Seminar by François Rincon (IRAP - Toulouse)

Abstract

For more than 40 years, the quest to understand how large-scale magnetic fields arise 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 manifests in both astrophysics and geophysics, and describe the current state of research. 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 nonlinear helical turbulent dynamos, which strongly point to an asymptotic ultimate regime of the large-scale solar dynamo at high magnetic Reynolds numbers.

In this regime, migrating “butterfly” dynamo waves naturally emerge that involve key helicity fluxes providing the elusive and long-sought-after synchronization 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 that are strongly sensitive to changes in kinetic and magnetic Reynolds numbers.

Overall, the results therefore also highlight the inherent limitations of the brute-force numerical modeling approach applied to this problem.