Michael Le Bars
email: lebars@irphe.univ-mrs.fr

Flows driven by libration, precession and tides in planetary cores

Understanding the flows in planetary cores, i.e. the large liquid iron oceans hidden in the central part of terrestrial planets, is a tremendous interdisciplinary challenge, at the frontier of fundamental fluid dynamics and planetary sciences. Beyond buoyancy driven flows that constitute the standard model for core fluid dynamics, an increasing amount of research has focused on the rotational dynamics of these spinning systems, periodically perturbed by tides, precession and libration. Although of small amplitude, those harmonic forcings are capable of exciting resonant instabilities in planetary cores, providing alternative routes towards turbulence and magnetic field generation. In this paper, I provide an overview of some recent works on this field, focusing on the mechanisms of tide and libration driven elliptical instabilities. Combined laboratory experiments and pioneering numerical simulations have allowed a full description of the stability and linear state of these flows, as well as the investigation of some convincing planetary applications. Open questions now remain regarding the non-linear saturation of the excited flows as well as their dynamo capability. These will undoubtedly be the focus of forthcoming research, in the context of intense activity in planetary exploration of our Solar System and others, that highlights the need to go beyond the standard convective models.

A large number of celestial bodies are made of a metallic, mostly iron, central core, surrounded by a solid silicate mantle. Examples include the Earth, the inner planets of the Solar System, the Moon and some of the large moons of Jupiter (Io, Europa, Ganymede), large asteroids (e.g. Vesta, as revealed by the recent Dawn mission [START_REF] Ct Russell | Dawn at vesta: Testing the protoplanetary paradigm[END_REF]), and presumably Super-Earths in extra-solar systems (see e.g. [START_REF] Valencia | Radius and structure models of the first super-earth planet[END_REF]). In all cases, the metal compound was liquid when the core formed during planet accretion, and the core has then remained liquid during some period whose duration depends on the planet size, the core chemical content, etc. For instance, the cores of the Earth and Moon are still partly liquid presently, as proven by seismic studies (e.g. [START_REF] Weber | Seismic Detection of the Lunar Core[END_REF]). Understanding the fluid dynamics of planetary cores, from their formation up to their present dynamical state, remains a tremendous challenge in planetary fluid dynamics, despite more than half a century of intense research. Beyond the challenge in fundamental fluid dynamics to understand these extraordinary flows involving turbulence, rotation and/or buoyancy effects at typical scales well beyond our day-to-day experience, a global knowledge of the involved processes is fundamental to a better global understanding of the dynamics of planets. Indeed, the flow driven by buoyancy and/or rotation in the core significantly influences the planet's thermal and orbital evolution because of heat advection, viscous dissipation, coupling with the overlying mantle, solidification, etc. Also, fluid motions in conducting liquid cores are the main mechanism for generating planetary magnetic fields through dynamo action, one of the essential ingredients for planetary habitability.

The main obstacle to quantitative modelling and understanding of planetary core flows stands in the extreme value of the involved physical dimensionless parameters. For instance, typical present-day flows at the Earth's core surface have a Reynolds number (measuring the relative importance of flow nonlinearity compared to viscous effects) Re = U R/ν 10 9 , where ν is the core viscosity, R its radius and U a typical velocity given by the measured drift of magnetic patterns at the core surface: core flows are thus highly turbulent. Another relevant dimensionless parameter is the Ekman number, which quantifies the relative importance of viscous and Coriolis forces: E = ν/ΩR 2 , where Ω is the planet rotation rate (spin). Earth is a fast rotator with E 10 -15 . Schematically, even with the most powerful computational tools, direct numerical simulation becomes barely feasible above Re 10 6 and/or below E 10 -6 , and at this level, a single computational run takes months for a few turnover times, which is not long enough to develop a converged statistical description of the flow. Studies relevant to planetary flows rely on the general principle of dynamical similitude and scaling laws, supported by asymptotic theory: rather than reproducing in a model the exact parameters of a planetary flow, the effort is focused on reaching the same dynamical regime, with the correct balance of forces. A systematic exploration of the parameter space then allows for the derivation of generic scaling laws that are extrapolated towards planetary scales and challenged against available data (e.g. [START_REF] Ulrich R Christensen | Energy flux determines magnetic field strength of planets and stars[END_REF]). In this approach, laboratory experiments are especially useful because they reach more extreme values of the relevant parameters and more extreme levels of turbulence than simulations. Besides, experiments allow for the systematic exploration of the parameter space using very long data acquisition. The drawbacks are of course the difficulty in data acquisition, as well as the limitations of accessible geometries and physics compared to simulations. Both approaches, underlined by theoretical analyses, are thus fully complementary.

