explanation blue bibcodes open ADS page with paths to full text
Author name code: klimchuk
ADS astronomy entries on 2022-09-14
author:"Klimchuk, James A." or author:"Klimchuk, Jim"
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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: Contribution of spicules to solar coronal emission
Authors: Sow Mondal, Shanwlee; Klimchuk, James A.; Sarkar, Aveek
2022arXiv220805240S Altcode:
Recent high-resolution imaging and spectroscopic observations have
generated renewed interest in spicules' role in explaining the hot
corona. Some studies suggest that some spicules, often classified as
type II, may provide significant mass and energy to the corona. Here
we use numerical simulations to investigate whether such spicules can
produce the observed coronal emission without any additional coronal
heating agent. Model spicules consisting of a cold body and hot tip
are injected into the base of a warm ($0.5$ MK) equilibrium loop with
different tip temperatures and injection velocities. Both piston- and
pressure-driven shocks are produced. We find that the hot tip cools
rapidly and disappears from coronal emission lines such as Fe XII $195$
and Fe XIV $274$. Prolonged hot emission is produced by pre-existing
loop material heated by the shock and by thermal conduction from
the shock. However, the shapes and Doppler shifts of synthetic line
profiles show significant discrepancies with observations. Furthermore,
spatially and temporally averaged intensities are extremely low,
suggesting that if the observed intensities from the quiet Sun and
active regions were solely due to type II spicules, one to several
orders of magnitude more spicules would be required than have been
reported in the literature. This conclusion applies strictly to the
ejected spicular material. We make no claims about emissions connected
with waves or coronal currents that may be generated during the ejection
process and heat the surrounding area.
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Title: The Coronal Veil
Authors: Malanushenko, A.; Cheung, M. C. M.; DeForest, C. E.; Klimchuk,
J. A.; Rempel, M.
2022ApJ...927....1M Altcode: 2021arXiv210614877M
Coronal loops, seen in solar coronal images, are believed to
represent emission from magnetic flux tubes with compact cross
sections. We examine the 3D structure of plasma above an active
region in a radiative magnetohydrodynamic simulation to locate volume
counterparts for coronal loops. In many cases, a loop cannot be linked
to an individual thin strand in the volume. While many thin loops are
present in the synthetic images, the bright structures in the volume
are fewer and of complex shape. We demonstrate that this complexity
can form impressions of thin bright loops, even in the absence of thin
bright plasma strands. We demonstrate the difficulty of discerning
from observations whether a particular loop corresponds to a strand in
the volume, or a projection artifact. We demonstrate how apparently
isolated loops could deceive observers, even when observations from
multiple viewing angles are available. While we base our analysis
on a simulation, the main findings are independent from a particular
simulation setup and illustrate the intrinsic complexity involved in
interpreting observations resulting from line-of-sight integration
in an optically thin plasma. We propose alternative interpretation
for strands seen in Extreme Ultraviolet images of the corona. The
"coronal veil" hypothesis is mathematically more generic, and
naturally explains properties of loops that are difficult to address
otherwise-such as their constant cross section and anomalously high
density scale height. We challenge the paradigm of coronal loops as
thin magnetic flux tubes, offering new understanding of solar corona,
and by extension, of other magnetically confined bright hot plasmas.
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Title: Impact of 3D Structure on Magnetic Reconnection
Authors: Daldorff, Lars K. S.; Leake, James E.; Klimchuk, James A.
2022ApJ...927..196D Altcode: 2022arXiv220204761D
Results from 2.5D and 3D studies of the onset and development of the
tearing instability are presented, using high-fidelity resistive
MHD simulations. A limited parameter study of the strength of the
reconnecting field (or shear angle) was performed. An initially simple
1D equilibrium was used, consisting of a modified force-free current
sheet, with periodic boundary conditions in all directions. In all
cases, the linear and nonlinear evolution led to a primary current
sheet between two large flux ropes. The global reconnection rate during
this later stage was analyzed in all simulations. It was found that
in 2.5D the primary current sheet fragmented owing to plasmoids, and
as expected, the global reconnection rate, calculated using multiple
methods, increases with the strength of the reconnecting field owing to
a stronger Alfvén speed. In 3D, the presence of interacting oblique
modes of the tearing instability complicates the simple 2.5D picture,
entangling the magnetic field of the inflow and introducing a negative
effect on the reconnection rate. The two competing effects of stronger
Alfvén speed and entangling, which both increase with the strength of
the reconnecting field, resulted in a decrease in the reconnection rate
with increasing reconnecting field. For all simulations, the 3D rates
were less than in 2.5D but suggest that as one goes to weak reconnecting
field (or strong guide field) the system becomes more 2.5D-like and the
2.5D and 3D rates converge. These results have relevance to situations
like nanoflare heating and flare current sheets in the corona.
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Title: Static and dynamic solar coronal loops with cross-sectional
area variations
Authors: Cargill, P. J.; Bradshaw, S. J.; Klimchuk, J. A.; Barnes,
W. T.
2022MNRAS.509.4420C Altcode: 2021arXiv211109339C; 2021MNRAS.tmp.2869C
The Enthalpy Based Thermal Evolution of Loops approximate model for
static and dynamic coronal loops is developed to include the effect
of a loop cross-sectional area which increases from the base of the
transition region (TR) to the corona. The TR is defined as the part of a
loop between the top of the chromosphere and the location where thermal
conduction changes from an energy loss to an energy gain. There are
significant differences from constant area loops due to the manner in
which the reduced volume of the TR responds to conductive and enthalpy
fluxes from the corona. For static loops with modest area variation the
standard picture of loop energy balance is retained, with the corona
and TR being primarily a balance between heating and conductive losses
in the corona, and downward conduction and radiation to space in the
TR. As the area at the loop apex increases, the TR becomes thicker
and the density in TR and corona larger. For large apex areas, the
coronal energy balance changes to one primarily between heating and
radiation, with conduction playing an increasingly unimportant role,
and the TR thickness becoming a significant fraction of the loop
length. Approximate scaling laws are derived that give agreement with
full numerical solutions for the density, but not the temperature. For
non-uniform areas, dynamic loops have a higher peak temperature and
are denser in the radiative cooling phase by of order 50 per cent than
the constant area case for the examples considered. They also show
a final rapid cooling and draining once the temperature approaches 1
MK. Although the magnitude of the emission measure will be enhanced
in the radiative phase, there is little change in the important
observational diagnostic of its temperature dependence.
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Title: Flows in Enthalpy-based Thermal Evolution of Loops
Authors: Rajhans, Abhishek; Tripathi, Durgesh; Bradshaw, Stephen J.;
Kashyap, Vinay L.; Klimchuk, James A.
2022ApJ...924...13R Altcode: 2021arXiv211003204R
Plasma-filled loop structures are common in the solar corona. Because
detailed modeling of the dynamical evolution of these structures is
computationally costly, an efficient method for computing approximate
but quick physics-based solutions is to rely on space-integrated 0D
simulations. The enthalpy-based thermal evolution of loops (EBTEL)
framework is a commonly used method to study the exchange of mass
and energy between the corona and transition region. EBTEL solves for
density, temperature, and pressure, averaged over the coronal part of
the loop, velocity at coronal base, and the instantaneous differential
emission measure distribution in the transition region. The current
single-fluid version of the code, EBTEL2, assumes that at all stages the
flows are subsonic. However, sometimes the solutions show the presence
of supersonic flows during the impulsive phase of heat input. It is thus
necessary to account for this effect. Here, we upgrade EBTEL2 to EBTEL3
by including the kinetic energy term in the Navier-Stokes equation. We
compare the solutions from EBTEL3 with those obtained using EBTEL2, as
well as the state-of-the-art field-aligned hydrodynamics code HYDRAD. We
find that the match in pressure between EBTEL3 and HYDRAD is better than
that between EBTEL2 and HYDRAD. Additionally, the velocities predicted
by EBTEL3 are in close agreement with those obtained with HYDRAD when
the flows are subsonic. However, EBTEL3 solutions deviate substantially
from HYDRAD's when the latter predicts supersonic flows. Using the
mismatches in the solution, we propose a criterion to determine the
conditions under which EBTEL can be used to study flows in the system.
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Title: Magnetic Reconnection in 3D vs. 2D and Dependence on Magnetic
Shear
Authors: Daldorff, Lars; Leake, James; Klimchuk, James
2021AGUFMSH25E2125D Altcode:
Magnetic reconnection plays a central role for heating the solar
coronal plasma including during flares as well as other places in
the heliosphere and beyond, where each event can be characterized by
its geometry and magnetic configurations, forming structures that we
can categorize to be more 2D or 3D. The difference in reconnection
rate between 2D and 3D simulations have been reported in multiple
studies over the years. We report on a study of the reconnection rate
using the resistive MHD code LaRe3D. We show that the rate depends
strongly on the existence and interaction of different tearing layers
(oblique tearing modes) within the current sheet. Such modes are only
present with a finite guide field and a spatial dependence in this
third direction. We find, as have others, that reconnection rates are
artificially high in 2D simulations, and we offer an explanation.
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Title: Distributed Space Telescopes Enabled by Constellation of
Small Satellites
Authors: Kamalabadi, Farzad; Lightsey, E.; Rabin, Douglas; Daw,
Adrian; D'Amico, Simone; Koenig, Adam; Chamberlin, Philip; Woods,
Thomas; Gupta, Subhanshu; Ekici, Eylem; Sample, John; Park, Hyeongjun;
Alexeenko, Alina; Hwang, John; Denis, Kevin; Klimchuk, James
2021AGUFM.A33C..03K Altcode:
New pathways to high-resolution sensing and imaging for a multitude
of high-priority scientific investigations are being realized by small
multi-spacecraft systems. Such pathfinder mission concepts circumvent
the limitations of conventional remote sensing/imaging systems by
utilizing multiple baselines, synthesized apertures, diffractive
optics, combined with computational imaging via interferometry,
tomography, or super-resolution. Regardless of the specific scientific
questions targeted, such small satellite constellation pathfinders
require technological breakthroughs in precision formation flying
and associated advances in guidance, navigation, and control;
proximity operations and associated autonomy and robust orbit
design with passive safety; innovations in sensor miniaturization;
inter-satellite communication; and sophisticated computational sensing
and reconstruction algorithms. We describe recent advances in such
enabling technologies in the context of a scalable ultra-high-resolution
spectral imaging mission for investigating the solar corona currently
under development by a multi-university consortium in collaboration
with NASA and under sponsorship by NSF.
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Title: Study of Type III Radio bursts in the Closed Corona and the
Solar Wind from Small-scale Reconnection: Observations
Authors: Chhabra, Sherry; Klimchuk, James; Gary, Dale
2021AGUFMSH24B..06C Altcode:
It is widely agreed that the ubiquitous presence of reconnection
events and the associated impulsive heating (nanoflares) are a strong
candidate for heating the magnetically closed corona. Whether nanoflares
accelerate energetic particles like fullsized flares is unknown. The
lack of strong emission in hard Xrays suggests that the quantity of
highly energetic particles is small. There could, however, be large
numbers of mildly energetic particles (~ 10 keV). Similarly, in the
context of the solar wind, these energetic particles can originate
from interchange reconnection, streamer tip reconnection, or turbulence
reconnection in the solar wind itself, in which case they stream away
from the Sun along the open field lines. To understand whether these
processes are efficient at accelerating particles, we search for the
type III radio bursts that they may produce. The timelag technique that
was developed to study subtle delays in light curves from different
EUV channels [Viall & Klimchuk 2012] can also be used to detect
subtle delays at different radio frequencies. We have modeled the
expected radio emission from mildly energetic particles propagating
in the closed corona and open corona/solar wind. The models were used
to test and calibrate the technique. We are currently applying the
technique to radio observations from VLA (Very Large Array), LOFAR
(LowFrequency Array), LWA, NM (Long Wavelength Array, New Mexico), and
the FIELDS experiment (encounters 1-6) to search for such signatures
of type IIIs. We also plan to investigate the relationship between
the bursts and activity on the Sun, such as the presence/absence of
active regions, relationship with their age etc. We will report the
results from our analysis.
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Title: Coronal Heating: A Coupled Multi-Scale Problem
Authors: Klimchuk, James
2021AGUFMSH13A..01K Altcode:
Despite substantial observational and theoretical progress, the heating
of the magnetically closed corona to temperatures 1000 times hotter than
the solar surface has yet to be fully explained. Like many problems in
Heliophysics, it involves an enormous range of spatial scales that are
coupled in fundamental ways. In this talk, I will review our current
understanding of how the coronal plasma is heated and how it responds
to produce the time variable spectrum of radiation that is an important
driver of space weather. I will emphasize that future progress requires
a more integrated approach than has typically been used.
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Title: Onset of Magnetic Reconnection in the Solar Corona
Authors: Leake, James; Klimchuk, James; Daldorff, Lars
2021AGUFMSH34C..01L Altcode:
Magnetic reconnection plays a vital role in the dynamics, heating,
and emission in the solar corona. It also plays a role in the onset
of transient events such as jets, spicules, filament eruptions, and
coronal mass ejections. One of the major questions still unanswered
is the nature of the onset of magnetic reconnection. How is it that
the solar corona can slowly build up magnetic energy and then rapidly
release it to manifest in the above phenomena? Recent theoretical
developments have considered the nature of the tearing instability in
dynamically thinning current sheets in terms of this rapid switch-on of
reconnection. We present a study of the onset of magnetic reconnection
in 3D dynamically evolving current sheets unstable to both oblique and
parallel modes of the tearing instability. While it is not currently
possible to realize the actual parameters in the solar corona such as
Lundquist number and current sheet thickness, we are able to perform
our study in a regime which has relevant ratios of tearing timescale
to dynamic timescale that allow us to make relevant conclusions about
the solar corona.
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Title: Signatures of Type III Solar Radio Bursts from Nanoflares:
Modeling
Authors: Chhabra, Sherry; Klimchuk, James A.; Gary, Dale E.
2021ApJ...922..128C Altcode: 2021arXiv210903355C
There is a wide consensus that the ubiquitous presence of magnetic
reconnection events and the associated impulsive heating (nanoflares)
are strong candidates for solving the solar coronal heating
problem. Whether nanoflares accelerate particles to high energies
like full-sized flares is unknown. We investigate this question by
studying the type III radio bursts that the nanoflares may produce on
closed loops. The characteristic frequency drifts that type III bursts
exhibit can be detected using a novel application of the time-lag
technique developed by Viall & Klimchuk (2012) even when there
are multiple overlapping events. We present a simple numerical model
that simulates the expected radio emission from nanoflares in an active
region, which we use to test and calibrate the technique. We find that
in the case of closed loops the frequency spectrum of type III bursts
is expected to be extremely steep such that significant emission is
produced at a given frequency only for a rather narrow range of loop
lengths. We also find that the signature of bursts in the time-lag
signal diminishes as: (1) the variety of participating loops within
that range increases; (2) the occurrence rate of bursts increases;
(3) the duration of bursts increases; and (4) the brightness of bursts
decreases relative to noise. In addition, our model suggests a possible
origin of type I bursts as a natural consequence of type III emission
in a closed-loop geometry.
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Title: Computing Emission Signatures from Coronal MHD Models Without
a Realistic Lower Atmosphere
Authors: Klimchuk, James; Knizhnik, Kalman; Uritsky, Vadim
2021AGUFMSH43A..08K Altcode:
The emission properties of the corona depend crucially on its energetic
and dynamic coupling with the lower atmosphere, involving processes like
thermal conduction and radiation cooling, chromospheric evaporation, and
draining/condensation. Incorporating these processes into an MHD model
is extremely challenging, because of the need to spatially resolve both
the transition region and current sheets involved in the heating, both
of which are thin and non-stationary. (New methods lesson the demands
on resolving the transition region, especially when the heating is
quasi steady.) We have developed a simple technique that can be applied
post facto to MHD simulations without a realistic lower atmosphere to
obtain useful approximations of the nonuniform and evolving radiation
spectrum. We demonstrate the technique with a simulation of an initially
uniform plasma/magnetic field system that is subjected to small-scale
driving at the photospheric boundary. Important results are obtained
concerning the nature of both the diffuse component of the corona and
bright coronal loops.
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Title: Signatures of Type III Radio Bursts from Small-scale
Reconnection Events in the Solar Wind
Authors: Chhabra, S.; Klimchuk, J.; Gary, D.; Psp/Fields Team
2021AAS...23812307C Altcode:
We look for evidence of energetic particles in the solar wind,
that could be produced by reconnection in the solar wind itself or
reconnection in the lower corona, where particles escape on open field
lines, from e.g. interchange reconnection.We expect reconnection to be
common at the current sheets that separate the thin magnetic strands
that make up the corona and solar wind, but whether it is efficient
at accelerating particles is an open question. Type III radio bursts
have been very frequently observed in the solar wind over the past
few decades. Energetic electron beams propagating along magnetic
field lines cause a bump-on-tail instability generating Langmuir
waves. Produced by their interaction with other particles and waves,
type III bursts exhibit a characteristic drift in frequency as they
propagate through the density gradient in the field. An interesting
question is whether there is a ubiquitous presence of type IIIs in the
radio 'background' observed. The radio background outside of clearly
identified bursts may actually be comprised of multiple overlapping
events. The time-lag technique that was developed to study subtle delays
in light curves from different EUV channels [Viall & Klimchuk 2012]
can also be used to detect subtle delays at different frequencies in
the radio background even when there is no hint in the individual light
curves. The FIELDS instrument onboard the Parker Solar Probe (PSP) is
utilized to investigate the solar wind for these signatures. We perform
a systematic study of the observed type III storms in Encounters 1-5,
to understand the signatures that can identify their presence and
the different features observed using the technique. Our findings are
then employed to study the times where no activity is visibly detected
by the instrument. We are currently analyzing multiple periods of no
visible activity and will report our findings.
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Title: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS)
Authors: Caspi, A.; Shih, A. Y.; Panchapakesan, S.; Warren, H. P.;
Woods, T. N.; Cheung, M.; DeForest, C. E.; Klimchuk, J. A.; Laurent,
G. T.; Mason, J. P.; Palo, S. E.; Seaton, D. B.; Steslicki, M.;
Gburek, S.; Sylwester, J.; Mrozek, T.; Kowaliński, M.; Schattenburg,
M.; The CubIXSS Team
2021AAS...23821609C Altcode:
The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) is a 6U
CubeSat proposed to NASA H-FORT. CubIXSS is motivated by a compelling
overarching science question: what are the origins of hot plasma in
solar flares and active regions? Elemental abundances are a unique
diagnostic of how mass and energy flow into and within the corona,
and CubIXSS addresses its science question through sensitive, precise
measurements of abundances of key trace ion species, whose spectral
signatures reveal the chromospheric or coronal origins of heated plasma
across the entire temperature range from ~1 to >30 MK. CubIXSS
measurements of the coronal temperature distribution and elemental
abundances directly address longstanding inconsistencies from prior
studies using instruments with limited, differing temperature and
composition sensitivities. <P />CubIXSS comprises two co-optimized
and cross-calibrated instruments that fill a critical observational
gap: <P />MOXSI, a novel diffractive spectral imager using a pinhole
camera and X-ray transmission diffraction grating for spectroscopy of
flares and active regions from 1 to 55 Å, with spectral and spatial
resolutions of 0.28-0.37 Å and 29-39 arcsec FWHM, respectively;
and <P />SASS, a suite of four spatially-integrated off-the-shelf
spectrometers for high-cadence, high-sensitivity X-ray spectra from
0.5 to 50 keV, with spectral resolution of 0.06-0.5 keV FWHM across
that range. <P />If selected for implementation, CubIXSS will launch
in late 2023 to mid-2024 to observe intense solar flares and active
regions during the rising phase and peak of the solar cycle. Its 1-year
prime mission is well timed with perihelia of Parker Solar Probe and
Solar Orbiter, and with the launches of complementary missions such
as the PUNCH Small Explorer. CubIXSS is a pathfinder for the next
generation of Explorer-class missions with improved capabilities for
SXR imaging spectroscopy. We present the CubIXSS motivating science
background, its suite of instruments and expected performances, and
other highlights from the completed Concept Study Report, including
novel analysis techniques to fully exploit the rich data set of CubIXSS
spectral observations.
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Title: Cross Sections of Coronal Loop Flux Tubes
Authors: Klimchuk, J. A.; DeForest, C.
2021AAS...23832808K Altcode:
Coronal loops reveal crucial information about the nature of
both coronal magnetic fields and coronal heating. The shape of the
corresponding flux tube cross section and how it varies with position
are especially important properties. They are a direct indication of
the expansion of the field and of the cross-field spatial distribution
of the heating. We have studied 20 loops using high spatial resolution
observations from the first flight of the Hi-C rocket experiment,
measuring the intensity and width as a function of position along
the loop axis. We find that intensity and width tend to either be
uncorrelated or to have a direct dependence, such that they increase
or decrease together. This implies that the flux tube cross sections
are approximately circular under the assumptions that the tubes have
non-negligible twist and that the plasma emissivity is approximately
uniform along the magnetic field. The shape need not be a perfect circle
and the emissivity need not be uniform within the cross section, but
sub-resolution patches of emission must be distributed quasi-uniformly
within an envelope that has an aspect ratio of order unity. This raises
questions about the suggestion that flux tubes expand with height,
but primarily in the line-of-sight direction so that the corresponding
(relatively noticeable) loops appear to have roughly uniform width,
a long-standing puzzle. It also casts doubt on the idea that most
loops correspond to simple warped sheets, although we leave open the
possibility of more complex manifold structures.
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Title: Nonthermal Velocity in the Transition Region of Active Regions
and Its Center-to-limb Variation
Authors: Ghosh, Avyarthana; Tripathi, Durgesh; Klimchuk, James A.
2021ApJ...913..151G Altcode: 2021arXiv210315081G
We derive the nonthermal velocities (NTVs) in the transition region
of an active region using the Si IV 1393.78 Å line observed by
the Interface Region Imaging Spectrograph and compare them with the
line-of-sight photospheric magnetic fields obtained by the Helioseismic
and Magnetic Imager on board the Solar Dynamics Observatory. The active
region consists of two strong field regions with opposite polarity,
separated by a weak field corridor that widened as the active region
evolved. The means of the NTV distributions in strong field regions
(weak field corridors) range between ∼18-20 (16-18) km s<SUP>-1</SUP>,
albeit the NTV maps show a much larger range. In addition, we identify
a narrow lane in the middle of the corridor with significantly reduced
NTV. The NTVs do not show a strong center-to-limb variation, albeit
they show somewhat larger values near the disk center. The NTVs are
well correlated with redshifts as well as line intensities. The results
obtained here and those presented in our companion paper on Doppler
shifts suggest two populations of plasma in the active region emitting
in Si IV. The first population exists in the strong field regions and
extends partway into the weak field corridor between them. We attribute
this plasma to spicules heated to ∼0.1 MK (often called type II
spicules). They have a range of inclinations relative to vertical. The
second population exists in the center of the corridor, is relatively
faint, and has smaller velocities, likely horizontal. These results
provide further insights into the heating of the transition region.
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Title: How Turbulent is the Magnetically Closed Corona?
Authors: Klimchuk, James A.; Antiochos, Spiro K.
2021FrASS...8...83K Altcode: 2021arXiv210512212K
We argue that the magnetically closed corona evolves primarily
quasi-statically, punctuated by many localized bursts of activity
associated with magnetic reconnection at a myriad of small current
sheets. The sheets form by various processes that do not involve
a traditional turbulent cascade whereby energy flows losslessly
through a continuum of spatial scales starting from the large scale
of the photospheric driving. If such an inertial range is a defining
characteristic of turbulence, then the magnetically closed corona is
not a turbulent system. It nonetheless has a complex structure that
bears no direct relationship to the pattern of driving.
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Title: The Heating of the Solar Corona
Authors: Viall, Nicholeen M.; De Moortel, Ineke; Downs, Cooper;
Klimchuk, James A.; Parenti, Susanna; Reale, Fabio
2021GMS...258...35V Altcode:
No abstract at ADS
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Title: High resolution soft X-ray spectroscopy and the quest for
the hot (5-10 MK) plasma in solar active regions
Authors: Del Zanna, Giulio; Andretta, Vincenzo; Cargill, Peter J.;
Corso, Alain J.; Daw, Adrian N.; Golub, Leon; Klimchuk, James A.;
Mason, Helen E.
2021FrASS...8...33D Altcode: 2021arXiv210306156D
We discuss the diagnostics available to study the 5--10 MK plasma in
the solar corona, which is key to understanding the heating in the
cores of solar active regions. We present several simulated spectra,
and show that excellent diagnostics are available in the soft X-rays,
around 100 Angstroms, as six ionisation stages of Fe can simultaneously
be observed, and electron densities derived, within a narrow spectral
region. As this spectral range is almost unexplored, we present an
analysis of available and simulated spectra, to compare the hot emission
with the cooler component. We adopt recently designed multilayers to
present estimates of count rates in the hot lines, with a baseline
spectrometer design. Excellent count rates are found, opening up
the exciting opportunity to obtain high-resolution spectroscopy of
hot plasma.
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Title: The Fascinating Phenomenon of Thermal Nonequilibrium
Authors: Klimchuk, James
2021cosp...43E.960K Altcode:
Thermal nonequilibrium (TNE) is an intriguing, counter-intuitive
situation in which the plasma contained in a coronal magnetic loop is
constantly evolving even though the heating is perfectly steady. It
is believed to be the explanation for prominences, coronal rain, and
long-period pulsating loops on the Sun, and there may be additional
applications in other astrophysical settings. In this talk, I will
attempt to peal away the mystery of TNE and provide some basic physical
insights. I will present two analytical formulae for predicting
whether TNE will occur, including the important role of asymmetries
in preventing TNE. Finally, I will explain how TNE is related to,
but different from, thermal instability.
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Title: Why do different current sheets reconnect differently?
Authors: Daldorff, L. K. S.; Leake, J. E.; Klimchuk, J. A.
2020AGUFMSH034..03D Altcode:
What variability can we expect for heating events from magnetic
reconnection in the solar corona is an important research
question. Complex flows in the photosphere entangle, twist, and compress
the magnetic field in the chromosphere and corona, resulting in a
complex forest of evolving current sheets. These sheets undergo tearing
and magnetic reconnection, the details of which depend on the properties
of the sheet, including its dimensions in all three directions, as
well as the shear angle of the field. The associated energy release
and heating can be very different for different sheets. We discuss
the possibilities with emphasis on the role played by the interaction
among the different tearing modes, parallel and oblique, that are
present in the system.
---------------------------------------------------------
Title: Transition Region Contribution to AIA Observations in the
Context of Coronal Heating
Authors: Schonfeld, S. J.; Klimchuk, J. A.
2020ApJ...905..115S Altcode: 2020arXiv200906759S
We investigate the ratio of coronal and transition region intensity
in coronal loops observed by the Atmospheric Imaging Assembly (AIA)
on the Solar Dynamics Observatory (SDO). Using Enthalpy-based Thermal
Evolution of Loops (EBTEL) hydrodynamic simulations, we model loops
with multiple lengths and energy fluxes heated randomly by events drawn
from power-law distributions with different slopes and minimum delays
between events to investigate how each of these parameters influences
observable loop properties. We generate AIA intensities from the corona
and transition region for each realization. The variations within and
between models generated with these different parameters illustrate
the sensitivity of narrowband imaging to the details of coronal
heating. We then analyze the transition region and coronal emission
from a number of observed active regions and find broad agreement
with the trends in the models. In both models and observations, the
transition region brightness is significant, often greater than the
coronal brightness in all six "coronal" AIA channels. We also identify
an inverse relationship, consistent with heating theories, between the
slope of the differential emission measure (DEM) coolward of the peak
temperature and the observed ratio of coronal to transition region
intensity. These results highlight the use of narrowband observations
and the importance of properly considering the transition region in
investigations of coronal heating.
---------------------------------------------------------
Title: Updates on the Fundamentals of Impulsive Energy Release in
the Corona Explorer (FIERCE) mission concept
Authors: Shih, A. Y.; Glesener, L.; Krucker, S.; Guidoni, S. E.;
Christe, S.; Reeves, K.; Gburek, S.; Caspi, A.; Alaoui, M.; Allred,
J. C.; Battaglia, M.; Baumgartner, W.; Dennis, B. R.; Drake, J. F.;
Goetz, K.; Golub, L.; Hannah, I. G.; Hayes, L.; Holman, G.; Inglis,
A.; Ireland, J.; Kerr, G. S.; Klimchuk, J. A.; McKenzie, D. E.; Moore,
C. S.; Musset, S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage,
S. L.; Schwartz, R.; Seaton, D. B.; Steslicki, M.; Woods, T. N.
2020AGUFMSH0480012S Altcode:
The Fundamentals of Impulsive Energy Release in the Corona Explorer
( FIERCE ) Medium-Class Explorer (MIDEX) mission concept addresses
the following science questions: <P />What are the physical origins
of space-weather events? <P />How are particles accelerated at the
Sun? <P />How is impulsively released energy transported throughout
the solar atmosphere? <P />How is the solar corona heated? <P />FIERCE
achieves its science objectives through co-optimized X-ray and extreme
ultraviolet (EUV) observations by the following instruments: <P />FOXSI,
a focusing hard X-ray spectroscopic imager that is able to capture the
full range of emission in flares and CMEs (e.g., faint coronal sources
near bright chromospheric sources) <P />THADIS, a high-resolution,
fast-cadence EUV imager that will not saturate for even intense flares
to follow dynamic changes in the configuration of plasma structures
<P />STC, a soft X-ray spectrometer that provides detailed thermal and
elemental composition diagnostics <P />If selected, FIERCE will launch
in 2025, near the peak of the next solar cycle, which is also well timed
with perihelia of Parker Solar Probe and Solar Orbiter . We describe the
status and latest updates of the mission concept since it was proposed
to NASA last year. We also highlight the anticipated science return
from co-observations with other observatories/instruments such as the
Expanded Owens Valley Solar Array (EOVSA) or the STIX instrument on
Solar Orbiter .
---------------------------------------------------------
Title: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS)
Authors: Caspi, A.; Shih, A. Y.; Warren, H.; Winebarger, A. R.; Woods,
T. N.; Cheung, C. M. M.; DeForest, C.; Klimchuk, J. A.; Laurent,
G. T.; Mason, J. P.; Palo, S. E.; Schwartz, R.; Seaton, D. B.;
Steslicki, M.; Gburek, S.; Sylwester, J.; Mrozek, T.; Kowaliński,
M.; Schattenburg, M.
2020AGUFMSH0480007C Altcode:
The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) is a 6U
CubeSat currently in a formulation phase under the 2019 NASA H-FORT
program. CubIXSS is motivated by a compelling overarching science
question: what are the origins of hot plasma in solar flares and active
regions? Elemental abundances are a unique diagnostic of how mass
and energy flow into and within the corona, and CubIXSS addresses
its science question through sensitive, precise measurements of
abundances of key trace ion species, whose spectral signatures reveal
the chromospheric or coronal origins of heated plasma across the
entire range of coronal temperatures, from ~1 to >30 MK. CubIXSS
measurements of the coronal temperature distribution and elemental
abundances directly address longstanding inconsistencies from prior
studies using instruments with limited, differing temperature and
composition sensitivities. <P />CubIXSS comprises two co-optimized and
cross-calibrated instruments that fill a critical observational gap:
<P />MOXSI, a novel diffractive spectral imager using a pinhole camera
and X-ray transmission diffraction grating to achieve spectroscopy of
flares and active regions from 1 to 55 Å, with spectral resolution of
0.24 Å FWHM and a spatial resolution of 25 arcsec FWHM; and <P />SASS,
a suite of four spatially-integrated off-the-shelf spectrometers for
high-cadence, high-sensitivity measurements of soft and hard X-rays,
from 0.5 to 50 keV, with spectral resolution from 0.06 to 0.5 keV
FWHM. <P />If selected for implementation, CubIXSS will launch in
mid-2023 to observe intense solar flares and active regions during
the rising phase of the solar cycle. Its nominal 1-year mission is
well timed with perihelia of Parker Solar Probe and Solar Orbiter,
and with the launches of complementary missions such as the PUNCH
Small Explorer. CubIXSS is also a pathfinder for the next generation
of Explorer-class missions with improved capabilities for SXR imaging
spectroscopy. We present the CubIXSS motivating science background, its
suite of instruments and expected performances, and other highlights
from the completed Concept Study Report, including novel analysis
techniques to fully exploit the rich data set of CubIXSS spectral
observations.
---------------------------------------------------------
Title: Signatures of Type III Solar Radio Bursts from Nanoflares:
Final Results
Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M.
2020AGUFMSH0430016C Altcode:
The heating mechanisms responsible for the million degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence o f reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate in
solving this problem [Klimchuk J.A., 2015 and references therein]. <P
/>Whether nanoflares accelerate energetic particles like full sized
flares is unknown. The lack of strong emission in hard X rays suggests
that the quantity of highly energetic particles is small. There could,
however, be large numbers of mildly energetic particles (~ 10 keV). We
investigate such particles by searching for the type III radio bursts
that they may produce. If energetic electron beams propagating along
magnetic field lines generate a bump on tail instability, they will
produce Langmuir waves, which can then interact with other particles
and waves to give rise to emission at the local plasma frequency and
its first harmonic. Type III radio bursts bursts are characteristically
known to exhibit high frequency drifts as the beam propagates through
a density gradient. The time lag technique that was developed to study
subtle delays in light curves from different EUV channels [Viall &
Klimchuk 2012] can also be used to detect subtle delays at different
radio frequencies. We have modeled the expected radio emission from
nanoflares, which we used to test and calibrate the technique. We will
present the final results of our modeling efforts along with results
from application of the technique to actual radio observations from VLA
(Very Large Array), MWA (Murchison Widefield Array) and seeking data
from LOFAR (Low Frequency Array) as well.We are also using data from the
PSP (Parker Solar Probe) to look for similar reconnection signatures in
the Solar Wind. Our goal is to determine whether nanoflares accelerate
energetic particles and to determine their properties. The results
will have important implications for both the particle acceleration
and reconnection physics.
---------------------------------------------------------
Title: Cross Sections of Coronal Loop Flux Tubes
Authors: Klimchuk, J. A.; DeForest, C.
2020AGUFMSH0370001K Altcode:
Coronal loops reveal crucial information about the nature of
both coronal magnetic fields and coronal heating. The shape of the
corresponding flux tube cross section and how it varies with position
are especially important properties. They are a direct indication of
the expansion of the field and of the cross-field spatial distribution
of the heating. We have studied 20 loops using high spatial resolution
observations from the first flight of the Hi-C rocket experiment,
measuring the intensity and width as a function of position along
the loop axis. We find that intensity and width tend to either be
uncorrelated or to have a direct dependence, such that they increase
or decrease together. This implies that the flux tube cross sections
are approximately circular under the assumptions that the tubes have
non-negligible twist and that the plasma emissivity is approximately
uniform along the magnetic field. The shape need not be a perfect circle
and the emissivity need not be uniform within the cross section, but
sub-resolution patches of emission must be distributed quasi-uniformly
within an envelope that has an aspect ratio of order unity. This raises
questions about the suggestion that flux tubes expand with height,
but primarily in the line-of-sight direction so that the corresponding
(relatively noticeable) loops appear to have roughly uniform width,
a long-standing puzzle. It also casts doubt on the idea that most
loops correspond to simple warped sheets, although we leave open the
possibility of more complex manifold structures.
---------------------------------------------------------
Title: Can nanoflare heating define the coronal loop size?
Authors: Uritsky, V. M.; Knizhnik, K.; Klimchuk, J. A.
2020AGUFMSH0370002U Altcode:
The idea of nanoflare heating first coined by T. Gold (1964) and further
developed by E. Parker (1972) has resulted in a dramatic paradigm shift
in our understanding of the Sun, by emphasizing the critical role of
the microscale physics in the formation of huge coronal structures such
as coronal loops. Nanoflares are small, short-living energy conversion
events in which the magnetic free energy is efficiently converted into
heat. While the outcome of a single nanoflare is negligibly small, their
cooperative contribution to the bulk coronal heating can be enormous. In
this talk, we present recent advances in the numerical modeling of the
nanoflare heating process. Our simulations demonstrate that densely
distributed nanoflare events occur naturally inside a topologically
complex coronal loop energized by photospheric vortices. These events
lead to a formation of visible loops of various sizes, with the largest
loop size imposed by the photospheric convection pattern and the
smaller-scale loops reflecting clusters of nanoflares. Observational
implications of our findings will be discussed.
---------------------------------------------------------
Title: Spectroscopic Constraints on the Dimension of Active Region
Loops Along the Line of Sight
Authors: Kucera, T. A.; Young, P. R.; Klimchuk, J. A.; DeForest, C.
2020AGUFMSH041..05K Altcode:
Understanding the cross sections of coronal loops and how they vary
along the loop is important both for understanding coronal heating
and how the loops are shaped by the coronal magnetic field. To better
address this question we have developed a new method to constrain the
dimension of loops along the line of sight by utilizing spectroscopic
observations. We apply this method to a cool (5.5<logT<6.2)
loop using data from the Hinode/EUV Imaging Spectrometer (EIS) with
supporting data from Solar Dynamic Observatory (SDO) and the Solar
TErrestrial RElations Observatory (STEREO), and discuss the results and
their limitations. Our results are consistent with circular loop cross
sections, but could also be consistent with aspect ratios of 2 or 3.
---------------------------------------------------------
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: Cross Sections of Coronal Loop Flux Tubes
Authors: Klimchuk, James A.; DeForest, Craig E.
2020ApJ...900..167K Altcode: 2020arXiv200715085K
Coronal loops reveal crucial information about the nature of
both coronal magnetic fields and coronal heating. The shape of the
corresponding flux tube cross section and how it varies with position
are especially important properties. They are a direct indication of
the expansion of the field and of the cross-field spatial distribution
of the heating. We have studied 20 loops using high spatial resolution
observations from the first flight of the Hi-C rocket experiment,
measuring the intensity and width as a function of position along
the loop axis. We find that intensity and width tend to either be
uncorrelated or to have a direct dependence, such that they increase
or decrease together. This implies that the flux tube cross sections
are approximately circular under the assumptions that the tubes have
nonnegligible twist and that the plasma emissivity is approximately
uniform along the magnetic field. The shape need not be a perfect circle
and the emissivity need not be uniform within the cross section, but
subresolution patches of emission must be distributed quasi-uniformly
within an envelope that has an aspect ratio of order unity. This raises
questions about the suggestion that flux tubes expand with height,
but primarily in the line-of-sight direction so that the corresponding
(relatively noticeable) loops appear to have roughly uniform width,
a long-standing puzzle. It also casts doubt on the idea that most
loops correspond to simple warped sheets, although we leave open the
possibility of more complex manifold structures.
---------------------------------------------------------
Title: The Significance of the Transition Region in AIA Channels:
Modeling and Observations
Authors: Schonfeld, S. J.; Klimchuk, J.
2020SPD....5121014S Altcode:
We investigate the relative contributions from the transition region
and corona of coronal loops observed by the Atmospheric Imaging
Assembly (AIA) on the Solar Dynamics Observatory (SDO). Using EBTEL
(Enthalpy-Based Thermal Evolution of Loops) hydrodynamic simulations,
we model loops with multiple lengths and energy fluxes heated randomly
by events drawn from power-law distributions with different slopes
and minimum event sizes to investigate how each of these parameters
influences observable loop properties. We generate AIA intensities from
the corona and transition region for each realization. The variations
within and between models generated with these different parameters
illustrate the sensitivity of narrowband imaging to the details of
coronal heating. We then analyze the transition region and coronal
emission from a number of observed active regions and find broad
agreement with the trends in the models. We find that in both models
and observations, the transition region brightness is significant,
often greater than the coronal brightness in all six "coronal" AIA
channels. These results highlight the use of narrowband observations
and the importance of properly considering the transition region in
investigations of coronal heating.
---------------------------------------------------------
Title: The Onset of 3D Magnetic Reconnection and Heating in the
Solar Corona
Authors: Leake, James E.; Daldorff, Lars K. S.; Klimchuk, James A.
2020ApJ...891...62L Altcode: 2020arXiv200102971L
Magnetic reconnection, a fundamentally important process in
astrophysics, is believed to be initiated by the tearing instability
of an electric current sheet, a region where magnetic field abruptly
changes direction. Recent studies have suggested that the amount of
magnetic shear in these structures is a critical parameter for the
switch-on nature of magnetic reconnection in the solar atmosphere,
at large spatial scales. We present results of visco-resistive
magnetohydrodynamic simulations of magnetic reconnection in 3D current
sheets with conditions appropriate to the solar corona. We follow the
evolution of the linear and nonlinear 3D tearing instability. We find
that, depending on the parameter space, magnetic shear can play a vital
role in the onset of significant energy release and plasma heating. Two
regimes in our study exist, dependent on whether the current sheet is
longer or shorter than the wavelength of the fastest growing mode, thus
determining whether subharmonics are present in the actual system. In
one parameter regime, where the fastest growing parallel mode has
subharmonics, the subsequent coalescence of 3D plasmoids dominates
the nonlinear evolution, with magnetic shear playing only a weak role
in the amount of energy released. In the second parameter regime,
where the fastest growing parallel mode has no subharmonics, only
strongly sheared current sheets, where 3D effects are strong enough,
show any significant energy release. We expect both regimes to exist
on the Sun, and so our results have important consequences for the
question of reconnection onset in various solar physics applications.
---------------------------------------------------------
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena throughout
the Universe
Authors: Ji, H.; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo,
F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.;
Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.;
Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus, W. H.;
Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.;
Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.;
Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
2020arXiv200400079J Altcode:
This white paper summarizes major scientific challenges and
opportunities in understanding magnetic reconnection and related
explosive phenomena as a fundamental plasma process.
---------------------------------------------------------
Title: Volume-filling Simulations of Coronal Loops Heated by
Nanoflares
Authors: Plowman, J.; Barnes, W.; Bradshaw, S. J.; Caspi, A.; DeForest,
C.; Klimchuk, J. A.
2019AGUFMSH53B3380P Altcode:
We present results of a coronal simulation consisting of loop strands
that fill the coronal volume in a self-consistent fashion. The
simulation is heated by a fully controllable 3D distribution, which
can be specified independent of the loop geometry and can include
nanoflares and continuous heating. The heating is then mapped to the
loop strands, and the physics of each strand are simulated using the
HYDRAD field-aligned hydrodynamics code. The simulation is applied
to a small example active region and used to produce synthetic AIA
data, which are then processed to produce a distribution of coronal
EUV brightening events. This distribution is then compared with that
found in the real AIA data for the same region, and we use the results
to determine if the observations are consistent with our prescribed
heating distribution.
---------------------------------------------------------
Title: Erratum: “The Role of Magnetic Helicity in Coronal Heating”
(<A href="https://doi.org/10.3847/1538-4357/ab3afd">2019, ApJ,
883, 26</A>)
Authors: Knizhnik, K. J.; Antiochos, S. K.; Klimchuk, J. A.; DeVore,
C. R.
2019ApJ...887..270K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Combined Next-Generation X-ray and EUV Observations with the
FIERCE Mission Concept
Authors: Shih, A. Y.; Glesener, L.; Christe, S.; Reeves, K.; Gburek,
S.; Alaoui, M.; Allred, J. C.; Baumgartner, W.; Caspi, A.; Dennis,
B. R.; Drake, J. F.; Goetz, K.; Golub, L.; Guidoni, S. E.; Inglis,
A.; Hannah, I. G.; Holman, G.; Hayes, L.; Ireland, J.; Kerr, G. S.;
Klimchuk, J. A.; Krucker, S.; McKenzie, D. E.; Moore, C. S.; Musset,
S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Seaton,
D. B.; Steslicki, M.; Woods, T. N.
2019AGUFMSH33A..08S Altcode:
While there have been significant advances in our understanding
of impulsive energy release at the Sun through the combination
of RHESSI X-ray observations and SDO/AIA EUV observations, there
is a clear science need for significantly improved X-ray and EUV
observations. These new observations must capture the full range
of emission in flares and CMEs (e.g., faint coronal sources near
bright chromospheric sources), connect the intricate evolution of
energy release with dynamic changes in the configuration of plasma
structures, and identify the signatures of impulsive energy release in
even the quiescent Sun. The Fundamentals of Impulsive Energy Release
in the Corona Explorer ( FIERCE ) MIDEX mission concept makes these
observations by combining the two instruments previously proposed on the
FOXSI SMEX mission concept - a focusing hard X-ray spectroscopic imager
and a soft X-ray spectrometer - with a high-resolution EUV imager that
will not saturate for even intense flares. All instruments observe at
high cadence to capture the initiation of solar transient events and
the fine time structure within events. FIERCE would launch in mid-2025,
near the peak of the next solar cycle, which is also well timed with
perihelions of Parker Solar Probe and Solar Orbiter.
---------------------------------------------------------
Title: The Sensitivity of AIA Observations to Coronal Heating
Parameters
Authors: Schonfeld, S.; Klimchuk, J. A.
2019AGUFMSH41F3323S Altcode:
We explore the effects of changing heating parameters in closed
coronal loops on the intensity of Atmospheric Imaging Assembly (AIA)
observations. Using EBTEL (Enthalpy-Based Thermal Evolution of Loops)
hydrodynamic simulations, we produce realizations of coronal loops
with a series of heating events randomly drawn from a power law
distribution. We repeat this procedure for multiple loop lengths,
heating intensities, and characteristic heating frequencies to
investigate how each of these parameters individually and in combination
influences observable loop properties. We generate AIA intensities from
the corona and transition region for each realization. The variations
within and between models generated with these different parameters
illustrate the sensitivity of narrowband imaging to the details of
coronal heating. Using insights from this analysis, we generate images
of observed coronal active regions using the GX Simulator SSW IDL
package and interpret the causes of discrepancies between the models
and observations.
---------------------------------------------------------
Title: Study of Type III Solar Radio Bursts in Nanoflares
Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M.
2019AGUFMSH23C3337C Altcode:
The heating mechanisms responsible for the million-degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence of reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate in
solving this problem [Klimchuk J.A., 2015 and references therein]. <P
/>Whether nanoflares accelerate energetic particles like full-sized
flares is unknown. The lack of strong emission in hard X-rays suggests
that the quantity of highly energetic particles is small. There could,
however, be large numbers of mildly energetic particles (~ 10 keV). We
investigate such particles by searching for the type III radio bursts
that they may produce. If energetic electron beams propagating along
magnetic field lines generate a bump-on-tail instability, they will
produce Langmuir waves, which can then interact with other particles
and waves to give rise to emission at the local plasma frequency and
its first harmonic. Type III bursts are characteristically known
to exhibit high frequency drifts as the beam propagates through a
density gradient. The time-lag technique that was developed to study
subtle delays in light curves from different EUV channels [Viall &
Klimchuk 2012] can also be used to detect subtle delays at different
radio frequencies. We have modeled the expected radio emission from
nanoflares, which we used to test and calibrate the technique. We are
applying the technique to actual radio observations from VLA (Very Large
Array), MWA (Murchison Widefield Array) and seeking data from LOFAR
(Low-Frequency Array) as well. We also plan to use data from the PSP
(Parker Solar Probe) to look for similar reconnection signatures in
the Solar Wind. Our goal is to determine whether nanoflares accelerate
energetic particles and to determine their properties. The results
will have important implications for both the particle acceleration
and reconnection physics.
---------------------------------------------------------
Title: The magnetic structure and electrodynamics of the emerging
solar wind
Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.;
Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.;
Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; Dudok de Wit, T.;
Eastwood, J. P.; Webster, J.; Farrell, W. M.; Fong, C.; Goetz, K.;
Goldstein, M. L.; Goodrich, K.; Harvey, P.; Horbury, T. S.; Howes,
G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck,
K. E.; Krasnoselskikh, V.; Krucker, S.; Laker, R.; Larson, D. E.;
MacDowall, R. J.; Maksimovic, M.; Malaspina, D.; Martinez Oliveros,
J. C.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F.;
Phan, T.; Pulupa, M.; Raouafi, N. E.; Salem, C. S.; Stansby, D.;
Stevens, M. L.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R.
2019AGUFMSH11A..05B Altcode:
Convection and rotation drive the solar dynamo and, ultimately,
provide the mechanical energy flux required to heat the solar corona
and accelerate the solar wind. However, the way in which energy is then
dissipated to heat the corona and wind are not well understood. Some
energization models invoke non-thermal energy flux imparted by plasma
Alfvén waves, while others rely on a carpet of small nano-flares as
energy input, however these models have been unconstrained by direct
measurements of the solar wind near its origin. Here we use in situ
measurements from the FIELDS instrument suite during the first solar
encounter (E1) at 35.7 solar radii (Rs) of the NASA Parker Solar
Probe (PSP) mission to reveal the magnetic structure and kinetics of
slow Alfvénic solar wind emerging from a small, equatorial coronal
hole. Our measurements show that, at solar minimum, the slow wind can
escape from above the low-lying, complex magnetic structures of the
equatorial streamer belt, carrying a magnetic field that is highly
dynamic, exhibiting polarity reversals on timescales from seconds
to hours. These rapidly oscillating field structures are associated
with clustered radial jets of plasma in which the energy flux is
dramatically enhanced and turbulence levels are higher. Time intervals
between groups of jets indicate a solar wind that is steady with a
mostly radial magnetic field and relatively low levels of Alfvénic
turbulent fluctuations. This 'quiet' wind however shows clear signatures
of plasma micro-instabilities associated with ion and electron beams
and velocity-space structure.
---------------------------------------------------------
Title: The Onset and Development of 3D Magnetic Reconnection in the
Solar Corona: New Insights
Authors: Klimchuk, J. A.; Daldorff, L. K. S.; Leake, J. E.; Knizhnik,
K. J.
2019AGUFMSH52A..08K Altcode:
Magnetic reconnection, one of the most important processes in the
universe, is believed to be initiated in most cases by the tearing
instability of an electric current sheet. Whether and how the tearing
develops nonlinearly and releases substantial amounts of magnetic
free energy depends on the relative growth rates of the different
normal modes that are allowed in the system. In the general case
where there is a guide field—in which the field rotates across the
sheet by a shear angle—there exist both parallel modes centered
on the sheet and oblique modes offset from it. We have performed a
series of resistive magnetohydrodynamic (MHD) simulations of weakly
perturbed, triply periodic current sheets with conditions appropriate
to the solar corona to study how the different modes evolve and
interact. We find three primary evolutionary paths: (1) a parallel
mode dominates and saturates at a level that releases only a small
amount of energy, but is large enough to choke off the growth of other
modes; (2) parallel and oblique modes grow at nearly the same rate
and interact violently as they become nonlinear, releasing a large
amount of energy; (3) subharmonics of the fastest growing parallel
mode induce a coalescence of islands, also releasing a large amount
of energy. Which path will be taken depends in a predictable way on
the length, thickness, and shear angle of the current sheet, as well
as the resistivity. A critical issue is whether the wavelength of the
fastest growing mode is longer or shorter than the current sheet,
i.e., whether subharmonics exist. We expect all three behaviors on
the Sun. These results have important implications for the question
of reconnection onset. Observed phenomena require the buildup of high
levels of magnetic stress before reconnection switches on to release
the stored magnetic energy. Physical details of these simulations will
be discussed in a companion presentation by Daldorff et al.
---------------------------------------------------------
Title: The Onset and Development of 3D Magnetic Reconnection in the
Solar Corona: Important Physical Details
Authors: Daldorff, L. K. S.; Leake, J. E.; Klimchuk, J. A.; Knizhnik,
K. J.
2019AGUFMSH53B3366D Altcode:
Magnetic reconnection, one of the most important processes in the
universe, is believed to be initiated in most cases by the tearing
instability of an electric current sheet. Whether and how the tearing
develops nonlinearly and releases substantial amounts of magnetic
free energy is a crucial unanswered question. Using a series of
resistive magnetohydrodynamic (MHD) simulations of weakly perturbed,
triply periodic current sheets with conditions appropriate to the
solar corona, we have found that the answer depends on the relative
growth rates of the different normal modes that are allowed in
the system. Dramatically different evolutionary paths are possible
depending on the length, thickness, and shear angle of the current
sheet, as well as the resistivity. The basic results are discussed in
the companion presentation by Klimchuk et al. Here, we delve into the
important physical details underpinning the complex behavior.
---------------------------------------------------------
Title: FIERCE Science: Expected Results From a High-Energy
Medium-Class Explorer
Authors: Glesener, L.; Shih, A. Y.; Christe, S.; Reeves, K.; Gburek,
S.; Alaoui, M.; Allred, J. C.; Baumgartner, W.; Caspi, A.; Dennis,
B. R.; Drake, J. F.; Golub, L.; Goetz, K.; Guidoni, S. E.; Hannah,
I. G.; Hayes, L.; Holman, G.; Inglis, A.; Ireland, J.; Kerr, G. S.;
Klimchuk, J. A.; Krucker, S.; McKenzie, D. E.; Moore, C. S.; Musset,
S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Seaton,
D. B.; Steslicki, M.; Woods, T. N.
2019AGUFMSH31C3313G Altcode:
A variety of individual X-ray and EUV instruments have probed
high-energy aspects of the Sun over the decades, each contributing
pieces to the puzzles of the energization, heating, and acceleration of
solar plasma and particles. But fundamental difficulties in sensitivity
and dynamic range impart big challenges in probing the details of
particle acceleration sites, understanding how eruptions and flares are
initiated, and tracking the intricacies of energy transfer as flares
evolve. The Fundamentals of Impulsive Energy Release in the Corona
Explorer ( FIERCE ) mission will make substantial leaps forward in
these scientific ventures by combining a variety of instruments into
one platform, each optimized to have high sensitivity and dynamic
range. FIERCE is a proposed NASA Heliophysics Medium-Class Explorer
that will investigate high-energy solar phenomena across a variety
of spectral and spatial dimensions. It combines hard X-ray imaging
spectroscopy (via focusing, for the first time for a solar-dedicated
spacecraft), spatially integrated soft X-ray spectroscopy, and fast,
high-resolution extreme ultraviolet imaging at coronal and flare
temperatures. FIERCE uses this array of instruments to make important
contributions toward probing the genesis of space weather events,
the acceleration of particles, the transport of flare energy, and the
heating of the corona. Here, we present some of the expected science
outcomes for the FIERCE observatory, concentrating on the ways in which
FIERCE can probe confined and eruptive events, particle acceleration
everywhere it may occur on the Sun, and the connections of solar
high-energy phenomena to the heliosphere.
---------------------------------------------------------
Title: The Distinction Between Thermal Nonequilibrium and Thermal
Instability
Authors: Klimchuk, James A.
2019SoPh..294..173K Altcode: 2019arXiv191111849K
For some forms of steady heating, coronal loops are in a state of
thermal nonequilibrium and evolve in a manner that includes accelerated
cooling, often resulting in the formation of a cold condensation. This
is frequently confused with thermal instability, but the two are
in fact fundamentally different. We explain the distinction and
discuss situations where they may be interconnected. Large-amplitude
perturbations, perhaps associated with MHD waves, likely play a role in
explaining phenomena that have been attributed to thermal nonequilibrium
but also seem to require cross-field communication.
---------------------------------------------------------
Title: Highly structured slow solar wind emerging from an equatorial
coronal hole
Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.;
Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.;
Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; de Wit, T. Dudok;
Eastwood, J. P.; Ergun, R. E.; Farrell, W. M.; Fong, C.; Goetz,
K.; Goldstein, M.; Goodrich, K. A.; Harvey, P. R.; Horbury, T. S.;
Howes, G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck,
K. E.; Krasnoselskikh, V. V.; Krucker, S.; Laker, R.; Larson, D. E.;
MacDowall, R. J.; Maksimovic, M.; Malaspina, D. M.; Martinez-Oliveros,
J.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F. S.;
Phan, T. D.; Pulupa, M.; Raouafi, N. E.; Salem, C.; Stansby, D.;
Stevens, M.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R.
