SOLAR ENERGETIC PARTICLES STUDIED FROM YOHKOH GAMMA-RAY OBSERVATIONS

The Yohkoh spacecraft observed a large flare (H-alpha importance 3B an d 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 accele rated to >1 MeV and >10 MeV, respectively, during the maximum phase of the flare. Assuming second-order Fermi stochastic acceleration, we de rive the Bessel function-type proton spectrum of alpha T = 0.029 from a ratio of neutron capture line to nuclear deexcitation line fluences and estimate several parameters describing the acceleration process. I n a case of first-order Fermi shock acceleration, the proton spectrum can not be derived from the gamma-ray lines. If the shock compression ratio (ratio of upstream to downstream bulk plasma flow velocities) an d 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 observe d Be-7 (429 keV) and Li-7 (478 keV) lines resulting from He-4 + He-4 r eactions from two flares on 27 October 1991 and 15 November 1991 (H-al pha importance 3B and GOES class X1.0). From the Be-7 and Li-7 line pr ofiles we suggest that the angular distribution of accelerated alpha p articles (He-4 nuclei) is peaked in a direction tangential to the phot osphere for the 27 October flare, whereas it is peaked in the downward direction for the 15 November flare.