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Author name code: antiochos
ADS astronomy entries on 2022-09-14
author:"Antiochos, Spiro K."
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Title: Advancing Theory and Modeling Efforts in Heliophysics
Authors: Guo, Fan; Antiochos, Spiro; Cassak, Paul; Chen, Bin; Chen,
Xiaohang; Dong, Chuanfei; Downs, Cooper; Giacalone, Joe; Haggerty,
Colby C.; Ji, Hantao; Karpen, Judith; Klimchuk, James; Li, Wen; Li,
Xiaocan; Oka, Mitsuo; Reeves, Katharine K.; Swisdak, Marc; Tu, Weichao
2022arXiv220903611G Altcode:
Heliophysics theory and modeling build understanding from fundamental
principles to motivate, interpret, and predict observations. Together
with observational analysis, they constitute a comprehensive scientific
program in heliophysics. As observations and data analysis become
increasingly detailed, it is critical that theory and modeling develop
more quantitative predictions and iterate with observations. Advanced
theory and modeling can inspire and greatly improve the design of
new instruments and increase their chance of success. In addition,
in order to build physics-based space weather forecast models, it is
important to keep developing and testing new theories, and maintaining
constant communications with theory and modeling. Maintaining a
sustainable effort in theory and modeling is critically important
to heliophysics. We recommend that all funding agencies join forces
and consider expanding current and creating new theory and modeling
programs--especially, 1. NASA should restore the HTMS program to its
original support level to meet the critical needs of heliophysics
science; 2. a Strategic Research Model program needs to be created to
support model development for next-generation basic research codes;
3. new programs must be created for addressing mission-critical theory
and modeling needs; and 4. enhanced programs are urgently required
for training the next generation of theorists and modelers.
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Title: Quantifying magnetic reconnection in the Solar corona
Authors: Aslanyan, Valentin; Pontin, David; Wyper, Peter; Antiochos,
Spiro; Scott, Roger; Higginson, Aleida
2022cosp...44.1495A Altcode:
Magnetic reconnection is understood to have important effects on
the dynamics of the Solar atmosphere, including those that lead
to the formation of the slow Solar wind. Of particular importance
is interchange reconnection between very long "open" field lines
emerging from coronal holes into the heliosphere and shorter "closed"
field lines between two points on the photosphere. We have used the
Adaptively Refined Magnetohydrodynamic Solver to perform a number of
simulations of the global corona with varying magnetic geometries,
from which we subsequently determine regions where reconnection has
taken place. Energy is injected into the magnetic field by plasma
flows at the photosphere which transport the footpoints of field
lines. We find that the total reconnected magnetic flux of numerous
localized vortices representing supergranules exceeds that of a global
differential rotation profile. We also find systematic differences in
the interchange reconnection rates based on the length of the closed
field lines involved. Our simulations show that shorter closed field
lines of pseudostreamers reconnect more readily than the longer field
lines of helmet streamers. Consequently, we predict smoother coronal
hole boundaries in the vicinity of pseudostreamers than other coronal
structures. We have identified signatures of these processes which
may be detected both remotely and in-situ by spacecraft such as the
Solar Dynamics Observatory, Parker Solar Probe, and Solar Orbiter.
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Title: Relating the variability of the middle corona to the structure
of the slow solar wind
Authors: Higginson, Aleida; DeVore, C. Richard; Antiochos, Spiro;
Viall, Nicholeen
2022cosp...44.1320H Altcode:
The recent revolution in heliospheric measurements, brought about by
NASAʼs Parker Solar Probe and ESAʼs Solar Orbiter, has shown that
processes in the middle corona can influence the structure and dynamics
of the solar wind across spatial scales. Understanding the formation
of the young solar wind structures currently being measured by Parker
Solar Probe and Solar Orbiter is now essential. Numerical calculations
have shown that magnetic field dynamics at coronal hole boundaries
in the middle corona, in particular interchange reconnection driven
by photospheric motions, can be responsible for the dynamic release
of structured slow solar wind, including along huge separatrix-web
(S-Web) arcs formed by pseudostreamers. Quantifying the plasma and
magnetic variability along the heliospheric current sheet and these
S-Web arcs is crucial to furthering our understanding of how coronal
magnetic field dynamics can influence the slow solar wind throughout the
heliosphere. Here we present fully dynamic, 3D numerical calculations of
a coronal hole boundary driven continuously by realistic photospheric
motions at its base. We consider our simulation results within the
context of Parker Solar Probe and Solar Orbiter, and make predictions
for the structure and variability of the young slow solar wind.
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Title: Driving of Heliospheric Structure and Dynamics by the Closed
Corona
Authors: Antiochos, Spiro; Schlenker, Michael; MacNeice, Peter;
Mason, Emily
2022cosp...44.1082A Altcode:
The heliosphere is observed to have structure and dynamics on a vast
range of spatial and temporal scales. Much of the observed dynamics is
turbulent in nature, and is believed to be generated primarily in situ
by stream-stream interactions for example; but, coherent structures
such as magnetic islands are also ubiquitous. These are especially
common near the Heliospheric Current Sheet (HCS) where the slow wind
is generally located. Since the HCS maps down to the Y-null at the
top of the streamer belt, it has long been proposed that interactions
between the closed and open flux are the origin of much of the HCS
dynamics and, perhaps, of the slow wind, itself. We investigate
thermal non-equilibrium (TNE) in the closed field as a possible
driver of HCS dynamics. TNE is a process by which condensations form
quasi-randomly in coronal loops as a result of spatial localization
of the coronal heating. We use 2.5D MHD simulations of the corona
and inner heliosphere that include a full thermodynamics treatment of
the plasma. Our calculations show that plasma dynamics in the closed
field couple to magnetic dynamics that end up driving reconnection in
the HCS. Our conclusion is that coronal heating very low in the solar
atmosphere, near the chromosphere, may well be responsible for the
quasi-periodic dynamics observed far out in the heliosphere. We discuss
the implications of our model for observations. This work was supported
by the NASA Living With a Star, GSFC/ISFM, and DRIVE Center Programs.
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Title: Driving Solar Eruptions by Flux Rope Emergence
Authors: Antiochos, Spiro; Linton, Mark; Leake, James
2022cosp...44.2405A Altcode:
Solar eruptive events (SEEs) consisting of a filament ejection, fast
coronal mass ejections (CME) and X-class flare are the most powerful
explosions in our solar system and the primary drivers of highly
destructive space weather at Earth and in interplanetary space. SEEs
are known to be due to the release of the free magnetic energy stored
in a filament channel; consequently, the formation of the filament
field is the fundamental origin of SEEs. Flux emergence has long been
observed to be a primary mechanism for filament channel formation, and
the emergence into the corona of a sub-photospheric twisted flux rope,
which leads to a filament channel, has been modeled by many authors. The
key point of the emergence models is that they make no assumptions on
the nature of the filament field, sheared arcade or twisted flux rope,
the channel forms self-consistently as a result of the interaction
between the corona and the convection zone. In previous work we showed
that the alignment of the subsurface twisted flux rope with respect
to a pre-existing coronal arcade plays the determining role in whether
eruption occurs or not. In this presentation we describe the effect of
the relative orientation of the coronal and subsurface fluxes on coronal
evolution, and demonstrate that a range of eruptive/non-eruptive
behaviors can occur depending on this parameter. We discuss the
implications of our results for understanding the eruption mechanism
and for interpreting observations. This work was supported by the NASA
Living With a Star, GSFC/ISFM, and DRIVE Center Programs.
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Title: The Role of Reconnection in the Onset of Solar Eruptions
Authors: Leake, James E.; Linton, Mark G.; Antiochos, Spiro K.
2022ApJ...934...10L Altcode: 2022arXiv220512957L
Solar eruptive events such as coronal mass ejections and eruptive
flares are frequently associated with the emergence of magnetic flux
from the convection zone into the corona. We use three-dimensional
magnetohydrodynamic numerical simulations to study the interaction
of coronal magnetic fields with emerging flux and determine the
conditions that lead to eruptive activity. A simple parameter study is
performed, varying the relative angle between emerging magnetic flux
and a preexisting coronal dipole field. We find that in all cases the
emergence results in a sheared magnetic arcade that transitions to a
twisted coronal flux rope via low-lying magnetic reconnection. This
structure, however, is constrained by its own outer field and so is
noneruptive in the absence of reconnection with the overlying coronal
field. The amount of this overlying reconnection is determined by
the relative angle between the emerged and preexisting fields. The
reconnection between emerging and preexisting fields is necessary
to generate sufficient expansion of the emerging structure so that
flare-like reconnection below the coronal flux rope becomes strong
enough to trigger its release. Our results imply that the relative
angle is the key parameter in determining whether the resultant
active regions exhibit eruptive behavior and is thus a potentially
useful candidate for predicting eruptions in newly emerging active
regions. More generally, our results demonstrate that the detailed
interaction between the convection zone/photosphere and the corona
must be calculated self-consistently in order to model solar eruptions
accurately.
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Title: Variability of the Reconnection Guide Field in Solar Flares
Authors: Dahlin, Joel T.; Antiochos, Spiro K.; Qiu, Jiong; DeVore,
C. Richard
2022ApJ...932...94D Altcode: 2021arXiv211004132D
Solar flares may be the best-known examples of the explosive conversion
of magnetic energy into bulk motion, plasma heating, and particle
acceleration via magnetic reconnection. The energy source for all
flares is the highly sheared magnetic field of a filament channel
above a polarity inversion line (PIL). During the flare, this
shear field becomes the so-called reconnection guide field (i.e.,
the nonreconnecting component), which has been shown to play a major
role in determining key properties of the reconnection, including the
efficiency of particle acceleration. We present new high-resolution,
three-dimensional, magnetohydrodynamics simulations that reveal the
detailed evolution of the magnetic shear/guide field throughout
an eruptive flare. The magnetic shear evolves in three distinct
phases: shear first builds up in a narrow region about the PIL, then
expands outward to form a thin vertical current sheet, and finally is
transferred by flare reconnection into an arcade of sheared flare loops
and an erupting flux rope. We demonstrate how the guide field may be
inferred from observations of the sheared flare loops. Our results
indicate that initially the guide field is larger by about a factor
of 5 than the reconnecting component, but it weakens by more than an
order of magnitude over the course of the flare. Instantaneously,
the guide field also varies spatially over a similar range along
the three-dimensional current sheet. We discuss the implications
of the remarkable variability of the guide field for the timing and
localization of efficient particle acceleration in flares.
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Title: The Dynamic Structure of Coronal Hole Boundaries
Authors: Aslanyan, V.; Pontin, D. I.; Scott, R. B.; Higginson, A. K.;
Wyper, P. F.; Antiochos, S. K.
2022ApJ...931...96A Altcode:
The boundaries of solar coronal holes are difficult to uniquely
define observationally but are sites of interest in part because the
slow solar wind appears to originate there. The aim of this article
is to explore the dynamics of interchange magnetic reconnection
at different types of coronal hole boundaries-namely streamers and
pseudostreamers-and their implications for the coronal structure. We
describe synthetic observables derived from three-dimensional
magnetohydrodynamic simulations of the atmosphere of the Sun in which
coronal hole boundaries are disturbed by flows that mimic the solar
supergranulation. Our analysis shows that interchange reconnection takes
place much more readily at the pseudostreamer boundary of the coronal
hole. As a result, the portion of the coronal hole boundary formed by
the pseudostreamer remains much smoother, in contrast to the highly
distorted helmet-streamer portion of the coronal hole boundary. Our
results yield important new insights on coronal hole boundary regions,
which are critical in coupling the corona to the heliosphere as the
formation regions of the slow solar wind.
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Title: The Dynamic Coupling of Streamers and Pseudostreamers to
the Heliosphere
Authors: Aslanyan, V.; Pontin, D. I.; Higginson, A. K.; Wyper, P. F.;
Scott, R. B.; Antiochos, S. K.
2022ApJ...929..185A Altcode: 2022arXiv220102388A
The slow solar wind is generally believed to result from the
interaction of open and closed coronal magnetic flux at streamers
and pseudostreamers. We use three-dimensional magnetohydrodynamic
simulations to determine the detailed structure and dynamics of
open-closed interactions that are driven by photospheric convective
flows. The photospheric magnetic field model includes a global dipole
giving rise to a streamer together with a large parasitic polarity
region giving rise to a pseudostreamer that separates a satellite
coronal hole from the main polar hole. Our numerical domain extends
out to 30R <SUB>⊙</SUB> and includes an isothermal solar wind,
so that the coupling between the corona and heliosphere can be
calculated rigorously. This system is driven by imposing a large set
of quasi-random surface flows that capture the driving of coronal
flux in the vicinity of streamer and pseudostreamer boundaries by
the supergranular motions. We describe the resulting structures and
dynamics. Interchange reconnection dominates the evolution at both
streamer and pseudostreamer boundaries, but the details of the resulting
structures are clearly different from one another. Additionally,
we calculate in situ signatures of the reconnection and determine
the dynamic mapping from the inner heliosphere back to the Sun for a
test spacecraft orbit. We discuss the implications of our results for
interpreting observations from inner heliospheric missions, such as
Parker Solar Probe and Solar Orbiter, and for space weather modeling
of the slow solar wind.
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Title: SynthIA: A Synthetic Inversion Approximation for the Stokes
Vector Fusing SDO and Hinode into a Virtual Observatory
Authors: Higgins, Richard E. L.; Fouhey, David F.; Antiochos, Spiro K.;
Barnes, Graham; Cheung, Mark C. M.; Hoeksema, J. Todd; Leka, K. D.;
Liu, Yang; Schuck, Peter W.; Gombosi, Tamas I.
2022ApJS..259...24H Altcode: 2021arXiv210812421H
Both NASA's Solar Dynamics Observatory (SDO) and the JAXA/NASA
Hinode mission include spectropolarimetric instruments designed
to measure the photospheric magnetic field. SDO's Helioseismic
and Magnetic Imager (HMI) emphasizes full-disk, high-cadence,
and good-spatial-resolution data acquisition while Hinode's Solar
Optical Telescope Spectro-Polarimeter (SOT-SP) focuses on high
spatial resolution and spectral sampling at the cost of a limited
field of view and slower temporal cadence. This work introduces a
deep-learning system, named the Synthetic Inversion Approximation
(SynthIA), that can enhance both missions by capturing the best of
each instrument's characteristics. We use SynthIA to produce a new
magnetogram data product, the Synthetic Hinode Pipeline (SynodeP),
that mimics magnetograms from the higher-spectral-resolution
Hinode/SOT-SP pipeline, but is derived from full-disk, high-cadence,
and lower-spectral-resolution SDO/HMI Stokes observations. Results
on held-out data show that SynodeP has good agreement with the
Hinode/SOT-SP pipeline inversions, including magnetic fill fraction,
which is not provided by the current SDO/HMI pipeline. SynodeP further
shows a reduction in the magnitude of the 24 hr oscillations present in
the SDO/HMI data. To demonstrate SynthIA's generality, we show the use
of SDO/Atmospheric Imaging Assembly data and subsets of the HMI data as
inputs, which enables trade-offs between fidelity to the Hinode/SOT-SP
inversions, number of observations used, and temporal artifacts. We
discuss possible generalizations of SynthIA and its implications for
space-weather modeling. This work is part of the NASA Heliophysics
DRIVE Science Center at the University of Michigan under grant NASA
80NSSC20K0600E, and will be open-sourced.
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Title: Correlated Spatio-temporal Evolution of Extreme-Ultraviolet
Ribbons and Hard X-Rays in a Solar Flare
Authors: Naus, S. J.; Qiu, J.; DeVore, C. R.; Antiochos, S. K.;
Dahlin, J. T.; Drake, J. F.; Swisdak, M.
2022ApJ...926..218N Altcode: 2021arXiv210915314N
We analyze the structure and evolution of ribbons from the M7.3
SOL2014-04-18T13 flare using ultraviolet images from the Interface
Region Imaging Spectrograph and the Solar Dynamics Observatory
(SDO)/Atmospheric Imaging Assembly (AIA), magnetic data from the
SDO/Helioseismic and Magnetic Imager, hard X-ray (HXR) images from the
Reuven Ramaty High Energy Solar Spectroscopic Imager, and light curves
from the Fermi/Gamma-ray Burst Monitor, in order to infer properties
of coronal magnetic reconnection. As the event progresses, two flare
ribbons spread away from the magnetic polarity inversion line. The
width of the newly brightened front along the extension of the ribbon
is highly intermittent in both space and time, presumably reflecting
nonuniformities in the structure and/or dynamics of the flare current
sheet. Furthermore, the ribbon width grows most rapidly in regions
exhibiting concentrated nonthermal HXR emission, with sharp increases
slightly preceding the HXR bursts. The light curve of the ultraviolet
emission matches the HXR light curve at photon energies above 25
keV. In other regions the ribbon-width evolution and light curves do
not temporally correlate with the HXR emission. This indicates that
the production of nonthermal electrons is highly nonuniform within
the flare current sheet. Our results suggest a strong connection
between the production of nonthermal electrons and the locally
enhanced perpendicular extent of flare ribbon fronts, which in turn
reflects the inhomogeneous structure and/or reconnection dynamics of
the current sheet. Despite this variability, the ribbon fronts remain
nearly continuous, quasi-one-dimensional features. Thus, although
the reconnecting coronal current sheets are highly structured, they
remain quasi-two-dimensional and the magnetic energy release occurs
systematically, rather than stochastically, through the volume of the
reconnecting magnetic flux.
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Title: Driving of Reconnection in the Heliospheric Current Sheet by
Thermal Nonequilibrium
Authors: Antiochos, Spiro; Schlenker, Michael; MacNeice, Peter;
Mason, Emily
2021AGUFMSH25F2155A Altcode:
Observations have shown that the coupling between the corona and
heliosphere is intrinsically dynamic with quasi-periodic density
structures ubiquitously seen in the Heliospheric Current Sheet (HCS)
that maps down to the top of the streamer belt. These structures have
been identified by several authors as due to magnetic reconnection
that produces magnetic islands in the HCS. Such islands have important
implications for understanding the origins of heliospheric energetic
particles and plasma/field variability. A key feature of the density
structures is their quasi-periodicity, on time scales of one to two
hours. We propose that the mechanism responsible for the periodicity
is thermal nonequilibrium (TNE), a process by which solar coronal
loops undergo quasi-periodic cycles of heating and cooling due to the
spatial localization of coronal heating near the loop base. Since the
requirement for TNE onset is that the loop length is large compared to
the scale of the heating, it is most likely to occur on the largest
coronal loops, those near the open-closed boundary. We use 2.5D MHD
numerical simulations to investigate the effect of TNE in the corona
and heliosphere of an axisymmetric helmet streamer and polar coronal
holes. As in the many 1D loop studies, we find that TNE occurs in
coronal loops with sufficiently large length, but in contrast to
previous studies, we find that the process also drives substantial
magnetic dynamics, especially near the top of the streamer where the
plasma beta becomes of order unity. From the simulation results we
determine predictions for spectroscopic and imaging observations of
both the hot and cool helmet streamer plasma and the solar wind near
to the streamer stalk. We conclude that TNE occurring in the largest
closed loops in the corona may explain several puzzling observations
of the corona and wind, such as the ubiquitous blue shifts observed
at the edges of active regions, and the quasi-periodic solar wind
blobs. This work was supported by the NASA Living With a Star Program.
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Title: How does photospheric driving effect helmet streamer
reconnection and HCS structure?
Authors: Higginson, Aleida; Antiochos, Spiro; DeVore, C. Richard
2021AGUFMSH25F2146H Altcode:
It is now well understood that the transition from coronal helmet
streamers to the corresponding current sheet in the heliosphere is a
region of increased magnetic field dynamics. The boundary between the
two structures is the preferred location for both interchange magnetic
reconnection and helmet streamer pinch-off reconnection, both of which
are believed to be important processes for the formation of the solar
wind. These magnetic field dynamics can leave behind imprints in the
solar wind in the form of heliospheric current sheet (HCS) blobs,
which have been observed across a continuum of temporal and spatial
scales in remote and in situ observations. As we begin to unravel
the unprecedented data returned from missions like Parker Solar Probe
(PSP) and Solar Orbiter (SO), it becomes more important than ever to
understand the dynamic processes occurring in this middle corona region
and to determine the subsequent effects on the solar wind. We present
here simulations of a helmet streamer - HCS system with an isothermal
solar wind driven by photospheric motions where we fully resolve the
entire separatrix system. We quantify the effect of photospheric driving
on the formation of the HCS blobs and make predictions for PSP, SO,
and future observations as these resulting structures propagate out
into the solar wind and heliosphere.
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Title: Correlated Spatio-temporal Evolution of Extreme-Ultraviolet
Ribbons and Hard X-rays in a Solar Flare
Authors: Naus, Stephen; Qiu, Jiong; Antiochos, Spiro; Dahlin, Joel;
DeVore, C. Richard; Drake, James; Swisdak, Marc
2021AGUFMSH23B..05N Altcode:
We analyzed the structure and evolution of flare ribbons in the solar
chromosphere to infer properties of coronal magnetic reconnection. We
used ultraviolet (UV) imaging observations of the M7.3 SOL2014-04-18T13
flare obtained by IRIS and SDO/AIA, magnetic data from SDO/HMI, and hard
X-ray (HXR) images from RHESSI and light curves from Fermi/GBM. Two
flare ribbons spread away from the magnetic polarity inversion line
as the event progressed. From high-resolution IRIS observations, we
measured the width of the newly brightening front along the extension
of the ribbon, which maps the feet of magnetic field lines reconnecting
in the corona. We find that the ribbon front is highly intermittent
in both space and time, presumably reflecting non-uniformities in the
structure and/or dynamics of the flare current sheet. Early in the
event, the ribbon fronts form and widen to near maximum thickness,
and the rate of change of reconnected magnetic flux rises to its peak
value. Subsequently, the flux change rate drops steeply, the UV light
curves continue to rise toward their maxima, and the HXR emissions rise
rapidly to their own maxima, which are simultaneous with those in the
UV. We find indirect evidence that well-resolved local peaks in the UV
ribbon-front widths are cospatial and cotemporal with the UV emissions
and poorly resolved HXR emissions. This result suggests that there is
a strong connection between the production of non-thermal electrons
and locally enhanced perpendicular extent of flare ribbon fronts, which
reflect the inhomogeneous structure and/or reconnection dynamics of the
flare current sheet in the corona. We discuss the implications of these
results for understanding the inhomogeneous structure and dynamics of
the coronal flare current sheet and the origin of the flare-accelerated
particles. NASA supported our research via the SOLFER DRIVE Center at
UMD, the H-GI program at MSU, and the H-ISFM program at GSFC.
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Title: Temporal Evolution of the Guide Field in Eruptive Flares
Authors: Dahlin, Joel; Antiochos, Spiro; Jiong, Qiu; DeVore, C. Richard
2021AGUFMSH23B..07D Altcode:
Solar flares are explosive space weather events that rapidly convert
stored magnetic energy into bulk motion, plasma heating, and particle
acceleration via magnetic reconnection. For all flares, the free energy
source is ultimately the highly sheared magnetic field of a filament
channel above a polarity inversion line. During the flare, the shear
field becomes the reconnection guide field, the strength of which is
widely believed to control the efficiency of reconnection-driven
particle acceleration. We present new high-resolution 3D MHD
simulations that calculate the evolution of the magnetic shear/guide
field throughout an eruptive flare. The magnetic shear evolves in three
distinct phases: shear first builds up in a narrow region about the PIL,
expands outward to drive the formation of a thin current sheet, and is
finally transferred by the flare reconnection into sheared post-flare
loops and erupting flux rope. We show that the guide field weakens
more than an order of magnitude over the course of the flare, and
instantaneously varies over a similar range along the three-dimensional
current sheet. We demonstrate how the guide field may be inferred
from observations of sheared post-flare loops. Interestingly, we find
that the number of plasmoids in the flare reconnecting current sheet
increases with weakening guide field, underscoring the important role
of the guide field in particle acceleration. We discuss implications
for observations by IRIS, SDO/AIA, and DKIST. This work was supported
by NASA via the SOLFER DRIVE Center at UMD, the H-ISFM program, and
the HGI program.
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Title: STITCH: A Subgrid-Scale Model for Energy Buildup in the
Solar Corona
Authors: Dahlin, Joel T.; DeVore, C. Richard; Antiochos, Spiro K.
2021arXiv211200641D Altcode:
The solar corona routinely exhibits explosive activity, in particular
coronal mass ejections and their accompanying eruptive flares, that have
global-scale consequences. These events and their smaller counterparts,
coronal jets, originate in narrow, sinuous filament channels. The
key processes that form and evolve the channels operate on still
smaller spatial scales and much longer time scales, culminating in a
vast separation of characteristic lengths and times that govern these
explosive phenomena. In this article, we describe implementation and
tests of an efficient subgrid-scale model for generating eruptive
structures in magnetohydrodynamics (MHD) coronal simulations. STITCH
-- STatistical InjecTion of Condensed Helicity -- is a physics-based,
reduced representation of helicity condensation: a process wherein
small-scale vortical surface convection forms ubiquitous current sheets,
and pervasive reconnection across the sheets mediates an inverse cascade
of magnetic helicity and free energy, thereby forming the filament
channels. STITCH abstracts these complex processes into a single
new term, in the MHD Ohm's law and induction equation, which directly
injects tangential magnetic flux into the low corona. We show that this
approach is in very good agreement with a full helicity-condensation
calculation that treats all of the dynamics explicitly, while enabling
substantial reductions in temporal duration especially, but also
in spatial resolution. In addition, we illustrate the flexibility of
STITCH at forming localized filament channels and at energizing complex
surface flux distributions that have sinuous boundaries. STITCH is
simple to implement and computationally efficient, making it a powerful
new technique for event-based, data-driven modeling of solar eruptions.
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Title: The Effect of Thermal Nonequilibrium on Helmet Streamers
Authors: Schlenker, Michael J.; Antiochos, Spiro K.; MacNeice, Peter
J.; Mason, Emily I.
2021ApJ...916..115S Altcode:
Solar loops in which the coronal heating scale is short compared to
the loop length are known to be susceptible to thermal nonequilibrium
(TNE). We investigate the effects of this process on the largest
loops in the corona, those of a helmet streamer. Our numerical study
uses a 2.5D MHD code that includes the full magnetic field dynamics
as well as the detailed plasma thermodynamics. The simulation model
is axisymmetric, consisting of an equatorial streamer belt and two
polar coronal holes. As in previous 1D loop studies, we find that TNE
occurs in coronal loops with sufficiently large length, but in contrast
to these studies, we find that the process also drives substantial
magnetic dynamics, especially near the top of the streamer where the
plasma beta becomes of order unity. From the simulation results we
determine predictions for spectroscopic and imaging observations of
both the hot and cool helmet streamer plasma. Simulations are preformed
using different scale heights for the heating and different numerical
resolution in order to determine the dependence of our findings
on these important parameters. We conclude that TNE in streamers
may explain several puzzling observations, such as the ubiquitous
blueshifts observed at the edges of active regions. We also discuss
the implications of our results for the solar wind.
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Title: Coupled Pseudostreamer/Helmet Streamer Eruptions
Authors: Wyper, P.; Antiochos, S.; DeVore, C.; Lynch, B.; Karpen,
J.; Kumar, P.
2021AAS...23832205W Altcode:
An important aspect of solar activity is the coupling between eruptions
and the surrounding coronal magnetic field topology. This coupling
determines the trajectory and morphology of the event and can even
trigger sympathetic eruptions from multiple sources. Here we report
on a numerical simulation of a new type of coupled eruption, in which
a large-scale coronal jet initiated by a pseudostreamer filament
eruption triggers a streamer-blowout coronal mass ejection (CME). The
initial pseudostreamer in our simulation is typical of many observed
pseudostreamers in that it separates an equatorial and polar coronal
hole and is associated with a broad S-Web arc in the heliosphere. Our
results show that the coupled eruption is a result of the enhanced
breakout reconnection that occurs above the erupting filament channel
as the jet is launched and progresses into the neighbouring helmet
streamer. This partially launches the jet along closed helmet streamer
field lines which blows out the streamer top to produce a classic
bubble-shaped CME. Another key finding is that the CME is strongly
deflected from the jet's initial trajectory and contains a mixture of
open and closed magnetic field lines. We present the detailed dynamics
of this new type of coupled eruption and discuss the implications of
this work for interpreting in-situ and remote-sensing observations
and for understanding CME formation and evolution in general.
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Title: Switch-on Shock and Nonlinear Kink Alfvén Waves in Solar
Coronal-Hole Jets
Authors: DeVore, C. R.; Karpen, J. T.; Antiochos, S. K.; Uritsky,
V. M.; Roberts, M. A.; Pariat, E.
2021AAS...23821322D Altcode:
It is generally accepted that solar coronal-hole jets are generated by
fast magnetic reconnection in the low corona, whether driven directly by
flux emergence from below or indirectly by instability onset above the
photosphere. In either case, twisted flux on closed magnetic field lines
reconnects with untwisted flux on neighboring open field lines. Some
of that twist is inherited by the newly reconnected open flux, which
rapidly relaxes due to magnetic tension forces that transmit the twist
impulsively into the outer corona and heliosphere. We suggest that the
transfer of twist launches switch-on MHD shock waves, which propagate
parallel to the ambient coronal magnetic field ahead of the shock
and convect a perpendicular component of magnetic field behind the
shock. In the frame moving with the shock front, the post-shock flow
is precisely Alfvénic in all three directions, whereas the pre-shock
flow is super-Alfvénic along the ambient magnetic field. Consequently,
there is a density enhancement across the shock front. Nonlinear kink
Alfvén waves are exact solutions of the time-dependent MHD equations
in the post-shock region when the ambient corona is uniform and the
magnetic field is straight. We report 3D spherical simulations of
coronal-hole jets driven by instability onset in the corona. The results
are consistent with the generation of MHD switch-on shocks trailed
predominantly by incompressible, irrotational, kink Alfvén waves. We
will discuss the implications of our results for understanding solar
jets and interpreting their heliospheric signatures in light of the
new data on S-bends (a.k.a. switchbacks) from Parker Solar Probe. Our
research is supported by NASA's H-ISFM program.
---------------------------------------------------------
Title: An Observational Study of a "Rosetta Stone" Solar Eruption
Authors: Mason, E. I.; Antiochos, Spiro K.; Vourlidas, Angelos
2021ApJ...914L...8M Altcode: 2021arXiv210509164M
This Letter reports observations of an event that connects all major
classes of solar eruptions: those that erupt fully into the heliosphere
versus those that fail and are confined to the Sun, and those that eject
new flux into the heliosphere, in the form of a flux rope, versus those
that eject only new plasma in the form of a jet. The event originated
in a filament channel overlying a circular polarity inversion line
and occurred on 2016 March 13 during the extended decay phase of the
active region designated NOAA 12488/12501. The event was especially
well observed by multiple spacecraft and exhibited the well-studied
null-point topology. We analyze all aspects of the eruption using Solar
Dynamics Observatory Atmospheric Imaging Assembly and Helioseismic
and Magnetic Imager, Solar-Terrestrial Relations Observatory Extreme
Ultraviolet Imager, and Solar and Heliospheric Observatory Large
Angle and Spectrometric Coronagraph (SOHO LASCO) imagery. One section
of the filament undergoes a classic failed eruption with cool plasma
subsequently draining onto the section that did not erupt, but a complex
structured coronal mass ejection/jet is clearly observed by SOHO/LASCO
C2 shortly after the failed filament eruption. We describe in detail
the slow buildup to eruption, the lack of an obvious trigger, and the
immediate reappearance of the filament after the event. The unique
mixture of major eruption properties observed during this event places
severe constraints on the structure of the filament channel field and,
consequently, on the possible eruption mechanism.
---------------------------------------------------------
Title: From Pseudostreamer Jets to Coronal Mass Ejections:
Observations of the Breakout Continuum
Authors: Kumar, P.; Karpen, J.; Antiochos, S.; Wyper, P.; DeVore,
C.; Lynch, B.
2021AAS...23832203K Altcode:
The magnetic breakout model, in which reconnection in the corona leads
to destabilization of a filament channel, explains numerous features
of eruptive solar events, from small-scale jets to global-scale
coronal mass ejections (CMEs). The underlying multipolar topology,
pre-eruption activities, and sequence of magnetic-reconnection onsets
(first breakout, then flare) of many observed fast CMEs/eruptive flares
are fully consistent with the model. Recently, we demonstrated that most
observed coronal-hole jets in fan/spine topologies also are induced by
breakout reconnection at the null point above a filament channel (with
or without a filament). For these two types of eruptions occurring in
similar topologies, the key question is, why do some events generate
jets while others form CMEs? We focused on the initiation of eruptions
in large bright points/small active regions that were located in
coronal holes and clearly exhibited null-point (fan/spine) topologies:
such configurations are referred to as pseudostreamers. We analyzed and
compared Solar Dynamics Observatory/Atmospheric Imaging Assembly, Solar
and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph
Experiment, and Reuven Ramaty High Energy Solar Spectroscopic Imager
observations of three events. Our analysis of the events revealed
two new observable signatures of breakout reconnection prior to the
explosive jet/CME outflows and flare onset: coronal dimming and the
opening up of field lines above the breakout current sheet. Most
key properties were similar among the selected erupting structures,
thereby eliminating region size, photospheric field strength, magnetic
configuration, and pre-eruptive evolution as discriminating factors
between jets and CMEs. We consider the factors that contribute to
the different types of dynamic behavior, and conclude that the main
determining factor is the ratio of the magnetic free energy associated
with the filament channel compared to the energy associated with the
overlying flux inside and outside the pseudostreamer dome.
---------------------------------------------------------
Title: Mind The Gap: Observing The Jet/CME Continuum In A Hybrid
Eruption
Authors: Mason, E.; Antiochos, S.; Vourlidas, A.
2021AAS...23821316M Altcode:
Coronal mass ejections, jets, prominence eruptions: solar eruptions are
an active field with a broad range of accepted phenomena, and an even
broader range of proposed mechanisms that cause the phenomena. This
talk reports the observations of an event that connects the major
eruption classes, and could provide a holistic explanation for all of
them. The event originated in a filament channel overlying a circular
polarity inversion line (PIL) and occurred on 2013 March 13 during the
extended decay phase of the active region designated (sequentially)
NOAA 12488/12501. This event was especially well-observed by multiple
spacecraft and was seen to have the well-studied null-point topology. We
analyze all aspects of the eruption using SDO AIA and HMI, STEREO-A,
and SOHO LASCO imagery. One section of the filament undergoes a
classic failed eruption with cool plasma subsequently draining onto
the section that did not erupt, but a complex structured CME/jet is
clearly observed by SOHO LASCO C2 shortly after the failed filament
eruption. We describe in detail the long, slow buildup to eruption;
the lack of an obvious trigger; and the immediate reappearance of
the filament after the event. The unique mixture of major eruption
properties that are observed in this event places severe constraints
on the structure of the filament channel field and, consequently,
on the possible eruption mechanism.
---------------------------------------------------------
Title: The 3D Dynamics of Flare Reconnection
Authors: Dahlin, J.; Antiochos, S.; Qiu, J.; DeVore, C.; Wyper, P.
2021AAS...23812710D Altcode:
Solar flares are explosive space weather events that rapidly convert
stored magnetic energy into bulk motion, plasma heating, and particle
acceleration. Understanding the structure and dynamics of the magnetic
reconnection that powers flares is critical for predicting the energy
release. In particular, the amount of energy transferred to energetic
particles is thought to be highly dependent on whether the reconnection
is primarily turbulent (e.g., plasmoids) or instead laminar. We present
new high-resolution MHD simulations of three-dimensional reconnection
in an eruptive flare and compare the results to recent data. Although
flare reconnection is challenging to observe directly in the corona,
highly detailed constraints on its dynamics can be obtained from
observations of flare ribbons that track the chromospheric footpoints
of newly reconnected field lines. The analogues of flare ribbons
in our simulations are identified by tracking discontinuous changes
in field-line magnetic connectivity due to the reconnection. In our
highest-resolution calculations, we find that these ribbon analogues
are highly structured and exhibit many 'whorl' patterns that are linked
to turbulent plasmoids in the reconnecting current sheet. Such flare
ribbon fine structure therefore reveals crucial information about the
fundamental turbulent vs. laminar nature of the reconnection critical
for understanding particle acceleration. We discuss implications for
SDO, IRIS, and GST observations of explosive flare energy release.
---------------------------------------------------------
Title: The Dynamic Formation of Pseudostreamers
Authors: Scott, R. B.; Pontin, D. I.; Antiochos, S. K.; DeVore, C. R.;
Wyper, P. F.
2021AAS...23832818S Altcode:
Streamers and pseudostreamers structure the corona at the largest
scales, as seen in both eclipse and coronagraph white-light
images. Their inverted-goblet appearance encloses broad coronal
loops at the Sun and tapers to a narrow radial stalk away from the
star. The streamer associated with the global solar dipole magnetic
field is long-lived, predominantly contains a single arcade of nested
loops within it, and separates opposite-polarity interplanetary
magnetic fields with the heliospheric current sheet anchored at
its apex. Pseudostreamers, on the other hand, are transient, enclose
double arcades of nested loops, and separate like-polarity fields with
a dense plasma sheet. We use numerical magnetohydrodynamic simulations
to calculate, for the first time, the formation of pseudostreamers in
response to photospheric magnetic-field evolution. Convective transport
of a minority-polarity flux concentration, initially positioned under
one side of a streamer, through the streamer boundary into the adjacent,
pre-existing coronal hole forms the pseudostreamer. Interchange
magnetic reconnection at the overlying coronal null point(s)
governs the development of the pseudostreamer above - and of a new,
satellite coronal hole behind - the moving minority polarity. The
reconnection dynamics liberate coronal-loop plasma that can escape
into the heliosphere along so-called separatrix-web ("S-Web") arcs,
which reach far from the heliospheric current sheet and the solar
equatorial plane, and can explain the origin of high-latitude slow
solar wind. We describe the implications of our results for in-situ and
remote-sensing observations of the corona and heliosphere as obtained,
most recently, by Parker Solar Probe and Solar Orbiter.
---------------------------------------------------------
Title: Relating the Variability of the Middle Corona to the Structure
of the Slow Solar Wind
Authors: Higginson, A.; Antiochos, S.; DeVore, C.
2021AAS...23822905H Altcode:
The recent revolution in heliospheric measurements, brought about
by NASA's Parker Solar Probe and ESA's Solar Orbiter, has shown that
processes in the middle corona can influence the structure and dynamics
of the solar wind across spatial scales. Understanding the formation
of the young solar wind structures currently being measured by Parker
Solar Probe and Solar Orbiter is now essential. Numerical calculations
have shown that magnetic field dynamics at coronal hole boundaries
in the middle corona, in particular interchange reconnection driven
by photospheric motions, can be responsible for the dynamic release
of structured slow solar wind, including along huge separatrix-web
(S-Web) arcs formed by pseudostreamers. Quantifying the plasma and
magnetic variability along these S-Web arcs is crucial to furthering
our understanding of how coronal magnetic field dynamics can influence
the slow solar wind throughout the heliosphere. Here we present fully
dynamic, 3D numerical calculations of a coronal hole boundary driven
continuously by realistic photospheric motions at its base. We consider
our simulation results within the context of Parker Solar Probe and
Solar Orbiter, and make predictions for the structure and variability
of the young slow solar wind.
---------------------------------------------------------
Title: How Turbulent is the Magnetically Closed Corona?
Authors: Klimchuk, James A.; Antiochos, Spiro K.
2021FrASS...8...83K Altcode: 2021arXiv210512212K
We argue that the magnetically closed corona evolves primarily
quasi-statically, punctuated by many localized bursts of activity
associated with magnetic reconnection at a myriad of small current
sheets. The sheets form by various processes that do not involve
a traditional turbulent cascade whereby energy flows losslessly
through a continuum of spatial scales starting from the large scale
of the photospheric driving. If such an inertial range is a defining
characteristic of turbulence, then the magnetically closed corona is
not a turbulent system. It nonetheless has a complex structure that
bears no direct relationship to the pattern of driving.
---------------------------------------------------------
Title: The Dynamic Formation of Pseudostreamers
Authors: Scott, Roger B.; Pontin, David I.; Antiochos, Spiro K.;
DeVore, C. Richard; Wyper, Peter F.
2021ApJ...913...64S Altcode:
Streamers and pseudostreamers structure the corona at the largest
scales, as seen in both eclipse and coronagraph white-light
images. Their inverted-goblet appearance encloses broad coronal loops
at the Sun and tapers to a narrow radial stalk away from the star. The
streamer associated with the global solar dipole magnetic field is
long-lived, predominantly contains a single arcade of nested loops
within it, and separates opposite-polarity interplanetary magnetic
fields with the heliospheric current sheet (HCS) anchored at its
apex. Pseudostreamers, on the other hand, are transient, enclose double
arcades of nested loops, and separate like-polarity fields with a
dense plasma sheet. We use numerical magnetohydrodynamic simulations
to calculate, for the first time, the formation of pseudostreamers in
response to photospheric magnetic-field evolution. Convective transport
of a minority-polarity flux concentration, initially positioned
under one side of a streamer, through the streamer boundary into the
adjacent preexisting coronal hole forms the pseudostreamer. Interchange
magnetic reconnection at the overlying coronal null point(s) governs the
development of the pseudostreamer above—and of a new satellite coronal
hole behind—the moving minority polarity. The reconnection dynamics
liberate coronal-loop plasma that can escape into the heliosphere
along so-called separatrix-web ("S-Web") arcs, which reach far from
the HCS and the solar equatorial plane, and can explain the origin
of high-latitude slow solar wind. We describe the implications of
our results for in situ and remote-sensing observations of the corona
and heliosphere as obtained, most recently, by Parker Solar Probe and
Solar Orbiter.
---------------------------------------------------------
Title: Fast and Accurate Emulation of the SDO/HMI Stokes Inversion
with Uncertainty Quantification
Authors: Higgins, Richard E. L.; Fouhey, David F.; Zhang, Dichang;
Antiochos, Spiro K.; Barnes, Graham; Hoeksema, J. Todd; Leka, K. D.;
Liu, Yang; Schuck, Peter W.; Gombosi, Tamas I.
2021ApJ...911..130H Altcode: 2021arXiv210317273H
The Helioseismic and Magnetic Imager (HMI) on board NASA's Solar
Dynamics Observatory produces estimates of the photospheric magnetic
field, which are a critical input to many space weather modeling and
forecasting systems. The magnetogram products produced by HMI and its
analysis pipeline are the result of a per-pixel optimization that
estimates solar atmospheric parameters and minimizes disagreement
between a synthesized and observed Stokes vector. In this paper,
we introduce a deep-learning-based approach that can emulate the
existing HMI pipeline results two orders of magnitude faster than the
current pipeline algorithms. Our system is a U-Net trained on input
Stokes vectors and their accompanying optimization-based Very Fast
Inversion of the Stokes Vector (VFISV) inversions. We demonstrate
that our system, once trained, can produce high-fidelity estimates of
the magnetic field and kinematic and thermodynamic parameters while
also producing meaningful confidence intervals. We additionally show
that despite penalizing only per-pixel loss terms, our system is able
to faithfully reproduce known systematic oscillations in full-disk
statistics produced by the pipeline. This emulation system could serve
as an initialization for the full Stokes inversion or as an ultrafast
proxy inversion. This work is part of the NASA Heliophysics DRIVE
Science Center (SOLSTICE) at the University of Michigan, under grant
NASA 80NSSC20K0600E, and will be open sourced.
---------------------------------------------------------
Title: A Model for the Coupled Eruption of a Pseudostreamer and
Helmet Streamer
Authors: Wyper, P. F.; Antiochos, S. K.; DeVore, C. R.; Lynch, B. J.;
Karpen, J. T.; Kumar, P.
2021ApJ...909...54W Altcode: 2021arXiv210101962W
A highly important aspect of solar activity is the coupling between
eruptions and the surrounding coronal magnetic field topology,
which determines the trajectory and morphology of the event and can
even lead to sympathetic eruptions from multiple sources. In this
paper, we report on a numerical simulation of a new type of coupled
eruption, in which a coronal jet initiated by a large pseudostreamer
filament eruption triggers a streamer-blowout coronal mass ejection
(CME) from the neighboring helmet streamer. Our configuration has a
large opposite-polarity region positioned between the polar coronal
hole and a small equatorial coronal hole, forming a pseudostreamer
flanked by the coronal holes and the helmet streamer. Further out,
the pseudostreamer stalk takes the shape of an extended arc in the
heliosphere. We energize the system by applying photospheric shear along
a section of the polarity inversion line within the pseudostreamer. The
resulting sheared-arcade filament channel develops a flux rope that
eventually erupts as a classic coronal-hole-type jet. However, the
enhanced breakout reconnection above the channel as the jet is launched
progresses into the neighboring helmet streamer, partially launching
the jet along closed helmet streamer field lines and blowing out the
streamer top to produce a classic bubble-like CME. This CME is strongly
deflected from the jet's initial trajectory and contains a mixture of
open and closed magnetic field lines. We present the detailed dynamics
of this new type of coupled eruption, its underlying mechanisms, and
the implications of this work for the interpretation of in situ and
remote-sensing observations.
---------------------------------------------------------
Title: Effects of Pseudostreamer Boundary Dynamics on Heliospheric
Field and Wind
Authors: Aslanyan, V.; Pontin, D. I.; Wyper, P. F.; Scott, R. B.;
Antiochos, S. K.; DeVore, C. R.
2021ApJ...909...10A Altcode:
Interchange reconnection has been proposed as a mechanism for the
generation of the slow solar wind, and a key contributor to determining
its characteristic qualities. In this paper we study the implications
of interchange reconnection for the structure of the plasma and field
in the heliosphere. We use the Adaptively Refined Magnetohydrodynamic
Solver to simulate the coronal magnetic evolution in a coronal topology
containing both a pseudostreamer and helmet streamer. We begin with
a geometry containing a low-latitude coronal hole that is separated
from the main polar coronal hole by a pseudostreamer. We drive the
system by imposing rotating flows at the solar surface within and
around the low-latitude coronal hole, which leads to a corrugation
(at low altitudes) of the separatrix surfaces that separate open from
closed magnetic flux. Interchange reconnection is induced both at the
null points and separators of the pseudostreamer, and at the global
helmet streamer. We demonstrate that a preferential occurrence of
interchange reconnection in the "lanes" between our driving cells leads
to a filamentary pattern of newly opened flux in the heliosphere. These
flux bundles connect to but extend far from the separatrix-web (S-Web)
arcs at the source surface. We propose that the pattern of granular
and supergranular flows on the photosphere should leave an observable
imprint in the heliosphere.
---------------------------------------------------------
Title: Particle acceleration in erupting 3D coronal mass ejections
in the breakout model
Authors: Xia, Qian; Zharkova, Valentina; Dahlin, Joel; Antiochos, Spiro
2021cosp...43E1005X Altcode:
We examine particle energisation in CMEs generated via the breakout
mechanism and explore both 2D and 3D MHD configurations. In the
breakout scenario, reconnection at a breakout current sheet (CS)
initiates the flux rope eruption by destabilizing the quasi-static force
balance. Reconnection at the flare CS triggers the fast acceleration
of the CME, which forms flare loops below and triggers particle
acceleration in flares. We present test-particle studies that focus on
two selected times during the impulsive and decay phases of the eruption
and obtain particle energy gains and spatial distributions. We find that
particles are accelerated more efficiently in the flare CS than in the
breakout CS even in the presence of large magnetic islands. The maximum
particle energy gain is estimated from the energization terms based on
the guiding-centre approximation. Particles are first accelerated in the
CSs (with or without magnetic islands) where Fermi-type acceleration
dominates. Accelerated particles escape to the interplanetary space
along open field lines rather than trapped in flux ropes, precipitate
into the chromosphere along the flare loops, or become trapped in the
flare loop top due to the magnetic mirror structure. Some trapped
particles are re-accelerated, either via re-injection to the flare
CS or through a local Betatron-type acceleration associated with
compression of the magnetic field. The energy gains of particles result
in relatively hard energy spectra during the impulsive phase. During
the gradual phase, the relaxation of the shear in the magnetic field
reduces the guiding magnetic field in the flare CS, which leads to a
decrease in particle energization efficiency.
---------------------------------------------------------
Title: The Role of 3D Reconnection in the Escape of Impulsive SEPs
Authors: Antiochos, Spiro; Masson, Sophie; DeVore, C. Richard
2021cosp...43E1004A Altcode:
It is widely accepted that impulsive solar energetic particle
(SEP) events are due to the escape into the interplanetary
medium of flare-accelerated particles produced by solar eruptive
events. According to the standard solar eruption model, however,
particles accelerated by flare reconnection should remain trapped in
the closed field lines of the flare loops and the flux rope comprising
the coronal mass ejection. To resolve this paradox, we performed fully
3D high-resolution MHD simulations of a CME/eruptive flare in a coronal
system that consists of a bipolar active region embedded in a background
global dipole field structured by solar wind. Our simulations show that
multiple magnetic reconnection episodes occur prior to and during the
CME eruption and its interplanetary propagation. In addition to the
episodes that build up the flux rope, reconnection between the open
field and the CME couples the closed corona to the open interplanetary
field. Flare-accelerated particles initially trapped in the CME thereby
gain access to the open interplanetary field along a trail blazed by
magnetic reconnection. A key difference between these 3D results and
our previous 2.5D calculations is that the interchange reconnection
allows accelerated particles to escape from deep within the CME
flux rope. We estimate the spatial extent of the particle-escape
channels. The relative timings between flare acceleration and release
of the energetic particles through CME/open-field coupling are also
determined. We discuss the implications of these results for Parker
Solar Probe and Solar Orbiter observations. This work was supported
by the NASA Living with a Star Program.
---------------------------------------------------------
Title: Understanding the Onset of CMEs/Eruptive Flares
Authors: Antiochos, Spiro; Linton, Mark
2021cosp...43E2397A Altcode:
The most important drivers of destructive space weather are the giant
solar eruptions consisting of a filament ejection, an intense X-ray
flare, and a fast coronal mass ejection (CME). These major eruptive
events drive space weather such as particle radiation throughout
interplanetary space, powerful geomagnetic storms, and ground-level
electric-power disruptions. Understanding the physical origin of
these major eruptive events is absolutely essential for developing an
eventual first-principles-based predictive capability. It is well-known
that solar eruptions are due to the explosive release of free magnetic
energy that is slowly built up in the corona, but, the exact mechanisms
are still intensely debated. The over-arching objectives of this
ISWAT Team are to advance our understanding of both the pre-eruption
magnetic field and of the onset mechanism. In order to decide between
competing theories for these processes, we use forward modeling: first
select several best-observed events for detailed study, then perform
the most comprehensive calculations possible for energy build up and
eruption onset using the various theories proposed for these processes,
and finally compare the results with the actual events to determine
which of the theories are most likely to be valid. We present results
from the Team's studies, and discuss prospects for future progress.
---------------------------------------------------------
Title: From Pseudostreamer Jets to Coronal Mass Ejections:
Observations of the Breakout Continuum
Authors: Kumar, Pankaj; Karpen, Judith T.; Antiochos, Spiro K.; Wyper,
Peter F.; DeVore, C. Richard; Lynch, Benjamin J.
2021ApJ...907...41K Altcode: 2020arXiv201107029K
The magnetic breakout model, in which reconnection in the corona leads
to destabilization of a filament channel, explains numerous features
of eruptive solar events, from small-scale jets to global-scale
coronal mass ejections (CMEs). The underlying multipolar topology,
pre-eruption activities, and sequence of magnetic-reconnection onsets
(first breakout, then flare) of many observed fast CMEs/eruptive flares
are fully consistent with the model. Recently, we demonstrated that most
observed coronal-hole jets in fan/spine topologies also are induced by
breakout reconnection at the null point above a filament channel (with
or without a filament). For these two types of eruptions occurring in
similar topologies, the key question is, why do some events generate
jets while others form CMEs? We focused on the initiation of eruptions
in large bright points/small active regions that were located in
coronal holes and clearly exhibited null-point (fan/spine) topologies:
such configurations are referred to as pseudostreamers. We analyzed and
compared Solar Dynamics Observatory/Atmospheric Imaging Assembly, Solar
and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph
Experiment, and Reuven Ramaty High Energy Solar Spectroscopic Imager
observations of three events. Our analysis of the events revealed
two new observable signatures of breakout reconnection prior to the
explosive jet/CME outflows and flare onset: coronal dimming and the
opening up of field lines above the breakout current sheet. Most
key properties were similar among the selected erupting structures,
thereby eliminating region size, photospheric field strength, magnetic
configuration, and pre-eruptive evolution as discriminating factors
between jets and CMEs. We consider the factors that contribute to
the different types of dynamic behavior, and conclude that the main
determining factor is the ratio of the magnetic free energy associated
with the filament channel compared to the energy associated with the
overlying flux inside and outside the pseudostreamer dome.
---------------------------------------------------------
Title: The Effect of 3D Complexity on the Flux Cancellation Model
Authors: Antiochos, S. K.; Dahlin, J.; DeVore, C. R.
2020AGUFMSH034..07A Altcode:
Explosive solar activity ranging from giant CMEs/eruptive flares to tiny
coronal hole jets is all believed to be due to the fast release of the
free magnetic energy stored in the highly stressed field of filament
channels. Understanding the formation and resulting topology of the
filament field, therefore, is absolutely essential for understanding
the physical mechanisms driving solar explosions. For example, ideal
eruption mechanisms such as the kink or torus instability require that
the filament field have a substantial amount of large-scale magnetic
twist. The most widely studied model for the formation of this twist
is so-called flux cancellation, in which the magnetic flux normal to
the photosphere is assumed to reconnect along a polarity inversion
line resulting in a twisted flux tube in the corona. A key feature of
the flux cancellation model is that the reconnection is systematic -
close to 2D in nature, so that a post-reconnection flux rope with a
globally coherent twist forms in the corona. The random motions of the
photosphere, however, are likely to induce complex structure to the
normal flux, in which case the reconnection will be far from systematic
and be fully 3D. We present preliminary calculations using the ARMS
MHD code of fully 3D flux cancellation. As in the previous work, we
start with a 2D-like sheared arcade, but then introduce some random
photospheric motions prior to the cancellation. We discuss the resulting
filament channel topology and compare it to the standard models. The
implications for both theories and observations of explosive solar
activity are discussed. <P />This work was supported in part by the
NASA LWS Program.
---------------------------------------------------------
Title: The Nature of Solar Flare Reconnection
Authors: Dahlin, J.; Antiochos, S. K.; Jiong, Q.; DeVore, C. R.;
Wyper, P. F.
2020AGUFMSH045..04D Altcode:
Solar flares are explosive space weather events that can, in the span of
only a few minutes, release well over 10^32 ergs of energy in the forms
of plasma heating, energetic particles, and bulk motion. Flares are
known to be driven by magnetic reconnection; consequently, determining
the structure and dynamics of flare reconnection is essential for
modeling and eventually predicting the energy release channels of
these events. In particular, the amount of energy that ends up as
energetic particles is believed to be highly dependent on whether the
reconnection is primarily turbulent or primarily laminar. We use the
well-proven adaptive mesh refinement capabilities of the ARMS MHD code
to perform new very-high-resolution simulations of three-dimensional
reconnection in an eruptive flare and compare the results to recent
data. Although flare reconnection is difficult to observe directly
in the corona, highly detailed constraints on its dynamics can be
obtained from observations of the so-called flare ribbons that track
the chromospheric footpoints of newly reconnected field lines. The
analogues of flare ribbons in our simulations are identified by tracking
discontinuous changes in field-line magnetic connectivity due to the
reconnection. We discuss the implications of the simulated ribbon
formation for the nature of flare reconnection. We also determine
how the time-evolving guide field in flares affects the formation of
the ribbons and, hence, the reconnection dynamics. We interpret our
results through an analytical model, and discuss implications for SDO,
IRIS, and GST observations of explosive flare energy release. <P />This
work was supported in part by the SolFER DRIVE Center and by the NASA
LWS Program.
---------------------------------------------------------
Title: Relating the Variability of the Middle Corona to the Structure
of the Slow Solar Wind
Authors: Higginson, A. K.; DeVore, C. R.; Antiochos, S. K.
2020AGUFMSH0300002H Altcode:
The recent revolution in heliospheric measurements, brought about
by NASA's Parker Solar Probe and ESA's Solar Orbiter, has shown that
processes in the middle corona can influence the structure and dynamics
of the solar wind across spatial scales. Understanding the formation
of the young solar wind structures currently being measured by Parker
Solar Probe and Solar Orbiter is now essential. Numerical calculations
have shown that magnetic field dynamics at coronal hole boundaries
in the middle corona, in particular interchange reconnection driven
by photospheric motions, can be responsible for the dynamic release
of structured slow solar wind, including along huge separatrix-web
(S-Web) arcs formed by pseudostreamers. Quantifying the plasma and
magnetic variability along these S-Web arcs is crucial to furthering
our understanding of how coronal magnetic field dynamics can influence
the slow solar wind throughout the heliosphere. Here we present fully
dynamic, 3D numerical calculations of a coronal hole boundary driven
continuously by realistic photospheric motions at its base. We consider
our simulation results within the context of Parker Solar Probe and
Solar Orbiter, and make predictions for the structure and variability
of the young slow solar wind.
---------------------------------------------------------
Title: Decoding the Pre-Eruptive Magnetic Field Configurations of
Coronal Mass Ejections
Authors: Patsourakos, S.; Vourlidas, A.; Török, T.; Kliem, B.;
Antiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou,
G.; Georgoulis, M. K.; Green, L. M.; Leake, J. E.; Moore, R.; Nindos,
A.; Syntelis, P.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J.
2020SSRv..216..131P Altcode: 2020arXiv201010186P
A clear understanding of the nature of the pre-eruptive magnetic
field configurations of Coronal Mass Ejections (CMEs) is required
for understanding and eventually predicting solar eruptions. Only
two, but seemingly disparate, magnetic configurations are considered
viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes
(MFR). They can form via three physical mechanisms (flux emergence,
flux cancellation, helicity condensation). Whether the CME culprit
is an SMA or an MFR, however, has been strongly debated for thirty
years. We formed an International Space Science Institute (ISSI) team to
address and resolve this issue and report the outcome here. We review
the status of the field across modeling and observations, identify
the open and closed issues, compile lists of SMA and MFR observables
to be tested against observations and outline research activities
to close the gaps in our current understanding. We propose that the
combination of multi-viewpoint multi-thermal coronal observations
and multi-height vector magnetic field measurements is the optimal
approach for resolving the issue conclusively. We demonstrate the
approach using MHD simulations and synthetic coronal images.
---------------------------------------------------------
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena in Solar and
Heliospheric Plasmas
Authors: Ji, H.; Karpen, J.; Alt, A.; Antiochos, S.; Baalrud, S.;
Bale, S.; Bellan, P. M.; Begelman, M.; Beresnyak, A.; Bhattacharjee,
A.; Blackman, E. G.; Brennan, D.; Brown, M.; Buechner, J.; Burch, J.;
Cassak, P.; Chen, B.; Chen, L. -J.; Chen, Y.; Chien, A.; Comisso,
L.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.; Dong, C. F.;
Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun, R.; Eyink,
G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.; Fujimoto,
K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo, F.; Hare,
J.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.; Le,
A.; Lebedev, S.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.;
Liu, W.; Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus,
W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sironi, L.; Sitnov, M.; Stanier, A.; Swisdak, M.; TenBarge,
J.; Tharp, T.; Uzdensky, D.; Vaivads, A.; Velli, M.; Vishniac, E.;
Wang, H.; Werner, G.; Xiao, C.; Yamada, M.; Yokoyama, T.; Yoo, J.;
Zenitani, S.; Zweibel, E.
2020arXiv200908779J Altcode:
Magnetic reconnection underlies many explosive phenomena in the
heliosphere and in laboratory plasmas. The new research capabilities in
theory/simulations, observations, and laboratory experiments provide the
opportunity to solve the grand scientific challenges summarized in this
whitepaper. Success will require enhanced and sustained investments
from relevant funding agencies, increased interagency/international
partnerships, and close collaborations of the solar, heliospheric,
and laboratory plasma communities. These investments will deliver
transformative progress in understanding magnetic reconnection and
related explosive phenomena including space weather events.
---------------------------------------------------------
Title: Trigger Shy? Flare-less Active Region Circular Prominence
Eruption
Authors: Mason, E.; Antiochos, S.; Vourlidas, A.
2020SPD....5121001M Altcode:
Prominence eruptions have been studied since the days of Skylab, and
generally fall into two categories based on their locations: quiet Sun
and active regions. Quiescent prominences are generally slow to grow and
take can days to erupt, with or without any evidence of energization
prior to eruption. By contrast, active region prominences generally
erupt on time scales of hours or minutes, and are often accompanied by
powerful flares. This study reports on an observation of an unusual
circular prominence eruption located in an active region which
occurs without any evidence of flaring as a trigger. The prominence
is under the dome surface of a raining null point topology, which was
part of the extended decay phase of the active region designated NOAA
12488/12501. One half of the prominence undergoes a partial eruption,
and the cool plasma subsequently drains onto the side which did not
erupt, followed by a poorly-structured CME observed by SOHO LASCO C2
shortly after the eruption. We analyze both the failed eruption and
secondary CME using SDO AIA, STEREO-A, and SOHO LASCO imagery. The
location of the null-point topology raises critical questions about
the role of open/closed boundaries in eruptive phenomena and CME
structure. The poor structure of the outflowing CME is likely the
result of the destruction of the flux rope through reconnection as
it passes through the null-point structure, and possibly through
additional overlying closed field. The eruption does not show a
trigger, but arcades and ribbons form over the erupted half of the
prominence. Taken together, the failed eruption presents eruption
characteristics of both a CME and a jet, with potential evidence of a
low-energy reconnection mechanism driving failed eruptions in highly
decayed but still topologically compact magnetic fields.
---------------------------------------------------------
Title: Magnetic Origins of Cool Plasma in the Sun's Corona
Authors: Mason, E.; Antiochos, S.; Viall, N.
2020AAS...23610606M Altcode:
Much of solar physics research focuses on two questions: how the
corona's temperature becomes hundreds of times hotter than the surface,
and how the slow solar wind forms. Among the most fascinating phenomena
produced by coronal heating is coronal rain, in which plasma undergoes
rapid cooling (from roughly 10<SUP>6</SUP> to 10<SUP>3</SUP> K),
condenses, and falls to the surface. One proposed rain origin theory,
thermal nonequilibrium (TNE), posits a height restriction in coronal
heating. By studying condensations, physicists hope to better understand
coronal heating. Solar wind is often subdivided into fast and slow
wind. The former originates in coronal hole regions; slow wind's source,
however, is still under debate. One leading theory postulates that
it comes from coronal hole boundaries, where magnetic field lines
frequently reconnect. This research investigates the origins and
dynamics of coronal rain via study of recently-discovered structures
called raining null-point topologies, or RNPTs. RNPTs — the first
identification and characterization of which comprise part of this work
— are decaying active regions situated near coronal hole boundaries,
between 50-150 Mm in height. They are host to long periods of continuous
coronal rain formation, and provide insight into coronal heating, slow
solar wind origins, and coronal dynamics. We focus on identifying and
analyzing RNPTs' observational characteristics. We process and analyze
RNPT data using both the Solar Dynamics Observatory Atmospheric Imaging
Assembly and the Helioseismic and Magnetic Imager. Potential field
source-surface extrapolations that model the magnetic field in the
corona aid in the interpretation of the structures' topology. Results
indicate that RNPTs experience two rain-forming mechanisms, TNE and
interchange reconnection. The interchange reconnection is posited to
power much of the early bursts of coronal rain, which constitutes a
new rain-formation mechanism and allows for plasma from closed loops
to escape into the slow solar wind. Observations also show evidence
of partial condensations, which condense but do not fully cool.
---------------------------------------------------------
Title: High-Resolution Three-Dimensional MHD Simulations of Plasmoid
Formation in Solar Flares
Authors: Dahlin, Joel; Antiochos, Spiro; DeVore, C. Richard
2020EGUGA..2210039D Altcode:
In highly conducting plasmas, reconnecting current sheets are
often unstable to the generation of plasmoids, small-scale magnetic
structures that play an important role in facilitating the rapid
release of magnetic energy and channeling that energy into accelerated
particles. There is ample evidence for plasmoids throughout the
heliosphere, from in situ observations of flux ropes in the solar
wind and planetary magnetospheres to remote-sensing imaging of plasma
'blobs' associated with explosive solar activity such as eruptive
flares and coronal jets. Accurate models for plasmoid formation and
dynamics must capture the large-scale self-organization responsible
for forming the reconnecting current sheet. However, due to the
computational difficulty inherent in the vast separation between
the global and current sheet scales, previous numerical studies have
typically explored configurations with either reduced dimensionality
or pre-formed current sheets. We present new three-dimensional MHD
studies of an eruptive flare in which the formation of the current
sheet and subsequent reconnection and plasmoid formation are captured
within a single simulation. We employ Adaptive Mesh Refinement (AMR)
to selectively resolve fine-scale current sheet dynamics. Reconnection
in the flare current sheet generates many plasmoids that exhibit highly
complex, three-dimensional structure. We show how plasmoid formation and
dynamics evolve through the course of the flare, especially in response
to the weakening of the reconnection "guide field" linked to the global
reduction of magnetic shear. We discuss implications of our results for
particle acceleration and transport in eruptive flares as well as for
observations by Parker Solar Probe and the forthcoming Solar Orbiter.
---------------------------------------------------------
Title: Acceleration of particles in different parts of erupting
coronal mass ejections
Authors: Zharkova, Valentina; Xia, Qian; Dahlin, Joel; Antiochos, Spiro
2020EGUGA..2220181Z Altcode:
We examine particle energisation in CMEs generated via the breakout
mechanism and explore both 2D and 3D MHD configurations. In the
breakout scenario, reconnection at a breakout current sheet (CS)
initiates the flux rope eruption by destabilizing the quasi-static
force balance. Reconnection at the flare CS triggers the fast
acceleration of the CME, which forms flare loops below and triggers
particle acceleration in flares. We present test-particle studies
that focus on two selected times during the impulsive and decay
phases of the eruption and obtain particle energy gains and spatial
distributions. We find that particles accelerated more efficiently
in the flare CS than in the breakout CS even in the presence of large
magnetic islands. The maximum particle energy gain is estimated from the
energization terms based on the guiding-center approximation. Particles
are first accelerated in the CSs (with or without magnetic islands)
where Fermi-type acceleration dominates. Accelerated particles escape
to the interplanetary space along open field lines rather than trapped
in flux ropes, precipitate into the chromosphere along the flare
loops, or become trapped in the flare loop top due to the magnetic
mirror structure. Some trapped particles are re-accelerated, either
via re-injection to the flare CS or through a local betatron-type
acceleration associated with compression of the magnetic field. The
energy gains of particles result in relatively hard energy spectra
during the impulsive phase. During the gradual phase, the relaxation of
the shear in magnetic field reduces the guiding magnetic field in the
flare CS, which leads to a decrease in particle energization efficiency.
---------------------------------------------------------
Title: Observations and Simulations of Reconnecting Current Sheets
in the Solar Corona
Authors: Antiochos, Spiro; Kumar, Pankaj; Jarpen, Judy; Dahlin, Joel
2020EGUGA..22.5597A Altcode:
Jets and mass ejections are ubiquitous features of the Sun's
corona. These explosive dynamics are all believed to be driven by
magnetic reconnection at two types of current sheets that form in the
solar atmosphere: those that form at magnetic null points and separatrix
surfaces, and those, such as the heliospheric current sheet, that form
as a result of a large expansion of a bipolar magnetic field. In our
breakout model, both types of current sheets are essential for the
explosive release of magnetic energy. We report on the first direct
observations of reconnection and island formation in a null-point
current sheet associated with a large coronal jet. The topology and
velocities of the islands are in excellent agreement with our numerical
simulations of coronal jets. We discuss the implications of the
observations and our models for understanding the energetic particles
produced by these events and their release into interplanetary space,
as well as the implications for observations by Solar Orbiter and the
Parker Solar Probe.This work was supported by the NASA Living With a
Star Program.
---------------------------------------------------------
Title: Particle Acceleration and Transport during 3D CME Eruptions
Authors: Xia, Qian; Dahlin, Joel T.; Zharkova, Valentina; Antiochos,
Spiro K.
2020ApJ...894...89X Altcode:
We calculate particle acceleration during coronal mass ejection
(CME) eruptions using combined magnetohydrodynamic and test-particle
models. The 2.5D/3D CMEs are generated via the breakout mechanism. In
this scenario a reconnection at the "breakout" current sheet (CS)
above the flux rope initiates the CME eruption by destabilizing a
quasi-static force balance. Reconnection at the flare CS below the
erupting flux rope drives the fast acceleration of the CME, which
forms flare loops below and produces the energetic particles observed
in flares. For test-particle simulations, two times are selected during
the impulsive and decay phases of the eruption. Particles are revealed
to be accelerated more efficiently in the flare CS rather than in the
breakout CS even in the presence of large magnetic islands. Particles
are first accelerated in the CSs (with or without magnetic islands)
by the reconnection electric field mainly through particle curvature
drift. We find, as expected, that accelerated particles precipitate into
the chromosphere, become trapped in the loop top by magnetic mirrors,
or escape to interplanetary space along open field lines. Some trapped
particles are reaccelerated, either via reinjection to the flare CS or
through a local Betatron-type acceleration associated with compression
of the magnetic field. The energetic particles produce relatively hard
energy spectra during the impulsive phase. During the gradual phase,
the relaxation of magnetic field shear reduces the guiding field
in the flare CS, which leads to a decrease in particle energization
efficiency. Important implications of our results for observations of
particle acceleration in the solar coronal jets are also discussed.
---------------------------------------------------------
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena throughout
the Universe
Authors: Ji, H.; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo,
F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.;
Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.;
Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus, W. H.;
Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.;
Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.;
Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
2020arXiv200400079J Altcode:
This white paper summarizes major scientific challenges and
opportunities in understanding magnetic reconnection and related
explosive phenomena as a fundamental plasma process.
---------------------------------------------------------
Title: Reconnection-Driven Energy Release in the Solar Corona
Authors: Antiochos, Spiro
2020APS..DPPP10002A Altcode:
The Sun's corona is characterized by bursts of energy release that are
most strikingly observed as intense X-Ray solar flares. The underlying
origin for this activity is that magnetic free energy builds up and
is released impulsively to the plasma in the form of heating, mass
motions, and/or particle acceleration. We present high-resolution
observations from NASA/ESA/JAXA space missions showing that the
energy buildup process appears to be similar for flaring activity
ranging across orders of magnitude in scale and energy. Furthermore,
the observations demonstrate conclusively that magnetic reconnection is
the energy release process. We also present very recent MHD numerical
simulations of solar flares that include self-consistently both the
energy buildup and explosive release. Our models show that current
sheet formation leading to reconnection and energy release occurs
almost continuously in the corona, but explosive energy release occurs
only when there is strong feedback between the reconnection and the
global ideal evolution. We discuss the mechanism for flare reconnection
onset and its 3D nature. Capturing accurately the multiscale feedback
inherent in flare reconnection remains as the greatest challenge to
understanding and eventually predicting these critically important space
weather events. <P />This work was supported by the NASA LWS Program.
---------------------------------------------------------
Title: From Jets to CMEs: Observations of the Breakout Continuum
Authors: Karpen, J. T.; Kumar, P.; Antiochos, S. K.; Wyper, P. F.;
DeVore, C. R.
2019AGUFMSH43D3354K Altcode:
The magnetic breakout model can explain a variety of eruptive solar
events, from jets to coronal mass ejections (CMEs). The breakout
model is consistent with many observed fast CMEs/eruptive flares, in
terms of the underlying multipolar topology, pre-eruption activities,
and sequence of reconnection onsets. We have also demonstrated that
most observed coronal-hole jets in fan-spine topologies are produced
by breakout and flare reconnection above a filament channel (with or
without a filament). For eruptions occurring in such topologies, the
key question is, why are some events jets while others form slow or fast
CMEs? We have analyzed SDO/AIA, LASCO, and RHESSI observations focusing
on the initiation of CMEs in large bright points (small active regions)
in coronal holes with clear fan-spine topologies. Our analysis revealed
pre-eruptive evidence for slow breakout reconnection before the onset
of jets, slow CMEs, and fast CMEs from these ARs. We find that this
continuum of activity is consistent with the breakout model of solar
eruptions, and explore the factors contributing to the different forms
of dynamic behavior.
---------------------------------------------------------
Title: First Detection of Plasmoids from Breakout Reconnection
Authors: Kumar, P.; Karpen, J.; Antiochos, S. K.; Wyper, P. F.;
DeVore, C. R.
2019AGUFMSH44A..08K Altcode:
Transient collimated plasma ejections (jets) occur frequently throughout
the solar corona, in active regions, quiet Sun, and coronal holes. Our
previous studies demonstrated that the magnetic breakout model explains
the triggering and evolution of these jets over a wide range of scales,
through detailed comparisons between our numerical simulations and
high-resolution observations. Here we report direct observations
of breakout reconnection during a small eruptive flare accompanied
by a filament eruption in the fan-spine topology of an embedded
bipole. Breakout reconnection operated in two distinct phases in this
event. The first narrow jet was launched by magnetic reconnection at
the breakout null without significant flare reconnection or a filament
eruption. In contrast, the second jet and release of cool filament
plasma were triggered by explosive breakout reconnection when the
leading edge of the rising flux rope formed by flare reconnection
beneath the filament encountered the preexisting breakout current
sheet. We observed plasma heating in the flare arcade and at the top of
the flux rope during the latter episode of breakout reconnection. For
the first time, we detected the formation and evolution of multiple
plasmoids with bidirectional flows in the breakout current sheet
originating at a deformed 3D null point. These observations provide
evidence for both models: the resistive kink for the first jet, and
the breakout model for the second explosive jet with filament eruption.
---------------------------------------------------------
Title: Determining the Transport of Magnetic Helicity in the Sun's
Atmosphere
Authors: Antiochos, S. K.; Schuck, P. W.
2019AGUFMSH41B..07A Altcode:
A critically important factor determining solar coronal activity is the
constraint of magnetic helicity conservation. Direct measurement of the
magnetic helicity in the coronal volume is difficult, but its value may
be estimated from measurements of the helicity transport rates through
the photosphere. We examine this transport for a topologically open
system such as the corona, in which the magnetic field has a nonzero
normal component at the boundaries, and derive a new formula for the
helicity transport rate at the boundaries. In addition, we derive new
expressions for helicity transport due to flux emergence/submergence
versus photospheric horizontal motions. The key new feature of our
formulas is that they are manifestly gauge invariant. We discuss the
physical interpretation of our results and their implications for using
photospheric vector magnetic and velocity field measurements to derive
the solar coronal helicity, which can then be used to constrain and
drive models for coronal activity. <P />This work was supported by
the NASA LWS Program.
---------------------------------------------------------
Title: Erratum: “The Role of Magnetic Helicity in Coronal Heating”
(<A href="https://doi.org/10.3847/1538-4357/ab3afd">2019, ApJ,
883, 26</A>)
Authors: Knizhnik, K. J.; Antiochos, S. K.; Klimchuk, J. A.; DeVore,
C. R.
2019ApJ...887..270K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simulations of Thermal Nonequilibrium in Raining Null-Point
Topologies
Authors: Antiochos, S. K.; Mason, E. I.; Viall, N. M.
2019AGUFMSH53B3381A Altcode:
Coronal heating and the origins of slow solar wind remain central
open questions of solar physics. The recent discovery of raining
null-point topologies allows study of regions that hold implications
for both questions. We present observations from SDO AIA that show
persistent coronal condensations in null-point topologies formed by
decaying active regions located near coronal hole boundaries. Coronal
rain - catastrophically-cooled plasma precipitating along flux tubes
- can be used as a tracer of several physical processes to provide
insight into local heating and cooling dynamics. The rain forms in
two observationally-distinct ways: along the lower spine and null,
and within the closed loops under the fan surface. The former is
attributed to interchange reconnection, while the latter is due
to thermal nonequilibrium (TNE). TNE is caused by height-dependent
footpoint heating, which creates a runaway cooling affect far from
the loop base and triggers condensation. Using the one-dimensional
HYDrodynamic and RADiation solver code (HYDRAD, Bradshaw & Mason
2003), we model asymmetric flux tubes with a large expansion factor
where the loop apex occurs near the null point. A broad parameter study
of heating scale heights and heating rates show the ranges within which
rain could occur, and point to highly restricted coronal heating scale
heights in the decaying active regions. This study provides predictions
for Parker Solar Probe and Solar Orbiter observations.
---------------------------------------------------------
Title: Numerical simulation of helical jets at active region
peripheries
Authors: Wyper, Peter F.; DeVore, C. Richard; Antiochos, Spiro K.
2019MNRAS.490.3679W Altcode: 2019MNRAS.tmp.2312W; 2019arXiv190909423W
Coronal jets are observed above minority-polarity intrusions throughout
the solar corona. Some of the most energetic ones occur on the periphery
of active regions where the magnetic field is strongly inclined. These
jets exhibit a non-radial propagation in the low corona as they
follow the inclined field, and often have a broad, helical shape. We
present a three-dimensional magnetohydrodynamic simulation of such
an active-region-periphery helical jet. We consider an initially
potential field with a bipolar flux distribution embedded in a highly
inclined magnetic field, representative of the field nearby an active
region. The flux of the minority polarity sits below a bald-patch
separatrix initially. Surface motions are used to inject free energy
into the closed field beneath the separatrix, forming a sigmoidal flux
rope that eventually erupts producing a helical jet. We find that a
null point replaces the bald patch early in the evolution and that the
eruption results from a combination of magnetic breakout and an ideal
kinking of the erupting flux rope. We discuss how the two mechanisms
are coupled, and compare our results with previous simulations of
coronal-hole jets. This comparison supports the hypothesis that the
generic mechanism for all coronal jets is a coupling between breakout
reconnection and an ideal instability. We further show that our results
are in good qualitative and quantitative agreement with observations
of active-region-periphery jets.
---------------------------------------------------------
Title: Estimating Coronal Helicity Injection from Photospheric
Measurements
Authors: Schuck, P. W.; Antiochos, S. K.
2019AGUFMSH43E3390S Altcode:
Magnetic helicity is one of the most important factors determining
solar coronal activity. Direct measurements of the helicity in the
corona are difficult, but its value may be estimated from measurements
of the helicity transport rates through the photosphere. However, the
accurate measurements of the electric and magnetic fields necessary to
compute helicity transport are usually available only over a limited
field of view of the photosphere corresponding to only one boundary
of a coronal volume. This adds additional complexity to computing
helicity transport into the corona which generally requires a closed
surface. We discuss the issues that must be addressed to accurately
compute helicity transport across the photosphere using a recently
developed manifestly gauge invariant helicity transport formula. We
present explicit helicity transport calculations for several coronal
field configurations in a Cartesian box subject to a driven boundary
on the bottom face corresponding to the photosphere. We discuss the
extension of these calculations to photospheric vector magnetic and
velocity field measurements in spherical geometry to derive the solar
coronal helicity, which can then be used to constrain and drive models
for coronal activity.
---------------------------------------------------------
Title: Three-Dimensional Numerical Studies of Plasmoid Formation in
Eruptive Flares
Authors: Dahlin, J.; Antiochos, S. K.; DeVore, C. R.
2019AGUFMSH12B..04D Altcode:
Solar flares are among the most energetic phenomena in the solar
system, notable in particular for generating non-thermal particles
that may comprise a large fraction of the released energy. In an
eruptive flare, the energy release process is generally understood
to be magnetic reconnection in a current sheet beneath an erupting
flux rope. In a highly conductive plasma such as the corona, current
sheets are unstable to the generation of plasmoids, small-scale
magnetic structures that play an important role in facilitating the
rapid release of magnetic energy and channeling that energy into
accelerated particles. Due to the computational difficulty inherent
in the vast separation in spatial scales between the global eruption
dynamics and the current sheet dissipation, previous numerical studies
have largely focused on cases with either reduced dimensionality or
highly idealized initial configurations. We present new numerical
studies of plasmoid formation in three dimensional MHD calculations of
a self-consistent eruptive flare. Using adaptive mesh refinement (AMR),
our calculations simultaneously capture both the global flare evolution
and the fine-scale generation and evolution of plasmoids in the thin
flare current sheet. We show how the evolution of plasmoid structure
of generated plasmoids in different phases of the flare and discuss
observational signatures and implications for particle acceleration.
---------------------------------------------------------
Title: Modeling the onset of solar eruptions in active regions
Authors: Leake, J. E.; Linton, M.; Antiochos, S. K.; Schuck, P. W.
2019AGUFMSH41F3320L Altcode:
We present results of a numerical investigation into the initiation
of solar coronal eruptions. Using numerical magnetohydrodynamic
(MHD) models, we investigate the role of magnetic reconnection between
emerging magnetic flux at the solar photosphere and pre-existing coronal
flux in the eruptivity likelihood of emerging solar active regions. Our
investigation covers a large range of spatial and temporal scales. We
analyze the photospheric and coronal magnetic field to investigate
how the relevant mechanisms that drive solar coronal eruptions can be
detected by current and future observations.
---------------------------------------------------------
Title: Relating the Structure and Dynamics of the Corona to the
Variability Ofthe Slow Solar Wind
Authors: Higginson, A. K.; Antiochos, S. K.; DeVore, C. R.
2019AGUFMSH11C3405H Altcode:
Recent coronagraph and in situ observations have shown that the slow
solar wind includes highly structured and dynamic outflow across
spatial scales, most likely due to magnetic reconnection processes in
the solar corona. In light of the recently launched Parker Solar Probe
and anticipated Solar Orbiter missions, understanding this temporal
and spatial variability has become essential. Numerical calculations
have shown that magnetic field dynamics at coronal hole boundaries,
in particular interchange reconnection driven by photospheric motions,
can be responsible for the dynamic release of structured slow solar
wind, including along huge separatrix-web (S-Web) arcs formed by
pseudostreamers. Quantifying the slow solar wind variability along these
S-Web arcs is crucial to furthering our understanding of how coronal
magnetic field dynamics can influence the plasma and magnetic field
throughout the heliosphere. Here we present for the first time, fully
dynamic, 3D numerical calculations of an S-Web arc driven continuously
by realistic photospheric motions at its base. We present an analysis
of the resulting magnetic field dynamics and subsequent plasma release,
both near and far from the heliospheric current sheet. We consider our
simulation results within the context of future Parker Solar Probe and
Solar Orbiter observations and make predictions for the structure and
variability of the slow solar wind.
---------------------------------------------------------
Title: First Detection of Plasmoids from Breakout Reconnection on
the Sun
Authors: Kumar, Pankaj; Karpen, Judith T.; Antiochos, Spiro K.; Wyper,
Peter F.; DeVore, C. Richard
2019ApJ...885L..15K Altcode: 2019arXiv190906637K
Transient collimated plasma ejections (jets) occur frequently
throughout the solar corona, in active regions, quiet Sun, and coronal
holes. Although magnetic reconnection is generally agreed to be the
mechanism of energy release in jets, the factors that dictate the
location and rate of reconnection remain unclear. Our previous studies
demonstrated that the magnetic breakout model explains the triggering
and evolution of most jets over a wide range of scales, through detailed
comparisons between our numerical simulations and high-resolution
observations. An alternative explanation, the resistive-kink model,
invokes breakout reconnection without forming and explosively expelling
a flux rope. Here we report direct observations of breakout reconnection
and plasmoid formation during two jets in the fan-spine topology of
an embedded bipole. For the first time, we observed the formation
and evolution of multiple small plasmoids with bidirectional flows
associated with fast reconnection in 3D breakout current sheets (BCSs)
in the solar corona. The first narrow jet was launched by reconnection
at the BCS originating at the deformed 3D null, without significant
flare reconnection or a filament eruption. In contrast, the second jet
and release of cool filament plasma were triggered by explosive breakout
reconnection when the leading edge of the rising flux rope formed by
flare reconnection beneath the filament encountered the preexisting
BCS. These observations solidly support both reconnection-driven jet
models: the resistive kink for the first jet, and the breakout model
for the second explosive jet with a filament eruption.
