Stereoscopic observations of the giant hard X-ray/gamma-ray solar flare on 1991 June 30 at 0255 UT |
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Nicole Vilmer Submitted: 2003-09-12 09:10
The hard X-ray/gamma-ray (HXR/GR) impulsive burst on 1991 June 30
(~ 0255 UT) was associated with a flare which occured
between 2° and 12° behind the east limb of the Sun.
The partially occulted HXR/GR emission from this flare was
detected at up to 100 MeV by three instruments on Earth-orbiting
spacecraft: the Burst and Transient Source Experiment
(BATSE) and the Energetic Gamma-Ray Experiment (EGRET)
on CGRO and by the Payload for High Energy Burst Spectroscopy
(PHEBUS) on GRANAT. As seen from the two spacecraft in Earth orbit,
the size of the burst corresponds to that of a
moderate electron-dominated GR event (Dingus et al. 1994, Vilmer et al 1999}.
However, this event is one of the giant flares reported by
Kane et al. (1995). It was observed by the Solar X-ray/Cosmic
Gamma-Ray Burst Experiment (GRB) on Ulysses, located 135° east
of the Earth-Sun line. GRB measured the total > 28 keV HXR emission from
the flare. In this paper we combine HXR observations by GRB and
BATSE in order to determine the time evolution of the power-law index gamma
of the photon spectrum of the partially occulted HXR emission seen by BATSE
and of the fraction R of the partially occulted to the total > 28 keV
emission. gamma decreased from ~5.4 to ~ 2.6 and
R varied from ~20% at the beginning of the event
down to ~1 at its maximum.
These results indicate
that the spatial distribution of the HXR sources was complex, and
evolved in the course of the event.
While the HXR emission detected by GRB was almost entirely produced
at the footpoints of this complex of loops by thick-target interactions,
a fraction of the HXR emission seen by BATSE likely originated in
the unocculted, low density, portion of the HXR emitting loops.
The data also show that a small fraction (~10%) of the
HXR emission detected by BATSE in Earth's orbit was radiated
by a thick-target source on the visible disk.
Authors: G. Trottet, R.A. Schwartz, K. Hurley, J.M. McTiernan, S.R. Kane, N. Vilmer
Projects: None
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Publication Status: Astronomy and Astrophysics, 403,1157, 2003
Last Modified: 2003-09-12 09:10
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What can be learned about competing acceleration models from multiwavelength observations? |
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Nicole Vilmer Submitted: 2002-07-19 07:45
We review the available evidence from various wavelength ranges, alone and in combination, bearing on solar particle acceleration. Radio, X-ray and gamma-ray observations yield direct information on solar ion and electron acceleration at the Sun. We describe the main spectral features in the
X/gamma domain, outline the means by which they yield information on accelerated particles, and summarise results obtained using them on numbers and energies of flare fast ions and electrons. Relative numbers and energy content
of electrons and ions may vary from flare to flare, and in the course of a single event. In general, both electronic and ionic species appear to embody significant fractions of the total flare energy and either can be dominant,
although there is great uncertainty over accelerated particle minimum energies.Rapid fluctuations in X/gamma-rays point to a fragmented accelerator, acting
on timescales of 100 ms or less, even after particle transport effects have been considered. Millimeter wave observations also reveal spatial fragmentation. Together
with distributions of overall event size, such fragmentation suggests a scale-invariant energy release process, such as would occur in a state of Self-Organised Criticality. There is good evidence from X/gamma and cm/mm observations for hardening of the electron distribution towards the MeV energy range. Intercomparisons of X/gamma rays and cm/mm wave observations emphasise the importance of MeV energy range electrons in the latter. 'Electron-rich' events, characterised by a hard electron population extending to relativistic energies, may occur during individual flares. Existing instrumental capabilities mean that the absence of gamma-ray lines does not rule out significant,
simultaneous ion acceleration. Radio observations indicate these spectral changes are associated with changes in spatial structure. Spatially resolved radio observations
indicate that primary particle acceleration takes place moderately high in the corona (107 to 108 m), and have recently been made to yield information
on accelerated electron pitch angle distribution. Throughout, we emphasise questions on which the unprecedented capabilities of the RHESSI mission will shed
new light.
Authors: Nicole Vilmer and Alexander L. MacKinnon
Projects:
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Publication Status: to be published in Lecture Notes in Physics
Last Modified: 2002-07-19 07:45
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