Resonant and Kelvin–Helmholtz instabilities on the magnetopause

Abstract

The Kelvin–Helmholtz instability (KHI) of magnetohydrodynamic surface waves on the magnetopause is examined and the resonant flow instability (RFI) of these waves is studied under the assumption that the plasma density decreases in the direction from the magnetosheath towards the magnetosphere. It is found that when a finite plasma temperature is taken into account, three different kinds of surface modes (labeled as “primary” modes and “β” modes on sides 1 and 2) may exist. The β modes remain almost unaffected by the flow in the corresponding rest frames and can be excited in certain velocity intervals which are determined. These modes do not interact with each other, but their interaction with primary modes leads to KHIs confined to narrow velocity intervals when the mode energies are of opposite signs. Only primary mode can be resonantly unstable with growth rates smaller than those typical for KHIs. Starting with the slow resonant instability the negative energy primary and β modes are subject to several RF and KH instabilities which are separated from each other by narrow velocity intervals of stability. The growth rates of these instabilities tend to increase with increasing velocity shear. Possible applications to, e.g., observations in the terrestrial magnetosphere, are discussed.