The Physics of Positron Annihilation in the Solar Atmosphere

We compile the most recently measured and calculated cross sections for pro- cesses relevant to annihilation of positrons in solar flares and present the final evaluated cross sections. We calculate the 511 keV annihilation line spectrum and relative strength of the 3! continuum for each of the annihilation processes, using a Monte Carlo code that simulates the thermalization of and positronium production by !MeV positrons. We calculate the thermally-averaged annihila- tion and positronium production rates and the positronium quenching rates. We apply the results to four specific environments (fully ionized, neutral, partially ionized and a non-LTE model of the quiet solar atmosphere), calculating the rela- tive strengths of each process and the combined total annihilation line spectrum. The results are compared with data obtained recently from the high spectral- resolution detectors of RHESSI. We find that positron annihilation in solar flares can occur in a wide variety of environments. The cross sections presented here are also useful for evaluation of positron annihilation in other environments such as the interstellar medium.