has been much interest in the use of solar sail
technology for advanced space mission concepts. We present here some
trajectories and orbits for a number of potential solar missions. These
missions utilize the power of solar sails in a number of different ways,
providing unique capabilities in the study of the Sun. The first mission
concept is a solar polar mission using a "parking orbit" above one of
the solar poles, the second is a multi-spacecraft mission designed
to map out the three-dimensional solar atmosphere using identical
spacecraft at different heliographic latitudes, and the third is a
STEREO slowdown mission designed to extend the lifetime of the STEREO
mission using small sails attached to the STEREO payloads to slow
down the rate of drift ahead and behind the Earth. Here, the first two
payloads are assumed to be inert masses with possible instrument and
spacecraft packages to be defined by the specific goals of any mission.
---------------------------------------------------------
Title: Relative Timing of Soft X-Ray Nonthermal Line Broadening and
Hard X-Ray Emission in Solar Flares
Authors: Alexander, David; Harra-Murnion, Louise K.; Khan, Josef I.;
Matthews, Sarah A.
1998ApJ...494L.235A Altcode:
The time development of both hard X-ray emission and soft X-ray
nonthermal line widths is important for an understanding of
energy transport in the flaring solar corona. In this Letter, we
investigate the relationship between the temporal behavior of these
two phenomena for a number of flares detected by instruments on the
Yohkoh spacecraft. We examine 10 flares, all occurring within 30° of
the limb, using data from the Bragg Crystal Spectrometer (BCS) and the
hard X-ray telescope (HXT). We find that the nonthermal velocity either
(1) exhibits a maximum prior to the first significant burst of hard
X-rays or (2) is already decaying from an earlier unobserved maximum
at the time of the first significant burst of hard X-rays. The decay of
the nonthermal velocity as it proceeds from its observed maximum shows
little evidence for a direct association with individual hard X-ray
bursts. These observations suggest that the nonthermal broadening
may be a direct consequence of the flare energy release process
rather than a by-product of the energy deposition. In addition, the
attainment of a maximum in the nonthermal line width very early in
the flare is more indicative that plasma turbulence is the source
of the observed broadening rather than hydrodynamic flows, such as
chromospheric evaporation.
---------------------------------------------------------
Title: A Spectral Analysis of the Masuda Flare Using Yohkoh Hard
X-Ray Telescope Pixon Reconstruction
Authors: Alexander, David; Metcalf, Thomas R.
1997ApJ...489..442A Altcode:
Masuda's discovery of a compact hard X-ray impulsive source at the apex
of a flaring coronal loop has received a great deal of recent attention
in the solar physics community. The Masuda flare, which occurred on 1992
January 13, exhibited evidence of energy deposition in a compact region
some distance above the soft X-ray loop, suggesting, to some authors,
a flare process similar to the classical model for two-ribbon flares
proposed by Shibata et al. These conclusions were made on the basis
of a maximum entropy method (MEM) reconstruction of the Yohkoh Hard
X-Ray Telescope (HXT) observations. Recently, a new approach has been
developed for reconstructing the spatial information from the HXT: that
of pixon reconstruction, proposed by Metcalf et al. <P />In this paper,
we apply the pixon reconstruction technique to the event of 1992 January
13 and determine the temporal and spectral characteristics of the
loop-top source. While our emphasis here is on the spectral properties
of the Masuda flare, we also provide a brief comparison between the
pixon reconstruction and that of MEM for the hard X-ray loop top. In
carrying out the comparison between the methods, we have applied recent
improvements to the instrument response functions and reconstruction
algorithms. We have also identified a previously unknown effect of weak
source suppression that was inherent in previous analyses and that
significantly compromised the ability to study weak sources of hard
X-ray emission in the presence of strong sources. The improved response
functions and the better flux estimation used in this paper reduce (but
do not eliminate) the effects of this suppression, and consequently,
it should be noted that the MEM analysis presented in this paper is
quite distinct from any that have been carried out previously. <P />Our
conclusions are that (a) a compact loop-top hard X-ray source exists
with an impulsive temporal profile spanning the peak of the flare; (b)
the loop-top source is nonthermal in nature at the peak of the flare;
(c) there is a distinct dearth of HXT LO channel emission, relative
to the higher energy channels, from the loop-top region, indicating
either a very hard spectrum or the presence of a low-energy cutoff
in the energetic electron spectrum; (d) the footpoint and loop-top
emission during the impulsive phase of the flare are produced by
two distinct particle populations; (e) following the main phase of
this flare, the loop top is clearly thermal in nature with a peak
temperature of ~40 MK that decreases with time as the event proceeds;
and (f) the disparity between the present pixon results and previous
MEM results is primarily due to the intrinsically better photometry
achieved by the pixon method and the avoidance of suppression effects
in the present analysis. These conclusions therefore support, in part,
those made in previous works, confirming the existence of an impulsive
source of hard X-rays in the corona above a flaring loop. Our analysis
does, however, allow for a more comprehensive understanding of the
temporal and spectral development of this event in the context of an
alternative reconstruction technique.
