Van Allen Radiation Belt
The Van Allen Radiation Belt is a torus of energetic charged particles around Earth, trapped by Earth's magnetic field. When the belts "overload", particles strike the upper atmosphere and fluoresce, causing the polar aurora. The presence of a radiation belt had been theorized prior to the Space Age and the belt's presence was confirmed by the Explorer I on January 31, 1958 and Explorer III missions, under Doctor James van Allen. The trapped radiation was first mapped out by Explorer IV and Pioneer III.
Qualitatively, it is very useful to view this belt as consisting of two belts around Earth, the inner radiation belt and the outer radiation belt. The particles are distributed such that the inner belt consists mostly of protons while the outer belt consists mostly of electrons. Within these belts are particles capable of penetrating ~1g/cm2 of shielding (1 millimetre of lead).
The term Van Allen Belts refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planets. The Sun does not support long-term radiation belts. The atmosphere limits the belts' particles to regions above 200-1000 km, while the belts do not extend past 7 RE. The belts are confined to an area which extends about 65° from the celestial equator.