Spatial-Temporal characteristics of ion beamlets in the plasma sheet boundary layer of magnetotail

摘要

The processes of nonadiabatic ion acceleration occurring in the vicinity of magnetic neutral lines produce highly accelerated (up to 2500 km/s) field-aligned ion beams (beamlets) with transient appearance streaming earthward in the plasma sheet boundary layer (PSBL) of the Earth’s magnetotail. Previous studies of these phenomena based on single spacecraft (s/c) missions supported the view that beamlets are temporal transients, since the typical time of a beamlet observation at a given s/c very rarely exceeds ～1–2 min. Now multipoint Cluster observations have led to a new understanding of these phenomena with a spatial rather than a temporal structure. On the basis of 3-year Cluster measurements made in the PSBL, we present statistical evaluation of the beamlet duration (at least 5–15 min) and confirm well-manifested localization of the beamlet along Z and in some cases along Y directions, i.e., approximately across the lobe magnetic field. Earlier results reporting shorter beamlet observations could be understood by invoking not only PSBL flapping motions but also of an additional effect revealed by Cluster: earthward propagation of kink-like perturbations along the beamlet filaments. Phase velocity of these perturbations is of the order of the local Alfven velocity (V ～ 600–1400 km/s) and related fast flappings of localized beamlet structures in the Y-Z direction significantly decreasing the time of their observation at a given spacecraft. Multipoint observations of beamlets revealed that they represent long-living (～5–15 min) plasma filaments elongated along the lobe magnetic field (～60–100R E) and strongly localized in direction perpendicular to the PSBL-lobe boundary (～0.2–0.7R E). In some cases, it was also possible to estimate the width of beamlet in dawn-dusk direction which was of the order of fractions of R E.