Solar proton events in solar activity cycles 21–24
It is shown that the number of solar proton events (SPEs) with proton energies (E) higher than 10 and 100 MeV in the current solar cycle (cycle 24) differs slightly from the number of the same events in earlier cycles (cycles 21–23), even though solar activity was low during the growth and maximum phase in cycle 24. A deficit was in this case observed for the most powerful GLE events, which are characterized by high proton fluxes with E > 100 MeV. The ratio of the number of SPEs with E > 10 and 100 MeV to the number of sunspots in cycle 24 doubled, compared to the same ratio in cycles 21–23, and the relative number of GLEs fell by more than half. The characteristics of flares and coronal mass ejections associated with proton events with E > 100 MeV in cycle 24 were virtually the same as the analogous parameters in cycle 23.