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Author name code: karpen
ADS astronomy entries on 2022-09-14
author:"Karpen, Judith T."
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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: Quasi-periodic Energy Release and Jets at the Base of Solar
Coronal Plumes
Authors: Kumar, Pankaj; Karpen, Judith T.; Uritsky, Vadim M.; Deforest,
Craig E.; Raouafi, Nour E.; Richard DeVore, C.
2022ApJ...933...21K Altcode: 2022arXiv220413871K
Coronal plumes are long, ray-like, open structures that have been
considered as possible sources of the solar wind. Their origin in
the largely unipolar coronal holes has long been a mystery. Earlier
spectroscopic and imaging observations revealed blueshifted plasma and
propagating disturbances (PDs) in plumes that are widely interpreted
in terms of flows and/or propagating slow-mode waves, but these
interpretations (flows versus waves) remain under debate. Recently we
discovered an important clue about plume internal structure: dynamic
filamentary features called plumelets, which account for most of the
plume emission. Here we present high-resolution observations from
the Solar Dynamics Observatory/Atmospheric Imaging Assembly and
the Interface Region Imaging Spectrograph that revealed numerous,
quasi-periodic, tiny jets (so-called jetlets) associated with
transient brightening, flows, and plasma heating at the chromospheric
footpoints of the plumelets. By analogy to larger coronal jets,
these jetlets are most likely produced within the plume base by
magnetic reconnection between closed and open flux at stressed 3D
null points. The jetlet-associated brightenings are in phase with
plumelet-associated PDs, and vary with a period of ~3-5 minutes, which
is remarkably consistent with the photospheric/chromospheric p-mode
oscillation. This reconnection at the open-closed boundary in the
chromosphere/transition region is likely modulated or driven by local
manifestations of the global p-mode waves. The jetlets extend upward
to become plumelets, contribute mass to the solar wind, and may be
sources of the switchbacks recently detected by the Parker Solar Probe.
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Title: Kink Oscillation of a Flux Rope During a Failed Solar Eruption
Authors: Kumar, Pankaj; Nakariakov, Valery M.; Karpen, Judith T.;
Richard DeVore, C.; Cho, Kyung-Suk
2022ApJ...932L...9K Altcode: 2022arXiv220503480K
We report a decaying kink oscillation of a flux rope during a confined
eruptive flare, observed off the solar limb by the Solar Dynamics
Observatory's Atmospheric Imaging Assembly (AIA), which lacked a
detectable white-light coronal mass ejection. The erupting flux rope
underwent kinking, rotation, and apparent leg-leg interaction during the
event. The oscillations were observed simultaneously in multiple AIA
channels at 304, 171, and 193 Å, indicating that multithermal plasma
was entrained in the rope. After reaching the overlying loops in the
active region, the flux rope exhibited large-amplitude, decaying kink
oscillations with an apparent initial amplitude of 30 Mm, a period of
about 16 minutes, and a decay time of about 17 minutes. We interpret
these oscillations as a fundamental standing kink mode of the flux
rope. The oscillation polarization has a clear vertical component,
while the departure of the detected waveform from a sinusoidal signal
suggests that the oscillation could be circularly or elliptically
polarized. The estimated kink speed is 1080 km s<SUP>-1</SUP>,
corresponding to an Alfvén speed of about 760 km s<SUP>-1</SUP>. This
speed, together with the estimated electron density in the rope from our
differential emission measure analysis, n <SUB> e </SUB> ≍ (1.5-2.0)
× 10<SUP>9</SUP> cm<SUP>-3</SUP>, yields a magnetic-field strength of
about 15 G. To the best of our knowledge, decaying kink oscillations of
a flux rope with nonhorizontal polarization during a confined eruptive
flare have not been reported before. These oscillations provide unique
opportunities for indirect measurements of the magnetic-field strength
in low-coronal flux ropes during failed eruptions.
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Title: Extension and validation of the pendulum model for longitudinal
solar prominence oscillations
Authors: Luna, M.; Terradas, J.; Karpen, J.; Ballester, J. L.
2022A&A...660A..54L Altcode: 2022arXiv220207957L
Context. Longitudinal oscillations in prominences are common phenomena
on the Sun. These oscillations can be used to infer the geometry and
intensity of the filament magnetic field. Previous theoretical studies
of longitudinal oscillations made two simplifying assumptions: uniform
gravity and semicircular dips on the supporting flux tubes. However, the
gravity is not uniform and realistic dips are not semicircular. <BR />
Aims: Our aim is to understand the effects of including the nonuniform
solar gravity on longitudinal oscillations and explore the validity
of the pendulum model with different flux-tube geometries. <BR />
Methods: We first derived the equation describing the motion of the
plasma along the flux tube including the effects of nonuniform gravity,
yielding corrections to the original pendulum model. We also computed
the full numerical solutions for the normal modes and compared them
with the new pendulum approximation. <BR /> Results: We find that
the nonuniform gravity introduces a significant modification in the
pendulum model. We also found a cut-off period; i.e., the longitudinal
oscillations cannot have a period longer than 167 min. In addition,
considering different tube geometries, the period depends almost
exclusively on the radius of curvature at the bottom of the dip. <BR />
Conclusions: We conclude that nonuniform gravity significantly modifies
the pendulum model. These corrections are important for prominence
seismology, because the inferred values of the radius of curvature
and minimum magnetic-field strength differ substantially from those
of the old model. However, we find that the corrected pendulum model
is quite robust and is still valid for noncircular dips.
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Title: Spectral Power-law Formation by Sequential Particle
Acceleration in Multiple Flare Magnetic Islands
Authors: Guidoni, S. E.; Karpen, J. T.; DeVore, C. R.
2022ApJ...925..191G Altcode: 2022arXiv220105564G
We present a first-principles model of pitch-angle and energy
distribution function evolution as particles are sequentially
accelerated by multiple flare magnetic islands. Data from
magnetohydrodynamic (MHD) simulations of an eruptive flare/coronal
mass ejection provide ambient conditions for the evolving particle
distributions. Magnetic islands, which are created by sporadic
reconnection at the self-consistently formed flare current sheet,
contract and accelerate the particles. The particle distributions are
evolved using rules derived in our previous work. In this investigation,
we assume that a prescribed fraction of particles sequentially "hops"
to another accelerator and receives an additional boost in energy
and anisotropy. This sequential process generates particle number
spectra that obey an approximate power law at mid-range energies
and presents low- and high-energy breaks. We analyze these spectral
regions as functions of the model parameters. We also present a fully
analytic method for forming and interpreting such spectra, independent
of the sequential acceleration model. The method requires only a few
constrained physical parameters, such as the percentage of particles
transferred between accelerators, the energy gain in each accelerator,
and the number of accelerators visited. Our investigation seeks to
bridge the gap between MHD and kinetic regimes by combining global
simulations and analytic kinetic theory. The model reproduces and
explains key characteristics of observed flare hard X-ray spectra as
well as the underlying properties of the accelerated particles. Our
analytic model provides tools to interpret high-energy observations for
missions and telescopes, such as RHESSI, FOXSI, NuSTAR, Solar Orbiter,
EOVSA, and future high-energy missions.
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Title: Can solar coronal plumelets precondition switchback events
in the wind?
Authors: Uritsky, Vadim; DeForest, Craig; Karpen, Judith; DeVore,
C. Richard; Kumar, Pankaj; Raouafi, Nour; Wyper, Peter
2021AGUFMSH24C..05U Altcode:
Filamentary structures and motions in plume images have been known
for many years (e.g., Raouafi & Stenborg (2014) and references
therein). Recently, we have presented the first in-depth quantitative
investigation of these structures, which we denoted plumelets (Uritsky
et al., 2021). Using an extended set of high-resolution, high-cadence
solar coronal images covering 40 hr of nearly continuous observations
of a typical solar coronal plume by SDO/AIA on 2016 July 23, we have
investigated the highly dynamic nature of the plumelets. The figure
below (courtesy NASA/SDO) provides an example of processing of a
high-resolution SDO/AIA image to reveal distinct plumelets within the
studied plume. Our analysis has demonstrated that the impulsive behavior
of the plumelets may dominate the large-scale behavior of the host
plume. The plumelets support persistent longitudinal fluctuations whose
typical period (35 minutes) is consistent with the peak-power period
of the solar p-modes, and the radial propagation speed (190240 km/s)
is in agreement with the characteristic speed of plasma outflows in a
typical coronal hole jet. Elsewhere (Kumar et al., 2021), we present
evidence for direct causal connection between the plumelets, jetlets,
and localized reconnection activity observed at the plume base. In this
talk, we focus on the stability and spatio-temporal correlation pattern
of the velocity field in a system of multiple coronal plumelets. Our
analysis reveals significant transient velocity shears at the interface
boundaries of adjacent plumelets. We argue that these shears could lead
to a localized onset of Kelvin Helmholtz instability in the downstream
plume plasma, which could introduce topological irregularities in the
frozen-in magnetic field and facilitate the formation of switchbacks and
other small-scale structures in the magnetically connected solar wind.
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Title: Small-Scale Solar Activity and its effect on the coronal
environment
Authors: Raouafi, Nour; Stenborg, Guillermo; Seaton, Daniel; DeForest,
Craig; Bale, Stuart; Horbury, Timothy; Kasper, Justin; Velli, Marco;
Karpen, Judith; Kumar, Pankaj; DeVore, C. Richard; Uritsky, Vadim
2021AGUFMSH25F2144R Altcode:
Careful analysis of solar observations reveals a myriad of small-scale
jetting activity (i.e., jetlets; Raouafi & Stenborg 2014). Jetlets
are miniature manifestations of the typical coronal jets observed
in both X-rays and extreme-ultraviolet (EUV) solar images. They are
the product of near-ubiquitous magnetic reconnection. Their role in
energy and mass transport to the solar corona and wind has not been
yet well established. Here we provide an overview of this phenomenon
and explore its role at the base of the corona and the young solar
wind. We conjecture that these small dynamic features might be the
source or at least one of the sources of the magnetic switchbacks
observed by the Parker Solar Probe.
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Title: Formation and Characteristics of Filament Threads in
Double-dipped Magnetic Flux Tubes
Authors: Guo, J. H.; Zhou, Y. H.; Guo, Y.; Ni, Y. W.; Karpen, J. T.;
Chen, P. F.
2021ApJ...920..131G Altcode: 2021arXiv210712181G
As one of the main formation mechanisms of solar filament formation, the
chromospheric evaporation-coronal condensation model has been confirmed
by numerical simulations to explain the formation of filament threads
very well in flux tubes with single dips. However, coronal magnetic
extrapolations indicated that some magnetic field lines might possess
more than one dip. It is expected that the formation process would
be significantly different in this case compared to a single-dipped
magnetic flux tube. In this paper, based on the evaporation-condensation
model, we study filament thread formation in double-dipped magnetic
flux tubes by numerical simulations. We find that only with particular
combinations of magnetic configuration and heating, e.g., concentrated
localized heating and a long magnetic flux tube with deep dips,
can two threads form and persist in a double-dipped magnetic flux
tube. Comparing our parametric survey with observations, we conclude
that such magnetically connected threads due to multiple dips are
more likely to exist in quiescent filaments than in active-region
filaments. Moreover, we find that these threads are usually shorter
than independently trapped threads, which might be one of the reasons
why quiescent filaments have short threads. These characteristics of
magnetically connected threads could also explain barbs and vertical
threads in quiescent filaments.
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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.
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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.
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Title: Magnetic Field Curvature In A Filament Channel Derived From
Oscillation Measurements And MHD Modeling
Authors: Kucera, T. A.; Luna, M.; Torok, T.; Muglach, K.; Downs, C.;
Sun, X.; Thompson, B.; Karpen, J.; Gilbert, H.
2021AAS...23811306K Altcode:
We have used measurements of repeated large amplitude longitudinal
oscillations (LALOs) in an active region filament to diagnose the
curvature of the magnetic field in the filament channel and compared the
results with predictions of an MHD flux-rope model based on magnetograms
of the region. In May and June of 2014 Active Region 12076 exhibited a
complex of filaments undergoing repeated oscillations over the course
of twelve days. The central filament channel exhibited emerging and then
canceling magnetic flux that resulted in multiple activations, filament
eruptions, and eight oscillation events, which we analyzed using GONG
H-alpha data. Luna and Karpen (2012) model LALOs as oscillations of
magnetized filament plasma moving along dipped magnetic field lines
with gravity as a restoring force. Under this model the period of these
oscillations can be used to estimate the curvature of the magnetic
field in the location of the filament threads. Utilizing this, we find
that the measured periods in the central filament ranging from 34-74
minutes should correspond to magnetic field curvatures of about 30-136
Mm. We also derive radii of curvature for the central filament channel
using a flux-rope model that is based on an SDO/HMI magnetogram of the
region. The rope is constructed using the analytic expressions by Titov
et al. (2018) and then numerically relaxed towards a force-free state in
the zero-beta MHD approximation, where gravity and thermal pressure are
neglected. For comparison, we also employ a nonlinear force-free field
(NLFFF) extrapolation of the active region. We compare the results
of these different ways of attempting to determine the field in the
filament channel.
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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: 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: Plumelets: Dynamic Filamentary Structures in Solar Coronal
Plumes
Authors: Uritsky, V. M.; DeForest, C. E.; Karpen, J. T.; DeVore,
C. R.; Kumar, P.; Raouafi, N. E.; Wyper, P. F.
2021ApJ...907....1U Altcode: 2020arXiv201205728U
Solar coronal plumes long seemed to possess a simple geometry
supporting spatially coherent, stable outflow without significant
fine structure. Recent high-resolution observations have challenged
this picture by revealing numerous transient, small-scale, collimated
outflows ("jetlets") at the base of plumes. The dynamic filamentary
structure of solar plumes above these outflows, and its relationship
with the overall plume structure, have remained largely unexplored. We
analyzed the statistics of continuously observed fine structure inside
a single representative bright plume within a mid-latitude coronal
hole during 2016 July 2-3. By applying advanced edge-enhancement and
spatiotemporal analysis techniques to extended series of high-resolution
images from the Solar Dynamics Observatory's Atmospheric Imaging
Assembly, we determined that the plume was composed of numerous
time-evolving filamentary substructures, referred to as "plumelets" in
this paper, that accounted for most of the plume emission. The number
of simultaneously identifiable plumelets was positively correlated
with plume brightness, peaked in the fully formed plume, and remained
saturated thereafter. The plumelets had transverse widths of 10 Mm and
intermittently supported upwardly propagating periodic disturbances with
phase speeds of 190-260 km s<SUP>-1</SUP> and longitudinal wavelengths
of 55-65 Mm. The characteristic frequency (≍ 3.3 mHz) is commensurate
with that of solar p-modes. Oscillations in neighboring plumelets
are uncorrelated, indicating that the waves could be driven by p-mode
flows at spatial scales smaller than the plumelet separation. Multiple
independent sources of outflow within a single coronal plume should
impart significant fine structure to the solar wind that may be
detectable by Parker Solar Probe and Solar Orbiter.
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Title: Semi-Analytical Hybrid Model of Sequential Particle
Acceleration in Flares
Authors: Guidoni, S. E.; Karpen, J. T.; DeVore, C. R.
2020AGUFMSH057..08G Altcode:
Understanding how particles are accelerated in flares has been a
long-sought goal in Heliophysics. It is currently impossible to
self-consistently unify flare models of particle energization over
magnetohydrodynamics (MHD) and kinetic regimes because they operate
at scales that differ by 10 orders of magnitude. Here, we describe
our efforts to bridge this theoretical gap by combining global flare
simulations and analytical kinetic theory. Simulation data provide
ambient conditions for the particle distributions to be evolved in
energy and pitch angle. L arge-scale islands created by sporadic
flare reconnection contract, energizing the ambient particles
mostly through the betatron mechanism and supplemented by the Fermi
mechanism. Hypothesized sequential boosts to the particle energy yield
power-law- like spectra. We have developed a fully analytical model
that characterizes the power-law properties (e.g., spectral indexes and
power law breaks) as functions of very few physical parameters. This
model explains key characteristics of observed flare hard X-ray spectra,
as well as the underlying accelerated-electron properties .
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Title: New Insights into the Dynamic Relationship between Jetlets
and Plumes
Authors: Kumar, P.; Karpen, J. T.; Uritsky, V. M.; DeForest, C.;
Raouafi, N. E.; DeVore, C. R.
2020AGUFMSH0240002K Altcode:
Plumes are among the most fascinating large-scale coronal structures,
but also are among the most puzzling and controversial features. They
are significantly denser and have lower flow speeds than the inter-plume
regions, and are rooted in regions of fine-scale, highly mixed magnetic
polarity within predominantly unipolar coronal holes. The advent of
high-resolution, high-cadence coronal observations from the Solar
Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA), coupled
with photospheric magnetograms from SDO's Helioseismic and Magnetic
Imager (SDO/HMI), has enabled detailed studies of plumes from their
footprints outward. In particular, the detection of small transient
outflows at the base of a few plumes led to the hypothesis that these
"jetlets" are the long-sought source of plume mass and energy that
sustain them for hours to weeks (Raouafi & Stenborg 2014). We
have analyzed high-cadence multiwavelength SDO/AIA data and SDO/HMI
magnetograms for a well-observed plume on 2016 July 3, focusing on
the activity at the base and the fine structure within the overlying
plume. In contrast to earlier studies, we used a noise-gating method
(DeForest 2017) to clean the AIA and HMI data that revealed in greater
detail the jetlets and other small-scale structures throughout the
plume. Our investigation revealed multiple quasi-periodic jetlets within
the multipolar footpoint region, throughout the period of observation,
as well as evolving filamentary structures above the jetlets. This
presentation will discuss the measured and derived jetlet properties,
the structural and dynamic connections between the jetlets and the
plume, and implications for the underlying physical processes. <P
/>DeForest C. E., Noise-gating to Clean Astrophysical Image Data,
ApJ, 838, 155 (2017) <P />Raouafi, N. E. & G. Stenborg, Role of
Transients in the Sustainability of Solar Coronal Plumes, ApJ, 787,
118 (2014)
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Title: Plumelets: Dynamic Filamentary Structures in Solar Plumes
Authors: Karpen, J. T.; Uritsky, V. M.; DeForest, C.; DeVore, C. R.;
Kumar, P.; Raouafi, N. E.; Wyper, P. F.
2020AGUFMSH0240003K Altcode:
Solar plumes long seemed to possess a simple geometry supporting
spatially coherent, stable outflow without significant fine
structure. Recent high-resolution observations have challenged
this picture by revealing numerous transient, small-scale,
collimated outflows ("jetlets") at the base of plumes (see Kumar et
al. presentation in this session). The dynamic filamentary structure
of solar plumes above these outflows, and its relationship with
the overall plume structure, have remained largely unexplored. We
report a statistical analysis of continuously observed fine structure
inside a bright plume within a mid-latitude coronal hole during 2016
July 2-3. By applying advanced edge-enhancement and spatiotemporal
analysis techniques to extended series of highresolution images
from the Solar Dynamics Observatory's Atmospheric Imaging Assembly,
we determined that the plume was composed of numerous time-evolving
bright filamentary substructures, referred to as "plumelets" in this
paper, that accounted for most of the plume emission. The number of
simultaneously identifiable plumelets varied over the observation
period, was positively correlated with plume brightness, and peaked
in the fully formed plume. The plumelets had transverse widths of
10 Mm and intermittently supported upwardly propagating periodic
disturbances with phase speeds of 190-260 km s<SUP>-1</SUP> and
longitudinal wavelengths of 55-65 Mm. The characteristic frequency
(3.5 mHz) is commensurate with that of solar p-modes. Oscillations
in neighboring plumelets are uncorrelated, indicating that the waves
could be driven by p-mode flows at spatial scales smaller than the
plumelet separation. Multiple independent sources of outflow within a
single coronal plume should impart significant fine structure to the
fast solar wind and be detectable by Parker Solar Probe at perihelion.
