Dynamics of the flow in a precessing ellipsoid

The flow induced by the precession inside the core of the Earth has often been considered as a potential source of energy for the Earth's magnetic field. We study the idealized flow inside a precessing ellipsoid using PIV measurements and numerical simulations. The viscous tilt-over solution splits into two different solutions in a specific range of precession frequencies, as predicted by Busse (1968). The hysteretic cycle between the two solutions is observed experimentally for the first time. For large tilt-over, each solution destabilises through triadic resonances related either to the eruption of boundary layers (critical shear instability, Lin, Marti & Noir 2015) or to the ellipiticity of the streamlines (elliptic instability, Kerswell 1993). Our study thus closes a long-lasting debate on the origin of the precession instability.