2019Natur.576..237B Altcode:
During the solar minimum, when the Sun is at its least active, the solar
wind<SUP>1,2</SUP> is observed at high latitudes as a predominantly fast
(more than 500 kilometres per second), highly Alfvénic rarefied stream
of plasma originating from deep within coronal holes. Closer to the
ecliptic plane, the solar wind is interspersed with a more variable slow
wind<SUP>3</SUP> of less than 500 kilometres per second. The precise
origins of the slow wind streams are less certain<SUP>4</SUP>; theories
and observations suggest that they may originate at the tips of helmet
streamers<SUP>5,6</SUP>, from interchange reconnection near coronal hole
boundaries<SUP>7,8</SUP>, or within coronal holes with highly diverging
magnetic fields<SUP>9,10</SUP>. The heating mechanism required to
drive the solar wind is also unresolved, although candidate mechanisms
include Alfvén-wave turbulence<SUP>11,12</SUP>, heating by reconnection
in nanoflares<SUP>13</SUP>, ion cyclotron wave heating<SUP>14</SUP>
and acceleration by thermal gradients<SUP>1</SUP>. At a distance of
one astronomical unit, the wind is mixed and evolved, and therefore
much of the diagnostic structure of these sources and processes
has been lost. Here we present observations from the Parker Solar
Probe<SUP>15</SUP> at 36 to 54 solar radii that show evidence of
slow Alfvénic solar wind emerging from a small equatorial coronal
hole. The measured magnetic field exhibits patches of large,
intermittent reversals that are associated with jets of plasma and
enhanced Poynting flux and that are interspersed in a smoother and
less turbulent flow with a near-radial magnetic field. Furthermore,
plasma-wave measurements suggest the existence of electron and ion
velocity-space micro-instabilities<SUP>10,16</SUP> that are associated
with plasma heating and thermalization processes. Our measurements
suggest that there is an impulsive mechanism associated with solar-wind
energization and that micro-instabilities play a part in heating, and
we provide evidence that low-latitude coronal holes are a key source
of the slow solar wind.
---------------------------------------------------------
Title: Spectroscopic Constraints on the Cross-sectional Asymmetry
and Expansion of Active Region Loops
Authors: Kucera, T. A.; Young, P. R.; Klimchuk, J. A.; DeForest, C. E.
2019ApJ...885....7K Altcode:
We explore the constraints that can be placed on the dimensions of
coronal loops out of the plane of the sky by utilizing spectroscopic
observations from the Hinode/EUV Imaging Spectrometer (EIS). The
usual assumption is that loop cross sections are circular. Changes in
intensity are assumed to be the result of changing density, filling
factor, and/or point of view. In this work we instead focus on the
possibility that the loop dimensions may be changing along the line of
sight while the filling factor remains constant. We apply these ideas
to two warm (5.5≲ {log}T({{K}})< 6.2) loops observed by EIS in
Active Region 11150 on 2011 February 6 with supporting observations
from Solar Dynamics Observatory's Atmospheric Imaging Assembly and
the Solar TErrestrial RElations Observatory-A's Extreme Ultraviolet
Imager. Our results are generally consistent with nonexpanding loops
but could also allow linear expansions of up to a factor of 2.5 along
a 40 Mm section of one loop and up to a factor of 3.9 in another loop,
both under the assumption that the filling factor is constant along
the loop. Expansions in the plane of the sky over the same sections of
the loops are 1.5 or less. For a filling factor of 1, the results of
the analysis are consistent with circular cross sections but also with
aspect ratios of 2 or greater. Count rate statistics are an important
part of the uncertainties, but the results are also significantly
dependent on radiometric calibration of EIS and the selection of the
loop backgrounds.
---------------------------------------------------------
Title: On Doppler Shift and Its Center-to-limb Variation in Active
Regions in the Transition Region
Authors: Ghosh, Avyarthana; Klimchuk, James A.; Tripathi, Durgesh
2019ApJ...886...46G Altcode: 2019arXiv191012033G
A comprehensive understanding of the structure of Doppler motions
in the transition region including the center-to-limb variation and
its relationship with the magnetic field structure is vital for the
understanding of mass and energy transfer in the solar atmosphere. In
this paper, we have performed such a study in an active region using
the Si IV 1394 Å emission line recorded by the Interface Region Imaging
Spectrograph and the line-of-sight photospheric magnetic field obtained
by the Helioseismic and Magnetic Imager onboard the Solar Dynamics
Observatory. The active region has two opposite polarity strong field
regions separated by a weak field corridor, which widened as the active
region evolved. On average, the strong field regions (corridor) show(s)
redshifts of 5-10 (3-9) km s<SUP>-1</SUP> (depending on the date of
observation). There is, however, a narrow lane in the middle of the
corridor with near-zero Doppler shifts at all disk positions, suggesting
that any flows there are very slow. The Doppler velocity distributions
in the corridor seem to have two components—a low velocity component
centered near 0 km s<SUP>-1</SUP> and a high-velocity component centered
near 10 km s<SUP>-1</SUP>. The high-velocity component is similar to
the velocity distributions in the strong field regions, which have just
one component. Both exhibit a small center-to-limb variation and seem
to come from the same population of flows. To explain these results,
we suggest that the emission from the lower transition region comes
primarily from warm type II spicules, and we introduce the idea of a
“chromospheric wall”—associated with classical cold spicules—to
account for a diminished center-to-limb variation.
---------------------------------------------------------
Title: The Role of Asymmetries in Thermal Nonequilibrium
Authors: Klimchuk, James A.; Luna, Manuel
2019ApJ...884...68K Altcode: 2019arXiv190509767K
Thermal nonequilibrium (TNE) is a fascinating situation that occurs
in coronal magnetic flux tubes (loops) for which no solution to
the steady-state fluid equations exists. The plasma is constantly
evolving even though the heating that produces the hot temperatures
does not. This is a promising explanation for isolated phenomena
such as prominences, coronal rain, and long-period pulsating loops,
but it may also have much broader relevance. As known for some time,
TNE requires that the heating be both (quasi-)steady and concentrated
at low coronal altitudes. Recent studies indicate that asymmetries are
also important, with large enough asymmetries in the heating and/or
cross-sectional area resulting in steady flow rather than TNE. Using
reasonable approximations, we have derived two formulae for quantifying
the conditions necessary for TNE. As a rough rule of thumb, the ratio
of apex to footpoint heating rates must be less than about 0.1, and
asymmetries must be less than about a factor of 3. The precise values
are case-dependent. We have tested our formulae with 1D hydrodynamic
loop simulations and find a very acceptable agreement. These results
are important for developing physical insight about TNE and assessing
how widespread it may be on the Sun.
---------------------------------------------------------
Title: Achievements of Hinode in the first eleven years
Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick;
Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks,
David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio;
Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra,
Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.;
Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao,
Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota,
Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu,
Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin;
Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young,
Peter R.
2019PASJ...71R...1H Altcode:
Hinode is Japan's third solar mission following Hinotori (1981-1982)
and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in
operation currently. Hinode carries three instruments: the Solar Optical
Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These
instruments were built under international collaboration with the
National Aeronautics and Space Administration and the UK Science and
Technology Facilities Council, and its operation has been contributed
to by the European Space Agency and the Norwegian Space Center. After
describing the satellite operations and giving a performance evaluation
of the three instruments, reviews are presented on major scientific
discoveries by Hinode in the first eleven years (one solar cycle long)
of its operation. This review article concludes with future prospects
for solar physics research based on the achievements of Hinode.
---------------------------------------------------------
Title: The Role of Magnetic Helicity in Coronal Heating
Authors: Knizhnik, K. J.; Antiochos, S. K.; Klimchuk, J. A.; DeVore,
C. R.
2019ApJ...883...26K Altcode: 2019arXiv190903768K
One of the greatest challenges in solar physics is understanding
the heating of the Sun’s corona. Most theories for coronal heating
postulate that free energy in the form of magnetic twist/stress is
injected by the photosphere into the corona where the free energy is
converted into heat either through reconnection or wave dissipation. The
magnetic helicity associated with the twist/stress, however, is expected
to be conserved and appear in the corona. In previous works, we showed
that the helicity associated with the small-scale twists undergoes
an inverse cascade via stochastic reconnection in the corona and
ends up as the observed large-scale shear of filament channels. Our
“helicity condensation” model accounts for both the formation
of filament channels and the observed smooth, laminar structure of
coronal loops. In this paper, we demonstrate, using helicity- and
energy-conserving numerical simulations of a coronal system driven
by photospheric motions, that the model also provides a natural
mechanism for heating the corona. We show that the heat generated by
the reconnection responsible for the helicity condensation process is
sufficient to account for the observed coronal heating. We study the
role that helicity injection plays in determining coronal heating and
find that, crucially, the heating rate is only weakly dependent on the
net helicity preference of the photospheric driving. Our calculations
demonstrate that motions with 100% helicity preference are least
efficient at heating the corona; those with 0% preference are most
efficient. We discuss the physical origins of this result and its
implications for the observed corona.
---------------------------------------------------------
Title: Frequency Agile Solar Radiotelescope
Authors: Bastian, Tim; Bain, H.; Bradley, R.; Chen, B.; Dahlin, J.;
DeLuca, E.; Drake, J.; Fleishman, G.; Gary, D.; Glesener, L.; Guo,
Fan; Hallinan, G.; Hurford, G.; Kasper, J.; Ji, Hantao; Klimchuk,
J.; Kobelski, A.; Krucker, S.; Kuroda, N.; Loncope, D.; Lonsdale,
C.; McTiernan, J.; Nita, G.; Qiu, J.; Reeves, K.; Saint-Hilaire, P.;
Schonfeld, S.; Shen, Chengcai; Tun, S.; Wertheimer, D.; White, S.
2019astro2020U..56B Altcode:
We describe the science objectives and technical requirements for a
re-scoped Frequency Agile Solar Radiotelescope (FASR). FASR fulfills
a long term community need for a ground-based, solar-dedicated, radio
telescope - a next-generation radioheliograph - designed to perform
ultra-broadband imaging spectropolarimetry.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener,
Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Gburek, Szymon;
Steslicki, Marek; Allred, Joel C.; Battaglia, Marina; Baumgartner,
Wayne H.; Drake, James; Goetz, Keith; Grefenstette, Brian; Hannah,
Iain; Holman, Gordon D.; Inglis, Andrew; Ireland, Jack; Klimchuk,
James A.; Ishikawa, Shin-Nosuke; Kontar, Eduard; Massone, Anna-maria;
Piana, Michele; Ramsey, Brian; Schwartz, Richard A.; Woods, Thomas N.;
Chen, Bin; Gary, Dale E.; Hudson, Hugh S.; Kowalski, Adam; Warmuth,
Alexander; White, Stephen M.; Veronig, Astrid; Vilmer, Nicole
2019AAS...23422501C Altcode:
The Focusing Optics X-ray Solar Imager (FOXSI), a SMEX mission concept
in Phase A, is the first-ever solar-dedicated, direct-imaging, hard
X-ray telescope. FOXSI provides a revolutionary new approach to
viewing explosive magnetic-energy release on the Sun by detecting
signatures of accelerated electrons and hot plasma directly in
and near the energy-release sites of solar eruptive events (e.g.,
solar flares). FOXSI's primary science objective is to understand the
mystery of how impulsive energy release leads to solar eruptions, the
primary drivers of space weather at Earth, and how those eruptions are
energized and evolve. FOXSI addresses three important science questions:
(1) How are particles accelerated at the Sun? (2) How do solar plasmas
get heated to high temperatures? (3) How does magnetic energy released
on the Sun lead to flares and eruptions? These fundamental physics
questions are key to our understanding of phenomena throughout
the Universe from planetary magnetospheres to black hole accretion
disks. FOXSI measures the energy distributions and spatial structure of
accelerated electrons throughout solar eruptive events for the first
time by directly focusing hard X-rays from the Sun. This naturally
enables high imaging dynamic range, while previous instruments have
typically been blinded by bright emission. FOXSI provides 20-100 times
more sensitivity as well as 20 times faster imaging spectroscopy
than previously available, probing physically relevant timescales
(<1 second) never before accessible. FOXSI's launch in July 2022
is aligned with the peak of the 11-year solar cycle, enabling FOXSI
to observe the many large solar eruptions that are expected to take
place throughout its two-year mission.
---------------------------------------------------------
Title: Constraints from Hinode/EIS on the Expansion of Active Region
Loops Along the Line of Sight
Authors: Kucera, Therese A.; Young, Peter R.; Klimchuk, James A.;
DeForest, Craig
2019AAS...23411706K Altcode:
We explore the constraints that can be placed on the dimensions of
coronal loops out of the plane of the sky by utilizing spectroscopic
observations from the Hinode/EUV Imaging Spectrometer (EIS). The
usual assumption is that loop cross sections are circular. Changes
in intensity not constant with the measured width are assumed to be
the result of changing density and/or filling factor. Here we instead
focus on the possibility that the loop dimensions may be changing along
the line of sight while the filling factor remains constant. We apply
these ideas to two cool (5.5<logT<6.2) loops observed by EIS with
supporting observations from Solar Dynamics Observatory's Atmospheric
Imaging Assembly (SDO/AIA) and the Solar TErrestrial RElations
Observatory-A's Extreme Ultraviolet Imager (STEREO-A/EUVI). Our
results are generally consistent with non-expanding loops, but allow
for line-of-sight expansion factors up to 3-4. The uncertainties are
sizable and are driven by count rate statistics, radiometric calibration
of EIS, and the selection of the loop backgrounds.
---------------------------------------------------------
Title: Ultrahigh-Resolution Imaging of the Solar Corona using a
Distributed Diffractive Telescope
Authors: Rabin, Douglas M.; Daw, Adrian N.; Denis, Kevin; Kamalabadi,
Farzad; Klimchuk, James A.
2019AAS...23410704R Altcode:
Several observational and theoretical considerations suggest that
energy is often released in the solar corona on small spatial scales
of order 100 km. It has been a longstanding goal of solar physics to
subject this hypothesis to direct observational test. However, extreme
ultraviolet (EUV) and soft x-ray (SXR) telescopes rarely approach
diffraction-limited performance because conventional reflective optics
of adequate size typically cannot be manufactured to the requisite
figure accuracy. Diffractive optics can overcome the angular-resolution
limitations of EUV/SXR mirrors. We describe a mission approach that
employs diffractive optics and small satellites flying in formation
to form a distributed solar telescope operating at EUV wavelengths.
---------------------------------------------------------
Title: Study of Type III Radio Bursts in Nanoflares
Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M.;
Gary, Dale E.
2019shin.confE..12C Altcode:
The heating mechanisms responsible for the million-degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence of reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate in
solving this problem [Klimchuk J.A., 2015 and references therein]. <P
/>Whether nanoflares accelerate energetic particles like full-sized
flares is unknown. The lack of strong emission in hard X-rays suggests
that the quantity of highly energetic particles is small. There could,
however, be large numbers of mildly energetic particles ( 10 keV). We
investigate such particles by searching for the type III radio bursts
that they may produce. If energetic electron beams propagating along
magnetic field lines generate a bump-on-tail instability, they will
produce Langmuir waves, which can then interact with other particles
and waves to give rise to emission at the local plasma frequency and
its first harmonic. Type III bursts are characteristically known
to exhibit high frequency drifts as the beam propagates through a
density gradient. The time-lag technique that was developed to study
subtle delays in light curves from different EUV channels [Viall &
Klimchuk 2012] can also be used to detect subtle delays at different
radio frequencies. We have modeled the expected radio emission from
nanoflares, which we used to test and calibrate the technique. We have
begun applying the technique to actual radio observations from VLA
(Very Large Array) and seeking data from MWA (Murchison Widefield Array)
as well. We also plan to use data from the PSP(Parker Solar Probe) to
look for similar reconnection signatures in the Solar Wind. Our goal is
to determine whether nanoflares accelerate energetic particles and to
determine their properties. The results will have important implications
for both the particle acceleration and reconnection physics."
---------------------------------------------------------
Title: Studying Coronal Heating with Data Driven Active Region
Modeling
Authors: Schonfeld, Samuel J.; Klimchuk, James
2019shin.confE.158S Altcode:
Successfully forecasting coronal emission requires an accurate
understanding of the mechanism(s) that heat the corona to temperatures
well above a million degrees Kelvin. We explore the relevance of a range
of coronal heating models by varying the physical parameters needed
to simulate EUV emission from an active region. We use GX Simulator
to generate synthetic images from non-linear force-free field (NLFFF)
magnetic field extrapolations that are filled with plasma using EBTEL
(Enthalpy-Based Thermal Evolution of Loops) hydrodynamic simulations
and compare these with AIA images. Modifying the heating function,
heating event recurrence time, and treatment of the transition region
allows us to quantify the effects of these parameters on the observed
coronal emission. We discuss the implications of these findings on
the nature of heating properties in the magnetically closed corona.
---------------------------------------------------------
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena throughout
the Universe
Authors: Ji, Hantao; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.;
Guo, F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte,
J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian,
A.; Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu,
W.; Longcope, D.; Louriero, N.; Lu, Q. -M.; Ma, Z. -W.; Matthaeus,
W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
2019BAAS...51c...5J Altcode: 2019astro2020T...5J
This is a group white paper of 100 authors (each with explicit
permission via email) from 51 institutions on the topic of magnetic
reconnection which is relevant to 6 thematic areas. Grand challenges
and research opportunities are described in observations, numerical
modeling and laboratory experiments in the upcoming decade.
---------------------------------------------------------
Title: ICSF: Intensity Conserving Spectral Fitting
Authors: Klimchuk, James A.; Patsourakos, Spiros; Tripathi, Durgesh
2019ascl.soft03007K Altcode:
ICSF (Intensity Conserving Spectral Fitting) "corrects" (x,y) data in
which the ordinate represents the average of a quantity over a finite
interval in the abscissa. A typical example is spectral data, where
the average intensity over a wavelength bin (the measured quantity)
is assigned to the center of the bin. If the profile is curved, the
average will be different from the discrete value at the bin center
location. ICSF, written in IDL and available separately and as part of
SolarSoft (ascl:1208.013), corrects the intensity using an iterative
procedure and cubic spline. The corrected intensity equals the "true"
intensity at bin center, rather than the average over the bin. Unlike
other methods that are restricted to a single fitting function,
typically a spline, ICSF can be used with any function, such as a
cubic spline or a Gaussian, with slight changes to the code.
---------------------------------------------------------
Title: Hard X-Ray Constraints on Small-scale Coronal Heating Events
Authors: Marsh, Andrew J.; Smith, David M.; Glesener, Lindsay;
Klimchuk, James A.; Bradshaw, Stephen J.; Vievering, Juliana; Hannah,
Iain G.; Christe, Steven; Ishikawa, Shin-nosuke; Krucker, Säm
2018ApJ...864....5M Altcode: 2018arXiv180802630M
Much evidence suggests that the solar corona is heated impulsively,
meaning that nanoflares may be ubiquitous in quiet and active regions
(ARs). Hard X-ray (HXR) observations with unprecedented sensitivity
>3 keV are now enabled by focusing instruments. We analyzed data
from the Focusing Optics X-ray Solar Imager (FOXSI) rocket and the
Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft to constrain
properties of AR nanoflares simulated by the EBTEL field-line-averaged
hydrodynamics code. We generated model X-ray spectra by computing
differential emission measures for homogeneous nanoflare sequences
with heating amplitudes H <SUB>0</SUB>, durations τ, delay times
between events t <SUB> N </SUB>, and filling factors f. The single
quiescent AR observed by FOXSI-2 on 2014 December 11 is well fit by
nanoflare sequences with heating amplitudes 0.02 erg cm<SUP>-3</SUP>
s<SUP>-1</SUP> <H <SUB>0</SUB> < 13 erg cm<SUP>-3</SUP>
s<SUP>-1</SUP> and a wide range of delay times and durations. We
exclude delays between events shorter than ∼900 s at the 90%
confidence level for this region. Three of five regions observed
by NuSTAR on 2014 November 1 are well fit by homogeneous nanoflare
models, while two regions with higher fluxes are not. Generally,
the NuSTAR count spectra are well fit by nanoflare sequences with
smaller heating amplitudes, shorter delays, and shorter durations than
the allowed FOXSI-2 models. These apparent discrepancies are likely
due to differences in spectral coverage between the two instruments
and intrinsic differences among the regions. Steady heating (t <SUB>
N </SUB> = τ) was ruled out with >99% confidence for all regions
observed by either instrument.
---------------------------------------------------------
Title: Shifting and broadening of coronal spectral lines by nanoflare
heating
Authors: López Fuentes, M.; Klimchuk, J. A.
2018BAAA...60..207L Altcode:
The heating of the solar corona by nanoflares is one of the most
succesful theories in recent times to explain the wide variety of
available observations. In a recent work we developed and analyzed
a model based on coronal loops formed by elemental magnetic strands
that interact with each other, reconnecting and consequently heating
the plasma. Each of these heating events (or cluster of linked events)
is considered a nanoflare. In a later article we showed that the model
reproduces the main statistical characteristics of observed coronal
loop lightcurves and the typical emission measure distributions obtained
from observations. One of the predictions of the model is the presence
of intense plasma flows in the heated strands. The summed radiative
emission of strands at different temperatures and velocities produce
spectral lines of coronal ions with characteristic broadenings and
Doppler shifts. In this work, we study the contribution to emission
of flows with different temperatures and velocities obtained with our
model and construct synthetic spectral lines that we plan to compare
in the future with observational results.
---------------------------------------------------------
Title: Study of Type III Radio Bursts in Nanoflares
Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M.
2018shin.confE..18C Altcode:
The heating mechanisms responsible for the million-degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence of reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate
in solving this problem [Klimchuk J.A., 2015 and references therein].
---------------------------------------------------------
Title: Power-Law Statistics of Driven Reconnection in the Magnetically
Closed Corona
Authors: Knizhnik, Kalman Joshua; Uritsky, Vadim M.; Klimchuk, James
A.; DeVore, C. Richard
2018tess.conf21164K Altcode:
Numerous observations have revealed that power-law distributions
are ubiquitous in energetic solar processes. Hard X-rays, soft
X-rays, extreme ultraviolet radiation, and radio waves all display
power-law frequency distributions. Since magnetic reconnection is
the driving mechanism for many energetic solar phenomena, it is
likely that reconnection events themselves display such power-law
distributions. In this work, we perform numerical simulations of the
solar corona driven by simple convective motions at the photospheric
level. Using temperature changes, current distributions, and Poynting
fluxes as proxies for heating, we demonstrate that energetic events
occurring in our simulation display power-law frequency distributions,
with slopes in good agreement with observations. We suggest that
the braiding-associated reconnection in the corona can be understood
in terms of a self-organized criticality model driven by convective
rotational motions similar to those observed at the photosphere.
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection in the Solar Corona
Authors: Leake, James Edward; Daldorff, Lars K. S.; Klimchuk, James
A.; Knizhnik, Kalman Joshua
2018tess.conf10418L Altcode:
Understanding why the release of magnetic energy on the Sun only
occurs after substantial stresses have been built up is a fundamental
question. We investigate the onset of magnetic reconnection in
the corona, initially focusing on the linear growth and non-linear
interaction of both parallel and oblique tearing modes in 3D sheared
coronal current sheets. We then study the critical parameters of
current sheet thickness and magnetic field angle. We also investigate
the release of magnetic energy in the corona by magnetic reconnection
driven by realistic surface motions.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener,
Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel C.; Chen,
Bin; Battaglia, Marina; Drake, James Frederick; Gary, Dale E.; Goetz,
Keith; Gburek, Szymon; Grefenstette, Brian; Hannah, Iain G.; Holman,
Gordon; Hudson, Hugh S.; Inglis, Andrew R.; Ireland, Jack; Ishikawa,
Shin-nosuke; Klimchuk, James A.; Kontar, Eduard; Kowalski, Adam F.;
Massone, Anna Maria; Piana, Michele; Ramsey, Brian; Schwartz, Richard;
Steslicki, Marek; Ryan, Daniel; Warmuth, Alexander; Veronig, Astrid;
Vilmer, Nicole; White, Stephen M.; Woods, Thomas N.
2018tess.conf40444C Altcode:
We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer
(SMEX) Heliophysics mission that is currently undergoing a Phase A
concept study. FOXSI will provide a revolutionary new perspective on
energy release and particle acceleration on the Sun. FOXSI's primary
instrument, the Direct Spectroscopic Imager (DSI), is a direct imaging
X-ray spectrometer with higher dynamic range and better than 10x the
sensitivity of previous instruments. Flown on a 3-axis-stabilized
spacecraft in low-Earth orbit, DSI uses high-angular-resolution
grazing-incidence focusing optics combined with state-of-the-art
pixelated solid-state detectors to provide direct imaging of solar hard
X-rays for the first time. DSI is composed of a pair of X-ray telescopes
with a 14-meter focal length enabled by a deployable boom. DSI has a
field of view of 9 arcminutes and an angular resolution of better than 8
arcsec FWHM; it will cover the energy range from 3 up to 50-70 keV with
a spectral resolution of better than 1 keV. DSI will measure each photon
individually and will be able to create useful images at a sub-second
temporal resolution. FOXSI will also measure soft x-ray emission down
to 0.8 keV with a 0.25 keV resolution with its secondary instrument,
the Spectrometer for Temperature and Composition (STC) provided by
the Polish Academy of Sciences. Making use of an attenuator-wheel and
high-rate-capable detectors, FOXSI will be able to observe the largest
flares without saturation while still maintaining the sensitivity to
detect X-ray emission from weak flares, escaping electrons, and hot
active regions. This presentation will cover the data products and
software that can be expected from FOXSI and how they could be used
by the community.
---------------------------------------------------------
Title: The Case for Spectroscopic Observations of Very Hot Plasmas
Authors: Klimchuk, James A.; Daldorff, Lars K. S.; Liu, Yi-Hsin;
Brosius, Jeffrey W.; Daw, Adrian Nigel; Leake, James Edward
2018tess.conf11003K Altcode:
Explosive magnetic energy release is responsible for many solar
phenomena, ranging from coronal heating, to jets, to CMEs and
flares. Despite its importance, many of the basic details of how this
works are still not well established. In order to make significant
progress, we must observe the actual energy release process. That means
measuring plasmas at high temperatures (> 5 MK). Most existing
coronal observations are of plasma that has cooled dramatically,
after vital information has been lost, or, worse yet, plasma that has
evaporated from the chromosphere and is only an indirect bi-product of
the energy release. Spectroscopic observations are especially valuable
because of their potential for diagnosing flows, temperatures, and
densities. Furthermore, only with spectroscopy can we disentangle the
disparate plasmas that are invariably present along optically-thin
lines-of-sight. I will discuss these points in more detail, review the
pros and cons of different wavelength regimes, and present preliminary
results on synthetic line profiles from a particle-in-cell (PIC)
simulation of magnetic reconnection.
---------------------------------------------------------
Title: The Role of Asymmetries in Thermal Non-Equilibrium
Authors: Klimchuk, James A.; Luna Bennasar, Manuel
2018tess.conf22205K Altcode:
Thermal non-equilibrium (TNE) is a fascinating situation that applies
when quasi-steady coronal heating, including impulsive heating with a
high repetition frequency, is concentrated at sufficiently low altitudes
in the corona. No equilibrium exists, and as the atmosphere "searches"
for one, it undergoes convulsions that typically involve the formation
of a cold condensation. This is the likely explanation of prominences
and coronal rain. Under the right conditions, a condensation does not
fully form, and it has been suggested that such conditions may be
rather common. Asymmetries in heating or flux tube cross-sectional
area can have a major impact on the behavior. If the asymmetries
are large enough, a steady flow develops rather than TNE. We here
present an analytical formula for predicting steady flow vs. TNE,
and we compare it with 1D hydrodynamic simulations.
---------------------------------------------------------
Title: Magnetic Braids in Eruptions of a Spiral Structure in the
Solar Atmosphere
Authors: Huang, Zhenghua; Xia, Lidong; Nelson, Chris J.; Liu, Jiajia;
Wiegelmann, Thomas; Tian, Hui; Klimchuk, James A.; Chen, Yao; Li, Bo
2018ApJ...854...80H Altcode: 2018arXiv180105967H
We report on high-resolution imaging and spectral observations of
eruptions of a spiral structure in the transition region, which
were taken with the Interface Region Imaging Spectrograph, and the
Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The eruption
coincided with the appearance of two series of jets, with velocities
comparable to the Alfvén speeds in their footpoints. Several pieces of
evidence of magnetic braiding in the eruption are revealed, including
localized bright knots, multiple well-separated jet threads, transition
region explosive events, and the fact that all three of these are
falling into the same locations within the eruptive structures. Through
analysis of the extrapolated 3D magnetic field in the region, we found
that the eruptive spiral structure corresponded well to locations
of twisted magnetic flux tubes with varying curl values along their
lengths. The eruption occurred where strong parallel currents,
high squashing factors, and large twist numbers were obtained. The
electron number density of the eruptive structure is found to be ∼3 ×
10<SUP>12</SUP> cm<SUP>-3</SUP>, indicating that a significant amount
of mass could be pumped into the corona by the jets. Following the
eruption, the extrapolations revealed a set of seemingly relaxed loops,
which were visible in the AIA 94 Å channel, indicating temperatures
of around 6.3 MK. With these observations, we suggest that magnetic
braiding could be part of the mechanisms explaining the formation of
solar eruption and the mass and energy supplement to the corona.
---------------------------------------------------------
Title: Power-law Statistics of Driven Reconnection in the Magnetically
Closed Corona
Authors: Knizhnik, K. J.; Uritsky, V. M.; Klimchuk, J. A.; DeVore,
C. R.
2018ApJ...853...82K Altcode: 2018arXiv180105245K
Numerous observations have revealed that power-law distributions
are ubiquitous in energetic solar processes. Hard X-rays, soft
X-rays, extreme ultraviolet radiation, and radio waves all display
power-law frequency distributions. Since magnetic reconnection is
the driving mechanism for many energetic solar phenomena, it is
likely that reconnection events themselves display such power-law
distributions. In this work, we perform numerical simulations of the
solar corona driven by simple convective motions at the photospheric
level. Using temperature changes, current distributions, and Poynting
fluxes as proxies for heating, we demonstrate that energetic events
occurring in our simulation display power-law frequency distributions,
with slopes in good agreement with observations. We suggest that
the braiding-associated reconnection in the corona can be understood
in terms of a self-organized criticality model driven by convective
rotational motions similar to those observed at the photosphere.
---------------------------------------------------------
Title: Dressing the Coronal Magnetic Extrapolations of Active Regions
with a Parameterized Thermal Structure
Authors: Nita, Gelu M.; Viall, Nicholeen M.; Klimchuk, James A.;
Loukitcheva, Maria A.; Gary, Dale E.; Kuznetsov, Alexey A.; Fleishman,
Gregory D.
2018ApJ...853...66N Altcode:
The study of time-dependent solar active region (AR) morphology and
its relation to eruptive events requires analysis of imaging data
obtained in multiple wavelength domains with differing spatial and
time resolution, ideally in combination with 3D physical models. To
facilitate this goal, we have undertaken a major enhancement of our
IDL-based simulation tool, GX_Simulator, previously developed for
modeling microwave and X-ray emission from flaring loops, to allow it
to simulate quiescent emission from solar ARs. The framework includes
new tools for building the atmospheric model and enhanced routines
for calculating emission that include new wavelengths. In this paper,
we use our upgraded tool to model and analyze an AR and compare the
synthetic emission maps with observations. We conclude that the modeled
magneto-thermal structure is a reasonably good approximation of the
real one.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission
Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.;
Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.;
Drake, J. F.; Gary, D. E.; Goetz, K.; Gburek, S.; Grefenstette, B.;
Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland,
J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.;
Massone, A. M.; Piana, M.; Ramsey, B.; Schwartz, R.; Steslicki, M.;
Turin, P.; Ryan, D.; Warmuth, A.; Veronig, A.; Vilmer, N.; White,
S. M.; Woods, T. N.
2017AGUFMSH44A..07C Altcode:
We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer
(SMEX) Heliophysics mission that is currently undergoing a Phase A
concept study. FOXSI will provide a revolutionary new perspective
on energy release and particle acceleration on the Sun. FOXSI is
a direct imaging X-ray spectrometer with higher dynamic range and
better than 10x the sensitivity of previous instruments. Flown
on a 3-axis-stabilized spacecraft in low-Earth orbit, FOXSI uses
high-angular-resolution grazing-incidence focusing optics combined
with state-of-the-art pixelated solid-state detectors to provide direct
imaging of solar hard X-rays for the first time. FOXSI is composed of
a pair of x-ray telescopes with a 14-meter focal length enabled by a
deployable boom. Making use of a filter-wheel and high-rate-capable
solid-state detectors, FOXSI will be able to observe the largest flares
without saturation while still maintaining the sensitivity to detect
x-ray emission from weak flares, escaping electrons, and hot active
regions. This mission concept is made possible by past experience with
similar instruments on two FOXSI sounding rocket flights, in 2012 and
2014, and on the HEROES balloon flight in 2013. FOXSI's hard X-ray
imager has a field of view of 9 arcminutes and an angular resolution
of better than 8 arcsec; it will cover the energy range from 3 up to
50-70 keV with a spectral resolution of better than 1 keV; and it will
have sub-second temporal resolution.
---------------------------------------------------------
Title: Anticipated Results from the FOXSI SMEX Mission
Authors: Shih, A. Y.; Christe, S.; Krucker, S.; Glesener, L.;
Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.;
Drake, J. F.; Gary, D. E.; Gburek, S.; Goetz, K.; Grefenstette, B.;
Gubarev, M.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.;
Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski,
A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Ryan, D.; Schwartz,
R.; Steslicki, M.; Turin, P.; Veronig, A.; Vilmer, N.; Warmuth, A.;
White, S. M.; Woods, T. N.
2017AGUFMSH43C..03S Altcode:
While there have been significant advances in our understanding
of impulsive energy release at the Sun since the advent of RHESSI
observations, there is a clear need for new X-ray observations that
can capture the full range of emission in flares (e.g., faint coronal
sources near bright chromospheric sources), follow the intricate
evolution of energy release and changes in morphology, and search
for the signatures of impulsive energy release in even the quiescent
Sun. The FOXSI Small Explorer (SMEX) mission, currently undergoing a
Phase A concept study, combines state-of-the-art grazing-incidence
focusing optics with pixelated solid-state detectors to provide
direct imaging of hard X-rays for the first time on a solar
observatory. FOXSI's X-ray observations will provide quantitative
information on (1) the non-thermal populations of accelerated electrons
and (2) the thermal plasma distributions at the high temperatures
inaccessible through other wavelengths. FOXSI's major science questions
include: Where are electrons accelerated and on what time scales? Where
do escaping flare-accelerated electrons originate? What is the energy
input of accelerated electrons into the chromosphere and corona? How
much do flare-like processes heat the corona above active regions? Here
we present examples with simulated observations to show how FOXSI's
capabilities will address and resolve these and other questions.
---------------------------------------------------------
Title: Conditions for Thermal Non-Equilibrium
Authors: Klimchuk, J. A.; Luna Bennasar, M.
2017AGUFMSH43A2798K Altcode:
Thermal non-equilibrium (TNE) is a fascinating phenomenon in which
perfectly steady heating produces inherently dynamic behavior in coronal
loops. Sometimes, but not always, a condensation of cold ( 10,000 K)
material is produced. This is the likely explanation of coronal rain,
prominences, and "pulsating loops," and has been suggested to have
additional application in active regions and even helmet streamers. A
primary requirement of TNE is that steady heating must be concentrated
at low altitudes in the corona. Another requirement is that asymmetries
in the heating and/or loop cross sectional area cannot be too great. We
present theoretical formulas for predicting whether TNE will occur and
whether the associated condensations will be full or incomplete. The
predictions are compared against hydrodynamic simulations.
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection
Authors: Daldorff, L. K. S.; Klimchuk, J. A.; Leake, J. E.; Knizhnik,
K. J.
2017AGUFMSH11B2454D Altcode:
A fundamental question concerning the release of the magnetic energy
on the Sun is why it occurs only after substantial stresses have built
up in the field. If reconnection were to occur readily, the released
energy would be insufficient to explain coronal heating, CMEs, flares,
jets, spicules, etc. How can we explain this switch-on property? What
is the physical nature of the onset conditions? One idea involves the
“secondary instability” of current sheets, which switches on when
the rotation of the magnetic field across a current sheet reaches
a critical angle. Such conditions would occur, for example, at the
boundaries of flux tubes that become tangled and twisted by turbulent
photospheric convection. Other ideas involve a critical thickness for
the current sheet. Our ultimate goal is to simulate the evolution of a
current sheet that is driven by shear flows at the photopheric boundary
and extends upward through the chromosphere and transition region and
into the corona. We report here on the status of our investigation
into this fundamental problem.
---------------------------------------------------------
Title: Nanoflare Heating: Observations and Theory
Authors: Klimchuk, James A.
2017arXiv170907320K Altcode:
This is a review of the observational and theoretical evidence for
nanoflare heating of the magnetically-closed corona.
---------------------------------------------------------
Title: Current Sheet Proliferation, Turbulence, and the Heating of
the Magnetically-Closed Corona
Authors: Klimchuk, James A.; Antiochos, Spiro K.
2017SPD....4830302K Altcode:
Electric current sheets in the solar corona are essential to many
theories of coronal heating and activity. They can form by a number
of mechanisms. The magnetic field is known to be very clumpy in the
photosphere, with approximately 100,000 elemental flux tubes in a
single active region. Convection causes the tubes to become twisted and
tangled, with current sheets forming unavoidably at their boundaries in
the corona. Partial reconnections of the tubes as well as a patchiness
of the reconnection process lead to a multiplication of the number of
distinct sheets. Quasi-ideal instabilities, such as kinking, multiply
the numbers even more. We conclude, therefore, that there will be a
proliferation of current sheets in the corona. An important question
is whether large-scale (active region size) models of the corona
need to take this complexity into account to successfully predict
the distribution of plasma and the resulting radiation. We discuss
the picture of current sheet proliferation and compare and contrast
it to MHD turbulence. We also discuss the implication of our results
for coronal observations.
---------------------------------------------------------
Title: Diagnosing Coronal Heating in a Survey of Active Regions
using the Time Lag Method
Authors: Viall, Nicholeen; Klimchuk, James A.
2017SPD....4840202V Altcode:
In this paper we examine 15 different active regions observed with the
Solar Dynamics Observatory and analyze their nanoflare properties using
the time lag method. The time lag method is a diagnostic of whether
the plasma is maintained at a steady temperature, or if it is dynamic,
undergoing heating and cooling cycles. An important aspect of our
technique is that it analyses both observationally distinct coronal
loops as well as the much more prevalent diffuse emission surrounding
them. Warren et al. (2012) first studied these same 15 active regions,
which are all quiescent and exhibit a broad range of characteristics,
including age, total unsigned magnetic flux, area, hot emission, and
emission measure distribution. We find that widespread cooling is a
generic property of both loop and diffuse emission from all 15 active
regions. However, the range of temperatures through which the plasma
cools varies between active regions and within each active region,
and only occasionally is there full cooling from above 7 MK to well
below 1 MK. We find that the degree of cooling is not well correlated
with slopes of the emission measure distribution measured by Warren et
al. (2012). We show that these apparently contradictory observations
can be reconciled with the presence of a distribution of nanoflare
energies and frequencies along the line of sight, with the average
delay between successive nanoflare events on a single flux tube being
comparable to the plasma cooling timescale. Warren, H. P., Winebarger,
A. R., & Brooks, D. H. 2012, ApJ, 759, 141
---------------------------------------------------------
Title: Constraints on Nonuniform Expansion in Coronal Loops
Authors: Kucera, Therese A.; DeForest, Craig; Klimchuk, James A.;
Young, Peter R.
2017SPD....4810608K Altcode:
We use measurements of coronal loop properties to constrain the
hypothesis that coronal loops expand differently in different
directions. A long standing problem in understanding coronal loops is
that although the magnetic field is expected to expand with altitude
and does indeed seem to do so on scales of active regions, individual
loops seem to have fairly uniform diameters along the length of the
loop. Malanushenko & Schrijver (2013) have suggested that loops
may be expanding, but with a non-circular cross section. In this
scenario a loop might have a constant width in the plane of the sky,
but expand along the line of sight. Furthermore, such loops might be
easier to see from the point of view that does not show expansion. We
use Hinode/EIS and SDO/AIA data to measure loop intensities, electron
densities, temperatures and dimensions in order to determine the extent
to which loops may be expanding along the line of sight.
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection: Tearing Instability in
Current Sheets with a Guide Field
Authors: Daldorff, Lars K. S.; Klimchuk, James A.; Leake, James E.;
Knizhnik, Kalman
2017SPD....4810616D Altcode:
Magnetic reconnection is fundamental to many solar phenomena, ranging
from coronal heating, to jets, to flares and CMEs. A poorly understood
yet crucial aspect of reconnection is that it does not occur until
magnetic stresses have built to sufficiently high levels for significant
energy release. If reconnection were to happen too soon, coronal heating
would be weak and flares would be small. As part of our program to study
the onset conditions for magnetic reconnection, we have investigated
the instability of current sheets to tearing. Surprisingly little work
has been done on this problem for sheets that include a guide field,
i.e., for which the field rotates by less than 180 degrees. This is the
most common situation on the Sun. We present numerical 3D resistive MHD
simulations of several sheets and show how the behavior depends on the
shear angle (rotation). We compare our results to the predictions of
linear theory and discuss the nonlinear evolution in terms of plasmoid
formation and the interaction of different oblique tearing modes. The
relevance to the Sun is explained.
---------------------------------------------------------
Title: Hard X-Ray Constraints on Small-Scale Coronal Heating Events
Authors: Marsh, Andrew; Smith, David M.; Glesener, Lindsay; Klimchuk,
James A.; Bradshaw, Stephen; Hannah, Iain; Vievering, Juliana;
Ishikawa, Shin-Nosuke; Krucker, Sam; Christe, Steven
2017SPD....4810614M Altcode:
A large body of evidence suggests that the solar corona is heated
impulsively. Small-scale heating events known as nanoflares may be
ubiquitous in quiet and active regions of the Sun. Hard X-ray (HXR)
observations with unprecedented sensitivity >3 keV have recently been
enabled through the use of focusing optics. We analyze active region
spectra from the FOXSI-2 sounding rocket and the NuSTAR satellite to
constrain the physical properties of nanoflares simulated with the
EBTEL field-line-averaged hydrodynamics code. We model a wide range
of X-ray spectra by varying the nanoflare heating amplitude, duration,
delay time, and filling factor. Additional constraints on the nanoflare
parameter space are determined from energy constraints and EUV/SXR data.
---------------------------------------------------------
Title: A Survey of Nanoflare Properties in Active Regions Observed
with the Solar Dynamics Observatory
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2017ApJ...842..108V Altcode:
In this paper, we examine 15 different active regions (ARs) observed
with the Solar Dynamics Observatory and analyze their nanoflare
properties. We have recently developed a technique that systematically
identifies and measures plasma temperature dynamics by computing time
lags between light curves. The time lag method tests whether the
plasma is maintained at a steady temperature, or if it is dynamic,
undergoing heating and cooling cycles. An important aspect of our
technique is that it analyzes both observationally distinct coronal
loops as well as the much more prevalent diffuse emission between
them. We find that the widespread cooling reported previously for NOAA
AR 11082 is a generic property of all ARs. The results are consistent
with impulsive nanoflare heating followed by slower cooling. Only
occasionally, however, is there full cooling from above 7 MK to well
below 1 MK. More often, the plasma cools to approximately 1-2 MK
before being reheated by another nanoflare. These same 15 ARs were
first studied by Warren et al. We find that the degree of cooling is
not well correlated with the reported slopes of the emission measure
distribution. We also conclude that the Fe xviii emitting plasma that
they measured is mostly in a state of cooling. These results support the
idea that nanoflares have a distribution of energies and frequencies,
with the average delay between successive events on an individual flux
tube being comparable to the plasma cooling timescale.
---------------------------------------------------------
Title: Exploring impulsive solar magnetic energy release and particle
acceleration with focused hard X-ray imaging spectroscopy
Authors: Christe, Steven; Krucker, Samuel; Glesener, Lindsay; Shih,
Albert; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel; Battaglia,
Marina; Chen, Bin; Drake, James; Dennis, Brian; Gary, Dale; Gburek,
Szymon; Goetz, Keith; Grefenstette, Brian; Gubarev, Mikhail; Hannah,
Iain; Holman, Gordon; Hudson, Hugh; Inglis, Andrew; Ireland, Jack;
Ishikawa, Shinosuke; Klimchuk, James; Kontar, Eduard; Kowalski, Adam;
Longcope, Dana; Massone, Anna-Maria; Musset, Sophie; Piana, Michele;
Ramsey, Brian; Ryan, Daniel; Schwartz, Richard; Stęślicki, Marek;
Turin, Paul; Warmuth, Alexander; Wilson-Hodge, Colleen; White, Stephen;
Veronig, Astrid; Vilmer, Nicole; Woods, Tom
2017arXiv170100792C Altcode:
How impulsive magnetic energy release leads to solar eruptions and how
those eruptions are energized and evolve are vital unsolved problems
in Heliophysics. The standard model for solar eruptions summarizes
our current understanding of these events. Magnetic energy in the
corona is released through drastic restructuring of the magnetic
field via reconnection. Electrons and ions are then accelerated by
poorly understood processes. Theories include contracting loops,
merging magnetic islands, stochastic acceleration, and turbulence at
shocks, among others. Although this basic model is well established,
the fundamental physics is poorly understood. HXR observations
using grazing-incidence focusing optics can now probe all of the key
regions of the standard model. These include two above-the-looptop
(ALT) sources which bookend the reconnection region and are likely
the sites of particle acceleration and direct heating. The science
achievable by a direct HXR imaging instrument can be summarized by the
following science questions and objectives which are some of the most
outstanding issues in solar physics (1) How are particles accelerated
at the Sun? (1a) Where are electrons accelerated and on what time
scales? (1b) What fraction of electrons is accelerated out of the
ambient medium? (2) How does magnetic energy release on the Sun lead
to flares and eruptions? A Focusing Optics X-ray Solar Imager (FOXSI)
instrument, which can be built now using proven technology and at modest
cost, would enable revolutionary advancements in our understanding of
impulsive magnetic energy release and particle acceleration, a process
which is known to occur at the Sun but also throughout the Universe.
---------------------------------------------------------
Title: Solving the Coronal Heating Problem using X-ray
Microcalorimeters
Authors: Christe, Steven; Bandler, Simon; DeLuca, Edward; Caspi,
Amir; Golub, Leon; Smith, Randall; Allred, Joel; Brosius, Jeffrey W.;
Dennis, Brian; Klimchuk, James
2017arXiv170100795C Altcode:
Even in the absence of resolved flares, the corona is heated to several
million degrees. However, despite its importance for the structure,
dynamics, and evolution of the solar atmosphere, the origin of this
heating remains poorly understood. Several observational and theoretical
considerations suggest that the heating is driven by small, impulsive
energy bursts which could be Parker-style "nanoflares" (Parker 1988)
that arise via reconnection within the tangled and twisted coronal
magnetic field. The classical "smoking gun" (Klimchuk 2009; Cargill et
al. 2013) for impulsive heating is the direct detection of widespread
hot plasma (T > 6 MK) with a low emission measure. In recent years
there has been great progress in the development of Transition Edge
Sensor (TES) X-ray microcalorimeters that make them more ideal for
studying the Sun. When combined with grazing-incidence focusing optics,
they provide direct spectroscopic imaging over a broad energy band
(0.5 to 10 keV) combined with extremely impressive energy resolution
in small pixels, as low as 0.7 eV (FWHM) at 1.5 keV (Lee 2015),
and 1.56 eV (FWHM) at 6 keV (Smith 2012), two orders of magnitude
better than the current best traditional solid state photon-counting
spectrometers. Decisive observations of the hot plasma associated
with nanoflare models of coronal heating can be provided by new solar
microcalorimeters. These measurements will cover the most important part
of the coronal spectrum for searching for the nanoflare-related hot
plasma and will characterize how much nanoflares can heat the corona
both in active regions and the quiet Sun. Finally, microcalorimeters
will enable to study all of this as a function of time and space in
each pixel simultaneously a capability never before available.
---------------------------------------------------------
Title: Unravelling the Components of a Multi-thermal Coronal Loop
using Magnetohydrodynamic Seismology
Authors: Krishna Prasad, S.; Jess, D. B.; Klimchuk, J. A.; Banerjee, D.
2017ApJ...834..103K Altcode: 2016arXiv161104011K; 2017ApJ...834..103P
Coronal loops, constituting the basic building blocks of the active Sun,
serve as primary targets to help understand the mechanisms responsible
for maintaining multi-million Kelvin temperatures in the solar and
stellar coronae. Despite significant advances in observations and
theory, our knowledge on the fundamental properties of these structures
is limited. Here, we present unprecedented observations of accelerating
slow magnetoacoustic waves along a coronal loop that show differential
propagation speeds in two distinct temperature channels, revealing
the multi-stranded and multithermal nature of the loop. Utilizing
the observed speeds and employing nonlinear force-free magnetic field
extrapolations, we derive the actual temperature variation along the
loop in both channels, and thus are able to resolve two individual
components of the multithermal loop for the first time. The obtained
positive temperature gradients indicate uniform heating along the loop,
rather than isolated footpoint heating.
---------------------------------------------------------
Title: Hard X-ray Detectability of Small-Scale Coronal Heating Events
Authors: Marsh, A.; Glesener, L.; Klimchuk, J. A.; Bradshaw, S. J.;
Smith, D. M.; Hannah, I. G.
2016AGUFMSH11D..06M Altcode:
The nanoflare heating theory predicts the ubiquitous presence
of hot ( >5 MK) plasma in the solar corona, but evidence for
this high-temperature component has been scarce. Current hard x-ray
instruments such as RHESSI lack the sensitivity to see the trace amounts
of this plasma that are predicted by theoretical models. New hard X-ray
instruments that use focusing optics, such as FOXSI (the Focusing Optics
X-ray Solar Imager) and NuSTAR (the Nuclear Spectroscopic Telescope
Array) can extend the visible parameter space of nanoflare "storms"
that create hot plasma. We compare active-region data from FOXSI and
NuSTAR with a series of EBTEL hydrodynamic simulations, and constrain
nanoflare properties to give good agreement with observations.
---------------------------------------------------------
Title: Turbulence, Current Sheet Proliferation, and the Heating of
the Magnetically-Closed Corona
Authors: Klimchuk, J. A.; Antiochos, S. K.; Dahlburg, R. B.
2016AGUFMSH33A..03K Altcode:
Turbulence plays an important role in heating and accelerating the
solar wind, and it has been proposed to also be important in heating
active regions and the quiet Sun. These regions are fundamentally
different from the sources of the solar wind, however, in that they are
magnetically closed and have a small plasma beta. We suggest that the
strong, line-tied magnetic field resists being distorted and inhibits
turbulence from developing. To test this idea, we performed a 3D MHD
simulation representing a solar active region being driven by slow
photospheric motions. The conditions said to be necessary for turbulence
are met, yet the system evolves quasi-statically up to the point where
a kink instability occurs. We conclude that the magnetically-closed
corona is not turbulent in the classical sense. There is no inertial
range of spatial scales where energy flows without dissipation through
a continuum of eddies. Rather, there is a quasi-static evolution that
is interrupted by localized and temporary bursts of turbulent behavior
associated with the tearing and reconnection of current sheets. Because
of a proliferation of current sheets, these episodes are widespread and
frequent, with many occurring at the same time within a single active
region. This picture is fundamentally different from MHD turbulence,
despite some similarities. In addition to the lack of an inertial
range, the amount of heating is not independent of the details of
the dissipation. On the contrary, it depends critically on the onset
conditions for tearing and reconnection.
---------------------------------------------------------
Title: Focusing Solar Hard X-rays: Expected Results from a FOXSI
Spacecraft
Authors: Glesener, L.; Christe, S.; Shih, A. Y.; Dennis, B. R.;
Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Ryan, D.; Inglis,
A. R.; Hannah, I. G.; Caspi, A.; Klimchuk, J. A.; Drake, J. F.;
Kontar, E.; Holman, G.; White, S. M.; Alaoui, M.; Battaglia, M.;
Vilmer, N.; Allred, J. C.; Longcope, D. W.; Gary, D. E.; Jeffrey,
N. L. S.; Musset, S.; Swisdak, M.
2016AGUFMSH13A2282G Altcode:
Over the course of two solar cycles, RHESSI has examined high-energy
processes in flares via high-resolution spectroscopy and imaging of
soft and hard X-rays (HXRs). The detected X-rays are the thermal
and nonthermal bremsstrahlung from heated coronal plasma and from
accelerated electrons, respectively, making them uniquely suited to
explore the highest-energy processes that occur in the corona. RHESSI
produces images using an indirect, Fourier-based method and has made
giant strides in our understanding of these processes, but it has also
uncovered intriguing new mysteries regarding energy release location,
acceleration mechanisms, and energy propagation in flares. Focusing
optics are now available for the HXR regime and stand poised to perform
another revolution in the field of high-energy solar physics. With
two successful sounding rocket flights completed, the Focusing Optics
X-ray Solar Imager (FOXSI) program has demonstrated the feasibility and
power of direct solar HXR imaging with its vastly superior sensitivity
and dynamic range. Placing this mature technology aboard a spacecraft
will offer a systematic way to explore high-energy aspects of the
solar corona and to address scientific questions left unanswered by
RHESSI. Here we present examples of such questions and show simulations
of expected results from a FOXSI spaceborne instrument to demonstrate
how these questions can be addressed with the focusing of hard X-rays.
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection: Tearing Instability in
Current Sheets with a Guide Field
Authors: Daldorff, L. K. S.; Klimchuk, J. A.; Knizhnik, K. J.
2016AGUFMSH51B2590D Altcode:
Magnetic reconnection is fundamental to many solar phenomena, ranging
from coronal heating, to jets, to flares and CMEs. A poorly understood
yet crucial aspect of reconnection is that it does not occur until
magnetic stresses have built to sufficiently high levels for significant
energy release. If reconnection were to happen too soon, coronal heating
would be weak and flares would be small. As part of our program to study
the onset conditions for magnetic reconnection, we have investigated
the instability of current sheets to tearing. Surprisingly little work
has been done on this problem for sheets that include a guide field,
i.e., for which the field rotates by less than 180 degrees. This is the
most common situation on the Sun. We present numerical 3D resistive MHD
simulations of several sheets and show how the behaviour depends on the
shear angle (rotation). We compare our results to the predictions of
linear theory and discuss the nonlinear evolution in terms of plasmoid
formation and the interaction of different oblique tearing modes. The
relevance to the Sun is explained.
---------------------------------------------------------
Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission
Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.;
Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen,
B.; Drake, J. F.; Gary, D. E.; Goetz, K.; Grefenstette, B.; Hannah,
I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa,
S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.;
Piana, M.; Ramsey, B.; Gubarev, M.; Schwartz, R. A.; Steslicki, M.;
Ryan, D.; Turin, P.; Warmuth, A.; White, S. M.; Veronig, A.; Vilmer,
N.; Dennis, B. R.
