  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
proposes a new 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. Supplementary material for this article is
supplied as a separate archive Nataf_Schaeffer_SupMat.zip, the related data is
displayed in document Nataf_Schaeffer_SupMat.pdf.
