Recent cusp and cleft results from interball Original Research Article

The Interball project has given important contributions to our understanding of the morphology and the physical processes in the cusp and cleft. Interball Tail and Magion-4 have performed more extensive measurements in the high altitude cusp than any previous spacecraft. Interball has also been a part in the ISTP program and data have been used in many multipoint studies. In this paper recent cusp and cleft studies based entirely or partly on Interball data will be reviewed. Interball data show that processes at high latitudes are very important for plasma entry into the magnetosphere. A case study for southward IMF conditions agrees with a model in which the mantle is populated via entry along open high-latitude field lines. A statistical study of events dominated by IMF By shows that merging in anti-parallel fields, rather than subsolar point reconnection, populates the mantle. Plasma entry also takes place through the turbulent boundary layer, TBL, a region of strong, Alfvenic ULF turbulence above the cusp and cleft. The TBL is almost always present. It extends tailward from the cusp and is proposed to be related to the magnetospheric sash. For the overall magnetosheath plasma entry into the magnetosphere the magnetotail boundary is probably more important than the cusp. The position of the cusp is controlled by the solar wind in a similar way as the low altitude cusp. The mid-altitude cusp was found to maintain its fine structure over periods of the order of one hour. A suprathermal proton population not previously described has been detected in the mid-altitude cusp.