Motion of Axisymmetric Magnetic Eddies with Swirl

Abstract

We consider the motion of axisymmetric magnetic eddies with swirl in ideal magnetohydrodynamic (MHD) flow. The magnetic field is assumed to be toroidal, while the velocity field has both toroidal and poloidal components. The contour-dynamics formu- lation by Hattori and Moffatt (2006) for the case without swirl is extended to include swirl velocity so that the cross helicity does not vanish in general. The strength of the vortex sheets that appear on the contours varies with time under the influence of the centrifugal force due to swirl and the magnetic tension due to the Lorentz force. Numerical simulation using the contour-dynamics formulation shows that there exist counter-propagating dipolar structures whose radius is given by a balance between the centrifu- gal force and the magnetic tension. These structures are well described by the steady solutions obtained by perturbation expansion. The effects of vorticity inside the eddy on the motion of eddies are also investigated.