Characteristics of the exterior cusp for steady southward interplanetary magnetic field: Interball observations

Abstract

High‐latitude observations of plasma flow across the magnetopause are made by the Interball‐Tail satellite near the polar cusp for steady southward and dawnward interplanetary magnetic field. Three‐dimensional proton distribution functions measured in this region contain the features predicted by the existing reconnection models. D‐shaped distributions in magnetospheric parts of the field lines with cutoff near zero velocity parallel to the magnetic field and tablet‐shaped distributions tailward from the cusp are observed and can be interpreted in terms of a gradually diminishing ion injection. Besides these predicted signatures, a new type of proton distribution with large pitch angles is also observed. We present observations of suprathermal ions and electrons with peaks at ∼ 90° pitch angles on the high‐latitude magnetospheric field lines adjacent to the magnetopause. These particles show a sharp change in fluxes on the magnetospheric side, indicating that they are probably quasi‐trapped in the cusp field. We also present the results of a simulation that uses the Toffoletto‐Hill model of the open magnetosphere to trace the particle transport from the magnetosheath to magnetospheric locations of Interball. The simulation successfully reproduces the main predicted features of distribution functions as well as distributions peaked at large pitch angles. The results show that for steady southward interplanetary magnetic field with a large dawnward component, the entry of magnetosheath plasma into the magnetosphere can be described in terms of open topology of field lines near the high‐altitude cusp.