60 research outputs found
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
Investigation of quasi-periodic varaiations in hard X-rays of solar flares
The aim of the present paper is to use quasi-periodic oscillations in hard
X-rays (HXRs) of solar flares as a diagnostic tool for investigation of
impulsive electron acceleration. We have selected a number of flares which
showed quasi-periodic oscillations in hard X-rays and their loop-top sources
could be easily recognized in HXR images. We have considered MHD standing waves
to explain the observed HXR oscillations. We interpret these HXR oscillations
as being due to oscillations of magnetic traps within cusp-like magnetic
structures. This is confirmed by a good correlation between periods of the
oscillations and the sizes of the loop-top sources. We argue that a model of
oscillating magnetic traps is adequate to explain the observations. During the
compressions of a trap particles are accelerated, but during its expansions
plasma, coming from chromospheric evaporation, fills the trap, which explains
the large number of electrons being accelerated during a sequence of strong
impulses. The advantage of our model of oscillating magnetic traps is that it
can explain both the impulses of electron acceleration and quasi-periodicity of
their distribution in time.Comment: 21 pages, 11 figures, 3 tables, submitted to Solar Physic
Multi-Thread Hydrodynamic Modeling of a Solar Flare
Past hydrodynamic simulations have been able to reproduce the high
temperatures and densities characteristic of solar flares. These simulations,
however, have not been able to account for the slow decay of the observed flare
emission or the absence of blueshifts in high spectral resolution line
profiles. Recent work has suggested that modeling a flare as an sequence of
independently heated threads instead of as a single loop may resolve the
discrepancies between the simulations and observations. In this paper we
present a method for computing multi-thread, time-dependent hydrodynamic
simulations of solar flares and apply it to observations of the Masuda flare of
1992 January 13. We show that it is possible to reproduce the temporal
evolution of high temperature thermal flare plasma observed with the
instruments on the \textit{GOES} and \textit{Yohkoh} satellites. The results
from these simulations suggest that the heating time-scale for a individual
thread is on the order of 200 s. Significantly shorter heating time scales (20
s) lead to very high temperatures and are inconsistent with the emission
observed by \textit{Yohkoh}.Comment: Submitted to Ap
Scaling laws of solar and stellar flares
In this study we compile for the first time comprehensive data sets of solar
and stellar flare parameters, including flare peak temperatures T_p, flare peak
volume emission measures EM_p, and flare durations t_f from both solar and
stellar data, as well as flare length scales L from solar data. Key results are
that both the solar and stellar data are consistent with a common scaling law
of EM_p ~ T_p^4.7, but the stellar flares exhibit ~250 times higher emission
measures (at the same flare peak temperature). For solar flares we observe also
systematic trends for the flare length scale L(T_p) ~ T_p^0.9 and the flare
duration t_F(T_p) ~ T_p^0.9 as a function of the flare peak temperature. Using
the theoretical RTV scaling law and the fractal volume scaling observed for
solar flares, i.e., V(L) ~ L^2.4, we predict a scaling law of EM_p ~ T_p^4.3,
which is consistent with observations, and a scaling law for electron densities
in flare loops, n_p ~ T_p^2/L ~ T_p^1.1. The RTV-predicted electron densities
were also found to be consistent with densities inferred from total emission
measures, n_p=(EM_p/q_V*V)^1/2, using volume filling factors of q_V=0.03-0.08
constrained by fractal dimensions measured in solar flares. Our results affect
also the determination of radiative and conductive cooling times, thermal
energies, and frequency distributions of solar and stellar flare energies.Comment: 9 Figs., (paper in press, The Astrophsycial Journal
X-ray Flares of EV Lac: Statistics, Spectra, Diagnostics
We study the spectral and temporal behavior of X-ray flares from the active
M-dwarf EV Lac in 200 ks of exposure with the Chandra/HETGS. We derive flare
parameters by fitting an empirical function which characterizes the amplitude,
shape, and scale. The flares range from very short (<1 ks) to long (10 ks)
duration events with a range of shapes and amplitudes for all durations. We
extract spectra for composite flares to study their mean evolution and to
compare flares of different lengths. Evolution of spectral features in the
density-temperature plane shows probable sustained heating. The short flares
are significantly hotter than the longer flares. We determined an upper limit
to the Fe K fluorescent flux, the best fit value being close to what is
expected for compact loops.Comment: 9 pages; 9 figures; latex/emulateapj style; Submitted to The
Astrophysical Journa
A Survey of High Contrast Stellar Flares Observed by Chandra
The X-ray light curves of pre-main sequence stars can show variability in the
form of flares altering a baseline characteristic activity level; the largest
X-ray flares are characterized by a rapid rise to more than 10 times the
characteristic count rate, followed by a slower quasi-exponential decay.
