Bursty bulk flows, the geomagnetic tail current sheet, and substorm timing

Abstract

Individual flow bursts in a bursty bulk flow event observed by the AMPTE/IRM satellite at R = (−12, −3, 1) R e , have been modelled by following large numbers of single particle orbits in a model of the geomagnetic tail current sheet containing both B z and B y components and a near-Earth neutral line. A flow burst modelled in the central plasma sheet, just after a substorm onset, implied there was a near-Earth neutral line 1 to 1 1 2 R e 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 | B y | component. With such a B y 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 B z reproduces some of the observed distribution function features, but not all of them, as well as the neutral line model.