The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed the positron-electron annihilation line at 511 keV produced during the 2002 July 23 solar flare. The shape of the line is consistent with annihilation in two vastly different solar environments. It can be produced by formation of positronium by charge exchange in flight with hydrogen in a quiet solar atmosphere at a temperature of ∼6000 K. However, the measured upper limit to the ratio (ratio of annihilation photons

more »

... n the positronium continuum to the number 3g/2g in the line) is only marginally consistent with what is calculated for this environment. The annihilation line can also be fitted by a thermal Gaussian having a width of keV (FWHM), indicating temperatures of 8.1 ‫ע‬ 1.1 ∼ K. The measured ratio does not constrain the density when the annihilation takes place in 5 (4-7) # 10 3g/2g such an ionized medium, although the density must be high enough to slow down the positrons. This would require the formation of a substantial mass of atmosphere at transition-region temperatures during the flare. Subject headings: Sun: flares -Sun: particle emission -Sun: X-rays, gamma rays L86 SHARE ET AL. Vol. 595