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Super Dual Auroral Radar Network (DARN) radars oftendetect a distinct transition in line-of-sight Doppler velocity spread, orspectral width, from <50ms–1 at lower latitude to >200ms–1 at higher latitude. They also detect a similar boundary, namely the range atwhich ionospheric scatter with large spectral width suddenly commences(i.e. without preceding scatter with low spectral width). The location andbehaviour of the spectral width boundary (SWB) (and scatter boundary) andthe open-closed magnetic field line boundary (OCB) are thought to be closelyrelated. The location of the nightside OCB can be inferred from the polewardedge of the auroral oval determined using energy spectra of precipitatingparticles measured on board Defence Meteorology Satellite Program (DMSP)satellites. Observations made with the Halley SuperDARN radar (75.5° S,26.6° W, geographic; –62.0°Λ) and the Tasman InternationalGeospace Environment Radar (TIGER) (43.4° S, 147.2° E; –54.5°Λ)are used to compare the location of the SWB with theDMSP-inferred OCB during 08:00 to 22:00 UT on 1 April 2000. This study interval waschosen because it includes several moderate substorms, whilst the Halleyradar provided almost continuous high-time resolution measurements of thedayside SWB location and shape, and TIGER provided the same in the nightsideionosphere. The behaviour of the day- and nightside SWB can be understoodin terms of the expanding/contracting polar cap model of high-latitudeconvection change, and the behaviour of the nightside SWB can also beorganised according to substorm phase. Previous comparisons with DMSP OCBshave proven that the radar SWB is often a reasonable proxy for the OCB from duskto just past midnight (Chisham et al., 2004). However, the present casestudy actually suggests that the nightside SWB is often a better proxy for thepoleward edge of Pedersen conductance enhanced by hot particle precipitationin the auroral zone. Simple modeling implies that the large spectral widths mustbe caused by ~10-km scale velocity fluctuations. Key words. Ionosphere (auroral ionosphere; ionospheremagnetosphereinteractions) – Magnetospheric physics(storms and substorms

Magnetic local time, substorm, and particle precipitation-related variations in the behaviour of SuperDARN Doppler spectral widths