Kelvin-Helmholtz driven modes of the magnetosphere

Ultralow frequency (ULF) waves in the magnetosphere are thought to be drive n by disturbances of the magnetopause caused by the flow in the magnetoshea th. In this paper a model showing how the trapping and excitation of these modes depends upon the shear flow and propagation angle is presented. The i deal magnetohydrodynamics (MHD) equations are used and the perturbations ar e assumed to be linear. A bounded, uniform magnetospheric cavity, with a fi nite plasma beta, separated by a vortex sheet from a semi-infinite, field-f ree, flowing magnetosheath is considered. It is shown that the bounded mode l allows the trapping and excitation of both fast and slow cavity modes, an d that unstable surface modes may also exist. Slow surface modes are unstab le only for a small interval of flow speed, becoming fast surface modes for higher flows. Slow cavity modes have small growth rates and are unlikely t o be significant observationally. It is shown that fast modes propagating q uasiparallel to the flow may be excited for realistic flow speeds, but that for nonparallel modes, much higher flows are required. Finally, an exact m ethod for predicting the onset of instability for fast modes is derived and is shown to occur at the coalescence of modes of opposite energy. (C) 1999 American Institute of Physics. [S1070-664X(99)01510-4].