Geotail Observations of Bursty Bulk Flows and Plasmoid Associated Flux Ropes

Dr. James Slavin
Laboratory for Extraterrestrial Physics
NASA Goddard Space Flight Center

Examination of Geotail measurements in the near tail (X>-30 Re) has revealed the presence of small flux ropes embedded within high-speed flows in the plasma sheet. A total of 35 flux rope events were identified in association with earthward directed bursty bulk flows (BBF) while 38 flux ropes were found during high-speed tailward flows between November 1998 and April 1999. We refer to these two populations as "BBF flux ropes" and "plasmoid flux ropes". The two classes of flux ropes are readily distinguished by the sense of their Bz perturbations; ΔBz is +/- the plasmoid events and -/+ for the BBF events. The mean temporal duration for both types of flux rope event was ~ 30 sec. In most instances, the flux ropes were observed to be simultaneous with or follow the onset of the earthward or tailward flow event by ~ 1-2 min. Application of the Lepping-Burlaga constant a (i.e., J = α B) flux rope model to these events determined that approximately 60% of both classes of flux ropes could be well modeled as force-free flux ropes. These models yielded a mean BBF and plasmoid flux rope diameters and core field intensities of 2.0 Re and 20 nT and 5.2 Re and 14 nT, respectively. BBF flux rope core field intensity was linearly correlated with IMF By, correlation coefficient = 0.65, but the core fields within plasmoid flux ropes was not well correlated with IMF By, cc = 0.13. The inclinations of the flux ropes were small relative to the GSM X-Y plane at 10 deg and 1 deg, respectively, for the BBF and plasmoid flux ropes. However, a wide range of azimuthal flux rope orientations were determined with only a weak preference for the +/- Y directions. The presence of such structures is interpreted as strong evidence for multiple X-lines reconnection in the near-tail.