ASTROPHYSICAL GYROKINETICS: KINETIC AND FLUID TURBULENT CASCADES IN MAGNETIZED WEAKLY COLLISIONAL PLASMAS

By A. Schekochihin, S. Cowley, William Dorland, Gregory W. Hammett, Gregory G. Howes, E. Quataert, Tomoya Tatsuno. 2007

690 citations recorded by the source corpus.

This paper presents a theoretical framework for understanding plasma turbulence in astrophysical plasmas. It is motivated by observations of electromagnetic and density fluctuations in the solar wind, interstellar medium and galaxy clusters, as well as by models of particle heating in accretion disks. All of these plasmas and many others have turbulent motions at weakly collisional and collisionless scales. The paper focuses on turbulence in a strong mean magnetic field. The key assumptions are that the turbulent fluctuations are small compared to the mean field, spatially anisotropic with respect to it and that their frequency is low compared to the ion cyclotron frequency. The turbulence is assumed to be forced at some system-specific outer scale. The energy injected at this scale has to be dissipated into heat, which ultimately cannot be accomplished without collisions. A kinetic cascade develops that brings the energy to collisional scales both in space and velocity. The nature of the kinetic cascade in various scale ranges depends on the physics of plasma fluctuations that exist there. There are four special scales that separate physically distinct regimes: the electron and ion gyroscales, the mean free path and the electron diffusion scale. In each of the scale ranges separated by these scales, the fully kinetic problem is systematically reduced to a more physically transparent and computationally tractable system of equations, which are derived in a rigorous way. In the “inertial range” above the ion gyroscale, the kinetic cascade separates into two parts: a cascade of Alfvénic fluctuations and a passive cascade of density and magnetic-field-strength fluctuations. The former are governed by the reduced magnetohydrodynamic (RMHD) equations at both the collisional and collisionless scales; the latter obey a linear kinetic equation along the (moving) field lines associated with the Alfvénic component (in the collisional limit, these compressive fluctuations become the slow and entropy modes of the conventional MHD). In the “dissipation range” below ion gyroscale, there are again two cascades: the kinetic-Alfvén-wave (KAW) cascade governed by two fluid-like electron reduced magnetohydrodynamic (ERMHD) equations and a passive cascade of ion entropy fluctuations both in space and velocity. The latter cascade brings the energy of the inertial-range fluctuations that was Landau-damped at the ion gyroscale to collisional scales in the phase space and leads to ion heating. The KAW energy is similarly damped at the electron gyroscale and converted into electron heat. Kolmogorov-style scaling relations are derived for all of these cascades. The relationship between the theoretical models proposed in this paper and astrophysical applications and observations is discussed in detail.

Subfields: Astronomy and Astrophysics, Computational Mechanics

View Paper: https://www.semanticscholar.org/paper/3438e5f43c653da9d59bffff84af27bd25a00e57

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Citations in the corpus

HEATING OF THE SOLAR CHROMOSPHERE AND CORONA BY ALFVÉN WAVE TURBULENCE (2011, background): “Schekochihin et al. (2009) considered the extension of the RMHD equations into the kinetic regime.”
DEPENDENCE OF SOLAR-WIND POWER SPECTRA ON THE DIRECTION OF THE LOCAL MEAN MAGNETIC FIELD (2009, background): “…fluctuations makes a transition from an Alfvén wave cascade to a kinetic Alfvén wave (KAW) cascade at the perpendicular wavenumber k⊥ρi 1 (Quataert 1998; Quataert & Gruzinov 1999; Leamon et al. 1999; Cranmer & van Ballegooijen 2003; Howes 2008; Howes et al. 2008; Schekochihin et al. 2008, 2007).”
Solar Wind Turbulence and the Role of Ion Instabilities (2013, methodology): “Chen et al. (2012b) measured the δB‖ fluctuations to be more anisotropic than the Alfvénic component in the fast solar wind, suggesting this as a possible reason why they are not heavily damped (Schekochihin et al. 2009).”
The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere (2019, background): “This has important implications, e.g., the level of anisotropy at kinetic scales is determined by the extent of the inertial range ( Goldreich & Sridhar 1995 ) , and the possible heating mechanisms there depend on the level of anisotropy ( Schekochihin et al. 2009 ) .”
A Model of Turbulence in Magnetized Plasmas: Implications for the Dissipation Range in the Solar Wind (2007, background): “…Alfvénic fluctuations are essentially fluid in nature, satisfy the Reduced MHD (RMHD) equations [Strauss, 1976], and remain undamped until their cascade reaches the perpendicular scale of the ion Larmor radius, k⊥ρi ∼ 1 [Quataert , 1998; Lithwick and Goldreich, 2001; Schekochihin et al., 2007].”
Nature of Kinetic Scale Turbulence in the Earth's Magnetosheath (2016, background): “These features can be interpreted as a cascade of kinetic Alfvén turbulence from ion to electron scales (Howes et al. 2008; Schekochihin et al. 2009; Chen et al. 2010b; Boldyrev & Perez 2012).”
Turbulent Motions and Shocks Waves in Galaxy Clusters simulated with AMR (2009, background): “Remarkably, the interplay between Cosmic Rays (CR) and turbulent magnetic fields may drive still poorly explored plasma processes that may potentially affect our simplified view of the IGM (Subramanian et al. 2006, Schekochihin et al.2007, Brunetti & Lazarian 2007, Guo & Oh 2008).”
Power and spectral index anisotropy of the entire inertial range of turbulence in the fast solar wind (2010, background): “This is the beginning of the effect of kinetic plasma physics close to the ion gyroscale (e.g. Schekochihin et al. 2009), as we show in the next section.”
SOLAR WIND TURBULENT SPECTRUM AT PLASMA KINETIC SCALES (2012, background): “he turbulent ﬂuctuations. Some authors argue that the electron inertial range is a whistler mode turbulence (Saito et al. 2010; Narita and Gary 2010), others suggest KAW turbulence (Bale et al. 2005; Schekochihin et al. 2009; Sahraoui et al. 2010; Salem et al. 2012), or a combination of both types o”
LOCATING THE “MISSING” BARYONS WITH EXTRAGALACTIC DISPERSION MEASURE ESTIMATES (2013, background): “However, it is also possible that the inner scale is set by the much smaller proton-gyro radius (Schekochihin et al. 2009).5 If rdiff < in, τ halosc would be reduced by the factor ∼ (rdiff/ in)1/3 as long as the scattering remained strong.”
Recent progress in astrophysical plasma turbulence from solar wind observations (2016, background): “…have the same scaling as each other is consistent with expectations for kinetic Alfvén turbulence, in which these two fields are directly coupled (Schekochihin et al. 2009; Boldyrev et al. 2013), but the spectral index of −2.8 is steeper than the pure cascade prediction of −7/3 (Equation 4.4)†.”