The characteristics of the magnetopause reconnection X‐line deduced from low‐altitude satellite observations of cusp ions

Abstract

We present an analysis of a “quasi‐steady” cusp ion dispersion signature observed at low altitudes. We reconstruct the field‐parallel part of the Cowley‐D ion distribution function, injected into the open LLBL in the vicinity of the reconnection X‐line. From this we find the field‐parallel magnetosheath flow at the X‐line was only 20 ± 60 km s−1, placing the reconnection site close to the flow streamline which is perpendicular to the magnetosheath field. Using interplanetary data and assuming the subsolar magnetopause is in pressure balance, we derive a wealth of information about the X‐line, including: the density, flow, magnetic field and Alfvén speed of the magnetosheath; the magnetic shear across the X‐line; the de‐Hoffman Teller speed with which field lines emerge from the X‐line; the magnetospheric field; and the ion transmission factor across the magnetopause. The results indicate that some heating takes place near the X‐line as the ions cross the magnetopause, and that sheath densities may be reduced in a plasma depletion layer. We also compute the reconnection rate. Despite its quasi‐steady appearance on an ion spectrogram, this cusp is found to reveal a large pulse of enhanced reconnection rate. Copyright 1994 by the American Geophysical Union.