BEAM HEATING IN SOLAR-FLARES - ELECTRONS OR PROTONS

The current status of electron and proton beam models, as candidates for the impulsive phase heating of solar flares, is discussed in relation to observational constraints and to theoretical difficulties. As far as the electron beam model is concerned, it is concluded that, while none of the observational data completely exclude it, many of the tests so far have been weak or ambiguous, and that there remain several theoretical problems to be resolved. Proton beam models are divided into high- and low-energy versions. In the former, protons of greater than 40 MeV produce hard X-rays directly by p - e bremsstrahlung, while in the latter they have to be produced thermally by some indirect means. Most of the problems of the electron beam model arise equally in these, but there are more severe problems in addition. Most specifically, unless a very unconventional geometry is invoked, the high-energy proton model will produce heating and hard X-rays either much too deep in the atmosphere or lacking in impulsive time scales as short as those observed. The 102-103 discrepancy between the predicted and observed gamma-ray flux remains unresolved and controversial. In the low-energy proton model, only protons around 1 MeV produce atmospheric heating at about the right depths, and there is as yet no convincing means for these to produce hard X-rays.