Nonlinear evolution of torsional Alfvén waves 

Soheil Vasheghani Farahani Submitted: 20120731 03:38
We study the efficiency of the energy transfer to shorter scales in the fieldaligned 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.
The evolution of upwardly propagating torsional Alfvén waves is considered in terms of the secondorder thin
fluxtube approximation in a straight untwisted and nonrotating magnetic fluxtube. The CohenKulsrud equation for weakly nonlinear torsional waves is derived. In the model, the effect of the cubic nonlinearity on the propagation of longwavelength axisymmetric torsional waves is compared with the similar effect that accompanies the propagation of plane linearlypolarised (shear) Alfvén waves of small amplitude.
The solution to the CohenKulsrud type equation for torsional waves shows that their evolution is independent
of the plasma?, which is in contrast to the shear Alfvén wave. In a finite? 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, onedimensional models of coronal heating and wave acceleration, which use shear Alfvén waves instead of torsional Alfvén waves, overestimate the efficiency of these processes.
Authors: S. Vasheghani Farahani, V. M. Nakariakov, E. Verwichte, and T. Van Doorsselaere
Projects: None

Publication Status: Accepted by A&A
Last Modified: 20120802 12:56



Nonlinear longwavelength tortionalAlfvén waves 

Soheil Vasheghani Farahani Submitted: 20110328 09:44
Aims: We investigate the nonlinear phenomena accompanyinglongwavelength torsional waves in solar and stellar coronae.
Methods: The second order thin fluxtube approximation is used todetermine perturbations of a straight untwisted and nonrotatingmagnetic fluxtube, nonlinearly induced by longwavelengthaxisymmetric magnetohydrodynamic waves of small, but finite amplitude.
Results: Propagating torsional waves induce compressible perturbationsoscillating with double the frequency of the torsional waves.In contrast with plane shear Alfvén waves, the amplitude ofcompressible perturbations is independent of the plasmaβ and isproportionalto the torsional wave amplitude squared. Standing torsional wavesinduce compressible perturbations of two kinds, thatgrow with the characteristic time inversely proportional to the soundspeed, and that oscillate at double the frequency of the inducingtorsional wave. The growing density perturbation saturates at thelevel, inversely proportional to the sound speed.
Authors: S. Vasheghani Farahani, V. M. Nakariakov, T. Van Doorsselaere, and E. Verwichte
Projects: None

Publication Status: A&A 526, A80 (2011)
Last Modified: 20110329 06:23




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