Van Allen Belts

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.