many solar processes show the same variation. Importantly for the Oxygen Nuclei, 16 to 200 MeV/nuc, Quiet Sun Figure 2.2 Geographic distribution of trapped oxygen ions during solar quiet time as measured by the Mass Spectrometer Telescope (MAST) on SAMPEX [13]. present case, this includes the incidence of energetic solar events, with maximum numbers of solar flares and CMEs occurring during active years. Solar events still occur during solar quiet times, but they occur less frequently. We have long associated solar flares with an increased flux of energetic particles, but the evidence appears to support that CMEs may be more important [1]. Solar events can be broadly characterized as being either gradual or impulsive. The gradual events produce a raised particle flux that decays slowly over several hours or even days, and have been correlated to CMEs. These events are proton-rich and can produce high-energy 30 MeV) proton fluences higher than 109 protons/cm2 2 accumulated over a few days. Gradual events are responsible for the majority of large proton fluence events, and occur at a frequency of about 10 per year during solar maximum conditions. Impulsive events are by definition of much shorter duration (hours at most), and are marked by increased fluences of heavy ions and low energy electrons. Impulsive events produce heavy ion fluences that can be orders of magnitude above the galactic cosmic ray background. These heavy ions have energies ranging from tens of MeV/nucleon to hundreds of GeV/nucleon, but at the upper end of this range the flux falls below the galactic cosmic ray background. Impulsive events may be associated with solar flare activity and are responsible for about 1000 small solar particle events per year at solar maximum [1,3]. Figure 2.3 shows solar event proton fluences for the last three solar cycles, superimposed over a plot of the sunspot number [1]. The cyclical variation of the sunspot number is readily apparent, as is the fact that most (but not all) high-fluence proton events occur duri