Evolution of our understanding of solar flare particle acceleration: (1942–1995)

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Abstract
We review the evidence for the acceleration of charged particles to relativistic energies associated with solar flares from 1942, the time of their discovery, to the present. First, we present a brief summary of early attempts to identify the mechanism which accelerates the charged particles in solar flares. Second, we describe observational progress made before 1970 which revealed additional properties of the solar flare particle acceleration process. Also, some relevant, significant pre‐1970 theoretical advances are mentioned. Third, the neutral high‐energy observational data, obtained since 1972, are reviewed as diagnostics for the properties of the accelerated electrons and ions. These provide constraints for acceleration theories. Finally, the capabilities of stochastic, direct electric field, and shock acceleration mechanisms are briefly discussed. The early concept that the particles are accelerated in a two‐phase process in which electrons are first accelerated to ∼100 keV, followed by a longer term process which accelerates ions to relativistic energies is not consistent with observations. Rather, ions and electrons are accelerated together and the basic acceleration mechanisms, i.e., stochastic, direct electric field, and shock acceleration may all be operative in a flare.

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