2016AGUFMSH13A2281C Altcode:
We present FOXSI (Focusing Optics X-ray Solar Imager), a recently
proposed Small Explorer (SMEX) mission that will provide a revolutionary
new perspective on energy release and particle acceleration on the
Sun. FOXSI is a direct imaging X-ray spectrometer with higher dynamic
range and better than 10x the sensitivity of previous instruments. Flown
on a 3-axis stabilized spacecraft in low-Earth orbit, FOXSI uses
high-angular-resolution grazing-incidence focusing optics combined
with state-of-the-art pixelated solid-state detectors to provide direct
imaging of solar hard X-rays for the first time. FOXSI is composed of
two individual x-ray telescopes with a 14-meter focal length enabled by
a deployable boom. Making use of a filter-wheel and high-rate-capable
solid-state detectors, FOXSI will be able to observe the largest flares
without saturation while still maintaining the sensitivity to detect
x-ray emission from weak flares, escaping electrons, and hot active
regions. This SMEX mission is made possible by past experience with
similar instruments on two sounding rocket flights, in 2012 and 2014,
and on the HEROES balloon flight in 2013. FOXSI will image the Sun
with a field of view of 9 arcminutes and an angular resolution of
better than 8 arcsec; it will cover the energy range from 3 to 100
keV with a spectral resolution of better than 1 keV; and it will have
sub-second temporal resolution.
---------------------------------------------------------
Title: The FIELDS Instrument Suite for Solar Probe Plus. Measuring
the Coronal Plasma and Magnetic Field, Plasma Waves and Turbulence,
and Radio Signatures of Solar Transients
Authors: Bale, S. D.; Goetz, K.; Harvey, P. R.; Turin, P.; Bonnell,
J. W.; Dudok de Wit, T.; Ergun, R. E.; MacDowall, R. J.; Pulupa,
M.; Andre, M.; Bolton, M.; Bougeret, J. -L.; Bowen, T. A.; Burgess,
D.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen,
C. H. K.; Choi, M. K.; Connerney, J. E.; Cranmer, S.; Diaz-Aguado, M.;
Donakowski, W.; Drake, J. F.; Farrell, W. M.; Fergeau, P.; Fermin, J.;
Fischer, J.; Fox, N.; Glaser, D.; Goldstein, M.; Gordon, D.; Hanson,
E.; Harris, S. E.; Hayes, L. M.; Hinze, J. J.; Hollweg, J. V.; Horbury,
T. S.; Howard, R. A.; Hoxie, V.; Jannet, G.; Karlsson, M.; Kasper,
J. C.; Kellogg, P. J.; Kien, M.; Klimchuk, J. A.; Krasnoselskikh,
V. V.; Krucker, S.; Lynch, J. J.; Maksimovic, M.; Malaspina, D. M.;
Marker, S.; Martin, P.; Martinez-Oliveros, J.; McCauley, J.; McComas,
D. J.; McDonald, T.; Meyer-Vernet, N.; Moncuquet, M.; Monson, S. J.;
Mozer, F. S.; Murphy, S. D.; Odom, J.; Oliverson, R.; Olson, J.;
Parker, E. N.; Pankow, D.; Phan, T.; Quataert, E.; Quinn, T.; Ruplin,
S. W.; Salem, C.; Seitz, D.; Sheppard, D. A.; Siy, A.; Stevens, K.;
Summers, D.; Szabo, A.; Timofeeva, M.; Vaivads, A.; Velli, M.; Yehle,
A.; Werthimer, D.; Wygant, J. R.
2016SSRv..204...49B Altcode: 2016SSRv..tmp...16B
NASA's Solar Probe Plus (SPP) mission will make the first in situ
measurements of the solar corona and the birthplace of the solar
wind. The FIELDS instrument suite on SPP will make direct measurements
of electric and magnetic fields, the properties of in situ plasma waves,
electron density and temperature profiles, and interplanetary radio
emissions, amongst other things. Here, we describe the scientific
objectives targeted by the SPP/FIELDS instrument, the instrument
design itself, and the instrument concept of operations and planned
data products.
---------------------------------------------------------
Title: Towards a Physics-Based Flare Irradiance Model
Authors: Hock-Mysliwiec, R. A.; Klimchuk, J. A.; Eparvier, F. G.;
Woods, T. N.; Balasubramaniam, K. S.
2016usc..confE..46H Altcode:
The Extreme UltraViolet (EUV) irradiance from solar flares is a critical
driver of short term variability in the Earth's upper atmosphere. The
EUV Variability Experiment (EVE) onboard NASA's Solar Dynamics
Observatory (SDO) has been making moderate spectral resolution (0.1
nm), high time cadence (10 s) measurements of the solar EUV irradiance
(5-105 nm) since 2010. A key observation from EVE is that flares of the
same magnitude at one wavelength (e.g. GOES XRS) have different peak
intensities and time profiles in other wavelengths. As it is impractical
to measure the entire EUV spectrum with sufficient spectral resolution
and temporal cadence to capture these differences for space weather
operations, the next generation of flare irradiance models must be
able to capture these variations. We have developed a framework for
a physics-based flare irradiance model based on the EBTEL model. At
present, this Multi-Strand Flare Irradiance Model (MS-FIM) is able to
predict EUV lightcurves over a range of coronal temperatures given the
lightcurves from two EVE lines as inputs. In this paper, we present an
overview of the Multi-Strand Flare Irradiance Model as well as initial
results showing its ability to predict the irradiances for a diverse
range of flares, including EUV late phase flares. We also describe
preliminary efforts to drive the model with parameters derived from
images of the flaring region instead of EUV lightcurves.
---------------------------------------------------------
Title: Signatures of Steady Heating in Time Lag Analysis of Coronal
Emission
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2016ApJ...828...76V Altcode: 2016arXiv160702008V
Among the multitude of methods used to investigate coronal heating,
the time lag method of Viall & Klimchuk is becoming increasingly
prevalent as an analysis technique that is complementary to those
that are traditionally used. The time lag method cross correlates
light curves at a given spatial location obtained in spectral bands
that sample different temperature plasmas. It has been used most
extensively with data from the Atmospheric Imaging Assembly on the
Solar Dynamics Observatory. We have previously applied the time lag
method to entire active regions and surrounding the quiet Sun and
created maps of the results. We find that the majority of time lags
are consistent with the cooling of coronal plasma that has been
impulsively heated. Additionally, a significant fraction of the
map area has a time lag of zero. This does not indicate a lack of
variability. Rather, strong variability must be present, and it must
occur in phase between the different channels. We have previously
shown that these zero time lags are consistent with the transition
region response to coronal nanoflares, although other explanations are
possible. A common misconception is that the zero time lag indicates
steady emission resulting from steady heating. Using simulated and
observed light curves, we demonstrate here that highly correlated
light curves at zero time lag are not compatible with equilibrium
solutions. Such light curves can only be created by evolution.
---------------------------------------------------------
Title: A Nanoflare-based Cellular Automaton Model and the Observed
Properties of the Coronal Plasma
Authors: López Fuentes, Marcelo; Klimchuk, James A.
2016ApJ...828...86L Altcode: 2016arXiv160703917L
We use the cellular automaton model described in López Fuentes &
Klimchuk to study the evolution of coronal loop plasmas. The model,
based on the idea of a critical misalignment angle in tangled magnetic
fields, produces nanoflares of varying frequency with respect to the
plasma cooling time. We compare the results of the model with active
region (AR) observations obtained with the Hinode/XRT and SDO/AIA
instruments. The comparison is based on the statistical properties
of synthetic and observed loop light curves. Our results show that
the model reproduces the main observational characteristics of the
evolution of the plasma in AR coronal loops. The typical intensity
fluctuations have amplitudes of 10%-15% both for the model and the
observations. The sign of the skewness of the intensity distributions
indicates the presence of cooling plasma in the loops. We also study
the emission measure (EM) distribution predicted by the model and obtain
slopes in log(EM) versus log(T) between 2.7 and 4.3, in agreement with
published observational values.
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection: Tearing Instability in
Current Sheets with a Guide Field
Authors: Daldorff, Lars K. S.; Klimchuk, James A.
2016shin.confE.110D Altcode:
Magnetic reconnection is fundamental to many solar phenomena, ranging
from coronal heating o jets, to flares and CMEs. A poorly understood yet
crucial aspect of reconnection is that it does not occur until magnetic
stresses have built to sufficiently high levels for significant energy
release. If reconnection were to happen too soon, coronal heating would
be weak and flares would be small. As part of our program to study
the onset conditions for magnetic reconnection, we have investigated
the instability of current sheets to tearing. Surprisingly little work
has been done on this problem for sheets that include a guide field,
i.e., for which the field rotates by less than 180 degrees. This is the
most common situation on the Sun. We present numerical 3D resistive MHD
simulations of several sheets and show how the behaviour depends on the
shear angle (rotation). We compare our results to the predictions of
linear theory and discuss the nonlinear evolution in terms of plasmoid
formation and the interaction of different oblique tearing modes. The
relevance to the Sun is explained.
---------------------------------------------------------
Title: Hard X-ray Detectability of Small-Scale Coronal Heating Events
Authors: Marsh, Andrew; Glesener, Lindsay; Klimchuk, James A.;
Bradshaw, Stephen; Smith, David; Hannah, Iain
2016SPD....4720204M Altcode:
The nanoflare heating theory predicts the ubiquitous presence
of hot (~>5 MK) plasma in the solar corona, but evidence for
this high-temperature component has been scarce. Current hard x-ray
instruments such as RHESSI lack the sensitivity to see the trace amounts
of this plasma that are predicted by theoretical models. New hard X-ray
instruments that use focusing optics, such as FOXSI (the Focusing
Optics X-ray Solar Imager) and NuSTAR (the Nuclear Spectroscopic
Telescope Array) can extend the visible parameter space of nanoflare
“storms” that create hot plasma. We compare active-region data from
FOXSI and NuSTAR with a series of EBTEL hydrodynamic simulations, and
constrain nanoflare properties to give good agreement with observations.
---------------------------------------------------------
Title: The Transition Region Response to a Coronal Nanoflare:
Forward Modeling and Observations in SDO/AIA
Authors: Viall, Nicholeen; Klimchuk, James A.
2016SPD....4720202V Altcode:
The corona and transition region (TR) are fundamentally coupled
through the processes of thermal conduction and mass exchange. Yet
the temperature-dependent emissions from the two locations behave
quite differently in the aftermath of an impulsive heating event such
as a coronal nanoflare. In this presentation, we use results from the
EBTEL hydrodynamics code to demonstrate that after a coronal nanoflare,
the TR is multithermal and the emission at all temperatures responds
in unison. This is in contrast to the coronal plasma, which cools
sequentially, emitting first at higher temperatures and then at lower
temperatures. We apply the time lag technique of Viall & Klimchuk
(2012) to the simulated Atmospheric Imaging Assembly (AIA) on the
Solar Dynamics Observatory emission and show that coronal plasma light
curves exhibit post-nanoflare cooling time lags, while TR light curves
show time lags of zero, as observed. We further demonstrate that time
lags of zero, regardless of physical cause, do not indicate a lack of
variability. Rather, strong variability must be present, and it must
occur in unison in the different channels. Lastly, we show that the
'coronal' channels in AIA can be dominated by bright TR emission. When
defined in a physically meaningful way, the TR reaches a temperature
of roughly 60% the peak temperature in a flux tube. The TR resulting
from impulsive heating can extend to 3 MK and higher, well within the
range of the 'coronal' AIA channels.
---------------------------------------------------------
Title: Science Objectives of the FOXSI Small Explorer Mission Concept
Authors: Shih, Albert Y.; Christe, Steven; Alaoui, Meriem; Allred,
Joel C.; Antiochos, Spiro K.; Battaglia, Marina; Buitrago-Casas,
Juan Camilo; Caspi, Amir; Dennis, Brian R.; Drake, James; Fleishman,
Gregory D.; Gary, Dale E.; Glesener, Lindsay; Grefenstette, Brian;
Hannah, Iain; Holman, Gordon D.; Hudson, Hugh S.; Inglis, Andrew R.;
Ireland, Jack; Ishikawa, Shin-Nosuke; Jeffrey, Natasha; Klimchuk, James
A.; Kontar, Eduard; Krucker, Sam; Longcope, Dana; Musset, Sophie; Nita,
Gelu M.; Ramsey, Brian; Ryan, Daniel; Saint-Hilaire, Pascal; Schwartz,
Richard A.; Vilmer, Nicole; White, Stephen M.; Wilson-Hodge, Colleen
2016SPD....47.0814S Altcode:
Impulsive particle acceleration and plasma heating at the Sun, from the
largest solar eruptive events to the smallest flares, are related to
fundamental processes throughout the Universe. While there have been
significant advances in our understanding of impulsive energy release
since the advent of RHESSI observations, there is a clear need for
new X-ray observations that can capture the full range of emission
in flares (e.g., faint coronal sources near bright chromospheric
sources), follow the intricate evolution of energy release and changes
in morphology, and search for the signatures of impulsive energy
release in even the quiescent Sun. The FOXSI Small Explorer (SMEX)
mission concept combines state-of-the-art grazing-incidence focusing
optics with pixelated solid-state detectors to provide direct imaging
of hard X-rays for the first time on a solar observatory. We present
the science objectives of FOXSI and how its capabilities will address
and resolve open questions regarding impulsive energy release at the
Sun. These questions include: What are the time scales of the processes
that accelerate electrons? How do flare-accelerated electrons escape
into the heliosphere? What is the energy input of accelerated electrons
into the chromosphere, and how is super-heated coronal plasma produced?
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection: Tearing Instability in
Current Sheets with a Guide Field
Authors: Daldorff, Lars K. S.; Klimchuk, James A.
2016SPD....4740207D Altcode:
Magnetic reconnection is fundamental to many solar phenomena, ranging
from coronal heating, to jets, to flares and CMEs. A poorly understood
yet crucial aspect of reconnection is that it does not occur until
magnetic stresses have built to sufficiently high levels for significant
energy release. If reconnection were to happen too soon, coronal heating
would be weak and flares would be small. As part of our program to study
the onset conditions for magnetic reconnection, we have investigated
the instability of current sheets to tearing. Surprisingly little work
has been done on this problem for sheets that include a guide field,
i.e., for which the field rotates by less than 180 degrees. This is the
most common situation on the Sun. We present numerical 3D resistive MHD
simulations of several sheets and show how the behaviour depends on the
shear angle (rotation). We compare our results to the predictions of
linear theory and discuss the nonlinear evolution in terms of plasmoid
formation and the interaction of different oblique tearing modes. The
relevance to the Sun is explained.
---------------------------------------------------------
Title: Comparing Loop Cross Sections Observed with Hi-C and AIA/SDO
Authors: Klimchuk, James A.; DeForest, Craig
2016SPD....47.0301K Altcode:
Many studies have reported coronal loop widths measured with AIA/SDO,
TRACE, and other data. For warm loops (T ~ 1 MK), the characteristic
diameter is about 1500 km. Sub-structure is likely to exist on smaller
scales, but the envelope of the "strands" has this typical size. Since
1500 km (2 arcsec) is not large compared to the spatial resolution of
the observations, there remained a question about whether the loops
are actually much thinner. To address this concern, we have measured
the widths of several loops observed at 193 A by both AIA and the Hi-C
rocket experiment. Hi-C has 3-6 times better spatial resolution, so if
the loops are substantially unresolved by AIA, it should be readily
apparent. We find that the measured widths are very similar. Small
differences (< 25%) are explainable by uncertainties in the point
spread functions. We conclude that previous measurements of loop
widths made by AIA and TRACE are essentially correct. We also find
little evidence for loop sub-structure at the resolution of Hi-C. The
individual strands that comprise loops are therefore smaller than 200
km. These results have important implications for coronal heating.
---------------------------------------------------------
Title: Intensity Conserving Spectral Fitting
Authors: Klimchuk, J. A.; Patsourakos, S.; Tripathi, D.
2016SoPh..291...55K Altcode: 2015SoPh..tmp..180K
The detailed shapes of spectral-line profiles provide valuable
information about the emitting plasma, especially when the plasma
contains an unresolved mixture of velocities, temperatures, and
densities. As a result of finite spectral resolution, the intensity
measured by a spectrometer is the average intensity across a wavelength
bin of non-zero size. It is assigned to the wavelength position at
the center of the bin. However, the actual intensity at that discrete
position will be different if the profile is curved, as it invariably
is. Standard fitting routines (spline, Gaussian, etc.) do not account
for this difference, and this can result in significant errors when
making sensitive measurements. We have developed an iterative procedure
that corrects for this effect. It converges rapidly and is very
flexible in that it can be used with any fitting function. We present
examples of cubic-spline and Gaussian fits and give special attention
to measurements of blue-red asymmetries of coronal emission lines.
---------------------------------------------------------
Title: Reconnection Between Twisted Flux Tubes - Implications for
Coronal Heating
Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R.; Klimchuk,
J. A.; Wyper, P. F.
2015AGUFMSH13B2439K Altcode:
The nature of the heating of the Sun's corona has been a long-standing
unanswered problem in solar physics. Beginning with the work of Parker
(1972), many authors have argued that the corona is continuously heated
through numerous small-scale reconnection events known as nanoflares. In
these nanoflare models, stressing of magnetic flux tubes by photospheric
motions causes the field to become misaligned, producing current sheets
in the corona. These current sheets then reconnect, converting the
free energy stored in the magnetic field into heat. In this work,
we use the Adaptively Refined MHD Solver (ARMS) to perform 3D MHD
simulations that dynamically resolve regions of strong current to study
the reconnection between twisted flux tubes in a plane-parallel Parker
configuration. We investigate the energetics of the process, and show
that the flux tubes accumulate stress gradually before undergoing
impulsive reconnection. We study the motion of the individual field
lines during reconnection, and demonstrate that the connectivity of the
configuration becomes extremely complex, with multiple current sheets
being formed, which could lead to enhanced heating. In addition, we
show that there is considerable interaction between the twisted flux
tubes and the surrounding untwisted field, which contributes further
to the formation of current sheets. The implications for observations
will be discussed. This work was funded by a NASA Earth and Space
Science Fellowship, and by the NASA TR&T Program.
---------------------------------------------------------
Title: Hard X-ray Detectability of Small Impulsive Heating Events
in the Solar Corona
Authors: Glesener, L.; Klimchuk, J. A.; Bradshaw, S. J.; Marsh, A.;
Krucker, S.; Christe, S.
2015AGUFMSH13B2440G Altcode:
Impulsive heating events ("nanoflares") are a candidate to supply the
solar corona with its ~2 MK temperature. These transient events can
be studied using extreme ultraviolet and soft X-ray observations,
among others. However, the impulsive events may occur in tenuous
loops on small enough timescales that the heating is essentially not
observed due to ionization timescales, and only the cooling phase is
observed. Bremsstrahlung hard X-rays could serve as a more direct and
prompt indicator of transient heating events. A hard X-ray spacecraft
based on the direct-focusing technology pioneered by the Focusing
Optics X-ray Solar Imager (FOXSI) sounding rocket could search for these
direct signatures. In this work, we use the hydrodynamical EBTEL code to
simulate differential emission measures produced by individual heating
events and by ensembles of such events. We then directly predict hard
X-ray spectra and consider their observability by a future spaceborne
FOXSI, and also by the RHESSI and NuSTAR spacecraft.
---------------------------------------------------------
Title: Nanoflare Heating of the Quiet Sun
Authors: Viall, N. M.; Klimchuk, J. A.
2015AGUFMSH31D..05V Altcode:
How the solar corona is heated to temperatures of over 1 MK, while
the photosphere below is only ~ 6000 K remains one of the outstanding
problems in all of space science. Solving this problem is crucial for
understanding Sun-Earth connections, and will provide new insight into
universal processes such as magnetic reconnection and wave-particle
interactions. We use a systematic technique to analyze the properties
of coronal heating throughout the solar corona using data taken
with the Atmospheric Imaging Assembly onboard the Solar Dynamics
Observatory. Our technique computes cooling times of the coronal plasma
on a pixel-by-pixel basis and has the advantage that it analyzes all
of the coronal emission, including the diffuse emission surrounding
distinguishable coronal features. We have already applied this technique
to 15 different active regions, and find clear evidence for dynamic
heating and cooling cycles that are consistent with the 'impulsive
nanoflare' scenario. What about the rest of the Solar corona? Whether
the quiet Sun is heated in a similar or distinct manner from active
regions is a matter of great debate. Here we apply our coronal heating
analysis technique to quiet Sun locations. We find areas of quiet
Sun locations that also undergo dynamic heating and cooling cycles,
consistent with impulsive nanoflares. However, there are important
characteristics that are distinct from those of active regions.
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection
Authors: Daldorff, L. K. S.; Klimchuk, J. A.
2015AGUFMSH13B2437D Altcode:
A fundamental question concerning magnetic energy release on the Sun is
why the release occurs only after substantial stresses have been built
up in the field. If reconnection were to occur readily, the released
energy would be insufficient to explain coronal heating, CMEs, flares,
jets, spicules, etc. How can we explain this switch-on property? What
is the physical nature of the onset conditions? One idea involves the
"secondary instability" of current sheets, which switches on when
the rotation of the magnetic field across a current sheet reaches
a critical angle. Such conditions would occur at the boundaries of
flux tubes that become tangled and twisted by turbulent photospheric
convection, for example. Other ideas involve a critical thickness
for the current sheet. We report here on the preliminary results of
our investigation of reconnect onset. Unlike our earlier work on the
secondary instability (Dahlburg, Klimchuk, and Antiochos 2005), we
treat the coupled chromosphere-corona system. Using the BATS-R-US MHD
code, we simulate a single current sheet in a sheared magnetic field
that extends from the chromosphere into the corona. Driver motions are
applied at the base of the model. The configuration and chromosphere are
both idealized, but capture the essential physics of the problem. The
advantage of this unique approach is that it resolves the current sheet
to the greatest extent possible while maintaining a realistic solar
atmosphere. It thus bridges the gap between"reconnection in a box"
studies and studies of large-scale systems such as active regions. One
question we will address is whether onset conditions are met first
in the chromosphere or corona. We will report on the work done on
the project.
---------------------------------------------------------
Title: The Details of Coronal Heating Matter!
Authors: Klimchuk, J. A.; Daldorff, L. K. S.
2015AGUFMSH13B2438K Altcode:
Understanding how the magnetically-closed corona is heated remains one
of the most important goals in Heliophysics. It is generally believed
that much or most of the heating involves the conversion of energy
that is stored in stressed magnetic fields. What makes the problem so
challenging is that the conversion process involves very small spatial
scales. Simulations of, e.g., solar active regions cannot resolve the
thin current sheets that separate the approximately 100,000 elemental
magnetic flux strands that comprise a real active region. Heating
in the simulations primarily takes the form of Ohmic dissipation of
currents that are far less structured. Thus, the heating is only a
proxy for the real heating mechanism. How good a proxy is it? Does
it have the essential properties of the real mechanism? We suggest
that the details of coronal heating matter. They determine not only
the temporal behavior and spatial distribution of the heating, but
also the total amount of energy release. We present some idealized
MHD simulations that demonstrate this last point.
---------------------------------------------------------
Title: Capabilities of a FOXSI Small Explorer
Authors: Inglis, A. R.; Christe, S.; Glesener, L.; Krucker, S.; Dennis,
B. R.; Shih, A.; Wilson-Hodge, C.; Gubarev, M.; Hudson, H. S.; Kontar,
E.; Buitrago Casas, J. C.; Drake, J. F.; Caspi, A.; Holman, G.; Allred,
J. C.; Ryan, D.; Alaoui, M.; White, S. M.; Saint-Hilaire, P.; Klimchuk,
J. A.; Hannah, I. G.; Antiochos, S. K.; Grefenstette, B.; Ramsey,
B.; Jeffrey, N. L. S.; Reep, J. W.; Schwartz, R. A.; Ireland, J.
2015AGUFMSH43B2456I Altcode:
We present the FOXSI (Focusing Optics X-ray Solar Imager) small explorer
(SMEX) concept, a mission dedicated to studying particle acceleration
and energy release on the Sun. FOXSI is designed as a 3-axis stabilized
spacecraft in low-Earth orbit making use of state-of-the-art grazing
incidence focusing optics, allowing for direct imaging of solar
X-rays. The current design being studied features three telescope
modules deployed in a low-inclination low-earth orbit (LEO). With a 15
meter focal length enabled by a deployable boom, FOXSI will observe
the Sun in the 3-50 keV energe range. The FOXSI imaging concept has
already been tested on two sounding rocket flights, in 2012 and 2014
and on the HEROES balloon payload flight in 2013. FOXSI will image
the Sun with an angular resolution of 5”, a spectral resolution of
0.5 keV, and sub-second temporal resolution using CdTe detectors. In
this presentation we investigate the science objectives and targets
which can be accessed from this mission. Because of the defining
characteristic of FOXSI is true imaging spectroscopy with high dynamic
range and sensitivity, a brand-new perspective on energy release on the
Sun is possible. Some of the science targets discussed here include;
flare particle acceleration processes, electron beams, return currents,
sources of solar energetic particles (SEPs), as well as understanding
X-ray emission from active region structures and the quiescent corona.
---------------------------------------------------------
Title: How Gas-dynamic Flare Models Powered by Petschek Reconnection
Differ from Those with Ad Hoc Energy Sources
Authors: Longcope, D. W.; Klimchuk, J. A.
2015ApJ...813..131L Altcode: 2015arXiv151005985L
Aspects of solar flare dynamics, such as chromospheric evaporation
and flare light curves, have long been studied using one-dimensional
models of plasma dynamics inside a static flare loop, subjected
to some energy input. While extremely successful at explaining
the observed characteristics of flares, all such models so
far have specified energy input ad hoc, rather than deriving it
self-consistently. There is broad consensus that flares are powered
by magnetic energy released through reconnection. Recent work has
generalized Petschek’s basic reconnection scenario, topological
change followed by field line retraction and shock heating, to permit
its inclusion in a one-dimensional flare loop model. Here we compare
the gas dynamics driven by retraction and shocking to those from more
conventional static loop models energized by ad hoc source terms. We
find significant differences during the first minute, when retraction
leads to larger kinetic energies and produces higher densities at
the loop top, while ad hoc heating tends to rarify the loop top. The
loop-top density concentration is related to the slow magnetosonic
shock, characteristic of Petschek’s model, but persists beyond
the retraction phase occurring in the outflow jet. This offers an
explanation for observed loop-top sources of X-ray and EUV emission,
with advantages over that provided by ad hoc heating scenarios. The
cooling phases of the two models are, however, notably similar to one
another, suggesting that observations at that stage will yield little
information on the nature of energy input.
---------------------------------------------------------
Title: Chromospheric Nanoflares as a Source of Coronal
Plasma. II. Repeating Nanoflares
Authors: Bradshaw, S. J.; Klimchuk, J. A.
2015ApJ...811..129B Altcode: 2016arXiv160306673B
The million degree plasma of the solar corona must be supplied by
the underlying layers of the atmosphere. The mechanism and location
of energy release, and the precise source of coronal plasma, remain
unresolved. In earlier work, we pursued the idea that warm plasma
is supplied to the corona via direct heating of the chromosphere by
nanoflares, contrary to the prevailing belief that the corona is heated
in situ and the chromosphere is subsequently energized and ablated by
thermal conduction. We found that single (low-frequency) chromospheric
nanoflares could not explain the observed intensities, Doppler-shifts,
and red/blue asymmetries in Fe xii and xiv emission lines. In the
present work, we follow up on another suggestion that the corona could
be powered by chromospheric nanoflares that repeat on a timescale
substantially shorter than the cooling/draining timescale. That is,
a single magnetic strand is re-supplied with coronal plasma before
the existing plasma has time to cool and drain. We perform a series
of hydrodynamic experiments and predict the Fe xii and xiv line
intensities, Doppler-shifts, and red/blue asymmetries. We find that
our predicted quantities disagree dramatically with observations and
fully developed loop structures cannot be created by intermediate- or
high-frequency chromospheric nanoflares. We conclude that the mechanism
ultimately responsible for producing coronal plasma operates above the
chromosphere, but this does not preclude the possibility of a similar
mechanism powering the chromosphere, extreme examples of which may
be responsible for heating chromospheric plasma to transition region
temperatures (e.g., type II spicules).
---------------------------------------------------------
Title: Loop observations and the coronal heating problem
Authors: López Fuentes, M. C.; Klimchuk, J. A.
2015BAAA...57..231L Altcode:
Coronal heating continues to be one of the fundamental problems of solar
physics. In recent years, instrumental advances and the availability of
data from space observatories produced important progress, imposing
restrictions to the models proposed. However, since the physical
processes occur at spatial scales below the present instrumental
resolution, definitive answers are still due. Since the corona is
strongly dominated by the magnetic field, active region plasma is
confined in closed structures or loops. These are the basic observable
blocks of the corona, so the analysis of their structure and evolution
is essential to understand the heating. In this report, mainly addressed
to astronomers not necessarily familiarized with the subject, we review
some of the proposed heating models and we pay special attention to
the sometimes confusing and apparently contradictory observations of
coronal loops. We discuss the consequences of these observations for
some of the heating models proposed, in particular those based on
impulsive events known as nanoflares.
---------------------------------------------------------
Title: Division II: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Scrijver, Karel J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
2015IAUTB..28..106V Altcode:
The Business Meeting of Commission 10 was held as part of the Business
Meeting of Division II (Sun and Heliosphere), chaired by Valentin
Martínez-Pillet, the President of the Division. The President of
Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took
the chair for the business meeting of C10. She summarised the activities
of C10 over the triennium and the election of the incoming OC.
---------------------------------------------------------
Title: Helicity Condensation During Reconnection of Twisted Flux
Tubes: Implications for Coronal Heating
Authors: Knizhnik, Kalman Joshua; Antiochos, Spiro K.; DeVore,
C. Richard; Klimchuk, James A.; Wyper, Peter F.
2015shin.confE..18K Altcode:
The nature of the heating of the Sun's corona has been a long-standing
unanswered problem in solar physics. Beginning with the work of Parker
(1972), many authors have argued that the corona is continuously
heated through numerous small-scale reconnection events known as
nanoflares. In these nanoflare models, braiding of magnetic flux tubes
by surface motions causes the field to become misaligned. The current
sheet separating the misaligned field eventually reconnects, converting
the energy stored in the magnetic field into heat. A major challenge
facing these models, however, is that the braiding required for this
process injects magnetic helicity into the corona, and helicity is
conserved under reconnection. In contrast, EUV and X-ray images of
coronal loops reveal invariably smooth, laminar structures. The
recently proposed helicity condensation model (Antiochos 2013)
resolves this difficulty, explaining how reconnection transports
helicity throughout the solar atmosphere and produces a smooth,
hot corona. In this model, reconnection between adjacent flux tubes,
twisted and tangled by surface convection, transports helicity to ever
larger scales, where it ultimately condenses in filament channels. The
reconnection that occurs throughout the solar atmosphere not only
results in a smooth corona, but its net effect is to convert much of
the magnetic energy injected by surface motions into heat. In this
work, we use the Adaptively Refined MHD Solver (ARMS) to perform 3D MHD
simulations that dynamically resolve regions of strong current to study
the reconnection between multiple twisted flux tubes in a plane-parallel
Parker configuration. We investigate the energetics of the process, and
show that the flux tubes accumulate stress gradually before undergoing
impulsive reconnection. We place constraints on the amount of heating
expected from such reconnection. Finally, we study the motion of the
individual field lines during the impulsive reconnection events.
---------------------------------------------------------
Title: How gas-dynamic flare models powered by Petschek reconnection
differ from those with ad hoc energy sources
Authors: Longcope, Dana; Klimchuk, Jim
2015shin.confE...9L Altcode:
Aspects of solar flare dynamics, such as chromospheric evaporation
and flare light-curves, have long been studied using one-dimensional
models of plasma dynamics inside a static flare loop, subjected to some
energy input. While extremely successful at explaining the observed
characteristics of flares, all such models so far have specified energy
input ad hoc, rather than deriving it self-consistently. There is broad
consensus that flares are powered by magnetic energy released through
reconnection. Recent work has generalized Petschek's basic reconnection
scenario, topological change followed by field line retraction and
shock heating, to permit its inclusion into a one-dimensional flare
loop model. Here we compare the gas dynamics driven by retraction and
shocking to those from more conventional static loop models energized
by ad hoc source terms. We find significant differences during the
first minute, when retraction leads to larger kinetic energies and
produces higher densities at the loop top, while ad hoc heating tends
to rarify the loop top. The loop-top density concentration is related
to the slow magnetosonic shock, characteristic of Petschek's model,
but persists beyond the retraction phase occurring in the outflow
jet. This offers an explanation of observed loop-top sources of X-ray
and EUV emission, with advantages over that provided by ad hoc heating
scenarios. The cooling phases of the two models are, however, notably
similar to one another, suggesting observations at that stage will
yield little information on the nature of energy input.
---------------------------------------------------------
Title: The Myth of Long Duration Flare Emission: Slow Heating or
Slow Cooling?
Authors: Qiu, Jiong; Longcope, Dana; Klimchuk, James A.
2015TESS....130214Q Altcode:
Long duration flare emissions lasting for a few hours are likely
governed by magnetic reconnection that continuously heats flare plasmas
in continuously formed flare loops. In this study, we confirm that
this process leads to the long-duration total emission for up to four
hours in a C2.9 flare on 2011 September 13. Observed by AIA, the flare
exhibits an ordered spread of flare UV ribbons along the polarity
inversion line, followed by the sequential formation of post-flare
loops in EUV emissions. We infer heating rates of thousands of flare
loops from the UV light curves at the flare foot-points, and model
the flare total emission with the 0d EBTEL model, which reproduces the
global evolution pattern of the long-duration flare EUV emissions as
the result of superposition of continuously formed and heated flare
loops. However, observations at single loop pixels also show long
duration EUV emission at 10 MK, long cooling time from 10 MK to 3 MK,
and later on very short duration of EUV emission at 1-2 MK. All of these
signatures cannot be produced by superposition of multiple impulsive
heating events. Our experiments, with both the 0d EBTEL model and a
1d hydrodynamic model, have demonstrated that a heating profile in
a single loop consisting of two parts, an intense impulsive heating
followed by a low-rate heating 1-2 orders of magnitude smaller that
is attenuated over 20-30 minutes, is required to produce the observed
time evolution signatures in a single loop. The total energy in the
gradual heating phase is comparable with that in the impulsive heating
phase in a flare loop. We discuss viable physical mechanisms for such
two-phase heating in a post-reconnection flare loop.
---------------------------------------------------------
Title: How gas-dynamic flare models powered by Petschek reconnection
differ from thosewith ad hoc energy sources
Authors: Longcope, Dana; Klimchuk, James A.
2015TESS....130212L Altcode:
Many aspects of solar flare dynamics including chromospheric evaporation
have been, for more than thirty years, studied using one-dimensional
models of static flaring loops. These models solve one-dimensional
gas-dynamic equations for the dynamics of plasma inside a static
loop, subjected to energy input through either non-thermal particles
or heating. While they have been extremely successful at explaining
the characteristics of emission observed in flares, none so far have
been developed in which the energy input is derived self-consistently
from the loop's dynamics. Instead the energy input is specified ad
hoc. According to another line of theoretical investigation, flares
derive their energy from magnetic energy released through fast magnetic
reconnection. In the model due originally to Petschek reconnection
occurring in a small diffusion region produces a bent flux tube whose
retraction generates fast flows (an outflow jet) and shocks where flow
energy is thermalized. In a recent line of work this scenario has been
generalized so it may be incorporated into a one-dimensional loop model
of the kind used so successfully in flare modeling. In this new model
the flaring loop itself undergoes the retraction and shock formation,
and thereby introduces the flare energy self-consistently. Here we
compare the gas dynamics driven by retraction and shocking to those from
more conventional static loop models. We find significant differences
during the first minute, when retraction produces high densities at
the loop top, while ad hoc heating tends to rarify the loop top.
---------------------------------------------------------
Title: Key Aspects of Coronal Heating
Authors: Klimchuk, James A.
2015TESS....120308K Altcode:
We highlight ten key aspects of coronal heating that must be understood
before we can consider the problem to be solved. (1) All coronal
heating is impulsive. (2) The details of coronal heating matter. (3)
The corona is filled with elemental magnetic stands. (4) The corona is
densely populated with current sheets. (5) The strands must reconnect
to prevent an infinite buildup of stress. (6) Nanoflares repeat with
different frequencies. (7) What is the characteristic magnitude of
energy release? (8) What causes the collective behavior responsible
for loops? (9) What are the onset conditions for energy release? (10)
Chromospheric nanoflares are not a primary source of coronal
plasma. Significant progress in solving the coronal heating problem
will require a coordination of approaches: observational studies,
field-aligned hydrodynamic simulations, large-scale and localized
3D MHD simulations, and possibly also kinetic simulations. There is
a unique value to each of these approaches, and the community must
strive to coordinate better.
---------------------------------------------------------
Title: Key aspects of coronal heating
Authors: Klimchuk, James A.
2015RSPTA.37340256K Altcode: 2014arXiv1410.5660K
We highlight 10 key aspects of coronal heating that must be understood
before we can consider the problem to be solved. (1) All coronal
heating is impulsive. (2) The details of coronal heating matter. (3)
The corona is filled with elemental magnetic stands. (4) The corona
is densely populated with current sheets. (5) The strands must
reconnect to prevent an infinite build-up of stress. (6) Nanoflares
repeat with different frequencies. (7) What is the characteristic
magnitude of energy release? (8) What causes the collective behaviour
responsible for loops? (9) What are the onset conditions for energy
release? (10) Chromospheric nanoflares are not a primary source of
coronal plasma. Significant progress in solving the coronal heating
problem will require coordination of approaches: observational studies,
field-aligned hydrodynamic simulations, large-scale and localized
three-dimensional magnetohydrodynamic simulations, and possibly
also kinetic simulations. There is a unique value to each of these
approaches, and the community must strive to coordinate better.
---------------------------------------------------------
Title: Intensity Conserving Spline Interpolation (ICSI): A New Tool
for Spectroscopic Analysis
Authors: Klimchuk, James A.; Patsourakos, Spiros; Tripathi, Durgesh
2015TESS....120309K Altcode: 2015arXiv150608102K
Spectroscopy is an extremely powerful tool for diagnosing astrophysical
and other plasmas. For example, the shapes of line profiles provide
valuable information on the distribution of velocities along
an optically thin line-of-sight and across the finite area of a
resolution element. A number of recent studies have measured the
asymmetries of line profiles in order to detect faint high-speed
upflows, perhaps associated with coronal nanoflares or perhaps
associated with chromospheric nanoflares and type II spicules. Over
most of the Sun, these asymmetries are very subtle, so great care
must be taken. A common technique is to perform a spline fit of the
points in the profile in order to extract information at a spectral
resolution higher than that of the original data. However, a fundamental
problem is that the fits do not conserve intensity. We have therefore
developed an iterative procedure called Intensity Conserving Spline
Interpolation that does preserve the observed intensity within each
wavelength bin. It improves the measurement of line asymmetries and
can also help with the determination of line blends.
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection
Authors: Daldorff, Lars K. S.; Klimchuk, James A.; van der Holst, Bart
2015TESS....110404D Altcode:
A fundamental question concerning magnetic energy release on the Sun is
why the release occurs only after substantial stresses have been built
up in the field. If reconnection were to occur readily, the released
energy would be insufficient to explain coronal heating, CMEs, flares,
jets, spicules, etc. How can we explain this switch-on property? What
is the physical nature of the onset conditions? One idea involves the
"secondary instability" of current sheets, which switches on when
the rotation of the magnetic field across a current sheet reaches
a critical angle. Such conditions would occur at the boundaries of
flux tubes that become tangled and twisted by turbulent photospheric
convection, for example. Other ideas involve a critical thickness
for the current sheet. We report here on the preliminary results of
our investigation of reconnect onset. Unlike our earlier work on the
secondary instability (Dahlburg, Klimchuk, and Antiochos 2005), we
treat the coupled chromosphere-corona system. Using the BATS-R-US MHD
code, we simulate a single current sheet in a sheared magnetic field
that extends from the chromosphere into the corona. Driver motions are
applied at the base of the model. The configuration and chromosphere are
both idealized, but capture the essential physics of the problem. The
advantage of this unique approach is that it resolves the current sheet
to the greatest extent possible while maintaining a realistic solar
atmosphere. It thus bridges the gap between "reconnection in a box"
studies and studies of large-scale systems such as active regions. One
question we will address is whether onset conditions are met first
in the chromosphere or corona. We will report on the work done on
the project.
---------------------------------------------------------
Title: Nanoflare Heating of the Quiet Sun
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2015TESS....121303V Altcode:
How the solar corona is heated to temperatures of over 1 MK, while
the photosphere below is only ~ 6000 K remains one of the outstanding
problems in all of space science. Solving this problem is crucial for
understanding Sun-Earth connections, and will provide new insight into
universal processes such as magnetic reconnection and wave-particle
interactions. We use a new systematic technique to analyze the
properties of coronal heating throughout the solar corona using data
taken with the Atmospheric Imaging Assembly onboard the Solar Dynamics
Observatory. Our technique computes cooling times of the coronal plasma
on a pixel-by-pixel basis and has the advantage that it analyzes all
of the coronal emission, including the diffuse emission surrounding
distinguishable coronal features. We have already applied this technique
to 15 different active regions, and find clear evidence for dynamic
heating and cooling cycles that are consistent with the 'impulsive
nanoflare' scenario. What about the rest of the Solar corona? Whether
the quiet Sun is heated in a similar or distinct manner from active
regions is a matter of great debate. In this paper, we apply our coronal
heating analysis technique to quiet Sun locations. We find that the
majority of the analyzed quiet Sun locations do undergo dynamic heating
and cooling cycles, consistent with impulsive nanoflares. However, there
are important characteristics that are distinct from those of active
regions.This research was supported by a NASA Guest Investigator grant.
---------------------------------------------------------
Title: Synthetic 3D modeling of active regions and simulation of
their multi-wavelength emission
Authors: Nita, Gelu M.; Fleishman, Gregory; Kuznetsov, Alexey A.;
Loukitcheva, Maria A.; Viall, Nicholeen M.; Klimchuk, James A.; Gary,
Dale E.
2015TESS....131204N Altcode:
To facilitate the study of solar active regions, we have created a
synthetic modeling framework that combines 3D magnetic structures
obtained from magnetic extrapolations with simplified 1D thermal
models of the chromosphere, transition region, and corona. To handle,
visualize, and use such synthetic data cubes to compute multi-wavelength
emission maps and compare them with observations, we have
undertaken a major enhancement of our simulation tools, GX_Simulator
(ftp://sohoftp.nascom.nasa.gov/solarsoft/packages/gx_simulator/),
developed earlier for modeling emission from flaring loops. The greatly
enhanced, object-based architecture, which now runs on Windows, Mac,
and UNIX platform, offers important new capabilities that include the
ability to either import 3D density and temperature distribution models,
or to assign to each individual voxel numerically defined coronal
or chromospheric temperature and densities, or coronal Differential
Emission Measure distributions. Due to these new capabilities, the
GX_Simulator can now apply parametric heating models involving average
properties of the magnetic field lines crossing a given voxel volume,
as well as compute and investigate the spatial and spectral properties
of radio (to be compared with VLA or EOVSA data), (sub-)millimeter
(ALMA), EUV (AIA/SDO), and X-ray (RHESSI) emission calculated from the
model. The application integrates shared-object libraries containing
fast free-free, gyrosynchrotron, and gyroresonance emission codes
developed in FORTRAN and C++, and soft and hard X-ray and EUV codes
developed in IDL. We use this tool to model and analyze an active
region and compare the synthetic emission maps obtained in different
wavelengths with observations.This work was partially supported
by NSF grants AGS-1250374, AGS-1262772, NASA grant NNX14AC87G, the
Marie Curie International Research Staff Exchange Scheme "Radiosun"
(PEOPLE-2011-IRSES-295272), RFBR grants 14-02-91157, 15-02-01089,
15-02-03717, 15-02-03835, 15-02-08028.
---------------------------------------------------------
Title: Two-dimensional Cellular Automaton Model for the Evolution
of Active Region Coronal Plasmas
Authors: López Fuentes, Marcelo; Klimchuk, James A.
2015ApJ...799..128L Altcode: 2016arXiv160703883L
We study a two-dimensional cellular automaton (CA) model for the
evolution of coronal loop plasmas. The model is based on the idea that
coronal loops are made of elementary magnetic strands that are tangled
and stressed by the displacement of their footpoints by photospheric
motions. The magnetic stress accumulated between neighbor strands
is released in sudden reconnection events or nanoflares that heat
the plasma. We combine the CA model with the Enthalpy Based Thermal
Evolution of Loops model to compute the response of the plasma to
the heating events. Using the known response of the X-Ray Telescope
on board Hinode, we also obtain synthetic data. The model obeys
easy-to-understand scaling laws relating the output (nanoflare
energy, temperature, density, intensity) to the input parameters
(field strength, strand length, critical misalignment angle). The
nanoflares have a power-law distribution with a universal slope of
-2.5, independent of the input parameters. The repetition frequency of
nanoflares, expressed in terms of the plasma cooling time, increases
with strand length. We discuss the implications of our results for
the problem of heating and evolution of active region coronal plasmas.
---------------------------------------------------------
Title: The Transition Region Response to a Coronal Nanoflare:
Forward Modeling and Observations in SDO/AIA
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2015ApJ...799...58V Altcode:
The corona and transition region (TR) are fundamentally coupled
through the processes of thermal conduction and mass exchange. It
is not possible to understand one without the other. Yet the
temperature-dependent emissions from the two locations behave quite
differently in the aftermath of an impulsive heating event such as a
coronal nanoflare. Whereas the corona cools sequentially, emitting
first at higher temperatures and then at lower temperatures, the
TR is multithermal and the emission at all temperatures responds in
unison. We have previously applied the automated time lag technique
of Viall & Klimchuk to disk observations of an active region
(AR) made by the Atmospheric Imaging Assembly (AIA) on the Solar
Dynamics Observatory. Lines of sight passing through coronal plasma
show clear evidence for post-nanoflare cooling, while lines of sight
intersecting the TR footpoints of coronal strands show zero time lag. In
this paper, we use the EBTEL hydrodynamics code to demonstrate that
this is precisely the expected behavior when the corona is heated by
nanoflares. We also apply the time lag technique for the first time
to off-limb observations of an AR. Since TR emission is not present
above the limb, the occurrence of zero time lags is greatly diminished,
supporting the conclusion that zero time lags measured on the disk are
due to TR plasma. Lastly, we show that the ”coronal” channels in AIA
can be dominated by bright TR emission. When defined in a physically
meaningful way, the TR reaches a temperature of roughly 60% the peak
temperature in a flux tube. The TR resulting from impulsive heating
can extend to 3 MK and higher, well within the range of the ”coronal”
AIA channels.
---------------------------------------------------------
Title: Intensity Conserving Spline Interpolation (ICSI): A New Tool
for Spectroscopic Analysis
Authors: Klimchuk, J. A.; Patsourakos, S.; Tripathi, D.
2014AGUFMSH13B4109K Altcode:
Spectroscopy is an extremely powerful tool for diagnosing astrophysical
and other plasmas. For example, the shapes of line profiles provide
valuable information on the distribution of velocities along
an optically thin line-of-sight and across the finite area of a
resolution element. A number of recent studies have measured the
asymmetries of line profiles in order to detect faint high-speed
upflows, perhaps associated with coronal nanoflares or perhaps
associated with chromospheric nanoflares and type II spicules. Over
most of the Sun, these asymmetries are very subtle, so great care
must be taken. A common technique is to perform a spline fit of the
points in the profile in order to extract information at a spectral
resolution higher than that of the original data. However, a fundamental
problem is that the fits do not conserve intensity. We have therefore
developed an iterative procedure called Intensity Conserving Spline
Interpolation that does preserve the observed intensity within each
wavelength bin. It improves the measurement of line asymmetries and
can also help with the determination of line blends.
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection in the Solar Atmosphere
Authors: Evans, R. M.; Klimchuk, J. A.; van der Holst, B.
2014AGUFMSH12A..02E Altcode:
A fundamental question concerning magnetic energy release on the Sun
is why the release occurs only after substantial stresses have been
built up in the field. If reconnection were to occur readily, then
the released energy would be much less than the energy required for
coronal heating, CMEs, flares, jets, spicules, etc. How can we explain
this switch-on property? What is the physical nature of the onset
conditions? One idea involves the secondary instability of current
sheets, which switches on when the rotation of the magnetic field
across a current sheet reaches a critical angle. Such conditions would
occur at the boundaries of flux tubes that become tangled and twisted
by turbulent photospheric convection, for example. Other ideas focus
on a critical thickness for the current sheet. We report here on the
preliminary results of our investigation of reconnection onset. Unlike
our earlier work on the secondary instability (Dahlburg, Klimchuk,
and Antiochos 2005), here we treat the coupled chromosphere-corona
system. Using the BATS-R-US MHD code (Toth et al. 2012), we simulate a
single current sheet in a sheared magnetic field that extends from the
chromosphere into the corona. Driver motions are applied at the base
of the model. The configuration and chromosphere are both idealized,
but capture the essential physics of the problem. The advantage of this
unique approach is that it resolves the current sheet to the greatest
extent possible while maintaining a realistic solar atmosphere. It thus
bridges the gap between reconnection in a box studies and studies of
large-scale systems such as active regions. One question we will address
is whether onset conditions are met first in the chromosphere or corona.
---------------------------------------------------------
Title: Hi-C Observations and the Structure of Coronal Loops
Authors: DeForest, C. E.; Klimchuk, J. A.
2014AGUFMSH31C..04D Altcode:
Despite nearly four decades of study since the launch of Skylab, the
physical structure of coronal loops remains an enigma. Loops are guided
by the magnetic field and, in the common EUV emission lines, appear
to be composed of stranded structures reminiscent of field lines. This
stranded structure appears to have constant or nearly-constant width,
at odds with naive understanding of flux tube behavior in a field
gradient. Possible explanations range from physical solutions such as
twisted magnetic structure or peculiar properties of separators and
quasi-separators, to observation effects that invoke finite resolution
or anisotropy of the field containing each strand. The uncertainty
affects many aspects of basic coronal physics, because some of the
possible explanations for stranded structure have strong implications
for other mysteries such as the anomalously tall scale height of the
EUV corona. The Hi-C EUV images are the highest resolution coronal
images to date, and offer new insights into the structure of coronal
loops. We present an overview of research to date, show results from
a detailed analysis of several dozen well-presented loops that are
visible in the Hi-C data set, and speculate on the implications for
the rest of the corona.
---------------------------------------------------------
Title: Emission Measure Distribution for Diffuse Regions in Solar
Active Regions
Authors: Subramanian, Srividya; Tripathi, Durgesh; Klimchuk, James A.;
Mason, Helen E.
2014ApJ...795...76S Altcode: 2014arXiv1409.1447S
Our knowledge of the diffuse emission that encompasses active regions
is very limited. In this paper we investigate two off-limb active
regions, namely, AR 10939 and AR 10961, to probe the underlying heating
mechanisms. For this purpose, we have used spectral observations from
Hinode/EIS and employed the emission measure (EM) technique to obtain
the thermal structure of these diffuse regions. Our results show that
the characteristic EM distributions of the diffuse emission regions peak
at log T = 6.25 and the coolward slopes are in the range 1.4-3.3. This
suggests that both low- as well as high-frequency nanoflare heating
events are at work. Our results provide additional constraints on the
properties of these diffuse emission regions and their contribution to
the background/foreground when active region cores are observed on-disk.
---------------------------------------------------------
Title: Are Chromospheric Nanoflares a Primary Source of Coronal
Plasma?
Authors: Klimchuk, J. A.; Bradshaw, S. J.
2014ApJ...791...60K Altcode: 2014arXiv1405.1708K
It has been suggested that the hot plasma of the solar corona comes
primarily from impulsive heating events, or nanoflares, that occur
in the lower atmosphere, either in the upper part of the ordinary
chromosphere or at the tips of type II spicules. We test this idea with
a series of hydrodynamic simulations. We find that synthetic Fe XII
(195) and Fe XIV (274) line profiles generated from the simulations
disagree dramatically with actual observations. The integrated line
intensities are much too faint; the blueshifts are much too fast; the
blue-red asymmetries are much too large; and the emission is confined
to low altitudes. We conclude that chromospheric nanoflares are not a
primary source of hot coronal plasma. Such events may play an important
role in producing the chromosphere and powering its intense radiation,
but they do not, in general, raise the temperature of the plasma to
coronal values. Those cases where coronal temperatures are reached
must be relatively uncommon. The observed profiles of Fe XII and Fe
XIV come primarily from plasma that is heated in the corona itself,
either by coronal nanoflares or a quasi-steady coronal heating
process. Chromospheric nanoflares might play a role in generating
waves that provide this coronal heating.
---------------------------------------------------------
Title: MHD modelling of coronal loops: injection of high-speed
chromospheric flows
Authors: Petralia, A.; Reale, F.; Orlando, S.; Klimchuk, J. A.
2014A&A...567A..70P Altcode: 2014arXiv1405.2198P
Context. Observations reveal a correspondence between chromospheric type
II spicules and bright upward-moving fronts in the corona observed in
the extreme-ultraviolet (EUV) band. However, theoretical considerations
suggest that these flows are probably not the main source of heating
in coronal magnetic loops. <BR /> Aims: We investigate the propagation
of high-speed chromospheric flows into coronal magnetic flux tubes and
the possible production of emission in the EUV band. <BR /> Methods: We
simulated the propagation of a dense 10<SUP>4</SUP> K chromospheric jet
upward along a coronal loop by means of a 2D cylindrical MHD model that
includes gravity, radiative losses, thermal conduction, and magnetic
induction. The jet propagates in a complete atmosphere including
the chromosphere and a tenuous cool (~0.8 MK) corona, linked through
a steep transition region. In our reference model, the jet initial
speed is 70 km s<SUP>-1</SUP>, its initial density is 10<SUP>11</SUP>
cm<SUP>-3</SUP>, and the ambient uniform magnetic field is 10 G. We
also explored other values of jet speed and density in 1D and different
magnetic field values in 2D, as well as the jet propagation in a hotter
(~1.5 MK) background loop. <BR /> Results: While the initial speed of
the jet does not allow it to reach the loop apex, a hot shock-front
develops ahead of it and travels to the other extreme of the loop. The
shock front compresses the coronal plasma and heats it to about
10<SUP>6</SUP> K. As a result, a bright moving front becomes visible
in the 171 Å channel of the SDO/AIA mission. This result generally
applies to all the other explored cases, except for the propagation in
the hotter loop. <BR /> Conclusions: For a cool, low-density initial
coronal loop, the post-shock plasma ahead of upward chromospheric
flows might explain at least part of the observed correspondence
between type II spicules and EUV emission excess. <P />Movies
associated to Figs. 3, 6, 7 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201323012/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection in the Solar Atmosphere
Authors: Evans, Rebekah Minnel; Klimchuk, James; van der Holst, Bart
2014shin.confE..65E Altcode:
A fundamental question concerning magnetic energy release on the Sun
is why the release occurs only after substantial stresses have been
built up in the field. If reconnection were to occur readily, then
the released energy would be much less than the energy required for
coronal heating, CMEs, flares, jets, spicules, etc. How can we explain
this switch-on property? What is the physical nature of the onset
conditions? One idea involves the secondary instability of current
sheets, which switches on when the rotation of the magnetic field
across a current sheet reaches a critical angle. Such conditions would
occur at the boundaries of flux tubes that become tangled and twisted
by turbulent photospheric convection, for example. Other ideas focus
on a critical thickness for the current sheet. We report here on the
preliminary results of our investigation of reconnection onset. Unlike
our earlier work on the secondary instability (Dahlburg, Klimchuk,
and Antiochos 2005), here we treat the coupled chromosphere-corona
system. Using the BATS-R-US MHD code (Toth et al. 2012), we simulate a
single current sheet in a sheared magnetic field that extends from the
chromosphere into the corona. Driver motions are applied at the base
of the model. The configuration and chromosphere are both idealized,
but capture the essential physics of the problem. The advantage of this
unique approach is that it resolves the current sheet to the greatest
extent possible while maintaining a realistic solar atmosphere. It thus
bridges the gap between reconnection in a box studies and studies of
large-scale systems such as active regions. One question we will address
is whether onset conditions are met first in the chromosphere or corona.
