Solar flare protons and alpha particles during the last three solar cycles

Abstract

Event-integrated fluxes of protons and alpha particles in solar-flare-associated particle events during solar cycle 21 (1976-1986) are determined from data obtained by detectors on board the IMP-7 and IMP-8 satellites. Sixty-three solar particle events with proton fluence (E 10 MeV) 10⁷ cm^-2 were identified from October 1972 to March 1987. The average omnidirectional flux of protons with kinetic energy 10 MeV for cycle 21, 64 cm^-2s^-1, is lower than the corresponding number for cycle 20 (92 cm^-2s^-1) based on satellite data and for the cycle 19 (378 cm^-2s^-1) based on lunar sample data. Six large events contributed 70% of the total proton fluence during solar cycle 21. Several events in early 1981 with high proton fluences could account for much of the high ⁵⁶Co radioactivities observed in the small-sized Salem meteorite. The event-averaged alpha-particle to proton ratio in the energy interval 1-10 MeV/nucleon varies from 0.006 to 0.05, with an average value of 0.02 for the whole cycle. The events during solar cycle 21 are characterized by softer spectra for both protons and alpha particles compared to those in earlier solar cycles. No definitive correlation exists between cycle-averaged solar flare proton fluxes and peak sunspot numbers. A comparison with long-term (million year) averaged data for these parameters, obtained from lunar sample data, shows that the contemporary solar flare proton spectra are characterized by softer spectra (lower R₀ values). A similar comparison cannot be made for the mean long-term averaged flux, as the contemporary average suffers from uncertainty due to statistics of single events. For this reason, the solar energetic particle activity for the last two solar cycles cannot be considered as representative of average solar particle activity over longer time periods.