Abstract:
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 eff ects of the ion beam alone and the combined eff ect
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 < 1 B 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 > 1 B .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.
