fl089.yoshimori01 Posted: 05-Dec-92 Updated: 31-Jul-93, 12-Dec-93, 7-Aug-94 Events specified: 15-Nov-91 flare
Collaboration: M. Yoshimori (Rikkyo University, X024862@JPNRKY00.BITNET), K. Kawabata, K. Ohki, K. Suga, K. Morimoto, T. Hiraoka and J. Sato
Scientific Questions: (a) What are positron production processes in solar flare ? (b) Where do positrons annihilate ? (c) Can we determine the density and temperature of the positron annihilation region ?
Motivation: Possible evidence of the positron annihilation line at 511 keV was obtained from gamma-ray spectral observation of the 1991 November 15 flare (X1.0 / 3B; location S12 W13). In most gamma-ray flares, the 511 keV line is hidden by strong electron bremsstrahlung continuum, and the detection of the 511 keV line is considered to be difficult. Theoretical studies suggestsed that the positrons are produced from the beta-decay of radioactive nuclei and positive pions resulting from nuclear interactions of accelerated ions on ambient material. The beta-decay radioactive nuclei are produced by low-energy ions ( 10 - 50 MeV/nuc), while the positive pions are primarily produced by high-energy protons (> 300 MeV). Since the production of 511 keV line requires that the positrons slow down and annihilate with electrons, there is a time delay between positron production and annihilation. The time profile of 511 keV line emission depends on the following two factors: (1) the relative production rate of the positron emitters and their half-lives and (2) the slowing down time of the positrons. From the light curve of 511 keV line emission we study which process contributes to the positron production and where positrons annihilate. Furthermore, spectral broadening allow us to set an upper limmit to the temperature of the annihilation region.
Required Observations: GRS and HXS spectral and light curve data.
Update 7-Aug-94
We have reached the goal of our paper, so we like to close the project.
The paper entitled
Scientific output:
We present the detailed time profile of the positron annihilation line emission from the 1991 Nov.15 flare and discuss both the origin of positron emitting nuclei and the annihilation region. Since only two positron annihilation line events were reported in the 21st solar cycle and the production process of the positron in solar flares has not yet been discussed in details, the present paper contributes to understanding of the positron production and annihilation during solar flares.
Abstract:
The Yohkoh gamma-ray spectrometer detected the positron annihilation line at 511 keV from the 1991 Nov.15 flare (X1.0/3B). The 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 exhibit long decay time compared with the electron bremsstrahlung and prompt nuclear gamma-ray lines. We come to the following conclusions using the detailed time profile data: (1) The main source of positrons is deexcitation of O-16 (6.025 MeV) by positron-electron pair emission, (2)beta-emitting nuclei of S-31, P-29, S-27, Al-26m, Al-26, Mg-23, Ne-19, and Na-21 are also important sources of positrons in the decay phase of the flare, (3) The density of the positron annihilation region in the photosphere is 10^16 cm^(-3) and (4) Most likely interpretation of the time profile is that at least 50 % of the positrons annihilates in coronal flare loops with density of 10^(12-13) cm^(-3) and with temperature of 1-3 MK.
Update 12-Dec-93
The paper entitled Positron Annihilation Line from the 15 November 1991 Flare by K.Kawabata, M.Yoshimori, K.Suga, K.Morimoto, T.Hiraoka, J.Sato and K.Ohki was accepted for publication in Ap. J. Suppl.
Abstract The gamma-ray spectrometer on Yohkoh has detected the positron annihilation line at 511 keV produced during 15 November 15, 1991 flare (X1.0/3B). The 511 keV line fluence, integrated over the time interval of 2:37:50 -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 nuclear 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 MeV) by positron-electron pair emission. (2) positive beta-ray emitting nuclei of S-31, P-29, S27, Al-26m, Al-26, Mg-23, Ne-19, and Na-21 are also important sources of positropns in the decay phase. (3) The density of the positron annililation region in the photosphere is 10^16 cm/cm^3. (4) Most likely interpretation of the time profile is that at least 50 % of positrons annihilate in coronal flare loops with density of 10^12-13/cm^3 and with temperature of 10^6 - 3x10^6 K.
Update 31-Jul-93
The gamma-ray spectrometer on Yohkoh detected the positron annihilation line at 511 keV during the 15 November 1991 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 a delay of the starting time and long decay time compared with the electron bremsstrahlung and prompt gamma-ray line components. 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 8 x 10**5 - 3 x 10**6 K. The number of >30 MeV protons impinging on the photosphere is estimated to be 1.5 x 10**32 and the energy of the protons is 9 x 10**27 erg. When we extrapolate the proton spectrum to lower energy, the total energy of energetic nuclei impinging on the photosphere is estimated to be 10**30 erg.
The paper entitled
Positron Annihilation Line from the 15 November 1991 Flare
by K.Kawabata, M.Yoshimori, K.Suga, K.Morimoto, T.Hiraoka, J.Sato and
K.Ohki
was submitted to Astrophys. J. Suppl. on 31 March, 1993.