Properties of Short-wavelength Oblique Alfven and Slow Waves

Abstract

Linear properties of kinetic Alfvén 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 , 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 Alfvén 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.

13 pages, 13 figures. This version is accepted and published in the Astrophysical Journal

References in corpus (1)

• Magnetohydrodynamic Slow Mode with Drifting He${}^{++}$: Implications for Coronal Seismology and the Solar Wind

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