Analysis of these high-contrast X-ray flares enables the study of the innermost
magnetic fields of pre-main sequence stars. We have scanned the ANCHORS
database of Chandra observations of star-forming regions to extend the study of
flare events on pre-main sequence stars both in sky coverage and in volume. We
developed a sample of 30 high-contrast flares out of the 14,000 stars of
various ages and masses available in ANCHORS at the start of our study.
Applying methods of time-resolved spectral analysis, we obtain the
temperatures, confining magnetic field strengths, and loop lengths of these
bright, energetic flares. The results of the flare analysis are compared to the
2MASS and Spitzer data available for the stars in our sample. We find that the
longest flare loop lengths (of order several stellar radii) are only seen on
stars whose IR data indicates the presence of disks. This suggests that the
longest flares may stretch all the way to the disk. Such long flares tend to be
more tenuous than the other large flares studied. A wide range of loop lengths
are observed, indicating that different types of flares may occur on disked
young stellar objects.Comment: 38 pages, 8 figures, 4 table
Simultaneous X-ray spectroscopy of YY Gem with Chandra and XMM-Newton
We report on a detailed study of the X-ray spectrum of the nearby eclipsing
spectroscopic binary YY Gem. Observations were obtained simultaneously with
both large X-ray observatories, XMM-Newton and Chandra. We compare the
high-resolution spectra acquired with the Reflection Grating Spectrometer
onboard XMM-Newton and with the Low Energy Transmission Grating Spectrometer
onboard Chandra, and evidence in direct comparison the good performance of both
instruments in terms of wavelength and flux calibration. The strongest lines in
the X-ray spectrum of YY Gem are from oxygen. Oxygen line ratios indicate the
presence of a low-temperature component (1-4 MK) with density n_e < 2 10^{10}
cm^-3. The X-ray lightcurve reveals two flares and a dip corresponding to the
secondary eclipse. An increase of the density during phases of high activity is
suggested from time-resolved spectroscopy. Time-resolved global fitting of the
European Photon Imaging Camera CCD spectrum traces the evolution of temperature
and emission measure during the flares. These medium-resolution spectra show
that temperatures > 10^7 K are relevant in the corona of YY Gem although not as
dominant as the lower temperatures represented by the strongest lines in the
high-resolution spectrum. Magnetic loops with length on the order of 10^9 cm,
i.e., about 5 % of the radius of each star, are inferred from a comparison with
a one-dimensional hydrodynamic model. This suggests that the flares did not
erupt in the (presumably more extended) inter-binary magnetosphere but are
related to one of the components of the binary.Comment: 15 pages, accepted for publication in A&
Energy Release During Slow Long Duration Flares Observed by RHESSI
Slow Long Duration Events (SLDEs) are flares characterized by long duration
of rising phase. In many such cases impulsive phase is weak with lack of
typical short-lasting pulses. Instead of that smooth, long-lasting Hard X-ray
(HXR) emission is observed. We analysed hard X-ray emission and morphology of
six selected SLDEs. In our analysis we utilized data from RHESSI and GOES
satellites. Physical parameters of HXR sources were obtained from imaging
spectroscopy and were used for the energy balance analysis. Characteristic time
of heating rate decrease, after reaching its maximum value, is very long, which
explains long rising phase of these flares.Comment: Accepted for publication in Solar Physic
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