Saturation level of Alfvén waves driven by Kelvin–Helmholtz instability

Abstract

Alfv,n waves (AWs) can play an important role in the macroscopic dynamics as well as in the microscopic wave-particle interaction in various magneto-plasma environments. A very wide observed range of the relative amplitude of magnetic fluctuations of AWs from lower than 10(-3) in the terrestrial magnetosphere up to similar to 1 in the solar wind implies the complexity of saturation mechanisms of AWs. Taking account of the ion-neutral collision damping in a partially ionized plasma, the saturation level of AWs driven by the Kelvin-Helmholtz (K-H) instability is investigated in this paper. The intensified magnetic field due to the excited AWs may result in the saturation of the excited AWs when the growth rate is balanced by the damping rate. An equation determining the saturation level of the AWs is obtained. The results show that, for a fixed-frequency wave, the saturation level of the AWs considerably increases as the ionization degree increases as well as the sheared-flow velocity. On the other hand, for a fixed ionization degree the saturation level of the AWs slightly increases as the plasma beta increases but decreases as the wave frequency increases. These results have potential importance for us to estimate the saturation level of AWs in space and astrophysical plasmas.