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86 MEMS and Microstructures in Aerospace Applications

which produce large, short-duration increases in the solar wind. Solar storms can
have a tremendous impact on the Earth’s magnetic field. When enhanced solar wind
associated with a solar storm reaches the Earth, it interacts strongly with the
geomagnetic field, producing an intense electromagnetic pulse. The electromag-
netic pulse can cause considerable damage not only to space hardware, but also to
Earth-based infrastructure, as evidenced by the failure in 1989 of the electrical
power grid serving Canada and the northeastern states of the U.S. Enhanced solar
wind can also ‘‘pump up’’ the radiation belts by injecting large numbers of particles.
The distribution of particles in the solar wind generally decreases with increas-
ing energy. At the same time, however, great variations in the particle energy
spectra have been observed from one solar-flare event to another. Measurements
indicate that 10 GeV is the upper energy limit of particles in the solar wind. Short-
duration flux increases of up to five orders of magnitude have been observed near
Earth following a solar event identified as a coronal mass ejection. 3
The Sun’s quiet phase typically lasts 4 years, and is characterized by a dimin-
ished solar wind and a reduction in the frequency of storms. During both the active
and quiet phases of the Sun the occurrence of solar storms is random and, therefore,
impossible to predict with certitude. Figure 5.1 illustrates that major solar storms
occur randomly during each solar cycle. Predictions of how many solar storms to
expect during a mission require the use of probabilistic techniques and can only be
4
stated within certain confidence levels. In general, long-term average predictions
are more reliable than short-term predictions.
Galactic cosmic rays (GCRs), whose origins are believed to be outside the solar
system, most likely in supernova millions of light years away, also contribute to the
radiation environment. Although the GCR flux is relatively low, GCRs consist of
fully ionized atoms, some of which have energies in the TeV range, making them
capable of penetrating most spacecraft as well as the Earth’s magnetosphere. Solar
wind, whose direction is opposite to that of the cosmic rays, partially attenuates the
cosmic ray flux. Therefore, during times of maximum solar activity, when solar
wind is at its most intense, the cosmic ray flux is reduced. Figure 5.2 shows the
relative fluxes of the nuclei that make up the cosmic rays and illustrates that there
are very few cosmic rays with nuclear charge greater than that of iron (Z ¼ 28). A
detailed description of the radiation environment in space is beyond the scope of
this book. Only a brief summary of the major aspects will be included here, and the
interested reader is referred to the literature for a more comprehensive exposition. 2,5

5.1.2 EARTH ORBITS
Predicting the radiation environment experienced by a spacecraft in orbit around the
Earth requires knowledge of orbital parameters, such as apogee, perigee, and angle
of inclination as well as launch date and mission duration. Some orbits are relatively
benign from a radiation exposure point-of-view, whereas others are quite severe.
For example, a spacecraft in a low-Earth equatorial orbit (LEO), where the radiation
environment is relatively benign, would be expected to survive for many years,
whereas in medium-Earth orbit (MEO), where the radiation belts are at their most

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