---------------------------------------------------------
Title: Flare Models of Magnetic Energy Release into Plasma Heating
and Particle Acceleration
Authors: Guidoni, S. E.; Karpen, J. T.; DeVore, C. R.; Longcope, D.
2020AGUFMSH045..02G Altcode:
Understanding how flare magnetic energy can be released at rates of
the order of 10<SUP>27-32</SUP> ergs/s has been a long-sought goal
in Heliophysics. Indirect observations of the lower solar corona
point to magnetic reconnection as the fundamental process that
converts free magnetic energy mainly into flows, heat, and particle
acceleration. The partitioning among these three energies is usually
inferred indirectly from subsequent radiation emitted by heated plasma
and energetic particles, but the uncertainties are large. T he energy
conversion to bulk motion and heat can be reasonably well described
with magnetohydrodynamic (MHD) models and simulations, while kinetic
models are better suited to study particle energization. However,
the scale separation between MHD and kinetic regimes in flares is
approximately 10 orders of magnitude. Therefore, it is currently
impossible to self-consistently unify flare models over all relevant
scales . <P />We present results of our analytical 1D model of the
super-A lfvé nic shortening of reconnected field lines (reconnection
outflow) and the consequent plasma heating by strong gas-dynamic
shocks formed by this fast retraction. We also describe our efforts
to bridge the theoretical gap between MHD and kinetic regimes by
combining global flare simulations and analytical kinetic theory to
produce power-law- like particle energy spectra. This model explains
key characteristics of observed flare hard X-ray spectra, as well as
the underlying accelerated-electron properties .
---------------------------------------------------------
Title: Using SDO/AIA to Understand the Thermal Evolution of Solar
Prominence Formation
Authors: Viall, Nicholeen M.; Kucera, Therese A.; Karpen, Judith T.
2020ApJ...905...15V Altcode:
We investigated the thermal properties of prominence formation using
time series analysis of Solar Dynamics Observatory's Atmospheric
Imaging Assembly (SDO/AIA) data. Here, we report the first time-lag
measurements derived from SDO/AIA observations of a prominence and its
cavity on the solar limb, made possible by AIA's different wave bands
and high time resolution. With our time-lag analysis, which tracks
the thermal evolution using emission formed at different temperatures,
we find that the prominence cavity exhibited a mixture of heating and
cooling signatures. This is in contrast to prior time-lag studies of
multiple active regions that chiefly identified cooling signatures
and very few heating signatures, which is consistent with nanoflare
heating. We also computed time lags for the same pairs of SDO/AIA
channels using output from a one-dimensional hydrodynamic model of
prominence material forming through thermal nonequilibrium (TNE). We
demonstrate that the SDO/AIA time lags for flux tubes undergoing TNE
are predicted to be highly complex, changing with time and location
along the flux tube, and are consistent with the observed time-lag
signatures in the cavity surrounding the prominence. Therefore, the
time-lag analysis is a sensitive indicator of the heating and cooling
processes in different coronal regions. The time lags calculated for
the simulated prominence flux tube are consistent with the behavior
deduced from the AIA data, thus supporting the TNE model of prominence
formation. Future investigations of time lags predicted by other models
for the prominence mass could be a valuable method for discriminating
among competing physical mechanisms.
---------------------------------------------------------
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: Solar Flare Energy Partitioning and Transport -- the Impulsive
Phase (a Heliophysics 2050 White Paper)
Authors: Kerr, Graham S.; Alaoui, Meriem; Allred, Joel C.; Bian,
Nicholas H.; Dennis, Brian R.; Emslie, A. Gordon; Fletcher, Lyndsay;
Guidoni, Silvina; Hayes, Laura A.; Holman, Gordon D.; Hudson, Hugh
S.; Karpen, Judith T.; Kowalski, Adam F.; Milligan, Ryan O.; Polito,
Vanessa; Qiu, Jiong; Ryan, Daniel F.
2020arXiv200908400K Altcode:
Solar flares are a fundamental component of solar eruptive events (SEEs;
along with solar energetic particles, SEPs, and coronal mass ejections,
CMEs). Flares are the first component of the SEE to impact our
atmosphere, which can set the stage for the arrival of the associated
SEPs and CME. Magnetic reconnection drives SEEs by restructuring the
solar coronal magnetic field, liberating a tremendous amount of energy
which is partitioned into various physical manifestations: particle
acceleration, mass and magnetic-field eruption, atmospheric heating,
and the subsequent emission of radiation as solar flares. To explain
and ultimately predict these geoeffective events, the heliophysics
community requires a comprehensive understanding of the processes that
transform and distribute stored magnetic energy into other forms,
including the broadband radiative enhancement that characterises
flares. This white paper, submitted to the Heliophysics 2050 Workshop,
discusses the flare impulsive phase part of SEEs, setting out the
questions that need addressing via a combination of theoretical,
modelling, and observational research. In short, by 2050 we must
determine the mechanisms of particle acceleration and propagation,
and must push beyond the paradigm of energy transport via nonthermal
electron beams, to also account for accelerated protons & ions
and downward directed Alfven waves.
---------------------------------------------------------
Title: Solar Flare Energy Partitioning and Transport -- the Gradual
Phase (a Heliophysics 2050 White Paper)
Authors: Kerr, Graham S.; Alaoui, Meriem; Allred, Joel C.; Bian,
Nicholas H.; Dennis, Brian R.; Emslie, A. Gordon; Fletcher, Lyndsay;
Guidoni, Silvina; Hayes, Laura A.; Holman, Gordon D.; Hudson, Hugh
S.; Karpen, Judith T.; Kowalski, Adam F.; Milligan, Ryan O.; Polito,
Vanessa; Qiu, Jiong; Ryan, Daniel F.
2020arXiv200908407K Altcode:
Solar flares are a fundamental component of solar eruptive events
(SEEs; along with solar energetic particles, SEPs, and coronal
mass ejections, CMEs). Flares are the first component of the SEE
to impact our atmosphere, which can set the stage for the arrival
of the associated SEPs and CME. Magnetic reconnection drives SEEs
by restructuring the solar coronal magnetic field, liberating a
tremendous amount of energy which is partitioned into various physical
manifestations: particle acceleration, mass and magnetic-field eruption,
atmospheric heating, and the subsequent emission of radiation as solar
flares. To explain and ultimately predict these geoeffective events,
the heliophysics community requires a comprehensive understanding of
the processes that transform and distribute stored magnetic energy
into other forms, including the broadband radiative enhancement that
characterises flares. This white paper, submitted to the Heliophysics
2050 Workshop, discusses the flare gradual phase part of SEEs, setting
out the questions that need addressing via a combination of theoretical,
modelling, and observational research. In short, the flare gradual phase
persists much longer than predicted so, by 2050, we must identify the
characteristics of the significant energy deposition sustaining the
gradual phase, and address the fundamental processes of turbulence
and non-local heat flux.
---------------------------------------------------------
Title: Filament Oscillations in Active Region 12076
Authors: Kucera, T. A.; Muglach, K.; Luna Bennasar, M.; Karpen, J.;
Thompson, B.; Gilbert, H.
2020AAS...23633004K Altcode:
We present an analysis of repeated large amplitude longitudinal
oscillations (LALO) in filaments in Active Region 12076 in May
and June of 2014. Most of the oscillations were associated with a
region of emerging and then canceling magnetic flux that resulted
in multiple activations and filament eruptions. We analyze twelve
separate oscillations that occur in a complex of filaments in the
active region over twelve days. Luna and Karpen (2012) model LALO
in filaments oscillations of magnetized filament plasma moving along
dipped magnetic field lines with gravity as a restoring force. Under
this model the period of these oscillations can be used to estimate
the curvature of the magnetic field in the location of the filament,
providing observationally derived values to compare with models of the
magnetic field in the active region corona. Periods ranged from 26-74
minutes, corresponding to magnetic field curvatures of about 20-130 Mm.
---------------------------------------------------------
Title: Heating and Eruption of a Solar Circular-ribbon Flare
Authors: Lee, Jeongwoo; Karpen, Judith T.; Liu, Chang; Wang, Haimin
2020ApJ...893..158L Altcode: 2020arXiv200805020L
We studied a circular-ribbon flare, SOL2014-12-17T04:51, with emphasis
on its thermal evolution as determined by the differential emission
measure (DEM) inversion analysis of the extreme ultraviolet (EUV)
images of the Atmospheric Imaging Assembly instrument on board the
Solar Dynamics Observatory. Both temperature and emission measure
start to rise much earlier than the flare, along with an eruption and
formation of a hot halo over the fan structure. In the main flare phase,
another set of ribbons forms inside the circular ribbon, and expands
as expected for ribbons at the footpoints of a postflare arcade. An
additional heating event further extends the decay phase, which is also
characteristic of some eruptive flares. The basic magnetic configuration
appears to be a fan-spine topology, rooted in a minority-polarity patch
surrounded by majority-polarity flux. We suggest that reconnection at
the null point begins well before the impulsive phase, when the null
is distorted into a breakout current sheet, and that both flare and
breakout reconnection are necessary in order to explain the subsequent
local thermal evolution and the eruptive activities in this confined
magnetic structure. Using local DEMs, we found a postflare temperature
increase inside the fan surface, indicating that the so-called EUV
late phase is due to continued heating in the flare loops.
---------------------------------------------------------
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: 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: 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: 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: Using SDO/AIA to Understand the Thermal Evolution of Solar
Prominence Formation
Authors: Viall, Nicholeen; Kucera, Therese; Karpen, Judith
2018csc..confE.124V Altcode:
We investigate prominence formation using time series analysis of
Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA)
data. We examine the thermal properties of forming prominences by
analyzing observed light curves using the same technique that we have
already successfully applied to active regions to diagnose heating
and cooling cycles. This technique tracks the thermal evolution using
emission formed at different temperatures, made possible by AIA's
different wavebands and high time resolution. We also compute the
predicted light curves in the same SDO/AIA channels of a hydrodynamic
model of thermal nonequilibrium formation of prominence material,
an evaporation-condensation model. In these models of prominence
formation, heating at the foot-points of sheared coronal flux-tubes
results in evaporation of material of a few MK into the corona followed
by catastrophic cooling of the hot material to form cool ( 10,000 K)
prominence material. We investigate prominences from different viewing
angles to evaluate possible line of sight effects. We demonstrate
that the SDO/AIA light curves for flux tubes undergoing thermal
nonequilibrium vary at different locations along the flux tube,
especially in the region where the condensate forms, and we compare
the predicted light curves with those observed.
---------------------------------------------------------
Title: Simulated Encounters of the Parker Solar Probe with a
Coronal-hole Jet
Authors: Roberts, Merrill A.; Uritsky, Vadim M.; DeVore, C. Richard;
Karpen, Judith T.
2018ApJ...866...14R Altcode: 2017arXiv171010323R
Solar coronal jets are small, transient, collimated ejections most
easily observed in coronal holes (CHs). The upcoming Parker Solar
Probe (PSP) mission provides the first opportunity to encounter CH
jets in situ near the Sun and examine their internal structure and
dynamics. Using projected mission orbital parameters, we have simulated
PSP encounters with a fully three-dimensional magnetohydrodynamic
(MHD) model of a CH jet. We find that three internal jet regions,
featuring different wave modes and levels of compressibility, have
distinct identifying signatures detectable by PSP. The leading
Alfvén wave front and its immediate wake are characterized by
trans-Alfvénic plasma flows with mild density enhancements. This
front exhibits characteristics of a fast switch-on MHD shock, whose
arrival is signaled by the sudden onset of large-amplitude transverse
velocity and magnetic-field oscillations highly correlated in space
and time. The trailing portion is characterized by supersonic but
sub-Alfvénic outflows of dense plasma with uncorrelated velocity and
magnetic-field oscillations. This compressible region contains most
of the jet’s mass. The volume between the immediate wake and dense
jet, the remote wake, mixes and transitions the characteristics of
the two other regions. In addition to probing each region separately,
we also simulate a corotational PSP-jet encounter. In this scenario,
the simulated spacecraft hovers over the jet-producing CH, as may
occur during the mission’s corotational phases, sampling each jet
region in turn. We estimate that PSP will encounter numerous CH jets
over the lifetime of the mission.
---------------------------------------------------------
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: GONG Catalog of Solar Filament Oscillations Near Solar Maximum
Authors: Luna, M.; Karpen, J.; Ballester, J. L.; Muglach, K.; Terradas,
J.; Kucera, T.; Gilbert, H.
2018ApJS..236...35L Altcode: 2018arXiv180403743L
We have cataloged 196 filament oscillations from the Global Oscillation
Network Group Hα network data during several months near the maximum
of solar cycle 24 (2014 January-June). Selected examples from the
catalog are described in detail, along with our statistical analyses of
all events. Oscillations were classified according to their velocity
amplitude: 106 small-amplitude oscillations (SAOs), with velocities
<10 {km} {{{s}}}<SUP>-1</SUP>, and 90 large-amplitude oscillations
(LAOs), with velocities >10 {km} {{{s}}}<SUP>-1</SUP>. Both SAOs
and LAOs are common, with one event of each class every two days on the
visible side of the Sun. For nearly half of the events, we identified
their apparent trigger. The period distribution has a mean value of
58 ± 15 minutes for both types of oscillations. The distribution
of the damping time per period peaks at τ/P = 1.75 and 1.25 for
SAOs and LAOs, respectively. We confirmed that LAO damping rates
depend nonlinearly on the oscillation velocity. The angle between the
direction of motion and the filament spine has a distribution centered
at 27° for all filament types. This angle agrees with the observed
direction of filament-channel magnetic fields, indicating that most
of the cataloged events are longitudinal (i.e., undergo field-aligned
motions). We applied seismology to determine the average radius of
curvature in the magnetic dips, R ≈ 89 Mm, and the average minimum
magnetic field strength, B ≈ 16 G. The catalog is available to the
community online and is intended to be expanded to cover at least 1
solar cycle.
---------------------------------------------------------
Title: Simulated Encounters of Parker Solar Probe with a Solar
Coronal Jet: Early Mission Predictions
Authors: Roberts, Merrill A.; Uritsky, Vadim M.; DeVore, C. Richard;
Karpen, Judith T.
2018tess.conf32101R Altcode:
Solar coronal jets are highly collimated, transient features typically
observed in coronal holes (CH). The upcoming Parker Solar Probe (PSP)
mission provides the first opportunity to encounter CH jets in situ,
shedding light on the internal structure and dynamics of these
features, and necessitating a detailed grasp of a jet's defining
characteristics for proper interpretation. We present simulated PSP
encounters with a CH jet, using projected PSP orbital parameters and
a fully three-dimensional MHD model of a CH jet using the Adaptively
Refined Magnetohydrodynamics Solver (ARMS) (Karpen et al. 2017; Roberts
et al. 2018, in review). Our results suggest that CH jets are internally
complex, with multi-scale, radially stratified internal structure that
evolves as the jet progresses through the heliosphere. We find that
three internal jet regions featuring different wave modes and levels
of compressibility (Uritsky et al. 2017) have distinct identifying
signatures which could be detected by PSP. We discuss the the features
of these simulated detections during the different PSP mission phases,
with a focus on detections that could be made during the early mission
perihelia.
---------------------------------------------------------
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: 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: Roles of Reconnection in the Solar Atmosphere
Authors: Karpen, Judith T.
2018tess.conf10801K Altcode:
Impulsive energy release on the Sun occurs on a vast range
of scales, from the nanoflares thought to heat much of the
corona to coronal mass ejections and eruptive flares. From
the highly collisional, neutral-dominated photosphere to the
rarefied, low-beta corona, reconnection at current sheets changes
large-scale magnetic connectivity, drives flows, and heats and
accelerates particles. However, unlike geospace and the solar wind,
in-situ observations of the reconnection process on the Sun are
impossible. High-resolution imaging and spectroscopy have yielded
strong evidence for the macroscopic effects of reconnection in the
chromosphere and corona, while numerical simulations have laid the
foundations for understanding how and where reconnection operates in
the solar atmosphere. I will discuss some of the best existing examples
of this fundamental process, and suggest ways to reach long-sought
closure between theory and observations of reconnection-driven solar
activity. <P />Judy Karpen is a solar physicist and Chief of the Space
Weather Laboratory at NASA/GSFC. Her research interests include the
origins of solar eruptions from coronal jets to CMEs, the formation
and evolution of solar prominences, flare particle acceleration,
and the physics of magnetic reconnection.
---------------------------------------------------------
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: 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: 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: 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: A New Paradigm for Flare Particle Acceleration
Authors: Guidoni, Silvina E.; Karpen, Judith T.; DeVore, C. Richard
2017SPD....4810202G Altcode:
The mechanism that accelerates particles to the energies required
to produce the observed high-energy impulsive emission and its
spectra in solar flares is not well understood. Here, we propose a
first-principle-based model of particle acceleration that produces
energy spectra that closely resemble those derived from hard X-ray
observations. Our mechanism uses contracting magnetic islands formed
during fast reconnection in solar flares to accelerate electrons, as
first proposed by Drake et al. (2006) for kinetic-scale plasmoids. We
apply these ideas to MHD-scale islands formed during fast reconnection
in a simulated eruptive flare. A simple analytic model based on the
particles’ adiabatic invariants is used to calculate the energy gain
of particles orbiting field lines in our ultrahigh-resolution, 2.5D,
MHD numerical simulation of a solar eruption (flare + coronal mass
ejection). Then, we analytically model electrons visiting multiple
contracting islands to account for the observed high-energy flare
emission. Our acceleration mechanism inherently produces sporadic
emission because island formation is intermittent. Moreover, a large
number of particles could be accelerated in each macroscopic island,
which may explain the inferred rates of energetic-electron production
in flares. We conclude that island contraction in the flare current
sheet is a promising candidate for electron acceleration in solar
eruptions. This work was supported in part by the NASA LWS and H-SR
programs..
---------------------------------------------------------
Title: A New Model for Flare Particle Acceleration
Authors: Guidoni, Silvina E.; DeVore, C. R.; Karpen, J. T.
2017shin.confE.118G Altcode:
The mechanism that accelerates particles to the energies required
to produce the observed high-energy impulsive emission and its
spectra in solar flares is not well understood. Here, we propose a
first-principle-based model of particle acceleration that produces
energy spectra that closely resemble those derived from hard X-ray
observations. Our mechanism uses contracting magnetic islands formed
during fast reconnection in solar flares to accelerate electrons, as
first proposed by Drake et al. (2006) for kinetic-scale plasmoids. We
apply these ideas to MHD-scale islands formed during fast reconnection
in a simulated eruptive flare. A simple analytic model based on the
particles' adiabatic invariants is used to calculate the energy gain
of particles orbiting field lines in our ultrahigh-resolution, 2.5D,
MHD numerical simulation of a solar eruption (flare + coronal mass
ejection). Then, we analytically model electrons visiting multiple
contracting islands to account for the observed high-energy flare
emission. In addition, the number of visited islands is related
to the spectrum high-energy break. Our acceleration mechanism
inherently produces sporadic emission because island formation is
intermittent. Moreover, a large number of particles could be accelerated
in each macroscopic island, which may explain the inferred rates
of energetic-electron production in flares. We conclude that island
contraction in the flare current sheet is a promising candidate for
electron acceleration in solar eruptions. This work was supported in
part by the NASA LWS and H-SR programs.
---------------------------------------------------------
Title: Reconnection-driven Magnetohydrodynamic Turbulence in a
Simulated Coronal-hole Jet
Authors: Uritsky, Vadim M.; Roberts, Merrill A.; DeVore, C. Richard;
Karpen, Judith T.
