Frequency distributions of solar proton events

Abstract

A study has been coordinated of 147 solar proton events (SPEs) with proton energies > 10 MeV and peak intensities > 10 protons cm-2 s-1 sr-1 "particle flux units (pfu)" at the Earth's orbit that were measured between 1976 and 1999. This study has been done in association with other related activities such as sunspot numbers, solar flare index, H-alpha solar flares, magnetic field of the Sun, high-speed solar wind streams, and galactic cosmic-rays. The time frequency and size distributions of the peak intensities of the SPEs have been obtained over the entire period and over the ascending-descending phases of each solar activity cycle. For the threshold intensity of > 10 pfu the differential size distributions have a power-law form with a slope of - 1.3 ± 0.2 with no evidence of any change with time, which is in agreement with the results of other authors in different time intervals. An updated catalogue of the solar proton events with energy > 10 MeV and peak flux > 10 pfu is presented. It is based on the Moscow University catalogue (Catalogue of Solar Proton Events 1987-1996, Moscow University, Moscow) for the period 1987-1996. The events are separated into two categories (ordinary and anomalous) with respect to their sources at the Sun, their peak intensity, and their ground level enhancements. The relation between high-energy H-alpha flares and proton events associated with neutron monitor enhancements is discussed in order to be useful for the determination of acceleration processes at the flare site and in very rare interplanetary magnetic conditions. © 2002 Elsevier Science Ltd. All rights reserved.