On the location of the source for the energetic electron layer at the polar magnetopause and its relation to magnetic reconnection

Abstract

The existence of a layer of energetic (≳20 keV) electrons along the magnetopause, extending from the polar cusp region to the distant magnetotail, has been established by measurements on various spacecraft. We have performed a statistical study of electron flux and spectral measurements above 0.5 MeV observed in different regions of the magnetopause accessible to HEOS 2, with the aim of identifying the source for this magnetopause electron layer. The measurements along the polar magnetotail (i.e., adjacent to the plasma mantle), along the mid‐latitude dayside magnetopause (i.e., adjacent to the dayside plasma boundary layer), and on both sides of the magnetosheath/polar cusp interface (i.e., in the exterior polar cusp and in the entry layer) have been separately analyzed and are statistically compared. We conclude that escape of electrons from the magnetosphere can be excluded as a significant source for the tail magnetopause layer on the basis of the observed intensities, of the spectra, and of the frequency of appearance of electrons outside those magnetopause regions which lie adjacent to the outer trapping zone. The electron intensities in the exterior polar cusp region are found to be correlated with the north/south component of the interplanetary magnetic field (IMF). High intensity levels are observed when the IMF is directed southward (BZ <0), and low levels when the IMF points northward (BZ >0). The electron flux in the exterior cusp region at times of southward IMF and at the near‐earth polar magnetotail at times of northward IMF is well correlated with the number of field lines carried by the solar wind towards the magnetosphere (expressed by the product of solar wind speed and interplanetary field magnitude). We interpret this as strong evidence that acceleration processes associated with magnetic reconnection occur at the magnetopause at those times. Local acceleration near the high‐latitude magnetopause appears therefore to be a prime source for the magnetopause electron layer.