---------------------------------------------------------
Title: Escape of Flare-accelerated Particles in Solar Eruptive Events
Authors: Masson, S.; Antiochos, S. K.; DeVore, C. R.
2019ApJ...884..143M Altcode: 2019arXiv190913578M
Impulsive solar energetic particle events are widely believed to be due
to the prompt escape into the interplanetary medium of flare-accelerated
particles produced by solar eruptive events. According to the standard
model for such events, however, particles accelerated by the flare
reconnection should remain trapped in the flux rope comprising the
coronal mass ejection. The particles should reach the Earth only
much later, along with the bulk ejecta. To resolve this paradox,
we have extended our previous axisymmetric model for the escape
of flare-accelerated particles to fully three-dimensional (3D)
geometries. We report the results of magnetohydrodynamic simulations
of a coronal system that consists of a bipolar active region embedded
in a background global dipole field structured by solar wind. Our
simulations show that multiple magnetic reconnection episodes occur
prior to and during the coronal mass ejection (CME) eruption and its
interplanetary propagation. In addition to the episodes that build up
the flux rope, reconnection between the open field and the CME couples
the closed corona to the open interplanetary field. Flare-accelerated
particles initially trapped in the CME thereby gain access to the open
interplanetary field along a trail blazed by magnetic reconnection. A
key difference between these 3D results and our previous calculations
is that the interchange reconnection allows accelerated particles to
escape from deep within the CME flux rope. We estimate the spatial
extent of the particle-escape channels. The relative timings between
flare acceleration and release of the energetic particles through
CME/open-field coupling are also determined. All our results compare
favorably with observations.
---------------------------------------------------------
Title: The Role of Magnetic Helicity in Coronal Heating
Authors: Knizhnik, K. J.; Antiochos, S. K.; Klimchuk, J. A.; DeVore,
C. R.
2019ApJ...883...26K Altcode: 2019arXiv190903768K
One of the greatest challenges in solar physics is understanding
the heating of the Sun’s corona. Most theories for coronal heating
postulate that free energy in the form of magnetic twist/stress is
injected by the photosphere into the corona where the free energy is
converted into heat either through reconnection or wave dissipation. The
magnetic helicity associated with the twist/stress, however, is expected
to be conserved and appear in the corona. In previous works, we showed
that the helicity associated with the small-scale twists undergoes
an inverse cascade via stochastic reconnection in the corona and
ends up as the observed large-scale shear of filament channels. Our
“helicity condensation” model accounts for both the formation
of filament channels and the observed smooth, laminar structure of
coronal loops. In this paper, we demonstrate, using helicity- and
energy-conserving numerical simulations of a coronal system driven
by photospheric motions, that the model also provides a natural
mechanism for heating the corona. We show that the heat generated by
the reconnection responsible for the helicity condensation process is
sufficient to account for the observed coronal heating. We study the
role that helicity injection plays in determining coronal heating and
find that, crucially, the heating rate is only weakly dependent on the
net helicity preference of the photospheric driving. Our calculations
demonstrate that motions with 100% helicity preference are least
efficient at heating the corona; those with 0% preference are most
efficient. We discuss the physical origins of this result and its
implications for the observed corona.
---------------------------------------------------------
Title: Determining the Transport of Magnetic Helicity and Free Energy
in the Sun’s Atmosphere
Authors: Schuck, Peter W.; Antiochos, Spiro K.
2019ApJ...882..151S Altcode: 2019arXiv190710598S
The most important factors determining solar coronal activity are
believed to be the availability of magnetic free energy and the
constraint of magnetic helicity conservation. Direct measurements of the
helicity and magnetic free energy in the coronal volume are difficult,
but their values may be estimated from measurements of the helicity and
free energy transport rates through the photosphere. We examine these
transport rates for a topologically open system such as the corona,
in which the magnetic fields have a nonzero normal component at the
boundaries, and derive a new formula for the helicity transport
rate at the boundaries. In addition, we derive new expressions
for helicity transport due to flux emergence/submergence versus
photospheric horizontal motions. The key feature of our formulas is
that they are manifestly gauge invariant. Our results are somewhat
counterintuitive in that only the lamellar electric field produced by
the surface potential transports helicity across boundaries, and the
solenoidal electric field produced by a surface stream function does
not contribute to the helicity transport. We discuss the physical
interpretation of this result. Furthermore, we derive an expression
for the free energy transport rate and show that a necessary condition
for free energy transport across a boundary is the presence of a closed
magnetic field at the surface, indicating that there are current systems
within the volume. We discuss the implications of these results for
using photospheric vector magnetic and velocity field measurements to
derive the solar coronal helicity and magnetic free energy, which can
then be used to constrain and drive models for coronal activity.
---------------------------------------------------------
Title: A Model for Energy Buildup and Eruption Onset in Coronal
Mass Ejections
Authors: Dahlin, J. T.; Antiochos, S. K.; DeVore, C. R.
2019ApJ...879...96D Altcode: 2019arXiv190513218D
Coronal mass ejections (CMEs) and eruptive flares (EFs) are the most
energetic explosions in the solar system. Their underlying origin is
the free energy that builds up slowly in the sheared magnetic field of a
filament channel. We report the first end-to-end numerical simulation of
a CME/EF, from zero-free-energy initial state through filament channel
formation to violent eruption, driven solely by the magnetic-helicity
condensation process. Helicity is the topological measure of linkages
between magnetic flux systems, and is conserved in the corona, building
up inexorably until it is ejected into interplanetary space. Numerous
investigations have demonstrated that helicity injected by small-scale
vortical motions, such as those observed in the photosphere, undergoes
an inverse cascade from small scales to large, “condensing” at
magnetic-polarity boundaries. Our new results verify that this process
forms a filament channel within a compact bipolar region embedded in
a background dipole field, and show for the first time that a fast CME
eventually occurs via the magnetic-breakout mechanism. We further show
that the trigger for explosive eruption is reconnection onset in the
flare current sheet that develops above the polarity inversion line:
this reconnection forms flare loops below the sheet and a CME flux
rope above, and initiates high-speed outward flow of the CME. Our
findings have important implications for magnetic self-organization
and explosive behavior in solar and other astrophysical plasmas,
as well as for understanding and predicting explosive solar activity.
---------------------------------------------------------
Title: New Insights into the 10 September 2017 Mega-Eruption
Authors: Karpen, Judith T.; Kumar, Pankaj; Antiochos, Spiro K.; Gary,
Dale E.; Dahlin, Joel
2019AAS...23431702K Altcode:
The X8.2 flare on 10 September 2017 was part of a well-observed,
extremely energetic solar eruption that has been intensely studied. Much
attention has been devoted to the striking appearance and persistence
of a current sheet behind the explosively accelerating CME. We focus
here on the unusual appearance of prominent emission features on either
side of the flare arcade, which were detected in microwave emissions by
NJIT's EOVSA before the peak impulsive phase. Our analysis combines the
results of 3D numerical simulations with observations by SDO, EOVSA,
and IRIS to decipher the underlying magnetic structure of the erupting
region and the initiation mechanism. The event originated in a complex
active region with a large-scale quadrupolar magnetic field punctuated
by many intrusions of minority polarity. We interpret the observed
microwave features as evidence of electron acceleration due to breakout
reconnection, and present compelling evidence for this conclusion.
---------------------------------------------------------
Title: Multiscale Helicity Condensation and Filament Channel Formation
Authors: DeVore, C. Richard; Antiochos, Spiro K.
2019AAS...23410602D Altcode:
Solar eruptive events ranging from small-scale jets to global-scale
coronal mass ejections are associated with filaments and their
underlying filament-channel magnetic structures. In previous work,
we have demonstrated that sheared-arcade filament channels can be
formed via the process of helicity condensation. Magnetic twist,
representing helicity, is transported across unipolar regions
in response to reconnection induced by small-scale, close-packed,
surface flows (e.g., the granulation or supergranulation) that possess
a vortical component of motion. The small-scale twists induced by
the flows inverse-cascade to the largest scales and boundaries of the
unipolar regions, i.e., to the polarity inversion lines (PILs). If the
flows have a preferred sense of rotation, clockwise or counter, they
inject a net helicity into the magnetic field, as well as transport
it so that it condenses into filament channels at the PILs. We now
have examined how the helicity condensation mechanism is modified
when the small-scale flows have no preferred sense of rotation, and
large-scale flows are solely responsible for introducing net helicity
into the corona. On the Sun, differential rotation is well-known to be a
prodigious generator of helicity. Our new simulation results show that
a large-scale shear flow produces structure with large-scale magnetic
twist, but this twist concentrates near the PILs to form filament
channels only when small-scale vortical flows also are present. We
conclude that the key role of the vortical flows is to transport the
injected net helicity and condense it at the PILs. The source of the
net helicity, on the other hand, can be flows at any scale. We refer
to this extended concept as multiscale helicity condensation: it is
a more general, hence more robust, explanation for the formation of
filament channels on the Sun. Our work was supported by NASA's H-ISFM,
H-SR, and LWS TR&T programs.
---------------------------------------------------------
Title: STITCH: A New Method for Generating Filament Channels and
Driving Solar Eruptions
Authors: Dahlin, Joel; Antiochos, Spiro K.; DeVore, C. Richard
2019AAS...23431703D Altcode:
We present a new formalism for generating eruptive magnetic structure
in MHD simulations of the solar corona. STITCH (STatistical InjecTion
of Condensed Helicity) derives from the helicity condensation model
of Antiochos (2013). In the helicity condensation model, small-scale
photospheric convection drives a reconnection-mediated inverse cascade
that concentrates energy and structure to form highly sheared filament
channels.Our recent 3D MHD calculations using more than 100 cyclonic
surface flows have demonstrated explosive solar eruptions driven by
helicity condensation. However, this manner of direct calculation of
small-scale flows and the resulting reconnection is prohibitively
expensive for use in data-driven event modeling or long-duration
magnetofrictional studies of the global solar magnetic field (Mackay
et al. 2014, 2018). Our new method, STITCH, directly injects the
tangential field (shear) resulting from statistically averaged,
sub-grid helicity condensation. Numerically, this represents a source
term in the induction equation, consisting of the curl of the vertical
field times a factor proportional to the cyclonic specific angular
momentum - a single free parameter. The new approach reproduces prior
calculations with small-scale flows at greatly reduced computational
expense. We present a variety of simulations with complex initial flux
distributions to demonstrate the flexibility of the model. STITCH is
both simple to implement and computationally inexpensive, making it
a useful new technique for event-based and data-driven modeling of
solar eruptions. This work was supported by the NASA LWS, NPP, H-SR
and ISFM programs.
---------------------------------------------------------
Title: Observations and Modelling of Condensation Formation at
Coronal Hole Boundaries
Authors: Mason, Emily; Antiochos, Spiro; Viall, Nicholeen; Macneice,
Peter; Bradshaw, Stephen
2019shin.confE..40M Altcode:
One of the primary mechanisms suggested for slow solar wind formation
is interchange reconnection. This tool for leveraging closed-loop
plasma into the heliosphere is believed to occur ubiquitously in the
corona, but has few definitive observational characteristics. We
present recent observations from SDO AIA and STEREO-A of frequent
condensations in small null-point topologies. These structures, termed
raining null-point topologies, result from decayed active regions
bordering on or entirely within coronal holes. These structures may have
unique S-Web fingerprints, aiding slow wind detection and prediction
capability. Our observations clearly show condensation formation at
the open-closed boundary, where interchange reconnection is widely
believed to occur. The condensations take the form of coronal rain,
catastrophically cooled plasma that is easy to track using remote
observations; their formation on apparently newly-opened coronal flux
tubes gives interchange reconnection a hallmark signature. We will also
present 1D hydrodynamic models for how these condensations can form
via interchange reconnection and thermal nonequilibrium. Due to the
nature of these null-point topologies and their proximity to coronal
holes, they share characteristics common to open-closed boundaries,
pseudostreamers, and active regions. Coordination between Parker Solar
Probe, DKIST, Solar Orbiter, etc. would provide a deeper understanding
of these ideal targets, which encompass such useful signatures for
slow wind investigation.
---------------------------------------------------------
Title: A New Method for Generating Filament Channels and Driving
Solar Eruptions
Authors: Dahlin, Joel T.; Antiochos, Spiro K.; DeVore, C. Richard
2019shin.confE.209D Altcode:
We present a new formalism for generating eruptive magnetic structure
in MHD simulations of the solar corona. STITCH (STatistical InjecTion
of Condensed Helicity) derives from the helicity condensation model
of Antiochos (2013). In the helicity condensation model, small-scale
photospheric convection drives a reconnection-mediated inverse cascade
that concentrates energy and structure to form highly sheared filament
channels. Our recent 3D MHD calculations using more than 100 cyclonic
surface flows have demonstrated explosive solar eruptions driven by
helicity condensation. However, this manner of direct calculation of
small-scale flows and the resulting reconnection is prohibitively
expensive for use in data-driven event modeling or long-duration
magnetofrictional studies of the global solar magnetic field (Mackay
et al. 2014, 2018). Our new method, STITCH, directly injects the
tangential field (shear) resulting from statistically averaged,
sub-grid helicity condensation. Numerically, this represents a source
term in the induction equation, consisting of the curl of the vertical
field times a factor proportional to the cyclonic specific angular
momentum - a single free parameter. The new approach reproduces prior
calculations with small-scale flows at greatly reduced computational
expense. We present a variety of simulations with complex initial flux
distributions to demonstrate the flexibility of the model. STITCH is
both simple to implement and computationally inexpensive, making it
a useful new technique for event-based and data-driven modeling of
solar eruptions. This work was supported by the NASA LWS, NPP, H-SR
and ISFM programs.
---------------------------------------------------------
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena throughout
the Universe
Authors: Ji, Hantao; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.;
Guo, F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte,
J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian,
A.; Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu,
W.; Longcope, D.; Louriero, N.; Lu, Q. -M.; Ma, Z. -W.; Matthaeus,
W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
2019BAAS...51c...5J Altcode: 2019astro2020T...5J
This is a group white paper of 100 authors (each with explicit
permission via email) from 51 institutions on the topic of magnetic
reconnection which is relevant to 6 thematic areas. Grand challenges
and research opportunities are described in observations, numerical
modeling and laboratory experiments in the upcoming decade.
---------------------------------------------------------
Title: Observations of Solar Coronal Rain in Null Point Topologies
Authors: Mason, E. I.; Antiochos, Spiro K.; Viall, Nicholeen M.
2019ApJ...874L..33M Altcode: 2019arXiv190408982M
Coronal rain is the well-known phenomenon in which hot plasma high in
the Sun’s corona undergoes rapid cooling (from ∼10<SUP>6</SUP> to
<10<SUP>4</SUP> K), condenses, and falls to the surface. Coronal rain
appears frequently in active region coronal loops and is very common
in post-flare loops. This Letter presents discovery observations,
which show that coronal rain is ubiquitous in the commonly occurring
coronal magnetic topology of a large (∼100 Mm scale) embedded bipole
very near a coronal hole boundary. Our observed structures formed when
the photospheric decay of active-region-leading-sunspots resulted in a
large parasitic polarity embedded in a background unipolar region. We
observe coronal rain to appear within the legs of closed loops well
under the fan surface, as well as preferentially near separatrices
of the resulting coronal topology: the spine lines, null point, and
fan surface. We analyze three events using SDO Atmospheric Imaging
Assembly observations in the 304, 171, and 211 Å channels, as well
as SDO Helioseismic and Magnetic Imager magnetograms. The frequency of
rain formation and the ease with which it is observed strongly suggests
that this phenomenon is generally present in null point topologies of
this size scale. We argue that these rain events could be explained
by the classic process of thermal nonequilibrium or via interchange
reconnection at the null; it is also possible that both mechanisms
are present. Further studies with higher spatial resolution data and
MHD simulations will be required to determine the exact mechanism(s).
---------------------------------------------------------
Title: Multiwavelength Study of Equatorial Coronal-hole Jets
Authors: Kumar, Pankaj; Karpen, Judith T.; Antiochos, Spiro K.; Wyper,
Peter F.; DeVore, C. Richard; DeForest, Craig E.
2019ApJ...873...93K Altcode: 2019arXiv190200922K
Jets (transient/collimated plasma ejections) occur frequently
throughout the solar corona and contribute mass/energy to the corona
and solar wind. By combining numerical simulations and high-resolution
observations, we have made substantial progress recently on determining
the energy buildup and release processes in these jets. Here we
describe a study of 27 equatorial coronal-hole jets using Solar Dynamics
Observatory/Atmospheric Imaging Assembly and Helioseismic and Magnetic
Imager observations on 2013 June 27-28 and 2014 January 8-10. Out of
27 jets, 18 (67%) are associated with mini-filament ejections; the
other nine (33%) do not show mini-filament eruptions but do exhibit
mini-flare arcades and other eruptive signatures. This indicates that
every jet in our sample involved a filament-channel eruption. From
the complete set of events, six jets (22%) are apparently associated
with tiny flux-cancellation events at the polarity inversion line, and
two jets (7%) are associated with sympathetic eruptions of filaments
from neighboring bright points. Potential-field extrapolations of
the source-region photospheric magnetic fields reveal that all jets
originated in the fan-spine topology of an embedded bipole associated
with an extreme ultraviolet coronal bright point. Hence, all our
jets are in agreement with the breakout model of solar eruptions. We
present selected examples and discuss the implications for the jet
energy buildup and initiation mechanisms.
---------------------------------------------------------
Title: Magnetic Helicity Condensation and the Solar Cycle
Authors: Mackay, Duncan H.; DeVore, C. Richard; Antiochos, Spiro K.;
Yeates, Anthony R.
2018ApJ...869...62M Altcode:
Solar filaments exhibit a global chirality pattern where
dextral/sinistral filaments, corresponding to negative/positive magnetic
helicity, are dominant in the northern/southern hemisphere. This pattern
is opposite to the sign of magnetic helicity injected by differential
rotation along east-west oriented polarity inversion lines, posing
a major conundrum for solar physics. A resolution of this problem is
offered by the magnetic helicity-condensation model of Antiochos. To
investigate the global consequences of helicity condensation for the
hemispheric chirality pattern, we apply a temporally and spatially
averaged statistical approximation of helicity condensation. Realistic
magnetic field configurations in both the rising and declining phases of
the solar cycle are simulated. For the helicity-condensation process,
we assume convective cells consisting of positive/negative vorticities
in the northern/southern hemisphere that inject negative/positive
helicity. The magnitude of the vorticity is varied as a free parameter,
corresponding to different rates of helicity injection. To reproduce the
observed percentages of dominant and minority filament chiralities, we
find that a vorticity of magnitude 2.5 × 10<SUP>-6</SUP> s<SUP>-1</SUP>
is required. This rate, however, is insufficient to produce the
observed unimodal profile of chirality with latitude. To achieve this, a
vorticity of at least 5 × 10<SUP>-6</SUP> s<SUP>-1</SUP> is needed. Our
results place a lower limit on the small-scale helicity injection
required to dominate differential rotation and reproduce the observed
hemispheric pattern. Future studies should aim to establish whether the
helicity injection rate due to convective flows and/or flux emergence
across all latitudes of the Sun is consistent with our results.
---------------------------------------------------------
Title: The role of small-scale photospheric motions in coronal
magnetic energy buildup and explosive release
Authors: Dahlin, Joel; Antiochos, Spiro; DeVore, C. Richard
2018csc..confE..62D Altcode:
CMEs/eruptive flares are spectacular examples of explosive solar
activity resulting from magnetic self-organization in the corona. Recent
theory and modeling studies have demonstrated a mechanism by which
small-scale stochastic flows (e.g., photospheric convection) trigger an
inverse cascade that concentrates coronal magnetic structure at polarity
inversion lines to form highly sheared filament channels. We report on
new 3D MHD simulations of an eruptive flare driven by this process of
'helicity condensation'. Energy buildup occurs in the form of a sheared
arcade that explosively erupts via magnetic breakout. Interestingly,
the magnetic shear above the PIL undergoes a three-phase evolution: an
initial increase in response to the driving followed by a decrease as
the magnetic structure expands outward, concluding with a sharp increase
upon the onset of flare reconnection and fast downflows. We discuss
implications of our results for SDO observations of CMEs/eruptive
flares. Our simulations are especially relevant to the many SDO
observations of eruptions from circular filament channels. We also
discuss future opportunities for data-driven modeling of the magnetic
energy build up leading to explosive solar activity, and for possible
application to space weather prediction. This work was supported by
the NASA LWS, H-SR and ISFM programs.
---------------------------------------------------------
Title: A Model for Coronal Hole Bright Points and Jets Due to Moving
Magnetic Elements
Authors: Wyper, P. F.; DeVore, C. R.; Karpen, J. T.; Antiochos, S. K.;
Yeates, A. R.
2018ApJ...864..165W Altcode: 2018arXiv180803688W
Coronal jets and bright points occur prolifically in predominantly
unipolar magnetic regions, such as coronal holes (CHs), where they
appear above minority-polarity intrusions. Intermittent low-level
reconnection and explosive, high-energy-release reconnection above
these intrusions are thought to generate bright points and jets,
respectively. The magnetic field above the intrusions possesses
a spine-fan topology with a coronal null point. The movement of
magnetic flux by surface convection adds free energy to this field,
forming current sheets and inducing reconnection. We conducted
three-dimensional magnetohydrodynamic simulations of moving magnetic
elements as a model for coronal jets and bright points. A single
minority-polarity concentration was subjected to three different
experiments: a large-scale surface flow that sheared part of the
separatrix surface only, a large-scale surface flow that also sheared
part of the polarity inversion line surrounding the minority flux,
and the latter flow setup plus a “flyby” of a majority-polarity
concentration past the moving minority-polarity element. We found that
different bright-point morphologies, from simple loops to sigmoids, were
created. When only the field near the separatrix was sheared, steady
interchange reconnection modulated by quasi-periodic, low-intensity
bursts of reconnection occurred, suggestive of a bright point with
periodically varying intensity. When the field near the polarity
inversion line was strongly sheared, on the other hand, filament
channels repeatedly formed and erupted via the breakout mechanism,
explosively increasing the interchange reconnection and generating
nonhelical jets. The flyby produced even more energetic and explosive
jets. Our results explain several key aspects of CH bright points and
jets, and the relationships between them.
---------------------------------------------------------
Title: Observations of Coronal Rain in Null Point Topologies
Authors: Mason, Emily; Antiochos, Spiro; MacNiece, Peter; Schlenker,
Michael
2018shin.confE..17M Altcode:
The most traditional example of time-dependent heating is the solar
flare. These explosive events are often accompanied by cascades of
coronal rain (CR), a phenomenon in which plasma at coronal temperatures
undergoes rapid cooling (from roughly 10^6 to below 10^4 K), condenses,
and falls to the surface. However, CR is not seen exclusively in flares;
this presentation reports multiple observations of rain forming and
precipitating along the legs of null point topologies (NPT) in the low
corona, from the spine, null point, and within the legs. We analyze
the events using SDO Atmospheric Imaging Assembly (AIA) observations
in the 304, 171, and 211 Å channels, as well as SDO Helioseismic and
Magnetic Imager (HMI) magnetograms; we also include 1D loop simulations
to model the small-scale dynamics driving the events. The frequency
of rain formation, and the ease with which these observations were
identified lead us to believe that this phenomenon is very common in
NPTs. These CR events could be explained via heating cutoff secondary
to magnetic reconnection, or by thermal nonequilibrium; it is also
possible that both mechanisms are present. Further study involving
higher spatial resolution data and a greater range of loop lengths
(i.e., pseudostreamers and helmet streamers) will be required to
constrain the drivers.
---------------------------------------------------------
Title: Relating the Structure and Dynamics of the Corona to the
Variability of the Slow Solar Wind
Authors: Higginson, Aleida Katherine; Antiochos, Spiro K.; Lynch,
Ben. J.; DeVore, C. Rick; Wyper, Peter F.
2018shin.confE..52H Altcode:
Recent coronagraph and in situ observations have shown that the slow
solar wind includes highly structured and dynamic outflow across
spatial scales, most likely due to magnetic reconnection processes
in the solar corona. As we prepare for Parker Solar Probe and Solar
Orbiter, understanding this temporal and spatial variability has
become essential. Numerical calculations have shown that magnetic
field dynamics at coronal hole boundaries, in particular interchange
reconnection driven by photospheric motions, can be responsible for the
dynamic release of structured slow solar wind, including along huge
separatrix-web (S-Web) arcs formed by pseudostreamers. Quantifying
the slow solar wind variability along these S-Web arcs is crucial to
furthering our understanding of how coronal magnetic field dynamics can
influence the plasma and magnetic field throughout the heliosphere. Here
we present for the first time, fully dynamic, 3D numerical calculations
of an S-Web arc driven continuously by realistic photospheric motions
at its base. We present an analysis of the resulting magnetic field
dynamics and subsequent plasma release, with a focus on quantifying
how the photospheric drivers affect the width of the separatrix arc in
the heliosphere. We consider our simulation results within the context
of future Parker Solar Probe and Solar Orbiter observations and make
predictions for the structure and variability of the slow solar wind.
---------------------------------------------------------
Title: Measuring the Free Energy and Helicity Leading to Solar
Eruptive Events
Authors: Antiochos, Spiro K.; Schuck, P. W.
2018shin.confE.151A Altcode:
The essential ingredients determining solar coronal activity are
believed to be the availability of magnetic free energy and the
constraint of magnetic helicity conservation. Direct measurements of the
helicity and magnetic free energy in the corona are difficult, but it
should be possible to infer them from measurements of the helicity and
free energy transport through the photosphere. We examine the rate of
change of helicity and free energy for a topological open system such as
the corona in which the magnetic fields have a non-zero normal component
at the boundaries and derive a new formula for the helicity transport
rate through the boundaries. A key feature of this formula is that it
is manifestly gauge invariant. The result is somewhat counter-intuitive
in that only the irrotational electric field transports helicity across
boundaries and the inductive electric field does not contribute. We
discuss the physical interpretation of our result and demonstrate
its application with instructive examples. Furthermore, we derive an
expression for the free energy flux, and show that a necessary condition
for free energy transport across a boundary is the presence of normal
electric currents at the boundary. We discuss the implications of
our results for using photospheric vector magnetic and velocity field
measurements to derive the solar coronal helicity and magnetic free
energy, which can then be used to constrain and drive space weather
models for coronal activity.
---------------------------------------------------------
Title: The Effect of Thermal Nonequilibrium in Streamers
Authors: Antiochos, Spiro; Schlenker, Michael; MacNeice, Peter;
Mason, Emily
2018cosp...42E..95A Altcode:
Thermal nonequilibrium (TNE) is the process in which a solar coronal
loop undergoes a nonsteady cycle of condensation formation due to the
spatial localization of coronal heating near the loop base. Since the
requirements for TNE onset is that the loop length is large compared
to the scale of the heating, we investigate the effects of TNE on
the largest loops in the corona, those of a helmet streamer. Our
numerical study uses a 2.5D MHD code that includes the full magnetic
field dynamics as well as the detailed plasma thermodynamics. As
in previous 1D loop studies, we find that TNE occurs in coronal
loops with sufficiently large length, but in contrast to 1D studies,
we find that the process also drives substantial magnetic dynamics,
especially near the top of the streamer where the plasma beta becomes of
order unity. From the simulation results we determine predictions for
spectroscopic and imaging observations of both the hot and cool helmet
streamer plasma. We conclude that TNE occurring in the largest closed
loops in the corona may explain several puzzling observations of the
corona, such as the ubiquitous blue shifts observed at the edges of
active regions. We also discuss the implications of our results for
the solar wind.This work was supported, in part, by the NASA Living
With a Star Program.
---------------------------------------------------------
Title: A model for coronal mass ejection energy buildup and eruption
onset
Authors: Dahlin, Joel T.; Antiochos, Spiro K.; DeVore, C. Richard
2018shin.confE.214D Altcode:
Determining the mechanism that drives coronal mass ejections is one of
the most important problems in all of space science. Understanding the
trigger for eruption onset is essential for accurate prediction of major
space weather events. Self-consistent modeling of the energy buildup
and resulting magnetic field configuration is vital for distinguishing
the role of ideal instabilities (e.g. the torus instability) versus
reconnection (e.g. magnetic breakout) in the onset of CMEs. We present
new 3D spherical MHD simulations in which the initial state is a minimum
energy potential field and the system is driven by small-scale motions
observed for photospheric convection. This simple, self-consistent
model drives large-scale energy build up through an inverse cascade of
magnetic helicity, forming a filament channel consistent with solar
observations of the pre-eruptive magnetic field. We show that energy
buildup continues until reconnection in the overlying magnetic field
destabilizes the configuration resulting in the ejection of a fast
CME. We conclude from these simulations that the trigger mechanism for
eruption onset is magnetic reconnection in the coronal field overlying
the filament.
---------------------------------------------------------
Title: Numerical Simulation of a Helical Active Region Jet
Authors: Wyper, Peter Fraser; DeVore, C. Richard; Antiochos, Spiro K.
2018shin.confE..65W Altcode:
Parker Solar Probe (PSP) promises to shed light on the origins of
solar wind variability. One major contributor to this variability is
expected to be coronal jets: high-speed ejections of plasma launched
by impulsive interchange reconnection above parasitic polarities. Due
to the higher field strengths at the edges of active regions, the
most energetic jets often occur there. Such jets are associated with
jet-like CMEs in coronagraphs and impulsive SEP events, making them
excellent candidates for detection by PSP. Building on our previous work
simulating coronal hole jets with filaments - the breakout-jet model -
we present a 3D MHD model of a helical active region jet generated by
the eruption of a small-scale filament channel. The jet is triggered by
interchange reconnection within a current layer formed around a magnetic
null point. Following a complex two-step eruption process, an extended
helical jet is formed by the transfer of twist from the filament channel
to open field lines. Our simulation results explain recent observations
of helical jets with complex base dynamics occurring at the periphery of
active regions. We gratefully acknowledge support from an RAS fellowship
(PFW) and by NASA's LWS and H-SR programs (CRD and SKA).
---------------------------------------------------------
Title: Multiwavelength Study of 24 Equatorial Coronal-Hole Jets
Authors: Kumar, Pankaj; Antiochos, Spiro; Karpen, Judy; DeForest,
Craig; DeVore, C. Richard; Wyper, Peter
2018cosp...42E1863K Altcode:
We studied 24 equatorial coronal-hole (ECH) jets using SDO/AIA and
HMI observations on 27-28 June 2013 and 8-10 January 2014. Out of 24
jets (i) 16 jets (67%) are associated with mini-filament eruptions;
(ii) 8 jets (34%) are triggered without mini-filament eruptions
but with mini-flare arcades and other CME-like signatures; (iii)
5 jets (21%) are apparently associated with tiny flux-cancellation
events at the polarity inversion line; (iv) 3 events are associated
with sympathetic eruptions of filaments from neighboring jet source
regions. The potential field extrapolations of the source regions
reveal that almost all jets occurred in the fan-spine topology, and
most of the events are in agreement with the breakout model of solar
jets. We will present selected examples of each type, and discuss the
implications for the jet energy-buildup and initiation mechanisms.
---------------------------------------------------------
Title: The Effect of Thermal Non-equilibrium on Helmet Streamers; yes
Authors: Schlenker, Michael John; Antiochos, Spiro
2018shin.confE.256S Altcode:
Thermal nonequilibrium is the well-known process in which a solar
coronal loop undergoes a nonsteady cycle of heating and cooling due to
the spatial localization of coronal heating near the loop base. Since
the requirements for thermal nonequilibrium onset is that the loop
length is large compared to the scale of the heating, we investigate
the effects of this process on the largest loops in the corona, those
of a helmet streamer. Our numerical study uses a 2.5D MHD code that
includes the full magnetic field dynamics as well as the detailed plasma
thermodynamics. The simulation model is axisymmetric and consists of the
magnetic field of a dipole at Sun center, which results in a streamer
belt centered about the equator and two polar coronal holes. As in
previous 1D loop studies, we find that thermal nonequilibrium occurs in
coronal loops with sufficiently large length, but in contrast to these
studies, we find that the process also drives substantial magnetic
dynamics, especially near the top of the streamer where the plasma
beta becomes of order unity. From the simulation results we determine
predictions for spectroscopic and imaging observations of both the
hot and cool helmet streamer plasma. Simulations are preformed using
different scale heights for the heating in order to determine the
dependence of our findings on this key parameter. The dependence of
the results on numerical resolution is also determined via a parameter
study. We conclude that thermal nonequilibrium occurring in the largest
closed loops in the corona may explain several puzzling observations of
the corona, such as the ubiquitous blue shifts observed at the edges
of active regions. We also discuss the implications of our results
for the solar wind.
---------------------------------------------------------
Title: The physics of thermal nonequilibrium
Authors: Karpen, Judy; Antiochos, Spiro
2018cosp...42E1690K Altcode:
The presence of cool, dense mass in the hot, rarefied solar corona,
in the form of prominences, has mystified scientists for over a
century. Its more fragmentary and dynamic manifestation, coronal rain,
was discovered more recently but has been equally perplexing. Several
processes have been proposed to explain this phenomenon: levitation
(bulk lifting of chromospheric mass into the corona), injection
(bulk expulsion of chromospheric mass), and evaporation-condensation
methods. This talk addresses the last category, which has received the
greatest quantitative scrutiny over the past 20 years, particularly
in the form of thermal nonequilibrium (TNE). Thermal nonequilibrium
has specific requirements and observable signatures, which have been
explored thoroughly with theoretical analyses, numerical simulations,
and comparison with known characteristics of prominences and coronal
rain. I will discuss the basic physical processes at the heart of
TNE, the parameter studies that have established the strengths and
limitations of this mechanism as applied to these solar phenomena,
the latest extensions to multidimensional magnetic geometries and more
realistic physics, and future research directions.
---------------------------------------------------------
Title: Relating the Structure and Dynamics of the Corona to the
Variability of the Slow Solar Wind
Authors: Higginson, Aleida Katherine; Antiochos, Spiro K.; DeVore,
C. Richard
2018tess.conf31705H Altcode:
Recent coronagraph and in situ observations have shown that the slow
solar wind includes highly structured and dynamic outflow across
spatial scales, most likely due to magnetic reconnection processes
in the solar corona. As we prepare for Parker Solar Probe and Solar
Orbiter, understanding this temporal and spatial variability has
become essential. Numerical calculations have shown that magnetic
field dynamics at coronal hole boundaries, in particular interchange
reconnection driven by photospheric motions, can be responsible for the
dynamic release of structured slow solar wind, including along huge
separatrix-web (S-Web) arcs formed by pseudostreamers. Quantifying
the slow solar wind variability along these S-Web arcs is crucial to
furthering our understanding of how coronal magnetic field dynamics can
influence the plasma and magnetic field throughout the heliosphere. Here
we present for the first time, fully dynamic, 3D numerical calculations
of an S-Web arc driven continuously by realistic photospheric motions
at its base. We present an analysis of the resulting magnetic field
dynamics and subsequent plasma release, both near and far from the
heliospheric current sheet. We consider our simulation results within
the context of future Parker Solar Probe and Solar Orbiter observations
and make predictions for the structure and variability of the slow
solar wind.
---------------------------------------------------------
Title: Magnetic Energy Buildup and Explosive Release
Authors: Antiochos, Spiro K.; Dahlin, Joel; DeVore, C. Richard
2018tess.conf22202A Altcode:
It is now generally accepted that major solar eruptions such as CMEs
and eruptive flares are due to the explosive release of magnetic
free energy stored in the corona; specifically, in the highly
stressed magnetic field that supports filaments and prominences. An
important observational finding in recent years is that the mechanisms
underlying these eruptions may be invariant over many decades in energy
release. We have proposed that the formation of the filament field and,
consequently, the free energy buildup, is due to an inverse cascade of
magnetic helicity injected into the corona by motions and flux emergence
at the photosphere. We present our latest 3D MHD numerical simulations
of the self-consistent energy buildup by helicity condensation and
eventual explosive energy release. The calculations are in a realistic
spherical domain that extends outward to 30 solar radii. We conclude
from these simulations that the onset for the eruption, the trigger
mechanism, is magnetic reconnection in the coronal field overlying the
filament. Our results demonstrate that solar eruptions are an amazing
example on cosmic scales of self-organization leading to catastrophic
dynamics. <P />This work was supported by the NASA LWS and HSR Programs.
---------------------------------------------------------
Title: Using Solar Wind Structures as a Rosetta Stone for
Understanding Solar Wind Formation
Authors: Viall, Nicholeen M.; Kepko, Larry; Antiochos, Spiro K.;
Higginson, Aleida Katherine; Vourlidas, Angelos; Lepri, Susan T.
2018tess.conf31702V Altcode:
In the inner heliosphere, the slow solar wind is often comprised of
mesoscale structures: structures with timescales of hours and length
scales of hundreds of mega meters. White light coronagraph data suggest
that these mesoscale structures are formed and embedded in the solar
wind within the first several solar radii above the solar surface,
which is still below even the closest approach of Parker Solar Probe at
nine solar radii. We argue that these mesoscale structures represent a
'Rosetta Stone' for using the embedded solar wind plasma signatures
to understand the fundamental release and acceleration of solar wind
plasma. We study events identified in data from current missions
to demonstrate how mesoscale structures can link dynamics observed
remotely in the lower corona with in situ observations. We discuss the
observations that Parker Solar Probe will make and how to capitalize
on this remote-to-in situ data connection.
---------------------------------------------------------
Title: Multilevel Numerical Simulations of Explosive Magnetic Energy
Release at the Sun
Authors: DeVore, C. Richard; Antiochos, Spiro K.; Karpen, Judith T.
2018tess.conf10417D Altcode:
Reconnection onset at current sheets and the resultant magnetic
energy release are important at the Sun (coronal heating, coronal mass
ejections, flares, jets) and at the Earth (magnetopause flux transfer
events, magnetotail substorms) and other magnetized planets. The
most dramatic consequences include highly explosive releases of
kinetic and thermal energy and of accelerated particles in solar
eruptions. We use the Adaptively Refined Magnetohydrodynamics Solver
(ARMS) to investigate self-consistent formation and reconnection
of current sheets in an initially potential, axisymmetric magnetic
field in which four flux systems are separated by a magnetic null
line. Stressing the equatorial flux system by applying shear flows
eventually leads to reconnection-driven onset of a coronal mass ejection
and eruptive flare due to the breakout mechanism (see figure). We
report ultrahigh-resolution simulations of this process that extend
our previous work (Karpen et al. 2012) by investigating grid-resolution
effects on the eruption. Each simulation conserves the injected magnetic
helicity, which we calculate analytically, extremely well, and the
maximum magnetic free energy stored prior to onset is essentially
identical, consistent with convergence of the results versus effective
Lundquist number. As expected, the number of null-point pairs created
in the current sheets and the kinetic energy released by the eruption
increase as the resolution improves. Somewhat counter-intuitively,
eruption initiation occurs progressively earlier at higher resolution,
due to the increasing aspect ratio (length to width) of the extended
flare current sheet; reconnection onset there triggers the transition
from slow to very fast outward expansion. We discuss the implications
of our work for understanding explosive energy release in the solar
atmosphere. <P />Our research was supported by NASA's Heliophysics SR,
LWS, and ISFM programs.
---------------------------------------------------------
Title: Structure and Dynamics of Helmet Streamer Coronal Rain (or,
Is this even the Right Haystack?)
Authors: Mason, Emily Irene; Schlenker, Michael; Antiochos, Spiro K.
2018tess.conf20542M Altcode:
One of the outstanding problems in heliophysics is the nature of the
process that heats the solar atmosphere to temperatures more than two
orders of magnitude larger than those at the Sun's surface. Physical
insight and critical constraints on this process can be obtained by
observing the structure and dynamics of coronal plasma. One of the most
intriguing forms of dynamics is coronal rain, a phenomenon in which
plasma at coronal temperatures undergoes rapid cooling (from roughly
106 to 103 K), condenses, and falls to the surface. Its origins are
not thoroughly understood, but proposed theories posit mechanisms
of either temporal or spatial variations in coronal heating. The
goal of this work is to determine which of these models, if either,
agrees with observations. Observation sources include SDO AIA, IRIS,
HAO K-Coronagraph, Proba2 SWAP, and LASCO C2. This presentation will
include the results of the investigation, and conclusions on the root
mechanisms producing these signatures.
---------------------------------------------------------
Title: Statistical Study of 24 Equatorial Coronal-Hole Jets
Authors: Kumar, Pankaj; Karpen, Judith T.; Antiochos, Spiro K.;
Fraser Wyper, Peter; DeVore, C. Richard; DeForest, Craig
2018tess.conf40805K Altcode:
To understand the trigger mechanisms of coronal-hole jets, we analysed
24 equatorial coronal-hole (ECH) jets using SDO/AIA and HMI observations
during 2013-2014. Out of 24 jets (i) 16 jets (67%) are associated
with mini-filament eruptions; (ii) 8 jets (34%) are triggered without
mini-filament eruptions but with mini-flare arcades and other CME-like
signatures; (iii) 5 jets (21%) are apparently associated with tiny
flux-cancellation events at the polarity inversion line; (iv) 3 events
are associated with sympathetic eruptions of filaments from neighboring
jet source regions. The potential field extrapolations of the source
regions reveal that almost all jets occurred in the fan-spine topology,
and most of the events are in agreement with the breakout model of solar
jets. We will present selected examples of each type, and discuss the
implications for the jet energy-buildup and initiation mechanisms.
---------------------------------------------------------
Title: Determining the physical mechanism for magnetic helicity
injection into the Sun's corona
Authors: Schuck, Peter W.; Antiochos, Spiro K.
2018tess.conf20340S Altcode:
Magnetic helicity is widely believed to play a major role in
solar activity, especially in CMEas/eruptive flares. Consequently,
understanding and measuring accurately the helicity injection into the
corona is critical for developing predictive models of major solar
eruptions. As with magnetic flux, there are two physically distinct
mechanisms for helicity injection: emergence through photosphere and
mass motions at the photosphere that shear and twist pre-existing
coronal magnetic field. We revisit the helicity transport equation and
derive an expression for this transport that rigorously distinguishes
between the two mechanisms. We discuss the application of our results
for observation of helicity build up in the corona and the implications
for developing predictive models of eruptive activity. <P />This work
was supported by the NASA Living with a Star Program
---------------------------------------------------------
Title: Evidence for the Magnetic Breakout Model in an Equatorial
Coronal-hole Jet
Authors: Kumar, Pankaj; Karpen, Judith T.; Antiochos, Spiro K.; Wyper,
Peter F.; DeVore, C. Richard; DeForest, Craig E.
2018ApJ...854..155K Altcode: 2018arXiv180108582K
Small, impulsive jets commonly occur throughout the solar corona,
but are especially visible in coronal holes. Evidence is mounting that
jets are part of a continuum of eruptions that extends to much larger
coronal mass ejections and eruptive flares. Because coronal-hole jets
originate in relatively simple magnetic structures, they offer an ideal
testbed for theories of energy buildup and release in the full range
of solar eruptions. We analyzed an equatorial coronal-hole jet observed
by the Solar Dynamics Observatory (SDO)/AIA on 2014 January 9 in which
the magnetic-field structure was consistent with the embedded-bipole
topology that we identified and modeled previously as an origin of
coronal jets. In addition, this event contained a mini-filament,
which led to important insights into the energy storage and release
mechanisms. SDO/HMI magnetograms revealed footpoint motions in the
primary minority-polarity region at the eruption site, but show
negligible flux emergence or cancellation for at least 16 hr before
the eruption. Therefore, the free energy powering this jet probably
came from magnetic shear concentrated at the polarity inversion line
within the embedded bipole. We find that the observed activity sequence
and its interpretation closely match the predictions of the breakout
jet model, strongly supporting the hypothesis that the breakout model
can explain solar eruptions on a wide range of scales.
---------------------------------------------------------
Title: A Breakout Model for Solar Coronal Jets with Filaments
Authors: Wyper, P. F.; DeVore, C. R.; Antiochos, S. K.
2018ApJ...852...98W Altcode: 2017arXiv171200134W
Recent observations have revealed that many solar coronal jets involve
the eruption of miniature versions of large-scale filaments. Such
“mini-filaments” are observed to form along the polarity inversion
lines of strong, magnetically bipolar regions embedded in open (or
distantly closing) unipolar field. During the generation of the jet,
the filament becomes unstable and erupts. Recently we described a model
for these mini-filament jets, in which the well-known magnetic-breakout
mechanism for large-scale coronal mass ejections is extended to
these smaller events. In this work we use 3D magnetohydrodynamic
simulations to study in detail three realizations of the model. We
show that the breakout-jet generation mechanism is robust and that
different realizations of the model can explain different observational
features. The results are discussed in relation to recent observations
and previous jet models.
---------------------------------------------------------
Title: Evidence for the Magnetic Breakout Model in AN Equatorial
Coronal-Hole Jet
Authors: Kumar, P.; Karpen, J.; Antiochos, S. K.; Wyper, P. F.;
DeVore, C. R.; DeForest, C. E.
2017AGUFMSH52B..02K Altcode:
We analyzed an equatorial coronal-hole jet observed by Solar Dynamic
Observatory (SDO)/AtmosphericImaging Assembly (AIA). The source-region
magnetic field structure is consistent withthe embedded-bipole topology
that we identified and modeled previously as a source of coronal
jets. Theinitial brightening was observed below a sigmoid structure
about 25 min before the onset of an untwisting jet.A circular magnetic
flux rope with a mini-filament rose slowly at the speed of ∼15 km/s ,
then accelerated(∼126 km/s) during the onset of explosive breakout
reconnection. Multiple plasmoids, propagating upward(∼135 km/s)
and downward (∼55 km/s ), were detected behind the rising flux rope
shortly before andduring explosive breakout reconnection. The jet
was triggered when the rising flux rope interacted with theoverlying
magnetic structures near the outer spine. This event shows a clear
evidence of reconnection not onlybelow the flux rope but also a breakout
reconnection above the flux rope. During the breakout reconnection,we
observed heating of the flux rope, deflection of loops near the
spine, and formation of multiple ribbons.The explosive breakout
reconnection destroyed the flux rope that produced an untwisting jet
with a speed of∼380 km/s . HMI magnetograms reveal the shear motion
at theeruption site, but do not show any significant flux emergence
or cancellation during or 2 hours before theeruption. Therefore, the
free energy powering this jet most likely originated in magnetic shear
concentratedat the polarity inversion line within the embedded bipole-a
mini-filament channel-possibly created by helicitycondensation. The
result of of a statistical study of multiple jets will also be
discussed.
---------------------------------------------------------
Title: First Demonstration of a Coronal Mass Ejection Driven by
Helicity Condensation
Authors: Dahlin, J. T.; Antiochos, S. K.; DeVore, C. R.
2017AGUFMSH52B..07D Altcode:
Understanding the mechanism for CMEs/eruptive flares is one of the
most important problems in all space science. Two classes of theories
have been proposed: ideal processes such as the torus instability, or
magnetic reconnection as in the breakout model. Previous simulations of
eruptions have used special assumptions, such as a particular initial
condition ripe for instability and/or particular boundary conditions
designed to induce eruption. We report on a simulation in which the
initial state is the minimum-energy potential field, and the system
is driven solely by the small-scale random motions observed for
photospheric convection. The only requirement on the system is that
the flows are sufficiently complex to induce pervasive and random
reconnection throughout the volume, as expected for coronal heating,
and a net helicity is injected into the corona, in agreement with the
observed hemispheric helicity preference. We find that as a result of
a turbulent-like cascade, the helicity "condenses" onto a polarity
inversion line forming a filament channel, which eventually erupts
explosively. We discuss the implications of this fully self-consistent
eruption simulation for understanding CMEs/flares and for interpreting
coronal observations. This work was supported by the NASA LWS and
SR Programs.
---------------------------------------------------------
Title: Self-Organization by Stochastic Reconnection: The Mechanism
Underlying CMEs/Flares
Authors: Antiochos, S. K.; Knizhnik, K. J.; DeVore, C. R.
2017AGUFMSH14B..01A Altcode:
The largest explosions in the solar system are the giant CMEs/flares
that produce the most dangerous space weather at Earth, yet may
also have been essential for the origin of life. The root cause of
CMEs/flares is that the lowest-lying magnetic field lines in the Sun's
corona undergo the continual buildup of stress and free energy that can
be released only through explosive ejection. We perform the first MHD
simulations of a coronal-photospheric magnetic system that is driven by
random photospheric convective flows and has a realistic geometry for
the coronal field. Furthermore, our simulations accurately preserve
the key constraint of magnetic helicity. We find that even though
small-scale stress is injected randomly throughout the corona, the net
result of "stochastic" coronal reconnection is a coherent stretching
of the lowest-lying field lines. This highly counter-intuitive
demonstration of self-organization - magnetic stress builds up
locally rather than spreading out to a minimum energy state - is the
fundamental mechanism responsible for the Sun's magnetic explosions
and is likely to be a mechanism that is ubiquitous throughout space
and laboratory plasmas. This work was supported in part by the NASA
LWS and SR Programs.
---------------------------------------------------------
Title: Is the S-Web the Secret to Observed Heliospheric Particle
Distributions?
Authors: Higginson, A. K.; Antiochos, S. K.; DeVore, C. R.; Daldorff,
L. K. S.; Wyper, P. F.; Ukhorskiy, A. Y.; Sorathia, K.
2017AGUFMSH22B..02H Altcode:
Particle transport in the heliosphere remains an unsolved problem
across energy regimes. Observations of slow solar wind show that plasma
escapes from the closed-field corona, but ends up far away from the
heliospheric current sheet, even though the release mechanisms are
expected to occur at the HCS. Similarly, some impulsive SEP events
have extreme longitudinal extents of 100 degrees or more. Recent
theoretical and numerical work has shown that interchange reconnection
near a coronal-hole corridor can release plasma from originally closed
magnetic field lines into a large swath spread across the heliosphere,
forming what is known as an S-Web arc. This is a promising mechanism
for explaining both the slow solar wind, with its large latitudinal
extent, and impulsive SEP particles, with their large longitudinal
extent. Here we compute, for the first time, the dynamics of the S-Web
when the photospheric driver is applied over a large portion of the
solar surface compared to the scale of the driving. We examine the
time scales for the interchange reconnection and compute the angular
extent of the plasma released, in the context of understanding both the
slow solar wind and flare-accelerated SEPs. We will make predictions
for Solar Orbiter and Parker Solar Probe and discuss how these new
measurements will help to both pinpoint the source of the slow solar
wind and illuminate the transport mechanisms of wide-spread impulsive
SEP events.
---------------------------------------------------------
Title: The Mechanism for the Energy Buildup Driving Solar Eruptive
Events
Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R.; Wyper, P. F.
2017ApJ...851L..17K Altcode: 2017arXiv171100166K
The underlying origin of solar eruptive events (SEEs), ranging
from giant coronal mass ejections to small coronal-hole jets, is
that the lowest-lying magnetic flux in the Sun’s corona undergoes
continual buildup of stress and free energy. This magnetic stress has
long been observed as the phenomenon of “filament channels:”
strongly sheared magnetic field localized around photospheric
polarity inversion lines. However, the mechanism for the stress
buildup—the formation of filament channels—is still debated. We
present magnetohydrodynamic simulations of a coronal volume that is
driven by transient, cellular boundary flows designed to model the
processes by which the photosphere drives the corona. The key feature
of our simulations is that they accurately preserve magnetic helicity,
the topological quantity that is conserved even in the presence
of ubiquitous magnetic reconnection. Although small-scale random
stress is injected everywhere at the photosphere, driving stochastic
reconnection throughout the corona, the net result of the magnetic
evolution is a coherent shearing of the lowest-lying field lines. This
highly counterintuitive result—magnetic stress builds up locally
rather than spreading out to attain a minimum energy state—explains
the formation of filament channels and is the fundamental mechanism
underlying SEEs. Furthermore, this process is likely to be relevant
to other astrophysical and laboratory plasmas.
---------------------------------------------------------
Title: Combining Remote and In Situ Observations with MHD models to
Understand the Formation of the Slow Solar Wind
Authors: Viall, N. M.; Kepko, L.; Antiochos, S. K.; Lepri, S. T.;
Vourlidas, A.; Linker, J.
2017AGUFMSH21C..05V Altcode:
Connecting the structure and variability in the solar corona to the
Heliosphere and solar wind is one of the main goals of Heliophysics
and space weather research. The instrumentation and viewpoints of
the Parker Solar Probe and Solar Orbiter missions will provide an
unprecedented opportunity to combine remote sensing with in situ data
to determine how the corona drives the Heliosphere, especially as it
relates to the origin of the slow solar wind. We present analysis of
STEREO coronagraph and heliospheric imager observations and of in
situ ACE and Wind measurements that reveal an important connection
between the dynamics of the corona and of the solar wind. We show
observations of quasi-periodic release of plasma into the slow solar
wind occurring throughout the corona - including regions away from the
helmet streamer and heliospheric current sheet - and demonstrate that
these observations place severe constraints on the origin of the slow
solar wind. We build a comprehensive picture of the dynamic evolution
by combining remote imaging data, in situ composition and magnetic
connectivity information, and MHD models of the solar wind. Our results
have critical implications for the magnetic topology involved in slow
solar wind formation and magnetic reconnection dynamics. Crucially,
this analysis pushes the limits of current instrument resolution and
sensitivity, showing the enormous potential science to be accomplished
with the Parker Solar Probe and Solar Orbiter missions.
---------------------------------------------------------
Title: The Effects of Thermal Non-Equilibrium on a Helmet Streamer
Authors: Schlenker, M.; Antiochos, S. K.; MacNeice, P. J.
2017AGUFMSH23D2691S Altcode:
We investigate the effects of localized heating on the evolution of the
plasma within helmet streamers. By implementing a sufficiently small
heating scale height, the process of thermal non-equilibrium triggers
the formation of coronal rain within the helmet streamer. We present
the comparative formation rates of coronal rain in simulations of 3
different grid resolutions. The heating scale height itself is also
varied to examine its affect on the rain that is observed. Lastly,
we present the evolution of plasma along particular field lines. Our
model shows that the thermal physics of the plasma and the dynamical
motions of the magnetic field work together to affect the creation rate
of coronal rain. This finding has wider implications and suggests that
the presence of coronal rain within a helmet streamer can drive the
process of magnetic reconnection above the cusp of the streamer. This
work was supported in part by the NASA LWS and SR Programs.
---------------------------------------------------------
Title: Filament Channel Formation, Eruption, and Jet Generation
Authors: DeVore, C. Richard; Antiochos, Spiro K.; Karpen, Judith T.
2017SPD....4810618D Altcode:
The mechanism behind filament-channel formation is a longstanding
mystery, while that underlying the initiation of coronal mass
ejections and jets has been studied intensively but is not yet firmly
established. In previous work, we and collaborators have investigated
separately the consequences of magnetic-helicity condensation (Antiochos
2013) for forming filament channels (Zhao et al. 2015; Knizhnik et
al. 2015, 2017a,b) and of the embedded-bipole model (Antiochos 1996)
for generating reconnection-driven jets (Pariat et al. 2009, 2010,
2015, 2016; Wyper et al. 2016, 2017). Now we have taken a first step
toward synthesizing these two lines of investigation. Our recent
study (Karpen et al. 2017) of coronal-hole jets with gravity and wind
employed an ad hoc, large-scale shear flow at the surface to introduce
magnetic free energy and form the filament channel. In this effort,
we replace the shear flow with an ensemble of local rotation cells,
to emulate the Sun’s ever-changing granules and supergranules. As in
our previous studies, we find that reconnection between twisted flux
tubes within the closed-field region concentrates magnetic shear and
free energy near the polarity inversion line, forming the filament
channel. Onset of reconnection between this field and the external,
unsheared, open field releases stored energy to drive the impulsive
jet. We discuss the results of our new simulations with implications
for understanding solar activity and space weather.
---------------------------------------------------------
Title: Evidence for the Magnetic Breakout Model in an Equatorial
Coronal-Hole Jet
Authors: Karpen, Judith T.; Kumar, Pankaj; Antiochos, Spiro K.; Wyper,
Peter; DeVore, C. Richard
2017SPD....4820303K Altcode:
We have analyzed an equatorial coronal-hole jet observed by
SDO/AIA on 09 January 2014. The source-region magnetic field
structure is consistent with the embedded-bipole topology that
we identified and modeled previously as a source of coronal jets
(Pariat et al. 2009, 2010, 2015, 2016; Karpen et al. 2017; Wyper et
al. 2016). Initial brightenings were observed below a small but distinct
“mini-filament” about 25 min before jet onset. A bright circular
structure, interpreted as magnetic flux rope (MFR), surrounded the
mini-filament. The MFR and filament rose together slowly at first,
with a speed of ∼15 km s<SUP>-1</SUP>. When bright footpoints
and loops appeared below, analogous to flare ribbons and arcade, the
MFR/mini-filament rose rapidly (∼126 km s<SUP>-1</SUP>), and a bright
elongated feature interpreted as a current sheet appeared between the
MFR and the growing arcade. Multiple plasmoids propagating upward
(∼135 km s<SUP>-1</SUP>) and downward (∼55 km s<SUP>-1</SUP>)
were detected in this sheet. The jet was triggered when the rising
MFR interacted with the overlying magnetic structure, most likely at
a stressed magnetic null distorted into a current sheet. This event
thus exhibits clear evidence of “flare” reconnection below the
MFR as well as breakout reconnection above it, consistent with the
breakout model for a wide range of solar eruptions (Antiochos et
al. 1999; Devore & Antiochos 2008; Karpen et al. 2012; Wyper
et al. 2017). Breakout reconnection destroyed the MFR and enabled
the entrained coronal plasma and mini-filament to escape onto open
field lines, producing an untwisting jet. SDO/HMI magnetograms reveal
small footpoint motions at the eruption site and its surroundings,
but do not show significant flux emergence or cancellation during or
1-2 hours before the eruption. Therefore, the free energy powering
this jet most likely originated in magnetic shear concentrated at the
polarity inversion line within the embedded bipole - a mini-filament
channel - possibly created by helicity condensation (Antiochos 2013;
Knizhnik et al. 2015, 2017).This work was supported in part by a grant
from the NASA H-SR program and the NASA Postdoctoral Program.
---------------------------------------------------------
Title: Current Sheet Proliferation, Turbulence, and the Heating of
the Magnetically-Closed Corona
Authors: Klimchuk, James A.; Antiochos, Spiro K.
2017SPD....4830302K Altcode:
Electric current sheets in the solar corona are essential to many
theories of coronal heating and activity. They can form by a number
of mechanisms. The magnetic field is known to be very clumpy in the
photosphere, with approximately 100,000 elemental flux tubes in a
single active region. Convection causes the tubes to become twisted and
tangled, with current sheets forming unavoidably at their boundaries in
the corona. Partial reconnections of the tubes as well as a patchiness
of the reconnection process lead to a multiplication of the number of
distinct sheets. Quasi-ideal instabilities, such as kinking, multiply
the numbers even more. We conclude, therefore, that there will be a
proliferation of current sheets in the corona. An important question
is whether large-scale (active region size) models of the corona
need to take this complexity into account to successfully predict
the distribution of plasma and the resulting radiation. We discuss
the picture of current sheet proliferation and compare and contrast
it to MHD turbulence. We also discuss the implication of our results
for coronal observations.
---------------------------------------------------------
Title: Solar Jetlets and Plumes
Authors: DeForest, Craig; Antiochos, Spiro K.; DeVore, C. Richard;
Karpen, Judith T.; Kumar, Pankaj; Raouafi, Nour-Eddine; Roberts,
Merrill; Uritsky, Vadim; Wyper, Peter
2017SPD....4830401D Altcode:
We present results of a careful deep-field (low-noise) analysis of
evolution and structure of solar plumes using multiple wavelength
channels from SDO/AIA. Using new noise-reduction techniques on
SDO/AIA images, we reveal myriad small, heating events that appear
to be the primary basis of plume formation and sustenance. These
events ("jetlets") comprise a dynamic tapestry that forms the more
distributed plume itself. We identify the "jetlets" with ejecta that
have been previously observed spectroscopically, and distinguish
them from the quasi-periodic slow mode waves that are seen as large
collective motions. We speculate that the jetlets themselves, which
are consistent with multiple interchange reconnection events near
the base of the plume, are the primary energy driver heating plasma
in the plume envelope.Solar polar (and low-latitude) plumes have been
analyzed by many authors over many years. Plumes are bright, persistent
vertical structures embedded in coronal holes over quasi-unipolar
magnetic flux concentrations. They are EUV-bright in the ~1MK lines,
slightly cooler (by ionization fraction) than the surrounding coronal
hole, persistent on short timescales of a few hours, and recurrent on
timescales of a few days. Their onset has been associated with large
X-ray jets, although not all plumes are formed that way. Plumes appear
to comprise myriad small "threads" or "strands", and may (or may not)
contribute significantly to the solar wind, though they have been
associated with myriad small, frequent eruptive ejection events.Our
results are new and interesting because they are the lowest-noise,
time-resolved observations of polar plumes to date; and they reveal
the deep association between small-scale magnetic activity and the
formation of the plumes themselves.
---------------------------------------------------------
Title: Driving Solar Eruptions via Helicity Condensation
Authors: Dahlin, Joel Timothy; Antiochos, Spiro K.; DeVore, C. Richard
2017SPD....4840605D Altcode:
One of the important questions in solar physics is, “How does
the Sun store and release energy in coronal mass ejections"? Key to
answering this question is understanding how the sun (a) stores magnetic
energy in the form of a solar filament and (b) suddenly releases this
energy as a coronal mass ejection. An important model for the energy
release is the ‘magnetic breakout’ - a positive-feedback mechanism
between filament ejection and magnetic reconnection. Recent theory and
numerical calculations have demonstrated that helicity injected into
the corona via photospheric driving can accumulate in the form of a
filament channel of strongly sheared magnetic fields that can provide
the free energy for a coronal mass ejection. We present preliminary
calculations that, for the first time, incorporate helicity injection
in a breakout topology to model a fully self-consistent eruption,
from filament formation to ejection.
---------------------------------------------------------
Title: Implications of the S-Web Model for Impulsive SEPs
Authors: Antiochos, Spiro K.; Higginson, Aleida K.; DeVore, C. Richard
2017SPD....4840403A Altcode:
One of the most important discoveries of the STEREO mission is that
impulsive Solar Energetic Particle (SEP) events frequently exhibit
large longitudinal spread in the heliosphere, up to 100 degrees
or more. This result is especially puzzling given the long-standing
observations that impulsive SEPs originate in highly localized regions
in the corona, angular extent less than one degree, and that the SEPs
frequently show so-called drop-outs, effectively ruling out diffusion
as a mechanism for the observed spread. We discuss the implications
of the S-Web slow solar wind model for the propagation of SEPs and
their distribution in the heliosphere. We present results from 3D MHD
simulations demonstrating that for commonly-observed coronal magnetic
topologies, the connectivity of the corona to heliosphere will be
quasi-singular, with small regions near the Sun dynamically connecting
to giant arcs in the heliosphere that span tens of degrees in both
latitude and longitude. We show that the S-Web model can account for
both SEP longitudinal spread and dropouts, and discuss implications
for observations from the upcoming Solar Orbiter and Solar Probe Plus
missions.This research was supported, in part, by the NASA LWS Program.
---------------------------------------------------------
Title: A Universal Model for Solar Eruptions
Authors: Wyper, Peter; Antiochos, Spiro K.; DeVore, C. Richard
2017SPD....4820302W Altcode:
We present a universal model for solar eruptions that encompasses
coronal mass ejections (CMEs) at one end of the scale, to coronal
jets at the other. The model is a natural extension of the Magnetic
Breakout model for large-scale fast CMEs. Using high-resolution
adaptive mesh MHD simulations conducted with the ARMS code, we show that
so-called blowout or mini-filament coronal jets can be explained as one
realisation of the breakout process. We also demonstrate the robustness
of this “breakout-jet” model by studying three realisations in
simulations with different ambient field inclinations. We conclude that
magnetic breakout supports both large-scale fast CMEs and small-scale
coronal jets, and by inference eruptions at scales in between. Thus,
magnetic breakout provides a unified model for solar eruptions. P.F.W
was supported in this work by an award of a RAS Fellowship and an
appointment to the NASA Postdoctoral Program. C.R.D and S.K.A were
supported by NASA’s LWS TR&T and H-SR programs.
---------------------------------------------------------
Title: Slow Solar Wind from S-Web Arcs
Authors: Higginson, Aleida K.; Antiochos, Spiro K.; DeVore, C. Richard;
Wyper, Peter; Zurbuchen, Thomas H.
2017SPD....4830106H Altcode:
A long-standing mystery posed by in-situ heliospheric observations is
the large angular extent of slow solar wind about the heliospheric
current sheet (HCS). Measurements of plasma composition strongly
imply that much of the slow wind consists of plasma from the closed
corona that escapes onto open field lines, presumably by field-line
opening or by interchange reconnection. Both of these processes are
expected to release closed-field plasma into the solar wind within and
immediately adjacent to the HCS. The recently proposed Separatrix-Web
(S-Web) Theory postulates that the observations of slow wind far from
the HCS can be explained by the dynamical interaction of open and
closed flux in regions of complex coronal-hole topology. We present
the first high-resolution, three-dimensional numerical simulations
of the dynamic S-Web. These simulations suggest that photospheric
motions at coronal-hole boundaries are responsible for the release
of slow solar wind plasma from the magnetically closed solar corona,
specifically through prolific interchange magnetic reconnection. The
location of this plasma once it is released into the solar wind depends
strongly on the geometry of the coronal-hole flux. We demonstrate
how the dynamics at the boundaries of narrow corridors of open flux
(coronal hole corridors) can create giant S-Web arcs of slow solar
wind at high latitudes in the heliosphere, far from the HCS, accounting
for the long-puzzling slow-wind observations.
---------------------------------------------------------
Title: Observational Tests of Slow Wind Theories
Authors: Antiochos, Spiro K.; Higginson, Aleida; DeVore, C. Richard
DeVore
2017shin.confE..75A Altcode:
Our theoretical understanding of the slow solar wind has undergone
a revolution during the past decade, due to the recognition that
topological complexity must be an essential feature of all slow wind
models. In this scene setting presentation I will briefly review the
slow wind models, emphasizing the distinguishing physics of each. A
key point is the role of dynamics, especially magnetic reconnection,
in the various theories. I will contrast the theories according to their
dynamics and then discuss how observations may be used to differentiate
between them. In particular, I will discuss how the future observations
expected from Solar Probe Plus and Solar Orbiter may finally reveal
the true origins of the slow solar wind. <P />This work was supported
by the NASA LWS and HSR Programs.
---------------------------------------------------------
Title: Formation of Heliospheric Arcs of Slow Solar Wind
Authors: Higginson, A. K.; Antiochos, S. K.; DeVore, C. R.; Wyper,
P. F.; Zurbuchen, T. H.
2017ApJ...840L..10H Altcode: 2017arXiv170108797H
A major challenge in solar and heliospheric physics is understanding
the origin and nature of the so-called slow solar wind. The Sun’s
atmosphere is divided into magnetically open regions, known as coronal
holes, where the plasma streams out freely and fills the solar system,
and closed regions, where the plasma is confined to coronal loops. The
boundary between these regions extends outward as the heliospheric
current sheet (HCS). Measurements of plasma composition strongly imply
that much of the slow wind consists of plasma from the closed corona
that escapes onto open field lines, presumably by field-line opening
or by interchange reconnection. Both of these processes are expected to
release closed-field plasma into the solar wind within and immediately
adjacent to the HCS. Mysteriously, however, slow wind with closed-field
plasma composition is often observed in situ far from the HCS. We use
high-resolution, three-dimensional, magnetohydrodynamic simulations
to calculate the dynamics of a coronal hole with a geometry that
includes a narrow corridor flanked by closed field and is driven by
supergranule-like flows at the coronal-hole boundary. These dynamics
produce giant arcs of closed-field plasma that originate at the
open-closed boundary in the corona, but extend far from the HCS and
span tens of degrees in latitude and longitude at Earth. We conclude
that such structures can account for the long-puzzling slow-wind
observations.
---------------------------------------------------------
Title: A universal model for solar eruptions
Authors: Wyper, Peter F.; Antiochos, Spiro K.; DeVore, C. Richard
2017Natur.544..452W Altcode:
Magnetically driven eruptions on the Sun, from stellar-scale coronal
mass ejections to small-scale coronal X-ray and extreme-ultraviolet
jets, have frequently been observed to involve the ejection of the
highly stressed magnetic flux of a filament. Theoretically, these
two phenomena have been thought to arise through very different
mechanisms: coronal mass ejections from an ideal (non-dissipative)
process, whereby the energy release does not require a change in the
magnetic topology, as in the kink or torus instability; and coronal
jets from a resistive process involving magnetic reconnection. However,
it was recently concluded from new observations that all coronal jets
are driven by filament ejection, just like large mass ejections. This
suggests that the two phenomena have physically identical origin and
hence that a single mechanism may be responsible, that is, either
mass ejections arise from reconnection, or jets arise from an ideal
instability. Here we report simulations of a coronal jet driven by
filament ejection, whereby a region of highly sheared magnetic field
near the solar surface becomes unstable and erupts. The results show
that magnetic reconnection causes the energy release via ‘magnetic
breakout’—a positive-feedback mechanism between filament ejection
and reconnection. We conclude that if coronal mass ejections and jets
are indeed of physically identical origin (although on different spatial
scales) then magnetic reconnection (rather than an ideal process)
must also underlie mass ejections, and that magnetic breakout is a
universal model for solar eruptions.
---------------------------------------------------------
Title: Dynamics of Coronal Hole Boundaries
Authors: Higginson, A. K.; Antiochos, S. K.; DeVore, C. R.; Wyper,
P. F.; Zurbuchen, T. H.
2017ApJ...837..113H Altcode: 2016arXiv161104968H
Remote and in situ observations strongly imply that the slow solar wind
consists of plasma from the hot, closed-field corona that is released
onto open magnetic field lines. The Separatrix Web theory for the slow
wind proposes that photospheric motions at the scale of supergranules
are responsible for generating dynamics at coronal-hole boundaries,
which result in the closed plasma release. We use three-dimensional
magnetohydrodynamic simulations to determine the effect of photospheric
flows on the open and closed magnetic flux of a model corona with
a dipole magnetic field and an isothermal solar wind. A rotational
surface motion is used to approximate photospheric supergranular driving
and is applied at the boundary between the coronal hole and helmet
streamer. The resulting dynamics consist primarily of prolific and
efficient interchange reconnection between open and closed flux. The
magnetic flux near the coronal-hole boundary experiences multiple
interchange events, with some flux interchanging over 50 times in one
day. Additionally, we find that the interchange reconnection occurs
all along the coronal-hole boundary and even produces a lasting change
in magnetic-field connectivity in regions that were not driven by
the applied motions. Our results show that these dynamics should be
ubiquitous in the Sun and heliosphere. We discuss the implications of
our simulations for understanding the observed properties of the slow
solar wind, with particular focus on the global-scale consequences of
interchange reconnection.
---------------------------------------------------------
Title: Coronal Jets Simulated with the Global Alfvén Wave Solar Model
Authors: Szente, J.; Toth, G.; Manchester, W. B., IV; van der Holst,
B.; Landi, E.; Gombosi, T. I.; DeVore, C. R.; Antiochos, S. K.
2017ApJ...834..123S Altcode:
This paper describes a numerical modeling study of coronal jets to
understand their effects on the global corona and their contribution
to the solar wind. We implement jets into a well-established
three-dimensional, two-temperature magnetohydrodynamic (MHD) solar
corona model employing Alfvén-wave dissipation to produce a realistic
solar-wind background. The jets are produced by positioning a compact
magnetic dipole under the solar surface and rotating the boundary plasma
around the dipole's magnetic axis. The moving plasma drags the magnetic
field lines along with it, ultimately leading to a reconnection-driven
jet similar to that described by Pariat et al. We compare line-of-sight
synthetic images to multiple jet observations at EUV and X-ray
bands, and find very close matches in terms of physical structure,
dynamics, and emission. Key contributors to this agreement are the
greatly enhanced plasma density and temperature in our jets compared
to previous models. These enhancements arise from the comprehensive
thermodynamic model that we use and, also, our inclusion of a dense
chromosphere at the base of our jet-generating regions. We further
find that the large-scale corona is affected significantly by the
outwardly propagating torsional Alfvén waves generated by our polar
jet, across 40° in latitude and out to 24 R<SUB>⊙</SUB>. We estimate
that polar jets contribute only a few percent to the steady-state
solar-wind energy outflow.
---------------------------------------------------------
Title: The Role of Magnetic Helicity in Structuring the Solar Corona
Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R.
2017ApJ...835...85K Altcode: 2016arXiv160706756K
Two of the most widely observed and striking features of the Sun's
magnetic field are coronal loops, which are smooth and laminar,
and prominences or filaments, which are strongly sheared. Loops are
puzzling because they show little evidence of tangling or braiding,
at least on the quiet Sun, despite the chaotic nature of the solar
surface convection. Prominences are mysterious because the origin of
their underlying magnetic structure—filament channels—is poorly
understood at best. These two types of features would seem to be quite
unrelated and wholly distinct. We argue that, on the contrary, they are
inextricably linked and result from a single process: the injection
of magnetic helicity into the corona by photospheric motions and the
subsequent evolution of this helicity by coronal reconnection. In this
paper, we present numerical simulations of the response of a Parker
(1972) corona to photospheric driving motions that have varying
degrees of helicity preference. We obtain four main conclusions:
(1) in agreement with the helicity condensation model of Antiochos
(2013), the inverse cascade of helicity by magnetic reconnection in the
corona results in the formation of filament channels localized about
polarity inversion lines; (2) this same process removes most complex
fine structure from the rest of the corona, resulting in smooth and
laminar coronal loops; (3) the amount of remnant tangling in coronal
loops is inversely dependent on the net helicity injected by the
driving motions; and (4) the structure of the solar corona depends
only on the helicity preference of the driving motions and not on
their detailed time dependence. We discuss the implications of our
results for high-resolution observations of the corona.
---------------------------------------------------------
Title: Electron Acceleration in Contracting Magnetic Islands during
Solar Flares
Authors: Borovikov, D.; Tenishev, V.; Gombosi, T. I.; Guidoni, S. E.;
DeVore, C. R.; Karpen, J. T.; Antiochos, S. K.
2017ApJ...835...48B Altcode:
Electron acceleration in solar flares is well known to be efficient at
generating energetic particles that produce the observed bremsstrahlung
X-ray spectra. One mechanism proposed to explain the observations is
electron acceleration within contracting magnetic islands formed by
magnetic reconnection in the flare current sheet. In a previous study,
a numerical magnetohydrodynamic simulation of an eruptive solar flare
was analyzed to estimate the associated electron acceleration due
to island contraction. That analysis used a simple analytical model
for the island structure and assumed conservation of the adiabatic
invariants of particle motion. In this paper, we perform the first-ever
rigorous integration of the guiding-center orbits of electrons in a
modeled flare. An initially isotropic distribution of particles is
seeded in a contracting island from the simulated eruption, and the
subsequent evolution of these particles is followed using guiding-center
theory. We find that the distribution function becomes increasingly
anisotropic over time as the electrons’ energy increases by up to
a factor of five, in general agreement with the previous study. In
addition, we show that the energized particles are concentrated on the
Sunward side of the island, adjacent to the reconnection X-point in
the flare current sheet. Furthermore, our analysis demonstrates that
the electron energy gain is dominated by betatron acceleration in the
compressed, strengthened magnetic field of the contracting island. Fermi
acceleration by the shortened field lines of the island also contributes
to the energy gain, but it is less effective than the betatron process.
---------------------------------------------------------
Title: Reconnection-Driven Coronal-Hole Jets with Gravity and
Solar Wind
Authors: Karpen, J. T.; DeVore, C. R.; Antiochos, S. K.; Pariat, E.
2017ApJ...834...62K Altcode: 2016arXiv160609201K
Coronal-hole jets occur ubiquitously in the Sun's coronal holes, at
EUV and X-ray bright points associated with intrusions of minority
magnetic polarity. The embedded-bipole model for these jets posits
that they are driven by explosive, fast reconnection between the
stressed closed field of the embedded bipole and the open field of
the surrounding coronal hole. Previous numerical studies in Cartesian
geometry, assuming uniform ambient magnetic field and plasma while
neglecting gravity and solar wind, demonstrated that the model is
robust and can produce jet-like events in simple configurations. We
have extended these investigations by including spherical geometry,
gravity, and solar wind in a nonuniform, coronal hole-like ambient
atmosphere. Our simulations confirm that the jet is initiated by the
onset of a kink-like instability of the internal closed field, which
induces a burst of reconnection between the closed and external open
field, launching a helical jet. Our new results demonstrate that the
jet propagation is sustained through the outer corona, in the form
of a traveling nonlinear Alfvén wave front trailed by slower-moving
plasma density enhancements that are compressed and accelerated by
the wave. This finding agrees well with observations of white-light
coronal-hole jets, and can explain microstreams and torsional Alfvén
waves detected in situ in the solar wind. We also use our numerical
results to deduce scaling relationships between properties of the
coronal source region and the characteristics of the resulting jet,
which can be tested against observations.
---------------------------------------------------------
Title: Turbulence, Current Sheet Proliferation, and the Heating of
the Magnetically-Closed Corona
Authors: Klimchuk, J. A.; Antiochos, S. K.; Dahlburg, R. B.
2016AGUFMSH33A..03K Altcode:
Turbulence plays an important role in heating and accelerating the
solar wind, and it has been proposed to also be important in heating
active regions and the quiet Sun. These regions are fundamentally
different from the sources of the solar wind, however, in that they are
magnetically closed and have a small plasma beta. We suggest that the
strong, line-tied magnetic field resists being distorted and inhibits
turbulence from developing. To test this idea, we performed a 3D MHD
simulation representing a solar active region being driven by slow
photospheric motions. The conditions said to be necessary for turbulence
are met, yet the system evolves quasi-statically up to the point where
a kink instability occurs. We conclude that the magnetically-closed
corona is not turbulent in the classical sense. There is no inertial
range of spatial scales where energy flows without dissipation through
a continuum of eddies. Rather, there is a quasi-static evolution that
is interrupted by localized and temporary bursts of turbulent behavior
associated with the tearing and reconnection of current sheets. Because
of a proliferation of current sheets, these episodes are widespread and
frequent, with many occurring at the same time within a single active
region. This picture is fundamentally different from MHD turbulence,
despite some similarities. In addition to the lack of an inertial
range, the amount of heating is not independent of the details of
the dissipation. On the contrary, it depends critically on the onset
conditions for tearing and reconnection.
---------------------------------------------------------
Title: Coronal and Heliospheric Impacts of Reconnection-driven
Coronal-Hole Jets, and Implications for Plume Formation
Authors: Karpen, J. T.; DeVore, C. R.; Antiochos, S. K.
2016AGUFMSH53A..04K Altcode:
Jets from coronal holes on the Sun have been observed for decades,
but the physical mechanism responsible for these events is still
debated. An important clue about their origin lies in their association
with small intrusions of minority polarity within the large-scale
open magnetic field, strongly suggesting that these jets are powered
by interchange reconnection between embedded bipoles (closed flux)
and the surrounding open flux (Antiochos 1996). Through computational
investigations of this embedded-bipole paradigm, we have demonstrated
that energetic, collimated, Alfvénic flows can be driven by explosive
reconnection between twisted closed flux of the minority polarity
and the unstressed external field (e.g., Pariat et al. 2009, 2010,
2015, 2016). Our recent numerical study (Karpen et al. 2016) explored
the dynamics and energetics of this process under the more realistic
conditions of spherical geometry, solar gravity, and an isothermal
solar wind out to 9 Rsun. We present results of an extension of this
simulation to 30 Rsun, which allows us to predict observable signatures
within the orbit of Solar Probe Plus (see Roberts et al. 2016,
this meeting). Coronal-hole jets also have been implicated in the
formation and maintenance of plumes (e.g., Raouafi & Stenborg
2014), but the physical relationship between the transient, narrow
jets and the diffuse, longer-lived plumes is far from understood. To
address this question, we analyze the mass density enhancements and
fluctuations from the Sun to the inner heliosphere, driven by both
slow and explosive reconnection in the embedded-bipole scenario and
the associated nonlinear Alfvén wave. Our preliminary results indicate
that a substantial ( 20%) density increase over background appears at
the moving location of the wave front as far as 12 Rsun. We present
the full spatial extent and temporal evolution of mass and momentum
after reconnection onset, as well as synthetic coronagraph images of
the perturbed corona and inner heliosphere, for comparison with AIA/SDO,
LASCO/SOHO, and SECCHI/STEREO observations of jets and plumes. Our goal
is to determine the contribution of individual reconnection-driven
jets to a plume. This research was supported by NASA's Living With
a Star Targeted Research and Technology and Heliophysics Supporting
Research programs.
---------------------------------------------------------
Title: Fundamental Physics of the Slow Solar Wind - What do we Know?
Authors: Ofman, L.; Abbo, L.; Antiochos, S. K.; Hansteen, V. H.;
Harra, L.; Ko, Y. K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.;
von Steiger, R.; Wang, Y. M.
2016AGUFMSH42A..01O Altcode:
Fundamental physical properties of the slow solar wind (SSW), such
as density, temperature, outflow speed, heavy ion abundances and
charges states were obtained from in-situ measurements at 1AU in
the past from WIND, ACE, and other spacecraft. Plasma and magnetic
field measurement are available as close as 0.3 AU from Helios data,
Spektr-R, and MESSENGER spacecraft. Remote sensing spectroscopic
measurements are available in the corona and below from SOHO/UVCS,
Hinode, and other missions. One of the major objectives of the Solar
Orbiter and Solar Probe Plus missions is to study the sources of the
SSW close to the Sun. The present state of understanding of the physics
of the SSW is based on the combination of the existing observations,
theoretical and numerical 3D MHD and multi-fluid models, that connect
between the SSW sources in the corona and the heliosphere. Recently,
hybrid models that combine fluid electrons and kinetic ions of the
expanding solar wind were developed, and provide further insights of the
local SSW plasma heating processes that related to turbulent magnetic
fluctuations spectra and kinetic ion instabilities observed in the
SSW plasma. These models produce the velocity distribution functions
(VDFs) of the protons and heavier ions as well as the ion anisotropic
temperatures. I will discuss the results of the above observations
and models, and review the current status of our understanding of
the fundamental physics of the SSW. I will review the open questions,
and discuss how they could be addressed with near future observations
and models.
---------------------------------------------------------
Title: Studying the thermodynamics of coronal jets through modeling-
and observational diagnostics techniques
Authors: Szente, J.; Manchester, W.; Landi, E.; Toth, G.; van der
Holst, B.; Gombosi, T. I.; DeVore, C. R.; Antiochos, S. K.
2016AGUFMSH21E2577S Altcode:
We present a comprehensive study of simulated and observed coronal jets
using EUV and soft X-ray narrow-band images and EUV high resolution
spectra. The goal of our study is to understand the thermodynamics
and time evolution of jets and their impact on the coronal plasma. We
simulate jets with a full 3D MHD coronal model with separate electron
and proton temperatures and heating due to Alfvén wave turbulence. Due
to the fast dynamics of the small-scale eruptive reconnections at the
footpoint of the jet, it is essential to undertake this effort with a
model with separate electron and proton temperatures to interpret the
observed signatures in EUV and soft X-ray bands. The obtained synthetic
images are compared to observations done by the instrumentations of
SDO, STEREO and Hinode space crafts. The turbulence in this model is
ideally suited to analyze the spectroscopic signatures, such as line
broadening. The 3-hour long simulation of jets interacting with the
global solar corona shows plasma responses potentially being observed
with the upcoming Solar Probe Plus mission.