---------------------------------------------------------
Title: A Comparison of the MEM and Pixon Algorithms for HXT Image
Reconstruction
Authors: Metcalf, Thomas R.; Alexander, David; Nitta, Nariaki;
Kosugi, Takeo
1997SPD....28.0217M Altcode: 1997BAAS...29Q.896M
Recently a workshop was held in Palo Alto, CA to discuss image
reconstruction for the Hard X-ray Telescope (HXT) on the Yohkoh
satellite. At the workshop, the participants concluded that a
detailed comparison of the primary reconstruction algorithms should
be undertaken. We will report on the results of a comparison of the
Maximum Entropy and Pixon algorithms using pseudo data. The comparison
will check photometric accuracy, speed, and image quality using a number
of test images. The test images utilized in the comparison will examine
a broad range of reconstruction problems, including the ability of the
algorithms to accurately reconstruct single sources, multiple sources
and loop-like features, as well as the ability to reconstruct weak
sources in the presence of spatially distinct bright sources.
---------------------------------------------------------
Title: Using the WWW to Make YOHKOH SXT Images Available to the
Public: The YOHKOH Public Outreach Project
Authors: Larson, M.; McKenzie, D.; Slater, T.; Acton, L.; Alexander,
D.; Freeland, S.; Lemen, J.; Metcalf, T.
1997SPD....28.0231L Altcode: 1997BAAS...29..898L
The Yohkoh Public Outreach Project (YPOP) is funded by NASA as one of
the Information Infrastructure Technology and Applications Cooperative
Agreement Teams to create public access to high quality Yohkoh SXT data
via the World Wide Web. These products are being made available to the
scientific research community, K-12 schools, and informal education
centers including planetaria, museums, and libraries. The project aims
to utilize the intrinsic excitement of the SXT data, and in particular
the SXT movies, to develop science learning tools and classroom
activities. The WWW site at URL: http://www.space.lockheed.com/YPOP/
uses a movie theater theme to highlight available Yohkoh movies in a
non-intimidating and entertaining format for non-scientists. The site
features lesson plans, 'solar' activities, slide shows and, of course,
a variety of movies about the Sun. Classroom activities are currently
undergoing development with a team of scientists and K-12 teachers
for distribution in late 1997. We will display the products currently
online, which include a solar classroom with activities for teachers,
background resources, and a virtual tour of our Sun.
---------------------------------------------------------
Title: Automated Identification of Soft X-ray Coronal Loops
Authors: Alexander, David; Metcalf, Thomas R.
1997SPD....28.0138A Altcode: 1997BAAS...29..886A
The presence of loop-like structures in the solar corona is clearly
evident in soft X-ray images such as those from the Soft X-ray Telescope
on board the Yohkoh satellite. These structures were first discovered
in Skylab data and are thought to represent the enhanced heating
of the coronal plasma confined in magnetic fluxtubes. In many cases
the heating is not confined to a single well defined fluxtube nor is
it always strong enough to dilineate the structure sharply against
the diffuse coronal background, which is itself presumably composed
of loop structures. In these cases it is often extremely difficult
to identify the structures involved in coronal activity. We have
developed a technique which uses a Pixon reconstruction of the soft
X-ray images to search specifically for loop-like structures. This
allows us to pick out faint loops against a strong background and to
identify multiple loop structures in bright regions. We are, therefore,
better able to address the heating of the solar corona both in the
diffuse and active regions.
---------------------------------------------------------
Title: Multi-Spectral Imaging of Coronal Activity.
Authors: Bagenal, Fran; Darnell, Tony; Burkepile, Joan; Hundhausen,
Art; Alexander, David
1997SPD....28.0146B Altcode: 1997BAAS...29Q.887B
By combining white light coronameter, Yohkoh soft x-ray and H-alpha
images from time intervals that encompass coronal mass ejections
(CMEs), we are able to examine the related evolution of structures
in the solar atmosphere. For example, what is the role of prominences
in pre-CME evolution of the corona; how well does the Pneumann &
Kopp model describe post-CME re-formation of coronal loops. On the
west limb, Yohkoh data show the structure of the corona before the CME
while prominences/filaments are revealed by H-alpha images. On the east
limb,the same data show x-ray-emitting loops that brighten and expand
after the CME. We have chosen examples of events from 1994 and 1995
that illustrate the use of such composite images. Composite white light,
Yohkoh and H-alpha images are also shown for the Whole Sun Month and we
discuss the possibilities of adding SOHO data as they become available.
