Origin of the high energy gamma-ray emission in the March 26, 1991 solar flare

Abstract

The solar flare on March 26, 1991 presents a unique case when high energy (up to 300 MeV) gamma radiation was registered in both, impulsive and delayed, phases of the flare. The radiation in the delayed phase has been attributed to neutral pions decay (1) analogous to the high energy gamma‐ray emission at the late stages of the solar flares on June 11 and June 15, 1991 (2), (3), (4). On the contrary, spectra of the emission in the impulsive phase of the March 26 flare definitely indicate a bremsstrahlung origin of this emission. From the position of the flare close to the center of the disc we conclude that the high energy gamma‐rays could be radiated only by moving upward electrons. We compare time profiles of the gamma‐ray and the microwave emissions and show that the high and the low energy electrons responsible for these emissions were accelerated in the same acts. We put forward arguments in favour of an acceleration of the electrons in the upper chromosphere or in the transition layer.