---------------------------------------------------------
Title: On the Origin of the Slow Solar Wind: Periodic Plasma Release
from Pseudostreamers
Authors: Viall, N. M.; Kepko, L.; Antiochos, S. K.
2016AGUFMSH54A..05V Altcode:
We present observations of quasi-periodic release of plasma from
pseudostreamers, and demonstrate that these observations place
severe constraints on the origin of both the slow solar wind and
pseudostreamer dynamics. Though quasi-periodic release of slow solar
wind plasma is routinely observed in remote white light images, such
plasma release is often associated with the tips of helmet streamers and
the heliospheric current sheet. Helmet streamers and the heliospheric
current sheet are natural locations for magnetic reconnection to
occur, both in the form of complete disconnections and interchange
reconnection. However, pseudostreamers are not associated with the
heliospheric current sheet, and are predicted by some models to have
steady solar wind release. In contrast, in the S-web model of solar
wind formation, pseudostreamers and their magnetic extensions into
the heliosphere are also locations where slow solar wind is released
sporadically through magnetic reconnection. We present the first
observations demonstrating that quasi-periodic plasma release occurs
in pseudostreamers as well. We build a comprehensive picture of the
dynamics by combining remote-sensing data with in situ composition
and magnetic connectivity information. Our results have critical
implications for the magnetic topology of pseudostreamers and for
their reconnection dynamics. This analysis pushes the limits of current
instrument resolution and sensitivity, showing the enormous potential
science to be accomplished with Solar Probe Plus and Solar Orbiter.
---------------------------------------------------------
Title: Achieving Consistent Vector Magnetic Field Measurements
from SDO/HMI
Authors: Schuck, P. W.; Antiochos, S. K.; Scherrer, P. H.; Hoeksema,
J. T.; Leka, K. D.; Barnes, G.
2016AGUFMSH31B2575S Altcode:
NASA's Solar Dynamics Observatory (SDO) is delivering vector magnetic
field observations of the full solar disk with unprecedented temporal
and spatial resolution; however, the satellite is in a highly inclined
geosynchronous orbit. The relative spacecraft-Sun velocity varies by ±3
km/s over a day which introduces significant orbital artifacts in the
Helioseismic Magnetic Imager (HMI) data. We have recently demonstrated
that the orbital artifacts contaminate all spatial and temporal scales
in the data and developed a procedure for mitigating these artifacts
in the Doppler data obtained from the Milne-Eddington inversions in the
HMI Pipeline. Simultaneously, we have found that the orbital artifacts
may be introduced by inaccurate estimates for the free-spectral ranges
(FSRs) of the optical elements in HMI. We describe our approach and
attempt to minimize orbital artifacts in the hmi.V_720 Dopplergram
series by adjusting the FSRs for the optical elements of HMI within
their measurement uncertainties of ±1%.
---------------------------------------------------------
Title: The Formation of Filament Channels in the Corona
Authors: Karpen, J.; Knizhnik, K. J.; DeVore, C. R.; Antiochos, S. K.
2016AGUFMSH43C2585K Altcode:
We investigate a new model for the formation of highly sheared filament
channels above photospheric polarity inversion lines (PILs). The
question of filament channel formation is a major problem in solar
physics, its significance stemming from the propensity of filament
channels to erupt in coronal mass ejections. The free energy released
in these eruptions was originally stored as filament channel shear,
indicating that filament channels are highly non-potential structures,
containing tremendous magnetic helicity. Since magnetic helicity is
conserved under magnetic reconnection in a high-Rm environment such as
the solar corona, this helicity must be injected at the photospheric
level. We present helicity-conserving numerical simulations that show,
for the first time, the formation of such highly sheared filament
channels as a result of photospheric helicity injection into a
coronal magnetic field containing both a PIL and a coronal hole
(CH). Remarkably, sheared filament channels form only at the PIL,
leaving the rest of the corona laminar and smooth. We show that this
result is in excellent agreement with observations, and follows directly
from the recently proposed helicity condensation model (Antiochos,
2013). Building on initial tests of this model performed by Knizhnik,
Antiochos & DeVore (2015, 2016), we show that the rate of helicity
injection drastically affects the timescale of filament channel
formation, and discuss the implications for observations.
---------------------------------------------------------
Title: The Dynamics of Open-Field Corridors
Authors: Viall, N. M.; Antiochos, S. K.; Higginson, A. K.; DeVore,
C. R.
2016AGUFMSH54A..06V Altcode:
The source of the slow solar wind and the origins of its dynamics
have long been major problems in solar/heliospheric physics. Due to
its observed location in the heliosphere, its plasma composition, and
its variability, the slow wind is widely believed to be due to the
release of closed-field plasma onto open field lines. In the S-Web
model the slow wind is postulated to result from the driving of the
open-closed boundary in the corona by the quasi-random photospheric
convective motions. A key feature of the model is the topological
complexity of the open field regions at the Sun, in other words,
the distribution and geometry of coronal holes. In particular, narrow
corridors of open field and even singular topologies are required in
order to account for the observed angular extent of the slow wind in
the heliosphere. We present the first calculations of the dynamics of
an open-field corridor driven by photospheric flows. The calculations
use our high-resolution MHD code and an isothermal approximation for
the coronal and solar wind plasma. We show that the corridor dynamics
do, in fact, result in the release of closed field plasma far from
the heliospheric current sheet, in agreement with observations and
as predicted by the S-Web model. The implications of our results for
understanding the corona-heliosphere connection and especially for
interpreting observations from the upcoming Solar Orbiter and Solar
Probe Plus missions will be discussed. This research was supported by
the NASA LWS programs.
---------------------------------------------------------
Title: Slow Solar Wind: Observations and Modeling
Authors: Abbo, L.; Ofman, L.; Antiochos, S. K.; Hansteen, V. H.;
Harra, L.; Ko, Y. -K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.;
von Steiger, R.; Wang, Y. -M.
2016SSRv..201...55A Altcode: 2016SSRv..tmp...34A
While it is certain that the fast solar wind originates from coronal
holes, where and how the slow solar wind (SSW) is formed remains an
outstanding question in solar physics even in the post-SOHO era. The
quest for the SSW origin forms a major objective for the planned future
missions such as the Solar Orbiter and Solar Probe Plus. Nonetheless,
results from spacecraft data, combined with theoretical modeling, have
helped to investigate many aspects of the SSW. Fundamental physical
properties of the coronal plasma have been derived from spectroscopic
and imaging remote-sensing data and in situ data, and these results
have provided crucial insights for a deeper understanding of the origin
and acceleration of the SSW. Advanced models of the SSW in coronal
streamers and other structures have been developed using 3D MHD and
multi-fluid equations.
---------------------------------------------------------
Title: A model for stealth coronal mass ejections
Authors: Lynch, B. J.; Masson, S.; Li, Y.; DeVore, C. R.; Luhmann,
J. G.; Antiochos, S. K.; Fisher, G. H.
2016JGRA..12110677L Altcode: 2016arXiv161208323L
Stealth coronal mass ejections (CMEs) are events in which there
are almost no observable signatures of the CME eruption in the low
corona but often a well-resolved slow flux rope CME observed in
the coronagraph data. We present results from a three-dimensional
numerical magnetohydrodynamics (MHD) simulation of the 1-2 June
2008 slow streamer blowout CME that Robbrecht et al. (2009) called
"the CME from nowhere." We model the global coronal structure using a
1.4 MK isothermal solar wind and a low-order potential field source
surface representation of the Carrington Rotation 2070 magnetogram
synoptic map. The bipolar streamer belt arcade is energized by simple
shearing flows applied in the vicinity of the helmet streamer's polarity
inversion line. The flows are large scale and impart a shear typical
of that expected from the differential rotation. The slow expansion
of the energized helmet streamer arcade results in the formation of a
radial current sheet. The subsequent onset of expansion-induced flare
reconnection initiates the stealth CME while gradually releasing the
stored magnetic energy. We present favorable comparisons between our
simulation results and the multiviewpoint SOHO-LASCO (Large Angle and
Spectrometric Coronagraph) and STEREO-SECCHI (Sun Earth Connection
Coronal and Heliospheric Investigation) coronagraph observations of
the preeruption streamer structure and the initiation and evolution
of the stealth streamer blowout CME.
---------------------------------------------------------
Title: A model for straight and helical solar jets. II. Parametric
study of the plasma beta
Authors: Pariat, E.; Dalmasse, K.; DeVore, C. R.; Antiochos, S. K.;
Karpen, J. T.
2016A&A...596A..36P Altcode: 2016arXiv160908825P
Context. Jets are dynamic, impulsive, well-collimated plasma events
that develop at many different scales and in different layers of
the solar atmosphere. <BR /> Aims: Jets are believed to be induced
by magnetic reconnection, a process central to many astrophysical
phenomena. Within the solar atmosphere, jet-like events develop in many
different environments, e.g., in the vicinity of active regions, as well
as in coronal holes, and at various scales, from small photospheric
spicules to large coronal jets. In all these events, signatures of
helical structure and/or twisting/rotating motions are regularly
observed. We aim to establish that a single model can generally
reproduce the observed properties of these jet-like events. <BR />
Methods: Using our state-of-the-art numerical solver ARMS, we present
a parametric study of a numerical tridimensional magnetohydrodynamic
(MHD) model of solar jet-like events. Within the MHD paradigm, we study
the impact of varying the atmospheric plasma β on the generation and
properties of solar-like jets. <BR /> Results: The parametric study
validates our model of jets for plasma β ranging from 10<SUP>-3</SUP>
to 1, typical of the different layers and magnetic environments of
the solar atmosphere. Our model of jets can robustly explain the
generation of helical solar jet-like events at various β ≤ 1. We
introduces the new result that the plasma β modifies the morphology of
the helical jet, explaining the different observed shapes of jets at
different scales and in different layers of the solar atmosphere. <BR
/> Conclusions: Our results enable us to understand the energisation,
triggering, and driving processes of jet-like events. Our model enables
us to make predictions of the impulsiveness and energetics of jets as
determined by the surrounding environment, as well as the morphological
properties of the resulting jets.
---------------------------------------------------------
Title: Achieving Consistent Vector Magnetic Field Measurements
from SDO/HMI
Authors: Schuck, P. W.; Scherrer, Phil; Antiochos, Spiro; Hoeksema,
Todd
2016usc..confE..71S Altcode:
NASA's Solar Dynamics Observatory (SDO) is delivering vector magnetic
field observations of the full solar disk with unprecedented temporal
and spatial resolution; however, the satellite is in a highly inclined
geosynchronous orbit. The relative spacecraft-Sun velocity varies by ±3
km/s over a day which introduces significant orbital artifacts in the
Helioseismic Magnetic Imager (HMI) data. We have recently demonstrated
that the orbital artifacts contaminate all spatial and temporal scales
in the data and developed a procedure for mitigating these artifacts
in the Doppler data obtained from the Milne-Eddington inversions in the
HMI Pipeline. Simultaneously, we have found that the orbital artifacts
may be introduced by inaccurate estimates for the free-spectral ranges
(FSRs) of the optical elements in HMI. We describe our approach and
attempt to minimize orbital artifacts in the hmi.V_720 Dopplergram
series by adjusting the FSRs for the optical elements of HMI within
their measurement uncertainties of ±1%. introduces major orbital
artifacts in the Helioseismic Magnetic Imager (HMI) data. We have
recently demonstrated that the orbital artifacts contaminate all spatial
and temporal scales in the data and developed a procedure for mitigating
these artifacts in the Doppler data obtained from the Milne-Eddington
inversions in the HMI Pipeline. Simultaneously, we have found that
the orbital artifacts may be introduced by inaccurate estimates for
the free-spectral ranges (FSRs) of the optical elements in HMI. We
describe our approach and attempt to minimize orbital artifacts in
the hmi.V_720 Dopplergram series by adjusting the FSRs for the optical
elements in HMI within their measurement uncertainties of ±1%.
---------------------------------------------------------
Title: The Source of the Slow Wind and the Origin of its Dynamics
Authors: Antiochos, S. K.; Higginson, A. K.; DeVore, C. R.
2016usc..confE..18A Altcode:
The origin of the slow solar wind has long been one of the major
unsolved problems in solar physics. Recently, we have proposed the
S-Web model in which the slow wind originates from a dense web of
separatrices and quasi-separatrix layers that form the boundary
between open and closed magnetic flux in the corona. The large-scale
dynamics of the photosphere and corona drive this S-Web, causing closed
field plasma to be released onto open field lines, which is observed
in the heliosphere as the slow wind. The S-Web model, therefore,
predicts that both the source and variability of the slow wind are
due to the dynamics of the open-closed magnetic field boundary. We
argue that two main processes drive these dynamics: photospheric
motions and thermal nonequilibrium. We present simulations showing
the form of the variability expected from the S-web dynamics and
discuss the implications of our calculations for understanding the
observed properties of the slow wind and especially for interpreting
SDO observations of coronal hole evolution. This work was supported
by the NASA LWS Program.
---------------------------------------------------------
Title: Coronal response to EUV jets modeled with the Alfvén Wave
Solar Model
Authors: Szente, Judith; Toth, Gabor; Manchester, Ward B., IV; van der
Holst, Bartholomeus; Landi, Enrico; Gombosi, Tamas; DeVore, Carl R.;
Antiochos, Spiro K.
2016usc..confE..72S Altcode:
We study the thermodynamics of jet phenomena with the use of multiple
wavelength SDO-AIA observations [e.g. Adams (2014) and Moore (2015)]
combined with advanced numerical simulations made with AWSoM coronal
model [van der Holst (2014)]. AWSoM provides a fully three-dimensional,
magnetohydrodynamic description of the solar atmosphere heated by the
dissipation of kinetic Alfvén waves in a self-consistent manner. In
addition, the model's multi-species thermodynamics with electron
heat conduction provides for the accurate construction of synthetic
line-of-sight images of phenomena. We implement our jets in the solar
wind with a magnetic dipole twisted about axis, resulting in EUV jets
similar in topology and dynamics as being observed. We show that the
coronal atmosphere responds at a large-scale as torsional Alfvén waves
propagate into the outer corona (up to 24 solar radii and 40 degrees in
latitude), introduced by the small-scale eruptive reconnection events
at the footpoint of the jet.
---------------------------------------------------------
Title: The Breakout Model for Coronal Jets with Filaments
Authors: Wyper, Peter Fraser; DeVore, C. Richard; Antiochos, Spiro
2016shin.confE.111W Altcode:
Coronal jets are impulsive, collimated plasma outflows originating low
in the solar corona. Many of these events exhibit broad, curtain-like
morphologies. Recently, Sterling et al. (2015) [doi:10.1038/nature14556]
reported that such jets are associated with the eruption of small
filaments and, therefore, are miniature versions of corona mass
ejections (CMEs). We present 3D simulations, performed with the
Adaptively Refined MHD Solver (ARMS), which demonstrate how the
magnetic breakout mechanism generates mini-CME-type jets in a compact
bipolar region energized by simple footpoint motions. Our numerical
model captures the formation of the strongly sheared pre-jet
filament structure, the post-jet flare-like loops and ribbons,
and the curtain-like untwisting dynamics observed higher in the
corona. Similar to large-scale breakout calculations (e.g. Karpen et
al. (2012) [doi:10.1088/0004-637X/760/1/81]) tearing and intermittent
reconnection also plays a role in the dynamics and naturally explains
the intermittent blob-like outflows observed in many jets. NASA
supported this research by awards to the NASA Postdoctoral Program
(P.F.W.) and the LWS TR&T and H-SR programs (C.R.D. & S.K.A.).
---------------------------------------------------------
Title: Slow Solar Wind at Mid-Latitudes Due to Photospheric Motions
Authors: Higginson, Aleida Katherine; Antiochos, S. K.; DeVore, C. R.;
Zurbuchen, T. H.
2016shin.confE..82H Altcode:
In-situ measurements of charge-state compositions and elemental
abundances show that slow wind plasma closely resembles the plasma in
the closed corona as determined by remote observations rather than the
plasma in coronal holes, which are known to be the source of the fast
solar wind. The likely origin of the slow solar wind, therefore, is
the release of closed field plasma onto open field lines. The S-Web
model predicts that photospheric motions at coronal-hole boundaries
are responsible for the transfer of plasma from closed magnetic field
to open magnetic field through reconnection or the opening of field
lines, or both. Our previous work showed that simple rotational motions
at a coronal boundary result primarily in prolific and efficient
interchange reconnection. On the Sun, when coronal-hole boundaries
become sufficiently complex, they map to locations in the heliosphere
far from the heliospheric current sheet. In such cases, the dynamics
of reconnection and/or opening can cause plasma to be released as
much as 30 degrees from the sheet. We performed a fully dynamic, 3D
MHD simulation of a complex coronal-hole boundary in an isothermal
solar wind. The quasi-steady open/closed boundary was disturbed by
introducing rotational motions. We discuss the reconnection that takes
place, the amount of closed-field plasma that is released, and the
implications of our results for understanding the origin of the slow
solar wind. <P />This work was supported by the NASA LWS Program.
---------------------------------------------------------
Title: The Variability of the S-Web
Authors: Antiochos, Spiro K.
2016shin.confE..75A Altcode:
In the S-Web model the slow wind originates from a dense web of
separatrices and quasi-separatrix layers that form the boundary between
open and closed flux. The dynamics of this S-Web causes closed field
plasma to be released onto open field lines, which is observed in the
heliosphere as the slow wind. The S-Web model, therefore, predicts
that the variability of the slow wind is due to the dynamics of the
open closed boundary. We argue that these dynamics are driven by two
main processes: direct driving by photospheric motions and thermal
nonequilibrium. we discuss the variability that would be expected from
each of these processes.
---------------------------------------------------------
Title: A Model for Filament Channel Formation in a Coronal Magnetic
Field
Authors: Knizhnik, Kalman J.; DeVore, C. Richard; Antiochos, Spiro K.
2016shin.confE.136K Altcode:
We investigate a new model for the formation of highly sheared filament
channels above photospheric polarity inversion lines (PILs). The
question of filament channel formation is a major problem in solar
physics, its significance stemming from the propensity of filament
channels to erupt in coronal mass ejections. The free energy released
in these eruptions was originally stored as filament channel shear,
indicating that filament channels are highly non-potential structures,
containing tremendous magnetic helicity. Since magnetic helicity is
conserved under magnetic reconnection in a high-Rm environment such as
the solar corona, this helicity must be injected at the photospheric
level. We present numerical simulations that show, for the first time,
the formation of such highly sheared filament channels as a result
of photospheric helicity injection into a coronal magnetic field
containing both a PIL and a coronal hole (CH). Remarkably, sheared
filament channels form only at the PIL, leaving the rest of the
corona laminar and smooth. We show that this result is in excellent
agreement with observations, and follows directly from the recently
proposed helicity condensation model (Antiochos, 2013). Building on
initial tests of this model performed by Knizhnik, Antiochos &
DeVore (2015), we show that that interchange reconnection between
open and closed magnetic fields drastically affects the CH boundary,
and discuss the implications of this result for observations.
---------------------------------------------------------
Title: Composition of Coronal Mass Ejections
Authors: Zurbuchen, T. H.; Weberg, M.; von Steiger, R.; Mewaldt,
R. A.; Lepri, S. T.; Antiochos, S. K.
2016ApJ...826...10Z Altcode:
We analyze the physical origin of plasmas that are ejected from the
solar corona. To address this issue, we perform a comprehensive analysis
of the elemental composition of interplanetary coronal mass ejections
(ICMEs) using recently released elemental composition data for Fe,
Mg, Si, S, C, N, Ne, and He as compared to O and H. We find that
ICMEs exhibit a systematic abundance increase of elements with first
ionization potential (FIP) < 10 eV, as well as a significant increase
of Ne as compared to quasi-stationary solar wind. ICME plasmas have a
stronger FIP effect than slow wind, which indicates either that an FIP
process is active during the ICME ejection or that a different type of
solar plasma is injected into ICMEs. The observed FIP fractionation
is largest during times when the Fe ionic charge states are elevated
above Q <SUB>Fe</SUB> > 12.0. For ICMEs with elevated charge states,
the FIP effect is enhanced by 70% over that of the slow wind. We argue
that the compositionally hot parts of ICMEs are active region loops that
do not normally have access to the heliosphere through the processes
that give rise to solar wind. We also discuss the implications of
this result for solar energetic particles accelerated during solar
eruptions and for the origin of the slow wind itself.
---------------------------------------------------------
Title: Streamer Blowout CME Initiation: Not Loss-of-Equilibrium,
Not Flux-Cancellation, Not the Kink Instability, and Not the Torus
Instability
Authors: Lynch, Benjamin J.; Masson, S.; Li, Y.; DeVore, C. R.;
Luhmann, J. G.; Antiochos, S. K.; Fisher, G. H.
2016shin.confE..49L Altcode:
We present results from a three-dimensional numerical
magnetohydrodynamics (MHD) simulation of the 2008 June 1-2 slow
streamer blowout CME that Robbrecht et al. [2009] called 'the CME from
nowhere.' <P />We investigate the CME initiation mechanism in detail,
showing definitely the eruption is *not* caused by loss-of-equilibrium,
flux-cancellation, the kink instability or the torus instability,
rather, the rising sheared arcade becomes a CME in the traditional sense
only when the eruptive flare reconnection occurs at the radial current
sheet, forming a flux rope structure *during* the eruption. <P />We
present favorable comparisons between our simulation results and the
multi-viewpoint SOHO-LASCO and STEREO-SECCHI coronagraph observations
of the pre-eruption streamer structure and the initiation and evolution
of the stealth streamer blowout CME. We also present synthetic in-situ
time series at r=15Rs of the plasma and field signatures of the flux
rope CME and show qualitative agreement to the ICME observed by STB
on 2008 June 6-7.
---------------------------------------------------------
Title: Achieving Consistent Doppler Measurements from SDO/HMI Vector
Field Inversions
Authors: Schuck, Peter W.; Antiochos, S. K.; Leka, K. D.; Barnes,
Graham
2016ApJ...823..101S Altcode: 2015arXiv151106500S
NASA’s Solar Dynamics Observatory is delivering vector magnetic
field observations of the full solar disk with unprecedented temporal
and spatial resolution; however, the satellite is in a highly
inclined geosynchronous orbit. The relative spacecraft-Sun velocity
varies by ±3 km s<SUP>-1</SUP> over a day, which introduces major
orbital artifacts in the Helioseismic Magnetic Imager (HMI) data. We
demonstrate that the orbital artifacts contaminate all spatial and
temporal scales in the data. We describe a newly developed three-stage
procedure for mitigating these artifacts in the Doppler data obtained
from the Milne-Eddington inversions in the HMI pipeline. The procedure
ultimately uses 32 velocity-dependent coefficients to adjust 10 million
pixels—a remarkably sparse correction model given the complexity of
the orbital artifacts. This procedure was applied to full-disk images
of AR 11084 to produce consistent Dopplergrams. The data adjustments
reduce the power in the orbital artifacts by 31 dB. Furthermore, we
analyze in detail the corrected images and show that our procedure
greatly improves the temporal and spectral properties of the data
without adding any new artifacts. We conclude that this new procedure
makes a dramatic improvement in the consistency of the HMI data and
in its usefulness for precision scientific studies.
---------------------------------------------------------
Title: Science Objectives of the FOXSI Small Explorer Mission Concept
Authors: Shih, Albert Y.; Christe, Steven; Alaoui, Meriem; Allred,
Joel C.; Antiochos, Spiro K.; Battaglia, Marina; Buitrago-Casas,
Juan Camilo; Caspi, Amir; Dennis, Brian R.; Drake, James; Fleishman,
Gregory D.; Gary, Dale E.; Glesener, Lindsay; Grefenstette, Brian;
Hannah, Iain; Holman, Gordon D.; Hudson, Hugh S.; Inglis, Andrew R.;
Ireland, Jack; Ishikawa, Shin-Nosuke; Jeffrey, Natasha; Klimchuk, James
A.; Kontar, Eduard; Krucker, Sam; Longcope, Dana; Musset, Sophie; Nita,
Gelu M.; Ramsey, Brian; Ryan, Daniel; Saint-Hilaire, Pascal; Schwartz,
Richard A.; Vilmer, Nicole; White, Stephen M.; Wilson-Hodge, Colleen
2016SPD....47.0814S Altcode:
Impulsive particle acceleration and plasma heating at the Sun, from the
largest solar eruptive events to the smallest flares, are related to
fundamental processes throughout the Universe. While there have been
significant advances in our understanding of impulsive energy release
since the advent of RHESSI observations, there is a clear need for
new X-ray observations that can capture the full range of emission
in flares (e.g., faint coronal sources near bright chromospheric
sources), follow the intricate evolution of energy release and changes
in morphology, and search for the signatures of impulsive energy
release in even the quiescent Sun. The FOXSI Small Explorer (SMEX)
mission concept combines state-of-the-art grazing-incidence focusing
optics with pixelated solid-state detectors to provide direct imaging
of hard X-rays for the first time on a solar observatory. We present
the science objectives of FOXSI and how its capabilities will address
and resolve open questions regarding impulsive energy release at the
Sun. These questions include: What are the time scales of the processes
that accelerate electrons? How do flare-accelerated electrons escape
into the heliosphere? What is the energy input of accelerated electrons
into the chromosphere, and how is super-heated coronal plasma produced?
---------------------------------------------------------
Title: Implications of L1 observations for slow solar wind formation
by solar reconnection
Authors: Kepko, L.; Viall, N. M.; Antiochos, S. K.; Lepri, S. T.;
Kasper, J. C.; Weberg, M.
2016GeoRL..43.4089K Altcode:
While the source of the fast solar wind is known to be coronal holes,
the source of the slow solar wind has remained a mystery. Long
time scale trends in the composition and charge states show strong
correlations between solar wind velocity and plasma parameters, yet
these correlations have proved ineffective in determining the slow
wind source. We take advantage of new high time resolution (12 min)
measurements of solar wind composition and charge state abundances
at L1 and previously identified 90 min quasiperiodic structures
to probe the fundamental timescales of slow wind variability. The
combination of new high temporal resolution composition measurements
and the clearly identified boundaries of the periodic structures
allows us to utilize these distinct solar wind parcels as tracers of
slow wind origin and acceleration. We find that each 90 min (2000 Mm)
parcel of slow wind has near-constant speed yet exhibits repeatable,
systematic charge state and composition variations that span the entire
range of statistically determined slow solar wind values. The classic
composition-velocity correlations do not hold on short, approximately
hourlong, time scales. Furthermore, the data demonstrate that these
structures were created by magnetic reconnection. Our results impose
severe new constraints on slow solar wind origin and provide new,
compelling evidence that the slow wind results from the sporadic
release of closed field plasma via magnetic reconnection at the boundary
between open and closed flux in the Sun's atmosphere.
---------------------------------------------------------
Title: Switch-on Shock and Nonlinear Kink Alfvén Waves in Solar
Polar Jets
Authors: DeVore, C. Richard; Karpen, Judith T.; Antiochos, Spiro K.;
Uritsky, Vadim
2016SPD....47.0309D Altcode:
It is widely accepted that solar polar jets are produced by fast
magnetic reconnection in the low corona, whether driven directly by
flux emergence from below or indirectly by instability onset above the
photosphere. In either scenario, twisted flux on closed magnetic field
lines reconnects with untwisted flux on nearby open field lines. Part
of the twist is inherited by the newly reconnected open flux, which
rapidly relaxes due to magnetic tension forces that transmit the twist
impulsively into the outer corona and heliosphere. We propose that this
transfer of twist launches switch-on MHD shock waves, which propagate
parallel to the ambient coronal magnetic field ahead of the shock
and convect a perpendicular component of magnetic field behind the
shock. In the frame moving with the shock front, the post-shock flow
is precisely Alfvénic in all three directions, whereas the pre-shock
flow is super-Alfvénic along the ambient magnetic field, yielding a
density enhancement at the shock front. Nonlinear kink Alfvén waves are
exact solutions of the time-dependent MHD equations in the post-shock
region when the ambient corona is uniform and the magnetic field is
straight. We have performed and analyzed 3D Cartesian and spherical
simulations of polar jets driven by instability onset in the corona. The
results of both simulations are consistent with the generation of
MHD switch-on shocks trailed predominantly by incompressible kink
Alfvén waves. It is noteworthy that the kink waves are irrotational,
in sharp contrast to the vorticity-bearing torsional waves reported
from previous numerical studies. We will discuss the implications of
the results for understanding solar polar jets and predicting their
heliospheric signatures. Our research was supported by NASA’s LWS
TR&T and H-SR programs.
---------------------------------------------------------
Title: The Breakout Model for Coronal Jets with Filaments
Authors: Wyper, Peter; DeVore, C. Richard; Antiochos, Spiro K.
2016SPD....4740203W Altcode:
Coronal jets are impulsive, collimated plasma outflows originating low
in the solar corona. Many of these events exhibit broad, curtain-like
morphologies with helical structure and motions. Recently, Sterling
et al. (2015) [doi:10.1038/nature14556] reported that such jets
are associated with the eruption of small filaments and, therefore,
are miniature versions of corona mass ejections (CMEs). This account
differs from the traditional picture of jets, in that internal flare
reconnection, rather than interchange reconnection with the external
ambient magnetic field, creates the bright loops observed at the
jet base. We present 3D simulations, performed with the Adaptively
Refined MHD Solver (ARMS), which demonstrate how the magnetic breakout
mechanism generates mini-CME-type jets in a compact bipolar region
energized by simple footpoint motions. Our numerical model captures
the formation of the strongly sheared pre-jet filament structure, the
post-jet flare-like loops and ribbons, and the curtain-like untwisting
dynamics observed higher in the corona. We will discuss the significance
of our new results for understanding solar EUV and X-ray jets and
CMEs in general. NASA supported this research by awards to the NASA
Postdoctoral Program (P.F.W.) and the LWS TR&T and H-SR programs
(C.R.D. & S.K.A.).
---------------------------------------------------------
Title: Simulations of Filament Channel Formation in a Coronal
Magnetic Field
Authors: Knizhnik, Kalman; DeVore, C. Richard; Antiochos, Spiro K.
2016SPD....4710301K Altcode:
A major unanswered problem in solar physics has been explaining the
presence of sheared filament channels above photospheric polarity
inversion lines (PILs) and the simultaneous lack of structure in the
‘loop’ portion of the coronal magnetic field. The shear inherent
in filament channels represents not only a form of magnetic energy,
but also magnetic helicity. As a result, models of filament channel
formation need to explain not only why helicity is observed above
PILs, but also why it is apparently not observed anywhere else in
the corona. Previous results (Knizhnik, Antiochos & DeVore,
2015) have suggested that any helicity injected into the coronal
field inverse-cascades in scale, a process known as magnetic helicity
condensation (Antiochos, 2013). In this work, we present high resolution
numerical simulations of photospheric helicity injection into a coronal
magnetic field that contains both a PIL and a coronal hole (CH). We show
conclusively that the inverse cascade of magnetic helicity terminates at
the PIL, resulting in the formation of highly sheared filament channels
and a smooth, untwisted corona. We demonstrate that even though magnetic
helicity is injected throughout the flux system, it accumulates only
at the PIL, where it manifests itself in the form of highly sheared
filament channels, while any helicity obtained by the CH is ejected
out of the system. We show that the formation of filament channels is
both qualitatively and quantitatively in agreement with observations
and discuss the implications of our simulations for observations.This
work was supported by the NASA Earth and Space Science Fellowship,
LWS TR&T and H-SR Programs.
---------------------------------------------------------
Title: Achieving Consistent Doppler Measurements from SDO/HMI Vector
Field Inversions
Authors: Schuck, Peter W.; Antiochos, Spiro K.; Leka, K. D.; Barnes,
Graham
2016SPD....47.1207S Altcode:
NASA’s Solar Dynamics Observatory is delivering vector magnetic
field observations of the full solar disk with unprecedented temporal
and spatial resolution; however, the satellite is in a highly inclined
geosynchronous orbit. The relative spacecraft-Sun velocity varies by
±3 km/s over a day which introduces major orbital artifacts in the
Helioseismic Magnetic Imager data. We demonstrate that the orbital
artifacts contaminate all spatial and temporal scales in the data. We
describe a newly-developed three stage procedure for mitigating these
artifacts in the Doppler data obtained from the Milne-Eddington
inversions in the HMI Pipeline. The procedure ultimately uses 32
velocity dependent coefficients to adjust 10 million pixels - a
remarkably sparse correction model given the complexity of the orbital
artifacts. This procedure was applied to full disk images of AR11084 to
produce consistent Dopplergrams. The data adjustments reduce the power
in the orbital artifacts by 31dB. Furthermore, we analyze in detail
the corrected images and show that our procedure greatly improves
the temporal and spectral properties of the data without adding any
new artifacts. We conclude that this new procedure makes a dramatic
improvement in the consistency of the HMI data and in its usefulness
for precision scientific studies.
---------------------------------------------------------
Title: A Model for Stealth Coronal Mass Ejections
Authors: Lynch, Benjamin J.; Masson, Sophie; Li, Yan; DeVore,
C. Richard; Luhmann, Janet; Antiochos, Spiro K.; Fisher, George H.
2016SPD....47.0616L Altcode:
Stealth coronal mass ejections (CMEs) are events in which there
are almost no observable signatures of the CME eruption in the low
corona but often a well-resolved slow flux rope CME observed in
the coronagraph data. We present results from a three-dimensional
numerical magnetohydrodynamics (MHD) simulation of the 2008 June 1-2
slow streamer blowout CME that Robbrecht et al. [2009] called “the
CME from nowhere.” We model the global coronal structure using a
1.4 MK isothermal solar wind and a low-order potential field source
surface representation of the Carrington Rotation 2070 magnetogram
synoptic map. The bipolar streamer belt arcade is energized by simple
shearing flows applied in the vicinity of the helmet streamer’s
polarity inversion line. The slow expansion of the energized
helmet-streamer arcade results in the formation of a radial current
sheet. The subsequent onset of expansion-driven flare reconnection
initiates the stealth CME while gradually releasing ~1.5E+30 erg of
stored magnetic energy over the 20+ hour eruption duration. We show
the energy flux available for flare heating and flare emission during
the eruption is approximately two orders of magnitude below the energy
flux required to heat the ambient background corona, thus confirming the
“stealth” character of the 2008 June 1-2 CME’s lack of observable
on disk signatures. We also present favorable comparisons between our
simulation results and the multi-viewpoint SOHO-LASCO and STEREO-SECCHI
coronagraph observations of the pre-eruption streamer structure and
the initiation and evolution of the stealth streamer blowout CME.
---------------------------------------------------------
Title: Structures in the Outer Solar Atmosphere
Authors: Fletcher, L.; Cargill, P. J.; Antiochos, S. K.; Gudiksen,
B. V.
2016mssf.book..231F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Reconnection Between Twisted Flux Tubes - Implications for
Coronal Heating
Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R.; Klimchuk,
J. A.; Wyper, P. F.
2015AGUFMSH13B2439K Altcode:
The nature of the heating of the Sun's corona has been a long-standing
unanswered problem in solar physics. Beginning with the work of Parker
(1972), many authors have argued that the corona is continuously heated
through numerous small-scale reconnection events known as nanoflares. In
these nanoflare models, stressing of magnetic flux tubes by photospheric
motions causes the field to become misaligned, producing current sheets
in the corona. These current sheets then reconnect, converting the
free energy stored in the magnetic field into heat. In this work,
we use the Adaptively Refined MHD Solver (ARMS) to perform 3D MHD
simulations that dynamically resolve regions of strong current to study
the reconnection between twisted flux tubes in a plane-parallel Parker
configuration. We investigate the energetics of the process, and show
that the flux tubes accumulate stress gradually before undergoing
impulsive reconnection. We study the motion of the individual field
lines during reconnection, and demonstrate that the connectivity of the
configuration becomes extremely complex, with multiple current sheets
being formed, which could lead to enhanced heating. In addition, we
show that there is considerable interaction between the twisted flux
tubes and the surrounding untwisted field, which contributes further
to the formation of current sheets. The implications for observations
will be discussed. This work was funded by a NASA Earth and Space
Science Fellowship, and by the NASA TR&T Program.
---------------------------------------------------------
Title: Slow Solar Wind: Observable Characteristics for Constraining
Modelling
Authors: Ofman, L.; Abbo, L.; Antiochos, S. K.; Hansteen, V. H.;
Harra, L.; Ko, Y. K.; Lapenta, G.; Li, B.; Riley, P.; Strachan, L.;
von Steiger, R.; Wang, Y. M.
2015AGUFMSH11F..03O Altcode:
The Slow Solar Wind (SSW) origin is an open issue in the post SOHO
era and forms a major objective for planned future missions such as
the Solar Orbiter and Solar Probe Plus.Results from spacecraft data,
combined with theoretical modeling, have helped to investigate many
aspects of the SSW. Fundamental physical properties of the coronal
plasma have been derived from spectroscopic and imaging remote-sensing
data and in-situ data, and these results have provided crucial insights
for a deeper understanding of the origin and acceleration of the
SSW.Advances models of the SSW in coronal streamers and other structures
have been developed using 3D MHD and multi-fluid equations.Nevertheless,
there are still debated questions such as:What are the source regions
of SSW? What are their contributions to the SSW?Which is the role
of the magnetic topology in corona for the origin, acceleration and
energy deposition of SSW?Which are the possible acceleration and heating
mechanisms for the SSW?The aim of this study is to present the insights
on the SSW origin and formationarisen during the discussions at the
International Space Science Institute (ISSI) by the Team entitled
”Slowsolar wind sources and acceleration mechanisms in the corona”
held in Bern (Switzerland) in March2014--2015. The attached figure will
be presented to summarize the different hypotheses of the SSW formation.
---------------------------------------------------------
Title: Capabilities of a FOXSI Small Explorer
Authors: Inglis, A. R.; Christe, S.; Glesener, L.; Krucker, S.; Dennis,
B. R.; Shih, A.; Wilson-Hodge, C.; Gubarev, M.; Hudson, H. S.; Kontar,
E.; Buitrago Casas, J. C.; Drake, J. F.; Caspi, A.; Holman, G.; Allred,
J. C.; Ryan, D.; Alaoui, M.; White, S. M.; Saint-Hilaire, P.; Klimchuk,
J. A.; Hannah, I. G.; Antiochos, S. K.; Grefenstette, B.; Ramsey,
B.; Jeffrey, N. L. S.; Reep, J. W.; Schwartz, R. A.; Ireland, J.
2015AGUFMSH43B2456I Altcode:
We present the FOXSI (Focusing Optics X-ray Solar Imager) small explorer
(SMEX) concept, a mission dedicated to studying particle acceleration
and energy release on the Sun. FOXSI is designed as a 3-axis stabilized
spacecraft in low-Earth orbit making use of state-of-the-art grazing
incidence focusing optics, allowing for direct imaging of solar
X-rays. The current design being studied features three telescope
modules deployed in a low-inclination low-earth orbit (LEO). With a 15
meter focal length enabled by a deployable boom, FOXSI will observe
the Sun in the 3-50 keV energe range. The FOXSI imaging concept has
already been tested on two sounding rocket flights, in 2012 and 2014
and on the HEROES balloon payload flight in 2013. FOXSI will image
the Sun with an angular resolution of 5”, a spectral resolution of
0.5 keV, and sub-second temporal resolution using CdTe detectors. In
this presentation we investigate the science objectives and targets
which can be accessed from this mission. Because of the defining
characteristic of FOXSI is true imaging spectroscopy with high dynamic
range and sensitivity, a brand-new perspective on energy release on the
Sun is possible. Some of the science targets discussed here include;
flare particle acceleration processes, electron beams, return currents,
sources of solar energetic particles (SEPs), as well as understanding
X-ray emission from active region structures and the quiescent corona.
---------------------------------------------------------
Title: The Dynamics of Coronal-Hole Boundaries
Authors: Higginson, A. K.; Antiochos, S. K.; DeVore, C. R.; Wyper,
P. F.; Zurbuchen, T.
2015AGUFMSH24A..06H Altcode:
The source of the slow solar wind at the Sun is the subject of intense
debate in solar and heliospheric physics. Because the majority of the
solar wind observed at Earth is slow wind, understanding its origin
is essential for understanding and predicting Earth's space weather
environment. In-situ and remote observations show that, compared to
the fast wind, the slow solar wind corresponds to higher freeze-in
temperatures, as indicated by charge-state ratios, and more corona-like
elemental abundances. These results indicate that the most likely source
for the slow wind is the hot plasma in the closed-field corona; however,
the release mechanism for the wind from the closed-field regions is
far from understood. Here we present the first fully dynamic, 3D MHD
simulations of a coronal-hole boundary driven by photospheric convective
flows. We determine in detail the opening and closing of coronal flux
due to photospheric motions at the base of a helmet streamer. These
changes should lead to the release of plasma from the closed magnetic
field at the edge of the streamer. Our analysis demonstrates that the
bulk of the release is due to interchange reconnection. We calculate
the effective of numerical Lundquist number on the dynamics and discuss
the implications of our results for theories of slow-wind origin,
in particular the S-Web model. We also discuss the implications of
our results for observations, in particular from the upcoming Solar
Orbiter and Solar Probe Plus missions. This work was supported by the
NASA SR&T and TR&T Programs.
---------------------------------------------------------
Title: Determining the Energy Transport Through the Photosphere into
Corona with HMI
Authors: Schuck, P. W.; Antiochos, S. K.
2015AGUFMSH31B2422S Altcode:
The underlying source of all solar activity, from the largest explosive
solar eruptive event to the quasi-steady radiation from quiet regions
is the transport of free magnetic energy into the chromosphere
and corona through the photosphere. There are two mechanisms for
this transport, the emergence of magnetic field and accompanying
electric currents, and the stressing of pre-emerged chromosphere and
coronal field by photospheric flows. Consequently, if we are ever to
understand solar activity it is essential that the flow of energy be
measured accurately. In principle, this can be estimated from the high
spatial and temporal resolution vector magnetograms obtained by HMI on
SDO. However, due to the artifacts introduced by the orbital motions of
SDO convolved with the finite-sampling of the relevant polarizations,
the data cannot be used for measuring the energy flux or helicity flux
accurately. We present recent work on the first successful attempt
to remove the orbital artifacts. We describe the procedure, which
consists of correcting each pixel separately for its bias and gain. We
show results for before and after our artifact removal procedures. The
method is being prepared for community use through the SDO pipeline. We
discuss the application of our methods to analyzing active regions
both with and without major eruptions. This work was supported by the
NASA LWS and R&A Programs.
---------------------------------------------------------
Title: The Origin and Development of Solar Eruptive Events
Authors: Antiochos, S. K.; DeVore, C. R.; Karpen, J. T.; Masson, S.
2015AGUFMSH11A2384A Altcode:
Solar eruptive events (SEE), which consist of fast coronal mass
ejections and intense flares, are the largest and most energetic form
of solar activity, and are the drivers of the most destructive space
weather throughout interplanetary space. Understanding the physical
origin of these giant magnetic explosions is absolutely essential for
any first-principles based space weather forecasting and, consequently,
is a core focus of the NASA LWS Program. We describe how magnetic
reconnection is responsible for the energy buildup that leads to SEEs,
how it drives the explosive energy release, and how it controls the
propagation of the event. Reconnection turns out to be especially
important for understanding the escape of high-energy particles into
the heliosphere. A key issue for numerical simulation of SEEs is the
effect of the resistivity model used by the simulation, because the
onset and subsequent development of reconnection inherently dependent on
the effective resistivity. We present the latest ultra-high numerical
resolution 2.5D simulations quantifying how the reconnection dynamics
scale with effective resistivity. We also present 3D simulations
demonstrating the complexities introduced by reconnection in a realistic
3D system. The implications of our results for interpreting observations
and for developing space weather capabilities will be described. This
work was supported by the NASA LWS Strategic Capability Program.
---------------------------------------------------------
Title: Shear-Driven Reconnection in Kinetic Models
Authors: Black, C.; Antiochos, S. K.; Germaschewski, K.; Karpen,
J. T.; DeVore, C. R.; Bessho, N.
2015AGUFMSH43A2432B Altcode:
The explosive energy release in solar eruptive phenomena is believed
to be due to magnetic reconnection. In the standard model for coronal
mass ejections (CME) and/or solar flares, the free energy for the
event resides in the strongly sheared magnetic field of a filament
channel. The pre-eruption force balance consists of an upward force
due to the magnetic pressure of the sheared field countered by a
downward tension due to overlying unsheared field. Magnetic reconnection
disrupts this force balance; therefore, it is critical for understanding
CME/flare initiation, to model the onset of reconnection driven by the
build-up of magnetic shear. In MHD simulations, the application of a
magnetic-field shear is a trivial matter. However, kinetic effects are
dominant in the diffusion region and thus, it is important to examine
this process with PIC simulations as well. The implementation of
such a driver in PIC methods is challenging, however, and indicates
the necessity of a true multiscale model for such processes in
the solar environment. The field must be sheared self-consistently
and indirectly to prevent the generation of waves that destroy the
desired system. Plasma instabilities can arise nonetheless. In the
work presented here, we show that we can control this instability
and generate a predicted out-of-plane magnetic flux. This material
is based upon work supported by the National Science Foundation under
Award No. AGS-1331356.
---------------------------------------------------------
Title: On Streamer-Blowout CMEs That Aren't Really CMEs: How the
Corona Makes Slow Flux Rope-Like ICMEs Without an Explosive Release
of Free Magnetic Energy
Authors: Lynch, B. J.; Masson, S.; Li, Y.; DeVore, C. R.; Luhmann,
J. G.; Antiochos, S. K.
2015AGUFMSH53A2459L Altcode:
We present a 3D numerical MHD simulation of the 2008 Jun 2 gradual
streamer blowout CME that had virtually no identifiable low coronal
signatures. We energize the field by simple footpoint shearing along
the source region's polarity inversion line and model the background
solar wind structure using an ~2MK isothermal wind and a low-order
potential field source surface representation of the CR2070 synoptic
magnetogram. Our results show that the CME "initiation" is obtained by
slowly disrupting the quasi-steady-state configuration of the helmet
streamer, resulting in the standard eruptive flare picture that ejects
the sheared/twisted fields -- very slowly and on a relatively large
scale -- with virtually no decrease in the global magnetic energy. We
obtain a relatively slow CME eruption of order the background solar
wind speed (Vcme ~ 300 km/s by 15 Rs). We argue that these very
slow, expansion-driven "eruptions" are merely the natural and gradual
response of the large-scale corona to the accumulation of global-scale
stress (e.g. differential rotation). We present comparisons of the CME
propagation through the corona (≤15Rs) in synthetic white-light images
derived from the simulation density structure with multi-spacecraft
coronagraph data from STEREO/SECCHI and SOHO/LASCO. We show a favorable
comparison between the simulation's ICME flux rope structure with the
in situ STEREO observations.
---------------------------------------------------------
Title: Filament Channel Formation via Magnetic Helicity Condensation
Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R.
2015ApJ...809..137K Altcode: 2014arXiv1411.5396K
A major unexplained feature of the solar atmosphere is the accumulation
of magnetic shear in the form of filament channels at photospheric
polarity inversion lines (PILs). In addition to free energy,
this shear represents magnetic helicity, which is conserved under
reconnection. In this paper we address the problem of filament channel
formation and show how filaments acquire their shear and magnetic
helicity. The results of three-dimensional (3D) simulations using
the Adaptively Refined Magnetohydrodynamics Solver are presented. Our
findings support the model of filament channel formation by magnetic
helicity condensation that was developed by Antiochos. We consider
the small-scale photospheric twisting of a quasi-potential flux system
that is bounded by a PIL and contains a coronal hole (CH). The magnetic
helicity injected by the small-scale photospheric motions is shown to
inverse cascade up to the largest allowable scales that define the
closed flux system: the PIL and the CH. This process produces field
lines that are both sheared and smooth, and are sheared in opposite
senses at the PIL and the CH. The accumulated helicity and shear flux
are shown to be in excellent quantitative agreement with the helicity
condensation model. We present a detailed analysis of the simulations,
including comparisons of our analytical and numerical results, and
discuss their implications for observations.
---------------------------------------------------------
Title: Understanding Magnetic Reconnection Drivers: Magnetic Field
Shear in Kinetic Models
Authors: Black, Carrie E.; Antiochos, Spiro K.; DeVore, C. Richard;
Germaschewski, Kai; Bessho, Naoki; Karpen, Judith T.
2015shin.confE..25B Altcode:
The explosive energy release in solar eruptive phenomena believed to
be due to magnetic reconnection. In the standard model for coronal
mass ejections (CME) and/or solar flares, the free energy for the
event resides in the strongly sheared magnetic field of a filament
channel. The pre-eruption force balance consists of an upward force
due to the magnetic pressure of the sheared field countered by a
downward tension due to overlying unsheared field. Magnetic reconnection
disrupts this force balance, therefore, it is critical for understanding
CME/flare initiation, to model the onset of reconnection driven by the
build-up of magnetic shear. In MHD simulations, the application of a
magnetic-field shear is a trivial matter. However, kinetic effects
are important in the diffusion region and thus, it is important to
examine this process with PIC simulations as well. The implementation
of such a driver in PIC methods is nontrivial, however, and indicates
the necessity of a true multiscale model for such processes in
the solar environment. The field must be sheared self-consistently
and indirectly to prevent the generation of waves that destroy the
desired system. Plasma instabilities can arise nonetheless. In the
work presented here, we show that we can control this instability
and generate a predicted out-of-plane magnetic flux. This material
is based upon work supported by the National Science Foundation under
Award No. AGS-1331356.
---------------------------------------------------------
Title: Erratum: “Numerical Simulations of Helicity Condensation
in the Solar Corona” <A href="/abs/2015ApJ...805...61Z">(2015,
ApJ, 805, 61)</A>
Authors: Zhao, L.; DeVore, C. R.; Antiochos, S. K.; Zurbuchen, T. H.
2015ApJ...807..113Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Helicity Condensation During Reconnection of Twisted Flux
Tubes: Implications for Coronal Heating
Authors: Knizhnik, Kalman Joshua; Antiochos, Spiro K.; DeVore,
C. Richard; Klimchuk, James A.; Wyper, Peter F.
2015shin.confE..18K Altcode:
The nature of the heating of the Sun's corona has been a long-standing
unanswered problem in solar physics. Beginning with the work of Parker
(1972), many authors have argued that the corona is continuously
heated through numerous small-scale reconnection events known as
nanoflares. In these nanoflare models, braiding of magnetic flux tubes
by surface motions causes the field to become misaligned. The current
sheet separating the misaligned field eventually reconnects, converting
the energy stored in the magnetic field into heat. A major challenge
facing these models, however, is that the braiding required for this
process injects magnetic helicity into the corona, and helicity is
conserved under reconnection. In contrast, EUV and X-ray images of
coronal loops reveal invariably smooth, laminar structures. The
recently proposed helicity condensation model (Antiochos 2013)
resolves this difficulty, explaining how reconnection transports
helicity throughout the solar atmosphere and produces a smooth,
hot corona. In this model, reconnection between adjacent flux tubes,
twisted and tangled by surface convection, transports helicity to ever
larger scales, where it ultimately condenses in filament channels. The
reconnection that occurs throughout the solar atmosphere not only
results in a smooth corona, but its net effect is to convert much of
the magnetic energy injected by surface motions into heat. In this
work, we use the Adaptively Refined MHD Solver (ARMS) to perform 3D MHD
simulations that dynamically resolve regions of strong current to study
the reconnection between multiple twisted flux tubes in a plane-parallel
Parker configuration. We investigate the energetics of the process, and
show that the flux tubes accumulate stress gradually before undergoing
impulsive reconnection. We place constraints on the amount of heating
expected from such reconnection. Finally, we study the motion of the
individual field lines during the impulsive reconnection events.
---------------------------------------------------------
Title: Understanding the Effects of 3D Reconnection in a Breakout CME
Authors: Antiochos, Spiro K.
2015shin.confE...5A Altcode:
Coronal mass ejections/solar flares are the most dramatic and most
energetic forms of solar energy release. Most theories for this energy
release postulate magnetic reconnection as the underlying physical
mechanism. In previous 2.5D calculations we showed how two different
types of reconnection both trigger the eruption and produce the bulk
of the energy release. The problem, however, is that reconnection
in 2D is tightly constrained and, consequently, leads to magnetic
topologies such as disconnected plasmoids that are not physical and not
observed. Consequently, we have extended our calculations to a fully
3D topology that includes a multi-polar coronal field suitable for a
breakout CME/eruptive flare near a coronal hole region. We performed
high-resolution 3D MHD numerical simulations with the Adaptively
Refined MHD Solver (ARMS). We show how the complex interactions
between the 3D flare and breakout reconnections reproduce all the
main observational features of CMEs/flares and impulsive SEPs. We
also discuss the implications of our results for understanding the
heliospheric signatures of CMEs and for further tests of the model. <P
/>This research was supported, in part, by the NASA SR&T and
TR&T Programs.
---------------------------------------------------------
Title: Simulations of Coronal-Hole Boundary Dynamics Near Helmet
Streamers
Authors: Higginson, Aleida Katherine; Antiochos, S. K.; DeVore, C. R.;
Zurbuchen, T. H.
2015shin.confE..19H Altcode:
The source of the slow solar wind at the Sun is the subject of intense
debate in solar and heliospheric physics. Because the majority of the
solar wind observed at Earth is slow wind, understanding its origin
is essential for understanding and predicting Earth's space weather
environment. In-situ and remote observations show that, compared to
the fast wind, the slow solar wind corresponds to higher freeze-in
temperatures, as indicated by charge-state ratios, and more corona-like
elemental abundances. These results indicate that the most likely
source for the slow wind is the hot plasma in the closed-field corona;
however, the release mechanism for the wind from the closed-field
regions is far from understood. Here we present fully dynamic, 3D MHD
simulations of a driven coronal-hole boundary. We determine in detail
the opening and closing of coronal flux due to photospheric motions
at the base of a helmet streamer. These changes should lead to the
release of plasma from the closed magnetic field at the edge of the
streamer. We discuss the implications of our results for theories of
slow-wind origin, in particular the S-Web model.
---------------------------------------------------------
Title: Structures in the Outer Solar Atmosphere
Authors: Fletcher, L.; Cargill, P. J.; Antiochos, S. K.; Gudiksen,
B. V.
2015SSRv..188..211F Altcode: 2014SSRv..tmp...52F; 2014arXiv1412.7378F
The structure and dynamics of the outer solar atmosphere are reviewed
with emphasis on the role played by the magnetic field. Contemporary
observations that focus on high resolution imaging over a range
of temperatures, as well as UV, EUV and hard X-ray spectroscopy,
demonstrate the presence of a vast range of temporal and spatial scales,
mass motions, and particle energies present. By focusing on recent
developments in the chromosphere, corona and solar wind, it is shown
that small scale processes, in particular magnetic reconnection, play
a central role in determining the large-scale structure and properties
of all regions. This coupling of scales is central to understanding
the atmosphere, yet poses formidable challenges for theoretical models.
---------------------------------------------------------
Title: Numerical Simulations of Helicity Condensation in the Solar
Corona
Authors: Zhao, L.; DeVore, C. R.; Antiochos, S. K.; Zurbuchen, T. H.
2015ApJ...805...61Z Altcode:
The helicity condensation model has been proposed by Antiochos to
explain the observed smoothness of coronal loops and the observed
buildup of magnetic shear at filament channels. The basic hypothesis
of the model is that magnetic reconnection in the corona causes the
magnetic stress injected by photospheric motions to collect only at
those special locations where prominences are observed to form. In this
work we present the first detailed quantitative MHD simulations of the
reconnection evolution proposed by the helicity condensation model. We
use the well-known ansatz of modeling the closed corona as an initially
uniform field between two horizontal photospheric plates. The system is
driven by applying photospheric rotational flows that inject magnetic
helicity into the corona. The flows are confined to a finite region
on the photosphere so as to mimic the finite flux system of a bipolar
active region, for example. The calculations demonstrate that, contrary
to common belief, opposite helicity twists do not lead to significant
reconnection in such a coronal system, whereas twists with the same
sense of helicity do produce substantial reconnection. Furthermore,
we find that for a given amount of helicity injected into the corona,
the evolution of the magnetic shear is insensitive to whether the
pattern of driving photospheric motions is fixed or quasi-random. In
all cases, the shear propagates via reconnection to the boundary of
the flow region while the total magnetic helicity is conserved, as
predicted by the model. We discuss the implications of our results
for solar observations and for future, more realistic simulations of
the helicity condensation process.
---------------------------------------------------------
Title: Filament Channel Formation Via Magnetic Helicity Condensation
Authors: Knizhnik, Kalman Joshua; Antiochos, Spiro K.; DeVore,
C. Richard
2015TESS....111305K Altcode:
A major unexplained feature of the solar atmosphere is the accumulation
of magnetic shear, in the form of filament channels, at photospheric
polarity inversion lines (PILs). In addition to free energy, this
shear also represents magnetic helicity, which is conserved under
reconnection. In this work, we address the problem of filament
channel formation and show how they acquire their shear and magnetic
helicity. The results of 3D simulations using the Adaptively Refined
Magnetohydrodynamics Solver (ARMS) are presented that support the
model of filament channel formation by magnetic helicity condensation
developed by Antiochos (2013). We consider the convective twisting of
a quasi-potential flux system that is bounded by a PIL and contains a
coronal hole (CH). The magnetic helicity injected by the small-scale
photospheric motions is shown to inverse-cascade up to the largest
allowable scales that defined the closed flux system: the PIL and
the CH. This process produces field lines that are both sheared and
smooth, and are sheared in opposite senses at the PIL and the CH. The
accumulated helicity and shear flux are shown to be in excellent
quantitative agreement with the helicity-condensation model. We present
a detailed analysis of the simulations, including comparisons of our
analytical and numerical results, and discuss their implications for
observations. Our research was supported by NASA's Earth and Space
Science Fellowship (K.J.K.) and Heliophysics Supporting Research
(S.K.A. and C.R.D.) programs.
---------------------------------------------------------
Title: Magnetic Reconnection Onset and Energy Release at Current
Sheets
Authors: DeVore, C. R.; Antiochos, Spiro K.
2015TESS....110201D Altcode:
Reconnection and energy release at current sheets are important at the
Sun (coronal heating, coronal mass ejections, flares, and jets) and at
the Earth (magnetopause flux transfer events and magnetotail substorms)
and other magnetized planets, and occur also at the interface between
the Heliosphere and the interstellar medium, the heliopause. The
consequences range from relatively quiescent heating of the ambient
plasma to highly explosive releases of energy and accelerated
particles. We use the Adaptively Refined Magnetohydrodynamics
Solver (ARMS) model to investigate the self-consistent formation and
reconnection of current sheets in an initially potential 2D magnetic
field containing a magnetic null point. Unequal stresses applied to the
four quadrants bounded by the X-line separatrix distort the potential
null into a double-Y-type current sheet. We find that this distortion
eventually leads to onset of fast magnetic reconnection across the
sheet, with copious production, merging, and ejection of magnetic
islands due to plasmoid instability. In the absence of a mechanism
for ideal instability or loss of equilibrium of the global structure,
however, this reconnection leads to minimal energy release. Essentially,
the current sheet oscillates about its force-free equilibrium
configuration. When the structure is susceptible to a large-scale
rearrangement of the magnetic field, on the other hand, the energy
release becomes explosive. We identify the conditions required for
reconnection to transform rapidly a large fraction of the magnetic free
energy into kinetic and other forms of plasma energy, and to restructure
the current sheet and its surrounding magnetic field dramatically. We
discuss the implications of our results for understanding heliophysical
activity, particularly eruptions, flares, and jets in the corona.Our
research was supported by NASA’s Heliophysics Supporting Research
and Living With a Star Targeted Research and Technology programs.
---------------------------------------------------------
Title: Understanding Magnetic Reconnection: The Physical Mechanism
Driving Space Weather
Authors: Black, Carrie; Antiochos, Spiro K.; Karpen, Judith T.;
Germaschewski, Kai; Bessho, Naoki
2015TESS....111004B Altcode:
The explosive energy release in solar eruptive events is believed to
be due to magnetic reconnection. In the standard model for coronal
mass ejections (CME) and/or solar flares, the free energy for the
event resides in the strongly sheared magnetic field of a filament
channel. The pre-eruption force balance consists of an upward
force due to the magnetic pressure of the sheared field countered
by the downward tension of the overlying unsheared field. Magnetic
reconnection disrupts this force balance. Therefore, to understand
CME/flare initiation, it is critical to model the onset of reconnection
driven by the build-up of magnetic shear. In MHD simulations, the
application of a magnetic-field shear is trivial. However, kinetic
effects are important in the diffusion region and thus, it is important
to examine this process with PIC simulations as well. The implementation
of such a driver in PIC methods is nontrivial, however, and indicates
the necessity of a true multiscale model for such processes in the
solar environment. The field must be sheared self-consistently and
indirectly to prevent the generation of waves that destroy the desired
system. In the work presented here, we show reconnection in an X-Point
geometry due to a velocity shear driver perpendicular to the plane of
reconnection.This material is based upon work supported by the National
Science Foundation under Award No. AGS-1331356 and NASA's Living With
a Star Targeted Research and Technology program.
---------------------------------------------------------
Title: Modeling Reconnection-driven Polar Jets from the Sun to
the Heliosphere
Authors: Karpen, Judith T.; DeVore, C. R.; Antiochos, Spiro K.
2015TESS....120303K Altcode:
Jets from coronal holes on the Sun have been observed in EUV and
white-light emissions since the launch of SOHO, but the physical
mechanism responsible for these events remains elusive. An important
clue about their origin lies in their association with small
intrusions of minority polarity within the large-scale open magnetic
field, strongly suggesting that these jets are powered by interchange
reconnection between embedded bipoles (closed flux) and the surrounding
open flux (Antiochos 1999). We have explored this model for polar jets
through a series of computational investigations of the embedded-bipole
paradigm. The results demonstrate that energetic, collimated, Alfvénic
flows can be driven by explosive reconnection between twisted closed
flux of the minority polarity and the unstressed external field (e.g.,
Pariat et al. 2009, 2010, 2015). Our previous studies were focused on
the dynamics and energetics of this process close to the solar surface,
utilizing Cartesian geometry without gravity or wind. In the present
study, we compare new simulations of reconnection-driven polar jets
in spherical geometry and an isothermal solar wind with Cartesian,
gravity- and wind-free simulations. Our new, more realistic simulations
strongly support the interchange reconnection model as the explanation
for observed polar jets. We pay particular attention to identifying
observable signatures and measuring the evolving mass, wave, and energy
fluxes as the jet extends toward heights comparable to the perihelion
of Solar Probe Plus.This research was supported by NASA's Living With
a Star Targeted Research and Technology program.
---------------------------------------------------------
Title: 3D Simulations of Helmet Streamer Dynamics and Implications
for the Slow Solar Wind
Authors: Higginson, Aleida K.; Antiochos, Spiro K.; DeVore, C. R.;
Zurbuchen, Thomas H.
2015TESS....110804H Altcode:
The source of the slow solar wind at the Sun is still an issue of
intense debate in solar and heliospheric physics. Because the majority
of the solar wind observed at Earth is slow wind, understanding its
origin is essential for understanding and predicting Earth’s space
weather environment. In-situ and remote observations show that, when
compared to the fast wind, the slow solar wind corresponds to higher
freeze-in temperatures, as indicated by charge-state ratios, and more
corona-like elemental abundance ratios. These results indicate that
the most likely source for the slow wind is the hot plasma in the
closed-field corona, but the release mechanism(s) for the wind from
the closed-field regions is far from understood. We perform fully
dynamic, 3D MHD simulations in order to the study the opening and
closing of the Sun’s magnetic field that leads to the escape of the
slow solar wind. In particular, we calculate the dynamics of helmet
streamers that are driven by photospheric motions such as supergranular
flows. We determine in detail the opening and closing of coronal flux,
and discuss the implications of our results for theories of slow wind
origin, especially the S-Web model. We also determine observational
signatures for the upcoming inner heliosphere missions Solar Orbiter
and Solar Probe Plus.This work was supported by the NASA SR&T and
TR&T Programs.
---------------------------------------------------------
Title: Flare Particle Escape in 3D Solar Eruptive Events
Authors: Antiochos, Spiro K.; Masson, Sophie; DeVore, C. R.
2015TESS....111404A Altcode:
Among the most important, but least understood forms of space weather
are the so-called Impulsive Solar Energetic Particle (SEP) events,
which can be especially hazardous to deep-space astronauts. These
energetic particles are generally believed to be produced by the
flare reconnection that is the primary driver of solar eruptive events
(SEE). A key point is that in the standard model of SEEs, the particles
should remain trapped in the coronal flare loops and in the ejected
plasmoid, the CME. However, flare-accelerated particles frequently
reach the Earth long before the CME does. In previous 2.5D calculations
we showed how the external reconnection that is an essential element
of the breakout model for CME initiation could lead to the escape of
flare-accelerated particles. The problem, however, is that in 2.5D this
reconnection also tends to destroy the plasmoid, which disagrees with
the observation that SEP events are often associated with well-defined
plasmoids at 1 AU known as “magnetic clouds”. Consequently,
we have extended our model to a fully 3D topology that includes a
multi-polar coronal field suitable for a breakout SEE near a coronal
hole region. We performed high-resolution 3D MHD numerical simulations
with the Adaptively Refined MHD Solver (ARMS). Our results demonstrate
that the model allows for the effective escape of energetic particles
from deep within an ejecting well-defined plasmoid. We show how the
complex interactions between the flare and breakout reconnection
reproduce all the main observational features of SEEs and SEPs. We
discuss the implications of our calculations for the upcoming Solar
Orbiter and Solar Probe Plus missions, which will measure SEEs and SEPs
near the Sun, thereby, mitigating propagation effects.This research
was supported, in part, by the NASA SR&T and TR&T Programs.
---------------------------------------------------------
Title: A Model for the Electrically Charged Current Sheet of a Pulsar
Authors: DeVore, C. R.; Antiochos, S. K.; Black, C. E.; Harding,
A. K.; Kalapotharakos, C.; Kazanas, D.; Timokhin, A. N.
2015ApJ...801..109D Altcode:
Global-scale solutions for the magnetosphere of a pulsar consist of
a region of low-lying, closed magnetic field near the star, bounded
by opposite-polarity regions of open magnetic field along which the
pulsar wind flows into space. Separating these open-field regions is
a magnetic discontinuity—an electric current sheet—consisting of
generally nonneutral plasma. We have developed a self-consistent model
for the internal equilibrium structure of the sheet by generalizing the
charge-neutral Vlasov/Maxwell equilibria of Harris and Hoh to allow for
net electric charge. The resulting equations for the electromagnetic
field are solved analytically and numerically. Our results show that the
internal thermal pressure needed to establish equilibrium force balance,
and the associated effective current-sheet thickness and magnetization,
can differ by orders of magnitude from the Harris/Hoh charge-neutral
limit. The new model provides a starting point for kinetic or fluid
investigations of instabilities that can cause magnetic reconnection
and flaring in pulsar magnetospheres.
---------------------------------------------------------
Title: Model for straight and helical solar jets. I. Parametric
studies of the magnetic field geometry
Authors: Pariat, E.; Dalmasse, K.; DeVore, C. R.; Antiochos, S. K.;
Karpen, J. T.
2015A&A...573A.130P Altcode:
Context. Jets are dynamic, impulsive, well-collimated plasma events
developing at many different scales and in different layers of
the solar atmosphere. <BR /> Aims: Jets are believed to be induced
by magnetic reconnection, a process central to many astrophysical
phenomena. Studying their dynamics can help us to better understand the
processes acting in larger eruptive events (e.g., flares and coronal
mass ejections) as well as mass, magnetic helicity, and energy transfer
at all scales in the solar atmosphere. The relative simplicity of
their magnetic geometry and topology, compared with larger solar active
events, makes jets ideal candidates for studying the fundamental role
of reconnection in energetic events. <BR /> Methods: In this study,
using our recently developed numerical solver ARMS, we present several
parametric studies of a 3D numerical magneto-hydrodynamic model of
solar-jet-like events. We studied the impact of the magnetic field
inclination and photospheric field distribution on the generation
and properties of two morphologically different types of solar jets,
straight and helical, which can account for the observed so-called
standard and blowout jets. <BR /> Results: Our parametric studies
validate our model of jets for different geometric properties of the
magnetic configuration. We find that a helical jet is always triggered
for the range of parameters we tested. This demonstrates that the 3D
magnetic null-point configuration is a very robust structure for the
energy storage and impulsive release characteristic of helical jets. In
certain regimes determined by magnetic geometry, a straight jet precedes
the onset of a helical jet. We show that the reconnection occurring
during the straight-jet phase influences the triggering of the helical
jet. <BR /> Conclusions: Our results allow us to better understand
the energization, triggering, and driving processes of straight and
helical jets. Our model predicts the impulsiveness and energetics of
jets in terms of the surrounding magnetic field configuration. Finally,
we discuss the interpretation of the observationally defined standard
and blowout jets in the context of our model, as well as the physical
factors that determine which type of jet will occur.
---------------------------------------------------------
Title: Implications of the S-Web for the Corona and Inner Heliosphere
Authors: Antiochos, S. K.
2014AGUFMSH21B4100A Altcode:
Decades of satellite observations have shown that the solar wind that
forms the heliosphere consists of two distinct types: the so-called
fast and slow. The fast wind originates back at the Sun in long-lived
open field regions observed as "coronal holes" in X-Ray images,
but the source of the slow wind has long been an issue of intense
debate. Due to its observed location in the heliosphere, its plasma
composition, and its variability, many models for the origin of the
slow wind postulate that it is a result of the release of closed-field
plasma onto open field lines. In the recent S-Web model we proposed
that the slow wind originates at a dynamic boundary region between
open and closed flux in the solar corona. Consequently, the detailed
structure of the open-closed magnetic boundary and, most important, the
dynamics of this boundary are essential for determining the properties
of the slow wind and of the heliosphere, in general. The three important
magnetic topologies of the open-closed boundary in the corona and their
consequences for the inner heliosphere will be discussed. Furthermore,
recent simulations will be presented revealing the dynamics of this
boundary and demonstrating that these dynamics can account for the
observed properties of the slow wind. The implications of our results
for understanding the corona-heliosphere connection and especially for
interpreting observations from the upcoming Solar Orbiter and Solar
Probe Plus missions will be discussed. This work was supported by the
NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: X-Point Reconnection from Shear Driving in Kinetic Simulations
Authors: Black, C.; Antiochos, S. K.; DeVore, C. R.; Germaschewski,
K.; Bessho, N.; Karpen, J. T.
2014AGUFMSH23A4154B Altcode:
The explosive energy release in solar eruptive phenomena such as
CMEs/eruptive flares and coronal jets is believed to be due to magnetic
reconnection. Magnetic free energy builds up slowly in the corona due
to footpoint stressing by the photospheric motions. Along with the free
energy, current sheets build up at coronal nulls, which eventually
triggers fast reconnection and explosive energy release. This basic
scenario has been modeled extensively by MHD simulations and applied
to both CMEs/eruptive flares and jets, but the reconnection itself
is well-known to be due to kinetic processes. Consequently, it is
imperative that shear-driven X-point reconnection be modeled in a
fully kinetic system so as to test and guide the MHD results. In MHD
simulations, the application of a magnetic-field shear at the system
boundary is a trivial matter, but this is definitely not the case
for a kinetic system, because the electric currents need to be fully
consistent with all the mass motions. We present the first results of
reconnection in a 2D X-Point geometry due to a velocity shear driver
perpendicular to the plane of reconnection. We compare the results
to high-resolution MHD simulations and discuss the implications for
coronal activity.
---------------------------------------------------------
Title: Vision for the Future of Lws TR&T
Authors: Schwadron, N.; Mannucci, A. J.; Antiochos, S. K.;
Bhattacharjee, A.; Gombosi, T. I.; Gopalswamy, N.; Kamalabadi, F.;
Linker, J.; Pilewskie, P.; Pulkkinen, A. A.; Spence, H. E.; Tobiska,
W. K.; Weimer, D. R.; Withers, P.; Bisi, M. M.; Kuznetsova, M. M.;
Miller, K. L.; Moretto, T.; Onsager, T. G.; Roussev, I. I.; Viereck,
R. A.
2014AGUFMSH33B..02S Altcode:
The Living With a Star (LWS) program addresses acute societal
needs for understanding the effects of space weather and developing
scientific knowledge to support predictive capabilities. Our society's
heavy reliance on technologies affected by the space environment,
an enormous number of airline customers, interest in space tourism,
and the developing plans for long-duration human exploration space
missions are clear examples that demonstrate urgent needs for space
weather models and detailed understanding of space weather effects and
risks. Since its inception, the LWS program has provided a vehicle
to innovate new mechanisms for conducting research, building highly
effective interdisciplinary teams, and ultimately in developing the
scientific understanding needed to transition research tools into
operational models that support the predictive needs of our increasingly
space-reliant society. The advances needed require broad-based
observations that cannot be obtained by large missions alone. The
Decadal Survey (HDS, 2012) outlines the nation's needs for scientific
development that will build the foundation for tomorrow's space weather
services. Addressing these goals, LWS must develop flexible pathways
to space utilizing smaller, more diverse and rapid development of
observational platforms. Expanding utilization of ground-based assets
and shared launches will also significantly enhance opportunities
to fulfill the growing LWS data needs. Partnerships between NASA
divisions, national/international agencies, and with industry will
be essential for leveraging resources to address increasing societal
demand for space weather advances. Strengthened connections to user
communities will enhance the quality and impact of deliverables from
LWS programs. Thus, we outline the developing vision for the future of
LWS, stressing the need for deeper scientific understanding to improve
forecasting capabilities, for more diverse data resources, and for
project deliverables that address the growing needs of user communities.
---------------------------------------------------------
Title: Simulations of Filament Channel Formation
Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R.
2014AGUFMSH23C..06K Altcode:
A major unexplained feature of the solar atmosphere is the
accumulation of magnetic shear, in the form of filament channels,
at photospheric polarity inversion lines (PILs). In addition to
free energy, this shear also represents magnetic helicity, which is
conserved under reconnection. In this work, we address the problem of
filament channel formation and show how they acquire their shear and
magnetic helicity. Results of 3D MHD simulations using the Adaptively
Refined MHD Solver (ARMS) are presented that support the magnetic
helicity-condensation model of filament-channel formation described
by Antiochos, 2013. We consider the supergranular twisting of a
quasi-potential flux system that is bounded by a PIL and contains a
coronal hole (CH). The magnetic helicity injected by the small-scale
photospheric motions is shown to inverse-cascade up to the largest
allowable scales that define the closed flux system: the PIL and
the CH boundary. This process produces field lines that are both
sheared and smooth, and are sheared in opposite senses at the PIL
and the CH. The accumulated helicity and shear flux are shown to be
in excellent quantitative agreement with the helicity-condensation
model. We present a detailed analysis of the simulation, including
comparisons of our analytical and numerical results, and discuss their
implications for observations.
---------------------------------------------------------
Title: Measuring Coronal Energy and Helicity Buildup with SDO/HMI
Authors: Schuck, P. W.; Antiochos, S. K.; Barnes, G.; Leka, K. D.
2014AGUFMSH44A..08S Altcode:
Solar eruptions are driven by energy and helicity transported through
the photosphere and into the corona. However, the mechanism by which
energy and helicity emerge from the solar interior to form the observed
coronal structures is poorly understood. SDO/HMI data are the first
space-based full-disk vector field observations of the Sun with
a near 100% duty cycle and, therefore, represent an unprecedented
opportunity to quantify the energy end helicity fluxes through the
photosphere. However, because of the SDO satellite's highly inclined
geostationary orbit, the relative velocity of the instrument varies by
±3~km/s which introduces major orbital artifacts. We have developed a
procedure for mitigating these artifacts and have applied this analysis
to AR11084 to produce a cleaned data set. Our analysis procedure is
described, in detail, and the results for AR11084 presented. We have
also recast the Berger and Field (1984) helicity transport equation
in manifestly gauge invariant form and derived the terms quantifying
the injection of helicity into the corona by the emergence of closed
field, versus helicity injection by the stressing of pre-emerged
flux. The plasma velocity fields in the photosphere, necessary for
computing energy and helicity fluxes are determined using an upgraded
version of DAVE4VM that incorporates the spherical geometry of the
solar images. We find that the bulk of the helicity into the corona is
injected by twisting motions, and we discuss the implications of our
results for understanding solar activity and especially for data-driven
modeling of solar eruptions.This work was supported, in part, by NASA
---------------------------------------------------------
Title: How Well Does the S-Web Theory Predict In-Situ Observations
of the Slow Solar Wind?
Authors: Young, A. K.; Antiochos, S. K.; Linker, J.; Zurbuchen, T.
2014AGUFMSH33A4124Y Altcode:
The S-Web theory provides a physical explanation for the origin and
properties of the slow solar wind, particularly its composition. The
theory proposes that magnetic reconnection along topologically complex
boundaries between open and closed magnetic fields on the sun releases
plasma from closed magnetic field regions into the solar wind at
latitudes away from the heliospheric current sheet. Such a wind
would have elevated charge states compared to the fast wind and an
elemental composition resembling the closed-field corona. This theory
is currently being tested using time-dependent, high-resolution,
MHD simulations, however comparisons to in-situ observations play
an essential role in testing and understanding slow-wind release
mechanisms. In order to determine the relationship between S-Web
signatures and the observed, slow solar wind, we compare plasma data
from the ACE and Ulysses spacecraft to solutions from the steady-state
models created at Predictive Science, Inc., which use observed magnetic
field distributions on the sun as a lower boundary condition. We discuss
the S-Web theory in light of our results and the significance of the
S-Web for interpreting current and future solar wind observations. This
work was supported, in part, by the NASA TR&T and SR&T programs.
---------------------------------------------------------
Title: Strategic Science to Address Current and Future Space
Weather Needs
Authors: Mannucci, A. J.; Schwadron, N.; Antiochos, S. K.;
Bhattacharjee, A.; Bisi, M. M.; Gopalswamy, N.; Kamalabadi, F.;
Pulkkinen, A. A.; Tobiska, W. K.; Weimer, D. R.; Withers, P.
2014AGUFMSM24A..09M Altcode:
NASA's Living With a Star (LWS) program has contributed a wealth of
scientific knowledge that is relevant to space weather and user needs. A
targeted approach to science questions has resulted in leveraging
new scientific knowledge to improve not only our understanding of the
Heliophysics domain, but also to develop predictive capabilities in key
areas of LWS science. This fascinating interplay between science and
applications promises to benefit both domains. Scientists providing
feedback to the LWS program are now discussing an evolution of the
targeted approach that explicitly considers how new science improves,
or enables, predictive capability directly. Long-term program goals
are termed "Strategic Science Areas" (SSAs) that address predictive
capabilities in six specific areas: geomagnetically induced currents,
satellite drag, solar energetic particles, ionospheric total electron
content, radio frequency scintillation induced by the ionosphere,
and the radiation environment. SSAs are organized around user needs
and the impacts of space weather on society. Scientists involved in
the LWS program identify targeted areas of research that reference
(or bear upon) societal needs. Such targeted science leads to new
discoveries and is one of the valid forms of exploration. In this
talk we describe the benefits of targeted science, and how addressing
societal impacts in an appropriate way maintains the strong science
focus of LWS, while also leading to its broader impacts.
---------------------------------------------------------
Title: A Model for the Formation of Filament Channels on the Sun
Authors: Knizhnik, Kalman Joshua; Antiochos, Spiro K.; DeVore,
C. Richard
2014shin.confE..63K Altcode:
A major unexplained feature of the solar atmosphere is the accumulation
of magnetic shear, in the form of filament channels, at photospheric
polarity inversion lines (PILs). In addition to free energy, this
shear also represents magnetic helicity, which is conserved under
reconnection. Consequently, the observations raise the question: Why
is the magnetic shear observed to be concentrated along PILs? Results
of 3D MHD simulations using the Adaptively Refined MHD Solver
(ARMS) are presented that support the magnetic-helicity condensation
model of filament channel formation (Antiochos 2013). In this work,
we consider the supergranular twisting of a quasi potential flux
system that is bounded by a PIL and contains a coronal hole (CH). The
magnetic helicity injected by the small-scale photospheric motions
is shown to inverse-cascade up to the largest allowable scales that
define the closed flux system: the PIL and the CH boundary. This
process produces field lines that are both sheared and smooth, and
are sheared in opposite senses at the PIL and the CH, in agreement
with Antiochos (2013). The accumulated helicity and shear flux are
shown to be in excellent quantitative agreement with the helicity
condensation model. We present a detailed analysis of the simulation,
including comparisons of our analytical and numerical results, and
discuss their implications for observations. This work was supported,
in part, by the NASA TR&T and SR&T programs.
---------------------------------------------------------
Title: Kinetic Simulations of the Electrically Charged Current Sheet
of a Pulsar
Authors: Black, Carrie; DeVore, C. Richard; Antiochos, Spiro
K.; Harding, Alice Kust; Kazanas, Demosthenes; Kalapotharakos,
Constantinos; Timokhin, Andrey
2014AAS...22412110B Altcode:
The pulsar magnetosphere is believed to comprise a volume of low-lying,
closed field about the magnetic equator, bounded by polar open-field
regions in which the pulsar wind flows into space. In the standard
global-scale models, a magnetic discontinuity (electric current
sheet) of nonneutral plasma separates these opposite-polarity open
fields. We use the particle-in-cell Plasma Simulation Code, PSC,
to examine the dynamics of a self-consistent model for the internal
structure of this sheet, in which the charge-neutral Vlasov/Maxwell
equilibria of Harris (1962) and Hoh (1966) are generalized to allow a
net electric charge. PSC accommodates both Maxwell (nonrelativistic)
and Jüttner/Synge (relativistic) distribution functions for the
electrons and positrons. Numerical equilibrium solutions to the 1D
Maxwell equations are initialized on the 2D PSC grid, supplemented by
periodic boundary conditions in the direction parallel to the sheet
and insulating-wall boundary conditions remote from the sheet in the
perpendicular direction. As is typical in kinetic studies of pair
plasmas, the particle thermal energy and the relative drift velocity
driving the current are assumed to be of order the rest energy and
the speed of light, respectively. In this limit, the Debye length,
skin depth, and Harris/Hoh current-sheet width are all comparable
to each other, rather than widely separated and arranged in order of
increasing size as generally occurs in nonrelativistic plasmas. The
qualitatively new feature of our pulsar simulations is the equilibrium
electric field, whose strength can be comparable to that of the
magnetic field in the relativistic limit. We expect its presence to
have profound consequences for the linear stability and nonlinear
evolution of charged pulsar current sheets, substantially modifying
the tearing and reconnection of the magnetic field. Exploratory PSC
simulations of magnetic reconnection in representative electrified
Harris/Hoh equilibria will be presented. The derivation, solution, and
analysis of the equilibrium Vlasov/Maxwell equations are discussed in
a companion paper at this conference (C. R. DeVore et al. 2014).This
work was supported by NASA GSFC’s Science Innovation Fund.
