Abstract
The velocity shear of ion beams observed in the polar cusp region can drive the kinetic Alfven modes unstable. A hot ion beam can excite both a resonant kinetic Alfven wave instability and a nonresonant coupled Alfven ion-acoustic wave instability. For the case of a cold ion beam only the latter instability is excited. For the altitude range of 5–7Re, velocity shearS≃0.04–1.0 is needed to excite the kinetic Alfven wave instabilities. HereS=(dVB/ωcBdx), whereVb is the streaming velocity,and ωcB is the gyrofrequency of the bean ions. The excited modes have frequencies, in the satellite frame of reference, in the ULF frequency range. The noise generated by the velocity shear-driven Alfven modes is electromagnetic in nature. These modes have a substantial component of parallel electric fields and, therefore, they can play an important role in the ionosphere-magnetosphere coupling process occurring in the polar cusp region.
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