2017ApJ...837..123U Altcode: 2016arXiv160703843U
Extreme-ultraviolet and X-ray jets occur frequently in magnetically open
coronal holes on the Sun, especially at high solar latitudes. Some of
these jets are observed by white-light coronagraphs as they propagate
through the outer corona toward the inner heliosphere, and it has
been proposed that they give rise to microstreams and torsional
Alfvén waves detected in situ in the solar wind. To predict and
understand the signatures of coronal-hole jets, we have performed a
detailed statistical analysis of such a jet simulated by an adaptively
refined magnetohydrodynamics model. The results confirm the generation
and persistence of three-dimensional, reconnection-driven magnetic
turbulence in the simulation. We calculate the spatial correlations
of magnetic fluctuations within the jet and find that they agree best
with the Müller-Biskamp scaling model including intermittent current
sheets of various sizes coupled via hydrodynamic turbulent cascade. The
anisotropy of the magnetic fluctuations and the spatial orientation
of the current sheets are consistent with an ensemble of nonlinear
Alfvén waves. These properties also reflect the overall collimated jet
structure imposed by the geometry of the reconnecting magnetic field. A
comparison with Ulysses observations shows that turbulence in the jet
wake is in quantitative agreement with that in the fast solar wind.
---------------------------------------------------------
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: 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: Probing Prominence Formation with Time Series Analysis of
Models and AIA Data
Authors: Kucera, T. A.; Viall, N. M.; Karpen, J. T.
2016AGUFMSH43C2583K Altcode:
We present a observational and modeling study of the formation and
dynamics of prominence plasma, using a time series analysis of data
from the Solar Dynamic Observatory's Atmospheric Imaging Assembly
(SDO/AIA). The analysis consists of a diagnosis of heating and cooling
events by comparing the time profiles of emission formed at different
temperatures and observed by different AIA bands. We apply this
analysis both to prominences observed by AIA and to model runs from
the thermal non-equilibrium model in which heating at the foot-points
of sheared coronal flux-tubes results in evaporation of hot (a few MK)
material into the corona and subsequent catastrophic cooling of the
hot material to form the cool ( 10,000 K) prominence material. We find
that both the data and model show characteristic heating and cooling
signatures that are significantly different from those seen in active
regions. Supported by NASA's Living with a Star program.
---------------------------------------------------------
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: Large-Amplitude Oscillations as a Probe of Solar Prominences
Authors: Luna Bennasar, M.; Karpen, J. T.; Gilbert, H. R.; Kucera,
T. A.; Muglach, K.
2016AGUFMSH41E..01L Altcode:
Large-amplitude oscillations in prominences are among the most
spectacular phenomena of the solar atmosphere. Such an oscillations
involve motions with velocities above 20 km/s, and large portions
of the filament that move in phase. These are triggered by energetic
disturbances as flares and jets. These oscillations are an excellent
tool to probe the not directly measurable filament morphology. In
addition, the damping of these motions can be related with the process
of evaporation of chromospheric plasma associated to coronal heating. In
these talk I will show recent observational and theoretical progress
on large-amplitude seismology on prominences.
---------------------------------------------------------
Title: Fly-Throughs of Simulated Solar Coronal Jets in Preparation
for Solar Probe Plus
Authors: Roberts, M. A.; Uritsky, V. M.; Karpen, J. T.; DeVore, C. R.
2016AGUFMSH54A..02R Altcode:
Coronal hole jets are highly collimated impulsive flows of plasma
that are observed within the open field of solar coronal holes. In
the low corona, the jets can be narrow spires or extended fans, and
many exhibit helical motions (e.g. Patsourakos et al. 2008). The jets
have been associated with an embedded dipole topology, consisting of a
fan-separatrix and a spine line emanating from a null point occurring
at the top of the dome shaped fan surface (Antiochos, 1998). With the
upcoming launch of Solar Probe Plus (SPP), the possibility to observe
these structures in situ will exist for the first time. With that
in mind, this study analyzes simulated coronal jets and examines the
signatures of virtual spacecraft fly-throughs based on SPP's projected
orbital parameters. Using the Adaptively Refined MHD Solver (ARMS),
our simulations take into account gravity, solar wind, and spherical
geometry to generate coronal jets by reconnection between a twisted
embedded bipole and the surrounding open field (e.g. Karpen et al. 2016,
in review). These new simulations confirm and extend previous Cartesian
studies of coronal jets based on this mechanism (Pariat et al. 2009,
2010, 2015), and our latest jet simulation for SPP extends out to 30
solar radii, well beyond the planned perapsis of SPP's later orbits. We
find that the interior structure of the jet generated in the dynamic,
compressible region just above the reconnection point maintains
its relative structure as the jet propagates outward into altitudes
sampled by SPP, where the jet's immediate wake becomes incompressible
and Alfvénic. This suggests that not only should SPP be able to
detect these jets, but also that the features measured by SPP could
be extrapolated back to gain knowledge about the plasma dynamics just
after jet onset. We also find that spatial correlations of the magnetic
field fluctuations inside the jet agree with the Müeller-Biskamp
model of MHD turbulence (2000), with intermittent two dimensional
current sheets as the primary energy dissipation structures.
---------------------------------------------------------
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: Using SDO/AIA to Understand the Thermal Evolution of Solar
Prominence Formation
Authors: Viall, Nicholeen; M.; Kucera, Therese T.; Karpen, Judith
2016usc..confE..49V Altcode:
In this study, we investigate prominence formation using time series
analysis of Solar Dynamics Observatory's Atmospheric Imaging Assembly
(SDO/AIA) data. We investigate the thermal properties of forming
prominences by analyzing observed light curves using the same technique
that we have already successfully applied to active regions to diagnose
heating and cooling cycles. This technique tracks the thermal evolution
using emission formed at different temperatures, made possible by
AIA's different wavebands and high time resolution. We also compute the
predicted light curves in the same SDO/AIA channels of a hydrodynamic
model of thermal nonequilibrium formation of prominence material,
an evaporation-condensation model. In these models of prominence
formation, heating at the foot-points of sheared coronal flux-tubes
results in evaporation of material of a few MK into the corona followed
by catastrophic cooling of the hot material to form cool ( 10,000 K)
prominence material. We demonstrate that the SDO/AIA light curves
for flux tubes undergoing thermal nonequilibrium vary at different
locations along the flux tube, especially in the region where the
condensate forms, and we compare the predicted light curves with those
observed. Supported by NASA's Living with a Star program.
---------------------------------------------------------
Title: The effects of magnetic-field geometry on longitudinal
oscillations of solar prominences: Cross-sectional area variation
for thin tubes
Authors: Luna, M.; Díaz, A. J.; Oliver, R.; Terradas, J.; Karpen, J.
2016A&A...593A..64L Altcode: 2016arXiv160702996L
Context. Solar prominences are subject to both field-aligned
(longitudinal) and transverse oscillatory motions, as evidenced by an
increasing number of observations. Large-amplitude longitudinal motions
provide valuable information on the geometry of the filament-channel
magnetic structure that supports the cool prominence plasma against
gravity. Our pendulum model, in which the restoring force is the gravity
projected along the dipped field lines of the magnetic structure, best
explains these oscillations. However, several factors can influence
the longitudinal oscillations, potentially invalidating the pendulum
model. <BR /> Aims: The aim of this work is to study the influence
of large-scale variations in the magnetic field strength along the
field lines, I.e., variations of the cross-sectional area along
the flux tubes supporting prominence threads. <BR /> Methods: We
studied the normal modes of several flux tube configurations, using
linear perturbation analysis, to assess the influence of different
geometrical parameters on the oscillation properties. <BR /> Results:
We found that the influence of the symmetric and asymmetric expansion
factors on longitudinal oscillations is small. <BR /> Conclusions:
We conclude that the longitudinal oscillations are not significantly
influenced by variations of the cross-section of the flux tubes,
validating the pendulum model in this context.
---------------------------------------------------------
Title: Three-Dimensional Simulations of Tearing and Intermittency
in Coronal Jets
Authors: Wyper, P. F.; DeVore, C. R.; Karpen, J. T.; Lynch, B. J.
2016ApJ...827....4W Altcode: 2016arXiv160700692W
Observations of coronal jets increasingly suggest that local
fragmentation and intermittency play an important role in the dynamics
of these events. In this work, we investigate this fragmentation in
high-resolution simulations of jets in the closed-field corona. We study
two realizations of the embedded-bipole model, whereby impulsive helical
outflows are driven by reconnection between twisted and untwisted
field across the domed fan plane of a magnetic null. We find that the
reconnection region fragments following the onset of a tearing-like
instability, producing multiple magnetic null points and flux-rope
structures within the current layer. The flux ropes formed within the
weak-field region in the center of the current layer are associated
with “blobs” of density enhancement that become filamentary
threads as the flux ropes are ejected from the layer, whereupon new
flux ropes form behind them. This repeated formation and ejection
of flux ropes provides a natural explanation for the intermittent
outflows, bright blobs of emission, and filamentary structure observed
in some jets. Additional observational signatures of this process
are discussed. Essentially all jet models invoke reconnection between
regions of locally closed and locally open field as the jet-generation
mechanism. Therefore, we suggest that this repeated tearing process
should occur at the separatrix surface between the two flux systems
in all jets. A schematic picture of tearing-mediated jet reconnection
in three dimensions is outlined.
---------------------------------------------------------
Title: Modeled Flare Hard X-ray Emission from Electrons Accelerated
in Simulated Large-scale Magnetic Islands
Authors: Guidoni, Silvina E.; Allred, Joel C.; Alaoui, Meriem; Holman,
Gordon D.; DeVore, C. Richard; Karpen, Judith T.
2016shin.confE.109G Altcode:
The mechanism that accelerates particles to the energies required to
produce the observed impulsive hard X-ray emission in solar flares
is not well understood. It is generally accepted that this emission
is produced by a non-thermal beam of electrons that collides with the
ambient ions as the beam propagates from the top of a flare loop to its
footpoints. Most current models that investigate this transport assume
an injected beam with an initial energy spectrum inferred from observed
hard X-ray spectra, usually a power law with a low-energy cutoff. In
our previous work (Guidoni et al. 2016), we proposed an analytical
method to estimate particle energy gain in contracting, large-scale,
2.5-dimensional magnetic islands, based on a kinetic model by Drake et
al. (2010). We applied this method to sunward-moving islands formed
high in the corona during fast reconnection in a simulated eruptive
flare. The overarching purpose of the present work is to test this
proposed acceleration model by estimating the hard X-ray flux resulting
from its predicted accelerated-particle distribution functions. To do
so, we have coupled our model to a unified computational framework that
simulates the propagation of an injected beam as it deposits energy and
momentum along its way (Allred et al. 2015). This framework includes the
effects of radiative transfer and return currents, necessary to estimate
flare emission that can be compared directly to observations. We will
present preliminary results of the coupling between these models.
---------------------------------------------------------
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: Modeling Flare Hard X-ray Emission from Electrons in
Contracting Magnetic Islands
Authors: Guidoni, Silvina E.; Allred, Joel C.; Alaoui, Meriem; Holman,
Gordon D.; DeVore, C. Richard; Karpen, Judith T.
2016SPD....47.0602G Altcode:
The mechanism that accelerates particles to the energies required to
produce the observed impulsive hard X-ray emission in solar flares
is not well understood. It is generally accepted that this emission
is produced by a non-thermal beam of electrons that collides with the
ambient ions as the beam propagates from the top of a flare loop to its
footpoints. Most current models that investigate this transport assume
an injected beam with an initial energy spectrum inferred from observed
hard X-ray spectra, usually a power law with a low-energy cutoff. In
our previous work (Guidoni et al. 2016), we proposed an analytical
method to estimate particle energy gain in contracting, large-scale,
2.5-dimensional magnetic islands, based on a kinetic model by Drake et
al. (2010). We applied this method to sunward-moving islands formed
high in the corona during fast reconnection in a simulated eruptive
flare. The overarching purpose of the present work is to test this
proposed acceleration model by estimating the hard X-ray flux resulting
from its predicted accelerated-particle distribution functions. To do
so, we have coupled our model to a unified computational framework that
simulates the propagation of an injected beam as it deposits energy and
momentum along its way (Allred et al. 2015). This framework includes the
effects of radiative transfer and return currents, necessary to estimate
flare emission that can be compared directly to observations. We will
present preliminary results of the coupling between these models.
---------------------------------------------------------
Title: Magnetic-island Contraction and Particle Acceleration in
Simulated Eruptive Solar Flares
Authors: Guidoni, S. E.; DeVore, C. R.; Karpen, J. T.; Lynch, B. J.
2016ApJ...820...60G Altcode: 2016arXiv160301309G
The mechanism that accelerates particles to the energies required
to produce the observed high-energy impulsive emission in solar
flares is not well understood. Drake et al. proposed a mechanism
for accelerating electrons in contracting magnetic islands formed
by kinetic reconnection in multi-layered current sheets (CSs). We
apply these ideas to sunward-moving flux ropes (2.5D magnetic islands)
formed during fast reconnection in a simulated eruptive flare. A simple
analytic model is used to calculate the energy gain of particles
orbiting the field lines of the contracting magnetic islands in our
ultrahigh-resolution 2.5D numerical simulation. We find that the
estimated energy gains in a single island range up to a factor of
five. This is higher than that found by Drake et al. for islands in
the terrestrial magnetosphere and at the heliopause, due to strong
plasma compression that occurs at the flare CS. In order to increase
their energy by two orders of magnitude and plausibly account for the
observed high-energy flare emission, the electrons must visit multiple
contracting islands. This mechanism should produce sporadic emission
because island formation is intermittent. Moreover, a large number of
particles could be accelerated in each magnetohydrodynamic-scale island,
which may explain the inferred rates of energetic-electron production
in flares. We conclude that island contraction in the flare CS is a
promising candidate for electron acceleration in solar eruptions.
---------------------------------------------------------
Title: High Resolution Simulations of Tearing and Flux-Rope Formation
in Active Region Jets
Authors: Wyper, P. F.; DeVore, C. R.; Karpen, J. T.
2015AGUFMSH21A2383W Altcode:
Observations of coronal jets increasingly suggest that local
fragmentation and the generation of small-scale structure plays an
important role in the dynamics of these events. In the magnetically
closed corona, jets most often occur near active regions and are
associated with an embedded-bipole topology consisting of a 3D magnetic
null point atop a domed fan separatrix surface at the base of a coronal
loop. Impulsive reconnection in the vicinity of the null point between
the magnetic fluxes inside and outside the dome launches the jet along
the loop. Wyper & Pontin 2014 showed that the 3D current layers
that facilitate such reconnection are explosively unstable to tearing,
generating complex flux-rope structures. Utilizing the adaptive
mesh capabilities of the Adaptively Refined Magnetohydrodynamics
Solver, we investigate the generation of such fine-scale structure
in high-resolution simulations of active-region jets. We observe the
formation of multiple flux-rope structures forming across the fan
separatrix surface and discuss the photospheric signatures of these
flux ropes and the associated local topology change. We also introduce
a new way of identifying such flux ropes in the magnetic field, based
on structures observed in the magnetic squashing factor calculated on
the photosphere. By tracking the position and number of new null points
produced by the fragmentation, we also show that the formation of flux
ropes can occur away from the main null region on the flanks of the
separatrix dome and that the jet curtain has a highly complex magnetic
structure. This work was funded through an appointment to the NASA
Postdoctoral Program and by NASA's Living With a Star TR&T program.
---------------------------------------------------------
Title: Reconnection-Driven Solar Polar Jets to be Encountered by
Solar Probe Plus: Simulated In Situ Measurements and Data Analysis
Authors: Uritsky, V. M.; Roberts, M. A.; Karpen, J. T.; DeVore, C. R.
2015AGUFMSH31C2439U Altcode:
Solar polar jets are observed to originate in regions within the
open field of solar coronal holes. These so called "anemone" regions
are associated with an embedded dipole topology, consisting of a
fan-separatrix and a spine line emanating from a null point occurring at
the top of the dome shaped fan surface (Antiochos 1996). In this study,
we analyze simulations using the Adaptively Refined MHD Solver (ARMS)
that take into account gravity, solar wind, and spherical geometry
to generate polar jets by reconnection between a twisted embedded
bipole and the surrounding open field (Karpen et al. 2015). These
simulations confirm and extend previous Cartesian studies of polar
jets based on this mechanism (Pariat et al. 2009, 2010, 2015), as
well as extending the analyses from our previous work (Roberts et
al. 2014,2015) out to radial distances that will be sampled by Solar
Probe Plus. Focusing on the plasma density, velocity, magnetic field,
and current density, we interpolate the adaptively gridded simulation
data onto a regular grid, and analyze the signatures that the jet
produces as it propagates outward from the solar surface into the
inner heliosphere. We also conduct simulated spacecraft fly-throughs
of the jet in several different velocity regimes, illustrating the
signatures that Solar Probe Plus may encounter in situ as the jet
propagates into the heliosphere. The trans-Alfvénic nature of the jet
front is confirmed by temporally differencing the plasma mass density
and comparing the result with the local Alfvén speed. Our analysis
confirms the presence of a reconnection driven magnetic turbulence
in the simulated plasma jet, finding spatial correlations of magnetic
fluctuations inside the jet to be in agreement with the scaling model
of MHD turbulence. The turbulence cascade is supported by multiscale
current sheets combined with filamentary structures representing fluid
vorticies. The spatial orientation of these current sheets, combined
with the anisotropy of the magnetic fluctuations, is indicative of
torsional Alfvén wave packets, consistent with the helical geometry of
the jet. This research was supported by NASA grant NNG11PL10A 670.036
to CUA/IACS (M.A.R. and V.M.U.) and NASA's Living With a Star Targeted
Research and Technology (J.T.K. and C.R.D.) program.
---------------------------------------------------------
Title: The Effects of Partial Ionization on Prominence Mass Formation
Authors: Karpen, J. T.; Olson, K.; DeVore, C. R.; Martinez Gomez,
D.; Sokolov, I.
2015AGUFMSH23D..02K Altcode:
The origin of the prominence mass has been an open question since this
cool plasma suspended in the hot corona was first discovered. We have
known for a long time that the mass must come from the chromosphere,
but it is unclear whether this mass is lifted bodily through
magnetic levitation, injected by reconnection-driven upflows, or
driven from the chromosphere by evaporation and then condensed. One
evaporation-condensation scenario, the thermal nonequilibrium (TNE)
model, is the most fully developed, quantitative model for the
prominence plasma to date. In the TNE scenario, localized heating
concentrated at the coronal loop footpoints produces chromospheric
evaporation, filling the flux tube with hot, dense plasma that
subsequently collapses radiatively to form cool condensations. Thus far
this model has been successful in explaining the key properties of the
long, persistent threads and small, highly dynamic, transient blobs in
prominences, the damping of large-amplitude field-aligned prominence
oscillations, the appearance of horn-shaped features above the cool
prominence in EUV images of coronal cavities, and coronal rain in
the ambient corona. To date, all studies of TNE have assumed that the
plasma is fully ionized, which is appropriate for the hot coronal gas
but unrealistic for the cool plasma below ~30,000 K. The energetics,
dynamics, and evolutionary time scales of the TNE process are expected
to be altered when the effects of ionization and recombination are
considered. We have modified ARGOS, our 1D hydrodynamic code with
adaptive mesh refinement, to include an equation of state that accounts
for the effects of partial ionization of the plasma over a wide range
of temperatures and densities. We will discuss the results of these
simulations and their comparison with our previous studies of TNE
in typical filament-supporting flux tubes. This work was partially
supported by NASA's LWS Strategic Capability program.
