96 research outputs found
Sunspot waves and flare energy release
We address a possibility of the flare process initiation and further
maintenance of its energy release due to a transformation of sunspot
longitudinal waves into transverse magnetic loop oscillations with initiation
of reconnection. This leads to heating maintaining after the energy release
peak and formation of a flat stage on the X-ray profile. We applied the
time-distance plots and pixel wavelet filtration (PWF) methods to obtain
spatio-temporal distribution of wave power variations in SDO/AIA data. To find
magnetic waveguides, we used magnetic field extrapolation of SDO/HMI
magnetograms. The propagation velocity of wave fronts was measured from their
spatial locations at specific times. In correlation curves of the 17 GHz (NoRH)
radio emission we found a monotonous energy amplification of 3-min waves in the
sunspot umbra before the 2012 June 7 flare. This dynamics agrees with an
increase in the wave-train length in coronal loops (SDO/AIA, 171 {\AA})
reaching the maximum 30 minutes prior to the flare onset. A peculiarity of this
flare time profile in soft X-rays (RHESSI, 3-25 keV) is maintaining the
constant level of the flare emission for 10 minutes after the short impulse
phase, which indicates at the energy release continuation. Throughout this
time, we found 30-sec period transverse oscillations of the flare loop in the
radio-frequency range (NoRH, 17 GHz). This periodicity is apparently related to
the transformation of propagating longitudinal 3-min waves from the sunspot
into the loop transverse oscillations. The magnetic field extrapolation showed
the existence of the magnetic waveguide (loop) connecting the sunspot with the
energy release region. A flare loop heating can be caused by the interaction
(reconnections) of this transversally oscillating waveguide with the underlying
twisted loops.Comment: 7 pages, 9 figure
Study of flare energy release using events with numerous type III-like bursts in microwaves
The analysis of narrowband drifting of type III-like structures in radio
bursts dynamic spectra allows to obtain unique information about primary energy
release mechanisms in solar flares. The SSRT spatially resolved images and a
high spectral and temporal resolution allow direct determination not only the
positions of its sources but also the exciter velocities along the flare loop.
Practically, such measurements are possible during some special time intervals
when the SSRT (about 5.7 GHz) is observing the flare region in two high-order
fringes; thus, two 1D scans are recorded simultaneously at two frequency bands.
The analysis of type III-like bursts recorded during the flare 14 Apr 2002 is
presented. Using-muliwavelength radio observations recorded by SSRT, SBRS,
NoRP, RSTN we study an event with series of several tens of drifting microwave
pulses with drift rates in the range from -7 to 13 GHz/s. The sources of the
fast-drifting bursts were located near the top of the flare loop in a volume of
a few Mm in size. The slow drift of the exciters along the flare loop suggests
a high pitch-anisotropy of the emitting electrons.Comment: 16 pages, 6 figures, Solar Physics, in press, 201
Diagnostics of electron beam properties from the simultaneous hard X-ray and microwave emission in the 10 March 2001 flare
Simultaneous simulation of HXR and MW emission with the same populations of
electrons is still a great challenge for interpretation of observations in real
events. In this paper we apply the FP kinetic model of precipitation of
electron beam with energy range from 12 keV to 1.2 MeV to the interpretation of
X-ray and microwave emissions observed in the flare of 10 March 2001. Methods.
