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

Abstract

Context. Both dynamo theory and observations of stellar large-scale magnetic fields suggest a change from nearly axisymmetric configurations at solar rotation rates to nonaxisymmetric configurations for rapid rotation. Aims. We seek to understand this transition using numerical simulations. Methods. We use three-dimensional simulations of turbulent magnetohydrodynamic convection in spherical shell wedges and considered rotation rates between 1 and 31 times the solar value. Results. We find a transition from axi- to nonaxisymmetric solutions at around 1.8 times the solar rotation rate. This transition coincides with a change in the rotation profile from antisolar- to solar-like differential rotation with a faster equator and slow poles. In the solar-like rotation regime, the field configuration consists of an axisymmetric oscillatory field accompanied by an m = 1 azimuthal mode (two active longitudes), which also shows temporal variability. At slow (rapid) rotation, the axisymmetric (nonaxisymmetric) mode dominates. The axisymmetric mode produces latitudinal dynamo waves with polarity reversals, while the nonaxisymmetric mode often exhibits a slow drift in the rotating reference frame and the strength of the active longitudes changes cyclically over time between the different hemispheres. In the majority of cases we find retrograde waves, while prograde waves are more often found from observations. Most of the obtained dynamo solutions exhibit cyclic variability either caused by latitudinal or azimuthal dynamo waves. In an activity-period diagram, the cycle lengths normalized by the rotation period form two different populations as a function of rotation rate or magnetic activity level. The slowly rotating axisymmetric population lies close to what in observations is called the inactive branch, where the stars are believed to have solar-like differential rotation, while the rapidly rotating models are close to the superactive branch with a declining cycle to rotation frequency ratio and an increasing rotation rate. Conclusions. We can successfully reproduce the transition from axi- to nonaxisymmetric dynamo solutions for high rotation rates, but high-resolution simulations are required to limit the effect of rotational quenching of convection at rotation rates above 20 times the solar value.

Magneto-hydrodynamical origin of eclipsing time variations in post-common-envelope binaries for solar mass secondaries

“…The decrease of differential rotation as the overall rotation rate is increased is consistent with earlier studies (e.g. Viviani et al 2018).…”

“…In this case, we obtain a Taylor number Ta= 2.55 × 10 9 , a Coriolis number Co= 59.7, a Reynolds number Re= 21.2, a magnetic Reynolds number Re M = 21.2 and a magnetic Prandtl number Pr M = 1. The latter lies still within the parameter space also explored by Viviani et al (2018) for the stellar dynamo. Figure 16 shows the near-surface radial magnetic field at the end of run20x.…”

“…However, the simulations in previous studies have already proven to be of extreme success in reproducing some of the solar phenomena (see e.g. Viviani et al 2018;Käpylä et al 2016Käpylä et al , 2013Käpylä et al , 2012. Further development of 3D MHD simulations of fully-convective stars will prove to be of great importance as we expect the Applegate mechanism to be an important tool for studying M dwarfs dynamos through eclipsing time variations.…”

Stellar dynamos with solar and antisolar differential rotations: Implications to magnetic cycles of slowly rotating stars

“…Indeed, Karak et al (2015) found an increase of the magnetic field in the anti-solar DR regime. Simulations of Warnecke (2018) in different parameter regimes seem to show a little increase of the magnetic field, in contrast, Viviani et al (2018) did not find significant increase. This remains a curiosity in the community whether the magnetic field indeed increases in the anti-solar DR regime and if so, then whether this is actually caused by the anti-solar DR.…”

On the Origin of Solar Torsional Oscillations and Extended Solar Cycle

“…Our model describes the dynamo processes distributed in the convection zone. Previously, it was argued that such type of distributed dynamo models provides satisfactory agreement with solar and stellar observations (Pipin 2015;Brandenburg 2018;Viviani et al 2018). Previous models of the torsional oscillations were constructed by coupling the dynamo equations with equations of the angular momentum transport (e.g.…”