Spectral Evolution of Energetic Protons in Solar Flares

Abstract

The energy spectrum of accelerated protons in solar flares assuming it is either a Bessel function or a power law, is usually deduced from the fluence ratio of the 2.223 MeV neutron capture line, which is of a delayed nature, to 4-7 MeV nuclear emissions. The spectral index obtained in this way is an integration over time. So far almost no detailed information on how the energetic particle spectrum changes with time is available. In this paper we develop a method to study this question and apply it to observations of Gamma-Ray Spectrometer/Solar Maximum Mission flares. For three intense gamma-ray line flares, with a temporal resolution of 16.384 s, we find for the first time that from the maximum (even from the rising phase) to the decay phase, the energy spectra of accelerated protons evolve from soft to hard. This gradual hardening with time, in contrast to the general soft-hard-soft variation of the spectra of energetic electrons, represents a new constraint for the acceleration mechanism in solar flares.