Alpha-Effect, Helicity and Angular Momentum Transport for a Magnetically Driven Turbulence in the Solar Convection Zone

Abstract

For a given field of magnetic fluctuations the dynamo-α, as well as the kinetic and current helicities, have been computed, assuming that turbulence is subject to magnetic buoyancy and global rotation. The sign of the dynamo-α is positive in the northern hemisphere and negative in the southern hemisphere. The kinetic helicity has just the same latitudinal distribution as the α-effect, indeed there is no minus sign between the dynamo-α and the kinetic helicity. Also the current helicity 〈j ⋅ B 〉 changes its sign at the equator. It is negative in the northern hemisphere and positive in the southern hemisphere. Our current helicities (due to fluctuations) and α-effects are thus always out of phase, this confirming a previous result of Keinigs (1983) and Radler and Seehafer (1990). The signs of the α-effect and both helicities correspond to the numerical simulations by Brandenburg and Schmitt (1998). We have also computed the turbulent angular momentum transport, which proves to be always inwards, as in hydrodynamic simulations without magnetic fields (Chan, 1999). Thus we can easily explain why, in the supergranulation zone, deeper layers appear to rotate faster than the solar surface plasma, or why in the solar tachocline at high-latitudes the angular velocity decreases outwards. The dynamo number derived from the observed current helicity reveals the α-effect to be rather small, unless the magnetic eddy diffusivity is not as low as the sunspot decay suggests (i.e., 1011 cm2 s−1).