ng solar active years. While solar particle events can be broadly classified as gradual or impulsive, individual events have their own very unique properties in terms of duration, particle fluence, energy spectrum, etc. Stassinopoulos et al. have presented a classification system where events are classified from small to extremely large, and solar cycles are classified from extremely mild to Event Fluences For Cycles 20 - 22 Cycle 20 Cycle 21 Cycle 22 > 10 MeV; D> 108 p/cm² 2 > 30 MeV; © > 107 p/cm² Zurich Smoothed Sunspot Number 10 10 Figure 2.3 Correlation of proton solar event fluence to sunspot number for solar cycles 20-22 [1]. Sunspot number is shown by the solid line plot, and proton solar event fluences by the bars. extremely severe [5]. It can be seen in Figure 2.3 that solar cycle 20 was in general extremely quiet with the exception of an extremely large solar event in August 1972; this solar cycle was classified as very mild. Solar cycle 21 was extremely mild, with no individual events of high- energy proton fluence > 109 protons/cm ². By contrast, solar cycle 22 had 8 events with high- energy proton fluence > 109 protons/cm2; this cycle is considered extremely severe [5]. Some events (such as the August 1972 event) are actually a series of closely-spaced events, where proton fluxes have not yet decayed to their original level before the next event occurs. The importance of individual large solar events cannot be overestimated - the August 1972 event by itself accounted for 84% of the total high-energy proton fluence received by spacecraft during the entire 20th solar cycle. Accurate models to predict worst-case particle fluence from solar events are crucial to appropriate parts selection and survivable spacecraft design. 2.2.2 Galactic Cosmic Rays Solar event particles are true transient particles in the sense that elevated fluxes of particles are observed only for a short time following an event (although recall that following a large event increased levels of trapped particles are observed and in so