877 research outputs found
The role of torsional Alfven waves in coronal heating
In the context of coronal heating, among the zoo of MHD waves that exist in
the solar atmosphere, Alfven waves receive special attention. Indeed, these
waves constitute an attractive heating agent due to their ability to carry over
the many different layers of the solar atmosphere sufficient energy to heat and
maintain a corona. However, due to their incompressible nature these waves need
a mechanism such as mode conversion (leading to shock heating), phase mixing,
resonant absorption or turbulent cascade in order to heat the plasma. New
observations with polarimetric, spectroscopic and imaging instruments such as
those on board of the japanese satellite Hinode, or the SST or CoMP, are
bringing strong evidence for the existence of energetic Alfven waves in the
solar corona. In order to assess the role of Alfven waves in coronal heating,
in this work we model a magnetic flux tube being subject to Alfven wave heating
through the mode conversion mechanism. Using a 1.5-dimensional MHD code we
carry out a parameter survey varying the magnetic flux tube geometry (length
and expansion), the photospheric magnetic field, the photospheric velocity
amplitudes and the nature of the waves (monochromatic or white noise spectrum).
It is found that independently of the photospheric wave amplitude and magnetic
field a corona can be produced and maintained only for long (> 80 Mm) and thick
(area ratio between photosphere and corona > 500) loops. Above a critical value
of the photospheric velocity amplitude (generally a few km/s) the corona can no
longer be maintained over extended periods of time and collapses due to the
large momentum of the waves. These results establish several constraints on
Alfven wave heating as a coronal heating mechanism, especially for active
region loops.Comment: 39 pages, 8 figures; http://stacks.iop.org/0004-637X/712/49
The State of Self-Organized Criticality of the Sun During the Last Three Solar Cycles. II. Theoretical Model
The observed powerlaw distributions of solar flare parameters can be
interpreted in terms of a nonlinear dissipative system in the state of
self-organized criticality (SOC). We present a universal analytical model of a
SOC process that is governed by three conditions: (i) a multiplicative or
exponential growth phase, (ii) a randomly interrupted termination of the growth
phase, and (iii) a linear decay phase. This basic concept approximately
reproduces the observed frequency distributions. We generalize it to a
randomized exponential-growth model, which includes also a (log-normal)
distribution of threshold energies before the instability starts, as well as
randomized decay times, which can reproduce both the observed occurrence
frequency distributions and the scatter of correlated parametyers more
realistically. With this analytical model we can efficiently perform
Monte-Carlo simulations of frequency distributions and parameter correlations
of SOC processes, which are simpler and faster than the iterative simulations
of cellular automaton models. Solar cycle modulations of the powerlaw slopes of
flare frequency distributions can be used to diagnose the thresholds and growth
rates of magnetic instabilities responsible for solar flares.Comment: Part II of Paper I: The State of Self-Organized Criticality of the
Sun During the Last Three Solar Cycles. I. Observation
Observations of Active Region Loops with the EUV Imaging Spectrometer on Hinode
Previous solar observations have shown that coronal loops near 1 MK are
difficult to reconcile with simple heating models. These loops have lifetimes
that are long relative to a radiative cooling time, suggesting quasi-steady
heating. The electron densities in these loops, however, are too high to be
consistent with thermodynamic equilibrium. Models proposed to explain these
properties generally rely on the existence of smaller scale filaments within
the loop that are in various stages of heating and cooling. Such a framework
implies that there should be a distribution of temperatures within a coronal
loop. In this paper we analyze new observations from the EUV Imaging
Spectrometer (EIS) on \textit{Hinode}. EIS is capable of observing active
regions over a wide range of temperatures (\ion{Fe}{8}--\ion{Fe}{17}) at
relatively high spatial resolution (1\arcsec). We find that most isolated
coronal loops that are bright in \ion{Fe}{12} generally have very narrow
temperature distributions ( K), but are not
isothermal. We also derive volumetric filling factors in these loops of
approximately 10%. Both results lend support to the filament models.Comment: Submitted to ApJ
Relationship between Hard and Soft X-ray Emission Components of a Solar Flare
X-ray observations of solar flares routinely reveal an impulsive high-energy
and a gradual low-energy emission component, whose relationship is one of the
key issues of solar flare study. The gradual and impulsive emission components
are believed to be associated with, respectively, the thermal and nonthermal
components identified in spectral fitting. In this paper, a prominent about 50
second hard X-ray (HXR) pulse of a simple GOES class C7.5 flare on 20 February
2002 is used to study the association between high energy, non-thermal and
impulsive evolution, and low energy, thermal and gradual evolution. We use
regularized methods to obtain time derivatives of photon fluxes to quantify the
time evolution as a function of photon energy, obtaining a break energy between
impulsive and gradual behavior. These break energies are consistent with a
constant value of about 11 keV in agreement with those found spectroscopically
between thermal and non-thermal components, but the relative errors of the
former are greater than 15% and much greater than the a few percent errors
found from the spectral fitting. These errors only weakly depend on assuming an
underlying spectral model for the photons, pointing to the current data being