---------------------------------------------------------
Title: Solar identification of solar-wind disturbances observed
at Ulysses
Authors: Lemen, J. R.; Acton, L. W.; Alexander, D.; Galvin, A. B.;
Harvey, K. L.; Hoeksema, J. T.; Zhao, X.; Hudson, H. S.
1996AIPC..382...92L Altcode:
The Ulysses polar passages are producing a unique set of observations of
solar-wind disturbances at high heliographic latitudes. In this paper
we use the Yohkoh soft X-ray telescope (SXT) to locate some of these
events, as defined by the Ulysses/SWICS data, in the solar corona. Of 8
events, we identify two with flares, three with front-side large arcade
events, two with far-side events, and one was not seen in the Ulysses
data. The arcade events generally resemble long-duration flares seen
in active regions, but are larger, slower, and cooler. We present
Yohkoh images of each of these events. In the large arcade events
(see Alexander et al., 1996, for a detailed look at one of them) the
magnetic morphology at the location of the Yohkoh arcade is generally
consistent with the development of a large system of loops. Some of
the identifications are ambiguous, and we summarize the reasons for
this. From the SWICS data we have obtained ionization temperatures for
several events, and find that they have no obvious pattern in relation
to the X-ray temperatures; this may be expected on the basis that the
interplanetary plasma cloud is physically distinct from the plasma
trapped in the corona. Soft X-ray observations of the solar corona
show occasional occurrences of large-scale brightenings in the form
of arcades of loops. Such structures have been known since Skylab
(e.g., Sturrock, 1980), and have a clear relationship with coronal
mass ejections (e.g., Kahler, 1977). We now may study this phenomenon
statistically with the much more comprehensive Yohkoh observations;
with Yohkoh movies we can also begin to extend our knowledge to the
three-dimensional development of the structures. At the same time
Ulysses has sampled the latitude dependence of the interplanetary
effects. With this paper we introduce this subject and provide a
preliminary listing of events from the passage of Ulysses through
high heliographic latitudes. The starting point of the present
survey is a list of interplanetary plasma clouds (IPC's) derived
from Ulysses/SWICS data. These are essentially the same as the events
termed CMEs by Gosling et al. (1994a, 1994b). For this identification
the presence of bidirectional streaming in the suprathermal electron
distribution is one of the main criteria. We note that there are no
direct coronagraph observations, however. The Yohkoh observations
were examined at the apparent time of origin of each Ulysses event,
resulting in some clear and some less-certain identifications. We
also studied the ionization temperatures of the IPC material as a
beginning step to give the identifications a physical basis. There
has been little study thus far of the Yohkoh soft X-ray observations
in relationship to CMEs, which we believe to be closely related to
the interplanetary disturbances. Hiei et al. (1993) reported the only
Yohkoh event yet studied in conjunction with white-light coronagraph
observations. However Klimchuk et al. (1994) showed that X-ray eruptive
phenomena with parameters similar to those of CMEs occur frequently at
the limb, and there have been several studies of individual eruptive
events (e.g., Watanabe et al., 1992). Presently there is no systematic
knowledge of the X-ray coronal counterparts of CMEs, and the survey
represented here is part of the effort to rectify this situation.
---------------------------------------------------------
Title: Yohkoh/SXT soft x-ray observations of sudden mass loss from
the solar corona
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Freeland, S. L.;
Lemen, J. R.; Harvey, K. L.
1996AIPC..382...88H Altcode:
With soft X-ray imaging we can study the entire coronal volume, except
for cold inclusions such as prominences, as a function of time. This
should allow us to observe the origins of coronal mass ejections. We
report here an initial survey of the Yohkoh/SXT observations at the
times of reported or apparent mass ejections: three LDE flare events
and two large-scale arcade formations. For each of the events we
can easily detect sudden coronal dimming, which we interpret as the
launch interval of a CME. In one of the flare events we have found
a well-defined plasma cloud, apparently formed from a set of loop
structures, which rises and disappears during the growth phase of the
flare emission. Its mass amounted to some 4×10<SUP>14</SUP> g with
a density of 3×10<SUP>8</SUP> cm<SUP>-3</SUP> and a temperature of
2.8 MK before its disappearance.
---------------------------------------------------------
Title: The solar origins of two high-latitude interplanetary
disturbances
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Harvey, K. L.;
Kahler, S. W.; Kurokawa, H.; Lemen, J. R.
