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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 the solar eruption on 15 June 1991 there exists a rich set of measurements, including in particular γ-ray and neutron observations of the COMPTEL instrument on board the COMPTON Gamma Ray Observatory (CGRO) satellite. The γ-ray and neutron emissions indicate prolonged acceleration of protons well after the impulsive phase of the flare. Some accelerated particles interact with solar matter and produce γ-rays and neutrons at the Sun, others escape into the interplanetary medium. Here we present the deduced energy spectrum of the interacting protons for a nearly one hour period in the gradual phase of the flare. From the low-energy γ-ray and neutron results of COMPTEL we determine the energy spectrum of the interacting protons in 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 the high-energy γ-ray measurements on board the GAMMA-1 satellite, the spectrum must steepen at a few hundred MeV and then become harder again to produce a detectable amount of GeV photons. Although the degree of steepening is composition dependent, the deduced spectrum of interacting protons is qualitatively invariant in the entire parameter region α/p ≤ 0.5.