Postdoctoral research position in geodynamics
Lyon Institute of Origins
In 2011, The Lyon Institute of Origins LabEx was selected following the first “Laboratory of Excellence” call for projects, part of the “Investissement d’Avenir” program for forward-looking research. It is one of 12 LabExes supported by the University of Lyon community of universities and establishments (COMUE). LIO brings together more than 200 elite researchers recruited throughout the word and forming 18 research teams from four laboratories in the Rhône-Alps region, all leaders in their fields, under the auspices of the University Claude Bernard Lyon 1 (UCBL), the Ecole Normale Supérieure de Lyon, and the CNRS. LIO’s goal is to explore questions about our origins, operating in a broad field of study that ranges from particle physics to geophysics, and includes cosmology, astrophysics, planetology and life.
Nonlinear Processes in Geosciences (NP)
Planetary and Solar System Sciences (PS)
Part of the work will take place (especially at the beginning) at the Laboratoire de Géologie de Lyon, on the
University Campus of Lyon I. The experimental setup will be assembled and tested in the fluid laboratory,
and reference experiments, without rotation, will be run. Other parts of the work will take place at the
Laboratoire de Physique of Ecole Normale Supérieure de Lyon, where a rotating platform is available for
the measurements in a rotating frame.
The concept of penetrative convection has been identified to play a role in various astrophysical and geo-
physical 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 of 0.1 mm/s are much smaller. The config-
uration 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 simultane-
ous 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.
Applicants must email a CV, a statement of interest, a letter of recommendation and con-
tact details for 2-3 references to firstname.lastname@example.org before January 31st, 2021.
Candidates on the short list will be informed by mid February. They will be interviewed in the
second half of February.