Nonlinear Kelvin-Helmholtz instability in FLR and Hall MHD

The solar wind meets different kinds of obstacles on its way through the solar system. One type of obstacles are planets, which possess no (strong) intrinsic magnetic field, but an atmosphere and ionosphere, and an "induced magnetosphere" due to the solar wind interaction. Venus and Mars are such unmagnetized planets. The interaction of the solar wind with an unmagnetized planet evokes different plasma processes in the vicinity of the planet. Pioneer Venus Orbiter, for example, observed wave-like irregularities on the dayside ionopause and possibly detached plasma structures above the ionopause. Such observations gave rise to the hypothesis that the Kelvin-Helmholtz instability could be able to develop at the boundary layer between the magnetosheath and the ionosphere and that it could lead to the detachment of so-called plasma clouds containing ionospheric particles. In general, the Kelvin-Helmholtz instability occurs at boundaries between two plasma layers with a relative horizontal motion to each other. A small perturbation at the interface between these two layers is able to grow, making the boundary unstable and turbulent. When the instability reaches its nonlinear stage, vortices might form, reconnection might be initiated and plasma structures might detach. These processes are responsible for the consideration of the Kelvin-Helmholtz instability as a loss process of ionospheric particles for unmagnetized planets. However, the mechanisms involved are currently not well understood. The main objectives of the proposed project are to study the properties and dynamics of the Kelvin-Helmholtz instability in the solar wind interaction with Venus and Mars and to determine if the instability contributes to the loss of particles from these planets. It will be investigated if there exists a connection between the instability, boundary waves and detached plasma structures, through which planetary material might escape, and if positive, how effective such a loss process might be. These objectives will be achieved (1) by developing a nonlinear, multi- fluid magnetohydrodynamic model including finite Larmor radius and Hall effects, (2) by conducting detailed theoretical studies on the Kelvin-Helmholtz instability and its consequences (i.e., vortices, plasma clouds), (3) by performing detailed data analysis of appropriate data from space missions, and (4) by comparing the theoretical results with these data. Thus, this project proposes a comprehensive approach, which combines theoretical studies and data analysis. The research in this proposed project will help to gain a better understanding of the properties and consequences of the Kelvin-Helmholtz instability in the solar wind interaction with Venus and Mars and of the contribution of the instability to the loss of planetary particles from Venus and Mars.

The main aim of this project was to study the evolution of the Kelvin-Helmholtz instability at boundary layers around Venus and Mars with a focus on its contribution to the loss of planetary particles. The instability, when developing on a boundary between the solar wind and the planet, forms waves, which can evolve into vortices on their way from the subsolar point to the terminator. There, these vortices can detach and form so-called plasma clouds containing planetary ions, which might then escape from the planet. To understand the various atmospheric and ionospheric loss processes is one of the key issues for understanding the evolution of planets. The main research question we wanted to answer in this project was if the Kelvin-Helmholtz instability plays a significant role as a loss process of atmosphere/ionospheric particles from unmagnetized planets. We found that the Kelvin-Helmholtz instability can in principal develop at the ionopauses and the induced magnetopauses around Venus and Mars under certain plasma conditions. However, in most of the cases the instability does not evolve into vortices along the boundary, due to very low growth rates resulting from highly stabilizing effects. The ionopause is basically always stable, the induced magnetopause might become unstable for solar maximum activity phases. This result reflects the lack of observations of vortex structures and plasma clouds from current spacecraft missions around Venus and Mars. We investigated different factors that might stabilize the boundary layer with respect to the Kelvin-Helmholtz instability, i.e. factors that decrease the growth rate of the instability. Our results show that the most prominent stabilizing influence follows from a high increase of the mass density across the boundary layer when approaching an unmagnetized planet. Another factor we included in our newly developed model is gravity. Compared to the large stabilization due to the density increase, gravity is seen to slightly decrease the growth rate additionally, but this effect is rather negligible. To study the influence of the orientation of the magnetic field on the evolution of the Kelvin-Helmholtz instability, we extended our model to be able to cope with arbitrary magnetic field configurations. A magnetic field component which is parallel to the boundary and the plasma flow decreases the growth rate even further. Additionally, our results suggest that magnetic reconnection can occur inside the Kelvin-Helmholtz vortices, when the magnetic field gets wrapped up. We conclude that the Kelvin-Helmholtz instability and its vortices can sporadically form at boundary layers around unmagnetized planets, but this is not a continuous process leading to a continuous loss of planetary particles. Thus, the instabilitys role as a loss process for atmospheric and ionospheric particles can be considered as negligible and was overestimated in the past by various studies dealing with this topic.