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88 MEMS and Microstructures in Aerospace Applications

6
2 10 10 5 10 4
3
−2
10 6 10 cm sec −1
51.6 inclination
Trapped electrons
Trapped protons
10 5 2 10 5
10 4
10 3
10 2
10
−2
1cm sec −1 10 6
10 5
L (Earth radii)
4 3 2 1 0 1 2 3 4 5 6 7 8 9
R ≈ 6380 km
E
FIGURE 5.3 Artist’s impression of the radiation belts. The protons on the left are separated
in the figure from the electrons on the right. (From C. Dyer, Space Radiation Environment
Dosimetry, NSREC Short Course, IEEE, 1998.)

the Earth. They take the form of ‘‘jug handles,’’ approaching closer to the Earth’s
surface near the North and South Poles. Heavy ions are also present in the belts, but
at much lower fluxes. Also, the outer belt, though dominated by electrons, is not
devoid of protons. Some protons in the belts have energies of hundreds of MeV,
making them very penetrating and, therefore, difficult to shield against. Most
electrons in the belts have energies below 10 MeV, so shielding on the spacecraft
is much more effective. Figure 5.4a shows the energy distributions as a function of
altitude for protons and Figure 5.4b that for electrons. The highest energy protons
and electrons have their maximum concentrations at about 1.5 Earth radii. To avoid
the high radiation exposure levels, most spacecraft orbits avoid this region.
As a result of the displacement of the Earth’s magnetic axis with respect to the
center of the Earth, the magnetic field in the South Atlantic is much weaker, allowing
protons and electrons to reach lower altitudes than at other locations on Earth. This
produces the well-known South Atlantic Anomaly (SAA) where the radiation belts
extend down to very low altitudes. Most spacecraft in LEO with large inclination will
pass through the SAA where they will accumulate most of their radiation dose.
Another characteristic of the Earth’s magnetic field is that the magnetic field lines
at the North and South Poles are perpendicular to the Earth’s surface and connected to
those emanating from the Sun. Therefore, the geomagnetic field does not deflect
cosmic rays and solar particles from the North and South Poles. As a result, there are
large fluxes of protons and heavy ions over both poles. Enhanced particle fluxes
associated with solar storms are first apparent on Earth at the poles and signal future
enhanced particle fluxes in the belts. Spacecraft in orbits that pass over the poles will
be directly exposed to high particle fluxes during solar storms.
Given that the ions in the radiation belts originate primarily in the Sun, it is not
surprising that the Sun’s activity also affects the structure of those radiation belts. In

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