Ref: SEMIN-2017-0031

Nonlinear evolution of torsional Alfven waves in magnetic flux

Shestov, S

Seminar presented at KU Leuven on 2017-01-26

Abstract: Torsional Alfven waves are azimuthal (rotational) perturbations of the plasma velocity accompanied by azimuthal components of the magnetic field. Linear torsional waves propagate along the magnetic field at the local Alfven speed. The existence of plane Alfven waves was theoretically predicted by Alfven in 1942, and since then the ubiquitous presence of Alfven waves was found in magnetospheric, space, cosmic and laboratory plasmas. In solar physics Alfven waves are mainly considered as a candidate for coronal heating, due to their ability to freely propagate from lower layers of the solar atmosphere to the corona. In addition, Alfven waves provide a mechanism for acceleration of energetic electrons, solar wind etc. The key element of the nonlinear evolution of Alfven waves (both plane and torsional) is the ponderomotive force, that produce gradients in the magnetic pressure, induces parallel plasma flows, and perturbs plasma density. Besides, the induced variations change local values of the Alfven speed, and cause the Alfven wave self-interaction resulting in steepening of the wave front. Nonlinear dynamics of Alfven waves is actively studied analytically and numerically, with the main emphasis put on the study of plane waves. However, in a plane wave the perpendicular scale should be much larger than the wavelength, which is rarely fulfilled in the case of the solar corona. Since corona is structured into magnetic tubes, represented by coronal loops or open field structures, coronal Aflven waves are likely to appear in the form of torsional, instead of plane perturbations. The aim of our work is to study by means of 3D MHD modeling the effects of nonlinear excitation of compressive perturbations by a torsional wave of finite amplitude. The simulations are performed with MPI-AMRVAC code in a cylindrical reference frame. We set up an equilibrium magnetic tube with background magnetic field parallel to the axis and perturb azimuthal components of velocity and magnetic field at the tube boundary. The observed dynamics in general agree with theory: torsional wave propagate along the tube with the Alfven speed, compressible perturbations are induced, that have double the spatial frequency. Indeed, we found several effects not considered before. For example, induced radial and parallel velocities depend on radial coordinate. The induced plasma motions generate a sausage wave, that propagates faster than the Alfven wave. Finally, wave profile steepening differs from the case of plane wave. In the talk we discuss main aspects of the torsional Alfven wave dynamics.

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The record appears in these collections:
Royal Observatory of Belgium > Astronomy & Astrophysics
Conference Contributions & Seminars > Seminars
Solar-Terrestrial Centre of Excellence

 Record created 2017-12-18, last modified 2017-12-18