I.1.4 The South Atlantic Anomaly

I.1.4.1. Overview

The South Atlantic Anomaly (see clause 9.2.1) produces an “island” of radiation and provides the only significant radiation encountered on low Earth orbits with altitudes below about 800 km and inclinations below about 40°.Figure I-3 shows the South Atlantic Anomaly at 400 km.

I.1.4.2. Anisotropy (the “EastWest effect”)

Because of the inclination of geomagnetic fieldlines with respect to the atmosphere here, particles reaching a point from the West have gyrated from higher altitude while those arriving from the East have gyrated from lower altitude. There are fewer coming from below because of atmospheric absorption and therefore an asymmetry in the fluxes results. This can be important in certain cases, including the International Space Station. The current standard AP-8 model does not treat this effect but models have been developed by NASA [RD.63] and BIRA [RD.64]. The ratio of the East and West peak fluxes is about 4,6 for 100 MeV protons in an ISS type orbit. Measurements from MIR are also available which are consistent with this ratio [RD.65].

I.1.4.3. Location of the South Atlantic Anomaly

The slow movement of the South Atlantic Anomaly as a result of shifts in the geomagnetic field has been clearly observed. This shift is essentially westward at a rate of 0,3°yr^-1 (~10° since the models were created) and account is taken of this figure for low Earth orbits when planning operations which involve a sensitivity to radiation (payload radiation background, astronaut EVA). Models including this shift capability are available [RD.14].