---------------------------------------------------------
Title: Modeling the response of the lower atmosphere to flare
reconnection
Authors: Longcope, Dana; Qiu, Jiong; Klimchuk, James A.
2014AAS...22412324L Altcode:
It has long been recognized that energy release in a solar flare gives
rise to ablation of material from the chromosphere (more commonly
called evaporation). The prevailing view is that energy is initially
transformed from stored magnetic energy by the process of magnetic
reconnection. In some models reconnection accelerates electrons, either
directly or indirectly, and these non-thermal electrons carry energy
to the chromospheric footpoints. In others the reconnection converts
magnetic energy into heat in the corona and thermal conduction carries
that heat to the chromosphere. While no comprehensive, self-consistent
model yet exists for the conversion of magnetic energy to non-thermal
electron energy, models of the conversion to heat, via slow magnetosonic
shocks, have been available since Petschek's 1964 paper. We present
a numerical model encompassing the conversion of magnetic energy to
shocks, to heat, and then to conduction-driven evaporation. We compare
its results to those of more traditional conduction-driven models where
reconnection is replaced by an ad hoc plasma heating. We consider, in
particular, observable signatures such as doppler shifts and formation
of flare ribbons.This work was supported by the NASA SR&T program.
---------------------------------------------------------
Title: A one-dimensional solar flare model capturing reconnection
energy release, evaporation, and gradually cooling post-flare loops
Authors: Longcope, Dana; Qiu, Jiong; Klimchuk, Jim
2014shin.confE..32L Altcode:
The most obvious signature of energy release in a solar flare is the
large amount of chromospheric material heated to coronal temperatures
through a process known (inaccurately) as chromospheric evaporation. The
thousand-fold increase in X-ray luminosity we associate with a
flare is due entirely to evaporation. The most successful models of
flare evaporation to date have come from one-dimensional flux tube
simulations. These have provided the best means of resolving the
very thin pre-flare transition region, and of easily accommodating
the perfect field-alignment of the energy transport by either
non-thermal electrons or thermal conduction. In traditional flux
tube models magnetic reconnection is represented by an ad hoc heating
term. This adds energy but no momentum, and represents only crudely
known models of magnetic reconnection. Here we present a new approach
which captures the physics of fast Petschek reconnection in a flux tube
simulation. Following its creation by localized reconnection within a
current sheet, the flux tube retracts under magnetic tension, converting
magnetic energy into bulk flows; this is the outflow jet. These flows
form a slow magnetosonic shock which heats the coronal plasma and
drives a conduction front into the chromosphere. Our one-dimensional
model captures the energy release, thermalization, and the evaporation
it drives. We find observable signatures of the interplay between
reconnection energy release and evaporation; signatures different
from those found in conventional flux tube models with ad hoc heating,
but similar to actual flare observations.
---------------------------------------------------------
Title: The Onset of Magnetic Reconnection in the Solar Atmosphere
Authors: Evans, Rebekah M.; Klimchuk, James A.; Van Der Holst, Bart
2014AAS...22432342E Altcode:
A fundamental question concerning magnetic energy release on the Sun
is why the release occurs only after substantial stresses have been
built up in the field. If reconnection were to occur readily, then
the released energy would be much less than the energy required for
coronal heating, CMEs, flares, jets, spicules, etc. How can we explain
this switch-on property? What is the physical nature of the onset
conditions? One idea involves the "secondary instability" of current
sheets, which switches on when the rotation of the magnetic field
across a current sheet reaches a critical angle. Such conditions would
occur at the boundaries of flux tubes that become tangled and twisted
by turbulent photopheric convection, for example. Other ideas focus
on a critical thickness for the current sheet. We report here on the
preliminary results of our investigation of reconnection onset. Unlike
our earlier work on the secondary instability (Dahlburg, Klimchuk,
and Antiochos 2005), here we treat the coupled chromosphere-corona
system. Using the BATS-R-US MHD code (Toth et al. 2012), we simulate a
single current sheet in a sheared magnetic field that extends from the
chromosphere into the corona. Driver motions are applied at the base
of the model. The configuration and chromosphere are both idealized,
but capture the essential physics of the problem. The advantage of this
unique approach is that it resolves the current sheet to the greatest
extent possible while maintaining a realistic solar atmosphere. It thus
bridges the gap between "reconnection in a box" studies and studies of
large-scale systems such as active regions. One question we will address
is whether onset conditions are met first in the chromosphere or corona.
---------------------------------------------------------
Title: Long Duration Flare Emission by Sequential Reconnection
and Heating
Authors: Qiu, Jiong; Longcope, Dana; Klimchuk, James A.
2014AAS...22412325Q Altcode:
Long duration flare emissions lasting for a few hours are likely
produced by magnetic reconnection that continuously forms flare loops
and heats plasma inside. In this study, we demonstrate that this
process leads to the long duration emission in a C2.9 flare on 2011
September 13. Observed by AIA, the flare exhibits an organized pattern
of evolution with UV brightenings in flare ribbons spreading along the
polarity inversion line, followed by sequential formation of post-flare
loops seen in EUV emissions. The spatially resolved observation of
flare ribbons can be used to infer heating rates insequentially formed
and heated flare loops, with which we synthesize flare emission in
these loops with hydrodynamic models. The 0d EBTEL model (Klimchuk
et al. 2008) efficiently computes meanproperties of thousands of
flare loops identified from flare ribbon signatures, and the synthetic
tempo-spatial evolution of the total emission is in reasonable agreement
with EUV observations. The 1d model applied on a few selected loops
reveals physics of the heating mechanism and along-the-loop dynamics,
particularly during the impulsive heating phase. During the four hours
of this event, the estimated total energy in the heating amounts to
2e30 erg, with the total reconnection flux about 1e21 Mx.
---------------------------------------------------------
Title: A Survey of Coronal Heating Properties in Solar Active Regions
Authors: Viall, Nicholeen; Klimchuk, James A.
2014AAS...22432315V Altcode:
We investigate the properties of coronal heating in solar active
regions (AR) by systematically analyzing coronal light curves
observed by the Atmospheric Imaging Assembly onboard the Solar Dynamics
Observatory. Our automated technique computes time-lags (cooling times)
on a pixel-by-pixel basis, and has the advantage that it allows us
to analyze all of the coronal AR emission, including the so-called
diffuse emission between coronal loops. We recently presented results
using this time-lag analysis on NOAA AR 11082 (Viall & Klimchuk
2012) and found that the majority of the pixels contained cooling
plasma along their line of sight. This result is consistent with
impulsive coronal nanoflare heating of both coronal loops and the
surrounding diffuse emission in the AR. Here we present the results
of our time-lag technique applied to a survey of 15 AR of different
magnetic complexity, total unsigned magnetic flux, size and age. We
show that the post-nanoflare cooling patterns identified in NOAA AR
11082 are identified throughout all of the active regions in this
survey, indicating that nanoflare heating is ubiquitous in solar
active regions. However, some details of the nanoflare properties,
such as the nanoflare energy, are different across these different
active regions.We thank the SDO/AIA team for the use of these data,
and the Coronal Heating ISSI team for helpful discussion of these
topics. This research was supported by a NASA Heliophysics GI.
---------------------------------------------------------
Title: Evidence for Impulsive Coronal Heating from EUNIS 2013
Authors: Daw, Adrian N.; Brosius, Jeffrey W.; Rabin, Douglas M.;
Landi, Enrico; Klimchuk, James A.
2014AAS...22431204D Altcode:
Pervasive, faint Fe XIX 592 Å line emission was observed in active
regions by the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS)
sounding rocket instrument on 23 April 2013. The broad spectral coverage
(303-370 Å, 527-635 Å) and unprecedented dynamic range of the EUNIS
observations includes emission lines of ionization stages from He I to
Fe XX, and thus a wide temperature range of 0.03 to 10 MK. Comparison
of observed line intensities with calculations demonstrates that the
Fe XIX emission, formed at temperatures around 8 MK, is evidence of
the faint hot emission predicted by impulsive heating models of the
solar corona (such as nanoflares).
---------------------------------------------------------
Title: Chromospheric Nanoflares
Authors: Klimchuk, James A.; Bradshaw, Stephen
2014AAS...22430206K Altcode:
The traditional view is that coronal plasma results from energy
release (reconnection, waves, etc.) that takes place above the
chromosphere, in the corona itself. However, this need not be the
case. Cool chromospheric plasma could be directly heated to high
temperatures and rise upward to fill the corona. We here investigate
this scenario in the context of impulsive nanoflares that occur in the
upper chromosphere. Such events could represent the sudden heating
of the tips of type II spicules, or they could take place in the
ordinary chromosphere, completely unrelated to spicules. The results
are general. We generate synthetic line profiles of Fe XIV (274) and
Fe XII (195) based on a series of simulations performed with the HYDRAD
1D hydro code, which includes non-equilibrium ionization. We find that
the profiles are strongly blue-shifted and/or have large asymmetries
that are grossly inconsistent with actual observations. The results
are in agreement with our previous analytical predictions (Klimchuk
2012). We conclude that most coronal plasma is a result of coronal
energy release, not chromospheric nanoflares or type II spicules.
---------------------------------------------------------
Title: MHD modeling of coronal loops: the transition region throat
Authors: Guarrasi, M.; Reale, F.; Orlando, S.; Mignone, A.; Klimchuk,
J. A.
2014A&A...564A..48G Altcode: 2014arXiv1402.0338G
Context. The expansion of coronal loops in the transition region
may considerably influence the diagnostics of the plasma emission
measure. The cross-sectional area of the loops is expected to depend
on the temperature and pressure, and might be sensitive to the heating
rate. <BR /> Aims: The approach here is to study the area response
to slow changes in the coronal heating rate, and check the current
interpretation in terms of steady heating models. <BR /> Methods: We
study the area response with a time-dependent 2D magnetohydrodynamic
(MHD) loop model, including the description of the expanding magnetic
field, coronal heating and losses by thermal conduction, and radiation
from optically thin plasma. We run a simulation for a loop 50 Mm
long and quasi-statically heated to about 4 MK. <BR /> Results: We
find that the area can change substantially with the quasi-steady
heating rate, e.g., by ~40% at 0.5 MK as the loop temperature varies
between 1 MK and 4 MK, and, therefore, affects the interpretation of
the differential emission measure vs. temperature (DEM(T)) curves. <P
/>The movie associated to Fig. 4 is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201322848/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Core and Wing Densities of Asymmetric Coronal Spectral
Profiles: Implications for the Mass Supply of the Solar Corona
Authors: Patsourakos, S.; Klimchuk, J. A.; Young, P. R.
2014ApJ...781...58P Altcode: 2013arXiv1312.4842P
Recent solar spectroscopic observations have shown that coronal spectral
lines can exhibit asymmetric profiles, with enhanced emissions at their
blue wings. These asymmetries correspond to rapidly upflowing plasmas
at speeds exceeding ≈50 km s<SUP>-1</SUP>. Here, we perform a study
of the density of the rapidly upflowing material and compare it with
that of the line core that corresponds to the bulk of the plasma. For
this task, we use spectroscopic observations of several active regions
taken by the Extreme Ultraviolet Imaging Spectrometer of the Hinode
mission. The density sensitive ratio of the Fe XIV lines at 264.78 and
274.20 Å is used to determine wing and core densities. We compute the
ratio of the blue wing density to the core density and find that most
values are of order unity. This is consistent with the predictions for
coronal nanoflares if most of the observed coronal mass is supplied
by chromospheric evaporation driven by the nanoflares. However,
much larger blue wing-to-core density ratios are predicted if most of
the coronal mass is supplied by heated material ejected with type II
spicules. Our measurements do not rule out a spicule origin for the
blue wing emission, but they argue against spicules being a primary
source of the hot plasma in the corona. We note that only about 40%
of the pixels where line blends could be safely ignored have blue wing
asymmetries in both Fe XIV lines. Anticipated sub-arcsecond spatial
resolution spectroscopic observations in future missions could shed
more light on the origin of blue, red, and mixed asymmetries.
---------------------------------------------------------
Title: Cross-Sectional Properties of Coronal Loops
Authors: West, Matthew; Zhukov, Andrei; Klimchuk, James
2014cosp...40E3620W Altcode:
In this work we assess if coronal loop cross sections observed in
EUV images are symmetrical or asymmetrical in nature. To do this, we
identified individual loop structures observed in EUV images taken
with the EUVI instruments on the STEREO satellites. To image loops
from two unique angles, we chose loops clearly discernible in both
EUVI imagers during the period when the satellites were separated by
approximately 90 degrees, allowing us to make observations of individual
loops from two unique vantage points. Preference was given to loops
which could be clearly identified in both satellites, especially
those which were not crossed by other bright structures or loops,
so reasonable background subtractions could be made. Once identified,
the images were co-aligned and straightened, using a spline routine,
for comparison. In total we identified 11 clearly discernible loops
and derived the standard deviation and widths for both perspectives
of the loop. It was found that within instrumental errors the loops
can be considered circular in nature.
---------------------------------------------------------
Title: EUV emission along observed coronal loops
Authors: Lopez Fuentes, Marcelo; Klimchuk, James
2014cosp...40E1872L Altcode:
Theories based on steady heating concentrated at the footpoints of
coronal loops predict a state of thermal nonequilibrium which produce,
according to numerical studies, highly asymmetric intensity profiles. In
this work we study a series of coronal loop observations obtained with
the Transition Region and Coronal Explorer (TRACE) and the Atmospheric
Imager Assembly (AIA) on board the Solar Dynamics Observatory (SDO)
to explore how the intensity varies with position along the loops. As
part of the data processing we apply a careful procedure to subtract
the background contribution to the loop intensity. We find that the
obtained intrinsic intensities have strong variations over short
distances, due in part to a residual effect of the intense nonuniform
background. Although it is not easy to separate the background
contribution from the proper loop intensity fluctuations, our results
show that in the majority of the cases the intensity structure is not
consistent with the predicted profiles.
---------------------------------------------------------
Title: Asymmetries in Coronal Spectral Lines and Emission Measure
Distribution
Authors: Tripathi, Durgesh; Klimchuk, James A.
2013ApJ...779....1T Altcode: 2013arXiv1310.0168T
It has previously been argued that (1) spicules do not provide enough
pre-heated plasma to fill the corona, and (2) even if they did,
additional heating would be required to keep the plasma hot as it
expands upward. Here we address whether spicules play an important
role by injecting plasma at cooler temperatures (<2 MK), which
then gets heated to coronal values at higher altitudes. We measure
red-blue asymmetries in line profiles formed over a wide range of
temperatures in the bright moss areas of two active regions. We derive
emission measure distributions from the excess wing emission. We find
that the asymmetries and emission measures are small and conclude
that spicules do not inject an important (dominant) mass flux into
the cores of active regions at temperatures >0.6 MK (log T >
5.8). These conclusions apply not only to spicules but also to any
process that suddenly heats and accelerates chromospheric plasma
(e.g., a chromospheric nanoflare). The traditional picture of coronal
heating and chromospheric evaporation appears to remain the most likely
explanation of the active region corona.
---------------------------------------------------------
Title: Can the Differential Emission Measure Constrain the Timescale
of Energy Deposition in the Corona?
Authors: Guennou, C.; Auchère, F.; Klimchuk, J. A.; Bocchialini,
K.; Parenti, S.
2013ApJ...774...31G Altcode: 2013arXiv1306.3114G
In this paper, the ability of the Hinode/EIS instrument to detect
radiative signatures of coronal heating is investigated. Recent
observational studies of active region cores suggest that both
the low and high frequency heating mechanisms are consistent with
observations. Distinguishing between these possibilities is important
for identifying the physical mechanism(s) of the heating. The
differential emission measure (DEM) tool is one diagnostic that
allows us to make this distinction, through the amplitude of the
DEM slope coolward of the coronal peak. It is therefore crucial to
understand the uncertainties associated with these measurements. Using
proper estimations of the uncertainties involved in the problem
of DEM inversion, we derive confidence levels on the observed DEM
slope. Results show that the uncertainty in the slope reconstruction
strongly depends on the number of lines constraining the slope. Typical
uncertainty is estimated to be about ±1.0 in the more favorable cases.
---------------------------------------------------------
Title: Ultraviolet and Extreme-ultraviolet Emissions at the Flare
Footpoints Observed by Atmosphere Imaging Assembly
Authors: Qiu, Jiong; Sturrock, Zoe; Longcope, Dana W.; Klimchuk,
James A.; Liu, Wen-Juan
2013ApJ...774...14Q Altcode: 2013arXiv1305.6899Q
A solar flare is composed of impulsive energy release events by magnetic
reconnection, which forms and heats flare loops. Recent studies have
revealed a two-phase evolution pattern of UV 1600 Å emission at the
feet of these loops: a rapid pulse lasting for a few seconds to a few
minutes, followed by a gradual decay on timescales of a few tens of
minutes. Multiple band EUV observations by the Atmosphere Imaging
Assembly further reveal very similar signatures. These two phases
represent different but related signatures of an impulsive energy
release in the corona. The rapid pulse is an immediate response of
the lower atmosphere to an intense thermal conduction flux resulting
from the sudden heating of the corona to high temperatures (we rule
out energetic particles due to a lack of significant hard X-ray
emission). The gradual phase is associated with the cooling of hot
plasma that has been evaporated into the corona. The observed footpoint
emission is again powered by thermal conduction (and enthalpy), but now
during a period when approximate steady-state conditions are established
in the loop. UV and EUV light curves of individual pixels may therefore
be separated into contributions from two distinct physical mechanisms to
shed light on the nature of energy transport in a flare. We demonstrate
this technique using coordinated, spatially resolved observations of
UV and EUV emissions from the footpoints of a C3.2 thermal flare.
---------------------------------------------------------
Title: Structure of solar coronal loops: from miniature to large-scale
Authors: Peter, H.; Bingert, S.; Klimchuk, J. A.; de Forest, C.;
Cirtain, J. W.; Golub, L.; Winebarger, A. R.; Kobayashi, K.; Korreck,
K. E.
2013A&A...556A.104P Altcode: 2013arXiv1306.4685P
<BR /> Aims: We use new data from the High-resolution Coronal Imager
(Hi-C) with its unprecedented spatial resolution of the solar corona
to investigate the structure of coronal loops down to 0.2”. <BR />
Methods: During a rocket flight, Hi-C provided images of the solar
corona in a wavelength band around 193 Å that is dominated by emission
from Fe xii showing plasma at temperatures around 1.5 MK. We analyze
part of the Hi-C field-of-view to study the smallest coronal loops
observed so far and search for the possible substructuring of larger
loops. <BR /> Results: We find tiny 1.5 MK loop-like structures that
we interpret as miniature coronal loops. Their coronal segments above
the chromosphere have a length of only about 1 Mm and a thickness of
less than 200 km. They could be interpreted as the coronal signature
of small flux tubes breaking through the photosphere with a footpoint
distance corresponding to the diameter of a cell of granulation. We
find that loops that are longer than 50 Mm have diameters of about 2”
or 1.5 Mm, which is consistent with previous observations. However, Hi-C
really resolves these loops with some 20 pixels across the loop. Even
at this greatly improved spatial resolution, the large loops seem to
have no visible substructure. Instead they show a smooth variation in
cross-section. <BR /> Conclusions: That the large coronal loops do not
show a substructure on the spatial scale of 0.1” per pixel implies that
either the densities and temperatures are smoothly varying across these
loops or it places an upper limit on the diameter of the strands the
loops might be composed of. We estimate that strands that compose the
2” thick loop would have to be thinner than 15 km. The miniature loops
we find for the first time pose a challenge to be properly understood
through modeling. <P />Appendices are available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Modeling the Line-of-sight Integrated Emission in the Corona:
Implications for Coronal Heating
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2013ApJ...771..115V Altcode: 2013arXiv1304.5439V
One of the outstanding problems in all of space science is uncovering
how the solar corona is heated to temperatures greater than 1 MK. Though
studied for decades, one of the major difficulties in solving this
problem has been unraveling the line-of-sight (LOS) effects in the
observations. The corona is optically thin, so a single pixel measures
counts from an indeterminate number (perhaps tens of thousands)
of independently heated flux tubes, all along that pixel's LOS. In
this paper we model the emission in individual pixels imaging the
active region corona in the extreme ultraviolet. If LOS effects are
not properly taken into account, erroneous conclusions regarding both
coronal heating and coronal dynamics may be reached. We model the corona
as an LOS integration of many thousands of completely independently
heated flux tubes. We demonstrate that despite the superposition of
randomly heated flux tubes, nanoflares leave distinct signatures in
light curves observed with multi-wavelength and high time cadence
data, such as those data taken with the Atmospheric Imaging Assembly
on board the Solar Dynamics Observatory. These signatures are readily
detected with the time-lag analysis technique of Viall & Klimchuk
in 2012. Steady coronal heating leaves a different and equally distinct
signature that is also revealed by the technique.
---------------------------------------------------------
Title: Where is Coronal Plasma Heated?
Authors: Klimchuk, James A.; Bradshaw, S.; Patsourakos, S.; Tripathi,
D.
2013SPD....4420006K Altcode:
The coupling between the chromosphere and corona is a question of
great current interest. It has long been understood that coronal mass
originates in the chromosphere and that the energy which powers the
corona flows up through the chromosphere. However, the details of
how this happens are now being questioned. In the traditional view,
“mechanical” energy flows into the corona in the form of waves
or gradually increasing magnetic stresses. The waves and stresses
dissipate and heat the plasma. The resulting downward thermal conduction
flux causes material to evaporate from the chromosphere and fill
the corona. If the heating is steady, an equilibrium is established
whereby radiation and thermal conduction balance the energy input. If
the heating is impulsive (a nanoflare), the evaporated plasma cools
and drains, only to reappear during the next event. In either case,
the heating occurs in the corona. A new idea is that the heating
occurs instead in the chromosphere. Cold plasma is directly heated
to coronal temperatures and then flows upward due to expansion and
perhaps also an ejection process. The hot tips of type II spicules
are one example, though spicules need not be involved. I will discuss
these two fundamentally different scenarios and the observational
predictions that they make. A comparison with actual observations
leads to the conclusion that only a small fraction of the hot plasma
in the corona comes from chromospheric heating. Most coronal plasma
is a consequence of heating that occurs in the corona itself.
---------------------------------------------------------
Title: UV and EUV Emissions at the Flare Foot-points Observed by AIA
Authors: Qiu, Jiong; Sturrock, Z.; Longcope, D.; Klimchuk, J. A.;
Liu, W.
2013SPD....44...53Q Altcode:
A solar flare is composed of impulsive energy release events by magnetic
reconnection, which forms and heats flare loops. Recent studies have
revealed a two-phase evolution pattern of UV 1600A emission at the
feet of these loops: a rapid pulse lasting for a few seconds to a
few minutes, followed by a gradual decay on timescales of a few tens
of minutes. Multiple band EUV observations by AIA further reveal
very similar signatures. These two phases represent different but
related signatures of an impulsive energy release in the corona. The
rapid pulse is an immediate response of the lower atmosphere to an
intense thermal conduction flux resulting from the sudden heating of
the corona to high temperatures (we rule out energetic particles due
to a lack of significant hard X-ray emission). The gradual phase is
associated with the cooling of hot plasma that has been evaporated
into the corona. The observed footpoint emission is again powered
by thermal conduction (and enthalpy), but now during a period when
approximate steady state conditions are established in the loop. UV
and EUV light curves of individual pixels may therefore be separated
into contributions from two distinct physical mechanisms to shed
light on the nature of energy transport in a flare. We demonstrate
this technique using coordinated, spatially resolved observations of
UV and EUV emission from the footpoints of a C3.2 thermal flare.
---------------------------------------------------------
Title: A Survey of Nanoflare Properties in Solar Active Regions
Authors: Viall, Nicholeen; Klimchuk, J. A.
2013SPD....44...16V Altcode:
We investigate the characteristics of coronal heating using a systematic
technique that analyzes the properties of nanoflares in active regions
(AR). Our technique computes cooling times, or time lags, using SDO/AIA
light curves of all of the coronal AR emission, including the so-called
diffuse emission. We recently presented results using this time-lag
analysis on NOAA AR 11082 (Viall & Klimchuk 2012). We found that
the majority of the pixels had cooling plasma along their line of sight,
consistent with impulsive coronal nanoflare heating. Additionally, our
results using the AIA 94 channel data showed that the nanoflare energy
is stronger in the AR core and weaker in the AR periphery. Are these
results representative of the nanoflare characteristics exhibited in
the majority of active regions, or is AR 11082 unique? Here we present
the time-lag results for a survey of active regions and determine
whether these nanoflare patterns are born out in other active regions
as well. This research was supported by the NASA Heliophysics Guest
Investigator program.
---------------------------------------------------------
Title: Understanding Coronal Heating by Comparing SDO/AIA Observations
with Modeled Light Curves
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2013enss.confE..18V Altcode:
An important signature of nanoflare heated coronal plasma is the
sudden appearance of the plasma at hot temperatures, followed by
a comparatively slow cooling and draining phase. This is due to
the impulsive nature of nanoflare heating and the heat conduction
and mass exchange between the corona and chromosphere. Identifying
such nanoflare signatures is complicated by the fact that the solar
corona is optically thin: many thousands of flux tubes which are
heated completely independently are contributing to the total emission
along a given line of sight. One approach has been to analyze isolated
features such as coronal loops; however the diffuse emission between
and around isolated features contribute as much, if not more to the
EUV coronal emission, and therefore is crucial to the understanding of
coronal heating. In this study we move beyond isolated features and
analyze all of the emission in an entire active region and quiet Sun
area. We investigate SDO/AIA light curves, systematically identifying
nanoflare signatures. We compare the observations with a model of the
corona as a line-of-sight integration of many thousands of completely
independently heated flux tubes. We consider that the emission from
these flux tubes may be due exclusively to impulsive nanoflare bursts,
quasi-steady heating, or a mix of both, depending on the cadence of
heat release. We demonstrate that despite the superposition of randomly
heated flux tubes, different distributions of nanoflare cadences produce
distinct signatures in light curves observed with multi-wavelength and
high time cadence data, such as those from SDO/AIA. We find that much
of the solar corona is heated through impulsive nanoflares.
---------------------------------------------------------
Title: Can the Differential Emission Measure diagnostic be used to
constrain the timescale of energy deposition in the corona?
Authors: Guennou, C.; Auchère, F.; Klimchuk, J. A.; Bocchialini,
K.; Parenti, S.
2013enss.confE..34G Altcode:
Differential emission measure (DEM) analysis is a widespread tool used
to diagnose the thermal properties of coronal plasmas. The slope of
the DEM distribution coolward of the coronal peak (near 3-4MK in active
regions) can be used to diagnose the timescale for the energy deposition
repeating on a given magnetic strand. Recent AR studies suggest that
some active region cores are consistent with low frequency heating
mechanisms, where the plasma cools completely before being reheated,
while other show consistency with high frequency energy deposition,
where rapid reheating causes the temperature to fluctuate about
a particular value. Distinguishing between these possibilities is
important for identifying the physical mechanism of the heating. It is
therefore crucial to understand the uncertainties in measurements of
observed DEM slopes. In this work, based on a probabilistic approach
and Monte Carlo simulations, we carefully assess the errors in the
slopes determined from EIS data. We consider both the random errors due
to photon counting statistics, and the systematic errors associated
with uncertainties in atomic physics and instrument calibration. The
technique developed provides all the solutions consistent with the data
and their associated probabilities. We demonstrate how the quality
and the accuracy of the inversion are affected by the presence of
noises and systematic errors, and we characterise the quality of the
DEM inversion and its statistical properties. From these results,
estimation of the uncertainties in the reconstructed slopes can be
derived, thereby allowing a proper interpretation of the degree of
agreement between observations and heating model predictions.
---------------------------------------------------------
Title: Where is Coronal Plasma Heated?
Authors: Klimchuk, James A.
2013enss.confE.105K Altcode:
The coupling between the chromosphere and corona is a question of
great interest at the moment. It has long been understood that
coronal mass originates in the chromosphere and that the energy
which powers the corona flows up through the chromosphere. However,
the details of how this happens are now being questioned. In the
traditional view, "mechanical" energy flows into the corona in the
form of waves or gradually increasing magnetic stresses. The waves or
stresses dissipate and heat the plasma. The resulting downward thermal
conduction flux causes material to evaporate from the chromosphere and
fill the corona. If the heating is steady, an equilibrium is established
whereby radiation and thermal conduction balance the energy input. If
the heating is impulsive (a nanoflare), the evaporated plasma cools
and drains, only to reappear during the next event. In either case,
the heating occurs in the corona. A new idea is that the heating
occurs instead in the chromosphere. Cold plasma is directly heated
to coronal temperatures and then flows upward due to expansion and
perhaps also an ejection process. The hot tips of type II spicules
are one example, though spicules need not be involved. I will discuss
these two fundamentally different scenarios and the observational
predictions that they make. A comparison with actual observations
leads to the conclusion that only a small fraction of the hot plasma
in the corona comes from chromospheric heating. Most coronal plasma
is a consequence of heating that occurs in the corona itself.
---------------------------------------------------------
Title: Diagnosing the Time Dependence of Active Region Core Heating
from the Emission Measure. II. Nanoflare Trains
Authors: Reep, J. W.; Bradshaw, S. J.; Klimchuk, J. A.
2013ApJ...764..193R Altcode: 2013arXiv1303.4466R
The time dependence of heating in solar active regions can be studied
by analyzing the slope of the emission measure distribution coolward
of the peak. In a previous study we showed that low-frequency heating
can account for 0% to 77% of active region core emission measures. We
now turn our attention to heating by a finite succession of impulsive
events for which the timescale between events on a single magnetic
strand is shorter than the cooling timescale. We refer to this
scenario as a "nanoflare train" and explore a parameter space of
heating and coronal loop properties with a hydrodynamic model. Our
conclusions are (1) nanoflare trains are consistent with 86% to 100%
of observed active region cores when uncertainties in the atomic
data are properly accounted for; (2) steeper slopes are found for
larger values of the ratio of the train duration Δ<SUB> H </SUB>
to the post-train cooling and draining timescale Δ<SUB> C </SUB>,
where Δ<SUB> H </SUB> depends on the number of heating events,
the event duration and the time interval between successive events
(τ<SUB> C </SUB>); (3) τ<SUB> C </SUB> may be diagnosed from the
width of the hot component of the emission measure provided that the
temperature bins are much smaller than 0.1 dex (4) the slope of the
emission measure alone is not sufficient to provide information about
any timescale associated with heating—the length and density of the
heated structure must be measured for Δ<SUB> H </SUB> to be uniquely
extracted from the ratio Δ<SUB> H </SUB>/Δ<SUB> C </SUB>.
---------------------------------------------------------
Title: Study of the EUV intensity variation along observed coronal
loops
Authors: Lopez Fuentes, M.; Klimchuk, J. A.
2013BAAA...56..399L Altcode:
The variation of intensity along coronal loops observed in X-rays and
EUV can be used to constrain different coronal heating theories. For
instance; thermal nonequilibrium that results from heating located at
the footpoints of the loops predicts an intensity structure that is
highly non uniform and asymmetric. We study the intensity variation
along coronal loops observed with the Transition Region and Coronal
Explorer (TRACE) in the 171 channel and find that loops present
pronounced spatial fluctuations. Although during the data processing we
carefully subtract the background contribution; part of the observed
fluctuations is due to a residual effect of this contribution. Even
allowing for the imperfect background subtraction; we conclude that
most of the analyzed observations are inconsistent with the intensity
profiles predicted by some numerical models.
---------------------------------------------------------
Title: The role of type II spicules in the upper solar atmosphere
Authors: Klimchuk, J. A.
2012JGRA..11712102K Altcode: 2012arXiv1207.7048K
We examine the suggestion that most of the hot plasma in the Sun's
corona comes from type II spicule material that is heated as it is
ejected from the chromosphere. This contrasts with the traditional view
that the corona is filled via chromospheric evaporation that results
from coronal heating. We explore the observational consequences of
a hypothetical spicule dominated corona and conclude from the large
discrepancy between predicted and actual observations that only a
small fraction of the hot plasma can be supplied by spicules (<2%
in active regions, <5% in the quiet Sun, and <8% in coronal
holes). The red-blue asymmetries of EUV spectral lines and the ratio
of lower transition region (LTR;T ≤ 0.1 MK) to coronal emission
measures are both predicted to be 2 orders of magnitude larger than
observed. Furthermore, hot spicule material would cool dramatically by
adiabatic expansion as it rises into the corona, so substantial coronal
heating would be needed to maintain the high temperatures that are
seen at all altitudes. We suggest that the corona contains a mixture
of thin strands, some of which are populated by spicule injections,
but most of which are not. A majority of the observed hot emission
originates in non-spicule strands and is explained by traditional
coronal heating models. However, since these models predict far too
little emission from the LTR, most of this emission comes from the bulk
of the spicule material that is only weakly heated and visible in He II
(304 Å) as it falls back to the surface.
---------------------------------------------------------
Title: The Role of Type II Spicules in the Upper Solar Atmosphere
Authors: Klimchuk, J. A.
2012AGUFMSH31B..07K Altcode:
We examine the suggestion that most of the hot plasma in the Sun's
corona comes from type II spicule material that is heated as it is
ejected from the chromosphere. This contrasts with the traditional view
that the corona is filled via chromospheric evaporation that results
from coronal heating. We explore the observational consequences of
a hypothetical spicule dominated corona and conclude from the large
discrepancy between predicted and actual observations that only a
small fraction of the hot plasma can be supplied by spicules (<2% in
active regions and <5% in the quiet Sun). The red-blue asymmetries
of EUV spectral lines and the ratio of lower transition region (LTR;
T<0.1 MK) to coronal emission measures are both predicted to be
2 orders of magnitude larger than observed. Furthermore, hot spicule
material would cool dramatically by adiabatic expansion as it rises
into the corona, so coronal heating would likely be required to maintain
the high temperatures that are seen at all altitudes. The necessity of
coronal heating seems inescapable. Traditional coronal heating models
predict far too little emission from the LTR, and we suggest that this
emission comes primarily from the bulk of the spicule material that
is heated to <0.1 MK and is visible in He II (304 A) as it falls
back to the surface.
---------------------------------------------------------
Title: The Role of Spicules in Explaining the Corona and Transition
Region
Authors: Klimchuk, J.
2012IAUSS...6E.107K Altcode:
A portion of the material in the newly-discovered type II spicules
is heated to coronal temperatures and contributes to the hot emission
that we observe. What fraction of the coronal plasma can be attributed
to spicules and what fraction must be explained by ordinary coronal
heating? Is the as yet unexplained bright emission from the lower
transition region (T < 0.1 MK) due to spicules? I will address
these questions and offer suggestions about future high-resolution
observations, both space and ground-based, that can provide important
information about this fascinating phenomenon.
---------------------------------------------------------
Title: Diagnosing the Time-Dependence of Active Region Core Heating
Using Emission Measures
Authors: Klimchuk, J. A.; Bradshaw, S. J.; Reep, J. W.
2012AGUFMSH42A..01K Altcode:
It is widely believed that the cross-field spatial scale of coronal
heating is small, so that the fundamental plasma structures
(loop strands) are spatially unresolved. We therefore must
appeal to diagnostic techniques that are not strongly affected by
spatial averaging. One valuable observable is the emission measure
distribution, EM(T), which indicates how much material is present
at each temperature. The slope of the distribution coolward of its
peak is related to the frequency of the presumed impulsive energy
release. Nanoflares that have a long delay before repeating on the same
loop strand give rise to shallow slopes, while nanoflares that repeat
with a timescale shorter than a cooling time (or truly steady heating)
give rise to steep slopes. Comparing recent Hinode observations with
hydrodynamic loop simulations, we find that about 36% of active region
cores are consistent with low-frequency nanoflares. The observational
uncertainties are large, however, so as many as 77% or as few as none
are consistent with low-frequency nanoflares when the uncertainties
are taken into account. Constraining the time dependence of the heating
is important for identifying the physical mechanism.
---------------------------------------------------------
Title: Nanoflare Heating of the Solar Corona: Comparing SDO/AIA
Observations with Modeled Light Curves
Authors: Viall, N. M.; Klimchuk, J. A.
2012AGUFMSH42A..03V Altcode:
A significant outstanding issue in current solar and astrophysical
research is that of the heating of the solar corona. Coronal plasma is
typically measured to be at temperatures near ~1-3 MK. Is the majority
of the coronal plasma maintained at these temperatures through a
form of quasi-steady heating, or is this simply a measure of the
average temperature of widely varying, impulsively heated coronal
plasma? Addressing even this basic question is complicated by the
fact that the corona is optically thin: many thousands of flux tubes
which are heated completely independently are contributing to the
total emission along a given line of sight. There is a large body of
work focused on the heating of isolated features - coronal loops in
active regions- which are impulsively heated, however understanding
of the diffuse emission between loops and the emission from the quiet
Sun are also crucial. Therefore in this study we move beyond isolated
features and analyze all of the emission in an entire active region and
quiet Sun area from all contributing flux tubes. We investigate light
curves systematically using SDO/AIA observations. We also model the
corona as a line-of-sight integration of many thousands of completely
independently heated flux tubes. The emission from these flux tubes
may be time dependent, quasi-steady, or a mix of both, depending on the
cadence of heat release. We demonstrate that despite the superposition
of randomly heated flux tubes, different distributions of nanoflare
cadences produce distinct signatures in light curves observed with
multi-wavelength and high time cadence data, such as those from
SDO/AIA. We discuss the quiet Sun and active region emission in the
context of these predicted nanoflare signatures.
---------------------------------------------------------
Title: Exploring Small Spatial Scales in the Transition Region
and Solar Corona with the Very High Angular Resolution Imaging
Spectrometer (VERIS)
Authors: Chua, D. H.; Korendyke, C. M.; Vourlidas, A.; Brown, C. M.;
Tun-Beltran, S.; Klimchuk, J. A.; Landi, E.; Seely, J.; Davila, J. M.;
Hagood, R.; Roberts, D.; Shepler, E.; Feldman, R.; Moser, J.; Shea, J.
2012AGUFMSH33A2217C Altcode:
Theoretical and experimental investigations of the transition region
and coronal loops point to the importance of processes occurring on
small spatial scales in governing the strong dynamics and impulsive
energy release in these regions. As a consequence, high spatial,
temporal, and temperature resolution over a broad temperature range,
and accuracy in velocity and density determinations are all critical
observational parameters. Current instruments lack one or more of these
properties. These observational deficiencies have created a wide array
of opposing descriptions of coronal loop heating and questions such
as whether or not the plasma within coronal loops is multi-thermal or
isothermal. High spectral and spatial resolution spectroscopic data
are absolutely required to resolve these controversies and to advance
our understanding of the dynamics within the solar atmosphere. We
will achieve this with the Very High Angular Resolution Imaging
Spectrometer (VERIS) sounding rocket payload. VERIS consists of an
off-axis paraboloid telescope feeding a very high angular resolution,
extreme ultraviolet (EUV) imaging spectrometer that will provide
the first ever, simultaneous sub-arcsecond (0.16 arcsecond/pixel)
spectra in bright lines needed to study plasma structures in the
transition region, quiet corona, and active region core. It will do
so with a spectral resolution of >5000 to allow Doppler velocity
determinations to better than 3 km/s. VERIS uses a novel two-element,
normal incidence optical design with highly reflective, broad wavelength
coverage EUV coatings to access a spectral range with broad temperature
coverage (0.03-15 MK) and density-sensitive line ratios. Combined with
Hinode Solar Optical Telescope (SOT) and ground based observatories,
VERIS will deliver simultaneous observations of the entire solar
atmosphere from the photosphere to the multi-million degree corona
at sub-arcsecond resolution for the first time ever, allowing us to
understand the missing link between chromospheric structures and the
corona. VERIS will be launched from White Sands Missile Range in early
2013. This paper presents a progress report on the VERIS payload and
a summary of observations planned to further our understanding of
the fine-scale structure of individual coronal loops and the heating
mechanisms operating within them.
---------------------------------------------------------
Title: Enthalpy-based Thermal Evolution of Loops. III. Comparison
of Zero-dimensional Models
Authors: Cargill, P. J.; Bradshaw, S. J.; Klimchuk, J. A.
2012ApJ...758....5C Altcode:
Zero-dimensional (0D) hydrodynamic models provide a simple and quick
way to study the thermal evolution of coronal loops subjected to
time-dependent heating. This paper presents a comparison of a number
of 0D models that have been published in the past and is intended to
provide a guide for those interested in either using the old models
or developing new ones. The principal difference between the models
is the way the exchange of mass and energy between corona, transition
region, and chromosphere is treated, as plasma cycles into and out of
a loop during a heating-cooling cycle. It is shown that models based
on the principles of mass and energy conservation can give satisfactory
results at some or, in the case of the Enthalpy-based Thermal Evolution
of Loops model, all stages of the loop evolution. Empirical models
can have significant difficulties in obtaining accurate behavior due
to invocation of assumptions incompatible with the correct exchange
of mass and energy between corona, transition region, and chromosphere.
---------------------------------------------------------
Title: Diagnosing the Time-dependence of Active Region Core Heating
from the Emission Measure. I. Low-frequency Nanoflares
Authors: Bradshaw, S. J.; Klimchuk, J. A.; Reep, J. W.
2012ApJ...758...53B Altcode: 2012arXiv1209.0737B
Observational measurements of active region emission measures contain
clues to the time dependence of the underlying heating mechanism. A
strongly nonlinear scaling of the emission measure with temperature
indicates a large amount of hot plasma relative to warm plasma. A
weakly nonlinear (or linear) scaling of the emission measure
indicates a relatively large amount of warm plasma, suggesting that
the hot active region plasma is allowed to cool and so the heating is
impulsive with a long repeat time. This case is called low-frequency
nanoflare heating, and we investigate its feasibility as an active
region heating scenario here. We explore a parameter space of heating
and coronal loop properties with a hydrodynamic model. For each model
run, we calculate the slope α of the emission measure distribution
EM(T)vpropT <SUP>α</SUP>. Our conclusions are: (1) low-frequency
nanoflare heating is consistent with about 36% of observed active
region cores when uncertainties in the atomic data are not accounted
for; (2) proper consideration of uncertainties yields a range in
which as many as 77% of observed active regions are consistent with
low-frequency nanoflare heating and as few as zero; (3) low-frequency
nanoflare heating cannot explain observed slopes greater than 3; (4)
the upper limit to the volumetric energy release is in the region of
50 erg cm<SUP>-3</SUP> to avoid unphysical magnetic field strengths;
(5) the heating timescale may be short for loops of total length
less than 40 Mm to be consistent with the observed range of slopes;
(6) predicted slopes are consistently steeper for longer loops.
---------------------------------------------------------
Title: Active Region Moss: Doppler Shifts from
Hinode/Extreme-ultraviolet Imaging Spectrometer Observations
Authors: Tripathi, Durgesh; Mason, Helen E.; Klimchuk, James A.
2012ApJ...753...37T Altcode: 2012arXiv1204.6550T
Studying the Doppler shifts and the temperature dependence of Doppler
shifts in moss regions can help us understand the heating processes
in the core of the active regions. In this paper, we have used an
active region observation recorded by the Extreme-ultraviolet Imaging
Spectrometer (EIS) on board Hinode on 2007 December 12 to measure the
Doppler shifts in the moss regions. We have distinguished the moss
regions from the rest of the active region by defining a low-density
cutoff as derived by Tripathi et al. in 2010. We have carried out a
very careful analysis of the EIS wavelength calibration based on the
method described by Young et al. in 2012. For spectral lines having
maximum sensitivity between log T = 5.85 and log T = 6.25 K, we find
that the velocity distribution peaks at around 0 km s<SUP>-1</SUP>
with an estimated error of 4-5 km s<SUP>-1</SUP>. The width of the
distribution decreases with temperature. The mean of the distribution
shows a blueshift which increases with increasing temperature and the
distribution also shows asymmetries toward blueshift. Comparing these
results with observables predicted from different coronal heating
models, we find that these results are consistent with both steady
and impulsive heating scenarios. However, the fact that there are a
significant number of pixels showing velocity amplitudes that exceed
the uncertainty of 5 km s<SUP>-1</SUP> is suggestive of impulsive
heating. Clearly, further observational constraints are needed to
distinguish between these two heating scenarios.
---------------------------------------------------------
Title: Evidence for Widespread Cooling in an Active Region Observed
with the SDO Atmospheric Imaging Assembly
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2012ApJ...753...35V Altcode: 2012arXiv1202.4001V
A well-known behavior of EUV light curves of discrete coronal loops
is that the peak intensities of cooler channels or spectral lines
are reached at progressively later times than hotter channels. This
time lag is understood to be the result of hot coronal loop plasma
cooling through these lower respective temperatures. However, loops
typically comprise only a minority of the total emission in active
regions (ARs). Is this cooling pattern a common property of AR coronal
plasma, or does it only occur in unique circumstances, locations, and
times? The new Solar Dynamics Observatory/Atmospheric Imaging Assembly
(SDO/AIA) data provide a wonderful opportunity to answer this question
systematically for an entire AR. We measure the time lag between pairs
of SDO/AIA EUV channels using 24 hr of images of AR 11082 observed
on 2010 June 19. We find that there is a time-lag signal consistent
with cooling plasma, just as is usually found for loops, throughout
the AR including the diffuse emission between loops for the entire 24
hr duration. The pattern persists consistently for all channel pairs
and choice of window length within the 24 hr time period, giving us
confidence that the plasma is cooling from temperatures of greater
than 3 MK, and sometimes exceeding 7 MK, down to temperatures lower
than ~0.8 MK. This suggests that the bulk of the emitting coronal
plasma in this AR is not steady; rather, it is dynamic and constantly
evolving. These measurements provide crucial constraints on any model
which seeks to describe coronal heating.
---------------------------------------------------------
Title: A cellular automaton model for coronal heating
Authors: López Fuentes, M. C.; Klimchuk, J. A.
2012IAUS..286..433L Altcode:
We present a simple coronal heating model based on a cellular automaton
approach. Following Parker's suggestion (1988), we consider the
corona to be made up of elemental magnetic strands that accumulate
magnetic stress due to the photospheric displacements of their
footpoints. Magnetic energy is eventually released in small scale
reconnection events. The model consists of a 2D grid in which strand
footpoints travel with random displacements simulating convective
motions. Each time two strands interact, a critical condition is
tested (as in self-organized critical models), and if the condition
is fulfilled, the strands reconnect and energy is released. We
model the plasma response to the heating events and obtain synthetic
observations. We compare the output of the model with real observations
from Hinode/XRT and discuss the implications of our results for
coronal heating.
---------------------------------------------------------
Title: Enthalpy-based Thermal Evolution of Loops. II. Improvements
to the Model
Authors: Cargill, P. J.; Bradshaw, S. J.; Klimchuk, J. A.
2012ApJ...752..161C Altcode: 2012arXiv1204.5960C
This paper develops the zero-dimensional (0D) hydrodynamic coronal loop
model "Enthalpy-based Thermal Evolution of Loops" (EBTEL) proposed by
Klimchuk et al., which studies the plasma response to evolving coronal
heating, especially impulsive heating events. The basis of EBTEL is
the modeling of mass exchange between the corona and transition region
(TR) and chromosphere in response to heating variations, with the key
parameter being the ratio of the TR to coronal radiation. We develop new
models for this parameter that now include gravitational stratification
and a physically motivated approach to radiative cooling. A number of
examples are presented, including nanoflares in short and long loops,
and a small flare. The new features in EBTEL are important for accurate
tracking of, in particular, the density. The 0D results are compared
to a 1D hydro code (Hydrad) with generally good agreement. EBTEL
is suitable for general use as a tool for (1) quick-look results of
loop evolution in response to a given heating function, (2) extensive
parameter surveys, and (3) situations where the modeling of hundreds
or thousands of elemental loops is needed. A single run takes a few
seconds on a contemporary laptop.
---------------------------------------------------------
Title: Nanoflare Evidence from Analysis of the X-Ray Variability of
an Active Region Observed with Hinode/XRT
Authors: Terzo, S.; Reale, F.; Miceli, M.; Kano, R.; Tsuneta, S.;
Klimchuk, J. A.
2012ASPC..455..245T Altcode: 2012arXiv1201.5482T
The heating of the solar corona is one of the big questions in
astrophysics. Rapid pulses called nanoflares are among the best
candidate mechanisms. The analysis of the time variability of coronal
X-ray emission is potentially a very useful tool to detect impulsive
events. We analyze the small-scale variability of a solar active
region in a high cadence Hinode/XRT observation. The dataset allows
us to detect very small deviations of emission fluctuations from the
distribution expected for a constant rate. We discuss the deviations
in the light of the pulsed-heating scenario.
---------------------------------------------------------
Title: Nanoflare Properties throughout Active Regions: Comparing
SDO/AIA Observations with Modeled Active Region Light Curves
Authors: Viall, Nicholeen; Klimchuk, J.
2012AAS...22030904V Altcode:
Coronal plasma in active regions is typically measured to be at
temperatures near 1-3 MK. Is the majority of the coronal plasma in
hydrostatic equilibrium, maintained at these temperatures through
a form of quasi-steady heating, or is this simply a measure of the
average temperature of widely varying, impulsively heated coronal
plasma? Addressing this question is complicated by the fact that
the corona is optically thin: many thousands of flux tubes which
are heated completely independently are contributing to the total
emission along a given line of sight. There is a large body of work
focused on the heating of isolated features - coronal loops - which are
impulsively heated, however it is the diffuse emission between loops
which often comprises the majority of active region emission. Therefore
in this study we move beyond isolated features and analyze all of the
emission in an entire active region from all contributing flux tubes. We
investigate light curves systematically using SDO/AIA observations. We
also model the active region corona as a line-of-sight integration
of many thousands of completely independently heated flux tubes. The
emission from these flux tubes may be time dependent, quasi-steady, or
a mix of both, depending on the cadence of heat release. We demonstrate
that despite the superposition of randomly heated flux tubes, different
distributions of nanoflare cadences produce distinct signatures in
light curves observed with multi-wavelength and high time cadence data,
such as those from SDO/AIA. We conclude that the majority of the active
region plasma is not maintained in hydrostatic equilibrium, rather
it is undergoing dynamic heating and cooling cycles. The observed
emission is consistent with heating through impulsive nanoflares,
whose energy is a function of location within the active region. <P
/>This research was supported by an appointment to the NASA Postdoctoral
Program at GSFC/NASA.
---------------------------------------------------------
Title: Understanding Coronal Heating with Emission Measure
Distributions
Authors: Klimchuk, James A.; Tripathi, D.; Bradshaw, S. J.; Mason,
H. E.
2012AAS...22042302K Altcode:
It is widely believed that the cross-field spatial scale of coronal
heating is small, so that the fundamental plasma structures
(loop strands) are spatially unresolved. We therefore must
appeal to diagnostic techniques that are not strongly affected by
spatial averaging. One valuable observable is the emission measure
distribution, EM(T), which indicates how much material is present
at each temperature. Using data from the Extreme-ultraviolet Imaging
Spectrograph on the Hinode mission, we have determined emission measure
distributions in the cores of two active regions. The distributions have
power law slopes of approximately 2.4 coolward of the peak. We compare
these slopes, as well as the amount of emission measure at very high
temperature, with the predictions of a series of models. The models
assume impulsive heating (nanoflares) in unresolved strands and take
full account of nonequilibrium ionization.
---------------------------------------------------------
Title: Division II: Sun and Heliosphere
Authors: Martínez Pillet, Valentín; Klimchuk, James A.; Melrose,
Donald B.; Cauzzi, Gianna; van Driel-Gesztelyi, Lidia; Gopalswamy,
Natchimuthuk; Kosovichev, Alexander; Mann, Ingrid; Schrijver,
Carolus J.
2012IAUTA..28...61M Altcode: 2012IAUTA..28...61P
The solar activity cycle entered a prolonged quiet phase that started
in 2008 and ended in 2010. This minimum lasted for a year longer
than expected and all activity proxies, as measured from Earth and
from Space, reached minimum values never observed before (de Toma,
2012). The number of spotless days from 2006 to 2009 totals 800, the
largest ever recorded in modern times. Solar irradiance was at historic
minimums. The interplanetary magnetic field was measured at values as
low as 2.9 nT and the cosmic rays were observed at records-high. While
rumors spread that the Sun could be entering a grand minimum quiet
phase (such as the Maunder minimum of the XVII century), activity
took over in 2010 and we are now well into Solar Cycle 24 (albeit,
probably, a low intensity cycle), approaching towards a maximum due
by mid 2013. In addition to bringing us the possibility to observe
a quiet state of the Sun and of the Heliosphere that was previously
not recorded with modern instruments, the Sun has also shown us how
little we know about the dynamo mechanism that drives its activity as
all solar cycle predictions failed to see this extended minimum coming.
---------------------------------------------------------
Title: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk,
James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
Vršnak, Bojan; Yan, Yihua
2012IAUTA..28...69V Altcode:
Commission 10 of the International Astronomical Union has more than
650 members who study a wide range of activity phenomena produced by
our nearest star, the Sun. Solar activity is intrinsically related
to solar magnetic fields and encompasses events from the smallest
energy releases (nano- or even picoflares) to the largest eruptions
in the Solar System, coronal mass ejections (CMEs), which propagate
into the Heliosphere reaching the Earth and beyond. Solar activity is
manifested in the appearance of sunspot groups or active regions, which
are the principal sources of activity phenomena from the emergence of
their magnetic flux through their dispersion and decay. The period
2008-2009 saw an unanticipated extended solar cycle minimum and
unprecedentedly weak polar-cap and heliospheric field. Associated with
that was the 2009 historical maximum in galactic cosmic rays flux since
measurements begun in the middle of the 20th Century. Since then Cycle
24 has re-started solar activity producing some spectacular eruptions
observed with a fleet of spacecraft and ground-based facilities. In
the last triennium major advances in our knowledge and understanding
of solar activity were due to continuing success of space missions as
SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched
by the breathtaking images of the solar atmosphere produced by the
Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the
framework of NASA's Living with a Star program. In August 2012, at the
time of the IAU General Assembly in Beijing when the mandate of this
Commission ends, we will be in the unique position to have for the
first time a full 3-D view of the Sun and solar activity phenomena
provided by the twin STEREO missions about 120 degrees behind and
ahead of Earth and other spacecraft around the Earth and ground-based
observatories. These new observational insights are continuously
posing new questions, inspiring and advancing theoretical analysis
and modelling, improving our understanding of the physics underlying
magnetic activity phenomena. Commission 10 reports on a vigorously
evolving field of research produced by a large community. The number
of refereed publications containing `Sun', `heliosphere', or a synonym
in their abstracts continued the steady growth seen over the preceding
decades, reaching about 2000 in the years 2008-2010, with a total of
close to 4000 unique authors. This report, however, has its limitations
and it is inherently incomplete, as it was prepared jointly by the
members of the Organising Committee of Commission 10 (see the names
of the primary contributors to the sections indicated in parentheses)
reflecting their fields of expertise and interest. Nevertheless, we
believe that it is a representative sample of significant new results
obtained during the last triennium in the field of solar activity.
---------------------------------------------------------
Title: Spectroscopic Diagnostics and Heating of Active Region Cores
Authors: Tripathi, D.; Mason, H. E.; Klimchuk, J. A.
