173 research outputs found
Coronal Seismology and the Propagation of Acoustic Waves Along Coronal Loops
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 4.35x10^9 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.Comment: To appear in the Dec. 1, 2004 issue of the Astrophysical Journa
The Dynamic Formation of Prominence Condensations
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 (Antiochos & Klimchuk 1991). Using a new
adaptive grid code, we simulate the complete growth of a condensation, and find
that after approx. 5,000 s it reaches a quasi-steady state. We show that the
size and the growth time of the condensation are in good agreement with data,
and discuss the implications of the model for coronal heating and SOHO/TRACE
observations.Comment: Astrophysical Journal latex file, 20 pages, 7 b-w figures (gif files
Post-flare UV light curves explained with thermal instability of loop plasma
In the present work we study the C8 flare occurred on September 26, 2000 at
19:49 UT and observed by the SOHO/SUMER spectrometer from the beginning of the
impulsive phase to well beyond the disappearance in the X-rays. The emission
first decayed progressively through equilibrium states until the plasma reached
2-3 MK. Then, a series of cooler lines, i.e. Ca x, Ca vii, Ne vi, O iv and Si
iii (formed in the temperature range log T = 4.3 - 6.3 under equilibrium
conditions), are emitted at the same time and all evolve in a similar way. Here
we show that the simultaneous emission of lines with such a different formation
temperature is due to thermal instability occurring in the flaring plasma as
soon as it has cooled below ~ 2 MK. We can qualitatively reproduce the relative
start time of the light curves of each line in the correct order with a simple
(and standard) model of a single flaring loop. The agreement with the observed
light curves is greatly improved, and a slower evolution of the line emission
is predicted, if we assume that the model loop consists of an ensemble of
subloops or strands heated at slightly different times. Our analysis can be
useful for flare observations with SDO/EVE.Comment: 24 pages, 7 figures, accepted for publicatio
Self-Organization of Reconnecting Plasmas to Marginal Collisionality in the Solar Corona
We explore the suggestions by Uzdensky (2007) and Cassak et al. (2008) that
coronal loops heated by magnetic reconnection should self-organize to a state
of marginal collisionality. We discuss their model of coronal loop dynamics
with a one-dimensional hydrodynamic calculation. We assume that many current
sheets are present, with a distribution of thicknesses, but that only current
sheets thinner than the ion skin depth can rapidly reconnect. This assumption
naturally causes a density dependent heating rate which is actively regulated
by the plasma. We report 9 numerical simulation results of coronal loop
hydrodynamics in which the absolute values of the heating rates are different
but their density dependences are the same. We find two regimes of behavior,
depending on the amplitude of the heating rate. In the case that the amplitude
of heating is below a threshold value, the loop is in stable equilibrium.
Typically the upper and less dense part of coronal loop is collisionlessly
heated and conductively cooled. When the amplitude of heating is above the
threshold, the conductive flux to the lower atmosphere required to balance
collisionless heating drives an evaporative flow which quenches fast
reconnection, ultimately cooling and draining the loop until the cycle begins
again. The key elements of this cycle are gravity and the density dependence of
the heating function. Some additional factors are present, including pressure
driven flows from the loop top, which carry a large enthalpy flux and play an
important role in reducing the density. We find that on average the density of
the system is close to the marginally collisionless value.Comment: accepted for publication in The Astrophysical Journal, 33 pages, 12
figure
Structure of solar coronal loops: from miniature to large-scale
We will use new data from the High-resolution Coronal Imager (Hi-C) with
unprecedented spatial resolution of the solar corona to investigate the
structure of coronal loops down to 0.2 arcsec. During a rocket flight Hi-C
provided images of the solar corona in a wavelength band around 193 A 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 a possible sub-structuring of larger loops.
We find tiny 1.5 MK loop-like structures that we interpret as miniature coronal
loops. These have length of the coronal segment above the chromosphere 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 loops that are longer than 50 Mm to have a diameter of about 2 arcsec or
1.5 Mm, 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 sub-structure.
Instead they show a smooth variation in cross-section. The fact that the large
coronal loops do not show a sub-structure at the spatial scale of 0.1 arcsec
per pixel implies that either the densities and temperatures are smoothly
varying across these loops or poses an upper limit on the diameter of strands
the loops might be composed of. We estimate that strands that compose the 2
arcsec 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 in terms of
modeling.Comment: Accepted for publication in A&A (Jun 19, 2013), 11 pages, 10 figure
A Contemporary View of Coronal Heating
Determining the heating mechanism (or mechanisms) that causes the outer
atmosphere of the Sun, and many other stars, to reach temperatures orders of
magnitude higher than their surface temperatures has long been a key problem.
