EVIDENCE FOR ENHANCED 3HE IN FLARE-ACCELERATED PARTICLES BASED ON NEW CALCULATIONS OF THE GAMMA-RAY LINE SPECTRUM

The He-3 abundance in impulsive solar energetic particle (SEP) events is enhanced up to several orders of magnitude compared to its photospheric value of [He-3]/[He-4] = 1-3. x. 10(-4). Interplanetary magnetic field and timing observations suggest that these events are related to solar flares. Observations of He-3 in flare-accelerated ions would clarify the relationship between these two phenomena. Energetic He-3 interactions in the solar atmosphere produce gamma-ray nuclear-deexcitation lines, both lines that are also produced by protons and a particles and lines that are essentially unique to He-3. Gamma-ray spectroscopy can, therefore, reveal enhanced levels of accelerated He-3. In this paper, we identify all significant deexcitation lines produced by He-3 interactions in the solar atmosphere. We evaluate their production cross sections and incorporate them into our nuclear deexcitation-line code. We find that enhanced He-3 can affect the entire gamma-ray spectrum. We identify gammaray line features for which the. yield ratios depend dramatically on the He-3 abundance. We determine the accelerated He-3/alpha ratio by comparing these ratios with flux ratios measured previously from the gamma-ray spectrum obtained by summing the 19 strongest flares observed with the Solar Maximum Mission Gamma-Ray Spectrometer. All six flux ratios investigated show enhanced He-3, confirming earlier suggestions. The He-3/alpha weighted mean of these new measurements ranges from 0.05 to 0.3 (depending on the assumed accelerated alpha/ proton ratio) and has a < 1. x. 10(-3) probability of being consistent with the photospheric value. With the improved code,. we can now exploit the full potential of gamma-ray spectroscopy to establish the relationship between flareaccelerated ions and He-3-rich SEPs.