Without magnetosphere the planet would be directly affected by energetic particles from space. However, there is a place on Earth poorly protected where the magnetic field has weakened considerably. This area is located off the coast of Brazil, it is called the South Atlantic Anomaly or SAA, it covers 7.8 million km2 (˜ 2780 km by 2780 km) and it continues to grow.
Geographic distribution of the geomagnetic field intensity. The SAA boundary is around B=28000 nT. The triangle indicates the SAA central region (Niterói-Brazil is near this region), where the geomagnetic field intensity is the lowest. The upper circle is the point of Chacaltaya-Bolivia and the lower point is Auger-Malargue-Argentina.
Geographical distribution of proton flux measured by the HEPAD ICARE instrument on-board the Argentinean satellite SAC-C shows an excess (up to 10 times) of protons with E>850 MeV in the SAA central region in comparison with the region outside of the SAA. These high energy protons are hard to be considered as Van Allen trapped protons, because the SAA models such as AP8 and several measurements of the trapped protons showed that their energies do not exceed 300 MeV.
The PAMELA collaboration. introduced a sub-cutoff in the rigidity, that is below the nominal Stormer rigidity cutoff in the SAA area. Downward going protons with energies above 200 MeV are also clearly seen at the latitudes between 400 S and 200 S (the SAA area). These are so-called ‘quasi-trapped particles’.