---------------------------------------------------------
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: 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: Analytic Method to Estimate Particle Acceleration in Flux Ropes
Authors: Guidoni, Silvina E.; Karpen, Judith T.; DeVore, C. Richard
2015shin.confE..20G Altcode:
The mechanism that accelerates particles to the energies required
to produce the observed high-energy emission in solar flares is not
well understood. Drake et al. (2006) proposed a kinetic mechanism
for accelerating electrons in contracting magnetic islands formed by
reconnection. In this model, particles that gyrate around magnetic
field lines transit from island to island, increasing their energy
by Fermi acceleration in those islands that are contracting. Based on
these ideas, we present an analytic model to estimate the energy gain of
particles orbiting around field lines inside a flux rope (2.5D magnetic
island). We calculate the change in the velocity of the particles as
the flux rope evolves in time. The method assumes a simple profile for
the magnetic field of the evolving island; it can be applied to any
case where flux ropes are formed. In our case, the flux-rope evolution
is obtained from our recent high-resolution, compressible 2.5D MHD
simulations of breakout eruptive flares. The simulations allow us to
resolve in detail the generation and evolution of large-scale flux
ropes as a result of sporadic and patchy reconnection in the flare
current sheet. Our results show that the initial energy of particles
can be increased by 2-5 times in a typical contracting island, before
the island reconnects with the underlying arcade. Therefore, particles
need to transit only from 3-7 islands to increase their energies by
two orders of magnitude. These macroscopic regions, filled with a large
number of particles, may explain the large observed rates of energetic
electron production in flares. We conclude that this mechanism is a
promising candidate for electron acceleration in flares, but further
research is needed to extend our results to 3D flare conditions.
---------------------------------------------------------
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: Simulated In Situ Measurements and Structural Analysis of
Reconnection-Driven Solar Polar Jets
Authors: Roberts, Merrill A.; Uritsky, Vadim M.; Karpen, Judith T.;
DeVore, C. R.
2015TESS....120302R Altcode:
Solar polar jets are observed to originate in regions within the open
field of solar coronal holes. These so called “anemone” regions
are associated with an embedded dipole topology, consisting of a
fan-separatrix and a spine line emanating from a null point occurring at
the top of the dome shaped fan surface (Antiochos 1998). In this study,
we analyze simulations using the Adaptively Refined MHD Solver (ARMS)
that take into account gravity, solar wind, and spherical geometry
to generate polar jets by reconnection between a twisted embedded
bipole and the surrounding open field (Karpen et al. 2015). These new
simulations confirm and extend previous Cartesian studies of polar jets
based on this mechanism (Pariat et al. 2009, 2010, 2015). Focusing on
the plasma density, velocity, and magnetic field, we interpolate the
adaptively gridded simulation data onto a regular grid, and analyze
the signatures that the jet produces as it propagates outward from the
solar surface. The trans-Alfvénic nature of the jet front is confirmed
by temporally differencing the plasma mass density and comparing the
result with the local Alfvén speed. We perform a preliminary analysis
of the magnetized plasma turbulence, and examine how the turbulence
affects the overall structure of the jet. We also conduct simulated
spacecraft fly-throughs of the jet, illustrating the signatures that
spacecraft such as Solar Probe Plus may encounter in situ as the jet
propagates into the heliosphere. These fly-throughs are performed
in several different velocity regimes to better model the changing
velocity of Solar Probe Plus relative to the Sun and its jets over
the course of the mission.This research was supported by NASA grant
NNG11PL10A 670.036 to CUA/IACS (M.A.R. and V.M.U.) and the Living With
a Star Targeted Research and Technology (J.T.K. and C.R.D.) 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: Does Flare Reconnection Occur Before or After Explosive
Coronal Mass Ejection Acceleration?
Authors: Guidoni, Silvina E.; Karpen, Judith T.; DeVore, C. R.;
Qiu, Jiong
2015TESS....110704G Altcode:
The mechanism for producing fast coronal mass ejections/eruptive flares
(CME/EFs) is hotly debated. Most models rely on ideal instability/loss
of equilibrium or magnetic reconnection; these two categories of models
predict different causal relationships between CMEs and flares. In
both cases, flare reconnection disconnects the bulk of the CME from
the Sun, but in the former models, flare reconnection onset is a
consequence of the fast outward motion of the CME while in the later
models reconnection is what causes the CME acceleration. Discriminating
between these models requires continuous, high-cadence observations and
state-of-the-art numerical simulations that enable the relative timing
of key stages in the events to be determined. With the advent of SDO,
STEREO, and massively parallel supercomputers, we are well poised to
tackle this major challenge to our understanding of solar activity. In
recent work (Karpen et al. 2012), we determined the timing and location
of triggering mechanisms for the breakout initiation model (Antiochos et
al. 1999), using ultra-high-resolution magnetohydrodynamic simulations
with adaptive mesh refinement and high-cadence analysis. This approach
enabled us to resolve as finely as possible the small scales of
magnetic reconnection and island formation in the current sheets,
within the global context of a large-scale solar eruption. We found
that the explosive acceleration of the fast CME occurs only after
the onset of rapid reconnection at the flare current sheet formed in
the wake of the rising CME flux rope. In the present work, we compare
flare reconnection rates, measured from flare ribbon UV brightenings
observed by SDO/AIA and magnetograms from SDO/HMI, with the height
evolution of CME fronts and cores, measured from STEREO/SECCHI
EUV and coronagraph images. We also calculate these quantities from
numerical simulations and compare them to observations, as a new test
of the breakout initiation model. This work was supported by NASA's
Heliophysics Supporting Research and Living With a Star Targeted
Research and Technology programs.
---------------------------------------------------------
Title: Investigating the Thermal Evolution of Solar Prominence
Formation
Authors: Kucera, Therese A.; Viall, Nicholeen M.; Karpen, Judith T.
2015TESS....120315K Altcode:
We present a study of prominence formation using time series analysis of
Solar Dynamics Observatory’s Atmospheric Imaging Assembly (SDO/AIA)
data. In evaporation-condensation models of prominence formation,
heating at the foot-points of sheared coronal flux-tubes results in
evaporation of hot (a few MK) material into the corona and subsequent
catastrophic cooling of the hot material to form the cool (~10,000 K)
prominence material. We present the results of a time-lag analysis
that tracks the thermal evolution using emission formed at different
temperatures. This analysis is made possible by AIA's many wavebands
and high time resolution, and it allows us to look for signs of the
evaporation-condensation process and to study the heating time scales
involved. Supported by NASA’s Living with a Star program.
---------------------------------------------------------
Title: Plasma Structure and Dynamics
Authors: Karpen, Judith T.
2015ASSL..415..237K Altcode:
Despite over a century of observations, the physical processes by
which prominence plasma forms and evolves remain controversial. In this
chapter we review the observational constraints on all mass formation
models, review the four leading models—injection, levitation,
evaporation-condensation, and magneto-thermal convection, describe the
strengths and weaknesses of each model, and point out opportunities for
future work. As needed, short tutorials are provided on fundamental
physical mechanisms and concepts not covered in other chapters,
including magnetic reconnection and energy balance in coronal loops.
---------------------------------------------------------
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: Analyses of Simulated Reconnection-Driven Solar Polar Jets
Authors: Roberts, M. A.; Uritsky, V. M.; Karpen, J. T.; DeVore, C. R.
2014AGUFMSH21B4123R Altcode:
Solar polar jets are observed to originate in regions within
the open field of solar coronal holes. These so called "anemone"
regions are generally accepted to be regions of opposite polarity,
and are associated with an embedded dipole topology, consisting
of a fan-separatrix and a spine line emanating from a null point
occurring at the top of the dome shaped fan surface. Previous
analysis of these jets (Pariat et al. 2009,2010) modeled using the
Adaptively Refined Magnetohydrodynamics Solver (ARMS) has supported
the claim that magnetic reconnection across current sheets formed at
the null point between the highly twisted closed field of the dipole
and open field lines surrounding it releases the energy necessary
to drive these jets. However, these initial simulations assumed a
"static" environment for the jets, neglecting effects due to gravity,
solar wind and the expanding spherical geometry. A new set of ARMS
simulations taking into account these additional physical processes
was recently performed. Initial results are qualitatively consistent
with the earlier Cartesian studies, demonstrating the robustness of
the underlying ideal and resistive mechanisms. We focus on density
and velocity fluctuations within a narrow radial slit aligned with
the direction of the spine of the jet, as well as other physical
properties, in order to identify and refine their signatures in the
lower heliosphere. These refined signatures can be used as parameters
by which plasma processes initiated by these jets may be identified
in situ by future missions such as Solar Orbiter and Solar Probe Plus.
---------------------------------------------------------
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: Onset of Flare Reconnection and Coronal Mass Ejection
Acceleration in Eruptive Events
Authors: Guidoni, S. E.; Karpen, J. T.; DeVore, C. R.; Qiu, J.
2014AGUFMSH23A4150G Altcode:
The mechanism for producing fast coronal mass ejections/eruptive flares
(CME/EFs) is hotly debated. Most models rely on ideal instability/loss
of equilibrium or magnetic reconnection; these two categories of models
predict different relationships between CMEs and flares. Discriminating
between them requires continuous, high-resolution observations and
state-of-the-art numerical simulations that enable the relative timing
of key stages in the events to be determined. With the advent of SDO,
STEREO, and massively parallel supercomputers, we are well poised to
tackle this major challenge to our understanding of solar activity. In
recent work (Karpen et al. 2012), we determined the timing and location
of triggering mechanisms for the breakout initiation model (Antiochos et
al. 1999), using ultra-high-resolution magnetohydrodynamic simulations
with adaptive mesh refinement and high-cadence analysis. This approach
enabled us to resolve as finely as possible the small scales of magnetic
reconnection and island formation in the current sheets, within the
global context of a large-scale solar eruption. We found that the
explosive acceleration of the fast CME occurs only after the onset of
rapid reconnection at the flare current sheet formed in the wake of
the rising CME flux rope. In the present work, we discriminate between
ideal and resistive mechanisms for fast CME/EFs using a combination
of state-of-the-art observations and simulations. We compare flare
reconnection rates, measured from flare ribbon UV brightenings observed
by SDO/AIA and magnetograms from SDO/HMI, with the height evolution of
CME fronts and cores, measured from STEREO/SECCHI EUV and coronagraph
images. We also calculate these quantities from numerical simulations
and compare them to observations, as a new test of the breakout
initiation model.
---------------------------------------------------------
Title: Observations and Implications of Large-Amplitude Longitudinal
Oscillations in a Solar Filament
Authors: Karpen, J. T.; Luna Bennasar, M.; Knizhnik, K. J.; Muglach,
K.; Gilbert, H. R.; Kucera, T. A.; Uritsky, V. M.; Asfaw, T. T.
2014AGUFMSH51C4171K Altcode:
On 20 August 2010 an energetic disturbance triggered large-amplitude
longitudinal oscillations in a large fraction of a nearby filament. The
triggering mechanism appears to be episodic jets connecting the
energetic event with the filament threads. We analyzed this periodic
motion to characterize the underlying physics of the oscillation as
well as the filament properties. The results support our previous
theoretical conclusions that the restoring force of large-amplitude
longitudinal oscillations is solar gravity, and the damping mechanism
is the ongoing accumulation of mass onto the oscillating threads. Based
on our previous work, we used the fitted parameters to determine the
magnitude and radius of curvature of the dipped magnetic field along
the filament, as well as the mass accretion rate onto the filament
threads. These derived properties are nearly uniform along the filament,
indicating a remarkable degree of homogeneity throughout the filament
channel. Moreover, the estimated mass accretion rate implies that the
footpoint heating responsible for the thread formation, according to
the thermal nonequilibrium model, agrees with previous coronal heating
estimates. We also estimated the magnitude of the energy released in
the nearby event by studying the dynamic response of the filament
threads, and concluded that the initiating event is likely to be a
microflare. Using a nonlinear force-free field extrapolation of the
photospheric magnetogram to estimate the coronal magnetic structure,
we determined the possible connectivity between the jet source and the
oscillating prominence segments. We will present the results of this
investigation and discuss their implications for filament structure
and heating. This work was supported by NASA's H-SR program.
---------------------------------------------------------
Title: Mass Flows in a Prominence Spine as Observed in EUV
Authors: Kucera, T. A.; Gilbert, H. R.; Karpen, J. T.
2014ApJ...790...68K Altcode:
We analyze a quiescent prominence observed by the Solar Dynamics
Observatory's Atmospheric Imaging Assembly (AIA) with a focus on
mass and energy flux in the spine, measured using Lyman continuum
absorption. This is the first time this type of analysis has been
applied with an emphasis on individual features and fluxes in a
quiescent prominence. The prominence, observed on 2010 September
28, is detectable in most AIA wavebands in absorption and/or
emission. Flows along the spine exhibit horizontal bands 5”-10” wide
and kinetic energy fluxes on the order of a few times 10<SUP>5</SUP>
erg s<SUP>-1</SUP>cm<SUP>-2</SUP>, consistent with quiet sun coronal
heating estimates. For a discrete moving feature we estimate a mass
of a few times 10<SUP>11</SUP> g. We discuss the implications of our
derived properties for a model of prominence dynamics, the thermal
non-equilibrium model.
---------------------------------------------------------
Title: Contracting magnetic islands in MHD simulations of flare
reconnection
Authors: Guidoni, Silvina E.; Karpen, Judith T.; DeVore, C. Richard
2014shin.confE..37G Altcode:
The mechanisms that accelerate ionized particles to the energies
required to produce the observed high-energy emission in solar flares
are not well understood. Drake et al. (2006) proposed a kinetic
mechanism for accelerating electrons in contracting magnetic
islands formed by reconnection. In this model, particles that
gyrate around magnetic field lines transit from island to island,
increasing their energy by Fermi acceleration in those islands that
are contracting. Macroscopic regions filled with a large number of
these small islands are required to achieve the large observed rates
of energetic electron production in flares, but at the moment it is
impossible to simulate sufficiently large-scale systems using kinetic
models. Our recent high-resolution, compressible MHD simulations of a
breakout eruptive flare (Karpen et al. 2012) allow us to resolve in
detail the generation and evolution of macroscopic magnetic islands
in the flare current sheet, and to study the Drake et al. mechanism
in a configuration that more closely represents the flare atmosphere
and structure. Based on the Drake et al. studies, we attempt to close
the gap between kinetic and fluid models by characterizing island
contractions in our simulations as the islands move away from the
main reconnection site toward the flare arcade. To that end, with our
null-tracking capabilities, we follow the creation and evolution of X-
and O-type (island) nulls that result from spatially and temporally
localized reconnection. Preliminary results show that the initial energy
of particles could be increased by 2-4 times in a typical contracting
island, before the island reconnects with the underlying arcade. We
conclude that this mechanism is a promising candidate for electron
acceleration in flares, but further research is needed to extend our
results to 3D flare conditions.
---------------------------------------------------------
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: Observations and Implications of Large-Amplitude
LongitudinalOscillations in a Solar Filament
Authors: Karpen, Judith T.; Luna, Manuel; Knizhnik, Kalman J.; Muglach,
Karin; Gilbert, Holly; Kucera, Therese A.; Uritsky, Vadim
2014AAS...22411106K Altcode:
On 20 August 2010 an energetic disturbance triggered large-amplitude
longitudinal oscillations in a large fraction of a nearby filament. The
triggering mechanism appears to be episodic jets connecting the
energetic event with the filament threads. We analyzed this periodic
motion to characterize the underlying physics of the oscillation as
well as the filament properties. The results support our previous
theoretical conclusions that the restoring force of large-amplitude
longitudinal oscillations is solar gravity, and the damping mechanism
is the ongoing accumulation of mass onto the oscillating threads. Based
on our previous work, we used the fitted parameters to determine the
magnitude and radius of curvature of the dipped magnetic field along
the filament, as well as the mass accretion rate onto the filament
threads. These derived properties are nearly uniform along the filament,
indicating a remarkable degree of homogeneity throughout the filament
channel. Moreover, the estimated mass accretion rate implies that the
footpoint heating responsible for the thread formation, according to
the thermal nonequilibrium model, agrees with previous coronal heating
estimates. We also estimated the magnitude of the energy released in
the nearby event by studying the dynamic response of the filament
threads, and concluded that the initiating event is likely to be a
microflare. We will present the results of this investigation and
discuss their implications for filament structure and heating. This
work was supported by NASA’s H-SR program.
---------------------------------------------------------
Title: Mass Flows in a Prominence Spine as Observed in EUV
Authors: Kucera, Therese A.; Gilbert, Holly; Karpen, Judith T.
2014AAS...22440804K Altcode:
We analyze a quiescent prominence observed by the Solar Dynamics
Observatory's Atmospheric Imaging Assembly with a focus on mass
and energy flows in the spine measured using Lyman continuum
absorption. This is the first time this sort of analysis has been
applied with an emphasis on individual features and flows in a quiescent
prominence. The prominence, observed on 2010 Sept. 28, is detectable in
most AIA wavebands in absorption and/or emission. Flows along the spine
exhibit horizontal bands 5-10 arcsec wide and kinetic energy fluxes
consistent with quiet sun coronal heating estimates. For a discrete
moving feature we estimate a mass of a few times 10^11 g. We discuss
the implications of our derived properties for models of prominence
dynamics, in particular the thermal non-equilibrium model. This project
was supported by NASA's LWS TR&T program.
---------------------------------------------------------
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: 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: Inferred Particle Acceleration by Contracting Magnetic Islands
in MHD Simulations of Flare Reconnection
Authors: Guidoni, Silvina; Karpen, Judith T.; DeVore, C. Richard
2014AAS...22410405G Altcode:
The mechanisms that accelerate ionized particles to the energies
required to produce the observed high-energy emission in solar flares
are not well understood. Drake et al. (2006) proposed a kinetic
mechanism for accelerating electrons in contracting magnetic
islands formed by reconnection. In this model, particles that
gyrate around magnetic field lines transit from island to island,
increasing their energy by Fermi acceleration in those islands that
are contracting. Macroscopic regions filled with a large number of
these small islands are required to achieve the large observed rates
of energetic electron production in flares, but at the moment it is
impossible to simulate sufficiently large-scale systems using kinetic
models. Our recent high-resolution, compressible MHD simulations of a
breakout eruptive flare (Karpen et al. 2012) allow us to resolve in
detail the generation and evolution of macroscopic magnetic islands
in the flare current sheet, and to study the Drake et al. mechanism in
a configuration that more closely represents the flare atmosphere and
structure. Based on the Drake et al. studies, we characterize island
contractions in our simulations as the islands move away from the
main reconnection site toward the flare arcade. To that end, with our
null-tracking capabilities, we follow the creation and evolution of X-
and O-type (island) nulls that result from spatially and temporally
localized reconnection. Preliminary results show that the initial energy
of particles could be increased by up to an order of magnitude in a
typical contracting island, before it reconnects with the underlying
arcade. We conclude that this mechanism is a promising candidate
for electron acceleration in flares, but further research is needed,
including extending our results to 3D flare conditions.
---------------------------------------------------------
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: Observations and Implications of Large-amplitude Longitudinal
Oscillations in a Solar Filament
Authors: Luna, M.; Knizhnik, K.; Muglach, K.; Karpen, J.; Gilbert,
H.; Kucera, T. A.; Uritsky, V.
2014ApJ...785...79L Altcode: 2014arXiv1403.0381L
On 2010 August 20, an energetic disturbance triggered large-amplitude
longitudinal oscillations in a nearby filament. The triggering mechanism
appears to be episodic jets connecting the energetic event with the
filament threads. In the present work, we analyze this periodic motion
in a large fraction of the filament to characterize the underlying
physics of the oscillation as well as the filament properties. The
results support our previous theoretical conclusions that the restoring
force of large-amplitude longitudinal oscillations is solar gravity,
and the damping mechanism is the ongoing accumulation of mass onto
the oscillating threads. Based on our previous work, we used the
fitted parameters to determine the magnitude and radius of curvature
of the dipped magnetic field along the filament, as well as the mass
accretion rate onto the filament threads. These derived properties are
nearly uniform along the filament, indicating a remarkable degree of
cohesiveness throughout the filament channel. Moreover, the estimated
mass accretion rate implies that the footpoint heating responsible
for the thread formation, according to the thermal nonequilibrium
model, agrees with previous coronal heating estimates. We estimate the
magnitude of the energy released in the nearby event by studying the
dynamic response of the filament threads, and discuss the implications
of our study for filament structure and heating.