Since the seminal analytical works of Roberts [START_REF] Harry | On the thermal instability of a rotating-fluid sphere containing heat sources[END_REF] and Busse [START_REF] Busse | Thermal instabilities in rapidly rotating systems[END_REF], 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. Besides, while planetary cores are mostly made of iron, they may also include some amount of light elements: convection may then be driven by thermal energy and light elements released during the solidification of this alloy. Early analytical works (e.g. [START_REF] Busse | Thermal instabilities in rapidly rotating systems[END_REF]), complemented by innovative laboratory experiments (e.g. [START_REF] Fh Busse | Laboratory simulation of thermal convection in rotating planets and stars[END_REF][START_REF] Aubert | A systematic experimental study of rapidly rotating spherical convection in water and liquid gallium[END_REF]) and numerous direct numerical simulations (e.g. [START_REF] Glatzmaier | A 3-dimensional self-consistent computer-simulation of a geomagnetic-field reversal[END_REF]), 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 the 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 convective flows in an electrically conducting fluid can generate a dynamo [START_REF] Glatzmaier | A 3-dimensional self-consistent computer-simulation of a geomagnetic-field reversal[END_REF]. Today the most advanced numerical models, supported by asymptotic theory and by a new generation of large-scale experiments, succeed in addressing the highly non-linear convective regimes (see e.g. [START_REF] Favier | Inverse cascade and symmetry breaking in rapidly rotating boussinesq convection[END_REF][START_REF] Guervilly | Large-scale vortices in rapidly rotating rayleigh-bénard convection[END_REF][START_REF] Stellmach | Approaching the asymptotic regime of rapidly rotating convection: Boundary layers versus interior dynamics[END_REF]), where the buoyant flows excited at small scale build up large vorticity structures, of importance for sustaining the dynamo effect [START_REF] Guervilly | Generation of magnetic fields by large-scale vortices in rotating convection[END_REF].

The convective dynamo model has proven successful to explain the Earth's magnetic field, its dipolar shape and amplitude, as well as the existence of polar reversals (see [START_REF] Paul | On the genesis of the earth's magnetism[END_REF] and references therein). The same model has then been applied to other planetary cores. However, its results are sometimes difficult to reconcile with available observational data, and its validity can be questioned in lots of planetary configurations. For instance, the evolution of the Earth's thermal state is still controversial, and the associated energy budget may appear as difficult to reconcile with a convective dynamo, especially before the onset of inner core growth [START_REF] Andrault | The deep earth may not be cooling down[END_REF]. Also, the small size of the Moon and Ganymede makes it difficult to maintain a sufficient temperature gradient to sustain convection and to explain their past and present magnetic field, respectively [START_REF] Dwyer | A long-lived lunar dynamo driven by continuous mechanical stirring[END_REF][START_REF] Bars | An impact-driven dynamo for the early moon[END_REF][START_REF] Gr Sarson | Magnetoconvection dynamos and the magnetic fields of io and ganymede[END_REF]. Besides, the unusually low amplitude of the magnetic field on Mercury is difficult to explain with the standard scaling laws derived from convective models [START_REF] Stanley | Thin shell dynamo models consistent with mercury's weak observed magnetic field[END_REF]. More generally, and even in planets where convection is present, it is of fundamental importance to also explore the role that other instabilities play in the organization of core flows.

