Vortical flow in the plasma sheet: Non-linear growth of flow burst surface wave?

In contrast to the simple conventional plasma flow convection governed by the Dungey Cycle, past studies have revealed that the plasma flows in the magnetotail region are more complicated, hosting high-speed bursty and meandering vortical flows. We have utilized magnetic field and plasma data from the Cluster mission to investigate a high speed earthward propagating flow burst with a peak velocity of ~530 km/s in the magnetotail plasma sheet (X_GSM ~ -17R_E) on 20 September 2002. In the vicinity of this flow burst, a vortical flow, whose plasma vectors are first directed tailward then earthward, is also observed. The plasma data shows that the plasma population in the vortical flow is likely to originate from the associated flow burst. In addition, the boundaries of both structures are also found to be tangential discontinuities, clearly surrounded by the ambient slow moving plasma sheet. Inside the vortical flow, there exists a region where plasma originating from the flow burst and ambient plasma sheet are mixed. The local segment of inbound boundary crossing of the vortical flow is shown to have a thickness that is non-uniform. Coupled with the flow evolution in the vortical flow, these characteristics are consistent to a boundary crossing of a vortical flow. The magnetic field on the flow burst is quasi-perpendicular to the large velocity shear (~460 km/s) across the flow burst boundary. These results suggest that the formation of vortical flow can arise from the development and subsequent growth of flow burst boundary wave as a result of Kelvin-Helmholtz instability. In summary, this article presents a detailed observational study of a vortical flow and the formation of which would serve as the first direct observational consequence of an excited and growing flow burst boundary wave. Continuous scattering of the detached vortices may play an important role in the braking mechanism of earthward propagating flow bursts.