G. Rudiger et al., Alpha-effect, helicity and angular momentum transport for a magnetically driven turbulence in the solar convection zone, SOLAR PHYS, 198(2), 2001, pp. 241-251
For a given field of magnetic fluctuations the dynamo-alpha, as well as the
kinetic and current helicities, have been computed. assuming that turbulen
ce is subject to magnetic buoyancy and global rotation. The sign of the dyn
amo-alpha is positive in the northern hemisphere and negative in the southe
rn hemisphere. The kinetic helicity has just the same latitudinal distribut
ion as the alpha -effect, indeed there is no minus sign between the dynamo-
alpha and the kinetic helicity. Also the current helicity [j . B] changes i
ts sign at the equator. It is negative in the northern hemisphere and posit
ive in the southern hemisphere. Our current helicities (due to fluctuations
) and alpha -effects are thus always out of phase, this confirming a previo
us result of Keinigs (1983) and Radler and Seehafer (1990). The signs of th
e alpha -effect and both helicities correspond to the numerical simulations
by Brandenburg and Schmitt (1998). We have also computed the turbulent ang
ular momentum transport, which proves to be always inwards, as in hydrodyna
mic simulations without magnetic fields (Chau, 1999). Thus we can easily ex
plain why, in the supergranulation zone, deeper layers appear to rotate fas
ter than the solar surface plasma, or why in the solar tachocline at high-l
atitudes the angular velocity decreases outwards. The dynamo number derived
from the observed current helicity reveals the alpha -effect to be rather
small, unless the magnetic eddy diffusivity is not as low as the sunspot de
cay suggests (i.e., 10(11) cm(2) s(-1)).