A comparison of energetic solar proton events during the declining phase of four solar cycles (cycles 19–22)

Abstract

Solar proton events having a flux of >10 MeV protons ≥10 [cm 2-sec-ster] −1 typically occur during the years around solar sunspot maximum. Using >10 MeV solar proton data acquired at the earth for the 19th–21st solar cycles we find that approximately one third of the events occur on the rising portion of the cycle, one third during a two-to-three year period after sunspot maximum, and one third during the remainder of the cycle. The 22nd solar cycles has been an exception with more than twice as many events during the two years following solar maximum as during the rising portion of the cycle. In addition, as of July 1994, there have been relatively few solar proton events during the final decline of cycle 22. A terminal episode of major solar proton events that typically occurs several years after solar maximum, such as those events of July 1961, August 1972 and April 1984 has not yet been observed during the 22nd cycle which, as of July 1994, now appears to be rapidly approaching solar minimum. A comparison will be made of the energetic solar proton events that occurred during the declining portion of the past three solar cycles with those that have occurred during the decline of the present 22nd solar cycle.