An MHD simulation study of the Kelvin-Helmholtz instability at the Martian ionopause

Abstract:

Because Mars has no intrinsic magnetic filed, the solar wind directly interacts with the planetary ionosphere. Under this circumstance, the planetary ionopause represents a density discontinuity surface and a velocity shear surface between the magnetized solar wind flow and the planetary ionosphere. The ionopause is subject to the Kelvin-Helmholtz (KH) instability [Amerstorfer et al., 2010], which is expected to play a role in removing ionospheric materials from the planet. In addition, the KH instability may cause a dawn-dusk asymmetry at the magnetopause because of the finite Larmor radius (FLR) effect of ions [Nagano, 1978]. At an ionopause, for the same reason, the KH instability may cause an asymmetry in the direction of the solar wind motional electric field.

Terada et al. [2002] pointed out using a global hybrid simulation that the KH instability at the Venusian ionopause develops asymmetrically through the acceleration of ionospheric ions in the direction of the solar wind motional electric field. It is known that the ion FLR effect, the gravitational stabilizing effect, the effect of the thickness of the boundary layer, etc. determine the initial growth of the KH instability. However, it was difficult to separately evaluate each contribution of these effects with the global hybrid simulation. Because of this, quantitative evaluations of these competing effects at the Martian ionopause have not yet been done.

In this study, to model a more realistic condition of Mars, we have performed simulation runs with a density gradient in the direction of the velocity shear (horizontal direction) in the Martian ionosphere. Our past simulations, which used a periodic boundary condition, had a peak of momentum at the ionopause in the initial condition, so that the growth rate of the KH instability can be over-estimated. This problem is partly resolved by employing this realistic condition. Accordingly, it is expected that the growth rate of the KH instability decreases near the subsolar point, but increases at high SZAs due to the horizontal density gradient compared to our previous results, resulting in a different atmospheric escape rate. In this presentation, initial results obtained from the numerical simulation that considers the horizontal density gradient will be presented.