Fluid Models for Kinetic Effects on Coherent Nonlinear Alfven Waves. I. Fundamental Theory

摘要

Collisionless regime kinetic models for coherent nonlinear Alfven wavedynamics are studied using fluid moment equations with an approximate closureanzatz. Resonant particle effects are modelled by incorporating an additionalterm representing dissipation akin to parallel heat conduction. Unlikecollisional dissipation, parallel heat conduction is presented by an integraloperator. The modified derivative nonlinear Schrodinger equation thus has aspatially nonlocal nonlinear term describing the long-time evolution of theenvelope of parallel-propagating Alfven waves, as well. Coefficients in thenonlinear terms are free of the 1/(1-beta) singularity usually encountered inprevious analyses, and have very a simple form which clarifies the physicalprocesses governing the large amplitude Alfvenic nonlinear dynamics. Thenonlinearity appears via coupling of an Alfvenic mode to a kinetic ion-acousticmode. Damping of the nonlinear Alfven wave appears via strong Landau damping ofthe ion-acoustic wave when the electron-to-ion temperature ratio is close tounity. For a (slightly) obliquely propagating wave, there are finite Larmorradius corrections in the dynamical equation. This effect depends on the angleof wave propagation relative to B_0 and vanishes for the limit of strictlyparallel propagation. Explicit magnetic perturbation envelope equationsamenable to further analysis and numerical solution are obtained. Implicationsof these models for collisionless shock dynamics are discussed.