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The magnetic activity of late-type stars is correlated with their rotationrates. Up to a certain limit, stars with smaller Rossby numbers, defined as therotation period divided by the convective turnover time, have higher activity.A more detailed look at this rotation-activity relation reveals that, ratherthan being a simple power law relation, the activity scaling has a shallowerslope for the low-Rossby stars than for the high-Rossby ones. We find that, forthe chromospheric CaII H&K activity, this scaling relation is well modelled bya broken two-piece power law. Furthermore, the knee-point of the relationcoincides with the axisymmetry to non-axisymmetry transition seen in both thespot activity and surface magnetic field configuration of active stars. Weinterpret this knee-point as a dynamo transition between dominating axi- andnon-axisymmetric dynamo regimes with a different dependence on rotation anddiscuss this hypothesis in the light of current numerical dynamo models.

A Knee-Point in the Rotation-Activity Scaling of Late-type Stars with a Connection to Dynamo Transitions