A transient narrow poleward extrusion from the diffuse aurora and the concurrent magnetotail activity

Abstract

We report observation of a transient narrow auroral feature extruding from the poleward boundary of the diffuse aurora on March 19, 2009. It moved westward and poleward initially to form part of a vortex pattern, followed by its equatorward‐dawnward retreat later. During this auroral activity, THEMIS satellites, projected near the same magnetic local time of the auroral feature, detected appreciable plasma flows, increase in the ratio of the ion energy over the electron energy, and some enhancements of electrostatic waves. The plasma flows were initially duskward‐earthward and changed to duskward‐tailward later. The overall development of the observed plasma flow pattern was detected during the equatorward‐dawnward retreat of the auroral feature when the Alfvén transit time between the magnetotail and the ionosphere is taken into account. This suggests that THEMIS satellites remotely sensed a counter‐clockwise flow vortex (viewed from above the equatorial plane) in the magnetotail with decreasing strength. We suggest that the process generating the auroral feature is related to the flow vortex in association with the depletion of the electron energy relative to the ion energy and wave‐particle interaction. An estimate of the possible associated current density is made. We provide reasoning for this auroral feature to be an auroral streamer and not a “failed” transpolar arc.