Transition from axi- to nonaxisymmetric dynamo modes in spherical convection models of solar-like stars

摘要

We seek to understand this transition using numerical simulations. We usethree-dimensional simulations of turbulent magnetohydrodynamic convection inspherical shell segments covering the full $360\degr$ longitudinal range andlatitudes within $\pm75\degr$. We consider angular velocities between one and31 times solar. We find a transition from axi- to nonaxisymmetric solutions ataround 1.8 time solar rotation that coincides with the simultaneous change ofthe rotation profiles from antisolar to solar-like ones. In the solar-likerotation regime, the field configuration consists of an axisymmetricoscillatory field accompanied with the first azimuthal mode $m=1$ (two activelongitudes), which also shows temporal variability. At slow (rapid) rotationthe axisymmetric (nonaxisymmetric) mode dominates. The axisymmetric modeproduces latitudinal dynamo waves with polarity reversals, while thenonaxisymmetric mode often exhibits a drift in the rotational reference frameand the strengths of the active longitudes change cyclically over time betweenthe different hemispheres. The latter two phenomena are azimuthal dynamo waves.In the majority of cases we find retrograde waves -- in disagreement withobservations. In an activity--period diagram, the cycle lengths normalized tothe rotation period fall on clearly distinct branches as a function of rotationor magnetic activity level. We can clearly identify the inactive branch, butthe active and superactive branches appear to be merged into one. As therotation rate increases, the length scale of convection is decreasing, and thesupercriticality of convection is decreasing. The results presented are thefirst step to understand dynamos in stars with vastly varying rotation rates,with many new findings, but they also clearly indicate the need forhigh-resolution simulations in a more supercritical regime with grids thatcover full spheres.