Auroral precipitation/ion upwelling as a driver of neutral density enhancement in the cusp

Soft electron precipitation in collaboration with ion upwelling is investigated as a possible driver of the cusp neutral density enhancement discovered by the CHAMP satellite. A time-dependent, three-fluid numerical model is used to simulate the vertical ionospheric and thermospheric response for this type of event. Particle data from the FAST satellite from a single favorable conjunction alignment event with CHAMP are input to the model. Results are given which suggest that altitude-dependent neutral density enhancements accompany ion upwelling that is driven by soft electron precipitation. The enhancement mechanism is summarized as follows. Sufficiently soft electron precipitation transfers energy to the ambient electron “gas”, which subsequently undergoes a thermal (upward) expansion. This establishes a vertical ambipolar electric field which pulls the ions upward. The momentum carried by up-flowing ions is significant and results show it to be capable of dragging neutral gas upward to create density structures above the F-region.

► CHAMP satellite has observed unexplained neutral density structures in the cusp region.
► Auroral precipitation/ion upwelling is examined as a driver of these density enhancements.
► Soft electron precipitation heats ambient electron gas which expands upwards.
► Ambipolar electric field draws ions upwards which in turn drag neutral gas.