Mean field solar surface dynamo in the presence of partially ionized plasmas and sub-surface shear layer

Abstract

A non-linear α − Ω dynamo in the partially ionized turbulent plasma in the presence of sub-surface velocity shear is studied with mean-field electrodynamics. Such a dynamo is probably operational in the near-surface region of the Sun, where the presence of both neutrals and the velocity shear (due to sub-surface shear layer in the rotation profile) is observationally well established. In particular, we show that the inclusion of ambipolar diffusion leads to a saturation of magnetic field amplitudes in the α − Ω dynamo. We also demonstrate that the temporal evolution of large-scale global magnetic fields follows the well-known pattern similar to the ‘butterfly’ diagram displayed by sunspots. As usual the velocity shear converts part of the poloidal into the toroidal magnetic field which in turn is regenerated largely by the combined kinetic plus Hall helicity, thus closing the dynamo loop. In addition, by allowing temporal variation in the helicity and ambipolar diffusion coefficient we are able to reproduce the grand-minimum type behaviour of the solar dynamo. Details of theoretical model along with numerical computations of dynamo equations in the partially ionized plasma are outlined. The solar surface dynamo model envisaged in this work could operate in conjunction with the global dynamo present in the bulk of the convection zone.