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Space Radiation Effects and Microelectromechanical Systems 87

Nuclear Composition of Galactic Cosmic Particles
Energy ~ 2 GeV/nuc. Normalized to Silicon = 10 6
10 10
H
10 9
He
10 8 7 C O Si
Relative flux (Si = 10 6 ) 10 6 5 4 3 Fe
10
10
10
10
2
10
Zr
Ba
10
1
Pt
Pb
10 0
10 −1 Individual elements Even-Z elements Element groups
10 −2
0 10 20 30 40 50 60 70 80 90 100
Nuclear charge (Z)
2
FIGURE 5.2 Relative abundances of galactic cosmic ray ions in interplanetary space. (From
J. Barth, Modelling Space Radiation Environments, IEEE, 1997.)
intense, the spacecraft might survive for only a few days. This dependence on
orbit is a result of the complex structure of the Earth’s magnetic field, which
determines the shape of the radiation belts and attenuates the flux of solar particles
and cosmic rays.
The magnetic field experienced by LEO spacecraft is dominated by the Earth’s
geomagnetic field, which may be assumed to be a bar magnet (dipole) located
within the Earth. The axis of the bar magnet is tilted by 118 with respect to the
Earth’s axis of rotation and is also displaced from the Earth’s center. The geomag-
netic field, which, to first order, is independent of azimuthal angle (latitude), does
vary significantly with both altitude and longitude. At a distance of about 5 Earth
radii is the ‘‘shock’’ region where the solar wind and the geomagnetic fields interact
strongly. Because magnetic field lines cannot cross, those from the Sun and the
Earth ‘‘repel’’ each other and the solar wind is redirected around the Earth. This
effectively shields the Earth from direct exposure to most solar particle radiation.
On the Earth’s ‘‘dark’’ side, solar wind has the shape of a cylinder with its axis
directed along a line extending from the Sun through the Earth. The distortion on
the ‘‘dark’’ side of the Earth extends to more than 100 Earth radii and is the region
where particles are injected into the radiation belts. 2
An important consequence of the interaction between the solar wind and the
Earth’s magnetic field is the presence of radiation belts, known as van Allen belts.
These radiation belts are regions containing high fluxes of charged particles sur-
rounding the Earth (and other planets with magnetic fields, such as Jupiter). For the
Earth, there is an inner belt of mostly protons and electrons located at approxi-
mately 1.5 Earth radii in the equatorial plane, and an outer belt dominated by
electrons at approximately 5 Earth radii. Figure 5.3 shows the two belts around

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