Signatures of the nightside open–closed magnetic field-line boundary during moderately disturbed conditions and ionospheric substorms

The comparatively low latitude of the Tasman International Geospace Environment Radar (TIGER) (147.2°E, 43.4°S, geographic; −54.6°Λ), a Southern Hemisphere HF SuperDARN radar, facilitates the observation of extensive backscatter from decametre-scale irregularities drifting in the auroral and polar cap ionosphere in the midnight sector. The radar often detects a persistent, sharp increase over ~90 km of range in line-of-sight Doppler velocity spread, or spectral width, from <50 m s−1 at lower latitude to >200 m s−1 at higher latitude. It was previously shown that for moderately disturbed conditions in the pre-midnight sector, the location of the spectral width boundary (SWB) corresponds to the poleward edge of the auroral oval, as determined from energy spectra of precipitating particles measured on board Defense Meteorology Satellite Program satellites. This implies the radar SWB is a proxy for the open–closed magnetic field-line boundary (OCB) under these particular conditions. Here we investigate whether the radar SWB is aligned with the satellite OCB under a broader range of geomagnetic conditions including small to moderate substorms occurring in the pre- and post-magnetic midnight sectors. The behaviour of the SWB can be reconciled with the spatial and temporal variations of energetic particle precipitation throughout the substorm cycle.