1996AIPC..382...84H Altcode:
Two extremely similar interplanetary forward/reverse shock events,
with bidirectional electron streaming, were detected by Ulysses in
1994 [Gosling et al., 1994]. Both events resulted in geomagnetic
storms and presumably were associated with coronal mass ejections. In
this paper we use the Yohkoh soft X-ray observations to characterize
the conditions in the lower corona at the times appropriate for the
launching of these two events. We find two strikingly different solar
events to be the likeliest candidates: an LDE flare on 20 Feb. 1994,
and a extremely large-scale arcade event on 14 April 1994.
---------------------------------------------------------
Title: Comparison of YOHKOH x-ray coronal events with ULYSSES
interplanetary events
Authors: Lemen, J. R.; Acton, L. W.; Alexander, D.; Galvin, A. B.;
Harvey, K. L.; Hoecksema, J. T.; Zhao, X.; Hudson, H.
1995sowi.conf...58L Altcode:
The Yohkoh soft X-ray telescope (SXT) has observed several
largescale eruptive events per year for the first three years
of observations (Aug. 1991 - Nov. 1994) Such events are most
prominent at high latitudes, but resemble long-duration flare
events seen in active regions. Some of the high-latitude events
have now been identified in the Ulysses/SWICS data base during the
Ulysses south polar passage. There are puzzling examples of solar
events with no interplanetary counterparts. A comparison of coronal
and interplanetary events can lead to better models for mapping
interplanetary disturbances back to their source location, especially
by combining Yohkoh morphology with three-dimensional representations
of the coronal magnetic field. In this paper we describe the parameters
of the hot plasma seen by SXT. There is clear evidence for non radial
motion in specific events. We present comparisons between the ionization
temperature of the interplanetary plasma with that observed at the
Sun in cases where this is possible.
---------------------------------------------------------
Title: Yohkoh/SXT soft x-ray observations of sudden mass loss from
the solar corona
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Freeland, S. L.;
Lemen, J. R.; Harvey, K. L.
1995sowi.confR..58H Altcode:
Direct X-ray observations allow us to estimate the hot coronal mass
before and after a flare or other disturbance of the type leading to
a coronal mass ejection. The sudden disappearance of a large coronal
structure (scale greater than 105 km) gives evidence that an ejection
has occurred, if the time scales are much shorter than the conductive
or radiative cooling times for such structures. A flare also typically
adds large amounts of new material to the corona via evaporation
resulting from the coronal energy release. This provides a competing
mechanism that makes the estimation of the total mass loss somewhat
difficult. We note that the X-ray observations have the advantage of
covering the entire corona rather than the limb regions unlike the
coronagraph observations. We have identified two examples of coronal
mass disappearances. before and during long duration flare events on
21 Feb. 1992 (on the E limb) and 13 Nov. 1994 (near disk center). In
latter case the total mass amounted to some 4 x 10<SUP>14</SUP> g with a
density of 3 x 10<SUP>8</SUP>cm<SUP>-3</SUP> and a temperature of 2.8 MK
before its disappearance. This corresponds to a radiative cooling time
of some 104 S. much longer than the observed time of disappearance. We
therefore suggest that these sudden mass disappearances correspond with
coronal mass ejections (CMEs), and suggest that further data analysis
will be able to confirm this by comparison with optical observations
of specific CMEs.
---------------------------------------------------------
Title: The solar origins of two high-latitude interplanetary
disturbances
Authors: Hudson, H. S.; Acton, L. W.; Alexander, D.; Harvey, K. L.;
Kurokawa, H.; Kahler, S.; Lemen, J. R.
1995sowi.confS..58H Altcode:
Two extremely similar interplanetary forward/reverse shock events,
with bidirectional electron streaming were detected by Ulysses in
1994. Ground-based and Yohkoh/SXT observations show two strikingly
different solar events that could be associated with them: an LDE flare
on 20 Feb. 1994, and a extremely large-scale eruptive event on 14 April
1994. Both events resulted in geomagnetic storms and presumably were
associated with coronal mass ejections. The sharply contrasting nature
of these solar events argues against an energetic causal relationship
between them and the bidirectional streaming events observed by Ulysses
during its S polar passage. We suggest instead that for each pair of
events. a common solar trigger may have caused independent instabilities
leading to the solar and interplanetary phenomena.
