The gamma-ray spectrometer on Yohkoh has detected the positron annihilation line at 511 keV produced during the 1991 November 15 flare (X1.0/3B). The 511 keV line fluence, integrated over the time interval of 22:37:50-22:38:14 UT, is (6.7 ± 2.2) photons cm−2 . The time profile of 511 keV line exhibited long decay time compared with the electron bremsstrahlung and prompt gamma-ray line components. From the analysis of time profile of the 511 keV line, we come to the following conclusions: (1) the main source of positrons is deexcitation of 16O *6.052 by e+ – e- pair emisson. (2) β+ −emitting nuclei of 31S, 29P, 27Si, 26mAl, 25Al,23Mg, 19Ne, and 21Na are also important sources of positrons in the decay phase. (3) The density of the positron annihilation region in the photosphere is 1016 cm−3 . (4) Most likely interpretation of the time profile is that at least 50% of positrons annihilate in coronal flare loops with a density of 1012−1013 cm−3 and with a temperature of 106 −3 × 106 K.
Subject headings: nuclear reactions, nucleosynthesis, abundances — Sun: flares — Sun: X-rays, gamma-rays