Substructures within a dipolarization front revealed by high-temporal resolution Cluster observations

Abstract

The dipolarization front (DF), usually observed near the leading edge of a bursty bulk flow (BBF), is thought to carry an intense current sufficient to modify the large-scale near-Earth magnetotail current system. However, the physical mechanism of the current generation associated with DFs is poorly understood. This is primarily due to the limitations of conventional plasma instruments which are unable to provide a sufficient number of unaliased 3-D distribution functions on the timescale of the DF, which usually travels past a spacecraft in only a few seconds. It is thus almost impossible to unambiguously determine the detailed plasma structure of the DF at the usual temporal resolution of such instruments. Here we present detailed plasma measurements using the Cluster Plasma Electron and Current Experiment and Cluster Ion Spectrometry-Composition and Distribution Function ion data for an event during which it was possible to observe the full pitch angle distribution at a cadence of 1/4 s. The observations clearly show details of plasma substructure within the DF, including the presence of field-aligned electron beams. In this event, the current density carried by the electron beam is much larger than the current obtained from the curlometer method. We also suggest that the field-aligned current around the DF obtained from the curlometer method may have been misinterpreted in previous studies. Our results imply that the nature of the DF current system needs to be revisited using high-resolution particle measurements, such as those observations shortly to be available from the Magnetospheric Multiscale mission.