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Gamma Ray Spectroscopy in the Pre-HESSI Era

G.H. Share and R.J. Murphy

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HESSI will provide the high-resolution gamma-ray spectroscopy not available in early missions. In spite of these spectral limitations the experiments on SMM, Yohkoh, GRANAT and the Compton Gamma Ray Observatory have provided data for fundamental discoveries over the past decades relating to particle acceleration, transport and energetics in flares and to the ambient abundance of the corona and chromosphere. These include: 1) enhancement in the concentration of low FIP elements where accelerated particles interact, 2) a new line ratio for deriving the spectra of accelerated particles <10 MeV, 3) energies in accelerated ions that exceed those in electrons for some flares, 4) a highly variable ion to electron ratio during flares, 5) concentration of 3He in flare-accelerated particles enhanced by a factor of >1000 over its photospheric value, 6) an accelerated alpha/p ratio >0.1 in several flares and evidence for high ambient 4He in some flares, 7) measurements of the positronium fraction and a temperature-broadened 511 keV line width, 8) new information on the directionality of electrons, protons, and heavy ions and/or on the homogeneity of the interaction region, and 9) the spectrum of broadened gamma-ray lines emitted by accelerated heavy ions that indicates Fe enhancements consistent with those observed in solar energetic particles. We discuss some of these and also new developments.