Helicity and dynamo action in magnetized stellar radiation zones

Abstract

Helicity and \alpha effect driven by the nonaxisymmetric Tayler instability of toroidal magnetic fields in stellar radiation zones are computed. In the linear approximation a purely toroidal field always excites pairs of modes with identical growth rates but with opposite helicity so that the net helicity vanishes. If the magnetic background field has a helical structure by an extra (weak) poloidal component then one of the modes dominates producing a net kinetic helicity anticorrelated to the current helicity of the background field. The mean electromotive force is computed with the result that the \alpha effect by the most rapidly growing mode has the same sign as the current helicity of the background field. The \alpha effect is found as too small to drive an \alpha^{2} dynamo but the excitation conditions for an \alpha\Omega dynamo can be fulfilled for weak poloidal fields. Moreover, if the dynamo produces its own \alpha effect by the magnetic instability then problems with its sign do not arise. For all cases, however, the \alpha effect shows an extremely strong concentration to the poles so that a possible \alpha\Omega dynamo might only work at the polar regions. Hence, the results of our linear theory lead to a new topological problem for the existence of large-scale dynamos in stellar radiation zones on the basis of the current-driven instability of toroidal fields.