2012decs.confE..92T Altcode:
It is widely believed that we are still far from spatially resolving
the fundamental plasma structures in solar corona. Therefore,
we must use spectroscopic diagnostic techniques such as emission
measure distribution (EM(T)) and Doppler shifts that are not affected
by spatial averaging. Using observations recorded by the Extreme
ultraviolet Imaging Spectrometer we have studies emission measure (EM)
distribution and Doppler shift in the moss and inter-moss regions. The
EM distributions obtained for moss regions cab be reproduced by
considering strong coronal condensation scenario suggesting bulk
downflow of the plasma. Doppler shift measurements for the moss regions
show that almost all the moss regions are red-shifted with velocities
up to 15km/s with mean velocity of 5 km/s. However, the uncertainty
on the Doppler shift was large. The EM distributions obtained for
inter-moss regions have power law slopes of approximately 2.4 coolward
of the peak. We compare the EM for inter-moss region with that obtained
from nanoflare model using EBTEL (Enthalpy-Based Thermal Evolution of
Loops). Our results suggest that the EM distribution for both the moss
as well as inter-moss regions and Doppler shift in the moss regions can
be explained by nanoflare heating. IRIS will provide a better account
of the Doppler shift in the moss regions, which will dramatically
enhance our understanding of the heating of active region core.
---------------------------------------------------------
Title: EBTEL: Enthalpy-Based Thermal Evolution of Loops
Authors: Klimchuk, J. A.; Patsourakos, S.; Cargill, P. J.
2012ascl.soft03007K Altcode:
Observational and theoretical evidence suggests that coronal heating
is impulsive and occurs on very small cross-field spatial scales. A
single coronal loop could contain a hundred or more individual strands
that are heated quasi-independently by nanoflares. It is therefore
an enormous undertaking to model an entire active region or the
global corona. Three-dimensional MHD codes have inadequate spatial
resolution, and 1D hydro codes are too slow to simulate the many
thousands of elemental strands that must be treated in a reasonable
representation. Fortunately, thermal conduction and flows tend to
smooth out plasma gradients along the magnetic field, so "0D models"
are an acceptable alternative. We have developed a highly efficient
model called Enthalpy-Based Thermal Evolution of Loops (EBTEL) that
accurately describes the evolution of the average temperature, pressure,
and density along a coronal strand. It improves significantly upon
earlier models of this type-in accuracy, flexibility, and capability. It
treats both slowly varying and highly impulsive coronal heating;
it provides the differential emission measure distribution, DEM(T),
at the transition region footpoints; and there are options for heat
flux saturation and nonthermal electron beam heating. EBTEL gives
excellent agreement with far more sophisticated 1D hydro simulations
despite using four orders of magnitude less computing time. It promises
to be a powerful new tool for solar and stellar studies.
---------------------------------------------------------
Title: The Pros and Cons of 1D vs. 3D Modeling
Authors: Klimchuk, James A.
2012decs.confE..25K Altcode:
Advances in computing capability have led to tremendous improvements in
3D modeling. Entire active regions are being simulated in what might
be described as a first principles way, in which plasma heating is
treated self consistently rather than through the specification of
heating functions. There are limitations to this approach, however,
as actual heating mechanisms on the Sun involve spatial scales orders
of magnitude smaller than what these simulations can resolve. Other
simulations begin to resolve these scales, but they only treat a
tiny volume and do not include the all important coupling with larger
scales or with other parts of the atmosphere, and so cannot be readily
compared with observations. Finally, 1D hydrodynamic models capture the
field-aligned evolution of the plasma extremely well and are ideally
suited for data comparison, but they treat the heating in a totally
ad hoc manner. All of these approaches have important contributions
to make, but we must be aware of their limitations. I will highlight
some of the strengths and weaknesses of each.
---------------------------------------------------------
Title: Determining the Typical Nanoflare Cadence in Active Regions:
Comparing SDO/AIA Observations with Modeled Active Region Light Curves
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2012decs.confE..40V Altcode:
Coronal plasma in active regions is typically measured to be at
temperatures near 1-3 MK. Is the majority of the coronal plasma in
hydrostatic equilibrium, maintained at these temperatures through a
form of quasi-steady heating, or is this simply a measure of the average
temperature of widely varying, impulsively heated coronal plasma which
is continually undergoing heating and cooling cycles? Addressing this
question is complicated by the fact that the corona is optically thin:
many thousands of strands which are heated completely independently are
contributing to the total emission along a given line of sight. There
is a large body of work focused on the heating of coronal loops,
which are impulsively heated, however it is the diffuse emission
between loops which often comprises the majority of active region
emission. Therefore, a different and necessary approach to analyzing
active region heating is to analyze all of the emission in an active
region, and account for emission along the line of sight from all of
the contributing strands. We investigate light curves systematically in
an entire active region using SDO/AIA observations. We also model the
active region corona as a line-of-sight integration of many thousands
of completely independently heated strands. The emission from these
flux tubes may be time dependent, quasi-steady, or a mix of both,
depending on the cadence of heat release on each strand. We examine a
full range of heat cadences from effectively steady (heat pulse repeat
time << plasma cooling time) to fully impulsive (heat pulse repeat
time >> plasma cooling time) and model the resulting emission
when superposing strands undergoing these differing heat cycles. We
demonstrate that despite the superposition of randomly heated strands,
different distributions of heat cadences produce distinct signatures
in light curves observed with multi-wavelength and high time cadence
data, such as those from the AIA telescopes on SDO. Using these model
predictions in conjunction with SDO/AIA observations, we evaluate the
typical cadence of heat release in different active regions and patterns
therein, which is a crucial constraint on coronal heating mechanisms.
---------------------------------------------------------
Title: The Origin of the EUV Late Phase: A Case Study of the C8.8
Flare on 2010 May 5
Authors: Hock, R. A.; Woods, T. N.; Klimchuk, J. A.; Eparvier, F. G.;
Jones, A. R.
2012arXiv1202.4819H Altcode:
Since the launch of NASA's Solar Dynamics Observatory on 2010 February
11, the Extreme ultraviolet Variability Experiment (EVE) has observed
numerous flares. One interesting feature observed by EVE is that a
subset of flares exhibit an additional enhancement of the 2-3 million
K emission several hours after the flare's soft X-ray emission. From
the Atmospheric Imaging Assembly (AIA) images, we observe that this
secondary emission, dubbed the EUV late phase, occurs in the same active
region as the flare but not in the same coronal loops. Here, we examine
the C8.8 flare that occurred on 2010 May 5 as a case study of EUV late
phase flares. In addition to presenting detailed observations from
both AIA and EVE, we develop a physical model of this flare and test
it using the Enthalpy Based Thermal Evolution of Loops (EBTEL) model.
---------------------------------------------------------
Title: Evidence of nanoflare heating in coronal loops observed with
Hinode/XRT and SDO/AIA
Authors: López Fuentes, M. C.; Klimchuk, J. A.
2012BAAA...55..103L Altcode:
We study a series of coronal loop lightcurves from X-ray and EUV
observations. In search for signatures of nanoflare heating, we analyze
the statistical properties of the observed lightcurves and compare them
with synthetic cases obtained with a 2D cellular-automaton model based
on nanoflare heating driven by photospheric motions. Our analysis shows
that the observed and the model lightcurves have similar statistical
properties. The asymmetries observed in the distribution of the
intensity fluctuations indicate the possible presence of widespread
cooling processes in sub-resolution magnetic strands.
---------------------------------------------------------
Title: Understanding Coronal Heating with Emission Measure
Distributions
Authors: Klimchuk, J. A.; Tripathi, D.; Bradshaw, S. J.; Mason, H. E.
2011AGUFMSH43F..03K Altcode:
It is widely believed that the cross-field spatial scale of coronal
heating is small, so that the fundamental plasma structures
(loop strands) are spatially unresolved. We therefore must
appeal to diagnostic techniques that are not strongly affected by
spatial averaging. One valuable observable is the emission measure
distribution, EM(T), which indicates how much material is present
at each temperature. Using data from the Extreme-ultraviolet Imaging
Spectrograph on the Hinode mission, we have determined emission measure
distributions in the cores of two active regions. The distributions
have power law slopes of approximately 2.4 coolward of the peak. We
compare these slopes, as well as the amount of emission measure at
very high temperature, with the predictions of a series of models. The
models assume impulsive heating (nanoflares) in unresolved strands and
take full account of nonequilibrium ionization. A variety of nanoflare
properties and initial conditions are considered. We also comment on the
selection of spectral lines for upcoming missions like Solar Orbiter.
---------------------------------------------------------
Title: Determining the Typical Nanoflare Cadence in Active Regions:
Modeling Light Curves of Active Regions
Authors: Viall, N. M.; Klimchuk, J. A.
2011AGUFMSH33B2057V Altcode:
Active region coronal loops visible at 1MK are likely composed of many
unresolved strands, heated by storms of impulsive nanoflares. Though
well-studied, these loops often contribute only a fraction of the total
emission in an active region; the degree to which the entire active
region is heated in the same manner as loops are is highly debated. Is
the majority of coronal active region plasma heated impulsively, or
is the majority of the heating quasi-steady? Addressing this question
is complicated by the fact that the corona is optically thin: many
thousands of strands which are heated completely independently
are contributing to the total emission along a given line of
sight. Furthermore, certain geometries preclude even the best background
subtraction methods from fully isolating the emission from even a
single coronal loop. Therefore, a different and necessary approach
to analyzing active region heating is to account for emission along
the line of sight from all of the contributing strands. We model the
active region corona as a line-of-sight integration of many thousands
of completely independently heated strands. The emission from these
flux tubes may be time dependent, quasi-steady, or a mix of both,
depending on the cadence of heat release on each strand. We examine a
full range of heat cadences from effectively steady (heat pulse repeat
time << plasma cooling time) to fully impulsive (heat pulse repeat
time >> plasma cooling time) and model the resulting emission
when superposing strands undergoing these differing heat cycles. We
demonstrate that despite the superposition of randomly heated strands,
different distributions of heat cadences produce distinct signatures
in light curves observed with multi-wavelength and high time cadence
data, such as those from the AIA telescopes on SDO. For this reason,
high time cadence spectral information for lines sensitive to the 1-10
MK range will be especially useful in future missions. Using these
model predictions, we evaluate the typical cadence of heat release
in different active regions and patterns therein, which is a crucial
constraint on coronal heating mechanisms.
---------------------------------------------------------
Title: New Solar Extreme-ultraviolet Irradiance Observations during
Flares
Authors: Woods, Thomas N.; Hock, Rachel; Eparvier, Frank; Jones,
Andrew R.; Chamberlin, Phillip C.; Klimchuk, James A.; Didkovsky,
Leonid; Judge, Darrell; Mariska, John; Warren, Harry; Schrijver,
Carolus J.; Webb, David F.; Bailey, Scott; Tobiska, W. Kent
2011ApJ...739...59W Altcode:
New solar extreme-ultraviolet (EUV) irradiance observations from the
NASA Solar Dynamics Observatory (SDO) EUV Variability Experiment provide
full coverage in the EUV range from 0.1 to 106 nm and continuously at
a cadence of 10 s for spectra at 0.1 nm resolution and even faster,
0.25 s, for six EUV bands. These observations can be decomposed into
four distinct characteristics during flares. First, the emissions
that dominate during the flare's impulsive phase are the transition
region emissions, such as the He II 30.4 nm. Second, the hot coronal
emissions above 5 MK dominate during the gradual phase and are highly
correlated with the GOES X-ray. A third flare characteristic in the
EUV is coronal dimming, seen best in the cool corona, such as the
Fe IX 17.1 nm. As the post-flare loops reconnect and cool, many of
the EUV coronal emissions peak a few minutes after the GOES X-ray
peak. One interesting variation of the post-eruptive loop reconnection
is that warm coronal emissions (e.g., Fe XVI 33.5 nm) sometimes exhibit
a second large peak separated from the primary flare event by many
minutes to hours, with EUV emission originating not from the original
flare site and its immediate vicinity, but rather from a volume of
higher loops. We refer to this second peak as the EUV late phase. The
characterization of many flares during the SDO mission is provided,
including quantification of the spectral irradiance from the EUV late
phase that cannot be inferred from GOES X-ray diagnostics.
---------------------------------------------------------
Title: Emission Measure Distribution and Heating of Two Active
Region Cores
Authors: Tripathi, Durgesh; Klimchuk, James A.; Mason, Helen E.
2011ApJ...740..111T Altcode: 2011arXiv1107.4480T
Using data from the Extreme-ultraviolet Imaging Spectrometer aboard
Hinode, we have studied the coronal plasma in the core of two active
regions. Concentrating on the area between opposite polarity moss, we
found emission measure distributions having an approximate power-law
form EMvpropT <SUP>2.4</SUP> from log T = 5.5 up to a peak at log T
= 6.55. We show that the observations compare very favorably with
a simple model of nanoflare-heated loop strands. They also appear
to be consistent with more sophisticated nanoflare models. However,
in the absence of additional constraints, steady heating is also a
viable explanation.
---------------------------------------------------------
Title: Patterns of Nanoflare Storm Heating Exhibited by an Active
Region Observed with Solar Dynamics Observatory/Atmospheric Imaging
Assembly
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2011ApJ...738...24V Altcode: 2011arXiv1106.4196V
It is largely agreed that many coronal loops—those observed at a
temperature of about 1 MK—are bundles of unresolved strands that are
heated by storms of impulsive nanoflares. The nature of coronal heating
in hotter loops and in the very important but largely ignored diffuse
component of active regions is much less clear. Are these regions also
heated impulsively, or is the heating quasi-steady? The spectacular
new data from the Atmospheric Imaging Assembly (AIA) telescopes on the
Solar Dynamics Observatory offer an excellent opportunity to address
this question. We analyze the light curves of coronal loops and the
diffuse corona in six different AIA channels and compare them with the
predicted light curves from theoretical models. Light curves in the
different AIA channels reach their peak intensities with predictable
orderings as a function the nanoflare storm properties. We show that
while some sets of light curves exhibit clear evidence of cooling
after nanoflare storms, other cases are less straightforward to
interpret. Complications arise because of line-of-sight integration
through many different structures, the broadband nature of the AIA
channels, and because physical properties can change substantially
depending on the magnitude of the energy release. Nevertheless, the
light curves exhibit predictable and understandable patterns consistent
with impulsive nanoflare heating.
---------------------------------------------------------
Title: Widespread Nanoflare Variability Detected with Hinode/X-Ray
Telescope in a Solar Active Region
Authors: Terzo, Sergio; Reale, Fabio; Miceli, Marco; Klimchuk, James
A.; Kano, Ryouhei; Tsuneta, Saku
2011ApJ...736..111T Altcode: 2011arXiv1105.2506T
It is generally agreed that small impulsive energy bursts called
nanoflares are responsible for at least some of the Sun's hot corona,
but whether they are the explanation for most of the multimillion-degree
plasma has been a matter of ongoing debate. We present here evidence
that nanoflares are widespread in an active region observed by the X-Ray
Telescope on board the Hinode mission. The distributions of intensity
fluctuations have small but important asymmetries, whether taken
from individual pixels, multipixel subregions, or the entire active
region. Negative fluctuations (corresponding to reduced intensity)
are greater in number but weaker in amplitude, so that the median
fluctuation is negative compared to a mean of zero. Using Monte Carlo
simulations, we show that only part of this asymmetry can be explained
by Poisson photon statistics. The remainder is explainable through
a tendency for exponentially decreasing intensity, such as would be
expected from a cooling plasma produced from a nanoflare. We suggest
that nanoflares are a universal heating process within active regions.
---------------------------------------------------------
Title: Heating of Active Regions by Impulsive Nanoflares
Authors: Viall, Nicholeen Mary; Klimchuk, James A.
2011shin.confE..57V Altcode:
It has been proposed that plasma on active regions may be contributing
plasma to the slow solar wind. If this is the case, then understanding
the heating and dynamics of active regions adds vital knowledge to
our understanding of the heating and acceleration of the slow solar
wind. It seems largely agreed that many coronal loops--those observed
at a temperature of about 1 MK--are bundles of unresolved strands that
are heated by storms of impulsive nanoflares. The nature of coronal
heating in hotter loops and in the very important but largely ignored
diffuse component of active regions is much less clear. Are these
regions also heated impulsively, or is the heating quasi steady? The
spectacular new data from the Atmospheric Imaging Assembly (AIA)
telescopes on the Solar Dynamics Observatory (SDO) offer an excellent
opportunity to address this question. We analyze the light curves
of coronal loops and the diffuse corona in 6 different AIA channels
and compare them with the predicted light curves from theoretical
models. Light curves in the different AIA channels reach their peak
intensities with predictable orderings as a function of the nanoflare
storm properties. These orderings, or time lags, are clearly exhibited
in loop observations in all channels. What is especially exciting
is that we identify these time lag patterns in observations of the
seemingly steady diffuse corona as well. We model the diffuse corona as
a line-of-sight integration of many thousands of completely independent,
impulsively heated strands. The time lags of the simulated and actual
observations are in excellent agreement. Our results suggest that
impulsive nanoflare heating is ubiquitous within active regions.
---------------------------------------------------------
Title: What Dominates the Coronal Emission Spectrum During the Cycle
of Impulsive Heating and Cooling?
Authors: Bradshaw, S. J.; Klimchuk, J. A.
2011ApJS..194...26B Altcode:
The "smoking gun" of small-scale, impulsive events heating the solar
corona is expected to be the presence of hot (>5 MK) plasma. Evidence
for this has been scarce, but has gradually begun to accumulate due to
recent studies designed to constrain the high-temperature part of the
emission measure distribution. However, the detected hot component
is often weaker than models predict and this is due in part to the
common modeling assumption that the ionization balance remains in
equilibrium. The launch of the latest generation of space-based
observing instrumentation on board Hinode and the Solar Dynamics
Observatory (SDO) has brought the matter of the ionization state of the
plasma firmly to the forefront. It is timely to consider exactly what
emission current instruments would detect when observing a corona heated
impulsively on small scales by nanoflares. Only after we understand
the full effects of nonequilibrium ionization can we draw meaningful
conclusions about the plasma that is (or is not) present. We have
therefore performed a series of hydrodynamic simulations for a variety
of different nanoflare properties and initial conditions. Our study
has led to several key conclusions. (1) Deviations from equilibrium
are greatest for short-duration nanoflares at low initial coronal
densities. (2) Hot emission lines are the most affected and are
suppressed sometimes to the point of being invisible. (3) For the many
scenarios we have considered, the emission detected in several of the
SDO-AIA channels (131, 193, and 211 Å) would be dominated by warm,
overdense, cooling plasma. (4) It is difficult not to create coronal
loops that emit strongly at 1.5 MK and in the range 2-6 MK, which
are the most commonly observed kind, for a broad range of nanoflare
scenarios. (5) The Fe XV (284.16 Å) emission in most of our models
is about 10 times brighter than the Ca XVII (192.82 Å) emission,
consistent with observations. Our overarching conclusion is that
small-scale, impulsive heating inducing a nonequilibrium ionization
state leads to predictions for observable quantities that are entirely
consistent with what is actually observed.
---------------------------------------------------------
Title: Patterns of Nanoflare Heating Exhibited by Active Regions
Observed with SDO/AIA
Authors: Viall, Nicholeen; Klimchuk, J.
2011SPD....42.2103V Altcode: 2011BAAS..43S.2103V
It seems largely agreed that many coronal loops---those observed at a
temperature of about 1 MK---are bundles of unresolved strands that are
heated by storms of impulsive nanoflares. The nature of coronal heating
in hotter loops and in the very important but largely ignored diffuse
component of active regions is much less clear. Are these regions also
heated impulsively, or is the heating quasi steady? The spectacular new
data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar
Dynamics Observatory (SDO) offer an excellent opportunity to address
this question. We analyze the light curves of coronal loops and the
diffuse corona in 6 different AIA channels and compare them with the
predicted light curves from theoretical models. Light curves in the
different AIA channels reach their peak intensities with predictable
orderings as a function of the nanoflare storm properties. These
orderings, or time lags, are clearly exhibited in loop observations in
all channels. What is especially exciting is that we identify these time
lag patterns in observations of the seemingly steady diffuse corona
as well. We model the diffuse corona as a line-of-sight integration
of many thousands of completely independent, impulsively heated
strands. The time lags of the simulated and actual observations are
in excellent agreement. Our results suggest that impulsive nanoflare
heating is ubiquitous within active regions. <P />This research was
supported through an appointment to the NASA Postdoctoral Program at
the Goddard Space Flight Center, administered by Oak Ridge Associated
Universities through a contract with NASA.
---------------------------------------------------------
Title: Radiative Signatures of the Coronal Heating and Cooling Cycle
Authors: Bradshaw, Stephen; Klimchuk, J.
2011SPD....42.0503B Altcode: 2011BAAS..43S.0503B
The 'smoking gun' of small-scale, impulsive heating in the non-flaring
solar corona is expected to be emission from plasma at temperatures
greater than 5 MK. Recent studies, designed to strongly constrain
the high temperature part of the emission measure distribution, have
begun to provide evidence for such a hot component to the emission
spectrum. However, it is significantly weaker than predicted by
numerical models. We propose that the discrepancy can be resolved by
dropping the common modeling assumption of ionization equilibrium. <P
/>The launches of Hinode and the Solar Dynamics Observatory (SDO)
have brought the matter of the ionization state of the plasma, which
lies at the interface between models and observations, firmly to the
forefront. <P />We present detailed, quantitative predictions for
the spectral emission that the instruments EIS and AIA would actually
detect, for a broad range of impulsive heating scenarios, derived from
a combination of numerical hydrodynamic and forward modeling. <P />We
demonstrate that a nonequilibrium ionization state, as induced by
small-scale, impulsive heating, leads to predictions for observable
quantities that are consistent with what is actually observed.
---------------------------------------------------------
Title: Are Spicules the Primary Source of Hot Coronal Plasma?
Authors: Klimchuk, James A.
2011SPD....42.1801K Altcode: 2011BAAS..43S.1801K
The recent discovery of Type II spicules has generated considerable
excitement. It has even been suggested that these ejections can account
for a majority of the hot plasma observed in the corona, thus obviating
the need for "coronal” heating. If this is the case, however, then
there should be observational consequences. We have begun to examine
some of these consequences and find reason to question the idea that
spicules are the primary source of hot coronal plasma.
---------------------------------------------------------
Title: SDO/AIA Light Curves and Implications for Coronal Heating:
Observations
Authors: Viall, N. M.; Klimchuk, J. A.
2010AGUFMSH41E..02V Altcode:
It seems largely agreed that many coronal loops---those observed at a
temperature of about 1 MK---are bundles of unresolved strands that are
heated by storms of impulsive nanoflares. The nature of coronal heating
in hotter loops and in the very important but largely ignored diffuse
component of active regions is much less clear. Is it also impulsive
or is it quasi steady? The spectacular new data from the Atmospheric
Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory
(SDO) offer an excellent opportunity to address this question. We
analyze the light curves of coronal loops and the diffuse corona in
6 different AIA channels and compare them with the predicted light
curves from theoretical models. Light curves in the different AIA
channels reach their peak intensities with predictable orderings as a
function the nanoflare storm properties. We show that while some sets of
light curves exhibit clear evidence of cooling after nanoflare storms,
other cases are less straightforward to interpret. Complications arise
because of line-of-sight integration through many different structures,
the broadband nature of the AIA channels, and because physical
properties can change substantially depending on the magnitude of the
energy release. Nevertheless, the light curves exhibit predictable and
understandable patterns. This presentation emphasizes the observational
aspects of our study. A companion presentation emphasizes the models.
---------------------------------------------------------
Title: Modeling the Secondary Flare Irradiance Measured by Solar
Dynamic Observatory (SDO) Extreme ultraviolet Variability Experiment
(EVE)
Authors: Hock, R. A.; Woods, T. N.; Klimchuk, J. A.; Eparvier, F. G.
2010AGUFMSH13A..05H Altcode:
NASA’s Solar Dynamic Observatory (SDO) launched on 11 February
2010 and normal operations for all three instruments began 1 May
2010. Since then numerous small and moderate (C- and M-class) flares
have been observed. One interesting feature observed by the Extreme
ultraviolet Variability Experiment (EVE) is the enhancement of 2-3
million K emission several hours after the flare’s soft x-ray
emission. From the Atmospheric Imaging Assembly (AIA) images, we can
tell that these secondary emissions occur in the same active region
as the flare but not in same coronal loops. Here, we examine the C8.8
flare that occurred on 5 May 2010. The flare occurred in Active Region
11069, a small magnetically complex region near the western limb of
the Sun. The gradual phase of the flare is clearly seen in both GOES
soft X-rays and the hot coronal emissions (>2 million K) measured
by EVE. The secondary flare emission starts 30 minutes after the peak
in gradual phase and slowly increases over an hour before decaying to
the pre-flare levels. It is most strongly seen in Fe XV and Fe XVI (2-3
million K). Using the Enthalpy-Based Thermal Evolution of Loops (EBTEL)
model, we are able calculate the EUV irradiance of a set of coronal
loops for a given heating function. This allows us to determine the
best-fit heating profile as a function of time for the C8.8 flare. The
heating profile for this event clearly shows that there are two separate
phases of heating. The first phase involves traditional post-flare
loops. The field reconnects after erupting, and the energy released
during the reconnection heats the plasma to very high temperatures. As
the loops cool, emissions are seen in progressively cooler lines from
10 to 1 million K. The second phase is very different. A large number
of coronal loops are heated only modestly-. The plasma in each loop,
instead of reaching 10 million K, reaches 3 million K. The heating
is also spread out over an hour generating the long secondary flare
emission profile. We discuss the nature of this secondary flare
emission, which seems to be an important component of many events.
---------------------------------------------------------
Title: SDO/AIA Light Curves and Implications for Coronal Heating:
Model Predictions
Authors: Klimchuk, J. A.; Viall, N. M.
2010AGUFMSH41E..03K Altcode:
It seems largely agreed that many coronal loops---those observed
at a temperature of about 1 MK---are bundles of unresolved strands
that are heated by storms of impulsive nanoflares. The nature of
coronal heating in hotter loops and in the very important but largely
ignored diffuse component of active regions is much less clear. Is
it also impulsive or is it quasi steady? The spectacular new data
from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar
Dynamics Observatory (SDO) offer an excellent opportunity to address
this question. We analyze the light curves of coronal loops and the
diffuse corona in 6 different AIA channels and compare them with the
predicted light curves from theoretical models. Light curves in the
different AIA channels reach their peak intensities with predictable
orderings as a function the nanoflare storm properties. We show that
while some sets of light curves exhibit clear evidence of cooling
after nanoflare storms, other cases are less straightforward to
interpret. Complications arise because of line-of-sight integration
through many different structures, the broadband nature of the AIA
channels, and because physical properties can change substantially
depending on the magnitude of the energy release. Nevertheless, the
light curves exhibit predictable and understandable patterns. This
presentation emphasizes the modeling aspects of our study. A companion
presentation emphasizes the observations.
---------------------------------------------------------
Title: Science Objectives for an X-Ray Microcalorimeter Observing
the Sun
Authors: Laming, J. Martin; Adams, J.; Alexander, D.; Aschwanden, M;
Bailey, C.; Bandler, S.; Bookbinder, J.; Bradshaw, S.; Brickhouse,
N.; Chervenak, J.; Christe, S.; Cirtain, J.; Cranmer, S.; Deiker, S.;
DeLuca, E.; Del Zanna, G.; Dennis, B.; Doschek, G.; Eckart, M.; Fludra,
A.; Finkbeiner, F.; Grigis, P.; Harrison, R.; Ji, L.; Kankelborg,
C.; Kashyap, V.; Kelly, D.; Kelley, R.; Kilbourne, C.; Klimchuk, J.;
Ko, Y. -K.; Landi, E.; Linton, M.; Longcope, D.; Lukin, V.; Mariska,
J.; Martinez-Galarce, D.; Mason, H.; McKenzie, D.; Osten, R.; Peres,
G.; Pevtsov, A.; Porter, K. Phillips F. S.; Rabin, D.; Rakowski, C.;
Raymond, J.; Reale, F.; Reeves, K.; Sadleir, J.; Savin, D.; Schmelz,
J.; Smith, R. K.; Smith, S.; Stern, R.; Sylwester, J.; Tripathi, D.;
Ugarte-Urra, I.; Young, P.; Warren, H.; Wood, B.
2010arXiv1011.4052L Altcode:
We present the science case for a broadband X-ray imager with
high-resolution spectroscopy, including simulations of X-ray spectral
diagnostics of both active regions and solar flares. This is part of
a trilogy of white papers discussing science, instrument (Bandler et
al. 2010), and missions (Bookbinder et al. 2010) to exploit major
advances recently made in transition-edge sensor (TES) detector
technology that enable resolution better than 2 eV in an array that
can handle high count rates. Combined with a modest X-ray mirror, this
instrument would combine arcsecondscale imaging with high-resolution
spectra over a field of view sufficiently large for the study of
active regions and flares, enabling a wide range of studies such as
the detection of microheating in active regions, ion-resolved velocity
flows, and the presence of non-thermal electrons in hot plasmas. It
would also enable more direct comparisons between solar and stellar
soft X-ray spectra, a waveband in which (unusually) we currently have
much better stellar data than we do of the Sun.
---------------------------------------------------------
Title: On the Isothermality of Solar Plasmas
Authors: Landi, E.; Klimchuk, J. A.
2010ApJ...723..320L Altcode:
Recent measurements have shown that the quiet unstructured solar corona
observed at the solar limb is close to isothermal, at a temperature
that does not appear to change over wide areas or with time. Some
individual active region loop structures have also been found to
be nearly isothermal both along their axis and across their cross
section. Even a complex active region observed at the solar limb has
been found to be composed of three distinct isothermal plasmas. If
confirmed, these results would pose formidable challenges to the current
theoretical understanding of the thermal structure and heating of the
solar corona. For example, no current theoretical model can explain
the excess densities and lifetimes of many observed loops if the loops
are in fact isothermal. All of these measurements are based on the
so-called emission measure (EM) diagnostic technique that is applied
to a set of optically thin lines under the assumption of isothermal
plasma. It provides simultaneous measurement of both the temperature and
EM. In this work, we develop a new method to quantify the uncertainties
in the technique and to rigorously assess its ability to discriminate
between isothermal and multithermal plasmas. We define a formal measure
of the uncertainty in the EM diagnostic technique that can easily be
applied to real data. We here apply it to synthetic data based on a
variety of assumed plasma thermal distributions and develop a method
to quantitatively assess the degree of multithermality of a plasma.
---------------------------------------------------------
Title: Evidence of Impulsive Heating in Active Region Core Loops
Authors: Tripathi, Durgesh; Mason, Helen E.; Klimchuk, James A.
2010ApJ...723..713T Altcode: 2010arXiv1009.0663T
Using a full spectral scan of an active region from the
Extreme-Ultraviolet Imaging Spectrometer (EIS) we have obtained emission
measure EM(T) distributions in two different moss regions within the
same active region. We have compared these with theoretical transition
region EMs derived for three limiting cases, namely, static equilibrium,
strong condensation, and strong evaporation from Klimchuk et al. The
EM distributions in both the moss regions are strikingly similar and
show a monotonically increasing trend from log T[K] = 5.15-6.3. Using
photospheric abundances, we obtain a consistent EM distribution for
all ions. Comparing the observed and theoretical EM distributions,
we find that the observed EM distribution is best explained by the
strong condensation case (EM<SUB>con</SUB>), suggesting that a downward
enthalpy flux plays an important and possibly dominant role in powering
the transition region moss emission. The downflows could be due to
unresolved coronal plasma that is cooling and draining after having
been impulsively heated. This supports the idea that the hot loops
(with temperatures of 3-5 MK) seen in the core of active regions are
heated by nanoflares.
---------------------------------------------------------
Title: A Simple Model for the Evolution of Multi-stranded Coronal
Loops
Authors: López Fuentes, M. C.; Klimchuk, J. A.
2010ApJ...719..591L Altcode: 2010arXiv1004.2061L
We develop and analyze a simple cellular automaton model that reproduces
the main properties of the evolution of soft X-ray coronal loops. We
are motivated by the observation that these loops evolve in three
distinguishable phases that suggest the development, maintenance,
and decay of a self-organized system. The model is based on the
idea that loops are made of elemental strands that are heated by the
relaxation of magnetic stress in the form of nanoflares. In this vision,
usually called the "Parker conjecture," the origin of stress is the
displacement of the strand footpoints due to photospheric convective
motions. Modeling the response and evolution of the plasma we obtain
synthetic light curves that have the same characteristic properties
(intensity, fluctuations, and timescales) as the observed cases. We
study the dependence of these properties on the model parameters and
find scaling laws that can be used as observational predictions of the
model. We discuss the implications of our results for the interpretation
of recent loop observations in different wavelengths.
---------------------------------------------------------
Title: Determining the Temperature Structure of Solar Coronal Loops
using their Temporal Evolution
Authors: Mulu, Fana; Winebarger, A. R.; Warren, H. P.; Aschwanden,
M. J.; Klimchuk, J. A.
2010AAS...21630001M Altcode:
Despite much progress toward understanding the dynamics of the
corona, the physical properties of coronal loops are not yet fully
understood. Recent investigations and observations from different
instruments have yielded contradictory results about the true physical
properties of coronal loops, specifically as to whether the observed
loops are isothermal structures or the convolution of several
multi-thermal strands. In this talk, we introduce a new technique
to determine if an observed loop is isothermal or multi-thermal. We
determine the evolution of ten selected loops in multiple filter
images from the Transition Region and Coronal Explorer (TRACE). Our
new technique calculates the delay, calculates a cooling time,
and determines if that cooling time is consistent with the observed
lifetime. If the observational lifetime of the loop agrees with the
calculated lifetime, then we can conclude that the loop is a single
"monolithic” structure that heats and cools as a homogeneous unit,
with isothermal temperature over the cross-section. If not, the
loop must be a bundle of multiple multi-thermal strands, all being
heated and cooling independently. In the second part of the talk,
we utilize the concept of nanoflare storms to understand the reason
behind the extended lifetimes. By simulating the observed light curves
of the loops using EBTEL (Enthalpy-Based Thermal Evolution of Loops),
we find that the longer observed lifetimes can be reproduced by using
a set of small-scale impulsively heated strands.
---------------------------------------------------------
Title: The Existence and Origin of Turbulence in Solar Active Regions
Authors: Klimchuk, James A.; Nigro, G.; Dahlburg, R. B.; Antiochos,
S. K.
2010AAS...21630205K Altcode:
It has been suggested that turbulence plays a fundamental role in the
heating of solar active regions, with its intermittent behavior being
the explanation of impulsive energy release (nanoflares). We know that
episodes of turbulence are produced in the final nonlinear stage of
the secondary instability of electric current sheets. However, these
current sheets must exist prior to the turbulence. Whether turbulence
can dynamically produce current sheets that would not otherwise be
present is a different and important question. <P />Turbulence occurs
freely in the solar wind and in other situations where the magnetic
field does not dominate. However, the magnetic field strongly resists
being distorted in line-tied, low-beta environments such as active
regions. Can turbulence develop naturally in these environments
without being driven by an instability? To answer this question,
we have performed a time-dependent MHD simulation of a slowly driven
system that does not contain current sheets and is stable to applied
perturbations. We find no evidence for bursty energy release, steep
spatial gradients, or power-law energy spectra that are the typical
signatures of turbulence. We conclude that the turbulence which occurs
in active regions is an important yet secondary process and not the
primary cause of heating.
---------------------------------------------------------
Title: Division II: Sun and Heliosphere
Authors: Melrose, Donald B.; Martinez Pillet, Valentin; Webb, David
F.; Bougeret, Jean-Louis; Klimchuk, James A.; Kosovichev, Alexander;
van Driel-Gesztelyi, Lidia; von Steiger, Rudolf
2010IAUTB..27..146M Altcode:
This report is on activities of the Division at the General Assembly
in Rio de Janeiro. Summaries of scientific activities over the past
triennium have been published in Transactions A, see Melrose et
al. (2008), Klimchuk et al. (2008), Martinez Pillet et al. (2008) and
Bougeret et al. (2008). The business meeting of the three Commissions
were incorporated into the business meeting of the Division. This
report is based in part on minutes of the business meeting, provided
by the Secretary of the Division, Lidia van Driel-Gesztelyi, and it
also includes reports provided by the Presidents of the Commissions
(C10, C12, C49) and of the Working Groups (WGs) in the Division.
---------------------------------------------------------
Title: Can Thermal Nonequilibrium Explain Coronal Loops?
Authors: Klimchuk, James A.; Karpen, Judy T.; Antiochos, Spiro K.
2010ApJ...714.1239K Altcode: 2009arXiv0912.0953K
Any successful model of coronal loops must explain a number of observed
properties. For warm (~1 MK) loops, these include (1) excess density,
(2) flat temperature profile, (3) super-hydrostatic scale height,
(4) unstructured intensity profile, and (5) 1000-5000 s lifetime. We
examine whether thermal nonequilibrium can reproduce the observations
by performing hydrodynamic simulations based on steady coronal heating
that decreases exponentially with height. We consider both monolithic
and multi-stranded loops. The simulations successfully reproduce
certain aspects of the observations, including the excess density,
but each of them fails in at least one critical way. Monolithic models
have far too much intensity structure, while multi-strand models
are either too structured or too long-lived. Our results appear to
rule out the widespread existence of heating that is both highly
concentrated low in the corona and steady or quasi-steady (slowly
varying or impulsive with a rapid cadence). Active regions would have a
very different appearance if the dominant heating mechanism had these
properties. Thermal nonequilibrium may nonetheless play an important
role in prominences and catastrophic cooling events (e.g., coronal rain)
that occupy a small fraction of the coronal volume. However, apparent
inconsistencies between the models and observations of cooling events
have yet to be understood.
---------------------------------------------------------
Title: Nanoflare heating of solar and stellar coronae
Authors: Klimchuk, James
2010cosp...38.2897K Altcode: 2010cosp.meet.2897K
A combination of observational and theoretical evidence suggests
that much, and perhaps most, of the Sun's corona is heated by small
unresolved bursts of energy called nanoflares. It seems likely that
stellar coronae are heated in a similar fashion. Nanoflares are here
taken to mean any impulsive heating that occurs within a magnetic flux
strand. Many mechanisms have this property, including waves, but we
prefer Parker's picture of tangled magnetic fields. The tangling is
caused by turbulent convection at the stellar surface, and magnetic
energy is released when the stresses reach a critical level. We suggest
that the mechanism of energy release is the "secondary instability"
of electric current sheets that are present at the boundaries between
misaligned strands. I will discuss the collective evidence for solar
and stellar nanoflares and hopefully present new results from the
Solar Dynamics Observatory that was just launched.
---------------------------------------------------------
Title: Nanoflares, spicules, and other small-scale dynamic phenomena
on the sun
Authors: Klimchuk, James
2010cosp...38.2831K Altcode: 2010cosp.meet.2831K
There is abundant evidence of highly dynamic phenomena occurring
on very small scales in the solar atmosphere. For example, the
observed properties of many coronal loops can only be ex-plained
if the loops are bundles of unresolved strands that are heated
impulsively by nanoflares. Type II spicules recently discovered by
Hinode are an example of small-scale impulsive events occurring in the
chromosphere. The existence of these and other small-scale phenomena
is not surprising given the highly structured nature of the magnetic
field that is revealed by photo-spheric observations. Dynamic phenomena
also occur on much larger scales, including coronal jets, flares,
and CMEs. It is tempting to suggest that these different phenomena are
all closely related and represent a continuous distribution of sizes
and energies. However, this is a danger-ous over simplification in
my opinion. While it is true that the phenomena all involve "magnetic
reconnection" (the changing of field line connectivity) in some form,
how this occurs depends strongly on the magnetic geometry. A nanoflare
resulting from the interaction of tangled mag-netic strands within a
confined coronal loop is much different from a major flare occurring
at the current sheet formed when a CME rips open an active region. I
will review the evidence for ubiquitous small-scale dynamic phenomena
on the Sun and discuss why different phenomena are not all fundamentally
the same.
---------------------------------------------------------
Title: A cellular automaton nanoflare model of coronal loops
Authors: Lopez Fuentes, Marcelo; Klimchuk, James
2010cosp...38.2833L Altcode: 2010cosp.meet.2833L
In Lopez-Fuentes, Klimchuk and Mandrini (2007) we found that the
evolution of soft X-ray loops can be separated into three main
phases that suggest the development, maintainance, and decay of a
self-organized system. Here, we present a cellular automaton model that
reproduces the main features of the observed evolution. The model is
based on the idea that loops are made of multiple unresolved strands
which have footpoints that are displaced by random photospheric motions
(Parker 1988). In this scenario, there is a continuous increase of
the magnetic stress between neighboring strands until a critical
stress is reached and the accumulated energy is suddenly released by
reconnection. Each of these reconnection "events" is associated with
a nanoflare. Using the EBTEL hydrodynamic code (Klimchuk, Patsourakos
and Cargill 2008) to model the plasma response we construct synthetic
light curves that we compare with the observations. We also study how
the properties of the light curves scale with the different parameters
of the model. Finally, we discuss how the present model can be used
to explain loop observations in different wavelengths.
---------------------------------------------------------
Title: XRT Detection of Hot Plasma in Active Regions and Nanoflare
Heating
Authors: Reale, F.; Klimchuk, J. A.; Parenti, S.; Testa, P.
2009ASPC..415..256R Altcode:
Nanoflares occurring in sub-resolution strands have been long invoked
as strong candidates for the heating of active region (AR) coronal
loops. However, the frequent occurrence of nanoflares requires the
steady presence of flare-hot plasma in the active region, which
has been difficult to detect so far. We report on the analysis of
multi-filter Hinode/XRT observations of an active region, which may
show the widespread presence of 10 MK plasma.
---------------------------------------------------------
Title: Coronal Loop Models and Those Annoying Observations! (Keynote)
Authors: Klimchuk, J. A.
2009ASPC..415..221K Altcode: 2009arXiv0904.1391K
It was once thought that all coronal loops are in static equilibrium,
but observational and modeling developments over the past decade
have shown that this is clearly not the case. It is now established
that warm (∼ 1 MK) loops observed in the EUV are explainable
as bundles of unresolved strands that are heated impulsively by
storms of nanoflares. A raging debate concerning the multi-thermal
versus isothermal nature of the loops can be reconciled in terms
of the duration of the storm. We show that short and long storms
produce narrow and broad thermal distributions, respectively. We also
examine the possibility that warm loops can be explained with thermal
nonequilibrium, a process by which steady heating produces dynamic
behavior whenever the heating is highly concentrated near the loop
footpoints. We conclude that this is not a viable explanation for
monolithic loops under the conditions we have considered, but that
it may have application to multi-stranded loops. Serious questions
remain, however.
---------------------------------------------------------
Title: The Existence and Origin of Turbulence in Solar Active Regions
Authors: Klimchuk, J. A.; Nigro, G.; Dahlburg, R. B.; Antiochos, S. K.
2009AGUFMSM42B..03K Altcode:
It has been suggested that turbulence plays a fundamental role in the
heating of solar active regions, with its intermittent behavior being
the explanation of impulsive energy release (nanoflares). We know that
episodes of turbulence are produced in the final nonlinear stage of
the secondary instability of electric current sheets. However, these
current sheets must exist prior to the turbulence. Whether turbulence
can dynamically produce current sheets that would not otherwise be
present is a different and important question. Turbulence occurs
freely in the solar wind and in other situations where the magnetic
field does not dominate. However, the magnetic field strongly resists
being distorted in line-tied, low-beta environments such as active
regions. Can turbulence develop naturally in these environments
without being driven by an instability? To answer this question,
we have performed a time-dependent MHD simulation of a slowly driven
system that does not contain current sheets and is stable to applied
perturbations. We find no evidence for bursty energy release, steep
spatial gradients, or power-law power spectra that are the typical
signatures of turbulence. We conclude that the turbulence which occurs
in active regions is an important yet secondary process and not the
primary cause of heating.
---------------------------------------------------------
Title: Explosive Instability and Coronal Heating
Authors: Dahlburg, R. B.; Liu, J. -H.; Klimchuk, J. A.; Nigro, G.
2009ApJ...704.1059D Altcode:
The observed energy-loss rate from the solar corona implies that
the coronal magnetic field has a critical angle at which energy is
released. It has been hypothesized that at this critical angle an
"explosive instability" would occur, leading to an enhanced conversion
of magnetic energy into heat. In earlier investigations, we have shown
that a shear-dependent magnetohydrodynamic process called "secondary
instability" has many of the distinctive features of the hypothetical
"explosive instability." In this paper, we give the first demonstration
that this "secondary instability" occurs in a system with line-tied
magnetic fields and boundary shearing—basically the situation
described by Parker. We also show that, as the disturbance due to
secondary instability attains finite amplitude, there is a transition
to turbulence which leads to enhanced dissipation of magnetic and
kinetic energy. These results are obtained from numerical simulations
performed with a new parallelized, viscoresistive, three-dimensional
code that solves the cold plasma equations. The code employs a Fourier
collocation—finite difference spatial discretization, and uses a
third-order Runge-Kutta temporal discretization.
---------------------------------------------------------
Title: Evidence of Widespread Hot Plasma in a Nonflaring Coronal
Active Region from Hinode/X-Ray Telescope
Authors: Reale, Fabio; Testa, Paola; Klimchuk, James A.; Parenti,
Susanna
2009ApJ...698..756R Altcode: 2009arXiv0904.0878R
Nanoflares, short and intense heat pulses within spatially unresolved
magnetic strands, are now considered a leading candidate to solve
the coronal heating problem. However, the frequent occurrence of
nanoflares requires that flare-hot plasma be present in the corona at
all times. Its detection has proved elusive until now, in part because
the intensities are predicted to be very faint. Here, we report on the
analysis of an active region observed with five filters by Hinode/X-Ray
Telescope (XRT) in 2006 November. We have used the filter ratio method
to derive maps of temperature and emission measure (EM) both in soft and
hard ratios. These maps are approximate in that the plasma is assumed
to be isothermal along each line of sight. Nonetheless, the hardest
available ratio reveals the clear presence of plasma around 10 MK. To
obtain more detailed information about the plasma properties, we have
performed Monte Carlo simulations assuming a variety of nonisothermal
EM distributions along the lines of sight. We find that the observed
filter ratios imply bi-modal distributions consisting of a strong cool
(log T ~ 6.3 - 6.5) component and a weaker (few percent) and hotter (6.6
< log T < 7.2) component. The data are consistent with bi-modal
distributions along all lines of sight, i.e., throughout the active
region. We also find that the isothermal temperature inferred from a
filter ratio depends sensitively on the precise temperature of the cool
component. A slight shift of this component can cause the hot component
to be obscured in a hard ratio measurement. Consequently, temperature
maps made in hard and soft ratios tend to be anti-correlated. We
conclude that this observation supports the presence of widespread
nanoflaring activity in the active region.
---------------------------------------------------------
Title: Spectroscopic Observations of Hot Lines Constraining Coronal
Heating in Solar Active Regions
Authors: Patsourakos, S.; Klimchuk, J. A.
2009ApJ...696..760P Altcode: 2009arXiv0903.3880P
Extreme-ultraviolet observations of warm coronal loops suggest that they
are bundles of unresolved strands that are heated impulsively to high
temperatures by nanoflares. The plasma would then have the observed
properties (e.g., excess density compared with static equilibrium)
when it cools into the 1-2MK range. If this interpretation is correct,
then very hot emission should be present outside of proper flares. It
is predicted to be very faint, however. A critical element for proving
or refuting this hypothesis is the existence of hot, yet faint plasmas
which should be at amounts predicted by impulsive heating models. We
report on the first comprehensive spectroscopic study of hot plasmas
in active regions (ARs). Data from the Extreme-ultraviolet Imaging
Spectrometer on Hinode were used to construct emission measure (EM)
distributions in quiescent ARs in the 1-5 MK temperature range. The
distributions are flat or slowly increasing up to approximately 3 MK
and then fall off rapidly at higher temperatures. We show that AR models
based on impulsive heating can reproduce the observed EM distributions
relatively well. Our results provide strong new evidence that coronal
heating is impulsive in nature.
---------------------------------------------------------
Title: Spectroscopic Observations of Hot Lines Constraining Coronal
Heating in Solar Active Regions
Authors: Patsourakos, Spiros; Klimchuk, J. A.
2009SPD....40.1211P Altcode:
EUV observations of warm coronal loops suggest that they are bundles of
unresolved strands that are heated impulsively to high temperatures by
nanoflares. The plasma would then have the observed properties (e.g.,
excess density compared to static equilibrium) when it cools into
the 1-2 MK range. If this interpretation is correct, then very hot
emission should be present outside of proper flares. It is predicted
to be vey faint, however. A critical element for proving or refuting
this hypothesis is the existence of hot, very faint plasmas which
should be at amounts predicted by impulsive heating. We report on
the first comprehensive spectroscopic study of hot plasmas in active
regions. Data from the EIS spectrometer on Hinode were used to construct
emission measure distributions in quiescent active regions in the 1-5 MK
temperature range. The distributions are flat or slowly increasing up to
approximately 3 MK and then fall off rapidly at higher temperatures. We
show that active region models based on impulsive heating can reproduce
the observed EM distributions relatively well. Our results provide
strong new evidence that coronal heating is impulsive in nature.
---------------------------------------------------------
Title: Observations of Nanoflare Produced Hot ( 10 Mk) Plasma
Authors: Klimchuk, James A.; Reale, F.; Testa, P.; Parenti, S.
2009SPD....40.1214K Altcode:
Indirect observational evidence suggests that some or most of
the corona is heated impulsively on sub-resolution scales by
nanoflares. Theoretical studies of possible heating mechanisms
also support this picture. However, the most direct evidence of
nanoflares---plasma hotter than 5 MK---has been difficult to obtain
because the emission is expected to be very faint. The reason is
two-fold: first, hot plasma cools very rapidly by thermal conduction;
and second, densities are small because chromospheric evaporation
has not had time to fill the corona. Recent observations from several
instruments have now provided strong evidence of hot plasma. We report
here on the detection of 10 MK plasma by the X-Ray Telescope (XRT)
on Hinode. We show that the intensity of the emission is consistent
with nanoflare models, but is extremely difficult to explain with
steady heating.
---------------------------------------------------------
Title: Models of Impulsively Heated Solar Active Regions
Authors: Airapetian, Vladimir; Klimchuk, J.
2009SPD....40.1202A Altcode:
A number of attempts to model solar active regions with steady coronal
heating have been modestly successful at reproducing the observed
soft X-ray emission, but they fail dramatically at explaining EUV
observations. Since impulsive heating (nanoflare) models can reproduce
individual EUV loops, it seems reasonable to consider that entire active
regions are impulsively heated. However, nanoflares are characterized
by many parameters, such as magnitude, duration, and time delay between
successive events, and these parameters may depend on the strength
of the magnetic field or the length of field lines, for example, so a
wide range of active region models must be examined. We have recently
begun such a study. Each model begins with a magnetic "skeleton”
obtained by extrapolating an observed photospheric magnetogram into
the corona. Field lines are populated with plasma using our highly
efficient hydro code called Enthalpy Based Thermal Evolution of Loops
(EBTEL). We then produce synthetic images corresponding to emission
line or broad-band observations. By determining which set of nanoflare
parameters best reproduces actual observations, we hope to constrain
the properties of the heating and ultimately to reveal the physical
mechanism. We here report on the initial progress of our study.
---------------------------------------------------------
Title: Hinode X-Ray Telescope Detection of Hot Emission from Quiescent
Active Regions: A Nanoflare Signature?
Authors: Schmelz, J. T.; Saar, S. H.; DeLuca, E. E.; Golub, L.;
Kashyap, V. L.; Weber, M. A.; Klimchuk, J. A.
2009ApJ...693L.131S Altcode: 2009arXiv0901.3122S
The X-Ray Telescope (XRT) on the Japanese/USA/UK Hinode (Solar-B)
spacecraft has detected emission from a quiescent active region
core that is consistent with nanoflare heating. The fluxes from 10
broadband X-ray filters and filter combinations were used to construct
differential emission measure (DEM) curves. In addition to the expected
active region peak at log T = 6.3-6.5, we find a high-temperature
component with significant emission measure at log T > 7.0. This
emission measure is weak compared to the main peak—the DEM is down
by almost three orders of magnitude—which accounts of the fact
that it has not been observed with earlier instruments. It is also
consistent with spectra of quiescent active regions: no Fe XIX lines
are observed in a CHIANTI synthetic spectrum generated using the XRT
DEM distribution. The DEM result is successfully reproduced with a
simple two-component nanoflare model.
---------------------------------------------------------
Title: Multi-wavelength observations and modelling of a canonical
solar flare
Authors: Raftery, C. L.; Gallagher, P. T.; Milligan, R. O.; Klimchuk,
J. A.
2009A&A...494.1127R Altcode: 2008arXiv0812.0311R
Aims: We investigate the temporal evolution of temperature, emission
measure, energy loss, and velocity in a C-class solar flare from
both observational and theoretical perspectives. <BR />Methods:
The properties of the flare were derived by following the systematic
cooling of the plasma through the response functions of a number of
instruments - the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI; >5 MK), GOES-12 (5-30 MK), the Transition Region and
Coronal Explorer (TRACE 171 Å; 1 MK), and the Coronal Diagnostic
Spectrometer (CDS; ~0.03-8 MK). These measurements were studied in
combination with simulations from the 0-D enthalpy based thermal
evolution of loops (EBTEL) model. <BR />Results: At the flare
onset, upflows of ~90 km s<SUP>-1</SUP> and low-level emission were
observed in Fe XIX, consistent with pre-flare heating and gentle
chromospheric evaporation. During the impulsive phase, upflows of
~80 km s<SUP>-1</SUP> in Fe XIX and simultaneous downflows of ~20 km
s<SUP>-1</SUP> in He I and O V were observed, indicating explosive
chromospheric evaporation. The plasma was subsequently found to reach
a peak temperature of ⪆13 MK in approximately 10 min. Using EBTEL,
conduction was found to be the dominant loss mechanism during the
initial ~300 s of the decay phase. It was also found to be responsible
for driving gentle chromospheric evaporation during this period. As
the temperature fell below ~8 MK, and for the next ~4000 s, radiative
losses were determined to dominate over conductive losses. The
radiative loss phase was accompanied by significant downflows of
≤40 km s<SUP>-1</SUP> in O V. <BR />Conclusions: This is the first
extensive study of the evolution of a canonical solar flare using
both spectroscopic and broad-band instruments in conjunction with a
0-D hydrodynamic model. While our results are in broad agreement with
the standard flare model, the simulations suggest that both conductive
and non-thermal beam heating play important roles in heating the flare
plasma during the impulsive phase of at least this event.
---------------------------------------------------------
Title: Division II: Sun and Heliosphere
Authors: Melrose, Donald B.; Martínez Pillet, Valentin; Webb, David
F.; van Driel-Gesztelyi, Lidia; Bougeret, Jean-Louis; Klimchuk,
James A.; Kosovichev, Alexander; von Steiger, Rudolf
2009IAUTA..27...73M Altcode:
Division II of the IAU provides a forum for astronomers and
astrophysicists studying a wide range of phenomena related to the
structure, radiation and activity of the Sun, and its interaction with
the Earth and the rest of the solar system. Division II encompasses
three Commissions, 10, 12 and 49, and four Working Groups.
