2. The Earth's trapped radiation environment

 

The resulting trajectories lie on toroidal surfaces, called drift shells, centred on the Earth's dipole centre. Particles confined to a drift shell can remain there for long periods, up to years for protons at altitudes of a few thousand kilometers, whence the term "trapped particles".

The population of charged particles stably trapped by the Earth's magnetic field consists mainly of protons with energies between 100 keV and several hundred MeV and electrons with energies between a few tens of keV and 10 MeV. There is also evidence for the existence of a narrow region centred around altitudes of about one Earth radius containing trapped heavy ions which are believed to be decelerated anomalous cosmic ray ions; the intensities of these heavy ions are several orders of magnitude below the intensities of trapped energetic protons in this region.

2.2 The trapped proton population

The energetic (above 10 MeV) trapped proton population is confined to altitudes below 20,000 km, while lower energy protons cover a wider region, with protons below 1 MeV reaching geosynchronous altitudes.

Figure 1 shows the distribution of trapped protons with energies above 10 MeV, as predicted by the NASA AP-8 MAX model (4), in invariant coordinate space.

Figure 1. Invariant coordinate map of the AP-8 MAX integral proton flux >10 MeV. The semi-circle represents the surface of the Earth, distances are expressed in Earth radii.

The region of space covered by higher energy protons diminishes with increasing energies and the location of the highest intensities moves inward.

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