Nonlinear evolution of torsional Alfvén waves

¹ Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B bus 2400, 3001 Leuven, Belgium
e-mail: soheil.vasheghanifarahani@wis.kuleuven.be
² Centre for Fusion, Space and Astrophysics, Physics Department, University of Warwick, Coventry CV4 7AL, UK
³ Central Astronomical Observatory of the Russian Academy of Sciences at Pulkovo, 196140 St Petersburg, Russia
⁴ Postdoctoral fellow of the FWO vlaanderen Belgium

Received: 9 May 2012
Accepted: 24 July 2012

Abstract

Aims. We study the efficiency of the energy transfer to shorter scales in the field-aligned direction – the parallel nonlinear cascade – that accompanies the propagation of torsional Alfvén waves along open magnetic fields in the solar and stellar coronae, and compare it with the same effects for the shear Alfvén wave. The evolution of the torsional Alfvén wave is caused by the back reaction of nonlinearly induced compressive perturbations on the Alfvén wave.

Methods. The evolution of upwardly propagating torsional Alfvén waves is considered in terms of the second-order thin flux-tube approximation in a straight untwisted and non-rotating magnetic flux-tube. The Cohen-Kulsrud equation for weakly nonlinear torsional waves is derived. In the model, the effect of the cubic nonlinearity on the propagation of long-wavelength axisymmetric torsional waves is compared with the similar effect that accompanies the propagation of plane linearly-polarised (shear) Alfvén waves of small amplitude.

Results. The solution to the Cohen-Kulsrud type equation for torsional waves shows that their evolution is independent of the plasma-β, which is in contrast to the shear Alfvén wave. In a finite-β plasma, the nonlinear evolution of torsional Alfvén waves is slower and the parallel nonlinear cascade is less efficient than those of shear Alfvén waves. These results have important implications for the analysis of possible heating of the plasma and its acceleration in the upper layers of solar and stellar coronae. In particular, one-dimensional models of coronal heating and wave acceleration, which use shear Alfvén waves instead of torsional Alfvén waves, over-estimate the efficiency of these processes.