Rayleigh-Taylor-Kelvin-Helmholtz combined instability at the magnetopause

The magnetopause separates the geomagnetic field from the interplanetary plasma and performs finite motions under the action of the solar wind pressure variable in time. Accelerations originating in this case result in that the necessary condition for the development of the Rayleigh-Taylor instability is formed at quite a definite motion phase. We can anticipate that the instability will develop during compression of the magnetosphere. It should be taken into account that the magnetopause is a potential tangential discontinuity. On the one hand, a plasma flow along the magnetopause results in a decrease in the Rayleigh-Taylor instability threshold. On the other hand, the Kelvin-Helmholtz instability threshold, typical of the tangential discontinuity, also decreases during the magnetosphere compression phase. Thus, if we speak about the magnetopause, it is natural and necessary to jointly consider both types of instability. Main information on the combined Rayleigh-Taylor-Kelvin-Helmholtz instability is presented, the dispersion equation determining the evolution of small perturbations is considered, and the possible geophysical applications to the theory (e.g., penetration of the solar plasma into the magnetosphere, excitation of global Pc5 oscillations) are indicated.