Bifurcated current sheets: Statistics from Cluster magnetometer measurements

摘要

We report on a statistical study of bifurcated current sheets in the near-Earth plasma sheet and the association of such current sheets with a near-Earth neutral line. Support for this interpretation is provided by a global MHD simulation generated for geomagnetic and solar wind conditions prevailing at the time when a bifurcated current sheet was previously identified in 2001 Cluster magnetometer data. A local bifurcation of the current sheet with spatial scales of 3–7 RE in Y and Z GSM was present in the MHD simulation at the ?19 RE apogee distance of Cluster. Magnetic field line tracing revealed a clear connection to a local neutral line at ～22 RE downtail. For our statistical study, criteria developed to characterize bifurcations were automatically applied to the magnetic field and particle observations of the four Cluster spacecraft for 1500 km and 5000 km separations. We found that bifurcations are detected ～25% of the time Cluster is in the current sheet. Thickness estimates from our statistical results reveal structures 1–3 RE in scale size (north-south), comparable with those present in our MHD simulation results. Our simulation and statistical results show that bifurcated current flows near the boundary of the plasma sheet. Local conditions at Cluster for our selected events are often consistent with the presence of a nearby neutral line. We propose that bifurcated current density profiles develop in association with small-scale (few RE) neutral lines that do not stretch across the entire magnetotail. Therefore bifurcated current sheets may well be a signature of magnetotail dynamics rather than an equilibrium current sheet configuration.