Utilizing Magnetospheric Multiscale (MMS) observation in the tail plasma sheet, we study the vorticity field (omega = del x V) of the plasma bulk (convective) velocity within the bursty bulk flow (BBF) in detailed. Two typical events are presented. In the event on 25 June 2017, E-y is the main component. In the other event on 6 July 2017, E-z is the main component. For both cases, the BBF electric field is dominated by the convective electric field (Ec = - V x B). Our case studies show clearly the existence of the convective vorticity field within the BBF. The vorticity field has prominent anisotropy (quantified by the anisotropic angle (theta aa = arctan(omega(perpendicular to)/omega(parallel to))). More often, the BBF omega field has stronger perpendicular vorticity (omega(perpendicular to)) than the parallel vorticity (omega(parallel to)). The dominance of vorticity by omega(perpendicular to)-dominating BBF is confirmed in the statistical sense. In particular, event on 25 June shows the significant evolution of the ion flux energy with the strength of the omega field. The strong omega field corresponds to the ion flux enhancement at high energy (above 10 keV), while the weak omega field corresponds to the ion flux enhancement at medium energy (2-5 keV). Investigation of the subset of channel from fast plasma investigation partial moment measurement reveals that the ion behaviors in the strong and decayed BBFs are distinctly different. The channeled ions form the narrow band distribution in the strong BBF but the multiple-layer distribution in the weak BBF. Finally, spectrum analysis indicates that the BBF omega(perpendicular to) and omega(parallel to) have a similar scaling about -2.0 (below 0.2 Hz).
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