Effects of parallel ion dynamics on drift-Alfvén vortices in plasmas

Abstract

Drift-Alfven vortices are investigated, taking into account the nonlinear ion dynamics parallel to the external magnetic field. It is found that the parallel ion motion restricts the vortex speed.Recently, a number of authors (Shukla 1985;Shukla, Yu & Varma 1985;Shukla, Yu & Stenflo 1986;Liu & Horton 1986;Mikhailovskii et al. 1987;Kaladze et al. 1987) have investigated the formation of drift-Alfven vortices in plasmas. However, all of these studies have been restricted to the twodimensional ion-fluid and three-dimensional electron-fluid motions. It has been found that drift-Alfven vortices can propagate with sub-Alfvenic velocity with no lower bound to their speed. In this paper we incorporate the nonlinear ion dynamics parallel to the external magnetic field and show that the sound term restricts the propagation speed of drift-Alfven vortices.If we represent the electromagnetic fields as E = -V<f>-zc-1 d t A z , B = VA Z x z (2)then the perpendicular (to the external magnetic field B o z) components of the electron-and ion-fluid velocities in the drift approximation (d t <i Q 4 = eB a /m l c, the ion gyrofrequency) are given respectively by %V• (4)where 0 is the scalar potential, A z is the parallel component of the vector potential, n e is the electron number density, T e is the electron temperature, e is the electronic charge and c is the speed of light. We have assumed cold ions and that (c/B 0 )zx V0. V > v iz d z . Furthermore, in view of the low-/? (= 8nn 0 T e /Bl <£ 1) approximation, compressional magnetic field effects are neglected.From the z component of Ampere's law, we find a relationship between the parallel component of the electron-and ion-fluid velocities and the parallel vector potential. We have