ISTerre seminar

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Angular momentum transport in stellar stably-stratified layers: impact of dynamo action (and waves)

Monday 13 October 2025 - 10h30

Florentin Daniel - Northwestern University
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It has recently been shown that a significant slowdown of many stars can be attributed to the emergence of a strong magnetic field within the radiative region, where heat is transferred through radiation in a stably stratified layer. In this presentation, I will describe how this transition can be understood as a subcritical bifurcation to small-scale turbulence in linearly stable flows. The turbulence is sustained by a nonlinear mean-field dynamo and can be observed down to relatively small differential rotation, arbitrarily far from the linear onset of any hydrodynamic instability. In this regime, turbulent fluctuations provide a diffusivity-free transfer of angular momentum that increases the transport generated by the magnetic field triggering the turbulence. Although the results obtained through direct numerical simulations exhibit many similarities with the so-called Tayler-Spruit dynamo, I will present a general simple nonlinear model that captures this scenario. It can be used as a universal description of the transition to turbulence in astrophysical flows, as long as it involves a competition between a large-scale dynamo and a small-scale magnetic instability. If time permits, I will describe how internal gravity waves could be used as realistic source of differential rotation in a stellar context, and could trigger the dynamo by presenting a numerical study of the Shear-Layer Oscillation. Similar to the quasi-biennial oscillation on Earth, this phenomenon is studied through a coupling between a convective zone and a stably-stratified one, showing a large scale oscillation in the latter developing on much longer time scales than the typical underlying turbulence of convection.

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Organizing team : Géodynamo

Amphithéâtre Killian, Maison des Géosciences, 38400 Saint Martin d'Hères

Informations de visio :

https://univ-grenoble-alpes-fr.zoom.us/j/91690538748?pwd=buNDS0rAZw9rG4vS1rWxX53zHYdQaw.1