inadequate to reduce the uncertainties rather than there being a problem
associated with an assumed model. The time derivative method is used to test
for the presence of a 'pivot energy' in this flare. Although these pivot
energies are marginally consistent with a constant value of about 9 keV, its
values in the HXR rise phase appear to be lower than those in the decay phase
Deterministically Driven Avalanche Models of Solar Flares
We develop and discuss the properties of a new class of lattice-based
avalanche models of solar flares. These models are readily amenable to a
relatively unambiguous physical interpretation in terms of slow twisting of a
coronal loop. They share similarities with other avalanche models, such as the
classical stick--slip self-organized critical model of earthquakes, in that
they are driven globally by a fully deterministic energy loading process. The
model design leads to a systematic deficit of small scale avalanches. In some
portions of model space, mid-size and large avalanching behavior is scale-free,
being characterized by event size distributions that have the form of
power-laws with index values, which, in some parameter regimes, compare
favorably to those inferred from solar EUV and X-ray flare data. For models
using conservative or near-conservative redistribution rules, a population of
large, quasiperiodic avalanches can also appear. Although without direct
counterparts in the observational global statistics of flare energy release,
this latter behavior may be relevant to recurrent flaring in individual coronal
loops. This class of models could provide a basis for the prediction of large
solar flares.Comment: 24 pages, 11 figures, 2 tables, accepted for publication in Solar
Physic
Modeling Evolving Coronal Loops with Observations from STEREO, Hinode, and TRACE
The high densities, long lifetimes, and narrow emission measure distributions
observed in coronal loops with apex temperatures near 1 MK are difficult to
reconcile with physical models of the solar atmosphere. It has been proposed
that the observed loops are actually composed of sub-resolution ``threads''
that have been heated impulsively and are cooling. We apply this heating
scenario to nearly simultaneous observations of an evolving post-flare loop
arcade observed with the EUVI/\textit{STEREO}, XRT/\textit{Hinode}, and
\textit{TRACE} imagers and the EIS spectrometer on \textit{HINODE}. We find
that it is possible to reproduce the extended loop lifetime, high electron
density, and the narrow differential emission measure with a multi-thread
hydrodynamic model provided that the time scale for the energy release is
sufficiently short. The model, however, does not reproduce the evolution of the
very high temperature emission observed with XRT. In XRT the emission appears
diffuse and it may be that this discrepancy is simply due to the difficulty of
isolating individual loops at these temperatures. This discrepancy may also
reflect fundamental problems with our understanding of post-reconnection
dynamics during the conductive cooling phase of loop evolution.Comment: Revised version submitted to ApJ in response to referee's comment
The Source of Three-minute Magneto-acoustic Oscillations in Coronal Fans
We use images of high spatial, spectral and temporal resolution, obtained
using both ground- and space-based instrumentation, to investigate the coupling
between wave phenomena observed at numerous heights in the solar atmosphere.
Intensity oscillations of 3 minutes are observed to encompass photospheric
umbral dot structures, with power at least three orders-of-magnitude higher
than the surrounding umbra. Simultaneous chromospheric velocity and intensity
time series reveal an 87 \pm 8 degree out-of-phase behavior, implying the
presence of standing modes created as a result of partial wave reflection at
the transition region boundary. An average blue-shifted Doppler velocity of
~1.5 km/s, in addition to a time lag between photospheric and chromospheric
oscillatory phenomena, confirms the presence of upwardly-propagating slow-mode
waves in the lower solar atmosphere. Propagating oscillations in EUV intensity
are detected in simultaneous coronal fan structures, with a periodicity of 172
\pm 17 s and a propagation velocity of 45 \pm 7 km/s. Numerical simulations
reveal that the damping of the magneto-acoustic wave trains is dominated by
thermal conduction. The coronal fans are seen to anchor into the photosphere in
locations where large-amplitude umbral dot oscillations manifest. Derived
kinetic temperature and emission measure time-series display prominent
out-of-phase characteristics, and when combined with the previously established
sub-sonic wave speeds, we conclude that the observed EUV waves are the coronal
counterparts of the upwardly-propagating magneto-acoustic slow-modes detected
in the lower solar atmosphere. Thus, for the first time, we reveal how the
propagation of 3 minute magneto-acoustic waves in solar coronal structures is a
direct result of amplitude enhancements occurring in photospheric umbral dots.Comment: Accepted into ApJ (13 pages and 10 figures
A Nonlinear Force-Free Magnetic Field Approximation Suitable for Fast Forward-Fitting to Coronal Loops. I. Theory
We derive an analytical approximation of nonlinear force-free magnetic field
solutions (NLFFF) that can efficiently be used for fast forward-fitting to
solar magnetic data, constrained either by observed line-of-sight magnetograms
and stereoscopically triangulated coronal loops, or by 3D vector-magnetograph
data. The derived NLFFF solutions provide the magnetic field components
, , , the force-free parameter
, the electric current density , and are
accurate to second-order (of the nonlinear force-free -parameter). The
explicit expressions of a force-free field can easily be applied to modeling or
forward-fitting of many coronal phenomena.Comment: Solar Physics (in press), 26 pages, 11 figure
- …