On energetic electrons (>38 keV) in the central plasma sheet: Data analysis and modeling

Abstract

The spatial distribution of >38 keV electron fluxes in the central plasma sheet (CPS) and the statistical relationship between the CPS electron fluxes and the upstream solar wind and interplanetary magnetic field (IMF) parameters are investigated quantitatively using measurements from the Geotail satellite (1998-2004) at geocentric radial distances of 9-30 RE in the night side. The measured electron fluxes increase with closer distance to the center of the neutral sheet, and exhibit clear dawn-dusk asymmetry, with the lowest fluxes at the dusk side and increasing toward the dawn side. The asymmetry persists along the Earth's magnetotail region (at least to Geotail's apogee of 30 RE during the period of interest). Both the individual case and the statistical analysis on the relationship between >38 keV CPS electron fluxes and solar wind and IMF properties show that larger (smaller) solar wind speed and southward (northward) IMF B(z) imposed on the magnetopause result in higher (lower) energetic electron fluxes in the CPS with a time delay of about 1 hour, while the influence of solar wind ion density on the energetic electrons fluxes is insignificant. Interestingly, the energetic electron fluxes at a given radial distance correlate better with IMF B(z) than with the solar wind speed. Based on these statistical analyses, an empirical model is developed for the first time to describe the 2-D distribution (along and across the Earth's magnetotail) of the energetic electron fluxes (>38 keV) in the CPS, as a function of the upstream solar wind and IMF parameters. The model reproduces the observed energetic electron fluxes well, with a correlation coefficient R equal to 0.86. Taking advantage of the time delay, full spatial distribution of energetic electron fluxes in the CPS can be predicted about 2 hours in advance using the real-time solar wind and IMF measurements at the L1 point: 1 hour for the solar wind to propagate to the magnetopause and a 1 hour delay for the best correlation. Such a prediction helps us to determine whether there are enough electrons in the CPS available to be transported inward to enhance the outer electron radiation belt.