Positron-emitter production in solar flares from ³He reactions

We treat in detail positron production from the decay of radioactive nuclei produced in nuclear reactions of accelerated ³He. Because of their large cross sections and low threshold energies, these reactions can significantly contribute to positron production in solar flares with accelerated-particle compositions enriched in ³He. The addition of these ³He reactions extends earlier calculations of positron production by accelerated protons and α-particles. ³He reactions not only add significantly to the total positron yield in flares, but can also yield a positron depth distribution that peaks higher in the solar atmosphere. We discuss the impact these reactions have on the analysis of the annihilation line observed with RHESSI from the 2002 July 23 flare. A significant contribution from ³He reactions expands the utility of the annihilation line as a sensitive tool for investigating the structure of the flaring solar atmosphere.