Coupled models of generation, emergence, and surface evolution of stellar magnetic flux
E. Isik, D. Schmitt, M. Schüssler
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
Last modified: Sun Apr 13 17:25:13 CEST 2008