---------------------------------------------------------
Title: CME Initiation Driven by Velocity-Shear Kinetic Reconnection
Simulations
Authors: Black, Carrie; Antiochos, Spiro K.; DeVore, C. Richard;
Karpen, Judith T.; Germaschewski, Kai
2014shin.confE..40B Altcode:
In the standard model for coronal mass ejections (CME) and/or solar
flares, the free energy for the event resides in the strongly sheared
magnetic field of a filament channel. The pre-eruption force balance
consists of an upward force due to the magnetic pressure of the sheared
field countered by a downward tension due to overlying unsheared
field. Magnetic reconnection is widely believed to be the mechanism
that disrupts this force balance, leading to explosive eruption. For
understanding CME/flare initiation, therefore, it is critical to model
the onset of reconnection that is driven by the build-up of magnetic
shear. In MHD simulations, the application of a magnetic-field shear
is a trivial matter. However, kinetic effects are important in the
diffusion region and thus, it is important to examine this process
with PIC simulations as well. The implementation of such a driver in
PIC methods is nontrivial, however, and indicates the necessity of a
true multiscale model for such processes in the solar environment. The
field must be sheared self-consistently and indirectly to prevent
the generation of waves that destroy the desired system. In the work
presented here, we discuss methods for applying a velocity shear
perpendicular to the plane of reconnection in a system with open
boundary conditions. This material is based upon work supported by
the National Science Foundation under Award No. AGS-1331356.
---------------------------------------------------------
Title: Numerical Simulation of a Slow Streamer-Blowout CME
Authors: Lynch, Benjamin J.; Masson, Sophie; Li, Yan; DeVore,
C. Richard; Luhmann, Janet; Antiochos, Spiro K.
2014AAS...22421816L Altcode:
We present a 3D numerical MHD simulation of the 2008 Jun 2 gradual
streamer blowout CME that had virtually no identifiable low coronal
signatures. We energize the field by simple footpoint shearing along
the source region's polarity inversion line and model the background
solar wind structure using an ∼2MK isothermal wind and a low-order
potential field source surface representation of the CR2070 synoptic
magnetogram. Our results show that the CME “initiation’’ is
obtained by slowly disrupting the quasi-steady-state configuration of
the helmet streamer, resulting in the standard eruptive flare picture
that ejects the sheared fields, but very slowly, on a relatively
large scale, and with very little magnetic energy release. We obtain a
relatively slow CME eruption of order the background solar wind speed
and argue that these slow streamer blowout CMEs (now also known as
“stealth CMEs’’) are simply at the lowest end of the CME energy
distribution. We present comparisons of the CME propagation through
the corona (≤15Rs) in synthetic white-light images derived from the
simulation density structure with multi-spacecraft coronagraph data
from STEREO/SECCHI and SOHO/LASCO.
---------------------------------------------------------
Title: Simulation of S-Web Corridor Dynamics
Authors: Young, Aleida Katherine; Antiochos, Spiro; DeVore, C.;
Zurbuchen, Thomas
2014shin.confE..64Y Altcode:
The higher average charge-state composition and bias towards heavier
elements (Zurbuchen et al. 1999) of the slow solar wind suggest that
its source is the release of coronal plasma from high-temperature,
closed-field regions. The S-Web (separatrix web) model for the source
of the slow solar wind is based on the conclusion that the apparent
multiple coronal holes observed within single-polarity regions are
connected by narrow corridors at scales smaller than the spatial
resolution of current measurements. Magnetic field lines from the
boundary of such a corridor map to the heliospheric current sheet, while
field lines from the interior of the corridor map to an arc extending
to high latitudes in the heliosphere (Antiochos et al. 2011). In
this work, we simulate the dynamics of an S-Web corridor using the
Adaptively Refined MHD Solver (ARMS). The objective is to quantify the
release of coronal plasma at high heliospheric latitudes and show that
the dynamics support the S-Web model as an explanation for the source
of the slow solar wind. We will present results from our efforts to
simulate open-field corridor dynamics, outline plans for further work,
and discuss implications for understanding the slow solar wind. This
work was supported, in part, by the NASA TR&T and SR&T programs.
---------------------------------------------------------
Title: The role of CME in the escape of solar energtic particles
Authors: Masson, Sophie; Antiochos, S. K.; DeVore, C. R.
2014shin.confE..81M Altcode:
Heliospheric manifestations of intense energy release linked to solar
activity include the impact at the Earth of energetic particles
accelerated during solar eruptions. Observationally, the magnetic
configuration of active regions, where solar eruptions occur, agrees
well with the standard model of eruption, consisting of a flare and a
coronal mass ejection (CME). According to the standard model, particles
accelerated at the flare reconnection site should remain trapped in the
CME. However, flare-accelerated particles frequently reach the Earth
long before the CME does. <P />We present a 3D model that explains how
flare-accelerated particles escape into the interplanetary magnetic
flux tubes during a solar eruption. Our model is based on results of
large-scale 3D MHD simulations of a breakout-CME erupting into the
heliosphere build by an isothermal solar wind. The simulations are
performed with the Adaptively Refined Mhd Solver (ARMS). We describe
the multiple reconnection episodes that occur during the evolution
of the event, and show that the CME magnetic flux reconnect with
the open magnetic field from the coronal hole nearby. Such a dynamic
implies that the flare-accelerated particles initially trapped in the
CME can now be release onto open field lines. Analyzing the dynamics
of the reconnected flux during the eruption, we evaluate the spatial
distribution of particles beams and find that the particle release can
occur over a wide-longitudinal range scaling to the size of the CME
front. We discuss the implications of results for CME/flare models
and for SEPs observations. <P />This work was supported, in part,
by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: A Model for the Formation of Filament Channels on the Sun
Authors: Knizhnik, Kalman J.; Antiochos, Spiro K.; DeVore, C. Richard
2014AAS...22440802K Altcode:
A major unexplained feature of the solar atmosphere is the accumulation
of magnetic shear, in the form of filament channels, at photospheric
polarity inversion lines (PILs). In addition to free energy, this
shear also represents magnetic helicity, which is conserved under
reconnection. Consequently, the observations raise the question: Why
is the magnetic shear observed to be concentrated along PILs? Results
of 3D MHD simulations using the Adaptively Refined MHD Solver (ARMS)
are presented that support the magnetic-helicity condensation model of
filament-channel formation (Antiochos 2013). In this work, we consider
the supergranular twisting of a quasi-potential flux system that
is bounded by a PIL and contains a coronal hole (CH). The magnetic
helicity injected by the small-scale photospheric motions is shown
to inverse-cascade up to the largest allowable scales that define the
closed flux system: the PIL and the CH boundary. This process produces
field lines that are both sheared and smooth, and are sheared in
opposite senses at the PIL and the CH, in agreement with Antiochos
(2013). The accumulated helicity and shear flux are shown to be
in excellent quantitative agreement with the helicity-condensation
model. We present a detailed analysis of the simulation, including
comparisons of our analytical and numerical results, and discuss
their implications for observations. This work was supported, in part,
by the NASA TR&T and SR&T programs.
---------------------------------------------------------
Title: Solar Polar Jets Driven by Magnetic Reconnection, Gravity,
and Wind
Authors: DeVore, C. Richard; Karpen, Judith T.; Antiochos, Spiro K.
2014AAS...22432351D Altcode:
Polar jets are dynamic, narrow, radially extended structures observed
in solar EUV emission near the limb. They originate within the open
field of coronal holes in “anemone” regions, which are intrusions of
opposite magnetic polarity. The key topological feature is a magnetic
null point atop a dome-shaped fan surface of field lines. Applied
stresses readily distort the null into a current patch, eventually
inducing interchange reconnection between the closed and open fields
inside and outside the fan surface (Antiochos 1996). Previously, we
demonstrated that magnetic free energy stored on twisted closed field
lines inside the fan surface is released explosively by the onset of
fast reconnection across the current patch (Pariat et al. 2009, 2010). A
dense jet comprised of a nonlinear, torsional Alfvén wave is ejected
into the outer corona along the newly reconnected open field lines. Now
we are extending those exploratory simulations by including the effects
of solar gravity, solar wind, and expanding spherical geometry. We find
that the model remains robust in the resulting more complex setting,
with explosive energy release and dense jet formation occurring in
the low corona due to the onset of a kink-like instability, as found
in the earlier Cartesian, gravity-free, static-atmosphere cases. The
spherical-geometry jet including gravity and wind propagates far more
rapidly into the outer corona and inner heliosphere than a comparison
jet simulation that excludes those effects. We report detailed analyses
of our new results, compare them with previous work, and discuss the
implications for understanding remote and in-situ observations of solar
polar jets.This work was supported by NASA’s LWS TR&T program.
---------------------------------------------------------
Title: A Model for the Electrically Charged Current Sheet of a Pulsar
Authors: DeVore, C. Richard; Antiochos, Spiro K.; Black, Carrie;
Harding, Alice Kust; Kalapotharakos, Constantinos; Kazanas,
Demosthenes; Timokhin, Andrey
2014AAS...22412111D Altcode:
Global-scale electromagnetohydrodynamic solutions for the magnetosphere
of a pulsar consist of a region of low-lying, closed magnetic field
near the star bounded by opposite-polarity regions of open magnetic
field along which the pulsar wind flows into space. Separating
these open-field regions is a magnetic discontinuity - an electric
current sheet - consisting of nonneutral plasma. We have developed
a self-consistent model for the internal structure of this sheet by
generalizing the charge-neutral Vlasov/Maxwell equilibria of Harris
(1962) and Hoh (1966) to allow a net electric charge. The resulting
equations for the electromagnetic field are identical for Maxwell
(nonrelativistic) and Jüttner/Synge (relativistic) distribution
functions of the particles. The solutions have a single sign of net
charge everywhere, with the minority population concentrated near
the current sheet and the majority population completely dominant
far from the sheet. As the fractional charge imbalance at the sheet
increases, for fixed relative drift speed and total thermal pressure
of the particles, both the electric- and magnetic-field strengths far
from the sheet increase. The electrostatic force acts to disperse the
charged particles from the sheet, so the magnetic force must increase
proportionately, relative to the charge-neutral case, to pinch the
sheet together and maintain the equilibrium. The charge imbalance
in the sheet that can be accommodated has an upper bound, which
increases monotonically with the relative drift speed. Implications
of the model for the steady-state structure of pulsar magnetospheres
will be discussed. The model also provides a rigorous starting point
for investigating electromagnetohydrodynamic and kinetic instabilities
that could lead to magnetic reconnection and current-sheet disruption
in pulsars. Exploratory particle-in-cell simulations of representative
equilibria are presented in a companion paper at this conference
(C. E. Black et al. 2014).This work was supported by NASA GSFC’s
Science Innovation Fund.
---------------------------------------------------------
Title: The Onset of Fast Magnetic Reconnection in Solar and Laboratory
Plasmas
Authors: Antiochos, Spiro K.; DeVore, C. Richard; Karpen, Judith T.;
Guidoni, Silvina
2014AAS...22440306A Altcode:
Magnetic reconnection is widely believed to be the physical process
underlying explosive activity in both solar and laboratory plasmas. The
question of what determines whether and when magnetic reconnection
will produce explosive energy release has long been one of the most
important problems in all plasma physics. We examine this problem using
numerical simulations of major solar eruptions, coronal mass ejections
and eruptive flares. These events are among the most energetic and the
best observed examples of the onset phenomenon. Our calculations show
that reconnection in the solar corona invariably exhibits two distinct
phases. First, we observe an initial slow growth characterized by the
appearance of an extended current sheet and magnetic islands, somewhat
analogous to resistive tearing. Eventually, however, the reconnection
transitions to an explosive phase characterized by well-developed
jets and further island formation. We discuss how these results
scale with numerical refinement level, i.e., effective Lundquist
number. We conclude that, at least for the case of solar plasmas,
fast reconnection onset requires an interaction between reconnection
and an ideal instability. We discuss the implications of our results
for observations of both solar and laboratory plasmas. This work was
supported in part by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: The Role of CMEs in the Escape of Solar Energetic Particles
Authors: Masson, Sophie; Antiochos, Spiro K.; DeVore, C. Richard
2014AAS...22421837M Altcode:
Heliospheric manifestations of intense energy release linked to
solar activity include the impact at Earth of energetic particles
accelerated during solar eruptions. Observationally, the magnetic
configuration of active regions, where solar eruptions occur, agrees
well with the standard model of eruption, consisting of a flare and a
coronal mass ejection (CME). According to the standard model, particles
accelerated at the flare reconnection site should remain trapped in the
CME. However, flare-accelerated particles frequently reach the Earth
long before the CME does. We present a 3D model that demonstrates
how flare-accelerated particles escape into interplanetary magnetic
flux tubes during a solar eruption. Our model is based on results
of large-scale 3D MHD simulations of a breakout CME erupting into
a heliospheric magnetic field that is opened by an isothermal solar
wind. The simulations are performed with the Adaptively Refined MHD
Solver (ARMS). We describe the multiple reconnection episodes that
occur during the evolution of the event, and show how CME magnetic
flux reconnects with the open field from a nearby coronal hole. This
reconnection allows flare-accelerated particles initially trapped in
the CME to escape onto open field lines. Analyzing the dynamics of
the reconnected flux during the eruption, we determine the spatial
distribution of particle beams originating in the CME flux rope. We
find that particle release can occur over a wide longitudinal range,
which heretofore has been a puzzling feature of SEP observations. We
discuss the implications of our results for CME/flare models and for
the origin and transport of SEPs.This work was supported, in part,
by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: CME Initiation Driven by Velocity-Shear Kinetic Reconnection
Simulations
Authors: Black, Carrie; Antiochos, Spiro K.; Karpen, Judith T.;
DeVore, C. Richard; Germaschewski, Kai
2014AAS...22440303B Altcode:
In the standard model for coronal mass ejections (CME) and/or solar
flares, the free energy for the event resides in the strongly sheared
magnetic field of a filament channel. The pre-eruption force balance
consists of an upward force due to the magnetic pressure of the sheared
field countered by a downward tension due to overlying unsheared
field. Magnetic reconnection is widely believed to be the mechanism
that disrupts this force balance, leading to explosive eruption. For
understanding CME/flare initiation, therefore, it is critical to model
the onset of reconnection that is driven by the build-up of magnetic
shear. In MHD simulations, the application of a magnetic-field shear
is a trivial matter. However, kinetic effects are important in the
diffusion region and thus, it is important to examine this process
with PIC simulations as well. The implementation of such a driver in
PIC methods is nontrivial, however, and indicates the necessity of a
true multiscale model for such processes in the solar environment. The
field must be sheared self-consistently and indirectly to prevent
the generation of waves that destroy the desired system. In the work
presented here, we discuss methods for applying a velocity shear
perpendicular to the plane of reconnection in a system with open
boundary conditions. This material is based upon work supported by
the National Science Foundation under Award No. AGS-1331356.
---------------------------------------------------------
Title: Simulation of S-Web Corridor Dynamics
Authors: Young, Aleida; Antiochos, Spiro K.; DeVore, C. Richard;
Zurbuchen, Thomas H.
2014AAS...22440203Y Altcode:
The higher average charge-state composition and bias towards heavier
elements (Zurbuchen et al. 1999) of the slow solar wind suggest that
its source is the release of coronal plasma from high-temperature,
closed-field regions. The S-Web (separatrix web) model for the source
of the slow solar wind is based on the conclusion that the apparent
multiple coronal holes observed within single-polarity regions are
connected by narrow corridors at scales smaller than the spatial
resolution of current measurements. Magnetic field lines from the
boundary of such a corridor map to the heliospheric current sheet, while
field lines from the interior of the corridor map to an arc extending
to high latitudes in the heliosphere (Antiochos et al. 2011). In
this work, we simulate the dynamics of an S-Web corridor using the
Adaptively Refined MHD Solver (ARMS). The objective is to quantify the
release of coronal plasma at high heliospheric latitudes and show that
the dynamics support the S-Web model as an explanation for the source
of the slow solar wind. We will present results from our efforts to
simulate open-field corridor dynamics, outline plans for further work,
and discuss implications for understanding the slow solar wind. This
work was supported, in part, by the NASA TR&T and SR&T programs.
---------------------------------------------------------
Title: Simulations of Emerging Magnetic Flux. II. The Formation
of Unstable Coronal Flux Ropes and the Initiation of Coronal Mass
Ejections
Authors: Leake, James E.; Linton, Mark G.; Antiochos, Spiro K.
2014ApJ...787...46L Altcode: 2014arXiv1402.2645L
We present results from three-dimensional magnetohydrodynamic
simulations of the emergence of a twisted convection zone flux tube
into a pre-existing coronal dipole field. As in previous simulations,
following the partial emergence of the sub-surface flux into the corona,
a combination of vortical motions and internal magnetic reconnection
forms a coronal flux rope. Then, in the simulations presented here,
external reconnection between the emerging field and the pre-existing
dipole coronal field allows further expansion of the coronal flux rope
into the corona. After sufficient expansion, internal reconnection
occurs beneath the coronal flux rope axis, and the flux rope erupts
up to the top boundary of the simulation domain (~36 Mm above the
surface). We find that the presence of a pre-existing field, orientated
in a direction to facilitate reconnection with the emerging field, is
vital to the fast rise of the coronal flux rope. The simulations shown
in this paper are able to self-consistently create many of the surface
and coronal signatures used by coronal mass ejection (CME) models. These
signatures include surface shearing and rotational motions, quadrupolar
geometry above the surface, central sheared arcades reconnecting with
oppositely orientated overlying dipole fields, the formation of coronal
flux ropes underlying potential coronal field, and internal reconnection
which resembles the classical flare reconnection scenario. This suggests
that proposed mechanisms for the initiation of a CME, such as "magnetic
breakout," are operating during the emergence of new active regions.
---------------------------------------------------------
Title: Global-scale Consequences of Magnetic-helicity Injection and
Condensation on the Sun
Authors: Mackay, Duncan H.; DeVore, C. Richard; Antiochos, Spiro K.
2014ApJ...784..164M Altcode:
In the recent paper of Antiochos, a new concept for the injection of
magnetic helicity into the solar corona by small-scale convective
motions and its condensation onto polarity inversion lines (PILs)
was developed. We investigate this concept through global simulations
of the Sun's photospheric and coronal magnetic fields, and compare the
results with the hemispheric pattern of solar filaments. Assuming that
the vorticity of the cells is predominantly counterclockwise/clockwise
in the northern/southern hemisphere, the convective motions inject
negative/positive helicity into each hemisphere. The simulations show
that: (1) on a north-south oriented PIL, both differential rotation
and convective motions inject the same sign of helicity, which matches
that required to reproduce the hemispheric pattern of filaments. (2) On
a high-latitude east-west oriented polar crown or subpolar crown PIL,
the vorticity of the cells has to be approximately 2-3 times greater
than the local differential-rotation gradient in order to overcome the
incorrect sign of helicity injection from differential rotation. (3)
In the declining phase of the cycle, as a bipole interacts with the
polar field, in some cases, helicity condensation can reverse the
effect of differential rotation along the east-west lead arm but not
in all cases. The results show that this newly developed concept of
magnetic helicity injection and condensation, in conjunction with
the mechanisms used in Yeates et al., is a viable explanation for the
hemispheric pattern of filaments. Future observational studies should
focus on examining the vorticity component within convective motions
to determine both its magnitude and latitudinal variation relative to
the differential-rotation gradient on the Sun.
---------------------------------------------------------
Title: A Model for the Electrically Charged Current Sheet of a Pulsar
Authors: DeVore, C. R.; Antiochos, S. K.; Black, C. E.; Harding,
A. K.; Kalapotharakos, C.; Kazanas, D.; Timokhin, A.
2014AAS...22315326D Altcode:
Global-scale electromagnetohydrodynamic solutions for the magnetosphere
of a pulsar consist of a region of low-lying, closed magnetic field
near the star bounded by opposite-polarity regions of open magnetic
field along which the pulsar wind flows into space. Separating
these open-field regions is a magnetic discontinuity - an electric
current sheet - consisting of nonneutral plasma. We have developed
a self-consistent model for the internal structure of this sheet by
generalizing the charge-neutral Vlasov/Maxwell equilibria of Harris
(1962) and Hoh (1966) to allow a net electric charge. The resulting
equations for the electromagnetic field are identical for Maxwell
(nonrelativistic) and Jüttner/Synge (relativistic) distribution
functions of the particles. The solutions have a single sign of net
charge everywhere, with the minority population concentrated near
the current sheet and the majority population completely dominant
far from the sheet. As the fractional charge imbalance at the sheet
increases, for fixed relative drift speed and total thermal pressure
of the particles, both the electric- and magnetic-field strengths far
from the sheet increase. The electrostatic force acts to disperse
the charged particles from the sheet, so the magnetic force must
increase proportionately, relative to the charge-neutral case, to
pinch the sheet together and maintain the equilibrium. The charge
imbalance in the sheet that can be accommodated has an upper bound,
which increases monotonically with the relative drift speed. In the
limit of maximum charge imbalance and field strength, the density
of majority particles asymptotically approaches a uniform value far
from the sheet, rather than falling exponentially to zero as in the
charge-neutral case. This model provides a rigorous starting point
for investigating electromagnetohydrodynamic and kinetic instabilities
that could lead to magnetic reconnection and current-sheet disruption
in pulsar magnetospheres. Exploratory particle-in-cell simulations of
some representative equilibria are presented in a companion paper at
this conference (C. E. Black et al. 2014). This work was supported by
NASA GSFC’s Science Innovation Fund.
---------------------------------------------------------
Title: Kinetic Simulations of the Electrically Charged Current Sheet
of a Pulsar
Authors: Black, Carrie; Antiochos, S. K.; DeVore, C. R.; Harding,
A. K.; Kalapotharakos, C.; Kazanas, D.; Timokhin, A.
2014AAS...22315327B Altcode:
The pulsar magnetosphere is believed to comprise a volume of low-lying,
closed field about the magnetic equator, bounded by polar open-field
regions in which the pulsar wind flows into space. In the standard
global-scale models, a magnetic discontinuity (electric current
sheet) of nonneutral plasma separates open field regions of opposite
polarity. We use the particle-in-cell Plasma Simulation Code, PSC,
to examine the dynamics of a self-consistent model for the internal
structure of this sheet, in which the charge-neutral Vlasov/Maxwell
equilibria of Harris (1962) and Hoh (1966) are generalized to allow a
net electric charge. PSC accommodates both Maxwell (nonrelativistic)
and Jüttner/Synge (relativistic) distribution functions for the
electrons and positrons. Numerical equilibrium solutions to the 1D
Maxwell equations are initialized on the 2D PSC grid, supplemented
by periodic boundary conditions in the direction parallel to the
sheet and insulating boundary conditions remote from the sheet in
the perpendicular direction. As is typical in kinetic studies of pair
plasmas, the particle thermal energy and the relative drift velocity
driving the current are assumed to be of order the rest energy and the
speed of light, respectively. In this limit, the Debye length, skin
depth, and Harris/Hoh width of the current sheet are all comparable
to each other, rather than widely separated and arranged in order of
increasing size as typically occurs in nonrelativistic plasmas. The
qualitatively new feature of our pulsar simulations is the equilibrium
electric field, whose strength can be comparable to that of the
magnetic field in the relativistic limit. We expect its presence to have
profound consequences for the linear stability and nonlinear evolution
of charged pulsar current sheets, especially with regard to tearing
and reconnection of the magnetic field. Exploratory PSC simulations
of magnetic reconnection in some representative “electrified
Harris/Hoh” equilibria will be presented. The derivation, solution,
and analysis of the equilibrium Vlasov/Maxwell equations are discussed
in a companion paper at this conference (C. R. DeVore et al. 2014). This
work was supported by NASA GSFC’s Science Innovation Fund.
---------------------------------------------------------
Title: Particle escape in the interplanetary medium: Link between
CME observations and MHD simulations
Authors: Masson, Sophie; Antiochos, Spiro; DeVore, C. Richard
2014cosp...40E2031M Altcode:
Among the more hazardous forms of space weather at Earth and in the
heliosphere are the intense solar energetic particle (SEP) bursts
associated with fast coronal mass ejections (CMEs) and eruptive
flares. A fundamental question to understand the origin and the
evolution of solar energetic particles is: How do solar energetic
particles escape the Sun? Answering this question is critical for
understanding how the corona couples dynamically to the heliosphere
during explosive events, and is fundamental to developing any future
forecasting capability for SEP events. The release onto open field
lines of energetic particles originating in the low corona is the bridge
connecting the acceleration site to the interplanetary propagation and
is, therefore, the key to reconciling remote and in-situ observations of
energetic particles. Recent multi-instrument studies showed that CMEs
are important factors that determine whether the energetic particles
escape into the heliosphere and partly define the spatial distribution
of particle flux. In order to understand how and why CMEs play a
crucial role in the particle escape, we must understand the dynamics
of the corona disturbed by a CME ejection. The details of the dynamics
can be studied through MHD simulations. To advance understanding,
it is pertinent to combine observations and simulations to develop
models that respect the observational constraints. Thus, first we will
describe the observational results, then discuss how MHD simulations
help demonstrate why CMEs are important for particle release.
---------------------------------------------------------
Title: Modeling Reconnection-Driven Solar Polar Jets with Gravity
and Wind
Authors: Karpen, Judith T.; DeVore, C. R.; Antiochos, S. K.
2013SPD....44...43K Altcode:
Solar polar jets are dynamic, narrow, radially extended structures
observed in EUV emission. They have been found to originate within
the open magnetic field of coronal holes in “anemone” regions,
which are generally accepted to be intrusions of opposite polarity. The
associated embedded-dipole topology consists of a spine line emanating
from a null point atop a dome-shaped fan surface. Previous work
(Pariat et al. 2009, 2010) has validated the idea that magnetic free
energy stored on twisted closed field lines within the fan surface
can be released explosively by the onset of fast reconnection between
the highly stressed closed field inside the null and the unstressed
open field outside (Antiochos 1996). The simulations showed that a
dense jet comprising a nonlinear, torsional Alfven wave is ejected
into the outer corona on the newly reconnected open field lines. While
proving the principle of the basic model, those simulations neglected
the important effects of gravity, the solar wind, and an expanding
spherical geometry. We introduce those additional physical processes in
new simulations of reconnection-driven jets, to determine whether the
model remains robust in the resulting more realistic setting, and to
begin establishing the signatures of the jets in the inner heliosphere
for comparison with observations. Initial results demonstrate explosive
energy release and a jet in the low corona very much like that in the
earlier Cartesian, gravity-free, static-atmosphere runs. We report
our analysis of the results, their comparison with previous work,
and their implications for observations. This work was supported by
NASA’s LWS TR&T program.Abstract (2,250 Maximum Characters):
Solar polar jets are dynamic, narrow, radially extended structures
observed in EUV emission. They have been found to originate within the
open magnetic field of coronal holes in “anemone” regions, which are
generally accepted to be intrusions of opposite polarity. The associated
embedded-dipole topology consists of a spine line emanating from a null
point atop a dome-shaped fan surface. Previous work (Pariat et al. 2009,
2010) has validated the idea that magnetic free energy stored on twisted
closed field lines within the fan surface can be released explosively
by the onset of fast reconnection between the highly stressed closed
field inside the null and the unstressed open field outside (Antiochos
1996). The simulations showed that a dense jet comprising a nonlinear,
torsional Alfven wave is ejected into the outer corona on the newly
reconnected open field lines. While proving the principle of the
basic model, those simulations neglected the important effects of
gravity, the solar wind, and an expanding spherical geometry. We
introduce those additional physical processes in new simulations of
reconnection-driven jets, to determine whether the model remains robust
in the resulting more realistic setting, and to begin establishing the
signatures of the jets in the inner heliosphere for comparison with
observations. Initial results demonstrate explosive energy release and
a jet in the low corona very much like that in the earlier Cartesian,
gravity-free, static-atmosphere runs. We report our analysis of the
results, their comparison with previous work, and their implications for
observations. This work was supported by NASA’s LWS TR&T program.
---------------------------------------------------------
Title: Helicity Condensation as the Origin of Coronal and Solar
Wind Structure
Authors: Antiochos, S. K.
2013ApJ...772...72A Altcode: 2012arXiv1211.4132A
Three of the most important and most puzzling features of the Sun's
atmosphere are the smoothness of the closed-field corona (the so-called
coronal loops), the accumulation of magnetic shear at photospheric
polarity inversion lines (PILs; filament channels), and the complex
dynamics of the slow wind. We propose that a single process, helicity
condensation, is the physical mechanism giving rise to all three
features. A simplified model is presented for how helicity is injected
and transported in the closed corona by magnetic reconnection. With
this model, we demonstrate that magnetic shear must accumulate at PILs
and coronal hole boundaries, and estimate the rate of shear growth
at PILs and the loss to the wind. Our results can account for many of
the observed properties of the corona and wind.
---------------------------------------------------------
Title: The Initiation of Coronal Mass Ejections (CMEs) by Dynamical
Magnetic Flux Emergence
Authors: Leake, James; Linton, M.; Antiochos, S. K.
2013SPD....44...46L Altcode:
We present results from 3D numerical MHD simulations, which show how the
partial emergence of twisted magnetic flux tubes from the convection
zone, and their interaction with background coronal magnetic fields,
leads to the formation of unstable magnetic configurations in the
corona. These unstable configurations are capable of initiating the
ejection of a flux rope. Our studies improve upon the traditional
approach of driving the magnetically-dominated corona with kinematic
boundary conditions by explicitly including the dynamic emergence
of flux through the convection zone and lower, pressure-dominated,
solar atmosphere. By showing that magnetic flux emergence is capable of
initiating coronal ejections, we can root these dynamic events in the
convection zone and hence to the source of solar activity, the solar
dynamo, which is a vital step in improving our understanding of space
weather.Abstract (2,250 Maximum Characters): We present results from
3D numerical MHD simulations, which show how the partial emergence
of twisted magnetic flux tubes from the convection zone, and their
interaction with background coronal magnetic fields, leads to the
formation of unstable magnetic configurations in the corona. These
unstable configurations are capable of initiating the ejection of a
flux rope. Our studies improve upon the traditional approach of driving
the magnetically-dominated corona with kinematic boundary conditions
by explicitly including the dynamic emergence of flux through the
convection zone and lower, pressure-dominated, solar atmosphere. By
showing that magnetic flux emergence is capable of initiating coronal
ejections, we can root these dynamic events in the convection zone
and hence to the source of solar activity, the solar dynamo, which is
a vital step in improving our understanding of space weather.
---------------------------------------------------------
Title: A Model for the Escape of Solar-flare-accelerated Particles
Authors: Masson, S.; Antiochos, S. K.; DeVore, C. R.
2013ApJ...771...82M Altcode: 2013arXiv1301.0654M
We address the problem of how particles are accelerated by solar
flares can escape into the heliosphere on timescales of an hour or
less. Impulsive solar energetic particle (SEP) bursts are generally
observed in association with so-called eruptive flares consisting
of a coronal mass ejection (CME) and a flare. These fast SEPs are
believed to be accelerated directly by the flare, rather than by
the CME shock. However, the precise mechanism by which the particles
are accelerated remains controversial. Regardless of the origin of
the acceleration, the particles should remain trapped in the closed
magnetic fields of the coronal flare loops and the ejected flux rope,
given the magnetic geometry of the standard eruptive-flare model. In
this case, the particles would reach the Earth only after a delay of
many hours to a few days (coincident with the bulk ejecta arriving at
Earth). We propose that the external magnetic reconnection intrinsic
to the breakout model for CME initiation can naturally account for
the prompt escape of flare-accelerated energetic particles onto open
interplanetary magnetic flux tubes. We present detailed 2.5-dimensional
magnetohydrodynamic simulations of a breakout CME/flare event with
a background isothermal solar wind. Our calculations demonstrate
that if the event occurs sufficiently near a coronal-hole boundary,
interchange reconnection between open and closed fields can occur. This
process allows particles from deep inside the ejected flux rope to
access solar wind field lines soon after eruption. We compare these
results to standard observations of impulsive SEPs and discuss the
implications of the model on further observations and calculations.
---------------------------------------------------------
Title: The Formation and Evolution of Coronal Flux Ropes Created by
Dynamical Flux Emergence
Authors: Linton, Mark; Leake, J. E.; Antiochos, S. K.
2013SPD....44..105L Altcode:
We investigate the formation and evolution of coronal flux ropes created
by the dynamical emergence of convection zone magnetic fields into
various pre-existing coronal magnetic fields. While we found earlier
that 2D dynamical flux emergence is not effective at creating coronal
flux ropes, either eruptive or stable, we find that in 3D the results
are dramatically different. We show that with a dipolar coronal field
in a non-reconnecting configuration, a 3D emerging flux rope can form
into a stable, prominence-like twisted coronal flux rope. In contrast,
we show that when the coronal dipole field is in a reconnecting
configuration, a 3D emerging flux rope will reconnect with it to form a
breakout quadrupolar field, and then an erupting flux rope. We therefore
conclude that, while 2D flux emergence is not an effective mechanism
for eruptive flux rope formation, 3D flux emergence is an effective
way to simultaneously create a breakout quadrupolar field and to emerge
the magnetic shear needed to drive a breakout coronal mass ejection.
---------------------------------------------------------
Title: Helicity Condensation as the Origin of Coronal and Solar
Wind Structure
Authors: Antiochos, Spiro K.; DeVore, C. R.
2013SPD....4410101A Altcode:
Three of the most important and most puzzling features of the Sun’s
atmosphere are the smoothness of the closed field corona (so-called
coronal loops), the accumulation of magnetic shear at photospheric
polarity inversion lines (filament channels), and the complex
dynamics of the slow wind. We propose that a single process, helicity
condensation, is the physical mechanism giving rise to all three
features. A simplified model is presented for how helicity is injected
and transported in the closed corona by magnetic reconnection. With
this model we demonstrate that magnetic shear must accumulate at PILs
and coronal hole boundaries, and estimate the rate of shear growth at
PILs and the loss to the wind. Our results can account for many of the
observed properties of the corona and wind. This work was supported
in part by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: The Effects of Solar Eruption Dynamics on Particle Escape
Authors: Masson, Sophie; Antiochos, S. K.; DeVore, C. R.
2013SPD....4420304M Altcode:
Magnetic reconnection in the solar atmosphere is believed to be the
driver of most solar and heliospheric activity; therefore, understanding
the structure and dynamics of the coronal magnetic field is central to
understanding this activity. Important heliospheric manifestations
of intense energy release linked to solar activity include the
impact at the Earth of energetic particles accelerated during solar
eruptions. Observationally, the magnetic configuration of active
regions where solar eruptions occur agrees well with the standard
model of eruption, consisting of a flare and a coronal mass ejection
(CME). According to the standard model, particles accelerated at the
flare reconnection site should remain trapped in the CME. However,
flare-accelerated particles frequently reach the Earth long before the
CME does. We present a 3D model that explains how flare-accelerated
particles escape onto interplanetary magnetic flux tubes during
a solar eruption. Our model is based on results from large-scale
3D MHD simulations of a breakout-CME erupting into a heliosphere
with an isothermal solar wind. The simulations are performed with
the Adaptively Refined Mhd Solver (ARMS). We describe the multiple
reconnection episodes that occur during the evolution of the event,
and show how they lead to the release of flare-accelerated particles
onto open field lines. Analyzing the dynamics of the reconnected
flux during the eruption, we evaluate the spatial distribution and
the timing of the particle beams injected into the heliosphere. We
discuss the implications of results for CME/flare models and for SEPs
observations. This work was supported, in part, by the NASA TR&T
and SR&T Programs.
---------------------------------------------------------
Title: CME Initiation Driven by Velocity-Shear Kinetic Reconnection
Simulations
Authors: Black, Carrie; Antiochos, S. K.; Karpen, J. T.; Germaschewski,
K.; DeVore, C. R.
2013SPD....44..104B Altcode:
In the standard model for coronal mass ejections (CME) and/or solar
flares, the free energy for the event resides in the strongly sheared
magnetic field of a filament channel. The pre-eruption force balance
consists of an upward force due to the magnetic pressure of the
sheared field balanced by a downward tension due to overlying unsheared
field. Magnetic reconnection is widely believed to be the mechanism
that disrupts this force balance, leading to explosive eruption. For
understanding CME/flare initiation, therefore, it is critical to model
the onset or reconnection that is driven by the buildup of magnetic
shear. In MHD simulations, the application of a magnetic field shear
is a trivial matter. However, kinetic effects are important in the
diffusion region and thus, it is important to examine this process
with PIC simulations as well. The implementation of such a driver in
PIC methods is nontrivial and indicates necessity of a true multiscale
model for such processes in the Solar environment. The field must be
sheared self-consistently/ indirectly to prevent the generation of
waves that destroy the desired system. In the work presented here,
we discuss methods for applying a velocity shear perpendicular to the
plane of reconnection for periodic and nonperiodic systems.
---------------------------------------------------------
Title: Simulation of S-Web Corridor Dynamics
Authors: Young, Aleida Katherine; Antiochos, S. K.; Karpen, J. T.;
DeVore, C. R.; Zurbuchen, T. H.
2013shin.confE...2Y Altcode:
Unlike the fast solar wind, the slow solar wind compositionally
resembles the corona. Its higher average charge state composition
and bias towards heavier elements (Zurbuchen et al., 1999) suggests
that the most likely source for the slow solar wind is the release
of closed-field coronal plasma. The S-Web (separatrix web) model for
the source of slow solar wind is based on the uniqueness conjecture,
which states that only one coronal hole can exist in a single-polarity
region on the Sun (Antiochos et al. 2007). The apparent multiple
coronal holes observed within single-polarity regions therefore must
be connected by narrow corridors at scales smaller than the spatial
resolution of current measurements of the photosphere. Magnetic field
lines from the boundary of such a corridor map to the heliospheric
current sheet, while field lines from the interior of the corridor map
to an arc extending to high latitudes in the heliosphere (Antiochos et
al. 2011). Magnetic reconnection along a narrow corridor is a possible
release mechanism for coronal plasma. In this work, we simulate
the dynamics of an S-Web corridor using the Adaptively Refined
MHD Solver (ARMS) to examine the effects of magnetic reconnection
along the corridor on the opening and closing of field lines at
high latitudes in the heliosphere. The objective is to quantify the
release of coronal plasma due to reconnection and show that these
dynamics support the S-Web model as an explanation for the source of
slow solar wind. We will present results from our initial efforts to
simulate open-field corridor dynamics, outline plans for further work,
and discuss implications for understanding the slow solar wind.
---------------------------------------------------------
Title: The ISS Space Plasma Laboratory: A Proposed Orbital Solar
Physics Simulation Lab
Authors: Antiochos, Spiro K.; DeVore, C. R.; Thompson, B. J.; Bering,
E. A., III; Edeen, G.; Carter, M.; Giambusso, M.; Olsen, C. S.;
Squire, J.; Larson, D.; McFadden, J. P.; Longmier, B.
2013shin.confE.162A Altcode:
We describe a proposed laboratory-experiment research program that will
answer several fundamental questions concerning the dynamical opening
and closing of the Sun's magnetic field - the defining property of
CMEs and eruptive flares. Our experiment is specifically designed to
address the key questions of the rate of reconnection in the topology of
a flare or heliospheric current sheet, its burstiness, and the energy
partition between thermal, kinetic, and particle. Of course, it seems
completely contradictory to use a laboratory experiment to study an open
magnetic system, because so far all laboratory plasmas have very solid
walls. The pioneering feature of our program is that the experiments
will be performed on the International Space Station (ISS). Only by
going into space can we obtain the open domain that is absolutely
essential for studying the opening and closing of coronal flux. Our
research program will provide the instrumentation infrastructure,
modeling and solar data expertise and initial scientific understanding
required to develop the VASIMR® VF-200 high powered plasma source
into a wall-less, orbiting ISS Space Plasma Laboratory (ISPL) national
facility. For example, the VF-200 exhaust will simulate conditions
in the solar corona during CMEs/eruptive flares by creating plasma
jets in open magnetic field geometries. Such a facility would measure
quantities in the plasma flow with the goal of measuring magnetic
reconnection and transport phenomena that should be similar in nature
to those occurring in the corona and solar wind. Our experiment will
capture all the effects inherent in a fully 3D magnetic system and
reproduce some of the physics occurring in the post initiation phase
of CMEs/eruptive flares. The Aurora Plasma Diagnostics Package (APDP)
will carry Langmuir probes, a retarding potential analyzer (RPA),
dc magnetometer, plasma wave detectors, Faraday cups, electrostatic
analyzers, solid state energetic particle telescope and Ar II and
broadband imagers.
---------------------------------------------------------
Title: A Model for the Escape of Solar-Flare Accelerated Particles
Authors: Masson, Sophie; Antiochos, S.; DeVore, C. R.
2013shin.confE.132M Altcode:
Magnetic reconnection in the solar atmosphere is believed to be the
driver of most solar active phenomena. Therefore, the structure and
dynamics of the coronal magnetic field are central to understanding
solar and heliospheric activity. Important heliospheric manifestations
of intense energy release linked to solar activity include the
impact at the Earth of energetic particles accelerated during solar
eruptions. Observationally, the magnetic configuration of active
regions, where solar eruptions occur, agrees well with the standard
model of eruption, consisting of a flare and a coronal mass ejection
(CME). According to the standard model, particles accelerated at the
flare reconnection site should remain trapped in the CME. However,
flare-accelerated particles frequently reach the Earth long before
the CME does. <P />We present a new model that may lead to injection
of energetic particles onto open interplanetary magnetic flux
tubes. Our model is based on results of 2.5D MHD simulations of a
large-scale coronal null-point topology with the outer spine opened to
interplanetary space by an isothermal solar wind. The simulations are
performed with the Adaptively Refined Mhd Solver (ARMS). We describe the
multiple reconnections that occur during the evolution of the event,
and show how they lead to the release of flare accelerated particles
onto open field lines. We discuss the implications of our results for
CME/flare models and for observations. <P />This work was supported,
in part, by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: The Magnetic Topology of Slow Wind Sources
Authors: Antiochos, Spiro K.
2013shin.confE..19A Altcode:
Due to its observed location in the heliosphere, its plasma
composition, and its variability, most models for the origins of the
slow wind postulate that it is a result of the release of closed-field
plasma onto open field lines. In particular, in the S-Web model the
slow wind originates at a dynamic boundary region between open and
closed flux in the solar corona. Consequently, the detailed topology
of the open-closed magnetic boundary is critically important for
determining the properties of the slow wind. We discuss the possible
magnetic topologies for the open-closed boundary. There are three main
topologies corresponding to three types of observed coronal structures:
helmet streamers, plumes/coronal jets, and plasma sheets (also known
as pseudostreamers). I present models for each of these topologies
and show that each is very different at the Sun. I also argue that
each will have different consequences for the observed properties of
the wind in the heliosphere.
---------------------------------------------------------
Title: Velocity-Shear Driven CME Initiation in Kinetic Reconnection
Simulations
Authors: Black, Carrie; Antiochos, Spiro K.; Karpen, Judith; DeVore,
C. Richard; Germaschewski, Kai
2013shin.confE..79B Altcode:
In the standard model for coronal mass ejections (CME) and/or solar
flares, the free energy for the event resides in the strongly sheared
magnetic field of a filament channel. The pre-eruption force balance
consists of an upward force due to the magnetic pressure of the
sheared field balanced by a downward tension due to overlying unsheared
field. Magnetic reconnection is widely believed to be the mechanism
that disrupts this force balance, leading to explosive eruption. For
understanding CME/flare initiation, therefore, it is critical to model
the onset or reconnection that is driven by the buildup of magnetic
shear. In MHD simulations, the application of a magnetic field shear
is a trivial matter. However, kinetic effects are important in the
diffusion region and thus, it is important to examine this process
with PIC simulations as well. The implementation of such a driver in
PIC methods is nontrivial and indicates necessity of a true multiscale
model for such processes in the Solar environment. The field must be
sheared self-consistently/ indirectly to prevent the generation of waves
that destroy the desired system. In the work presented here, we discuss
preliminary results from a shear driven system in a 2.5D PiC simulation.
---------------------------------------------------------
Title: Helicity transport through the photosphere
Authors: Schuck, P. W.; Antiochos, S. K.; Linton, M.
2013AGUSMSH53C..04S Altcode:
Solar eruptions are driven by energy and helicity transported through
the photosphere and into the Corona. However, the mechanism by which
helicity is transferred from the solar dynamo to coronal structures
is pooly understood. We recast the Berger and Field (1984) helicity
transport equation in manifestly gauge invariant form and examine
the individual terms leading to the transport of helicity through
the emergence of closed field, and twisting and tangling of potential
fields. These theoretical results are applied to erupting active regions
observed by SDO/HMI. The plasma velocity fields in the photosphere,
necessary for computing energy and helicity fluxes are determined
using an upgraded version of DAVE4VM that incorporates the spherical
geometry of the solar images. We find that the bulk of the helicity
into the corona is injected by twisting motions, and we discuss the
implications of our results for understanding coronal activity.
---------------------------------------------------------
Title: The Structure and Dynamics of the Corona—Heliosphere
Connection
Authors: Antiochos, Spiro K.; Linker, Jon A.; Lionello, Roberto;
Mikić, Zoran; Titov, Viacheslav; Zurbuchen, Thomas H.
2013mspc.book..169A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simulation of S-Web Corridor Dynamics
Authors: Young, A. K.; Antiochos, S. K.; Karpen, J.; DeVore, C. R.;
Zurbuchen, T.
2012AGUFMSH53A2267Y Altcode:
The S-Web (separatrix web) model for the source of slow solar
wind is based on the uniqueness conjecture, which states that only
one coronal hole can exist in a single-polarity region on the Sun
(Antiochos et al. 2007). The apparent multiple coronal holes observed
within single-polarity regions therefore must be connected by narrow
corridors at scales smaller than the spatial resolution of current
measurements of the photosphere. Magnetic field lines from the boundary
of such a corridor map to the heliospheric current sheet, while field
lines from the interior of the corridor map to an arc extending to high
latitudes in the heliosphere (Antiochos et al. 2011). In this work, we
simulate the dynamics of an S-Web corridor using the Adaptively Refined
MHD Solver (ARMS), to examine the effects of magnetic reconnection
along the corridor on the opening and closing of field lines at high
latitudes in the heliosphere. The objective is to show that these
dynamics support the S-Web model as an explanation for the source of
slow solar wind. We will present results from our initial efforts to
simulate open-field corridor dynamics, outline plans for further work,
and discuss implications for understanding the slow solar wind.
---------------------------------------------------------
Title: The Magnetic Topology of Slow Wind Sources
Authors: Antiochos, S. K.
2012AGUFMSH52A..05A Altcode:
Due to its observed location in the heliosphere, plasma composition,
and variability, most models for the origins of the slow wind
postulate that it is a result of the release of closed-field plasma
onto open field lines. In particular, in the S-Web model the slow
wind originates at a dynamic boundary region between open and closed
flux in the solar corona. Consequently, the detailed topology
of the open-closed magnetic boundary is critically important for
determining the properties of the slow wind. We discuss the possible
magnetic topologies for the open-closed boundary. There are three main
topologies corresponding to three types of observed coronal structures:
helmet streamers, plumes/coronal jets, and plasma sheets (also known
as pseudostreamers). I present models for each of these topologies and
show that each is very different at the Sun. I also argue that each
will have very different consequences for the observed properties of
the wind in the heliosphere. This work was funded in part by the NASA
TR&T and SR&T Programs.
---------------------------------------------------------
Title: Kinetic Reconnection Simulations for CME Initiation Driven
by Velocity-Shear
Authors: Black, C.; Antiochos, S. K.; Karpen, J.; DeVore, C. R.;
Germaschewski, K.
2012AGUFMSH51A2213B Altcode:
In the standard model for coronal mass ejections (CME) and/or solar
flares, the free energy for the event resides in the strongly sheared
magnetic field of a filament channel. The pre-eruption force balance
consists of an upward force due to the magnetic pressure of the
sheared field balanced by a downward tension due to overlying unsheared
field. Magnetic reconnection is widely believed to be the mechanism
that disrupts this force balance, leading to explosive eruption. For
understanding CME/flare initiation, therefore, it is critical to model
the onset or reconnection that is driven by the buildup of magnetic
shear. In MHD simulations, the application of a magnetic field shear
is a trivial matter. However, kinetic effects are important in the
diffusion region and thus, it is important to examine this process
with PIC simulations as well. The implementation of such a driver in
PIC methods is nontrivial. The field must be sheared self-consistently/
indirectly to prevent the generation of waves that destroy the desired
system. In the work presented here, we discuss methods for applying
a velocity shear perpendicular to the plane of reconnection for a
nonperiodic system. We also discuss the implementation of boundary
conditions that are open to electric currents that flow through the
system boundary. C.B. is supported through an appointment to the NASA
Postdoctoral Program at GSFC, administered by Oak Ridge Associated
Universities through a contract with NASA.
---------------------------------------------------------
Title: The Structure and Dynamics of the Corona—Heliosphere
Connection
Authors: Antiochos, Spiro K.; Linker, Jon A.; Lionello, Roberto;
Mikić, Zoran; Titov, Viacheslav; Zurbuchen, Thomas H.
2012SSRv..172..169A Altcode: 2011SSRv..tmp..371A; 2011SSRv..tmp..224A; 2011SSRv..tmp..148A;
2011SSRv..tmp...79A
Determining how the heliospheric magnetic field and plasma connect
to the Sun's corona and photosphere is, perhaps, the central problem
in solar and heliospheric physics. For much of the heliosphere,
this connection appears to be well understood. It is now generally
accepted that so-called coronal holes, which appear dark in X-rays
and are predominantly unipolar at the photosphere, are the sources
of quasi-steady wind that is generally fast, >500 km/s, but can
sometimes be slow. However, the connection to the Sun of the slow,
non-steady wind is far from understood and remains a major mystery. We
review the existing theories for the sources of the non-steady wind and
demonstrate that they have difficulty accounting for both the observed
composition of the wind and its large angular extent. A new theory is
described in which this wind originates from the continuous opening and
closing of narrow open field corridors in the corona, which give rise
to a web of separatrices (the S-Web) in the heliosphere. Note that
in this theory the corona—heliosphere connection is intrinsically
dynamic, at least for this type of wind. Support for the S-Web model
is derived from MHD solutions for the corona and wind during the time
of the August 1, 2008 eclipse. Additionally, we perform fully dynamic
numerical simulations of the corona and heliosphere in order to test
the S-Web model as well as the interchange model proposed by Fisk
and co-workers. We discuss the implications of our simulations for
the competing theories and for understanding the corona—heliosphere
connection, in general.
---------------------------------------------------------
Title: The Mechanisms for the Onset and Explosive Eruption of Coronal
Mass Ejections and Eruptive Flares
Authors: Karpen, J. T.; Antiochos, S. K.; DeVore, C. R.
2012ApJ...760...81K Altcode:
We have investigated the onset and acceleration of coronal mass
ejections (CMEs) and eruptive flares. To isolate the eruption physics,
our study uses the breakout model, which is insensitive to the energy
buildup process leading to the eruption. We performed 2.5D simulations
with adaptive mesh refinement that achieved the highest overall
spatial resolution to date in a CME/eruptive flare simulation. The
ultra-high resolution allows us to separate clearly the timing of
the various phases of the eruption. Using new computational tools, we
have determined the number and evolution of all X- and O-type nulls
in the system, thereby tracking both the progress and the products
of reconnection throughout the computational domain. Our results show
definitively that CME onset is due to the start of fast reconnection
at the breakout current sheet. Once this reconnection begins, eruption
is inevitable; if this is the only reconnection in the system, however,
the eruption will be slow. The explosive CME acceleration is triggered
by fast reconnection at the flare current sheet. Our results indicate
that the explosive eruption is caused by a resistive instability,
not an ideal process. Moreover, both breakout and flare reconnections
begin first as a form of weak tearing characterized by slowly evolving
plasmoids, but eventually transition to a fast form with well-defined
Alfvénic reconnection jets and rapid flux transfer. This transition
to fast reconnection is required for both CME onset and explosive
acceleration. We discuss the key implications of our results for
CME/flare observations and for theories of magnetic reconnection.
---------------------------------------------------------
Title: Existence of two MHD reconnection modes in a solar 3D magnetic
null point topology
Authors: Pariat, Etienne; Antiochos, Spiro; DeVore, C. Richard;
Dalmasse, Kévin
2012cosp...39.1450P Altcode: 2012cosp.meet.1450P
Magnetic topologies with a 3D magnetic null point are common in
the solar atmosphere and occur at different spatial scales: such
structures can be associated with some solar eruptions, with the
so-called pseudo-streamers, and with numerous coronal jets. We have
recently developed a series of numerical experiments that model
magnetic reconnection in such configurations in order to study and
explain the properties of jet-like features. Our model uses our
state-of-the-art adaptive-mesh MHD solver ARMS. Energy is injected
in the system by line-tied motion of the magnetic field lines in a
corona-like configuration. We observe that, in the MHD framework, two
reconnection modes eventually appear in the course of the evolution of
the system. A very impulsive one, associated with a highly dynamic and
fully 3D current sheet, is associated with the energetic generation
of a jet. Before and after the generation of the jet, a quasi-steady
reconnection mode, more similar to the standard 2D Sweet-Parker model,
presents a lower global reconnection rate. We show that the geometry of
the magnetic configuration influences the trigger of one or the other
mode. We argue that this result carries important implications for
the observed link between observational features such as solar jets,
solar plumes, and the emission of coronal bright points.
---------------------------------------------------------
Title: Generation of plasma flows and waves during the development
of coronal jets
Authors: Pariat, Etienne; Antiochos, Spiro; DeVore, C. Richard
2012cosp...39.1449P Altcode: 2012cosp.meet.1449P
No abstract at ADS
---------------------------------------------------------
Title: The effect of magnetic topology on the escape of flare
accelerated particles
Authors: Masson, Sophie; Antiochos, S. K.; DeVore, C. R.
2012shin.confE..84M Altcode:
Magnetic reconnection in the solar atmosphere is believed to be the
driver of most solar active phenomena. Therefore, the structure and
dynamics of the coronal magnetic field are central to understanding
solar and heliospheric activity. This study investigates the dynamics
of the magnetic topology when a CME interacts with the interplanetary
magnetic field.Our model is based on results of 2.5D MHD simulations of
a large-scale coronal null-point topology with the outer spine opened to
interplanetary space by an isothermal solar wind. The simulations are
performed with the Adaptively Refined Mhd Solver (ARMS). We describe
the multiple reconnections that occur during the evolution of the
event. These dynamics of the magnetic field during solar eruption
provide a new model for the injection of flare-accelerated particles
from the closed coronal field of the CME onto open interplanetary flux
tubes. Wediscussthe implications of our results for CME/flare models
and for observations. This work was supported, in part, by the NASA
TR&T and SR&T
---------------------------------------------------------
Title: Investigating the Sources of Solar Activity via Flux Emergence
Simulations
Authors: Linton, Mark George; Leake, James; Schuck, Peter; Antiochos,
Spiro
2012shin.confE..41L Altcode:
Solar coronal activity, and the space weather it drives, is
generatedby the dynamics and energy release of interacting coronal
magneticfields. These fields are created far below the solar surface,
at thebase of the convection zone. They then rise to the surface and
emergethrough the photosphere and chromosphere to create the dynamic
corona.However, as much of this process can not be observed, many
importantquestions about how this flux reaches and populates the corona
haveyet to be solved. Theory and numerical simulations are powerful
toolsto address these fundamental issues, and this presentation will
focuson several of these unsolved problems in flux emergence which
arecurrently being investigated with these tools.Current models
show that magnetic flux should neither be able torise to the solar
surface, nor to emerge into the corona unless itis highly twisted,
yet the majority of emerged magnetic regions areobserved to have very
little twist. A related issue is that manysuccessful models of coronal
mass ejection initiation requireenergized, sheared magnetic field to
emerge into the corona todrive these solar eruptions, yet flux emergence
simulations showthat it is primarily twisted field which emerges to form
asimple arcade, rather than sheared field. This presentation willreview
the evidence behind these conundrums, and presentresults from ongoing
investigations which show how these issuesmay be solved.This work was
supported by the ONR and by the NASA LWS and HGIprograms.
---------------------------------------------------------
Title: The effect of magnetic topology on the escape of
flare-accelerated particles
Authors: Masson, Sophie; Antiochos, S. K.; DeVore, C. R.
2012shin.confE..20M Altcode:
Magnetic reconnection in the solar atmosphere is believed to be the
driver of most solar active phenomena. Therefore, the structure and
dynamics of the coronal magnetic field are central to understanding
solar and heliospheric activity. Important heliospheric manifestations
of intense energy release linked to solar activity include the
impact at the Earth of energetic particles accelerated during solar
eruptions. Observationally, the magnetic configuration of active
regions, where solar eruptions occur, agrees well with the standard
model of eruption, consisting of a flare and a coronal mass ejection
(CME). According to the standard model, particles accelerated at the
flare reconnection site should remain trapped in the CME. However,
flare-accelerated particles frequently reach the Earth long before the
CME does.We present a new model that may lead to injection of energetic
particles onto open interplanetary magnetic flux tubes. Our model is
based on results of 2.5D MHD simulations of a large-scale coronal
null-point topology with the outer spine opened to interplanetary
space by an isothermal solar wind. The simulations are performed with
the Adaptively Refined Mhd Solver (ARMS). We describe the multiple
reconnections that occur during the evolution of the event, and show
how they lead to the release of flare accelerated particles onto open
field lines. We discuss the implications of our results for CME/flare
models and for observations.This work was supported, in part, by the
NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: Coronal jets in an inclined coronal magnetic field : a
parametric 3D MHD study
Authors: Dalmasse, K.; Pariat, E.; Antiochos, S. K.; DeVore, C. R.
2012EAS....55..201D Altcode:
X-ray solar coronal jets are short-duration, fast, well collimated
plasma brightenings occurring in the solar corona. To explain and
understand the processes driving the jets, one must be able to model an
explosive release of free energy. Magnetic reconnection is believed
to play a key role in the generation of these energetic bursting
events. The model of jets that we have been developing is based on
a magnetic field constructed by embedding a vertical magnetic dipole
in a uniform open magnetic field. In this study, we investigate the
influence of the inclination of the open field on the properties of the
jet using numerical simulations. We will show that the inclination of
the open field is of critical importance for the properties of the jet
such as the energy released. We conclude that the characteristics of
the open field at the time of observations are a central criterion that
must be taken into account and reported on in observational studies.
---------------------------------------------------------
Title: Understanding Solar Flares
Authors: Antiochos, Spiro K.; Karpen, J. T.; DeVore, C. R.
2012AAS...22041001A Altcode:
Solar flares and their associated coronal mass ejections are the
most energetic explosions in the solar system. The largest events
pose the greatest space weather dangers to life and civilization,
and are of extreme importance to human space exploration. They also
provide the best opportunity to study the universal processes of
magnetic reconnection and particle acceleration that underlie most
solar activity. The two great mysteries of solar flares are: how can so
much energy be released so quickly, and how can such a large fraction
(50% or more) end up in energetic particles. We present results from
recent numerical modeling that sheds new light on these mysteries. These
calculations use the highest spatial resolution yet achieved in order
to resolve the flare dynamics as clearly as possible. We conclude from
this work that magnetic island formation is the defining property of
magnetic reconnection in the solar corona, at least, in the large-scale
current sheet required for a solar flare. Furthermore, we discuss the
types of future observations and modeling that will be required to solve
definitively the solar flare mysteries. <P />This work was supported,
in part, by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: Comparison of Prominence Structures with Instances of Flux
Rope CMEs in STEREO Data
Authors: Rager, Amy; Thompson, B. J.; Antiochos, S. K.; Thernisien,
A.; Thompson, W. T.
2012AAS...22020004R Altcode:
STEREO A and B CME data have been visually searched for instances
of flux ropes, signified by a concave outward cavity feature in
the COR1 coronagraph. The flux rope events selected were observed
by both spacecraft, and also had visible prominences in both EUVI-A
and EUVI-B. The appearance of a flux rope was compared to the angle
of the inferred magnetic neutral line of the CME to discover if a
relationship existed. The GCS CME flux rope model was fit to the COR1
data, allowing for a clearer representation of the flux rope structure
to compare with the magnetic neutral line.
---------------------------------------------------------
Title: The Effect of Magnetic Topology on the Escape of
Flare-accelerated Particles
Authors: Masson, Sophie; Antiochos, S. K.; DeVore, C. R.
2012AAS...22051606M Altcode:
Magnetic reconnection in the solar atmosphere is believed to be the
driver of most solar active phenomena. Therefore, the structure and
dynamics of the coronal magnetic field are central to understanding
solar and heliospheric activity. Important heliospheric manifestations
of intense energy release linked to solar activity include the
impact at the Earth of energetic particles accelerated during solar
eruptions. Observationally, the magnetic configuration of active
regions, where solar eruptions occur, agrees well with the standard
model of eruption, consisting of a flare and a coronal mass ejection
(CME). According to the standard model, particles accelerated at the
flare reconnection site should remain trapped in the CME. However,
flare-accelerated particles frequently reach the Earth long before
the CME does. <P />We present a new model that may lead to injection
of energetic particles onto open interplanetary magnetic flux
tubes. Our model is based on results of 2.5D MHD simulations of a
large-scale coronal null-point topology with the outer spine opened to
interplanetary space by an isothermal solar wind. The simulations are
performed with the Adaptively Refined Mhd Solver (ARMS). We describe the
multiple reconnections that occur during the evolution of the event,
and show how they lead to the release of flare accelerated particles
onto open field lines. We discuss the implications of our results for
CME/flare models and for observations. <P />This work was supported,
in part, by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: Global network of slow solar wind
Authors: Crooker, N. U.; Antiochos, S. K.; Zhao, X.; Neugebauer, M.
2012JGRA..117.4104C Altcode: 2012JGRA..11704104C
The streamer belt region surrounding the heliospheric current sheet
(HCS) is generally treated as the primary or sole source of the
slow solar wind. Synoptic maps of solar wind speed predicted by the
Wang-Sheeley-Arge model during selected periods of solar cycle 23,
however, show many areas of slow wind displaced from the streamer
belt. These areas commonly have the form of an arc that is connected to
the streamer belt at both ends. The arcs mark the boundaries between
fields emanating from different coronal holes of the same polarity
and thus trace the paths of belts of pseudostreamers, i.e., unipolar
streamers that form over double arcades and lack current sheets. The
arc pattern is consistent with the predicted topological mapping of
the narrow open corridor or singular separator line that must connect
the holes and, thus, consistent with the separatrix-web model of the
slow solar wind. Near solar maximum, pseudostreamer belts stray far
from the HCS-associated streamer belt and, together with it, form a
global-wide web of slow wind. Recognition of pseudostreamer belts as
prominent sources of slow wind provides a new template for understanding
solar wind stream structure, especially near solar maximum.
---------------------------------------------------------
Title: The Role of Topology in the Energetics of the Coupled Solar
Atmosphere
Authors: Antiochos, Spiro K.
2012decs.confE..43A Altcode:
The defining physical property of the solar atmosphere is that
the magnetic field dominates the plasma. This property implies that
magnetic topology plays the central role in determining the structure
and energetics of the atmosphere. For example, the formation of observed
coronal loops, the explosive energy release in flares and coronal
mass ejections, and the creation of the solar wind are all controlled
by the topological constraints imposed on the plasma by the magnetic
field. In this presentation I discuss how magnetic topology leads to
the formation of the structures and dynamics observed in the solar
atmosphere including the wind. Not surprisingly, the most important
process for driving the dynamics is magnetic reconnection, which acts
to break many of the topological constraints. Reconnection, however,
preserves some of the topology, in particular, helicity. This turns
out to have major implications for the coupled atmosphere. In this
presentation, I will also discuss the implications of the topological
constraints on observations from SDO and Hinode. This work was
supported, in part, by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: Forced Magnetic Reconnection at an X-point: Comparative Fluid
and Fully Kinetic Studies
Authors: Wang, L.; Antiochos, S. K.; Bessho, N.; Bhattacharjee, A.;
Black, C.; DeVore, C. R.; Dorelli, J.; Karpen, J. T.
2011AGUFMSM23B2049W Altcode:
We have undertaken a challenge problem of investigating current sheet
formation and the resulting magnetic reconnection at an X-point of
an initially potential field by a suite of MHD, Hall MHD, and fully
electromagnetic PIC codes, all with the same initial conditions. Our
goals are to investigate the similarities and differences between
the various physical models, and to seek suitable parameterization
of kinetic effects in the fluid models. We use two types of forcing:
(i) shearing flows at the boundaries, and (ii) pressure perturbations
imposed in two spatial domains on opposite sides of the initial
separatrix. In both cases the system is driven slowly compared to the
characterstic Alfven speed, and the forcing is far from the initial
separatrices. While both the fluid and PIC models show current sheet
formation and magnetic reconnection, the reconnection onset, the rate,
and the energy released show significant differences. We will present
scaling results in the fluid as well as PIC simulations, and discuss
reasons for the differences between them. We also discuss possible
extensions of the MHD model in order to reconcile it with the PIC
model. This challenge problem is carried out under the auspices of
a Focus Team in the NASA Living With a Star Targeted Research and
Technology Program.
---------------------------------------------------------
Title: Magnetohydrodynamic Simulations of Current-Sheet Formation
and Reconnection at a Magnetic X Line
Authors: DeVore, C. R.; Antiochos, S. K.; Karpen, J. T.; Black, C.
2011AGUFMSH43A1923D Altcode:
Phenomena ranging from the quiescent heating of the ambient plasma to
the highly explosive release of energy and acceleration of particles in
flares are conjectured to result from magnetic reconnection at electric
current sheets in the Sun's corona. We are investigating numerically,
using a macroscopic magnetohydrodynamic (MHD) model with adaptive mesh
refinement, the formation and reconnection of a current sheet in an
initially potential 2D magnetic field containing a null. Subjecting
this simple configuration to unequal stresses in the four quadrants
bounded by the X-line separatrix distorts the potential null into a
double-Y-line current sheet. We find that even small distortions of the
magnetic field induce the formation of a tangential discontinuity in
the high-beta region around the null. A continuously applied stress
eventually leads to the onset of fast magnetic reconnection across
the sheet, with copious production, merging, and ejection of magnetic
islands. We compare the current-sheet development and evolution
for three cases: quasi-ideal MHD with numerical resistivity only;
uniformly resistive MHD; and MHD with an embedded kinetic reconnection
model. Analogous kinetic simulations using particle-in-cell (PIC)
methods to investigate the small-scale dynamics of the system also
are being pursued (C. Black et al., this meeting). Our progress
toward understanding this simple system will be reported, as will the
implications of our results for the dynamic activity associated with
coronal current sheets and for general multiscale modeling of magnetized
plasmas in the Heliosphere. Our research was supported by NASA.
---------------------------------------------------------
Title: Current-Sheet Formation and Reconnection at a Magnetic X Line
in Particle-in-Cell Simulations
Authors: Black, C.; Antiochos, S. K.; Hesse, M.; Karpen, J. T.;
DeVore, C. R.; Zenitani, S.; Kuznetsova, M. M.
2011AGUFMSH43A1919B Altcode:
The integration of kinetic effects into macroscopic numerical
models is currently of great interest to the heliophysics community,
particularly in the context of magnetic reconnection. Reconnection
governs the large-scale energy release and topological rearrangement
of magnetic fields in a wide variety of laboratory, heliophysical, and
astrophysical systems. We are examining the formation and reconnection
of current sheets in a simple, two-dimensional X-line configuration
using high-resolution particle-in-cell (PIC) simulations. The initial
minimum-energy, potential magnetic field is perturbed by excess
thermal pressure introduced into the particle distribution function
far from the X line. Subsequently, the relaxation of this added stress
leads self-consistently to the development of a current sheet that
reconnects for imposed stress of sufficient strength. We compare the
time-dependent evolution and final state of our PIC simulations with
macroscopic magnetohydrodynamic simulations assuming both uniform and
localized electrical resistivities (C. R. DeVore et al., this meeting),
as well as with force-free magnetic-field equilibria in which the amount
of reconnection across the X line can be constrained to be zero (ideal
evolution) or optimal (minimum final magnetic energy). We will discuss
implications of our results for understanding magnetic-reconnection
onset and cessation at kinetic scales in dynamically formed current
sheets, such as those occurring in the solar corona and terrestrial
magnetotail. This research was supported by NASA.
---------------------------------------------------------
Title: A Numerical Simulation for the Origins of Solar Magnetic
Structure
Authors: Zhao, L.; Antiochos, S. K.; DeVore, C. R.
2011AGUFMSH43A1930Z Altcode:
We investigate numerically a new model for the origin of the solar
coronal magnetic field structure observed in filament channels and in
the complex structures of the slow wind. Using the Adaptively Refined
Magnetohydrodynamic Solver (ARMS), we perform a series of numerical
experiments to study the evolution of magnetic helicity injected into
the solar corona by photospheric motions. Our simulation domain consists
of a Cartesian box with an initially uniform vertical magnetic field
and a low-beta plasma with uniform pressure and density. This system is
driven by imposing flow patterns at the top and bottom boundary planes
corresponding to the twisting motions expected from the quasi-random
photospheric motions. We consider a variety of flow patterns made up
of twist arranged in regular geometric orders (e.g. four twists in a
quadrilateral arrangement, seven twists in a hexagonal), which generate
sets of twisted flux tubes in the interior of the simulation box, the
corona. This driving twist injects both energy and helicity into the
coronal field. Depending upon the sense of the applied twist, we can
inject either positive or negative helicity. If helicity of the same
sign is injected into each of the flux tubes (the co-helicity case),
we expect that the twist magnetic-field component of neighboring flux
tubes will be oppositely directed and, therefore, will reconnect;
on the other hand, if helicity of opposite signs is injected into
neighboring flux tubes (the counter-helicity case), reconnection will
not occur. This conjecture is confirmed by our simulations. We also have
found generally that in co-helicity cases the reconnection indeed occurs
and leads to a state in which the twist propagates to the largest scale:
essentially, the individual flux tubes merge into one large twisted
tube, with the twist concentrated at its outer boundary. We discuss
the implications of our results for the evolution of coronal helicity
and for the formation of filament channels on the Sun and slow-wind
structures in the Heliosphere. Our research was sponsored by NASA.
---------------------------------------------------------
Title: Ion-neutral Coupling in Solar Prominences
Authors: Gilbert, H. R.; DeVore, C. R.; Karpen, J. T.; Kucera, T. A.;
Antiochos, S. K.; Kawashima, R.
2011AGUFMSH13B1953G Altcode:
Coupling between ions and neutrals in magnetized plasmas is
fundamentally important to many aspects of heliophysics, including our
ionosphere, the solar chromosphere, the solar wind interaction with
planetary atmospheres, and the interface between the heliosphere and
the interstellar medium. Ion-neutral coupling also plays a major role
in the physics of solar prominences. By combining theory, modeling,
and observations we are working toward a better understanding of the
structure and dynamics of partially ionized prominence plasma. Two
key questions are addressed in the present work: 1) what physical
mechanism(s) sets the cross-field scale of prominence threads? 2)
Are ion-neutral interactions responsible for the vertical flows and
structure in prominences? We present initial results from a study
investigating what role ion-neutral interactions play in prominence
dynamics and structure. This research was supported by NASA.
---------------------------------------------------------
Title: The Magnetic Connectivity of the Sun to the Heliosphere
Authors: Antiochos, S. K.
2011AGUFMSH43F..05A Altcode:
A prime research focus of the upcoming Solar Probe Plus and Solar
Orbiter missions will be to determine how the heliospheric magnetic
field and plasma connect to the Sun's corona and photosphere. For much
of the heliosphere this connection appears to be well understood. The
quasi-steady fast wind emanates from so-called coronal holes,
which appear dark in X-rays and are predominantly unipolar at the
photosphere. However, the connection to the Sun of the slow, non-steady
wind is far from understood and remains a major mystery. We review the
existing theories for the sources of the non-steady wind and demonstrate
that they have difficulty accounting for both the observed composition
of the wind and its large angular extent. A new theory is described in
which this wind originates from the continuous opening and closing of
narrow open field corridors in the corona, which gives rise to a web of
separatrices (the S-Web) in the heliosphere. Note that in this theory
the corona - heliosphere connection is intrinsically dynamic, at least,
for this type of wind. We present numerical simulations of the model
and describe observational tests. We discuss the implications of our
results for the competing slow wind theories and for understanding the
corona - heliosphere connection, in general. This work was supported,
in part, by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: Consequences of the Breakout Model for Particle Acceleration
in CMEs and Flares
Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R.
2011AGUFMSH51E..01A Altcode:
The largest and most efficient particle accelerators in the solar system
are the giant events consisting of a fast coronal mass ejection (CME)
and an intense X-class solar flare. Both flares and CMEs can produce
10<SUP>32</SUP> ergs or more in nonthermal particles. Two general
processes are believed to be responsible: particle acceleration at the
strong shock ahead of the CME, and reconnection-driven acceleration
in the flare current sheet. Although shock acceleration is relatively
well understood, the mechanism by which flare reconnection produces
nonthermal particles is still an issue of great debate. We address
the question of CME/flare particle acceleration in the context of
the breakout model using 2.5D MHD simulations with adaptive mesh
refinement (AMR). The AMR capability allows us to achieve ultra-high
numerical resolution and, thereby, determine the detailed structure and
dynamics of the flare reconnection region. Furthermore, we employ newly
developed numerical analysis tools for identifying and characterizing
magnetic nulls, so that we can quantify accurately the number and
location of magnetic islands during reconnection. Our calculations
show that flare reconnection is dominated by the formation of magnetic
islands. In agreement with many other studies, we find that the number
of islands scales with the effective Lundquist number. This result
supports the recent work by Drake and co-workers that postulates
particle acceleration by magnetic islands. On the other hand, our
calculations also show that the flare reconnection region is populated
by numerous shocks and other indicators of strong turbulence, which
can also accelerate particles. We discuss the implications of our
calculations for the flare particle acceleration mechanism and for
observational tests of the models. This work was supported, in part,
by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: CMEs with multiple reconnection sites : A model for energetic
particle injection
Authors: Masson, S.; Antiochos, S. K.; DeVore, C. R.
2011AGUFMSH43A1931M Altcode:
Magnetic reconnection in the solar atmosphere is believed to be the
driver of most solar explosive phenomena. Therefore, the structure and
dynamics of the coronal magnetic field are central to understanding
solar and heliospheric activity. Important heliospheric manifestations
of intense energy release linked to solar activity include the
impact at the Earth of energetic particles accelerated during solar
eruptions. Observationally, the magnetic configuration of active
regions where solar eruptions occur, agrees well with the standard
model of an eruption consisting of a flare and a coronal mass ejection
(CME). According to the standard model, particles accelerated at the
flare reconnection site should remain trapped in the CME. However,
flare-accelerated particles frequently reach the Earth long before
the CME does. We present a new model that may lead to injection of
energetic particles onto open magnetic flux tubes connecting to the
Earth. Our model is based on the well-known 2.5D breakout topology,
which has a coronal null point (null line) and a four-flux system. A
key new addition, however, is that we include an isothermal solar
wind. Depending on the location of the open flux with respect to
the null point, we find that the flare reconnection can consist of
two distinct phases. At first, the flare reconnection involves only
closed field, but if the eruption occurs close to the open field,
we find a second phase involving interchange reconnection between
open and closed. We argue that this second reconnection episode is
responsible for the injection of flare-accelerated particles into
the interplanetary medium. We will report on our recent work toward
understanding how flare particles escape to the heliosphere. This work
uses high-resolution 2.5D MHD numerical simulations performed with
the Adaptively Refined MHD Solver (ARMS). This research was supported,
in part, by the NASA SR&T and TR&T Programs.
---------------------------------------------------------
Title: Numerical Simulation of a "Stealth" CME: Why Slow and Simple
is Not Mysterious
Authors: Lynch, B. J.; Li, Y.; Antiochos, S. K.; DeVore, C. R.;
Luhmann, J. G.; Fisher, G. H.
2011AGUFMSH43A1937L Altcode:
The stereoscopic viewing and improvements in coronagraph observations
by STEREO/SECCHI and low corona EUV and X-ray observations at multiple
wavelengths by STEREO, Hinode, and SDO -- combined with this solar
minimum's exceptionally low activity -- have given rise to the
community's interest in so-called "stealth" CMEs. A "stealth" CME is
one in which there are almost no low coronal signatures of the CME
eruption but often a very well resolved slow, flux-rope like eruption
seen in the coronagraph data. The fact that the in situ observations
of "stealth" CMEs have shown many of the signatures of magnetic clouds
(including the interplanetary flux rope structure) poses the question,
"Just how different these events are from normal CMEs?" We present a
3D numerical MHD simulation of the 2008 Jun 2 gradual streamer blowout
CME which had virtually no identifiable low coronal signatures. We
energize the field by simple footpoint shearing along the source
region's polarity inversion line (PIL) and model the background solar
wind structure using an ~2MK isothermal wind and a low-order PFSS
representation of the CR2070 synoptic magnetogram. Our results will
show that the CME "initiation" is obtained by slowly disrupting the
quasi-steady-state configuration of the helmet streamer, resulting in
the standard eruptive flare picture (albeit, on a large scale) that
ejects the sheared fields and lowers the magnetic energy stored in
filament channel. We obtain a relatively slow CME eruption and argue
that these "stealth" CMEs are no different than the standard quasi-2D
picture but are simply at the low end of the CME energy distribution. We
will show preliminary comparisons between the simulation results and
the coronagraph observations of the low coronal evolution of the CME.
---------------------------------------------------------
Title: Erratum: "Tests of Dynamical Flux Emergence as
a Mechanism for Coronal Mass Ejection Initiation" <A
href="/abs/2010ApJ...722..550L">(2010, ApJ, 722, 550)</A>
Authors: Leake, James E.; Linton, Mark G.; Antiochos, Spiro K.
2011ApJ...741..125L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Preface
Authors: Lewis, W. S.; Antiochos, S. K.; Drake, J. F.
2011SSRv..160....1L Altcode: 2011SSRv..tmp..259L; 2011SSRv..tmp..294L; 2011SSRv..tmp..241L
No abstract at ADS
---------------------------------------------------------
Title: The S-Web Hypothesis and the Slow Solar Wind
Authors: Linker, Jon A.; Lionello, Roberto; Titov, Viacheslav S.;
Mikic, Zoran; Antiochos, Spiro
2011shin.confE.160L Altcode:
The origin of the slow solar wind is controversial. A successful theory
must explain the plasma composition and angular extent of the slow wind,
as well as its frequent asymmetry with respect to the heliospheric
current sheet. Recently, a new idea has been put forward for the
origin of the slow wind, dubbed the S-Web model. The name comes from
high-resolution MHD calculations that have revealed that coronal hole
boundaries are not smooth, but are highly corrugated with a web of
separatrices and quasi-separatrix layers. These are regions that are
likely to be susceptible to interchange reconnection. In this talk we
describe the origin of this idea, how it may explain key features of the
slow solar wind, and further calculations/observational tests that may
help confirm or refute this idea. <P />Work supported by NASA and NSF.
---------------------------------------------------------
Title: Current Sheet Formation and Reconnection at a Magnetic X Line
Authors: DeVore, C. Richard; Antiochos, S. K.
2011SPD....42.1820D Altcode: 2011BAAS..43S.1820D
Phenomena ranging from the quiescent heating of the ambient plasma to
the highly explosive release of energy and acceleration of particles in
flares are conjectured to result from magnetic reconnection at electric
current sheets in the Sun's corona. We are investigating numerically
the formation and eventual reconnection of a current sheet in an
initially potential 2D magnetic field containing a null. Subjecting
this simple configuration to unequal stresses in the four quadrants
bounded by the X-line separatrix distorts the potential null into
a double-Y-line current sheet. Although the gas pressure is finite
in our simulations, so that the plasma beta is infinite at the null,
we find that even small distortions of the magnetic field induce the
formation of a tangential discontinuity there. This result is well known
to occur in the zero-beta, force-free limit; surprisingly, it persists
into the high-beta regime where, in principle, a small plasma pressure
inhomogeneity could balance all of the magnetic stress. In addition
to working to understand the dynamical details of this ideal process,
we are examining the effect of resistive dissipation on the development
of the current sheet and are seeking to determine the critical condition
for fast-reconnection onset in the sheet. Our progress on understanding
these issues, and the implications for the dynamic activity associated
with current sheets in the solar corona, will be reported at the
conference. We gratefully acknowledge NASA sponsorship of our research.
---------------------------------------------------------
Title: CME Onset and Take-off
Authors: Antiochos, Spiro K.; Karpen, J. T.; DeVore, C. R.
2011SPD....42.1302A Altcode: 2011BAAS..43S.1302A
For understanding and eventually predicting coronal mass
ejections/eruptive flares, two critical questions must be answered:
What is the mechanism for eruption onset, and what is the mechanism
for the rapid acceleration? We address these questions in the context
of the breakout model using 2.5D MHD simulations with adaptive mesh
refinement (AMR). The AMR capability allowed us to achieve ultra-high
numerical resolution and, thereby, determine the influence of the
effective Lundquist number on the eruption. Our calculations show that,
at least, for the breakout model, the onset of reconnection external
to the highly-sheared filament channel is the onset mechanism. Once
this reconnection turns on, eruption is inevitable. However, as long
as this is the only reconnection in the system, the eruption remains
slow. We find that the eruption undergoes an abrupt "take-off" when the
flare reconnection below the erupting plasmoid develops significant
reconnection jets. We conclude that in fast CMEs, flare reconnection
is the primary mechanism responsible for both flare heating and CME
acceleration. We discuss the implications of these results for SDO
observations and describe possible tests of the model. <P />This work
was supported, in part, by the NASA TR&T and SR&T Programs.
---------------------------------------------------------
Title: Constraints on Coronal Mass Ejection Evolution from in Situ
Observations of Ionic Charge States
Authors: Gruesbeck, Jacob R.; Lepri, Susan T.; Zurbuchen, Thomas H.;
Antiochos, Spiro K.
2011ApJ...730..103G Altcode:
We present a novel procedure for deriving the physical properties of
coronal mass ejections (CMEs) in the corona. Our methodology uses
in situ measurements of ionic charge states of C, O, Si, and Fe in
the heliosphere and interprets them in the context of a model for the
early evolution of interplanetary CME (ICME) plasma, between 2 and 5 R
<SUB>sun</SUB>. We find that the data are best fit by an evolution that
consists of an initial heating of the plasma, followed by an expansion
that ultimately results in cooling. The heating profile is consistent
with a compression of coronal plasma due to flare reconnection jets and
an expansion cooling due to the ejection, as expected from the standard
CME/flare model. The observed frozen-in ionic charge states reflect
this time history and, therefore, provide important constraints for the
heating and expansion timescales, as well as the maximum temperature
the CME plasma is heated to during its eruption. Furthermore, our
analysis places severe limits on the possible density of CME plasma in
the corona. We discuss the implications of our results for CME models
and for future analysis of ICME plasma composition.
---------------------------------------------------------
Title: Magnetic Topology of Coronal Hole Linkages
Authors: Titov, V. S.; Mikić, Z.; Linker, J. A.; Lionello, R.;
Antiochos, S. K.
2011ApJ...731..111T Altcode: 2010arXiv1011.0009T
In recent work, Antiochos and coworkers argued that the boundary between
the open and closed field regions on the Sun can be extremely complex
with narrow corridors of open flux connecting seemingly disconnected
coronal holes from the main polar holes and that these corridors may be
the sources of the slow solar wind. We examine, in detail, the topology
of such magnetic configurations using an analytical source surface model
that allows for analysis of the field with arbitrary resolution. Our
analysis reveals three new important results. First, a coronal hole
boundary can join stably to the separatrix boundary of a parasitic
polarity region. Second, a single parasitic polarity region can produce
multiple null points in the corona and, more important, separator
lines connecting these points. It is known that such topologies are
extremely favorable for magnetic reconnection, because they allow
this process to occur over the entire length of the separators rather
than being confined to a small region around the nulls. Finally, the
coronal holes are not connected by an open-field corridor of finite
width, but instead are linked by a singular line that coincides with
the separatrix footprint of the parasitic polarity. We investigate
how the topological features described above evolve in response to
the motion of the parasitic polarity region. The implications of our
results for the sources of the slow solar wind and for coronal and
heliospheric observations are discussed.
