Figure 1.1 The radiation environments of the International Space Station (adapted from Baker4). The figure shows the three regions of space around Earth where penetrating radiation occurs. The inner and outer radiation belts each have an electron and ion component.

(LEO satellites) can fly under the inner belt in most places, they cannot avoid it within the SAA. (Note: except for those in equatorial orbits, which largely avoid the SAA.) Based on data from the Mir space station, cosmonauts and astronauts normally accumulate about half of their total radiation dose during the 2 to 5 percent of the time they spend in the SAA. High-inclination LEO satellites also encounter the polar caps, which are accessible to solar energetic particles. Figure 1.2 shows locations of radiation-induced upsets ("hits") suffered by computer memory in a polar-orbiting LEO satellite (UOSAT-2). The hits generate a pointillistic map of the SAA and the horns of the belts at LEO altitude. A final point to note regarding the belts, especially the outer belt, is that their intensity varies with time. The outer electron belt shows variations synchronized with distinctive and relatively common solar wind conditions known as high-speed solar wind streams. During such conditions, the intensity of energetic electrons can increase by many orders of magnitude. Space physicists call times of elevated intensities of energetic electrons highly relativistic electron events (HRE events).

Penetrating particle radiation from the Sun takes the form of solar particle events (SPEs), which typically last several days to a week. Because penetrating SPEs are mainly composed of protons generated by solar storms, they share the statistical properties of these storms. They exhibit a quasi-11-year cycle loosely synchronized with the solar activity cycle. During the last solar cycle (cycle 22), 20 SPEs were officially designated as such by NOAA's Space Environment Center (SEC). (Section 2.1 includes the criterion SEC uses to declare an SPE to be in progress.) The SEC, one of NOAA's eight national centers for environmental prediction (NCEPs), is responsible for the space environment. SPEs constitute the high-altitude acute radiation hazard. The geomagnetic field shields low-latitude LEO satellites from SPEs. Shielding ceases, however, at altitudes above about 4 Earth radii (1 Earth radius, or Re = 6,370 km) or at LEO latitudes (geomagnetic) above about 60 degrees. Geomagnetic storms, which are terrestrial responses to solar storms, weaken this shielding and allow solar energetic particles to penetrate to lower altitudes and latitudes. Figure 1.1 portrays the situation during a storm, in which solar energetic particles fill the space above 50 degrees geomagnetic latitude. In rough numbers, one large SPE can impart to a high-

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