168 research outputs found
Quantitative analysis of flare accelerated electrons through their hard X-ray and microwave radiation
Hard X-ray and microwave modelling that takes into account the temporal evolution of the electron spectrum as well as the inhomogeneity of the magnetic field and the ambient medium in the radio source is presented. This method is illustrated for the June 29 1980 10:41 UT event. The implication on the process of acceleration/injection is discussed
High energy particles accelerated during the large solar flare of 1990 May 24: X/γ-ray observations
The PHEBUS experiment aboard GRANAT observed γ-ray line emission and γ-ray continuum above 10 MeV from the 24 May, 1990 solar flare. Observations and interpretation of the high-energy continuum have been discussed previously. Here we re-examine these, combining the PHEBUS observations above 10 MeV with calculations of the pion decay continuum to quantitatively constrain the accelerated ion energy distribution at energies above 300 MeV. The uncertainty in the determination of the level of the primary electron bremsstrahlung as well as the lack of measurements on the γ-ray emission above 100 MeV combine to allow rather a wide range of energy distribution parameters (in terms of the number of protons above 30 MeV, the spectral index of the proton distribution and the high energy cut-off of the energetic protons). Nevertheless we are able to rule out some combinations of these parameters. Using the additional information provided by the γ-ray line observations we discuss whether it is possible to construct a consistent picture of the ions which are accelerated in a wide energy range during this flare. Our findings are discussed with respect to previous works on the spectrum of energetic protons in the 10 MeV to GeV energy range
Temporal evolution of an energetic electron population in an inhomogeneous medium: Application to solar hard X-ray bursts
Energetic electrons accelerated during solar flares can be studied through the hard X-ray emission they produce when interacting with the solar ambient atmosphere. In the case of the non thermal hard X-ray emission, the instanteous X-ray flux emitted at one point of the atmosphere is related to the instantaneous fast electron spectrum at that point. A hard X-ray source model then requires the understanding of the evolution in space and time of the fast particle distribution. The physical processes involved here are energy losses due to Coulomb collisions and pitch angle scattering due to both collisions and magnetic field gradients
Radio Spectral Evolution of an X-ray Poor Impulsive Solar Flare: Implications for Plasma Heating and Electron Acceleration
We present radio and X-ray observations of an impulsive solar flare that was
moderately intense in microwaves, yet showed very meager EUV and X-ray
emission. The flare occurred on 2001 Oct 24 and was well-observed at radio
wavelengths by the Nobeyama Radioheliograph (NoRH), the Nobeyama Radio
Polarimeters (NoRP), and by the Owens Valley Solar Array (OVSA). It was also
observed in EUV and X-ray wavelength bands by the TRACE, GOES, and Yohkoh
satellites. We find that the impulsive onset of the radio emission is
progressively delayed with increasing frequency relative to the onset of hard
X-ray emission. In contrast, the time of flux density maximum is progressively
delayed with decreasing frequency. The decay phase is independent of radio
frequency. The simple source morphology and the excellent spectral coverage at
radio wavelengths allowed us to employ a nonlinear chi-squared minimization
scheme to fit the time series of radio spectra to a source model that accounts
for the observed radio emission in terms of gyrosynchrotron radiation from
MeV-energy electrons in a relatively dense thermal plasma. We discuss plasma
heating and electron acceleration in view of the parametric trends implied by
the model fitting. We suggest that stochastic acceleration likely plays a role
in accelerating the radio-emitting electrons.Comment: 22 pages, 10 figure
Submillimeter and X-ray observations of an X Class flare
The GOES X1.5 class flare that occurred on August 30,2002 at 1327:30 UT is
one of the few events detected so far at submillimeter wavelengths. We present
a detailed analysis of this flare combining radio observations from 1.5 to 212
GHz (an upper limit of the flux is also provided at 405 GHz) and X-ray.
