Analytic meanfield α^{2}dynamo with a forcefree corona
Abstract
Context. Stellar dynamos are affected by boundary conditions imposed by stellar coronae. Under some approximations, it is possible to find analytical solutions. Interior dynamo models often consider a currentfree corona without taking into account the constraints imposed by the presence of currents in the corona.
Aims: We aim to analytically evaluate the effect of coronal currents and of an outer boundary condition on the efficiency of an α^{2} dynamo. We intend to estimate the change in geometry and dynamo excitation numbers with respect to the currentfree case.
Methods: We analytically solved the turbulent dynamo induction equation for a homogeneous, nonmirror symmetric turbulence in a spherical domain surrounded by a linear forcefree corona with the mean magnetic field B satisfying ∇ × B = βB.
Results: The dynamo number is a decreasing function of β. Moreover, if the current is parallel to the field (β > 0), the dynamo number is smaller than in the forcefree case. In contrast, for (β < 0), the dynamo number is greater than in the forcefree case.
Conclusions: Currents in the corona need to be taken into account because they affect the condition for excitation of a dynamo.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 September 2017
 DOI:
 10.1051/00046361/201731330
 arXiv:
 arXiv:1706.02223
 Bibcode:
 2017A&A...605A..33B
 Keywords:

 dynamo;
 stars: coronae;
 stars: magnetic field;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 Accepted for publication in A&