Deriving the Ionospheric Electric Field from the Bulk Motion of Radar
Aurora in the E-region
Abstract
In the auroral E-region strong electric fields can create an environment
characterized by fast plasma drifts. These fields lead to strong Hall
currents which trigger small-scale plasma instabilities that evolve into
turbulence. Radio waves transmitted by radars are scattered off of this
turbulence, giving rise to the ‘radar aurora’. However, the Doppler
shift from the scattered signal does not describe the F-region plasma
flow, the ExB drift imposed by the magnetosphere. Instead, the radar
aurora Doppler shift is typically limited by nonlinear processes to not
exceed the local ion-acoustic speed of the E-region. This being stated,
recent advances in radar interferometry enable the tracking of the bulk
motion of the radar aurora, which can be quite different and is
typically larger than the motion inferred from the Doppler shift
retrieved from turbulence scatter. We argue that the bulk motion
inferred from the radar aurora tracks the motion of turbulent source
regions (provided by the aurora). This allows us to retrieve the
electric field responsible for the motion of field tubes involved with
auroral precipitation, since the precipitating electrons have to
ExB-drift. Through a number of case studies, as well as a statistical
analysis, we demonstrate that, as a result, the radar aurora bulk motion
is closely associated with the high-latitude convection electric field.
We conclude that, while still in need of further refinement, the method
of tracking structures in the radar aurora has the potential to provide
estimates of the ionospheric electric field that are consistent with
nature.