ISEE OBSERVATIONS OF LOW-LATITUDE BOUNDARY-LAYER FOR NORTHWARD INTERPLANETARY MAGNETIC-FIELD - IMPLICATIONS FOR CUSP RECONNECTION

We present a study of the formation of the low-latitude boundary layer for northward interplanetary magnetic field that provides evidence fo r the reconnection poleward of the region of the cusp. Velocity distri butions as observed by the joint Los AIamos/Max-Planck-Institut (MPE) Garching Fast Plasma Experiment on the ISEE spacecraft reveal two type s of low-latitude boundary layer plasma: heated magnetosheath plasma w ith little or no hot magnetospheric component in the outer boundary la yer and a mixture of magnetosheath and magnetospheric plasmas in the i nner boundary layer. The observed plasma characteristics can be explai ned by the process of magnetic reconnection poleward of the region of the cusp. The outer boundary layer that contains heated magnetosheath plasma and little or no hot magnetospheric component appears to be for med by reconnection between magnetosheath and. lobe field lines polewa rd of one cusp. It is identified to be on open field lines with one en d in the ionosphere and the other one in the solar wind. The magnetosh eath plasma entering the outer boundary layer is heated and accelerate d at the cusp reconnection site and is then transported to the low lat itudes along the magnetic field. The hot plasma from the magnetosphere is severely depleted but is not necessarily completely absent since t hey can escape to the magnetosheath on open field lines with a finite time being required for total depletion to occur. The inner boundary l ayer is identified to be on closed field lines that have become closed by reconnection of the open end of the flux tube poleward of the seco nd cusp. The inner boundary layer contains a mixture of magnetosheath and magnetospheric plasmas since the hot magnetospheric plasma can dri ft on to these closed field lines even if the hot plasma was completel y absent when the field line was open. Thus when the interplanetary fi eld is strongly northward, two boundary layers are formed, one on open field lines and one on closed field lines, by the sequential action o f reconnection beyond the polar cusp.