task 2: The fluid dynamics of heterogeneous core convection

State-of-the-art and objectives: Since the seminal analytical works of Roberts (1968) and Busse (1970), most research on the fluid dynamics of planetary cores has focused on convection. Indeed, buoyancy is naturally present in planetary cores because of both radiogenic and primordial heat, and thermal convection takes place providing the temperature profile is super-adiabatic. Also, while planetary cores are made mostly of iron, they may also include some amounts of light elements: convection may then be driven by energy and light elements release during the solidification of this alloy. Early analytical works (e.g. Busse 1970), complemented by innovative laboratory experiments (e.g. Busse & Carrigan 1976, Aubert et al. 2001) and numerous direct numerical simulations (e.g. Glatzmaier & Roberts 1995), have provided a clear picture of the convective flow organization in a rapidly rotating spherical shell, at least for moderate values of the Ekman number and of supercritical Rayleigh number (i.e. the ratio between buoyancy and diffusive forces). One of the most significant outcomes of this research was to demonstrate that those flows in an electrically conducting fluid can generate a dynamo. The present magnetic field of Earth most likely results from such motions, predominantly driven by the gravitational energy release related to the inner core growth (Glatzmaier & Roberts 1995). Today, the most advanced numerical models, supported by asymptotic theory and by a new generation of large-scale experiments, begin to succeed in addressing the highly non-linear convective regimes (see e.g. Stellmach et al. 2014), where the buoyant flows excited at small scale build up large vorticity structures, of importance for sustaining a dynamo effect.

In parallel to this race towards the more extreme convective regimes, we propose to investigate an additional complexity of the convective flows at the planetary scale that has recently come into light for the Earth, and may completely change our global understanding of its core dynamics. A growing amount of evidences, coming from the latest investigations in mineral physics (e.g. Hirose et al. 2013) as well as from the latest interpretations of the geomagnetic field fluctuations (Buffet 2014), suggests the presence of a stratified fluid layer at the top of the Earth’s liquid core. Taking into account the uncertainties in all parameters (e.g. the core-mantle boundary heat flux), the depth of this stratified layer is still largely unknown, with estimates ranging from tens to hundreds of kilometres. But no matter what the effective size of this layer is, it obviously has a non-negligible effect on the underlying convective flow that must be damped by this thermal and momentum blanket, and on the magnetic field. Besides, following studies on the dynamics of other systems where such a two-layer organisation is naturally present (i.e. troposphere/stratosphere in planetary atmospheres, convective/radiative zones of stars), the stratified layer also has its own dynamics: underlying turbulence patterns excite internal-inertial waves sustained by stratification and rotation (e.g. Ansong and Sutherland 2010, Alvan et al. 2014), and the non-linear interactions of those waves generate large-scale flows, responsible for instance for the so-called Quasi-Biennial Oscillations of the equatorial zonal wind in the Earth’s tropical stratosphere (Lindzen and Holton 1968), and possibly for the misalignment of hot Jupiters around hot stars, one of the many “observational mysteries” of these exotic systems (Rogers et al. 2012). The study of the dynamical effects of a stratified layer at the top of liquid planetary cores is thus groundbreaking and perfectly timely (see e.g. Vidal and Schaeffer 2015). We will take part in the upcoming competitive research on the subject by developing innovative laboratory experiments and pioneering numerical simulations, going beyond the first results that we recently obtained (Le Bars et al. 2015a, Lecoanet et al. 2015, https://www.youtube.com/watch?v=kO3g7O-_tuY).

Ongoing research projects:

• Dynamics of mixed convective--stably-stratified fluid, a numerical approach in cartesian geometry: L.-A. Couston, D. Lecoanet, B. Favier and M. Le Bars.
• Flows in a mixed turbulent / stratified domain, an experimental approach: P. Léard, P. Le Gal and M. Le Bars
• Dynamics of mixed convective--stably-stratified fluid, a numerical approach in spherical geometry: M. Bouffard, B. Favier and M. Le Bars.
• Zonostrophic turbulence and interactions with a stratified layer: D. Lemasquerier, B. Favier and M. Le Bars.

Open positions for this project: check out projects listed here