Although the observations of radio emission up to 212 GHz indicates that
relativistic electrons with energies of a few MeV were accelerated, no
significant hard X-ray emission was detected by RHESSI above ~ 250 keV. Images
at 12--20 and 50--100 keV reveal a very compact, but resolved, source of about
~ 10" x 10". EUV TRACE images show a multi-kernel structure suggesting a
complex (multipolar) magnetic topology. During the peak time the radio spectrum
shows an extended flatness from ~ 7 to 35 GHz. Modeling the optically thin part
of the radio spectrum as gyrosynchrotron emission we obtained the electron
spectrum (spectral index delta, instantaneous number of emitting electrons). It
is shown that in order to keep the expected X-ray emission from the same
emitting electrons below the RHESSI background at 250 keV, a magnetic field
above 500 G is necessary. On the other hand, the electron spectrum deduced from
radio observations >= 50 GHz is harder than that deduced from ~ 70 - 250 keV
X-ray data, meaning that there must exist a breaking energy around a few
hundred keV. During the decay of the impulsive phase, a hardening of the X-ray
spectrum is observed which is interpreted as a hardening of the electron
distribution spectrum produced by the diffusion due to Coulomb collisions of
the trapped electrons in a medium with an electron density of n_e ~ 3E10 - 5E10
cm-3.Comment: Accpeted in Astronomy & Astrophysics. 9 Pages, 6 Figures ADDED
REFERENCE
A burst with double radio spectrum observed up to 212 GHz
We study a solar flare that occurred on September 10, 2002, in active region
NOAA 10105 starting around 14:52 UT and lasting approximately 5 minutes in the
radio range. The event was classified as M2.9 in X-rays and 1N in H\alpha.
Solar Submillimeter Telescope observations, in addition to microwave data give
us a good spectral coverage between 1.415 and 212 GHz. We combine these data
with ultraviolet images, hard and soft X-rays observations and full-disk
magnetograms. Images obtained from Ramaty High Energy Solar Spectroscopic
Imaging data are used to identify the locations of X-ray sources at different
energies and to determine the X-ray spectrum, while ultra violet images allow
us to characterize the coronal flaring region. The magnetic field evolution of
the active region is analyzed using Michelson Doppler Imager magnetograms. The
burst is detected at all available radio-frequencies. X-ray images (between 12
keV and 300 keV) reveal two compact sources and 212 GHz data, used to estimate
the radio source position, show a single compact source displaced by 25" from
one of the hard X-ray footpoints. We model the radio spectra using two
homogeneous sources, and combine this analysis with that of hard X-rays to
understand the dynamics of the particles. Relativistic particles, observed at
radio wavelengths above 50 GHz, have an electron index evolving with the
typical soft-hard-soft behaviour.Comment: Submitted to Solar Physics, 20 pages, 8 fugure
On the origin of 140 GHz emission from the 4 July 2012 solar flare
The sub-THz event observed on the 4 July 2012 with the Bauman Moscow State
Technical University Radio Telescope RT-7.5 at 93 and 140~GHz as well as
Kislovodsk and Mets\"ahovi radio telescopes, Radio Solar Telescope Network
(RSTN), GOES, RHESSI, and SDO orbital stations is analyzed. The spectral flux
between 93 and 140 GHz has been observed increasing with frequency. On the
basis of the SDO/AIA data the differential emission measure has been
calculated. It is shown that the thermal coronal plasma with the temperature
above 0.5~MK cannot be responsible for the observed sub-THz flare emission. The
non-thermal gyrosynchrotron mechanism can be responsible for the microwave
emission near ~GHz but the observed millimeter spectral characteristics are
likely to be produced by the thermal bremsstrahlung emission from plasma with a
temperature of about 0.1~MK.Comment: 18 pages, 6 figure
H-Alpha and Hard X-Ray Observations of a Two-Ribbon Flare Associated with a Filament Eruption
We perform a multi-wavelength study of a two-ribbon flare on 2002 September
29 and its associated filament eruption, observed simultaneously in the H-alpha
line by a ground-based imaging spectrograph and in hard X-rays by RHESSI. The
flare ribbons contain several H-alpha bright kernels that show different
evolutional behaviors. In particular, we find two kernels that may be the
footpoints of a loop. A single hard X-ray source appears to cover these two
