fl061.feldman03 Posted: 08-Aug-92 Updated: 11-Mar-93 Events specified: N/A
Proposers: U. Feldman, E. Hiei, K. Phillips, C. Brown, J. Lang
We propose to make a study of very impulsive flares as observed by the Bragg Crystal Spectrometer on Yohkoh. These flares will be selected from GOES X-ray light-curves, with rise and fall times of the order of a minute or so, with intensities in the mid- to low-C range. BCS spectra over the durations of these events will be analysed for temperature determinations. Initial indications are that, not only the X-ray light-curves show rapid variations but also temperature as determined, e.g., from the Fe XXIV satellite j to Fe XXV resonance line (w) intensity ratios. We will comment on possible mechanisms for the heating and cooling of such flare plasmas.
Update 11-Mar-93
Thirty-eight very impulsive flares observed with the Bragg Crystal Spectrometer and Soft X-ray Telescope on Yohkoh are discussed. They were identified from GOES soft X-ray light-curves over a seven-month period, October 1991 -- April 1992, at the beginning of the Yohkoh mission. All had X-ray importance C1 or greater. The total durations, measured at the half-maximum peak counting rate levels, range from 22~s to about 3~minutes, and the $e$-folding rise times from as small as 7~s to about 1~minute. Electron temperatures derived from Fe XXV spectra decline immediately after the intensity maximum in the Fe XXV resonance line, implying that energy into the flare plasma is reduced or possibly ceases after this time. Images of these flares show that the main emitting regions consist of tiny point-like sources (v<3x10**24 cm3); they are contained within fainter loop structures, generally near the loop tops. This is contrary to the appearance if thermal conduction were a signficant energy loss mechanism; the classical value of conductivity is large enough to ensure that the loop temperature and therefore X-ray brightness are practically uniform over almost the entire loop length. A previous study suggested that mass loss along field lines is likely to be unimportant. We deduce, then, that the flare plasma most probably cools by radiation. If so, a lower limit for electron density n= 3x10**12 cm**-3 is imposed, which is only compatible with observed emission measures if the flare volume V=3x10**23 cm3, equivalent to a sphere of 400 km in diameter, or 0.5 arc sec as seen from the earth. The paper on the Short Duration Flares by Feldman et al. (tbb subj fl061) is about to be finished. A final version should be ready by mid May 1993.