me cases can produce new trapped particle belts). In contrast, galactic cosmic rays (GCR) form a background component of radiation that shows a slow cyclical variation with solar activity. GCRs are composed of very highly energetic protons and heavy ions that come from outside the solar system. These particles must fight against the solar wind to enter the solar system and are therefore at their maximum intensity at solar minimum and drop off a factor of 2 to 10 at solar maximum. The cyclical variation of GCRs is shown in Figure 2.4, which is a plot of 25-95 MeV/nucleon He flux over a 20-year period as measured by the IMP-8 satellite [15]. The spikes on this plot are due to increased heavy ion fluences from solar events. Note that these spikes are more likely to occur at solar maximum, when the baseline GCR heavy ion flux is lowest. He: 25-95 MeV/nuc 1975.0 1980.0 1985.0 1990.0 1995.0 Figure 2.4 Solar cycle variation of heavy ions (in this case He nuclei) as measured by the Cosmic Ray Telescope (CRT) aboard the IMP-8 satellite [15]. Note the spikes in He flux caused by solar events. The particle composition of GCRs is shown in Figure 2.5 [16]. Protons comprise about 83% of the GCR flux, He nuclei (alpha particles) account for 13%, 3% are electrons, and the remaining 1% are heavier nuclei. Even though they are not very abundant, heavy ions are very important to SEE because they deposit the most energy per unit pathlength, as discussed in later sections. Note that beyond Fe, the heavy ion flux drops dramatically. This turns out to be important, because the energy deposited by an ion per unit pathlength depends on its atomic number. Ions heavier than Fe are more ionizing, but are much less abundant. Therefore, radiation hardening microelectronics SO that they don't experience SEE from ions up to Fe can result in low SEE rates. This is sometimes referred to as the "iron threshold." GCRs that come into contact with the near-Earth environment encounter Earth's geomagnetic field. Because they are SO energetic (tens of M