Multisatellite measurements of electron phase space density gradients in the Earth's inner and outer magnetosphere

Abstract

The variability of the intensity of energetic (MeV) electrons in the Earth's radiation belts has been a renewed area of interest in recent years, yet a major outstanding question is the origin of these particles. In the current study we attempt to differentiate between local stochastic heating and radial diffusion or transport by investigating whether the plasma sheet contains a sufficient population of energetic electrons to supply relativistic electron enhancements in the Earth's radiation belts by radial transport alone or whether one needs to invoke other more localized acceleration processes. We have ordered the equatorial/central plasma sheet electron data from several satellites (GPS (∼4 RE), LANL geosynchronous (GEO) (∼6.6 RE), POLAR (∼8 RE), and CLUSTER (∼18 RE)) in phase space density (PSD). By combining these measurements at a particular value of the first adiabatic invariant, we are able to examine the radial profile of PSD. We present a case study of data from three successive CLUSTER orbits centered on a small to moderate (DST = −40 nT) geomagnetic storm to investigate the storm dynamics of the electron radial phase space gradient in the magnetotail. Our results indicate a steep outward PSD gradient to near geosynchronous orbit (∼trapping boundary) followed by a fairly flat to moderately rising outward PSD gradient into the plasmasheet near 18 RE, with CLUSTER PSDs highly dependent on the vicinity of the current sheet. We conclude that for this case, there appears to be a sufficient source of electrons to account for the populations of electrons in the inner magnetosphere (i.e., a positive radial gradient). However, evidence of a local PSD peak located between our GPS and POLAR radial positions suggests we cannot rule out local acceleration mechanisms.