On charge neutrality in the cusp region

Following reconnection of the magnetic field lines in the cusp region, the magnetosheath electrons which travel along a reconnected magnetic flux tube to the ionosphere overtake the slower moving ions. The problem is analogous to the problem of charge neutrality for a bunch of accelerated electrons moving in the cold magnetospheric plasma. The charge neutrality condition leads to the formation of an oncoming flux of cold background electrons moving against the electron bunch. To accelerate the cold electrons, an electric double layer appears at the foreboundary of the electron bunch. At a rather large distance from the Earth, the density of the magnetosheath electrons injected into the magnetosphere is comparable with that of the background plasma. In this case, electrons can not separate from the ions, and the particles in this region propagate in a regime of ambipolar diffusion. This may explain impulsive enhancements of energetic ions observed in the cusp region preceding the appearance of 100 eV electrons. It also provides an alternative approach to the problem of cusp identification.