Whistler Mode Waves Excited by Anisotropic Hot Electrons With a Drift Velocity in Earth's Magnetosphere: Linear Theory

Abstract

With a linear theoretical model, we have investigated the properties of whistler waves excited by anisotropic hot electrons with a drift velocity parallel to the background magnetic field, which is usually neglected in previous studies. It is found that a finite drift velocity can significantly change the properties of excited whistler waves, resulting in distinct properties for parallel and antiparallel propagating waves. In the high-beta regime, as the drift velocity increases, the frequency of parallel propagating whistler waves increases, while that of antiparallel propagating waves is found to decline. So parallel and antiparallel propagating whistler waves appear in different frequency bands. However, the growth rate of parallel wave is always smaller than that of antiparallel wave and falls below 10(-2)Omega(e) for large drift velocities (v(d)/v(th) > 1.5), in which case the parallel wave may be too weak to be observed. Generally, the growth rate of whistler waves in both directions is enhanced with the increasing anisotropy or proportion of hot electrons. In the low-beta regime, the trends of the frequency and linear growth rate of excited whistler waves are quite similar to those in the high-beta regime. But with the increase of the drift velocity, the wave normal angle of parallel propagating whistler waves gradually declines until reaching 0, while that of antiparallel propagating waves continues to increase. Our study may be helpful to understand various whistler mode spectra observed in the Earth's magnetosphere.