FREQUENCY DISTRIBUTIONS OF SOLAR GAMMA RAY EVENTS RELATED AND NOT RELATED WITH SPEs IN 1980–1995 R. PÉREZ ENRIQUEZ * and L. I. MIROSHNICHENKO † Instituto de Geofísica UNAM, 04510, México, D.F., México (Received 10 July 1998; accepted 13 April 1999) Abstract. The advent of new and better instruments in space has resulted in a considerable in- crease in the number of solar gamma-ray events (GRE) detected. In this paper, we analyze available SMM/GRS and GRANAT/PHEBUS data on the hard X-ray and gamma-ray events, and their as- sociations with solar proton events (SPE) at the Earth’s orbit, for the observation period of 1980– 1995. About 58% of the GREs under study were found to be SPE-related ones. Size (frequency) distributions have been obtained, for the first time, for the events with different types of emissions (bremsstrahlung, narrow GR lines, positron annihilation line, neutron capture line, SPEs, etc.). We discuss the possible relationships between size distributions implied by the parameter correlation. The distribution for GR events turns out to be generally harder than that for X-ray bursts. The GREs involving energetic particles in space are shown to have a harder frequency distribution in comparison with that for GREs without detectable SPEs. There is also a tendency for the GREs with highest fluences to be related with SPEs. Finally, no correlation seems to exist between the GRL fluence and maximum flux of >10 MeV protons near the Earth. 1. Introduction The occurrence of solar gamma-ray events (GRE) is, perhaps, one of the most dramatic manifestations of the presence of nuclear reactions of energetic particles in the solar atmosphere. Therefore, their observation, together with the direct detection of the particles themselves in interplanetary space, constitutes the most clear evidence of the existence of the acceleration process at the Sun. This opens up again, in particular, in the context of a newly arising paradigm of particle accel- eration in different sources at or near the Sun (e.g., Reames, 1996, and references therein). Solar flare GR emission is produced by accelerated electrons and ions inter- acting with the ambient solar atmosphere. The main components of this emis- sion are (e.g., Ramaty and Mandzhavidze, 1994): electron bremsstrahlung, which dominates at energies of the photons ≤1 MeV, and at energies of ∼10–50 MeV; nuclear gamma ray line (GRL) emission (∼1–10 MeV) and pion decay emission (>50 MeV). Gamma rays provide important information on many aspects of the Sun’s physics, including a fundamental problem of particle acceleration in solar * Campus UNAM-Juriquilla, 73200, Juriquilla, Qro. † On leave from IZMIRAN, Troitsk, Moscow Region, 142092, Russia. Solar Physics 188: 169–185, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.