ISTerre - Institut des Sciences de la Terre

current post-doc proposals

2020-03-12T15:21:35Z

Zonal winds

Dominique JAULT, Henri-Claude NATAF, Philippe CARDIN — 2019-10-15T13:36:40Z

Zonal winds are axisymmetric flows in rotating geophysical systems such as atmopheres, oceans or cores. We'd like to understand the physical mechanism responsible of their presence, their amplitude and their geometry.
The zonal flows are east-west wind (along lines parallel to the equator of a sphere) which turns around the axis of rotation of the planet. An image of Jupiter gives us a clear representation of what they are.

NASA

Jupiter observed by Voyager, NASA

We mainly focus in deep layer systems where the rotation (through the Coriolis forces) imposes a geometrical constraint on the flow, known as geostrophic constraint. The zonal flows are quite peculiar because they cannot originate from radial forcing such as boyancy forces but are produced by non linear interaction of the flow in presence of rotation. Result of rotating turbulence, these flows are difficult to compute.

Equatorial view of the longitudinal velocities (in Reynolds number) in rapidly rotating convection showing multiple azimuthal jets. from Guervilly et al (GJI 2017)

In ISTerre, we built an experimental set up, called ZoRo (for Zonal flows in Rotating fluids), to study zonal flows. We study the flow of aire embedded inside a rotating spheroidal cavity (up to 30Hz). In order to measure the flow, we develop a velocimetry method based on pertubations of acoustic modes.

Photograp of the experimental setup - Max Solazzo

Quasigeostrophy

Dominique JAULT, Henri-Claude NATAF, Nicolas GILLET, Philippe CARDIN — 2019-10-15T13:22:06Z

We expect Earth rotation to make the motions in the core columnar, nearly invariant in the direction parallel to the rotation axis. Geodynamo simulations nicely reproduce this feature.

Assuming columnar geometry (quasi-geostrophic hypothesis), we find that the motions depend only on the spatial coordinates in a plane perpendicular to the rotation axis, making the problem two-dimensional and reducing its size significantly.

There is a large body of research on quasi-geostrophic flows that has been developed specifically to better understand motions in thin fluid layers (Earth atmosphere and oceans). We use this approximation to compute dynamical regimes of thermal convection instead in a rapidly rotating sphere.
We have been able to reproduce turbulent convection for very low viscosity and to exhibit a regime where the small diffusivities do not control the dynamics. Interestingly, in this new regime, we discover that thermal convection becomes subcritical and consequently shuts down abruptly, which could explain rapid extinction of some ancient planetary dynamos (Mars, Moon…).

We have also obtained a new set of reduced quasi-geostrophic momentum equations valid in deep shells, such as planetary cores, including their equatorial regions. We generalize these equations to non spherical bodies in order to document the pressure coupling between the fluid and its solid boundaries. We thus obtain very good approximations of the slowest inertial modes. We are also pairing the quasi-geostrophic equation with the induction equation (for the magnetic field) in order to model the secular variation of the Earth's magnetic field.

Deep Earth

Dominique JAULT, Henri-Claude NATAF, Philippe CARDIN, Renaud DEGUEN — 2019-10-10T09:25:50Z

One of the main objective of the group is to understand the dynamics and evolution of the deep interior of Earth and terrestrial planets. We use for this a combination of theoretical and numerical modelling, fluid dynamics experiments, and high pressure mineral physics experiments.

Topics currently under investigation include:
– the generation of magnetic field in planetary cores through dynamo effect
– accretion and core/mantle differentiation of terrestrial planets
– physical properties of the mantle and core of terrestrial planets (including exoplanets)
– convection in a crystallizing mantle and magma oceans
– Earth's inner core crystallisation and dynamics
– ...

Mineral Physics

Terrestrial planets are constituted of different layers, mainly a silicate mantle surrounding a metallic core. With increasing depth, pressure and temperature increase, modifying the physical properties of the materials, such as density, crystallographic structure, etc … These changes strongly affect the dynamic of the whole planet. To constrain the actual state of a planet, as well as its evolution from its early stage, it is mandatory to perform measurements on materials under planetary interiors conditions. Mineral Physics approach aim at using different experimental tools, such as Diamond Anvil Cell, Multi-Anvil press or Laser-driven Shock Compression, to unravel density, structure, phase diagram or transport properties of geomaterials under extreme conditions. The explored conditions will span from few GPa, for the Moon's interior, to several hundreds of GPa for large exoplanets.

