Accelerated Particle Composition and Energetics and Ambient Abundances from Gamma‐Ray Spectroscopy of the 1991 June 4 Solar Flare

Abstract

The Oriented Scintillation Spectrometer Experiment (OSSE) on board the Compton Gamma Ray Observatory observed the 1991 June 4 X12+ solar flare, one of the most intense nuclear gamma-ray line flares observed to date. Using these OSSE observations, we have derived time profiles 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 flare site. The main results are (1) the nuclear reactions associated with the impulsive phase of the flare 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 α/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 flare progressed; (5) there is evidence for a temporal change in the composition of the flare plasma; (6) the ratio of electron bremsstrahlung to the flux in narrow γ-ray lines decreased as the flare 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 flare.