... 12a. Inner Belt . ... The inner radiation belt, discovered by Explorers 1 and 3, owes its existence to the extraordinary stability of trapped orbits near the Earth. ... If these are electrically charged, e.g. electrons or ions, they will often end up trapped by the Earth's magnetic field. ... The energetic protons which then materialize are grabbed by the Earth's magnetic field, often on trapped orbits which do not return to the atmosphere, in which the proton can stay trapped for a rather long time...