Abstracts Category : Other

Stellar Magnetism in Theory and Observation

by Elizabeth Cole

The universe is populated with magnetically active stars. This magnetic activity is thought to be generated by dynamos operating in turbulent stellar convection zones, a process by which kinetic energy is converted into magnetic energy. The solar dynamo is but one dynamo type possible for stars. Rapidly rotating late-type stars are observed to have large spots, activity cycles, flip-flops, and active longitudes, all indicating a different dynamo mechanism may be responsible. Numerical simulations provide a tool for better understanding some of the mechanisms responsible for these dynamos. In this thesis, direct numerical simulations in spherical wedges are used to study dynamo mechanisms in the stellar convection zone. These spherical wedges are used to investigate the dependence of the resulting magnetic field on input parameters such as the density stratification and rotation rate. Mean-field models are used to evaluate the assumption that the wedges can be used to approximate full spheres. As rotation increases, differential rotation decreases in the models, in agreement with observations where more rapidly rotating stars have smaller estimates for differential rotation. As density stratification approaches more realistic values, a lat- itudinal dynamo wave with equatorward propagation is found. The impact of the domain size in the azimuthal direction on the results is explored. When the domain size is increased to 2pi in the azimuthal direction, a non-axisymmetric m = 1 mode is excited. This non-axisymmetry is reminiscent of the field configurations of rapidly rotating late-type stars. The azimuthal dynamo wave rotates nearly independently of latitude and depth, and its rotation rate is slower than that of the mean rotation of the model. This azimuthal dynamo can provide a possible explanation for the observed rotational difference of spots from the mean rotation observed on stars. The wedges use the perfect conductor boundary conditions at the latitudinal boundary to compensate for the omission of polar regions due to the time step becoming prohibitively small there. Simple mean-field models with only a latitudinal extent and perfectly conducting boundaries do not oscillate when the model is extended to the poles. Thus oscillations near the polar region may be an artifact of the boundary condition. However, when the alpha effect from mean-field dynamo theory and magnetic diffusivity are concentrated towards lower latitudes, oscillatory solutions with equatorward migration are found. When sufficient shear is added, oscillatory solutions are again found, and the Parker-Yoshimura rule for latitudinal dynamo wave propagation is obeyed. It is concluded that numerical simulations where the alpha effect and diffusivity are found to be stronger at lower latitudes and simulations with sufficient shear are considered good approximations of full spheres. These numerical simulations are put into context with stellar observations. Two young solar analogs are selected, V352 Canis Majoris and LQ Hydrae. V352 CMa is