Decelerated magnetosheath plasma flow at high latitudes behind the cusp region: Interball Tail observations

On May 25, 1996 the Interball Tail spacecraft was moving through the northern hemisphere of the high-latitude magnetosphere on its outbound trajectory. It successively crossed lobe field lines followed by the high latitude magnetopause, then entering the magnetosheath proper near the cusp region covering magnetic local time from 8h20m to 9h30m at magnetic latitudes of about 77O. IMF observed by WIND was northward during the time interval of interest and favorable for reconnection at high latitude magnetopause. The well-defined De Hoffmann Teller frame and stress balance indicate that the magnetopause was a rotational discontinuity with ongoing reconnection. After the magnetopause crossing, the spacecraft observed decelerated magnetosheath flow in the sub-Alfvénic regime. A gradually increasing of the flow velocity is observed and the plasma flow regime changed from sub-Alfvénic through Alfvénic to super-Alfvénic one. We explain these results by direct passing of the Interball Tail through the secondary stagnation point, which has been predicted by theoretical and semiempirical models of the high latitude magnetopause.