Gamma Rays from Solar Flares

Abstract

We review recent results obtained from the analysis of the solar flare gamma ray line emission: (a) The gamma ray derived ambient elemental abundances show that the First Ionization Potential (FIP) effect already sets in at relatively low altitudes in the solar atmosphere. (b) The composition of the flare accelerated particles that produce the gamma rays exhibit heavy element and 3He abundance enhancements that are typical for impulsive flares. Unlike the solar energetic particle (SEP) observations in interplanetary space, the gamma ray method allows us to trace the time development of these enhancements. (c) Solar flare gamma ray spectroscopy provides the most direct measure of the abundances of the two very high FIP elements, He and Ne, in subcoronal regions leading to somewhat higher abundances than the generally accepted values. (d) The high intensities of the act lines observed from a number of flares imply a high (≳ 0.1) ambient He/H and/or accelerated α/p. (e) There are indications for the isotopic fractionation of He from the photosphere to corona that has important implications on the mechanism of solar wind acceleration, the protosolar deuterium abundance and Galactic chemical evolution.