Observations of particle accelerator in solar flares

Solar flares provide several examples of non‐thermal particle acceleration. This paper reviews the information gained about these processes via X‐ray and γ‐ray astronomy, which can presently distinguish among three separate particle‐acceleration processes at the sun: An impulsive acceleration of ≳20 keV electrons, a gradual accelerator of ≳20 keV electrons, and a gradual accelerator of ≳10 MeV ions. The acceleration energy efficiency (total particle energy divided by total flare energy) of any of these mechanisms cannot be less than about 0.1%, although the gradual acceleration does not occur in every flare. The observational material suggests that both the impulsive and gradual accelerations take place preferentially in closed magnetic‐field structures, but that the electrons decay in these traps before they can escape. The ions escape very efficiently, on the other hand.