We develop a model which connects the missing link between deep-seated dynamos and the evolving surface flux in cool stars. The link, which is hitherto not included in any dynamo model, is the buoyant rise of magnetic flux tubes from the dynamo layer throughout the entire convection zone. We choose toroidal flux tubes with a spatial probability distribution determined by the mean toroidal magnetic field generated by a cyclic dynamo. As a first example, we use a thin-layer alpha-omega dynamo (Schüssler & Schmitt 1989) with Sun-like shear in the convective overshoot region. We carry out numerical simulations of the rise of Parker-unstable flux tubes (Caligari et al. 1995), which in turn determine the latitudes and the tilt angles of the emerging flux loops. This information is then put into a surface flux transport model (Baumann et al. 2004, 2006) with Sun-like differential rotation, meridional flow, and turbulent supergranular diffusion. In this part, we simulate the evolution of bipolar magnetic regions, which emerge with a Sun-like area distribution and with tilt angles and emergence latitudes determined by rising flux tubes. The figure on the left panel shows a comparison of the generated toroidal field pattern in the overshoot region (contours) and the emerging flux tubes on the surface (dots), for a star having solar internal structure but rotating 2.7 times faster than the Sun, hence representing somewhat a "younger Sun". The time-latitude diagram on the right-hand panel shows the surface evolution of longitudinally averaged magnetic flux density. The polar fields are about 20-30 times stronger than in the Sun, and the cyclic dynamo is no longer visible in the variation of magnetic flux integrated over the entire surface.

Reference

A coupled model of magnetic flux generation and transport in stars, Isik, E.; Schmitt, D.; Schüssler, M., Astron. Nachr., 328, 1111