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

The magnetopause separates the geomagnetic field from the interplanetary plasma and makes finite motions under the action of solar wind pressure varying in time. The resulting accelerations provide the necessary condition for developing Rayleigh-Taylor instability at well defined phase of motion. The instability would be expected to develop at the magnetosphere compression phase. It should be taken into account that the magnetopause is a tangential discontinuity. On the one hand, a plasma flow along the magnetopause causes a decrease in Rayleigh-Taylor instability threshold. On the other hand, the threshold of Kelvin-Helmholtz instability which is typical of the tangential discontinuity also decreases at the magnetosphere compression phase. Thus, we should jointly take into account both types of instabilities where the magnetosphere is concerned. The study presents basic information on combined Rayleigh-Taylor-Kelvin-Helmholtz instability. The dispersion equation determining the evolution of small perturbations is given, and possible geophysical applications of the theory are pointed out (solar plasma penetration into the magnetosphere, excitation of Pc5 global oscillations, etc.).