Statistical properties of tail plasma sheet electrons above 40 keV

Abstract

We use data from four tail seasons (2001-2004) of the Cluster spacecraft to study statistical features of energetic electrons (40-400 keV) in the magnetotail plasma sheet in combination with plasma parameters and indices of geomagnetic activity. We find that the horizontal magnetic field magnitude best orders the energetic electron observations. Using this method we observe a statistical flux gradient toward the neutral sheet. The fluxes also increase with an increase in the vertical magnetic field component (dipolarization). The intensity of supra-thermal electrons is observed to increase strongly as function of plasma temperature. Although the energetic electron fluxes also appear to increase strongly with geomagnetic activity this is found to be mainly due to the increase in plasma temperature with increasing Kp. Investigation of neutral sheet fluxes and phase space densities at fixed first adiabatic invariant (μ) indicates that adiabatic heating can explain observed flux increases during field dipolarization. The spectral slope in the supra-thermal range is observed to be independent of geomagnetic activity (Kp), while there is a significant local time dependence, with harder spectra observed at dawn compared to the dusk side. Copyright 2008 by the American Geophysical Union.