Kinetic cascade beyond magnetohydrodynamics of solar wind turbulence in two-dimensional hybrid simulations

Abstract

The nature of solar wind turbulence in the dissipation range at scales much smaller than the large MHD scales remains under debate. Here a two-dimensional model based on the hybrid code abbreviated as A.I.K.E.F. is presented, which treats massive ions as particles obeying the kinetic Vlasov equation and massless electrons as a neutralizing fluid. Up to a certain wavenumber in the MHD regime, the numerical system is initialized by assuming a superposition of isotropic Alfvén waves with amplitudes that follow the empirically confirmed spectral law of Kolmogorov. Then turbulence develops and energy cascades into the dispersive spectral range, where also dissipative effects occur. Under typical solar wind conditions, weak turbulence develops as a superposition of normal modes in the kinetic regime. Spectral analysis in the direction parallel to the background magnetic field reveals a cascade of left-handed Alfvén/ion-cyclotron waves up to wave vectors where their resonant absorption sets in, as well as a continuing cascade of right-handed fast-mode and whistler waves. Perpendicular to the background field, a broad turbulent spectrum is found to be built up of fluctuations having a strong compressive component. Ion-Bernstein waves seem to be possible normal modes in this propagation direction for lower driving amplitudes. Also signatures of short-scale pressure-balanced structures (very oblique slow-mode waves) are found.

Intermittency, nonlinear dynamics and dissipation in the solar wind and astrophysical plasmas

“…Such codes have shown evolution of proton-scale current sheets in a highly turbulent initial value problem [37] for the case of an out-of-plane guide field. Similar numerical models have also demonstrated stronger Vlasov wave activity [127], when the mean magnetic field lies in the plane of activity.…”

Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities

“…A background magnetic field B 0 can lead to an anisotropic cascade (Goldreich & Sridhar 1995), and, in fact, most astrophysical plasmas are permeated by such a magnetic field. Theoretical considerations, numerical simulations, and solar-wind observations indicate that the turbulent cascadeespecially for large-scale noncompressive fluctuations-preferentially transfers energy to large k ⊥ , and to a lesser extent to large k P in a magnetized plasma, where k ⊥ (k P ) is the wavenumber in the direction perpendicular (parallel) to B 0 (Montgomery & Turner 1981;Oughton et al 1994Oughton et al , 1998Cho & Vishniac 2000;Sahraoui et al 2010;Chen et al 2011;Narita et al 2011;He et al 2012;Salem et al 2012;Verscharen et al 2012). We focus our treatment on the outer scales of the turbulence at which the frequencies (linear and nonlinear) are W p , where W º q B m c p p 0 p is the proton gyrofrequency, q p and m p are the charge and the mass of a proton, and c is the speed of light.…”

Ion-Scale Turbulence in the Inner Heliosphere: Radial Dependence

“…The setup for the ion-scale plasma turbulence is detailed in Verscharen et al (2012) and Comişel et al (2014aComişel et al ( , 2015. Our simulations have been conducted for a wider range of ion beta from β i = 0.05 to β i = 2.0, covering nearly two orders of magnitude.…”

“…By using a MHD model of the solar wind expansion, we propose a mapping of the radial profile of the solar wind velocity into the time evolution. We then perform local hybrid simulations on ion-scale plasma turbulence, where the typical extension of the simulation box is a few thousand kilometers, e.g., Valentini et al (2010), Verscharen et al (2012), Comişel et al (2014aComişel et al ( , 2015. The mapping recipe based on the radial dependence of the plasma beta is finally applied to the results of the wave evolution into turbulence as well as to the wave vector anisotropy development.…”