---------------------------------------------------------
Title: Commission 10: Solar Activity
Authors: Klimchuk, James A.; van Driel-Gesztelyi, Lidia; Schrijver,
Carolus J.; Melrose, Donald B.; Fletcher, Lyndsay; Gopalswamy,
Natchimuthuk; Harrison, Richard A.; Mandrini, Cristina H.; Peter,
Hardi; Tsuneta, Saku; Vršnak, Bojan; Wang, Jing-Xiu
2009IAUTA..27...79K Altcode: 2008arXiv0809.1444K
Commission 10 deals with solar activity in all of its forms,
ranging from the smallest nanoflares to the largest coronal mass
ejections. This report reviews scientific progress over the roughly
two-year period ending in the middle of 2008. This has been an exciting
time in solar physics, highlighted by the launches of the Hinode and
STEREO missions late in 2006. The report is reasonably comprehensive,
though it is far from exhaustive. Limited space prevents the inclusion
of many significant results. The report is divided into the following
sections: Photosphere and chromosphere; Transition region; Corona and
coronal heating; Coronal jets; flares; Coronal mass ejection initiation;
Global coronal waves and shocks; Coronal dimming; The link between low
coronal CME signatures and magnetic clouds; Coronal mass ejections in
the heliosphere; and Coronal mass ejections and space weather. Primary
authorship is indicated at the beginning of each section.
---------------------------------------------------------
Title: Static and Impulsive Models of Solar Active Regions
Authors: Patsourakos, S.; Klimchuk, J. A.
2008ApJ...689.1406P Altcode: 2008arXiv0808.2745P
The physical modeling of active regions (ARs) and of the global corona
is receiving increasing interest lately. Recent attempts to model ARs
using static equilibrium models were quite successful in reproducing AR
images of hot soft X-ray (SXR) loops. They however failed to predict
the bright extreme-ultraviolet (EUV) warm loops permeating ARs: the
synthetic images were dominated by intense footpoint emission. We
demonstrate that this failure is due to the very weak dependence of
loop temperature on loop length which cannot simultaneously account for
both hot and warm loops in the same AR. We then consider time-dependent
AR models based on nanoflare heating. We demonstrate that such models
can simultaneously reproduce EUV and SXR loops in ARs. Moreover, they
predict radial intensity variations consistent with the localized core
and extended emissions in SXR and EUV AR observations, respectively. We
finally show how the AR morphology can be used as a gauge of the
properties (duration, energy, spatial dependence, and repetition time)
of the impulsive heating.
---------------------------------------------------------
Title: Highly Efficient Modeling of Dynamic Coronal Loops
Authors: Klimchuk, J. A.; Patsourakos, S.; Cargill, P. J.
2008ApJ...682.1351K Altcode: 2007arXiv0710.0185K
Observational and theoretical evidence suggests that coronal heating
is impulsive and occurs on very small cross-field spatial scales. A
single coronal loop could contain a hundred or more individual strands
that are heated quasi-independently by nanoflares. It is therefore an
enormous undertaking to model an entire active region or the global
corona. Three-dimensional MHD codes have inadequate spatial resolution,
and one-dimensional (1D) hydrodynamic codes are too slow to simulate
the many thousands of elemental strands that must be treated in
a reasonable representation. Fortunately, thermal conduction and
flows tend to smooth out plasma gradients along the magnetic field,
so zero-dimensional (0D) models are an acceptable alternative. We
have developed a highly efficient model called "enthalpy-based thermal
evolution of loops" (EBTEL), which accurately describes the evolution
of the average temperature, pressure, and density along a coronal
strand. It improves significantly on earlier models of this type—in
accuracy, flexibility, and capability. It treats both slowly varying
and highly impulsive coronal heating; it provides the time-dependent
differential emission measure distribution, DEM(T), at the transition
region footpoints; and there are options for heat flux saturation and
nonthermal electron beam heating. EBTEL gives excellent agreement with
far more sophisticated 1D hydrodynamic simulations despite using 4
orders of magnitude less computing time. It promises to be a powerful
new tool for solar and stellar studies.
---------------------------------------------------------
Title: Hot Spectral Emissions in Quiescent Active Regions and
Nanoflare Heating
Authors: Patsourakos, S.; Klimchuk, J. A.
2008AGUSMSP43C..02P Altcode:
A leading candidate for the heating of active region (AR) coronal loops
is the nanoflare model. This model treats coronal loops as collections
of impulsively heated sub-resolution strands and explains several
key observational aspects of warm (1-2 MK) coronal loops. However,
the basic requirement of this model is that the strands initially reach
very high temperatures of several MK before they cool down to canonical
coronal temperatures. Therefore, the detection of hot plasmas in AR
loops represents a stringest test of the nanoflare model. Previous
work has shown that the best way to observe the postulated hot
plasmas is by the means of spectroscopic observations in hot lines
(T > 3 MK). The emission is predicted to be quite faint, but the
EIS spectrometer onboard Hinode has sufficient sensitivity to allow us
to perform such a test for the first time. We will present an analysis
of the emission characteristics of quiescent coronal loops in a number
of hot lines spanning approximately 3-12 MK (Ni XVII, Ca XV, Fe XVII,
Ca XVII, Fe XXIII). We will show that hot plasmas are ubiquitous over
entire active regions, and we will compare the measured intensities
of both hot and warm lines with predictions of nanoflare models.
---------------------------------------------------------
Title: Are Constant Loop Widths an Artifact of the Background and
the Spatial Resolution?
Authors: López Fuentes, M. C.; Démoulin, P.; Klimchuk, J. A.
2008ApJ...673..586L Altcode: 2007arXiv0704.0637L
We study the effect of the coronal background in the determination
of the diameter of EUV loops, and we analyze the suitability of
the procedure followed in a previous paper for characterizing their
expansion properties. For the analysis we create different synthetic
loops, and we place them on real backgrounds from data obtained
with the Transition Region and Coronal Explorer (TRACE). We apply to
these loops the same procedure followed in our previous works, and we
compare the results with real loop observations. We demonstrate that
the procedure allows us to distinguish constant width loops from loops
that expand appreciably with height, as predicted by simple force-free
field models. This holds even for loops near the resolution limit. The
procedure can easily determine when loops are below the resolution
limit and therefore not reliably measured. We find that small-scale
variations in the measured loop width are likely due to imperfections
in the background subtraction. The greatest errors occur in especially
narrow loops and in places where the background is especially bright
relative to the loop. We stress, however, that these effects do not
impact the ability to measure large-scale variations. The result that
observed loops do not expand systematically with height is robust.
---------------------------------------------------------
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: Division II: Sun and Heliosphere
Authors: Webb, David F.; Melrose, Donald B.; Benz, Arnold O.; Bogdan,
Thomas J.; Bougeret, Jean-Louis; Klimchuk, James A.; Martinez-Pillet,
Valentin
2007IAUTB..26..101W Altcode:
Division II provides a forum for astronomers studying a wide range of
problems related to the structure, radiation and activity of the Sun,
and its interaction with the Earth and the rest of the solar system.
---------------------------------------------------------
Title: Explosive Instability and Coronal Heating
Authors: Dahlburg, R. B.; Liu, J.; Klimchuk, J. A.; Nigro, G.
2007AGUFMSH44A1726D Altcode:
The observed energy loss rate from the solar corona implies that
the coronal magnetic field has a critical angle at which energy is
released. It has been hypothesized that at this critical angle an
"explosive instability" would occur, leading to an enhanced conversion
of magnetic energy into heat. In earlier investigations we have shown
that a shear-dependent process called "secondary instability" could
account for many of the distinctive features of the hypothetical
"explosive instability." Here we show that this "secondary
instability" can occur in a system with line-tied magnetic fields
and boundary shearing. We also show that, as the disturbance due to
secondary instability attains finite amplitude, there is a transition
to turbulence which leads to enhanced dissipation of magnetic and
kinetic energy. Furthermore, after each dissipative burst, the system
is able to reform itself so that a subsequent burst can occur. These
results are obtained from numerical simulations performed with a new
parallelized, viscoresistive, three-dimensional code that solves the
cold plasma equations. The code employs a Fourier collocation -- finite
difference spatial discretization, and uses a third-order Runge-Kutta
temporal discretization.
---------------------------------------------------------
Title: The Cross-Field Thermal Structure of Coronal Loops from
Triple-Filter TRACE Observations
Authors: Patsourakos, S.; Klimchuk, J. A.
2007ApJ...667..591P Altcode:
The highly suppressed thermal transport across the magnetic field
in the solar corona makes the determination of the cross-field
thermal distribution within coronal loops a powerful diagnostic
of the properties of the heating process itself. The cross-field
thermal structure is currently being strongly debated. Spectroscopic
observations with high temperature fidelity but low spatial resolution
indicate that some observed loops are multithermal, whereas imaging
observations with high spatial resolution but low temperature fidelity
indicate more isothermal conditions. We report here on triple filter
observations of coronal loops made by the Transition Region and Coronal
Explorer (TRACE), which has the best spatial resolution currently
available. We tested the isothermal hypothesis using the emission
measure loci technique and found that the loops are consistent with
an isothermal plasma near 1.5 MK only if a generous estimate of
the photometric uncertainties is used. A more restrictive estimate
based on discussions with the TRACE experimenters rules out the
isothermal hypothesis. The observations are much better explained by
a multithermal plasma with significant emission measure throughout the
range 1-3 MK. The details of the emission measure distribution are not
well defined, however. Future subarcsecond spectroscopic observations
covering a wide range of temperatures are the most promising means of
unlocking the thermal structure of the corona.
---------------------------------------------------------
Title: Modeling Active Regions with Steady and Impulsive Heating
Authors: Patsourakos, Spiros; Klimchuk, J.
2007AAS...210.9124P Altcode: 2007BAAS...39..208P
There has been considerable recent interest in constructing physical
models of active regions (ARs) and the global coronal. Models based on
static equilibrium theory are quite successful at reproducing soft X-ray
(SXR) images of active regions. They however fail to predict the warm
( 1 MK) loops that are seen to permeate ARs in the EUV. Instead, the
synthetic EUV images are dominated by intense footpoint emission. We
demonstrate that the failure of static models to predict EUV loops is
associated with the very weak dependence of loop temperature on loop
length in models that are based on a single heating mechanism and
that match the SXR observations. The models predict either SXR loops
or EUV loops, but not both. We therefore consider time-dependent AR
models based on nanoflare heating. We demonstrate that such models
can simultaneously reproduce both SXR and EUV loops. Moreover, they
explain the general tendency for SXR emission to dominate in the cores
of ARs and EUV emission to dominate in the periphery. We finally show
how the properties of nanoflares (energy, duration, spatial dependence,
repetition time) can affect the AR morphology. <P />Research supported
by NASA and ONR.
---------------------------------------------------------
Title: Coronal Loops Really Do Have Constant Cross Sections!
Authors: Klimchuk, James A.; Lopez Fuentes, M.; Demoulin, P.
2007AAS...210.9111K Altcode: 2007BAAS...39..205K
The observation that coronal loops do not expand systematically
with height has been one of the more intriguing puzzles in solar
physics. Simple force-free magnetic field models based on extrapolated
magnetograms predict a much larger expansion than is observed. It
has been suggested that the cross section uniformity is an artifact
of inadequate spatial resolution, complex background emission, or
both. For example, loops that are everywhere thinner than the instrument
point spread function (PSF) would be seen to have a nearly constant
thickness even if they actually expand. We have argued previously
that actual loops are wide enough to rule out this possibility. Our
present work also rules out the background emission as a possible
explanation. We have simulated TRACE observations in the following
manner. We constructed synthetic loops with both uniform and expanding
cross sections, convolved them with the PSF, and placed them on actual
TRACE images. We then measured the widths of the loops using the same
technique used in our earlier studies of real observations. We find
that expanding loops can be readily distinguished from loops with
a constant cross section. Thus, the enigma remains! We tentatively
suggest that constant cross sections are a consequence of the complex
internal structure of loops (e.g., loops as bundles of tangled elemental
strands). We are confident that this can explain the observed symmetry
of loops, but whether it can also explain the lack of systematic
expansion with height is not at all clear.
---------------------------------------------------------
Title: Energy Release in Tangled Magnetic Fields
Authors: Klimchuk, James A.; DeVore, C. R.
2007AAS...210.5303K Altcode: 2007BAAS...39..164K
A highly promising picture of coronal heating, first advocated by
G. Parker, involves elemental magnetic flux tubes that become tangled
by the shuffling motions of photospheric convection. The tubes must
“reconnect” in the corona in order to avoid a monotonic increase
of magnetic stresses. The associated release of energy heats the
plasma. In earlier work, we showed that a mechanism called the secondary
instability is the likely mechanism of energy release (Dahlburg,
Klimchuk, and Antiochos, 2005). The instability occurs within the
electric current sheets that separate misaligned tubes and switches
on only after the misalignment angle reaches a critical value. It is
significant that this angle matches the angle implied by the observed
heating requirements of the corona. Though very encouraging, our initial
MHD simulations were highly idealized. We have therefore performed new
simulations that relax several assumptions by including the effects of
finite current sheet size, photospheric line-tying, and time-dependent
shearing. These fully 3D simulations of interacting flux tubes provide
further evidence for the fundamental role of the secondary instability
in coronal heating. <P />Work supported by NASA and ONR.
---------------------------------------------------------
Title: The Temporal Evolution of Coronal Loops Observed by GOES SXI
Authors: López Fuentes, M. C.; Klimchuk, J. A.; Mandrini, C. H.
2007ApJ...657.1127L Altcode: 2006astro.ph.11338L
We study the temporal evolution of coronal loops using data from the
Solar X-Ray Imager (SXI) on board the Geosynchronous Operational
Environmental Satellite 12 (GOES-12). This instrument provides
continuous soft X-ray observations of the solar corona at a high
temporal cadence permitting detailed study of the full lifetime of
each of several coronal loops. The observed light curves suggest
three evolutionary phases: rise, main, and decay. The durations and
characteristic timescales of these phases [I/(dI/dt), where I is the
loop intensity] are much longer than a cooling time and indicate that
the loop-averaged heating rate increases slowly, reaches a maintenance
level, and then decreases slowly. This suggests that a single heating
mechanism operates for the entire lifetime of the loop. For monolithic
(uniformly filled) loops, the loop-averaged heating rate is the
intrinsic energy release rate of the heating mechanism. For loops that
are bundles of impulsively heated strands, it relates to the frequency
of occurrence of individual heating events, or nanoflares. We show
that the timescale of the loop-averaged heating rate is proportional
to the timescale of the observed intensity variation, with a constant
of proportionality of approximately 1.5 for monolithic loops and 1.0
for multistranded loops. The ratios of the radiative to conductive
cooling times in the loops are somewhat less than 1, putting them
intermediate between the values measured previously for hotter and
cooler loops. This provides further support for a trend suggesting
that all loops are heated in a similar way.
---------------------------------------------------------
Title: Division II: Sun and Heliosphere
Authors: Webb, David F.; Melrose, Donald B.; Benz, Arnold O.; Bogdan,
Thomas J.; Bougeret, Jean-Louis; Klimchuk, James A.; Martinez Pillet,
Valentin
2007IAUTA..26...69W Altcode:
Division II of the IAU provides a forum for astronomers studying a wide
range of phenomena related to the structure, radiation and activity
of the Sun, and its interaction with the Earth and the rest of the
solar system. Division II encompasses three Commissions, 10, 12 and
49, and four working groups. During the last triennia the activities
of the division involved some reorganization of the division and its
working groups, developing new procedures for election of division and
commission officers, promoting annual meetings from within the division
and evaluating all the proposed meetings, evaluating the division's
representatives for the IAU to international scientific organizations,
and participating in general IAU business.
---------------------------------------------------------
Title: Commission 10: Solar Activity
Authors: Melrose, Donald B.; Klimchuk, James A.; Benz, A. O.; Craig,
I. J. D.; Gopalswamy, N.; Harrison, R. A.; Kozlovsky, B. Z.; Poletto,
G.; Schrijver, K. J.; van Driel-Gesztelyi, L.; Wang, J. -X.
2007IAUTA..26...75M Altcode:
Commission 10 aims at the study of various forms of solar activity,
including networks, plages, pores, spots, fibrils, surges, jets,
filaments/prominences, coronal loops, flares, coronal mass ejections
(CMEs), solar cycle, microflares, nanoflares, coronal heating etc.,
which are all manifestation of the interplay of magnetic fields and
solar plasma. Increasingly important is the study of solar activities
as sources of various disturbances in the interplanetary space
and near-Earth "space weather".Over the past three years a major
component of research on the active Sun has involved data from the
RHESSI spacecraft. This review starts with an update on current and
planned solar observations from spacecraft. The discussion of solar
flares gives emphasis to new results from RHESSI, along with updates on
other aspects of flares. Recent progress on two theoretical concepts,
magnetic reconnection and magnetic helicity is then summarized, followed
by discussions of coronal loops and heating, the magnetic carpet
and filaments. The final topic discussed is coronal mass ejections
and space weather.The discussions on each topic is relatively brief,
and intended as an outline to put the extensive list of references
in context.The review was prepared jointly by the members of the
Organizing Committee, and the names of the primary contributors to
the various sections are indicated in parentheses.
---------------------------------------------------------
Title: Coronal Heating
Authors: Klimchuk, J. A.; López Fuentes, M. C.
2006AIPC..848...55K Altcode:
Coronal heating is one of the most challenging and important problems
in astrophysics. We here review some of the critical aspects of the
problem and suggest that the most likely explanation is Parker's
long-standing idea of nanoflares occurring in magnetic fields that
become tangled by photospheric convection. An exciting new development
is the identification of the “secondary instability” as the likely
mechanism of energy release. We also present some preliminary new
results indicating that the rise, main, and decay phases of the soft
X-ray light curves of observed coronal loops might be associated
with the development, maintenance, and destruction of self-organized
critical systems.
---------------------------------------------------------
Title: Nonthermal Spectral Line Broadening and the Nanoflare Model
Authors: Patsourakos, S.; Klimchuk, J. A.
2006ApJ...647.1452P Altcode:
A number of theoretical and observational considerations suggest
that coronal loops are bundles of unresolved, impulsively heated
strands. This “nanoflare” model, as it is sometimes called,
predicts high-speed evaporative upflows, which might be revealed as
nonthermal broadening of spectral line profiles. We have therefore
generated synthetic line profile observations based on one-dimensional
hydrodynamic simulations for comparison with actual observations. The
predicted profiles for Ne VIII (770.4 Å), a transition region line,
and Mg X (624.9 Å), a warm coronal line, have modest broadening that
agrees well with existing observations. The predicted profiles for
Fe XVII (254.87 Å), a hot line that will be observed by the Extreme
Ultraviolet Imaging Spectrometer (EIS) on the Solar-B mission, are
somewhat broader and are also consistent with the limited number of hot
line observations that are currently available. Moreover, depending on
the properties of the assumed nanoflare and other parameters of the
simulation, the Fe XVII profile can have distinctive enhancements in
the line wing. This indicates a powerful diagnostic capability that
can be exploited once Solar-B is launched.
---------------------------------------------------------
Title: Summary of JD3: Solar Active Regions and 3D Magnetic Structure
Authors: Klimchuk, J. A.
2006IAUJD...3E..57K Altcode:
In this summary, I will attempt to synthesize many of the individual
results presented in JD3 into a more coherent picture of solar active
regions and 3D magnetic structure. I will offer an assessment of our
current state of understanding and suggest how we might best proceed
to make further progress on this important topic.
---------------------------------------------------------
Title: Heating of the Magnetically Closed Corona
Authors: Klimchuk, J. A.; López Fuentes, M. C.; DeVore, C. R.
2006ESASP.617E...8K Altcode: 2006soho...17E...8K
No abstract at ADS
---------------------------------------------------------
Title: Testing Nanoflare Heating in Coronal Loops With Observations
From the Extreme Ultraviolet Imaging Spectrometer On-board the
SOLAR-B Mission
Authors: Patsourakos, Spiros; Klimchuk, J. A.
2006SPD....37.0124P Altcode: 2006BAAS...38..219P
A number of theoretical and observational considerations suggest
that coronal loops are bundles of unresolved, impulsively heated
strands. This "nanoflare" model, as it is sometimes called,
predicts high-speed evaporative upflows, which might be revealed as
non-thermal broadening of spectral line profiles. We have therefore
generated synthetic line profile observations based on 1D hydrodynamic
simulations of nanoflare heated loop bundles.We will show that hot lines
(T>5MK) hold the imprints of the heating process via the development
of distinct enhancements in the line wings. These signatures do not
appear in the profiles of cooler lines, which is fully consistent
with existing observations. We will demonstrate how the spectra of hot
lines from the Extreme Ultraviolet Imaging Spectrometer (EIS) on-board
the upcoming SOLAR-B mission can be used to test the basic nanoflare
picture and perhaps even to pinpoint the properties of the nanoflares,
such as their energy content, duration, and spatial dependence. We
will present sample observing programs for studying nanoflare heating
in coronal loops that utilize EIS and other instrumentation on-board
SOLAR-B and STEREO.Research supported by NASA and ONR.
---------------------------------------------------------
Title: Why Are Coronal Loops So Symmetric?
Authors: Klimchuk, James A.; Lopez Fuentes, M. C.; Demoulin, P.
2006SPD....37.1706K Altcode: 2006BAAS...38..246K
Coronal loops are observed to be very symmetric in the sense that the
two legs have a comparable thickness. Magnetic flux tubes in magnetic
field extrapolation models are typically much less symmetric. We
have quantified these differences using 171 A images from TRACE and
magnetograms from MDI/SOHO. For a sample of 20 different loops, we found
the linear force-free field that best matches the observed loop. We
then measured the plane-of-the-sky widths of the loops and corresponding
flux tubes and computed footpoint-to-footpoint expansion factors (i.e.,
asymmetry ratios). The mean expansion factor of the flux tubes is 2.62,
whereas the mean expansion factor of the loops is only 1.35. Note that
these expansion factors are different from the footpoint-to-midpoint
expansion factors that we have presented previously.Evidence suggests
that the coronal magnetic field is comprised elemental flux strands
that are tangled by turbulent convection. These strands are so small
that many tens of them are contained within a single TRACE loop. We
suggest that this fine structure is a critical missing ingredient of
the extrapolation models and that a combination of footpoint shuffling
and coronal reconnection can explain the observed loop symmetry. This
has important implications for coronal heating.Research supported by
NASA and the Office of Naval Research.
---------------------------------------------------------
Title: On the Temperature-Emission Measure Distribution in Stellar
Coronae
Authors: Cargill, Peter J.; Klimchuk, James A.
2006ApJ...643..438C Altcode:
Strong peaks in the emission measure-temperature (EM-T ) distributions
in the coronae of some binary stars are associated with the presence of
hot (10<SUP>7</SUP> K), dense (up to 10<SUP>13</SUP> cm<SUP> -3</SUP>)
plasma. These peaks are very reminiscent of those predicted to arise in
an impulsively heated solar corona. A coronal model comprised of many
impulsively heated strands is adapted to stellar parameters. It is shown
that the properties of the EM-T distribution can be accounted for in
general terms provided the emission comes from many very small loops
(length under 10<SUP>3</SUP> km) with intense magnetic fields (1 kG)
distributed across part of the surface of the star. The heating requires
events that generally dissipate between 10<SUP>26</SUP> and 10<SUP>
28</SUP> ergs, which is in the range of solar microflares. This implies
that such stars must be capable of generating regions of localized
intense magnetic fields.
---------------------------------------------------------
Title: The Magnetic Structure of Coronal Loops Observed by TRACE
Authors: López Fuentes, M. C.; Klimchuk, J. A.; Démoulin, P.
2006ApJ...639..459L Altcode: 2006ApJ...639..459F; 2005astro.ph..7462L
Previous studies have found that coronal loops have a nearly uniform
thickness, which seems to disagree with the characteristic expansion
of active region magnetic fields. This is one of the most intriguing
enigmas in solar physics. We here report on the first comprehensive
one-to-one comparison of observed loops with corresponding magnetic flux
tubes obtained from cotemporal magnetic field extrapolation models. We
use EUV images from TRACE, magnetograms from the MDI instrument on
SOHO, and linear force-free field extrapolations satisfying b.nabla
XB=αB, with α equal to a constant. For each loop, we find the
particular value of α that best matches the observed loop axis and
then construct flux tubes using different assumed cross sections at
one footpoint (circle and ellipses with different orientations). We
find that the flux tubes expand with height by typically twice as much
as the corresponding loops. We also find that many flux tubes are much
wider at one footpoint than the other, whereas the corresponding loops
are far more symmetric. It is clear that the actual coronal magnetic
field is more complex than the models we have considered. We suggest
that the observed symmetry of loops is related to the tangling of
elemental magnetic flux strands produced by photospheric convection.
---------------------------------------------------------
Title: On Solving the Coronal Heating Problem
Authors: Klimchuk, James A.
2006SoPh..234...41K Altcode: 2005astro.ph.11841K
The question of what heats the solar corona remains one of the
most important problems in astrophysics. Finding a definitive
solution involves a number of challenging steps, beginning with an
identification of the energy source and ending with a prediction
of observable quantities that can be compared directly with actual
observations. Critical intermediate steps include realistic modeling
of both the energy release process (the conversion of magnetic stress
energy or wave energy into heat) and the response of the plasma
to the heating. A variety of difficult issues must be addressed:
highly disparate spatial scales, physical connections between the
corona and lower atmosphere, complex microphysics, and variability
and dynamics. Nearly all of the coronal heating mechanisms that have
been proposed produce heating that is impulsive from the perspective
of elemental magnetic flux strands. It is this perspective that must
be adopted to understand how the plasma responds and radiates. In our
opinion, the most promising explanation offered so far is Parker's
idea of nanoflares occurring in magnetic fields that become tangled
by turbulent convection. Exciting new developments include the
identification of the "secondary instability" as the likely mechanism
of energy release and the demonstration that impulsive heating in
sub-resolution strands can explain certain observed properties of
coronal loops that are otherwise very difficult to understand. Whatever
the detailed mechanism of energy release, it is clear that some form
of magnetic reconnection must be occurring at significant altitudes
in the corona (above the magnetic carpet), so that the tangling does
not increase indefinitely. This article outlines the key elements of
a comprehensive strategy for solving the coronal heating problem and
warns of obstacles that must be overcome along the way.
---------------------------------------------------------
Title: DC coronal heating and the nonlinear evolution of current
sheets
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
2006AdSpR..37.1342D Altcode:
Recent theoretical developments have re-awakened interest
in the role of electric current sheets in DC coronal heating
[Parker. Astrophys. J. 330, 474, 1988; Priest et al. Astrophys. J. 576,
522, 2002]. Dahlburg et al. [Dahlburg et al. Adv. Space Res. 32,
1029, 2003; Dahlburg et al. Astrophys. J. 622, 1191, 2005] reported
the existence of a "secondary instability" that could explain the
required "switch-on" effect required for adequate energy storage. This
ideal, three-dimensional instability also provided a straightforward
explanation for the subsequent fast release of energy, as the
rapid growth of the mode eventually results in a state of turbulent
magnetic reconnection. Earlier studies of the secondary instability
were limited to systems with relatively simple perturbations, viz.,
resistive stability eigenmodes. A current sheet in the Sun is likely
to be subject to much more complex perturbations involving a waves
of various wavelengths and amplitudes. We describe the evolution
of three-dimensional electric current sheets disturbed by random
3D perturbations. We find that the significant characteristics of
secondary instability are also observed in this case. The numerical
results are compared to solar observations.
---------------------------------------------------------
Title: Coronal loops as self-organized critical systems
Authors: López Fuentes, M. C.; Klimchuk, J. A.
2006BAAA...49..108L Altcode:
We developed a numerical model that explains the evolution of coronal
loops observed with GOES-SXI (see Lopez Fuentes, Klimchuk and Mandrini,
ApJ, 2006, in press) in terms of Self-organized Critical Systems
(SOC). We are inspired by the idea originally proposed by Parker (1988,
ApJ, 330, 474), that coronal loops are made of elemental magnetic
strands that wrap around each other due to photospheric convection. In
our code the magnetic strength between neighbor strands increase until a
threshold is reach and strands reconnect, releasing energy and heating
the plasma. The number and intensity of these release events increase
and a critical steady-state is reached. At some point, the photospheric
dispersion makes the “feeding” mechanism inefficient and the loop
decays. We model the plasma response and we obtain synthetic light
curves that qualitatively reproduce the observed loop evolution.
---------------------------------------------------------
Title: Magnetic structure and observed width of coronal loops
Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A.; Demoulin, P.
2006cosp...36.2575L Altcode: 2006cosp.meet.2575L
Previous studies have found that coronal loops have a nearly uniform
thickness which seems to disagree with the characteristic expansion
of active region magnetic fields This is one of the most intriguing
enigmas in solar physics We here report on the first comprehensive
one-to-one comparison of observed loops with corresponding magnetic
flux tubes obtained from cotemporal magnetic field extrapolation models
We use EUV images from TRACE magnetograms from the MDI instrument
on SOHO and linear force-free field extrapolations For each loop we
find the particular value of the force-free parameter alpha that best
matches the observed loop axis and then construct flux tubes using
different assumed cross sections at one footpoint circle and ellipses
with different orientations We find that the flux tubes expand with
height by typically twice as much as the corresponding loops We also
find that many flux tubes are much wider at one footpoint than the
other whereas the corresponding loops are far more symmetric It is
clear that the actual coronal magnetic field is more complex than the
models we have considered We suggest that the observed symmetry of
loops is related to the tangling of elemental magnetic flux strands
produced by photospheric convection
---------------------------------------------------------
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: Coronal heating and the need for high-resolution observations
Authors: Klimchuk, J.
2006cosp...36.2524K Altcode: 2006cosp.meet.2524K
Despite excellent progress in recent years in understanding coronal
heating there remain many crucial questions that are still unanswered
Limitations in the observations are one important reason Both
theoretical and observational considerations point to the importance
of small spatial scales impulsive energy release strong dynamics and
extreme plasma nonuniformity As a consequence high spatial resolution
broad temperature coverage high temperature fidelity and sensitivity
to velocities and densities are all critical observational parameters
Current instruments lack one or more of these properties and this
has led to considerable ambiguity and confusion In this talk I will
discuss recent ideas about coronal heating and emphasize that high
spatial resolution observations especially spectroscopic observations
are needed to make major progress on this important problem
---------------------------------------------------------
Title: Study of coronal loops observed by GOES-SXI
Authors: Lopez-Fuentes, M. C.; Mandrini, C. H.; Klimchuk, J. A.
2006cosp...36.2549L Altcode: 2006cosp.meet.2549L
We study the temporal evolution of coronal loops using data from
the Solar X-ray Imager SXI on board the Geosynchronous Operational
Environmental Satellite 12 GOES-12 This instrument has the advantage
of providing continuous soft X-ray observations of the solar corona at
a high temporal cadence which allows us to follow the full lifetime
of a set of coronal loops from their brightening to their decay From
the observed light curves we can divide the evolution of the loops in
three phases rise main and decay For each of these phases we compute
the corresponding evolutionary timescales and since we have full time
coverage the real loop lifetime Using data in different filters we
derive temperature and density averages The values found place SXI
loops halfway between the typical ranges of physical parameters for
loops observed by the Soft X-ray Telescope Yohkoh SXT and those for
loops observed by the Transition Region and Coronal Explorer TRACE
We compute radiative and conductive cooling times which turn out to
be much shorter than the evolutionary timescales of the loops These
results confirm previous findings Porter and Klimchuk 1995 based on
observations covering partially the loop temporal evolution Our results
can be interpreted in terms of two alternative coronal heating scenarios
quasi-static heating of monolithic uniform loop structures or impulsive
heating nanoflaring of multiple-stranded loops We present arguments
based on recent observations and loop modelling that support the idea
---------------------------------------------------------
Title: Evolution of coronal loops
Authors: López Fuentes, M. C.; Mandrini, C. H.; Klimchuk, J. A.
2006BAAA...49..107L Altcode:
We study the temporal evolution of coronal loops using data from the
Solar X-ray Imager (SXI) on board the Geosynchronous Operational
Environmental Satellite 12 (GOES-12). The observed light curves
show three distinct evolutionary phases: rise, main, and decay. The
durations and characteristic timescales of these phases are much
longer than the cooling times, suggesting two possible scenarios:
(1) loops are monolithic and evolve quasi-statically; or (2) they
are made of unresolved strands that are impulsively heated. We show
that the timescale of the loop-averaged heating rate is proportional
to the timescale of the observed intensity variation. The constant
of proportionality is approximately 1.5 for case (1) and 1.0 for case
(2). The ratios of the radiative to conductive cooling times place these
loops intermediate between previously measured hotter (Yohkoh-SXT)
and cooler (TRACE) loops. Our results help to complete a trend that
seems to support the impulsive heating hypothesis.
---------------------------------------------------------
Title: Observing the Solar atmosphere with the Extreme Ultraviolet
Imaging Spectrometer on Solar B
Authors: Korendyke, C. M.; Brown, C.; Dere, K.; Doschek, G.; Klimchuk,
J.; Landi, E.; Mariska, J.; Warren, H.; Lang, J.
2005AGUFMSH41B1124K Altcode:
The Extreme Ultraviolet Imaging Spectrometer (EIS) is part of the
instrument complement on the Solar B satellite, scheduled for launch
in the summer of 2006. The instrument has been calibrated and is
presently mounted on the spacecraft. EIS is the most sensitive EUV
solar spectrometer to be flown. The instrument is the first of a new
generation of two optical element, solar spectrographs. Preliminary
results from the laboratory focussing and calibration of the
instrument will be shown. The instrument wavelength coverage includes
reasonably bright spectral lines emitted by plasmas from 0.1 to 20 MK
in temperature. The wavelength range also provides coronal density
diagnostics. Temperature, density and velocity diagnostics will be
discussed. An example observing program for exploring active region
evolution and dynamics will be discussed.
---------------------------------------------------------
Title: Why We Need Imaging Spectroscopy
Authors: Klimchuk, J. A.
2005AGUFMSH44A..01K Altcode:
Despite excellent progress in recent years in understand fundamental
solar physics problems such as coronal heating, solar wind acceleration,
flares, and coronal mass ejections, there remain many crucial questions
that are still unanswered. Limitations in the observations are one
important reason. Both theoretical and observational considerations
point to the importance of small spatial scales, impulsive energy
release, strong dynamics, and extreme plasma nonuniformity. As a
consequence, high spatial resolution, broad temperature coverage,
high temperature fidelity, and sensitivity to velocities and densities
are all critical observational parameters. Current instruments lack
one or more of these properties, and this has led to considerable
ambiguity and confusion. In this talk, I will discuss some of the
ways that spectroscopic information is vital to further progress,
giving particular emphasis to the coronal heating problem. I will
argue that a high-speed imaging spectrometer must be a top priority
instrument for the future.
---------------------------------------------------------
Title: Coronal Loop Heating by Nanoflares: The Impact of the
Field-aligned Distribution of the Heating on Loop Observations
Authors: Patsourakos, S.; Klimchuk, J. A.
2005ApJ...628.1023P Altcode:
Nanoflares occurring at subresolution strands with repetition times
longer than the coronal cooling time are a promising candidate
for coronal loop heating. To investigate the impact of the spatial
distribution of the nanoflare heating on loop observables, we compute
hydrodynamic simulations with several different spatial distributions
(uniform, loop top, randomly localized, and footpoint). The outputs
of the simulations are then used to calculate density and temperature
diagnostics from synthetic TRACE and SXT observations. We find that
the diagnostics depend only weakly on the spatial distribution of the
heating and therefore are not especially useful for distinguishing among
the different possibilities. Observations of the very high temperature
plasmas that are present only in the earliest stages of nanoflares
can shed more light on the field-aligned distribution of the heating.
---------------------------------------------------------
Title: Are Coronal Loops Self-organized Critical Systems?
Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A.; Mandrini, C. H.
2005AGUSMSP14A..06L Altcode:
In Lopez-Fuentes et al. 2004 we studied a set of loops observed by
GOES/SXI and found that loop evolution can be separated in three
phases (rise, main and decay). We found that the time scales of all
three phases are long compared to a cooling time. In this work we
explore whether this evolution is consistent with the development
of a self-organized critical (SOC) system. We compare the observed
soft X-ray light curves with light curves predicted by a simple SOC
model, and we examine how the rise and decay times are related to the
properties of the system driver. The physical picture we have in mind
is the shuffling of elemental flux tubes by photospheric motions, as
first advocated by Parker. Work funded by NASA and ONR. Lopez Fuentes,
M.C., Mandrini, C.H., & Klimchuk, J.A., 2004, American Astronomical
Society Meeting Abstracts, 204, 5602L
---------------------------------------------------------
Title: Coronal Loop Heating by Nanoflares: Non-thermal Velocities
Authors: Patsourakos, S.; Klimchuk, J. A.
2005AGUSMSP41A..06P Altcode:
Spectroscopic observations show non-negligible non-thermal velocities
under coronal conditions. These motions place tight constraints on any
coronal heating mechanism that should be able to reproduce them. We
calculate the non-thermal velocities predicted by the nanoflare
model. We perform 1D time-dependent hydrodynamic simulations of
nanoflares occurring at sub-resolution strands, that make up the
observed coronal loops and calculate profiles for representative
spectral lines. We show that: (1) the calculated non-thermal velocities
compare favorably with observations of cool and warm spectral
lines and (2) the profiles of hot lines, that would be available in
observations from the Extreme Ultraviolet Imaging Spectrometer (EIS)
spectrometer onboard the SOLAR-B mission, can exhibit significant
blue-wing asymmetries which can be used as a monitor of nanoflare
properties. Research supported by NASA and ONR.
---------------------------------------------------------
Title: The Origin of High-Speed Motions and Threads in Solar
Prominences
Authors: Karpen, J.; Antiochos, S.; Klimchuk, J.
2005AGUSMSP21B..02K Altcode:
Prominences are among the most spectacular manifestations of both
quiescent and eruptive solar activity, yet the origins of their
magnetic-field and plasma structures remain poorly understood. We
have made steady progress toward a comprehensive model of prominence
formation and evolution with our sheared 3D arcade model for the
magnetic field and our thermal nonequilibrium model for the cool,
dense material suspended in the corona. According to the thermal
nonequilibrium model, condensations form readily along long,
low-lying magnetic field lines if the heating is localized near the
chromosphere. In most cases this process yields a dynamic cycle in
which condensations repetitively form, stream along the field line,
and ultimately disappear by falling onto the nearest footpoint. Two
key observed features were not adequately explained by our earlier
simulations of thermal nonequilibrium, however: the thread-like
(i.e., elongated) horizontal structure and high-speed motions of many
condensations. Here we discuss how simple modifications to our model
largely eliminate these discrepancies, strengthening the case for
thermal nonequilibrium as the origin of prominence condensations and
for low-twist models of prominence magnetic structure. This work was
supported by NASA and ONR.
---------------------------------------------------------
Title: Coronal Loop Heating by Nanoflares: The Influence of the
Field-aligned Distribution of the Heating on Observables
Authors: Patsourakos, S.; Klimchuk, J. A.
2005AGUSMSP41A..05P Altcode:
We investigate the effect of the spatial distribution of nanoflare
heating on loop observables. We perform 1D time-dependent hydrodynamic
simulations of nanoflares occurring at sub-resolution strands, that make
up the observed coronal loops. The simulations use different spatial
forms for the nanoflare heating (randomly localized, footpoint, uniform
loop top). The outputs of the simulations are then used to calculate
diagnostics from synthetic TRACE and SXT observations. We find that
the diagnostics depend only weakly on the spatial distribution of the
heating, and therefore are not especially useful for distinguishing
among the different possibilities. We propose that the best way to
study the field-aligned spatial distribution of nanoflare heating is
to observe the very high temperature plasmas that are present only in
the earliest stages of an event. Research supported by NASA and ONR.
---------------------------------------------------------
Title: Highly Efficient Modeling of Dynamic Coronal Loops
Authors: Klimchuk, J. A.; Patsourakos, S.; Cargill, P. J.
2005AGUSMSP14A..03K Altcode:
It now seems clear that many coronal loops, especially those observed
by TRACE and EIT, are inherently dynamic and composed of large numbers
of impulsively-heated strands. Modeling these loops in full detail is
extremely challenging, and modeling entire active regions or the whole
Sun is completely out of the question unless approximate techniques are
used. We have developed a simplified set of equations that is remarkably
accurate at describing the evolution of the thermodynamic variables
(T, P, n, v) averaged along the magnetic field of an individual
strand. The equations can be solved ten thousand times more quickly
than the full 1D hydro equations. This "0D" model relaxes two key
assumptions of Cargill's (1994) nanoflare model: (1) the heating can
have any time-dependent profile and need not be instantaneous; and
(2) thermal conduction cooling and radiation cooling occur together
at all times, in varying proportions. We here describe the essential
features of the model and show examples of how well it works.
---------------------------------------------------------
Title: An Explanation for the “Switch-On” Nature of Magnetic Energy
Release and Its Application to Coronal Heating
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
2005ApJ...622.1191D Altcode:
A large class of coronal heating theories postulate that the random
mixing of magnetic footpoints by photospheric motions leads to the
formation of current sheets in the corona and, consequently, to energy
release there via magnetic reconnection. Parker pointed out that in
order for this process to supply the observed energy flux into the
corona, the stress in the coronal magnetic field must have a fairly
specific value at the time that the energy is released. In particular,
he argued that the misalignment between reconnecting flux tubes must
be roughly 30° in order to match the observed heating. No physical
origin for this number was given, however. In this paper we propose
that secondary instability is the mechanism that “switches on” the
energy release when the misalignment angle in the corona reaches the
correct value. We calculate both the three-dimensional linear and fully
nonlinear development of the instability in current sheets corresponding
to various misalignment angles. We find that no secondary instability
occurs for angles less than about 45°, but for larger angles the
instability grows at a rapid rate, and there is an explosive release
of energy. We compare our results with the observed properties of the
corona and discuss the implications for future observations.
---------------------------------------------------------
Title: The Effect of the Spatial Distribution of Nanoflare Heating
on Loop Observables
Authors: Patsourakos, S.; Klimchuk, J. A.
2004ESASP.575..297P Altcode: 2004soho...15..297P
No abstract at ADS
---------------------------------------------------------
Title: Coronal Seismology and the Propagation of Acoustic Waves
along Coronal Loops
Authors: Klimchuk, J. A.; Tanner, S. E. M.; De Moortel, I.
2004ApJ...616.1232K Altcode: 2004astro.ph.12085K
We use a combination of analytical theory, numerical simulation, and
data analysis to study the propagation of acoustic waves along coronal
loops. We show that the intensity perturbation of a wave depends
on a number of factors, including dissipation of the wave energy,
pressure and temperature gradients in the loop atmosphere, work action
between the wave and a flow, and the sensitivity properties of the
observing instrument. In particular, the scale length of the intensity
perturbation varies directly with the dissipation scale length (i.e.,
damping length) and the scale lengths of pressure, temperature, and
velocity. We simulate wave propagation in three different equilibrium
loop models and find that dissipation and pressure and temperature
stratification are the most important effects in the low corona where
the waves are most easily detected. Velocity effects are small and
cross-sectional area variations play no direct role for lines of
sight that are normal to the loop axis. The intensity perturbation
scale lengths in our simulations agree very well with the scale
lengths we measure in a sample of loops observed by TRACE. The median
observed value is 4.35×10<SUP>9</SUP> cm. In some cases the intensity
perturbation increases with height, which is likely an indication of
a temperature inversion in the loop (i.e., temperature that decreases
with height). Our most important conclusion is that thermal conduction,
the primary damping mechanism, is accurately described by classical
transport theory. There is no need to invoke anomalous processes to
explain the observations.
---------------------------------------------------------
Title: Coronal Seismology and the Propagation of Acoustic Waves
Along Coronal Loops
Authors: Klimchuk, J. A.; Tanner, S. E.; De Moortel, I.
2004AGUFMSH24A..06K Altcode:
We use a combination of analytical theory, numerical simulation, and
data analysis to study the propagation of acoustic waves along coronal
loops. We show that the intensity perturbation of a wave depends
on a number of factors, including dissipation of the wave energy,
pressure and temperature gradients in the loop atmosphere, work action
between the wave and a flow, and the sensitivity properties of the
observing instrument. In particular, the scale length of the intensity
perturbation varies directly with the dissipation scale length (i.e.,
damping length) and the scale lengths of pressure, temperature, and
velocity. We simulate wave propagation in three different equilibrium
loop models and find that dissipation and pressure and temperature
stratification are the most important effects in the low corona where
the waves are most easily detected. Velocity effects are small, and
cross-sectional area variations play no direct role for lines-of-sight
that are normal to the loop axis. The intensity perturbation scale
lengths in our simulations agree very well with the scale lengths we
measure in a sample of loops observed by TRACE. The median observed
value is 4.35×10<SUP>9</SUP> cm. In some cases the intensity
perturbation increases with height, which is likely an indication of
a temperature inversion in the loop (i.e., temperature that decreases
with height). Our most important conclusion is that thermal conduction,
the primary damping mechanism, is accurately described by classical
transport theory. There is no need to invoke anomalous processes to
explain the observations.
---------------------------------------------------------
Title: A Model for Bright Extreme-Ultraviolet Knots in Solar Flare
Loops
Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A.
2004ApJ...614.1022P Altcode:
EUV observations often indicate the presence of bright knots in flare
loops. The temperature of the knot plasma is of the order of 1 MK,
and the knots themselves are usually localized somewhere near the loop
tops. We propose a model in which the formation of EUV knots is due
to the spatial structure of the nonflare active region heating. We
present the results of a series of one-dimensional hydrodynamic,
flare-loop simulations, which include both an impulsive flare heating
and a background, active region heating. The simulations demonstrate
that the formation of the observed knots depends critically on
the spatial distribution of the background heating during the decay
phase. In particular, the heating must be localized far from the loop
apex and have a magnitude comparable to the local radiative losses of
the cooling loop. Our results, therefore, provide strong constraints
on both coronal heating and postflare conditions.
---------------------------------------------------------
Title: Bright EUV Knots in Solar Flare Loops: Constraints on Coronal
Heating
Authors: Patsourakos, S.; Antiochos, S.; Klimchuk, J.
2004AAS...204.8705P Altcode: 2004BAAS...36Q.819P
EUV observations often indicate the presence of bright knots in flare
loops. The temperature of the knot plasma is of order 1MK, and the
knots themselves are usually localized somewhere near the loop tops. We
propose a model in which the formation of EUV knots is due to the
spatial structure of the non-flare active region heating. We present
the results of a series of 1D hydrodynamic, flare-loop simulations,
which include both an impulsive flare heating and a background, active
region heating. The simulations demonstrate that the formation of the
observed knots depends critically on the spatial distribution of the
background heating during the decay phase. In particular, the heating
must: (1) be localized, (2) be situated far from the loop apex and (3)
have a magnitude comparable with the local radiative losses of the
cooling loop. Our results, therefore, provide strong constraints on
both coronal heating and post-flare conditions. <P />Research supported
by NASA and ONR.
---------------------------------------------------------
Title: Evolution of Coronal Loops Observed by GOES-SXI
Authors: Lopez Fuentes, M. C.; Mandrini, C. H.; Klimchuk, J. A.
2004AAS...204.5602L Altcode: 2004BAAS...36R.761L
Several years ago, Porter and Klimchuk (PK, 1995) studied a collection
of coronal loops observed by the SXT instrument on Yohkoh and found
that they have evolutionary timescales of typically 10<SUP>4</SUP>
to 10<SUP>5</SUP> s. These very long timescales are roughly one or
two orders of magnitude greater than the corresponding cooling times
and therefore have important implications for loop heating. However,
PK observed each loop for only a single spacecraft orbit ( 1 hr),
and it has remained a question whether loops actually persist for as
long as the ”instantaneous” evolutionary timescales suggest. Perhaps
loops turn on suddenly, remain nearly constant for a modest period, then
decay on a cooling timescale. <P />We have begun to address the general
issue of loop evolution using images from the Soft X-ray Imager (SXI)
on board the GOES-12 satellite. This instrument has a coarser spatial
resolution than TRACE or SXT, but has the advantage of continuous
temporal coverage. Since GOES-12 is in geosynchronous orbit, there is
no spacecraft night. We present here the light curves of several loops
and compare them with the radiative cooling times inferred from the
observed temperature and emission measure. We discuss heating rate
requirements related to the observed turn on and maintenance of the
loops, and we consider the implications for nanoflare versus steady
heating scenarios. <P />This work is funded by NASA and the Office
of Naval Research. <P />Porter, L. J., and Klimchuk, J. A. 1995, ApJ,
454, 499
---------------------------------------------------------
Title: Acoustic Wave Interpretation of Propagating Intensity
Disturbances in Coronal Loops
Authors: Klimchuk, J. A.; Tanner, S. E. M.; De Moortel, I.
2004AAS...204.9503K Altcode: 2004BAAS...36..826K
Intensity disturbances have been observed by TRACE and EIT to propagate
upward along the legs of long active region coronal loops. The
periodic nature and speed of these disturbances suggest that they
are traveling acoustic waves. It is being debated, however, whether
the damping of the perturbations is consistent with the acoustic
wave interpretation. We here examine this issue in detail with a
combination of numerical simulation, analytical theory, and improved
analysis of the observations. Using our state-of-the-art 1D hydro code,
we simulate the propagation of waves generated at the base of model
coronal loops. We consider static equilibrium loops having constant and
expanding cross-section, and an equilibrium loop with steady flow. We
show that the amplitude of the intensity perturbation is affected by
a number of factors: wave dissipation (direct plasma heating), work
done by the wave on the flow, pressure stratification, nonuniform
temperature, and temperature-dependent sensitivity of the observing
instrument. We compare our theoretical results with intensity scale
lengths measured in a sample of loops observed by TRACE. <P />Research
supported by NASA and ONR.
---------------------------------------------------------
Title: Comments on `Possible Role of MHD Waves in Heating the Solar
Corona' by Dwivedi and Pandey
Authors: Klimchuk, J. A.; Porter, L. J.; Sturrock, P. A.
2004SoPh..221...47K Altcode:
We comment on the recent paper by Dwivedi and Pandey (Solar Physics216,
59, 2003). Parts of that paper closely reproduce, without reference,
material that we had published previously, while other parts that
deviate from our earlier analysis contain several critical flaws. We
show that magnetoacoustic waves are capable of heating the corona with
a modest enhancement in the coefficient of compressive viscosity.
---------------------------------------------------------
Title: Nanoflare Heating of the Corona Revisited
Authors: Cargill, Peter J.; Klimchuk, James A.
2004ApJ...605..911C Altcode:
The radiative signatures of the nanoflare model for coronal heating are
investigated. If an observed coronal loop is assumed to consist of many
small strands that cannot be distinguished spatially by EUV or X-ray
observations, we are able to calculate differential emission-measure
profiles and filling factors for a range of heating models. In this
picture the strands undergo continual heating and cooling, leading
to a corona comprising strands with a broad range of temperatures and
densities. Thus, observations over a range of temperatures will show
a multithermal coronal structure. The cyclical heating-cooling leads
inevitably to loops that are underdense and overdense at high and low
temperatures, respectively, compared to what would be expected from
static equilibrium models, and in addition, we show that differential
emission-measure profiles with shallow slopes can be obtained, as
reported in recent observations. The differences between filling
factors that can be seen by broadband and narrowband instruments are
explored. Loops with broadband filling factors near unity can still have
small narrowband factors, and the narrowband factor is shown to be a
strong function of the local temperature. Nanoflare energy distributions
that are constant, flat, or power laws are considered. Power laws lead
to wide distributions of temperatures and densities in the corona,
and steep power laws lead to larger filling factors.
---------------------------------------------------------
Title: The Inability of Steady-Flow Models to Explain the
Extreme-Ultraviolet Coronal Loops
Authors: Patsourakos, S.; Klimchuk, J. A.; MacNeice, P. J.
2004ApJ...603..322P Altcode:
Recent observations from the Transition Region and Coronal Explorer
(TRACE) and the EUV Imaging Telescope (EIT) show that warm (T~1-1.5 MK)
EUV coronal loops in active regions generally have enhanced densities,
enhanced pressure scale heights, and flat filter ratio (temperature)
profiles in comparison with the predictions of static-equilibrium
theory. It has been suggested that mass flows may explain these
discrepancies. We investigate this conjecture using one-dimensional
hydrodynamic simulations of steady flows in coronal loops. The flows
are driven by asymmetric heating that decreases exponentially along
the loop from one footpoint to the other. We find that a sufficiently
large heating asymmetry can produce density enhancements consistent
with a sizable fraction of the observed loops, but that the pressure
scale heights are smaller than the corresponding gravitational scale
heights, and that the filter ratio profiles are highly structured,
in stark contrast to the observations. We conclude that most warm EUV
loops cannot be explained by steady flows. It is thus likely that the
heating in these loops is time dependent.
---------------------------------------------------------
Title: An Observational Test for Coronal Heating Models
Authors: van Driel-Gesztelyi, L.; Démoulin, P.; Mandrini, C. H.;
Harra, L. K.; Klimchuk, J. A.
2004IAUS..219..473V Altcode: 2003IAUS..219E..97V
We correlate the evolution of the mean X-ray flux emission measure
and temperature (Yohkoh SXT & BCS) with the magnetic flux density
(SOHO/MDI) in active region NOAA 7978 from its birth throughout its
decay for five solar rotations. We show that these plasma parameters
together with other quantities deduced from them such as the density
and the pressure follow power-law relationships with the mean magnetic
flux density (bar{B}). We derive the dependence of the mean coronal
heating rate on the magnetic flux density. We use the obtained scaling
laws of coronal loops in thermal equilibrium to derive observational
estimates of the scaling of the coronal heating with bar{B}. These
results are used to test the validity of coronal heating models. We
find that models invoking stochastic buildup of energy current layers
and MHD turbulence are in best agreement with the observations. This
narrows down the range of possible models retained by previous results
obtained for individual coronal loops as well as for the global coronal
emission of the Sun and cool stars.
---------------------------------------------------------
Title: Coronal Loop Heating by Nanoflares: Some Observational
Implications
Authors: Patsourakos, S.; Klimchuk, J. A.
2004hell.conf...35P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: DC coronal heating and the nonlinear evolution of current
sheets
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
2004cosp...35.2721D Altcode: 2004cosp.meet.2721D
Recent theoretical developments have re-awakened interest in the role
of electric current sheets in DC coronal heating (Parker 1988; Priest
et al. 2002). Dahlburg et al. (2003; 2004) reported the existence
of a “secondary instability” that could explain the required
“switch-on” effect required for adequate energy storage. This
ideal, three-dimensional instability also provided a straightforward
explanation for the subsequent fast release of energy, as the
rapid growth of the mode eventually results in a state of turbulent
magnetic reconnection. Earlier studies of the secondary instability
were limited to systems with relatively simple perturbations, viz.,
resistive stability eigenmodes. A current sheet in the Sun is likely
to be subject to much more complex perturbations involving a waves
of various wavelengths and amplitudes. We describe the evolution
of three-dimensional electric current sheets disturbed by random 3D
perturbations. We find that the significant characteristics of secondary
instability are also observed in this case. The relative importance
of subharmonic interactions, i.e., coalescence instability, will also
be discussed. R. B. Dahlburg, J. A. Klimchuk and S. K. Antiochos
Adv. Space Phys. 32, 1029 (2003). R. B. Dahlburg, J. A. Klimchuk
and S. K. Antiochos Astrophys. J.. submitted, (2004). E. N. Parker
Astrophys. J. 330, 474 (1988). E. R. Priest, J. F. Heyvaerts and
A. M. TItle, Astrophys. J. 576, 533 (2002).
---------------------------------------------------------
Title: Linear Force Free Field Models of Observed Coronal Loops
Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A.
