What sets the magnetic field strength and cycle period in solar-type stars?

G. Guerrero, B. Zaire, P. K. Smolarkiewicz, E. M. de Gouveia Dal Pino, A. G. Kosovichev, N. N. Mansour

Published 2018-10-18Version 1

Observations of stellar magnetism indicate that stars of types F, G and K have magnetic activity similar to that observed in the Sun. Two fundamental properties of stellar magnetic fields have been determined for a significant number of stars with different Rossby numbers (Ro), namely the magnetic field strength and the magnetic cycle period. The field strength exhibits two regimes: 1) for fast rotation the field strength is independent of Ro, 2) for slow rotation the field decays with Ro with power law dependence. For the magnetic cycle period two regimes of activity, the so called active and inactive branches, have been identified. For both of them, the longer the rotation period, the longer the activity cycle. In this paper we present results of global dynamo simulations which show that the rotational shear in both, the tachocline and the near-surface layer, plays a fundamental role in explaining these observational properties. The results are consistent with non-linear alpha-Omega dynamos with different modes and levels of complexity. The magnetic field strength can be explained from results of a mean-field analysis. The power-law decay of the field strength in slow rotating stars is attributed to the decrease of the near-surface radial shear with Ro and to the meridional motions that advect the poloidal magnetic field out of the surface layers. In agreement with the observations, the magnetic cycle period increases with the rotational period. All the periods are consistent with the active branch of activity. However, a bifurcation is observed for Ro ~1. It separates a regime where the oscillatory dynamo operates in the convection zone, from a regime where the tachocline has a predominant role. In the latter, the periods are believed to result from the periodic energy exchange between the dynamo and the magneto-shear instabilities developing in the tachocline region.