What RHESSI Annihilation-Radiation Observations Tell Us About The Flaring Solar Atmosphere

The electron-positron annihilation line and continuum below the line provide information on the density, temperature, and ionization state of the medium in which the positrons annihilate. The positrons originate from radioactive decay of nuclei and from pions created when flare-accelerated ions interact deep in the solar atmosphere and photosphere. The width of line has been found to be highly variable from flare-flare and during individual flares; during the 2003 October 28 flare the line width changed from ~ 6 keV to 1 keV in 2 - 3 minutes. This suggests that the temperature in the high density region where the line formed changed rapidly from ≥ 2 ×105 K to < 104 K. There is evidence that this change might be associated with cooling of small regions near the photosphere late in the flare. We review RHESSI observations of annihilation radiation in five solar flares and implications for the state of the flaring solar atmosphere. This work was supported by NASA DPR 10049 and NNG04ED181.