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, 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 10Ω for large drift velocities (v/v>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 zero, while that of antiparallel propagating waves continues to
increase. Our study may help people to better understand various
whistler-mode spectra observed in the Earth’s magnetosphere.