Large eddy simulations in MHD: The rise of counter-rotating vortices at the magnetopause

A study of the magnetohydrodynamic (MHD) development of coherent structures in compressible, inhomogeneous, mixing layers due to the velocity shear instability is reported. The non-linear evolution of the original vorticity sheet is computed with 3-D large eddy simulations (LES) of temporal mixing layers tailored to represent distinctive conditions at the terrestrial magnetopause. We find that the boundary layer is characterized by the growth of large-scale vortices and becomes a site of mass mixing and enhanced plasma diffusion. In MHD the Lorentz force and its associated baroclinic term, together with the ordinary baroclinic term, and stratified entropy across the mixing layer, conspire to hinder vorticity flux conservation. In our LES the non-conservation of vorticity becomes manifest after ∼ one rollover time when in addition to vortices with positive rotation (the same as the original vorticity sheet) other coherent structures with strong negative vorticity also arise, a noteworthy effect examined here. It is found that the vorticity is concentrated in cores of both signs with absolute values ∼ 4-5 x ωi, (maximum vorticity of the initial shear layer). Concomitant with 3-D vortex stretching, the kinetic helicity also rises at vorticity cores. Furthermore, high temperature occurs in the cores, ∼ 3xTi (magnetospheric temperature) correlated with local density depletion, ∼ 0.4xni (magnetospheric density), while gas and magnetic pressure remain close to surrounding values. The study is intended as a contribution to the understanding of solar wind interaction with the magnetosphere during periods of northward interplanetary magnetic field.