EVIDENCE FOR PROLONGED ACCELERATION BASED ON A DETAILED ANALYSIS OF THE LONG-DURATION SOLAR GAMMA-RAY FLARE OF JUNE 15, 1991

Gamma-ray emission extending to energies greater than 2 GeV and lastin g at least for two hours as well as 0.8-8.1 MeV nuclear line emission lasting 40 min were observed with very sensitive telescopes aboard the GAMMA and CGRO satellites for the well-developed post-flare loop form ation phase of the 3B/X12 flare on June 15, 1991. We undertook an anal ysis of optical, radio, cosmic-ray, and other data in order to identif y the origin of the energetic particles producing these unusual gamma- ray emissions. The analysis yields evidence that the gamma-rays and ot her emissions, observed well after the impulsive phase of the flare, a ppear to be initiated by prolonged nonstationary particle acceleration directly during the late phase of the flare rather than by a long-ter m trapping of energetic electrons and protons accelerated at the onset of the flare. We argue that such an acceleration, including the accel eration of protons up to GeV energies, can be caused by a prolonged po st-eruptive energy release following a coronal mass ejection (CME), wh en the magnetic field above the active region, strongly disturbed by t he CME eruption, relaxes to its initial state through magnetic reconne ction in the coronal vertical current sheet.