---------------------------------------------------------
Title: Large-amplitude longitudinal oscillations in solar prominences
Authors: Luna, Manuel; Karpen, Judith; Díaz, Antonio; Knizhnik,
Kalman; Muglach, Karin; Gilbert, Holly; Kucera, Therese
2014IAUS..300..155L Altcode:
Large-amplitude longitudinal (LAL) prominence oscillations consist of
periodic mass motions along a filament axis. The oscillations appear
to be triggered by an energetic event, such as a microflare, subflare,
or small C-class flare, close to one end of the filament. Observations
reveal speeds of several tens to 100 km/s, periods of order 1 hr,
damping times of a few periods, and displacements equal to a significant
fraction of the prominence length. We have developed a theoretical model
to explain the restoring force and the damping mechanism. Our model
demonstrates that the main restoring force is the projected gravity in
the flux tube dips where the threads oscillate. Although the period
is independent of the tube length and the constantly growing mass,
the motions are strongly damped by the steady accretion of mass onto
the threads. We conclude that the LAL movements represent a collective
oscillation of a large number of cool, dense threads moving along
dipped flux tubes, triggered by a nearby energetic event. Our model
yields a powerful seismological method for constraining the coronal
magnetic field strength and radius of curvature at the thread locations.
---------------------------------------------------------
Title: Observational Study of Large Amplitude Longitudinal
Oscillations in a Solar Filament
Authors: Knizhnik, Kalman; Luna, Manuel; Muglach, Karin; Gilbert,
Holly; Kucera, Therese; Karpen, Judith
2014IAUS..300..428K Altcode: 2013arXiv1310.7657K
On 20 August 2010 an energetic disturbance triggered damped
large-amplitude longitudinal (LAL) oscillations in almost an entire
filament. In the present work we analyze this periodic motion in
the filament to characterize the damping and restoring mechanism of
the oscillation. Our method involves placing slits along the axis
of the filament at different angles with respect to the spine of the
filament, finding the angle at which the oscillation is clearest, and
fitting the resulting oscillation pattern to decaying sinusoidal and
Bessel functions. These functions represent the equations of motion
of a pendulum damped by mass accretion. With this method we determine
the period and the decaying time of the oscillation. Our preliminary
results support the theory presented by Luna and Karpen (2012) that
the restoring force of LAL oscillations is solar gravity in the tubes
where the threads oscillate, and the damping mechanism is the ongoing
accumulation of mass onto the oscillating threads. Following an earlier
paper, we have determined the magnitude and radius of curvature of
the dipped magnetic flux tubes hosting a thread along the filament,
as well as the mass accretion rate of the filament threads, via the
fitted parameters.
---------------------------------------------------------
Title: Prominence Mass Supply and the Cavity
Authors: Schmit, Donald J.; Gibson, S.; Luna, M.; Karpen, J.; Innes, D.
2013ApJ...779..156S Altcode: 2013arXiv1311.2382S
A prevalent but untested paradigm is often used to describe the
prominence-cavity system: the cavity is under-dense because it
is evacuated by supplying mass to the condensed prominence. The
thermal non-equilibrium (TNE) model of prominence formation offers
a theoretical framework to predict the thermodynamic evolution of
the prominence and the surrounding corona. We examine the evidence
for a prominence-cavity connection by comparing the TNE model with
diagnostics of dynamic extreme ultraviolet (EUV) emission surrounding
the prominence, specifically prominence horns. Horns are correlated
extensions of prominence plasma and coronal plasma which appear
to connect the prominence and cavity. The TNE model predicts that
large-scale brightenings will occur in the Solar Dynamics Observatory
Atmospheric Imaging Assembly 171 Å bandpass near the prominence that
are associated with the cooling phase of condensation formation. In
our simulations, variations in the magnitude of footpoint heating
lead to variations in the duration, spatial scale, and temporal offset
between emission enhancements in the other EUV bandpasses. While these
predictions match well a subset of the horn observations, the range of
variations in the observed structures is not captured by the model. We
discuss the implications of our one-dimensional loop simulations for
the three-dimensional time-averaged equilibrium in the prominence
and the cavity. Evidence suggests that horns are likely caused by
condensing prominence plasma, but the larger question of whether this
process produces a density-depleted cavity requires a more tightly
constrained model of heating and better knowledge of the associated
magnetic structure.
---------------------------------------------------------
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: Are Flare Quasi-periodic Pulsations Signatures of Intermittent
Reconnection?
Authors: Guidoni, Silvina; Karpen, J. T.; DeVore, C. R.
2013SPD....44...85G Altcode:
Flare quasi-periodic pulsations (QPPs) have been observed over a
vast energy spectrum, from radio to hard x-rays. The periodicities
of these fine structures range from tens of milliseconds to tens
of seconds and suggest highly structured but intermittent energy
release. In some cases, the sources of microwaves and thermal hard
x-rays are situated near the apex of the flare loop arcades and are not
stationary. Although it is unclear whether all the observed varieties
of QPPs can be explained via a single, unified process, our recent
high-resolution simulations of a breakout eruptive flare (Karpen et
al. 2012) indicate that spatially and temporally localized reconnection
is a plausible candidate for these bursts of radiation. With our
null-tracking capabilities, we follow the creation and evolution of X-
and O-type nulls in the flare current sheet and characterize their
periodicity. QPPs located at the apex of the flare arcade may result
from the interaction of downward-moving islands in the sheet with
the arcade below. Each island is composed of highly twisted magnetic
field lines that comprise a single reconnected flux tube. Upon arrival
at the top of the flare loops, secondary reconnection events between
the island and the arcade produce discrete energy release events that
could be related to observed QPPs in that region. Different regimes of
current-sheet reconnection (slow/fast), island sizes, rates of island
coalescence, and rates of reconnection between islands and arcades may
all help to explain the variety of energy and time scales exhibited by
the flare QPPs.Abstract (2,250 Maximum Characters): Flare quasi-periodic
pulsations (QPPs) have been observed over a vast energy spectrum,
from radio to hard x-rays. The periodicities of these fine structures
range from tens of milliseconds to tens of seconds and suggest highly
structured but intermittent energy release. In some cases, the sources
of microwaves and thermal hard x-rays are situated near the apex of
the flare loop arcades and are not stationary. Although it is unclear
whether all the observed varieties of QPPs can be explained via a
single, unified process, our recent high-resolution simulations of a
breakout eruptive flare (Karpen et al. 2012) indicate that spatially and
temporally localized reconnection is a plausible candidate for these
bursts of radiation. With our null-tracking capabilities, we follow
the creation and evolution of X- and O-type nulls in the flare current
sheet and characterize their periodicity. QPPs located at the apex of
the flare arcade may result from the interaction of downward-moving
islands in the sheet with the arcade below. Each island is composed of
highly twisted magnetic field lines that comprise a single reconnected
flux tube. Upon arrival at the top of the flare loops, secondary
reconnection events between the island and the arcade produce discrete
energy release events that could be related to observed QPPs in that
region. Different regimes of current-sheet reconnection (slow/fast),
island sizes, rates of island coalescence, and rates of reconnection
between islands and arcades may all help to explain the variety of
energy and time scales exhibited by the flare QPPs.
---------------------------------------------------------
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: 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: 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: Mechanisms of Eruptive Flares and Coronal Mass Ejections
Authors: Karpen, J.
2012AGUFMSH53B..05K Altcode:
The initiation of solar eruptions - fast coronal mass ejections
(CMEs) and eruptive flares - is one of the most important problems
in heliophysics, dynamically driving space weather and heliospheric
evolution. Two principal physical processes have been proposed for
the eruption onset: ideal loss of equilibrium or instability, and
magnetic reconnection. Our breakout scenario provides an intuitively
straightforward mechanism for fast-CME/eruptive-flare initiation
that involves reconnection both outside and inside a sheared filament
channel. Over the past decade, both 2.5D and 3D magnetohydrodynamic
simulations have demonstrated the basic feasibility of this model,
which also is supported by numerous observed eruptions. To understand
in detail the mechanisms for CME/flare onset and rapid acceleration, we
have explored the breakout model recently with a 2.5D simulation using
adaptive mesh refinement, with the highest spatial resolution achieved
to date. The ultra-high resolution allows us to separate clearly the
timing of the key phases of the event. Our results demonstrate clearly
that eruption becomes inevitable after fast breakout reconnection
starts, and that strong CME acceleration is due to the start of fast
flare reconnection at the flare current sheet. Most important, we
have identified a resistive instability as the trigger mechanism for
the eruption of fast breakout CMEs. I will discuss our research, its
implications for CME/flare models and observations, and instrumental
requirements for further progress.
---------------------------------------------------------
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 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: The Effects of Wave Escape on Fast Magnetosonic Wave Turbulence
in Solar Flares
Authors: Pongkitiwanichakul, Peera; Chandran, Benjamin D. G.; Karpen,
Judith T.; DeVore, C. Richard
2012ApJ...757...72P Altcode:
One of the leading models for electron acceleration in solar flares
is stochastic acceleration by weakly turbulent fast magnetosonic waves
("fast waves"). In this model, large-scale flows triggered by magnetic
reconnection excite large-wavelength fast waves, and fast-wave energy
then cascades from large wavelengths to small wavelengths. Electron
acceleration by large-wavelength fast waves is weak, and so the
model relies on the small-wavelength waves produced by the turbulent
cascade. In order for the model to work, the energy cascade time for
large-wavelength fast waves must be shorter than the time required for
the waves to propagate out of the solar-flare acceleration region. To
investigate the effects of wave escape, we solve the wave kinetic
equation for fast waves in weak turbulence theory, supplemented
with a homogeneous wave-loss term. We find that the amplitude of
large-wavelength fast waves must exceed a minimum threshold in order
for a significant fraction of the wave energy to cascade to small
wavelengths before the waves leave the acceleration region. We evaluate
this threshold as a function of the dominant wavelength of the fast
waves that are initially excited by reconnection outflows.
---------------------------------------------------------
Title: The Effects of Magnetic-field Geometry on Longitudinal
Oscillations of Solar Prominences
Authors: Luna, M.; Díaz, A. J.; Karpen, J.
2012ApJ...757...98L Altcode: 2012arXiv1207.6358L
We investigate the influence of the geometry of the solar filament
magnetic structure on the large-amplitude longitudinal oscillations. A
representative filament flux tube is modeled as composed of a cool
thread centered in a dipped part with hot coronal regions on either
side. We have found the normal modes of the system and establish
that the observed longitudinal oscillations are well described with
the fundamental mode. For small and intermediate curvature radii
and moderate to large density contrast between the prominence and the
corona, the main restoring force is the solar gravity. In this full wave
description of the oscillation a simple expression for the oscillation
frequencies is derived in which the pressure-driven term introduces
a small correction. We have also found that the normal modes are
almost independent of the geometry of the hot regions of the tube. We
conclude that observed large-amplitude longitudinal oscillations are
driven by the projected gravity along the flux tubes and are strongly
influenced by the curvature of the dips of the magnetic field in which
the threads reside.
---------------------------------------------------------
Title: An Explanation For Large-amplitude Longitudinal Oscillations
In Prominences
Authors: Karpen, Judith T.; Luna Bennasar, M.
2012AAS...22031003K Altcode:
Large amplitude longitudinal (LAL) oscillations, consisting of periodic
motions of prominence material along a filament axis, are rare but
quite dramatic. The oscillations appear to be triggered by an energetic
event, such as a microflare, subflare, or small C-class flare, close
to a filament. Observations reveal speeds of several tens to 100 km/s,
periods of order 1 hr, damping in a few periods, and displacements
that are a significant fraction of the prominence length. We have
developed the first self-consistent model for these oscillations that
explains the restoring force and damping mechanism. We investigated
the oscillations of multiple threads in our recent simulation (Luna et
al. 2012), in which they form in long, dipped flux tubes through the
thermal nonequilibrium process. The oscillation properties predicted by
our simulations agree with the observed LAL behavior. In addition, our
analytic model for the oscillations demonstrates that the restoring
force is the projected gravity in the tube. Although the period
is independent of the tube length and the constantly growing mass,
the motions are strongly damped by the steady accretion of mass onto
the threads. These suggest that a nearby impulsive event drives the
existing prominence threads along their supporting tubes, away from
the heating deposition site, without destroying them. As is also the
case for newly formed condensations, the subsequent oscillations occur
because the displaced threads reside in magnetic concavities with large
radii of curvature. Our model yields a powerful seismological method
for constraining the coronal magnetic field and radius of curvature of
dips. Furthermore, these results indicate that the magnetic structure is
most consistent with the sheared-arcade model for filament channels. We
conclude that the LAL movements represent a collective oscillation of
a large number of cool, dense threads moving along dipped flux tubes,
triggered by a small, nearby energetic event.
---------------------------------------------------------
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: The Effects Of B/L-dependent Heating On The Formation And
Evolution Of A Multi-threaded Prominence
Authors: Karpen, Judith T.; Luna, M.; DeVore, C.
2012AAS...22020203K Altcode:
We have developed a comprehensive, multi-threaded, three-dimensional
model of the plasma dynamics and energetics of a prominence and its
overlying arcade (Luna et al. 2012). In this model, the basic magnetic
structure is that of two interacting sheared arcades, while the cool
condensations comprising the prominence are formed by the well-studied
thermal nonequilibrium mechanism. In a given filament-channel flux
tube, the mass is evaporated from the chromosphere by heating localized
near the footpoints, and condenses in the form of transient blobs or
a persistent thread. Our previous studies of thermal nonequilibrium
used steady or impulsive heating functions with no dependence on
local physical variables. However, parametric active-region models
with steady heating proportional to B/L, where B is the flux-tube
magnetic field strength at the heated footpoint and L is the flux-tube
length, yield the best agreement with observations (e.g., Schrijver et
al. 2004). We have determined the effects of this active-region heating
function on our model for the formation and evolution of prominence
mass. We have also expanded the range of our computational domain to
include more of the overlying arcade (the so-called “cavity”), and
have increased the number of selected flux tubes from 125 to 533. We
will illustrate the time-dependent plasma behavior produced by the
B/L heating function with synthetic images in several AIA passbands,
and compare the resulting prominence properties with those predicted
by our model with flux-tube-independent heating.
---------------------------------------------------------
Title: Large-amplitude Longitudinal Oscillations in a Solar Filament
Authors: Luna, M.; Karpen, J.
2012ApJ...750L...1L Altcode: 2012arXiv1203.5027L
We have developed the first self-consistent model for the observed
large-amplitude oscillations along filament axes that explains
the restoring force and damping mechanism. We have investigated
the oscillations of multiple threads formed in long, dipped flux
tubes through the thermal nonequilibrium process, and found that
the oscillation properties predicted by our simulations agree
with the observed behavior. We then constructed a model for the
large-amplitude longitudinal oscillations that demonstrates that the
restoring force is the projected gravity in the tube where the threads
oscillate. Although the period is independent of the tube length and the
constantly growing mass, the motions are strongly damped by the steady
accretion of mass onto the threads by thermal nonequilibrium. The
observations and our model suggest that a nearby impulsive event
drives the existing prominence threads along their supporting tubes,
away from the heating deposition site, without destroying them. The
subsequent oscillations occur because the displaced threads reside
in magnetic concavities with large radii of curvature. Our model
yields a powerful seismological method for constraining the coronal
magnetic field and radius of curvature of dips. Furthermore, these
results indicate that the magnetic structure is most consistent with
the sheared-arcade model for filament channels.
---------------------------------------------------------
Title: Formation and Evolution of a Multi-threaded Solar Prominence
Authors: Luna, M.; Karpen, J. T.; DeVore, C. R.
2012ApJ...746...30L Altcode: 2012arXiv1201.3559L
We investigate the process of formation and subsequent evolution of
prominence plasma in a filament channel and its overlying arcade. We
construct a three-dimensional time-dependent model of an intermediate
quiescent prominence suitable to be compared with observations. We
combine the magnetic field structure of a three-dimensional sheared
double arcade with one-dimensional independent simulations of many
selected flux tubes, in which the thermal nonequilibrium process
governs the plasma evolution. We have found that the condensations
in the corona can be divided into two populations: threads and
blobs. Threads are massive condensations that linger in the flux tube
dips. Blobs are ubiquitous small condensations that are produced
throughout the filament and overlying arcade magnetic structure,
and rapidly fall to the chromosphere. The threads are the principal
contributors to the total mass, whereas the blob contribution is
small. The total prominence mass is in agreement with observations,
assuming reasonable filling factors of order 0.001 and a fixed number
of threads. The motion of the threads is basically horizontal, while
blobs move in all directions along the field. We have generated
synthetic images of the whole structure in an Hα proxy and in two
EUV channels of the Atmospheric Imaging Assembly instrument on board
Solar Dynamics Observatory, thus showing the plasma at cool, warm,
and hot temperatures. The predicted differential emission measure of
our system agrees very well with observations in the temperature range
log T = 4.6-5.7. We conclude that the sheared-arcade magnetic structure
and plasma behavior driven by thermal nonequilibrium fit the abundant
observational evidence well for typical intermediate prominences.
---------------------------------------------------------
Title: The Effect of Wave Escape on Fast-wave Turbulence in Solar
Flares
Authors: Pongkitiwanichakul, P.; Chandran, B. D.; DeVore, C. R.;
Karpen, J. T.
2011AGUFMSH41A1915P Altcode:
One candidate for particle acceleration in solar flares is stochastic
acceleration by plasma waves. This idea is often linked with resonant
interactions that require high-frequency waves. Wave turbulence can
provide high-frequency waves from low-frequency waves that are generated
when outflows from a magnetic reconnection site high in the corona
encounter magnetic loops lower in the corona. Many previous works
have considered the coupled processes of stochastic acceleration and
wave turbulence together. To the best of our knowledge, these works
have all neglected the loss of wave energy as waves propagate out of
the solar-flare acceleration region. In this work, we investigate
the effects of wave propagation on wave turbulence. We determine
the conditions needed for flares to generate high-frequency waves
via wave-wave interactions involving compressive fast magnetosonic
waves. We find that wave loss sets the minimum threshold on the
amplitude of low-frequency waves that must be reached in order for a
significant fraction of the wave energy to cascade to high frequencies
before the waves escape. We evaluate this threshold as a function of
the correlation length of the low-frequency waves.
---------------------------------------------------------
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: 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: 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: Parker Lecture - Prominences: the key to understanding solar
activity
Authors: Karpen, Judith T.
2011SPD....42.1101K Altcode: 2011BAAS..43S.1101K
Prominences are spectacular manifestations of both quiescent and
eruptive solar activity. The largest examples can be seen with the naked
eye during eclipses, making prominences among the first solar features
to be described and catalogued. Steady improvements in temporal and
spatial resolution from both ground- and space-based instruments have
led us to recognize how complex and dynamic these majestic structures
really are. Their distinguishing characteristics - cool knots and
threads suspended in the hot corona, alignment along inversion lines in
the photospheric magnetic field within highly sheared filament channels,
and a tendency to disappear through eruption - offer vital clues as to
their origin and dynamic evolution. Interpreting these clues has proven
to be contentious, however, leading to fundamentally different models
that address the basic questions: What is the magnetic structure
supporting prominences, and how does so much cool, dense plasma
appear in the corona? <P />Despite centuries of increasingly detailed
observations, the magnetic and plasma structures in prominences are
poorly known. Routine measurements of the vector magnetic field in and
around prominences have become possible only recently, while long-term
monitoring of the underlying filament-channel formation process remains
scarce. The process responsible for prominence mass is equally difficult
to establish, although we have long known that the chromosphere is
the only plausible source. As I will discuss, however, the motions and
locations of prominence material can be used to trace the coronal field,
thus defining the magnetic origins of solar eruptions. A combination of
observations, theory, and numerical modeling must be used to determine
whether any of the competing theories accurately represents the physics
of prominences. I will discuss the criteria for a successful prominence
model, compare the leading models, and present in detail one promising,
comprehensive scenario for prominence formation and evolution that
could answer the two questions posed above.