Accretion and core/mantle differentiation of terrestrial planets

Geomagnetic data analysis

Dominique JAULT, Henri-Claude NATAF, Nicolas GILLET, Philippe CARDIN — 2019-09-14T13:37:05Z

The Swarm mission of ESA is today central to the efforts carried by the geomagnetic community. It will allow a better description of external magnetic fields, and thus open a window towards the dynamics in the core at interannual (to monthly ?) periods. This evolution occurs at the same time a new generation of geodynamo simulations arises. It is timely to combine the above two aspects through data assimilation studies. Because we only have access to a limited amount of geophysical constraints (the field at the core surface), this requires to develop reduced models able to reproduce the rapid dynamics in the core. This is the rationale under the development of magnetic quasi-geostrophic equations, or of stochastic models anchored to geodynamo statistics.

Our efforts are supported by the Centre National d'Etudes Spatiales (CNES). In this context, we propose several products to the community:

– the geomagnetic data assimilation tool pygeodyn;

– the web platform webgeodyn for core surface flow and magnetic field visualization;

– the magnetic field model COV-OBS that covers eras of observatory and satellites magnetic records (1840-2020, coll. DTU Space, Copenhagen).

Current internship proposals

2019-04-16T12:35:34Z

– Topographic & buoyancy effects in turbulent boundary layers
supervisors: David Cébron & Nathanaël Schaeffer
contact: david.cebron univ-grenoble-alpes.fr
level: Master
duration: 4-6 months

– Modélisation 2D d'écoulements diphasiques en géodynamique interne: différentiation noyau/manteau, noyau interne, océans magmatiques
supervisors: R. Deguen, D. Cébron, G. Morard
contact: guillaume.morard univ-grenoble-alpes.fr
level: Master
duration: 4-6 months

– Dynamics in Earth's core from interannual to millenial time-scales
supervisors: Nicolas Gillet, Dominique Jault
contact: nicolas.gillet univ-grenoble-alpes.fr
level: Master
duration: 4-6 months

– construction d'un TP numérique d'analyse de données magnétiques satellitaires
supervisors: Nicolas Gillet, Mathieu Istas
contact: nicolas.gillet univ-grenoble-alpes.fr
level: Master
duration: 4-6 months

– Mélange d'un noyau liquide planétaire
supervisors: David Cébron (ISTerre)
contact: david.cebron univ-grenoble-alpes.fr
level: Stage de Master
duration: 3-6 months

– Turbulent boundary stress in the lunar core
supervisors: David Cébron (ISTerre)
contact: david.cebron univ-grenoble-alpes.fr
level: Stage de Master
duration: 3-6 months

– Liquid silicates under extreme conditions: implications for magma oceans differentiation
supervisors: Guillaume Morard (ISTerre) ; Jean-Alexis Hernandez (ESRF) ; Alessandra Ravasio (LULI, Ecole Polytechnique
contact: guillaume.morard univ-grenoble-alpes.fr
level: Stage de M2 Recherche
duration: 4-6 months (March-July 2022)

Décès de Denys Schmitt

Dominique JAULT, Franck PLUNIAN, Henri-Claude NATAF, Philippe CARDIN — 2015-08-27T13:10:18Z

C'est avec une grande tristesse que nous vous faisons part du décès de Denys Schmitt, survenu le 10 août 2015, à l'âge de 65 ans. Denys avait rejoint l'équipe géodynamo d'ISTerre en 2005, et était parti à la retraite en octobre 2012. C'est un collègue et un ami cher que nous perdons.

Polytechnicien, Denys Schmitt avait choisi de consacrer son travail à la recherche scientifique. A l'Institut Néel de Grenoble, où il a effectué la plus grande partie de sa carrière, il est devenu un spécialiste incontesté des propriétés magnétiques des composés intermétalliques de terre rare, avec plus de 200 publications à son actif (voir l'hommage que lui rend l'Institut Néel).

Mais Denys a voulu explorer d'autres mondes et, inspiré -entre autres- par une conférence de Philippe Cardin, il a choisi de contribuer à mieux comprendre l'origine du champ magnétique de la Terre et des planètes. Denys a demandé à rejoindre l'équipe Géodynamo en 2005 et y est vite devenu indispensable.

Denys a tout d'abord entrepris une étude des modes inertiels dans un sphéroide en rotation, recourant pour cela à un formalisme mathématique délicat. Il a ensuite joué un rôle important dans l'exploitation de l'expérience DTS. Il a découvert dans les mesures de cette expérience des modes magnéto-Coriolis qu'il a analysés et modélisés. Inspiré par ces observations, il a proposé un mécanisme très original pour rendre compte de la dérive vers l'Ouest de certaines structures magnétiques à la surface du noyau terrestre : des modes magnéto-Coriolis quasi-diffusifs.

