Propagation Of Torsional Alfven Waves Research Articles

Periodic variations of the Hα profile width in the chromosphere of coronal holes as a possible indicator of Alfvén waves

We analyze the oscillations of the Hα profile width based on our observations of the chromosphere at the base of solar coronal holes. The maximum oscillation amplitude averaged over ten time series is 64 m 0 A. Direct calculations show that this value cannot be reached through temperature oscillations, because the periodic intensity fluctuations observed during our experiment did not exceed 5%, corresponding to Hα profile broadening only by 1.5–2 m A. We hypothesize that the observed variations can result from the propagation of torsional Alfven waves in the chromosphere of coronal holes.

Torsional Alfvén waves in partially ionized solar plasma: effects of neutral helium and stratification

Ion-neutral collisions may lead to the damping of Alfven waves in chromospheric and prominence plasmas. Neutral helium atoms enhance the damping in certain temperature interval, where the ratio of neutral helium and neutral hydrogen atoms is increased. Therefore, the height-dependence of ionization degrees of hydrogen and helium may influence the damping rate of Alfven waves. We aim to study the effect of neutral helium in the damping of Alfven waves in stratified partially ionized plasma of the solar chromosphere. We consider a magnetic flux tube, which is expanded up to 1000 km height and then becomes vertical due to merging with neighboring tubes, and study the dynamics of linear torsional Alfven waves in the presence of neutral hydrogen and neutral helium atoms. We start with three-fluid description of plasma and consequently derive single-fluid magnetohydrodynamic (MHD) equations for torsional Alfven waves. Thin flux tube approximation allows to obtain the dispersion relation of the waves in the lower part of tubes, while the spatial dependence of steady-state Alfven waves is governed by Bessel type equation in the upper part of tubes. Consecutive derivation of single-fluid MHD equations results in a new Cowling diffusion coefficient in the presence of neutral helium which is different from previously used one. We found that shorter-period (< 5 s) torsional Alfven waves damp quickly in the chromospheric network due to ion-neutral collision. On the other hand, longer-period (> 5 s) waves do not reach the transition region as they become evanescent at lower heights in the network cores. Propagation of torsional Alfven waves through the chromosphere into the solar corona should be considered with caution: low-frequency waves are evanescent due to the stratification, while high-frequency waves are damped due to ion neutral collisions.

Nonlinear evolution of torsional Alfvén waves

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 Alfven waves along open magnetic fields in the solar and stellar coronae, and compare it with the same effects for the shear Alfven wave. The evolution of the torsional Alfven wave is caused by the back reaction of nonlinearly induced compressive perturbations on the Alfven wave.Methods. The evolution of upwardly propagating torsional Alfven 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) Alfven 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 Alfven wave. In a finite-β plasma, the nonlinear evolution of torsional Alfven waves is slower and the parallel nonlinear cascade is less efficient than those of shear Alfven 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 Alfven waves instead of torsional Alfven waves, over-estimate the efficiency of these processes.

Cutoff‐free Propagation of Torsional Alfven Waves along Thin Magnetic Flux Tubes

Propagation of torsional Alfv\'en waves along magnetic flux tubes has been extensively studied for many years but no conclusive results regarding the existence of a cutoff frequency for these waves have been obtained. The main purpose of this paper is to derive new wave equations that describe the propagation of linear torsional Alfv\'en waves along thin and isothermal magnetic flux tubes, and use these wave equations to demonstrate that the torsional wave propagation is not affected by any cutoff frequency. It is also shown that this cutoff-free propagation is independent of different choices of the coordinate systems and wave variables adopted in the previous studies. A brief discussion of implications of this cutoff-free propagation of torsional tube waves on theories of wave heating of the solar and stellar atmospheres is also given.

Propagation of torsional Alfven waves in an inhomogeneous plasma

In ideal MHD, the behaviour of the torsional Alfven wave in a cylindrical, inhomogeneous plasma depends on the m spectrum of the wave, where m is the azimuthal mode number. It is well known that a pure m=0 mode is phase-mixed by a density gradient and that modes with integer m are coupled to the fast wave at an Alfven resonance layer. The author considers the propagation of a narrow, magnetically guided beam in an inhomogeneous, current-free plasma. Such a beam contains an infinite spectrum of m components. It is shown that a guided beam is decoupled from the fast wave, despite the fact that omega lies in the Alfven continuum, and it is not phase-mixed by the density gradient. The relation omega /kz= nu A(x,y) describing modes with zero or integer m is therefore not appropriate for a wave containing an infinite spectrum of m components.

The propagation of torsion along flux tubes subject to dynamical nonequilibrium

It is noted that the dynamical nonequilibrium of close-packed flux tubes is driven by the torsion in the individual tubes. Because of this, whenever tubes with the same sense of twisting come into contact, there is reconnection of their azimuthal field components. The reconnection consumes the local torsion, and this causes the propagation of torsional Alfven waves into the region from elsewhere along the tubes. The formal problem of the propagation of the torsion along twisted flux tubes is presented, along with some of the basic physical properties worked out in the limit of small torsion. It is noted that in tubes with finite twisting the propagation of torsional Alfven waves can be a more complicated phenomenon. Application to the sun suggests that the propagation of torsion from below the visible surface up into the corona is an important energy supply to the corona for a period of perhaps 10-20 hours after the emergence of the flux tubes through the surface of the sun, bringing up torsion from depths of 10,000 km or more. Torsion is of course continually furnished by the manipulation and shuffling of the field by the convection.

Investigation of plasma-magnetic waves in an inhomogeneous cylindrical plasma

The effects of a resistivity gradient on the propagation of torsional Alfven-waves in a current carrying plasma is investigated experimentally and theoretically.