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Nonlinear evolution of torsional Alfvén waves  

Soheil Vasheghani Farahani   Submitted: 2012-07-31 03:38

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. 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. The solution to the Cohen-Kulsrud 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, 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.

Authors: S. Vasheghani Farahani, V. M. Nakariakov, E. Verwichte, and T. Van Doorsselaere
Projects: None

Publication Status: Accepted by A&A
Last Modified: 2012-08-02 12:56
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Nonlinear long-wavelength tortionalAlfvén waves  

Soheil Vasheghani Farahani   Submitted: 2011-03-28 09:44

Aims: We investigate the nonlinear phenomena accompanyinglong-wavelength torsional waves in solar and stellar coronae.

Methods: The second order thin flux-tube approximation is used todetermine perturbations of a straight untwisted and non-rotatingmagnetic flux-tube, nonlinearly induced by long-wavelengthaxisymmetric 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 plasma-β 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: 2011-03-29 06:23
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Long-wavelength torsional modes of solar coronal plasma structures  

Soheil Vasheghani Farahani   Submitted: 2010-04-30 02:24

We consider the effects of the magnetic twist and plasma rotation on the propagation of torsional m=0 perturbations of cylindrical plasma structures (straight magnetic flux tubes) in the case when the wavelength is much longer than the cylinder diameter. The second order thin flux tube approximation is used to derive dispersion relations and phase relations in linear long-wavelength axisymmetric magnetohydrodynamic waves in uniformly twisted and rotating plasma structures. Asymptotic dispersion relations linking phase speeds with the plasma parameters are derived. When twist and rotation are both present, the phase speed of torsional waves depends upon the direction of the wave propagation, and also the waves are compressible. The phase relations show that in a torsional wave the density and azimuthal magnetic field perturbations are in phase with the axial magnetic field perturbations and anti-phase with tube cross-section perturbations. In a zero-eta non-rotating plasma cylinder confined by the equilibrium twist, the density perturbation is found to be about 66, mathrm{percent} of the amplitude of the twist perturbation in torsional waves.

Authors: S.Vasheghani Farahani, V.M. Nakariakov, and T. Van Doorsselaere

Publication Status: Published in Astronomy & Astrophysics
Last Modified: 2010-09-05 11:15
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Propagating Transverse Waves in Soft X-ray Coronal Jets  

Soheil Vasheghani Farahani   Submitted: 2009-04-01 08:28

Aims. The theoretical model for magnetohydrodynamic (MHD) modes guided by a field-aligned plasma cylinder with a steady flow is adapted to interpret transverse waves observed in solar coronal hot jets, discovered with Hinode/XRT in terms of fast magnetoacoustic kink modes. Methods. Dispersion relations for linear magnetoacoustic perturbations of a plasma jet of constant cross-section surrounded by static magnetised plasma are used to determine the phase and group speeds of guided transverse waves and their relationship with the physical parameters of the jet and the background plasma. The structure of the perturbations in the macroscopic parameters of the plasma inside and outside the jet, and the phase relations between them are also established. Results. We obtained a convenient expansion for the long wave-length limit of the phase and group speeds and have shown that transverse waves observed in soft-X-ray solar coronal jets are adequately described in terms of fast magnetoacoustic kink modes by a magnetic cylinder model, which includes the effect of a steady flow. In the observationally determined range of parameters, the waves are not found to be subject to either the Kelvin-Helmholtz instability or the negative energy wave instability, and hence they are likely to be excited at the source of the jet.

Authors: S.Vasheghani Farahani, T. Van Doorsselaere, E. Verwichte, and V.M. Nakariakov
Projects: Hinode/XRT

Publication Status: Published in A&A
Last Modified: 2010-09-05 11:14
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Abstracts by Author
Nonlinear evolution of torsional Alfven waves
Nonlinear long-wavelength tortional Alfvén waves
Long-wavelength torsional modes of solar coronal plasma structures
Propagating Transverse Waves in Soft X-ray Coronal Jets

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