Exploring the ultimate regime of the solar dynamo
Abstract: For more than 40 years, the quest to understand how large-scale magnetic fields emerge from turbulent flows in rotating astrophysical systems, such as the Sun, has been a major focus of computational astrophysics research.
In this talk, I will first introduce the general (hard) problem of the dynamo effect in fluids, how it translates in both astrophysics and geophysics, and describe the current state of affairs. I will then present a parameter scan and phenomenological analysis of new maximally simplified and highly optimized three-dimensional cartesian magnetohydrodynamic simulations of large-scale non-linear helical turbulent dynamos, which strongly point to an asymptotic ultimate regime of the large-scale solar dynamo at large magnetic Reynolds numbers. In this regime, migrating “butterfly” dynamo waves naturally emerge that involve key helicity fluxes providing the elusive and long sought-for synchronisation between hemispheres. I will finally show that all “realistic” spherical global simulations of the solar dynamo to date lie in highly non-asymptotic turbulent magnetohydrodynamic regimes strongly sensitive to changes in kinetic and magnetic Reynolds numbers. Overall, the results therefore also highlight the hard limitations of the brute-force numerical modelling approach applied to this.