Global hybrid simulation of the Kelvin–Helmholtz instability at the Venus ionopause

Abstract

[1] The Kelvin-Helmholtz (K-H) instability at the Venus ionopause is investigated using a two-dimensional global hybrid (particle ions, massless fluid electrons) model for the case where the interplanetary field is oriented transverse to the flow. In order to selfconsistently study the kinetic processes at the Venus ionopause, we calculate the entire Venus ionopause-solar wind interaction region kinetically, including the ionosphere, ionopause transition layer, magnetosheath, and solar wind regions, by applying boundaryfitted coordinates to the particle-in-cell code. Calculations are done for the unmagnetized ionospheric condition. It is found that the distribution of ionopause surface waves generated by the K-H instability exhibits a clear asymmetry between hemispheres of upward and downward solar wind motional electric fields. In the hemisphere where the motional electric field points away from the ionosphere, our two-dimensional model observes the development of the instability from the subsolar location, where the Venus ionopause has been thought to be stable due to small velocity shear and stabilizing effects such as gravity stabilization. Ionospheric filaments (streamers) are highly elongated in this hemisphere, and perturbations on the scale of the planetary radius are observed at the flank of the ionotail. In the opposite hemisphere, the instability develops only in a limited region between solar zenith angles (SZA) of $40°and 90°, and the surface of the ionotail appears rather smooth except for some special cases. The wavelength of the dominant mode is very long (2000-3000 km) in this hemisphere due to the finite Larmor radius (FLR) stabilizing effect. We show that the process operating at the ionopause is not a localized one but is important for the dynamics of the ionosphere and other regions. The asymmetrical momentum transport across the ionopause yields an asymmetrical convection pattern of the ionosphere. Our model also shows the dynamic nature of the interaction. The dynamic ion removal process associated with the K-H instability is found to play a significant role in the ion escape from the planet (O + removal rate of approximately several times 10 25 particles per second).

Unusual nonlinear waves in the Venusian magnetosheath

“…The observations presented here also demonstrate that conditions are favorable for the generation of KHI at the ionopause. Hybrid simulations of the KHI by Terada et al [2002] have shown the existence of large filaments or streamers of plasma within the magnetosheath. These filaments tend to occur at all SZAs in the hemisphere in which the motional electric field possesses a component pointing away from the planet, while in the case when the electric field is directed downward the filaments were observed only at high SZA.…”

“…Hence these observations took place in the E up hemisphere, which, as mentioned above, may result in the formation of filamentary structures [Terada et al, 2002].…”

“…To explain this distribution, it is also important to consider the asymmetry in the magnetosheath caused by the solar wind motional electric field. Hybrid simulations of the KHI [Terada et al, 2002] have shown that for the hemisphere in which the motional electric field points away from the planet, filaments of plasma are generated at all SZAs, while in the opposite hemisphere such structures are typically observed at higher SZAs. Figure 8 shows the relationship between the solar zenith angle of the observed NRSs with altitude.…”

“…It should also be noted that these sizes are similar in magnitude to the dimensions of the magnetosheath itself. In this calculation, the antisunward flow component of ions in the ionosphere, typically of the order of a few kilometers per second [Terada et al, 2002;Knudsen and Miller, 1992] has been neglected, since it would not significantly affect the size estimates. [15] The second method assumes that the NRSs stand in space as the spacecraft crosses them.…”

“…Using two-dimensional global hybrid simulations, Terada et al [2002] showed that the KHI could result in the existence of highly elongated ionospheric filaments or streamers in the hemisphere for which the motional solar wind electric field is directed away from the ionosphere. However, the motional electric field alone is not sufficient to provide the observed particle velocities, and wave-particle interactions are expected to influence the particle trajectories [Pérez-de-Tejada et al, 2010].…”