Study of the thermal/nonthermal electric field model in solar flares using spectral and spatial X-ray data from the Compton gamma ray observatory and Yohkoh

We have analyzed a solar flare in terms of a model in which electric fields produce both thermal emission from Joule-heated electrons and nonthermal emission from runaway electrons. The flare was observed by the Compton Gamma. Ray Observatory Burst and Transient Source Experiment (BATSE), the Yohkoh satellite, and GOES. The measured BATSE hard X-ray spectra were fitted with functions derived from the electric field model, thus allowing values for the temperature [(4-5) X 10(7) K] and emission measure [(1 X 10(46))-(2 X 10(47)) cm(-3)] of the thermal source to be determined. These are qualitatively consistent with the characteristics of the loop-top emission observed with the Yohkoh Hard X-ray Telescope. Furthermore, the hard X-ray footpoint emission observed by Yohkoh had flatter spectra than the loop-top emission, as predicted by the model. We found that the effects of changes in the nonthermal parameters on the model spectra are not unique; it is possible to establish two of the three nonthermal parameters only if the third is determined by other means. Using Yohkoh images in conjunction with the fits to BATSE spectra, we calculated upper limits on the length of the region in which the electric field causes runaway electron acceleration. We find that this distance must be shorter than the length of the flaring loop (i.e., less than 3 x 10(9) cm).