Trapping of charged particles, especially fast electrons, within the geomagnetic field has been examined. Since the lines of the geomagnetic field converge towards the earth they act as magnetic mirrors in which fast electrons can oscillate back and forth. If the reflection points are in the upper atmosphere the electrons are scattered by the air molecules. Since the atmospheric density decreases exponentially outwards, this scattering occurs more or less near the mirror points. Due to this scattering process the mirror points move downwards. In a steady state, where particles are injected at constant rate, the density of the electrons below the altitude of injection varies inversely proportional to the air density. Lifetimes of several days to several years are possible depending on the location of the mirror points. Relativistic electrons might be artificially injected by a satellite carrying a low-current electron accelerator.
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