kernels and to move across the magnetic neutral line. We explain this as a
result of the merging of two footpoint sources that show gradually asymmetric
emission owing to an asymmetric magnetic topology of the newly reconnected
loops. In one of the H-alpha kernels, we detect a continuum enhancement at the
visible wavelength. By checking its spatial and temporal relationship with the
hard X-ray emission, we ascribe it as being caused by electron beam
precipitation. In addition, we derive the line-of-sight velocity of the
filament plasma based on the Doppler shift of the filament-caused absorption in
the H-alpha blue wing. The filament shows rapid acceleration during the
impulsive phase. These observational features are in principal consistent with
the general scenario of the canonical two-ribbon flare model.Comment: 15 pages, 5 figures, accepted for publication in Ap
Electron-Electron Bremsstrahlung Emission and the Inference of Electron Flux Spectra in Solar Flares
Although both electron-ion and electron-electron bremsstrahlung contribute to
the hard X-ray emission from solar flares, the latter is normally ignored. Such
an omission is not justified at electron (and photon) energies above
keV, and inclusion of the additional electron-electron bremsstrahlung in
general makes the electron spectrum required to produce a given hard X-ray
spectrum steeper at high energies.
Unlike electron-ion bremsstrahlung, electron-electron bremsstrahlung cannot
produce photons of all energies up to the maximum electron energy involved. The
maximum possible photon energy depends on the angle between the direction of
the emitting electron and the emitted photon, and this suggests a diagnostic
for an upper cutoff energy and/or for the degree of beaming of the accelerated
electrons.
We analyze the large event of January 17, 2005 observed by RHESSI and show
that the upward break around 400 keV in the observed hard X-ray spectrum is
naturally accounted for by the inclusion of electron-electron bremsstrahlung.
Indeed, the mean source electron spectrum recovered through a regularized
inversion of the hard X-ray spectrum, using a cross-section that includes both
electron-ion and electron-electron terms, has a relatively constant spectral
index over the range from electron kinetic energy keV to MeV. However, the level of detail discernible in the recovered electron
spectrum is not sufficient to determine whether or not any upper cutoff energy
exists.Comment: 7 pages, 5 figures, submitted to Astrophysical Journa
Statistical Evidence for Contributions of Flares and Coronal Mass Ejections to Major Solar Energetic Particle Events
Solar energetic particle (SEP) events are related to flares and coronal mass
ejections (CMEs). This work is a new investigation of statistical relationships
between SEP peak intensities - deka-MeV protons and near-relativistic electrons
- and characteristic quantities of the associated solar activity. We consider
the speed of the CME and quantities describing the flare-related energy
release: peak flux and fluence of soft X-ray (SXR) emission, fluence of
microwave emission. The sample comprises 38 SEP events associated with strong
SXR bursts (classes M and X) in the western solar hemisphere between 1997 and
2006, and where the flare-related particle acceleration is accompanied by radio
bursts indicating electron escape to the interplanetary space. The main
distinction of the present statistical analysis from earlier work is that
besides the classical Pearson correlation coefficient the partial correlation
coefficients are calculated in order to disentangle the effects of correlations
between the solar parameters themselves. The classical correlation analysis
shows the usual picture of correlations with broad scatter between SEP peak
intensities and the different parameters of solar activity, and strong
correlations between the solar activity parameters themselves. The partial
correlation analysis shows that the only parameters that affect significantly
the SEP intensity are the CME speed and the SXR fluence. The SXR peak flux and
the microwave fluence have no additional contribution. We conclude that these
findings bring statistical evidence that both flare acceleration and CME shock
acceleration contribute to the deka-MeV proton and near-relativistic electron
populations in large SEP events.Comment: The final publication is available at Springer via
http://dx.doi.org/10.1007/s11207-014-0628-
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