EISCAT OBSERVATIONS OF UNUSUAL FLOWS IN THE MORNING SECTOR ASSOCIATEDWITH WEAK SUBSTORM ACTIVITY

A discussion is given of plasma flows in the dawn and nightside high-l atitude ionospheric regions during substorms occurring on a contracted auroral oval, as observed using the EISCAT CP-4-A experiment. Support ing data from the PACE radar, Greenland magnetometer chain, SAMNET mag netometers and geostationary satellites are compared to the EISCAT obs ervations. On 4 October 1989 a weak substorm with initial expansion ph ase onset signatures at 0030 UT, resulted in the convection reversal b oundary observed by EISCAT (at approximately 0415 MLT) contracting rap idly poleward, causing a band of elevated ionospheric ion temperatures and a localised plasma density depletion. This polar cap contraction event is shown to be associated with various substorm signatures; Pi2 pulsations at mid-latitudes, magnetic bays in the mid-night sector and particle injections at geosynchronous orbit. A similar event was obse rved on the following day around 0230 UT (approximately 0515 MLT) with the unusual and significant difference that two convection reversals were observed, both contracting poleward. We show that this feature is not an ionospheric signature of two active reconnection neutral lines as predicted by the near-Earth neutral model before the plasmoid is ' 'pinched off'', and present two alternative explanations in terms of ( 1) viscous and lobe circulation cells and (2) polar cap contraction du ring northward IMF. The voltage associated with the anti-sunward flow between the reversals reaches a maximum of 13 kV during the substorm e xpansion phase. This suggests it to be associated with the polar cap c ontraction and caused by the reconnection of open flux in the geomagne tic tail which has mimicked ''viscous-like'' momentum transfer across the magnetopause.