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Astron. Astrophys. 320, 620-630 (1997)
Yohkoh observations of flares with flat hard X-ray spectra
F. Fárník 1,
H. Hudson 2 and
T. Watanabe 3
1 Astronomical Institute, 251 65
Ond
ejov, Czech Republic
2 Institute for Astronomy, University of Hawaii, Honolulu,
HI 96822, USA
3 National Astronomical Observatory, Mitaka, Tokyo 181,
Japan
Received 5 June 1996 / Accepted 9 September 1996
Abstract
A series of flares with exceptionally hard spectral indices in the
hard X-ray band occurred on 3 October 1993. The non-thermal
bremsstrahlung spectra may extend to a few keV in these events, one of
which was detectable in the Yohkoh Bragg Crystal
Spectrometer at 7 keV as well as by the hard X-ray instruments at
higher energies. We present Yohkoh soft and hard X-ray
imaging, spectroscopy and energetics analysis of these events, with
the idea that flares with such flat spectra (power-law as hard as 1.98
below 33 keV) might differ appreciably from ordinary flares. The
series of events is strongly homologous, with no systematic variations
in structure over a period of 3.5 hours except for jet-like ejecta
accompanying Type III/V bursts. Unlike other hard events, these flares
are large (footpoint separation about 3 ![[FORMULA]](img1.gif)
km) and therefore well resolved by the Yohkoh
imaging instruments. The time variations match the Neupert
effect. The hard and soft X-ray images also show footpoint brightening
and loop filling. The spikes with the flattest spectra have the
weakest Neupert-effect signature, but no perceptible time delays
between the hard X-ray time profile and the soft X-ray time
derivative. These events do not produce superhot emission but are
probably microwave-rich. We find no evidence for large-scale magnetic
reconnection in the development of these flares. We note two
discrepancies between the observations and the existing numerical
hydrodynamic models of flare energetics, and suggest that rapid spike
events of this type provide good tests of such models.
Key words: Solar
flares 
X-rays, gamma
rays
radio radiation
magnetic fields
Contents
- 1. Introduction
- 2. Observations
- 3. Hard X-ray and soft X-ray timing
- 4. High-energy continuum
- 5. Low-energy continuum
- 6. Image development
- 7. Relationship to magnetic fields
- 8. The coronal structure
- 9. Energetics
- 10. Conclusions
- Acknowledgements
- References
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998
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