Nightside Ionospheric Electrodynamics Associated with Substorms: PFISR and THEMIS ASI Observations

Abstract

Observations from the newly available Poker Flat Incoherent Scatter Radar have been used to study the ionospheric electrodynamics during the substorm expansion phase. Substorm events selected have been divided into three categories based on onset location relative to the radar, and repeatable features have been identified. West of onset, westward flows increased in response to the passage of the westward traveling surge (WTS) poleward of them. The enhanced flows occurred within the Sub-Auroral Polarization Streams region of downward Region 2 currents, which extend equatorward from near the equatorward boundary of electron precipitation. Strong E region ionization increase associated with the WTS occurred poleward of the enhanced westward flows, suggesting that poleward Pedersen currents fed by the downward Region 2 field-aligned current (FAC) further equatorward contribute at least partially to closure of the upward FAC of the WTS. An enhanced eastward flow channel developed east of onset. E region ionization decreased poleward of this flow peak, implying formation of downward FAC, and further increased equatorward of the flow peak, suggesting strengthening of preexisting Region 2 upward FAC. These observations indicate that the downward FAC formed at onset at least partially closes via the upward Region 2 currents. The enhanced eastward flows decreased after passage of the auroral bulge, and the reduced E region ionization increased again. When onset occurred within the radar fov, the thin breakup arc of onset occurred at the center of the Harang reversal, while if the optical onset was poleward of the radar fov, the Harang reversal was observed after the onset. A 2-D picture of the evolution of ionospheric electrodynamics was generated by synthesizing observations from the three categories, which provides a schematic description of the relationship between Region 2 and substorm expansion electrodynamics.