For the low altitude region near the Earth (250-1000 km), where the human activity, both commercial as well as scientific, has greatly increased over the last decades, it is particularly important to accurately model the radiation environment. As we briefly noted in Chapter 1, measurements carried out since the 1950s have shown the existence of radiation belts, containing energetic particles trapped in the geomagnetic field, so-called Earth’s Radiation Belts (ERB). The radiation belts consist principally of electrons of up to a few MeV and of protons of up to several hundred MeV.