| Speaker: Michael Proctor Affiliation: University of Cambridge Time: 2 pm Thursday, 19 March, 2020 Location: 303-257 |
| The solar magnetic field displays features on a wide range of lengthscales including spatial and temporal coherence on scales considerably larger than the chaotic convection which generates the field. Explaining how the Sun generates and sustains such large-scale magnetic field has been a major challenge of dynamo theory for many decades. Traditionally the ‘mean-field’ approach, utilising the well-known alpha effect, has been used to explain the generation of large-scale field from small-scale turbulence. However, with the advent of increasingly high-resolution computer simulations there is doubt as to whether the mean-field method is applicable under solar conditions. Models such as the ‘shear dynamo’ provide an alternative mechanism for the generation of large-scale field. In our first study we showed that while coherent magnetic field was possible under kinematic conditions (where the kinetic energy is far greater than magnetic energy), the saturated state typically displayed a destruction of large-scale field and a transition to a small-scale state. More recent work has shown that the quenching of large-scale field in this way is not the only regime possible in the saturated state of this model. Across a range of simulations we find quasi-cyclic behaviour where large- scale field is preserved and oscillates between two preferred lengthscales. In this regime the kinetic and magnetic energies can be of a similar order of magnitude. These results demonstrate that the shear dynamo may have some relevance as a model for the solar dynamo. |