Remote Sensing of the Geomagnetic Tail Current Sheet Topology Using Energetic Ions: Neutral Lines versus Weak Field Regions

Abstract

Individual flow bursts in a bursty bulk flow event observed by the AMPTE/IRM satellite at R = (-12, -3, 1)Re, have been modeled by following large numbers of single particle orbits in a model of the geomagnetic tail current sheet containing both Bz and By components and a near Earth neutral line. A flow burst modeled in the central plasma sheet, just after a substorm onset, implied there was a near-Earth neutral line 1 to 1/2 Re tailward of the satellite. An earlier flow burst at the plasma sheet edge, 1-2 minutes before the substorm onset, implied that at this earlier time the neutral line was already formed and closer to the satellite. To match the centroid of the observations, it was necessary that the source population was strongly earthward and duskward flowing, probably originating from the distant current sheet, and that there must have been a relatively large |By| component. With such a By component, it is interesting that a secondary feature of the observed distribution can also be explained qualitatively. During the time that we see the need for a strong earthward flow from a distant source, ground measurements indicate a significant increase in magnetospheric convection. A model with weak but non-reversing Bz reproduces some of the observed distribution function features, but not all of them, as well as the neutral line model. However, a secondary ridge seen in the observations implies a mixing of neutral line and weak field topologies, with such mixing occurring on a 1 minute time scale.