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

Abstract

[1] 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 ∼10 weaker than the anisotropies of the outer electron radiation belt.Citation: Borovsky, J. E., and M. H. Denton (2011), A survey of the anisotropy of the outer electron radiation belt during high-speed-stream-driven storms,

The proton and electron radiation belts at geosynchronous orbit: Statistics and behavior during high‐speed stream‐driven storms

“…In Figure (second panel) the superposed logarithmic averages of the number densities of the radiation belt protons (blue) and electrons (red) are plotted for the storms. The electron density shows (1) a decay before the storm, (2) a dropout near the onset of the storm, (3) a recovery of density early in the storm, and then (4) reaching a constant density, four evolutionary stages that are familiar from the studies of the electron radiation belt at geosynchronous orbit with SOPA [ Borovsky and Denton , , ]. In Figure (second panel) the superposed average of the density of radiation belt protons (blue) shows a constant density prior to storm onset and a shift to higher density during the storms.…”

“…As seen in Figures and , the radiation belt electrons during high‐speed stream‐driven storms show distinct density dropouts [cf. Freeman , ; Nagai , ; Onsager et al , ; Green et al , ; Borovsky and Denton , ; Morley et al , ] followed by density recoveries [ Borovsky and Denton , , , ; Denton et al , ]. Figures and will show that those electron density dropout and recovery features are much more abrupt than they appear in Figure .…”

“…In Figure (third panel) the superposed logarithmic averages of the radiation belt temperatures (spectral hardnesses) for protons and electrons are plotted. The electrons (red) show a constant temperature during the density decay before the storm, a drop in temperature when the radiation belt density recovers, and a slow heating during the duration of the storm, all signatures that are familiar from the studies of the electron radiation belt at geosynchronous orbit with SOPA [ Borovsky and Denton , , ]. The superposed average of the proton temperature shows an increase commencing before storm onset and a return to average values shortly after storm onset.…”

“…Further, the systems‐science coupling of the outer electron radiation belt to other plasma populations of the magnetosphere such as the plasma sheet and ring current [ Ebihara et al , ; Jordanova , ], the outer plasmasphere [ Borovsky and Steinberg , ; Borovsky and Denton , ], the plasmaspheric drainage plume [ Borovsky et al , ], substorm injection electrons [ Friedel et al , ], and waves driven by those populations such as ULF waves [ Ozeke et al , ], chorus [ Meredith et al , ; Summers et al , ], and electromagnetic ion cyclotron (EMIC) [ Ukhorskiy et al , ; Lazutin et al , ] has been considered; how the outer proton radiation belt fits into the coupled system has been less well considered. Of particular relevance for the present study, the evolution of the outer electron radiation belt through high‐speed stream‐driven (corotating interaction region (CIR)‐driven) storms has been repeatedly investigated [ Paulikas and Blake , ; Belian et al , ; Borovsky et al , ; Lam , ; Miyoshi and Kataoka , ; Kataoka and Miyoshi , ; Borovsky and Denton , , , , , ; McPherron et al , ; Denton et al , ], but the evolution of the outer proton radiation belt has not been studied.…”

Density and temperature of energetic electrons in the Earth's magnetotail derived from high-latitude GPS observations during the declining phase of the solar cycle

“…Fritz et al, 2003;Borovsky and Denton, 2011) the method of mapping GPS measurements described here is considered valid given numerous previous observations which indicate the electrons are quasiisotropic in the more distant magnetotail (e.g. Hones et al, 1968;Retzler and Simpson, 1969;Sarris et al, 2006).…”