Soucieux de donner toutes leurs chances aux jeunes chercheurs, Denys a pris sa retraite en octobre 2012, avec plein de projets en tête, associés à ses multiples passions.

Apportant une vision nouvelle et un grand sens du collectif, Denys Schmitt a beaucoup contribué à la dynamique de l'équipe Géodynamo. Ses contributions scientifiques et ses qualités humaines sont saluées au-delà des frontières. Sa disparition prématurée nous affecte profondément.

Ses publications avec l'équipe géodynamo

– D. Schmitt, P. Cardin, P. La Rizza, and H. C. Nataf, "Magneto-Coriolis Waves in a spherical Couette flow experiment", European Journal of Mechanics B/Fluids, 37, 10-22 (2013).
– A. Figueroa, N. Schaeffer, H. C. Nataf, and D. Schmitt, "Modes and instabilities in magnetized spherical Couette flow", J. Fluid Mech., 716, 445-469 (2013).
– D. Schmitt, "Quasi-Free-Decay magnetic modes in planetary cores", GAFD 106, 660-680 (2012).
– D. Brito, T. Alboussière, P. Cardin, N. Gagnière, D. Jault, P. La Rizza, J.-P. Masson, H. C. Nataf, and D. Schmitt, "Zonal shear and super-rotation in a magnetized spherical Couette-ﬂow experiment", Phys. Rev. E 83, 066310(17) (2011).
– T. Alboussière, P. Cardin, F. Debray, P. La Rizza, J.-P. Masson, F. Plunian, A. Ribeiro, and D. Schmitt, "Experimental evidence of Alfvén wave propagation in a Gallium alloy", Physics of Fluids 23, 096601(10) (2011).
– D. Schmitt, "Magneto-inertial waves in a rotating sphere", GAFD 104, 135 (2010).
– E. J. Kinast, S. R. de Oliveira Neto, E. G. Santos, O. Isnard, D. Schmitt, M. A. Gusmão, J. B. M. da Cunha, and C. A. dos Santos, "Magnetic phase diagram of the quasi-two-dimensional FexCo1-xTa2O6 compounds", Phys. Rev. B (2010).
– E. J. Kinast, C. A. dos Santos, D. Schmitt, O. Isnard, M. A. Gusmão, and J. B. M. da Cunha, "Magnetic structure of the quasi-two-dimensional compound CoTa2O6", J. Alloys Comp. 491, 41 (2010).
– J. A. Blanco, B. Fak, E. Ressouche, B. Grenier, M. Rotter, D. Schmitt, J. A. Rodriguez-Velamazan, J. Campo, and P. Lejay, "Magnetic ﬁeld and temperature dependence of the amplitude-modulated magnetic structure of PrNi2Si2 determined by single-crystal neutron diffraction", Phys. Rev. B 82, 054414 (2010).
– D. Schmitt, T. Alboussière, D. Brito, P. Cardin, N. Gagnière, D. Jault, and H. C. Nataf, "Rotating spherical Couette flow in a dipolar magnetic field : experimental study of magneto-inertial waves", J. Fluid Mech. 604, 175 (2008).
– H. C. Nataf, T. Alboussière, D. Brito, P. Cardin, N. Gagnière, D. Jault, and D. Schmitt, "Rapidly rotating spherical Couette flow in a dipolar magnetic field : An experimental study of the mean axisymmetric flow", PEPI 170, 60 (2008).
– M. El Massalami, M. Amara, R. M. Galera, D. Schmitt, and H. Takeya, "Magnetoelasticity and H-T phase diagram of TbNi2B2C", Phys. Rev. B 76, 104410 (2007).
– D. Schmitt, "Numerical study of viscous modes in a rotating spheroid", J. Fluid Mech. 567, 399 (2006).
– H. C. Nataf, T. Alboussière, D. Brito, P. Cardin, N. Gagnière, D. Jault, J.-P. Masson, and D. Schmitt, "Experimental study of super-rotation in a magnetostrophic spherical Couette flow", GAFD 100, 281 (2006).
– M. El Massalami, R. M. Galera, D. Schmitt, B. Ouladdiaf, and H. Takeya, "Stability of the modulated states in ErNi2B2C ; A neutron diffraction study of chemically doped samples", Phys. Rev. B 72, 144521 (2005).
– H. Suzuki, H. Abe, R. M. Galera, D. Schmitt, and A. Rogalev, "Sm L3 X-ray magnetic circular dichroism in SmMn2Ge2", Physica B 345, 217 (2004).
– H. Suzuki, D. Gignoux, D. Schmitt, T. Shigeoka, P. C. Canfield, and C. Detlefs, "Polar measurement of metamagnetic processes in X-Y tetragonal systems", J. Magn. Magn. Mater. 272-276, 459 (2004).
– D. Schmitt and D. Jault, "Numerical study of a rotating fluid in a spheroidal container", J. Comput. Phys. 197, 671 (2004).

