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Session 56 - The Sun.
Oral session, Tuesday, January 14
We describe observations of positron annihilation radiation observed in solar flares observed from 1980 to 1989 by the Gamma-Ray Spectrometer (GRS) on NASA's Solar Maximum Mission satellite. Positrons arise in flares from decay of radioactive isotopes and pions produced by charged particles interacting with ambient material. The \gamma-ray emission is delayed in time from the original interaction due to the intrinsic decay time and the time it takes the positron to slow down and annihilate. Annihilation can occur directly or through formation of positronium. A 3\gamma continuum comes from annihilation of the triplet state of positronium while the 511 keV line comes from either free annihilation or annihilation from the singlet state (2 \gamma-rays). The width of the line is dependent on temperature and the 3\gamma/2\gamma ratio is dependent on both temperature and density in the annihilation region. The GRS observed 24 flares in which the 511 keV line was detected with at least 99.5% confidence. The line was detected at >12\sigma in seven of the flares. We made fits to determine both the width of the line and the 3\gamma/2\gamma ratio for these flares. Line widths below \sim10 keV (FWHM) cannot be determined for flares due to the modest energy resolution of the detectors. In one flare, however, the measured width is \sim 20 keV suggesting temperatures of \sim 3 \times 10^6 K. The 3\gamma/2\gamma ratios for these flares are all below what the GRS measured for the annihilation feature from the Galaxy, also reflecting the higher temperatures of the flare plasma. In one or possibly two of the flares the ratio is even smaller than expected from high temperature; this suggests that annihilation took place at a local density of at least \sim10^14 cm^-3. This work is supported under NASA DPR W-18995.
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