Prompt acceleration of ? 30 MeV per nucleon ions in solar flares

Abstract

Observation of prompt γ-rays in solar flares requires that ions be accelerated to >30 MeV nucl-1 in ≲ 2 s. A model for prompt acceleration is developed. The energy release is assumed to occur in a flaring loop with the energy release region being ≲ 104 km in dimensions and with an Alfven speed υ A ≃ 3 × 103 km s-1. The acceleration is assumed to occur in two steps. The second-step acceleration from ≃ ɛ T = 1/2m p υA 2 nucl-1 to ≳ 30 MeV nucl-1 is attributed to stochastic acceleration by hydromagnetic turbulence which is found to be fast enough under conditions which are not extreme. Main emphasis is placed on the first step, called preacceleration, to ɛ T ≃ 100 keV nucl-1. Preacceleration mechanisms which involve accelerating a small fraction of ions from the tail of a Maxwellian distribution are unacceptable because they would lead to enormous abundance anomalies. Preacceleration is attributed either to localized heating of ions to ≃ 109 K or to acceleration by potential electric fields. The latter mechanism is favoured and some theoretical ideas are outlined based on observations of reconnection in the Earth's magnetotail. Whether energetic ions are prompt, delayed or unobservable depends only on the rate at which the stochastic acceleration proceeds. The second-step acceleration of electrons, invoked to account for a harder microwave component, is predicted to be slower by a factor ≃ 3 than for ≃ 30 MeV nucl-1 ions.