A theoretical model for the generation of Kinetic Alfvén waves in the Earth's Magnetosphere by Ion Beam and Velocity Shear

摘要

A generation mechanism for kinetic Alfvén waves (KAWs) by ion-beam and velocity shear is discussed. For this, a three-component plasma model, consisting of cold background ions, hot electrons, and hot ion beams is considered. The model is very general in the sense that all of the three species have drifting Maxwellian distributions, nonuniform streaming, and velocity shear, and can be applied to magnetospheric regions where velocity shear is present. The effects of the ion beam alone and the combined effect of the ion beam as well as the velocity shear in exciting the kinetic Alfvén waves are discussed. It is found that the ion beam alone can excite these kinetic Alfvén waves. However, in the presence of an anti-parallel ion beam and positive shear, the wave growth is much larger as compared to the ion-beam case alone. For a set of plasma parameters, waves are excited for λΒ <1 for the case of the ion beam alone, whereas for the combined case (an anti-parallel ion beam and positive velocity shear), these waves are excited for λΒ >1 .The present model is applied to the polar cusp/ auroral region of the Earth's magnetosphere, and it can explain several characteristic properties of the observed ultra-low-frequency waves. The mechanism presented here can excite the kinetic Alfvén waves up to a frequency of ≈ 65 mHz, which can explain the ultra-low-frequency waves observed in the auroral/polar cusp region.