Large parallel electric fields, currents, and density cavities in dispersive Alfvén waves above the aurora

Abstract

Large parallel electric fields (E∥ > 10 mV/m) are sometimes observed with intense filamentary field-aligned currents (J ∥ > 10 μA/m2 mapped to 100-km altitude) on transverse scales where ΚχC/ωpe ≈ 1 above the auroral oval. The ratio of the transverse electric (Eχ) and magnetic field (Bγ) variations is approximately the local Alfvén speed and increases with decreasing transverse scale in a manner consistent with inertial Alfvén wave dispersion. These fields are typically observed in association with density cavities. Statistically, it is shown that the depth of the cavities observed over the altitude range from 350 to 4175 km by the Fast Auroral Snapshot (FAST) satellite typically lies between 30 and 50% of the background plasma with 90% depletions being observed for 1% of the recorded events. The wave currents embedded within these cavities exceed 100 μA/m2 for 2% of the ensemble and are typically found over transverse widths of ∼2πλe Using a fluid description of the plasma, we show statistically that electron pressure gradients provide a probable means for balancing the large Epar; found in these structures with a likely but unknown contribution from anomalous resistivity. Furthermore, we find that the ratio of the average drift speed of the bulk electron plasma (vd) to the electron thermal speed (ve) inside these structures is vd/ve ≥ 0.1 and that in at least 2% of cases veh05d/ve ≥ 1.0. On the basis of recent simulation results and the Bohm criterion for double layer formation, these statistics indicate that the large E∥ sometimes observed in density cavities may occur in double layers driven by the Alfvén wave current. Copyright 2007 by the American Geophysical Union.