Astrophysical gyrokinetics: kinetic and fluid turbulent cascades in magnetized weakly collisional plasmas

摘要

We present a theoretical framework for plasma turbulence in astrophysicalplasmas (solar wind, interstellar medium, galaxy clusters, accretion disks).The key assumptions are that the turbulence is anisotropic with respect to themean magnetic field and frequencies are low compared to the ion cyclotronfrequency. The energy injected at the outer scale scale has to be convertedinto heat, which ultimately cannot be done without collisions. A KINETICCASCADE develops that brings the energy to collisional scales both in space andvelocity. Its nature depends on the physics of plasma fluctuations. In each ofthe physically distinct scale ranges, the kinetic problem is systematicallyreduced to a more tractable set of equations. In the "inertial range" above theion gyroscale, the kinetic cascade splits into a cascade of Alfvenicfluctuations, which are governed by the RMHD equations at both the collisionaland collisionless scales, and a passive cascade of compressive fluctuations,which obey a linear kinetic equation along the moving field lines associatedwith the Alfvenic component. In the "dissipation range" between the ion andelectron gyroscales, there are again two cascades: the kinetic-Alfven-wave(KAW) cascade governed by two fluid-like Electron RMHD equations and a passivephase-space cascade of ion entropy fluctuations. The latter cascade brings theenergy of the inertial-range fluctuations that was damped by collisionlesswave-particle interaction at the ion gyroscale to collisional scales in thephase space and leads to ion heating. The KAW energy is similarly damped at theelectron gyroscale and converted into electron heat. Kolmogorov-style scalingrelations are derived for these cascades. Astrophysical and space-physicalapplications are discussed in detail.