Onsets and Flapping Studied Using a Dynamic Harris Sheet Model

Abstract

We have developed a nonlinear fitting routine for the Harris current sheet based on magnetic data, and applied it to 71 plasma sheet crossings by Cluster. In most cases this simple fit appeared to represent the magnetic field well as a function of Z on the slow timescale of the spacecraft moving through the plasma sheet. However, it is expected that motion of the plasma sheet during such a passage will cause some blurring, possibly giving a larger half-width than is really the case. Sometimes, large plasma sheet flapping takes place and on a rapid timescale. The same technique, applied during flapping, shows a narrower current sheet than is encountered on the slow timescale. We do not interpret this to mean that the current sheet is narrower during flapping than at other times, but simply that flapping translates the current sheet rapidly so that an accurate profile is obtained. However, comparison with CIS data suggests that results from the simplest Harris sheet model, applied during flapping with only vertical motion and Bx magnetic field, do not properly match velocities suggested by the plasma moments. A three-dimensional Harris sheet model has thus been developed, and initial results using this are compared to those from the simpler routine. For some spacecraft separations, it was also possible to use the curlometer technique to obtain J and compare to that expected for the Harris sheet. We also present results from an event on August 21, 2002, when Cluster was ideally placed at substorm onset, showing fast plasma flows and motion of the reconnection region through the constellation, taking place at the same time as poleward expansion of the substorm electrojet as observed in central Canada. In this case, but not generally, initiation of flapping motion appeared to be related to substorm onset. Finally, we show flapping results during an unusual Ps 6/omega band event observed from central Canada on July 27, 2003, where the tail data suggest bifurcation of the current sheet in the morning sector during the event.