Van Allen Radiation Belt - Inner Belt

Inner Belt

While protons form one radiation belt, trapped electrons present two distinct structures, the inner and outer belt. The inner electron Van Allen Belt extends typically from an altitude of 1.2 to 3 Earth radii (L values of 1 to 3). In certain cases when solar activity is stronger or in geographical areas such as the South Atlantic Anomaly (SAA), the inner boundary may go down to roughly 200 kilometers above the Earth's surface. The inner belt contains high concentrations of electrons in the range of hundreds of keV and energetic protons with energies exceeding 100 MeV, trapped by the strong (relative to the outer belts) magnetic fields in the region.

It is believed that proton energies exceeding 50 MeV in the lower belts at lower altitudes are the result of the beta decay of neutrons created by cosmic ray collisions with nuclei of the upper atmosphere. The source of lower energy protons is believed to be proton diffusion due to changes in the magnetic field during geomagnetic storms.

Due to the slight offset of the belts from Earth's geometric center, the inner Van Allen belt makes its closest approach to the surface at the South Atlantic Anomaly.