Journée scientifique organisée en son hommage

Une journée très pluridisciplinaire pour rendre hommage à Denys Schmitt et échanger sur les thématiques scientifiques qu'il a développées à l'Institut Néel puis à ISTerre a été co-organisée par ISTerre et l'Institut Néel le mardi 15 mars 2016, 10h-15h.

Programme

9h45 : accueil

10h00 : introduction de la journée

10h15 : Denys Schmitt physicien du solide, Rafik Ballou, Institut Néel

10h35 : souvenirs et témoignages

10h50 : Denys Schmitt géophysicien, Philippe Cardin, ISTerre

11h10 : effets quadrupolaires dans les intermétalliques de terres rares, Mehdi Amara, Institut Néel

11h30 : magnétisme aux différentes échelles, Jésus Blanco, Univ. Oviédo

12h00 : pause déjeuner

13h40 : précession et nutations : quand les pôles nous jouent des tours, Jérôme Noir, ETH-Zürich

14h00 : modes dans le noyau de la Terre, Dominique Jault, ISTerre

14h20 : expériences dynamo et DTS, Henri-Claude Nataf, ISTerre

14h40 : ondes d'Alfvén, Thierry Alboussière, ENS-Lyon

15h00 : fin de la journée

Death of Denys Schmitt

2015-08-27T13:10:18Z

It is with great sadness that we share with you the death of Denys Schmitt, who passed away on August 10, 2015, at the age of 65. Denys joined the ISTerre geodynamo team in 2005, and retired in October 2012. It is a colleague and a dear friend that we are losing.

A graduate student, Denys Schmitt had chosen to devote his work to scientific research. At the Institut Néel in Grenoble, where he spent most of his career, he became an undisputed specialist in the magnetic properties of rare earth intermetallic compounds, with more than 200 publications to his credit (see the tribute paid to him by the Institut Néel).

But Denys wanted to explore other worlds and, inspired - among other things - by a conference given by Philippe Cardin, he chose to contribute to a better understanding of the origin of the Earth's magnetic field and the planets. Denys asked to join the Geodynamo team in 2005 and quickly became indispensable.

Denys first undertook a study of inertial modes in a rotating spheroid, using a delicate mathematical formalism for this purpose. He then played an important role in the exploitation of the DTS experiment. In the measurements of this experiment, he discovered magneto-Coriolis modes that he analyzed and modelled. Inspired by these observations, he proposed a very original mechanism to account for the westward drift of certain magnetic structures on the surface of the Earth's nucleus : quasi-diffusive magneto-Coriolis modes.

Concerned about giving young researchers every opportunity, Denys retired in October 2012, with many projects in mind, associated with his many passions.

Bringing a new vision and a great sense of collective spirit, Denys Schmitt has contributed greatly to the dynamics of the Geodynamo team. His scientific contributions and human qualities are recognized beyond borders. His premature death is deeply affecting us.

His publications

Scientific day organized in his honour

A very multidisciplinary day to pay tribute to Denys Schmitt and discuss the scientific themes he developed at the Institut Néel and then at ISTerre was co-organized by ISTerre and the Institut Néel on Tuesday, March 15, 2016, 10am-15pm.

Programme

9h45 : reception desk
10h00 : introduction of the day
10h15 : Denys Schmitt solid-state physicist, Rafik Ballou, Institut Néel
— > podcast
10h35 : memories and testimonies
10h50 : Denys Schmitt geophysicist, Philippe Cardin, ISTerre
— > podcast
11h10 : quadrupolar effects in rare earth intermetallics, Mehdi Amara, Institut Néel
— > podcast
11h30 : magnetism at different scales, Jésus Blanco, Univ. Oviédo
— > podcast
12h00 : lunch

13h40 : precession and nutations : when the poles play tricks on us, Jérôme Noir, ETH-Zürich
— > podcast
14h00 : modes in the Earth's core, Dominique Jault, ISTerre
— > podcast
14h20 : dynamo and DTS experiments, Henri-Claude Nataf, ISTerre
— > podcast
14h40 : Alfvén waves, Thierry Alboussière, ENS-Lyon
— > podcast
15h00 : end of the day

PhD position : Dynamo capable flows driven by coupled mechanical forcings and topographic effect

