Wind survey of high-speed bulk flows and field-aligned beams in the near-Earth plasma sheet

[1] We have surveyed all high-speed (> 250 km/s) flows detected by Wind during its 17 perigee passes across the near-Earth (x(GSE) = -25 to 0 R-E) plasma sheet in the period between 1995 and 1997. By classifying high-speed flow events based on their ion distribution characteristics rather than their plasma moments or the regions in which these flows were detected, we found that most (if not all) high-speed flow ion distributions fall into two distinct categories: bulk flows and field-aligned beams. We show that bulk flows are not simply the low-latitude counterparts of field-aligned beams. Field-aligned beams have sharp cutoffs at low energies, occur within relatively steady magnetic field and plasma conditions, and are detected more often away from the neutral sheet. On the other hand, high-speed bulk flows are well represented by a single drifting population and have their peak occurrence rate at the neutral sheet. Bulk flows are generally accompanied by large magnetic field fluctuations and sudden increase of energetic (up to 0.5 MeV) particle fluxes. They are often (but not always) associated with magnetic field dipolarization and plasma temperature enhancements. Little or no temperature enhancements are observed in cases where the spacecraft resides near the neutral sheet before the arrival of bursty bulk flows, suggesting that temperature enhancements seen in other bulk flow events may in part be a spatial effect instead of true heating of the plasma. Bulk flows are perpendicular to the magnetic field when detected at the neutral sheet but have a large field-aligned component at higher magnetic latitudes. Field-aligned bulk flows and field-aligned beams are similar in terms of their velocity moments and may occur at the same magnetic latitudes but are easily distinguishable based on their ion distributions. We have used our categorization of bulk flow and field-aligned beam events to search for the optimal moment-based selection criteria to distinguish bulk flows from beams. We found that no single moment-based parameter or threshold can serve to cleanly separate beam from bulk flow distributions because a range of values of these parameters exists where both types of fast flows are observed. We found perpendicular flow speed v(perpendicular to) > 250 km/s and plasma beta(xy) (based on the x and y components of the magnetic field) > 2 to be the optimal moment-based bulk flow selection criteria because they eliminate similar to95% of beam events while retaining similar to60% of bulk flow events. Finally, a previously reported dawn-dusk asymmetry in the occurrence of bursty bulk flows, with most events occurring in the premidnight sector, is confirmed by our survey. Field-aligned beams, on the other hand, have no dawn-dusk bias.