Deduced spectrum of interacting protons accelerated after the impulsive phase of the 15 June 1991 solar flare

In June 1991 the Sun produced a famous series of six powerful flares. For t he solar eruption on 15 June 1991 there exists a rich set of measurements, including in particular gamma-ray and neutron observations of the COMPTEL i nstrument on board the COMPTON Gamma Ray Observatory (CGRO) satellite. The gamma-ray and neutron emissions indicate prolonged acceleration of protons well after the impulsive phase of the flare. Some accelerated particles int eract with solar matter and produce gamma-rays and neutrons at the Sun, oth ers escape into the interplanetary medium. Here we present the deduced ener gy spectrum of the interacting protons for a nearly one hour period in the gradual phase of the flare. From the low-energy gamma-ray and neutron resul ts of COMPTEL we determine the energy spectrum of the interacting protons i n the 10-200 MeV range. We find that the COMPTEL data are consistent with a proton power law spectrum with index 3.3 +/- 0.1. To simultaneously fit th e high-energy gamma-ray measurements on board the GAMMA-1 satellite, the sp ectrum must steepen at a few hundred MeV and then become harder again to pr oduce a detectable amount of GeV photons. Although the degree of steepening is composition dependent, the deduced spectrum of interacting protons is q ualitatively invariant in the entire parameter region alpha/p less than or equal to 0.5.