Polar‐Interball coordinated observations of plasma and magnetic field characteristics in the regions of the northern and southern distant cusps

Abstract

The structure of the high‐latitude magnetosphere near the polar cusps is studied by using coordinated observations from the Polar and Interball satellites. While Polar surveyed the northern polar cusp region, Interball sampled the high‐altitude region in the vicinity of the southern cusp in similar local times. A “magnetic turbulent region” that is distinct from the magnetosphere and magnetosheath and consists of recurrent small‐scale events is observed in both hemispheres. The small‐scale events often contain a steep ramp followed by a wave train. The events are characterized by a steep increase in the ion energy followed by dispersed velocity filter features. Kinetic and fluid characteristics of ions and a test of the Walen relation show that the events are associated with reconnection processes. S‐shaped polarization of the magnetic field variations is a characteristic feature in many cases indicating the existence of intermediate shocks. The change in the low‐energy cutoff of injected ions indicates that the reconnection site is about a few Earth radii from Polar. Reflected ions on the reconnected magnetospheric field lines, which are not predicted by simple reconnection scenarios, are also observed. Coexistence of transmitted and reflected ions gives rise to a “stagnant” plasma on the magnetospheric field lines. Characteristics of ions and electrons strongly support the suggestion that the field lines threading the plasma in these regions, which extend from the magnetosheath to the magnetosphere, are connected, at least at one end, with the Earth. In other words, the boundary layer is located, at least partly, on reconnected field lines. In contrast, a turbulent region in the indentation of the cusps has been thought to be in the magnetosheath and not to be connected with the Earth. Although the events observed by both satellites have a certain similarity, the probability of conjugated events being recorded by both satellites is small. It is believed that the visible similarity between the observations from the two hemispheres is due to structures that are characteristic features of a single transient reconnection event.