Gamma-Ray Spectral Characteristics of Solar Flares

Abstract

Yohkoh observed three impulsive gamma-ray flares with different spectral characteristics: a narrow line flare on 27 October 1991, a broad line flare on 15 November 1991 and a continuum flare on 3 December 1991. The narrow gamma-ray lines are produced by accelerated proton interactions with heavy nuclei in the solar atmosphere, the broad gamma-ray lines by accelerated heavy nuclei interactions with protons in the solar atmosphere (reverse interactions) and the continuum by relativistic electron bremsstrahlung. The gamma-ray count spectrum of the flare (X6.1/3B) on 27 October 1991 is shown in Fig. 1. Several strong narrow lines (neutron capture line and deexcitation lines) and broad lines are apparent. These lines result from accelerated particles of the coronal abundances [1]. Fermi acceleration is most probable, because it is most effective in the early stage of the acceleration and the accelerated particle abundances are similar to the source abundances. The gamma-ray count spectrum of the flare (X1.5/3B) on 15 November 1991 is shown in Fig. 2. The spectrum shows broad C and 0 lines and a complex of broad Fe, Mg, Ne and Si lines without detectable narrow lines. Since the acceleration rate of the shock acceleration is proportional to the ratio of square root of mass to charge, heavy nuclei are accelerated in a different manner than protons. Assuming a two-stage acceleration process, Fermi acceleration (first stage) plus shock acceleration (second stage), we can explain the observed high variabilities of the ratio of heavy nuclei to proton abundances (the ratio varies from 0.3 to 0.01)[3]. The gamma-ray the electron bremsstrahlung continuum. This type of specturm is called an electron-dominated event [3]. Since betatron acceleration is more effective for electrins than ions, this spectrum suggests the possibility that betatron acceleration was dominant in this flare.