On the incidence of Kelvin-Helmholtz instability for mass exchange process at the Earth’s magnetopause

Abstract. Due to the velocity shear imposed by the solar wind flowing around the magnetosphere, the magnetopause flanks are preferred regions for the development of a Kelvin-Helmholtz instability. Since its efficiency for momentum transfer across the magnetopause has already been established, we investigate its efficiency for mass transfer. Using nonresistive magnetohydrodynamic simulations to describe the magnetic field shape in the instability region, we use test-particle calculations to analyse particle dynamics. We show that the magnetopause thickness and the instability wave-length are too large to lead to nonadiabatic motion of thermal electrons from the magnetosphere. On the other hand, the large mass of H⁺, He⁺ and O⁺ ions leads to such nonadiabatic motion and we thus propose the Kelvin-Helmholtz instability as a mechanism for either magnetospheric ion leakage into the magnetosheath or solar wind ion entry in the magnetosphere. Test-particle calculations are performed in a dimensionless way to discuss the case of each type of ion. The crossing rate is of the order of 10%. This rate is anti-correlated with shear velocity and instability wavelength. It increases with the magnetic shear. The crossing regions at the magnetopause are narrow and localized in the vicinity of the instability wave front. As a Kelvin-Helmholtz instability allows for mass transfer through the magnetopause without any resistivity, we propose it as an alternate process to reconnection for mass transfer through magnetic boundaries.

Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; MHD waves and instabilities) – Space plasma physics (numerical simulation studies)