Van Allen Belts
- Zones of ionised particles surrounding the Earth. The Van Allen Belts (called after US scientist James Van Allen, born 1914) were among the first scientific discoveries of the space age, as they were found by the first US satellite, Explorer 1. There are two main Van Allen belts, one lying mainly 3000–6000km above sea level at the equator and the other 20,000–30,000km above sea level at the equator. The particles are trapped there by the Earth’s magnetic field, which brings them into the Earth’s upper atmosphere in the polar regions, where their interaction with molecules of air causes the aurora. The inner belt consists mainly of protons and the outer of electrons. Within the inner belt a layer of heavier cosmic ray nuclei was discovered in 1993.