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90 MEMS and Microstructures in Aerospace Applications

particular, solar storms compress the belts on the side facing the Sun, forcing them
to lower altitudes while at the same time populating them with additional charged
particles. Particularly intense storms have been known to produce an extra radiation
belt that lasts for several months in the ‘‘slot’’ region between the inner and outer
electron belts. At distances greater than 5 Earth radii, the azimuthal component of
the Earth’s magnetic field is highly nonuniform and its mathematical description is
very complex. The shifting of the geomagnetic fields as a result of solar storms will
modify the radiation environment experienced by a spacecraft in Earth orbit;
particularly those close to the edges of the radiation belts.
Most Earth orbits fall into one of three categories — LEO, highly elliptical orbit
(HEO), and geostationary orbit (GEO). Medium-Earth orbits (MEOs) are generally
avoided because they are in radiation belts where the high-radiation fluxes severely
limit mission lifetimes. The radiation exposure in each of these orbits is very
different due to the combined effects of geomagnetic shielding and the presence
of the radiation belts.
LEOs in the equatorial plane typically have an altitude of only a few hundred
kilometers (300 km for the Space Shuttle) and, therefore, spend most of their time
below the radiation belts. At that height they are also shielded against solar particles
and cosmic rays by the Earth’s magnetosphere. As the angle of inclination in-
creases, the orbits pass through the ‘‘horn’’ regions of the belts located at high
latitudes. There the belts dip down closer to the Earth’s surface and the particle flux
is enhanced. The SAA is part of the southern ‘‘horn’’ region, and spacecraft in LEO
regularly pass through it, obtaining a significant boost to their total radiation
exposure. For orbit inclinations close to 908, spacecraft pass near the magnetic
poles where the magnetosphere is ineffective at shielding against solar particles and
cosmic rays. Therefore, low altitude and low inclination orbits are much more
benign than high altitude and high inclination orbits.
HEOs typically have their apogee near GEO (36,000 km) and their perigee near
LEO (300 km). Therefore, spacecraft pass through the radiation belts twice per orbit
where they experience high fluences of protons and electrons. Beyond the belts,
spacecrafts are exposed for extended periods of time to cosmic rays and particles
expelled during solar storms. HEOs are among the most severe from a radiation
standpoint.
Spacecraft in GEO are exposed to the outer edges of the electron belts and to
particles originating in cosmic rays and solar events. The magnetosphere provides
some shielding against cosmic rays and solar particles, but storms on the Sun can
compress the magnetosphere and reduce the effective particle attenuation. Com-
ponents that are not well shielded will acquire a significant dose from the relatively
low-energy electrons in the belts and from solar storms. In addition, the energy
spectrum at GEO is considerably ‘‘harder’’ than in LEO because of the presence of
high-energy galactic cosmic rays.
Launch date and mission duration must be factored into any calculation of
radiation exposure in Earth orbit, particularly for orbits with high angles of inclin-
ation that approach the polar regions. For example, if launch date and mission
duration occur entirely during a period of low solar activity where the Earth’s

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