Properties of Short-wavelength Oblique Alfven and Slow Waves

TL;DR

This paper investigates the linear properties of kinetic Alfvén and slow waves in plasma, revealing their distinct polarization, helicity, and pressure characteristics, which aid in identifying these modes in solar-terrestrial plasma observations.

Contribution

It provides new dispersion relations and polarization transition points for KAWs and KSWs, enhancing mode discrimination in plasma turbulence studies.

Findings

KAW frequency can reach and exceed ion cyclotron frequency.

KSW frequency remains below ion cyclotron frequency.

Distinct polarization and helicity properties help differentiate modes.

Abstract

Linear properties of kinetic Alfv\'{e}n waves (KAWs) and kinetic slow waves (KSWs) are studied in the framework of two-fluid magnetohydrodynamics. We obtain the wave dispersion relations that are valid in a wide range of the wave frequency $\omega $ and plasma-to-magnetic pressure ratio $\beta $. The KAW frequency can reach and exceed the ion cyclotron frequency at ion kinetic scales, whereas the KSW frequency remains sub-cyclotron. At $\beta \sim 1$, the plasma and magnetic pressure perturbations of both modes are in anti-phase, so that there is nearly no total pressure perturbations. However, these modes exhibit also several opposite properties. At high $% \beta $, the electric polarization ratios of KAWs and KSWs are opposite at the ion gyroradius scale, where KAWs are polarized in sense of electron gyration (right-hand polarized) and KSWs are left-hand polarized. The magnetic helicity $\sigma \sim 1$ for KAWs and $\sigma \sim -1$ for KSWs, and the ion Alfv\'{e}n ratio $R_{Ai}\ll 1$ for KAWs and $R_{Ai}\gg 1$ for KSWs. We also found transition wavenumbers where KAWs change their polarization from left- to right-hand. These new properties can be used to discriminate KAWs and KSWs when interpreting kinetic-scale electromagnetic fluctuations observed in various solar-terrestrial plasmas. This concerns, in particular, identification of modes responsible for kinetic-scale pressure-balanced fluctuations and turbulence in the solar wind.