Sustained magneto-shear instabilities in the solar tachocline

We present nonlinear three-dimensional simulations of the stably-stratified portion of the solar tachocline in which the rotational shear is maintained by mechanical forcing. When a broad toroidal field profile is specified as an initial condition, a clam-shell instability ensues which is similar to the freely-evolving cases studied previously. After the initial nonlinear saturation, the residual mean fields are apparently too weak to sustain the instability indefinitely. However, when a mean poloidal field is imposed in addition to the rotational shear, a statistically-steady state is achieved in which the clam-shell instability is operating continually. This state is characterized by a quasi-periodic exchange of energy between the mean toroidal field and the instability mode with a longitudinal wavenumber m=1. This quasi-periodic behavior has a timescale of several years and may have implications for tachocline dynamics and field emergence patterns throughout the solar activity cycle.Comment: 5 pages, 3 figures (eps format). Fig. 3 also in jpg format. Submitted to Astrophysical Journal Letter

A theoretical study of the build-up of the Sun's polar magnetic field by using a 3D kinematic dynamo model

We develop a three-dimensional kinematic self-sustaining model of the solar dynamo in which the poloidal field generation is from tilted bipolar sunspot pairs placed on the solar surface above regions of strong toroidal field by using the SpotMaker algorithm, and then the transport of this poloidal field to the tachocline is primarily caused by turbulent diffusion. We obtain a dipolar solution within a certain range of parameters. We use this model to study the build-up of the polar magnetic field and show that some insights obtained from surface flux transport (SFT) models have to be revised. We present results obtained by putting a single bipolar sunspot pair in a hemisphere and two symmetrical sunspot pairs in two hemispheres.We find that the polar fields produced by them disappear due to the upward advection of poloidal flux at low latitudes, which emerges as oppositely-signed radial flux and which is then advected poleward by the meridional flow. We also study the effect that a large sunspot pair, violating Hale's polarity law would have on the polar field. We find that there would be some effect---especially if the anti-Hale pair appears at high latitudes in the mid-phase of the cycle---though the effect is not very dramatic.Comment: 18 pages, 18 figures, published in Ap