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R.J. Murphy, G.H. Share (NRL)
The X12 solar flare that occurred on 1991 June 11 was well-observed by instruments on the Compton Gamma Ray Observatory (CGRO). This was an exceptional flare because emission up to a GeV was observed by EGRET up to 8 hours after the impulsive phase. The spectrum suggested a pion-decay origin. The COMPTEL data indicate that the accelerated-particle spectrum at energies greater than ~30 MeV hardened as the flare progressed; this study compared gamma-ray lines and 8--30 MeV gamma rays. We discuss observations of this flare with the Oriented Scintillation Spectroscopy Experiment (OSSE). OSSE detected gamma rays from 50 keV to >100 MeV and neutrons >16 MeV. We use the flux ratio in the 6.13 MeV 16O and 1.634 MeV 20Ne lines to determine whether spectral hardening also occurred in the ~5--20 MeV energy band. The OSSE data at high energy appear to confirm a hardening of the spectrum based on the time histories observed in >16 MeV neutrons and gamma rays, and in the e+-e- annihilation line: 1) the peak of the >16 MeV gamma-ray emission occurred ~900 s after that of the line emission and had a very hard spectrum suggestive of a pion-decay origin; 2) the neutrons from the June 11 flare also arrived significantly later after the peak of the line emission than those from other flares observed by OSSE; and 3) there was an increase in the flux of 0.511 MeV annihilaition line photons coincident with the peak in high-energy emission. These facts suggest that there was a second, delayed, ion acceleration episode in the June 11 flare in which the particle spectrum was much harder than that producing the bulk of the nuclear line emission. Such a harder spectrum is relatively more efficient in producing pions and neutrons than excited nuclei. In these respects this flare is similar to those observed on 1982 June 3 and 1984 April 24 by the SMM spectrometer in Cycle 21. We also discuss detailed spectroscopic studies that enable us to infer information on the elemental abundances and spectra of accelerated particles and the ambient abundances in the flare region.
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