The east-west effect in solar proton flux measurements in geostationary orbit: A new GOES capability

[1] Since 1998, the GOES system has made eastward and westward observations of multi-MeV solar proton fluxes. The gyrocenters of the fluxes observed looking westward (eastward) lie outside (inside) geostationary orbit. Due to this “east-west effect,” eastward observations of 4.2–82 MeV protons vary with respect to their westward equivalents. At times of high solar wind dynamic pressure (Pdyn > 10 nPa), the “inside” and “outside” fluxes are approximately equal. As Pdyn decreases to ∼1 nPa and the ring current decreases, the “inside” fluxes decrease as much as an order of magnitude with respect to the “outside” fluxes. Under low Pdyn, the “inside” fluxes exhibit short-lived (1–3 hr) increases, sometimes to the levels of the “outside” fluxes, during periods of enhanced AE index activity. This association suggests that magnetotail topologies associated with substorms enhance the access of solar protons to lower L shells under low Pdyn.