High energy particles accelerated during the large solar flare of 1990 May 24: X/γ-ray observations

Abstract

The PHEBUS experiment aboard GRANAT observed γ -ray line emission and γ -ray continuum above 10 MeV from the 24 May, 1990 solar flare. Observations and interpretation of the high-energy continuum have been discussed previously. Here we re-examine these, combining the PHEBUS observations above 10 MeV with calculations of the pion decay continuum to quantitatively constrain the accelerated ion energy distribution at energies above 300 MeV. The uncertainty in the determination of the level of the primary electron bremsstrahlung as well as the lack of measurements on the γ -ray emission above 100 MeV combine to allow rather a wide range of energy distribution parameters (in terms of the number of protons above 30 MeV, the spectral index of the proton distribution and the high energy cut-off of the energetic protons). Nevertheless we are able to rule out some combinations of these parameters. Using the additional information provided by the γ -ray line observations we discuss whether it is possible to construct a consistent picture of the ions which are accelerated in a wide energy range during this flare. Our findings are discussed with respect to previous works on the spectrum of energetic protons in the 10 MeV to GeV energy range.