THEORETICAL-STUDY OF THE SPATIALLY-RESOLVED SOFT-X-RAY VERSUS HARD X-RAY RELATIONSHIP IN SOLAR-FLARES

We have carried out a series of calculations of the hard (>30 keV) and soft (1-3 keV) X-ray time histories for various parts of a solar hare loop using hydrodynamic simulations of the electron-heated flare atmo sphere. We have considered two typical cases: the strongly evaporated flare in which the injected energetic electron flux is large and the i nitial atmospheric density is small, and the weakly evaporated flare i n which the electron flux is small and the initial atmospheric density is large. The spatially unresolved Neupert effect that the time deriv ative of the soft X-ray time history is proportional to the hard X-ray time history holds for these models; however, in both cases, we find that the footpoint soft X-ray time history is not proportional to the hard X-ray time history at the same location. The model cannot explain recent observations in which a correlation between the soft and hard X-ray time histories (and not a ''derivative'' relation) at the flare footpoint was observed. Furthermore, the loop-top soft X-ray source de cays much earlier than indicated by observations. In order for these m odel predictions to be consistent with observations, the footpoint sof t X-ray source may have to be nonthermal, excited by low-energy electr ons. Also, continuous heating of the plasma at the top of the loop is required in addition to that from energetic electrons.