Acceleration of >40 keV Electrons in Near-Earth Magnetotail Reconnection Events

Abstract

Reconnection of magnetic field lines have been invoked as an acceleration mechanism producing significant amounts of super-thermal electrons in the high energy range. However, in a recent paper by Åsnes et al., 2008, energetic particle generation during geomagnetic active times was shown to be mainly caused by plasma sheet heating rather than reconnection driven acceleration. To examine this discrepancy we present observations from an ensemble of near-Earth reconnection events observed by the Cluster spacecraft near apogee in the years 2001–2004, and compare electron fluxes with values obtained during the surrounding time intervals and statistical results obtained in the same region in the plasma sheet. We find that observations in the proximity of the X-line only sometimes yield high fluxes of energetic electrons. The maximum flux level is always observed near the neutral sheet, and typically occurs when the distribution is near Maxwellian. It appears that although reconnection immediately heats the cold inflowing plasma, this acceleration is typically only sufficient to bring the electron fluxes up to a level approximate to the pre-existing plasma sheet levels.