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Accelerated Particle Composition and Energetics and Ambient Abundances from Gamma-Ray Spectroscopy of the 1991 June 4 Solar Flare

R.J. Murphy, G.H. Share, J.E. Grove, W.N. Johnson, R.L. Kinzer, J.D. Kurfess, M.S. Strickman, and G.V. Jung

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The Oriented Scintillation Spectrometer Experiment (OSSE) on board the COMPTON Gamma Ray Observatory observed the 1991 June 4 X12+ solar flare, one of the most intense nuclear gamma-ray line flares observed to date. Using these OSSE observations, we have derived time profiles of the various components of gamma-ray emission and obtained information about the accelerated particle spectra and composition and about the ambient plasma at the flare site. The main results are: (1) the nuclear reactions associated with the impulsive phase of the flare continued for at least 2 hours and resulted from ions that were probably continuously accelerated rather than impulsively accelerated and trapped; (2) the total energy in these accelerated ions exceeded the energy in >0.1 MeV electrons; (3) the accelerated alpha/proton ratio was closer to 0.5 than to 0.1; (4) there is evidence for a decrease of the accelerated heavy ion-to-proton ratio as the flare progressed (5) there is evidence for a temporal change in the composition of the flare plasma; (6) the ratio of electron bremsstrahlung to the flux in narrow gamma-ray lines decreased as the flare progressed; (7) the high-energy (>16 MeV) component of the electron spectrum was much more impulsive than the lower-energy ~MeV component; (8) a model-dependent upper limit of 2.3 x 10^-5 was obtained for the photospheric 3He/H abundance ratio; and (9) energetic ions may have been present for several hours prior to and following the impulsive phase of the flare.