780 research outputs found
Indeterminacy and instability in Petschek reconnection
We explain two puzzling aspects of Petschek's model for fast reconnection. One is its failure to occur in plasma simulations with uniform resistivity. The other is its inability to provide anything more than an upper limit for the reconnection rate. We have found that previously published analytical solutions based on Petschek's model are structurally unstable if the electrical resistivity is uniform. The structural instability is associated with the presence of an essential singularity at the X-line that is unphysical. By requiring that such a singularity does not exist, we obtain a formula that predicts a specific rate of reconnection. For uniform resistivity, reconnection can only occur at the slow, Sweet-Parker rate. For nonuniform resistivity, reconnection can occur at a much faster rate provided that the resistivity profile is not too flat near the X-line. If this condition is satisfied, then the scale length of the nonuniformity determines the reconnection rate
Topological model of the anemone microflares in the solar chromosphere
Context: The chromospheric anemone microflares, which were discovered by
Hinode satellite about a decade ago, are specific transient phenomena starting
from a few luminous ribbons on the chromospheric surface and followed by an
eruption upward. While the eruptive stage was studied in sufficient detail, a
quantitative theory of formation of the initial multi-ribbon structure remains
undeveloped until now. Aims: We construct a sufficiently simple but general
model of the magnetic field sources that is able to reproduce all the observed
types of luminous ribbons by varying only a single parameter. Methods: As a
working tool, we employed the Gorbachev-Kel'ner-Somov-Shvarts (GKSS) model of
the magnetic field, which was originally suggested about three decades ago to
explain fast ignition of the magnetic reconnection over considerable spatial
scales by tiny displacements of the magnetic sources. Quite unexpectedly, this
model turns out to be efficient for the description of generic multi-ribbon
structures in the anemone flares as well. Results: As follows from our
numerical simulation, displacement of a single magnetic source (sunspot) with
respect to three other sources results in a complex transformation from three
to four ribbons and then again to three ribbons, but with an absolutely
different arrangement. Such structures closely resemble the observed patterns
of emission in the anemone microflares.Comment: LaTeX, aa documentclass, 4 pages, 4 EPS figures, submitted to
Astronomy & Astrophysics; v2: 6 pages, 5 EPS figures, text substantially
extended and modified, 1 figure replaced and 1 figure added, Appendix added;
v3: minor textual correction
On the Triggering of M-Class Solar Flare due to Loop-loop Interaction in AR NOAA 10875
We present multiwavelength analysis of an M7.9 /1N solar flare which occurred
on 27 April 2006 in AR NOAA 10875. The flare was triggered due to the
interaction of two loop systems. GOES soft X-ray and TRACE 195 {\AA} image
sequences show the observational evidences of 3-D X-type loop-loop interaction
with converging motion at the interaction site. We found the following
characteristics during the loop- loop interaction: (i) a short
duration/impulsive flare obeying the Neupart effect, (ii) double peak structure
in radio flux profiles (in 4.9 and 8.8 GHz), (iii) quasi-periodic oscillations
in the radio flux profiles for the duration of \sim3 min, (iv) absence of CME
and type III radio burst. The above characteristics observed during the flare
are in agreement with the theory and simulation of current loop coalescence by
Sakai et al. (1986). These are unique multiwavelength observations, which
provide the evidences of loop-loop interaction and associated triggering of
solar flare without CME.Comment: 7 pages, 4 figures, to be appear in the Proc. of the 1st
Asian-Pacific Solar Phys. Meetin
A coronal explosion on the flare star CN Leonis
We present simultaneous high-temporal and high-spectral resolution
observations at optical and soft X-ray wavelengths of the nearby flare star CN
Leo. During our observing campaign a major flare occurred, raising the star's
instantaneous energy output by almost three orders of magnitude. The flare
shows the often observed impulsive behavior, with a rapid rise and slow decay
in the optical and a broad soft X-ray maximum about 200 seconds after the
optical flare peak. However, in addition to this usually encountered flare
phenomenology we find an extremely short (~2 sec) soft X-ray peak, which is
very likely of thermal, rather than non-thermal nature and temporally coincides
with the optical flare peak. While at hard X-ray energies non-thermal bursts
are routinely observed on the Sun at flare onset, thermal soft X-ray bursts on
time scales of seconds have never been observed in a solar nor stellar context.
Time-dependent, one-dimensional hydrodynamic modeling of this event requires an
extremely short energy deposition time scale of a few seconds to reconcile
theory with observations, thus suggesting that we are witnessing the results of
a coronal explosion on CN Leo. Thus the flare on CN Leo provides the
opportunity to observationally study the physics of the long-sought
"micro-flares" thought to be responsible for coronal heating.Comment: 7 pages, accepted by A&
Investigation of quasi-periodic variations in hard X-rays of solar flares. II. Further investigation of oscillating magnetic traps
In our recent paper (Solar Physics 261, 233) we investigated quasi-periodic
oscillations of hard X-rays during impulsive phase of solar flares. We have
come to conclusion that they are caused by magnetosonic oscillations of
magnetic traps within the volume of hard-X-ray (HXR) loop-top sources. In the
present paper we investigate four flares which show clear quasi-periodic
sequences of HXR pulses. We also describe our phenomenological model of
oscillating magnetic traps to show that it can explain observed properties of
HXR oscillations. Main results are the following: 1. We have found that
low-amplitude quasi-periodic oscillations occur before impulsive phase of some
flares. 2. We have found that quasi-period of the oscillations can change in
some flares. We interpret this as being due to changes of the length of
oscillating magnetic traps. 3. During impulsive phase a significant part of the
energy of accelerated (non-thermal) electrons is deposited within the HXR
loop-top source. 4. Our analysis suggests that quick development of impulsive
phase is due to feedback between pulses of the pressure of accelerated
electrons and the amplitude of magnetic-trap oscillation. 5. We have also
determined electron number density and magnetic filed strength for HXR loop-top
sources of several flares. The values fall within the limits of cm, gauss.Comment: 18 pages, 14 figures, submitted to Solar Physic
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