ENTRY OF THE POLAR SPACECRAFT INTO THE POLAR CUSP UNDER NORTHWARD IMFCONDITIONS

On May 29, 1996 from 0200 to 0800 UT the solar wind dynamic pressure w as high ranging from 6 to 8 nPa and the interplanetary field was almos t due northward, ranging from 10 to 15 nT in BZ GSM. Even at apogee th e POLAR spacecraft should not have entered the magnetosheath according to recent scaling laws. However, the magnetic field was greatly depre ssed below the value expected indicating the presence of significant p lasma energy density throughout the high latitude magnetosphere surrou nding the cusp. The presence of this plasma is confirmed by the plasma instrumentation on board the spacecraft. While the entry into this ne arly stagnant plasma was gradual, the exit on to polar cap field lines was abrupt. We interpret these observations in terms of the post-cusp reconnection of the strongly northward interplanetary magnetic field.