For decades the problem has been known as the coronal heating problem, but it
is now clear that `coronal heating' cannot be treated or explained in isolation
and that the heating of the whole solar atmosphere must be studied as a highly
coupled system. The magnetic field of the star is known to play a key role,
but, despite significant advancements in solar telescopes, computing power and
much greater understanding of theoretical mechanisms, the question of which
mechanism or mechanisms are the dominant supplier of energy to the chromosphere
and corona is still open. Following substantial recent progress, we consider
the most likely contenders and discuss the key factors that have made, and
still make, determining the actual (coronal) heating mechanism (or mechanisms)
so difficult
How to use magnetic field information for coronal loop identification?
The structure of the solar corona is dominated by the magnetic field because
the magnetic pressure is about four orders of magnitude higher than the plasma
pressure. Due to the high conductivity the emitting coronal plasma (visible
e.g. in SOHO/EIT) outlines the magnetic field lines. The gradient of the
emitting plasma structures is significantly lower parallel to the magnetic
field lines than in the perpendicular direction. Consequently information
regarding the coronal magnetic field can be used for the interpretation of
coronal plasma structures. We extrapolate the coronal magnetic field from
photospheric magnetic field measurements into the corona. The extrapolation
method depends on assumptions regarding coronal currents, e.g. potential fields
(current free) or force-free fields (current parallel to magnetic field). As a
next step we project the reconstructed 3D magnetic field lines on an EIT-image
and compare with the emitting plasma structures. Coronal loops are identified
as closed magnetic field lines with a high emissivity in EIT and a small
gradient of the emissivity along the magnetic field.Comment: 14 pages, 3 figure
Shocks and Thermal Conduction Fronts in Retracting Reconnected Flux Tubes
We present a model for plasma heating produced by time-dependent, spatially
localized reconnection within a flare current sheet separating skewed magnetic
fields. The reconnection creates flux tubes of new connectivity which
subsequently retract at Alfv\'enic speeds from the reconnection site. Heating
occurs in gas-dynamic shocks which develop inside these tubes. Here we present
generalized thin flux tube equations for the dynamics of reconnected flux
tubes, including pressure-driven parallel dynamics as well as temperature
dependent, anisotropic viscosity and thermal conductivity. The evolution of
tubes embedded in a uniform, skewed magnetic field, following reconnection in a
patch, is studied through numerical solutions of these equations, for solar
coronal conditions. Even though viscosity and thermal conductivity are
negligible in the quiet solar corona, the strong gas-dynamic shocks generated
by compressing plasma inside reconnected flux tubes generate large velocity and
temperature gradients along the tube, rendering the diffusive processes
dominant. They determine the thickness of the shock that evolves up to a
steady-state value, although this condition may not be reached in the short
times involved in a flare. For realistic solar coronal parameters, this
steady-state shock thickness might be as long as the entire flux tube. For
strong shocks at low Prandtl numbers, typical of the solar corona, the
gas-dynamic shock consists of an isothermal sub-shock where all the compression
and cooling occur, preceded by a thermal front where the temperature increases
and most of the heating occurs. We estimate the length of each of these
sub-regions and the speed of their propagation.Comment: 39 pages (AASTeX: 29 pages of text, 10 figures), accepted for
publication in the Astrophysical Journa
Profiles of heating in turbulent coronal magnetic loops
Context: The location of coronal heating in magnetic loops has been the
subject of a long-lasting controversy: does it occur mostly at the loop
footpoints, at the top, is it random, or is the average profile uniform?
Aims: We try to address this question in model loops with MHD turbulence and
a profile of density and/or magnetic field along the loop.
Methods: We use the ShellAtm MHD turbulent heating model described in Buchlin
& Velli (2006), with a static mass density stratification obtained by the
HydRad model (Bradshaw & Mason 2003). This assumes the absence of any flow or
heat conduction subsequent to the dynamic heating.
Results: The average profile of heating is quasi-uniform, unless there is an
expansion of the flux tube (non-uniform axial magnetic field) or the variation
of the kinetic and magnetic diffusion coefficients with temperature is taken
into account: in the first case the heating is enhanced at footpoints, whereas
in the second case it is enhanced where the dominant diffusion coefficient is
enhanced.
Conclusions: These simulations shed light on the consequences on heating
profiles of the complex interactions between physical effects involved in a
non-uniform turbulent coronal loop.Comment: 9 pages, 8 figure
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