Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...183.6803z&link_type=abstract
American Astronomical Society, 183rd AAS Meeting, #68.03; Bulletin of the American Astronomical Society, Vol. 25, p.1396
Astronomy and Astrophysics
Astronomy
Scientific paper
We are analyzing solar flares that show evidence for strong stationary Ca XIX emission at the start of impulsive hard X-rays. An example is an M3.3 event that occurred at 0857 UT on 1992 September 6 in active region AR 7270. The flare was observed in Ca XIX by Yohkoh BCS and simultaneously in hard X-rays by CGRO BATSE. A strong stationary component of Ca XIX emission was already present at the start of impulsive hard X-ray emission, indicating a temperature T ~ 10 times 10(6) K and an emission measure EM ~ 7 times 10(47) cm(-3) . Simultaneous Yohkoh SXT Be-filter images imply a simple loop structure for the main soft X-ray source, with a characteristic cross-sectional area of A ~ 10(17) cm(2) and a half-length L ~ 10(9) cm. The detection of strong stationary soft X-ray emission before the peak of hard X-rays cannot be explained easily by the thick-target driven chromospheric evaporation model. To explain these observations, we adopt a DC-electric field model in which preflare thermal Ca XIX emission is produced by current heating dominating early in the flare, and nonthermal HXR emission is produced by electron runaway acceleration dominating during the impulsive phase. We present a novel method for deriving the strength and temporal variation of the electric field. This work was supported by NASA contracts NAS5-31235 and NAS-32064.
Dennis Brian R.
Mariska John T.
Zarro Dominic M.
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