Magnetic flux emergence across the Hertzsprung-Russell diagram
Final Report Abstract
The rise of magnetic flux to the stellar surface is an essential ingredient to a class of dynamo models (flux-transport dynamo) explaining the magnetic cycle of the Sun and solar-like stars. This rise is usually assumed to be instantaneous compared to the cycle length of the order of 10 years. The aim of the project was the investigation of the rise of magnetic flux tubes in stellar interiors in three-dimensional simulations. The project employed the compressible MHD-code NIRVANA for the global simulations of flux tubes. We studied the rise time of magnetic flux tubes as a function of rotation period and magnetic field strength. Since the independent variables - rotation and activity - are linked to stellar age and mass, we basically explore a certain region of the Hertzsprung-Russell diagram. We found scaling relations for the relative rise time, i.e. normalized with the rotation period, as functions of the angular velocity and the Alfven velocity (magnetic field strength). Axisymmetric simulations confirmed an earlier finding, while the non-axisymmetric solutions show a different scaling because of the effect of magnetic tension. In the non-axisymmetric case, local loops form out of the torus-like initial magnetic field configuration. The magnetic tension is stronger in those loops than in an axisymmetric field. The scaling relation was then used in an actual flux-transport dynamo to account for the delay between the amplification of the magnetic fields in the deep interior of the star and the emergence of active regions on its surface. The delay is a function of the magnetic field itself. Interestingly, a new regime of dynamo action with relatively weak fields was found as a result of this non-linear behavior.
Publications
- (2017) 3D simulations of rising magnetic flux tubes in a compressible rotating interior: The effect of magnetic tension. A&A (Astronomy & Astrophysics) 607 A1
Fournier, Y.; Arlt, R.; Ziegler, U.; Strassmeier, K. G.
(See online at https://doi.org/10.1051/0004-6361/201629989) - Dynamics of the rise of magnetic flux tubes in stellar interiors. PhD thesis, Potsdam University (2016)
Fournier, Y.
