Lessons from Solar Cycle 22 Regarding the Radiation Hazards for Life in Space from Solar Particle Events

Solar energetic particle events (SPE) in 1989, in the midst of Solar Cycle 22, provided some of the highest fluxes of charged particles observed at the Earth since the beginning of the satellite era. Data from the 1989 events have added substantially to our knowledge of the characteristics of SPE including their peak intensity, spectral variation, impulsiveness, and contribution to total fluxes over time scales of days and months. Some of the 1989 events were primarily single large pulses while others consisted of a series of pulses lasting over two weeks. Observations and understanding of the solar source of SPE and the effects of interplanetary space in modulating the flux of particles seen near the earth have improved since the very large events of Solar Cycle 19 in the late 1950's to early 1960's and the large isolated events of August 1972. We have found that the range of intensity of large SPE forms a continuous distribution and that “anomalously large events” are probably a part of that distribution. It is not yet clear that there is an upper limit to the size of the largest events. We can use the 1989 events, along with the older data, to make an improved estimate of the probability of occurrence of events larger than those that have been observed in the modern era. Looking ahead to the next solar cycle and beyond, it is not clear whether we have consistently improved our ability to plan and forecast the occurrence of SPE beyond the capability that existed prior to 1989. The variation of SPE fluxes through Solar Cycle 22 was not the same as the variation through the other recent cycles, yet the variation in Cycle 22 is not beyond the range of possibility that might have been expected based on the earlier data. Taken as a whole, the observations may have implications and provide guidance on the range of biological effects that need to be considered as a result of SPE during future long-duration space missions.