A huge amount of energy is stored in the rotational motion of planets (spin and orbit), and one could thus rely on this reservoir to sustain intense core flows. For instance, the rotational energy of the Earth-Moon system is approximately 1.7 × 10 29 J, while the power necessary to sustain the present-day magnetic field of Earth is approximately 10 11 W. Hence, less than 8% of the available rotation energy is necessary to sustain the dynamo over the age of Earth. The question is, How can the system extract energy from its rotation to drive intense fluid flows? If planets were perfectly non-deformable systems rotating with a perfectly constant rotation vector, their fluid layers would behave rigidly and rotate as solid bodies. This is never the case. The rotation of a real celestial body is always perturbed by gravitational interactions with its companions, which generate periodic perturbations of its shape (i.e. tides), of the direction of its rotational vector (i.e. precession), and of its rotation rate (i.e. libration and length-of-day variation). Those three types of perturbations are generically called harmonic or mechanical forcings. Malkus [START_REF] Malkus | Precessional torques as the cause of geomagnetism[END_REF][START_REF] Malkus | Precession of the Earth as the cause of geomagnetism[END_REF][START_REF] Malkus | An experimental study of global instabilities due to the tidal (elliptical) distortion of a rotating elastic cylinder[END_REF] was the first to highlight the relevance of those harmonic forcings for planetary core flows, but his work was at that time largely rejected, owing to a misunderstanding on the associated energy balances, as later elucidated by [START_REF] Kerswell | Upper bounds on the energy dissipation in turbulent precession[END_REF]: critiques indeed focused upon establishing the energetic irrelevance of the laminar response to mechanical forcing, rather than considering the fully turbulent case, which is significantly more energetic and thus more relevant for planetary bodies.

The key point is that small mechanical forcings do not provide the energy to drive the flows: they play the role of conveyers that extract part of the available rotational energy and convert it into intense fluid motions, generated by rotational fluid instabilities. Since the re-establishment of Malkus' seminal ideas in the late 90's, the fluid dynamics driven by mechanical forcing have been the subject of a growing interest in the fluid dynamics and planetary sciences communities, combining analytical, experimental and numerical studies (e.g. [START_REF] Busse | Mean zonal flows generated by librations of a rotating spherical cavity[END_REF][START_REF] Noir | Experimental evidence of inertial waves in a precessing spheroidal cavity[END_REF][START_REF] Tilgner | Precession driven dynamos[END_REF][START_REF] Zhang | On fluid motion in librating ellipsoids with moderate equatorial eccentricity[END_REF]). In this paper, I provide an overview of some recent research and established results in this domain, followed by a personal prospect of needed future works, focusing specifically on the elliptical instabilities driven in planetary cores by tides and libration.

II. TIDE AND LIBRATION DRIVEN ELLIPTICAL INSTABILITIES: WHAT DO WE KNOW?

Planetary cores, like any rotating fluid, support eigenmodes of oscillation called "inertial modes", whose restoring force is the Coriolis force and whose frequencies in the rotating frame of reference range between plus and minus twice the spin frequency. Those modes are usually damped by viscosity; but they can be resonantly excited by the small, yet regular, harmonic forcings of libration, precession and tides (see e.g. the review [START_REF] Michael | Flows driven by libration, precession, and tides[END_REF]). Let us consider for instance a planet with an orbiting moon along an elliptical orbit (figure 1), both having a solid mantle and liquid core. Gravitational interactions produce tides on the planet and moon, i.e. give an ellipsoidal shape to all layers, including surfaces and core-mantle boundaries. Most large planets are non-synchronised, i.e. their spin and the moon's orbital rate are different: so the rotation rate of the fluid core and of its tidal distortion are different. As a result, the base flow in the planet core, in the frame of reference where the elliptical deformation is stationary, consists in a rotation along two-dimensional elliptical streamlines. On the contrary, most moons are synchronized, i.e. their spin equals their orbital rate. Hence, they always show the same side to their planet, their tidal distortion is frozen in their mantle, and the rigid ellipsoidal moon rotates as a whole at a constant rate. But this is only true on average: due to the eccentricity of its orbit, the moon's orbital rate varies along the orbit following Kepler's third law, and a restoring torque affects the moon's spin, which actually undergoes small oscillations around its mean value, called librations. The moon's core base flow in the frame of reference where the elliptical deformation is stationary then consists in oscillations along two-dimensional elliptical streamlines. More information about the astrophysical complexities behind this very schematic view can be found for instance in [START_REF] Cébron | Elliptical instability in terrestrial planets & moons[END_REF]. Here we simply notice that the two configurations described above have the basic ingredients for exciting the generic "elliptical instability", described for instance in [START_REF] Kerswell | Elliptical instability[END_REF]: as is well known for unbounded vortices in various contexts ranging from wakes to turbulence, such elliptical base flows can non-linearly resonate with two inertial modes of the rotating fluid, giving rise to three-dimensional flows and turbulence.

