fl181.yoshimori06 Posted: 31-May-94 Updated: 18-Feb-95, 20-Oct-95 Events specified: Flare of 27-Oct-91 05:40 UT
M.Yoshimori (Rikkyo Univ., E-mail: yosimori@rikkyo.ac.jp), K.Suga, K.Morimoto, T.Hiraoka, T.Matsuda, K.Kawabata, and J.Sato
Yohkoh observed a strong gamma-ray emission from the 1991 Oct.27 flare (X6.1/3B). The proton spectrum in the 10 - 200 MeV range is derived from the ratio of the neutron-capture line (2.22 MeV) fluence to the C(4.44MeV) + O (6.13 MeV) deexcitation line fluence.Assuming two acceleration mechanisms of second-order Fermi process (stochastic particle interactions with turbulent Alfven waves) and shock acceleration (particles gain energies each time they cross the shock front), we can obtain the proton spectra with a spectralparameter aT = 0.029 (Bessel function) for the stochastic acceleration and with a power-law index of 1.8 and turnover energy of 100 MeV for the shock acceleration. We can estimate the particle acceleration times and compare them with the spike durationof gamma-ray emission. The directivity of accelerated ions is discussed from the Be and Li gamma-ray line profiles observed from the 1991 November 15 flare (X1.0/3B), while the directivity of accelerated electrons is discussed from the center-to-limb variations in the power-law indices of hard X-ray spectra. The former result indicates thepossibility of downward beaming of accelerated ions. The latter result indicatesthe weak limb hardening which implies the anisotropic emission. The present results is to be presented in the 8th International STP Symposium in June,1994.
Required Observations: GRS and HXS spectral and light curve data.
Update 20-Oct-95
We have reached the goal of the proposal (fl181) posted on 31-May-94.
Title: Solar Energetic Particles Studied from Yohkoh Gamma-Ray Observations
Authors: M.Yoshimori, K.Morimoto, K.Suga, T.Hiraoka, T.Matsuda, K.Kawabata and J.Sato
Abstract: The Yohkoh spacecraft observed a large flare (H-alpha importance 3B and GEOS class X6.1) on 27 October 1991. The flare showed strong gamma-ray emission above 1 MeV. The gamma-ray spectrum, which extends to 10 MeV/nuc, consists of electron bremsstrahlung continuum and several gamma-ray lines. The gamma-ray observation indicates that electrons and protons were accelerated to >1 MeV and >10 MeV, respectively, during the maximum phase of the flare. Assuming second-order Fermi stochastic acceleration, we derive the Bessel function-type proton spectrum of aT=0.029 from a ratio of neutron capture line to nuclear deexcitation line fluences and estimate several parameters describing the acceleration process. In a case of first-order Fermi shock acceleration, the proton spectrum can not be derived from the gamma-ray lines. If the shock compression ratio (ratio of upstream to downstream bulk plasma flow velocities) and the spectral characteristic energy are taken to be 1.8 and 100 MeV, respectively, the shock acceleration gives the proton spectrum similar to that obtained from second-order Fermi stochastic acceleration. It is not possible to determine the acceleration mechanism from the gamma-ray lines alone. Moreover, the Yohkoh hard X-ray spectrometer observed Be(429 keV) and Li(478 keV) lines resulting from He + He reactions from two flares on 27 October 1991 and 15 November 1991. From the Be and Li line profiles, we suggest that the angular distribution of accelerated He nuclei is peaked in a direction tangential to the photosphere for the 27 October flare, whereas it is peaked in the downward direction for the 15 November flare.
This paper was accepted for publication in J. Geomag. Geoelectr. special issue.
Update 18-Feb-95
Yohkoh observed a large flare on 27 October 1991. the flare showed strong gamma-ray emission above 1 MeV. The gamma-ray spectrum consists of bremsstrahlung continuum and several gamma-ray lines. We derive a proton spectrum from a ratio of neutron-capture line to nuclear deexcitation line fluences and discuss particle acceleration mechanism to explain the derived proton spectrum. We assume two accleration mechanisms, second-order Fermi stochastoic acceleration and first-order Fermi shock acceleration, and estimate acceleration parameters. Moreover, the Yohko hard X-ray spectrometer detected Be (429 keV) and Li (478 keV) lines resulting from He-He reactions from two flares on 27 October 1991 and 15 November 1991. We study the directivity of accelerated He nuclei from the two gamma-ray line profiles. We suggest the possibility that the angular distribution of He nuclei is peaked in a direction tangential to the photosphere for the October 27 flare, while it is peaked in the downward direction for the November 15 flare.
The present result was given in the 8th International STP Symposium in June 1994 and was submitted to Journal of Geomagnetism and Geoelectricity (JGG).