ISEE observations of low‐latitude boundary layer for northward interplanetary magnetic field: Implications for cusp reconnection

Abstract

We present a study of the formation of the low‐latitude boundary layer for northward interplanetary magnetic field that provides evidence for the reconnection poleward of the region of the cusp. Velocity distributions as observed by the joint Los Alamos/Max‐Planck‐Institut (MPE) Garching Fast Plasma Experiment on the ISEE spacecraft reveal two types of low‐latitude boundary layer plasma: heated magnetosheath plasma with little or no hot magnetospheric component in the outer boundary layer and a mixture of magnetosheath and magnetospheric plasmas in the inner boundary layer. The observed plasma characteristics can be explained 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 formed by reconnection between magnetosheath and lobe field lines poleward of one cusp. It is identified to be on open field lines with one end in the ionosphere and the other one in the solar wind. The magnetosheath plasma entering the outer boundary layer is heated and accelerated at the cusp reconnection site and is then transported to the low latitudes along the magnetic field. The hot plasma from the magnetosphere is severely depleted but is not necessarily completely absent since they can escape to the magnetosheath on open field lines, with a finite time being required for total depletion to occur. The inner boundary layer 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 second cusp. The inner boundary layer contains a mixture of magnetosheath and magnetospheric plasmas since the hot magnetospheric plasma can drift onto these closed field lines, even if the hot plasma was completely absent when the field line was open. Thus, when the interplanetary field is strongly northward, two boundary layers are formed, one on open field lines and one on closed field lines, by the sequential action of reconnection beyond the polar cusp.