Kinetic Alfvén wave revisited

Abstract

Abstract. We develop a series of new analytical expressions describing the physical properties of the kinetic Alfv6n wave. The wave becomes strongly compressive when k_7_ • is of the order of the ion inertial length. Thus, in a low-/3 plasma, the kinetic Alfv6n wave can be compressive at values of k ñ for which the dispersion relation departs only slightly from that of the usual MHD Alfvfin wave. The compression is accompanied by a magnetic field fluctuation •Bll such that the total pressure perturbation •Ptot m 0. Thus the wave undergoes transit-time damping as well as Landau damping; the two effects are comparable if the ion thermal speed is of the order of the Alfv6n speed. We find that the transverse electric field is elliptically polarized but rotating in the electron sense; this surprising behavior of the polarization of the Alfv6n branch was discovered numerically by Gary [1986]. We derive a new dispersion relation which explicitly shows how the kinetic Alfvfin wave takes on some properties of the large-k ñ limit of the slow mode. We also derive approximate dispersion relations valid for a multi-ion plasma with differential streaming. We suggest that the kinetic Alfvfin wave may be responsible for the flattening of density fluctuation spectra observed at large wavenumbers in the corona and in the solar wind. We also find that our derived properties of the kinetic Alternatively, large values of kñ could be the consequence of a turbulent cascade. In any case, at large values of k z, the Alfvfin mode, which is normally noncompressive, will become the kinetic Alfv6n wave, which is compressive. Harmon [1989] has suggested that the compressibility may be recognizable in the power spectra of coronal density fluctuations, which are derived from a variety of radio sounding techniques. Indeed, 14,811

Kinetic scale density fluctuations in the solar wind

“…The flattening at 0.1 < kρ i < 1 has been observed previously, both at 1 AU [26][27][28][29] and in the near-Sun solar wind [30,31] and has been attributed to either pressure anisotropy instabilities [32] or the increased compressive nature of kinetic Alfvén wave turbulence at the ion gyroscale [33][34][35][36]. In particular, Harmon and Coles [35] and Chandran et al [36] modeled the spectrum as a sum of density fluctuations passive to the Alfvénic turbulence, which dominate at large scales, and active density fluctuations from kinetic Alfvén wave (KAW) turbulence, which dominate at small scales.…”

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

“…We refer to that paper for a discussion of what is known about the slow mode wave in the kinetic regime for various values of the ion plasma beta and the electron-to-ion temperature ratio in a Maxwellian plasma. 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).…”

Non‐Gaussian Radio‐Wave Scattering in the Interstellar Medium

“…In the meantime one could perhaps look for a signature of the compressive effects in the density spectrum inferred from the pulsar scintillations data on scales $10 8 -10 10 cm. A likely signature would be a flattening of the spectrum on these scales: a flattening of this type was observed in the electron density spectrum of the solar wind (Coles & Harmon 1989), where it was, in fact, attributed to kinetic Alfvén wave effects (Hollweg 1999). …”