Properties of Short-Wavelength Oblique Alfvén 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 ω and plasma-to-magnetic pressure ratio β. The KAW frequency can reach and exceed the ion cyclotron frequency at ion kinetic scales, whereas the KSW frequency remains sub-cyclotron. At β ∼ 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 β, 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 σ ∼ 1 for KAWs and σ ∼ −1 for KSWs, and the ion Alfvén ratio R Ai ≪ 1 for KAWs and R Ai ≫ 1 for KSWs. We also found transition wavenumbers where KAWs change their polarization from leftto 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.

Multi-scale analysis of compressible fluctuations in the solar wind

“…To overcome this, a scale-dependent propagation direction is needed as well as the other quantities such as helicity and cross-correlations. Alternatively, the Alfvén ratio is another possibility which can distinguish between KAW and KSWs without the need for the propagation direction (Zhao et al, 2014). Competing interests.…”

“…Ion Bernstein waves (IBWs) exhibit positive correlations between n e and B, while KAW/KSWs both exhibit anticorrelations Zhao et al, 2014). The electron density is estimated from the spacecraft potential Kellogg and Horbury, 2005), which has a sufficient time resolution to study proton kinetic scales.…”

“…The electron density is estimated from the spacecraft potential Kellogg and Horbury, 2005), which has a sufficient time resolution to study proton kinetic scales. Meanwhile, magnetic helicity can be used to differentiate between KAWs and KSWs which have opposite helicities and polarisations (Zhao et al, 2014). Typically in the solar wind, B and n e are anti-correlated on fluid scales , and also down to kinetic scales (Kellogg and Horbury, 2005;Yao et al, 2011), which have been interpreted as KSWs or pressure-balanced structures (the undamped oblique limit of the KSWs).…”

Kinetic Slow Mode in the Solar Wind and Its Possible Role in Turbulence Dissipation and Ion Heating

“…Observational evidence for the kinetic Alfvén mode in the solar wind is reviewed by Podesta (2013), and the related multi-fluid theory by Hollweg (1999). The properties of kinetic Alfvén and kinetic slow modes were also studied recently in the framework of the isotropic two-fluid theory by Zhao et al (2014). A comparison of the kinetic Alfvén mode between the two-fluid and gyrokinetic models was presented by Yang et al (2014) in special plasma limits where the plasma dispersion function in gyrokinetic dispersion relations can be expanded in large and small arguments; dispersions were adopted from Howes et al (2006).…”

Kinetic Alfvén turbulence below and above ion cyclotron frequency

“…Therefore, for the spectrum of the high-frequency kinetic-scale Alfvénic turbulence, one needs the linear relations for high-frequency kinetic Alfvén wave (KAW). Recently, Zhao et al (2014b) derived the linear KAW responses at parallel length-scales extending down to the ion inertial length and below, with corresponding frequencies extending to ω ∼ ω ci and above. Below we use these results to obtain the highfrequency Alfvénic turbulent spectra and scalings.…”