---------------------------------------------------------
Title: A Model for the Sources of the Slow Solar Wind
Authors: Antiochos, S. K.; Mikić, Z.; Titov, V. S.; Lionello, R.;
Linker, J. A.
2011ApJ...731..112A Altcode: 2011arXiv1102.3704A
Models for the origin of the slow solar wind must account for two
seemingly contradictory observations: the slow wind has the composition
of the closed-field corona, implying that it originates from the
continuous opening and closing of flux at the boundary between open
and closed field. On the other hand, the slow wind also has large
angular width, up to ~60°, suggesting that its source extends far
from the open-closed boundary. We propose a model that can explain
both observations. The key idea is that the source of the slow wind
at the Sun is a network of narrow (possibly singular) open-field
corridors that map to a web of separatrices and quasi-separatrix
layers in the heliosphere. We compute analytically the topology of an
open-field corridor and show that it produces a quasi-separatrix layer
in the heliosphere that extends to angles far from the heliospheric
current sheet. We then use an MHD code and MDI/SOHO observations of
the photospheric magnetic field to calculate numerically, with high
spatial resolution, the quasi-steady solar wind, and magnetic field
for a time period preceding the 2008 August 1 total solar eclipse. Our
numerical results imply that, at least for this time period, a web of
separatrices (which we term an S-web) forms with sufficient density
and extent in the heliosphere to account for the observed properties
of the slow wind. We discuss the implications of our S-web model for
the structure and dynamics of the corona and heliosphere and propose
further tests of the model.
---------------------------------------------------------
Title: The Evolution of Open Magnetic Flux Driven by Photospheric
Dynamics
Authors: Linker, Jon A.; Lionello, Roberto; Mikić, Zoran; Titov,
Viacheslav S.; Antiochos, Spiro K.
2011ApJ...731..110L Altcode:
The coronal magnetic field is of paramount importance in solar and
heliospheric physics. Two profoundly different views of the coronal
magnetic field have emerged. In quasi-steady models, the predominant
source of open magnetic field is in coronal holes. In contrast, in the
interchange model, the open magnetic flux is conserved, and the coronal
magnetic field can only respond to the photospheric evolution via
interchange reconnection. In this view, the open magnetic flux diffuses
through the closed, streamer belt fields, and substantial open flux is
present in the streamer belt during solar minimum. However, Antiochos
and coworkers, in the form of a conjecture, argued that truly isolated
open flux cannot exist in a configuration with one heliospheric current
sheet—it will connect via narrow corridors to the polar coronal
hole of the same polarity. This contradicts the requirements of the
interchange model. We have performed an MHD simulation of the solar
corona up to 20 R <SUB>sun</SUB> to test both the interchange model
and the Antiochos conjecture. We use a synoptic map for Carrington
rotation 1913 as the boundary condition for the model, with two small
bipoles introduced into the region where a positive polarity extended
coronal hole forms. We introduce flows at the photospheric boundary
surface to see if open flux associated with the bipoles can be moved
into the closed-field region. Interchange reconnection does occur in
response to these motions. However, we find that the open magnetic
flux cannot be simply injected into closed-field regions—the flux
eventually closes down and disconnected flux is created. Flux either
opens or closes, as required, to maintain topologically distinct open-
and closed-field regions, with no indiscriminate mixing of the two. The
early evolution conforms to the Antiochos conjecture in that a narrow
corridor of open flux connects the portion of the coronal hole that
is nearly detached by one of the bipoles. In the later evolution,
a detached coronal hole forms, in apparent violation of the Antiochos
conjecture. Further investigation reveals that this detached coronal
hole is actually linked to the extended coronal hole by a separatrix
footprint on the photosphere of zero width. Therefore, the essential
idea of the conjecture is preserved, if we modify it to state that
coronal holes in the same polarity region are always linked, either
by finite width corridors or separatrix footprints. The implications
of these results for interchange reconnection and the sources of the
slow solar wind are briefly discussed.
---------------------------------------------------------
Title: On tether-cutting reconnection in sheared coronal arcades
Authors: Lynch, B. J.; Li, Y.; Antiochos, S. K.; DeVore, C. R.;
Fisher, G. H.
2010AGUFMSH31A1791L Altcode:
We present preliminary numerical magnetohydrodynamic (MHD)
simulation results of 3-dimensional bipolar sheared arcades and
their susceptibility to eruption in a spherical geometry. The MHD
simulations are run using the Adaptively Refined MHD Solver (ARMS)
developed at the Naval Research Laboratory. The initially potential
magnetic field is energized via highly concentrated shearing flows
parallel to the polarity inversion line (PIL) of an idealized,
elongated decayed active region. The tether-cutting reconnection
deep in the sheared field core is generated by applying converging
flows towards the active region PIL that compress magnetic fields
with oppositely directed r-components together setting up a moderate
guide-field reconnection scenario at the lower boundary. We present
three cases that test the amount of tether-cutting reconnection required
to cause the sheared field to go unstable and erupt, parameterized
as the relative length of the converging flow profiles to the active
region PIL. Our hypothesis is that, due to the overlying background
field of the bipolar arcade, the sheared field will remain stable for
a relatively small amount of tether-cutting reconnection and when the
converging flows cover a substantial portion of the active region PIL,
the bipolar arcade could experience a sort of eruption. We will discuss
observational consequences of the eruption process and discuss future
work, e.g. comparing these results with multipolar magnetic breakout
eruptions.
---------------------------------------------------------
Title: High-Resolution Numerical Simulations of Breakout Coronal
Mass Ejections
Authors: DeVore, C. R.; Karpen, J. T.; Antiochos, S. K.
2010AGUFMSM31B1875D Altcode:
We have conducted high-resolution numerical simulations of the
gradual energization, initiation of eruption, and expansion into
the inner heliosphere of coronal mass ejections. The critical
triggering process underlying the eruption is the onset of magnetic
reconnection. Reconnection at the deformed null point high in the corona
(at the ‘breakout’ current sheet) reconfigures the restraining
field overlying the eruptive core, accelerating the rise of the magnetic
structure; that between the nearly vertical legs of the field above the
polarity inversion lines (at the ‘flare’ current sheet) partially
detaches flux from the Sun and provides a further impulse to the
outward motion of the ejecta. To investigate these processes in detail,
we assumed an axisymmetric (2.5D) spherical geometry and exploited
the adaptive mesh refinement capabilities of our Adaptively Refined
MHD Solver (ARMS) simulation model to achieve unprecedentedly high
resolution of all current structures as they develop dynamically. As
the maximum refinement level increases, the current sheets exhibit
increasingly fine-scaled structure, with ever greater numbers of
magnetic islands forming, dividing, recombining, and streaming along
the sheets to their termini. The macroscopic properties of the ejecta,
such as the kinetic energy and radial velocity of the CME, on the other
hand, depend only weakly on the grid refinement level and the resultant
numerical resistivity. This demonstrates convergence of the results
toward the high-conductivity regime of the solar corona. In addition
to describing these findings, we will report our progress on adding a
kinetic-scale resistivity model to the global simulations. This work
has been supported by the NASA HTP, SR&T, and LWS programs.
---------------------------------------------------------
Title: Multiscale Modeling of Solar Coronal Magnetic Reconnection
Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R.
2010AGUFMSM31B1873A Altcode:
Magnetic reconnection is widely believed to be the primary process
by which the magnetic field releases energy to plasma in the Sun's
corona. For example, in the breakout model for the initiation of
coronal mass ejections/eruptive flares, reconnection is responsible
for the catastrophic destabilizing of magnetic force balance in the
corona, leading to explosive energy release. A critical requirement
for the reconnection is that it have a "switch-on' nature in that the
reconnection stays off until a large store of magnetic free energy
has built up, and then it turn on abruptly and stay on until most
of this free energy has been released. We discuss the implications
of this requirement for reconnection in the context of the breakout
model for CMEs/flares. We argue that it imposes stringent constraints
on the properties of the flux breaking mechanism, which is expected
to operate in the corona on kinetic scales. We present numerical
simulations demonstrating how the reconnection and the eruption depend
on the effective resistivity, i.e., the effective Lundquist number,
and propose a model for incorporating kinetic flux-breaking mechanisms
into MHD calculation of CMEs/flares. This work has been supported by
the NASA HTP, SR&T, and LWS programs. High-resolution simulation
of a breakout CME showing details of the reconnection region (Karpen
et al 2010).
---------------------------------------------------------
Title: Constraints on CME evolution from in situ observations of
ionic charge states
Authors: Gruesbeck, J. R.; Lepri, S. T.; Zurbuchen, T.; Antiochos,
S. K.
2010AGUFMSH23B1837G Altcode:
We present a novel analysis of the expansion properties of Coronal Mass
Ejections (CMEs) in the inner corona. This methodology uses measurements
of ionic charge states of C, O, Si, and Fe from the Advanced Composition
Explorer (ACE) and interprets them in the context of a quantitative
ionization model. All observed charge state distributions exhibit
signatures of substantial heating, yet to varying degrees, reflecting
peculiar properties of the frozen-in ions despite a common expansion
profile. For example, the vast majority of ICMEs exhibit bi-modal
Fe charge state distributions. Using a plasma heating and expansion
model, in-situ charge states are translated into constraints on the
heating and expansion profiles of the CME plasma. We find that CMEs
are first heated up to ~3 MK near the Sun and with a maximum electron
density between ~5e9 and 8.5e9 cm^-3 then followed by rapid expansion
and cooling, These CMEs exhibit frozen-in charge states qualitatively
and quantitatively consistent with our observations.
---------------------------------------------------------
Title: Tests of Dynamical Flux Emergence as a Mechanism for Coronal
Mass Ejection Initiation
Authors: Leake, James E.; Linton, Mark G.; Antiochos, Spiro K.
2010ApJ...722..550L Altcode: 2010arXiv1007.5484L
Current coronal mass ejection (CME) models set their lower boundary to
be in the lower corona. They do not calculate accurately the transfer
of free magnetic energy from the convection zone to the magnetically
dominated corona because they model the effects of flux emergence
using kinematic boundary conditions or simply assume the appearance of
flux at these heights. We test the importance of including dynamical
flux emergence in CME modeling by simulating, in 2.5D, the emergence
of sub-surface flux tubes into different coronal magnetic field
configurations. We investigate how much free magnetic energy, in
the form of shear magnetic field, is transported from the convection
zone to the corona, and whether dynamical flux emergence can drive
CMEs. We find that multiple coronal flux ropes can be formed during flux
emergence, and although they carry some shear field into the corona,
the majority of shear field is confined to the lower atmosphere. Less
than 10% of the magnetic energy in the corona is in the shear field,
and this, combined with the fact that the coronal flux ropes bring up
significant dense material, means that they do not erupt. Our results
have significant implications for all CME models which rely on the
transfer of free magnetic energy from the lower atmosphere into the
corona but which do not explicitly model this transfer. Such studies
of flux emergence and CMEs are timely, as we have new capabilities
to observe this with Hinode and the Solar Dynamics Observatory, and
therefore to test the models against observations.
---------------------------------------------------------
Title: Formation and Reconnection of Three-dimensional Current Sheets
in the Solar Corona
Authors: Edmondson, J. K.; Antiochos, S. K.; DeVore, C. R.; Zurbuchen,
T. H.
2010ApJ...718...72E Altcode:
Current-sheet formation and magnetic reconnection are believed to be
the basic physical processes responsible for much of the activity
observed in astrophysical plasmas, such as the Sun's corona. We
investigate these processes for a magnetic configuration consisting
of a uniform background field and an embedded line dipole, a topology
that is expected to be ubiquitous in the corona. This magnetic system
is driven by a uniform horizontal flow applied at the line-tied
photosphere. Although both the initial field and the driver are
translationally symmetric, the resulting evolution is calculated
using a fully three-dimensional (3D) magnetohydrodynamic simulation
with adaptive mesh refinement that resolves the current sheet and
reconnection dynamics in detail. The advantage of our approach is
that it allows us to directly apply the vast body of knowledge gained
from the many studies of two-dimensional (2D) reconnection to the
fully 3D case. We find that a current sheet forms in close analogy to
the classic Syrovatskii 2D mechanism, but the resulting evolution is
different than expected. The current sheet is globally stable, showing
no evidence for a disruption or a secondary instability even for aspect
ratios as high as 80:1. The global evolution generally follows the
standard Sweet-Parker 2D reconnection model except for an accelerated
reconnection rate at a very thin current sheet, due to the tearing
instability and the formation of magnetic islands. An interesting
conclusion is that despite the formation of fully 3D structures at small
scales, the system remains close to 2D at global scales. We discuss
the implications of our results for observations of the solar corona.
---------------------------------------------------------
Title: Magnetic Topology of Coronal Hole Linkages
Authors: Titov, Viacheslav S.; Mikic, Zoran; Linker, Jon A.; Lionello,
Roberto; Antiochos, Spiro
2010shin.confE.120T Altcode:
In recent work, Antiochos et al. (2007) argued that the boundary between
the open and closed field regions on the Sun can be extremely complex
with narrow corridors of open flux connecting seemingly disconnected
coronal holes from the main polar holes, and that these corridors
may be the sources of the slow solar wind. We examine, in detail, the
topology of such magnetic configurations using an analytical source
surface model that allows for analysis of the field with arbitrary
resolution. Our analysis reveals three important new results: First,
a coronal hole boundary can include the separatrix boundary of a
parasitic polarity region. Second, a single parasitic polarity region
can produce multiple null points in the corona and, more important,
separator lines connecting these points. Such topologies are extremely
favorable for magnetic reconnection, because it can now occur over
the entire length of the separators rather than being confined to
a small region around the nulls. Finally, the coronal holes are not
connected by an open-field corridor of finite width, but instead are
linked by a singular line that coincides with the separatrix footprint
of the parasitic polarity. We investigate how the topological features
described above evolve in response to motion of the parasitic polarity
region. The implications of our results for the sources of the slow
wind and for coronal and heliospheric observations are discussed.
---------------------------------------------------------
Title: A numerical simulation for the origins of solar magnetic
structure
Authors: Zhao, Liang; Antiochos, Spiro K.; DeVore, C. Richard
2010shin.confE.117Z Altcode:
We investigate numerically a new model for the origin of the solar
coronal magnetic field structure observed in filament channels and in
the complex structures of the slow wind. Using the adaptively Refined
Magnetohydrodynamic Solver (ARMS), we perform a series of numerical
experiments to study the evolution of magnetic helicity injected into
the solar corona by photospheric motions. Our simulation domain consists
of a Cartesian box with an initially uniform vertical magnetic field
and a low-beta plasma with uniform pressure and density. This system is
driven by imposing a flow pattern at the top and bottom boundary planes
corresponding to the twisting motions expected from the quasi-random
photospheric motions. We consider a variety of flow patterns made up
of twist arranged in regular geometric orders, (i.e. four twists in a
quadrilateral arrangement, 7 twists in a hexagonal), that generate a
set of twisted flux tubes in the interior of the simulation box, the
corona. Note that this driving twist injects both energy and helicity
into the coronal field. Depending on the sense of the applied twist,
we can inject either positive or negative helicity. If helicity of the
same sign is injected into each of the flux tubes (co-helicity case),
we expect that the twist magnetic-field component of neighboring flux
tubes will be oppositely directed and, therefore, will reconnect,
but if opposite sign helicity is injected into neighboring flux
tubes, reconnection will not occur. We tested this conjecture with
our simulations and found that for the co-helicity case reconnection
did occur and led to a state in which the twist propagated to the
largest scale. Essentially the tubes merged into one large twisted
flux tube. For the opposite helicity case, on the other hand, we
found that reconnection did not occur, and the tubes remained as
distinct structures. We discuss the implications of our results for
the evolution of coronal helicity and for the formation of magnetic
structures in the Sun.
---------------------------------------------------------
Title: Formation and Reconnection of Three-Dimensional Current Sheets
in the Solar Corona
Authors: Edmondson, Justin K.; Antiochos, S. K.; DeVore, C. R.;
Zurbuchen, T. H.
2010shin.confE.111E Altcode:
Current-sheet formation and magnetic reconnection are believed to
be the basic physical processes responsible for much of the activity
observed in astrophysical plasmas, such as interchange reconnection at
the boundaries between coronal holes and helmet streamers in the Sun's
corona. We investigate these processes for a magnetic configuration
consisting of a uniform background field and an embedded line dipole,
a topology that is expected to be ubiquitous in the corona. This
magnetic system is driven by a uniform horizontal flow applied
at the line-tied photosphere. Although both the initial field and
the driver are translationally symmetric, the resulting evolution
is calculated using a fully three-dimensional magnetohydrodynamic
(3D MHD) simulation with adaptive mesh refinement that resolves the
current sheet and reconnection dynamics in detail. The advantage of
our approach is that it allows us to apply directly the vast body
of knowledge gained from the many studies of 2D reconnection to the
fully 3D case. We find that a current sheet forms in close analogy to
the classic Syrovatskii 2D mechanism, but the resulting evolution is
different than expected. The current sheet is globally stable, showing
no evidence for a disruption or a secondary instability even for aspect
ratios as high as 80:1. The global evolution generally follows the
standard Sweet-Parker 2D reconnection model except for an accelerated
reconnection rate at a very thin current sheet, due to the tearing
instability and the formation of magnetic islands. An interesting
conclusion is that despite the formation of fully 3D structures at small
scales, the system remains close to 2D at global scales. We discuss
the implications of our results for observations of the solar corona.
---------------------------------------------------------
Title: Symmetric Coronal Jets: A Reconnection-controlled Study
Authors: Rachmeler, L. A.; Pariat, E.; DeForest, C. E.; Antiochos,
S.; Török, T.
2010ApJ...715.1556R Altcode:
Current models and observations imply that reconnection is a key
mechanism for destabilization and initiation of coronal jets. We evolve
a system described by the theoretical symmetric jet formation model
using two different numerical codes with the goal of studying the
role of reconnection in this system. One of the codes is the Eulerian
adaptive mesh code ARMS, which simulates magnetic reconnection through
numerical diffusion. The quasi-Lagrangian FLUX code, on the other hand,
is ideal and able to evolve the system without reconnection. The ideal
nature of FLUX allows us to provide a control case of evolution without
reconnection. We find that during the initial symmetric and ideal phase
of evolution, both codes produce very similar morphologies and energy
growth. The symmetry is then broken by a kink-like motion of the axis
of rotation, after which the two systems diverge. In ARMS, current
sheets formed and reconnection rapidly released the stored magnetic
energy. In FLUX, the closed field remained approximately constant
in height while expanding in width and did not release any magnetic
energy. We find that the symmetry threshold is an ideal property of the
system, but the lack of energy release implies that the observed kink is
not an instability. Because of the confined nature of the FLUX system,
we conclude that reconnection is indeed necessary for jet formation
in symmetric jet models in a uniform coronal background field.
---------------------------------------------------------
Title: Interpreting Small-Scale Structure from High Resolution Global
MHD Simulations
Authors: Mikic, Zoran; Titov, V. S.; Linker, J. A.; Lionello, R.;
Riley, P.; Antiochos, S.
2010AAS...21640503M Altcode: 2010BAAS...41..889M
High resolution 3D MHD simulations of the solar corona are beginning
to reveal how small-scale structures in the magnetic field interact
with the global structure of the corona and solar wind. In particular,
it has become evident that the detailed characteristics of coronal
holes, especially their equatorial extensions, may be related to the
source of the slow solar wind. Using structural analysis based on the
squashing factor Q (Titov et al. 2002, 2008; Titov 2007) we show how
small-scale structure in the magnetic field is related to the structure
of the streamer belt. These results have led to a new interpretation
of the source of the slow solar wind. <P />Research supported by NASA's
Heliospheric Theory and Living With a Star Programs, and NSF/CISM.
---------------------------------------------------------
Title: The Existence and Origin of Turbulence in Solar Active Regions
Authors: Klimchuk, James A.; Nigro, G.; Dahlburg, R. B.; Antiochos,
S. K.
2010AAS...21630205K Altcode:
It has been suggested that turbulence plays a fundamental role in the
heating of solar active regions, with its intermittent behavior being
the explanation of impulsive energy release (nanoflares). We know that
episodes of turbulence are produced in the final nonlinear stage of
the secondary instability of electric current sheets. However, these
current sheets must exist prior to the turbulence. Whether turbulence
can dynamically produce current sheets that would not otherwise be
present is a different and important question. <P />Turbulence occurs
freely in the solar wind and in other situations where the magnetic
field does not dominate. However, the magnetic field strongly resists
being distorted in line-tied, low-beta environments such as active
regions. Can turbulence develop naturally in these environments
without being driven by an instability? To answer this question,
we have performed a time-dependent MHD simulation of a slowly driven
system that does not contain current sheets and is stable to applied
perturbations. We find no evidence for bursty energy release, steep
spatial gradients, or power-law energy spectra that are the typical
signatures of turbulence. We conclude that the turbulence which occurs
in active regions is an important yet secondary process and not the
primary cause of heating.
---------------------------------------------------------
Title: Formation and Reconnection of Three-Dimensional Current Sheets
in the Solar Corona
Authors: Edmondson, Justin K.; Antiochos, S. K.; DeVore, C.; Velli,
M.; Zurbuchen, T. H.
2010AAS...21640701E Altcode: 2010BAAS...41..859E
Current-sheet formation and magnetic reconnection are believed to
be the basic physical processes responsible for much of the activity
observed in astrophysical plasmas, such as interchange reconnection at
the boundaries between coronal holes and helmet streamers in the Sun's
corona. We investigate these processes for a magnetic configuration
consisting of a uniform background field and an embedded line dipole,
a topology that is expected to be ubiquitous in the corona. This
magnetic system is driven by a uniform horizontal flow applied
at the line-tied photosphere. Although both the initial field and
the driver are translationally symmetric, the resulting evolution
is calculated using a fully three-dimensional magnetohydrodynamic
(3D MHD) simulation with adaptive mesh refinement that resolves the
current sheet and reconnection dynamics in detail. The advantage of
our approach is that it allows us to apply directly the vast body
of knowledge gained from the many studies of 2D reconnection to the
fully 3D case. We find that a current sheet forms in close analogy to
the classic Syrovatskii 2D mechanism, but the resulting evolution is
different than expected. The current sheet is globally stable, showing
no evidence for a disruption or a secondary instability even for aspect
ratios as high as 80:1. The global evolution generally follows the
standard Sweet-Parker 2D reconnection model except for an accelerated
reconnection rate at a very thin current sheet, due to the tearing
instability and the formation of magnetic islands. An interesting
conclusion is that despite the formation of fully 3D structures at small
scales, the system remains close to 2D at global scales. We discuss
the implications of our results for observations of the solar corona.
---------------------------------------------------------
Title: Sympathetic Coronal Mass Ejections Originating in Complex
Source Regions
Authors: DeVore, C. Richard; Antiochos, S. K.
2010AAS...21640613D Altcode: 2010BAAS...41..882D
We are investigating numerically the initiation of coronal mass
ejections (CMEs) in scenarios in which two bipolar active regions,
side by side, spawn solar eruptions. If the overall topology is
more complex than bipolar, with a magnetic null and separatrices
dividing the coronal field into two or more flux systems, then the
configuration is susceptible to magnetic-breakout eruptions. Previously,
we found that stressing the polarity inversion line (PIL) separating
the two active regions eventually initiates an eruption that breaks
open the central arcade of the combined structure, while stressing
the interior PIL of either active region initiates an eruption in
a side arcade of the configuration. Stressing both interior PILs
of the active regions together raises the possibility of initiating
paired sympathetic eruptions, either simultaneous or delayed, in the
remote side arcades. We will report our progress on simulating and
understanding these more complex scenarios for CME initiation and
their observational implications. Our research is supported by NASA.
---------------------------------------------------------
Title: Three-dimensional Modeling of Quasi-homologous Solar Jets
Authors: Pariat, E.; Antiochos, S. K.; DeVore, C. R.
2010ApJ...714.1762P Altcode:
Recent solar observations (e.g., obtained with Hinode and STEREO)
have revealed that coronal jets are a more frequent phenomenon than
previously believed. This higher frequency results, in part, from the
fact that jets exhibit a homologous behavior: successive jets recur at
the same location with similar morphological features. We present the
results of three-dimensional (3D) numerical simulations of our model
for coronal jets. This study demonstrates the ability of the model to
generate recurrent 3D untwisting quasi-homologous jets when a stress is
constantly applied at the photospheric boundary. The homology results
from the property of the 3D null-point system to relax to a state
topologically similar to its initial configuration. In addition, we find
two distinct regimes of reconnection in the simulations: an impulsive
3D mode involving a helical rotating current sheet that generates the
jet and a quasi-steady mode that occurs in a 2D-like current sheet
located along the fan between the sheared spines. We argue that these
different regimes can explain the observed link between jets and plumes.
---------------------------------------------------------
Title: A Model for the Sources of the Slow Solar Wind
Authors: Antiochos, Spiro K.; Mikic, Z.; Lionello, R.; Titov, V.;
Linker, J.
2010AAS...21640521A Altcode: 2010BAAS...41..892A
Models for the origin of the slow solar wind must account for
two seemingly contradictory observations: The slow wind has the
composition of the closed-field corona, implying that it originates at
the open-closed field boundary layer, but it also has large angular
width, up to 40 degrees. We propose a model that can explain both
observations. The key idea is that the source of the slow wind at the
Sun is a network of narrow (possibly singular) open-field corridors
that map to a web of separatrices and quasi-separatrix layers in
the heliosphere. We calculate with high numerical resolution, the
quasi-steady solar wind and magnetic field for a Carrington rotation
centered about the August 1, 2008 total solar eclipse. Our numerical
results demonstrate that, at least for this time period, a web of
separatrices (S-web) forms with sufficient density and extent in
the heliosphere to account for the observed properties of the slow
wind. We discuss the implications of our S-web model for the structure
and dynamics of the corona and heliosphere, and propose further tests
of the model. <P />This work was supported, in part, by the NASA HTP,
TR&T and SR&T programs.
---------------------------------------------------------
Title: Reconnection Onset in the Breakout Model for CME Initiation
Authors: Karpen, Judith T.; DeVore, C. R.; Antiochos, S. K.
2010AAS...21640605K Altcode: 2010BAAS...41..880K
Fast coronal mass ejections (CMEs) are the most massive explosions
in the heliosphere, and the primary drivers of geoeffective
space weather. Although it is generally agreed that magnetic
reconnection is the key to fast CME initiation, different models
incorporate reconnection in different ways. One promising model ---
the breakout scenario --- involves reconnection in two distinct yet
interconnected locations: breakout reconnection ahead of the CME, and
flare reconnection behind it. We will discuss what we have learned
about the early evolution of breakout and flare reconnection from
recent high-resolution 2.5D adaptively refined MHD simulations of
CME initiation, including the evolving properties of the breakout and
flare current sheets, the conditions that trigger reconnection onset
in each sheet, the ensuing positive feedback between breakout and flare
reconnections, and implications for electron acceleration in flares.
---------------------------------------------------------
Title: Tests of Dynamical Flux Emergence as a Mechanism for CME
Initiation
Authors: Leake, James E.; Linton, M.; Antiochos, S.
2010AAS...21631402L Altcode: 2010BAAS...41..893L
We present an investigation of whether the emergence of sheared flux
into the corona can initiate coronal mass ejections (CMEs) in two
dimensional geometries. Observations indicate that both flux emergence
and sheared magnetic fields are correlated with CME eruptions. <P
/>Numerical models of CME eruptions can recreate certain observational
features of CMEs using simple two dimensional geometries. These
models simply assume the appearance of flux in the corona and do not
self-consistently calculate a process for the flux emergence. These
simulations do not, therefore, address the significant question of
whether and how newly emerged, sheared magnetic flux can rise from
its origins in the high beta convection zone to the low corona where
it is required to drive CME models. <P />We study this problem by
simulating the dynamical emergence of twisted magnetic flux tubes from
a high beta convection zone into the low beta corona in CME eruptive
configurations. We focus here on the breakout CME model, which injects
sheared flux at a low beta lower boundary. The presence of this shear
energy can drive eruptions in quadrupolar coronal configurations. We
simulate, in 2D, the emergence of flux tubes with a range of twist and
shear profiles into field-free, dipolar and quadrupolar coronas. In all
cases we find that the dense plasma which is entrained in the emerging
flux ropes and the inability of shear energy to reach sufficient heights
in the corona prevent eruptions. CME models becomes significantly
more difficult when the self-consistent dynamics of flux emergence are
taken into account. These results present strong difficulties for all
CME models. We suggest mechanisms for the removal of dense matter from
emerging flux tubes which will allow the eruption of coronal magnetic
field structures.
---------------------------------------------------------
Title: Interchange Reconnection and Coronal Hole Dynamics
Authors: Edmondson, J. K.; Antiochos, S. K.; DeVore, C. R.; Lynch,
B. J.; Zurbuchen, T. H.
2010ApJ...714..517E Altcode:
We investigate the effect of magnetic reconnection between open and
closed fields, often referred to as "interchange" reconnection, on the
dynamics and topology of coronal hole boundaries. The most important and
most prevalent three-dimensional topology of the interchange process
is that of a small-scale bipolar magnetic field interacting with a
large-scale background field. We determine the evolution of such a
magnetic topology by numerical solution of the fully three-dimensional
MHD equations in spherical coordinates. First, we calculate the
evolution of a small-scale bipole that initially is completely inside
an open field region and then is driven across a coronal hole boundary
by photospheric motions. Next the reverse situation is calculated in
which the bipole is initially inside the closed region and driven
toward the coronal hole boundary. In both cases, we find that the
stress imparted by the photospheric motions results in deformation
of the separatrix surface between the closed field of the bipole and
the background field, leading to rapid current sheet formation and to
efficient reconnection. When the bipole is inside the open field region,
the reconnection is of the interchange type in that it exchanges open
and closed fields. We examine, in detail, the topology of the field as
the bipole moves across the coronal hole boundary and find that the
field remains well connected throughout this process. Our results,
therefore, provide essential support for the quasi-steady models of
the open field, because in these models the open and closed flux are
assumed to remain topologically distinct as the photosphere evolves. Our
results also support the uniqueness hypothesis for open field regions
as postulated by Antiochos et al. On the other hand, the results argue
against models in which open flux is assumed to diffusively penetrate
deeply inside the closed field region under a helmet streamer. We
discuss the implications of this work for coronal observations.
---------------------------------------------------------
Title: Can Thermal Nonequilibrium Explain Coronal Loops?
Authors: Klimchuk, James A.; Karpen, Judy T.; Antiochos, Spiro K.
2010ApJ...714.1239K Altcode: 2009arXiv0912.0953K
Any successful model of coronal loops must explain a number of observed
properties. For warm (~1 MK) loops, these include (1) excess density,
(2) flat temperature profile, (3) super-hydrostatic scale height,
(4) unstructured intensity profile, and (5) 1000-5000 s lifetime. We
examine whether thermal nonequilibrium can reproduce the observations
by performing hydrodynamic simulations based on steady coronal heating
that decreases exponentially with height. We consider both monolithic
and multi-stranded loops. The simulations successfully reproduce
certain aspects of the observations, including the excess density,
but each of them fails in at least one critical way. Monolithic models
have far too much intensity structure, while multi-strand models
are either too structured or too long-lived. Our results appear to
rule out the widespread existence of heating that is both highly
concentrated low in the corona and steady or quasi-steady (slowly
varying or impulsive with a rapid cadence). Active regions would have a
very different appearance if the dominant heating mechanism had these
properties. Thermal nonequilibrium may nonetheless play an important
role in prominences and catastrophic cooling events (e.g., coronal rain)
that occupy a small fraction of the coronal volume. However, apparent
inconsistencies between the models and observations of cooling events
have yet to be understood.
---------------------------------------------------------
Title: A Numerical Investigation of Unsheared Flux Cancelation
Authors: Karpen, J. T.; Antiochos, S. K.; DeVore, C. R.; Linton, M. G.
2010ASSP...19..518K Altcode: 2010mcia.conf..518K
Cancelation of magnetic flux in the solar photosphere and chromosphere
has been linked observationally and theoretically to a broad range
of solar activity phenomena, from filament channel formation to CME
initiation. Because cancelation is typically measured at only a single
layer in the atmosphere and only in the radial (line of sight) component
of the magnetic field, the actual processes behind its observational
signature are not fully understood. We have used our 3D MHD code with
adaptive mesh refinement, ARMS, to investigate numerically the physics
of flux cancelation, beginning with the simplest possible configuration:
a subphotospheric Lundquist flux tube surrounded by a potential field
in a gravitationally stratified atmosphere. Cancelation is driven by a
two-cell circulation pattern imposed in the convection zone, in which
the flows converge and form a downdraft at the polarity inversion line
(PIL). We present and compare the results of 2D and 3D simulations of
cancelation of initially unsheared flux - to our knowledge, these are
the first such calculations in which the computational domain extends
below the photosphere. The 2D simulation produces a flattened flux rope
(plasmoid) whose axis remains centered along the PIL about 1650km above
the photosphere, without rising higher into the corona by the end of
the run (10,000 s). Our calculations also show that 3D cancelation in an
arcade geometry does not produce a fully disconnected flux tube in the
corona, in contrast to the 2D results. Rather, most of the reconnected
field stays rooted in the photosphere and is gradually submerged by
the downdrafts at the PIL. An interchange-like instability develops
above the region where the converging flows are driven, breaking the
horizontal symmetry along the PIL. This generates an alternating
pattern of magnetic shear (magnetic field component aligned with
the PIL), which ultimately produces systematic footpoint shuffling
through reconnection across the folds of the convoluted PIL. These
simulations demonstrate the importance of considering the effects of
submergence, as well as the full 3D configuration of the magnetic
field and atmosphere, in determining the physical processes behind
flux cancelation on the Sun. A paper describing this work has been
submitted to the Astrophysical Journal (January 2009).
---------------------------------------------------------
Title: Reconnection-Driven Dynamics of Coronal-Hole Boundaries
Authors: Edmondson, J. K.; Lynch, B. J.; Antiochos, S. K.; De Vore,
C. R.; Zurbuchen, T. H.
2009ApJ...707.1427E Altcode:
We investigate the effect of magnetic reconnection on the boundary
between open and closed magnetic field in the solar corona. The magnetic
topology for our numerical study consists of a global dipole that
gives rise to polar coronal holes and an equatorial streamer belt,
and a smaller active-region bipole embedded inside the closed-field
streamer belt. The initially potential magnetic field is energized
by a rotational motion at the photosphere that slowly twists the
embedded-bipole flux. Due to the applied stress, the bipole field
expands outward and reconnects with the surrounding closed flux,
eventually tunneling through the streamer boundary and encountering the
open flux of the coronal hole. The resulting interchange reconnection
between closed and open field releases the magnetic twist and free
energy trapped inside the bipole onto open field lines, where they
freely escape into the heliosphere along with the entrained closed-field
plasma. Thereafter, the bipole field relaxes and reconnects back down
into the interior of the streamer belt. Our simulation shows that the
detailed properties of magnetic reconnection can be crucial to the
coronal magnetic topology, which implies that both potential-field
source-surface and quasi-steady magnetohydrodynamic models may often
be an inadequate description of the corona and solar wind. We discuss
the implications of our results for understanding the dynamics of the
boundary between open and closed field on the Sun and the origins of
the slow wind.
---------------------------------------------------------
Title: The Existence and Origin of Turbulence in Solar Active Regions
Authors: Klimchuk, J. A.; Nigro, G.; Dahlburg, R. B.; Antiochos, S. K.
2009AGUFMSM42B..03K Altcode:
It has been suggested that turbulence plays a fundamental role in the
heating of solar active regions, with its intermittent behavior being
the explanation of impulsive energy release (nanoflares). We know that
episodes of turbulence are produced in the final nonlinear stage of
the secondary instability of electric current sheets. However, these
current sheets must exist prior to the turbulence. Whether turbulence
can dynamically produce current sheets that would not otherwise be
present is a different and important question. Turbulence occurs
freely in the solar wind and in other situations where the magnetic
field does not dominate. However, the magnetic field strongly resists
being distorted in line-tied, low-beta environments such as active
regions. Can turbulence develop naturally in these environments
without being driven by an instability? To answer this question,
we have performed a time-dependent MHD simulation of a slowly driven
system that does not contain current sheets and is stable to applied
perturbations. We find no evidence for bursty energy release, steep
spatial gradients, or power-law power spectra that are the typical
signatures of turbulence. We conclude that the turbulence which occurs
in active regions is an important yet secondary process and not the
primary cause of heating.
---------------------------------------------------------
Title: Investigating the Topology of the “Disconnection” of
Coronal Holes
Authors: Titov, V. S.; Mikic, Z.; Linker, J. A.; Antiochos, S. K.;
Lionello, R.
2009AGUFMSH41B1665T Altcode:
Using a potential-field-source-surface approximation, we construct
an exact analytical model to describe the intrusion of a magnetic
flux spot from the closed-field region into the polar coronal hole
(CH). The spot, which has an opposite polarity compared to the
surrounding field, moves across a local bulge in the CH, eventually
detaching it into a separate minor CH. We show that the formation
of a magnetic minimum point, its subsequent degeneration into a null
point, and its bifurcation into a pair of nulls, plays a key role in
this process. The separatrix field lines that emanate from the nulls
form an interface between the open and closed field structures. This
implies that the corresponding MHD evolution must involve magnetic
reconnection to accommodate the redistribution of their magnetic
fluxes. We anticipate that the reconnection outflows along the open
part of the separatrix field lines may serve as a source of slow solar
wind. Work supported by NASA and the Center for Integrated Space Weather
Modeling (an NSF Science and Technology Center). Topological skeleton
of the magnetic field in the neighborhood of a detached minor coronal
hole; the skeleton consists of separatrix field lines emanating from
two magnetic null points. The gray-shaded photospheric distribution of
the squashing factor depicts the corresponding footprints of separatrix
surfaces and quasi-separatrix layers.
---------------------------------------------------------
Title: Implications of the Deep Minimum for Slow Solar Wind Origin
Authors: Antiochos, S. K.; Mikic, Z.; Lionello, R.; Titov, V. S.;
Linker, J. A.
2009AGUFMSH11A1502A Altcode:
The origin of the slow solar wind has long been one of the most
important problems in solar/heliospheric physics. Two observational
constraints make this problem especially challenging. First, the slow
wind has the composition of the closed-field corona, unlike the fast
wind that originates on open field lines. Second, the slow wind has
substantial angular extent, of order 30 degrees, which is much larger
than the widths observed for streamer stalks or the widths expected
theoretically for a dynamic heliospheric current sheet. We propose
that the slow wind originates from an intricate network of narrow
(possibly singular) open-field corridors that emanate from the polar
coronal hole regions. Using topological arguments, we show that these
corridors must be ubiquitous in the solar corona. The total solar
eclipse in August 2008, near the lowest point of the Deep Minimum,
affords an ideal opportunity to test this theory by using the ultra-high
resolution Predictive Science's (PSI) eclipse model for the corona and
wind. Analysis of the PSI eclipse model demonstrates that the extent
and scales of the open-field corridors can account for both the angular
width of the slow wind and its closed-field composition. We discuss the
implications of our slow wind theory for the structure of the corona
and heliosphere at the Deep Minimum and describe further observational
and theoretical tests. This work has been supported by the NASA HTP,
SR&T, and LWS programs.
---------------------------------------------------------
Title: Fluxon modeling of breakout CMEs
Authors: Rachmeler, L. A.; Deforest, C. E.; DeVore, C. R.; Antiochos,
S. K.
2009AGUFMSH41B1675R Altcode:
The pivotal element of the classic breakout model of CME initiation
is reconnection that occurs above inner magnetic field sheared by
rotation. We research this model with the FLUX code both with and
without reconnection. Without reconnection an eruption occurs after
several turns have been injected into the active region. The resultant
expansion or eruption is more like a kink-unstable flux rope than a
classic breakout CME. By varying whether and where reconnection is
allowed, we determine the location of magnetic free energy release in
the breakout model.
---------------------------------------------------------
Title: Impulsive Reconnection in the Sun's Atmosphere
Authors: Antiochos, Spiro
2009APS..DPPJM9001A Altcode:
Recent high-resolution observations from the Hinode mission show
dramatically that the Sun's atmosphere is filled with explosive activity
ranging from chromospheric explosions that reach heights of Mm, to
coronal jets that can extend to solar radii, to giant coronal mass
ejections (CME) that reach the edge of the heliosphere. The driver for
all this activity is believed to be 3D magnetic reconnection. From the
large variation observed in the temporal behavior of solar activity,
it is clear that reconnection in the corona must take on a variety of
distinct forms. The explosive nature of jets and CMEs requires that the
reconnection be impulsive in that it stays off until a substantial store
of free energy has been accumulated, but then turns on abruptly and
stays on until much of this free energy is released. The key question,
therefore, is what determines whether the reconnection is impulsive
or not. We present some of the latest observations and numerical
models of explosive and non-explosive solar activity. We argue that,
in order for the reconnection to be impulsive, it must be driven by
a quasi-ideal instability. We discuss the generality of our results
for understanding 3D reconnection in other contexts.
---------------------------------------------------------
Title: Observational Signatures of Reconnection-Driven Changes in
the Open-Closed Magnetic Field Boundary
Authors: Lynch, Benjamin J.; Edmondson, J. K.; Li, Y.; Luhmann, J. G.;
Antiochos, S. K.; DeVore, C. R.
2009shin.confE.134L Altcode:
We will present some recent MHD modeling results of small scale
transient phenomena in the solar corona. We start with a multipolar
magnetic topology which is the simplest non-trivial field distribution
in 3D, resulting from essentially a pair of magnetic dipole sources,
and has sufficient complexity to give rise to a wide variety of
transient phenomena for different specific boundary flows and flux
distributions. We slowly energize the fields associated with a bipole
near the boundary of the coronal helmet streamer belt and observe the
interaction between the flux systems and the open and closed field
boundary. During the resulting interchange reconnection magnetic
fieldline connectivity can change drastically with very little
associated energy release. We present these results in the context
of the Antiochos et al (2007) conjecture of narrow channels of open
fields, and show that in our MHD simulations, opening some amount of
field associated with the active region flux system necessarily opens
such a channel defined by the separatrix boundary. Additionally, we
examine some of consequences and potentially observable signatures of
these small scale transient openings and compare them to well known
coronal signatures such as
---------------------------------------------------------
Title: Current Sheet Formation, Stability, and Reconnection Dynamics
in MHD
Authors: Edmondson, Justin K.; Antiochos, Spiro K.; DeVore, C. Richard;
Zurbuchen, Thomas H.
2009shin.confE.188E Altcode:
Current sheet formation is a necessary consequence of the evolution of
the multi-polar magnetic field topologies that are ubiquitous throughout
the solar corona. We present a very high-resolution study of 3D MHD
current sheet formation and the resulting reconnection dynamics in an
environment appropriate for the corona. The initial field consists
of a translationally invariant, potential field with a null-point
topology (i.e., 4-flux systems) and a low-beta plasma. A finite-extent,
3D Syrovatskii-type current sheet forms as a result of stressing of
this system by a uniform, incompressible flow applied at the line-tied
photospheric boundary. The system is assumed to be ideal, except for the
presence of numerical resistivity. The fully 3-D evolution is calculated
with very high resolution (9x refinement across the full extent of
the current sheet) using the Adaptively Refined MHD Solver (ARMS). The
initial evolution of this computationally-intensive simulation results
in a current sheet with a nearly 40-to-1 aspect ratio, a significant
fraction of the system characteristic length, that unexpectedly
appears to be stable. In addition, up to this point in the evolution
any magnetic reconnection that we observe is of the slow Sweet-Parker
type. We expect, however, that as we continue stressing the field,
the current sheet will become unstable and develop explosive dynamics.
---------------------------------------------------------
Title: Do Closed Field Regions Contribute Plasma to the Slow Solar
Wind?
Authors: Linker, Jon A.; Lionello, Roberto; Mikic, Zoran; Titov,
Viacheslav S.; Antiochos, Spiro
2009shin.confE.140L Altcode:
Composition differences between the fast and slow solar wind suggest
that the slow solar wind plasma has a different origin than the fast
wind. A natural way that a bifurcation in the plasma properties could
arise is if the slow wind plasma originates from previously closed field
regions in the corona. I this talk I will discuss arguments both for
and against this idea, and I will illustrate mechanisms by which the
streamer belt can be opened as part of the slow evolution of the corona.
---------------------------------------------------------
Title: Small Bipoles Interacting with a Coronal Hole: MHD Simulations
Authors: Lionello, Roberto; Linker, Jon A.; Mikic, Zoran; Titov,
Viacheslav S.; Antiochos, Spiro
2009shin.confE.128L Altcode:
Coronal holes are known to be the source of the fast wind and are
also believed to play a key role in the formation of the slow wind;
consequently, their evolution is critical for understanding how
the heliospheric magnetic field connects to the Sun. In the context
of field reversal, the Fisk model postulates that open flux can be
transported out of coronal holes into the closed field region through
interchange reconnection with small loops associated with parasitic
polarities. This scenario is supported by in-situ observations,
which seem to favor interchange reconnection as the only mechanism
responsible for field reversal. However, it is hard to reconcile with
theoretical results on the topology of coronal holes. To determine the
feasibility of this mechanism, we have used our 3D MHD algorithm in
spherical coordinates to study the interaction of the magnetic field of
two bipoles with a coronal hole. The model uses a polytropic treatment
for the energy equation and includes a self-consistent solar wind. We
have prescribed as magnetic flux distribution at the lower boundary,
a smoothed Kitt Peak magnetogram for Carrington Rotation 1913 (late
August 1996), to which we have added two small bipoles. After reaching
a relaxed state with well-defined coronal holes and a close field
region inside a helmet streamer, we have introduced surface flows,
which evolve the magnetic flux distribution at the boundary. We have
investigated the reconfiguration of the coronal fields in response to
these motions; in particular we show what happens to the open flux in
the system as the bipoles move from the coronal holes into the closed
field region. We have found no evidence that open flux can be injected
into closed-field regions. Portions of coronal holes that may appear to
have been detached are actually still connected to the main coronal hole
through zero-width corridors. We conclude that interchange reconnection,
by itself, does not produce the open-closed field mixture postulated by
the Fisk model. On the other hand, the magnetic topology of the coronal
hole boundary becomes so complex that some of the essential features
of the model, in particular the open field diffusion, may prove to be
an effective approximation for capturing the magnetic dynamics.
---------------------------------------------------------
Title: Rotation of Coronal Mass Ejections during Eruption
Authors: Lynch, B. J.; Antiochos, S. K.; Li, Y.; Luhmann, J. G.;
DeVore, C. R.
2009ApJ...697.1918L Altcode:
Understanding the connection between coronal mass ejections (CMEs)
and their interplanetary counterparts (ICMEs) is one of the most
important problems in solar-terrestrial physics. We calculate
the rotation of erupting field structures predicted by numerical
simulations of CME initiation via the magnetic breakout model. In
this model, the initial potential magnetic field has a multipolar
topology and the system is driven by imposing a shear flow at the
photospheric boundary. Our results yield insight on how to connect
solar observations of the orientation of the filament or polarity
inversion line (PIL) in the CME source region, the orientation of the
CME axis as inferred from coronagraph images, and the ICME flux rope
orientation obtained from in situ measurements. We present the results
of two numerical simulations that differ only in the direction of the
applied shearing motions (i.e., the handedness of the sheared-arcade
systems and their resulting CME fields). In both simulations, eruptive
flare reconnection occurs underneath the rapidly expanding sheared
fields transforming the ejecta fields into three-dimensional flux
rope structures. As the erupting flux ropes propagate through the low
corona (from 2 to 4 R <SUB>sun</SUB>) the right-handed breakout flux
rope rotates clockwise and the left-handed breakout flux rope rotates
counterclockwise, in agreement with recent observations of the rotation
of erupting filaments. We find that by 3.5 R <SUB>sun</SUB> the average
rotation angle between the flux rope axes and the active region PIL is
approximately 50°. We discuss the implications of these results for
predicting, from the observed chirality of the pre-eruption filament
and/or other properties of the CME source region, the direction and
amount of rotation that magnetic flux rope structures will experience
during eruption. We also discuss the implications of our results for
CME initiation models.
---------------------------------------------------------
Title: Current Sheet Formation and Reconnection Dynamics in the
Solar Corona
Authors: Edmondson, Justin K.; Antiochos, S. K.; DeVore, C.; Zurbuchen,
T. H.
2009SPD....40.1305E Altcode:
Current sheet formation is a necessary consequence of the evolution
of the multi-polar magnetic field topologies that are ubiquitous
throughout the solar corona. We present a very high-resolution study
of 3D MHD current sheet formation and the resulting reconnection
dynamics in an environment appropriate for the corona. The initial
field consists of a translationally invariant, potential field with
a null-point topology (i.e., 4-flux systems) and a low-beta plasma. A
finite-extent, 3D Syrovatskii-type current sheet forms as a result of
stressing of this system by a uniform, incompressible flow applied
at the line-tied photospheric boundary. The system is assumed to be
ideal, except for the presence of numerical resistivity. The fully
3-D evolution is calculated with very high resolution (9x and 10x
refinement across the full extent of the current sheet) using the
Adaptively Refined MHD Solver (ARMS). The initial evolution of this
computationally-intensive simulation results in a current sheet with
a nearly 30-to-1 aspect ratio, a significant fraction of the system
characteristic length, that unexpectedly appears to be stable. In
addition, up to this point in the evolution any magnetic reconnection
that we observe is of the slow Sweet-Parker type. We expect, however,
that as we continue stressing the field, the current sheet will become
unstable and develop explosive dynamics. We discuss the implications
of our results on coronal structure and activity, such as heating and
eruptions. <P />This work has been supported, in part, by the NASA
HTP and SR&T programs.
---------------------------------------------------------
Title: Generation of Homologous Coronal Jets
Authors: Pariat, Etienne; Antiochos, S. K.; DeVore, C. R.
2009SPD....40.3201P Altcode:
Recent solar observations (e.g. Hinode & STEREO) have revealed
that coronal jets are a more frequent phenomenon than previously
believed. This higher frequency results, in part, from the fact that
jets exhibit a homologous behavior; successive jets re-occur at the
same location. <P />We present the results of 3D numerical simulations
of our model for coronal jets. The simulations were performed with our
state-of-art adaptive mesh MHD solver ARMS. The basic idea of the model
is that a jet is due to the release of twist as a closed field region
undergoes interchange reconnection with surrounding open field. If a
stress is constantly applied at the photospheric boundary we demonstrate
that our model of jets is able to reproduce the observed homologous
property. In addition, we find that two regimes of reconnection can
occur in the simulations. This result has important implications for
the observed link between jets and plumes. <P />This work was supported
by the NASA Theory and SR&T Programs.
---------------------------------------------------------
Title: Current Sheet Formation and Reconnection Dynamics in the
Solar Corona
Authors: Edmondson, Justin K.; Antiochos, S. K.; DeVore, C.; Zurbuchen,
T. H.
2009SPD....40.1401E Altcode:
Current sheet formation is a necessary consequence of the evolution
of the multi-polar magnetic field topologies that are ubiquitous
throughout the solar corona. We present a very high-resolution study
of 3D MHD current sheet formation and the resulting reconnection
dynamics in an environment appropriate for the corona. The initial
field consists of a translationally invariant, potential field with
a null-point topology (i.e., 4-flux systems) and a low-beta plasma. A
finite-extent, 3D Syrovatskii-type current sheet forms as a result of
stressing of this system by a uniform, incompressible flow applied
at the line-tied photospheric boundary. The system is assumed to be
ideal, except for the presence of numerical resistivity. The fully
3-D evolution is calculated with very high resolution (9x and 10x
refinement across the full extent of the current sheet) using the
Adaptively Refined MHD Solver (ARMS). The initial evolution of this
computationally-intensive simulation results in a current sheet with
a nearly 30-to-1 aspect ratio, a significant fraction of the system
characteristic length, that unexpectedly appears to be stable. In
addition, up to this point in the evolution any magnetic reconnection
that we observe is of the slow Sweet-Parker type. We expect, however,
that as we continue stressing the field, the current sheet will become
unstable and develop explosive dynamics. We discuss the implications
of our results on coronal structure and activity, such as heating and
eruptions. <P />This work has been supported, in part, by the NASA
HTP and SR&T programs.
---------------------------------------------------------
Title: Simulations of Flare Reconnection in Breakout Coronal Mass
Ejections
Authors: DeVore, C. Richard; Karpen, J. T.; Antiochos, S. K.
2009SPD....40.2007D Altcode:
We report 3D MHD simulations of the flare reconnection in the
corona below breakout coronal mass ejections (CMEs). The initial
setup is a single bipolar active region imbedded in the global-scale
background dipolar field of the Sun, forming a quadrupolar magnetic
configuration with a coronal null point. Rotational motions applied to
the active-region polarities at the base of the atmosphere introduce
shear across the polarity inversion line (PIL). Eventually, the
magnetic stress and energy reach the critical threshold for runaway
breakout reconnection, at which point the sheared core field erupts
outward at high speed. The vertical current sheet formed by the
stretching of the departing sheared field suffers reconnection that
reforms the initial low-lying arcade across the PIL, i.e., creates the
flare loops. Our simulation model, the Adaptively Refined MHD Solver,
exploits local grid refinement to resolve the detailed structure and
evolution of the highly dynamic current sheet. We are analyzing the
numerical experiments to identify and interpret observable signatures
of the flare reconnection associated with CMEs, e.g., the flare loops
and ribbons, coronal jets and shock waves, and possible origins of solar
energetic particles. <P />This research was supported by NASA and ONR.
---------------------------------------------------------
Title: 2D and 3D Numerical Simulations of Flux Cancellation
Authors: Karpen, Judith T.; DeVore, C.; Antiochos, S. K.; Linton, M. G.
2009SPD....40.0902K Altcode:
Cancellation of magnetic flux in the solar photosphere and
chromosphere has been linked observationally and theoretically to a
broad range of solar activity, from filament channel formation to CME
initiation. Because this phenomenon is typically measured at only a
single layer in the atmosphere, in the radial (line of sight) component
of the magnetic field, the actual processes behind this observational
signature are ambiguous. It is clear that reconnection is involved in
some way, but the location of the reconnection sites and associated
connectivity changes remain uncertain in most cases. We are using
numerical modeling to demystify flux cancellation, beginning with
the simplest possible configuration: a subphotospheric Lundquist flux
tube surrounded by a potential field, immersed in a gravitationally
stratified atmosphere, spanning many orders of magnitude in plasma
beta. In this system, cancellation is driven slowly by a 2-cell
circulation pattern imposed in the convection zone, such that the tops
of the cells are located around the beta=1 level (i.e., the photosphere)
and the flows converge and form a downdraft at the polarity inversion
line; note however that no flow is imposed along the neutral line. We
will present the results of 2D and 3D MHD-AMR simulations of flux
cancellation, in which the flux at the photosphere begins in either
an unsheared or sheared state. In all cases, a low-lying flux rope is
formed by reconnection at the polarity inversion line within a few
thousand seconds. The flux rope remains stable and does not rise,
however, in contrast to models which do not include the presence of
significant mass loading.
---------------------------------------------------------
Title: On The 3D Structure of the Pre- and After CME Coronal
Streamer Belt
Authors: Kramar, Maxim; Davila, J.; Xie, H.; Antiochos, S.
2009SPD....40.2211K Altcode:
We select several CME events and reconstruct the 3D coronal streamer
belt configurations for the periods of time before and after the
corresponded CMEs. The reconstructions are based on STEREO COR1
observations and made by using a regularized tomography technique
(Kramar et al. 2009). For some CME we found noticeable changes in the
streamer belt structure. Particularly, for a slow CME on June 1, 2008
(Robbrecht et al. 2009) we found that for a longitudinal range of about
10 degrees in Carrington longitude centered at the CME location, the
height of the overlying streamer belt was significantly reduced. This
reduction in height persisted for at least 14 days. The reconstruction
of the streamer belt before and after the CME allows direct estimate
of the mass lost. Also it was found that positions of this and some
others CME correspond to regions in the streamer belt where the latter
has a double structure (i.e. splitted into two parts).
---------------------------------------------------------
Title: The Origins of Magnetic Structure in the Corona and Wind
Authors: Antiochos, Spiro K.
2009SPD....40.3104A Altcode:
One of the most important and most puzzling features of the coronal
magnetic field is that it appears to have smooth magnetic structure with
little evidence for non-potentiality except at two special locations:
photospheric polarity inversions lines, (non-potentiality observed as
a filament channel) and coronal hole boundaries, (observed as the slow
solar wind). This characteristic feature of the closed-field corona is
highly unexpected given that its magnetic field is continuously tangled
by photospheric motions. Although reconnection can eliminate some of
the injected structure, it cannot destroy the helicity, which should
build up to produce observable complexity. I propose that an inverse
cascade process transports the injected helicity from the interior of
closed flux regions to their boundaries, inversion lines and coronal
holes, creating both filament channels and the slow wind. We describe
how the helicity is injected and transported and calculate the relevant
rates. I argue that one process, helicity transport, can explain both
the observed lack and presence of structure in the coronal magnetic
field. <P />This work has been supported by the NASA HTP, SR&T,
and LWS programs.
---------------------------------------------------------
Title: A Model for Solar Polar Jets
Authors: Pariat, E.; Antiochos, S. K.; DeVore, C. R.
2009ApJ...691...61P Altcode:
We propose a model for the jetting activity that is commonly observed
in the Sun's corona, especially in the open-field regions of polar
coronal holes. Magnetic reconnection is the process driving the jets
and a relevant magnetic configuration is the well known null-point
and fan-separatrix topology. The primary challenge in explaining
the observations is that reconnection must occur in a short-duration
energetic burst, rather than quasi-continuously as is implied by the
observations of long-lived structures in coronal holes, such as polar
plumes. The key idea underlying our model for jets is that reconnection
is forbidden for an axisymmetrical null-point topology. Consequently,
by imposing a twisting motion that maintains the axisymmetry, magnetic
stress can be built up to high levels until an ideal instability
breaks the symmetry and leads to an explosive release of energy via
reconnection. Using three-dimensional magnetohydrodynamic simulations,
we demonstrate that this mechanism does produce massive, high-speed
jets driven by nonlinear Alfvén waves. We discuss the implications
of our results for observations of the solar corona.
---------------------------------------------------------
Title: Reconnection-Driven Changes of the Open-Closed Coronal Magnetic
Field Boundary
Authors: Lynch, B. J.; Edmondson, J. K.; Li, Y.; Luhmann, J. G.;
Antiochos, S. K.; DeVore, C. R.; Zurbuchen, T. H.
2008AGUFMSH51A1598L Altcode:
We present recent 3D MHD simulation results with the ARMS code
that show the dynamic evolution of the coronal helmet streamer belt
boundary via magnetic reconnection processes. We start with an initial
multipolar PFSS configuration with a coronal null point and associated
topological features located under the streamer belt, well within
the closed-field region. As this configuration is slowly energized
via rotational shearing flows, volumetric currents form along the
separatrix boundary and during the subsequent field evolution are
compressed to a thin current sheet until the numerical resistivity
mimics the onset of magnetic reconnection. The reconnection scenario
is analogous to the coronal breakout-reconnection in CME modeling,
but here, once the closed streamer belt flux has been transferred out
of the way there is interchange reconnection that opens the outer-spine
fieldline and creates a narrow channel of open field surrounding the AR
sepratrix boundary. This topological evolution is best understood in
the context of the Antiochos (2007) conjecture about the existence of
"coronal hole canals". In addition, we will discuss the implication of
the MHD simulation results for creating slow, unstructured, streamer
blob-like transients and as one of the potential mechanisms operating
at the open-closed magnetic field boundary that could lead to the
formation of the slow solar wind. This work is supported, in part,
by NSF ATM-0621725 and NASA NNX08AJ04G.
---------------------------------------------------------
Title: Topological Origins of the Slow Solar Wind
Authors: Antiochos, S.
2008AGUFMSH43B..07A Altcode:
Although the slow solar wind has been studied for decades with both
in situ and remote sensing observations, its origin is still a matter
of intense debate. In the standard quasi-steady model, the slow wind
is postulated to originate near coronal hole boundaries that define
topologically well-behaved separatrices between open and closed field
regions. In the interchange model, on the other hand, the slow wind
is postulated to originate on open flux that is dynamically diffusing
throughout the seemingly closed-field corona. We argue in favor of
the quasi-steady scenario and propose that the slow wind is due to two
effects: First, the open-closed boundary is highly complex due to the
complexity of the photospheric flux distribution. Second, this boundary
is continuously driven by the transport of magnetic helicity from the
closed field region into the open. The implications of this model for
the structure and dynamics of the corona and slow wind are discussed,
and observational tests of the model are presented. This work has been
supported, in part, by the NASA LWS, HTP, and SR&T programs.
---------------------------------------------------------
Title: Dynamic Instability Leading to Increased Interchange
Reconnection Rates
Authors: Edmondson, J. K.; Antiochos, S. K.; Zurbuchen, T. H.
2008AGUFMSH51B1605E Altcode:
Interchange reconnection is widely believed to play an important
role in coronal magnetic field dynamics. In this investigation
we investigate the 3D dynamics of interchange reconnection by
extending the concept of a magnetic null-point to a null-volume,
the so-called "acute-cusp field" configuration. The acute-cusp
field geometry is characterized by high-beta plasma confined with
favorable curvature, surrounded by a low-beta environment. First,
we construct an initial translationally-symmetric potential field
configuration. This configuration contains the required topological
characteristics of four separate flux systems in the perpendicular
plane. We then drive the system by a slow, incompressible, uniform
flow at the boundary. The resulting evolution is calculated by solving
numerically the MHD equations in full 3D Cartesian coordinates using
the Adaptively Refined MHD Solver developed at the U.S. Naval Research
Laboratory. Field shearing along the topological boundaries changes
the shape of the acute-cusp field surface separating the high and low
plasma beta regions. An extended, 2D current sheet is generated by the
photospheric driving. We discuss the effect of 3D perturbations on the
current sheet dynamics and on the rate of the resulting interchange
reconnection. Finally, we discuss the implications of our simulations
for coronal observations. This work has been supported, in part,
by the NASA HTP and SR&T programs.
---------------------------------------------------------
Title: Topologically driven coronal dynamics - a mechanism for
coronal hole jets
Authors: Müller, D. A. N.; Antiochos, S. K.
2008AnGeo..26.2967M Altcode:
Bald patches are magnetic topologies in which the magnetic field is
concave up over part of a photospheric polarity inversion line. A
bald patch topology is believed to be the essential ingredient
for filament channels and is often found in extrapolations of the
observed photospheric field. Using an analytic source-surface model to
calculate the magnetic topology of a small bipolar region embedded in
a global magnetic dipole field, we demonstrate that although common
in closed-field regions close to the solar equator, bald patches are
unlikely to occur in the open-field topology of a coronal hole. Our
results give rise to the following question: What happens to a bald
patch topology when the surrounding field lines open up? This would
be the case when a bald patch moves into a coronal hole, or when
a coronal hole forms in an area that encompasses a bald patch. Our
magnetostatic models show that, in this case, the bald patch topology
almost invariably transforms into a null point topology with a spine
and a fan. We argue that the time-dependent evolution of this scenario
will be very dynamic since the change from a bald patch to null point
topology cannot occur via a simple ideal evolution in the corona. We
discuss the implications of these findings for recent Hinode XRT
observations of coronal hole jets and give an outline of planned
time-dependent 3-D MHD simulations to fully assess this scenario.
---------------------------------------------------------
Title: 3D Numerical Simulation of a New Model for Coronal Jets
Authors: Pariat, E.; Antiochos, S.; DeVore, C. R.; Patsourakos, S.
2008ESPM...12.3.28P Altcode:
Recent solar observations with STEREO and HINODE have revealed evidence
of twisting motions during the evolution of coronal jets. Furthermore,
the observations indicate that some jets achieve near-Alfvenic
velocities. Most models of jet are not capable of explaining these
new observational features. In addition, the impulsiveness of jets,
manifested as a brief, violent energy release phase in contrast to a
slow, quasi-static energy storage phase storage, is an issue not easily
addressed. <P />We will present the results of 3D numerical simulations
of our model for coronal jets. The simulations were performed with our
state-of-art adaptive mesh MHD solver ARMS. The basic idea of the model
is that a jet is due to the release of magnetic twist when a closed
field region undergoes interchange reconnection with surrounding open
field. The fast reconnection between open and closed field results
in the generation of nonlinear Alfven waves that propagate along
the open field, accelerating plasma upward. We will show how the new
stereoscopically-observed features of jets can be explained by the
results of our numerical simulations
---------------------------------------------------------
Title: Topological Evolution of a Fast Magnetic Breakout CME in
Three Dimensions
Authors: Lynch, B. J.; Antiochos, S. K.; DeVore, C. R.; Luhmann,
J. G.; Zurbuchen, T. H.
2008ApJ...683.1192L Altcode:
We present the extension of the magnetic breakout model for CME
initiation to a fully three-dimensional, spherical geometry. Given the
increased complexity of the dynamic magnetic field interactions in three
dimensions, we first present a summary of the well known axisymmetric
breakout scenario in terms of the topological evolution associated
with the various phases of the eruptive process. In this context,
we discuss the analogous topological evolution during the magnetic
breakout CME initiation process in the simplest three-dimensional
multipolar system. We show that an extended bipolar active region
embedded in an oppositely directed background dipole field has all the
necessary topological features required for magnetic breakout, i.e.,
a fan separatrix surface between the two distinct flux systems, a pair
of spine field lines, and a true three-dimensional coronal null point
at their intersection. We then present the results of a numerical MHD
simulation of this three-dimensional system where boundary shearing
flows introduce free magnetic energy, eventually leading to a fast
magnetic breakout CME. The eruptive flare reconnection facilitates the
rapid conversion of this stored free magnetic energy into kinetic energy
and the associated acceleration causes the erupting field and plasma
structure to reach an asymptotic eruption velocity of gtrsim1100 km
s<SUP>-1</SUP> over an ~15 minute time period. The simulation results
are discussed using the topological insight developed to interpret
the various phases of the eruption and the complex, dynamic, and
interacting magnetic field structures.
---------------------------------------------------------
Title: STEREO SECCHI Stereoscopic Observations Constraining the
Initiation of Polar Coronal Jets
Authors: Patsourakos, S.; Pariat, E.; Vourlidas, A.; Antiochos, S. K.;
Wuelser, J. P.
2008ApJ...680L..73P Altcode: 2008arXiv0804.4862P
We report on the first stereoscopic observations of polar coronal jets
made by the EUVI/SECCHI imagers on board the twin STEREO spacecraft. The
significantly separated viewpoints (~11°) allowed us to infer the 3D
dynamics and morphology of a well-defined EUV coronal jet for the first
time. Triangulations of the jet's location in simultaneous image pairs
led to the true 3D position and thereby its kinematics. Initially the
jet ascends slowly at ≈10-20 km s<SUP>-1</SUP> and then, after an
apparent "jump" takes place, it accelerates impulsively to velocities
exceeding 300 km s<SUP>-1</SUP> with accelerations exceeding the solar
gravity. Helical structure is the most important geometrical feature
of the jet which shows evidence of untwisting. The jet structure
appears strikingly different from each of the two STEREO viewpoints:
face-on in one viewpoint and edge-on in the other. This provides
conclusive evidence that the observed helical structure is real and
does not result from possible projection effects of single-viewpoint
observations. The clear demonstration of twisted structure in polar jets
compares favorably with synthetic images from a recent MHD simulation of
jets invoking magnetic untwisting as their driving mechanism. Therefore,
the latter can be considered as a viable mechanism for the initiation
of polar jets.
---------------------------------------------------------
Title: Homologous Confined Filament Eruptions via Magnetic Breakout
Authors: DeVore, C. Richard; Antiochos, Spiro K.
2008ApJ...680..740D Altcode:
We describe magnetohydrodynamic simulations of a bipolar active
region embedded in the Sun's global background field and subjected
to twisting footpoint displacements concentrated near its polarity
inversion lines to produce strong magnetic shear. The dipole
moments of the active region and background field are antiparallel,
so that the initially potential magnetic field contains a coronal
null. This configuration supports magnetic breakout eruptions in our
simulations that exhibit three novel features. First, the eruptions
are multiple and homologous: the flare reconnection following each
eruption reforms the magnetic null, setting the stage for a subsequent
episode of breakout reconnection and eruption driven by the ongoing
footpoint motions. Second, the eruptions are confined; that is, their
rapidly rising, moderately sheared field lines do not escape the Sun
but instead come to rest in the outer corona, comprising a large
coronal loop formed by reconnection during the rise phase. Third,
the most strongly sheared field lines of the active region are quite
flat prior to eruption, expand upward sharply during the event, and
lose most of their shear through reconnection with overlying flux,
while lower lying field lines survive the eruption and recover their
flat configuration within a few hours. These behaviors are consistent
with filament disappearance followed by reformation in place. We also
find that the upward motion of the erupting sheared flux exhibits a
distinct three-phase acceleration profile. All of these features of
our simulations—homology, confinement, reformation, and multiphase
acceleration—are well established aspects of solar eruptions.
---------------------------------------------------------
Title: Coronal Heating and Structure
Authors: Antiochos, S. K.
2008AGUSMSP33A..03A Altcode:
The existence of the Sun's million-degree corona is one of the oldest
and most challenging problems in all space physics. It is generally
accepted that the solar magnetic field is responsible for both
the heating and the structure of coronal plasma, but the physical
mechanisms are still not clearly understood. Gene Parker has made
many seminal contributions to solving the coronal heating problem,
in particular, his widely-used nano-flare model. Parker argued that
in closed field regions the complex motions of the photosphere must
lead to the formation of fine-scale electric currents in the corona
and, consequently, to continual bursts of magnetic reconnection. We
discuss the implications of these ideas for understanding the observed
features of the corona. We show that the type of reconnection proposed
by Parker may well account for all the well-known observations of both
the closed and open field corona, and we discuss the implications of
our results for upcoming NASA missions. This work was supported by
the NASA HTP and TR&T programs.
---------------------------------------------------------
Title: On the Rotation of "Flux Rope" CMEs During Eruption
Authors: Lynch, B. J.; Li, Y.; Luhmann, J. G.; Antiochos, S. K.;
DeVore, C. R.
2008AGUSMSP24A..04L Altcode:
We present an analysis of the dynamics of 3-dimensional sheared
arcade magnetic breakout eruptions that become flux rope CMEs during
the eruptive flare reconnection process. We compare the results of
two otherwise identical numerical simulations, differing only in
the direction of the applied shearing motions (i.e. the handedness
of the sheared arcade systems and their resulting CME fields). The
right-handed flux rope rotates clockwise and the left-handed flux rope
rotates counter-clockwise as they propagate through the low-corona from
2-4 Rs. We characterize the average rotation angle of their respective
axes relative to their active region neutral lines and discuss the
forces acting on the erupting structures. The simulation results are
then placed in the context of recent observational work linking the CME
source region neutral line orientation, the overlying helmet streamer
belt orientation, flux-rope orientation inferred from coronagraph
halo-CME observations, and 1 AU measurements of the associated magnetic
cloud orientation. BJL acknowledges NSF SHINE grant ATM-0621725.
---------------------------------------------------------
Title: 3D Numerical Simulation and Stereoscopic Observations of
Coronal Jets
Authors: Pariat, E.; Antiochos, S. K.; Patsourakos, S.; DeVore, C. R.
2008AGUSMSP53A..05P Altcode:
Recent solar observations have revealed that coronal jets are a more
frequent phenomenon than previously believed. It is widely accepted
that magnetic reconnection is the fundamental mechanism that gives
rise to the jets. The improved spatial and temporal resolution of
the STEREO observations in combination with stereoscopy yields new
insights into the origins of coronal jets, and provides detailed data
that can be used to test and refine models. We present the results
of 3D numerical simulations of our model for coronal jets. The
simulations were performed with our state-of-art adaptive mesh MHD
solver ARMS. The basic idea of the model is that a jet is due to
the release of twist as a closed field region undergoes interchange
reconnection with surrounding open field. The photospheric driven
evolution of the structure results in the generation of a non linear
Alfven wave along the open fields. Using stereoscopic EUVI images,
we reveal the presence of such twisted structure in a coronal jet
event. This work was supported, in part, by NASA and ONR.
---------------------------------------------------------
Title: Understanding the Initiation of Polar Coronal Jets with
STEREO/SECCHI Stereoscopic Observations
Authors: Vourlidas, A.; Patsourakos, S.; Pariat, E.; Antiochos, S.
2008AGUSMSH23A..02V Altcode:
Polar coronal jets are collimated transient ejections of plasma
occurring in polar coronal holes. The kinematics and mostly the 3D
morphology of jets place strong constraints on the physical mechanism(s)
responsible for their initiation, and were not accessible before
the STEREO mission. We report on the first stereoscopic observations
of polar coronal jets made by the EUVI/SECCHI imagers on-board the
twin STEREO spacecraft at spacecraft separations of ~ 11° and ~
45°. Triangulations of the jet locations in simultaneous image pairs
led to the true 3D position and thereby their kinematics. The most
important geometrical feature of the observed jets is helical structures
showing evidence of untwisting. The jet structure appear strikingly
different from each of the two STEREO viewpoints: face-on in the
one viewpoint and edge-on in the other. This provides solid evidence
that the observed helical structure is real and not resulting from
possible projection effects of single viewpoint observations. The clear
demonstration of twisted structure in polar jets compares favorably
with synthetic images from a recent MHD simulation of jets invoking
magnetic untwisting as their driving mechanism.
---------------------------------------------------------
Title: Simulated Coronal Mass Ejections Originating in Complex
Source Regions
Authors: DeVore, C.; Antiochos, S. K.
2008AGUSMSP24A..03D Altcode:
Previously, we have investigated numerically the initiation of
coronal mass ejections (CMEs) in the simplest possible solar sources:
a single bipolar active region embedded in the Sun's global background
field. If the overall topology is more complex than bipolar, with
a magnetic null and separatrices dividing the coronal field into
two or more flux systems, then the configuration is susceptible to
magnetic-breakout eruptions. Breakout reconnection across the null
allows the overlying field to be pushed aside, after which the highly
stressed field below undergoes a fast, free, ideal expansion into the
outer corona and heliosphere. We are now beginning to address more
complex scenarios in which two bipolar active regions, side by side,
spawn breakout CMEs. A CME originating at the polarity inversion
line (PIL) separating the two active regions breaks open the central
arcade of the combined configuration, and is analogous to an eruption
occurring at the interior PIL of a single active region embedded in
a background field. We also are investigating single CMEs originating
at the interior PIL of either active region in the new scenario, i.e.,
in either side arcade of the configuration. Furthermore, stressing both
interior PILs of the active regions contemporaneously gives rise to
the possibility of paired sympathetic eruptions, either simultaneous or
delayed, originating in the two distant side arcades. We will report our
progress on simulating and understanding these more complex scenarios
for CME initiation. NASA and ONR support our research.
---------------------------------------------------------
Title: Modeling Coronal Jets with FLUX
Authors: Rachmeler, L. A.; Pariat, E.; Antiochos, S. K.; Deforest,
C. E.
2008AGUSMSP43B..01R Altcode:
We report on a comparative study of coronal jet formation with and
without reconnection using two different simulation strategies. Coronal
jets are features on the solar surface that appear to have some
properties in common with coronal mass ejections, but are less
energetic, massive, and broad. Magnetic free energy is built up over
time and then suddenly released, which accelerates plasma outward in
the form of a coronal jet. We compare results from the ARMS adaptive
mesh and FLUX reconnection-less codes to study the role of reconnection
in this system. This is the first direct comparison between FLUX and
a numerical model with a 3D spatial grid.
---------------------------------------------------------
Title: A Mechanism for Coronal Hole Jets
Authors: Mueller, D. A. N.; Antiochos, S. K.
2008arXiv0804.3995M Altcode:
Bald patches are magnetic topologies in which the magnetic field is
concave up over part of a photospheric polarity inversion line. A
bald patch topology is believed to be the essential ingredient
for filament channels and is often found in extrapolations of the
observed photospheric field. Using an analytic source-surface model to
calculate the magnetic topology of a small bipolar region embedded in
a global magnetic dipole field, we demonstrate that although common
in closed-field regions close to the solar equator, bald patches are
unlikely to occur in the open-field topology of a coronal hole. Our
results give rise to the following question: What happens to a bald
patch topology when the surrounding field lines open up? This would
be the case when a bald patch moves into a coronal hole, or when
a coronal hole forms in an area that encompasses a bald patch. Our
magnetostatic models show that, in this case, the bald patch topology
almost invariably transforms into a null point topology with a spine
and a fan. We argue that the time-dependent evolution of this scenario
will be very dynamic since the change from a bald patch to null point
topology cannot occur via a simple ideal evolution in the corona. We
discuss the implications of these findings for recent Hinode XRT
observations of coronal hole jets and give an outline of planned
time-dependent 3D MHD simulations to fully assess this scenario.
---------------------------------------------------------
Title: Condensation Formation by Impulsive Heating in Prominences
Authors: Karpen, J. T.; Antiochos, S. K.
2008ApJ...676..658K Altcode:
Our thermal nonequilibrium model for prominence formation provides
an explanation for the well-observed presence of predominantly
dynamic, cool, dense material suspended in the corona above filament
channels. According to this model, condensations form readily along
long, low-lying magnetic field lines when heating is localized near
the chromosphere. Often this process yields a dynamic cycle in which
condensations repeatedly form, stream along the field, and ultimately
disappear by falling onto the nearest footpoint. Our previous studies
employed only steady heating, as is consistent with some coronal
observations, but many coronal heating models predict transient
episodes of localized energy release (e.g., nanoflares). Here we
present the results of a numerical investigation of impulsive heating
in a model prominence flux tube and compare the outcome with previous
steady-heating simulations. We find that condensations form readily
when the average interval between heating events is less than the
coronal radiative cooling time (~2000 s). As the average interval
between pulses decreases, the plasma evolution more closely resembles
the steady-heating case. The heating scale and presence or absence
of background heating also determine whether or not condensations
form and how they evolve. Our results place important constraints
on coronal heating in filament channels and strengthen the case for
thermal nonequilibrium as the process responsible for the plasma
structure in prominences.
---------------------------------------------------------
Title: Comparison of Heliospheric In Situ Data with the Quasi-steady
Solar Wind Models
Authors: Lepri, S. T.; Antiochos, S. K.; Riley, P.; Zhao, L.;
Zurbuchen, T. H.
2008ApJ...674.1158L Altcode:
The standard theory for the solar-heliospheric magnetic field is the
so-called quasi-steady model in which the field is determined by the
observed magnetic flux at the photosphere and the balance between
magnetic and plasma forces in the corona. In this model, the solar
magnetic flux that opens to the heliosphere can increase or decrease as
the photospheric flux evolves. The most sophisticated implementation
of the quasi-steady theory is the SAIC model, which solves the fully
time-dependent 3D MHD equations for the corona and wind until a
steady state is achieved. In order to test the quasi-steady theory,
we compare the 3D MHD model with observations of the heliospheric flux
using multipoint measurements from the VHM instrument on the Ulysses
spacecraft from 1991 to 2005 and from magnetic field measurements from
various spacecraft at L1 compiled into the OMNI data set from 1976
through 2005. We also compare the observations to the predictions
of the potential-field source-surface model, an older and simpler
implementation of the quasi-steady theory. During solar maximum, ICMEs
significantly disturb the heliospheric magnetic field, making our
comparisons difficult. We find that the MHD model compares well with
the general trends of the observed heliospheric fluxes. Variations on
short timescales, presumably due to local effects, are missed by the
model, but the long-term evolution is well matched. The model disagrees
with observations most when Ulysses is in slow wind or ICME-related
flows. The model underestimates the flux at solar maximum; however, this
is to be expected, given the large number of ICMEs in the heliosphere
at this time. We discuss the possible sources of discrepancy between
the observations and the quasi-steady models.
---------------------------------------------------------
Title: The role of magnetic reconnection in solar activity
Authors: Antiochos, Spiro
2008cosp...37..102A Altcode: 2008cosp.meet..102A
The central challenge in solar/heliospheric physics is to understand
how the emergence and transport of magnetic flux at the photosphere
drives the structure and dynamics that we observe in the corona and
heliosphere. This presentation focuses on the role of magnetic
reconnection in determining solar/heliospheric activity. We
demonstrate that two generic properties of the photospheric magnetic
and velocity fields are responsible for the ubiquitous reconnection
in the corona. First, the photospheric velocities are complex, which
leads to the injection of energy and helicity into the coronal magnetic
fields and to the efficient formation of small-scale structure. Second,
the flux distribution at the photosphere is multi-polar, which implies
that topological discontinuities and, consequently, current sheets,
must be present in the coronal magnetic field. We present numerical
simulations showing that photospherically-driven reconnection is
responsible for the heating and dynamics of coronal plasma, and for
the topology of the coronal/heliospheric magnetic field. The work was
supported by the NASA HTP, SR&T, and TR&T Programs.
---------------------------------------------------------
Title: 3D numerical simulation and stereoscopic observations of
coronal jets.
Authors: Pariat, Etienne; Antiochos, Spiro; Patsourakos, Spiro;
DeVore, C. R.
2008cosp...37.2354P Altcode: 2008cosp.meet.2354P
Recent solar observations have revealed that coronal jets are a more
frequent phenomenon than previously believed. It is widely accepted that
magnetic reconnection is the fundamental mechanism that gives rise to
the jets. The improved spatial and temporal resolution of the STEREO
observations in combination with stereoscopy yields new insights into
the origins of coronal jets, and provides detailed data that can be
used to test and refine models. We present the results of a 3D numerical
simulation of our model for coronal jets. The simulations were performed
with our state-of-art adaptive mesh MHD solver ARMS. The basic idea
of the model is that a jet is due to the release of twist as a closed
field region undergoes interchange reconnection with surrounding open
field. The photospheric driven evolution of the structure results in
the generation of nonlinear Alfven waves propagating along the open
field, which drive the jet flows. Using stereoscopic EUVI images,
we reveal the presence of such twisted structure in a coronal jet
event. This work was supported, in part, by NASA and ONR.
---------------------------------------------------------
Title: Breakout coronal mass ejections from solar active regions
Authors: DeVore, C. Richard; Lynch, Benjamin; MacNeice, Peter; Olson,
Kevin; Antiochos, Spiro
2008cosp...37..706D Altcode: 2008cosp.meet..706D
We are performing magnetohydrodynamic simulations of single bipolar
active regions (ARs) embedded in the Sun's global background field
and of pairs of ARs interacting with each other. The magnetic flux
near the polarity inversion lines (PILs) of the ARs is subjected to
twisting footpoint displacements that introduce strong magnetic shear
between the two polarities and gradually inflate the coronal volume
occupied by the AR fields. If the initially current-free coronal field
contains a magnetic null, then it is vulnerable to eruptions triggered
by magnetic breakout, which reconnects aside the previously restraining
field lines overhead. The sheared core flux promptly expands outward
at the Alfven speed, opening the magnetic field in the vicinity of
the PIL. Flare reconnection below the ejecta, across the vertical
current sheet thus established, thereafter reforms the magnetic-null
configuration above the AR. This reformation sets the stage for
subsequent homologous episodes of breakout reconnection and eruption,
if the energizing footpoint motions are sustained. The magnetic flux
and energy of an isolated AR, relative to those of the background
field, determine whether the eruption is confined or ejective, as the
sheared flux either comes to rest in the corona or escapes the Sun to
interplanetary space, respectively. In the latter case, the field lines
accompanying the coronal mass ejection can comprise a weakly twisted
"magnetic bottle" as readily as a strongly twisted flux rope, both
of which are observed routinely in situ. The latest developments in
this research will be reported. In particular, we will emphasize the
observational signatures inferred from the simulations that could be
sought in STEREO data, such as multiple three-dimensional views, EUV
brightenings at reconnection sites, and coronal dimmings in regions
of strong expansion. Our research is sponsored by NASA and ONR.