---------------------------------------------------------
Title: Formation and Evolution of a Multi-Threaded Prominence with
Different Heating Scenarios
Authors: Luna Bennasar, Manuel; Karpen, J.; DeVore, C. R.
2011SPD....42.0703L Altcode: 2011BAAS..43S.0703L
Solar prominences are cool and dense plasma suspended in the
million-degree solar corona. Recent observations reveal that prominences
are composed of fine and highly dynamic threads aligned with the local
magnetic field. We have constructed a 3D time-dependent model of a
prominence combining a magnetic field structure with 1D independent
simulations of many flux tubes. The 3D magnetic field is taken
from an adaptive MHD simulation of a sheared double-arcade filament
channel. Using the thermal non-equilibrium model we study different
parametrization of their heating function and influence on the evolution
of the plasmas. With the results of our simulations we produce synthetic
emission images of the filament channel and the overlying loops. We show
the evolving properties of our model and compare the results with data
from the AIA instrument onboard the recently launched SDO satellite.
---------------------------------------------------------
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: 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: Formation of a multi-threaded prominence
Authors: Luna Bennasar, M.; Karpen, J. T.; DeVore, C. R.
2010AGUFMSH51A1662L Altcode:
Solar prominences are cool and dense plasma suspended in the
million-degree solar corona. Recent observations reveal that prominences
are composed of fine and highly dynamic threads aligned with the local
magnetic field. We have constructed a 3D time-dependent model of a
prominence combining a magnetic field structure with 1D independent
simulations of many flux tubes. The 3D magnetic field is taken from an
adaptive MHD simulation of a sheared double-arcade filament channel. We
use the thermal non-equilibrium model for the plasma evolution. We study
different parametrization of the heating function, and with the results
of our simulations we produce synthetic emission images of the whole
prominence and the overlying loops. We show the evolving properties
of our model and compare the results with data from telescope onboard
satellites SOHO, STEREO, Hinode, and the recently launched SDO.
---------------------------------------------------------
Title: Formation of a multi-thread prominence
Authors: Luna Bennasar, Manuel; Karpen, J. T.; DeVore, C. R.
2010AAS...21640512L Altcode: 2010BAAS...41Q.891L
Solar prominences are composed of many threads of cool and dense plasma
suspended in the million-degree solar corona. We have constructed a 3D
time-dependent model of a prominence from 1D independent simulations
of many flux tubes. The 3D magnetic field is taken from an adaptive MHD
simulation of a sheared double-arcade filament channel. We will show the
evolving properties of our model prominence and compare the results with
data from telescopes onboard the SOHO, STEREO, and Hinode satellites.
---------------------------------------------------------
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: Physics of Solar Prominences: II—Magnetic Structure and
Dynamics
Authors: Mackay, D. H.; Karpen, J. T.; Ballester, J. L.; Schmieder,
B.; Aulanier, G.
2010SSRv..151..333M Altcode: 2010SSRv..tmp...32M; 2010arXiv1001.1635M
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations
and models, (2) the dynamics of prominence plasmas (formation and
flows), (3) Magneto-hydrodynamic (MHD) waves in prominences and (4)
the formation and large-scale patterns of the filament channels in
which prominences are located. Finally, several outstanding issues in
prominence research are discussed, along with observations and models
required to resolve them.
---------------------------------------------------------
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: 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: 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: Understanding Warm Coronal Loops
Authors: Klimchuk, J. A.; Karpen, J. T.; Patsourakos, S.
2007AGUFMSH51C..05K Altcode:
One of the great mysteries of coronal physics that has come to light
in the last few years is the discovery that warm (~ 1 MK) coronal loops
are much denser than expected for quasi-static equilibrium. It has been
shown that the excess density can be explained if loops are bundles
of unresolved strands that are heated impulsively and quasi-randomly
to very high temperatures. This picture of nanoflare heating predicts
that neighboring strands of different temperature should coexist and
therefore that loops should have multi-thermal cross sections. In
particular, emission should be produced at temperatures hotter than 2
MK. Such emission is sometimes but not always seen, however. We offer
two possible explanations for the existence of over-dense warm loops
without corresponding hot emission: (1) loops are bundles of nanoflare
heated strands, but a significant fraction of the nanoflare energy takes
the form of a nonthermal electron beam rather then direct heating;
(2) loops are bundles of strands that undergo thermal nonequilibrium
that results when steady heating is sufficiently concentrated near
the footpoints. We verify these possibilities with numerical hydro
simulations. Time permitting, we will show FeXVII line profile
observations from EIS/Hinode that support the existence of nanoflare
heating. Work supported by NASA and ONR.
---------------------------------------------------------
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: Multidimensional Simulations of Filament Channel Structure
and Evolution
Authors: Karpen, J. T.
2007ASPC..369..525K Altcode:
Over the past decade, the NRL Solar Theory group has made steady
progress toward formulating a comprehensive model of filament-channel
structure and evolution, combining the results of our sheared 3D arcade
model for the magnetic field with our thermal nonequilibrium model
for the cool, dense material suspended in the corona. We have also
discovered that, when a sheared arcade is embedded within the global
dipolar field, the resulting stressed filament channel can erupt through
the mechanism of magnetic breakout. Our progress has been largely
enabled by the development and implementation of state-of-the-art 1D
hydrodynamic and 3D magnetohydrodynamic (MHD) codes to simulate the
field-aligned plasma thermodynamics and large-scale magnetic-field
evolution, respectively. Significant questions remain, however,
which could be answered with the advanced observations anticipated
from Solar-B. In this review, we summarize what we have learned from
our simulations about the magnetic and plasma structure, evolution,
and eruption of filament channels, and suggest key observational
objectives for Solar-B that will test our filament-channel and
CME-initiation models and augment our understanding of the underlying
physical processes.
---------------------------------------------------------
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: 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: 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: The Reconnection and Microscale (RAM) probe
Authors: Golub, Leon; Bookbinder, Jay A.; DeLuca, Edward E.; Karpen,
Judith T.
2005SPIE.5901..281G Altcode:
Hot magnetized plasmas - typified by the solar corona - are
ubiquitous throughout the universe. The physics governing the
dynamics of such plasmas takes place on remarkably small spatial
and temporal scales, while both the cause activity and the response
occur on large spatial scales. Thus both high resolution and large
fields of view are needed. Observations from SMM, Yohkoh, EIT and
TRACE show that typical solar active region structures range in
temperature from 0.5 to 10 MK, and up to 40MK in flares, implying
the need for broad temperature coverage. The RAM S-T Probe consists
of a set of imaging and spectroscopic instruments that will enable
definitive studies of fundamental physical processes that govern
not only the solar atmosphere but much of the plasma universe. Few
problems in astrophysics have proved as resistant to solution as the
microphysics that results in the production of high-energy particles
in hot magnetized plasmas. Theoretical models have focused in recent
years on the various ways in which energy may be transported to the
corona, and there dissipated, through the reconnection of magnetic
fields. Theory implies that the actual dissipation of energy in the
corona occurs in spatially highly localized regions, and there is
observational support for unresolved structures with filling factors
0.01 - 0.001 in dynamic coronal events.
---------------------------------------------------------
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: 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: 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: 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: The High Resolution Imager on the Reconnection and Microscale
(RAM) Mission
Authors: Bookbinder, J. A.; DeLuca, E. E.; Golub, L.; Weber, M.;
Karpen, J. T.
2003SPD....34.2404B Altcode: 2003BAAS...35..853B
Hot, magnetized plasmas such as the solar corona have the property that
much of the physics governing its activity takes place on remarkably
small spatial and temporal scales, while the response to this activity
occurs on large scales. Future progress on the challenging solar
physics issues of eruptive flares, coronal heating and the initial of
the solar wind requires observations on spatial and temporal scales
relevant to the observable signatures of the underlying physical
processes. These spatial and temporal domains - in the relevant
temperature regimes - have been heretofore inaccessible to direct
observations from Earth, with the result that theoretical efforts have
relied heavily on extrapolations from more accessible regimes. <P />The
RAM Solar-Terrestrial Probe consists of a set of carefully selected
imaging and spectroscopic instruments that enable definitive studies of
the dynamics and energetics of the solar corona. We present an overview
of the synergism inherent in the RAM instrument suite, with emphasis
on the rationale for, and the capability of, its high-resolution imager.
---------------------------------------------------------
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: Why do we need high-resolution observations of the Sun?
Authors: Karpen, Judith T.
2003SPIE.4853..453K Altcode:
To make progress on major unsolved problems in solar physics (e.g.,
coronal heating, eruptive flare/CME initiation, solar wind initiation),
we must observe on scales relevant to the underlying physical processes
and their signatures. In this review I discuss the factors determining
the structure of magnetic fields and plasmas in the Sun’s outer
atmosphere, the key observable signatures of the relevant processes and
properties, and the instrumental capabilities necessary to detect and
measure these signatures. The primary emphasis is on state-of-the-art
theoretical and numerical predictions, which often are the only means
by which we can estimate the complex time-dependent evolution of the
underlying physical mechanisms and their local and global effects on
the corona.
---------------------------------------------------------
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: EUV Line Emission from Coronal Loop Models in Thermal
Non-equilibrium
Authors: Lanza, A. F.; Spadaro, D.; Lanzafame, A. C.; Karpen, J. T.
2002ASPC..277..521L Altcode: 2002sccx.conf..521L
No abstract at ADS
---------------------------------------------------------
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: 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: 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: 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: 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: 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: 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: 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: 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: Slow Solar Wind Formation
Authors: Einaudi, G.; Boncinelli, P.; Dahlburg, R. B.; Karpen, J. T.
2000AdSpR..25.1931E Altcode:
We review our recent research on the formation of the slow solar wind
(Einaudi et al. 1998), which we model as a magnetized wake. During its
evolution we find the formation of traveling plasmoids, turbulence
production, and streamwise filamentation. Turbulence reduces the
acceleration and destroys the plasmoids
---------------------------------------------------------
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: Formation of the slow solar wind in streamers
Authors: Karpen, J. T.
1999AIPC..471...47K Altcode: 1999sowi.conf...47K
We have investigated a magnetohydrodynamic mechanism which accounts
self-consistently for the variability, latitudinal extent, and bulk
acceleration of the slow solar wind. Our model represents a streamer
beyond the underlying coronal helmet as a neutral sheet embedded
in a plane fluid wake, characterized by two parameters which vary
with distance from the Sun: the ratio of the cross-stream velocity
scale to the neutral sheet width (δ), and the ratio of the typical
Alfvén velocity to the typical flow speed far from the neutral sheet
(A). Depending on the values of these parameters, our linear theory
predicts that this system responds to perturbations with three
kinds of instability: a streaming tearing instability, and two
ideal fluid instabilities with different cross-stream symmetries
(varicose and sinuous). In the magnetically-dominated region near
the helmet cusp, the steamer is resistively and ideally unstable,
evolving from tearing-type reconnection in the linear regime to a
nonlinear varicose fluid instability. Travelling magnetic islands
are formed which are similar to “blobs” recently revealed by the
Large-Angle Spectroscopic COronagraph (LASCO) on the joint ESA/NASA
Solar and Heliospheric Observatory (SOHO). Past the Alfvén point,
the tearing mode is suppressed but an ideal sinuous fluid mode can
develop, producing additional acceleration up to typical slow-wind
speeds and substantial broadening of the wake. Farther from the Sun,
the streamer becomes highly turbulent, thus slowing the acceleration
and producing strong filamentation throughout the core of the wake.
---------------------------------------------------------
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: Formation of the slow solar wind in a coronal streamer
Authors: Einaudi, Giorgio; Boncinelli, Paolo; Dahlburg, Russell B.;
Karpen, Judith T.
1999JGR...104..521E Altcode:
We have investigated a magnetohydrodynamic mechanism that accounts for
several fundamental properties of the slow solar wind, in particular
its variability, latitudinal extent, and bulk acceleration. In view of
the well-established association between the streamer belt and the slow
wind, our model begins with a simplified representation of a streamer
beyond the underlying coronal helmet: a neutral sheet embedded in a
plane fluid wake. This wake-neutral sheet configuration is characterized
by two parameters that vary with distance from the Sun: the ratio of
the cross-stream velocity scale to the neutral sheet width, and the
ratio of the typical Alfvén velocity to the typical flow speed far
from the neutral sheet. Depending on the values of these parameters,
our linear theory predicts that three kinds of instability can develop
when this system is perturbed: a tearing instability and two ideal
fluid instabilities with different cross-stream symmetries (varicose
and sinuous). In the innermost, magnetically dominated region beyond
the helmet cusp, we find that the streamer is resistively and ideally
unstable, evolving from tearing-type reconnection in the linear regime
to a nonlinear varicose fluid instability. Traveling magnetic islands
are formed which are similar to features recently revealed by the
large-angle spectroscopic coronagraph on the joint European Space
Agency/NASA Solar and Heliospheric Observatory (SOHO) [Brueckner et
al., 1995]. During this process, the center of the wake is accelerated
and broadened slightly. Past the Alfvén point, where the kinetic
energy exceeds the magnetic energy, the tearing mode is suppressed,
but an ideal sinuous fluid mode can develop, producing additional
acceleration up to typical slow wind speeds and substantial broadening
of the wake. Farther from the Sun, the system becomes highly turbulent
as a result of the development of ideal secondary instabilities, thus
halting the acceleration and producing strong filamentation throughout
the core of the wake. We discuss the implications of this model for the
origin and evolution of the slow solar wind, and compare the predicted
properties with current observations from SOHO.
---------------------------------------------------------
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: Acceleration of the slow solar wind
Authors: Dahlburg, R. B.; Karpen, J. T.; Einaudi, G.; Bonicelli, P.
1998ESASP.421..199D Altcode: 1998sjcp.conf..199D
No abstract at ADS
---------------------------------------------------------
Title: Effects of MHD Instabilities on the Structure of the Slow
Solar Wind
Authors: Karpen, J.; Dahlburg, R.
1997SPD....28.0113K Altcode: 1997BAAS...29R.881K
Recent LASCO observations of the streamer belt reveal a continual
outflow of material, often in the form of discrete “blobs" (Sheeley et
al. 1997, ApJ, in press). These features first appear above the cusps
of helmet streamers as density enhancements ~ 1 R<SUB>sun</SUB> in size,
which then expand while accelerating away from the Sun at velocities of
~ 50 - 400 km s(-1) . Wavy structure along streamers also is observed
to evolve with time. We have explored the possibility of explaining
the formation of these time-dependent structures through resistive
and ideal instabilities occurring in a system comprised of a single
current sheet embedded in a wake-type flow. Our linear analysis of this
system in both sub- and super-Alfvenic regimes has identified three
modes (Dahlburg et al. 1997, Phys. Plasmas, submitted): a varicose,
resistive mode; a varicose, ideal mode; and a sinuous, ideal mode. Wang
et al. (1988, Solar Phys. 117, 157) used the terms streaming tearing
mode, streaming sausage mode, and streaming kink mode, respectively,
to describe the same instabilities in a different context. The ideal
modes are of particular interest as they grow much more rapidly than
the resistive mode (for typical coronal Lundquist numbers) and are
driven by the free energy of the surrounding fast solar wind. To
study the development of observable structures by this mechanism,
we performed 2D and 3D nonlinear simulations initialized with small
velocity and magnetic field perturbations defined by the linear results,
as well as with random noise. We will discuss the growth and saturation
of the unstable modes, and present predictions of growth times and
characteristic lengths scaled to the coronal regime for comparison with
the LASCO observations of evolving streamer morphologies. Funding for
this work was provided by ONR, the NASA Space Physics Theory Program,
DoD and NASA's High Performance Computing and Communications Programs,
and the NASA Numerical Aerodynamic Simulation Program.
---------------------------------------------------------
Title: Reconnection in adjoining coronal helmet streamers
Authors: Dahlburg, R. B.; Karpen, J. T.
1997AdSpR..19.1887D Altcode:
Complex magnetic and plasma structures observed in the coronal streamer
belt (Crooker et al., 1993; Woo 1994) might arise from the instabilities
and evolution of multiple current sheets formed by adjoining coronal
helmet streamers. Previously we examined the static triple current
sheet (TCS), and found that three linearly unstable modes exist, two of
which are potentially observable by the LASCO instrument onboard SOHO
(Dahlburg and Karpen 1995). Here we investigate the variations created
in this model by the inclusion of wake flows, which have been observed
in coronal streamers. Our principal finding is that the structure of
the modes is changed significantly by the Alfvénic and sub-Alfvénic
wake flow, while their growth rates are not.
---------------------------------------------------------
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: 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: The Triple Current Sheet Model for Adjoining Helmet Streamers
Authors: Karpen, Judith T.; Dahlburg, Russell B.
1996ASPC...95..333K Altcode: 1996sdit.conf..333K
No abstract at ADS
---------------------------------------------------------
Title: A triple current sheet model for adjoining coronal helmet
streamers
Authors: Dahlburg, R. B.; Karpen, J. T.
1995JGR...10023489D Altcode:
The highly structured magnetic field and plasma properties observed
in the heliospheric extension of the coronal streamer belt have been
interpreted as evidence for multiple current sheets originating at
coronal helmet streamers. We explore the linear stability of a simple
case: a triple current sheet, as would exist above two neighboring
helmets of the same polarity. The behavior of the triple current
sheet when perturbed by small disturbances can be described (in the
incompressible limit) by MHD Orr-Sommerfeld and Squire equations,
which we solve with a Chebyshev-τ method. We show the velocity
and magnetic fields which characterize the three unstable modes and
describe the modal dependence on fieldwise wavenumber and current sheet
separation. At long wavelengths an unexpected phenomenon occurs: two
modes degenerate into unstable traveling modes. We also explore the
three-dimensional behavior and the modal variation with both large
and small values of the resistivity and viscosity. We conclude that
the magnetic topology in closely packed streamers is susceptible
to instabilities with growth times of the order of hours. Our
predictions indicate that the resultant plasmoid structures should be
observable with the large angle and spectrometric coronagraph (LASCO)
and ultraviolet coronagraph spectrometer (UVCS) instruments on the
upcoming Solar and Heliospheric Observatory (SOHO) mission.
---------------------------------------------------------
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: Stability of the Triple Current Sheet
Authors: Dahlburg, R. B.; Karpen, J. T.
1995SPD....26.1012D Altcode: 1995BAAS...27..979D
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: Turbulent transition in solar surges
Authors: Dahlburg, R. B.; Karpen, J. T.
1994SSRv...70...93D Altcode:
It has been suggested that a surge can be modelled as a jet travelling
in a sheared magnetic field, and that the transition to turbulence of
this “MHD tearing jet” can explain several key observed features. In
this paper we present our preliminary results of the transition to
turbulencevia secondary instabilities of the MHD tearing jet. Our
results confirm that turbulent transition can decelerate the surge, with
decay times which compare well with surge data. Furthermore, we find
that the turbulent MHD tearing jet forms magnetic field-aligned velocity
filaments similar to those often observed in the surge flow field.
---------------------------------------------------------
Title: Transition to Turbulence in Solar Surges
Authors: Dahlburg, Russell B.; Karpen, Judith T.
1994ApJ...434..766D Altcode:
Transition to turbulence in magnetohydrodynamic (MHD) tearing jets has
been invoked as a mechanism underlying some of the complex behavior
observed in solar surges, including deceleration of the upflowing
plasma and temporal correlations with types I and III radio bursts. In
this paper we investigate a possible mechanism for this transition:
three-dimensional secondary instabilities on two-dimensional saturated
states. We find through linear analysis that these MHD configurations
-- in particular, the tearing jet -- are secondarily unstable,
with the dominant energy transfer from the one-dimensional field
into the 3-dimensional fields. Using nonlinear simulations, we also
investigate the system evolution after the secondary modes attain finite
amplitude. When the tearing jet transitions to turbulence, the total
kinetic energy drops rapidly corresponding to the deceleration of the
jet. The electric field grows rapidly as the primary mode saturates
and the three-dimensional secondary mode develops, and then decays
quickly as the tearing jet becomes turbulent, providing a possible
explanation for the finite duration of the associated meter-wave
bursts. The electric field decays as the magnetic and velocity fields
both decay. The system is dominated at late times by spanwise modes,
which strongly resemble the magnetic field-aligned filamentary flows
characteristic of many surges.
