The Van Allen radiation belts are regions of energetic charged particles trapped around Earth by its magnetic field. Particle motion combines gyration around a field line, repeated magnetic reflection between hemispheres and slower drift around the planet. The inner belt is especially associated with energetic protons, while energetic electrons are prominent in the outer belt; their boundaries and intensities vary.
The discovery grew from cosmic-ray detector measurements aboard Explorer 1 and follow-up observations by Explorer 3 in 1958. A Geiger counter sometimes returned unexpectedly low counts because intense radiation saturated the detector, rather than because particles were absent. Repeated orbital observations established a trapped population associated with Earth's magnetosphere.[ref]
About Explorer 1 by NASA. Cosmic-ray experiment, Explorer 1/3 observations and trapped radiation; JPL mission account, detector saturation
A magnetic field can redirect a particle without doing work on it, so trapping alone does not explain where its energy came from. Measuring particle energy spectra together with waves and fields is necessary to investigate acceleration and losses. This distinction matters both for space-plasma physics and for the radiation exposure of spacecraft electronics.
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The Van Allen radiation belts are two layers of charged particles held in place by Earth's magnetic field. Named after American physicist James Van Allen, who discovered them in 1958 using data from the first successful U.S. satellite, Explorer 1, these belts comprise high-energy electrons and protons, primarily originating from the solar wind and cosmic rays.