Physics of the Solar Cycle: New Views

Abstract

Presently there are two schools of thought viz., turbulent dynamo and MHD oscillation mechanisms that explain the solar cycle and activity phenomena. Both the mechanisms are critically examined and fundamental difficulties are presented. By keeping in mind the more advantages of having MHD oscillation mechanism, compared to the turbulent dynamo mechanism, the following new ideas on the genesis of the solar cycle and activity phenomena are presented. The inevitability of the most likely existence of a combined poloidal and toroidal magnetic field structure in the solar interior is proposed. Owing to the suitable poloidal part of the steady field structure, the Alfven wave perturbations of long periods (~ 22 years) that excite in the solar core travel first to the poles in both the hemispheres and later reach the equator. While traveling towards the surface, the Alfven wave perturbations along the weak poloidal field structure in turn perturb the embedded strong toroidal field structure producing sunspots, especially in the convective envelope, that travel to the surface due to buoyancy along isorotational contours. With a realistic density structure of the solar interior, the computation of Alfven wave travel times along different field lines of the poloidal field structure [1] yields almost similar periods ( ~ 22 yr) explaining the constancy of 22 yr periodicity of the odd degree modes obtained from the Spherical Harmonic Fourier analysis of the surface magnetic field. The observed quasi-periodicities of solar activity indices in the range of 1-5 years are explained as due to the Alfven wave perturbation of the strong toroidal field structure. The variation of the long period solar cycle and activity phenomena such as the Maunder and the grand minima is explained to be due to the coupling of long period poloidal and toroidal MHD oscillations.