2003AGUFMSH42B0515L Altcode:
Although active region magnetic fields have an overall expansion
with height, soft X-ray and EUV loops are observed to have nearly
constant cross sections. To investigate this apparent discrepancy, we
have compared coronal loops observed by TRACE in the 171 A band with
corresponding magnetic flux tubes obtained from linear force-free
extrapolations (Demoulin et al. 1997) of nearly concurrent MDI
magnetograms. The flux tubes were determined using a procedure that
varies the force-free parameter alpha and searches for the field line
that most closely coincides with the observed loop axis. Once the axis
field line is identified, we construct flux tubes from the model field
using a variety of assumed footpoint shapes and orientations. Our
detailed comparison confirms the mystery of constant loop cross
sections. Although the expansion of the extrapolated flux tube as seen
projected onto the plane-of-the-sky varies depending on the footpoint
shape and orientation, it is always considerably greater than the
expansion of the corresponding TRACE loop. Furthermore, the extrapolated
flux tubes are often highly asymmetric (with one leg much wider than
the other), in stark contast to the TRACE loops. These results imply
that the magnetic structure and possibly also the heating of coronal
loops are more complex than we currently understand. Extrapolations of
observed photospheric fields are very useful, but direct measurement
of the coronal field may be necessary to make fundamental progress on
this important problem. This work was funded by NASA and the Office
of Naval Research.
---------------------------------------------------------
Title: The Non-flare Emisson Measure Above 5 MK Observed By RHESSI
and SXI
Authors: McTiernan, J. M.; Klimchuk, J. A.
2003AGUFMSH21B0162M Altcode:
Since RHESSI was launched in February 2002, it has observed thousands
of solar flares. It also observes solar emission above 3 keV when there
are no observeable flares present. In this work we present measurements
of the non-flare Temperature and Emission Measure for the period
from October 2002 through July 2003. The temperature is relatively
stable in the 6 - 8 MK range (this is not surprising considering
that RHESSI cannot reliably measure temperatures less than about
5 MK). The Emission Measure varies in the range from approximately
1.0e49 to 1.0e50, with higher values associated with periods of more
solar activity. Since RHESSI is an imaging spectrometer, we locate the
source of the emission when possible. Preliminary results show that
the source can be associated with active regions. We also present
comparisons with SXI data, and measure the Differential Emission
Measure for the range from 1 MK to 10 MK.
---------------------------------------------------------
Title: Coronal energy release via ideal three-dimensional instability
three-dimensional instability
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
2003AdSpR..32.1029D Altcode:
It is widely believed that most coronal phenomena involve the
release of free energy that is stored within stressed magnetic field
configurations. The availability of sufficient free energy to explain
everything from coronal heating to flares and coronal mass ejections
is well established. How this energy is released remains a major
puzzle. Observations reveal that an important property of the energy
release mechanism is its "switch on" character. The mechanism must
remain dormant for long periods of time to allow the magnetic stresses
to build, then it must operate very vigorously once it finally turns
on. We discuss a mechanism called the "secondary instability" which
exhibits this behavior. It is essentially an ideal instability of the
thin twisted magnetic flux tubes that form from the resistive tearing
of current sheets. We relate the mechanism to the coronal heating
idea of Parker in which the coronal magnetic field becomes tangled by
random motions of the photospheric footpoints. Global energy balance
considerations imply that magnetic energy dissipation occurs at a
particular angle in the field, and the secondary instability offers
the first quantitative explanation for why this should be. It thus
places Parker's popular idea on a much firmer physical footing.
---------------------------------------------------------
Title: Constraints on the Magnetic Field Geometry in Prominences
Authors: Karpen, J. T.; Antiochos, S. K.; Klimchuk, J. A.; MacNeice,
P. J.
2003ApJ...593.1187K Altcode:
This paper discusses constraints on the magnetic field geometry of solar
prominences derived from one-dimensional modeling and analytic theory
of the formation and support of cool coronal condensations. In earlier
numerical studies we identified a mechanism-thermal nonequilibrium-by
which cool condensations can form on field lines heated at their
footpoints. We also identified a broad range of field line shapes
that can support condensations with the observed sizes and lifetimes:
shallowly dipped to moderately arched field lines longer than several
times the heating scale. Here we demonstrate that condensations formed
on deeply dipped field lines, as would occur in all but the near-axial
regions of twisted flux ropes, behave significantly differently than
those on shallowly dipped field lines. Our modeling results yield
a crucial observational test capable of discriminating between two
competing scenarios for prominence magnetic field structure: the flux
rope and sheared-arcade models.
---------------------------------------------------------
Title: Hydrodynamic Simulations of Longitudinal Intensity Oscillations
Observed in Coronal Loops by TRACE
Authors: Tanner, S. E.; Klimchuk, J. A.; Hood, A. W.; De Moortel, I.
2003SPD....34.0406T Altcode: 2003BAAS...35..811T
Propagating intensity disturbances are often observed by TRACE in
large coronal loops located at the perimeters of active regions
(e.g., De Moortel et al., 2002, Solar Phys., 209, 61). On average,
the disturbances have periods of 280 s, propagation speeds of 120
km s<SUP>-1</SUP>, intensity amplitudes of 4%, and surprisingly
small damping (detection) lengths of 9000 km. In addition, there
is a positive correlation between damping length and period. The
preliminary interpretation of these disturbances is that they are
rapidly dissipating slow magneto-acoustic waves. <P />To investigate
this interpretation more rigorously, we have performed a series of
detailed coronal loop simulations using our 1D hydrodynamic code,
ARGOS. We generate waves in the loop by imposing a spatially localized
oscillating force at the loop footpoint, using a range of different
oscillation periods. We here report on the results of our study and,
in particular, whether the damping lengths have the properties observed
by TRACE. <P />This work was supported by NASA and ONR.
---------------------------------------------------------
Title: Linear force free field models of observed coronal loops
Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A.
2003SPD....34.0105L Altcode: 2003BAAS...35R.805L
Although active region magnetic fields have an overall expansion
with height, soft X-ray and EUV loops are observed to have nearly
constant cross sections. To investigate this apparent discrepancy, we
have compared coronal loops observed by TRACE in the 171 A band with
corresponding magnetic flux tubes obtained from linear force-free
extrapolations (Demoulin et al. 1997) of nearly concurrent MDI
magnetograms. The flux tubes were determined using a procedure that
varies the force-free parameter alpha and searches for the field line
that most closely coincides with the observed loop axis. Once the
axis field line is identified, we construct flux tubes from the model
field using a variety of assumed footpoint shapes and orientations. <P
/>Our detailed comparison confirms the mystery of constant loop cross
sections. Although the expansion of the extrapolated flux tube as seen
projected onto the plane-of-the-sky varies depending on the footpoint
shape and orientation, it is always considerably greater than the
expansion of the corresponding TRACE loop. This result implies that the
magnetic structure and possibly also the heating of coronal loops are
more complex than we currently understand. <P />This work was funded
by NASA and the Office of Naval Research.
---------------------------------------------------------
Title: The Non-flare Solar Temperature and Emission Measure Observed
by RHESSI
Authors: McTiernan, J. M.; Klimchuk, J. A.
2003SPD....34.1808M Altcode: 2003BAAS...35..841M
Since RHESSI was launched in February 2002, it has observed thousands
of solar flares. It also observes solar emission above 3 keV when
there are no observeable flares present. In this work we present
measurements of the non-flare Temperature and Emission Measure for
the period from October 2002 through May 2003. Preliminary results
indicate that the temperature is relatively stable in the 6 - 8 MK
range (this is not surprising considering that RHESSI cannot reliably
measure temperatures less than about 5 MK). The emission measure varies
in the range from approximately 1.0e49 to 1.0e50, with higher values
associated with periods of more solar activity. <P />Since RHESSI is an
imaging spectrometer, we will locate the source of the emission when
possible. We also will discuss comparisons with TRACE and SXI data,
and the possiblilty of measuring the Differential Emission Measure
for the range from 1 MK to 10 MK, using the multiple data sets.
---------------------------------------------------------
Title: Are All Coronal Loops Heated by Nanoflares?
Authors: Klimchuk, J. A.; Patsourakos, S.; Winebarger, A. R.
2003SPD....34.1006K Altcode: 2003BAAS...35R.825K
Observations from TRACE, SOHO, and Yohkoh have revealed new details
of coronal loops that make them more mysterious than ever. One of
the biggest puzzles concerns the loop density. Hot (> 2 MK) loops
observed by Yohkoh tend to be under dense compared to the predictions
of equilibrium theory, while warm ( 1 MK) loops observed by TRACE
and EIT tend to be over dense. Some over dense loops can be explained
by steady heating that is concentrated near one or both of the loop
legs, but a majority of these loops cannot. <P />We here consider
the possibility that observed loops are comprised of large numbers
of unresolved strands that are heated impulsively and randomly by
nanoflares. The loops appear quasi-steady even though the individual
sub-strands are highly time dependent. When the strands are hot,
they cool primarily by thermal conduction and are under dense, but
when they are warm, they cool primarily by radiation and are over
dense. Since Yohkoh and TRACE are sensitive to different strands,
we might expect them to observe the under and over densities that
they do. <P />To evaluate the feasibility of this universal model
of coronal loops, we have performed 1D hydrodynamic simulations of
impulsively heated strands and compared them with observations from
Yohkoh and TRACE. The results are encouraging in many respects, but
difficulties remain. In this presentation, we discuss the successes
and failures of the model. <P />This work was supported by NASA and ONR.
---------------------------------------------------------
Title: Coronal Energy Release via Explosive Three-Dimensional
Instability
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
2003SPD....34.0107D Altcode: 2003BAAS...35..806D
It is widely believed that most coronal phenomena involve the release
of magnetic free energy that is stored within stressed magnetic field
configurations. The availability of sufficient free energy to explain
everything from coronal heating to flares and coronal mass ejections
is well established, but how this energy is released remains a major
puzzle. Observations reveal that an important property of the energy
release mechanism is its “switch on" character. The mechanism must
remain dormant for long periods of time to allow the magnetic stresses
build, then it must operate very vigorously once it finally turns on. <P
/>We discuss a mechanism called the “secondary instability" which
exhibits this behavior. It is essentially the ideal kinking of thin
twisted magnetic flux tubes that form from the resistive instability
of current sheets. We relate the mechanism to the coronal heating
idea of Parker in which the coronal magnetic field becomes tangled by
random motions of the photospheric footpoints. Global energy balance
considerations imply that magnetic energy dissipation occurs at a
particular angle in the field, and the secondary instability offers
the first quantitative explanation for why this should be. It thus
places Parker's popular idea on a much firmer physical footing. <P
/>This research was funded by NASA.
---------------------------------------------------------
Title: Can Steady-state Mass Flows Explain the Non-hydrostatic Cool
EUV Coronal Loops in Active Regions?
Authors: Patsourakos, S.; Klimchuk, J. A.
2003SPD....34.1009P Altcode: 2003BAAS...35..826P
Recent EIT/TRACE observations of cool (≈ 1-1.5 MK) EUV coronal loops
in active regions showed that these loops are very often characterized
by greatly enhanced pressure scale-heights and densities compared to
the predictions of static equilibrium theory. It has been suggested
that mass flows may explain these over-dense and over-pressure
loops. We investigate this conjecture by the means of 1D hydrodynamic
simulations of steady-state mass flows in coronal loops. The mass flows
in our calculations are driven by asymmetric heating that decreases
exponentially along the loop from one footpoint all way to the
other. By considering several representative cases for the magnitude
and the length scale of the applied asymmetric heating, we determine
how steady-state mass flows affect the thermodynamic structure of
coronal loops and assess whether they can lead to enhanced pressure
scale-heights and densities. Research supported in part by NASA and ONR.
---------------------------------------------------------
Title: The Long-Term Evolution of AR 7978: The Scalings of the
Coronal Plasma Parameters with the Mean Photospheric Magnetic Field
Authors: van Driel-Gesztelyi, L.; Démoulin, P.; Mandrini, C. H.;
Harra, L.; Klimchuk, J. A.
2003ApJ...586..579V Altcode:
We analyze the evolution of the fluxes observed in X-rays and correlate
them with the magnetic flux density in active region (AR) NOAA 7978
from its birth throughout its decay, for five solar rotations. We
use Solar and Heliospheric Observatory Michelson Doppler Imager
(MDI) data, together with Yohkoh Soft X-Ray Telescope (SXT) and
Yohkoh Bragg Crystal Spectrometer (BCS) data, to determine the global
evolution of the temperature and the emission measure of the coronal
plasma at times when no significant brightenings were observed. We
show that the mean X-ray flux and derived parameters, temperature
and emission measure (together with other quantities deduced from
them, such as the density and the pressure), of the plasma in the AR
follow power-law relationships with the mean magnetic flux density
(B). The exponents (b) of these power-law functions (aB<SUP>b</SUP>)
are derived using two different statistical methods, a classical
least-squares method in log-log plots and a nonparametric method,
which takes into account the fact that errors in the data may not be
normally distributed. Both methods give similar exponents, within
error bars, for the mean temperature and for both instruments (SXT
and BCS); in particular, b stays in the range [0.27, 0.31] and [0.24,
0.57] for full-resolution SXT images and BCS data, respectively. For
the emission measure, the exponent b lies in the range [0.85, 1.35]
and [0.45, 1.96] for SXT and BCS, respectively. The determination of
such power-law relations, when combined with the results from coronal
heating models, can provide us with powerful tools for determining the
mechanism responsible for the existence of the high-temperature corona.
---------------------------------------------------------
Title: The Long-Term Evolution of AR 7978: Testing Coronal Heating
Models
Authors: Démoulin, P.; van Driel-Gesztelyi, L.; Mandrini, C. H.;
Klimchuk, J. A.; Harra, L.
2003ApJ...586..592D Altcode:
We derive the dependence of the mean coronal heating rate on the
magnetic flux density. Our results are based on a previous study of
the plasma parameters and the magnetic flux density (B) in the active
region NOAA 7978 from its birth to its decay, throughout five solar
rotations using the Solar and Heliospheric Observatory Michelson
Doppler Imager, Yohkoh Soft X-Ray Telescope (SXT), and Yohkoh Bragg
Crystal Spectrometer (BCS). We use the scaling laws of coronal loops
in thermal equilibrium to derive four observational estimates of the
scaling of the coronal heating with B (two from SXT and two from
BCS observations). These results are used to test the validity of
coronal heating models. We find that models based on the dissipation
of stressed, current-carrying magnetic fields are in better agreement
with the observations than models that attribute coronal heating to
the dissipation of MHD waves injected at the base of the corona. This
confirms, with smaller error bars, previous results obtained for
individual coronal loops, as well as for the global coronal emission
of the Sun and cool stars. Taking into account that the photospheric
field is concentrated in thin magnetic flux tubes, both SXT and BCS
data are in best agreement with models invoking a stochastic buildup
of energy, current layers, and MHD turbulence.
---------------------------------------------------------
Title: Riding the solar wind
Authors: Klimchuk, James A.
2003Natur.421..894K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: How to test coronal heating models?
Authors: Mandrini, C. H.; Démoulin, P.; van Driel-Gesztelyi, L.;
Klimchuk, J. A.; Harra, L. K.
2003BAAA...46....5M Altcode:
We have tested coronal heating models following two different
approaches. In the first case, we compared the dependence of the
coronal heating rate predicted by theoretical models with the
observed one, deriving the scalings of parameters, such as: the
density, temperature and intensity of the coronal magnetic field,
with the length of magnetic field lines. To do so, we combined density
and temperature measurements for 47 coronal loops with magnetic field
models for 14 active regions. In the second case, we analyzed the long
term evolution of an active region observed during seven rotations
on the solar disk and we determined the dependence of the observed
heating rate with the magnetic field density (bar{B}), after finding
the scalings of plasma parameters with bar{B}. In both cases, we found
that models based on the dissipation of stressed, current-carrying
magnetic fields (called direct current models) are in better agreement
with observations than models that attribute coronal heating to the
dissipation of MHD waves injected at the base of the corona (called
alternate current models). Taking into account that the photospheric
field is concentrated in thin magnetic flux tubes, observations are
in best agreement with models invoking a stochastic buildup of energy,
current layers and MHD turbulence, within direct current models.
---------------------------------------------------------
Title: Coronal arcs and magnetic structure of the solar corona
Authors: López Fuentes, M. C.; Klimchuk, J. A.
2003BAAA...46....2L Altcode:
Since the strength of the coronal magnetic field decreases with distance
from the photosphere, it is expected that coronal magnetic structures
expand with height. Nevertheless, loops observed in EUV and Soft X-rays
have constant cross section (Klimchuk 2000). In this study we compare
a set of TRACE loops with flux tubes obtained from the force-free
extrapolation of the coronal magnetic field. As boundary condition
we use MDI magnetograms cotemporal with the TRACE observations. The
observed width of the coronal loops in the studied set is constant. On
the other hand, the corresponding modeled flux tubes expand with
distance from the photosphere. To compare with observed quantities, we
compute the width of the flux tubes as seen from the line of sight. In
contrast with the nearly constant thickness of the observed loops,
the width of the modeled flux tubes is highly variable. These results
imply that the magnetic structure of the corona and the mechanism of
coronal heating are much more complex than currently understood.
---------------------------------------------------------
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: The mystery of Coronal Loops
Authors: Klimchuk, J. A.
2003BAAA...46....2K Altcode:
Coronal loops, beautiful plasma structures that outline arched magnetic
fields, are often referred to as the fundamental building blocks of
the million degree solar corona. Despite their importance and years
of intensive study, coronal loops are in many ways enigmatic. In
particular, recent observations have forced us to abandon our old belief
that they are static structures maintained by steady heating. In this
talk we review the status of coronal loop physics and describe an
emerging paradigm of nanoflare heating.
---------------------------------------------------------
Title: Bright Knots in EUV Post-flare Loops : TRACE Observations
and 1D Hydrodynamic Modeling
Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A.
2002AGUFMSH21C..04P Altcode:
EUV post-flare loops often possess bright knots along them. Some
examples of such post-flare loops seen by TRACE will be shown, along
with a brief outline of their properties. We will then present the
results of a series of 1D hydrodynamic simulations of flaring loops,
which employ different heating functions for the impulsive and decay
phase of the simulated flares. It will be demonstrated that the creation
of these knots depends crucially on the spatio-temporal distribution of
the heating during the decay phase. This provides strong constraints
on both post-flaring conditions and AR loop heating. We will finally
briefly outline how SDO instrumentation could improve our knowledge
of this topic. Research supported in part by NASA and ONR.
---------------------------------------------------------
Title: Scaling Laws for Solar and Stellar Coronae
Authors: Klimchuk, James A.
2002ASPC..277..321K Altcode: 2002sccx.conf..321K
No abstract at ADS
---------------------------------------------------------
Title: Fuzzy hot post-flare loops versus sharp cool post-flare loops
Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A.
2002ESASP.505..207P Altcode: 2002solm.conf..207P; 2002IAUCo.188..207P
By using high spatial resolution TRACE EUV observations we show that hot
(≍2 MK) post-flare loops are fuzzier than the cooler (≍1 MK) ones. A
simple 0d model of a cooling loop arcade, where different loops in the
arcade start to cool down at slightly different initial conditions,
is sufficient to reproduce qualitatively the observed behavior of the
EUV post-flare loops.
---------------------------------------------------------
Title: Hydrodynamic models of transiently heated coronal loops
Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.;
Antiochos, S. K.; Klimchuk, J. A.; MacNeice, P. J.
2002ESASP.505..583S Altcode: 2002solm.conf..583S; 2002IAUCo.188..583S
We investigate the hydrodynamic behaviour of coronal loops
undergoing transient heating. We adopt a 1-D loop model with space-
and time-dependent heating, concentrated near the chromospheric
footpoints. The timescale of heating variations is comparable with the
radiative cooling time of the coronal plasma (~10<SUP>3</SUP>s). We
use a new numerical code that has a fully adaptive grid, in order to
properly resolve the chromospheric-coronal transition region sections of
the loop. We simulate here the hydrodynamics of a loop with different
effective gravity (i.e., loop geometry) and heating terms. We describe
the temporal behaviour of the various physical quantities along the
loop (plasma density, temperature, flow velocity), showing that the
increase in heating produces a chromospheric evaporation, or a siphon
flow if the loop heating is taken to be significantly different at
the two footpoints, followed by long-lasting downflows with velocities
of a few km s<SUP>-1</SUP> during the quiescent phases in between the
episodic heatings. Moreover, in the case of considerable increase in
heating, a catastrophic cooling of the loop plasma can occur, giving
rise to downflows of several tens of km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: 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: Hot versus cool coronal loops
Authors: Patsourakos, S.; Klimchuk, J. A.; Antiochos, S. K.
2002AAS...200.0209P Altcode: 2002BAAS...34..640P
EUV and SXR observations show respectively that cool (1 MK) loops are
finer and maybe more dynamic than hotter (2 MK) ones. Whether this
reflects a fundamental difference in the properties of the heating
mechanism in action in each loop class is not yet clear. We will address
some aspects of this issue by combining EUV and SXR observations of
such loops with eventually hydrodynamic simulations of a nano-flare
heated corona. Research supported in part by ONR and NASA.
---------------------------------------------------------
Title: An Explanation for the “Switch On" Character of Magnetic
Energy Release
Authors: Klimchuk, J. A.; Dahlburg, R. B.; Antiochos, S. K.
2002AAS...200.1607K Altcode: 2002BAAS...34..668K
It is widely believed that most coronal phenomena involve the release
of magnetic free energy that is stored in stressed magnetic field
configurations. The availability of sufficient free energy to explain
everything from coronal heating to flares and coronal mass ejections
is well established, but how this energy is released remains a major
puzzle. Observations reveal that an important property of the energy
release mechanism is its “switch on" character. The mechanism must
remain dormant for long periods of time to allow the magnetic stresses
to build, then it must operate very vigorously once it finally turns
on. We discuss a mechanism called the “secondary instability" which
exhibits this behavior. It is essentially the ideal kinking of thin
twisted magnetic flux tubes that form from the restive tearing of
current sheets. We relate the mechanism to the coronal heating idea
of Parker in which the coronal magnetic field becomes tangled by
random motions of the photospheric footpoints. Global energy balance
considerations imply that magnetic energy dissipation occurs at a
particular angle in the field, and the secondary instability offers
the first quantitative explanation for why this should be. It thus
places Parker's popular idea on a much firmer physical footing.
---------------------------------------------------------
Title: Coronal loops
Authors: Klimchuk, James
2002ocnd.confE..17K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations and Modeling of Solar Coronal Loops
Authors: Klimchuk, J.
2002cosp...34E1208K Altcode: 2002cosp.meetE1208K
Coronal loops are often described as the fundamental building blocks
of solar and stellar coronae. Clearly, therefore, a comprehensive
understanding of coronae requires an explanation of the nature and
origin of these loops, including the mechanism of their heating. Certain
general aspects of coronal loops are reasonably well understood. For
example, we know that the plasma is structured by the magnetic field
and that strongly heated flux tubes tend to be hotter and denser than
weakly heated flux tubes. Some observations suggest that loops are in
quasi-static equilibrium, and scaling laws have been used to describe
the relationships among physical variables and to test competing
theories of coronal heating. Other observations raise serious doubts
about whether the quasi-static description is valid. At this point,
we cannot say with any certainty whether loops are isothermal or
multithermal (i.e., monolithic structures or collections of unresolved
strands) or whether they are heated steadily or in a highly episodic
fashion (e.g., by nanoflares). This talk will address what we can learn
about these important questions from a combination of observations
and theoretical modeling.
---------------------------------------------------------
Title: Modeling the coronal loop of an X-ray bright point
Authors: McMullen, R.; Longcope, D.; McKenzie, D.; Kankelborg, C.;
Klimchuk, J.
2002ocnd.confE..28M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observation and Theory of Coronal Loop Structure
Authors: Klimchuk, J. A.
2002mwoc.conf...65K Altcode:
Following up on an initial study of 10 soft X-ray loops observed
by Yohkoh (Klimchuk et al. 1992), we have carefully examined 43
additional Yohkoh loops and 24 EUV loops observed by TRACE, and
we confirm our original finding that most coronal loops have a
nearly uniform thickness. This implies that: 1. the magnetic field
in these loops expands with height much less than standard coronal
models would predict; and 2. the shape of the loop cross section is
approximately circular. We have investigated whether these surprising
results can be explained by locally enhanced twist in the field,
so that observed loops correspond to twisted coronal flux tubes. Our
approach is to construct numerical models of fully three-dimensional
force-free magnetic fields. To resolve the internal structure of an
individual loop embedded within a much larger dipole configuration,
we use a nonuniform numerical grid of size 609 times 513 times 593, the
largest ever applied to a solar problem, to our knowledge. Our models
indicate that twist does indeed promote circular cross sections in the
corona, even when the footpoint cross section is irregular. However,
twist does not seem to be a likely explanation for the observed minimal
expansion with height.
---------------------------------------------------------
Title: Coronal energy release via explosive magnetic reconnection
Authors: Dahlburg, R.; Klimchuk, J.; Antiochos, S.
2002cosp...34E1264D Altcode: 2002cosp.meetE1264D
It is widely believed that most coronal phenomena involve the release
of magnetic free energy that is stored within stressed magnetic field
configurations. The availability of sufficient free energy to explain
everything from coronal heating to flares and coronal mass ejections
is well established, but how this energy is released remains a major
puzzle. Observations reveal that an important property of the energy
release mechanism is its "switch on" character. The mechanism must
remain dormant for long periods of time to allow the magnetic stresses
to build, then it must operate very vigorously once it finally turns
on. We discuss a mechanism called the "secondary instability" which
exhibits this behavior. It is essentially the ideal kinking of thin
twisted magnetic flux tubes that form from the resistive tearing of
current sheets. We relate the mechanism to the coronal heating idea
of Parker in which the coronal magnetic field becomes tangled by
random motions of the photospheric footpoints. Global energy balance
considerations imply that magnetic energy dissipation occurs at a
particular angle in the field, and the secondary instability offers
the first quantitative explanation for why this should be. It thus
places Parker's popular idea on a much firmer physical footing.
---------------------------------------------------------
Title: On the Correlation between Coronal and Lower Transition Region
Structures at Arcsecond Scales
Authors: Vourlidas, A.; Klimchuk, J. A.; Korendyke, C. M.; Tarbell,
T. D.; Handy, B. N.
2001ApJ...563..374V Altcode:
We compare the morphology of active region structures observed in
the 171 Å (T~9×10<SUP>5</SUP> K) and Lyα (T~2×10<SUP>4</SUP> K)
lines. The coronal data were obtained by the Transition Region and
Coronal Explorer (TRACE) in support of the Very High Angular Resolution
Ultraviolet Telescope (VAULT) sounding rocket launch, which acquired
subarcsecond resolution images of an active region in the Lyα line,
on 1999 May 7. Using a pair of calibrated, nearly simultaneous images,
we find that: (i) a very good correlation exists between the Lyα and
171 Å intensities in the TRACE moss regions, (ii) we can identify
several identical structures in some (but not all) moss areas, and
(iii) the correlations are greatly reduced at the footpoints of the
171 Å large-scale loops. We derive a lower limit for the Lyα emission
measure, under the assumption of effectively optically thin emission,
and compare it to the 171 Å emission measure. As in previous studies,
we find an excess of Lyα material compared to the amount expected
for a thermal conduction-dominated corona-chromosphere transition
region, even for structures that appear to be identical in the two
wavelengths. This result implies that some other mechanism besides
classical heat conduction from the corona must contribute to the
observed Lyα intensities. The observations do not support the idea
of a physically distinct cool loop component within active regions.
---------------------------------------------------------
Title: Spectroscopic Diagnostics of Nanoflare-heated Loops
Authors: Klimchuk, J. A.; Cargill, P. J.
2001ApJ...553..440K Altcode:
To evaluate the usefulness of spectroscopic techniques for diagnosing
realistic solar plasmas and to better understand the physical origin of
coronal heating, we have simulated observations of model coronal loops
that are heated randomly and impulsively by nanoflares. We find that
the emission measures, densities, and filling factors that are inferred
from spectral line intensities (EM<SUB>s</SUB>, n<SUB>s</SUB>, and
φ<SUB>s</SUB>, respectively) are generally an excellent representation
of the properties of the nanoflare-heated plasma. To better than 25%
in most cases, EM<SUB>s</SUB> indicates the amount of material present
in the ΔlogT=0.3 temperature interval centered on the peak of the line
contribution function, n<SUB>s</SUB> indicates the average density of
this material, and φ<SUB>s</SUB> indicates the fraction of the total
volume that the material occupies. Measurements with lithium-like lines
are much less accurate, however. We provide diagnostic values and line
intensities for many different spectral lines that can be compared
directly with observations from the Coronal Diagnostic Spectrometer
and Solar Ultraviolet Measurements of Emitted Radiation instruments
on SOHO and from the future Extreme Ultraviolet Imaging Spectrometer
instrument on Solar-B. Such comparisons will provide the first ever
rigorous test of the nanoflare concept, which has enormous implications
for understanding the mechanism of coronal heating.
---------------------------------------------------------
Title: Ultra-High Resolution Observations of the Upper Chromosphere:
First Results From the NRL VAULT Sounding Rocket Payload
Authors: Vourlidas, A.; Korendyke, C. M.; Dere, K. P.; Klimchuk, J. A.
2001AGUSM..SP61A03V Altcode:
The Very high resolution Advanced ULtraviolet Telescope (VAULT) is
a new spectroscopic imaging instrument. The instrument was launched
on May 7, 1999 as a sounding rocket payload. The goal of the first
VAULT flight was to obtain sub-arcsecond images of the Sun in the
light of Lya (1216 Å). VAULT directly imaged an active region plage,
fliaments and the fine structures in the supergranule boundaries and
network with the unprecented spatial resolution of 0.33 arcseconds. We
present the VAULT images and the first results from the comparison of
the Lya data to observations from other instruments and in particular
with a sequence of TRACE 171 Å images taken during the VAULT flight.
---------------------------------------------------------
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: Observational Signatures of Nanoflare-Heated Loops
Authors: Klimchuk, J. A.; Cargill, P. J.
2001AGUSM..SP52B01K Altcode:
After many years of intense study, the heating of the corona remains
a largely unsolved problem. A number of ideas have been proposed
involving the episodic release of small bursts of energy, termed
“nanoflares" by Parker based on his rough estimate of the energies
involved. Adopting this concept, we investigate a model whereby
individual coronal loops like those observed by Yohkoh, SOHO, and
TRACE are comprised of many thousands of unresolved strands that are
heated randomly and independently by nanoflares. The strands cool
by thermal conduction and radiation until they are heated again by
the next event. We simulate the observation of these model loops by
broadband and spectroscopic instruments such as SXT, CDS, SUMER, and
EIS. By varying the nanoflare parameters (primarily the mean energy
of the events) and comparing with actual observations, we are able to
test the model and constrain the parameters. We find that many real
loops are compatible with the model, especially those hotter than 2 MK,
but that many others are not. These latter loops are also incompatible
with steady heating, and we suggest that they contain coronal material
that is injected into the loop by a mechanism not directly related to
coronal heating (e.g., not chromospheric evaporation).
---------------------------------------------------------
Title: Properties of EUV and X-ray emission in solar active regions
Authors: Matthews, S. A.; Klimchuk, J. A.; Harra, L. K.
2001A&A...365..186M Altcode:
Using observations from the Coronal Diagnostic Spectrometer (CDS) on
SoHO and the Soft X-ray Telescope (SXT) on Yohkoh we investigate how the
spatial properties of active region emission observed in the EUV and
X-ray range varies with temperature. We examine the contrast per unit
area of the EUV emission from a number of active regions, and employ
correlation techniques and Fourier methods with which we obtain the two
dimensional power spectrum of the intensity distribution for a number of
images in emission lines formed at different temperatures. Integrating
this over polar angle we find isotropic power-law behaviour at all
temperatures in a number of topologically different active regions,
with a tendency for flatter spectra at lower temperatures. The
existence of power-law spectra indicates that there is no preferred
length scale within the regions, at least not a resolvable one, while
flatter spectra at lower temperatures indicate that the structures are
relatively smaller in this temperature range, possibly providing support
for the idea of a multi-component transition region (TR). Implications
for various heating models are discussed.
---------------------------------------------------------
Title: Theory of Coronal Mass Ejections
Authors: Klimchuk, J. A.
2001GMS...125..143K Altcode:
Coronal mass ejections (CMEs) are extremely important phenomena,
both for understanding the evolution of the global corona and for
understanding and predicting space weather. Despite this importance,
the physical explanation of CMEs remains largely confused. A variety
of theoretical models have been proposed, and we here attempt to
organize them according to a classification scheme that identifies and
differentiates their essential physical attributes. We propose five
distinct classes of models, which we present with the aid of simple
analogues involving springs, ropes, and weights. We also indicate how
some of the models appear to be inconsistent with certain observations.
---------------------------------------------------------
Title: Twisted Coronal Magnetic Loops
Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.
2000ApJ...542..504K Altcode:
Observed coronal loops have a surprisingly uniform thickness that
cannot be easily understood in terms of standard coronal magnetic
field models. We investigate the possibility that the uniform
thickness can be explained by locally enhanced twist in the field,
so that observed loops correspond to twisted coronal flux tubes. Our
approach is to construct numerical models of fully three-dimensional
force-free magnetic fields. To resolve the internal structure of an
individual loop embedded within a much larger dipole configuration,
we use a nonuniform numerical grid of size 609×513×593, the largest
ever applied to a solar problem to our knowledge. Our models show that
twist promotes circular cross sections in loops. Such cross sections are
typically assumed, and have recently been verified from observations,
but their physical cause has been heretofore unexplained.
---------------------------------------------------------
Title: Observation and theory of coronal loop structure.
Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.; Watko, J. A.
2000BAAS...32R.809K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observation and Theory of Coronal Loop Structure
Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.; Watko, J. A.
2000SPD....31.0144K Altcode:
We have carefully examined 43 soft X-ray loops observed by Yohkoh and
24 EUV loops observed by TRACE and find that the large majority have
a nearly uniform thickness. This implies that: 1. the magnetic field
in these loops expands with height much less than standard coronal
models would predict; and 2. the shape of the loop cross section is
approximately circular. We have investigated whether these surprising
results can be explained by locally enhanced twist in the field,
so that observed loops correspond to twisted coronal flux tubes. Our
approach is to construct numerical models of fully three-dimensional
force-free magnetic fields. To resolve the internal structure of an
individual loop embedded within a much larger dipole configuration,
we use a nonuniform numerical grid of size 609 x 513 x 593, the largest
ever applied to a solar problem, to our knowledge. Our models indicate
that twist does indeed promote circular cross sections in the corona,
even when the footpoint cross section is irregular. However, twist does
not seem to be a likely explanation for the observed minimal expansion
with height. This work was supported by the NASA Sun-Earth Connection
Theory and Guest Investigator Programs.
---------------------------------------------------------
Title: Cross-Sectional Properties of Coronal Loops
Authors: Klimchuk, J. A.
2000SoPh..193...53K Altcode:
Careful examination of 43 soft X-ray loops observed by Yohkoh
has revealed a number of interesting properties of the loop cross
section. First, the loops tend to be only slightly (≈ 30%) wider at
their midpoints than at their footpoints, implying less-than-expected
expansion of the magnetic field. Second, the variation of width along
each loop tends to be modest, implying that the cross section has an
approximately circular shape. And third, cross-axis intensity profiles
tend to be singly-peaked and simple, implying that the cross section
is approximately uniformly filled on resolvable scales. We conclude
that the energy which heats the plasma is either dissipated axially
symmetrically on a scale equal to a loop diameter (≈ 11 000 km)
or else is dissipated with any spatial structure, but on a scale
much smaller than a loop diameter, and then transported laterally in
an axisymmetric fashion (perhaps via conduction along chaotic field
lines). In their present form, none of the theoretical ideas concerning
the magnetic structure and heating of loops are obviously capable of
explaining all of the observed properties.
---------------------------------------------------------
Title: Width Variations along Coronal Loops Observed by TRACE
Authors: Watko, J. A.; Klimchuk, J. A.
2000SoPh..193...77W Altcode:
We have measured width variations along coronal loops observed by TRACE
in the 171, 195, and 284 Å bandpasses. The loops are not significantly
thicker in the middle compared to near the footpoints, and there is
no correlation between the footpoint-to-midpoint expansion and the
loop length. This applies to both post-flare and non-flare loops. The
observations conflict with our present understanding of active region
magnetic fields, and they have important implications for the structure
and heating of the corona.
---------------------------------------------------------
Title: Three-dimensional Stereoscopic Analysis of Solar Active Region
Loops. II. SOHO/EIT Observations at Temperatures of 1.5-2.5 MK
Authors: Aschwanden, Markus J.; Alexander, David; Hurlburt, Neal;
Newmark, Jeffrey S.; Neupert, Werner M.; Klimchuk, J. A.; Gary,
G. Allen
2000ApJ...531.1129A Altcode:
In this paper we study the three-dimensional structure of hot
(T<SUB>e</SUB>~1.5-2.5 MK) loops in solar active region NOAA
7986, observed on 1996 August 30 with the Extreme-ultraviolet
Imaging Telescope (EIT) on board the Solar and Heliospheric
Observatory (SOHO). This complements a first study (Paper I) on
cooler (T<SUB>e</SUB>~1.0-1.5 MK) loops of the same active region,
using the same method of Dynamic Stereoscopy to reconstruct the
three-dimensional geometry. We reconstruct the three-dimensional
coordinates x(s), y(s), z(s), the density n<SUB>e</SUB>(s), and
temperature profile T<SUB>e</SUB>(s) of 35 individual loop segments
(as a function of the loop coordinate s) using EIT 195 and 284 Å
images. The major findings are as follows. (1) All loops are found
to be in hydrostatic equilibrium, in the entire temperature regime
of T<SUB>e</SUB>=1.0-2.5 MK. (2) The analyzed loops have a height of
2-3 scale heights, and thus only segments extending over about one
vertical scale height have sufficient emission measure contrast for
detection. (3) The temperature gradient over the lowest scale height
is of order dT/ds~1-10 K km<SUP>-1</SUP>. (4) The radiative loss
rate is found to exceed the conductive loss rate by about two orders
or magnitude in the coronal loop segments, implying that the loops
cannot be in quasi-static equilibrium, since standard steady-state loop
models show that radiative and conductive losses are comparable. (5) A
steady state could only be maintained if the heating rate E<SUB>H</SUB>
matches exactly the radiative loss rate in hydrostatic equilibrium,
requiring a heat deposition length λ<SUB>H</SUB> of the half density
scale height λ. (6) We find a correlation of p~L<SUP>-1</SUP> between
loop base pressure and loop length, which is not consistent with the
scaling law predicted from steady-state models of large-scale loops. All
observational findings indicate consistently that the energy balance
of the observed EUV loops cannot be described by steady-state models.
---------------------------------------------------------
Title: Magnetic Field and Plasma Scaling Laws: Their Implications
for Coronal Heating Models
Authors: Mandrini, C. H.; Démoulin, P.; Klimchuk, J. A.
2000ApJ...530..999M Altcode:
In order to test different models of coronal heating, we have
investigated how the magnetic field strength of coronal flux
tubes depends on the end-to-end length of the tube. Using
photospheric magnetograms from both observed and idealized
active regions, we computed potential, linear force-free, and
magnetostatic extrapolation models. For each model, we then
determined the average coronal field strength, <B>, in
approximately 1000 individual flux tubes with regularly spaced
footpoints. Scatter plots of <B> versus length, L, are
characterized by a flat section for small L and a steeply declining
section for large L. They are well described by a function of the form
log=C<SUB>1</SUB>+C<SUB>2</SUB>logL+C<SUB>3</SUB>/2log(L<SUP>2</SUP>+S<SUP>2</SUP>),
where C<SUB>2</SUB>~0, -3<=C<SUB>3</SUB><=-1, and 40<=S<=240
Mm is related to the characteristic size of the active region. There
is a tendency for the magnitude of C<SUB>3</SUB> to decrease as the
magnetic complexity of the region increases. The average magnetic
energy in a flux tube, <B<SUP>2</SUP>>, exhibits a similar
behavior, with only C<SUB>3</SUB> being significantly different. For
flux tubes of intermediate length, 50<=L<=300 Mm, corresponding
to the soft X-ray loops in a study by Klimchuk & Porter (1995),
we find a universal scaling law of the form ~L<SUP>δ</SUP>, where
δ=-0.88+/-0.3. By combining this with the Klimchuk & Porter result
that the heating rate scales as L<SUP>-2</SUP>, we can test different
models of coronal heating. We find that models involving the gradual
stressing of the magnetic field, by slow footpoint motions, are in
generally better agreement with the observational constraints than are
wave heating models. We conclude, however, that the theoretical models
must be more fully developed and the observational uncertainties must
be reduced before any definitive statements about specific heating
mechanisms can be made.
---------------------------------------------------------
Title: The spatial distribution of EUV emission in active regions
Authors: Matthews, S. A.; Klimchuk, J. A.; Harra, L. K.
2000ssls.work...53M Altcode:
The full version of this paper will be published elsewhere. We give
here only an extended abstract.
---------------------------------------------------------
Title: Test on the parameter dependence of coronal heating models
Authors: Démoulin, P.; Mandrini, C. H.; Klimchuk, J. A.
2000ssls.work...85D Altcode:
The motivation of this work has been to provide observational
constraints on coronal heating models by testing their predictions
for the heating rate as a function of several physical parameters. In
Mandrini et al. (1999), we have investigated how the magnetic field
strength, , of coronal flux tubes depends on the end-to-end length,
L, of the tube. For flux tubes of intermediate length, 50 ≤ L ≤
300 Mm, corresponding to the soft X-ray loops in a study by Klimchuk
& Porter (1995), we find a universal scaling law of the form
∝L<SUP>δ</SUP>, where δ= -0.88±0.3. By combining this with
the Klimchuk & Porter result that the heating rate scales as
L<SUP>-2</SUP>, we can test different models of coronal heating. We
find that models involving the gradual stressing of the magnetic field,
by slow footpoint motions, are in generally better agreement with the
observational constraints than are wave heating models.
---------------------------------------------------------
Title: Properties of Transition Region and Coronal Loops
Authors: Matthews, S. A.; Klimchuk, J. A.; Harra-Murnion, L. K.
1999ESASP.446..489M Altcode: 1999soho....8..489M
The magnetic field plays a vital role in governing the size, shape
and dynamics of loops in the solar atmosphere and as such it seems
reasonable to ask whether differences in the spatial distribution
of these structures are indicative of differences in the form
of the heating. Using observations from the Coronal Diagnostic
Spectrometer (CDS) on SoHO and the Soft X-ray Telescope (SXT) on
Yohkoh we investigate how the spatial distribution of EUV and X-ray
emission in active regions varies with temperature. We examine various
correlations between emission at different temperatures, the variation
of contrast with temperature and employ Fourier methods to obtain
the two dimensional power spectrum of the intensity distribution for
a number of lines at different temperatures. Integrating this over
polar angle we find isotropic power-law behaviour at all temperatures
in a number of topologically different active regions, with a tendency
for flatter spectra at lower temperatures. The existence of power-law
spectra indicates that there is no preferred length scale within the
regions. Flatter spectra at lower temperatures are consistent with
emission predominantly from smaller scale structures such as low-lying
loops or footpoints.
---------------------------------------------------------
Title: Temperature and density in a polar plume - measurements
from CDS/SOHO
Authors: Young, P. R.; Klimchuk, J. A.; Mason, H. E.
1999A&A...350..286Y Altcode:
A detailed analysis of a particularly intense polar plume observed
on the 25th of October, 1996, by the Coronal Diagnostic Spectrometer
(CDS) on board the Solar and Heliospheric Observatory (SOHO) is
presented. Above the limb, emission measure distributions derived for
both the plume and a section of coronal hole background are found to
be sharply peaked at approximately 1.0-1.1 million degrees in both
regions. The temperature rises with height in the background, but no
evidence is found for a rising temperature in the plume. The density
of the background is approximately 10(8) electrons/cm(3) and falls
with height. In the plume the density is between 3.8 and 9.5x 10(8)
electrons/cm(3) , and exhibits no decrease with height up to 70 000
km. The plume base is visible on the solar surface and shows a strong
brightening lying directly below the main body of the plume. This
brightening has a temperature of 2 000 000 K, and a density of
2.5-5.6x 10(9) electrons/cm(3) . Images from lines formed at different
temperatures suggest that the morphology of the base is consistent with
an emerged bipole in a region of unipolar magnetic flux. A measurement
of the Mg/Ne relative abundance is made at two transition region
brightenings at the base of the plume. An enhancement of only 1.5 is
found over the photospheric value. Considerations of the geometry of
both the high temperature brightening at the base of the plume and
the off-limb section give filling factors of 0.5 and 1.0, respectively.
---------------------------------------------------------
Title: Magnetic Field Scaling Laws and Their Implications for
Coronal Heating
Authors: Klimchuk, J. A.; Demoulin, P.; Mandrini, C. H.
1999AAS...194.2304K Altcode: 1999BAAS...31..861K
Ever since it was realized, some 60 years ago, that the solar
corona is two orders of magnitude hotter than the underlying
photosphere, scientists have puzzled over the reason for these
extreme conditions. A number of plausible ideas have been proposed,
including the dissipation of MHD waves (AC models) and the dissipation
of stressed, current-carrying magnetic fields (DC models), but it has
proved difficult to establish which, if any, is correct. One approach
to answering this fundamental question is to determine scaling laws
relating the heating rate to observable physical parameters. Klimchuk
& Porter (1995, Nature, 377, 131) showed that the heating rate
varies inversely with the square of the length of coronal loops observed
by Yohkoh. To compare this with the predictions of coronal heating
theories, it is necessary to know also how the magnetic field strength
in the loops varies with their length. By computing magnetic field
extrapolation models based on both observed and synthetic distributions
of active region surface fields, we have found that B ~ ( L(2) + S(2)
)(c/2) , where B is the coronal field strength averaged along a loop,
L is the loop length, S is the characteristic size of the active region,
and -3 <= c <= -1, depending on the complexity of the region. More
importantly, for the range of loop lengths studied by Klimchuk &
Porter, 50 < L < 300 Mm, there is a universal scaling law of
the form B ~ L(delta ,) where delta = -0.98 +/- 0.3. The details of
these results will be presented, and their implications for theories
of coronal heating will be discussed. It will be shown that DC models
are in better agreement with the observations than are AC models. This
work was supported in part by NASA grant W-19,200.
---------------------------------------------------------
Title: The Solar Flotilla
Authors: Kahler, S. W.; Klimchuk, J. A.; Szabo, A.; Galvin, A. B.
1999AAS...194.6507K Altcode: 1999BAAS...31R.928K
The Solar Flotilla is one of two candidate missions for the NASA
Roadmap which would use a multispacecraft mission to explore the
inner heliosphere. The Solar Flotilla will make particles and fields
measurements at 10 or more points around the Sun within a distance
of 0.2 to 0.4 AU. Identically instrumented microsatellites will be
placed in orbit after a journey to Mercury where they will be injected
into three principal orbit planes by 3-body interactions with the
planet. Solar Flotilla measurements will address the question of
how the solar output of plasma and magnetic flux varies with space
and time, similar to the far better understood variations of radiant
energy outputs, which can be measured remotely. Fundamental questions
of magnetic flux and helicity expulsion can only be studied with
such measurements. Additionally, mass flows and their variations
can be understood on a global scale with measurements from the Solar
Flotilla. The radial and azimuthal variations of turbulence, waves,
and particle temperatures and abundances all relate to the heating of
the solar corona and the acceleration of the solar wind. The spatial
extents and shapes of interplanetary shocks and energetic particle
distributions can be studied and used for improved space weather
predictions. Together with imaging instruments at 1 AU we can explore
the interplanetary extensions of short time-scale coronal features
such as blobs and coronal mass ejections and long time-scale features
such as plumes and streamers. The number of the spacecraft, their
near-solar environment and great distance from Earth pose significant
technological problems for operations and communications to be solved
before deployment of the mission. We discuss further benefits and
requirements of the proposed mission.
---------------------------------------------------------
Title: The Structure of Solar Prominences
Authors: Antiochos, S. K.; DeVore, C. R.; Klimchuk, J. A.
1999AAS...194.3102A Altcode: 1999BAAS...31Q.868A
With the advent of new high-spatial and high-temporal resolution
observations from SOHO and TRACE, prominences/filaments have once again
become a major focus of study for solar physics. Prominences/filaments
are also important for their role in space weather. They yield
key information on the type of magnetic structure that leads to
eruptive flares and coronal mass ejections. We present results from
our calculations of the 3D magnetic structure of prominences and the
origin of the prominence mass. We show that many of the well-know
features of their global structure, such as the prominence legs and
barbs, the inverse polarity, and the sinistral-dextral property,
can be easily understood as due to the geometry of a sheared bipolar
field. Both fully time-dependent 3D MHD simulations and 3D force-free
field equilibrium calculations demonstrate this conclusion. Furthermore,
we discuss results showing that the magnetic structure of a sheared
3D bipole leads naturally to the formation of cool condensations and
to their observed motions. (*) This work is supported by NASA and ONR.
---------------------------------------------------------
Title: Three-dimensional Stereoscopic Analysis of Solar Active
Region Loops. I. SOHO/EIT Observations at Temperatures of (1.0-1.5)
× 10<SUP>6</SUP> K
Authors: Aschwanden, Markus J.; Newmark, Jeffrey S.; Delaboudinière,
Jean-Pierre; Neupert, Werner M.; Klimchuk, J. A.; Gary, G. Allen;
Portier-Fozzani, Fabrice; Zucker, Arik
1999ApJ...515..842A Altcode:
The three-dimensional structure of solar active region NOAA 7986
observed on 1996 August 30 with the Extreme-Ultraviolet Imaging
Telescope (EIT) on board the Solar and Heliospheric Observatory
(SOHO) is analyzed. We develop a new method of dynamic stereoscopy to
reconstruct the three-dimensional geometry of dynamically changing
loops, which allows us to determine the orientation of the mean
loop plane with respect to the line of sight, a prerequisite to
correct properly for projection effects in three-dimensional loop
models. With this method and the filter-ratio technique applied
to EIT 171 and 195 Å images we determine the three-dimensional
coordinates [x(s), y(s), z(s)], the loop width w(s), the electron
density n<SUB>e</SUB>(s), and the electron temperature T<SUB>e</SUB>(s)
as a function of the loop length s for 30 loop segments. Fitting the
loop densities with an exponential density model n<SUB>e</SUB>(h)
we find that the mean of inferred scale height temperatures,
T<SUP>λ</SUP><SUB>e</SUB>=1.22+/-0.23 MK, matches closely that of EIT
filter-ratio temperatures, T<SUP>EIT</SUP><SUB>e</SUB>=1.21+/-0.06
MK. We conclude that these cool and rather large-scale loops (with
heights of h~30-225 Mm) are in hydrostatic equilibrium. Most of the
loops show no significant thickness variation w(s), but we measure
for most of them a positive temperature gradient (dT/ds>0) across
the first scale height above the footpoint. Based on these temperature
gradients we find that the conductive loss rate is about 2 orders of
magnitude smaller than the radiative loss rate, which is in strong
contrast to hot active region loops seen in soft X-rays. We infer a
mean radiative loss time of τ<SUB>rad</SUB>~40 minutes at the loop
base. Because thermal conduction is negligible in these cool EUV
loops, they are not in steady state, and radiative loss has entirely
to be balanced by the heating function. A statistical heating model
with recurrent heating events distributed along the entire loop can
explain the observed temperature gradients if the mean recurrence time
is <~10 minutes. We computed also a potential field model (from
SOHO/MDI magnetograms) and found a reasonable match with the traced
EIT loops. With the magnetic field model we determined also the height
dependence of the magnetic field B(h), the plasma parameter β(h),
and the Alfvén velocity v<SUB>A</SUB>(h). No correlation was found
between the heating rate requirement E<SUB>H0</SUB> and the magnetic
field B<SUB>foot</SUB> at the loop footpoints.
---------------------------------------------------------
Title: The Dynamic Formation of Prominence Condensations
Authors: Antiochos, S. K.; MacNeice, P. J.; Spicer, D. S.; Klimchuk,
J. A.
1999ApJ...512..985A Altcode: 1998astro.ph..8199A
We present simulations of a model for the formation of a prominence
condensation in a coronal loop. The key idea behind the model is that
the spatial localization of loop heating near the chromosphere leads
to a catastrophic cooling in the corona. Using a new adaptive grid
code, we simulate the complete growth of a condensation and find that
after ~5000 s it reaches a quasi-steady state. We show that the size
and growth time of the condensation are in good agreement with data
and discuss the implications of the model for coronal heating and for
observations of prominences and the surrounding corona.
---------------------------------------------------------
Title: A Model for Solar Coronal Mass Ejections
Authors: Antiochos, S. K.; DeVore, C. R.; Klimchuk, J. A.
1999ApJ...510..485A Altcode: 1998astro.ph..7220A
We propose a new model for the initiation of a solar coronal mass
ejection (CME). The model agrees with two properties of CMEs and
eruptive flares that have proved to be very difficult to explain with
previous models: (1) very low-lying magnetic field lines, down to the
photospheric neutral line, can open toward infinity during an eruption;
and (2) the eruption is driven solely by magnetic free energy stored
in a closed, sheared arcade. Consequently, the magnetic energy of the
closed state is well above that of the posteruption open state. The key
new feature of our model is that CMEs occur in multipolar topologies
in which reconnection between a sheared arcade and neighboring flux
systems triggers the eruption. In this “magnetic breakout” model,
reconnection removes the unsheared field above the low-lying, sheared
core flux near the neutral line, thereby allowing this core flux to
burst open. We present numerical simulations that demonstrate our
model can account for the energy requirements for CMEs. We discuss
the implication of the model for CME/flare prediction.
---------------------------------------------------------
Title: Report on new mission concept study: Stereo X-Ray Corona
Imager mission
Authors: Liewer, Paulett C.; Davis, John M.; de Jong, E. M.; Gary,
G. A.; Klimchuk, James A.; Reinert, Richard P.
1998SPIE.3442...53L Altcode:
Studies of the 3D structure and dynamics of the solar corona
have been severely limited by the constraint of single viewpoint
observations. The Stereo X-Ray Coronal Imager (SXCI) mission will send
a single instrument, an X-ray telescope, into deep space expressly to
record stereoscopic images of the solar corona. The SXCI spacecraft
will be inserted into an approximately 1 ZAU heliocentric orbit leading
Earth by approximately 25 degrees at the end of nine months. The SXCI
x-ray telescope forms one element of a stereo pair, the second element
being an identical x-ray telescope in Earth orbit placed there as part
of the NOAA GOES program. X-ray emission is a powerful diagnostic of
the corona and its magnetic fields, and 3D information on the coronal
magnetic structure would be obtained by combining the data from the
two x-ray telescopes. This information can be used to address the major
solar physics questions of (1) what causes explosive coronal events such
as coronal mass ejections, eruptive flares and prominence eruptions and
(2) what causes the transient heating of coronal loops. Stereoscopic
views of the optically thin corona will resolve some ambiguities
inherent in single line-of-sight observations. Triangulation gives 3D
solar coordinates of features which can be seen in the simultaneous
images form both telescopes. As part of this study, tools were
developed for determining the 3D geometry of coronal features using
triangulation. Advanced technologies for visualization and analysis of
stereo images were tested. Results of mission and spacecraft studies
are also reported.
---------------------------------------------------------
Title: Prominence Formation by Localized Heating
Authors: Dahlburg, Russell B.; Antiochos, Spiro K.; Klimchuk, James A.
1998ApJ...495..485D Altcode:
We describe a model for the formation of the cool condensed material
that comprises a coronal filament or prominence. Numerical calculations
are presented which demonstrate that large condensations form in
a coronal loop if the loop satisfies two key requirements: (1) the
loop heating must be localized near the chromospheric footpoints,
and (2) the loop must have a dipped geometry in order to support the
prominence condensation against gravity. We calculate one-dimensional
equilibrium solutions for the equations of force and energy balance
assuming optically thin radiative losses and a parameterized form for
the coronal heating. This physical situation is modeled as a boundary
value problem, which we solve numerically using a B-spline collocation
scheme. The relation of our solutions to the well-known loop scaling
laws is discussed, and the implications of our model for active region
and quiescent prominences are discussed.
