Solar Energetic Particles Studied from Yohkoh Gamma-Ray Observations

Abstract

The Yohkoh spacecraft observed a large flare (Ha importance 3B and GOES (Geosynchronous Operational Environmental Satellite) class X6.1) on 27 October 1991. The flare showed strong gamma-ray emission above 1 MeV. The gamma-ray spectrum, which extends to 10 MeV/nuc, consists of electron bremsstrahlung continuum and several gamma-ray lines. The gamma-ray observation indicates that electrons and protons were accelerated to >1 MeV and >10 MeV, respectively, during the maximum phase of the flare. Assuming second-order Fermi stochastic acceleration, we derive the Bessel function-type proton spectrum of αT = 0.029 from a ratio of neutron capture line to nuclear deexcitation line fluences and estimate several parameters describing the acceleration process. In a case of first-order Fermi shock acceleration, the proton spectrum can not be derived from the gamma-my lines. If the shock compression ratio (ratio of upstream to downstream bulk plasma flow velocities) and the spectral characteristic energy are taken to be 1.8 and 100 MeV, respectively, the shock acceleration gives the proton spectrum similar to that obtained from second-order Fermi stochastic acceleration. It is not possible to determine the acceleration mechanism from the gamma-ray lines alone. Moreover, the Yohkoh hard X-ray spectrometer observed ⁷Be (429 keV) and ⁷Li (478 keV) lines resulting from ⁴He+ ⁴He reactions from two flares on 27 October 1991 and 15 November 1991 (H_α importance 3B and GOES class X1.0). From the ⁷Be and ⁷Li line profiles we suggest that the angular distribution of accelerated alpha particles (⁴He nuclei) is peaked in a direction tangential to the photosphere for the 27 October flare, whereas it is peaked in the downward direction for the 15 November flare.