The Explorer I satellite was NASA's first scientific satellite launched on 31.01.58. It carried the radiation detection experiment designed by Dr. Van Allen and discovered the Van Allen Radiation Belt.
Trapped radiations belts: Protons and electrons trapped in the Van Allen belts represent an important contribution to radiation exposure in Low Earth Orbit – LEO, such as for astronauts on the ISS.
The rotation of the Earth's molten iron core creates electric currents that produce magnetic field lines around the Earth similar to those associated with an ordinary bar magnet. This magnetic field extends several thousand kilometres out from the surface of the Earth.
The interaction of the charged particles of the solar wind magnetic field forms a shock front around which the particles are deflected like water around the bow of a ship. The solar wind compresses and confines the magnetic field on the side toward the Sun and stretches it out into a long tail on the night side. The cavity formed by this process is called the "magnetosphere". This cavity shelters the surface of the Earth from constant bombardment by charged particles.
Not all of the particles are deflected by the magnetosphere, however, and some become trapped in the Earth's magnetic field. The particles are contained in one of two doughnut-shaped magnetic rings surrounding the Earth called the Van Allen radiation belts. The inner belt contains a fairly stable population of protons. The outer belt contains mainly electrons. The charged particles which compose the belts circulate along the Earth's magnetic lines of force. These lines of force extend from the area above the equator to the North pole, to the South Pole, and then circle back to the Equator.