POSITRON-ANNIHILATION RADIATION FROM THE 1991 NOVEMBER 15 FLARE

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 O-16*(6.052) by e(+) - e(-) pair emission. (2) beta(+)-emitting nuclei of S-31, 29P, Si-27, Al-26m, Al-25, Mg-23, Ne-19, and Na-21 are also important sources of positrons in the decay phase. (3) The density of the positron annihilation region in the photosphere is 10(16) 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 10(12)-10(13) cm(-3) and with a temperature of 10(6)-3 X 10(6) K.