David CÉBRON, Dominique JAULT — 2014-05-26T12:22:38Z

– Grant : Allocation MRE
– Fields : Geophysics, Fluid Mechanics
– Openings : Academia, applied research

It is a commonly accepted hypothesis that buoyancy drives planetary dynamos. Indeed, on Earth, the prevalent model is that the current magnetic field comes from thermochemical convective motions within the conducting liquid core. However, the validity of this model in certain planets can be questioned, as for instance the Early Moon or Earth, Ganymede, Mercury and certain asteroid such as Vesta. Moreover, the short magnetic cycle observed in certain hot-Jupiter systems, as -boo, are thought to be due to tidal forces, but the underlying physical mechanism remains unclear.

Alternative mechanisms, based on mechanical forcings (precession/nutation, libration, or tides), have been proposed (e.g. Malkus 1968), but their dynamo capabilities remains unknown. . Besides, even in planets where the dynamo is of convective origin, additional driving mechanisms may significantly modify the organization of fluid motions, and then heat transport or intern dissipation. In particular, topographic effects (i.e. pressure torques generated by boundaries on a fluid layer) are expected to play a key role in planetary liquid cores and subsurface oceans flow dynamics. However, their study requires to consider a non-spherical fluid layer, which is a real difficulty for theoretical calculations, as well as efficient numerical simulations (spectral code). Even a small departure from sphere can lead to vigorous large scale flows, which remains thus badly known.

We propose here to tackle this problem by considering two cases : (i) fluid layers of terrestrial planets, where the core-mantle boundary is rather ellipsoidal than spherical, and (ii) the stars, or gaseous planets, where the boundary is a free surface. We aim at combining theoretical approaches, allowing the study of the linear regime of the considered flows, with numerical simulations, allowing to characterize the fate of these flows in the non-linear, or even turbulent, regime.

The theoretical approach relies on both local and global stability methods. Generalizing the recent work of Hattori&Fukumoto (2012), the local analysis will extend these works to arbitrary unsteady 3D flows. The global stability study will first rely on polynomial modes (large deformation of ellipsoidal celestial bodies), and then on gravito-inertial modes (small deformations of arbitrary shape). The formers will allow to revisit the classical results of ellipsoidal figures of equilibrium (Chandrasekhar, 1969) in a dynamical framework, whereas the latters will allow to predict possible triadic resonances on a weakly deformed free surface. These calculations may lead to collaborations with W. Herreman (LIMSI, Orsay) or M. Rieutord (IRAP, Toulouse).

Concerning the numerical simulations, we will rely on the code XSHELLS (spherical), developed at ISTerre by N. Schaeffer (Schaeffer, 2013). Extending XSHELLS to ellipsoids will allow to benefit from its tremendous efficiency to study flows well beyond the laminar regime considered by the few existing studies (Ernst-Hullerman et al. 2013 ; Cébron&Hollerbach 2014), i.e. flows closer from the geophysical and astrophysical regimes. This is an important challenging extension given that no code can currently solve efficiently this kind of flows in such geometries, which are much closer from planets and stars. Two approaches are envisaged. First, we can describe a simple deformation of the sphere using a metric (and then rewrite the equations in this new generalized coordinates system). Second, we can keep the spherical shape and perturbate the flow to mimic an arbitrary topography.

This PhD thesis is directly linked with the general topic “topographic coupling” currently investigated within the group Geodynamo. It will thus be possible, for the PhD student, to collaborate on various projects currently developed in the group, either theoretical ones (with D. Jault) or experimental ones (EDEN project, with H.-C. Nataf).

At the frontier between fluid mechanics and geophysics, this work will lead the PhD student to code in C/C++ and/or Python, and to use computer algebra code ; the required skills range thus from code development to the use of mathematical methods used in fluid mechanics.

References

– Cébron & Hollerbach, 2014. Elliptical instability driven dynamo in sphere. In prep.
– Chandrasekhar, 1969. Ellipsoidal figures of equilibrium. Yale University Press ; 1st edition : 253p.
– Ernst-Hullermann, Harder, Hansen, 2013. Finite volume simulations of dynamos in ellipsoidal planets. Geophys. J. Int. 195:1395-1405.
– Hattori&Fukumoto, 2012. Effects of axial flow on the stability of a helical vortex tube. Phys. Fluids. 24, 054102.
– Malkus, 1968. Precession of the Earth as the cause of geomagnetism. Science 160:259-264.
– Schaeffer, 2013. Efficient spherical harmonic transforms aimed at pseudospectral numerical simulations. Geochem. Geophys. Geosyst., 14, 751–758.