From an analytical point of view, the elliptical instability can be tackled by two different approaches. The global approach consists in decomposing the fluid motions as the elliptical base flow plus some inertial modes, then looking for resonances. Instability takes place when the non-linear interaction between the base flow and some inertial mode "A" excites an inertial mode "B", while the non-linear interaction between the base flow and mode B reinforces mode A. This is possible only when some resonance conditions are fulfilled: the frequency of the tides or libration forcing must be equal to the difference between modes A and B frequencies, and the azimuthal wavenumber of the forcing (i.e. 2 for the elliptical flows considered) must be equal to the difference between modes A and B azimuthal wavenumbers. Note that the former condition imposes a restriction on the possible range of exciting tide or libration frequencies, since inertial mode frequencies range between plus and minus twice the spin frequency in the spin frame of reference. In complement to the global approach, the local approach quantifies the threshold and growth rate of the elliptical instability. Following the well-known Wentzel-Kramers-Brillouin method, it consists of looking for short wavelength, plane wave perturbations on the base flow. Additional complexities of planetary interest, such as the presence of a stable density profile and/or of an ambient magnetic field, can be straightforwardly introduced. Generic analytical formulae are obtained, in the range of frequencies where elliptical instability is possible [START_REF] Cébron | Elliptical instability in terrestrial planets & moons[END_REF]: the growth rate is proportional to the amplitude of the elliptical forcing (i.e. the product of the streamline ellipticity times the differential rotation of the fluid v.s. the elliptical distortion), minus the dissipative effects coming from viscous dissipation, and possibly from Joule dissipation. The effect of a stable density profile is more complex, depending on the specific shape of gravitational isopotentials and isopycnals: a stratification can then either enhance or inhibit the elliptical instability.

These analytical results have been validated by experimental investigations, including those based on the two setups presented in figure 2 [START_REF] Noir | Experimental study of libration-driven zonal flows in non-axisymmetric containers[END_REF][START_REF] Michael | Tidal instability in stellar and planetary binary systems[END_REF][START_REF] Am Grannan | Experimental study of global-scale turbulence in a librating ellipsoid[END_REF][START_REF] Am Grannan | The turbulent response of planetary interior fluid layers to tidal and librational forcings[END_REF]. Figure 2a shows a laboratory model of a rotating planet, tidally deformed by an orbiting moon. It consists of a hollow sphere of radius 10cm, cast in a silicone gel that is both deformable and transparent. The sphere is filled with water and set in rotation about its vertical axis at a constant angular velocity, up to 180rpm. In addition, to generate a tidal deformation on the rotating fluid, two vertical cylindrical rollers are applied symmetrically on the sphere and rotate independently at a constant angular velocity, up to ±180rpm. With this system, Ekman numbers down to 5×10 -6 can be reached. In order to excite elliptical instability at this (relatively) large Ekman number in comparison with planets, the amplitude of the tidal deformation β is exaggerated, up to 10 -1 , while the corresponding planetary values are on the order of 10 -7 -10 -4 . Figure 2b shows a laboratory model of a librating planet. The working tank consists of an hollow ellipsoid of typical radius 10cm, machined cast from acrylic, and filled with water. Here also, the harmonic forcing is replicated using two motors. The first motor rotates a turntable and super-structure at a constant angular velocity of 30rpm, while the second superimposes a sinusoidal oscillation of the working tank. As in the tides experiments, the relatively large value of the Ekman number 2 × 10 -5 is compensated for by an exagerated amplitude of harmonic forcing, with an ellipsoidal deformation β = 0.34 and a libration angle up to 140 o . Through a systematic exploration of the accessible parameter range, changing the amplitude and frequencies of the harmonic forcings, those two set-ups have validated the main conclusions of the analytical global and local approaches, using simple flow visualization: namely, the existence of a limited range of excitation frequency, the existence within this range of various flow resonances, and the validity of the threshold and growth rate formulae [START_REF] Noir | Experimental study of libration-driven zonal flows in non-axisymmetric containers[END_REF][START_REF] Michael | Tidal instability in stellar and planetary binary systems[END_REF][START_REF] Am Grannan | Experimental study of global-scale turbulence in a librating ellipsoid[END_REF][START_REF] Am Grannan | The turbulent response of planetary interior fluid layers to tidal and librational forcings[END_REF][START_REF] Bars | Coriolis effects on the elliptical instability in cylindrical and spherical rotating containers[END_REF]. Recently, particle image velocimetry measurements in the spin frame of reference have quantitatively validated the expected analytical base flow as well as the "global" mechanism for resonance by 1. Schematic representation of a planet -moon system with an elliptic orbit (top view), illustrating the tidal distortion of the planet, the frozen-in ellipsoidal shape of the moon, and the libration induced by the gravitational torque (see more details in e.g. [START_REF] Noir | Experimental study of libration-driven zonal flows in non-axisymmetric containers[END_REF]). All angles and dimensions have been exaggerated for clarity purpose.

explicitly exhibiting the superimposition of two inertial modes [START_REF] Am Grannan | Experimental study of global-scale turbulence in a librating ellipsoid[END_REF][START_REF] Am Grannan | The turbulent response of planetary interior fluid layers to tidal and librational forcings[END_REF].

