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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 $T \approx 10 \times 10^6$~K and an emission measure $EM \approx 7 \times 10^{47}$~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 $A \sim 10^{17}$~cm$^2$ and a half-length $L \sim 10^9$~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.
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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.
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This work was supported by NASA contracts NAS5-31235 and NAS-32064.
