I.4 Geomagnetic shielding

The Earth’s magnetic field partially shields nearEarth space from solar energetic particles and cosmic rays, an effect known as geomagnetic shielding. However, these particles can easily reach polar regions and high altitudes such as the geostationary orbit. Geomagnetic shielding of protons is computed on the basis of the trajectory in geomagnetic B, L space (see Clause 5).

Stassinopoulos and King [RD.80] developed a model which has total cutoff at L = 5. It assumes that no protons can penetrate to lower values. It can be shown that this model corresponds to a quiet magnetosphere vertical cutoff model excluding protons of E < 200 MeV from L < 5 Earthradii. This model is adequate for most cases. However, in reality protons of lower energy can penetrate below L = 5 with nonvertical arrival directions, especially in a disturbed magnetosphere where the geomagnetic shielding is weakened. For westward arrival at the L = 5 geomagnetic equator in a disturbed magnetosphere, the energy cutoff can be as low 30 MeV.

For engineering purposes, geomagnetic cutoff is usually not applied to orbits spending more than 50 % of the orbit period above L = 5. Geomagnetic cutoff is usually applied to orbits spending more than 75 % of their time below L = 5.