---------------------------------------------------------
Title: In-situ and Numerical Modeling Prospects for Multipoint ICME
Observations Featuring the 22 May 2007 STEREO Event
Authors: Lynch, B. J.; Li, Y.; Huttunen, K. E.; Antiochos, S. K.;
DeVore, C. R.; Luhmann, J. G.
2007AGUFMSH51B..04L Altcode:
We will present recent 3D MHD simulation results of eruptive
flux-rope formation via the magnetic breakout mechanism to provide a
theoretical/modeling context for multipoint in-situ observations. Many
generic observational CME properties are reproduced by this idealized
eruption as it propagates through the low corona (~10 R\odot). Recently,
STEREO observed a "classic" flux-rope ICME on 22 May 2007 with different
in- situ field and plasma signatures seen at each spacecraft. The
same event was also seen by WIND and ACE, arriving at L1 arriving a
few hours before either STEREO A or B. We will describe a preliminary
analysis of the large-scale heliospheric structure of the ICME flux-rope
utilizing the linear, force-free cylinder model fits to the in- situ
magnetic field rotations in order to determine the ICME's relative
simplicity and/or consistency with the standard Magnetic Cloud cartoon
picture. A comparison between the data, the derived in-situ cylinder
orientations at three spacecraft, and the CME large-scale magnetic
structure inferred from the MHD simulation results in the low corona
show a reasonable qualitative agreement. BJL would like to acknowledge
support from NSF ATM-0621725 as a SHINE Postdoc, and thank the STEREO,
WIND and ACE teams for making their data readily available.
---------------------------------------------------------
Title: Comparison of 3D Numerical Simulations with STEREO Observations
of Coronal Jets
Authors: Pariat, E.; Patsourakos, S.; Antiochos, S. K.; DeVore, C. R.
2007AGUFMSH41B..03P Altcode:
Recent solar observations have revealed that coronal jets are a more
frequent phenomenon than previously believed. It is widely accepted that
magnetic reconnection is the fundamental mechanism that gives rise to
the jets. The improved spatial and temporal resolution of the STEREO
observations in combination with stereoscopy yields new insights into
the origins of coronal jets, and provides detailed data that can be
used to test and refine models. We present the results of a 3D numerical
simulation of our model for coronal jets. The simulations were performed
with our state-of-art adaptive mesh MHD solver ARMS. The basic idea
of the model is that a jet is due to the release of twist as a closed
field region undergoes interchange reconnection with surrounding open
field. We compare the structure and dynamics of the simulated jet with
actual EUVI observations, focusing on how the reconfiguration of the
3D magnetic field explains observed properties of the jet. We also
discuss possible signatures for STEREO of twisted structures within
jets. Finally, we discuss the implications of our simulations for
future stereoscopic observations with STEREO. This work was supported,
in part, by NASA and ONR.
---------------------------------------------------------
Title: Solar Reconnection
Authors: Antiochos, S. K.
2007AGUFMSH41C..01A Altcode:
High spatial and temporal resolution observations from SOHO, TRACE,
Hinode, and STEREO prove dramatically that the photosphere is never
simple and the corona is never quiet. The photosphere exhibits a
constantly evolving, multipolar flux distribution on scales ranging from
the magnetic carpet to active region complexes. The corona exhibits
brightenings and jetting on a vast range of temporal and spatial
scales: from small transient spicules, to long-lived coronal loops,
to giant coronal mass ejections. We present theoretical and numerical
results demonstrating that magnetic reconnection is the physical
process underlying all of this activity. These results also show that
the topology of the solar field is the key to understanding why solar
activity exhibits such an apparently wide variety of forms. Conservation
of magnetic helicity turns out to be the critical condition that
distinguishes between the different types of reconnection in the solar
corona. We discuss the implications of our results for interpreting the
latest observations from Hinode and STEREO. This work was supported,
in part, by NASA, ONR, and the NSF.
---------------------------------------------------------
Title: A Model for Coronal Hole Jets
Authors: Antiochos, S. K.; Pariat, E.; DeVore, C.
2007AGUFMSH21B..01A Altcode:
The recent observations from XRT on Hinode show dramatically that
coronal hole are populated with intense X-ray jets that can reach
heights of solar radii. These jets appear to originate from closed
magnetic-field regions inside the holes; consequently, a natural
explanation for these jets is that they are due to interchange
reconnection between the open field of the hole and the closed field
of an embedded bipole. This type of interchange reconnection has long
been postulated as the driver, not only of coronal jets, but also
for the solar wind itself. We argue, however, that the explosive
nature of the jets imposes severe requirements on the reconnection
that are not easily satisfied by realistic 3D models. In particular,
the reconnection must have a "switch-on" nature in that it stays off
until a substantial store of free energy has been accumulated, but
then turns on abruptly and stays on until much of this free energy is
released. We discuss the possible magnetic topologies of an embedded
bipole in an open field region and present recent 3D simulations of a
model in which interchange reconnection does, indeed, yield a large
burst of energy release. We also discuss the implications of these
results for the Hinode observations. This work was supported, in part,
by NASA, ONR, and the NSF.
---------------------------------------------------------
Title: Comparison of Heliospheric In-Situ Data with the Quasi-Steady
Solar Wind Models
Authors: Lepri, S. T.; Antiochos, S. K.; Riley, P.; Zhao, L.;
Zurbuchen, T. H.
2007AGUFMSH21A0296L Altcode:
The standard theory for the solar-heliospheric magnetic field is the
so-called quasi-steady model in which the field is determined by the
observed magnetic flux at the photosphere and the balance between
magnetic and plasma forces in the corona. In this model, the solar
magnetic flux that opens to the heliosphere can increase or decrease
as the photospheric flux evolves. One of the most sophisticated
implementations of the quasi-steady theory is the SAIC model, which
solves the fully time-dependent 3D MHD equations for the corona and wind
until a steady state is achieved. In order to test the quasi-steady
theory, we compare the 3-D MHD model with observations of the
heliospheric flux using multi-point measurements from the VHM instrument
on the Ulysses spacecraft from 1991 through 2005 and from magnetic field
measurements from various spacecraft at L1 compiled into the OMNI data
set from 1976 through 2005. We also compare the observations to the
predictions of the potential-field source-surface model, an older and
simpler implementation of the quasi-steady theory. During solar maximum,
ICMEs significantly disturb the heliospheric magnetic field, making our
comparisons difficult. We find that the MHD model compares well with
the general trends of the observed heliospheric fluxes. Variations
on short timescales, presumably due to local effects, are missed by
the model, but the long-term evolution is well matched. The model
disagrees with observations most when Ulysses is in slow wind or ICME-
related flows. The model underestimates the flux at solar maximum;
however, this is to be expected, given the large number of ICMEs
in the heliosphere at this time. We discuss the possible sources of
discrepancy between the observations and the quasi-steady models.
---------------------------------------------------------
Title: Structure and Dynamics of the Sun's Open Magnetic Field
Authors: Antiochos, S. K.; DeVore, C. R.; Karpen, J. T.; Mikić, Z.
2007ApJ...671..936A Altcode: 2007arXiv0705.4430A
The solar magnetic field is the primary agent that drives solar
activity and couples the Sun to the heliosphere. Although the details
of this coupling depend on the quantitative properties of the field,
many important aspects of the corona-solar wind connection can be
understood by considering only the general topological properties of
those regions on the Sun where the field extends from the photosphere
out to interplanetary space, the so-called open field regions that are
usually observed as coronal holes. From the simple assumptions that
underlie the standard quasi-steady corona-wind theoretical models, and
that are likely to hold for the Sun as well, we derive two conjectures
as to the possible structure and dynamics of coronal holes: (1) coronal
holes are unique in that every unipolar region on the photosphere can
contain at most one coronal hole, and (2) coronal holes of nested
polarity regions must themselves be nested. Magnetic reconnection
plays the central role in enforcing these constraints on the field
topology. From these conjectures we derive additional properties for
the topology of open field regions, and propose several observational
predictions for both the slowly varying and transient corona/solar wind.
---------------------------------------------------------
Title: Homologous Confined Filament Eruptions via Magnetic Breakout
Authors: DeVore, C. R.; Antiochos, S. K.
2007AAS...210.2901D Altcode: 2007BAAS...39..137D
We have performed numerical simulations of a bipolar active region
embedded in a dipolar background field and subjected to twisting
footpoint displacements concentrated near its polarity inversion
lines. These displacements preserve the initial radial flux distribution
at the inner surface of our spherical domain while introducing strong
magnetic shear between the region’s two polarity concentrations. The
dipole moments of the active region and the background field are
antiparallel and aligned with the Sun’s polar axis, so that the
initially potential magnetic field has a null point in the corona above
the equator. This configuration is vulnerable to magnetic breakout
eruptions, which occur in our MHD simulations and exhibit three novel
features not previously found in our studies of coronal mass ejection
initiation. First, the eruptions are multiple and homologous, i.e.,
as a consequence of the flare reconnection that follows each eruption,
the coronal null point reforms above the equator, setting the stage
for the subsequent onset of a new episode of breakout reconnection
and eruption driven by the ongoing footpoint motions. Second, the
eruptions are confined, that is, the highest lying, rapidly rising,
strongly sheared field lines of the active region do not escape the Sun
but instead come to rest in the outer corona well above the reforming
null point, forming a large transequatorial loop. Third, the lowest
lying, strongly sheared field lines of the active region are very flat
prior to the eruption and, after expanding upward sharply during the
event, return to their flat configuration within just a few hours,
consistent with filament disappearance and prompt reformation. All
of these features of our simulations - homology, confinement, and
reformation - are commonly observed aspects of the Sun’s eruptive
activity. NASA and ONR sponsored this research.
---------------------------------------------------------
Title: A Mechanism for Coronal Hole Jets
Authors: Mueller, Daniel; Antiochos, S. K.
2007AAS...210.9117M Altcode: 2007BAAS...39..206M
Bald patches are magnetic topologies in which the magnetic field is
concave up over part of a photospheric polarity inversion line. A
bald patch topology is believed to be the essential ingredient
for filament channels and is often found in extrapolations of the
observed photospheric field. We demonstrate that although common in
closed field regions, bald patches are unlikely to occur in the open
field topology of a coronal hole. We use an analytic source-surface
model to calculate the magnetic topology of a small "active region"
dipole embedded in a central magnetic dipole field. While bald patches
readily occur in closed-field regions, we show that there is only
a highly limited parameter range for them to form in open-field <P
/>regions. Furthermore, the inclusion of a finite gas pressure and
solar wind is likely to destroy even this limited parameter range for
the existence of bald patches in coronal holes. Our results give rise
to the following question: What happens to a bald patch topology when
the surrounding field lines open up? This would be the case when a bald
patch moves into a coronal hole, or when a coronal hole forms in an
area that encompasses a bald patch. Our magnetostatic models show that,
in this case, the bald patch topology almost invariably transforms
into a null point topology with a spine and a fan. We argue that the
time-dependent evolution of this scenario will be very dynamic since
the change from a bald patch to <P />null point topology cannot occur
via a simple ideal evolution in the corona. We discuss the implications
of these findings for recent Hinode XRT observations of coronal hole
jets and give an outline of planned time-dependent 3D MHD simulations
to fully assess this scenario. <P />This work was supported in part
by NASA and ONR.
---------------------------------------------------------
Title: An MHD Simulation of an Emerging Bipole in the Presence of
the Solar Wind
Authors: Allred, J. C.; MacNeice, P. J.; Antiochos, S. K.
2006AGUFMSH33B0409A Altcode:
We report on an MHD simulation of a bipole emerging from the
solar surface in the presence of the solar wind. The initial field
configuration is obtained from a source surface model, with the field
at the solar surface defined by a dipole and a bipolar region near the
polar axis. We use a 2.5 dimensional MHD code to model the evolution of
the bipole in the presence of the solar wind and show that the emerging
bipole creates an embedded coronal hole at the location of the spine
of the patch of opposite polarity of the emerged bipole. This result
is in contrast to source surface models which predict an X-point along
the spine. We study the size and properties of the new coronal hole.
---------------------------------------------------------
Title: Constraints on the Sun-Heliosphere Magnetic Connection
Authors: Antiochos, S. K.
2006AGUFMSH21B..04A Altcode:
The solar magnetic field is the primary agent that drives solar
activity and couples the Sun to the Heliosphere. Although the details
of this coupling depend on the quantitative properties of the field,
many important aspects of the corona - solar wind connection can be
understood by considering only the topological properties of those
regions on the Sun where the field extends from the photosphere out to
interplanetary space, the so-called open field regions that are often
observed as "coronal holes". From the well-known assumptions that
underlie the standard quasi-steady corona-wind theoretical models,
and that are likely to hold for the Sun, as well, we derive several
constraints on the possible topology and dynamics of coronal open
and closed field regions. We show how magnetic reconnection plays
the central role in establishing these constraints. We discuss the
implications of our results on observations, and make a number of
predictions for the upcoming LWS missions. This work was supported by
the LWS TR&T Program and is part of the research by the Focus Team
on Connecting the Sun to the Heliosphere.
---------------------------------------------------------
Title: Energetics and Dynamics of Bipolar and Multipolar CME Source
Regions
Authors: Lynch, B. J.; Antiochos, S. K.; DeVore, C. R.; Luhmann, J. G.
2006AGUFMSH31B..04L Altcode:
We present results of a numerical experiment which tests the
Aly-Sturrock limit in a fully 3-dimensional, spherical geometry. We
compare two common magnetic configurations corresponding to bipolar
and multipolar "active region" arcades with identical photospheric
normal field distributions and applied shearing flows. The bipolar
response is a smooth expansion of the stressed fields, void of any
explosive behavior, whereas the multipolar configuration results in
the rapid expulsion of the low-lying sheared field via the magnetic
breakout mechanism for CME initiation. The critical nature of the
oppositely-directed overlying field and its topological consequences
is discussed in the context of the breakout model.
---------------------------------------------------------
Title: Solar Prominence Merging
Authors: Aulanier, Guillaume; DeVore, C. Richard; Antiochos, Spiro K.
2006ApJ...646.1349A Altcode:
In a recent paper, we described MHD simulations of the interaction
between a pair of distinct prominences formed by the photospheric
line-tied shearing of two separated dipoles. One case was typical of
solar observations of prominence merging, in which the prominences
have the same axial field direction and sign of magnetic helicity. For
that configuration, we reported the formation of linkages between the
prominences due to magnetic reconnection of their sheared fields. In
this paper, we analyze the evolution of the plasma-supporting
magnetic dips in this configuration. As the photospheric flux is
being progressively sheared, dip-related chromospheric fibrils and
high-altitude threads form and develop into the two prominences, which
undergo internal oscillations. As the prominences are stretched farther
along their axes, they come into contact and their sheared fluxes
pass each other, and new dips form in the interaction region. The
distribution of these dips increasingly fills the volume between
the prominences, so that the two progenitors gradually merge into
a single prominence. Our model reproduces typical observational
properties reported from both high-cadence and daily observations at
various wavelengths. We identify the multistep mechanism, consisting
of a complex coupling between photospheric shear, coronal magnetic
reconnection without null points, and formation of quasi bald patches,
that is responsible for the prominence merging through dip creation. The
resulting magnetic topology differs significantly from that of a
twisted flux tube.
---------------------------------------------------------
Title: Modeling Free Energy & Reconnection in the Corona
Authors: Welsch, Brian; DeVore, C.; Antiochos, S. K.
2006SPD....37.0908W Altcode: 2006BAAS...38..237W
The injection and storage of magnetic free energy into the coronal
magnetic field is an essential component of the widely accepted
“storage & release” paradigm of solar flares and CMEs.A central
role in many models of both the storage and release phases is played by
magnetic reconnection --- which can form an unstable structure (e.g.,
by flux cancellation) and/or reduce confinement allowing a metastable
structure to erupt (as in the breakout model).To investigate changes in
magnetic energy and field line connectivity in the presence of shearing,
convergence, and flux cancellation, we have used the ARMS code, a 3-D,
flux-corrected transport MHD code with adaptive mesh refinement, to
simulate the evolution of coronal magnetic fields driven by prescribed
photospheric motions.Here, we present the preliminary results of our
investigations, and outline directions for future studies.This work was
supported by ONR, NASA's SEC Theory program, and by a grant of computer
time from the DOD High Performance Computing Modernization Program.
---------------------------------------------------------
Title: Sympathetic Breakout Coronal Mass Ejections
Authors: DeVore, C. R.; Antiochos, S. K.
2006SPD....37.0906D Altcode: 2006BAAS...38..236D
Several instances of multiple, apparently coordinated solar eruptive
events have been reported, in which the close temporal association of
the eruptions suggests a possible causal link between them. Variously
called "global" or "sympathetic" coronal mass ejections (CMEs),
the low-coronal structures where the eruptions originate in some
cases neighbor each other, while in others they are more remote with
no obvious magnetic connections joining them.In the breakout model
for CMEs, eruption occurs due to the onset of reconnection across a
coronal magnetic null shared by multiple flux systems. The resultant
reconfiguration of the field overlying the participating structures
loosens the restraining forces on one or more of them, initiating
the accelerating outward expansion that becomes the CME. In a simple
three-arcade quadrupolar geometry, an eruption in either side lobe
depletes the overlying fields of both side lobes as they reconnect
with each other. This raises the possibility of either an immediate or
prompt second CME in the far side lobe; the two cases are distinguished
according to whether the second CME precedes or follows the reconnection
of a significant amount of its restraining flux. In the aftermath of
a solitary eruption in either side lobe, flare reconnection closes the
opened field and reforms the lobe, eventually also depleting the middle
lobe of its own overlying field. This process can produce a delayed
second CME in the middle lobe of the configuration.We are concluding
an analysis of the magnetic free energies available to power these
scenarios - immediate, prompt, and delayed - for sympathetic CMEs in
simple breakout geometries. The results will be presented. We also
are beginning a simulation study of susceptible configurations to
verify and understand the dynamics of sympathetic eruptive events. Our
progress will be reported.This research was supported by NASA and ONR.
---------------------------------------------------------
Title: Impulsive Heating And Thermal Nonequilibrium In Prominences
Authors: Karpen, Judith T.; Antiochos, S. K.
2006SPD....37.0203K Altcode: 2006BAAS...38Q.221K
Prominences are among the most spectacular manifestations of both
quiescent and eruptive solar activity, yet the origins of their
magnetic-field and plasma structures remain poorly understood. We
have made steady progress toward a comprehensive model of prominence
formation and evolution with our sheared 3D arcade model for
the magnetic field and our thermal nonequilibrium model for the
cool, dense material suspended in the corona. According to the
thermal nonequilibrium model, condensations form readily in long,
low-lying magnetic flux tubes if the heating is localized near the
chromosphere. Our previous studies established the effects of steady
heating in flux tubes of different geometries. In some cases this
process yields a dynamic cycle in which condensations repetitively form,
stream along the field line, and ultimately disappear by falling onto
the nearest footpoint; in others, static condensations grow as long as
the heating continues. Here we will discuss the effects of impulsive
heating, as indicated by many coronal-heating models, on the formation
and evolution of prominence plasmas.This work was supported by NASA
and ONR.
---------------------------------------------------------
Title: A Transient Heating Model for the Structure and Dynamics of
the Solar Transition Region
Authors: Spadaro, D.; Lanza, A. F.; Karpen, J. T.; Antiochos, S. K.
2006ApJ...642..579S Altcode:
Understanding the structure and dynamics of the Sun's transition
region has been a major challenge to scientists since the Skylab
era. In particular, the characteristic shape of the emission measure
distribution and the Doppler shifts observed in EUV emission lines
have thus far resisted all theoretical and modeling efforts to explain
their origin. Recent observational advances have revealed a wealth
of dynamic fine-scale structure at transition-region temperatures,
validating earlier theories about the existence of such cool structure
and explaining in part why static models focusing solely on hot,
large-scale loops could not match observed conditions. In response
to this newly confirmed picture, we have investigated numerically the
hydrodynamic behavior of small, cool magnetic loops undergoing transient
heating spatially localized near the chromospheric footpoints. For
the first time we have successfully reproduced both the observed
emission measure distribution over the entire range logT=4.7-6.1 and
the observed temperature dependence of the persistent redshifts. The
closest agreement between simulations and observations is obtained with
heating timescales of the order of 20 s every 100 s, a length scale of
the order of 1 Mm, and energy deposition within the typical range of
nanoflares. We conclude that small, cool structures can indeed produce
most of the quiet solar EUV output at temperatures below 1 MK.
---------------------------------------------------------
Title: A numerical simulation of an asymmetric breakout model for CMEs
Authors: MacNeice, P.; Gao, J.; Antiochos, S.
2006AGUSMSH52A..04M Altcode:
We made a high resolution numerical simulation on an asymmetric
'magnetic breakout' model for CMEs. A fast CME was generated. The
dynamics of the system is controlled by two reconnection events,
i.e. the breakout reconnection at the outer x-point and the flare
reconnection at the newly formed inner x-point. The CME is triggered
by the former event and is accelerated by the latter event.
---------------------------------------------------------
Title: The Dynamics of Magnetic Reconnection in the Solar Atmosphere
Authors: Antiochos, Spiro
2006APS..APR.E3001A Altcode:
Magnetic reconnection is widely believed to play the central role in the
interaction between matter and magnetic field in the Sun's corona and,
therefore, to underlie most solar activity. Direct plasma heating due
to reconnection has been proposed as the process that produces both the
quasi-steady and the flare hot corona. Reconnection-driven flows have
been proposed as the explanation for transient dynamic phenomena ranging
from the smallest spicule to giant surges and sprays. Reconnection
has also been proposed as the origin of the electron beams in flares,
and numerous authors have argued that it is the mechanism responsible
for the origin of coronal mass ejections and prominence/filament
eruptions. In fact, it is difficult to find a solar phenomenon that
has not been blamed on magnetic reconnection! On the other hand,
there is surprisingly scarce direct evidence for reconnection in
coronal observations. In this talk, I will present both the latest
observations and 3D models for reconnection-driven dynamics in the
corona and attempt to reconcile the data with theory.
---------------------------------------------------------
Title: CME Theory and Models
Authors: Forbes, T. G.; Linker, J. A.; Chen, J.; Cid, C.; Kóta, J.;
Lee, M. A.; Mann, G.; Mikić, Z.; Potgieter, M. S.; Schmidt, J. M.;
Siscoe, G. L.; Vainio, R.; Antiochos, S. K.; Riley, P.
2006SSRv..123..251F Altcode: 2006SSRv..tmp...59F
This chapter provides an overview of current efforts in the theory and
modeling of CMEs. Five key areas are discussed: (1) CME initiation;
(2) CME evolution and propagation; (3) the structure of interplanetary
CMEs derived from flux rope modeling; (4) CME shock formation in the
inner corona; and (5) particle acceleration and transport at CME driven
shocks. In the section on CME initiation three contemporary models are
highlighted. Two of these focus on how energy stored in the coronal
magnetic field can be released violently to drive CMEs. The third
model assumes that CMEs can be directly driven by currents from below
the photosphere. CMEs evolve considerably as they expand from the
magnetically dominated lower corona into the advectively dominated
solar wind. The section on evolution and propagation presents two
approaches to the problem. One is primarily analytical and focuses on
the key physical processes involved. The other is primarily numerical
and illustrates the complexity of possible interactions between the
CME and the ambient medium. The section on flux rope fitting reviews
the accuracy and reliability of various methods. The section on shock
formation considers the effect of the rapid decrease in the magnetic
field and plasma density with height. Finally, in the section on
particle acceleration and transport, some recent developments in
the theory of diffusive particle acceleration at CME shocks are
discussed. These include efforts to combine self-consistently the
process of particle acceleration in the vicinity of the shock with
the subsequent escape and transport of particles to distant regions.
---------------------------------------------------------
Title: DC coronal heating and the nonlinear evolution of current
sheets
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
2006AdSpR..37.1342D Altcode:
Recent theoretical developments have re-awakened interest
in the role of electric current sheets in DC coronal heating
[Parker. Astrophys. J. 330, 474, 1988; Priest et al. Astrophys. J. 576,
522, 2002]. Dahlburg et al. [Dahlburg et al. Adv. Space Res. 32,
1029, 2003; Dahlburg et al. Astrophys. J. 622, 1191, 2005] reported
the existence of a "secondary instability" that could explain the
required "switch-on" effect required for adequate energy storage. This
ideal, three-dimensional instability also provided a straightforward
explanation for the subsequent fast release of energy, as the
rapid growth of the mode eventually results in a state of turbulent
magnetic reconnection. Earlier studies of the secondary instability
were limited to systems with relatively simple perturbations, viz.,
resistive stability eigenmodes. A current sheet in the Sun is likely
to be subject to much more complex perturbations involving a waves
of various wavelengths and amplitudes. We describe the evolution
of three-dimensional electric current sheets disturbed by random
3D perturbations. We find that the significant characteristics of
secondary instability are also observed in this case. The numerical
results are compared to solar observations.
---------------------------------------------------------
Title: The Origin of High-Speed Motions and Threads in Prominences
Authors: Karpen, J. T.; Antiochos, S. K.; Klimchuk, J. A.
2006ApJ...637..531K Altcode:
Prominences are among the most spectacular manifestations of both
quiescent and eruptive solar activity, yet the origins of their
magnetic-field and plasma structures remain poorly understood. We
have made steady progress toward a comprehensive model of prominence
formation and evolution with our sheared three-dimensional arcade
model for the magnetic field and our thermal nonequilibrium model for
the cool, dense material suspended in the corona. According to the
thermal nonequilibrium model, condensations form readily along long,
low-lying magnetic field lines when the heating is localized near
the chromosphere. In most cases this process yields a dynamic cycle
in which condensations repetitively form, stream along the field,
and ultimately disappear by falling onto the nearest footpoint. Two
key observed features were not adequately explained by our earlier
simulations of thermal nonequilibrium, however: the threadlike
(i.e., elongated) horizontal structure and high-speed motions of
many condensations. In this paper we discuss how simple modifications
to the radiative loss function, the heating scale, and the geometry
of our model largely eliminate these discrepancies. In particular,
condensations in nearly horizontal flux tubes are most likely to
develop both transient high-speed motions and elongated threads. These
results strengthen the case for thermal nonequilibrium as the origin
of prominence condensations and support low-twist models of prominence
magnetic structure.
---------------------------------------------------------
Title: CME Theory and Models
Authors: Forbes, T. G.; Linker, J. A.; Chen, J.; Cid, C.; Kóta, J.;
Lee, M. A.; Mann, G.; Mikić, Z.; Potgieter, M. S.; Schmidt, J. M.;
Siscoe, G. L.; Vainio, R.; Antiochos, S. K.; Riley, P.
2006cme..book..251F Altcode:
This chapter provides an overview of current efforts in the theory and
modeling of CMEs. Five key areas are discussed: (1) CME initiation;
(2) CME evolution and propagation; (3) the structure of interplanetary
CMEs derived from flux rope modeling; (4) CME shock formation in the
inner corona; and (5) particle acceleration and transport at CME driven
shocks. In the section on CME initiation three contemporary models are
highlighted. Two of these focus on how energy stored in the coronal
magnetic field can be released violently to drive CMEs. The third
model assumes that CMEs can be directly driven by currents from below
the photosphere. CMEs evolve considerably as they expand from the
magnetically dominated lower corona into the advectively dominated
solar wind. The section on evolution and propagation presents two
approaches to the problem. One is primarily analytical and focuses on
the key physical processes involved. The other is primarily numerical
and illustrates the complexity of possible interactions between the
CME and the ambient medium. The section on flux rope fitting reviews
the accuracy and reliability of various methods. The section on shock
formation considers the effect of the rapid decrease in the magnetic
field and plasma density with height. Finally, in the section on
particle acceleration and transport, some recent developments in
the theory of diffusive particle acceleration at CME shocks are
discussed. These include efforts to combine self-consistently the
process of particle acceleration in the vicinity of the shock with
the subsequent escape and transport of particles to distant regions.
---------------------------------------------------------
Title: A Study of the Global Heliospheric Magnetic Flux Using In-Situ
Data and the SAIC MHD Model
Authors: Lepri, S. T.; Antiochos, S. K.; Riley, P.
2005AGUFMSH13B..08L Altcode:
There has been considerable controversy in recent years over the
slow evolution of the Sun's open field, which extends out to become
the heliospheric magnetic field. In the standard solar model (e.g.,
Wang and Sheeley [1993]) the open flux can increase or decrease in
response to the emergence or cancellation of magnetic flux at the
photosphere in relation to coronal holes. In the Fisk et al. [1999a,
1999b] model, on the other hand, the open flux is conserved and evolves
primarily via interchange reconnection with closed fields. This model
predicts no long-term variations in the amount of heliospheric flux. We
compare and test these theories by measuring the behavior of the
open magnetic flux in the global heliospheric magnetic field. Using
multi-point measurements from the VHM instrument on the Ulysses
spacecraft and from the MAG instrument on the ACE spacecraft, we
analyze in-situ radial magnetic field data and compare the behavior
to that predicted by the updated SAIC MHD model. During solar maximum,
ICMEs significantly disturb the heliospheric magnetic field, making our
comparisons difficult. We examine the radial component of the magnetic
field in data from 1991 though the present in order to determine the
variability of the open flux and hence the evolution of corona holes.
---------------------------------------------------------
Title: Magnetic Reconnection Models of Prominence Formation
Authors: Welsch, B. T.; DeVore, C. R.; Antiochos, S. K.
2005ApJ...634.1395W Altcode:
To investigate the hypothesis that prominences form by magnetic
reconnection between initially distinct flux systems in the solar
corona, we simulate coronal magnetic field evolution when two flux
systems are driven together by boundary motions. In particular, we
focus on configurations similar to those in the quiescent prominence
formation model of Martens & Zwaan. We find that reconnection
proceeds very weakly, if at all, in configurations driven with global
shear flows (i.e., differential rotation); reconnection proceeds much
more efficiently in similar configurations that are driven to collide
directly, with converging motions along the neutral line that lead to
flux cancellation; reconnected fields from this process can exhibit
sheared, dipped field lines along the neutral line, consistent with
prominence observations. Our field configurations do not possess the
“breakout” topology, and eruptions are not observed, even though a
substantial amount of flux is canceled in some runs.
---------------------------------------------------------
Title: Prominence Formation by Thermal Nonequilibrium in the
Sheared-Arcade Model
Authors: Karpen, J. T.; Tanner, S. E. M.; Antiochos, S. K.; DeVore,
C. R.
2005ApJ...635.1319K Altcode:
The existence of solar prominences-cool, dense, filamented plasma
suspended in the corona above magnetic neutral lines-has long been an
outstanding problem in solar physics. In earlier numerical studies
we identified a mechanism, thermal nonequilibrium, by which cool
condensations can form in long coronal flux tubes heated locally above
their footpoints. To understand the physics of this process, we began by
modeling idealized symmetric flux tubes with uniform cross-sectional
area and a simplified radiative-loss function. The present work
demonstrates that condensations also form under more realistic
conditions, in a typical flux tube taken from our three-dimensional MHD
simulation of prominence magnetic structure produced by the sheared
arcade mechanism. We compare these results with simulations of an
otherwise identical flux tube with uniform cross-sectional area,
to determine the influence of the overall three-dimensional magnetic
configuration on the condensation process. We also show that updating
the optically thin radiative loss function yields more rapidly varying,
dynamic behavior in better agreement with the latest prominence
observations than our earlier studies. These developments bring us
substantially closer to a fully self-consistent, three-dimensional
model of both magnetic field and plasma in prominences.
---------------------------------------------------------
Title: Observational Implications of 3D Breakout
Authors: Lynch, B. J.; Antiochos, S. K.; DeVore, C. R.; Zurbuchen,
T. H.
2005AGUFMSH12A..02L Altcode:
We present the latest numerical MHD simulations of the breakout model
for coronal mass ejections in 3-dimensions. We will emphasize features
of the model and simulation results that are uniquely associated with
"magnetic breakout", and briefly review the more generic features that
are common to almost all CME initiation models. Specifically, we will
focus on the multi-polar topology and the breakout reconnection at the
distorted null-point high in the corona, as the process responsible for
the eruption and will attempt to draw some conclusions about possible
observational signatures. We will also quantify the energetics of the
eruption and show the restraining overlying field is critical in order
to build up sufficient energy for a catastrophic, fast eruption of
low-lying sheared flux. This work was supported by NASA and ONR. BJL
acknowledges NASA GSRP NGT5-05453.
---------------------------------------------------------
Title: The Topology of Magnetic Reconnection in the Sun's Corona
Authors: Antiochos, S. K.
2005AGUFMSM12A..07A Altcode:
It has long been recognized that magnetic topology plays the critical
role in reconnection at the magnetopause. The essential requirement
for reconnection is that the system has a multi-flux topology, and the
location of the reconnection is at the boundaries between the different
flux systems, specifically separator lines and null points. On the other
hand, it is commonly believed that reconnection driven by photospheric
motions can occur more-or-less anywhere in the Sun's corona. We present
both theoretical and 3D numerical simulation results arguing that,
in fact, the magnetosphere and corona are physically similar in that
topology strongly restricts how and where coronal reconnection can
occur. We also discuss important differences between the reconnection
in the corona and magnetosphere. This work was supported, in part,
by ONR and NASA.
---------------------------------------------------------
Title: A Mechanism for the Emergence of Magnetic U-Loops and Flux
Cancellation on the Sun
Authors: Magara, T.; Antiochos, S. K.; DeVore, C. R.; Linton, M. G.
2005ESASP.596E..74M Altcode: 2005ccmf.confE..74M
No abstract at ADS
---------------------------------------------------------
Title: The Breakout Model for CME Initiation in 3-Dimensions
Authors: Lynch, B. J.; Antiochos, S. K.; de Vore, C. R.; Zurbuchen,
T. H.
2005ESASP.592..297L Altcode: 2005soho...16E..44L; 2005ESASP.592E..44L
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Free Energies of Breakout Coronal Mass Ejections
Authors: DeVore, C. Richard; Antiochos, Spiro K.
2005ApJ...628.1031D Altcode:
A critical issue in understanding and eventually predicting coronal
mass ejections (CMEs) is determining the magnetic free energy that
can drive the explosive eruption. We present calculations of this
free energy for the breakout CME model, which postulates that the
preeruption magnetic topology is a multipolar field with a null point
in the corona. Using analytical and numerical methods, we determine the
free energies for two broad families of photospheric flux distributions,
parameterized by the radius of the coronal null and the degree to which
flux is concentrated near the poles and equator. The available CME
energy attains a broad maximum for distributions whose potential null
resides between about 1.25 and 1.75 solar radii, and falls off toward
zero as the null approaches the surface or moves out to infinity. These
results may explain the wide range of energies observed for CMEs and
their associated flares. We find that concentrating the surface flux to
narrower latitude bands near the poles and equator, on the other hand,
has little effect on the available energy. Our mathematical approach
currently is restricted to spherically axisymmetric systems. Its
generalization to fully three-dimensional fields might provide the
foundation of a first-principles forecasting technique for solar
eruptions.
---------------------------------------------------------
Title: Solar Prominence Interactions
Authors: DeVore, C. Richard; Antiochos, Spiro K.; Aulanier, Guillaume
2005ApJ...629.1122D Altcode:
We report numerical simulations of the formation, interaction, and
magnetic reconnection between pairs of solar prominences within the
sheared-arcade model. Our experiments consider the four possible basic
combinations of chiralities (identical or opposite) and axial magnetic
fields (aligned or opposed) between the participating prominences. When
the topology of the global flux system comprising the prominences
and arcades is bipolar, so that a single polarity inversion line is
shared by the two structures, then identical chiralities necessarily
imply aligned axial fields, while opposite chiralities imply opposed
axial fields. In the former case, external magnetic reconnections
forming field lines linking the two prominences occur; in the latter,
such reconnections are disfavored, and no linkage takes place. These
results concur with empirical rules for prominence interactions. When
the topology instead is quadrupolar, so that a second polarity
inversion line crossing the first lies between the prominences,
then the converse relation holds between chirality and axial-field
alignment. External reconnections forming linking field lines now occur
between prominences with opposite chiralities; they also occur, but
result only in footpoint exchanges, between prominences with identical
chiralities. These findings conflict with the accepted empirical rules
but may not have been tested in observations to date. All of our model
prominences, especially those that undergo linking reconnections,
contain substantial magnetic shear and twist. Nevertheless, none
exhibits any sign of onset of instability or loss of equilibrium that
might culminate in an eruption.
---------------------------------------------------------
Title: Solar cycle-dependent helicity transport by magnetic clouds
Authors: Lynch, B. J.; Gruesbeck, J. R.; Zurbuchen, T. H.; Antiochos,
S. K.
2005JGRA..110.8107L Altcode: 2005JGRA..11008107L
Magnetic clouds observed with the Wind and ACE spacecraft are fit with
the static, linear force-free cylinder model to obtain estimates of
the chirality, fluxes, and magnetic helicity of each event. The fastest
magnetic clouds (MCs) are shown to carry the most flux and helicity. We
calculate the net cumulative helicity which measures the difference
in right- and left-handed helicity contained in MCs over time. The
net cumulative helicity does not average to zero; rather, a strong
left-handed helicity bias develops over the solar cycle, dominated
by the largest events of cycle 23: Bastille Day 2000 and 28 October
2003. The majority of MCs ("slow" events, <V<SUB>r</SUB>> <
500 km/s) have a net cumulative helicity profile that appears to be
modulated by the solar activity cycle. This is far less evident for
"fast" MC events (<V<SUB>r</SUB>> ≥ 500 km/s), which were
disproportionately left-handed over our data set. A brief discussion
about the various solar sources of CME helicity and their implication
for dynamo processes is included.
---------------------------------------------------------
Title: A Mechanism for the Flux Cancellation Caused by Emerging
Magnetic U-Loops in the Sun
Authors: Magara, T.; Antiochos, S. K.; DeVore, C. R.; Linton, M. G.
2005AGUSMSH51C..01M Altcode:
We used three-dimensional MHD simulation to study the evolution of
U-shaped magnetic field lines (U-loops) in a flux cancellation region
on the Sun. Emergence of U-loops is thought to be a process for causing
flux cancellation at the solar surface, although the physical mechanism
for this process is not obvious because the mass tends to accumulate
at the dipped part of U-loops thereby reducing the buoyancy. Our flux
emergence simulation reveals that a temporary siphon flow plays a key
role in enhancing the buoyancy of the dipped part of U-loops and helps
it emerge into the solar atmosphere against the gravity. By applying
a model of emerging U-loops to an observed flux cancellation region,
we study a possible configuration of magnetic field lines related to
flux cancellation.
---------------------------------------------------------
Title: 3D Numerical Simulations of the Breakout Model
Authors: Choe, G. S.; Cheng, C. Z.; Lee, J.; Lynch, B. J.; Antiochos,
S. K.; DeVore, C. R.; Zurbuchen, T. H.
2005AGUSMSP43C..02C Altcode:
We present the continuing progress of the numerical simulations of
the breakout model for coronal mass ejection initiation. To validate
the 3D spherical ARMS code we have run the 2.5D breakout problem and
compare the eruption to the published 2D results. The ARMS 2.5D CME
also forms a large magnetic island ahead of the erupting plasmoid due
to the code's excellent maintenance of equatorial symmetry. Progress
on the fully 3D breakout problem is also discussed. To build up enough
magnetic free energy for an eruption the active region field must
be strong with a steep gradient near the polarity inversion line and
the shear must be highly concentrated there. This requires adaptive
griding techniques. In the current simulation, the active region to
background field ratio is 20-to-1 and the neutral line is long compared
to the active region width. We present the evolution of this topology
under Br-conserving shearing flow and discuss implications for a 3D
eruption. This work is supported by NASA and ONR. BJL is supported by
NASA GSRP grant NGT5-50453.
---------------------------------------------------------
Title: Magnetic Flux Tube Reconnection: Tunneling Versus Slingshot
Authors: Linton, M. G.; Antiochos, S. K.
2005ApJ...625..506L Altcode: 2005astro.ph..1473L
The discrete nature of the solar magnetic field as it emerges
into the corona through the photosphere indicates that it exists
as isolated flux tubes in the convection zone and will remain as
discrete flux tubes in the corona until it collides and reconnects
with other coronal fields. Collisions of these flux tubes will in
general be three-dimensional and will often lead to reconnection,
both rearranging the magnetic field topology in fundamental ways
and releasing magnetic energy. With the goal of better understanding
these dynamics, we carry out a set of numerical experiments exploring
fundamental characteristics of three-dimensional magnetic flux tube
reconnection. We first show that reconnecting flux tubes at opposite
extremes of twist behave very differently: in some configurations,
low twist tubes slingshot while high twist tubes tunnel. We then
discuss a theory explaining these differences: by assuming helicity
conservation during the reconnection one can show that at high twist,
tunneled tubes reach a lower magnetic energy state than slingshot tubes,
whereas at low twist the opposite holds. We test three predictions
made by this theory. (1) We find that the level of twist at which the
transition from slingshot to tunnel occurs is about 2-3 times higher
than predicted on the basis of energetics and helicity conservation
alone, probably because the dynamics of the reconnection play a large
role as well. (2) We find that the tunnel occurs at all flux tube
collision angles predicted by the theory. (3) We find that the amount
of magnetic energy a slingshot or a tunnel reconnection releases agrees
reasonably well with the theory, although at the high resistivities we
have to use for numerical stability, a significant amount of magnetic
energy is lost to diffusion, independent of reconnection.
---------------------------------------------------------
Title: The Role of Magnetic Helicity in Coronal Mass Ejections
Authors: Phillips, A. D.; MacNeice, P. J.; Antiochos, S. K.
2005ApJ...624L.129P Altcode:
We investigate the factors responsible for initiating coronal mass
ejections (CMEs), specifically, the role of magnetic helicity. Using
numerical simulations of the breakout model for CMEs, we show that
eruption occurs at a fixed magnitude of free energy in the corona,
independent of the value of helicity. Almost identical eruptions are
obtained for both large and zero-helicity cases. Furthermore, the
eruption can actually lead to an increase in the helicity remaining
in the corona. These results argue strongly against recent models that
postulate a critical helicity buildup and shedding as the determining
factors for CME initiation.
---------------------------------------------------------
Title: The Origin of High-Speed Motions and Threads in Solar
Prominences
Authors: Karpen, J.; Antiochos, S.; Klimchuk, J.
2005AGUSMSP21B..02K Altcode:
Prominences are among the most spectacular manifestations of both
quiescent and eruptive solar activity, yet the origins of their
magnetic-field and plasma structures remain poorly understood. We
have made steady progress toward a comprehensive model of prominence
formation and evolution with our sheared 3D arcade model for the
magnetic field and our thermal nonequilibrium model for the cool,
dense material suspended in the corona. According to the thermal
nonequilibrium model, condensations form readily along long,
low-lying magnetic field lines if the heating is localized near the
chromosphere. In most cases this process yields a dynamic cycle in
which condensations repetitively form, stream along the field line,
and ultimately disappear by falling onto the nearest footpoint. Two
key observed features were not adequately explained by our earlier
simulations of thermal nonequilibrium, however: the thread-like
(i.e., elongated) horizontal structure and high-speed motions of many
condensations. Here we discuss how simple modifications to our model
largely eliminate these discrepancies, strengthening the case for
thermal nonequilibrium as the origin of prominence condensations and
for low-twist models of prominence magnetic structure. This work was
supported by NASA and ONR.
---------------------------------------------------------
Title: Coronal Mass Ejections: the Most Powerful Drivers of the
Sun-Earth System
Authors: Antiochos, S. K.
2005AAS...206.2001A Altcode: 2005BAAS...37..461A
A large Coronal Mass Ejection (CME) can consist of billions of tonnes
of matter, along with entangled magnetic field, erupting from the
Sun at speeds well over 1,000 km/s. These giant disruptions of the
solar atmosphere drive the most destructive space weather at Earth and
throughout the solar system. Furthermore, CMEs are the most dramatic
example of how slowly-evolving processes on the Sun can conspire to
produce explosive activity. Understanding their origin has long been
a central objective for astrophysical research. This talk will present
some of the latest observations and theories for CMEs and discuss the
outstanding challenges to modeling and predicting their initiation. <P
/>This work was supported in part by NASA and ONR.
---------------------------------------------------------
Title: New Frontiers/Hale Prize Lecture: Coronal Mass Ejections,
the Most Powerful Drivers of the Sun-Earth System
Authors: Antiochos, S. K.
2005AGUSM.U15A..01A Altcode:
A large Coronal Mass Ejection (CME) can consist of billions of tonnes
of matter, along with entangled magnetic field, erupting from the
Sun at speeds well over 1,000 km/s. These giant disruptions of the
solar atmosphere drive the most destructive space weather at Earth and
throughout the solar system. Furthermore, CMEs are the most dramatic
example of how slowly-evolving processes on the Sun can conspire to
produce explosive activity. Understanding their origin has long been
a central objective for space physics research. This talk will present
some of the latest observations and theories for CMEs and discuss the
outstanding challenges to modeling and predicting their initiation. This
work was supported in part by NASA and ONR.
---------------------------------------------------------
Title: An Explanation for the “Switch-On” Nature of Magnetic Energy
Release and Its Application to Coronal Heating
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
2005ApJ...622.1191D Altcode:
A large class of coronal heating theories postulate that the random
mixing of magnetic footpoints by photospheric motions leads to the
formation of current sheets in the corona and, consequently, to energy
release there via magnetic reconnection. Parker pointed out that in
order for this process to supply the observed energy flux into the
corona, the stress in the coronal magnetic field must have a fairly
specific value at the time that the energy is released. In particular,
he argued that the misalignment between reconnecting flux tubes must
be roughly 30° in order to match the observed heating. No physical
origin for this number was given, however. In this paper we propose
that secondary instability is the mechanism that “switches on” the
energy release when the misalignment angle in the corona reaches the
correct value. We calculate both the three-dimensional linear and fully
nonlinear development of the instability in current sheets corresponding
to various misalignment angles. We find that no secondary instability
occurs for angles less than about 45°, but for larger angles the
instability grows at a rapid rate, and there is an explosive release
of energy. We compare our results with the observed properties of the
corona and discuss the implications for future observations.
---------------------------------------------------------
Title: Prominence Formation Processes
Authors: Welsch, B. T.; DeVore, C. R.; Antiochos, S. K.
2005HiA....13..127W Altcode:
Martens and Zwaan (ApJ v. 558 872) have proposed a prominence/ filament
formation model in which differential rotation drives reconnection
between two initially unconnected active regions to form helical field
lines that support mass and are held down by overlying field. Using
an MHD solver with adaptive refinement we simulated this process by
imposing a shear flow meant to mimic differential rotation on two
bipolar flux distributions meant to mimic distinct active regions. In
some runs the flux systems are initially potential while in others
they have been twisted by footpoint rotation to inject helicity prior
to imposing the shear flow. The resulting structures are studied to
understand the role of helicity in the formation of prominence-like
structures.
---------------------------------------------------------
Title: 3D Breakout: Preliminary Results
Authors: Lynch, B. J.; Antiochos, S. K.; DeVore, C. R.; Zurbuchen,
T. H.
2004AGUFMSH21B0400L Altcode:
We present preliminary results of the breakout model for solar
coronal mass ejections in a global-scale 3D topology. Starting with a
background dipole field, we use a series of point dipole sources create
a latitudinally extended ( ∼ 100<SUP>o</SUP>) delta-spot active region
configuration with a null point high in the corona. This is the natural
3D extension of our very successful 2.5D case. Magnetic free energy is
added to the initial field configuration via two compressible vortex
flows that preserve B<SUB>r</SUB> at the surface and concentrate the
shear near the central neutral line of the AR flux system. Reconnection
at the null point removes the restraining overlying flux allowing
rapid, unstable expansion of the innermost sheared field. We examine
the evolution of the system and discuss its implications for a fully 3D
breakout eruption. We also discuss observational tests of the breakout
model that can be performed with the unique viewing capabilities of
STEREO. This work is supported in part by NASA and ONR.
---------------------------------------------------------
Title: Simulating The Breakout Model In An Asymmetric Configuration
Authors: Gao, J.; MacNeice, P.; Antiochos, S.
2004AGUFMSH13A1151G Altcode:
In a recent paper, MacNeice et al (Ap.J. v614) presented the first
complete MHD simulation of the `Breakout' model for CME initation. They
used an idealized magnetic topology in 2.5D, with an initial symmetry
plane at the equator. We have modified this configuration to test
the breakout model in an initial topology without this symmetry. We
examine the differences in the two simulations, experiment with
applying the driving shear to the different flux systems in this
complex configuration, and study the implications for the evolution
of magnetic helicity.
---------------------------------------------------------
Title: The Effects of Topology on Magnetic Reconnection
Authors: Antiochos, S. K.; Devore, R.; Karpen, J. T.
2004AGUFMSM43B..06A Altcode:
Magnetic reconnection is widely believed to be the dominant process by
which plasma and magnetic field exchange energy in the cosmos. Although
certain aspects of reconnection are universal, the nature of the
process depends strongly on the particular topology of the reconnecting
system. In the Earth's magnetosphere, the topology is fixed -- a
four flux system with a pair of nulls and separators. In the Sun's
corona, on the other hand, the topology can vary greatly depending on
the complexity of the active region. We argue that the usual coronal
topology is a two-flux system with an isolated 3D null, but four flux
systems that are topologically equivalent to the magnetosphere are
possible. We contrast and compare the dynamics of reconnection for
these two topologies. We present both theoretical models and fully
3D simulations using ARMS, the NRL adaptively-refined MHD solver. The
implications of the results for observations will be discussed. This
work was supported in part by NASA and ONR.
---------------------------------------------------------
Title: Variability of the Heliospheric Magnetic Flux
Authors: Lepri, S. T.; Antiochos, S. K.; Zurbuchen, T. H.
2004AGUFMSH31A1164L Altcode:
There has been considerable controversy in recent years over the
slow evolution of the Sun's open field, which extends out to become
the heliospheric magnetic field. In the standard solar model [e.g.,
Wang and Sheeley, 1993] the open flux can increase or decrease in
response to the emergence or cancellation of magnetic flux at the
photosphere. In particular, the appearance of a new active region can
lead to the formation of a new coronal hole, or the growth of an old
one, which should be detectable as a long-term increase in the radial
component of the magnetic field throughout the heliosphere. In the
Fisk et al. [1999a, 1999b] model, on the other hand, the open flux
is conserved and evolves primarily via interchange reconnection with
closed field. This model predicts no long-term variations in the amount
of heliospheric flux. To compare and test these competing theories,
we measure the behavior of the open magnetic flux in the global
heliospheric magnetic field. Using multi-point measurements from both
the MAG instrument on the Advanced Composition Explorer (ACE) and the
VHM instrument on the Ulysses spacecraft, we analyze in-situ magnetic
field data. In particular, we examine the radial component of the
magnetic field in data from 1998 through 2004 in order to determine
the variablity of the open flux and its relation to variations in the
area of coronal holes. We describe the implications of our results for
the two theories. This work has been supported in part by NSF and NASA.
---------------------------------------------------------
Title: Observable Properties of the Breakout Model for Coronal
Mass Ejections
Authors: Lynch, B. J.; Antiochos, S. K.; MacNeice, P. J.; Zurbuchen,
T. H.; Fisk, L. A.
2004ApJ...617..589L Altcode:
We compare the “magnetic breakout” model for coronal mass ejections
(CMEs) with observed general properties of CMEs by analyzing in detail
recent high-resolution MHD simulations of a complete breakout CME. The
model produces an eruption with a three-part plasma density structure
that shows a bright circular rim outlining a dark central cavity in
synthetic coronagraphic images of total brightness. The model also
yields height-time profiles similar to most three-part CMEs, but
the eruption speed by 2.5 R<SUB>solar</SUB> is of order the Alfvén
speed, indicative of a fast CME. We show that the evolution of the
posteruptive flare loop and chromospheric ribbons determined from
the model are in agreement with observations of long-duration flares,
and we propose an explanation for the long-standing observation that
flares have an impulsive and gradual phase. A helical magnetic flux
rope is generated during eruption and is consistent with a large class
of interplanetary CME observations. The magnetic fields in this flux
rope are well approximated by the Lundquist solution when the ejecta
are at 15 R<SUB>solar</SUB> and beyond. Furthermore, the interior
density structure of the magnetic flux rope appears to have some
of the basic features of an “average” magnetic cloud profile at 1
AU. Future simulation improvements and more stringent observational
tests are discussed.
---------------------------------------------------------
Title: A Numerical Study of the Breakout Model for Coronal Mass
Ejection Initiation
Authors: MacNeice, P.; Antiochos, S. K.; Phillips, A.; Spicer, D. S.;
DeVore, C. R.; Olson, K.
2004ApJ...614.1028M Altcode:
A leading theory for the initiation of coronal mass ejections (CMEs)
is the breakout model, in which magnetic reconnection above a filament
channel is responsible for disrupting the coronal magnetic field. We
present the first simulations of the complete breakout process
including the initiation, the plasmoid formation and ejection, and
the eventual relaxation of the coronal field to a more potential
state. These simulations were performed using a new numerical code
that solves the numerical cavitation problems that prevented previous
simulations from calculating a complete ejection. Furthermore, the
position of the outer boundary in the new simulations is increased
out to 30 R<SUB>solar</SUB>, which enables determination of the full
structure and dynamics of the ejected plasmoid. Our results show that
the ejection occurs at a speed on the order of the coronal Alfvén speed
and hence that the breakout model can produce fast CMEs. Another key
result is that the ejection speed is not sensitive to the refinement
level of the grid used in the calculations, which implies that, at least
for the numerical resistivity of these simulations, the speed is not
sensitive to the Lundquist number. We also calculate, in detail, the
helicity of the system and show that the helicity is well conserved
during the breakout process. Most of the helicity is ejected from
the Sun with the escaping plasmoid, but a significant fraction (of
order 10%) remains in the corona. The implications of these results
for observation and prediction of CMEs and eruptive flares is discussed.
---------------------------------------------------------
Title: A Model for Bright Extreme-Ultraviolet Knots in Solar Flare
Loops
Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A.
2004ApJ...614.1022P Altcode:
EUV observations often indicate the presence of bright knots in flare
loops. The temperature of the knot plasma is of the order of 1 MK,
and the knots themselves are usually localized somewhere near the loop
tops. We propose a model in which the formation of EUV knots is due
to the spatial structure of the nonflare active region heating. We
present the results of a series of one-dimensional hydrodynamic,
flare-loop simulations, which include both an impulsive flare heating
and a background, active region heating. The simulations demonstrate
that the formation of the observed knots depends critically on
the spatial distribution of the background heating during the decay
phase. In particular, the heating must be localized far from the loop
apex and have a magnitude comparable to the local radiative losses of
the cooling loop. Our results, therefore, provide strong constraints
on both coronal heating and postflare conditions.
---------------------------------------------------------
Title: Thermal and Nonthermal Emission in Solar Flares
Authors: Warren, Harry P.; Antiochos, Spiro K.
2004ApJ...611L..49W Altcode:
The observation that in many flares there is a linear correlation
between the peak soft X-ray emission and the time-integrated nonthermal
emission-the Neupert effect-indicates a strong link between particle
acceleration and chromospheric evaporation. In this Letter we consider
the hydrodynamic response of impulsively heated flare loops. We
find that the peak soft X-ray flux should scale approximately as
E<SUP>1.75</SUP>/V<SUP>0.75</SUP>L<SUP>0.25</SUP>, where E is the total
input energy, V is the flare volume, and L is the loop length. This
scaling is not consistent with the linear relationship implied by the
Neupert effect unless there are additional correlations between the
input energy and the other parameters of the flare.
---------------------------------------------------------
Title: Bright EUV Knots in Solar Flare Loops: Constraints on Coronal
Heating
Authors: Patsourakos, S.; Antiochos, S.; Klimchuk, J.
2004AAS...204.8705P Altcode: 2004BAAS...36Q.819P
EUV observations often indicate the presence of bright knots in flare
loops. The temperature of the knot plasma is of order 1MK, and the
knots themselves are usually localized somewhere near the loop tops. We
propose a model in which the formation of EUV knots is due to the
spatial structure of the non-flare active region heating. We present
the results of a series of 1D hydrodynamic, flare-loop simulations,
which include both an impulsive flare heating and a background, active
region heating. The simulations demonstrate that the formation of the
observed knots depends critically on the spatial distribution of the
background heating during the decay phase. In particular, the heating
must: (1) be localized, (2) be situated far from the loop apex and (3)
have a magnitude comparable with the local radiative losses of the
cooling loop. Our results, therefore, provide strong constraints on
both coronal heating and post-flare conditions. <P />Research supported
by NASA and ONR.
---------------------------------------------------------
Title: Flux Collision Models of Prominence Formation, or Breaking
Up is Hard to Do
Authors: Welsch, B. T.; DeVore, C. R.; Antiochos, S. K.
2004AAS...204.5505W Altcode: 2004BAAS...36..761W
To investigate the hypothesis that the prominences form by magnetic
reconnection between initially distinct flux systems above the solar
photosphere, we employ the ARMS code, a 3D, flux-corrected transport
MHD code with adaptive mesh refinement, to simulate magnetic field
evolution when two flux systems are driven to collide by photospheric
boundary motions. In particular, we focus on driving configurations
similar to the prominence model of Martens and Zwaan (2001). <P />We
find that: 1) reconnection proceeds only weakly, if at all, in typical
active region configurations driven with differential-rotation-like
shear, which leads to glancing collisions; 2) reconnection proceeds
efficiently in configurations that are driven to collide directly,
with converging motions along the neutral line; and 3) reconnected
fields from this process can exhibit sheared, dipped field lines along
the neutral line, consistent with prominence observations. <P />As
our field configurations do not posses the “breakout” topology,
eruptions are not observed. <P />This work was supported by ONR,
NASA's SEC Theory program, and by a grant of computer time from the
DOD High Performance Computing Modernization Program at the ERDC MSRC.
---------------------------------------------------------
Title: The Sheared-Arcade Model for Solar Prominences
Authors: DeVore, C. R.; Antiochos, S. K.
2004AAS...204.5504D Altcode: 2004BAAS...36..761D
The structure and stability of the magnetic field play critical
roles in the formation, evolution, and eventual eruption of solar
prominences. We have shown previously that a three-dimensional coronal
arcade with strong localized shear exhibits several characteristic
properties measured or inferred from prominence observations. These
include alignment with the polarity inversion line of the photospheric
field, inverse magnetic polarity in the body of the prominence, the
necessary restraining overlying arcade field, formation of helical
fields at high shear, and linkage of formerly distinct prominences
where they come into contact and their magnetic fields reconnect. Our
studies also suggested that the resulting structures are very stable,
showing no tendency to erupt violently as solar prominences frequently
do. <P />We now are extending these investigations by including in the
model the corona's expanding spherical geometry and its gravitationally
stratified mass density and thermal pressure. Our expectation is that
topologically bipolar prominence structures will be found to rise
to greater heights than in our previous cartesian studies, but still
will be unable to attain the free energy needed to open the field. A
multipolar structure in a breakout configuration, on the other hand,
in principle could approach its free-energy threshold and then
erupt once the breakout reconnection commences. This outcome would
be qualitatively different from our prior results. Progress on these
fronts and the implications for our understanding of prominences will
be reported. <P />This research was supported by NASA and ONR.
---------------------------------------------------------
Title: The Topology Of Solar Eruptions
Authors: Antiochos, S. K.; DeVore, C. R.; MacNeice, P. J.
2004AAS...204.2706A Altcode: 2004BAAS...36..694A
Understanding the physical mechanisms responsible for coronal mass
ejections (CME)/eruptive flares is essential for advancing both our
knowledge of major solar activity and our capability for forecasting
space weather. The topology of the coronal magnetic field plays the
essential role in most theories for these major solar eruptions. We
discuss the role of 3D effects in the `breakout model' for CMEs. We
argue that the 3D topology of the magnetic field is critical for
understanding the magnetic reconnection that leads to eruption and
understanding the subsequent development of the ejected plasmoid. Both
theoretical and numerical results on breakout simulations will be
presented. From these results, we derive predictions that can test the
validity of the model with the observations expected from the upcoming
STEREO and SOLAR-B missions. <P />This work was supported in part by
NASA and ONR.
---------------------------------------------------------
Title: Magnetic Cloud Net Cumulative Helicity During Solar Cycle 23
Authors: Lynch, B. J.; Gruesbeck, J. R.; Zurbuchen, T. H.; Antiochos,
S. K.
2004AAS...204.3803L Altcode: 2004BAAS...36..712L
Nine years of magnetic clouds (MCs) from the WIND and ACE spacecraft
(1995-2003) are analyzed using the static, linear, force-free cylinder
model. The net cumulative helicity is defined as the difference
between the running totals of right- and left-handed MC helicities
calculated from the model. This net helicity is a time-dependent
quantity that appears to be approximately sinusoidal with a solar
cycle like period. There is a right-handed bias in the helicity
carried by MCs during solar minimum, and much stronger left-handed
bias in the helicity transported during the rising phase and solar
maximum, even though there are not drastic differences in the overall
number of right- and left-handed clouds. The Bastille Day 2000 MC
event is the largest contributor to this left-handed transition. <P
/>Monte Carlo simulations of random magnetic cloud sequences from
the observed helicity distribution show that the magnitude of the
observed left-handed net helicity bias during solar maximum is only
expected 2-4 % of the time if right- and left-handed events are equally
likely. Possible sources of CME helicity are discussed. <P />This work
is supported by NASA, ONR, and NSF. BJL is supported by a NASA GSRP
fellowship NGT5-50453.
---------------------------------------------------------
Title: Prominence formation through thermal nonequilibrium in a
sheared arcade
Authors: Karpen, J. T.; Tanner, S. E. M.; Antiochos, S. K.; DeVore,
C. R.
2004AAS...204.5502K Altcode: 2004BAAS...36R.760K
We have shown, over the past few years, that both static and dynamic
prominence condensations can be formed through steady but unequal
localized heating in long coronal loops (Antiochos et al. 1999,
2000; Karpen et al. 2001, 2003). Theoretical analyses and numerical
simulations with ARGOS, our 1D hydrodynamic code with adaptive mesh
refinement, have revealed the behavior of this thermal nonequilibrium
mechanism under a wide range of solar conditions. Previously we
identified several key parameters governing the existence and
characteristics of the condensations: the ratio of loop length to
heating scale, the loop apex height, the heating imbalance, and (for
dipped fieldlines only) the dip slopes. These earlier calculations
assumed a constant cross-sectional area throughout the flux tube,
but on the Sun we expect the areas to be highly nonuniform. <P />To
test this condensation process under more realistic conditions,
we used our sheared 3D arcade model of the prominence magnetic field
(DeVore & Antiochos 2000) to define the geometry of the model flux
tube in a set of calculations with ARGOS. We selected representative
field lines capable of supporting condensations from the DeVore &
Antiochos 3D MHD simulation, measured the flux tube area at intervals
along these lines, and derived 5th order polynomial fits to the height
and area that were easily recomputed upon regridding. For comparison,
constant cross-section “control" loops also were set up with the
same height variations. These field lines were subjected to localized
heating near the footpoints, as before, and subsequent developments
were monitored. We have explored the effects of uniform vs. nonuniform
area, changing the heating imbalance, and altering the radiative loss
function. Results from this study will be compared with our previous
work and with prominence observations. <P />This work was supported
by NASA and ONR.
---------------------------------------------------------
Title: DC coronal heating and the nonlinear evolution of current
sheets
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
2004cosp...35.2721D Altcode: 2004cosp.meet.2721D
Recent theoretical developments have re-awakened interest in the role
of electric current sheets in DC coronal heating (Parker 1988; Priest
et al. 2002). Dahlburg et al. (2003; 2004) reported the existence
of a “secondary instability” that could explain the required
“switch-on” effect required for adequate energy storage. This
ideal, three-dimensional instability also provided a straightforward
explanation for the subsequent fast release of energy, as the
rapid growth of the mode eventually results in a state of turbulent
magnetic reconnection. Earlier studies of the secondary instability
were limited to systems with relatively simple perturbations, viz.,
resistive stability eigenmodes. A current sheet in the Sun is likely
to be subject to much more complex perturbations involving a waves
of various wavelengths and amplitudes. We describe the evolution
of three-dimensional electric current sheets disturbed by random 3D
perturbations. We find that the significant characteristics of secondary
instability are also observed in this case. The relative importance
of subharmonic interactions, i.e., coalescence instability, will also
be discussed. R. B. Dahlburg, J. A. Klimchuk and S. K. Antiochos
Adv. Space Phys. 32, 1029 (2003). R. B. Dahlburg, J. A. Klimchuk
and S. K. Antiochos Astrophys. J.. submitted, (2004). E. N. Parker
Astrophys. J. 330, 474 (1988). E. R. Priest, J. F. Heyvaerts and
A. M. TItle, Astrophys. J. 576, 533 (2002).
---------------------------------------------------------
Title: Reconnection in solar flares and coronal mass ejections
Authors: Antiochos, S.
2004cosp...35.2381A Altcode: 2004cosp.meet.2381A
Magnetic reconnection is widely believed to be the physical process
underlying much of solar activity. We argue that it plays two critical
roles in eruptive flares/coronal mass ejections (CME). Reconnection is
the trigger mechanism that initiates the eruption, and reconnection is
the relaxation process by which the post-eruption field relaxes down to
a quasi-potential closed state. We discuss the distinguishing physical
features of these two types of reconnection and their implications
for theory. Some of the latest results of both observations and
numerical simulations of reconnection in CMEs/eruptive flares will be
presented. This work is supported in part by NASA and ONR.
---------------------------------------------------------
Title: The role of flux emergence as a driver of coronal mass
ejections
Authors: Magara, T.; Antiochos, S. K.; Luhmann, J. G.
2003AGUFMSH42B0512M Altcode:
Recently it has been suggested that coronal mass ejections (CMEs) are
closely related to the interaction between different magnetic domains
formed in the corona. For example, a so-called breakout model of CMEs
shows that a core domain field which is enhanced by shearing motions in
the photosphere interacts with the overlying field, and this weakens
the confining effect of the overlying field and eventually enables
the core domain field to erupt outwards. In this study, we take the
subphotospheric dynamics into this model and see how flux emergence
affects the breakout process. Our work is based on 3-dimensional
resistive MHD simulations in which we initially set a potential bipolar
field above the photosphere and place a magnetic flux tube below the
photosphere. The flux tube then emerges into the photosphere and starts
to interact with the bipolar field. As the flux tube expands into the
corona, a current layer develops around the interface between emerging
magnetic field and preexisting coronal field. We focus on its structure
and evolution because that current layer plays a crucial role in a
breakout of emerging magnetic field. To see this, we apply a locally
enhanced resistivity around the current layer and study the magnetic
reconnection between emerging field and preexisting field.
---------------------------------------------------------
Title: The Coronal Magnetic Field Predicted by the Breakout Model
Authors: Antiochos, S. K.
2003AGUFMSH41A..02A Altcode:
Although there has been intense theoretical work in the past decade
on coronal mass ejections/eruptive flares, the mechanism for their
initiation is far from accepted. The problem is that the triggering of
these events is believed to be magnetically-driven and to take place
in the corona, but until very recently, the field there has not been
observed directly. We discuss how the new developments in coronal
field instrumentation will allow us to determine the mechanism for
CME/eruptive flare initiation. We focus, in particular, on the so-called
breakout model in which reconnection in the corona is postulated to
be the triggering process. First, we present the latest numerical
results on breakout. Then, we determine the predictions of the model
for coronal magnetic observations, and discuss definitive tests of
breakout that will be possible with the proposed new instrumental
capabilities. This work was supported in part by ONR and NASA.
---------------------------------------------------------
Title: Comparison of the Breakout Model With Flare-Loop and Ionic
Composition Data
Authors: Lynch, B. J.; MacNeice, P. J.; Antiochos, S. K.; Zurbuchen,
T. H.
2003AGUFMSH22B..08L Altcode:
We discuss our ongoing analysis of the observational properties of
Breakout coronal mass ejections. Presented are quantitative analyses
of the post-eruption evolution that produces the commonly observed
flare-loop arcades at the limb and the spreading flare ribbons at
disk center. The simulation dynamics are compared to observational
results of long-duration flares. We also present a novel application
of the ionic charge freeze-in analyses to the numerical simulation
output. This post-processing allows us to 'predict' the heavy ion
charge states (O7+/O6+) from the simulation density, temperature, and
velocities. Due to limitations in the energy accounting and initial
conditions of the MHD simulation, we do not obtain the observed O7+/O6+
values, but we emphasize the potential of this method and show how it
can be used to study relative variation in charge state composition
throughout the ejecta volume.
---------------------------------------------------------
Title: Coronal energy release via ideal three-dimensional instability
three-dimensional instability
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
2003AdSpR..32.1029D Altcode:
It is widely believed that most coronal phenomena involve the
release of free energy that is stored within stressed magnetic field
configurations. The availability of sufficient free energy to explain
everything from coronal heating to flares and coronal mass ejections
is well established. How this energy is released remains a major
puzzle. Observations reveal that an important property of the energy
release mechanism is its "switch on" character. The mechanism must
remain dormant for long periods of time to allow the magnetic stresses
to build, then it must operate very vigorously once it finally turns
on. We discuss a mechanism called the "secondary instability" which
exhibits this behavior. It is essentially an ideal instability of the
thin twisted magnetic flux tubes that form from the resistive tearing
of current sheets. We relate the mechanism to the coronal heating
idea of Parker in which the coronal magnetic field becomes tangled by
random motions of the photospheric footpoints. Global energy balance
considerations imply that magnetic energy dissipation occurs at a
particular angle in the field, and the secondary instability offers
the first quantitative explanation for why this should be. It thus
places Parker's popular idea on a much firmer physical footing.
---------------------------------------------------------
Title: Constraints on the Magnetic Field Geometry in Prominences
Authors: Karpen, J. T.; Antiochos, S. K.; Klimchuk, J. A.; MacNeice,
P. J.
2003ApJ...593.1187K Altcode:
This paper discusses constraints on the magnetic field geometry of solar
prominences derived from one-dimensional modeling and analytic theory
of the formation and support of cool coronal condensations. In earlier
numerical studies we identified a mechanism-thermal nonequilibrium-by
which cool condensations can form on field lines heated at their
footpoints. We also identified a broad range of field line shapes
that can support condensations with the observed sizes and lifetimes:
shallowly dipped to moderately arched field lines longer than several
times the heating scale. Here we demonstrate that condensations formed
on deeply dipped field lines, as would occur in all but the near-axial
regions of twisted flux ropes, behave significantly differently than
those on shallowly dipped field lines. Our modeling results yield
a crucial observational test capable of discriminating between two
competing scenarios for prominence magnetic field structure: the flux
rope and sheared-arcade models.
---------------------------------------------------------
Title: Constraints on Active Region Coronal Heating
Authors: Antiochos, S. K.; Karpen, J. T.; DeLuca, E. E.; Golub, L.;
Hamilton, P.
2003ApJ...590..547A Altcode:
We derive constraints on the time variability of coronal heating from
observations of the so-called active region moss by the Transition
Region and Coronal Explorer (TRACE). The moss is believed to be due to
million-degree emission from the transition regions at the footpoints
of coronal loops whose maximum temperatures are several million
degrees. The two key results from the TRACE observations discussed in
this paper are that in the moss regions one generally sees only moss,
not million-degree loops, and that the moss emission exhibits only weak
intensity variations, ~10% over periods of hours. TRACE movies showing
these results are presented. We demonstrate, using both analytic and
numerical calculations, that the lack of observable million-degree
loops in the moss regions places severe constraints on the possible
time variability of coronal heating in the loops overlying the moss. In
particular, the heating in the hot moss loops cannot be truly flarelike
with a sharp cutoff, but instead must be quasi-steady to an excellent
approximation. Furthermore, the lack of significant variations in
the moss intensity implies that the heating magnitude is only weakly
varying. The implications of these conclusions for coronal heating
models will be discussed.
---------------------------------------------------------
Title: Internal structure of magnetic clouds: Plasma and composition
Authors: Lynch, B. J.; Zurbuchen, T. H.; Fisk, L. A.; Antiochos, S. K.
2003JGRA..108.1239L Altcode:
A comprehensive analysis of magnetic clouds observed by the Advanced
Composition Explorer (ACE) spacecraft from February 1998 to July 2001
is presented. The magnetic field data from the MAG instrument is fit
with the cylindrically symmetric, linear force-free model and the
fit parameter distributions are examined. This magnetic field model
enables us to map plasma data from the SWEPAM and SWICS instruments to
a position within the model cylinder. A superposed epoch analysis of all
our magnetic cloud events is used to construct diameter cuts through an
"average" cloud profile in any desired plasma, elemental composition,
or charge state quantity. These diameter cuts are found to have
nontrivial structure and there appears to be significant composition
and structural differences between clouds of different speeds. The
slow magnetic clouds (<V<SUB>rad</SUB>> < 500 km/s) have an
almost constant proton density profile whereas the fast magnetic cloud
(<V<SUB>rad</SUB>> ≥ 500 km/s) profile is depleted throughout
with symmetric dips and a local maximum at the cloud center. The fast
magnetic clouds have a slightly higher N<SUB>α</SUB>/N<SUB>p</SUB>
ratio than the slow clouds. Both the fast and slow events have enhanced
oxygen and iron charge states compared to the slow solar wind. The fast
events have a slightly increased O<SUP>7+</SUP>/O<SUP>6+</SUP> average
profile and a much stronger Fe<SUP>≥16+</SUP>/Fe<SUB>total</SUB>
profile than the slow events. We briefly discuss the implications for
physical conditions at the Sun, the role these coronal mass ejections
(CMEs) may play in transporting magnetic flux, and the application of
our structure results to the current flux rope CME modeling effort.
---------------------------------------------------------
Title: Coronal Hole Topology
Authors: Antiochos, S. K.
2003SPD....34.0102A Altcode: 2003BAAS...35..805A
A key problem for Solar/Heliospheric physics is understanding the
structure and dynamics of the open magnetic field regions on the Sun,
the so-called coronal holes. There has been considerable debate in
recent years on the nature of coronal hole evolution, in particular,
on how closed and open field regions interact. We use the source surface
model to investigate in detail the topology of the solar coronal field
in cases where closed-field active regions interact with the boundaries
of coronal holes. These studies lead us to the conjecture that in any
continuously connected polarity region on the photosphere, there can
be at most one coronal hole, but this open field region is likely
to be highly complex with narrow corridors of open flux connecting
apparently isolated coronal holes. We discuss both the observational
and theoretical implications of our conjecture for the Sun - Heliosphere
connection. <P />This work was supported in part by NASA and ONR.
---------------------------------------------------------
Title: Coronal Mass Ejection Breakout with Adaptive Mesh Refinement
Authors: Lynch, B. J.; MacNeice, P. J.; Antiochos, S. K.; Zurbuchen,
T. H.
2003SPD....34.0513L Altcode: 2003BAAS...35S.816L
We compare the magnetic breakout model for solar coronal mass ejections
(CMEs) of Antiochos, DeVore, & Klimchuck [1] to coronagraph and
in-situ observations. We study the effect of adaptive mesh refinement
(AMR) on the density structures and coronagraph dynamics predicted by
the model. Synthetic coronagraph images are shown to reproduce the dark
cavity and bright rim features of 3-part CME observations. Height-time
plots are generated and compared with LASCO observations. The breakout
model creates a flux-rope like structure during eruption that shares
many properties with interplanetary magnetic cloud observations,
e.g. internal fields that can be well described by a linear, force-free
cylinder. This work is supported by ONR and NASA. <P />[1] Antiochos,
S. K., C. R. DeVore, J. A. Klimchuck, 1999, ApJ 510, pp. 485-493.
---------------------------------------------------------
Title: Effects of nonuniform flux tube area on prominence formation
through thermal nonequilibrium
Authors: Karpen, J. T.; Tanner, S. E. M.; Antiochos, S. K.
2003SPD....34.0414K Altcode: 2003BAAS...35R.812K
We have developed a dynamic model of prominence formation in which
steady but unequal footpoint heating causes a dynamic cycle of
chromospheric evaporation, condensation, motion, and destruction
(Antiochos et al. 1999a, 2000; Karpen et al. 2001, 2002). We have
performed 1D hydrodynamic simulations with varying geometries and
other properties to determine the limits of this mechanism under solar
conditions. In previous studies we identified several key parameters
that dictate the existence and characteristics of this cyclic process:
the ratio of loop length to heating scale height, the loop apex height,
the heating asymmetry, and dip depth. For those idealized calculations,
the cross-sectional area of the flux tube was assumed to be constant. On
the Sun, however, we expect the flux tube areas to be highly nonuniform,
narrowing where the flux is constrained by stronger adjacent fields
and expanding where neighboring fields are weaker. <P />To determine
the effects of varying cross-sectional area on the evaporation and
condensation processes at the core of our prominence formation model,
we performed a set of 1D calculations with ARGOS, our 1D hydrodynamic
code with adaptive mesh refinement. Representative field lines capable
of supporting prominence condensations were selected from the 3D
sheared-arcade model of the prominence magnetic field (DeVore &
Antiochos 2000); the flux tube area was measured at intervals along
these field lines and fit by a smooth analytic function suited for our
computational approach. For comparison, “control" loops also were set
up with the same 1D loop geometry but with constant cross-section. As in
our earlier calculations, these field lines were subjected to steady,
localized heating at the footpoints and subsequent developments were
monitored. Results from this study will be presented in the context
of our previous studies and compared with prominence observations,
as a critical test of our model. <P />This work was supported by NASA
and ONR.
---------------------------------------------------------
Title: Coronal Energy Release via Explosive Three-Dimensional
Instability
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
2003SPD....34.0107D Altcode: 2003BAAS...35..806D
It is widely believed that most coronal phenomena involve the release
of magnetic free energy that is stored within stressed magnetic field
configurations. The availability of sufficient free energy to explain
everything from coronal heating to flares and coronal mass ejections
is well established, but how this energy is released remains a major
puzzle. Observations reveal that an important property of the energy
release mechanism is its “switch on" character. The mechanism must
remain dormant for long periods of time to allow the magnetic stresses
build, then it must operate very vigorously once it finally turns on. <P
/>We discuss a mechanism called the “secondary instability" which
exhibits this behavior. It is essentially the ideal kinking of thin
twisted magnetic flux tubes that form from the resistive instability
of current sheets. We relate the mechanism to the coronal heating
idea of Parker in which the coronal magnetic field becomes tangled by
random motions of the photospheric footpoints. Global energy balance
considerations imply that magnetic energy dissipation occurs at a
particular angle in the field, and the secondary instability offers
the first quantitative explanation for why this should be. It thus
places Parker's popular idea on a much firmer physical footing. <P
/>This research was funded by NASA.
---------------------------------------------------------
Title: The Energetics of Breakout Coronal Mass Ejections
Authors: DeVore, C. R.; Antiochos, S. K.
2003SPD....34.0517D Altcode: 2003BAAS...35..817D
A key obstacle to understanding fast coronal mass ejections lies in
slowly accumulating sufficient magnetic energy to power an explosive
eruption of the structure, rather than a gradual expansion, and then
rapidly releasing the stored energy when a threshold is crossed and
the event is triggered. In the breakout model, a low-lying stressed
field is restrained by an overlying coronal field containing an
embedded null. The abrupt transition to explosive behavior occurs
when reconnection at the null lowers the energy required to open
the remnant restraining flux below the free energy stored in the
stressed field. <P />We investigate the energetics of opening the
coronal magnetic field under two possible extremes of evolution:
(1) complete reconnection at the null produces a "maximally closed"
final state, whose free energy is the lower bound for opening the
structure; (2) zero reconnection at the null produces a "maximally
open" final state, whose free energy is an upper bound. We calculate
the energies of these states for coronal fields that are potential
everywhere except at discrete current sheets, where the magnetic
stresses are continuous and the field is force-free. Varying the
relative amounts of flux in the inner arcade and the overlying field
shifts the location of the potential null. In axisymmetric spherical
geometries, the free energies of the "maximally closed" states can
be less than 2.5% of the energy of the initial configuration, and
as small as 125% of the initial energy of the inner arcade, with the
null positioned at 1.4 and 1.6 Rs, respectively. These findings change
little as the surface distributions of arcade and overlying flux are
increasingly concentrated, and all energy-minimizing states have a net
amount of open overlying flux. Results for fully three-dimensional,
nonaxisymmetric breakout configurations will be presented. <P />This
research was supported in part by NASA and ONR.
---------------------------------------------------------
Title: A Model for Prominence Formation
Authors: Welsch, B. T.; DeVore, C. R.; Antiochos, S. K.; Linton, M. G.
2003SPD....34.0413W Altcode: 2003BAAS...35..812W
The essential features of a model prominence configuration include:
dipped or helical field lines that are capable of supporting mass
against gravity; sheared field lines that run nearly parallel to
the photospheric polarity inversion line (PIL); and overlying field
lines that restrain the sheared field lines. <P />To determine how
reconnection might generate such field configurations, we have used
the ARMS code, an MHD solver with adaptive refinement, to model the
interaction of two active regions subjected to a shear flow, meant
to approximate differential rotation. <P />We present preliminary
results from two cases, one in which the model active region fields
were potential prior to shearing, and one in which the active region
fields were “spun up” prior to shearing, to approximate active region
fields possessing twist at their emergence. <P />This work was supported
in part by NASA, DoD's HPCMP, and AFOSR's Solar-MURI program.
---------------------------------------------------------
Title: A Transient Heating Model for Coronal Structure and Dynamics
Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.;
Antiochos, S. K.; Klimchuk, J. A.; MacNeice, P. J.
2003ApJ...582..486S Altcode:
A wealth of observational evidence for flows and intensity variations in
nonflaring coronal loops leads to the conclusion that coronal heating
is intrinsically unsteady and concentrated near the chromosphere. We
have investigated the hydrodynamic behavior of coronal loops undergoing
transient heating with one-dimensional numerical simulations in which
the timescale assumed for the heating variations (3000 s) is comparable
to the coronal radiative cooling time and the assumed heating location
and scale height (10 Mm) are consistent with the values derived from
TRACE studies. The model loops represent typical active region loops:
40-80 Mm in length, reaching peak temperatures up to 6 MK. We use ARGOS,
our state-of-the-art numerical code with adaptive mesh refinement, in
order to resolve adequately the dynamic chromospheric-coronal transition
region sections of the loop. The major new results from our work are
the following: (1) During much of the cooling phase, the loops exhibit
densities significantly larger than those predicted by the well-known
loop scaling laws, thus potentially explaining recent TRACE observations
of overdense loops. (2) Throughout the transient heating interval,
downflows appear in the lower transition region (T~0.1 MK) whose key
signature would be persistent, redshifted UV and EUV line emission,
as have long been observed. (3) Strongly unequal heating in the two
legs of the loop drives siphon flows from the more strongly heated
footpoint to the other end, thus explaining the substantial bulk flows
in loops recently observed by the Coronal Diagnostic Spectrometer and
the Solar Ultraviolet Measurement of Emission Radiation instrument. We
discuss the implications of our studies for the physical origins of
coronal heating and related dynamic phenomena.
---------------------------------------------------------
Title: Helicity Generation and Evolution in Coronal Mass Ejections
Authors: Spicer, Daniel S.; MacNiece, Peter; Antiochos, Spiro; Finn,
John M.
2003IAUJD...3E...5S Altcode:
A key issue in current modeling of coronal mass ejections is the role
of helicity. A number of authors have argued that CMEs are somehow
the result of the accumulation of too much helicity in the solar
corona -- the so-called helicity charging concept. We present results
demonstrating that this concept is incorrect and that the only essential
ingredient for eruption is magnetic free energy. We also show that
the Taylor conjecture is not valid for the pre-eruption corona. On
the other hand A Taylor-like evolution does seem to be applicable
to the post-eruption flux rope in the far outer corona. Results from
numerical simulations of the CME breakout model will be presented that
include magnetic helicity evolution. Other diagnostic information
such as the total magnetic energy total kinetic energy and local
magnetic energy density are also tracked and compared with global
and local helicity. Boundary conditions that allow for finite and
zero helicity injection into the computational domain will also be
presented and compared
---------------------------------------------------------
Title: Theoretical Energy Analysis of Reconnecting Twisted Magnetic
Flux Tubes
Authors: Linton, M. G.; Antiochos, S. K.
