Abstract
Abstract. An auroral westward flow channel (AWFC) is a latitudinally narrow channel of unstable F-region plasma with intense westward drift in the dusk-to-midnight sector ionosphere. AWFCs tend to overlap the equatorward edge of the auroral oval, and their life cycle is often synchronised to that of substorms: they commence close to substorm expansion phase onset, intensify during the expansion phase, and then decay during the recovery phase. Here we define for the first time the relationship between an AWFC, large-scale field-aligned current (FAC), the ring current, and plasmapause location. The Tasman International Geospace Environment Radar (TIGER), a Southern Hemisphere HF SuperDARN radar, observed a jet-like AWFC during ~08:35 to 13:28 UT on 7 April 2001. The initiation of the AWFC was preceded by a band of equatorward expanding ionospheric scatter (BEES) which conveyed an intense poleward electric field through the inner plasma sheet. Unlike previous AWFCs, this event was not associated with a distinct substorm surge; rather it occurred during an interval of persistent, moderate magnetic activity characterised by AL~−200 nT. The four Cluster spacecraft had perigees within the dusk sector plasmasphere, and their trajectories were magnetically conjugate to the radar observations. The Waves of High frequency and Sounder for Probing Electron density by Relaxation (WHISPER) instruments on board Cluster were used to identify the plasmapause location. The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) EUV experiment also provided global-scale observations of the plasmapause. The Cluster fluxgate magnetometers (FGM) provided successive measurements specifying the relative location of the ring current and filamentary plasma sheet current. An analysis of Iridium spacecraft magnetometer measurements provided estimates of large-scale ionospheric FAC in relation to the AWFC evolution. Peak flows in the AWFC were located close to the peak of a Region 2 downward FAC, located just poleward of the plasmapause. DMSP satellite observations confirmed the AWFC was located equatorward of the nightside plasmasheet, sometimes associated with ~10 keV ion precipitation.
Highlights
The circulation of plasma at high latitudes is controlled by the solar wind-magnetosphere-ionosphere interaction, and subsequent internal reconfigurations of the magnetosphere, whereas the circulation of plasma in the mid-latitude ionosphere is approximately co-rotational
This study reports a comprehensive set of ground-based and satellite measurements to help provide insights into the relationship between the band of equatorward expanding ionospheric scatter (BEES), auroral westward flow channel (AWFC), plasmapause, and the Region 1 and 2 field-aligned current (FAC), and other phenomena
We summarise the main results as follows: 1. The Tasman International Geospace Environment Radar (TIGER) SuperDARN radar observed an AWFC during ∼08:35 to 13:28 UT in the dusk-to-midnight sector (∼19:00–23:00 MLT)
Summary
The circulation of plasma at high latitudes is controlled by the solar wind-magnetosphere-ionosphere interaction, and subsequent internal reconfigurations of the magnetosphere, whereas the circulation of plasma in the mid-latitude ionosphere is approximately co-rotational. There is often a sharp delineation between the equatorward limit of highlatitude magnetospheric convection and the approximately co-rotational mid-latitude regime This feature is known as the flow reversal boundary (FRB) (Huang et al, 2001), and should not be confused with the convection reversal. Defining the relationship between AWFCs and the main population of PJ/SAIDs and SAPS occurring further equatorward will require a latitudinal chain of sub-auroral radars making continuous observations throughout the entire substorm cycle. Defining this morphology is important because of its implications for the development of field-aligned current (FAC) and polarisation fields within the inner magnetosphere, and energisation of the radiation belts and the RC during major storms. The polar orbiting Defence Meteorological Satellite Program (DMSP) satellites placed the AWFC in the context of auroral oval boundaries
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