SOLARNET Conference: The Many Scales of the Magnetic Sun

A widely accepted explanation for the origin of the ubiquitous small-scale magnetic field observed on the solar surface is the presence of a small-scale dynamo (SSD) operating in the sub-surface layers of the Sun. To shed light on the functioning of this SSD, a number of numerical studies of a realistic solar atmosphere have been carried out in the past two decades, greatly improving our knowledge on how an SSD operates. Yet, it must not be overlooked that characteristics of these simulations like Reynolds numbers or the magnetic Prandtl number are orders of magnitudes different from the real Sun. To widen the perspective on the Sun itself, we started to investigate SSD action in other cool main-sequence stars. Hydro and magneto-hydrodynamic simulations of a small partial volume encompassing the surface layers of F5V, G2V, K2V, and K8V main-sequence stars are carried out with the radiative magneto-hydrodynamic code CO5BOLD, investigating how SSD action can amplify a feeble seed magnetic field. Keeping the flow parameters similar for all models, they all reach the saturation level at about the same magnetic to kinetic energy ratio, irrespectively of the growth rate during the kinematic phase. While in a first phase of the simulation, the Poynting flux into the computational domain in the upflows through the bottom boundary is kept strictly zero, this boundary condition is changed in a second phase to allow for a partial replenishment of the magnetic field that is pumped out of the computational domain. In this second phase, strong kG magnetic flux concentrations form at the stellar surface, which appear as magnetic bright features (MBFs). The K2V and G2V models display the highest number of MBFs with about twice the magnetic flux in MBFs compared to models K8V and F5V.