Sunspot numbers and proton events in solar cycles 19 to 24

Abstract

In this study we compare the mean annual sunspot numbers for the six most recent solar cycles (19 to 24) with the number of solar energetic proton events occurring in those years. Though most of the individual cycles exhibit quite strong correlations, when they are combined into a generic solar cycle the result is very significant (ρ = 0.98). For cycles 21 to 24 (when spacecraft observations have been regularly available), we also investigate the variation in the source location of the solar energetic proton events in relation to four peak flux thresholds in orders of magnitude from ≥10 to ≥10,000 pfu. For helio-latitudes within ±40° (the range within which active regions usually occur) there is negligible variation in the helio-longitude of the source regardless of the peak flux threshold. However, the effect on the helio-longitude of varying the peak flux threshold is very significant: the higher the threshold the closer is the median source longitude to 0°, the variation being almost uniform from ≥10 pfu (32°W) to ≥10,000 pfu (2°W). Finally, of the 10 events in cycles 19 to 24 with peak flux ≥10,000 pfu, all occurred between years 4 and 8 after solar minimum, all were the result of M- or X-class flares with associated halo CMEs and shock fronts, and the three most intense events (≥40,000 pfu) were all related to X-class flares which occurred in the longitude range 10–28°E. These results support and extend previous work by the cited authors, and have implications for solar-terrestrial relations and the effects of space weather within geospace.