Shear flow vortices in magnetospheric plasmas

Abstract

Theory and numerical simulations are used to investigate the nonlinear evolution of vortices generated by the Kelvin–Helmholtz (KH) instability of sheared plasma flows in the Earth’s magnetosphere. The extent of broadening of the shear flow, and the energy and enstrophy exchange between the shear flow and KH vortices, is characterized. A new stationary vortex street solution is found, and two distinct phases of the nonlinear dynamics are identified. The first involves a transient phase in which burst-like pulsations of the flow lead to a rapid dissipation of enstrophy. After the transient phase, an asymptotic state is reached that corresponds to a periodic chain of pairs of monopolar vortices. The consequences of the model results for the dynamics of field line resonances (FLRs) in the Earth’s magnetosphere are discussed, and it is shown, in particular, that broadening of the flow correlates well with observations of periodic reforming of FLR structures.