Solar Flare

Accelerated Particle Composition and Energetics and Ambient Abundances from Gamma-Ray Spectroscopy of the 1991 June 4 Solar Flare
R. J. Murphy, G. H. Share, J. E. Grove, W. N. Johnson, R. L. Kinzer, J. D. Kurfess and M. S. Strickman E.O. Hulburt Center for Space Research, Code 7650, Naval Research Lab., Washington, DC 20375, U.S.A. G. V. Jung Universities Space Research Association, Washington, DC 20024, U.S.A. Accepted for publication in the Astrophysical Journal
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The Oriented Scintillation Spectrometer Experiment (OSSE) on board the COMPTON Gamma Ray Observatory observed the 1991 June 4 X12+ solar are, one of the most intense nuclear gamma-ray line ares observed to date. Using these OSSE observations, we have derived time pro les of the various components of gamma-ray emission and obtained information about the accelerated particle spectra and composition and about the ambient plasma at the are site. The main results are: (1) the nuclear reactions associated with the impulsive phase of the are continued for at least 2 hours and resulted from ions that were probably continuously accelerated rather than impulsively accelerated and trapped; (2) the total energy in these accelerated ions exceeded the energy in >0.1 MeV electrons; (3) the accelerated /proton ratio was closer to 0.5 than to 0.1; (4) there is evidence for a decrease of the accelerated heavy ion-to-proton ratio as the are progressed (5) there is evidence for a temporal change in the composition of the are plasma; (6) the ratio of electron bremsstrahlung to the ux in narrow -ray lines decreased as the are progressed; (7) the high-energy (>16 MeV) component of the electron spectrum was much more impulsive than the lower-energy MeV component; (8) a model-dependent upper limit of 2.3 10?5 was obtained for the photospheric 3He/H abundance ratio; and (9) energetic ions may have been present for several hours prior to and following the impulsive phase of the are.
Subject headings: Sun: ares | Sun: X-rays, gamma rays | Sun: abundances | acceleration of particles
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1. INTRODUCTION
Prior to the launch of the COMPTON Gamma Ray Observatory (CGRO), most solar -ray line measurements had been made with the Solar Maximum Mission, Hinotori, Yohkoh, and GRANAT -ray spectrometers (e.g., Chupp 1987; Yoshimori 1989; Yoshimori 1994; Vilmer 1994). With the launch of the CGRO in April of 1991, a new resource for high-energy solar measurements became available. In early June, less than two months after launch, active region 6659 rotated onto the disk and produced some of the largest X-ray ares ever recorded. On June 1, AR 6659 appeared at the east limb and produced an X12+ are. The peak of this are was missed by the CGRO instruments because the satellite was in the South Atlantic Anomaly (SAA), but it was observed by the PHEBUS experiment on GRANAT (Barat et al. 1994). As there was a high probability for subsequent intense ares, a solar Target of Opportunity was declared for the Oriented Scintillation Spectrometer Experiment (OSSE) on CGRO which is sensitive to rays from 0.04 to >150 MeV. The Sun was then observed for roughly 30 { 40 minutes during each orbit and, on June 4, AR 6659 produced a second X12+ are while OSSE was viewing the Sun. The are was one of the most intense nuclear -ray line ares observed to date; most instruments, from the soft X-ray to the -ray bands, saturated at its peak. The NOAA Solar-Geophysical Data Comprehensive Report gives the H are location as N30E70. This location corresponds to a heliocentric angle of 74:5 at the time of the are. Based on the count rate of > 20 keV X-rays observed from the June 4 are by the Solar X-Ray/Cosmic Gamma-Ray Burst Experiment on Ulysses, Kane et al. (1995) derived a peak >20 keV electron injection rate of 5.7 1038 electrons s?1 and an energy dissipation rate of 2.5 1031 ergs s?1 . Given its long duration, they pointed out that the material and energy resources of the active region appeared to be inadequate for the production of such a are. Ramaty et al. (1994) used data from the BATSE Charged Particle Detectors (CPD; the only CGRO detectors sensitive to MeV rays not saturated at the peak of the are) to monitor the electron bremsstrahlung time pro le and estimate the magnetic eld strength in the coronal portion of the are loop. Using the EGRET Total Absorption Shower Counter (TASC), Bertsch et al. (1997) measured -ray line uxes starting after the impulsive peak of the are. The are also produced >100 MeV neutrons (Takahashi et al. 1991; Muraki et al. 1992; Struminsky et al. 1994) and possibly even >10 GeV neutrons (Chiba et al. 1992). Energetic particles from the are were observed in interplanetary space by the EPAC experiment on Ulysses (McDonald et al. 1995) and the are was observed at 17, 35 and 80 GHz by radiometers at the Nobeyama Radio Observatory in Japan (Ramaty et al. 1994). The combined interplanetary modulation e ects of the June 4 are and the other intense June ares resulted in the lowest celestial cosmic-ray ux at earth in almost 40 years (Webber and Lockwood 1993; Van Allen 1993) and strong radio emission generated