Decay of MHD-scale vortices mediated by parasitic electron dynamics

While the Hall-MHD description separating ion and electron fluid velocities does produce various significant results out of the scope of traditional MHD framework, it seems necessary sometimes to step forward to include finite electron inertia effects in order to resolve the important issue of cross scale coupling down to the electron-scale phenomena. Since full particle simulations are still costly, the two fluid model that takes the finite electron inertia effects into account by modifying the induction equation from Hall-MHD equations is useful in studying large scale MHD scale dynamics while resolving the electron scales. In this study, we have performed two-dimensional simulations of the Kelvin-Helmholtz instability at the magnetopause-like situation. The magnetic field is assumed to be perpendicular to the simulation plane and to the sheared flow. The instability grows and the highly rolled-up vortex appears in the non-linear stage as in MHD calculations. What is striking, however, is that, in the presence of electron inertia effects, smaller vortices grow within the MHD scale vortex and eventually the MHD vortex decays. This decay is seen persistently throughout the range of ion-to-electron mass ratio (25-400) and throughout the range of the initial shear layer thickness (2-6 times the ion inertia length). The implication of the results to the plasma transport across the tail-flank is discussed.