---------------------------------------------------------
Title: 3D-Stereoscopic Analysis of Solar Active Region Loops Observed
with SOHO/EIT
Authors: Aschwanden, M. J.; Newmark, J. S.; Delaboudiniere, J. -P.;
Neupert, W. M.; Klimchuk, J. A.; Gary, G. Allen; Portier-Fozzani,
F.; Zucker, A.
1998cee..workE..19A Altcode:
The three-dimensional (3D) structure of solar active region NOAA
7986 observed on 1996 August 30 with the Extrem-ultraviolet Imaging
Telescope (EIT) onboard the Solar and Heliospheric Observatory
(SoHO) is analyzed. We develop a new method of Dynamic Stereoscopy to
reconstruct the 3D geometry of dynamically changing loops, which allows
us to determine the orientation of the loop plane with respect to
the line-of-sight, a prerequisite to correct properly for projection
effects in 3D loop models. With this method and the filter-ratio
technique applied to EIT 171 angle and 195 angle images we determine
the 3D coordinates [x(s),y(s),z(s)], the loop width w(s), the electron
density n_e(s), and the electron temperature T_e(s) as function of
the loop length s for 30 loop segments. Fitting the loop densities
with an exponential density model n_e(h) we find that the mean of
inferred scale height temperatures, T_e<SUP>lambda</SUP> = 1.22 plus or
minus 0.23 MK, matches closely that of EIT filter-ratio temperatures,
T_e<SUP>EIT</SUP> = 1.21 plus or minus 0.06 MK. We conclude that these
cool and rather large-scale loops (with heights of h ~30-225 Mm), which
dominate the EIT 171 angle images, are in hydrostatic equilibrium. Most
of the loops show no significant thickness variation w(s), but we can
measure for most of them a positive temperature gradient (dT/ds >
0) across the first scale height above the footpoint. Based on these
temperature gradients we find that the conductive loss rate is about
two orders of magnitude smaller than the radiative loss rate, which is
in strong contrast to hot active region loops seen in SXR. We infer a
mean radiative loss time of τ<SUB>rad</SUB> ~40 minutes. For steady
state models, the heating rate has to balance the radiative loss,
i.e. the heating rate has to scale with the squared density (E_H
propto n_e^2). From potential-field extrapolations we determine also
the magnetic field strength B(s), the plasma beta-parameter beta(s),
and the Alfven velocity v_A(s) along the loops, and discuss the findings
in the context of coronal heating models.
---------------------------------------------------------
Title: Theory of spicules, jets, plumes and other solar eruptions
Authors: Klimchuk, J. A.
1998ESASP.421..233K Altcode: 1998sjcp.conf..233K
No abstract at ADS
---------------------------------------------------------
Title: A Self-consistent Model for the Resonant Heating of Coronal
Loops: The Effects of Coupling with the Chromosphere
Authors: Ofman, L.; Klimchuk, J. A.; Davila, J. M.
1998ApJ...493..474O Altcode:
We present the first model of resonant heating of coronal loops that
incorporates the dependence of the loop density on the heating rate. By
adopting the quasi-static equilibrium scaling law ρ ~ Q<SUP>5/7</SUP>,
where ρ is the density and Q is the volumetric heating rate, we
are able to approximate the well-known phenomena of chromospheric
evaporation and chromospheric condensation, which regulate the coronal
density. We combine this scaling law with a quasi-nonlinear MHD model
for the resonant absorption of Alfvén waves in order to study the
spatial and temporal dependence of the heating. We find that the heating
is concentrated in multiple resonance layers, rather than in the single
layer of previous models, and that these layers drift throughout the
loop to heat the entire volume. These newfound properties are in much
better agreement with coronal observations.
---------------------------------------------------------
Title: A Self-Consistent Model for the Resonant Heating of Coronal
Loops: the Effects of Coupling with the Chromosphere
Authors: Klimchuk, J. A.; Ofman, L.; Davila, J. M.
1997SPD....28.0504K Altcode: 1997BAAS...29..909K
The physical nature of coronal heating remains one of the great problems
of solar physics. One of the several theories that are being pursued
is the resonant absorption of MHD waves. While promising in several
respects, this theory has suffered from a glaring deficiency: the
computed heating is incompatible with both the assumed density and
the observed structure of coronal loops. We present the first model
of resonant heating of coronal loops that incorporates the dependence
of the loop density on the heating rate. By adopting the quasi-static
equilibrium scaling law rho ~ Q(5/7) , where rho is the density and Q is
the volumetric heating rate, we are able to approximate the well-known
phenomena of chromospheric evaporation and chromospheric condensation,
which regulate the coronal density. We combine this scaling law with
a linearized MHD model for the resonant absorption of Alfven waves
to study the spatial and temporal dependence of the heating. We find
that the heating is concentrated in multiple resonance layers, rather
than the single layer of previous models, and that these layers drift
throughout the loop to heat the entire volume. These new properties
are in much better agreement with coronal observations, including
recent observations from the CDS and EIT instruments on SOHO, as well
as earlier observations from the SXT instrument on Yohkoh.
---------------------------------------------------------
Title: A Nanoflare Explanation for the Heating of Coronal Loops
Observed by Yohkoh
Authors: Cargill, P. J.; Klimchuk, J. A.
1997ApJ...478..799C Altcode:
The nanoflare model of Cargill (1994a) is used to model active
region loops observed by the Yohkoh Soft X-ray Telescope (SXT). Using
observed information concerning the dimensions and energy-loss rate
of each loop, a range of loop models with different temperatures,
emission measures, and filling factors is generated. For hot loops
(T > 4 × 10<SUP>6</SUP> K), it is shown that filling factors less
than 0.1 can fit the data, although the uncertainties can be quite
large. For cool loops (T ~ 2 × 10<SUP>6</SUP> K), the model cannot
reproduce the observed temperature and emission measure for any value
of the filling factor. Earlier work of Porter & Klimchuk suggested
that some of these loops cannot be explained by a steady state heating
model either. It is proposed that there may exist two distinct classes
of loops and that coronal material is injected into the cooler loops
by a mechanism that is not directly related to heating (e.g., not
chromospheric evaporation).
---------------------------------------------------------
Title: The Occurrence Rate of Soft X-Ray Flares as a Function of
Solar Activity
Authors: Feldman, U.; Doschek, G. A.; Klimchuk, J. A.
1997ApJ...474..511F Altcode:
In this paper we investigate the occurrence rate of soft X-ray
solar flares observed by the Geostationary Operational Environmental
Satellites (GOES). The analysis includes all flares classified as equal
to or brighter than A1 and covers the time period from 1993 November
to 1995 July. We find a power-law relationship between the number of
flares per hour and peak X-ray brightness in the 1-8 Å range. The
average power-law index for dN/dF, where N is the number of events
per hour and F, the GOES flux, is about -1.88 +/- 0.21. A similar
result was found from previous work based on uncollimated GOES-type
observations concerning flares brighter than about C2. This index is
independent of the background flux level (which is related to the solar
activity level) to within our statistical uncertainties. We obtain the
FWHM distribution of flare lifetimes from our sample and find that the
distribution is independent of X-ray brightness class. We extrapolate
the soft X-ray flare occurrence rate obtained for the Sun to other
very active solar-like stars.
---------------------------------------------------------
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: Magnetic Reconnection Following Coronal Mass Ejections
Authors: Klimchuk, J. A.
1996AAS...188.3306K Altcode: 1996BAAS...28..868K
It is well known that large-scale soft X-ray arcades form and grow
in the aftermath of coronal mass ejections. This phenomenon is
usually interpreted as evidence for magnetic reconnection occurring
at progressively greater heights along the vertical current sheet
that is created when the coronal field is stretched open by the
eruption. Closed magnetic loops are formed and heated by the
reconnection, and they subsequently fill with hot, dense plasma
evaporated from the chromosphere. The loops then cool and fade from
view, but as they do, new hot loops are created above. In this way the
arcade slowly grows, shell by shell. This picture is very appealing and
seems to agree qualitatively with many aspects of the observations,
but is it correct? We have studied the disk event of 1993 Jan 26 and
found a startling inconsistency: the rate at which the arcade plasma
is observed to be heated is two orders of magnitude smaller than the
rate at which energy is expected to be extracted from the magnetic
field. Is the standard interpretation fundamentally wrong? We suggest
that it is essentially correct, but that only a small fraction ( ~ 1%)
of the open magnetic field actually reconnects. We discuss a possible
reason for this involving 3D effects, as well as the interesting
consequences that it has for the detailed magnetic structure of fully
formed coronal arcades.
---------------------------------------------------------
Title: Broadband Imaging Spectroscopy with the Solar Radio Telescope
Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Hudson, H. S.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1996ASPC...93..430B Altcode: 1996ress.conf..430B
No abstract at ADS
---------------------------------------------------------
Title: The Heating of Soft X-ray Coronal Loops
Authors: Klimchuk, J. A.; Porter, L. J.
1996mpsa.conf...39K Altcode: 1996IAUCo.153...39K
No abstract at ADS
---------------------------------------------------------
Title: Post-Eruption Arcades and 3-D Magnetic Reconnection (Invited)
Authors: Klimchuk, James A.
1996ASPC..111..319K Altcode: 1997ASPC..111..319K
Soft X-ray emitting arcades are often observed to form and grow in
the aftermath of coronal eruptions. They are generally interpreted
as evidence for magnetic reconnection occurring at progressively
greater heights along a vertical current sheet that is created by the
eruption. This "standard model" explains many of the morphological
aspects of the observations very well. The author has examined the
energetics of an event observed by Yohkoh and finds that the rate
of magnetic energy conversion expected from the model exceeds the
observed rate of arcade plasma heating by perhaps as much as a factor
of 100. He takes this as evidence that reconnection is being inhibited,
and suggests that the cause may be three-dimensional effects involving
the interlinking of field lines.
---------------------------------------------------------
Title: Soft X-Ray Loops and Coronal Heating
Authors: Porter, Lisa J.; Klimchuk, James A.
1995ApJ...454..499P Altcode:
We have measured the temperatures, pressures, and lengths of 47
nonflaring coronal loops observed by the Soft X-Ray Telescope on the
Yohkoh satellite. The median temperature is 5.7 x 10<SUP>6</SUP> K,
and the median pressure is 1.6 x 10<SUP>16</SUP> cm<SUP>-3</SUP> K. We
have carefully examined the possible random and systematic errors in
the measurements and have found, through simulated observations, that
the errors produced by photon statistical noise can sometimes greatly
exceed the values given by commonly used error expressions, derived
here and elsewhere. Furthermore, the measurements are not normally
distributed and therefore are not amenable to standard statistical
analysis. We have used nonparametric methods to look for statistical
relationships and find that temperature and length are uncorrelated
and that pressure varies inversely with length to approximately the
first power. <P />The observed lifetimes of the loops are much longer
than their computed cooling times, suggesting that the loops are in a
state of quasi-static equilibrium. This has allowed us to use simple
scaling law theory to infer that the volumetric heating rate in the
loops (averaged along the loop axis) varies inversely with length to
approximately the second power. This is an important constraint for
distinguishing among competing theories of coronal heating, and we
discuss the results in the context of three specific models.
---------------------------------------------------------
Title: Solar Rotation Stereoscopy in Microwaves
Authors: Aschwanden, Markus J.; Lim, Jeremy; Gary, Dale E.; Klimchuk,
James A.
1995ApJ...454..512A Altcode:
We present here the first stereoscopic altitude measurements of active
region sources observed at microwave frequencies (10-14 GHz The active
region NOAA 7128 was observed with the Owens Valley Radio Observatory
(OVRO) on 1992 April 13, 14, 15, and 16 as it passed through the central
meridian. From white-light data of the underlying sunspot we determined
the rotation rate of the active region, which was found to have a
relative motion of dL/dt = +0°.240 day<SUP>-1</SUP> with respect to the
standard photospheric differential rotation rate. Based on this rotation
rate we determine for the microwave sources stereoscopic altitudes of
3.3-11.0 Mm above the photosphere. The altitude spectrum h(v) of the
right circular polarization (RCP) main source shows a discontinuity
at 12 GHz and can be satisfactorily fitted with a dipole model with a
transition from the second to the third harmonic level at 12 GHz. The
dominance of the third harmonic for frequencies above 12 GHz occurs
because the second harmonic level drops below the transition region, at
a height of 2.6±0.6 Mm according to the microwave data. The altitude
spectrum h(v) serves also to invert the temperature profile T(h)
from the optically thick parts of the radio brightness temperature
spectrum T<SUB>B</SUB>(ν[h]). The microwave emission in both circular
polarizations can be modeled with gyroresonance emission, with x-mode
for RCP and o-mode in LCP, with the same harmonics at each frequency,
but different emission angles in both modes. The contributions from
free-free emission are negligible in both polarizations, based on the
peak emission measure of EM ≍ 6 × 10<SUP>28</SUP> cm<SUP>-5</SUP>
observed in soft X-rays by Yohkoh/SXT. <P />This study demonstrates
that the height dependence of the coronal magnetic field B(h) and
the plasma temperature T(h) in an active region can be inverted from
the stereoscopic altitude spectra h(v) and the observed brightness
temperature spectra T<SUB>B</SUB>(ν).
---------------------------------------------------------
Title: Scaling of heating rates in solar coronal loops
Authors: Klimchuk, James A.; Porter, Lisa J.
1995Natur.377..131K Altcode:
THE gas of the solar corona is at a temperature of several million
degrees, orders of magnitude hotter than the underlying photosphere. The
nature of the physical process that heats the solar corona (and the
coronae of solar-type stars more generally) has been a long-standing
puzzle. A number of plausible heating mechanisms have been proposed,
but observations have so far been unable to discriminate between
them<SUP>1</SUP>. Here we show that coronal heating exhibits scaling
properties that should provide a powerful diagnostic of the underlying
mechanism. The coronal magnetic field organizes the coronal plasma
into loop-like features, which form the basic structural elements of
the corona<SUP>2</SUP>. We demonstrate that the pressures and lengths
of the coronal loops are statistically related, suggesting that the
heating rate scales inversely with approximately the square of the loop
length. Existing coronal heating theories make different predictions
about what this scaling should be, and a model<SUP>3-4</SUP> of energy
dissipation by stressed coronal magnetic fields appears at present to
be the most consistent with our observational result.
---------------------------------------------------------
Title: A Comparison of Active Region Temperatures and Emission
Measures Observed in Soft X-Rays and Microwaves and Implications
for Coronal Heating
Authors: Klimchuk, J. A.; Gary, D. E.
1995ApJ...448..925K Altcode:
We have determined active region temperatures and emission measures
using both broad-band soft X-ray images from the Yohkoh satellite and
spatially and spectrally resolved microwave data from the Owens Valley
Radio Observatory (OVRO). This work differs from previous work in that
the microwave temperatures and emission measures are directly measured
from the microwave spectrum, and are not model-dependent. The soft
X-ray temperatures and emission measures are ≍2.5 times greater
than the corresponding microwave values, on average. Detailed
error analysis indicates that the temperature differences are real,
but that the emission measure differences may not be. <P />We have
simulated Yohkoh and OVRO observations of idealized plane-parallel and
nested-loop coronal models. The plane-parallel model reproduces the
observed temperature differences if the coronal temperature decreases
exponentially with height from a maximum value of 4 × 10<SUP>6</SUP>
K at the base to an asymptotic value of ≍10<SUP>6</SUP> K. The
nested-loop model, which assumes quasi-static loop equilibrium, also
reproduces the observed temperature differences and indicates that the
volumetric coronal heating rate varies inversely with loop length to a
power greater than 2. Both models predict microwave emission measures
that are larger than observed. We suggest that a more complex model
is required to explain the observed emission measures and that more
than one coronal heating mechanism may be operative in solar active
regions. <P />We present derivations of the temperature and emission
measure uncertainties that result from random and systematic errors in
the Yohkoh observations. The expression for the random error emission
measure uncertainty is different from that used previously and is
especially important for observations of nonflaring plasmas.
---------------------------------------------------------
Title: The Magnetic Field of Solar Prominences
Authors: Antiochos, S. K.; Klimchuk, J. A.; Dahlburg, R. B.
1995SPD....26..717A Altcode: 1995BAAS...27..969A
No abstract at ADS
---------------------------------------------------------
Title: A Solar Radio Telescope for the Future: Science Summary from
the SRT Workshop
Authors: Gary, D. E.; Bastian, T. S.; Hudson, H. S.; Hurford, G. J.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1995SPD....26..801G Altcode: 1995BAAS...27..971G
No abstract at ADS
---------------------------------------------------------
Title: The Cross Sectional Properties of Coronal Loops
Authors: Klimchuk, J. A.
1995SPD....26..705K Altcode: 1995BAAS...27Q.966K
No abstract at ADS
---------------------------------------------------------
Title: A Solar Radio Telescope for the Future: Strawman Concept from
the SRT Workshop
Authors: Hurford, G. J.; Bastian, T. S.; Gary, D. E.; Hudson, H. S.;
Klimchuk, J. A.; Petrosian, V.; White, S. M.
1995SPD....26..802H Altcode: 1995BAAS...27..971H
No abstract at ADS
---------------------------------------------------------
Title: The Possible Role of MHD Waves in Heating the Solar Corona
Authors: Porter, Lisa J.; Klimchuk, James A.; Sturrock, Peter A.
1994ApJ...435..482P Altcode:
The possible role of waves in the heating of the solar corona has
been investigated. A general dispersion relation has been derived for
waves propagating in a homogeneous plasma subject to dissipation by
viscosity and thermal conduction. The dissipation mechanisms have been
incorporated self-consistently into the equations, and no assumptions
about the strength of the damping have been made. Solutions of the
sixth-order dispersion relation provide information on how the damping
of both slow and fast mode waves depends upon the plasma density,
temperature, field strength, and angle of propagation relative to the
background magnetic field. We provide a detailed comparison to the
standard approach, which is to solve for the wave quantities in the
absence of dissipation and then to use these quantities in expressions
for the heating due to viscosity and thermal conduction.
---------------------------------------------------------
Title: The Possible Role of High-Frequency Waves in Heating Solar
Coronal Loops
Authors: Porter, Lisa J.; Klimchuk, James A.; Sturrock, Peter A.
1994ApJ...435..502P Altcode:
We investigate the role of high-frequency waves in the heating of
solar active region coronal loops. We assume a uniform background
magnetic field, and we introduce a density stratification in a
direction perpendicular to this field. We focus on ion compressive
viscosity as the damping mechanism of the waves. We incorporate
viscosity self-consistently into the equations, and we derive a
dispersion relation by adopting a slab model, where the density inside
the slab is greater than that outside. Such a configuration supports
two types of modes: surface waves and trapped body waves. In order
to determine under what conditions these waves may contribute to the
heating of active regions, we solve our dispersion relation for a
range of densities, temperatures, magnetic field strengths, density
ratios, wavevector magnitudes, wavevector ratios, and slab widths. We
find that surface waves exhibit very small damping, but body waves can
potentially damp at rates needed to balance radiative losses. However,
the required frequencies of these body waves are very high. For
example, the wave frequency must be at least 5.0/s for a slab density
of 10<SUP>9,5</SUP>/cc, a slab temperature of 10<SUP>6,5</SUP> K, a
field strength of 100 G, and a density ratio of 5. For a slab density
of 10<SUP>10</SUP>/cc, this frequency increases to 8.8/s. Although
these frequencies are very high, there in no observational evidence
to rule out their existence, and they may be generated both below the
corona and at magnetic reconnection sites in the corona. However, we
do find that, for slab densities of 10<SUP>10</SUP>/cc or less, the
dissipation of high-frequency waves will be insufficient to balance
the radiative losses if the magnetic field strength exceeds roughly
200 G. Because the magnetic field is known to exceed 200 G in many
active region loops, particularly low-lying loops and loops emanating
from sunspots, it is unlikely that high-frequency waves can provide
sufficient heating in these regions.
---------------------------------------------------------
Title: Asymptotic Forms for the Energy of Force-free Magnetic Field
Configurations of Translational Symmetry
Authors: Sturrock, P. A.; Antiochos, S. K.; Klimchuk, J. A.;
Roumeliotis, G.
1994ApJ...431..870S Altcode:
It is known from computer calculations that if a force-free
magnetic field configuration is stressed progressively by footpoint
displacements, the configuration expands and approaches the open
configuration with the same surface flux distribution and the
energy of the field increases progressively. For configurations of
translational symmetry, it has been found empirically that the energy
tends asymptotically to a certain functional form. It is here shown
that analysis of a simple model of the asymptotic form of force-free
fields of translational symmetry leads to and therefore justifies
this functional form. According to this model, the field evolves in
a well-behaved manner with no indication of instability or loss of
equilibrium.
---------------------------------------------------------
Title: The Magnetic Field of Solar Prominences
Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J. A.
1994ApJ...420L..41A Altcode:
A model is presented which accounts for the formation of coronal
magnetic field lines with the appropriate 'dipped' structure to
support prominences. The critical ingredients of the model are that the
prominence magnetic field is a truly three-dimensional structure with
significant variation along the prominence length, and the magnetic
field is strongly sheared near the photospheric neutral line. Numerical
calculations are presented which demonstrate that these two features
lead to dip formation. In addition our model is able to account for the
long-puzzling observation of inverse polarity in quiescent prominences.
---------------------------------------------------------
Title: The Asymptotic Behavior of Force-Free Magnetic-Field
Configurations
Authors: Sturrock, P. A.; Klimchuk, J. A.; Roumeliotis, G.; Antiochos,
S. K.
1994ASPC...68..219S Altcode: 1994sare.conf..219S
No abstract at ADS
---------------------------------------------------------
Title: Coronal Eruptions Observed by YOHKOH
Authors: Klimchuk, J. A.; Acton, L. W.; Harvey, K. L.; Hudson, H. S.;
Kluge, K. L.; Sime, D. G.; Strong, K. T.; Watanabe, Ta.
1994xspy.conf..181K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Interplanetary Consequences of Transient Coronal Events
Authors: Watanabe, Ta.; Kojima, M.; Kozuka, Y.; Tsuneta, S.; Lemen,
J. R.; Hudson, H.; Joselyn, J. A.; Klimchuk, J. A.
1994xspy.conf..207W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Photospheric Magnetic Field Measurement Errors and the Inferred
Properties of Coronal Magnetic Fields
Authors: Klimchuk, James A.; Canfield, Richard C.
1994ASPC...68..233K Altcode: 1994sare.conf..233K
No abstract at ADS
---------------------------------------------------------
Title: Eruptive-Prominence Related Coronal Disturbances Observed
with YOHKOH SXT
Authors: Watanabe, T.; Kozuka, Y.; Ohyama, M.; Kojima, M.; Yamaguchi,
K.; Watari, S.; Tsuneta, S.; Joselyn, J. A.; Harvey, K. L.; Acton,
L. W.; Klimchuk, J. A.
1994step.conf...85W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal/Interplanetary Disturbances Associated with a Solar
Filament Disappearance on September 28, 1991
Authors: Watanabe, T.; Kozuka, Y.; Ohyama, M.; Kojima, M.; Yamaguchi,
K.; Watari, S.; Tsuneta, S.; Joselyn, J. A.; Harvey, K. L.; Acton,
L. W.; Klimchuk, J. A.
1994step.conf...89W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Collisional Damping of Magnetoacoustic Waves in the Solar
Corona
Authors: Porter, Lisa A.; Sturrock, Peter A.; Klimchuk, James A.
1993BAAS...25.1203P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Structure of Prominence Magnetic Fields
Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J. A.
1993BAAS...25.1206A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Comparison of Coronal Temperatures and Emission Measures
Determined from X-Ray and Microwave Observations
Authors: Klimchuk, J. A.; Gary, D. E.
1993BAAS...25.1179K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Static and dynamic loop models and their observational
signatures.
Authors: Klimchuk, James A.
1992ESASP.348..167K Altcode: 1992cscl.work..167K
The magnetically-closed regions of the outer solar atmosphere can be
studied in terms of one-dimensional hydrodynamic loop models. These
regions include the bright plasma loops that are readily visible in
EUV and X-ray images as well as the fainter, more diffuse-appearing
plasma that surrounds them. The author reviews the basic theoretical
properties of static, steady-state, and time-dependent loop models,
and relates these properties to observations of emission measure
distributions and Doppler shifts. Both existing observations and future
observations from the Solar and Heliospheric Observatory (SOHO) are
considered. In particular, the dependence of the models on the form
of the energy input is emphasized.
---------------------------------------------------------
Title: Thickness Variations along Coronal Loops Observed by the Soft
X-Ray Telescope on YOHKOH
Authors: Klimchuk, James A.; Lemen, James R.; Feldman, Uri; Tsuneta,
Saku; Uchida, Yutaka
1992PASJ...44L.181K Altcode:
It has been suggested that observed coronal loops have constant
thicknesses. However, if plasma loops coincide with magnetic loops,
then we might expect many loops to be significantly broader at their
tops than at their footpoints (since, on average, magnetic fields
must diverge with height in the solar corona). It is important to
understand how the thicknesses of loops vary along their lengths, since
such a variation is related to the distribution of electric currents
in the corona and is therefore relevant to solar flares and coronal
heating. We here present preliminary results of our investigation of
thickness variations along coronal loops observed with the Soft X-ray
Telescope (SXT) on board the Yohkoh satellite.
---------------------------------------------------------
Title: Coronal/Interplanetary Disturbances Associated with
Disappearing Solar Filaments
Authors: Watanabe, Takashi; Kozuka, Yukio; Ohyama, Masamitsu; Kojima,
Masayoshi; Yamaguchi, Kisuke; Watari, Shin-Ichi; Tsuneta, Saku;
Joselyn, Jo A.; Harvey, Karen L.; Acton, Loren W.; Klimchuk, James. A.
1992PASJ...44L.199W Altcode:
We discuss two examples of coronal/interplanetary disturbances
associated with the disappearance of a 35(deg) long quiescent filament
occurring near the solar disk center on 1991 September 28 (McAllister
et al. 1992, Publ. Astron. Soc. Japan, 44, L205) and with a 25(deg)
long eruptive prominence at the eastern solar limb taking place on
1991 November 7. Bright soft X-ray arcades were observed for both
cases with the Yohkoh SXT, about 2--3 hr after the onset of each
Hα event. For the erupting prominence on November 7, the arcade
did not appear before the prominence reached a height of about 0.3
solar radii above the limb. This suggests that magnetic reconnection
occurred below the relevant Hα structures. A transient coronal hole
was formed in the immediate vicinity of the disappearing filament on
September 28. Formation of the new coronal hole is suggested to be a
cause of the filament disappearance. An interplanetary disturbance was
detected by radio scintillation (IPS) observations immediately after
the filament disappeared.
---------------------------------------------------------
Title: A Model for the Magnetic Fields of Solar Prominences
Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J.
1992AAS...180.1205A Altcode: 1992BAAS...24..748A
No abstract at ADS
---------------------------------------------------------
Title: Thickness Variations Along Coronal Loops Observed by Yohkoh
Authors: Klimchuk, J. A.; Kluge, K.; Lemen, J. R.; Feldman, U.;
Uchida, Y.
1992AAS...180.2304K Altcode: 1992BAAS...24Q.760K
No abstract at ADS
---------------------------------------------------------
Title: Cylindrically Symmetric Force-free Magnetic Fields
Authors: Porter, Lisa J.; Klimchuk, James A.; Sturrock, Peter A.
1992ApJ...385..738P Altcode:
The magnetofrictional method was used to study the energy buildup
in stressed coronal fields possessing cylindrical symmetry. Four
different nonlinear, force-free magnetic-field configurations were
examined. It was determined that, in all cases, a reasonable amount
of twist in the field lines can produce enough free magnetic energy
to power a typical flare. Furthermore, it was found that the rate of
energy buildup is enhanced if the greatest twist and/or the magnetic
flux is concentrated closer to the neutral line. It is thought that
the open-field configuration (a configuration for which the field
lines extend to infinity and the current is confined to a current
sheet separating outgoing and incoming field lines) is the limiting
state as one imposes infinite shear. The results of this work do not
contradict this theory once numerical errors are taken into account.
---------------------------------------------------------
Title: The Practical Application of the Magnetic Virial Theorem
Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E.
1992ApJ...385..327K Altcode:
The magnetic energy of solar active regions is computed via the
application of the magnetic virial theorem together with vector
magnetograph data. In order to investigate how errors in the vector
magnetograph measurements produce errors in the virial theorem
energy, the effects of realistic errors on known magnetic fields are
simulated numerically. These include errors due to random polarization
noise, crosstalk between different polarization signals, systematic
polarization bias, and seeing-induced crosstalk. Analytical expressions
for the energy errors which apply under certain idealized conditions are
derived. The results are useful for evaluating the ability of vector
magnetographs to provide suitable data for the accurate determination
of magnetic energies using the virial theorem.
---------------------------------------------------------
Title: Three-dimensional Force-free Magnetic Fields and Flare
Energy Buildup
Authors: Klimchuk, J. A.; Sturrock, P. A.
1992ApJ...385..344K Altcode:
The 'magneto-frictional' method is used to compute fully 3D models
of force-free magnetic fields. Beginning with a potential field
produced by a point dipole buried below the solar surface, the magnetic
footpoints at the photosphere are displaced to investigate the buildup
of magnetic energy. Reasonable footpoint shearing displacements are
found to be able to increase the total magnetic energy by at least
one-third. The energy buildup is greater when the shearing displacements
are concentrated closer to the magnetic neutral line. Roughly half
of the energy buildup is free magnetic energy. The absolute quantity
of free magnetic energy (10 exp 30-33 ergs, depending on the scaling
of the models) is sufficient to explain solar flares. No evidence for
'loss of equilibrium' was found.
---------------------------------------------------------
Title: A Model for the Formation of Solar Prominences
Authors: Antiochos, S. K.; Klimchuk, J. A.
1991ApJ...378..372A Altcode:
A model for the formation of prominence condensations in hot coronal
loops is proposed. Previous studies have concentrated on cooling the
hot plasma by decreasing the coronal heating rate. The difficulty
with such models is that when the heating decreases, most of the
loop mass is lost by draining onto the chromosphere. It is argued
that a prominence condensation is likely to be due to an increase in
the heating. The key idea of the model is that the heating increase is
spatially dependent so that it is localized nearer to the chromospheric
footpoints than to the loop midpoint. Results are presented of
numerical simulations of hot loops that are initially heated uniformly,
and then undergo heating increases that are concentreated away from
the loop midpoint. The temperature at the midpoint first increases,
but eventually it collapses to chromospheric values as a result of
chromospheric evaporation. Hence, a curious result is obtained, that
increasing the heating causes cooling. The resulting densities and
time scales agree well with observations. The implications of this
model for coronal heating and prominence structure are discussed.
---------------------------------------------------------
Title: The Practical Application of the Magnetic Virial Therom:
Analytical Results
Authors: Rhoads, J. E.; Klimchuk, J. A.; Canfield, R. C.
1991BAAS...23.1055R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Practical Application of the Magnetic Virial Theorm:
Simulated Magnetograph Observations
Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E.
1991BAAS...23.1031K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simulated SXT Observations of Coronal Loops
Authors: Dixon, W. W.; Klimchuk, J. A.; Sturrock, P. A.; Lemen, J. R.
1991LNP...387..297D Altcode: 1991fpsa.conf..297D
We have simulated the appearance of two static coronal loops as they
might be observed by the Soft X-ray Telescope onboard the Solar-A
spacecraft. One loop corresponds to a non-flaring active region loop,
and the other corresponds to a post-flare loop. We find that the
loops have fundamentally different appearances: the quiescent loop is
brightest at its apex, while the hotter post-flare loop is brightest
at, or near, its base (depending on the particular X-ray filter assumed
for the observations).
---------------------------------------------------------
Title: The Practical Application of the Magnetic Virial Theorem
Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E.
1991LNP...387..219K Altcode: 1991fpsa.conf..219K
We have performed simulated vector magnetograph observations to study
the effects of random and systematic magnetic field measurement errors
on the magnetic energies that may be estimated using the virial theorem.
---------------------------------------------------------
Title: Episodic Coronal Heating
Authors: Sturrock, P. A.; Dixon, W. W.; Klimchuk, J. A.; Antiochos,
S. K.
1990ApJ...356L..31S Altcode:
A study is made of the observational consequences of the hypothesis
that there is no steady coronal heating, the solar corona instead
being heated episodically, such that each short burst of heating
is followed by a long period of radiative cooling. The form of the
resulting contribution to the differential emission measure (DEM), and
to a convenient related function (the differential energy flux, DEF) is
calculated. Observational data for the quiet solar atmosphere indicate
that the upper branch of the DEM, corresponding to temperatures above
100,000 K, can be interpreted in terms of episodic energy injection
at coronal temperatures.
---------------------------------------------------------
Title: Shear-induced Inflation of Coronal Magnetic Fields
Authors: Klimchuk, James A.
1990ApJ...354..745K Altcode:
Using numerical models of force-free magnetic fields, the shearing
of footprints in arcade geometries leading to an inflation of the
coronal magnetic field was examined. For each of the shear profiles
considered, all of the field lines become elevated compared with the
potential field. This includes cases where the shear is concentrated
well away from the arcade axis, such that B<SUB>z</SUB>, the component
of field parallel to the axis, increases outward to produce an inward
B<SUB>z</SUB> squared/8 pi magnetic pressure gradient force. These
results contrast with an earlier claim, shown to be incorrect, that
field lines can sometimes become depressed as a result of shear. It is
conjectured that an inflation of the entire field will always result
from the shearing of simple arcade configurations. These results have
implications for prominence formation, the interplanetary magnetic flux,
and possibly also coronal holes.
---------------------------------------------------------
Title: Flare Energy Buildup and the Stressing of 3-D Coronal
Magnetic Fields
Authors: Klimchuk, J. A.; Sturrock, P. A.
1990BAAS...22..900K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Cylindrically-Symmetric Force-Free Magnetic Fields
Authors: Porter, L. J.; Klimchuk, J. A.; Sturrock, P. A.
1990BAAS...22..853P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Force-free Magnetic Fields: Is There a “Loss of Equilibrium”?
Authors: Klimchuk, J. A.; Sturrock, P. A.
1989ApJ...345.1034K Altcode:
This paper examines concept in solar physics that is known as loss of
equilibrium in which a sequence of force-free magnetic fields, said to
represent a possible quasi-static evolution of solar magnetic fields,
reaches a critical configuration beyond which no acceptable solution
of the prescribed form exists. This concept is used to explain eruptive
phenomena ranging from solar flares to coronal mass ejections. Certain
sequences of force-free configurations are discussed that exhibit
a loss of equilibrium, and it is argued that the concept is devoid
of physical significance since each sequence is defined a way that
does not represent an acceptable thought experiment. For example, the
sequence may be defined in terms of a global constraint on the boundary
conditions, or the evolution of the sequence may require the creation
of mgnetic flux that is not connected to the photosphere and is not
present in the original configuration. The global constraints typically
occur in using the so-called generating function method. An acceptance
thought experiment is proposed to specify the field configuration
in terms of photospheric boundary conditions comprising the normal
component of the field and the field-line connectivity. Consider a
magnetic-field sequence that, when described in terms of a generating
function, exhibits a loss of equilibrium and show that, when one instead
defines the sequence in terms of the corresponding boundary conditions,
the sequence is well behaved.
---------------------------------------------------------
Title: Episodic Coronal Heating and the Solar Differential Emission
Measure
Authors: Sturrock, P. A.; Klimchuk, J. A.; Antiochos, S. K.
1989BAAS...21R1186S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Formation of Solar Prominences
Authors: Antiochos, S. K.; Klimchuk, J. A.
1989BAAS...21.1185A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Shear-induced inflation of coronal magnetic fields
Authors: Klimchuk, James A.
1989STIN...9014178K Altcode:
Using numerical models of force-free magnetic fields, the shearing
of footprints in arcade geometries leading to an inflation of the
coronal magnetic field was examined. For each of the shear profiles
considered, all of the field lines become elevated compared with the
potential field. This includes cases where the shear is concentrated
well away from the arcade axis, such that B<SUB>z</SUB>, the component
of field parallel to the axis, increases outward to produce an inward
B<SUB>z</SUB>squared/8 pi magnetic pressure gradient force. These
results contrast with an earlier claim, shown to be incorrect, that
field lines can sometimes become depressed as a result of shear. It is
conjectured that an inflation of the entire field will always result
from the shearing of simple arcade configurations. These results have
implications for prominence formation, the interplanetary magnetic flux,
and possibly also coronal holes.
---------------------------------------------------------
Title: Shear-Induced Inflation of Coronal Magnetic Fields
Authors: Klimchuk, J. A.
1989BAAS...21..864K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic properties of Civ Doppler shift patterns
Authors: Klimchuk, James A.
1989SoPh..119...19K Altcode:
The relationship between Doppler shift patterns observed in the
transition region and magnetic field patterns observed in the
photosphere is studied using coaligned pairs of CIV Dopplergrams
and FeI magnetograms. Categories of magnetic features are defined -
including neutral lines, unipolar regions, strong field regions, weak
field regions, and magnetic boundaries - and from these, magnetic
associations are determined for 159 V<SUB>0</SUB> lines separating
areas of relative blueshift and redshift observed in and around active
regions. The cases are subdivided on the basis of whether blueshifts
or redshifts are observed on the side of the V<SUB>0</SUB> line nearest
the solar limb.
---------------------------------------------------------
Title: Force-Free Magnetic Fields: Is there a "Loss of Equilibrium?"
Authors: Klimchuk, J. A.; Sturrock, P. A.
1989BAAS...21R.855K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Largescale Magnetic Field Phenomena
Authors: Harrison, R. A.; Bentley, R. D.; Brosius, J.; Dwivedi,
B. N.; Jardine, M.; Klimchuk, J. A.; Kundu, M. R.; Pearce, G.; Saba,
J.; Sakurai, T.; Schmahl, E. J.; Schmelz, J.; Sime, D. G.; Steele,
C. D. C.; Sun, M. T.; Tappin, S. J.; Waljeski, K.; Wang, A. H.; Wu,
S. T.
1989tnti.conf....1H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal Magnetic Fields Produced by Photospheric Shear
Authors: Klimchuk, J. A.; Sturrock, P. A.; Yang, W. -H.
1988ApJ...335..456K Altcode:
The magnetofrictional method for computing force-free fields is used
to examine the evolution of the magnetic field of a line dipole,
when there is relative shearing motion between the two polarities. It
is found that the energy of the sheared field can be arbitrarily
large compared with the potential field. It is also found that it is
possible to fit the magnetic energy, as a function of shear amplitude,
by a simple functional form. The fit parameters depend only on the
distribution of normal field in the photosphere and the form of the
shearing displacement. They show that the energy is relatively more
enhanced if the shear occurs: (1) where the normal field is strongest;
and/or (2) in the inner region of the dipole, near the axis; and/or
(3) over a large fraction of the dipole area.
---------------------------------------------------------
Title: Force-free magnetic fields: Is there a loss of equilibrium
Authors: Klimchuk, James A.; Sturrock, Peter A.
1988STIN...8921717K Altcode:
This paper examines concept in solar physics that is known as loss of
equilibrium in which a sequence of force-free magnetic fields, said to
represent a possible quasi-static evolution of solar magnetic fields,
reaches a critical configuration beyond which no acceptable solution
of the prescribed form exists. This concept is used to explain eruptive
phenomena ranging from solar flares to coronal mass ejections. Certain
sequences of force-free configurations are discussed that exhibit
a loss of equilibrium, and it is argued that the concept is devoid
of physical significance since each sequence is defined a way that
does not represent an acceptable thought experiment. For example, the
sequence may be defined in terms of a global constraint on the boundary
conditions, or the evolution of the sequence may require the creation
of magnetic flux that is not connected to the photosphere and is not
present in the original configuration. The global constraints typically
occur in using the so-called generating function method. An acceptable
thought experiment is proposed to specify the field configuration
in terms of photospheric boundary conditions comprising the normal
component of the field and the field-line connectivity. Consider a
magnetic-field sequence that, when described in terms of a generating
function, exhibits a loss of equilibrium and show that, when one instead
defines the sequence in terms of the corresponding boundary conditions,
the sequence is well behaved.
---------------------------------------------------------
Title: Heating-related Flows in Cool Solar Loops
Authors: Klimchuk, J. A.; Mariska, J. T.
1988ApJ...328..334K Altcode:
The authors have investigated the effects of spatial and temporal
variations in the heating of cool loop models in an attempt
to explain the net redshifts that are observed on the Sun. In
none of the situations considered are the induced flows able to
satisfactorily reproduce the observations. In the case of asymmetric
heating, the end-to-end flows can be as fast as 20 km s<SUP>-1</SUP>,
but the downflowing leg is neither appreciably faster nor appreciably
brighter than the upflowing leg; no net redshift is produced. In the
case of symmetric heating, the downflows can also be large, but they are
restricted to temperatures that are well below 10<SUP>5</SUP>K. Neither
situation would give rise to the ⪆7 km s<SUP>-1</SUP> disk-averaged
redshifts seen in emission lines of species like C IV.
---------------------------------------------------------
Title: Coronal magnetic fields produced by photospheric shear
Authors: Klimchuk, James A.; Sturrock, Peter A.; Yang, Wei-Hong
1988cmfp.book.....K Altcode:
The magneto-frictional method for computing force-free fields examines
the evolution of the magnetic field of a line dipole, when there is
relative shearing motion between the two polarities. The energy of the
sheared field can be arbitrarily large compared with the potential
field. It is possible to fit the magnetic energy, as a function
of shear amplitude, by a simple functional form. The fit parameters
depend only on the distribution of normal field in the photosphere and
the form of the shearing displacement. The energy is relatively more
enhanced if the shear occurs: (1) where the normal field is strongest;
(2) in the inner region of the dipole, near the axis; or (3) over a
large fraction of the dipole area.
---------------------------------------------------------
Title: Magnetic properties of C 4 Doppler shift patterns
Authors: Klimchuk, James A.
1988STIN...8915062K Altcode:
The relationship between Doppler shift patterns observed in the
transition region and magnetic field patterns observed in the
photosphere is studied using coaligned pairs C IV Dopplergrams and Fe
I magnetograms. Categories of magnetic features are defined--including
neutral lines, unipolar regions, strong field regions, weak field
regions, and magnetic boundaries--and from these, magnetic associations
are determined for 159 V0 lines separating areas of relative blueshift
and redshift observed in and nearby to active regions. The cases
are subdivided on the basis of whether blueshifts or redshifts are
observed on the side of the V0 line nearest the limb. Two of the main
results are that V0 lines associated with neutral lines tend to have
limbward blueshifts, while V0 lines associated with unipolar regions
tend to have limbward redshifts. These and other results provide
supportive evidence for the active region model proposed recently by
Klimchuk, in which relative redshifts occur where strong vertical
fields penetrate the surface, and relative blueshifts occur where
these same fields have spread out to become horizontal. It is likely
that the relative blueshifts correspond to absolute Doppler shifts of
very small amplitude, possibly even redshifts.
---------------------------------------------------------
Title: Coronal Magnetic Fields Produced by Photospheric Shear
Authors: Klimchuk, J. A.; Sturrock, P. A.; Yang, W. -H.
1988BAAS...20..716K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Large-Scale Dynamics and Magnetic Structure of Solar
Active Regions
Authors: Klimchuk, James A.
1987ApJ...323..368K Altcode:
The author has studied sets of carefully coaligned C IV Dopplergrams,
photospheric magnetograms, and Hα filtergrams to infer the flow
properties of active regions and the relationship of these flows
to the active region magnetic fields. The combined data show that
active regions can be naturally divided into three basic parts:
strong field regions, weak field corridors between strong fields of
opposite polarity, and surrounding weak field areas. An idealized
topological model in which the vertical fields of strong field regions
diverge very rapidly with height to become essentially horizontal in
the adjacent low-lying areas is proposed. The picture is similar to
canopy structures.
---------------------------------------------------------
Title: A Numerical Study of the Nonlinear Thermal Stability of
Solar Loops
Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T.
1987ApJ...320..409K Altcode:
A time-dependent numerical model is used to investigate the nonlinear
thermal stability of static loops of various heights. Simulations show
that the instability of a hot state with loop heights of less than about
1000 km is physically significant, with an initially hot atmosphere
in low-lying compact loops evolving to an extended atmosphere with
temperatures far below 100,000 K. Results also show that high-lying
loops are stable to all reasonable perturbations, including those of
large initial amplitude and long wavelength. The simulation results
suggest that low-lying compact loops should not be common to the sun,
and that cool loops with temperatures near 100,000 K must be formed
in the cool state initially and cannot evolve from preexisiting loops.
---------------------------------------------------------
Title: A numerical study of the thermal stability of solar loops.
Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T.
1987NASCP2483..113K Altcode: 1987tphr.conf..113K
An important property of all loops is their thermal stability. If low
lying hot loops were thermally unstable, for example, a great majority
of the low loops on the Sun might be expected to be cool. How small
perturbations evolve in low lying, linearly unstable hot loops was
determined and how high lying, linearly stable hot loops respond to
large amplitude disturbances such as might be expected on the Sun were
examined. Only general descriptions and results are given.
---------------------------------------------------------
Title: The Magnetic and Velocity Structure Adjacent to Solar Active
Regions
Authors: Athay, R. Grant; Klimchuk, J. A.
1987ApJ...318..437A Altcode:
Results from a number of earlier papers relating velocity patterns
observed in the C IV line at 154.8 nm to photospheric magnetic-field
patterns are combined to develop a qualitative model of the
magnetic-field geometry outside of the strong field areas of active
regions. The motion is assumed to originate at the crests of magnetic
arcades and flow downward along field lines, which are assumed to
be elliptical in shape with the major axis in the photosphere. It is
found that the ratio of the major axis to the minor axis of the ellipse
must be less than two for fields under 100 G. Also, it is concluded
that the magnetic neutral surfaces defined by the loci of horizontal
field lines are often tilted at a large angle to the vertical at the
altitude of the C IV emission.
---------------------------------------------------------
Title: Heating Related Flows in Cool Loops
Authors: Klimchuk, J. A.; Mariska, J. T.
1987BAAS...19..932K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: C IV Doppler shifts observed in active region filaments.
Authors: Klimchuk, J. A.
1986NASCP2442..183K Altcode: 1986copp.nasa..183K
The Doppler shift properties of 21 active region filaments have
been studied using C IV Dopplergram data. Most are associated with
corridors of weak magnetic field that separate opposite polarity strong
fields seen in photospheric magnetograms. A majority of the filaments
are relatively blue shifted, although several lie very close to the
dividing lines between relative blue and red shift. Only one filament
in our sample is clearly red shifted. A new calibration procedure
for Dopplergrams indicates that sizable zero point offsets are often
required. The center-to-limb behavior of the resulting absolute
Doppler shifts suggests that filament flows are usually quite small
(<3 km/s). It is possible that they vanish.
---------------------------------------------------------
Title: Numerical simulations of a siphon mechanism for quiescent
prominence formation.
Authors: Poland, A. I.; Mariska, J. T.; Klimchuk, J. A.
1986NASCP2442...57P Altcode: 1986copp.nasa...57P
Quiescent prominences represent a significant challenge to our
understanding of the flow of mass and energy in the outer layers of
the solar atmosphere. A small number of quiescent prominences contain
as much mass as the entire corona (Athay, 1976). The problem then is
how to get that much material into the relatively small volume of
a prominence and maintain it at a temperature of 10,000 K in close
proximity to material at one million K. The thermal insulation to
conduction provided by the magnetic field explains the disparate
temperatures. The mass source problem is less well understood. One
method for supplying mass to the prominence is to siphon it from the
chromosphere. The siphon mechanism begins with a magnetic loop that
evolves into a configuration with a gravitational well, such as that
described by Kippenhahn and Schluter (1957). This could be formed, for
example, by a twist in the magnetic field. A gravitational well could
also be formed by a condensation induced sag in the field. This could
further enhance the condensation process. Once this well has formed,
or as it is forming, the material in the well area of the loop must
cool and condense to the point where radiative losses exceed any
heat input. Additional material must also flow into the well from
the underlying chromosphere to supply the mass required to form the
prominence. One example from a series of numerical simulations that were
performed to study the formation of quiescent prominences is presented.
---------------------------------------------------------
Title: A numerical study of the thermal stability of low-lying
coronal loops.
Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T.
1986NASCP2442..389K Altcode: 1986copp.nasa..389K
The nonlinear evolution of loops that are subjected to a variety
of small but finite perturbations was studied. Only the low-lying
loops are considered. The analysis was performed numerically using a
one-dimensional hydrodynamical model developed at the Naval Research
Laboratory. The computer codes solve the time-dependent equations
for mass, momentum, and energy transport. The primary interest is
the active region filaments, hence a geometry appropriate to those
structures was considered. The static solutions were subjected to a
moderate sized perturbation and allowed to evolve. The results suggest
that both hot and cool loops of the geometry considered are thermally
stable against amplitude perturbations of all kinds.
---------------------------------------------------------
Title: Magnetic Shear. IV. Hale Regions 16740, 16815, and 16850
Authors: Athay, R. G.; Klimchuk, J. A.; Jones, H. P.; Zirin, H.
1986ApJ...303..884A Altcode:
Dopplergrams made in C IV 1548 A are studied for evidence of velocity
shear near H-alpha dark filaments and for large-scale flow convergent on
active regions. The three regions studied support earlier conclusions
that shear is a common property of active regions and that active
regions may be the foci of converging plasma flow. Flow patterns
near filaments show divergence or convergence as well as shear. Also
the sense of the shear can be either cyclonic or anticyclonic. No
preference is noted for convergence or divergence or for a particular
sense of shear, and there appears to be no correlation between the
sense of the shear and the sign of the velocity gradient normal to
the filament. The close association of H-alpha dark filaments with
shear lines leads to the suggestion that the filaments may arise from
a cooling instability induced by the Bernoulli effect.
---------------------------------------------------------
Title: A Numerical Study of the Stability of Low-Lying Solar Loops
Authors: Mariska, J. T.; Klimchuk, J. A.; Antiochos, S. K.
1986BAAS...18Q.708M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Large-Scale Dynamics and Structure of Solar Active Regions
Observed in C IV
Authors: Klimchuk, J. A.
1986BAAS...18R.702K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Large-scale structure and dynamics of solar active regions
observed in the far ultraviolet
Authors: Klimchuk, James Andrew
1985PhDT.......145K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Large-Scale Structure and Dynamics of Solar Active Regions
Observed in the Far Ultraviolet.
Authors: Klimchuk, J. A.
1985PhDT.........6K Altcode:
In this thesis we examine high resolution Dopplergrams in CIV (1548) for
the purpose of understanding the large -scale dynamics of solar active
regions and the closely related problem of magnetic structure. The
Dopplergrams are compared with photospheric magnetograms and H(,(alpha))
filtergrams in order to determine the relationship of the flows to
the overall magnetic field topology. These observations sample three
different heights in the atmosphere and can potentially be used to map
the field from the photosphere up through the transition region. Vector
properties of the flow and field are inferred from center-to-limb
variations in the measured Doppler shift and longitudinal field
component. We find that spatial correlations between features seen in
Dopplergrams, magnetograms, and filtergrams are quite close, and that
active regions can be naturally divided into three basic parts: strong
field regions, weak field corridors between opposite polarities, and
surrounding weak field areas. Strong field regions are relatively red
shifted and contain magnetic fields that penetrate the photosphere in a
nearly vertical fashion. Corridors and surrounding areas are relatively
blue shifted, on the other hand, and contain fields that are mostly
horizontal in the upper photosphere and chromosphere. The transition
from vertical to horizontal field appears to be quite sharp and implies
a magnetic topology that diverges very rapidly with height. Sizable
uncertainties in the Dopplergram interpretation have prevented us
from describing the transition region flows unambiguously. We can,
however, identify two possible scenarios that are consistent with the
data. In the most likely of the two, the flows vanish within corridors
and surrounding areas, and they produce constant (across the disk)
absolute red shifts of about 18 km/s within strong field regions. The
physical causes of this last result are unclear, and it remains an
important unsolved problem.
---------------------------------------------------------
Title: Observed Associations Between CIV Doppler Shifts and
Photospheric Magnetic Fields in Active Regions
Authors: Klimchuk, J. A.
1984BAAS...16..532K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Outflow from the sun's polar corona
Authors: Orrall, F. Q.; Rottman, G. J.; Klimchuk, J. A.
1983ApJ...266L..65O Altcode:
New observations of systematic Doppler shifts of EUV resonance lines
formed both in the low corona and transition region are reported. They
were made with an improved, high-resolution, stable, rocket-borne
spectrometer flown on 1981 November 23. The chord of the solar disk
scanned by the spectrometer crossed the north polar cap hole and its
low-latitude extension, and also a compact low-latitude hole near
sun center. Within both holes, the lines were systematically shifted
to shorter wavelengths relative to the rest of the solar disk. These
observations strengthen the association of negative Doppler shifts
with coronal holes and indicate that this 'blueshift signature',
previously observed only within small low-latitude holes, is also
characteristic of the low-density polar corona. The mean relative
blueshift measured in lambda 625 Mg X (T = 10 to the 6.15 power K)
within the polar hole was about 8 km/s.
---------------------------------------------------------
Title: Measurements of outflow from the base of solar coronal holes
Authors: Rottman, G. J.; Orrall, F. Q.; Klimchuk, J. A.
1982ApJ...260..326R Altcode:
New evidence is presented that EUV emission lines formed at the levels
of the base of the corona and the transition region are systematically
shifted to shorter wavelengths within coronal holes relative to the
rest of the solar disk, and that moreover this shift increases with
height in the atmosphere. Measurements were made with a rocket-borne
EUV spectrometer having high spectroscopic resolution and stability
flown on July 15, 1980. Repeated measurements were made along a chord
of the solar disk that crossed a compact coronal hole near sun center
identified on gamma 10830 He I spectroheliograms. The maximum measured
shift corresponded to a velocity of 12 km/sec in gamma 625 Mg X and 7
km/sec in gamma 629 O V. If these velocities correspond to a true mass
flux, they provide important data on the acceleration of coronal plasma
in open magnetic field regions. These observed Doppler displacements
are a strong and significant signature of coronal holes, now measured
on three rocket flights.
---------------------------------------------------------
Title: Measurement of systematic outflow from the solar transition
region underlying a coronal hole
Authors: Rottman, G. J.; Klimchuk, J. A.; Orrall, F. Q.
1981ApJ...247L.135R Altcode:
This letter presents measurements of small Doppler shifts in the line
center position of 629A OV obtained with a new high-resolution EUV
spectrometer flown aboard a sounding rocket. A major result is the
detection of an apparent systematic outflow (relative to the quiet
sun) of approximately 3 km/s average and 5 km/s maximum in the solar
transition region underlying a well-defined low-latitude coronal
hole. This is reminiscent of a similar apparent outflow observed by
Cushman and Rense in the coronal line 303A Si XI. The hypothesis that
this is evidence for acceleration of the high speed solar wind deep
in the transition region and inner corona is explored briefly.
---------------------------------------------------------
Title: EUV Observations of High-Speed Downflows Over Sunspots
Authors: Klimchuk, J. A.; Rottman, G. J.
1981BAAS...13..914K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: EUV Observations of Solar Mass Loss from the Lower Solar
Atmosphere
Authors: Rottman, G. J.; Orrall, F. Q.; Klimchuk, J. A.
1981BAAS...13..812R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mass Flux within Coronal Holes
Authors: Orrall, F. Q.; Rottman, G. J.; Klimchuk, J.
1980BAAS...12..919O Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Velocity Fields Observed in Coronal Holes and the Underlying
Transition Region
Authors: Rottman, G. J.; Klimchuk, J. A.; Orrall, F. Q.
1980BAAS...12..919R Altcode:
No abstract at ADS
