Abstract
High-time resolution CUTLASS observations and ground-based magnetometers have been employed to study the occurrence of vortical flow structures propagating through the high-latitude ionosphere during magnetospheric substorms. Fast-moving flow vortices (~800 m s-1) associated with Hall currents flowing around upward directed field-aligned currents are frequently observed propagating at high speed (~1 km s-1) azimuthally away from the region of the ionosphere associated with the location of the substorm expansion phase onset. Furthermore, a statistical analysis drawn from over 1000 h of high-time resolution, nightside radar data has enabled the characterisation of the bulk properties of these vortical flow systems. Their occurrence with respect to substorm phase has been investigated and a possible generation mechanism has been suggested.Key words: Ionosphere (auroral ionosphere; electric fields and currents) · Magnetospheric physics (storms and substorms)
Highlights
The study of highly time-dependent ionospheric electric ®elds that arise as a result of the substorm process and their coupling with magnetospheric source regions remains an area of lively discussion and debate
Ionospheric convection velocities are provided by CUTLASS, which comprises a pair of High Frequency (HF) coherent radars located at Hankasalmi, Finland and 3ykkvibñr in Iceland and form part of the international SuperDARN chain (Greenwald et al, 1995)
Substorm expansion phase onset/intensi®cations described in the text are indicated with dashed vertical lines. a X components; b Y components magnetic ®eld observed in the high latitude Scandinavian sector
Summary
The study of highly time-dependent ionospheric electric ®elds that arise as a result of the substorm process and their coupling with magnetospheric source regions remains an area of lively discussion and debate. By incorporating the ground magnetic perturbations observed by an extended magnetometer network, the transient features were interpreted as being due to ionospheric current vortices associated with ®eld aligned current pairs propagating azimuthally through the ®eld-of-view of an almost meridionally aligned beam. Both coherent and incoherent scatter radars have been employed previously to study similar propagating vortical ionosphericows such as auroral W-bands in the substorm recovery phase (LuÈ hr and Schlegel, 1994; Wild et al, 1999) and dayside travelling convection vortex events (LuÈ hr et al, 1996) the relatively short lifetimes of the substorm expansion phase phenomena observed by Yeoman and LuÈ hr (1997) required high-time resolution observations. By performing a statistical study of all available CUTLASS high-time resolution data the relationship between APVC occurrence and substorm phase is examined
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