2002ApJ...581..703L Altcode:
It has been shown that twisted magnetic flux tubes reconnect in a
number of different ways: either bouncing, tunneling, slingshotting,
or merging. Here we present an analytical theory to predict under
what conditions the bounce, tunnel, and slingshot will occur. This
theory calculates the energy of the reconnected state relative to
that of the initial state, subject to the restriction that helicity is
conserved during the interaction. Comparison of this energy change for
each of these interactions then indicates which is most energetically
favorable. In addition to providing potentially important predictive
capabilities, for example, for solar or magnetospheric flux-tube
reconnection, this also provides an intuitive explanation for why the
tunnel interaction occurs.
---------------------------------------------------------
Title: Hydrodynamics of coronal loops undergoing transient heating
Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.;
Antiochos, S. K.; MacNeice, P. J.
2002ASPC..277..597S Altcode: 2002sccx.conf..597S
No abstract at ADS
---------------------------------------------------------
Title: Bright Knots in EUV Post-flare Loops : TRACE Observations
and 1D Hydrodynamic Modeling
Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A.
2002AGUFMSH21C..04P Altcode:
EUV post-flare loops often possess bright knots along them. Some
examples of such post-flare loops seen by TRACE will be shown, along
with a brief outline of their properties. We will then present the
results of a series of 1D hydrodynamic simulations of flaring loops,
which employ different heating functions for the impulsive and decay
phase of the simulated flares. It will be demonstrated that the creation
of these knots depends crucially on the spatio-temporal distribution of
the heating during the decay phase. This provides strong constraints
on both post-flaring conditions and AR loop heating. We will finally
briefly outline how SDO instrumentation could improve our knowledge
of this topic. Research supported in part by NASA and ONR.
---------------------------------------------------------
Title: Coronal Canals
Authors: Antiochos, S. K.
2002AGUFMSH21C..03A Altcode:
As the first mission of LWS, one of SDO's most important goals will
be to determine how the plasma and magnetic structures observed on
the Sun connect to those observed in the heliosphere. In this paper,
we show that the topology of the coronal open field, (and consequently
the geometry of the heliospheric current sheet), is likely to be much
more complex than previously believed. Using the source-surface model,
we calculate the response of a polar coronal hole to the slow emergence
of a high-latitude bipolar active region. We show that at a critical
point in the active region growth, the coronal hole boundary must jump
discontinuously to form a narrow channel of open field that encircles
the trailing polarity spot. Contrary to a common misconception, the open
field region does not form a new coronal hole that is disconnected from
the polar one, but remains as one continuously connected region. The
boundary of the open field region, however, acquires enormous structure,
with canals that extend down to low latitudes. We argue that these
open-field canals are the explanation for the wide-spread belief that
open field emanates from closed field regions. We also discuss how
magnetic reconnection would, in fact, lead to the type of structure
produced by the source-surface model,and how SDO would observe the
canals on the Sun. This work was supported in part by NASA and ONR.
---------------------------------------------------------
Title: Fuzzy hot post-flare loops versus sharp cool post-flare loops
Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A.
2002ESASP.505..207P Altcode: 2002solm.conf..207P; 2002IAUCo.188..207P
By using high spatial resolution TRACE EUV observations we show that hot
(≍2 MK) post-flare loops are fuzzier than the cooler (≍1 MK) ones. A
simple 0d model of a cooling loop arcade, where different loops in the
arcade start to cool down at slightly different initial conditions,
is sufficient to reproduce qualitatively the observed behavior of the
EUV post-flare loops.
---------------------------------------------------------
Title: Hydrodynamic models of transiently heated coronal loops
Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.;
Antiochos, S. K.; Klimchuk, J. A.; MacNeice, P. J.
2002ESASP.505..583S Altcode: 2002solm.conf..583S; 2002IAUCo.188..583S
We investigate the hydrodynamic behaviour of coronal loops
undergoing transient heating. We adopt a 1-D loop model with space-
and time-dependent heating, concentrated near the chromospheric
footpoints. The timescale of heating variations is comparable with the
radiative cooling time of the coronal plasma (~10<SUP>3</SUP>s). We
use a new numerical code that has a fully adaptive grid, in order to
properly resolve the chromospheric-coronal transition region sections of
the loop. We simulate here the hydrodynamics of a loop with different
effective gravity (i.e., loop geometry) and heating terms. We describe
the temporal behaviour of the various physical quantities along the
loop (plasma density, temperature, flow velocity), showing that the
increase in heating produces a chromospheric evaporation, or a siphon
flow if the loop heating is taken to be significantly different at
the two footpoints, followed by long-lasting downflows with velocities
of a few km s<SUP>-1</SUP> during the quiescent phases in between the
episodic heatings. Moreover, in the case of considerable increase in
heating, a catastrophic cooling of the loop plasma can occur, giving
rise to downflows of several tens of km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Sheared magnetic fields and eruptive phenomena: prominences,
flares, and coronal mass ejections
Authors: Antiochos, Spiro K.
2002ESASP.505..219A Altcode: 2002solm.conf..219A; 2002IAUCo.188..219A
The underlying cause of all the giant manifestations of solar activity
- CMEs, eruptive flares and filament ejections - is the disruption of
a force balance between the upward pressure of the strongly sheared
field of a filament channel and the downward tension of overlying
field that is quasi-potential. A key point is that the upward
pressure cannot increase rapidly, because the magnetic shear/twist
is produced by the slow photospheric evolution (shear flows and/or
flux emergence). Therefore, explosive events such as flares and
CMEs must be due fundamentally to the catastrophic removal of the
downward magnetic tension. Recent theory and simulation have focused on
magnetic reconnection as the mechanism for the removal of the magnetic
tension. This paper discusses critically the recent models for magnetic
disruptions in the Sun's corona.
---------------------------------------------------------
Title: Advances and prospects in solar theory
Authors: Antiochos, Spiro K.
2002ESASP.505Q....A Altcode: 2002solm.confQ....A; 2002IAUCo.188Q....A
Theoretical solar physics has undergone a revolution during the
last decade, transitioning from a field in which the bulk of the
work consisted of developing semi-ideal models of highly simplified
systems, to fully 3D models using large-scale simulations and actual
observations as input. We are now close to developing comprehensive
theories for many of the outstanding problems in solar physics. Based
on results presented at the Santorini Conference on Magnetic Coupling
of the Solar Atmosphere, this paper highlights some of the recent
solar theory progress and discusses the likely opportunities for new
advances. Of course, it is not possible to describe even the gist of
these results in this brief review, therefore, I merely attempt to
organize some of them into a coherent structure and provide a personal
perspective. The natural organizing structure is to consider the major
theoretical problems that are now preventing us from developing a
comprehensive model for how the magnetic field couples the interior
to the atmosphere and, thereby, produces solar activity.
---------------------------------------------------------
Title: Coronal Magnetic Field Relaxation by Null-Point Reconnection
Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R.
2002ApJ...575..578A Altcode:
We derive the minimum energy state resulting from complete magnetic
reconnection in a translationally or axisymmetric MHD system,
in the limit of a low plasma beta and high magnetic Reynolds
number-conditions appropriate to the solar corona. The results are
necessary for determining the amount of energy that can be liberated
by reconnection and, hence, are important for understanding coronal
heating and other forms of solar activity. The key difference between
our approach and previous work is that because of line tying at
the high-beta photosphere, reconnection is limited to occur only at
magnetic null points initially present in the system. We find that under
these circumstances the minimum energy state is not the usual linear
force-free field but a state in which the nonpotential component of
the field is distributed uniformly on equal flux surfaces. We discuss
the implications of our results for the Sun's corona and for laboratory
plasmas.
---------------------------------------------------------
Title: Hydrodynamic simulations of coronal loops subject to transient
heating
Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.;
MacNeice, P. J.; Antiochos, S. K.; Klimchuk, J. A.
2002ESASP.508..331S Altcode: 2002soho...11..331S
We investigate the hydrodynamic behaviour of coronal loops
undergoing transient heating. We adopt a 1-D loop model with space-
and time-dependent heating, concentrated near the chromospheric
footpoints. The timescale of heating variations is comparable with the
radiative cooling time of the coronal plasma (~10<SUP>3</SUP>s). We
use a new numerical code that has a fully adaptive grid, in order to
properly resolve the chromospheric-coronal transition region sections of
the loop. We simulate here the hydrodynamics of a loop with different
effective gravity (i.e., loop geometry) and heating terms. We describe
the temporal behaviour of the various physical quantities along the loop
(plasma density,temperature, flow velocity), showing that the increase
in heating produces a chromospheric evaporation, or a siphon flow if
the loop heating is taken to be significantly different at the two
footpoints, followed by long-lasting downflows with velocities of a few
km s<SUP>-1</SUP> during the quiescent phases in between the episodic
heatings. Moreover, in the case of considerable increase in heating,
a thermal instability can occur during the cooling phase of the loop
plasma, giving rise to downflows of several tens of km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Reconnection of Twisted Magnetic Flux Tubes
Authors: Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K.
2002AAS...200.8802L Altcode: 2002BAAS...34..789L
We present 3D MHD simulations of the collision and reconnection of
pairs of twisted, isolated magnetic flux tubes at various collision
angles. Such reconnection is likely to be an important source of
energy release in solar flares, and could play a role in coronal
mass ejection dynamics. We show that the dynamics of the reconnection
depends strongly on the collision angle between the tube axes and on
the relative sign of twist of the tubes. The most energetic interaction
is a slingshot interaction, analogous to the reconnection often seen in
2D simulations. But, depending on the configuration, the tubes can also
bounce without reconnecting, merge into a single tube, or tunnel through
each other. We will discuss these various interactions, the topological
changes they bring about, and the magnetic energy released. In addition
we will present an analytical model which explains some of the results,
in particular the tunnel and slingshot interactions, in terms of a
simple energy calculation based on helicity conservation. This work
was supported by NASA and ONR grants, an ITP-NSF grant, and a grant
of computer time from the DoD/HPC Program.
---------------------------------------------------------
Title: Hot versus cool coronal loops
Authors: Patsourakos, S.; Klimchuk, J. A.; Antiochos, S. K.
2002AAS...200.0209P Altcode: 2002BAAS...34..640P
EUV and SXR observations show respectively that cool (1 MK) loops are
finer and maybe more dynamic than hotter (2 MK) ones. Whether this
reflects a fundamental difference in the properties of the heating
mechanism in action in each loop class is not yet clear. We will address
some aspects of this issue by combining EUV and SXR observations of
such loops with eventually hydrodynamic simulations of a nano-flare
heated corona. Research supported in part by ONR and NASA.
---------------------------------------------------------
Title: An Explanation for the “Switch On" Character of Magnetic
Energy Release
Authors: Klimchuk, J. A.; Dahlburg, R. B.; Antiochos, S. K.
2002AAS...200.1607K Altcode: 2002BAAS...34..668K
It is widely believed that most coronal phenomena involve the release
of magnetic free energy that is stored in stressed magnetic field
configurations. The availability of sufficient free energy to explain
everything from coronal heating to flares and coronal mass ejections
is well established, but how this energy is released remains a major
puzzle. Observations reveal that an important property of the energy
release mechanism is its “switch on" character. The mechanism must
remain dormant for long periods of time to allow the magnetic stresses
to build, then it must operate very vigorously once it finally turns
on. We discuss a mechanism called the “secondary instability" which
exhibits this behavior. It is essentially the ideal kinking of thin
twisted magnetic flux tubes that form from the restive tearing of
current sheets. We relate the mechanism to the coronal heating idea
of Parker in which the coronal magnetic field becomes tangled by
random motions of the photospheric footpoints. Global energy balance
considerations imply that magnetic energy dissipation occurs at a
particular angle in the field, and the secondary instability offers
the first quantitative explanation for why this should be. It thus
places Parker's popular idea on a much firmer physical footing.
---------------------------------------------------------
Title: Constraints placed by thermal nonequilibrium on the topology
of prominence magnetic fields
Authors: Karpen, J.; Antiochos, S. K.; MacNeice, P.
2002AAS...200.3719K Altcode: 2002BAAS...34..698K
We have developed a dynamic model of prominence formation in which
steady but unequal footpoint heating causes a dynamic cycle of
chromospheric evaporation, condensation, motion, and destruction
[Antiochos et al. 1999a, 2000, ApJ; Karpen et al. 2001, ApJ]. We have
performed 1D hydrodynamic simulations with varying geometries and
other properties to determine the limits of this mechanism under solar
conditions. In previous studies we identified three key parameters that
dictate the existence and characteristics of this cyclic process: the
ratio of loop length to heating scale height, the loop apex height, and
the heating asymmetry. Here we discuss our latest calculations, in which
we studied the role of the depth of field-line dips -- a feature common
to most magnetic-field configurations proposed for prominences. In
long fluxtubes with dips deeper than roughly f * H<SUB>g</SUB>, where
f measures the heating imbalance between footpoints and H<SUB>g</SUB>
is the gravitational scale height, condensations form, quickly fall
to the bottom of the dip, and remain there while steadily accreting
mass. Therefore, strongly dipped loops are not capable of supporting
the observed counterstreaming flows along prominence spines. This
places stringent limitations on flux rope models [e.g., Rust &
Kumar 1994, SolPhys], as only the least twisted field lines close
to the axis pass this test. For our shear-based model of prominence
fields [Antiochos et al. 1999b, ApJ], a larger subset of field lines can
support prominences formed by thermal nonequilibrium: for the case shown
(f=0.25), fluxtubes longer than ~80 Mm, lower than ~100 Mm at the apex,
or less deeply dipped than ~25 Mm meet the requirements. This work
was supported by NASA and ONR.
---------------------------------------------------------
Title: Active Region Loop Heating
Authors: Antiochos, S. K.; Karpen, J. T.; DeLuca, E. E.; Golub, L.;
Hamilton, P.
2002AAS...200.1606A Altcode: 2002BAAS...34..668A
A long-standing unresolved question in solar physics is whether the
heating in coronal loops is steady or impulsive. X-ray observations
of high-temperature loops (T > 2 x 10<SUP>6</SUP> K) tend to
show quasi-steady structures, (evolution slow compared to cooling
time scales), whereas theoretical models strongly favor impulsive
heating. We present simulations of impulsively heated loops using
our adaptive-mesh-refinement code ARGOS, and compare the results with
TRACE observations of the transition regions of high-temperature active
region loops. From this comparison, we deduce that the heating in the
core of active regions is quasi-steady rather than impulsive. These
results pose a formidable challenge to developing theoretical models
for the heating. This work was supported in part by NASA and ONR.
---------------------------------------------------------
Title: ACE Magnetic Clouds - Distributions and Statistics
Authors: Lynch, B. J.; Zurbuchen, T. H.; Fisk, L. A.; Antiochos, S. K.
2002AGUSMSH21A..05L Altcode:
We present a comprehensive analysis of magnetic cloud events observed
by the Advanced Composition Explorer spacecraft. We fit the standard
cylindrically symmetric force-free constant alpha magnetic field
model to each event and examine the model parameter distributions. In
general, our parameter distributions agree with results of previous
studies. Based on significant differences in many plasma and composition
quantities we distinguish between fast (< V<SUB>rad</SUB> >
>= 500 km/s) and slow (< V<SUB>rad</SUB> > < 500 km/s)
magnetic clouds and note the differences in their size and axial
field strength distributions. In addition, we examine some physical
properties of these clouds such as average Fe charge state, axial
current, and magnetic flux. The cloud orientation evolution during
solar cycle 23 is considered and compared to the corresponding time
period of solar cycle 21. We also compare our magnetic cloud frequency
to the variation in sunspot number and the variation in the dipole
and quadrupole moments of the photospheric magnetic field.
---------------------------------------------------------
Title: Simulations of Interactions and Magnetic Reconnection Between
Solar Filaments
Authors: DeVore, C. R.; Antiochos, S. K.; Aulanier, G.
2002AAS...200.3720D Altcode: 2002BAAS...34..698D
It has long been known that pairs of filaments near each other on the
Sun's disk sometimes come into contact and interact. Under favorable
conditions, the two structures apparently link up to form a single,
larger filament. When conditions are unfavorable, on the other hand,
the filaments appear to avoid each other and retain their distinct
identities. Recent ground-based observational studies have shown
that a key requirement for linkage to occur is that the two filaments
possess the same chirality, or handedness. We have performed detailed
numerical experiments of pairs of interacting filaments within the
sheared-arcade model. In this model, the filament plasma resides in
the magnetic hammock formed in a strongly sheared field held down by an
overlying arcade. We considered four cases: like or unlike chirality of
the two filaments, and like or unlike polarity of the vertical magnetic
fields at their approaching ends. Only the case of like chirality and
unlike polarity produces any significant reconfiguration. The magnetic
structure is substantially modified, with reconnected field lines
extending over the entire combined length of the filaments. Low, closed
arcade fields form in the reconnection zone, forcing the newly linked
filament fields above them to rise and form a magnetic 'aneurysm.' Our
simple, bipolar configuration relaxes to a new equilibrium, consistent
with those cases in which the linked structure is observed to persist
stably after the interaction has passed. In the much more complex
magnetic environment of the solar corona, on the other hand, newly
linked filaments with such aneurysms sometimes are observed to erupt
promptly and violently. The removal of the restraining arcade fields, by
reconnection with the external field of the corona, is likely necessary
for eruption to occur. This research was supported by NASA and ONR.
---------------------------------------------------------
Title: Magnetic Reconnection in Solar Flares and Coronal Mass
Ejections
Authors: Antiochos, Spiro
2002APS..APRK12003A Altcode:
Magnetic reconnection is, perhaps, the most important process in space
plasmas for transferring energy from magnetic fields to matter. In
particular, reconnection is believed to be the underlying driver
of the giant explosive releases of magnetic energy in the Sun's
atmosphere that are observed as a solar flare and/or coronal mass
ejection (CME) event. First, I will present compelling evidence for
reconnection in flares and CMEs from the spectacular observations
by TRACE and SOHO. Next the recent advances in reconnection theory
will be reviewed, and their implications for flare/cme models will
be discussed. Simulations will be presented of the recently developed
"breakout" model for eruptive flares/CMEs, in which reconnection plays
the key role in initiating the event. This work was supported in part
by NASA and ONR.
---------------------------------------------------------
Title: Prominence Magnetic Dips in Three-Dimensional Sheared Arcades
Authors: Aulanier, G.; DeVore, C. R.; Antiochos, S. K.
2002ApJ...567L..97A Altcode:
We calculate the distribution of field-line dips in the
three-dimensional sheared arcade model for prominence/filament magnetic
fields. We consider both moderately and highly sheared configurations
computed by fully time-dependent three-dimensional MHD simulations
in which the field was relaxed to a static equilibrium end state. In
agreement with previous low spatial resolution measurements of the
magnetic field inside prominences, we find that for all configurations,
the field in the great majority of the calculated dips exhibits inverse
polarity. But for each configuration we also find well-defined narrow
regions with stable dips of normal polarity. These tend to be located
on the edges of the filament ends and at the top of the central part
of the prominence. This distinctive mixture of normal/inverse polarity
dips that we find in sheared arcades is not likely to be present in
twisted flux rope prominence models. Therefore, our results provide a
rigorous and unique observational test that can distinguish between
the two classes of models, as well as new predictions for future
high spatial resolution spectropolarimetric observations of filaments
and prominences.
---------------------------------------------------------
Title: CME/Flare energy release
Authors: Antiochos, S.
2002cosp...34E.772A Altcode: 2002cosp.meetE.772A
The most spectacular and most energetic manifestations of solar
activity are the giant disruptions of the Sun's magnetic field that
give rise to CMEs and eruptive flares. These phenomena are important
both for their impact on space weather and their implications for
basic space physics. The SOHO and TRACE missions have given us new
insights into the physical mechanisms that give rise to eruptive
flares/CMEs and their associated filament ejections. The current
theories and modeling of CMEs/flares will be reviewed. In particular,
we will describe a recently developed model, "magnetic breakout", which
postulates that the interaction of neighboring flux systems via magnetic
reconnection leads to the sudden release of magnetic stress stored in
strong fields lying near the bottom of the solar atmosphere. Both 2.5D
and 3D numerical simulations of breakout will be presented. The model
proposes a general mechanism for explosive energy release, which should
be applicable to many astrophysical plasmas. This work was supported,
in part, by NASA and ONR.
---------------------------------------------------------
Title: Importance of topology
Authors: Antiochos, Spiro
2002ocnd.confE...1A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal energy release via explosive magnetic reconnection
Authors: Dahlburg, R.; Klimchuk, J.; Antiochos, S.
2002cosp...34E1264D Altcode: 2002cosp.meetE1264D
It is widely believed that most coronal phenomena involve the release
of magnetic free energy that is stored within stressed magnetic field
configurations. The availability of sufficient free energy to explain
everything from coronal heating to flares and coronal mass ejections
is well established, but how this energy is released remains a major
puzzle. Observations reveal that an important property of the energy
release mechanism is its "switch on" character. The mechanism must
remain dormant for long periods of time to allow the magnetic stresses
to build, then it must operate very vigorously once it finally turns
on. We discuss a mechanism called the "secondary instability" which
exhibits this behavior. It is essentially the ideal kinking of thin
twisted magnetic flux tubes that form from the resistive tearing of
current sheets. We relate the mechanism to the coronal heating idea
of Parker in which the coronal magnetic field becomes tangled by
random motions of the photospheric footpoints. Global energy balance
considerations imply that magnetic energy dissipation occurs at a
particular angle in the field, and the secondary instability offers
the first quantitative explanation for why this should be. It thus
places Parker's popular idea on a much firmer physical footing.
---------------------------------------------------------
Title: Numerical Simulation and Analytical Prediction of the
Reconnection of Twisted Flux Tubes
Authors: Linton, M. G.; Antiochos, S. K.; Dahlburg, R. B.
2001AGUFMSH11C0725L Altcode:
Observations of the solar corona show that some flares appear to be
caused by the collision and reconnection of magnetic flux tubes. We
investigate this possibility by studying, both numerically and
analytically, the interaction and reconnection of colliding pairs of
twisted flux tubes. In particular, we present an analytical model
based on various parameters of flux tube collisions, such as tube
twist, length and collision angle, which predicts whether a flux tube
pair will reconnect once and slingshot or reconnect twice and tunnel
(see Linton et al. ApJ 2001 533, 905). We then test these predictions
against numerical simulations of the same flux tube interactions, and
find good agreement between the two. This analytical model, therefore,
should be a useful tool in the analysis and prediction of solar flare
events due to flux tube collisions. This work was supported by NASA
and ONR grants, and a grant of computer time from the DoD/HPC Program.
---------------------------------------------------------
Title: On the Time Variability of Coronal Heating
Authors: Antiochos, S. K.; Karpen, J. T.; DeLuca, E. E.; Golub, L.;
Hamilton, P.
2001AGUFMSH11A0690A Altcode:
We derive constraints on the time variability of coronal heating from
observations of the so-called active-region moss by the Transition
and Coronal Explorer (TRACE). The moss is believed to be due to
million-degree emission from the transition regions at the footpoints
of coronal loops whose maximum temperatures are several million
degrees. The key point of the TRACE observations is that in the
moss regions one generally sees only moss, and not million degree
loops. TRACE movies showing this result will be presented. We will
demonstrate using both analytic and numerical calculations, that the
lack of observable million-degree loops in the moss regions places
severe constraints on the possible time variability of coronal heating
in the loops overlying the moss. In particular, the heating in the hot
moss loops cannot be truly flare-like with a sharp cutoff, but instead,
must be quasi-steady to an excellent approximation. The implications
of this result for coronal heating models will be discussed. This work
was supported in part by NASA and ONR
---------------------------------------------------------
Title: Hydrodynamics of coronal loops subject to transient heating
Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.;
MacNeice, P. J.; Antiochos, S. K.
2001ESASP.493..367S Altcode: 2001sefs.work..367S
No abstract at ADS
---------------------------------------------------------
Title: The Magnetic Origins of Coronal Mass Ejections
Authors: Antiochos, S.
2001hell.confE..14A Altcode:
The most spectacular and most energetic manifestations of solar
activity are the giant disruptions of the Sun's magnetic field that
give rise to coronal mass ejections (CME)/ eruptive flares. These
events are also the main drivers of geoeffective space weather,
producing disturbances ranging from intense particle storms to
electric power disruptions. CMEs/eruptive flares are also, perhaps,
the most interesting form of solar activity from the viewpoint of
basic MHD physics and present a great challenge to theory. Recent
observations by the SOHO and TRACE missions of CMEs/flares and their
associated prominence eruptions have given us new insights into the
physical mechanism for CME/eruptive flare initiation. We will first
review some of the latest observations and theories. Then, we will
describe a recently developed model, "magnetic breakout", which
appears to explain many of the important features of CME/eruptive
flares. The basic idea underlying breakout is that the interaction
of neighbouring flux systems in the Sun's corona leads to a positive
feedback between magnetic reconnection and outward expansion of the
coronal magnetic field, and thereby, an explosive energy release. Both
2.5D and 3D numerical simulations of breakout will be presented. The
model describes a general mechanism for explosive eruptions, which
should be applicable to many astrophysical plasmas.
---------------------------------------------------------
Title: Reconnection of Twisted Flux Tubes as a Function of Contact
Angle
Authors: Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K.
2001ApJ...553..905L Altcode:
The collision and reconnection of magnetic flux tubes in the solar
corona has been proposed as a mechanism for solar flares and in some
cases as a model for coronal mass ejections. We study this process by
simulating the collision of pairs of twisted flux tubes with a massively
parallel, collocation, viscoresistive, magnetohydrodynamic code using up
to 256×256×256 Fourier modes. Our aim is to investigate the energy
release and possible global topological changes that can occur in
flux-tube reconnection. We have performed a number of simulations for
different angles between the colliding flux tubes and for either co-
or counterhelicity flux tubes. We find the following four classes of
interaction: (1) bounce (no appreciable reconnection), (2) merge,
(3) slingshot (the most efficient reconnection), and (4) tunnel (a
double reconnection). We will describe these four classes of flux-tube
reconnection and discuss in what range of parameter space each class
occurs and the implications our results have for models of flares and
coronal mass ejections.
---------------------------------------------------------
Title: Reconnection of Twisted Magnetic Flux Tubes as a Solar Flare
Mechanism
Authors: Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K.
2001AGUSM..SP42A11L Altcode:
The collision and reconnection of magnetic flux tubes in the solar
corona is often proposed as a mechanism for solar flares. We study this
process by simulating the collision of pairs of twisted flux tubes with
a massively parallel collocation viscoresistive MHD code using up to
256 x 256 x 256 Fourier modes. Our aim is to investigate the energy
release and the possible global topological changes which occur in
flux tube reconnection. We have performed a number of simulations for
different angles between the colliding flux tubes and for either co-
or counter-helicity flux tubes. We find the following four classes of
reconnection can occur: 1) bounce (no appreciable reconnection), 2)
merge, 3) slingshot (the classical reconnection picture), and 4) tunnel
(a double reconnection). We will describe these four classes of flux
tube reconnection, discuss in what range of parameter space each class
occurs, and discuss the implications our results have for models of
flares. This work was supported by an NRC/NRL Postdoctoral Fellowship,
the NASA SECTP, and a grant of computer time from the DoD/HPC Program.
---------------------------------------------------------
Title: Origin and Evolution of Coronal Condensations
Authors: Karpen, J. T.; Antiochos, S. K.; MacNeice, P. J.
2001AGUSM..SP61A02K Altcode:
The existence of cool plasma high in the solar corona was first
established a century ago. In addition to the well-studied phenomenon
of prominences, persistent knots and episodic downflows of cool plasma
commonly denoted `coronal rain' have been observed in Hα , EUV, and UV
spectral lines. Our recent 1D hydrodynamic simulations of localized,
steady heating near the footpoints of long coronal loops produce
dynamic condensations which form, flow, and fall onto the nearest
chromosphere over the course of tens of hours (Antiochos et al. 2000,
Karpen et al. 2001). In low-lying loops, this process yields condensed
knots with dimensions and velocities consistent with high-resolution
observations of counterstreaming flows along prominence spines (Zirker
et al. 1998). Similar condensations develop even in high ( ~100,000
km) model loops, although they are small, short-lived, and form at
irregular intervals. In order to explain the broader phenomenon of
coronal condensations beyond prominences, however, we must investigate
the effects of temporally varying, localized footpoint heating on the
plasma dynamics in a range of active-region and quiet-Sun loops. We
will discuss the results of a series of 1D numerical simulations with
spatially and temporally variable heating, their observable signatures,
and how well they reproduce observations by SOHO and TRACE of `coronal
rain' and coronal condensations (e.g., Brekke 1999; Schrijver 2001).
---------------------------------------------------------
Title: The Role of Magnetic Flux Ropes in CMEs
Authors: Antiochos, S. K.
2001AGUSM..SH41C05A Altcode:
A commonly-held belief is that prominences/filaments are magnetic flux
ropes, consisting of a globally twisted structure with a number of
turns. These ropes are presumed to form either by emerging pre-made
from the photosphere, or via reconnection at the bottom boundary
of the corona. The claim is that filament eruption and coronal mass
ejections occur because the twist/helicity of the flux rope becomes
too large. We will argue that this so-called paradigm is merely an
unfortunate misconception. The key point is that flux ropes do not
produce CMEs but, instead, CMEs produce flux ropes. We will present
both observational and theoretical results, which demonstrate that
the pre-eruption topology is that of a differentially sheared arcade
and that the flux ropes observed in the upper corona by LASCO, for
example, are only a by-product of the eruption. We will discuss the
implications of these results for the STEREO and SOLAR-B missions. This
work supported in part by NASA and ONR.
---------------------------------------------------------
Title: Are Magnetic Dips Necessary for Prominence Formation?
Authors: Karpen, J. T.; Antiochos, S. K.; Hohensee, M.; Klimchuk,
J. A.; MacNeice, P. J.
2001ApJ...553L..85K Altcode:
The short answer: No.
---------------------------------------------------------
Title: Structure and Stability of Multipolar Prominence Magnetic
Fields
Authors: DeVore, C. R.; Antiochos, S. K.
2001AGUSM..SH41C09D Altcode:
We have previously proposed and simulated a differential shear model
for the formation of prominence magnetic fields. For very strong shears
and highly stressed fields, reconnections between the prominence and the
overlying arcade produce helical structures resembling flux ropes. The
resulting configurations relaxed to evidently stable equilibria, showing
no sign of imminent eruption, in the simple bipolar geometries we
considered. Recently, we have turned our attention to more magnetically
complex situations involving multipolar magnetic fields. These include
an interacting two-prominence scenario, in which the ends of a pair
of bipolar prominences come into contact and reconnect; and a single
prominence in a 'breakout' topology, in which the overlying arcade can
reconnect with an exterior field, thereby loosening the restraining
forces holding down the prominence. Results from these simulations
will be presented, and their implications for our understanding of
prominence stability and eruption will be discussed.
---------------------------------------------------------
Title: Evidence for Magnetic Reconnection in CMEs and Eruptive Flares
Authors: Antiochos, S. K.
2001AGUSM..SM22A01A Altcode:
Magnetic reconnection is believed to be the most important process
in the Sun's corona for transferring energy from magnetic field to
plasma; hence, reconnection plays the central role in theories for
all types of solar activity ranging from the smallest spicules to
giant CMEs. In particular, reconnection has long been thought to be
the mechanism for the heating and formation of closed flare loops and
the ejected plasmoid/flux rope. Observational and theoretical results
will be presented, which will make a compelling case for this type of
reconnection. Reconnection has also been proposed as the initiation
process for CMEs and eruptive flares. Theoretical models for this
type of reconnection will be discussed. The observational evidence
in this case is less definitive. I will demonstrate, however, that
flare observations by TRACE also give strong support for the type of
reconnection expected in the “breakout” model for CME initiation. This
work was supported in part by NASA and ONR.
---------------------------------------------------------
Title: Extreme-Ultraviolet Transition-Region Line Emission during
the Dynamic Formation of Prominence Condensations
Authors: Lanza, Antonino F.; Spadaro, Daniele; Lanzafame, Alessandro
C.; Antiochos, Spiro K.; MacNeice, Peter J.; Spicer, Daniel S.;
O'Mullane, Martin G.
2001ApJ...547.1116L Altcode:
We calculated the emission expected in EUV transition-region lines
during the process of dynamic formation of prominence condensations
in coronal loops, as predicted by the thermal nonequilibrium model of
Antiochos et al. We selected some lines emitted by ions of carbon and
oxygen because they are among the most intense and representative
in the temperature range corresponding to the solar transition
region. We present and discuss the principal characteristics of
the line intensities and profiles synthesized from the hydrodynamic
model at different times during the loop evolution. The ionization
balance is computed in detail and the deviations from the ionization
equilibrium caused by plasma flows and variations of temperature
and density are accounted for. The atomic physics is treated
using the latest atomic coefficients and the collisional-radiative
theory approach. The synthesized carbon and oxygen lines exhibit
a behavior significantly dependent on the variations of the plasma
parameters inside the magnetic flux tube and therefore are suitable
observational signatures of the processes giving rise to prominence
condensations. In particular, a sizeable increase of line intensity as
well as small blueshifts are expected from the loop footpoints during
the first part of the evaporation phase that fills the loop with the
material which subsequently condenses into the prominence. Once the
condensation appears, line intensities decrease in the footpoints and
simultaneously increase at the transition regions between the cool
plasma of the condensation and the coronal portion of the loop. Line
shifts are quite small in our symmetric model, and during most of
the condensation's lifetime, the nonthermal widths are relatively
small. These results can be compared with detailed ultraviolet
observations of filament/prominence regions obtained by recent space
missions in order to test the model proposed for the formation of
solar prominences.
---------------------------------------------------------
Title: EUV line emission during the dynamic formation of prominence
condensations
Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Antiochos,
S. K.; O'Mullane, M. G.
2001MmSAI..72..591S Altcode:
This contribution is a short summary of a paper recently submitted
to Astrophysical Journal. We calculated the emission expected in
EUV transition region lines during the process of dynamic formation
of prominence condensations in coronal loops, as predicted by the
thermal non-equilibrium model proposed by Antiochos et al. We present
and discuss the principal characteristics of the line intensities and
profiles synthesized from the hydrodynamic model at different times
during the loop evolution.
---------------------------------------------------------
Title: Determination of Flare Heating and Cooling Using the Transition
Region and Coronal Explorer
Authors: Antiochos, S. K.; DeLuca, E. E.; Golub, L.; McMullen, R. A.
2000ApJ...542L.151A Altcode:
We describe how the Transition Region and Coronal Explorer 171 Å
observations can be used to determine the properties of flare-loop
heating. The key point is that the evolution of a loop transition region
(TR) is much easier to measure quantitatively than the bulk flare plasma
because the TR emission originates from an unobscured source with simple
geometry. We derive general analytic expressions for the evolution
of a flare-loop TR that, in principle, permit a determination of the
heating function from the observations. These results are compared with
observations of the 1998 September 20 flare. We find that the observed
evolution of the flare ribbons is in good agreement with our model for
the evaporative cooling of flare loops and that the heating in these
loops is incompatible with the assumption of spatial uniformity.
---------------------------------------------------------
Title: Twisted Coronal Magnetic Loops
Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.
2000ApJ...542..504K Altcode:
Observed coronal loops have a surprisingly uniform thickness that
cannot be easily understood in terms of standard coronal magnetic
field models. We investigate the possibility that the uniform
thickness can be explained by locally enhanced twist in the field,
so that observed loops correspond to twisted coronal flux tubes. Our
approach is to construct numerical models of fully three-dimensional
force-free magnetic fields. To resolve the internal structure of an
individual loop embedded within a much larger dipole configuration,
we use a nonuniform numerical grid of size 609×513×593, the largest
ever applied to a solar problem to our knowledge. Our models show that
twist promotes circular cross sections in loops. Such cross sections are
typically assumed, and have recently been verified from observations,
but their physical cause has been heretofore unexplained.
---------------------------------------------------------
Title: The Topology and Evolution of the Bastille Day Flare
Authors: Aulanier, G.; DeLuca, E. E.; Antiochos, S. K.; McMullen,
R. A.; Golub, L.
2000ApJ...540.1126A Altcode:
On 1998 July 14, a class M3 flare occurred at 12:55 UT in AR 8270
near disk center. Kitt Peak line-of-sight magnetograms show that the
flare occurred in a δ spot. Mees vector magnetograms show a strong
shear localized near a portion of the closed neutral line around the
parasitic polarity of the δ spot. Observations of the flare in 171,
195, and 1600 Å have been obtained by TRACE, with ~=40 s temporal
and 0.5" spatial resolutions. They reveal that small-scale preflare
loops above the sheared region expanded and disappeared for more than
1 hr before flare maximum. During the flare, bright loops anchored in
bright ribbons form and grow. This occurs while large-scale dimmings,
associated with large expanding loops, develop on both sides of
the active region. This suggests that the flare was eruptive and
was accompanied by a coronal mass ejection (CME). Magnetic field
extrapolations reveal the presence of a null point in the corona, with
its associated “spine” field line, and its “fan” surface surrounding
the parasitic polarity. We show that while the whole event occurs,
the intersections of the “fan” and the “spine” with the photosphere
brighten and move continuously. The interpretation of the event shows
that the magnetic evolution of the eruptive flare is strongly coupled
with its surrounding complex topology. We discuss evidence supporting a
“magnetic breakout” process for triggering this eruptive flare. We
finally conclude that multipolar fields cannot be neglected in the
study and modeling of the origin of CMEs in the corona.
---------------------------------------------------------
Title: Dynamical Formation and Stability of Helical Prominence
Magnetic Fields
Authors: DeVore, C. Richard; Antiochos, Spiro K.
2000ApJ...539..954D Altcode:
We numerically simulated an initially bipolar magnetic field subjected
to shear motions concentrated near and parallel to the photospheric
polarity inversion line. The simulations yield three principal results:
(1) For footpoint displacements comparable to the bipole's depth,
the sheared core field acquires a dipped geometry that can support
cool prominence material against gravity. This confirms previous
force-free equilibrium models for forming dipped prominence fields
by differential shear and extends them to much larger applied shears
and time-dependent dynamics with dissipation. (2) At larger shears,
we discover a new mechanism for forming the helical magnetic fields
of prominences. It entails a two-step process of magnetic reconnection
in the corona. First, flux in the sheared core reconnects with flux in
the unsheared, restraining arcade, producing new pairs of interlinked
field lines. Second, as these interlinked fields continue to be sheared,
they are brought together and reconnect again, producing helical field
threading and enveloping the body of the prominence. This mechanism
can account for the twist that is often observed in both quiescent
and erupting prominences. (3) Even for very large shears, the dipped,
helical structure settles into an apparently stable equilibrium,
despite the substantial amount of reconnection and twist in the magnetic
field. We conclude that neither a kink instability of the helical core
field, nor a tether-cutting instability of the restraining arcade,
is operating in our low-lying model prominence. This concurs with both
observations and a theoretical model for prominence stability.
---------------------------------------------------------
Title: The Thermal Nonequilibrium of Prominences
Authors: Antiochos, S. K.; MacNeice, P. J.; Spicer, D. S.
2000ApJ...536..494A Altcode:
We present numerical simulations and analytic theory for the thermal
nonequilibrium of solar coronal flux tubes that have a stretched-out,
dipped geometry, appropriate for a prominence/filament. Our simulations
indicate that if the heating in such a flux tube is localized near the
chromosphere, then condensations appear which undergo a continuous cycle
of formation, motion, and destruction, even though the heating and all
other imposed conditions on the loop are purely time independent. We
show how this nonsteady evolution can be understood in terms of simple
scaling-law theory. The implications of thermal nonequilibrium for
observations of the solar corona are discussed. We argue that the model
can explain both the formation of prominence condensations and recent
observations of their dynamics.
---------------------------------------------------------
Title: Are dipped field lines required for prominence formation?
Authors: Karpen, J. T.; Antiochos, S. K.; MacNeice, P. J.
2000BAAS...32Q.809K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Reconnection of orthogonal magnetic flux tubes.
Authors: Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K.
2000BAAS...32R.810L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observation and theory of coronal loop structure.
Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.; Watko, J. A.
2000BAAS...32R.809K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Formation and Stability of Helical Prominence Magnetic Fields
Authors: DeVore, C. R.; Antiochos, S. K.
2000SPD....31.1401D Altcode: 2000BAAS...32R.846D
We have numerically simulated the evolution of initially bipolar
magnetic fields, subjected to shear motions concentrated near and
directed parallel to the polarity inversion line at the photosphere. The
simulations yield three principal results: (1) Footpoint displacements
comparable to the bipole's depth produce a dipped geometry in the
sheared core flux, capable of supporting condensed prominence material
against gravity. This confirms and extends the previous results of
force-free field models. (2) For much larger displacements, a new
mechanism for helical field formation ensues. A two-step reconnection
process acts first to reconnect the sheared core flux with flux from the
overlying arcade, and then to reconnect pairs of the newly formed field
lines with each other. The resultant helical field threads and envelops
the body of the prominence. (3) Even for very large displacements,
the dipped, helical structure finds a stable equilibrium. Despite the
substantial amounts of reconnection and twist in the field, it shows
no sign of eruption due to kink instability or tether-cutting. The
results suggest that even helical prominence configurations in simple,
bipolar topologies are immune to eruption, and do not lead to coronal
mass ejections; a multipolar, break-out topology may be essential. This
research was supported by NASA and ONR.
---------------------------------------------------------
Title: Observation and Theory of Coronal Loop Structure
Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.; Watko, J. A.
2000SPD....31.0144K Altcode:
We have carefully examined 43 soft X-ray loops observed by Yohkoh and
24 EUV loops observed by TRACE and find that the large majority have
a nearly uniform thickness. This implies that: 1. the magnetic field
in these loops expands with height much less than standard coronal
models would predict; and 2. the shape of the loop cross section is
approximately circular. We have investigated whether these surprising
results can be explained by locally enhanced twist in the field,
so that observed loops correspond to twisted coronal flux tubes. Our
approach is to construct numerical models of fully three-dimensional
force-free magnetic fields. To resolve the internal structure of an
individual loop embedded within a much larger dipole configuration,
we use a nonuniform numerical grid of size 609 x 513 x 593, the largest
ever applied to a solar problem, to our knowledge. Our models indicate
that twist does indeed promote circular cross sections in the corona,
even when the footpoint cross section is irregular. However, twist does
not seem to be a likely explanation for the observed minimal expansion
with height. This work was supported by the NASA Sun-Earth Connection
Theory and Guest Investigator Programs.
---------------------------------------------------------
Title: Reconnection of Orthogonal Magnetic Flux Tubes
Authors: Linton, M. G.; Dahlburg, R. B.; Antiochos, S. K.
2000SPD....31.0150L Altcode:
The interaction and reconnection of magnetic flux tubes in the solar
atmosphere may be the underlying energy release mechanism in some
types of flares and coronal mass ejections. To study this mechanism,
we present 3-D MHD simulations of reconnection due to the collision
of two orthogonal, twisted flux tubes. For identical tubes there exist
four possible interaction configurations. For the optimal reconnection
configuration, where the field lines in the reconnection region are
initially antiparallel, the flux tubes tunnel. For the intermediate
cases where these field lines are initially orthogonal, the tubes
reconnect to form two new flux tubes. Finally for the case where these
field lines are initially parallel, the two tubes bounce and do not
reconnect. We discuss the implications of these results for flux tube
interactions in the solar atmosphere, in particular the implications
for coronal mass ejection initiation, and for compact flares. This
work is supported by an NRC/NRL postdoctoral grant and by a grant of
time on the DoD HPC program.
---------------------------------------------------------
Title: Magnetic Energy Relaxation by Null-Point Reconnection
Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R.
2000SPD....31.0149A Altcode: 2000BAAS...32..809A
We derive the minimum energy state resulting from complete magnetic
reconnection in a 2.5D MHD system, in the limit of low plasma beta
and high magnetic Reynold's number --- appropriate, in particular,
to the solar corona. The results are useful for determining the
amount of energy that can be liberated by reconnection and, hence, are
important for understanding coronal heating and other forms of solar
activity. The key difference between our approach and previous work
is that reconnection is assumed to occur only at magnetic null points
initially present in the system. We find that the minimum energy state
is not the usual linear force-free field, but a state in which magnetic
stress is distributed uniformly on equal flux surfaces. Our results
are especially important for physical systems such as the solar corona
in which the field is line-tied at the high-beta photosphere and the
volume of the system is infinite, but the results are also valid for
general configurations with flux surfaces as boundaries. We discuss
the implications of this work for the Sun's corona and for laboratory
plasmas. This work was funded in part by ONR and NASA.
---------------------------------------------------------
Title: Are Dipped Field Lines Required for Prominence Formation?
Authors: Karpen, J. T.; Antiochos, S. K.; MacNeice, P. J.
2000SPD....31.0147K Altcode: 2000BAAS...32..809K
Previous studies of prominence formation have been focussed exclusively
on flux systems with dipped geometries (e.g., Antiochos and Klimchuk
1991; Antiochos et al. 1999), under the assumption that long-lived
prominences must contain locations where the cool, dense plasma
can be collected and suspended well above the photosphere. Within
one such configuration, localized asymmetric heating at the base of
long dipped field lines has been shown to yield a continual cycle of
formation, motion, and destruction of cool, dense plasma (Antiochos,
MacNeice, and Spicer 2000), thus reproducing the counterstreaming
flows recently observed along prominence “spines" (Zirker, Engvold,
and Martin 1998). In view of this discovery of the dynamical nature of
prominences, however, we speculate that the presence of dips may not be
necessary. Rather, thermal non-equilibrium in flux tubes with flat or
modestly peaked topologies might be able to produce the same cycle of
condensation and destruction that occurs along dipped field lines. We
have tested this hypothesis by performing a series of 1D hydrodynamic
simulations with ARGOS, an adaptively refined high-order Godunov solver
(see Antiochos et al. 1999). The results, comparison with observations,
and their implications for prominence formation and lifecycle will be
discussed. This work has been supported in part by NASA and ONR.
---------------------------------------------------------
Title: The Topology and Evolution of the Bastille Day Flare Observed
by TRACE
Authors: Aulanier, G.; Antiochos, S. K.; DeLuca, E. E.; McMullen,
R. A.; Golub, L.
2000SPD....31.1402A Altcode: 2000BAAS...32..846A
On July 14, 1998, a class M3 flare occurred at 12:55 UT in AR 8270
near disc center. Kitt Peak line-of-sight magnetograms show that the
flare occurred in a δ -spot. Mees vector magnetograms show a strong
shear localized near a portion of the closed neutral line around the
parasitic polarity of the δ -spot. Observations of the flare in 171
Angstroms, 195 Angstroms and 1600 Angstroms have been obtained by TRACE,
with ~= 40 s temporal and 0.5 arcsec spatial resolutions. They reveal
that small-scale pre-flare loops above the sheared region expanded and
disappeared for more than one hour before flare maximum. During the
flare, bright loops anchored in bright ribbons form and grow. This
occurs while large-scale dimmings, associated with large expanding
loops, develop on both sides of the AR. This suggests that the
flare was eruptive, and was accompanied by a coronal mass ejection
(CME). Magnetic field extrapolations reveal the presence of a null
point in the corona, with its associated “spine” field line, and
its “fan” surface surrounding the parasitic polarity. We show that
while the whole event occurs, the intersections of the “fan” and the
“spine” with the photosphere brighten and move continuously. The
interpretation of the event shows that the magnetic evolution of
the eruptive flare is strongly coupled with its surrounding complex
topology. We discuss evidence supporting a “magnetic breakout”
process for triggering this eruptive flare. We finally conclude that
multipolar fields cannot be neglected in the study and modeling of
the origin of CMEs in the corona. This work is supported, at SAO by
a NASA contract to Lockheed-Martin, and at NRL by NASA and ONR.
---------------------------------------------------------
Title: Theory of solar prominences
Authors: Antiochos, S. K.
2000ssls.work...73A Altcode:
We discuss the physical mechanisms for the formation, the gravitational
support, and the eruption of solar prominences and filaments. We show
that the formation of cool mass in the corona is due to the thermal
non-equilibrium of coronal loops that are heated preferentially near
their footpoints. The gravitational support can be understood as a
direct consequence of the differential shearing of a three-dimensional
magnetic field. Finally, the eruption can be explained by the
"breakout" model, in which magnetic reconnection leads to the explosive
expansion of the sheared prominence field. We present results from
numerical simulations which confirm each of these three fundamental
mechanisms. Based on this work, we conclude that we are now approaching
a true understanding of the basic physics of solar prominences.
---------------------------------------------------------
Title: The Dynamics of Prominence Condensations
Authors: MacNeice, P.; Spicer, D. S.; Antiochos, S. K.
1999ESASP.448..459M Altcode: 1999mfsp.conf..459M; 1999ESPM....9..459M
No abstract at ADS
---------------------------------------------------------
Title: A Model for Prominence Mass and Dynamics
Authors: MacNeice, P.; Spicer, D. S.; Antiochos, S.
1999ESASP.446..457M Altcode: 1999soho....8..457M
Solar prominences and filaments are observed to consist of a collection
of small H-alpha condensations or knots. We address two key issues
concerning these condensations: (1) the mechanism for their formation,
and (2) the origin of their observed motions. Recently, Zirker, Martin
and co-workers have found that filament condensations appear to move
approximately horizontally with velocities of order 5 - 10 km/s. These
observations provide important constraints on the magnetic structure
of prominences. We propose that the condensations form due to a lack
of thermal equilibrium in the magnetic flux tubes threading through
the prominence corona. We use LOOPAMR a new 1D high-order Godunov code
that includes both thermal conduction and radiation with PARAMESH,
a fully adaptive mesh refinement tool, to simulate for the first time
the complete formation process of a prominence condensation. We show
that both the origin and the observed velocities are explained by our
model. In addition we discuss the implication of our model for coronal
heating and for the global structure of prominences.
---------------------------------------------------------
Title: Variation of Thermal Structure with Height of a Solar Active
Region Derived from SOHO CDS and YOHKOH BCS Observations
Authors: Sterling, Alphonse C.; Pike, C. D.; Mason, Helen E.; Watanabe,
Tetsuya; Antiochos, Spiro K.
1999ApJ...524.1096S Altcode:
We present observations of NOAA solar Active Region 7999 when it was
near the west solar limb on 1996 December 2 and 3, using data from
the Coronal Diagnostic Spectrometer (CDS) experiment on the SOHO
satellite. Ratios of intensities of 2 MK material (as observed in
CDS Fe XVI images) to 1 MK material (from CDS Mg IX images) indicate
that there is a drop in the ratio of the hotter to the cooler material
with height in the region, up to an altitude of about 10<SUP>5</SUP>
km. At low altitudes the relative amount of 2 MK emission measure to
1 MK emission measure ranges from 8 to 10, while the ratio is minimum
near 10<SUP>5</SUP> km, ranging from 1.3 to 3.5. The decrease with
height of the CDS ratio qualitatively resembles the decrease in S
XV election temperature with height (measurable up to ~85,000 km) in
the same active region obtained from the Bragg crystal spectrometer
instrument on Yohkoh. The CDS images indicate that the highest S
XV temperatures and largest CDS ratios correspond to regions of
microflares, and somewhat lower S XV temperatures and CDS ratios
correspond to diffuse regions. Above 10<SUP>5</SUP> km, the trend of
the CDS ratios changes, either increasing or remaining approximately
constant with height. At these altitudes the CDS images show faint,
large-scale diffuse structures.
---------------------------------------------------------
Title: Working Group 5: Prominences and Coronal Mass Ejections
Authors: Kucera, T.; Antiochos, S. K.
1999ESASP.446...97K Altcode: 1999soho....8...97K
No abstract at ADS
---------------------------------------------------------
Title: An Eruptive Flare Observed by TRACE as a Test for the Magnetic
Authors: Aulaneir, G.; Deluca, E. E.; Golub, L.; McMullen, R. A.;
Karpen, J. T.; Antiochos, S. K.
1999ESASP.446..135A Altcode: 1999soho....8..135A
No abstract at ADS
---------------------------------------------------------
Title: Shear-driven Reconnection in Chromospheric Eruptions: 3D
Numerical Simulations
Authors: Karpen, J. T.; DeVore, C. R.; Antiochos, S. K.
1999AAS...194.3108K Altcode: 1999BAAS...31..869K
Magnetic reconnection has been implicated in nearly all forms of
solar activity. As observations continue to reveal such activity at
ever smaller scales, the chromospheric transients known as explosive
events and microjets have emerged as perhaps the most likely examples
of reconnection at work on the Sun. Although reconnection has been
studied for decades, only recently has it become feasible to explore
the behavior of interacting flux systems under conditions even remotely
approximating the solar environment. In particular, most analytic
and numerical treatments to date have been two-dimensional and highly
idealized in terms of assumed symmetries and boundary conditions. Our
earlier 2.5D calculations of reconnecting arcades driven by footpoint
motions demonstrated that reconnection can account for the key features
of (and differences between) explosive events and microjets, most
notably the characteristic jets and bidirectional flows. Encouraged
by these results, we have begun to investigate three-dimensional
reconnection at chromospheric/transition region heights between adjacent
bipoles with a new, fully 3D, FCT-based code developed for massively
parallel supercomputers under the NASA HPCC program. The dynamic and
energetic consequences of shear-driven 3D reconnection between paired
bipoles, and comparisons between our simulation results and SOHO and
TRACE observations of chromospheric eruptions, will be presented. (*)
This work is supported by NASA and ONR.
---------------------------------------------------------
Title: The Structure of Solar Prominences
Authors: Antiochos, S. K.; DeVore, C. R.; Klimchuk, J. A.
1999AAS...194.3102A Altcode: 1999BAAS...31Q.868A
With the advent of new high-spatial and high-temporal resolution
observations from SOHO and TRACE, prominences/filaments have once again
become a major focus of study for solar physics. Prominences/filaments
are also important for their role in space weather. They yield
key information on the type of magnetic structure that leads to
eruptive flares and coronal mass ejections. We present results from
our calculations of the 3D magnetic structure of prominences and the
origin of the prominence mass. We show that many of the well-know
features of their global structure, such as the prominence legs and
barbs, the inverse polarity, and the sinistral-dextral property,
can be easily understood as due to the geometry of a sheared bipolar
field. Both fully time-dependent 3D MHD simulations and 3D force-free
field equilibrium calculations demonstrate this conclusion. Furthermore,
we discuss results showing that the magnetic structure of a sheared
3D bipole leads naturally to the formation of cool condensations and
to their observed motions. (*) This work is supported by NASA and ONR.
---------------------------------------------------------
Title: The Dynamic Formation of Prominence Condensations
Authors: Antiochos, S. K.; MacNeice, P. J.; Spicer, D. S.; Klimchuk,
J. A.
1999ApJ...512..985A Altcode: 1998astro.ph..8199A
We present simulations of a model for the formation of a prominence
condensation in a coronal loop. The key idea behind the model is that
the spatial localization of loop heating near the chromosphere leads
to a catastrophic cooling in the corona. Using a new adaptive grid
code, we simulate the complete growth of a condensation and find that
after ~5000 s it reaches a quasi-steady state. We show that the size
and growth time of the condensation are in good agreement with data
and discuss the implications of the model for coronal heating and for
observations of prominences and the surrounding corona.
---------------------------------------------------------
Title: The role of magnetic reconnection in solar activity
Authors: Antiochos, S. K.; DeVore, C. R.
1999GMS...199..113A Altcode: 1998astro.ph..9161A
We argue that magnetic reconnection plays the determining role in many
of the various manifestations of solar activity. In particular, it is
the trigger mechanism for the most energetic of solar events, coronal
mass ejections and eruptive flares. We propose that in order to obtain
explosive eruptions, magnetic reconnection in the corona must have an
“on-off” nature, and show that reconnection in a sheared multi-polar
field configuration does have this property. Numerical simulation
results which support this model are presented, and implications for
coronal mass ejections/eruptive flare prediction are discussed.
---------------------------------------------------------
Title: The Role of Helicity in Magnetic Reconnection: 3D Numerical
Simulations
Authors: Antiochos, Spiro K.; DeVore, C. Richard
1999GMS...111..187A Altcode: 1999astro.ph..1039A; 1999mhsl.conf..187A
We demonstrate that conservation of global helicity plays only a
minor role in determining the nature and consequences of magnetic
reconnection in the solar atmosphere. First, we show that observations
of the solar coronal magnetic field are in direct conflict with
Taylor's theory. Next, we present results from three-dimensional
MHD simulations of the shearing of bipolar and multi-polar coronal
magnetic fields by photospheric footpoint motions, and discuss the
implications of these results for Taylor's theory and for models of
solar activity. The key conclusion of this work is that significant
magnetic reconnection occurs only at very specific locations and,
hence, the Sun's magnetic field cannot relax completely down to the
minimum energy state predicted by conservation of global helicity.
---------------------------------------------------------
Title: A Model for Solar Coronal Mass Ejections
Authors: Antiochos, S. K.; DeVore, C. R.; Klimchuk, J. A.
1999ApJ...510..485A Altcode: 1998astro.ph..7220A
We propose a new model for the initiation of a solar coronal mass
ejection (CME). The model agrees with two properties of CMEs and
eruptive flares that have proved to be very difficult to explain with
previous models: (1) very low-lying magnetic field lines, down to the
photospheric neutral line, can open toward infinity during an eruption;
and (2) the eruption is driven solely by magnetic free energy stored
in a closed, sheared arcade. Consequently, the magnetic energy of the
closed state is well above that of the posteruption open state. The key
new feature of our model is that CMEs occur in multipolar topologies
in which reconnection between a sheared arcade and neighboring flux
systems triggers the eruption. In this “magnetic breakout” model,
reconnection removes the unsheared field above the low-lying, sheared
core flux near the neutral line, thereby allowing this core flux to
burst open. We present numerical simulations that demonstrate our
model can account for the energy requirements for CMEs. We discuss
the implication of the model for CME/flare prediction.
---------------------------------------------------------
Title: The Magnetic Topology of Solar Activity
Authors: Antiochos, Spiro K.
1998APS..DPP.B1M05A Altcode:
Solar activity can appear in many forms, ranging from small spicules
with size scales < 1000 km and energies < 10^24 ergs, to giant
coronal mass ejections and eruptive flares with scales of order the
solar radius and energies up to 10^33 ergs. The underlying driver for
all this dynamics is the Sun's magnetic field. The high-beta plasma
below the photosphere generates and stresses magnetic field which
emerges through the surface and, in turn, energizes the plasma in the
low-beta corona. We describe the recent observations of violent solar
activity and present a general theoretical framework for understanding
them. We argue that strong activity requires two key features:
the magnetic field must have a multi-polar topology, and magnetic
reconnection in the solar corona must have a bursty nature. We present
results from numerical simulations that show how these features lead
to the observed dynamics. (This work was supported in part by NASA
and ONR.)
---------------------------------------------------------
Title: The Magnetic Topology of Solar Eruptions
Authors: Antiochos, S. K.
1998ApJ...502L.181A Altcode: 1998astro.ph..6030A
We present an explanation for the well-known observation that
complexity of the solar magnetic field is a necessary ingredient for
strong activity such as large eruptive flares. Our model starts with
the standard picture for the energy buildup--a highly sheared, newly
emerged magnetic field near the photospheric neutral line held down by
an overlying unsheared field. Previously, we proposed the key new idea
that magnetic reconnection between the unsheared field and neighboring
flux systems decreases the amount of the overlying field and, thereby,
allows the low-lying sheared flux to “break out.” In this Letter,
we show that a bipolar active region does not have the necessary
complexity for this process to occur, but a delta sunspot has the
right topology for magnetic breakout. We discuss the implications of
these results for observations from SOHO and TRACE.
---------------------------------------------------------
Title: LOOPREF: A Fluid Code for the Simulation of Coronal Loops
Authors: Defainchtein, Rosalinda; Antiochos, Spiro; Spicer, Daniel
1998nasa.reptW....D Altcode:
This report documents the code LOOPREF. LOOPREF is a semi-one
dimensional finite element code that is especially well suited to
simulate coronal-loop phenomena. It has a full implementation of
adaptive mesh refinement (AMR), which is crucial for this type of
simulation. The AMR routines are an improved version of AMR1D. LOOPREF's
versatility makes is suitable to simulate a wide variety of problems. In
addition to efficiently providing very high resolution in rapidly
changing regions of the domain, it is equipped to treat loops of
variable cross section, any non-linear form of heat conduction, shocks,
gravitational effects, and radiative loss.
---------------------------------------------------------
Title: LOOPREF: A Fluid Code for the Simulation of Coronal Loops
Authors: Defainchtein, Rosalinda; Antiochos, Spiro; Spicer, Daniel
1998nasa.reptU....D Altcode:
This report documents the code LOOPREF.LOOPREF is a semi-one dimensional
finite element code that is especially well suited to simulate
coronal-loop phenomena. It has a full implementation of adaptive mesh
refinement (AMR), which is crucial for this type of simulation. The
AMR routines are an improved version of AMR1D, an AMR code that is
posted on the World Wide Web, and documented in NASA's Contractor
Report. LOOPREF's versatility makes is suitable to simulate a wide
variety of problems. In addition to efficiently providing very high
resolution in rapidly changing regions of the domain, it is equipped
to treat loops of variable cross section, any non-linear form of heat
conduction, shocks, gravitational effects, and radiative loss.
---------------------------------------------------------
Title: Dynamic Responses to Magnetic Reconnection in Solar Arcades
Authors: Karpen, Judith T.; Antiochos, Spiro K.; Richard DeVore, C.;
Golub, Leon
1998ApJ...495..491K Altcode:
We present a numerical simulation of the interaction between two line
dipoles through magnetic reconnection in the lower solar atmosphere,
a process believed to be the origin of many manifestations of solar
activity. This work differs from previous studies in that the field
is sheared asymmetrically and that the dipoles have markedly unequal
field strengths. This calculation already yielded one key discovery,
denoted reconnection driven current filamentation, as described in a
previous Astrophysical Journal letter. In this paper we focus on the
chromospheric and coronal dynamics resulting from the shear-driven
reconnection of unequal dipoles, discuss the important implications for
chromospheric eruptions, compare our calculation with high-resolution
Normal Incidence X-Ray Telescope observations of a surge, and contrast
our results with the predictions of “fast reconnection” models.
---------------------------------------------------------
Title: Prominence Formation by Localized Heating
Authors: Dahlburg, Russell B.; Antiochos, Spiro K.; Klimchuk, James A.
1998ApJ...495..485D Altcode:
We describe a model for the formation of the cool condensed material
that comprises a coronal filament or prominence. Numerical calculations
are presented which demonstrate that large condensations form in
a coronal loop if the loop satisfies two key requirements: (1) the
loop heating must be localized near the chromospheric footpoints,
and (2) the loop must have a dipped geometry in order to support the
prominence condensation against gravity. We calculate one-dimensional
equilibrium solutions for the equations of force and energy balance
assuming optically thin radiative losses and a parameterized form for
the coronal heating. This physical situation is modeled as a boundary
value problem, which we solve numerically using a B-spline collocation
scheme. The relation of our solutions to the well-known loop scaling
laws is discussed, and the implications of our model for active region
and quiescent prominences are discussed.
---------------------------------------------------------
Title: An analysis of the unresolved fine structure model for the
solar transition region
Authors: Lanza, A. F.; Spadaro, D.; Antiochos, S. K.
1998MmSAI..69..695L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic flux tube tunneling
Authors: Dahlburg, R. B.; Antiochos, S. K.; Norton, D.
1997PhRvE..56.2094D Altcode:
We present numerical simulations of the collision and subsequent
interaction of orthogonal magnetic flux tubes. The simulations were
carried out using a parallelized spectral algorithm for compressible
magnetohydrodynamics. It is found that, under a wide range of
conditions, the flux tubes can “tunnel” through each other,
a behavior not previously seen in studies of either vortex tube or
magnetic flux tube interactions. Two conditions must be satisfied for
tunneling to occur: the magnetic field must be highly twisted with a
field line pitch >>1, and the Lundquist number must be somewhat
large, >=2880. An examination of magnetic field lines suggests
that tunneling is due to a double-reconnection mechanism. Initially
orthogonal field lines reconnect at two specific locations, exchange
interacting sections, and “pass” through each other. The implications
of these results for solar and space plasmas are discussed.
---------------------------------------------------------
Title: The Implications of 3D for Solar MHD Modelling
Authors: Antiochos, S. K.; Dahlburg, R. B.
1997SoPh..174....5A Altcode:
The effects of three-dimensionality on the modelling of solar magnetic
fields are described. We focus on two processes that are believed to
play an important role in coronal heating - the braiding of field
lines by photospheric motions and the reconnection of colliding
flux tubes. First, it is shown that a proper treatment of boundary
conditions at the photosphere in 3D entails qualitatively new physical
processes that are not present in 2D. The numerical resolution of even
simple boundary velocity patterns in 3D leads to obstacles which have no
counterpart in the 2D case. We conclude that adaptive mesh refinement is
necessary for capturing the essential 3D physics of a braiding motion at
the photosphere. Next, the effects of 3D on magnetic reconnection are
discussed. Reconnection in 3D can lead to an evolution of interacting
flux tubes, magnetic tunneling, that is not only impossible in lower
dimensionality, but is strikingly counterintuitive. The implications
of these results for the structure of the solar magnetic field and
for coronal heating are described.
---------------------------------------------------------
Title: The Solar-B Mission
Authors: Antiochos, Spiro; Acton, Loren; Canfield, Richard; Davila,
Joseph; Davis, John; Dere, Kenneth; Doschek, George; Golub, Leon;
Harvey, John; Hathaway, David; Hudson, Hugh; Moore, Ronald; Lites,
Bruce; Rust, David; Strong, Keith; Title, Alan
1997STIN...9721329A Altcode:
Solar-B, the next ISAS mission (with major NASA participation), is
designed to address the fundamental question of how magnetic fields
interact with plasma to produce solar variability. The mission has
a number of unique capabilities that will enable it to answer the
outstanding questions of solar magnetism. First, by escaping atmospheric
seeing, it will deliver continuous observations of the solar surface
with unprecedented spatial resolution. Second, Solar-B will deliver the
first accurate measurements of all three components of the photospheric
magnetic field. Solar-B will measure both the magnetic energy driving
the photosphere and simultaneously its effects in the corona. Solar-B
offers unique programmatic opportunities to NASA. It will continue an
effective collaboration with our most reliable international partner. It
will deliver images and data that will have strong public outreach
potential. Finally, the science of Solar-B is clearly related to the
themes of origins and plasma astrophysics, and contributes directly
to the national space weather and global change programs.
---------------------------------------------------------
Title: The Solar-B Mission
Authors: Antiochos, Spiro K.
1997SPD....28.1102A Altcode: 1997BAAS...29..915A
Solar-B is the ISAS mission follow-on to the highly successful
Japan/US/UK Yohkoh (Solar-A) collaboration. The overall science
goal of Solar-B is to determine the magnetic origins of the solar
variability that Yohkoh and SOHO have been observing, and that
drive the Sun-Earth connection. The mission complement consists of
an optical vector magnetograph and spectrograph, an X-ray telescope,
and an XUV spectroheliograph. This coordinated instrument package will
answer many of the outstanding questions in solar physics. We know
that processes hidden deep within the Sun generate surface activity
in an 11-year cycle. Emerging magnetic field topology can reveal the
workings of this dynamo process. Solar-B will observe the emerging
magnetic field and, for the first time, its twist in detail over large
regions of the Sun. Measurement of the solar“constant" shows the Sun
to be less luminous at sunspot cycle minima. Extremely small scale
features in the solar photosphere cause the solar cycle changes in
the luminosity. Solar-B will make the first observations with spatial
resolution, wavelength coverage, and sampling adequate to determine the
role of these features in the long-term solar luminosity changes. The
solar UV and X-radiation originates in the chromosphere and corona,
where temperatures rise to over one million degrees and where the
plasma is highly dynamic, often erupting into the heliosphere. Solar-B
will open a new window on the underlying causes of coronal heating and
eruptions by providing the first accurate measurements of the Sun's
magnetic field and electric currents, simultaneous with detailed
observations of the coronal dynamics. Programmatically, Solar-B
represents a unique opportunity for NASA to participate with a highly
reliable partner in a frontier-probe class mission at the cost of a
MIDEX. Solar-B also represents an ideal opportunity for continuing
the Solar-Terrestrial Probe Line of the Sun-Earth Connections Theme.
---------------------------------------------------------
Title: Magnetic fluxtube reconnection
Authors: Dahlburg, R. B.; Antiochos, S. K.
1997AdSpR..19.1781D Altcode:
We present the results of 3D numerical simulations of initially discrete
magnetic fluxtubes interacting via magnetic reconnection. The initial
topology consists of two orthogonal fluxtubes. Each fluxtube has a
uniform twist, force-free magnetic field specified by the Gold-Hoyle
model. The fluxtubes are then forced together by an initial flow
configuration consisting of two superimposed stagnation point flows. We
observe three distinct types of interaction, which depend on the twist
and on the Lundquist numbers, between the fluxtubes. For low twist the
fluxtubes experience an elastic collision. For a higher twist complete
reconnection is observed. If the Lundquist numbers are raised fluxtube
tunneling occurs.
---------------------------------------------------------
Title: A Model for Coronal Mass Ejections
Authors: Antiochos, S. K.
1996AAS...189.5603A Altcode: 1996BAAS...28R1346A
Coronal mass ejections (CME) consist of huge eruptions of solar
coronal plasma and magnetic field, and are now known to be the main
drivers of geomagnetic disturbances. The energy source for CMEs must
be magnetic since the plasma beta in the corona is observed to be
low, and the gravitational energy can only increase as a result of
eruption. Although there has been a great deal of work on coronal mass
ejections in recent years, we argue that none of the previous models
is able to satisfy the observational constraints, in particular, the
result that the eruption begins at very low heights in the corona. For
the case of the most energetic events which are usually accompanied
by large eruptive flares, the magnetic field must blow open all the
way down to the chromosphere. In this paper we propose a new model for
CMEs. The key feature of our model is that magnetic reconnection occurs
above the erupting flux, rather than below as in all previous models. We
present theoretical arguments and numerical simulations demonstrating
that our model can explain the observed opening of field lines down
to the chromosphere. This work was supported in part by NASA and ONR.
---------------------------------------------------------
Title: STEREO: a solar terrestrial event observer mission concept
Authors: Socker, Dennis G.; Antiochos, S. K.; Brueckner, Guenter E.;
Cook, John W.; Dere, Kenneth P.; Howard, Russell A.; Karpen, J. T.;
Klimchuk, J. A.; Korendyke, Clarence M.; Michels, Donald J.; Moses,
J. Daniel; Prinz, Dianne K.; Sheely, N. R.; Wu, Shi T.; Buffington,
Andrew; Jackson, Bernard V.; Labonte, Barry; Lamy, Philippe L.;
Rosenbauer, H.; Schwenn, Rainer; Burlaga, L.; Davila, Joseph M.; Davis,
John M.; Goldstein, Barry; Harris, H.; Liewer, Paulett C.; Neugebauer,
Marcia; Hildner, E.; Pizzo, Victor J.; Moulton, Norman E.; Linker,
J. A.; Mikic, Z.
1996SPIE.2804...50S Altcode:
A STEREO mission concept requiring only a single new spacecraft has been
proposed. The mission would place the new spacecraft in a heliocentric
orbit and well off the Sun- Earth line, where it can simultaneously view
both the solar source of heliospheric disturbances and their propagation
through the heliosphere all the way to the earth. Joint observations,
utilizing the new spacecraft and existing solar spacecraft in earth
orbit or L1 orbit would provide a stereographic data set. The new
and unique aspect of this mission lies in the vantage point of the
new spacecraft, which is far enough from Sun-Earth line to allow an
entirely new way of studying the structure of the solar corona, the
heliosphere and solar-terrestrial interactions. The mission science
objectives have been selected to take maximum advantage of this new
vantage point. They fall into two classes: those possible with the
new spacecraft alone and those possible with joint measurements using
the new and existing spacecraft. The instrument complement on the new
spacecraft supporting the mission science objectives includes a soft
x-ray imager, a coronagraph and a sun-earth imager. Telemetry rate
appears to be the main performance determinant. The spacecraft could
be launched with the new Med-Lite system.
---------------------------------------------------------
Title: A Study of the Unresolved Fine-Structure Model for the Solar
Transition Region
Authors: Spadaro, D.; Lanza, A. F.; Antiochos, S. K.
1996ApJ...462.1011S Altcode:
The unresolved fine-structure (UFS) model for the lower transition
region was proposed by Feldman as an explanation for a number of
puzzling observational results: specifically, the small filling factor
of this region, the inability of the observations to resolve the
temperature structure, and the observation of persistent redshifted
UV emission lines even near the solar limb. It was hypothesized that
opacity effects may be able to explain the redshift observations. We
consider a simple model for the UFS consisting of a plasma sphere
undergoing expansion and contraction due to a time-varying heating. We
calculate in detail the line profile of the well-observed C IV 1548
Å line. Our calculations include the effects of both nonequilibrium
ionization and radiative transfer. We find that although the model can
reproduce some of the features of the observations, such as the line
widths, the effect of finite optical depth is to produce a blueshifted
peak for the emission line, contrary to observations. The physical
origins of this blueshift are discussed. We conclude that unless
the pressures of the UFS are significantly higher than the typical
pressures assumed for the lower transition region, opacity effects
are unlikely to explain the observations.
---------------------------------------------------------
Title: Reconnection in the Solar Corona: the Effects of Slow
Footpoint Motions
Authors: Karpen, J. T.; Antiochos, S. K.; DeVore, C. R.
1996AAS...188.8605K Altcode: 1996BAAS...28Q.964K
Previous simulations of magnetic reconnection in both symmetric and
asymmetric topologies (Karpen, Antiochos, & DeVore 1995, ApJ,
450, 422; ApJL, 460, L73) demonstrated that shear-driven reconnection
can reproduce several fundamental features of chromospheric eruptions
(e.g., spicules, surges, and the HRTS explosive events). In asymmetric
systems, moreover, the random nature of the reconnection yields numerous
current sheets over a large but well-defined volume resembling a coronal
loop in profile, a phenomemon which we denoted Reconnection Driven
Current Filamentation. However, in these calculations the field was
subjected to footpoint shearing much stronger than typical photospheric
motions. In this talk we will discuss the response of the symmetric
topology to footpoint motions approximately an order of magnitude
slower, commensurate with typical photospheric flow speeds. The
finite-difference simulation was performed with a new, parallelized
version of our 2.5-dimensional FCT-based code (MAG25D), which solves
the ideal compressible MHD equations with complex boundary conditions;
numerical diffusivity alone provided localized reconnection. We find
that reconnection proceeds in an uneven manner, as the field around the
initial X point oscillates aperiodically between vertical and horizontal
current sheet formations, while the larger-scale surrounding field
rises and falls. The greater separation between the driver and the
characteristic plasma (e.g., the Alfven transit) time scales reveals
a variety of behaviors ranging from rapid bursts of reconnection to
the slow “breathing" of the large-scale stressed field. In addition,
we will explore the implications of these results for the applicability
of fast (Petschek) reconnection models to the solar atmosphere.
---------------------------------------------------------
Title: Reconnection Between Open and Closed Fields in the Solar Corona
Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R.
1996AAS...188.8606A Altcode: 1996BAAS...28..964A
The effects of shear-driven magnetic reconnection in a 2.5D quadrupolar
magnetic topology have been calculated previously (Karpen, Antiochos
& DeVore 1995, ApJL, 460, L73). A quadrupolar topology in the solar
corona corresponds to the interaction of two sheared closed magnetic
arcades of opposite polarity. The key result of the previous work
is that the reconnection proceeds by the creation of a long current
sheet and the sporadic formation of magnetic islands along that
sheet. This leads to the creation of numerous current sheets over a
large volume of the post-reconnection field. Magnetic reconnection has
also been frequently proposed, however, as the origin of the heating
and acceleration in coronal hole regions. The relevant topology
in this case must be that of a closed magnetic arcade and an open
flux system. In addition, reconnection of open field should be more
appropriate for phenomena such as eruptive flares and coronal mass
ejections. Consequently, we have simulated numerically the interaction
of a closed bipolar arcade and an open field flux system. A major
difference between this simulation and the previous case is that
the boundary conditions at the top of the simulation box can play an
important role in the evolution of the reconnection region. We present
the results of our simulations, and contrast them the results for the
quadrupole case. In addition, we discuss the physical reason for the
creation of long current sheets during the reconnection process.
---------------------------------------------------------
Title: Reconnection Driven Current Filament in Solar Arcades
Authors: Karpen, Judith T.; Antiochos, Spiro K.; DeVore, C. Richard
1996ApJ...460L..73K Altcode:
We present numerical simulations of the interaction between two
bipoles through magnetic reconnection in the lower solar atmosphere,
a process believed to be the origin of many manifestations of solar
activity. The present work differs from previous studies in that the
field is sheared asymmetrically and that the bipoles have markedly
unequal field strengths. Our key discovery is that, under such common
circumstances, reconnection leads to an apparently random distribution
of shear in the magnetic field, resulting in numerous current sheets
throughout the volume occupied by the reconnected field lines. To our
knowledge, this is the first example of a numerical simulation yielding
current sheets over a large but well-defined volume of the corona,
resembling a coronal loop in profile. In this Letter, we demonstrate
this process of reconnection-driven current filamentation and discuss
ramifications for coronal heating and structure.
---------------------------------------------------------
Title: The Nature of Magnetic Reconnection in the Corona
Authors: Antiochos, S. K.; Karpen, J. T.; DeVore, C. R.
1996ASPC..111...79A Altcode: 1997ASPC..111...79A
No abstract at ADS
---------------------------------------------------------
Title: Solar Drivers of Space Weather
Authors: Antiochos, Spiro K.
1996ASPC...95....1A Altcode: 1996sdit.conf....1A
No abstract at ADS
---------------------------------------------------------
Title: Reconnection of antiparallel magnetic flux tubes
Authors: Dahlburg, R. B.; Antiochos, S. K.
1995JGR...10016991D Altcode:
Many examples of solar activity, such as large two-ribbon flares
and prominence eruptions, are widely believed to involve the fast
reconnection of magnetic flux tubes. Because of the difficulties
associated with calculating the evolution of three-dimensional (3-D)
flux tubes, however, the details of the energy-release process are
poorly understood. In this paper we describe our first attempts to
shed light on this important process. We describe the results of
3-D numerical simulations of initially distinct magnetic flux tubes
interacting via magnetic reconnection. As a typical case, we consider an
initial magnetic field given by a compact support function distribution
so that the initial topology consists of two antiparallel flux tubes. We
then impose an initial velocity field on this system which causes
the flux tubes to move toward each other. As a result of this initial
velocity, the tubes first flatten against each other and an electric
current sheet begins to develop at the interface between them. After
approximately 10 Alfven times we observe a burst of reconnection. The
turbulent kinetic energy rises dramatically as two reconnection jets
form, which are aligned parallel to the initial field. The reconnection
phase lasts for approximately 20 Alfven times, by which time the central
region of the initial tubes has been completely dissipated so that the
system now consists of four tubes that are relatively widely separated
and hence stop interacting. We find that the excitation of small-scale
spatial structure in the flow field depends critically on the value of
the Lundquist numbers. Compressible effects are insignificant for this
particular case of flux tube reconnection. The numerical simulations
are carried out using a three-dimensional explicit Fourier collocation
algorithm for solving the viscoresistive equations of compressible
magnetohydrodynamics. We also report on the performance of a new
parallelized version of the code.
---------------------------------------------------------
Title: The Role of Magnetic Reconnection in Chromospheric Eruptions
Authors: Karpen, Judith T.; Antiochos, Spiro K.; DeVore, C. Richard
1995ApJ...450..422K Altcode:
We investigate the hypothesis that all chromospheric eruptions
are manifestations of a common magnetohydrodynamic phenomenon
occurring on different scales: the acceleration of chromospheric
plasma driven by localized magnetic reconnection. Our approach is
to perform 2.5-dimensional numerical simulations of shear-induced
reconnection in a potential magnetic field with a central X-point
above the photosphere, embedded in a model chromosphere with solar
gravity and numerical resistivity. Calculations with two values of
the footpoint displacement were performed by applying a localized
body-force duration twice as long in one case as in the other; after
the shearing was discontinued, the system was allowed to relax for
an additional interval. For the stronger shear, the initial X-point
lengthens upward into a current sheet which reconnects gradually for a
while but then begins to undergo multiple tearing. Thereafter, several
magnetic islands develop in sequence, move toward the ends of the sheet,
and disappear through reconnection with the overlying or underlying
field. During the relaxation stage, a new quasi-equilibrium state
arises with a central magnetic island. We also performed a reference
calculation with the stronger shear but with greatly reduced numerical
resistivity along the boundary where the X-point and subsequent current
sheet are located. This simulation confirmed our expectations for
the system evolution in the ideal limit: the current sheet becomes
much longer, without significant reconnection. For the weaker shear,
a much shorter sheet forms initially which then shrinks smoothly through
reconnection to yield an X-point relocated above its original position,
quite distinct from the final state of the strong-shear case. After
reviewing the dynamics and plasma properties as well as the evolving
magnetic topology, we conclude that geometry, shear strength, and local
resistivity must determine the dynamic signatures of chromospheric
eruptions. Our model reproduces such fundamental observed features
as intermittency and large velocities, as well as the approximately
concurrent appearance of oppositely directed flows. We also find that
reconnection in the vertical current sheet is more consistent with
Sweet-Parker reconnection theory, while the rapid interaction between
the magnetic islands and the background field better approximates the
Petschek process.
---------------------------------------------------------
Title: Asymptotic Analysis of Force-free Magnetic Fields of
Cylindrical Symmetry
Authors: Sturrock, P. A.; Antiochos, S. K.; Roumeliotis, G.
1995ApJ...443..804S Altcode:
It is known from computer calculations that if a force-free
magnetic-field configuration is stressed progressively by footpoint
displacements, the configuration expands and approaches the open
configuration with the same surface flux distribution, and, in the
process, the energy of the field increases progressively. Analysis
of a simple model of force-free fields of cylindrical symmetry leads
to simple asymptotic expressions for the extent and energy of such a
configuration. The analysis is carried through for both spherical and
planar source surfaces. According to this model, the field evolves
in a well-behaved manner with no indication of instability or loss
of equilibrium.
---------------------------------------------------------
Title: The Magnetic Field of Solar Prominences
Authors: Antiochos, S. K.; Klimchuk, J. A.; Dahlburg, R. B.
1995SPD....26..717A Altcode: 1995BAAS...27..969A
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric Reconnection in Asymmetric Topologies
Authors: Karpen, J. T.; de Vore, C. R.; Antiochos, S. K.
1995SPD....26..507K Altcode: 1995BAAS...27..958K
No abstract at ADS
---------------------------------------------------------
Title: Parallel Computation of Fluxtube Reconnection
Authors: Norton, D.; Dahlburg, R. B.; Antiochos, S. K.
1995SPD....26.1006N Altcode: 1995BAAS...27..978N
No abstract at ADS
---------------------------------------------------------
Title: Cooling of Solar Flare Plasmas. I. Theoretical Considerations
Authors: Cargill, Peter J.; Mariska, John T.; Antiochos, Spiro K.