---------------------------------------------------------
Title: The Effects of Kelvin-Helmholtz Instability on Resonance
Absorption Layers in Coronal Loops
Authors: Karpen, Judith T.; Dahlburg, Russell B.; Davila, Joseph M.
1994ApJ...421..372K Altcode:
One of the long-standing uncertainties in the wave-resonance theory
of coronal heating is the stability of the resonance layer. The wave
motions in the resonance layer produce highly localized shear flows
which vary sinusoidally in time with the resonance period. This
configuration is potentially susceptible to the Kelvin-Helmholtz
instability (KHI), which can enhance small-scale structure and turbulent
broadening of shear layers on relatively rapid ideal timescales. We
have investigated numerically the response of a characteristic velocity
profile, derived from resonance absorption models, to finite fluid
perturbations comparable to photospheric fluctuations. We find that
the KHI primarily should affect long (approximately greater than 6 x
10<SUP>4</SUP> km) loops where higher velocity flows (M approximately
greater than 0.2) exist in resonance layers of order 100 km wide. There,
the Kelvin-Helmholtz growth time is comparable to or less than the
resonance quarter-period, and the potentially stabilizing magnetic
effects are not felt until the instability is well past the linear
growth stage. Not only is the resonance layer broadened by the KHI,
but also the convective energy transport out of the resonance layer
is increased, thus adding to the efficiency of the wave-resonance
heating process. In shorter loops, e.g., those in bright points and
compact flares, the stabilization due to the magnetic field and the
high resonance frequency inhibit the growth of the Kelvin-Helmholtz
instability beyond a minimal level.
---------------------------------------------------------
Title: The Role of MHD Shear Layers in Solar Surges
Authors: Karpen, J. T.; Dahlburg, R. B.
1993BAAS...25.1212K 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: The Effects of Kelvin-Helmholtz Instability on Resonance
Absorption Layers in Coronal Loops
Authors: Karpen, J. T.; Dahlberg, R. B.; Davila, J. M.
1992AAS...180.5507K Altcode: 1992BAAS...24..820K
No abstract at ADS
---------------------------------------------------------
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: 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: Dynamic modeling of the solar atmosphere
Authors: Mariska, J. T.; Dahlburg, R. B.; Karpen, J. T.; Picone, J. M.
1990EOSTr..71..791M Altcode:
A brief review is presented of work done over the last eight years
investigating the fundamental physics of plasmas and magnetic fields
under conditions similar to those that are thought to be present in the
outer layers of the solar atmosphere, including the transition region
and the corona. The models used to study the coronal structures and
the thermal instability in the solar atmosphere are discussed. The
results of studies of magnetic energy release in the corona and MHD
turbulence in the solar wind are examined.
---------------------------------------------------------
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: 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 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: Nonlocal Thermal Transport in Solar Flares. II. Spectroscopic
Diagnostics
Authors: Karpen, Judith T.; Cheng, Chung-Chieh; Doschek, George A.;
DeVore, C. Richard
1989ApJ...338.1184K Altcode:
Physical parameters obtained for a flaring solar atmosphere in an
earlier paper are used here to predict time-dependent emission-line
profiles and integrated intensities as a function of position for
two spectral lines commonly observed during solar flares: the X-ray
resonance lines of Ca XIX and Mg XI. Considerations of ionization
nonequilibrium during the rise phase of the flare are addressed,
and the effects on the predicted spectral-line characteristics are
discussed. It is concluded that some spectroscopic diagnostics favor
the nonlocal model, but other long-standing discrepancies between the
numerical models and the observations remain unresolved.
---------------------------------------------------------
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: 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: Compressible dynamic alignment.
Authors: Dahlburg, R. B.; Picone, J. M.; Karpen, J. T.
1989GMS....54...95D Altcode: 1989sspp.conf...95D
Dynamic alignment has been proposed to account for correlations
between the magnetic and velocity fields of the solar wind. This
dynamic alignment problem is part of a more general class of problems,
related to self-organization in compressible magnetohydrodynamic (MDH)
turbulence, which is not yet well understood. In previous work the
authors demonstrated that dynamic alignment occurs in two-dimensional
compressible turbulent magnetofluids (Dahlburg et al., 1988). In this
paper they discuss numerical simulations which further the understanding
of dynamic alignment in compressible MHD.
---------------------------------------------------------
Title: Preflare activity.
Authors: Priest, E. R.; Gaizauskas, V.; Hagyard, M. J.; Schmahl, E. J.;
Webb, D. F.; Cargill, P.; Forbes, T. G.; Hood, A. W.; Steinolfson,
R. S.; Chapman, G. A.; Deloach, A. C.; Gary, G. A.; Jones, H. P.;
Karpen, J. T.; Martres, M. -J.; Porter, J. G.; Schmieder, B.; Smith,
J. B., Jr.; Toomre, J.; Woodgate, B.; Waggett, P.; Bentley, R.;
Hurford, G.; Schadee, A.; Schrijver, J.; Harrison, R.; Martens, P.
1989epos.conf....1P Altcode:
Contents: 1. Introduction. 2. Magnetohydrodynamic
instability. 3. Preflare magnetic and velocity fields. 4. Coronal
manifestations of preflare activity.
---------------------------------------------------------
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: Growth of correlation in compressible two-dimensional
magnetofluid turbulence
Authors: Dahlburg, R. B.; Picone, J. M.; Karpen, J. T.
1988JGR....93.2527D Altcode:
Spectral transfer has been proposed as the primary mechanism for
generating outward propagating Alfvén waves in the solar wind. This
process has been investigated extensively for incompressible
magnetofluids, but the issue of whether it occurs in compressible
magnetofluids such as the solar wind remains unresolved. We report the
results of direct numerical simulations of nonisentropic, compressible,
two-dimesnional magnetohydrodynamic turbulence which indicate that for
systems with finite initial cross-helicity the correlation between the
fluctuating velocity field and the fluctuating magnetic field grows
as a function of time. We support the interpretation of this growth
of correlation as a turbulent process by examination of modal wave
number spectra.
---------------------------------------------------------
Title: Nonlinear Thermal Instability in the Solar Transition Region
Authors: Karpen, Judith T.; Picone, Michael; Dahlburg, Russell B.
1988ApJ...324..590K Altcode:
The authors extend their earlier investigation on the initiation and
evolution of the radiation-driven thermal instability in an atmosphere
similar to the solar transition region. They find that the ultimate
state of the medium is highly sensitive to the evolving modal content of
the perturbation: both the initial modal composition and the extent of
mode coupling determine the final structure of the atmosphere. Previous
calculations showed that saturation of the instability occurs quite
early, preventing appreciable condensation, when excessive heat is
added either directly or by conversion of kinetic energy into internal
energy. No anologous perturbation-amplitude threshold exists for
the present calculations, in which the internal energy is perturbed
locally in coincidence with the density while the global values remain
unperturbed. The results show that the condensation process generates
highly rotational flows during and after the transition to a new
stable state.
---------------------------------------------------------
Title: Nonlocal Thermal Transport in Solar Flares
Authors: Karpen, Judith T.; DeVore, C. Richard
1987ApJ...320..904K Altcode:
A flaring solar atmosphere is modeled assuming classical thermal
transport, locally limited thermal transport, and nonlocal thermal
transport. The classical, local, and nonlocal expressions for the
heat flux yield significantly different temperature, density, and
velocity profiles throughout the rise phase of the flare. Evaporation
of chromospheric material begins earlier in the nonlocal case than in
the classical or local calculations, but reaches much lower upward
velocities. Much higher coronal temperatures are achieved in the
nonlocal calculations owing to the combined effects of delocalization
and flux limiting. The peak velocity and momentum are roughly the same
in all three cases. A more impulsive energy release influences the
evolution of the nonlocal model more than the classical and locally
limited cases.
---------------------------------------------------------
Title: Effects of Compressibility on Dynamic Alignment in the
Solar Wind
Authors: Picone, J. M.; Dahlburg, R. B.; Karpen, J. T.
1987BAAS...19.1123P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A search for forerunner activity associated with coronal
mass ejections
Authors: Karpen, Judith T.; Howard, Russell A.
1987JGR....92.7227K Altcode:
The possible existence of energetic disturbances in the corona
significantly before the associated surface events has profound
implications for the location and mechanisms of prevent coronal energy
storage. Precursor activity associated with coronal mass ejections
(CMEs), if it exists, would reflect the evolution and magnitude of the
energy release for failure of magnetic equilibrium characterizing the
interval before and during the events. Jackson and Hildner (JH) studied
21 CMEs observed with the Skylab white-light coronagraph, and found
a low-density plateau rimming each event for which good coverage was
available. They concluded that this “forerunner” material could not be
explained by mere translation of the overlying coronal plasma. Jackson
further inferred from the Skylab data that the forerunner must start
moving significantly before the onset of the associated CME, thus
identifying this phenomenon as a form of precursor activity. We have
performed a systematic search for forerunners using the white-light
coronagraph observations obtained with the Solwind instrument on
board the P78-1 satellite. Forty-four bright, well-observed events
were selected and analyzed, employing selection criteria and analysis
methods similar to those used by JH. Approximately half of these events
either do not exhibit low-density plateaus in front or are questionable
(e.g., a frontal plateau only appears intermittently). Based on our
analysis of the remaining CMEs, we conclude that identification of
the forerunner as a distinct entity probably is not warranted, and
that the low-density plasma is an integral part of the CME itself.
---------------------------------------------------------
Title: Nonlocal Thermal Transport in Solar Flares
Authors: Karpen, J. T.; DeVore, C. R.
1987BAAS...19..922K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamic Alignment in Compressible Magnetofluids
Authors: Dahlburg, R. B.; Picone, J. M.; Karpen, J. T.
1987BAAS...19..939D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nonlinear Evolution of Radiation-driven Thermally Unstable
Fluids
Authors: Dahlburg, R. B.; DeVore, C. R.; Picone, J. M.; Mariska,
J. T.; Karpen, J. T.
1987ApJ...315..385D Altcode:
The nonlinear evolution of a radiation-driven thermally unstable planar
fluid is simulated numerically using a semiimplicit finite-difference
algorithm. When the equilibrium state of the fluid is perturbed
by random initial excitation of the velocity field, dense, cool,
two-dimensional structures are found to form in a rarer, warmer
surrounding medium. The nonlinear phase of evolution is characterized
by the turbulent contraction of the condensed region, accompanied by a
significant increase in the amount of energy radiated. It is found that,
if the random velocity perturbation has a sufficiently large amplitude,
the fluid will not form condensed structures. Finally, the relationship
of these results to observations of the solar chromosphere, transition
region, and corona is discussed.
---------------------------------------------------------
Title: Nonlinear aspects of planar condensational instability
Authors: Dahlburg, R. B.; DeVore, C. R.; Picone, J. M.; Mariska,
J. T.; Karpen, J. T.
1987STIN...8723565D Altcode:
The numerical simulation of the nonlinear evolution of a radiation
driven thermally unstable planar fluid, using a semi-implicit finite
difference algorithm is discussed. When the equilibrium state of the
fluid is perturbed by random initial excitation of the velocity field,
dense, cool, two dimensional structures are observed forming in a
rarer, warmer, surrounding medium. The nonlinear phase of evolution
is characterized by the turbulent contraction of the condensed
region, accompanied by a significant increase in the amount of energy
radiated. If the random velocity perturbation has a sufficiently large
amplitude, the fluid will not form condensed structures. Finally, the
relationship of these results to observations of the solar chromosphere,
transition region and corona is discussed.
---------------------------------------------------------
Title: A Search for Forerunner Activity Associated with Coronal
Mass Ejections
Authors: Karpen, J. T.; Howard, R. A.
1987sowi.conf..242K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Preflare magnetic and velocity fields
Authors: Hagyard, M. J.; Gaizauskas, V.; Chapman, G. A.; Deloach,
A. C.; Gary, G. A.; Jones, H. P.; Karpen, J. T.; Martres, M. -J.;
Porter, J. G.; Schmeider, B.
1986epos.conf.1.16H Altcode: 1986epos.confA..16H
A characterization is given of the preflare magnetic field, using
theoretical models of force free fields together with observed field
structure to determine the general morphology. Direct observational
evidence for sheared magnetic fields is presented. The role of this
magnetic shear in the flare process is considered within the context
of a MHD model that describes the buildup of magnetic energy, and the
concept of a critical value of shear is explored. The related subject
of electric currents in the preflare state is discussed next, with
emphasis on new insights provided by direct calculations of the vertical
electric current density from vector magnetograph data and on the role
of these currents in producing preflare brightenings. Results from
investigations concerning velocity fields in flaring active regions,
describing observations and analyses of preflare ejecta, sheared
velocities, and vortical motions near flaring sites are given. This
is followed by a critical review of prevalent concepts concerning the
association of flux emergence with flares
---------------------------------------------------------
Title: Nonlocal Thermal Transport in Solar Flares
Authors: Karpen, J. T.; DeVore, C. R.
1986BAAS...18..898K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nonlocal Thermal Transport in the Solar Wind
Authors: DeVore, C. R.; Karpen, J. T.
1986BAAS...18.1041D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Response of an Emerging Flux Tube to a Current-driven
Instability
Authors: Karpen, J. T.; Boris, J. P.
1986ApJ...307..826K Altcode:
Emerging magnetic flux plays an important role in the development of
active regions on the sun and, perhaps, in the subsequent activation of
flares. However, the energy input that produces preflare brightenings
and flares probably does not come from the flux emergence itself
but from one or more associated energy-releasing processes - likely
candidates include magnetic reconnection and various current-driven
plasma micro-instabilities. Here the interplay between the changing
physical characteristics of an emerging magnetic-flux tube and the onset
and evolution of a representative 'bump-on-tail' plasma current-driven
instability is investigated. The microinstability heats the ambient
material, thus changing the macroscopic characteristics of the plasma
in which the model and current-driven instability occurs.
---------------------------------------------------------
Title: High-Resolution X-Ray Spectra of Solar Flares. VIII. Mass
Upflow in the Large Flare of 1980 November 7
Authors: Karpen, J. T.; Doschek, G. A.; Seely, J. F.
1986ApJ...306..327K Altcode:
The large flare of November 7, 1980 provides a unique opportunity to
investigate the upward-moving plasma seen during the early stages of
many flares. Soft X-ray spectroscopic data obtained by the Solar Flare
X-ray (SOLFLEX) instruments on board the Air Force P78-1 satellite
have been used to determine the spatial extent, turbulent velocity,
temperature, and emission measure of the blueshifted and stationary
plasmas, as well as the upward velocity of the blueshifted component
alone. Two geometries are considered in calculating the resultant
mass and energy balance. In addition, coincident hard X-ray data
was acquired from the HXRBS instrument on board the SMM satellite
to determine the relative timing and enertics of the hard and soft
X-ray flare plasmas. These results are compared with the predictions
of the chromospheric evaporation hypothesis. It is concluded that
electron-induced evaporation plays a minor role in this flare, and that
another mechanism must account for the observed blueshifted emission.
---------------------------------------------------------
Title: A Search for Forerunners in the SOLWIND Coronagraph Images
Authors: Karpen, J. T.; Howard, R. A.
1986BAAS...18..677K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Preflare activity.
Authors: Priest, E. R.; Gaizauskas, V.; Hagyard, M. J.; Schmahl, E. J.;
Webb, D. F.; Cargill, P.; Forbes, T. G.; Hood, A. W.; Steinolfson,
R. S.; Chapman, G. A.; Deloach, A. C.; Gary, G. A.; Jones, H. P.;
Karpen, J. T.; Martres, M. -J.; Porter, J. G.; Schmieder, B.; Smith,
J. B., Jr.; Toomre, J.; Woodgate, B.; Waggett, P.; Bentley, R.;
Hurford, G.; Schadee, A.; Schrijver, J.; Harrison, R.; Martens, P.
1986NASCP2439....1P Altcode:
Contents: 1. Introduction: the preflare state - a review of previous
results. 2. Magnetohydrodynamic instability: magnetic reconnection,
nonlinear tearing, nonlinear reconnection experiments, emerging flux and
moving satellite sunspots, main phase reconnection in two-ribbon flares,
magnetic instability responsible for filament eruption in two-ribbon
flares. 3. Preflare magnetic and velocity fields: general morphology of
the preflare magnetic field, magnetic field shear, electric currents in
the preflare active region, characterization of the preflare velocity
field, emerging flux. 4. Coronal manifestations of preflare activity:
defining the preflare regime, specific illustrative events, comparison
of preflare X-rays and ultraviolet, preflare microwave intensity and
polarization changes, non-thermal precursors, precursors of coronal
mass ejections, short-lived and long-lived HXIS sources as possible
precursors.
---------------------------------------------------------
Title: The role of nonlocal heat conduction in solar flares.
Authors: Karpen, Judith T.; DeVore, C. Richard
1986lasf.conf..416K Altcode: 1986lasf.symp..416K
The temperature scale height in the solar atmosphere, particularly
in the transition region, may be comparable to or smaller than the
collisional mean free paths of a substantial fraction of the electron
population. The authors have modelled a flaring solar atmosphere with
classical heat transport and with a nonlocal formulation of thermal
transport which is valid for both shallow and steep temperature
gradients. They conclude that nonlocal thermal transport can strongly
affect the physical characteristics of the transition region and
chromosphere during flares, with both flux limiting and delocalization
playing important roles.
---------------------------------------------------------
Title: A search for precursor activity associated with coronal mass
ejections, using white-light coronagraph observations obtained with
the SOLWIND instrument on board the Air Force P78-1 satellite
Authors: Karpen, J. T.
1985nrl..reptQ....K Altcode:
Large-scale coronal disturbances preceding solar flares or other
surface activity and resulting in mass ejection, are perhaps the
most intriguing and least understood manifestations of preflare/pre
mass ejection activity. The existence of energetic disturbances in
the corona significantly before the associated surface events has
profound implications for the location and mechanism of preflare
energy storage, as well as the evolution and magnitude of the energy
release or magnetic disequilibrium characterizing the interval before
and during the mass ejection. A systematic search for forerunners is
performed using the white light coronagraph observations obtained with
the WOLWIND instrument on board the P78-1 satellite. 44 bright, well
observed events are selected and analyzed. In comparing the SOLWIND
difference images to the excess mass contours, we find that the 2 sigma
contour level used to define the forerunner front is readily apparent
in the images. In fact, this level generally outlines the leading edge
of the visible event. If the contour plots of the SOLWIND events are
made with the same (linear) contour spacing, a forerunner plateau is
visible in both the CME itself and nearby affected coronal features,
e.g., a neighboring streamer that has been pushed aside. If contour
levels with power law spacing similar to the density distribution of
the background corona are chosen, however, the forerunner plateau
disappears. Therefore, we conclude that the forerunner phenomenon
is an integral part of the coronal mass ejection itself and not a
manifestation of precursor activity.
---------------------------------------------------------
Title: Nonlinear Numerical Simulation of Planar Thermal Instability
Authors: Dahlburg, R. B.; DeVore, C. R.; Picone, J. M.; Karpen, J. T.;
Mariska, J. T.
1985BAAS...17..833D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Two-Dimensional Nonlinear Numerical Simulations of Thermal
Instability in the Solar Atmosphere
Authors: Picone, J. M.; Dahlburg, R. B.; DeVore, C. R.; Karpen, J. T.;
Mariska, J. T.
1985BAAS...17..843P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nonlocal Thermal Transport in the Solar Atmosphere
Authors: Karpen, J. T.; DeVore, C. R.
1985BAAS...17..630K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Detailed studies of the dynamics and energetics of coronal
bullets
Authors: Karpen, J. T.; Oran, E. S.; Boris, J. P.
