xes as high as 105 protons/cm2/sec for protons with energy > 30 MeV [1]. Protons with these energies are easily able to penetrate shielding and impinge on electronics within spacecraft. The altitude at which proton flux peaks depends on the proton energy, with high energy (>30 MeV) protons being cut off by around 3.5 Earth radii, but lower energy protons existing throughout the slot region. Probably the most important region for proton SEE is the South Atlantic Anomaly (SAA), a region off the east coast of South America with greatly increased proton flux at altitudes less than 1000-2000 km. The SAA exists because of the difference between the Earth's geographic spin axis and its magnetic axis, which causes a localized region of lower magnetic field off the Argentine coast [1]. During passes through the SAA, the flux of energetic (>30 MeV) protons can be more than 104 times as intense than at equivalent altitudes over other regions of the Earth [12]. The SAA is illustrated in Figure 2.1, which shows flux contours for protons with energy > 30 MeV as a function of latitude and longitude at altitudes of 500 km (Fig. 2.1a), 1000 km (Fig. 2.1b), and 3000 km (Fig. 2.1c) [1]. At low altitudes, the SAA is highly localized, and as altitude increases, the SAA becomes less distinct until at 3000 km (Fig. 2.1c) the normal Van Allen belt structure re-emerges. 2.1.2 Heavy Ions The Van Allen belts are predominantly composed of trapped electrons and protons, but it is now well accepted that heavy ions are also trapped by the Earth's magnetic field. The origin of these particles is thought to be anomalous cosmic rays, which are neutral interstellar particles that drift into the solar system, become ionized by the solar wind and accelerated to 10's of MeV/nucleon, and are subsequently trapped by the magnetosphere [1]. Trapped heavy ions (e.g., He, C, N, O, and Ne) have been measured by the SAMPEX spacecraft, and an example geographic distribution is shown in Figure 2.2 for oxygen ions [13]. Similar results have been Integral Proton