What Sets the Magnetic Field Strength and Cycle Period in Solar-type Stars?
Résumé
Two fundamental properties of stellar magnetic fields have been determined by observations for solar-like 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, it is independent of {{Ro}}, and (2) for slow rotation, it decays with {{Ro}} following a power law. For the magnetic cycle period, two regimes of activity, the active and inactive branches, have also been identified. For both of them, the longer the rotation period, the longer the activity cycle. Using global dynamo simulations of solar-like stars with Rossby numbers between ∼0.4 and ∼2, this paper explores the relevance of rotational shear layers in determining these observational properties. Our results, consistent with nonlinear {α }2{{Ω }} dynamos, show that the total magnetic field strength is independent of the rotation period. Yet at surface levels, the origin of the magnetic field is determined by {{Ro}}. While for {{Ro}}≲ 1, it is generated in the convection zone, for {{Ro}}≳ 1, strong toroidal fields are generated at the tachocline and rapidly emerge toward the surface. In agreement with the observations, the magnetic cycle period increases with the rotational period. However, a bifurcation is observed for {{Ro}}∼ 1, separating a regime where oscillatory dynamos operate mainly in the convection zone from the regime where the tachocline has a predominant role. In the latter, the cycles are believed to result from the periodic energy exchange between the dynamo and the magneto-shear instabilities developing in the tachocline and the radiative interior.