Combined CUTLASS, EISCAT and ESR observations of ionospheric plasma flows at the onset of an isolated substorm

On August 21st 1998, a sharp southward turning of the IMF, following on fro m a 20 h period of northward directed magnetic field, resulted in an isolat ed substorm over northern Scandinavia and Svalbard. A combination of high t ime resolution and large spatial scale measurements from an array of cohere nt scatter and incoherent scatter ionospheric radars, ground magnetometers and the Polar UVI imager has allowed the electrodynamics of the impulsive s ubstorm electrojet region during its first few minutes of evolution at the expansion phase onset to be studied in great detail. At the expansion phase onset the substorm onset region is characterised by a strong enhancement o f the electron temperature and UV aurora. This poleward expanding auroral s tructure moves initially at 0.9 km s(-1) poleward, finally reaching a latit ude of 72.5 degrees. The optical signature expands rapidly westwards at sim ilar to 6 km s(-1), whilst the eastward edge also expands eastward at simil ar to 0.6 km s(-1) Typical flows of 600 m s(-1) and conductances of 2 S wer e measured before the auroral activation, which rapidly changed to similar to 100 m s(-1) and 10-20 S respectively at activation. The initial flow res ponse to the substorm expansion phase onset is a flow suppression, observed up to some 300 km poleward of the initial region of auroral luminosity, im posed over a time scale of less than 10 s. The high conductivity region of the electrojet acts as an obstacle to the flow, resulting in a region of lo w-electric held, but also low conductivity poleward of the high-conductivit y region. Rapid flows are observed at the edge of the high-conductivity reg ion, and subsequently the high how region develops, flowing around the expa nding auroral feature in a direction determined by the flow pattern prevail ing before the substorm intensification. The enhanced electron temperatures associated with the substorm-disturbed region extended some 2 degrees furt her poleward than the UV auroral signature associated with it.