1984ApJ...287..396K Altcode:
Coronal bullets are small ejecta of cool, dense plasma observed to
accelerate through the solar atmosphere from 20 to 450 km/s. The
NRL Dynamic Flux Tube Model has been used to simulate the evolving
physical properties of these dynamic events. The present calculations
utilize an adaptive-gridding technique to resolve the fine structure
within and around the bullets. In this work, an identification was
made of a component of shocked plasma which piles up ahead of the
bullet and eventually dominates both the dynamics and heating of the
original bullet mass. The observational consequences of this shocked
component are discussed in terms of the available HRTS EUV data,
and suggestions are made for optimizing future observations of this
phenomenon. An investigation has also been conducted of the structure
of the bullet material visible in EUV spectral lines and the observable
characteristics of the EUV-emitting plasma. Finally, the most likely
mechanisms for accelerating the bullets, as well as favorable sites
of origin are evaluated.
---------------------------------------------------------
Title: Numerical simulations of loops heated to solar flare
temperatures. III - Asymmetrical heating
Authors: Cheng, C. -C.; Doschek, G. A.; Karpen, J. T.
1984ApJ...286..787C Altcode:
A numerical model is defined for asymmetric full solar flare loop
heating and comparisons are made with observational data. The Dynamic
Flux Tube Model is used to describe the heating process in terms of
one-dimensional, two fluid conservation equations of mass, energy
and momentum. An adaptive grid allows for the downward movement of
the transition region caused by an advancing conduction front. A
loop 20,000 km long is considered, along with a flare heating system
and the hydrodynamic evolution of the loop. The model was applied to
generating line profiles and spatial X-ray and UV line distributions,
which were compared with SMM, P78-1 and Hintori data for Fe, Ca and
Mg spectra. Little agreement was obtained, and it is suggested that
flares be treated as multi-loop phenomena. Finally, it is concluded
that chromospheric evaporation is not an effective mechanism for
generating the soft X-ray bursts associated with flares.
---------------------------------------------------------
Title: The Large Flare November 7, 1980: A Test of Chromospheric
Evaporation Theories?
Authors: Karpen, J. T.; Doschek, G. A.; Feldman, U.
1984BAAS...16.1003K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Response of an Emerging Flux Tube to a Current-Driven
Instability
Authors: Karpen, J. T.; Boris, J. P.
1983BAAS...15R.971K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simulations of the Preflare State in Single Flux Tubes:
Response to Non-Impulsive Heating Functions
Authors: Karpen, J. T.; Cheng, C. C.; Oran, E. S.; Boris, J. P.
1983BAAS...15..710K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Numerical Simulations of Solar Flare Hydrodynamics:
Asymmetrical Heatings
Authors: Cheng, C. C.; Karpen, J. T.; Doschek, G. A.; Boris, J. P.
1983BAAS...15Q.708C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The dynamics of accelerating coronal bullets
Authors: Karpen, J. T.; Oran, E. S.; Mariska, J. T.; Boris, J. P.;
Brueckner, G. E.
1982ApJ...261..375K Altcode:
Results are presented of computer simulations of the jets
that accelerate through the corona at velocities of 50 to 400
km/s. Particular emphasis is placed on the sensitivity of the induced
acceleration to the form in which energy is put into the system. A
comparison is made between the observed and predicted physical
characteristics of the high-velocity bullets; the potential contribution
of the bullets to the mass and energy balance of the solar corona is
considered. It is found that the velocity and temperature evolution
of the bullets can be modeled successfully by assuming energy input
in the form of an external force, pushing continuously on the ejected
material. From the physical characteristics of the model bullets and the
energy input required to reproduce the observations, it is concluded
that the bullets may constitute a significant fraction of the coronal
mass flux but only a negligible component of the coronal energy budget.
---------------------------------------------------------
Title: The Role of Betatron Acceleration in Complex Solar Bursts
Authors: Karpen, J. T.
1982SoPh...77..205K Altcode:
The betatron mechanism was proposed by Brown and Hoyng (1975) as a
means of producing the continuous, quasi-periodic electron acceleration
which may occur in long-lasting hard X-ray events. In the present
work, two pertinent facets of the betatron model are investigated:
The possibility that the multiplicity characteristic of complex
impulsive bursts is due to the betatron process; and the possibility
that some or all of the second-stage emission during two-stage
bursts can be attributed to betatron acceleration. To test for the
pattern of X-ray spectral behavior predicted by the betatron model,
a number of multiply-impulsive events (cf., Karpen et al., 1979) and
two-stage bursts (cf., Frost and Dennis, 1971) were selected from the
OSO-5 hard X-ray spectrometer data for in-depth analysis. The purely
impulsive emissions show no signs of the effects of betatron action,
thus eliminating this process as a potential source of impulsive-phase
multiplicity. However, the spectral characteristics determined during
the first few minutes of the second stage are found to be consistent
with the predictions of the betatron model for the majority of the
two-stage events studied. The betatron-acceleration mechanism thus is
proposed as a common second-stage phenomenon, closely associated with
the diverse phenomena at other wavelengths which characterize this phase
of emission. The physical significance of the source parameters derived
according to the model-fitting procedure are discussed in detail, and
the role of the betatron process is evaluated in the broader context
of present-day concepts of the second stage.
---------------------------------------------------------
Title: Detailed Structure and Energetics of Accelerating Coronal
Bullets
Authors: Karpen, J. T.; Oran, E. S.; Boris, J. P.; Mariska, J. T.
1982BAAS...14..622K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Relationships between the energetics of impulsive and gradual
emissions from solar flares.
Authors: Crannell, C. J.; Karpen, J. T.; Thomas, R. J.
1982ApJ...253..975C Altcode:
The gradual soft X-ray emissions associated with a homogeneous set
of solar flares have been investigated in the context of a thermal
model proposed to explain the impulsive components. The parametric
techniques which successfully characterized the hard X-ray and microwave
observations are employed in an event-by-event analysis to test for
quantitative and correlative relationships between the impulsive and
gradual emissions. The results of this investigation are consistent
with the hypothesis that the hard X-ray and microwave emissions are
produced by bulk heating of a common thermal source. The quantitative
relationships require an additional source to explain the soft X-ray
observations, consistent with previous results. Correlations between
the energetics of the impulsive and gradual emissions, identified in
the present work, provide the first clear evidence that their energizing
mechanisms are related.
---------------------------------------------------------
Title: Coordinated X-ray, optical and radio observations of flaring
activityon YZ Canis Minoris.
Authors: Kahler, S.; Golub, L.; Harnden, F. R.; Liller, W.; Seward,
F.; Vaiana, G.; Lovell, B.; Davis, R. J.; Spencer, R. E.; Whitehouse,
D. R.; Feldman, P. A.; Viner, M. R.; Leslie, B.; Kahn, S. M.; Mason,
K. O.; Davis, M. M.; Crannell, C. J.; Hobbs, R. W.; Schneeberger,
T. J.; Worden, S. P.; Schommer, R. A.; Vogt, S. S.; Pettersen, B. R.;
Coleman, G. D.; Karpen, J. T.; Giampapa, M. S.; Hege, E. K.; Pazzani,
V.; Rodono, M.; Romeo, G.; Chugainov, P. F.
1982ApJ...252..239K Altcode:
The YZ Canis Minoris (Gliese 285), a late-type dwarf star with
Balmer emission (dM4.5e), is a member of the UV Ceti class of flare
stars. Obtaining good X-ray observations of a dMe star flare is
important not only for understanding the physics of flares but also for
testing current ideas regarding the similarity between stellar and solar
flares. The Einstein X-ray Observatory has made it possible to conduct
X-ray observations of dMe stars with unprecedented sensitivity. A
description is presented of the results of a program of ground-based
optical and radio observations of YZ CMi coordinated with those of
the Einstein Observatory. The observations were carried out as part
of a coordinated program on October 25, 26, and 27, 1979, when YZ CMi
was on the dawn side of the earth. Comprehensive observational data
were obtained of an event detected in all three wavelength regions on
October 25, 1979.
---------------------------------------------------------
Title: The Dynamics of Accelerating Coronal Bullets
Authors: Karpen, J. T.; Oran, E. S.; Boris, J. P.; Mariska, J. T.;
Brueckner, G. E.
1981BAAS...13..913K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Role of Betatron Acceleration in Complex Solar Bursts
Authors: Karpen, J. T.
1980BAAS...12..914K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamic Spectral Characteristics of Thermal Models for Solar
Hard X-Ray Bursts
Authors: Brown, J. C.; Craig, I. J. D.; Karpen, J. T.
1980SoPh...67..143B Altcode:
The dynamic spectral characteristics of the thermal model for solar
hard X-ray bursts recently proposed by Brown et al. (1979) (BMS) are
investigated. It is pointed out that this model, in which a single
source is heated impulsively and cooled by anomalous conduction across
an ion-acoustic turbulent thermal front, predicts that the total source
emission measure should rise as the temperature falls. This prediction,
which is common to all conductively cooled single-source models, is
contrary to observations of many simple spike bursts. It is proposed,
therefore, that the hard X-ray source may consist of a distribution
of many small impulsively-heated kernels, each cooled by anomalous
conduction, with lifetimes shorter than current burst data temporal
resolution. In this case the dynamic spectra of bursts are governed
by the dynamic evolution of the kernel production process, such as
magnetic-field dissipation in the tearing mode. An integral equation
is formulated, the solution of which yields information on this kernel
production process, from dynamic burst spectra, for any kernel model.
---------------------------------------------------------
Title: Origin of the Soft X-Ray Emission from Impulsive Solar Flares
Authors: Crannell, C. J.; Karpen, J. T.; Thomas, R. J.
1980BAAS...12R.527C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Origin of Multiply-Impulsive Emission from Solar Flares.
Authors: Karpen, J. T.
1980PhDT.........2K Altcode:
Over the past twenty years, our understanding of solar flares
has been augmented greatly by the advent of rocket-, balloon-, and
satellite-borne instrumentation dedicated to observations of the Sun. In
particular, the use of spacecraft-borne detectors has permitted coverage
of the shorter-wavelength regions of the electromagnetic spectrum,
inaccessible to ground-based facilities. Hard X-ray emission from solar
flares provides direct evidence of the role of energetic electrons in
these powerful explosions. Analyses of flare-associated hard X-rays,
in conjunction with coincident coverage at other wavelengths, have
contributed much of our current understanding of the basic energizing
processes and resultant particle acceleration which characterize the
flare phenomenon. During the previous solar maximum, the hard X -ray
burst spectrometer on board the OSO-5 satellite observed hundreds of
hard X-ray events on the Sun, in the energy range 14 to 254 keV. The
1.8-second temporal resolution of the detector enabled detailed studies
of the evolution of burst intensity with time, as well as the spectral
evolution, and was comparable to the resolution of most solar radio
observatories operating at that time. The analysis and interpretation
of a set of complex X-ray bursts, selected from the OSO-5 data, are
presented in this work. The multiply-impulsive events were chosen
on the basis of morphological characteristics: each event appears to
consist of a number of overlapping spikes, with no apparent gradual
component of significance. The two -stage events were selected on
the basis of both morphological characteristics and association with
the appropriate phenomena at other wavelengths. Concident microwave
and meter-wave radio, soft X-ray, H-alpha, interplanetary particle,
and magnetographic data were obtained from several observatories, to
aid in developing comprehensive and self-consistent pictures of the
physical processes underlying the complex bursts. The present research
is focussed on two specific aspects of the multiple-spike and two-stage
bursts: (1)To look for the causes of multiplicity in complex impulsive
events; and (2)To compare the impulsive emissions with associated
gradual emissions, to pinpoint the basic processes which are applicable
to each phase alone. The investigation is concentrated on the hard
X-ray and microwave emissions, with reference to associated meter-wave
phenomena were appropriate. The X-ray and microwave radiations are
bremsstrahlung and gyrosynchrotron, respectively, from the electrons
accelerated in the relevant regions of the solar atmosphere. Together,
they are used to deduce the characteristics of the source: electron
density, temperature or spectral index of the electron distribution,
magnetic -field strength, and area. The hard X-ray emissions alone
are used to determine the parent electron spectrum and its evolution
throughout an event, to search for correlated variations in spectral
parameters which may indicate the underlying acceleration mechanism. The
main conclusions of this work are: (1)The multiplicity of the impulsive
events studied requires at least two distinct causes. On the basis of
derived source parameters, the bursts fall into two categories: events
whose component spikes apparently originate in one location, and events
in which groups of spikes appear to come from separate regions which
flare sequentially. (2)The origin of multiplicity in the case of a
single source region remains unidentified. Progress was made, however,
in critical evaluation of postulated explanations. One hypothesis,
which attributes intensity variations to betatron acceleration
of electrons in a magnetic trap, was tested. It was found that the
purely impulsive emissions show no signs of betatron acceleration, thus
ruling out this mechanism as a candidate for inducing multiply-spiked
structure. The second-stage emissions of several complex bursts also
were tested, with results differing markedly from the analysis of
the impulsive bursts. The majority of the two-stage bursts exhibited
spectral behaviour consistent with the betatron model, for the first
few minutes of the second stage. Therefore, it appears that betatron
acceleration may be an integral feature of the early stages of the
second-stage emission, for many two-stage bursts.
---------------------------------------------------------
Title: On the origin of multiply-impulsive emission from solar flares
Authors: Karpen, Judith Tobi
1980PhDT.......185K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spectral evolution of multiply impulsive solar bursts.
Authors: Karpen, J. T.; Crannell, C. J.; Frost, K. J.
1979ApJ...234..370K Altcode:
Some results from the analysis of a set of multiply impulsive hard X-ray
and microwave solar bursts are presented, showing that some bursts
can exhibit widely different magnetic-field strengths at different
times. Two categories of microwave spectral behavior are identified:
those events during which the microwave turnover frequency (MTF) and
spectral shape (SS) remain the same from peak to peak, and those during
which the MTF and SS change significantly. These categories correspond
to two classes of multiply impulsive bursts: those for which the
emission can be characterized by a constant magnetic field and therefore
a single source region, and those in which groups of component spikes
appear to originate in regions of different magnetic-field strengths,
corresponding to separate source regions which flare sequentially. With
regard to the latter type, examples are presented, the discrete flaring
regions are examined, and their spatial separations are estimated.
---------------------------------------------------------
Title: Applicability of Betatron Acceleration to Two-Stage Hard-ray
Events
Authors: Karpen, J. T.; Frost, K. J.; Brown, J.
1979BAAS...11Q.436K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nucleosynthesis of <SUP>7</SUP>Li in flares on UV Ceti stars.
Authors: Karpen, J. T.; Worden, S. P.
1979A&A....71...92K Altcode:
The possible production of Li-7 by nuclear reactions in UV Ceti flares
has been considered. By utilizing solar observations and theory, a
relationship is derived between flare energy and production rates for
Li-7; approximately 100 erg of total flare energy is found to denote
the formation of a Li-7 atom. Based on this value and best estimates
of UV Ceti-type flare rates, it is concluded that less than 10% of
the Li-7 observed in the intestellar medium may have been produced
by this mechanism. Formation of significant amounts of interstellar
deuterium by this method is ruled out.
---------------------------------------------------------
Title: Spectral evolution of multiply-impulsive solar bursts
Authors: Karpen, J. T.; Crannell, C. J.; Frost, K. J.
1978PhDT.........8K Altcode:
Hard X-ray and microwave observations of multiply-impulsive solar
bursts, identified in the OSO-5 data were analyzed. Spectra in both
frequency ranges were used to determine whether or not the source
properties change from peak to peak within individual bursts. Two
categories of microwave spectral behavior were identified: those
events during which the microwave turnover frequency and spectral shape
remain the same from peak to peak, and those during which the turnover
frequency and spectral shape change significantly. These categories
correspond to two classes of multiply-impulsive bursts: those for
which the emission can be characterized by a constant magnetic field
and therefore a single source region, in which case the multiplicity
may be due to modulation of the emission process; and those in which
groups of component spikes appear to originate in regions of different
magnetic-field strengths, corresponding to separate source regions
which flare sequentially. Examples of the latter type of events are
presented. The discrete flaring regions are analyzed and their spatial
separations estimated.
---------------------------------------------------------
Title: Spectral Evolution of Multiply-Impulsive Solar Bursts.
Authors: Karpen, J. T.; Crannell, C. J.; Frost, K. J.
1978BAAS...10..455K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated X-ray, optical, and radio observations of YZ
Canis Minoris.
Authors: Karpen, J. T.; Crannell, C. J.; Hobbs, R. W.; Maran, S. P.;
Moffett, T. J.; Bardas, D.; Clark, G. W.; Hearn, D. R.; Li, F. K.;
Markert, T. H.; McClintock, J. E.; Primini, F. A.; Richardson, J. A.;
Cristaldi, S.; Rodono, M.; Galasso, D. A.; Magun, A.; Nelson, G. J.;
Slee, O. B.; Chugajnov, P. F.; Chugainov, P. F.; Efimov, Yu. S.;
Shakhovskoj, N. M.; Shakhovskoy, N. M.; Viner, M. R.; Venugopal,
V. R.; Spangler, S. R.; Kundu, M. R.; Evans, D. S.
1977ApJ...216..479K Altcode:
We report coordinated X-ray, optical, and radio observations of the
flare star YZ CMi, including the first occasion on which such a star has
been monitored in all three spectral regions simultaneously. Thirty-one
minor optical flares and 11 radio events were recorded. No major
optical flares greater than 3 magnitudes were observed during the
program. Although no flare- related X-ray emission was observed, the
measured upper limits in this band enable meaningful comparisons with
published flare-star models. Three of the five models predicting the
relative X-ray to optical or radio flare luminosities are in serious
disagreement with the observations. For the largest optical flare with
coincident X-ray co1verage, the 3 a upper limit on X-ray emission in
the 0.15-0.8 keV band is about 9 x 1028 ergs 5 - , corresponding to a
ratio of X-ray to B-band luminosity of <0.3. Based on the present
results, the fraction of the galactic component of the diffuse soft
X-ray background contributed by UV Ceti-type flare stars is <9 x
H, where H is the mean density of interstellar hydrogen within a few
hundred parsecs of the Sun. Subject headings: radio sources: variable -
stars: flare - stars: individual - X-rays: bursts
---------------------------------------------------------
Title: Coordinated X-ray optical and radio observations of YZ
Canis Minoris
Authors: Karpen, J. T.; Crannell, C. J.; Hobbs, R. W.; Maran, S. P.;
Moffett, T. J.; Bardas, D.; Clark, G. W.; Hearn, D. R.; Li, F. K.;
Markert, T. M.
1976STIN...7715945K Altcode:
Coordinated X ray, optical, and radio observations of the flare
star YZ CMi are reported. Twenty-two minor optical flares and twelve
radio events were recorded. No major optical flares, greater than 3
magnitudes, were observed. Although no flare related X ray emission
was observed, the measured upper limits in this band enable meaningful
comparisons with published flare star models. Three of the five models
predicting the relative X ray to optical or radio flare luminosities
are in serious disagreement with the observations. For the largest
optical flare with coincident X ray coverage, the 3 sigma upper limit
on X ray emission in the 0.15 to 0.8 keV band is 8.7 x 10 to the 28th
power erg/s, corresponding to a ratio of X ray to B-band luminosity of
less than 0.3. Based on the present results, the contribution of the
flares UV Ceti flare stars to the galactic component of the diffuse
soft X ray background is less than 0.2 percent.
---------------------------------------------------------
Title: Coordinated X-Ray, Optical, and Radio Observations of YZ
Canis Minoris
Authors: Crannell, C. J.; Hobbs, R. W.; Karpen, J.; Maran, S. P.;
Moffett, T. J.; Bardas, D.; Clark, G. W.; Hearn, D.; Li, F. K.;
Markert, T. H.; McClintock, J. E.; Primini, F. A.; Richardson, J. A.;
Spangler, S. R.
1976BAAS....8..448C Altcode:
No abstract at ADS
