Dynamic regimes in planetary cores: $\tau $–$\ell $ diagrams

Planetary cores are the seat of rich and complex fluid dynamics, in which the effects of rotation and magnetic field combine. The equilibria governing the strength of the magnetic field produced by the dynamo effect, the organisation and amplitude of the flow, and those of the density field, remain debated despite remarkable progress made in their numerical simulation. This paper describes an approach based on the explicit consideration of the variation of time scales $\tau $ with spatial scales $\ell $ for the different physical phenomena involved. The $\tau $–$\ell $ diagrams thus constructed constitute a very complete graphic summary of the dynamics of the object under study. We highlight the role of the available convective power in controlling this dynamics, together with the relevant force balance, for which we derive a very telling $\tau $–$\ell $ translation. Several scenarios are constructed and discussed for the Earth’s core, shedding new light on the width of convective columns, and on the force equilibria to be considered. A QG-MAC scenario adapted from [Aubert, 2019] gives a good account of the observations. A diversion to Venus reveals the subtlety and relativity of the notion of “fast rotator”. We discuss scaling laws and their validity domain, and illustrate “path strategies”. A complete toolbox is provided, allowing everyone to construct a $\tau $–$\ell $ diagram of a numerical simulation, a laboratory experiment, a theory, or a natural object.