The drawbacks in experiments are of course the difficulty in data acquisition (e.g. PIV remains up-to-now limited to two-dimensional measurements in a single equatorial plane), as well as the limitations of accessible geometries and physics (e.g. how to make a radial gravity field in a spherical geometry in the laboratory?). Hence, laboratory approaches have been fruitfully complemented by numerical simulations. The great difficulty here stands in the specific geometry necessary to excite elliptical instability driven either by tides or libration. Indeed, most existing numerical tools for studying planetary cores dynamics assume an axisymmetry of the system around its rotation axis in order to use a fast and powerful spherical harmonics decomposition. Accounting for the ellipsoidal shape of flow streamlines in planetary cores necessitates either tricky boundary conditions [START_REF] Favier | Non-linear evolution of tidally forced inertial waves in rotating fluid bodies[END_REF], "virtual" body forces to locally deform otherwise circular streamlines [START_REF] Cébron | Tidally driven dynamos in a rotating sphere[END_REF], or the use of alternative, less efficient, numerical methods. Over the last 6 years, significant results have been obtained using finite elements and spectral elements methods, addressing the full ellipsoidal geometry of planetary cores with Ekman numbers down to E = 5 × 10 -5 [START_REF] Favier | Generation and maintenance of bulk turbulence by libration-driven elliptical instability[END_REF][START_REF] Cébron | A systematic numerical study of the tidal instability in a rotating triaxial ellipsoid[END_REF][START_REF] Cébron | Libration driven elliptical instability[END_REF] (see figure 3). Beyond further validating analytical results regarding base flow, mode coupling, threshold and growth rate, those simulations have tackled configurations of planetary interest, but out of reach of laboratory investigations: for instance, the existence in real planets of polar flattening [START_REF] Cébron | A systematic numerical study of the tidal instability in a rotating triaxial ellipsoid[END_REF], of a radial stratification in subadiabatic cores, or of convection driven by radial gravity in superadiabatic cores [START_REF] Cébron | Tidal instability in a rotating and differentially heated ellipsoidal shell[END_REF].

