The fratricide of αΩ dynamos by their α² siblings

¹ Nordita, AlbaNova University Center, Roslagstullsbacken 23, 10691 Stockholm, Sweden
e-mail: hubbard@nordita.org
² Department of Astronomy, Stockholm University, 10691 Stockholm, Sweden

Received: 13 February 2011
Accepted: 2 September 2011

Abstract

Context. Helically forced magneto-hydrodynamic shearing-sheet turbulence can support different large-scale dynamo modes, although the αΩ mode is generally expected to dominate because it is the fastest growing one. In an αΩ dynamo, most of the field amplification is produced by the shear. As differential rotation is an ubiquitous source of shear in astrophysics, such dynamos are believed to be the source of most astrophysical large-scale magnetic fields.

Aims. We study the stability of oscillatory migratory αΩ type dynamos in turbulence simulations.

Methods. We use shearing-sheet simulations of hydromagnetic turbulence that is helically forced at a wavenumber that is about three times larger than the lowest wavenumber in the domain so that both αΩ and α² dynamo action is possible.

Results. After initial dominance and saturation, the αΩ mode is found to be destroyed by an orthogonal α² mode sustained by the helical turbulence alone. We show that there are at least two processes through which this transition can occur.

Conclusions. The fratricide of αΩ dynamos by its α² sibling is discussed in the context of grand minima of stellar activity. However, the genesis of αΩ dynamos from an α² dynamo has not yet been found.