What are the Causes of the Formation of the Sub-Alfvenic Flows at the High Latitude Magnetopause

We study magnetopause crossings made by the Interball Tail spacecraft at high latitudes under various interplanetary conditions. When the IMF mostly northward the Interball Tail observes quasi steady state reconnection signatures at the high latitude magnetopause, which include a well-defined de Hoffman-Teller frame, satisfaction of stress balance (Walen relations) and D-shaped ion velocity distributions. Under variable or southward IMF the high latitude magnetopause is a tangentional discontinuity. However, in certain conditions, just after the magnetopause crossing, irrespective of the IMF orientation, decelerate magnetosheath flows are observed in the magnetosheath region adjacent to the high latitude magnetopause. This leads to formation of the region where the sub-Alfvenic flow at high latitudes exists. We suggest that in some cases the dipole tilt plays an important role in the formation of the sub-Alfvenic flows, although in some cases formation the depletion layer is responsible for observation of the sub-Alfvenic flows at the high latitude magnetopause.