Particle acceleration and wave emissions associated with the formation of auroral cavities and enhancements

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Abstract
Observations from DE 1 and electrostatic particle simulations are combined in an effort to provide a unified model for (nightside) auroral particle acceleration and wave emissions and their association with plasma cavities and enhancements. The observations show that enhanced electron precipitation during inverted‐V events is associated with broadband electrostatic bursts (BEB), increased upward field‐aligned currents, and density enhancements. These regions are flanked by return current regions where the density is depleted (i.e., by plasma cavities). Perpendicular acceleration of ambient plasma ions can occur in both upward and return current regions. It is shown through the simulations that these processes are integrally related and are not independent of each other. The free energy for the auroral particle acceleration can be provided by energetic ion beams in the plasma sheet boundary layer with nonzero perpendicular energy. The perpendicular energy allows charge separation between the beam ions and costreaming electrons to occur. The resultant space charge fields accelerate electrons on the same field lines as the costreaming electrons downward toward the ionosphere, without the beam ions actually propagating down to auroral altitudes. Ambient plasma electrons on adjacent field lines are accelerated upward, forming a return current. Because these currents are spatially separate, a perpendicular electrostatic shock develops which accelerates the plasma ions across the field lines in an effort to close the currents. This acceleration creates ion conics in velocity space and, in coordinate space, plasma cavities are formed in the return current regions and plasma enhancements in the enhanced positive current regions. Strong broadband electrostatic waves are generated with spectral maxima being generated near the local electron plasma frequency by the accelerated electrons and near the lower hybrid frequency by the accelerated ions, similar to that observed in BEB.