Positron annihilation in solar flares

The gamma-ray line at 0.51 MeV originates from the annihilation of positrons. When a fraction of the positrons annihilate from bound states of positronium, the 0.51 MeV line is accompanied by a continuum of 3-gamma annihilation radiation at energies up to 0.51 MeV. We present accurate calculations of the rates of free annihilation and positronium formation in a solar flare plasma, and we also discuss positronium formation by charge exchange. The observability of the 3-gamma annihilation is increased by the inherent delay in the production and slowing down time of the positrons. We conclude that such radiation could be detected at times late in solar gamma-ray events when the continuum and prompt line emissions have essentially disappeared.