The theoretical HXR and MW emissions were calculated by using the distribution
functions of electron beams found by solving time-dependent Fokker-Planck
approach in a converging magnetic field (Zharkova at al., 2010; Kuznetsov and
Zharkova, 2010) for anisotropic scattering of beam electrons on the ambient
particles in Coloumb collisions and Ohmic losses. The simultaneous observed HXR
photon spectra and frequency distribution of MW emission and polarization were
fit by those simulated from FP models which include the effects of electric
field induced by beam electrons and precipitation into a converging magnetic
loop. Magnetic field strengths in the footpoints on the photosphere were
updated with newly calibrated SOHO/MDI data. The observed HXR energy spectrum
above 10 keV is shown to be a double power law which was fit precisely by the
photon HXR spectrum simulated for the model including the self-induced electric
field but without magnetic convergence. The MW emission simulated for different
models of electron precipitation revealed a better fit to the observed
distribution at higher frequencies for the models combining collisions and
electric field effects with a moderate magnetic field convergence of 2. The MW
simulations were able to reproduce closely the main features of the MW emission
observed at higher frequencies.Comment: 17 pages, 10 figures in press; A&A 201
Radioheliograph observations of microwave bursts with zebra structures
The so-called zebra structures in radio dynamic spectra, specifically their
frequencies and frequency drifts of emission stripes, contain information on
the plasma parameters in the coronal part of flare loops. This paper presents
observations of zebra structures in a microwave range. Dynamic spectra were
recorded by Chinese spectro-polarimeters in the frequency band close to the
working frequencies of the Siberian Solar Radio Telescope. The emission sources
are localized in the flare regions, and we are able to estimate the plasma
parameters in the generation sites using X-ray data. The interpretation of the
zebra structures in terms of the existing theories is discussed. The conclusion
has been arrived that the preferred generation mechanism of zebra structures in
the microwave range is the conversion of plasma waves to electromagnetic
emission on the double plasma resonance surfaces distributed across a flare
loop.Comment: 18 pages, 7 figure
Broadband microwave burst produced by electron beams
Theoretical and experimental study of fast electron beams attracts a lot of
attention in the astrophysics and laboratory. In the case of solar flares the
problem of reliable beam detection and diagnostics is of exceptional
importance. This paper explores the fact that the electron beams moving oblique
to the magnetic field or along the field with some angular scatter around the
beam propagation direction can generate microwave continuum bursts via
gyrosynchrotron mechanism. The characteristics of the microwave bursts produced
by beams differ from those in case of isotropic or loss-cone distributions,
which suggests a new tool for quantitative diagnostics of the beams in the
solar corona. To demonstrate the potentiality of this tool, we analyze here a
radio burst occurred during an impulsive flare 1B/M6.7 on 10 March 2001 (AR
9368, N27W42). Based on detailed analysis of the spectral, temporal, and
spatial relationships, we obtained firm evidence that the microwave continuum
burst is produced by electron beams. For the first time we developed and
applied a new forward fitting algorithm based on exact gyrosynchrotron formulae
and employing both the total power and polarization measurements to solve the
inverse problem of the beam diagnostics. We found that the burst is generated
by a oblique beam in a region of reasonably strong magnetic field ( G) and the burst is observed at a quasi-transverse viewing angle. We
found that the life time of the emitting electrons in the radio source is
relatively short, s, consistent with a single reflection
of the electrons from a magnetic mirror at the foot point with the stronger
magnetic field. We discuss the implications of these findings for the electron
acceleration in flares and for beam diagnostics.Comment: Astrophysical Journal, accepted: 26 pages, 8 figure
Where is the chromospheric response to conductive energy input from a hot pre-flare coronal loop?
Before the onset of a flare is observed in hard X-rays there is often a pro- longed pre-flare or pre-heating phase with no detectable hard X-ray emission but pronounced soft X-ray emission suggesting that energy is being released and deposited into the corona and chromosphere already at this stage. This work analyses the temporal evolution of coronal source heating and the chromospheric response during this pre-heating phase to investigate the origin and nature of early energy release and transport during a solar flare. Simultaneous X-ray, EUV, and microwave observations of a well observed flare with a prolonged pre-heating phase are analysed to study the time evolution of the thermal emission and to determine the onset of particle acceleration. During the 20 minutes duration of the pre-heating phase we find no hint of accelerated electrons, neither in hard X-rays nor in microwave emission. However, the total energy budget during the pre-heating phase suggests that energy must be supplied to the flaring loop to sustain the observed temperature and emission measure. Under the assumption of this energy being transported toward the chromosphere via thermal conduc- tion, significant energy deposition at the chromosphere is expected. However, no detectable increase of the emission in the AIA wavelength channels sensitive to chromospheric temperatures is observed. The observations suggest energy release and deposition in the flaring loop before the onset of particle acceleration, yet a model in which energy is conducted to the chromosphere and subsequent heating of the chromosphere is not supported by the observations
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