Van Allen
radiation belts, which surround the Earth, consist of energetic particles
trapped in the Earth’s dipole-like magnetic field. These belts were discovered
by James A. Van Allen and co-workers in 1958 using data transmitted by the U.S.
Explorer satellite. The Van Allen belts are most intense over the Equator and
are effectively absent above the poles.
There are
two radiation belts surrounding the Earth. No real gap exists between the two
zones. They actually merge with the flux of charged particles showing two
regions of maximum density.
Inner Zone: The inner belt, which extends from about 1-3 Earth radii in the
equatorial plane. The inner region is centred approximately 3000 km above the
terrestrial surface. It consists largely of highly energetic protons, with
energy exceeding 30 MeV and electrons of low energy of the order of 1 MeV. The
peak intensity of these protons is approximately 20,000 particles per second
crossing a spherical area of one sq. cm in all directions. It is believed that
the protons of the inner belt originate from the decay of neutrons produced
when high energy cosmic rays from outside the solar system collide with atoms
and molecules of Earth’s atmosphere.
Outer Zone: The outer belt, which extends from about 3-9 Earth radii in the
equatorial plane, consists mostly of electrons with energies below 0.1 MeV. The
origin of these electrons is via injection from the outer magnetosphere. Unlike
the inner belt, the outer belt is very dynamic, changing on time-scales of a
few hours in response to perturbations emanating from the outer magnetosphere.
