The Earth actually has two radiation belts of different origins. The inner ring is called the Van Allen belt after its discoverer James Van Allen of the University of Iowa. This atmospheric radiation was first detected by Geiger counters onboard the Explorer 1 satellite during the International Geophysical Year and was substantiated by data from later Explorer satellites.
The Van Allen belt extends above the equator at an altitude of about 4,000 miles (6437 kilometers). This belt is populated by very energetic protons in the 10-100 MeV range (a byproduct of collisions by cosmic rays with atoms of the atmosphere). The cosmic radiation has a rather low intensity (comparable to starlight) and only by accumulating particles over the span of years does the inner belt reach its high intensity. These particles can readily penetrate spacecraft and prolonged exposure can damage instruments and be a hazard to astronauts.
The space probes Pioneer 3 and 4 detected the outer radiation belt. It is nowadays seen as part of the plasma trapped in the magnetosphere. The name radiation belt is usually applied to the more energetic part of the plasma population (e.g., ions of about 1 MeV of energy). The more numerous lower-energy particles are known as the "ring current," since they carry the current responsible for magnetic storms. Most of the ring current energy resides in the ions (typically with 0.05 MeV), but energetic electrons can also be found.
In 1990, the satellite Combined Release and Radiation Effects Satellite (CRRES) discovered a third radiation belt located between the inner and outer Van Allen belts. Also, around May 8, 1998, there were a series of large, solar disturbances that caused a new radiation belt to form in the so-called “slot region” between the inner and outer van Allen belts. The new belt eventually disappeared once the solar activity subsided.