Page 109 - Intro to Space Sciences Spacecraft Applications

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Introduction to Space Sciences and Spacecraft Applications
systems. The ionosphere may also be altered, which can cause intermp-
tion of normal communication channels. The increased intensity of
charged particles can affect unprotected systems in space and on the
ground, causing overloads, processing or other electronic errors, and
increased radiation hazards to humans in space. An associated increase in
the motions of trapped charged particles, described next, can also cause an
increase in atmospheric density which may affect satellite lifetimes.

The presence of areas of radiation around the earth was discovered by Dr.
James Van Allen using instruments aboard the early U.S. V-2 test flights
and, later, Explorer series satellites. It was found that some of the charged
particles present in space were able to enter the magnetosphere and become
trapped within the geomagnetic field. The gainfloss mechanics of this phe-
nomenon is still not completely understood, but the levels, dispersion, and
behavior of the trapped particles have become well described.
Early representations of “belts” of radiation surrounding the earth are
better described by distributions of particles within the magnetosphere
which follow closely the geomagnetic lines of flux. Areas of different
concentrations exist due to the types of particles (protons and electrons)
and associated energy levels. More energetic particles are generally
trapped closer to the earth, and the major concentrations of particles occur
around the equator where the minimum value of magnetic field flux
occurs. Figure 4-1 1 shows this distribution of trapped particles around the
earth in terms of particle types and energy levels (1E2 indicates 1 x lo2
MeV or mega-electron volts of energy). In some regions, the level of radi-
ation due to these trapped particles is sufficient to be disruptive to space-
craft operations and hazardous to humans.
Although caught in the earth’s magnetic field, these particles are far
from stationary. Motion of these energetic charged particles is a combina-
tion of spiraling around the magnetic field lines while bouncing from pole
to pole along the lines. A slower drift around the equator also occurs.
When these particles interact with the ionosphere as they spiral in towards
the earth in the vicinity of the poles, they produce the auroral phenomena
known as the “northern lights” in the northern hemisphere. A more seri-
ous effect of these motions is the contribution to atmospheric heating by
particle precipitation in the auroral zones and by the electric currents
flowing at high and low latitudes.