Evolution of Kelvin-Helmholtz instability at boundary layers on Venus

Abstract:

Two-dimensional MHD simulation was performed to study the evolution of Kelvin-Helmholtz (KH) instability on Venusian ionopause in response to the strong sheared velocity flow in presence of the in-plane magnetic field parallel to the direction of the flow. The physical behavior as well as the trigger condition and occurrence condition for highly rolled-up vortex are characterized through several principle parameters, including Alfven Mach number on the upper side of the layer, the ratio of density and increase and the ratio of in-plane magnetic field between the two sides of the layer, et al. The Key result from our simulations is that both of the density increase and in-plane magnetic component on the boundary layer play a role of stabilizing the instability. In the high density increase cases, the value of final total magnetic energy in the quasi-steady status is much more than that of the initial status and dependent on the ratio of density increase, which is quite distinct from that with low density increase. The nonlinear development of case with high density increase and uniform magnetic field is of interest that a single magnetic island forms before the instability saturation. In the non-linear development phase, a new magnetic island arises associated with magnetic reconnection occurring inside the narrow high rolled up density region, combining the pre-existing magnetic island together to form a quasi-steady two island pattern. This pattern subsequently persists for a long period until the two magnetic islands die away because of the strong magnetic tension, instead of a steady pattern with almost uniform magnetic field.