Evidence for Enhanced Accelerated 3He in Solar Flares from Gamma-Ray Spectroscopy

The ³He abundance in impulsive solar energetic particle (SEP) events observed in interplanetary space is enhanced by up to several orders of magnitude compared to the photosphere. Observations of the associated interplanetary magnetic fields and timing suggest these events are related to active-region solar flares. We therefore expect to see an enhanced abundance of ³He in the flare-accelerated ions that interact in the solar atmosphere. Interactions of flare-accelerated ³He with the solar atmosphere produce numerous gamma-ray nuclear-deexcitation lines, both lines that are the same as those produced by protons and alpha particles and lines that are unique to ³He. Gamma-ray spectroscopy can therefore reveal the presence of enhanced ³He in the interacting flare-accelerated ions. We identify all significant lines produced by energetic ³He interactions with the major elements of the solar atmosphere, evaluate their production cross sections, and incorporate them into our nuclear deexcitation-line code. We use the code to calculate gamma-ray spectra and show that enhanced ³He can affect the entire spectrum and that there are gamma-ray features whose fluxes depend dramatically on the accelerated ³He abundance. Comparing these calculations with measurements of narrow gamma-ray line fluxes from flares observed with SMM/GRS, we find that flare-accelerated ³He is enhanced above its photospheric/coronal value, similar to the enhancements seen in impulsive SEP events, confirming earlier work. We discuss this result and also new fits to RHESSI flare data using a different technique in which calculated spectral "templates" are directly fit to the data. In contrast to simply comparing narrow-line fluxes, such an approach allows all of the data to be used simultaneously. This will improve the significance and reliability of the ³He abundance determination and insure that all relevant flare parameters are self-consistently determined.