Analytic solution of an oscillatory migratory α² stellar dynamo

¹ Nordita, KTH Royal Institute of Technology and Stockholm University, 10691 Stockholm, Sweden
e-mail: brandenb@nordita.org
² JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80303, USA
³ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA
⁴ Department of Astronomy, Stockholm University, 10691 Stockholm, Sweden

Received: 8 November 2016
Accepted: 1 December 2016

Abstract

Context. Analytic solutions of the mean-field induction equation predict a nonoscillatory dynamo for homogeneous helical turbulence or constant α effect in unbounded or periodic domains. Oscillatory dynamos are generally thought impossible for constant α.

Aims. We present an analytic solution for a one-dimensional bounded domain resulting in oscillatory solutions for constant α, but different (Dirichlet and von Neumann or perfect conductor and vacuum) boundary conditions on the two boundaries.

Methods. We solve a second order complex equation and superimpose two independent solutions to obey both boundary conditions.

Results. The solution has time-independent energy density. On one end where the function value vanishes, the second derivative is finite, which would not be correctly reproduced with sine-like expansion functions where a node coincides with an inflection point. The field always migrates away from the perfect conductor boundary toward the vacuum boundary, independently of the sign of α.

Conclusions. The obtained solution may serve as a benchmark for numerical dynamo experiments and as a pedagogical illustration that oscillatory migratory dynamos are possible with constant α.