Solar torsional oscillations due to the magnetic quenching of the Reynolds stress.
Abstract
The solar torsional oscillations are considered as the response of the Reynolds stress to the timedependent dynamoinduced magnetic field. This picture is opposite to the so far accepted idea that it is the largescale Lorentz force which directly drives the temporal variations of the surface rotation profile. Here, the 'magnetic quenching'' of the components of the Reynolds stress  viscosity tensor and {LAMBDA}effect  is the basic reason for the cyclic rotation law. In order to produce the suppressing magnetic field it was necessary to construct a turbulent dynamo. Its site is the overshoot region, with the αeffect existing only in an equatorial domain. The produced butterfly diagram is shown in Fig.5. Mainly the toroidal field quenches the turbulent Reynolds stress deep in the convection zone. For a simplified model we find indeed that an observable flow pattern of 12m/s appears with the correct frequency at the solar surface. The pattern can be interpreted as a wave originating at 30deg and vanishing at the equator. The phase relation with respect to the magnetic field does, however, not meet the observations. A more complete model of the solar overshoot dynamo works with turbulence intensities of 20m/s and turnover times from mixing length theory. The complete Reynolds tensor is applied. The magnetic diffusivity below the overshoot domain is put to 10^10^cm^2^/s. Then the surface value of the `torsional oscillation' increases to values up to 3m/s and the phase relation between magnetic cycle and torsional oscillations is correct. The amplitude of the oscillations proves to depend strongly on the magnetic Prandtl number. The results indicate that the value of the turbulent viscosity should not be smaller than 10^12^cm^2^/s.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 August 1996
 Bibcode:
 1996A&A...312..615K
 Keywords:

 MAGNETOHYDRODYNAMICS;
 TURBULENCE;
 SUN: ROTATION;
 SUN: MAGNETIC FIELDS