A survey of the anisotropy of the outer electron radiation belt during high-speed-stream-driven storms

Electron measurements on board six spacecraft in geosynchronous orbit are superposed-epoch analyzed for 42 high-speed-stream-driven storms. Using pitch angle-resolved fluxes in the range 30 keV to 1.7 MeV, the evolution of the outer electron radiation belt and the suprathermal tail of the electron plasma sheet are studied. The outer electron radiation belt exhibits perpendicular-dominated anisotropies on the dayside and parallel-dominated anisotropies on the nightside consistent with shell splitting in a distorted magnetosphere. The magnitudes of the radiation-belt anisotropies are weak prior to storm onset and become very large during the storms. The magnitudes of the anisotropies lessen with time as the storm ages and the radiation belt heats, probably owing to a weakening of the magnetic field distortion as the storm ages. When a calm before the storm occurs, the dayside radiation belt approaches isotropy, probably owing to pitch angle scattering in the outer plasmasphere that fills during the calm. If no calm before the storm occurs, the dayside radiation belt is strongly perpendicular dominated. The local-time pattern of anisotropy in storms is very different for the suprathermal tail of the electron plasma sheet, which tends to be perpendicular on the nightside and isotropic elsewhere. The magnitudes of the anisotropies of the suprathermal tail are a factor of similar to 10 weaker than the anisotropies of the outer electron radiation belt.