1995ApJ...439.1034C Altcode:
Theoretical models of the cooling of flare plasma are reexamined. By
assuming that the cooling occurs in two separate phase where conduction
and radiation, respectively, dominate, a simple analytic formula
for the cooling time of a flare plasma is derived. Unlike earlier
order-of-magnitude scalings, this result accounts for the effect of
the evolution of the loop plasma parameters on the cooling time. When
the conductive cooling leads to an 'evaporation' of chromospheric
material, the cooling time scales L<SUP>5/6</SUP>/p<SUP>1/6</SUP>, where
the coronal phase (defined as the time maximum temperature). When
the conductive cooling is static, the cooling time scales as
L<SUP>3/4</SUP>n<SUP>1/4</SUP>. In deriving these results, use was made
of an important scaling law (T proportional to n<SUP>2</SUP>) during
the radiative cooling phase that was forst noted in one-dimensional
hydrodynamic numerical simulations (Serio et al. 1991; Jakimiec et
al. 1992). Our own simulations show that this result is restricted
to approximately the radiative loss function of Rosner, Tucker, &
Vaiana (1978). for different radiative loss functions, other scaling
result, with T and n scaling almost linearly when the radiative loss
falls off as T<SUP>-2</SUP>. It is shown that these scaling laws are
part of a class of analytic solutions developed by Antiocos (1980).
---------------------------------------------------------
Title: Mass flows in coronal loops
Authors: Antiochos, Spiro K.
1994SSRv...70..143A Altcode:
Although static loop models are often used to describe the structure
of coronal loops, it is evident on both observational and theoretical
grounds that mass motions play a crucial role in the physics of the
corona and transition region. First we review the observations of
emission-line broadening and wavelength shifts, which imply the presence
of random motions and systematic downflows in coronal loops. Some
discrepancies in the observations are discussed. It is argued that
velocities due to gas pressure gradients are the most likely explanation
for the observed flows. A number of models that have been proposed for
these motions are reviewed. The implications of the various models on
observations of the corona and transition region by SOHO are discussed.
---------------------------------------------------------
Title: Observational evidence for non-equilibrium ionization in the
solar corona
Authors: Spadaro, D.; Leto, P.; Antiochos, S. K.
1994SSRv...70..207S Altcode:
We investigate whether temperature sensitive EUV line ratios can be
used as observational signatures for the presence of non-equilibrium
ionization in transition region plasma. We compute the total
intensity of some EUV lines of carbon and oxygen expected from
coronal loop models with a steady-state flow and which are known to
have significant departures from ionization equilibrium, selecting
lines whose intensity ratios are useful for deducing the electron
temperature in the coronal plasma. We calculate the intensity ratios
with and without the approximation of ionization equilibrium, in order
to determine the effects of any deviations from equilibrium on the
numerical values of the line ratios examined.
---------------------------------------------------------
Title: Asymptotic Forms for the Energy of Force-free Magnetic Field
Configurations of Translational Symmetry
Authors: Sturrock, P. A.; Antiochos, S. K.; Klimchuk, J. A.;
Roumeliotis, G.
1994ApJ...431..870S Altcode:
It is known from computer calculations that if a force-free
magnetic field configuration is stressed progressively by footpoint
displacements, the configuration expands and approaches the open
configuration with the same surface flux distribution and the
energy of the field increases progressively. For configurations of
translational symmetry, it has been found empirically that the energy
tends asymptotically to a certain functional form. It is here shown
that analysis of a simple model of the asymptotic form of force-free
fields of translational symmetry leads to and therefore justifies
this functional form. According to this model, the field evolves in
a well-behaved manner with no indication of instability or loss of
equilibrium.
---------------------------------------------------------
Title: Coronal Structures Observed in X-rays (NIXT) and H_alpha Surges
Authors: Schmieder, B.; Mouradian, Z.; Golub, L.; Antiochos, S.
1994kofu.symp..317S Altcode:
Ground-based coordinated observations with the Multichannel subtractive
double pass spectrograph (MSDP) and the heliograph in Meudon allowed
us to portray the chromospheric intensity and velocity fields below
coronal structures observed with the Normal Incidence X-ray Telescope
(NIXT). On July 11, 1991 (eclipse day) we have identified in AR 6713
(N38 W 42) the X-ray signatures of the network, subflares, filaments
and surges. The largest H_alpha surge has only weak emission in
X-ray, while a weak H_alpha feature corresponds to a very bright x-ray
subflare. We calculate the emission measures of these events and give
some constraints on the triggering mechanisms of surges.
---------------------------------------------------------
Title: Observational Tests for Nonequilibrium Ionization in the
Solar Corona
Authors: Spadaro, D.; Leto, P.; Antiochos, S. K.
1994ApJ...427..453S Altcode:
Nonequilibrium ionization may be produced by a variety of processes
in the solar corona, for example, by mass flows through the large
temperature gradients of the transition region or by impulsive heating
and cooling. Any deviation from equilibrium ionization would have a
strong effect on the radiation from the corona and on the interpretation
of solar observations; hence, it is important to determine observational
signatures of nonequilibrium. The temperature-sensitive line ratios can
be used as such signatures. We examine the line ratios: C IV I(1548.2
A)/I(312.4 A), O IV I(789.4 A)/I(554.4 A), O V I(629.7 A)/I(172.2 A),
O VI I(1031.9 A)/I(173.0 A) and O VI I(1031.9 A)/I(150.1 A). These
line ratios are calculated for four coronal loop models that have
a steady flow and that are known to have significant departures
from equilibrium ionization. Our results indicate that, in general,
nonequilibrium causes a considerable reduction in the line ratios,
more than an order of magnitude in the downflowing leg of the loop
model with the largest mass flows. We find that the C IV line ratio is
the most sensitive to nonequilibrium. We discuss the implications of
our results for observations, specifically, the observations expected
from the upcoming SOHO mission.
---------------------------------------------------------
Title: Comparison between Cool and Hot Plasma Behaviors of Surges
Authors: Schmieder, B.; Golub, L.; Antiochos, S. K.
1994ApJ...425..326S Altcode:
Ground-based coordinated observations with the Multichannel Subtractive
Double Pass spectrograph (MSDP) allowed us to obtain chromospheric
intensity and velocity field maps below coronal structures during the
launch of the NIXT payload on 1991 July 11 (eclipse day). A large
H-alpha ejection in AR 6713 (N38 W40) was detected during the NIXT
flight. However, only a low level of X-ray emission was associated
with this event. In contrast, bright X-ray emission associated with a
subflare was observed in a nearby active region, but with only a weak
associated ejection in H-alpha. A discussion of both of these events
gives strong constraints on the triggering mechanisms of surges.
---------------------------------------------------------
Title: The physics of coronal closed-field structures
Authors: Antiochos, Spiro K.
1994AdSpR..14d.139A Altcode: 1994AdSpR..14Q.139A
The properties of closed coronal loops are reviewed. First we
discuss the main features of the static, hot loop models. In these
models thermal conduction plays the dominant role in determining the
temperature and density structure. Next the cool loop models and
their implications for solar observations are discussed. Finally,
some new theoretical results on coronal abundances are presented. It
is argued that chromospheric evaporation, which is a basic feature of
the hot models, can account for the observed anomalies in the coronal
element abundances.
---------------------------------------------------------
Title: A Numerical Study of the Sudden Eruption of Sheared Magnetic
Fields
Authors: Roumeliotis, George; Sturrock, Peter A.; Antiochos, Spiro K.
1994ApJ...423..847R Altcode:
We investigate the quasi-static evolution of an idealized magnetic
configuration in the solar corona that is subjected to photospheric
shearing motions. The initial, unsheared field in our calculations
is a magnetic dipole located at the center of the Sun. The assumed
photospheric shearing motions are latitude-dependent and antisymmetric
about the equator. The quasi-static evolution of the coronal field
is calculated using the magneto-frictional method. A key difference
between our study and previous work is that the outer computational
boundary is placed exceedingly far from the solar surface where the
shearing motions are applied. This is achieved by writing the basic
equations of the magneto-frictional method in terms of the logarithm
of radial distance. We find that initially, the coronal magnetic field
expands steadily as the footpoint displacement is increased. However,
when the footpoint displacement exceeds a certain critical amount,
the qualitative behavior of the evolving field suddenly changes,
so that the outward expansion of the field lines becomes a much more
rapidly increasing function of the footpoint displacement. We propose
that this sudden transition to a regime with very sensitive dependence
on boundary conditions plays an important role in the onset of eruptive
phenomena in the solar atmosphere.
---------------------------------------------------------
Title: Sleuthing the Dynamo: HST/FOS Observations of UV Emissions
of Solar-Type Stars in Young Clusters
Authors: Ayres, T.; Basri, G.; Simon, T.; Stauffer, J.; Stern, R.;
Antiochos, S.; Bookbinder, J.; Brown, A.; Doschek, G.; Linsky, J.;
Ramsey, L.; Walter, F.
1994ASPC...64...53A Altcode: 1994csss....8...53A
No abstract at ADS
---------------------------------------------------------
Title: A Far-Ultraviolet Flare on a Pleiades G Dwarf
Authors: Ayres, T. R.; Stauffer, J. R.; Simon, Theodore; Stern, R. A.;
Antiochos, S. K.; Basri, G. S.; Bookbinder, J. A.; Brown, A.; Doschek,
G. A.; Linsky, J. L.; Ramsey, L. W.; Walter, F. M.
1994ApJ...420L..33A Altcode:
The Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) recorded
a remarkable transient brightening in the C IV lambda lambda 1548,50
emissions of the rapidly rotating Pleiades G dwarf H II 314. On the one
hand the 'flare' might be a rare event luckily observed; on the other
hand it might be a bellwether of the coronal heating in very young
solar-mass stars. If the latter, flaring provides a natural spin-down
mechanism through associated sporadic magnetospheric mass loss.
---------------------------------------------------------
Title: The Magnetic Field of Solar Prominences
Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J. A.
1994ApJ...420L..41A Altcode:
A model is presented which accounts for the formation of coronal
magnetic field lines with the appropriate 'dipped' structure to
support prominences. The critical ingredients of the model are that the
prominence magnetic field is a truly three-dimensional structure with
significant variation along the prominence length, and the magnetic
field is strongly sheared near the photospheric neutral line. Numerical
calculations are presented which demonstrate that these two features
lead to dip formation. In addition our model is able to account for the
long-puzzling observation of inverse polarity in quiescent prominences.
---------------------------------------------------------
Title: The Asymptotic Behavior of Force-Free Magnetic-Field
Configurations
Authors: Sturrock, P. A.; Klimchuk, J. A.; Roumeliotis, G.; Antiochos,
S. K.
1994ASPC...68..219S Altcode: 1994sare.conf..219S
No abstract at ADS
---------------------------------------------------------
Title: Coronal Structures Observed in X-Rays (NIXT) and Hα Surges
Authors: Schmieder, B.; Mouradian, Z.; Golub, L.; Antiochos, S.
1994emsp.conf..159S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Current Sheet Formation in Complex Solar Coronal Fields
Authors: Benka, Steve G.; Antiochos, Spiro K.
1993AAS...183.5903B Altcode: 1993BAAS...25.1386B
We discuss the formation of current singularities and reconnection in
magnetic fields with complex 3D topology. First, we argue that since
the photospheric field is observed to consist of a complicated mixture
of positive and negative polarity regions, the coronal magnetic field
must, in general, contain a large number of separatrix surfaces and
null points. Using numerical simulations, we calculate the effect of
photospheric stressing on such a field. As initial conditions in the
numerical model, we assume a cylindrically-symmetric potential field
consisting of a small dipole field imbedded in a background larger
dipole; hence, there are three polarity regions on the photosphere. In
the corona the field has a hemisperical separatrix surface with a
null point at the apex of this surface. The initial field is then
stressed by footpoint motions at the photosphere that have the
form of a vortical flow of finite width. Results are discussed for
two different photospheric locations of this flow, one in which the
flow is centered on the symmetry axis so that the system retains its
cylindrical symmetry, and one in which the flow is offset from the
symmetry axis. The results of these simulations are discussed, in
particular, the nature of reconnection in a true 3D geometry. We find
that in both cases current sheets form at the separatrix. We argue
that this mechanism for current sheet formation may play a central
role in coronal heating.
---------------------------------------------------------
Title: The Structure of Prominence Magnetic Fields
Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J. A.
1993BAAS...25.1206A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal Current Sheet Formation: 3D Simulations
Authors: Benka, S. G.; Antiochos, S. K.; Zalesak, S. T.; Spicer, D. S.
1993BAAS...25.1207B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Reconnection of Magnetic Flux Tubes
Authors: Dahlburg, R. B.; Antiochos, S. K.
1993BAAS...25.1199D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Asymptotic Forms for the Energy of Force Free Magnetic-Field
Configurations
Authors: Sturrock, P. A.; Roumeliotis, G.; Antiochos, S.
1993BAAS...25.1218S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Kelvin-Helmholtz Instability in Photospheric Flows:
Effects on Coronal Heating and Structure
Authors: Karpen, Judith T.; Antiochos, Spiro K.; Dahlburg, Russell B.;
Spicer, Daniel S.
1993ApJ...403..769K Altcode:
A series of hydrodynamic numerical simulations has been used to
investigate the nonlinear evolution of driven, subsonic velocity
shears under a range of typical photospheric conditions. These
calculations show that typical photospheric flows are susceptible to
the Kelvin-Helmholtz instability (KHI), with rapid nonlinear growth
times that are approximately half of a typical granule lifetime. The
KHI produces vortical structures in intergranule lanes comparable
to a typical fluxule radius; this is precisely the correct scale for
maximum power transfer to the corona.
---------------------------------------------------------
Title: Three-Dimensional Magnetic Reconnection in a Coronal Neutral
Sheet
Authors: Dahlburg, R. B.; Antiochos, S. K.; Zang, T. A.
1993ASSL..183..611D Altcode: 1993pssc.symp..611D
No abstract at ADS
---------------------------------------------------------
Title: Secondary instability in three-dimensional magnetic
reconnection
Authors: Dahlburg, R. B.; Antiochos, S. K.; Zang, T. A.
1992PhFlB...4.3902D Altcode:
We consider the transition to turbulence in three-dimensional
reconnection of a magnetic neutral sheet. We find that the
transition can occur via a three-step process. First, the sheet
undergoes the usual tearing instability. Second, the tearing mode
saturates to form a two-dimensional quasi-steady state. Third,
this secondary equilibrium is itself unstable when it is perturbed
by three-dimensional disturbances. Most of this paper is devoted to
the analysis and simulation of the three-dimensional linear stability
properties of the two-dimensional saturated tearing layer. The numerical
simulations are performed with a semi-implicit, pseudospectral-Fourier
collocation algorithm. We identify a three-dimensional secondary linear
stability which grows on the ideal timescale. An examination of the
modal energetics reveals that the largest energy transfer is from the
mean field to the three-dimensional field, with the two-dimensional
field acting as a catalyst.
---------------------------------------------------------
Title: The physics of solar prominences.
Authors: Antiochos, Spiro K.
1992ESASP.348..201A Altcode: 1992cscl.work..201A
The outstanding questions on the formation of quiescent prominences are
discussed. One key issue is identified to be the formation of dips in
coronal magnetic field lines. A model is presented which can account
for such dipped field lines. The critical ingredients of the model are
that (a) the prominence magnetic field is a truly three dimensional
structure with significant variation along the prominence length, and
(b) the magnetic field has strong shear concentrated at the photospheric
neutral line. Simulations are presented which demonstrate that these
two features lead to dip formation, and that the geometry of the dips
are such that inverse polarity prominences can be explained. Another
key issue is identified to be the formation of prominence condensations
on dipped field lines. It is argued that a spatially-varying coronal
heating rate which is maximum near the chromosphere can explain these
condensations.
---------------------------------------------------------
Title: A Model for the Magnetic Fields of Solar Prominences
Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J.
1992AAS...180.1205A Altcode: 1992BAAS...24..748A
No abstract at ADS
---------------------------------------------------------
Title: Secondary Instability in 3d Neutral Sheets
Authors: Dahlburg, R. B.; Antiochos, S. K.; Zang, A.
1992AAS...180.5504D Altcode: 1992BAAS...24..819D
No abstract at ADS
---------------------------------------------------------
Title: Dynamics of Solar Coronal Magnetic Fields
Authors: Dahlburg, R. B.; Antiochos, S. K.; Zang, T. A.
1991ApJ...383..420D Altcode:
A 3D time-dependent numerical simulation of the foot-point stressing
of coronal magnetic field was developed in order to relate coronal
activity with the stressing of the coronal magnetic field by foot-point
motions at the photosphere. The results of the simulation did not reveal
magnetic reconnection, kinking, or the formation of concave-up magnetic
field lines suitable for prominence formation. It is concluded that,
contrary to many models, photospheric twisting of a single arcade does
not lead to the type of processes required to explain solar activity.
---------------------------------------------------------
Title: Coronal Current-Sheet Formation: The Effect of Asymmetric
and Symmetric Shears
Authors: Karpen, Judith T.; Antiochos, Spiro K.; DeVore, C. R.
1991ApJ...382..327K Altcode:
A 2.5D numerical code is used to investigate the results of an
asymmetric shear imposed on a potential quadrupolar magnetic field
under two sets of atmospheric boundary conditions - a low-beta plasma
with line tying at the base, similar to the line-tied analytic model,
and a hydrostatic-equilibrium atmosphere with solar gravity, typical
of the observed photosphere-chromosphere interface. The low-beta
simulation confirms the crucial role of the line-tying assumption in
producting current sheets. The effects of a symmetric shear on the same
hydrostatic-equilibrium atmosphere is examined, using more grid points
to improve the resolution of the current structures which form along
the flux surfaces. It is found that true current sheets do not form
in the corona when a more realistic model is considered. The amount
of Ohmic dissipation in the thick currents is estimated to be two to
four orders of magnitude below that required to heat the corona. It
is concluded that magnetic topologies of the type examined here do
not contribute significantly to coronal heating.
---------------------------------------------------------
Title: Nonequilibrium Ionization Effects in Asymmetrically Heated
Loops
Authors: Spadaro, D.; Antiochos, Spiro K.; Mariska, J. T.
1991ApJ...382..338S Altcode:
The effects of nonequilibrium ionization on magnetic loop models with
a steady siphon flow that is driven by a nonuniform heating rate are
investigated. The model developed by Mariska (1988) to explain the
observed redshifts of transition region emission lines is examined,
and the number densities of the ions of carbon and oxygen along the
loop are computed, with and without the approximation of ionization
equilibrium. Considerable deviations from equilibrium were found. In
order to determine the consequences of these nonequilibrium effects
on the characteristics of the EUV emission from the loop plasma, the
profiles and wavelength positions of all the important emission lines
due to carbon and oxygen were calculated. The calculations are in broad
agreement with Mariska's conclusions, although they show a significant
diminution of the Doppler shifts, as well as modifications to the line
widths. It is concluded that the inclusion of nonequilibrium effects
make it more difficult to reproduce the observed characteristics of
the solar transition region by means of the asymmetric-heating models.
---------------------------------------------------------
Title: Magnetic Reconnection in Three Dimensions
Authors: Dahlberg, R. B.; Antiochos, S. K.; Zang, T. A.
1991BAAS...23.1467D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Model for the Formation of Solar Prominences
Authors: Antiochos, S. K.; Klimchuk, J. A.
1991ApJ...378..372A Altcode:
A model for the formation of prominence condensations in hot coronal
loops is proposed. Previous studies have concentrated on cooling the
hot plasma by decreasing the coronal heating rate. The difficulty
with such models is that when the heating decreases, most of the
loop mass is lost by draining onto the chromosphere. It is argued
that a prominence condensation is likely to be due to an increase in
the heating. The key idea of the model is that the heating increase is
spatially dependent so that it is localized nearer to the chromospheric
footpoints than to the loop midpoint. Results are presented of
numerical simulations of hot loops that are initially heated uniformly,
and then undergo heating increases that are concentreated away from
the loop midpoint. The temperature at the midpoint first increases,
but eventually it collapses to chromospheric values as a result of
chromospheric evaporation. Hence, a curious result is obtained, that
increasing the heating causes cooling. The resulting densities and
time scales agree well with observations. The implications of this
model for coronal heating and prominence structure are discussed.
---------------------------------------------------------
Title: Report of the solar physics panel
Authors: Withbroe, George L.; Fisher, Richard R.; Antiochos, Spiro;
Brueckner, Guenter; Hoeksema, J. Todd; Hudson, Hugh; Moore, Ronald;
Radick, Richard R.; Rottman, Gary; Scherrer, Philip
1991spsi....1...67W Altcode:
Recent accomplishments in solar physics can be grouped by the
three regions of the Sun: the solar interior, the surface, and the
exterior. The future scientific problems and areas of interest involve:
generation of magnetic activity cycle, energy storage and release,
solar activity, solar wind and solar interaction. Finally, the report
discusses a number of future space mission concepts including: High
Energy Solar Physics Mission, Global Solar Mission, Space Exploration
Initiative, Solar Probe Mission, Solar Variability Explorer, Janus,
as well as solar physics on Space Station Freedom.
---------------------------------------------------------
Title: The Effects of the Kelvin-Helmholtz Instability on Photospheric
Flows
Authors: Karpen, J. T.; Antiochos, S. K.; Dahlburg, R. B.; Spicer,
D. S.
1991BAAS...23.1059K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Effects of Strong Shear on Solar Coronal Magnetic Fields
Authors: DeVore, C. R.; Antiochos, S. K.
1991BAAS...23.1058D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Model for the Anomalous Elemental Abundances of the Solar
Corona and Wind
Authors: Antiochos, S. K.
1991BAAS...23.1046A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar astronomy
Authors: Rosner, Robert; Noyes, Robert; Antiochos, Spiro K.; Canfield,
Richard C.; Chupp, Edward L.; Deming, Drake; Doschek, George A.;
Dulk, George A.; Foukal, Peter V.; Gilliland, Ronald L.
1991aap..reptR....R Altcode:
An overview is given of modern solar physics. Topics covered include
the solar interior, the solar surface, the solar atmosphere, the Large
Earth-based Solar Telescope (LEST), the Orbiting Solar Laboratory, the
High Energy Solar Physics mission, the Space Exploration Initiative,
solar-terrestrial physics, and adaptive optics. Policy and related
programmatic recommendations are given for university research and
education, facilitating solar research, and integrated support for
solar research.
---------------------------------------------------------
Title: The Effects of Nonequilibrium Ionization on the Radiative
Losses of the Solar Corona
Authors: Spadaro, D.; Zappala, R. A.; Antiochos, S. K.; Lanzafame,
G.; Noci, G.
1990ApJ...362..370S Altcode: 1990ApJ...362R.370S
The emissivity of the ions of carbon and oxygen has been recalculated
for a set of solar coronal loop models with a steady state siphon
flow. The ion densities were calculated from the plasma velocities,
temperatures, and densities of the models, and large departures from
equilibrium were found. For purposes of comparison, the emissivity
was calculated with and without the approximation of ionization
equilibrium. Considerable differences in the radiative loss function
Lambda(T) curve between equilibrium and nonequilibrium conditions
were found. The nonequilibrium Lambda(T) function was then used to
solve again the steady state flow equations of the loop models. The
differences in the structure of these models with respect to the models
calculated adopting the Lambda(T) curve in equilibrium are discussed.
---------------------------------------------------------
Title: Numerical simulation of solar coronal magnetic fields
Authors: Dahlburg, Russell B.; Antiochos, Spiro K.; Zang, T. A.
1990nasa.rept.....D Altcode:
Many aspects of solar activity are believed to be due to the
stressing of the coronal magnetic field by footpoint motions at
the photosphere. The results are presented of a fully spectral
numerical simulation which is the first 3-D time dependent simulation
of footpoint stressing in a geometry appropriate for the corona. An
arcade is considered that is initially current-free and impose a smooth
footpoint motion that produces a twist in the field of approx 2 pi. The
footprints were fixed and the evolution was followed until the field
relaxes to another current-free state. No evidence was seen for any
instability, either ideal or resistive and no evidence for current
sheet formation. The most striking feature of the evolution is that in
response to photospheric motions, the field expands rapidly upward to
minimize the stress. The expansion has two important effects. First,
it suppresses the development of dips in the field that could support
dense, cool material. For the motions assumed, the magnetic field does
not develop a geometry suitable for prominence formation. Second, the
expansion inhibits ideal instabilities such as kinking. The results
indicate that simple stearing of a single arcade is unlikely to lead
to solar activity such as flares or prominences. Effects are discussed
that might possibly lead to such activity.
---------------------------------------------------------
Title: On the Formation of Current Sheets in the Solar Corona
Authors: Karpen, Judith T.; Antiochos, Spiro K.; DeVore, C. Richard
1990ApJ...356L..67K Altcode:
Several theoretical studies have proposed that, in response to
photospheric footpoint motions, current sheets can be generated in
the solar corona without the presence of a null point in the initial
potential magnetic field. A fundamental assumption in these analyses,
commonly referred to as the line-tying assumption, is that all coronal
field lines are anchored to a boundary surface representing the top of
the dense, gas pressure-dominated photosphere. It is shown here that
line-typing cannot be applied indiscriminately to dipped coronal fields,
and that the conclusions of the line-tied models are incorrect. To
support the theoretical arguments, the response of a dipped potential
magnetic field in a hydrostatic-equilibrium atmosphere to shearing
motions of the footpoints is studied, using a 2.5-dimensional MHD
code. The results show that, in the absence of artificial line-tying
conditions, a current sheet indeed does not form at the location of
the dip. Rather, the dipped magnetic field rises, causing upflows of
photospheric and chromospheric plasma.
---------------------------------------------------------
Title: Episodic Coronal Heating
Authors: Sturrock, P. A.; Dixon, W. W.; Klimchuk, J. A.; Antiochos,
S. K.
1990ApJ...356L..31S Altcode:
A study is made of the observational consequences of the hypothesis
that there is no steady coronal heating, the solar corona instead
being heated episodically, such that each short burst of heating
is followed by a long period of radiative cooling. The form of the
resulting contribution to the differential emission measure (DEM), and
to a convenient related function (the differential energy flux, DEF) is
calculated. Observational data for the quiet solar atmosphere indicate
that the upper branch of the DEM, corresponding to temperatures above
100,000 K, can be interpreted in terms of episodic energy injection
at coronal temperatures.
---------------------------------------------------------
Title: The Effect of Nonequilibrium Ionization on Ultraviolet Line
Shifts in the Solar Transition Region
Authors: Spadaro, D.; Noci, G.; Zappala, R. A.; Antiochos, S. K.
1990ApJ...355..342S Altcode:
The line profiles and wavelength positions of all the important
emission lines due to carbon were computed for a variety of steady
state siphon flow loop models. For the lines from the lower ionization
states (C II-C IV) a preponderance of blueshifts was found, contrary
to the observations. The lines from the higher ionization states can
show either a net red- or blueshift, depending on the position of the
loop on the solar disk. Similar results are expected for oxygen. It
is concluded that the observed redshifts cannot be explained by the
models proposed here.
---------------------------------------------------------
Title: The Formation of Current Sheets in the Solar Corona:
Asymmetric Shears
Authors: Karpen, J. T.; Antiochos, S. K.; DeVore, C. R.
1990BAAS...22..869K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Structure and Dynamics of Solar Coronal Magnetic Fields
Authors: Antiochos, S. K.; Dahlburg, R. B.; Zang, T.
1990BAAS...22..869A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Coronal Magnetic Field Evolution
Authors: Dahlburg, R. B.; Antiochos, S. K.; Zang, T. A.
1990BAAS...22..851D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mass Flows and the Ionization States of Coronal Loops: Erratum
Authors: Noci, G.; Spadaro, D.; Zappala, R. A.; Antiochos, S. K.
1990ApJ...349..678N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Structures and flows in coronal loops
Authors: Antiochos, Spiro K.
1990GMS....58..203A Altcode:
In the present consideration of the plasma flows in solar loops, the
field is approximated as completely rigid, since the plasma beta in
the corona is low and observed motion time-scales are much longer
than Alfven time scales. It is noted that the widely-used static
models are less than valid, since any asymmetry in loop geometry,
coronal heating, or chromospheric boundary condition will lead to a
'siphon' flow along the loop. Attention given to whether such flows
can account for the observed redshifts, as well as to the possible
importance of nonequilibrium ionization in these models. Impulsive
heating may be able to generate the observed redshifts.
---------------------------------------------------------
Title: Heating of the corona by magnetic singularities
Authors: Antiochos, Spiro K.
1990MmSAI..61..369A Altcode:
Theoretical models of current-sheet formation and magnetic heating in
the solar corona are examined analytically. The role of photospheric
connectivity in determining the topology of the coronal magnetic field
and its equilibrium properties is explored; nonequilibrium models of
current-sheet formation (assuming an initially well connected field)
are described; and particular attention is given to models with
discontinuous connectivity, where magnetic singularities arise from
smooth footpoint motions. It is shown that current sheets arise from
connectivities in which the photospheric flux structure is complex,
with three or more polarity regions and a magnetic null point within
the corona.
---------------------------------------------------------
Title: The Evolution of a Sheared Potential Magnetic Field in the
Solar Corona
Authors: Karpen, J. T.; Antiochos, S. K.; DeVore, C. R.
1990IAUS..142..309K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Episodic Coronal Heating and the Solar Differential Emission
Measure
Authors: Sturrock, P. A.; Klimchuk, J. A.; Antiochos, S. K.
1989BAAS...21R1186S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Evolution of Twisted Flux-Tubes in the Solar Corona
Authors: Dahlburg, R. B.; Antiochos, S. K.; Picone, J. M.
1989BAAS...21.1111D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Formation of Solar Prominences
Authors: Antiochos, S. K.; Klimchuk, J. A.
1989BAAS...21.1185A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Effect of Coronal Elemental Abundances on the Radiative
Loss Function
Authors: Cook, J. W.; Cheng, C. -C.; Jacobs, V. L.; Antiochos, S. K.
1989ApJ...338.1176C Altcode:
The solar photosphere and corona abundances tabulated by Meyer
(1985) and the chromospheric abundances given by Murphy (1985) are
used here to recalculate radiative loss functions for equilibrium,
low-density, optically thin plasmas. Results from a representative
standard photospheric abundance set and from coronal and chromospheric
abundance sets showing depletions of up to a factor of four in
certain elemental abundances are compared. A significant difference
is found for both the coronal and chromospheric abundance sets, with
the peak of the radiative loss curve shifted closer to 10 to the 6th
K than to the standard 2 x 10 to the 5th K found from photospheric
abundances. Consequences of these new calculations, in particular for
the cool loop model of Antiochos and Noci (1986), are discussed.
---------------------------------------------------------
Title: An Episodic Model of Coronal Heating
Authors: Sturrock, P. A.; Antiochos, S. K.
1989BAAS...21R.829S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nonlinear Thermal Instability in Magnetized Solar Plasmas
Authors: Karpen, Judith T.; Antiochos, Spiro K.; Picone, J. Michael;
Dahlburg, Russell B.
1989ApJ...338..493K Altcode:
The radiation-driven thermal instability might explain the formation and
maintenance of cool dense regions embedded in a hotter more rarefied
plasma. Structures of this type often are observed in astrophysical
environments such as the solar corona or the interstellar medium. In
the present work, the response of a magnetized solar transition-region
plasma to a spatially random magnetic-field perturbation is simulated,
where the magnetic field is perpendicular to the computational plane. It
is found that the presence of the magnetic field, the value of the
plasma beta, and the heating process significantly influence the number
and size of the condensations as well as the evolutionary time scale.
---------------------------------------------------------
Title: A Model for the Heating of the Transition Region
Authors: Antiochos, S. K.; Dere, K. P.
1989BAAS...21..841A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mass Flows and the Ionization State of Coronal Loops
Authors: Noci, G.; Spadaro, D.; Zappala, R. A.; Antiochos, S. K.
1989ApJ...338.1131N Altcode:
A basic assumption in the analysis of EUV and X-ray solar emission
is that the plasma is in ionization equilibrium. The effects of mass
flows on the ionization state of solar plasma have been investigated
in order to check the validity of ionization equilibrium. Solar
coronal loop models with a steady state flow as described by Antiochos
(1984) are considered. The number densities of carbon ions have been
determined for four loop models that cover a range of densities and
flow velocities. The results show evidence of nonequilibrium ionization
effects even for velocities of only a few km/s at the loop top and
10 times less at the base, with densities ranging from 10 to the
8th to 10 to the 10th/cu cm between the top and the footpoints. The
importance of these results for the analysis of EUV and X-ray solar
emission is discussed.
---------------------------------------------------------
Title: The evolution of a sheared potential magnetic field in a
gravity stratified atmosphere.
Authors: Karpen, J. T.; Antiochos, S. K.
1989BAAS...21R1027K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Thermal instability in magnetized solar plasma.
Authors: Karpen, J. T.; Antiochos, S. K.; Picone, J. M.; Dahlburg,
R. B.
1989GMS....54...99K Altcode: 1989sspp.conf...99K
In astrophysical plasmas such as the solar corona or the interstellar
medium, the radiation-driven thermal instability might explain
the formation of cool, dense regions embedded in a hotter, more
rarefied medium. In the present work, the authors extend their
previous investigation of this phenomenon by simulating the response
of a magnetized solar transition-region plasma to a spatially random
magnetic-field perturbation, where the magnetic field is perpendicular
to the computational plane. This investigation has determined the
effects of varying the plasma β and the heating mechanism.
---------------------------------------------------------
Title: Spectral simulation of coronal processes.
Authors: Dahlburg, R. B.; Antiochos, S. K.; Picone, J. M.; Zang, T. A.
1989BAAS...21.1028D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric explosions.
Authors: Doschek, G. A.; Antiochos, S. K.; Antonucci, E.; Cheng,
C. -C.; Culhane, J. L.; Fisher, G. H.; Jordan, C.; Leibacher, J. W.;
MacNiece, P.; McWhirter, R. W. P.; Moore, R. L.; Rabin, D. M.; Rust,
D. M.; Shine, R. A.
1989epos.conf..303D Altcode:
The work of this team addressed the question of the response and
relationship of the flare chromosphere and transition region to the
hot coronal loops that reach temperatures of about 10<SUP>7</SUP>K
and higher. Flare related phenomena such as surges and sprays were
also discussed. The team members debate three main topics: 1) whether
the blue-shifted components of X-ray spectral lines are signatures of
"chromospheric evaporation"; 2) whether the excess line broadening of UV
and X-ray lines is accounted for by "convective velocity distribution"
in evaporation; and 3) whether most chromospheric heating is driven by
electron beams. These debates illustrated the strengths and weaknesses
of our current observations and theories.
---------------------------------------------------------
Title: Magnetic topology and current sheet formation.
Authors: Antiochos, S. K.
1989sasf.confP.277A Altcode: 1988sasf.conf..277A; 1989IAUCo.104P.277A
The author describes a mechanism for coronal heating. The basic idea is
that since the photospheric flux is observed to consist of a complex
pattern of positive and negative polarity regions, the topology of
the coronal magnetic field (in particular the connectivity) must be
discontinuous over a complex network of surfaces and magnetic null
points in the corona. Consequently, photospheric motions of the field
line footpoints, even if arbitrarily smooth, result in discontinuous
stressing of the field. This produces coronal current sheets,
reconnection at the null points, and rapid heating.
---------------------------------------------------------
Title: Sub-sonic mass flows and ionization state in coronal loops
Authors: Noci, G.; Spadaro, D.; Zappala, R. A.; Antiochos, S. K.
1989MmSAI..60...55N Altcode:
The effects of subsonic mass flows on the ionization state of the
solar plasma inside magnetic loops are studied. Motions along the
magnetic field lines from one footpoint of the loop to the other are
considered in order to investigate the effects of the motion through
positive and negative temperature gradients. The number densities of
carbon ions are determined for some loop models that cover a range of
densities and flow velocities. The results show that deviations from
ionization equilibrium can occur in coronal loops with a steady-state
subsonic flow from one footpoint to the other. The deviations depend
on the electron density and flow velocity. The importance of these
results for the analysis of EUV and X-ray solar emission is discussed.
---------------------------------------------------------
Title: LASCO: A wide-field white light and spectrometric coronagraph
for SOHO
Authors: Michels, D. J.; Schwenn, R.; Howard, R. A.; Bartoe, J. -D. F.;
Antiochos, S. K.; Brueckner, G. E.; Cheng, C. -C.; Dere, K. P.;
Doschek, G. A.; Mariska, J. T.
1988sohi.rept...55M Altcode:
The scientific objectives of the LASCO (light and spectrometric
coronagraph) project in the SOHO (solar and heliospheric observatory)
mission are described. These include investigation of mechanisms
for heating of the corona and acceleration of the solar wind, causes
of coronal transients, and their role in development of large scale
coronal patterns and interplanetary disturbances. The distribution
and properties of dust particles, including those released from
sun-grazing comets are investigated. Interactions of coronal plasma
with the dust are studied. The corona is analyzed spectroscopically
by a high-resolution scanning, imaging interferometer. The spectral
profiles of three emission lines and one Fraunhofer line are measured
for each picture point, giving temperatures, velocities, turbulent
motions and volume densities. Polarization analysis yields the direction
of coronal magnetic fields.
---------------------------------------------------------
Title: On the Formation of Coronal Current Sheets Without Null Points
Authors: Antiochos, S. K.; Karpen, J. T.
1988BAAS...20.1029A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Effects of Magnetic Topology on Coronal Heating
Authors: Antiochos, S. K.
1988BAAS...20Q.681A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Numerical Study of the Nonlinear Thermal Stability of
Solar Loops
Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T.
1987ApJ...320..409K Altcode:
A time-dependent numerical model is used to investigate the nonlinear
thermal stability of static loops of various heights. Simulations show
that the instability of a hot state with loop heights of less than about
1000 km is physically significant, with an initially hot atmosphere
in low-lying compact loops evolving to an extended atmosphere with
temperatures far below 100,000 K. Results also show that high-lying
loops are stable to all reasonable perturbations, including those of
large initial amplitude and long wavelength. The simulation results
suggest that low-lying compact loops should not be common to the sun,
and that cool loops with temperatures near 100,000 K must be formed
in the cool state initially and cannot evolve from preexisiting loops.
---------------------------------------------------------
Title: A numerical study of the thermal stability of solar loops.
Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T.
1987NASCP2483..113K Altcode: 1987tphr.conf..113K
An important property of all loops is their thermal stability. If low
lying hot loops were thermally unstable, for example, a great majority
of the low loops on the Sun might be expected to be cool. How small
perturbations evolve in low lying, linearly unstable hot loops was
determined and how high lying, linearly stable hot loops respond to
large amplitude disturbances such as might be expected on the Sun were
examined. Only general descriptions and results are given.
---------------------------------------------------------
Title: Effect on the Radiative Loss Function of Coronal Elemental
Abundances
Authors: Cook, J. W.; Cheng, C. -C.; Antiochos, S. K.
1987BAAS...19..931C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Topology of Force-free Magnetic Fields and Its Implications
for Coronal Activity
Authors: Antiochos, Spiro K.
1987ApJ...312..886A Altcode:
The topological constraints on coronal magnetic fields are
considered. For a field that is initially well-behaved and undergoes
deformation by well-behaved ideal MHD motions, it is shown that
the topology of the field lines in the corona can be determined at
all times solely from the footpoint positions on the photospheric
boundary. This result implies that the topology and, consequently,
the history of the footpoint motions impose no further constraints on
the field beyond those already included in the connectivity boundary
conditions, so that there is no reason to expect a lack of equilibrium
for fields that are initially well-behaved and evolve by ideal MHD. On
the other hand, nonideal processes such as reconnection are bound to
occur in the solar corona, and these may lead to magnetic topologies
that have no well-bahaved Euler potentials. Hence Parker's hypothesis
that footpoint motions lead to the formation of current sheets is still
likely to be correct, but only if nonideal processes are included. The
effects of reconnection on magnetic topology and the implications for
coronal activity are discussed.
---------------------------------------------------------
Title: Theory of Cool Loops and the Dividing Line (Invited review)
Authors: Antiochos, Spiro K.
1987LNP...291..283A Altcode: 1987csss....5..283A
Static models for coronal loops have been widely used to interpret
observations of the coronae of cool stars. Although these models have
been successful in explaining several features of the observations;
they have been unsuccessful in accounting for two key features: (a) in
dwarf stars they do not agree with the observed form of the differential
emission measure at low temperatures, T < 10<SUP>5</SUP> K; and (b)
in certain giant stars they do not agree with the lack of emission at
high temperatures, T > 10<SUP>5</SUP> K (the so-called dividing
line). It appears that in high gravity stars there is more cool
material than the standard models of the transition region predict;
whereas in low gravity stars there is less hot material than the loop
models predict.
---------------------------------------------------------
Title: Chromospheric explosions
Authors: Doschek, G. A.; Antiochos, S. K.; Antonucci, E.; Cheng,
C. -C.; Culhane, J. L.; Fisher, G. H.; Jordan, C.; Leibacher, J. W.;
MacNeice, P.; McWhirter, R. W. P.
1986epos.conf..4.1D Altcode: 1986epos.confD...1D
Three issues relative to chromospheric explosions were debated. (1)
Resolved: The blue-shifted components of x-ray spectral lines are
signatures of chromospheric evaporation. It was concluded that
the plasma rising with the corona is indeed the primary source of
thermal plasma observed in the corona during flares. (2) Resolved:
The excess line broading of UV and X-ray lines is accounted for by a
convective velocity distribution in evaporation. It is concluded that
the hypothesis that convective evaporation produces the observed
X-ray line widths in flares is no more than a hypothesis. It is
not supported by any self-consistent physical theory. (3) Resolved:
Most chromospheric heating is driven by electron beams. Although it
is possible to cast doubt on many lines of evidence for electron
beams in the chromosphere, a balanced view that debaters on both
sides of the question might agree to is that electron beams probably
heat the low corona and upper chromosphere, but their direct impact
on evaporating the chromosphere is energetically unimportant when
compared to conduction. This represents a major departure from the
thick-target flare models that were popular before the Workshop.
---------------------------------------------------------
Title: Topological constraints and the existence of force-free fields.
Authors: Antiochos, S. K.
1986NASCP2442..419A Altcode: 1986copp.nasa..419A
A fundamental problem in plasma theory is the question of the existence
of MHD equilibria. The issue of topological constraints is of crucial
importance for the problem of the existence of equilibria. Heuristic
methods are used to discuss the coronal wrapping pattern. It is
concluded that for a given set of footpoint positions the wrapping
pattern in the corona is completely fixed. The topological constraints
are included in the boundary conditions on the Euler potentials and
impost no additional restrictions on possible equilibria. Although
this does not prove that equilibria always exist, it does show that
the force-free problem is not overdetermined and that existence of
equilibria is still an open question.
---------------------------------------------------------
Title: A numerical study of the thermal stability of low-lying
coronal loops.
Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T.
1986NASCP2442..389K Altcode: 1986copp.nasa..389K
The nonlinear evolution of loops that are subjected to a variety
of small but finite perturbations was studied. Only the low-lying
loops are considered. The analysis was performed numerically using a
one-dimensional hydrodynamical model developed at the Naval Research
Laboratory. The computer codes solve the time-dependent equations
for mass, momentum, and energy transport. The primary interest is
the active region filaments, hence a geometry appropriate to those
structures was considered. The static solutions were subjected to a
moderate sized perturbation and allowed to evolve. The results suggest
that both hot and cool loops of the geometry considered are thermally
stable against amplitude perturbations of all kinds.
---------------------------------------------------------
Title: Force-free Magnetic Fields: The Magneto-frictional Method
Authors: Yang, W. H.; Sturrock, P. A.; Antiochos, S. K.
1986ApJ...309..383Y Altcode:
The problem under discussion is that of calculating magnetic field
configurations in which the Lorentz force j x B is everywhere zero,
subject to specified boundary conditions. We choose to represent
the magnetic field in terms of Clebsch variables in the form B =
grad alpha x grad beta. These variables are constant on any field
line so that each field line is labeled by the corresponding values
of alpha and beta. When the field is described in this way, the most
appropriate choice of boundary conditions is to specify the values
of alpha and beta on the bounding surface. We show that such field
configurations may be calculated by a magneto-frictional method. We
imagine that the field lines move through a stationary medium, and
that each element of magnetic field is subject to a frictional force
parallel to and opposing the velocity of the field line. This concept
leads to an iteration procedure for modifying the variables alpha and
beta, that tends asymptotically towards the force-free state. We apply
the method first to a simple problem in two rectangular dimensions,
and then to a problem of cylindrical symmetry that was previously
discussed by Barnes and Sturrock (1972). In one important respect,
our new results differ from the earlier results of Barnes and Sturrock,
and we conclude that the earlier article was in error.
---------------------------------------------------------
Title: The Differential Emission Measure of Transiently Heated
Coronal Loops
Authors: Antiochos, S. K.; Sturrock, P. A.
1986BAAS...18..901A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Dividing Line for Stellar Coronae
Authors: Antiochos, S. K.; Haisch, B. M.; Stern, R. A.
1986ApJ...307L..55A Altcode:
The authors describe a possible explanation for the observation that
late-type stars falling in a certain region of the H-R diagram exhibit
no X-ray emission and, hence, appear not to have coronae. The basic
idea of the authors' model is that due to the low surface gravity
that characterizes the stars without X-ray emission, a hot (T >
10<SUP>6</SUP>K) corona is thermally unstable and spontaneously
cools down to chromospheric temperatures. The key parameter that
determines the outer atmospheric structure is shown to be the ratio
of the gravitational scale height of plasma at T = 10<SUP>5</SUP>K to
the maximum height of closed magnetic field lines in the corona.
---------------------------------------------------------
Title: Modeling of Coronal X-Ray Emission from Active Cool
Stars. I. Hyades Cluster
Authors: Stern, R. A.; Antiochos, S. K.; Harnden, F. R., Jr.
1986ApJ...305..417S Altcode:
X-ray pulse height spectra of the most active cool stars in the Hyades
cluster obtained with the Einstein IPC cannot be satisfactorily
fitted using isothermal thin plasma emission models. Addition of a
second isothermal component provides acceptable fits. However, a more
physically meaningful set of coronal parameters is provided by models
which consist of an ensemble of loops wih a single maximum temperature,
but with the temperature distribution within the loop determined by
loop physics. Such models have been successfully fitted to the IPC
pulse height spectra. Constraints on loop parameters are discussed
for the F-G dwarfs BD + 14 deg 693, BD + 14 deg 690, BD + 15 deg 640,
and 71 Tau. Models with a large variation of loop cross section from
base to top do not fit the data. A consistent physical description is
an ensemble of small high-pressure loops of similar maximum temperature
which dominate the coronal X-ray spectrum.
---------------------------------------------------------
Title: A Numerical Study of the Stability of Low-Lying Solar Loops
Authors: Mariska, J. T.; Klimchuk, J. A.; Antiochos, S. K.
1986BAAS...18Q.708M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Structure of the Static Corona and Transition Region
Authors: Antiochos, S. K.; Noci, G.
1986ApJ...301..440A Altcode:
Static models of coronal loops are investigated. For loops that are
low-lying with heights above the chromosphere below about 5000 km, it
is shown that a new type of solution appears to the static equations,
in addition to the well-known coronal loop solution. The new solution
is characterized by a maximum plasma temperature less than about
100,000 K. The structure and properties of these cool solutions
are discussed. The differential emission measure Q(T) expected for a
magnetic arcade, which must naturally contain both hot and cool loops,
is calculated. It is shown that the cool loops have a dramatic effect
on the form of Q(T) in the lower transition region. In particular,
they can account for the observed rise in Q at low T, which has long
been thought to be incompatible with the static-loop model. Finally,
the implications of the cool loops on other observations of both the
solar and stellar coronae and transition regions are discussed.
---------------------------------------------------------
Title: Chromospheric explosions.
Authors: Doschek, G. A.; Antiochos, S. K.; Antonucci, E.; Cheng,
C. -C.; Culhane, J. L.; Fisher, G. H.; Jordan, C.; Leibacher, J. W.;
MacNiece, P.; McWhirter, R. W. P.; Moore, R. L.; Rabin, D. M.; Rust,
D. M.; Shine, R. A.
1986NASCP2439....4D Altcode:
The work of this team addressed the question of the response and
relationship of the flare chromosphere and transition region to the
hot coronal loops that reach temperatures of about 10<SUP>7</SUP>K
and higher. Flare related phenomena such as surges and sprays are
also discussed. The team members debated three main topics: 1. whether
the blue-shifted components of X-ray spectral lines are signatures of
"chromospheric evaporation"; 2. whether the excess line broadening of UV
and X-ray lines is accounted for by "convective velocity distribution"
in evaporation; and 3. whether most chromospheric heating is driven
by electron beams.
---------------------------------------------------------
Title: Modeling of Coronal X-Ray Emission from Active Cool Stars
Authors: Stern, R. A.; Antiochos, S. K.; Harnden, F. R., Jr.
1986LNP...254..216S Altcode: 1986csss....4..216S
X-ray pulse-height spectra of the most active cool stars in the
Hyades cluster obtained with the Einstein IPC cannot be modeled using
isothermal thin plasma emission. Addition of a second isothermal
component provides acceptable fits. However, a more physically
meaningful set of coronal parameters is provided by models which
consist of an ensemble of loops with a single maximum temperature,
but with the temperature distribution within the loop determined
by loop physics. Such models have been successfully fit to the IPC
pulse-height spectra. Constraints on loop parameters are discussed
for four F-G dwarfs in the Hyades.
---------------------------------------------------------
Title: On the topology of force-free magnetic fields.
Authors: Antiochos, S. K.
1986BAAS...18..853A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Thermal stability of static coronal loops. I - Effects of
boundary conditions
Authors: Antiochos, S. K.; Shoub, E. C.; An, C. -H.; Emslie, A. G.
1985ApJ...298..876A Altcode: 1985STIN...8522330A
The linear stability of static coronal-loop models undergoing thermal
perturbations was investigated. The effect of conditions at the
loop base on the stability properties of the models was considered in
detail. The question of appropriate boundary conditions at the loop base
was considered and it was concluded that the most physical assumptions
are that the temperature and density (or pressure) perturbations
vanish there. However, if the base is taken to be sufficiently deep
in the chromosphere, either several chromospheric scale heights or
several coronal loop lengths in depth, then the effect of the boundary
conditions on loop stability becomes negligible so that all physically
acceptable conditions are equally appropriate. For example, one could
as well assume that the velocity vanishes at the base. The growth
rates and eigenmodes of static models in which gravity is neglected
and in which the coronal heating is a relatively simple function,
either constant per-unit mass or per-unit volume were calculated. It
was found that all such models are unstable with a growth rate of the
order of the coronal cooling time. The physical implications of these
results for the solar corona and transition region are discussed.
---------------------------------------------------------
Title: The Effect of Gravity on the X-Ray and UV Emission of Cool
Stars
Authors: Antiochos, S. K.
1985BAAS...17..570A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Summary proceedings of the Standford Workshop on Solar Flare
Prediction held in Paris on 28 February - 1 March 1985
Authors: Antiochos, S. K.; Bai, T.; Sturrock, P. A.
1985STIN...8623543A Altcode:
A workshop on The Prediction of Solar Activity was held at Meudon
Observatory in France in June 1984. During that meeting, a number
of participants from the United States expressed interest in meeting
together to discuss this topic with a view to exploring what actions
might be taken to improve our predictive capability. This document
contains abstracts of presentations made at the meeting.
---------------------------------------------------------
Title: The Structure of Transition Region Loops
Authors: Antiochos, S. K.
1985BAAS...17..631A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Differential Emission Measure in the Lower Transition
Region
Authors: Antiochos, S. K.
1984BAAS...16..928A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic flare model of γ-ray bursts
Authors: Liang, E. P.; Antiochos, S. K.
1984Natur.310..121L Altcode:
The thermal synchrotron (TS) interpretation of γ-ray burst continuum
spectra<SUP>1</SUP> has recently gained support from the analysis of
an expanding database<SUP>2-5</SUP>. However, this interpretation
requires an emission region which is hot (kT~0.2-1mc<SUP>2</SUP>),
optically very thin (nh <~ 10<SUP>21</SUP> cm<SUP>-2</SUP>) with
a highly super-Eddington flux F ~ 10<SUP>30</SUP> erg cm<SUP>-2</SUP>
s<SUP>-1</SUP> (»F<SUB>Edd</SUB> = 10<SUP>25</SUP> M/M<SUB>solar</SUB>)
for a 10-km neutron star. This picture is similar to that first
proposed for the 5 March 1979 event<SUP>6-8</SUP>. In addition there
are hints that the emission layer is very dense (n<SUB>e</SUB>
<= 10<SUP>24</SUP>-10<SUP>26</SUP> cm<SUP>-3</SUP>) and thin
(h <= 10<SUP>-3</SUP> cm). For example, events which show
simultaneous redshifted 511-keV annihilation lines and low energy
self-absorption (see Fig. 1) allow us to estimate a lower limit in
the range 10<SUP>23</SUP>-10<SUP>26</SUP> cm<SUP>-1</SUP> (ref. 9)
to the pair density, provided that the annihilation region coincides
with the synchrotron source. To maintain the pair population close
to maxwellian, the collision excitation rate into higher Landau
levels<SUP>9</SUP> (that is, the pitch-angle scattering rate) must
exceed the cyclotron decay rates. Coulomb scattering between pairs
and protons is too slow. Even if collective processes whose rates
are close to the electron plasma frequency, or scattering by heavy
ions (for example, Z = 26, rates ~ Z<SUP>2</SUP>), are invoked, a
particle density n >= 10<SUP>25</SUP> cm<SUP>-3</SUP> is still
needed. Both arguments point towards a dense but thin emitting
sheet with h <~ 10<SUP>-3</SUP>-10<SUP>-5</SUP> cm. The severe
energetics and persistence of such a hot, dense, thin emitting layer
prompted us to consider a picture in which the emission regions lie at
the foot points of reconnecting magnetic loops, powered by downward
impinging electromagnetic waves. We now examine the structure of the
emitting sheet, and the generation, propagation and coupling of the
electromagnetic energy fluxes to the surface layer, and show that
a flare-like model can account for most of the general features of
γ-ray bursts.
---------------------------------------------------------
Title: A dynamic model for the solar transition region
Authors: Antiochos, S. K.
1984ApJ...280..416A Altcode:
A model is developed for the lower transition region that can account
for the persistent and ubiquitous redshifts that are observed in the
UV emission lines formed at these temperatures. It is shown that
these shifts are not likely to be due either to falling spicular
material or to steady-state siphon flows. The model consists of two
key ingredients. The redshifted radiation originates from a minority
of flux tubes which have higher gas pressures than their surroundings,
and consequently have their transition regions situated below the
transition regions of their surroundings. The coronal heating in these
loops is impulsive in nature, and this is responsible for the transient
mass flows. The studies, therefore, favor theories for coronal heating
which involve flare-like magnetic-energy release. Previously announced
in STAR as N83-29163
---------------------------------------------------------
Title: The Effects of Gravity on the Stability of Coronal Loops
Authors: Antiochos, S. K.
1984BAAS...16..404A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Erratum - a Giant X-Ray Flare in the Hyades
Authors: Stern, R. A.; Underwood, J. H.; Antiochos, S. K.
1983ApJ...275L..25S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated Einstein and IUE observations of a disparitions
brusques type flare event and quiescent emission from Proxima
Centauri.
Authors: Haisch, B. M.; Linsky, J. L.; Bornmann, P. L.; Stencel,
R. E.; Antiochos, S. K.; Golub, L.; Vaiana, G. S.
1983ApJ...267..280H Altcode:
The Einstein Imaging Particle Counter observed a major X-ray flare
in its entirety during a 5-hr period of simultaneous observations,
with the IUE, of the dM5e flare star Proxima Centauri in August,
1980. The detailed X-ray light curve, temperature determinations
during various intervals, and UV line fluxes obtained before, during,
and after the flare indirectly indicate a 'two-ribbon flare' prominence
eruption. The calculated ratio of coronal to bolometric luminosity for
the event is about 100 times the solar ratio. The Proxima Cen corona
is analyzed in the context of static loop models, in light of which
it is concluded that less than 6% of the stellar surface seems to be
covered by X-ray emitting active regions.
---------------------------------------------------------
Title: On the Thermal Stability of Coronal Loops
Authors: Antiochos, S. K.
1983BAAS...15..704A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A giant X-ray flare in the Hyades.
Authors: Stern, R. A.; Underwood, J. H.; Antiochos, S. K.
1983ApJ...264L..55S Altcode:
The authors have observed a giant stellar flare in the Hyades binary
HD 27130 = VB 22 = BD +16°577 with the Einstein Observatory. The
peak X-ray luminosity of the flare is greater than 10<SUP>31</SUP>erg
s<SUP>-1</SUP>, at least several thousand times brighter than the
most intense solar flares. The ratio of flare peak to quiescent X-ray
luminosity is ≡35. HD 27130, first detected as an X-ray source in the
central Hyades survey of Stern et al., recently has been determined
to be a double-lined eclipsing binary with a period of 5.6 days. The
primary is a G dwarf, and the secondary is a K dwarf. The temperature
estimated for the flare (≡4×10<SUP>7</SUP>K) and the form of the
flare decay suggest that it is solar-like. It is suggested that giant
flares may be typical of young or rapidly rotating systems.
---------------------------------------------------------
Title: A dynamic model for the transition region
Authors: Antiochos, S. K.
1982STIN...8329163A Altcode:
We develop a model for the lower transition region that can account
for the persistent and ubiquitous redshifts that are observed in
the UV emission lines formed at these temperatures. We show that
these shifts are not likely to be due either to falling spicular
material or to steady-state siphon flows. Our model consists of two
key ingredients. The redshifted radiation originates from a minority
of flux tubes which have higher gas pressures than their surroundings,
and consequently have their transition regions situated below the
transition regions of their surroundings. The coronal heating in these
loops is impulsive in nature, and this is responsible for the transient
mass flows. Our studies, therefore, favor theories for coronal heating
which involve flare-like magnetic-energy release.
---------------------------------------------------------
Title: Implications of Solar Flare Observations on Stellar X-Ray
Flares
Authors: Antiochos, S. K.; Haisch, B. M.; Stern, R. A.
1982BAAS...14..864A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the thermal stability of coronal loop plasma
Authors: Antiochos, S. K.; Emslie, A. G.; Shoub, E. C.; An, C. H.
1982STIN...8234327A Altcode:
The stability to thermal perturbation of static models of coronal
loops is considered including the effects of cool, radiatively
stable material at the loop base. The linear stability turns out to
be sensitive only to the boundary conditions assumed on the velocity
at the loop base. The question of the appropriate boundary conditions
is discussed, and it is concluded that the free surface condition
(the pressure perturbation vanishes), rather than the rigid wall
(the velocity vanishes), is relevant to the solar case. The static
models are found to be thermally unstable, with a growth time of the
order of the coronal cooking time. The physical implications of these
results for the solar corona and transition region are examined.
---------------------------------------------------------
Title: International Ultraviolet Explorer observations of hyades
stars.
Authors: Zolcinski, M. C. S.; Antiochos, S. K.; Stern, R. A.; Walker,
A. B. C.
1982ApJ...258..177Z Altcode:
A description is presented of International Ultraviolet Explorer (IUE)
satellite observations of transition region and chromospheric emission
from a group of Hyades dwarfs which are strong X-ray emitters as seen
in a survey conducted by Stern et al. (1981). Short-wavelength spectra
(1175-2000 A) and long-wavelength spectra (1900-3200 A) have been
obtained. Although the IUE sensitivity limit did not make it possible
to detect emission lines in three stars, the presence of chromospheres
and transition regions could be confirmed in BD +15 deg 640, 70 Tau, BD
+14 deg 693, and BD +16 deg 592. The differential emission measure has
been plotted as a function of temperature for the four considered stars.
---------------------------------------------------------
Title: Erratum - Stellar Coronae in the Hyades - a Soft X-Ray Survey
with the Einstein Observatory
Authors: Stern, R. A.; Zolcinski, M. C.; Antiochos, S. K.; Underwood,
J. H.
1982ApJ...258..904S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Are Coronal Loops Stable?
Authors: Antiochos, S. K.
1982BAAS...14..623A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The cooling and condensation of flare coronal plasma
Authors: Antiochos, S. K.; Sturrock, P. A.
1982ApJ...254..343A Altcode:
A model is investigated for the decay of flare heated coronal
loops in which rapid radiative cooling at the loop base creates
strong pressure gradients which, in turn, generate large (supersonic)
downward flows. The important features of this model which distinguish
it from previous models of flare cooling are: (1) Most of the thermal
energy of the coronal plasma may be lost by mass motion rather than
by conduction or coronal radiation. (2) Flare loops are not isobaric
during their decay phase, and large downward velocities are present
near the footpoints. (3) The differential emission measure has a
strong temperature dependence. These results can account for recent
observations of compact flare loops that are not consistent with the
previous cooling models.
---------------------------------------------------------
Title: An X-ray flare in the Hyades binary HD 27130.
Authors: Stern, R. A.; Underwood, J. H.; Antiochos, S. K.
1982SAOSR.392B.101S Altcode: 1982csss....2..101S
No abstract at ADS
---------------------------------------------------------
Title: The differential emission measure of dynamic coronal loops.
Authors: Antiochos, S. K.
1982SAOSR.392B.115A Altcode: 1982csss....2..115A
No abstract at ADS
---------------------------------------------------------
Title: Progress report of an IUE survey of the Hyades star cluster.
Authors: Zolcinski, M. C.; Kay, L.; Antiochos, S.; Stern, R.; Walker,
A. B. C.
1982NASCP2238..239Z Altcode: 1982auva.nasa..239Z; 1982NASCP2338..239Z; 1982IUE82......239Z
To date 11 of the brightest X-Ray stars (F-K dwarfs) in the Hyades
have been observed with the IUE satellite with the short wavelength
spectrograph. The IUE results and the X-Ray observations from the Hyades
survey with the Einstein Observatory were combined. The differential
emission measure function was estimated for each of the 7 stars which
showed evidence of emission lines. Constraints on stellar atmospheric
parameters (chromospheric pressure, coronal temperature and filling
factor were derived. The implications of these results in the context
of loop models for the corona and transition region (TR) of these
stars are discussed.
---------------------------------------------------------
Title: The differential emission measure of dynamic coronal loops
Authors: Antiochos, S. K.
1981STIN...8220087A Altcode:
The effects of time dependent phenomena, such as flare energization
and decay, on the temperature and density structure of the transition
region and, in particular, on the form of the differential emission
measure are studied. It is found that unlike the case of the static
models, the form of the differential emission measure can be used to
determine the important physical mechanisms in the dynamic models.
---------------------------------------------------------
Title: Stellar coronae in the hyades : a soft X-ray survey with
the EinsteinObservatory.
Authors: Stern, R. A.; Zolcinski, M. Ch.; Antiochos, S. K.; Underwood,
J. H.
1981ApJ...249..647S Altcode:
An X-ray survey of the central region of the Hyades cluster demonstrates
that soft X-ray emission is a common property of the stars in the
cluster. Half of the 85 stars surveyed are detected above a sensitivity
threshold of 10 to the 28.5th ergs/s at the Hyades distance of 45
pc. The high incidence of X-ray emission and range of observed X-ray
luminosities indicate that stellar coronas produce the observed X-ray
emission, with a typical X-ray luminosity for solar-type Hyades of 10
to the 29th ergs/s. The use of coronal scaling laws is found to yield
reasonable values of maximum coronal temperatures and the fraction of
stellar surface covered for the Hyades coronas.
---------------------------------------------------------
Title: The cooling and condensation of flare coronal plasma
Authors: Antiochos, S. K.; Sturrock, P. A.
1981STIN...8127029A Altcode:
A model is investigated for the decay of flare heated coronal loops
in which rapid radiative cooling at the loop base creates strong
pressure gradients which, in turn, generate large (supersonic)
downward flows. The coronal material cools and 'condenses' onto
the flare chromosphere. The features which distinguish this model
from previous models of flare cooling are: (1) most of the thermal
energy of the coronal plasma may be lost by mass motion rather than
by conduction or coronal radiation; (2) flare loops are not isobaric
during their decay phase, and large downward velocities are present
near the footpoints; (3) the differential emission measure q has a
strong temperature dependence.
---------------------------------------------------------
Title: On the Thermal Stability of Coronal Loop Plasma
Authors: Antiochos, S. K.; Emslie, A. G.
1981BAAS...13..555A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Structure of the Lower Transition Region
Authors: Antiochos, S. K.
1981BAAS...13..835A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Structure of a Force-Free Coronal Loop
Authors: Wear, K. A.; Antiochos, S. K.; Emslie, A. G.; Sturrock, P. A.
1981BAAS...13..542W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: X-Ray Flare in HD 27130
Authors: Stern, R. A.; Underwood, J. H.; Antiochos, S. K.; McClure, R.
1981IAUC.3585....2S Altcode:
R. A. Stern and J. H. Underwood, Jet Propulsion Laboratory; and
S. K. Antiochos, Stanford University, guest observers with the Einstein
Observatory, write: "The 0.3-6.0-nm x-ray flux from the Hyades binary
system HD 27130 underwent a 40-fold increase to 10**24 J/s shortly
before 1980 Sept. 20d06h13m UT. This flare decayed with an e-foldlng
time of ~ 2500 s. HD 27130 has been determined recently to be a
double-lined eclipsing binary with a period of 5.6 days. The primary
is a main-sequence G star, while the secondary is probably a K dwarf
(R. McClure, private communication). Monitoring of this system for
evidence of optical flaring or unusual spectral characteristics would
be valuable."
---------------------------------------------------------
Title: The Structure of a Force Free Magnetic Flux Tube
Authors: Wear, K. A.; Antiochos, S. K.; Sturrock, P. A.
1981BAAS...13..915W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Numerical studies of the energy balance in coronal loops.
Authors: Underwood, J. H.; Antiochos, S. K.; Vesecky, J. F.
1981ASIC...68..227U Altcode: 1981spss.conf..227U
A numerical method is applied to treat the energy balance of
quasi-static solar coronal loops, which have been observed to
persist for periods much greater than the radiative cooling time. The
quasi-static loop model employed takes into account gravity, density-,
temperature- or position-dependent energy input, an accurate form of
the radiative losses and variable loop cross-sectional area, under
assumptions of energy input by coronal heating balanced by radiative
and conductive losses, an optically thin plasma, energy conduction
along the field lines only and hydrostatic equilibrium. Computations of
an emission measure function for various distributions of the energy
input and loop geometries are then presented which show that little
information on the location of the energy input may be gained from
spectral line intensity measurements integrated over a single loop.
---------------------------------------------------------
Title: Results from the central Hyades survey.
Authors: Stern, R. A.; Underwood, J. H.; Zolcinski, M. C.; Antiochos,
S. K.
1981ASIC...68..137S Altcode: 1981spss.conf..137S
Results of a soft X-ray survey of the central 5 deg of the Hyades
star cluster made with the Einstein Observatory Imaging Proportional
Counter are presented. Virtually all of the late F and early G stars
in the cluster were detected at an X-ray luminosity of greater than
10 to the 28.5 erg/sec, although only about 50% of the stars in the
cluster as a whole were detected. Plots of X-ray luminosity against
B-V index indicate that the typical solar-type star in the Hyades is
emitting soft X-rays at a level approximately 30 times that of the
active sun. Histograms of the X-ray to bolometric luminosity ratio
reveal a gradual distinction between late A-early F stars, which are
expected to possess little or no convective envelope, and solar-type
(F8-G5) stars, with convective outer atmospheres. Results confirm the
dependence of stellar coronal activity on rotation, and establish the
prevalence of stellar coronae throughout the main sequence and the
giant regions of the H-R diagram.
---------------------------------------------------------
Title: The evolution of active region loop plasma
Authors: Krall, K. R.; Antiochos, S. K.
1980ApJ...242..374K Altcode:
The adjustment of coronal active-region loops to changes in their
heating rate is investigated numerically. The one-dimensional
hydrodynamic equations are solved subject to boundary conditions in
which heat flux-induced mass exchange between coronal and chromospheric
components is allowed. The calculated evolution of physical parameters
suggests that (1) mass supplied during chromospheric evaporation
is much more effective in moderating coronal temperature excursions
than when downward heat flux is dissipated by a static chromosphere,
and (2) the method by which the chromosphere responds to changing
coronal conditions can significantly influence coronal readjustment
time scales. Observations are cited which illustrate the range of
possible fluctuations in the heating rates.
---------------------------------------------------------
Title: Radiative-dominated cooling of the flare corona and transition
region.
Authors: Antiochos, S. K.
1980ApJ...241..385A Altcode:
Recent observations of some compact flares indicate that the
differential emission measure, q, of flare coronal and transition
region plasma has a much steeper dependence on temperature than in
nonflare regions. It is noted that this result is not compatible
with models for a flare loop in which conduction to the chromosphere
dominates the cooling, even in the case where the loop has a large
divergence in its cross-sectional area. Only by a combination of many
loops is it possible to reproduce the observations. Hence, models in
which radiation dominates the evolution are investigated. It is found
that the radiative models predict that q varies as T to the power (l +
1) where l measures the dependence of the radiative loss coefficient
on temperature. It is concluded that the radiative models are also
incapable of explaining the observations (unless, again, a combination
of many loops is postulated) and it is suggested that large mass
motions with velocities of the order of the sound speed may be required.
---------------------------------------------------------
Title: Study of the Chromospheres, Coronae, and Transitions Regions
of Main Sequence Stars in the Hyades
Authors: Zolcinski, M. -C.; Antiochos, S. K.; Walker, A. B. C.; Stern,
R. A.; Underwood, J. H.
1980BAAS...12..872Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Differential Emission Measure of Coronal Loops
Authors: Antiochos, S. K.; Underwood, J. H.; Vesecky, J. F.
1980BAAS...12..792A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stellar Coronae in the Hyades
Authors: Stern, R.; Underwood, J.; Zolcinski, M.; Antiochos, S.
1980BAAS...12..801S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The minimum flux corona; theory or concept
Authors: Underwood, J. H.; Antiochos, S. K.
1980STIN...8034328U Altcode:
The reply to the criticisms of the minimum flux theory is
discussed. These criticisms are correct in substance, as well as
in detail. Counter arguments that the minimum flux corona theory is
untenable, because of errors in its formulation, are presented.
---------------------------------------------------------
Title: On the Thermal Stability of Coronal Loop Plasma
Authors: Antiochos, S. K.; Emslie, A. G.
1980BAAS...12..519A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A model of active prominences
Authors: Antiochos, S. K.
1980ApJ...236..270A Altcode:
A one-dimensional numerical model of active loop prominences
is investigated. The model includes the effects of gravity,
the geometry of the magnetic field, and conduction losses to the
chromosphere. Calculations indicate that, as originally proposed
by Goldsmith, the thermal instability mechanism is, by itself,
sufficient to account for the appearance of bright H-alpha knots
in postflare loops. Under certain conditions, initial perturbations
in the loop temperature and density profiles of small, but finite,
amplitude (approximately 5%) and large size scale (greater than or
approximately equal to 10 to the 9th cm) can grow into condensations
with temperature and density differences of over an order of magnitude
and size scales of less than 10 to the 8th cm. In agreement with
observations, the conditions that must be satisfied are such that loop
prominence systems are likely to occur only in large flares and such
that knots preferentially form at the tops of loops. The velocities,
densities, and lifetimes calculated for the loop material are also
in agreement with observations. It is concluded that in order for
H-alpha knots to occur, heating of some form must continue into the
late cooling phase of a flare loop, and that this heating is more
intense near the loop base than near the apex.
---------------------------------------------------------
Title: The Einstein Central Hyades Survey - a Progress Report
Authors: Stern, R.; Underwood, J. H.; Zolcinski, M. C.; Antiochos, S.
1980SAOSR.389..127S Altcode: 1980csss....1..127S
No abstract at ADS
---------------------------------------------------------
Title: The thermal X-ray flare plasma
Authors: Moore, R.; McKenzie, D. L.; Svestka, Z.; Widing, K. G.; Dere,
K. P.; Antiochos, S. K.; Dodson-Prince, H. W.; Hiei, E.; Krall, K. R.;
Krieger, A. S.
1980sfsl.work..341M Altcode: 1980sofl.symp..341M
Following a review of current observational and theoretical knowledge
of the approximately 10 to the 7th K plasma emitting the thermal soft
X-ray bursts accompanying every H alpha solar flare, the fundamental
physical problem of the plasma, namely the formation and evolution of
the observed X-ray arches, is examined. Extensive Skylab observations
of the thermal X-ray plasmas in two large flares, a large subflare and
several compact subflares are analyzed to determine plasma physical
properties, deduce the dominant physical processes governing the plasma
and compare large and small flare characteristics. Results indicate
the density of the thermal X-ray plasma to be higher than previously
thought (from 10 to the 10th to 10 to the 12th/cu cm for large to
small flares), cooling to occur radiatively as much as conductively,
heating to continue into the decay phase of large flares, and the
mass of the thermal X-ray plasma to be supplied primarily through
chromospheric evaporation. Implications of the results for the basic
flare mechanism are indicated.
---------------------------------------------------------
Title: Steady State Condensation of Coronal Flare Plasma
Authors: Antiochos, S. K.; Sturrock, P. A.
1979BAAS...11..697A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Numerical modeling of quasi-static coronal loops. I. Uniform
energy input.
Authors: Vesecky, J. F.; Antiochos, S. K.; Underwood, J. H.
1979ApJ...233..987V Altcode:
A quasi-static numerical model for coronal loops is considered for
the case of a uniform energy input per unit volume into the loops. A
line dipole model is used to represent the loop magnetic field, and
the variations in loop cross section observed in X-ray photographs are
parameterized by the ratio between the cross-sectional areas at the loop
apex and base. The results of numerical modeling indicate that for an
area ratio greater than unity, increases in the area ratio of a loop
with a given length and apex area cause a general rise in electron
density and a fall in the temperature gradient, leading to large
increases in the differential emission factor at high temperatures. The
differential function obtained is significantly different from that
predicted by analytical models; however, analytical predictions for
the temperature-electron density relations are comparable to numerical
results. It is also concluded that even a symmetrical loop may have
a maximum temperature away from the apex.
---------------------------------------------------------
Title: The EINSTEIN Central Hyades Survey: A Progress Report.
Authors: Stern, R.; Underwood, J. H.; Zolcinski, M.; Antiochos, S.
1979BAAS...11..781S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The stability of solar coronal loops.
Authors: Antiochos, S. K.
1979ApJ...232L.125A Altcode:
The stability of the 'quasi-static' models of coronal loops was
examined. It was found that all models in which the heat flux at
the base of the loop is assumed to vanish are unstable to the growth
of thermal perturbations. The growth rates and the profiles of the
unstable modes indicate that the instability involves primarily the
low-temperature, transition-region plasma. The models can be made stable
only by assuming that the heat flux at the base of the loop is large,
of the order of 13% of the maximum flux in the loop. The results imply
that the transition region must be intrinsically dynamic.
---------------------------------------------------------
Title: Radiative dominated cooling of the flare corona and transition
region
Authors: Antiochos, S. K.
1979STIN...7932145A Altcode:
Models in which radiation dominates cooling flare loops are
investigated. The radiative models are found to predict a differential
emission measure (Q) proportional to T to the (l+1) power, where
l measures the dependence of the radiative loss coefficient on
temperature, lamda (T) approximately T to the (-l) power. It is
concluded that the radiative models are incapable of explaining the
observed temperature dependence of Q for flare coronal and transitional
plasma. The models suggest that large mass motions (velocities of the
order of the sound speed) may be required.
---------------------------------------------------------
Title: The evolution of soft X-ray emitting flare loops.
Authors: Antiochos, S. K.; Krall, K. R.
1979ApJ...229..788A Altcode:
We have constructed a numerical model for a cooling flare loop in which
the complete set of single-fluid equations in a one-dimensional geometry
(i.e., parallel to the magnetic field) is solved. Both evaporative
and static boundary conditions for the chromosphere-corona interface
have been developed. This model is used to investigate the effects
of initial temperature and density, loop geometry, and boundary
conditions on the form of the plasma evolution and the soft X-ray
emission. The results are then compared with Skylab S-056 observations
of the 1973 August 9 flare. For this comparison, and under the present
assumptions, we conclude that even highly compact flares must have a
multiloop structure similar to large flares, and that both radiative and
conductive cooling are necessary to explain the observations. The data
appear to be consistent with the predicted emission from a combination
of evaporative cooling loops.
---------------------------------------------------------
Title: The analysis of high spatial resolution UV and X-ray images
by computational modeling
Authors: Vesecky, J. F.; Antiochos, S. K.; Underwood, J. H.
1978clus.nasa..118V Altcode:
Very high resolution stereoscopic images of high temperature loop
structures observed at UV and X-ray wavelengths in the solar corona can
be used to understand physical processes in the corona. An existing
computational model is described and sample results are given to
demonstrate that computational modeling of coronal structures can
indeed take advantage of very high resolution images. The sample
results include the run of temperature and number density along a
typical loop and the variation of the differential emission measure
with temperature. The integration of the differential emission measure
with temperature along a column commensurate with an instrument's
spatial resolution is the relevant parameter obtained from UV and
X-ray observations. The effects of loop geometry and energy input
are examined.
---------------------------------------------------------
Title: Evolution of the coronal and transition-zone plasma in a
compact flare: the event of 1973 August 9.
Authors: Underwood, J. H.; Antiochos, S. K.; Feldman, U.; Dere, K. P.
1978ApJ...224.1017U Altcode:
X-ray and extreme ultraviolet observations of a compact flare were
analyzed to determine the relative importance of radiation, thermal
conduction, and 'evaporation' in the evolution of the temperature and
density structure of the plasma. In the event studied (1973 August 9),
the electron density was relatively high (5 x 10 to the eleventh to 1 x
10 to the twelfth) and radiation was evidently an important energy-loss
and cooling mechanism. The light curves of ultraviolet lines formed at
temperatures between 10 to the fifth to 10 to the seventh K indicate a
time-varying emission measure gradient, and hence temperature gradient,
during the flare. Radiative instability evidently played an important
role in determining the steepness of these gradients during the rise
and fall phases, and caused strong downward motions of material during
the cooling phase. Toward the end of the event, the coronal electron
density decreased and the temperature gradient relaxed toward that
expected from a conduction-dominated plasma. For this flare, evaporative
cooling did not appear to be a significant factor.
---------------------------------------------------------
Title: Comments on the "minimum flux corona" concept.
Authors: Antiochos, S. K.; Underwood, J. H.
1978A&A....68L..19A Altcode:
Hearn's (1975) models of the energy balance and mass loss of
stellar coronae, based on a 'minimum flux corona' concept, are
critically examined. First, it is shown that the neglect of the
relevant length scales for coronal temperature variation leads to an
inconsistent computation of the total energy flux F. The stability
arguments upon which the minimum flux concept is based are shown to be
fallacious. Errors in the computation of the stellar wind contribution
to the energy budget are identified. Finally we criticize Hearn's (1977)
suggestion that the model, with a value of the thermal conductivity
modified by the magnetic field, can explain the difference between
solar coronal holes and quiet coronal regions.
---------------------------------------------------------
Title: Models of Stellar Coronae.
Authors: Antiochos, S. K.; Underwood, J. H.; Vesecky, J. F.
1978BAAS...10..510A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Numerical Simulations of the Decay Phase of Compact Flares.
Authors: Krall, K. R.; Antiochos, S. K.
1978BAAS...10..442K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Evaporative cooling of flare plasma.
Authors: Antiochos, S. K.; Sturrock, P. A.
1978ApJ...220.1137A Altcode:
We investigate a one-dimensional loop model for the evaporative
cooling of the coronal flare plasma. The important assumptions are that
conductive losses dominate radiative cooling and that the evaporative
velocities are small compared with the sound speed. We calculate the
profile and evolution of the temperature and verify the accuracy of
our assumptions for plasma parameters typical of flare regions. The
model is in agreement with soft X-ray observations on the evolution of
flare temperatures and emission measures. The effect of evaporation
is to greatly reduce the conductive heat flux into the chromosphere
and to enhance the EUV emission from the coronal flare plasma.
---------------------------------------------------------
Title: Thermal Instability in Post-Flare Plasmas.
Authors: Antiochos, S. K.
1977PhDT.........2A Altcode:
The cooling of post flare plasmas was investigated and a one dimensional
model developed for active loop prominences, taking into consideration
motion and heat fluxes parallel to the magnetic field. Included in
the model are the effects of gravity, the geometry of the field, and
conduction losses to the chromosphere. Calculations using the computer
code developed and a two-step time differencing scheme indicate that
the non-uniform cooling of the post-flare corona can be understood as
a direct consequence of the temperature and density dependence of the
radiative losses from a high-temperature solar plasma.
---------------------------------------------------------
Title: Observations of a Radiatively Cooling Subflare.
Authors: Antiochos, S. K.; Underwood, J. H.; Feldman, U.
1977BAAS....9..329A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Thermal instability in post-flare plasmas
Authors: Antiochos, Spiro Kosta
1977PhDT.......104A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Thermal instability in post-flare plasmas
Authors: Antiochos, S. K.
1976STIN...7716975A Altcode:
The cooling of post-flare plasmas is discussed and the formation
of loop prominences is explained as due to a thermal instability. A
one-dimensional model was developed for active loop prominences. Only
the motion and heat fluxes parallel to the existing magnetic fields
are considered. The relevant size scales and time scales are such
that single-fluid MHD equations are valid. The effects of gravity,
the geometry of the field and conduction losses to the chromosphere
are included. A computer code was constructed to solve the model
equations. Basically, the system is treated as an initial value problem
(with certain boundary conditions at the chromosphere-corona transition
region), and a two-step time differencing scheme is used.
---------------------------------------------------------
Title: Evaporative cooling of flare plasma
Authors: Antiochos, S. K.; Sturrock, P. A.
1976STIN...7714971A Altcode:
A one-dimensional loop model for the evaporative cooling of the coronal
flare plasma was investigated. Conductive losses dominated radiative
cooling, and the evaporative velocities were small compared to the sound
speed. The profile and evolution of the temperature were calculated. The
model was in agreement with soft X-ray observations on the evolution
of flare temperatures and emission measures. The effect of evaporation
was to greatly reduce the conductive heat flux into the chromosphere
and to enhance the EUV emission from the coronal flare plasma.
---------------------------------------------------------
Title: An Evaporative Model of Flare Loops.
Authors: Antiochos, S. K.; Sturrock, P. A.
1976BAAS....8R.555A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Dynamics of Active Loop Prominences.
Authors: Antiochos, S. K.
1976BAAS....8..502A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Influence of magnetic field structure on the conduction
cooling of flare loops.
Authors: Antiochos, S. K.; Sturrock, P. A.
1976SoPh...49..359A Altcode:
A simple model facilitates calculation of the influence of magnetic
field configuration on the conduction cooling rate of a hot post-flare
coronal plasma. The magnetic field is taken to be that produced
by a line dipole or point dipole at an arbitrary depth below the
chromosphere. For the high temperatures (T ≳ 10<SUP>7</SUP> K)
produced by flares, the plasma may remain static and isobaric. The
influence of the field is such as to increase the heat flux (per unit
area) into the chromosphere, but to decrease the total conduction
cooling of the flare plasma. This leads to a significant enhancement
of the total energy radiated by the flare plasma.
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Title: Thermal Instability in Loop Prominence Systems
Authors: Antiochos, S. K.; Sturrock, P. A.
1975BAAS....7..472A Altcode:
No abstract at ADS
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Title: Periodicity in the Radiofrequency Spectrum of the Pulsar
CP 0328
Authors: Sturrock, P. A.; Antiochos, S.; Switzer, P.; Vallée, J.
1972ApJ...171L..27S Altcode:
Long-term averaging of a sequence of wide-band radiofrequency
spectra of CP 0328 reveals a periodicity not apparent in the original
spectra. This may be caused by a mechanism intrinsic to the source,
or by a propagation mechanism distinct from ordinary scintillation.
