Small‐scale magnetic fields on late‐type M‐dwarfs

We performed kinematic studies of the evolution of small-scale magneticfields in the surface layers of M-dwarfs. We solved the inductionequation for a prescribed velocity field, magnetic Reynolds number ReM,and boundary conditions in a Cartesian box, representing a volumecomprising the optically thin stellar atmosphere and the uppermost partof the optically thick convective envelope. The velocity field isspatially and temporally variable, and stems from detailedradiation-hydrodynamics simulations of convective flows in aproto-typical late-type M-dwarf (Teff =2800pun {K}, logg =5.0, solarchemical composition, spectral type ~M6). We find dynamo action for ReM>= 400. Growth time scales of the magnetic field are comparable tothe convective turn-over time scale (~ 150pun {sec}). The convectivevelocity field concentrates the magnetic field in sheets and tubularstructures in the inter-granular downflows. Scaling from solarconditions suggests that field strengths as high as 20pun{kG} might bereached locally. Perhaps surprisingly, ReM is of order unity in thesurface layers of cooler M-dwarfs, rendering the dynamo inoperative. Inall studied cases we find a rather low spatial filling factor of themagnetic field.