Postdoctoral position: "Penetrative convection: experiments on a rotating table " - LPENSL (UMR 5672) et LGL-TPE (UMR 5276)

Research project

The concept of penetrative convection has been identified to play a role in various astrophysical and geophysical contexts. On Earth, a typical example of penetrative convection consists in the interaction between the convective troposphere and the stably stratified upper atmosphere. Another occurrence of penetrative convection is related to the dynamics of the interior of the Earth. The core is known to be in a convective state, vigorous enough to sustain the Earth's magnetic field by dynamo action. However, there could be a stably stratified layer at the top of the core that may have been there since the origin of the Earth. Seismology does not bring a definite answer, with a few researchers claiming that seismic velocities are slightly slower than expected in a well-mixed core in the top 100~km or less of the core. Others argue that crystallization of the inner core or exsolution within the core should bring light elements to the top of the core. It has also been argued that the secular cooling of the Earth should allow light elements of the lower mantle to be incorporated in the top of the core. Finally, current estimates of the thermal conductivity of the core would lead to an excessive heat flux out of the core for a fully convective core.

Experimental studies can help answer key questions relative to penetrative convection. In the last example above, it is important to determine what kind of motion can exist in a potentially stable layer. This top layer, and its dynamics, are linked to the magnetic observations. The picture from the observations is rather blurred and restricted to large scales, but can be used to discriminate between flow models. The experimental task is made more difficult due to the large effect of Coriolis forces in the core. Compared to the ocean or atmosphere, Coriolis forces are much stronger because the typical velocities -- 10-4 m/s -- are much smaller. The configuration creates a rich environment in terms of physical processes: convection, gravity waves due to the stable stratification and inertial waves from rotation. Our mixed group of geophysicists and physicists is particularly expert in these phenomena.

Some experimental results can be found in the literature, but very few have been carried out in a rotating frame. In the physics laboratory, we have a rotating table (2 m diameter, 60 rpm maximum) which can host an experimental setup devoted to penetrative convection. We have actually already been running preliminary experiments, showing that measurements are possible and will bring interesting results. The setup will use the peculiar property of liquid water between 0°C and 4°C where it has a negative thermal expansion coefficient. Maintaining the bottom of a water tank at 0°C and the top at 25°C generates a lower convective region (between 0°C and 4°C) while the upper part is stably stratified. The measurements consist in simultaneous velocimetry (PIV) and temperature (LIF) measurements on various plane laser sheets. Moving continuously the measurement planes will allow us to extract a 3D picture of the flow.

The post-doctoral researcher will participate in the setup and testing of the experimental rig. She or he will perform measurements and extract velocity and temperature fields and use the results in the geophysical context, bringing expertise in the interpretation of geomagnetic data concerning the top of the Earth's core. She or he will have a PhD in fluid mechanics, potentially gravity or inertial waves, and experience, or a keen interest, in the dynamics of the Earth. She or he will interact with members of the physics laboratory and members of the geology laboratory.