Statistical analysis of pitch angle distribution of radiation belt energetic electrons near the geostationary orbit: CRRES observations

A statistical analysis of energetic radiation belt electron pitch angle distributions (PADs) at the radial distances of 6 R-E and 6.6 R-E is performed on the basis of the pitch angle resolved flux observations from the Medium Electrons A (MEA) instrument onboard the Combined Release and Radiation Effects Satellite (CRRES). While previous studies of Vampola (1998) and Gannon et al. (2007) have used CRRES MEA data to investigate the general variations in electron PAD at particular energies, in this study we present a detailed statistical analysis of electron PADs including the dependence on electron kinetic energy, magnetic local time (MLT), and the level of geomagnetic activity. By fitting the measured PADs with a power law function of sine of local pitch angle, the power law index n that relates to the category of radiation belt electron PAD is quantified in detail as a function of electron kinetic energy, MLT interval, and geomagnetic index Kp. Statistical averaged n values vary considerably with respect to MLT, ranging from n similar to 0 within 0000-0400 MLT to n similar to 1.5 within 1200-1600 MLT, because of the MLT dependence of wave scattering and the effects associated with drift shell splitting and magnetopause shadowing. Drift shell splitting and magnetopause shadowing result in often observed negative values of n. At lower energies of a few hundred keV the pitch angle distributions are more flat than at MeV energies, which is consistent with faster pitch angle scattering at low energies by chorus waves. These quantitative results of radiation belt electron PAD, consistent with the previous studies by Vampola (1998) and Gannon et al. (2007), provide further insight into the global dynamics of energetic radiation belt electrons near the geostationary orbit and also are useful for inferring electron phase space densities and assimilating their radial profiles using omnidirectional electron flux measurements.