EVIDENCE for ENHANCED ³HE in FLARE-ACCELERATED PARTICLES BASED on NEW CALCULATIONS of the GAMMA-RAY LINE SPECTRUM

The ³He abundance in impulsive solar energetic particle (SEP) events is enhanced up to several orders of magnitude compared to its photospheric value of [³He]/[⁴He] = 1-3 ×10^-4. Interplanetary magnetic field and timing observations suggest that these events are related to solar flares. Observations of ³He in flare-accelerated ions would clarify the relationship between these two phenomena. Energetic ³He interactions in the solar atmosphere produce gamma-ray nuclear-deexcitation lines, both lines that are also produced by protons and α particles and lines that are essentially unique to ³He. Gamma-ray spectroscopy can, therefore, reveal enhanced levels of accelerated ³He. In this paper, we identify all significant deexcitation lines produced by ³He interactions in the solar atmosphere. We evaluate their production cross sections and incorporate them into our nuclear deexcitation-line code. We find that enhanced ³He can affect the entire gamma-ray spectrum. We identify gamma-ray line features for which the yield ratios depend dramatically on the ³He abundance. We determine the accelerated ³He/α 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 ³He, confirming earlier suggestions. The ³He/α weighted mean of these new measurements ranges from 0.05 to 0.3 (depending on the assumed accelerated α/proton ratio) and has a <1 ×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 flare-accelerated ions and ³He-rich SEPs.