Satisfyingly, none of these additional complexities significantly challenges the existence of the tide and libration driven elliptical instabilities in planetary configurations: on the basis of the validated scaling laws, extrapolated towards real planetary values, their presence has thus been quantitatively envisaged in several planets. To cite a few, elliptical instability is expected in most known Super-Earths [START_REF] Cébron | Elliptical instability in terrestrial planets & moons[END_REF], since planets detected up to now in extra-solar systems are especially close to their stars, hence especially deformed elliptically. The same conclusion applies in our Solar System to Io, significantly deformed by the nearby giant planet Jupiter [START_REF] Kerswell | Tidal instability as the source for Io's magnetic signature[END_REF]. The resulting three-dimensional core flow is then expected to induce a magnetic field from the ambient Jovian magnetic field, if not generating its own dynamo, hence contributing to the surprising magnetic signature detected by the Galileo space mission [START_REF] Lacaze | Magnetic field induced by elliptical instability in a rotating spheroid[END_REF][START_REF] Herreman | On the effects of an imposed magnetic field on the elliptical instability in rotating spheroids[END_REF]. Elliptical instability is also strongly expected in the Moon's core during the Late Heavy Bombardment (4.1-3.8 Gy ago), when the meteorites impacts were large enough to desynchronise the Moon, inducing either differential rotation between the liquid core and the elliptical distortion, or at least large amplitude libration [START_REF] Bars | An impact-driven dynamo for the early moon[END_REF]. The resulting threedimensional flow is then expected to have produced a lunar dynamo, explaining part of the surprising magnetic signature recorded in rock samples, brought back by Apollo space missions and recently re-analysed (see e.g. [START_REF] Garrick-Bethell | Early lunar magnetism[END_REF]). On Mars, an innovative scenario has been proposed by [START_REF] Arkani-Hamed | Tidal excitation of elliptical instability in the martian core: Possible mechanism for generating the core dynamo[END_REF], where a large asteroid captured on a retrograde orbit may have excited a tide driven elliptical instability, hence a dynamo, during hundreds of millions of years before colliding with the planet. The situation for the Earth is more controversial, because present estimates of its core deformation place it at the edge of instability threshold, in the absence of magnetic field. Two points are however worth being mentioned: the early Earth, which rotated faster and was more deformed by the then closer Moon, was clearly unstable to the tide driven elliptical instability; and evidence suggesting that Earth's magnetic field already existed 4Gy ago [START_REF] John A Tarduno | A hadean to paleoarchean geodynamo recorded by single zircon crystals[END_REF], i.e. largely before the onset time of inner core crystallisation, is difficult to explain through a convectively driven dynamo alone [START_REF] Andrault | The deep earth may not be cooling down[END_REF][START_REF] Labrosse | Thermal evolution of the core with a high thermal conductivity[END_REF]. A tidal instability in the Earth's core, responsible for the geodynamo, is conceivable, and the global energy budget of the Earth's rotational dynamics provides an additional argument for this. Indeed, models supported by "lunar laser ranging" measurements indicate that 3.7TW is continuously injected from the Earth-Moon-Sun orbital system into the Earth, while 0.2TW is dissipated into the Earth's atmosphere and mantle, 1TW in the deep ocean, and 1.5 to 2TW in shallow seas [START_REF] Wunsch | Vertical mixing, energy, and the general circulation of the oceans[END_REF]. Hence, 0.5 to 1TW of the dissipated rotational power is still missing in the current energy budget: it may very well be continuously injected into the outer core by the excitation of a tide driven elliptical instability, where it can fulfill or can have fulfilled the energy requirements of the geodynamo estimated to range between 0.1 to 2TW [START_REF] Christensen | Power requirement of the geodynamo from ohmic losses in numerical and laboratory dynamos[END_REF].

III. TIDE AND LIBRATION DRIVEN ELLIPTICAL INSTABILITIES: WHAT DO WE NEED TO

KNOW?

Schematically, one can claim today that the mechanisms and thresholds of elliptical instabilities driven by tides and libration are well known (see [START_REF] Michael | Flows driven by libration, precession, and tides[END_REF] and references therein). But at least two very challenging points remain to be tackled to validate those mechanically driven flows for planetary applications.

First, the saturation process of the excited elliptical instability remains unknown, giving rise either to large cycles of growth, saturation and collapse (as first reported by [START_REF] Malkus | An experimental study of global instabilities due to the tidal (elliptical) distortion of a rotating elastic cylinder[END_REF]), or to sustained bulk-filling turbulence (see e.g. [START_REF] Am Grannan | Experimental study of global-scale turbulence in a librating ellipsoid[END_REF]). Explaining these complex behaviors is very challenging and reflects in several aspects the intense research activity in rotating turbulence [START_REF] Leslie | On near resonances and symmetry breaking in forced rotating flows at moderate rossby number[END_REF][START_REF] Clark | Quantifying resonant and near-resonant interactions in rotating turbulence[END_REF]: it is clearly beyond the scope of this paper. But without claiming exhaustivity or mathematical rigor, one can suggest here, from very simple considerations, two plausible physical mechanisms in connection with the two observed types of saturation. In the first mechanism, excited inertial modes, once reaching a sufficient amplitude, can non-linearly self-interact and give rise to a quasigeostrophic flow, which perturbs the rotating base flow and detunes the excited modes, inducing the collapse of the whole resonant scaffolding. In the second mechanism, each of the first resonating inertial modes, once reaching a sufficient amplitude, could act as a seed to non-linearly excite two additional inertial modes, hence priming an energy cascade of triadic resonances. Such a cascade has been described recently around internal-wave attractors [START_REF] Brouzet | Energy cascade in internalwave attractors[END_REF], in the closely related context of the parametric subharmonic instability of internal gravity waves [START_REF] Staquet | Internal gravity waves: from instabilities to turbulence[END_REF], another vivid research area in geophysical fluid mechanics (e.g. [START_REF] Bourget | Experimental study of parametric subharmonic instability for internal plane waves[END_REF][START_REF] Bruce R Sutherland | The wave instability pathway to turbulence[END_REF][START_REF] Bourget | Finite-size effects in parametric subharmonic instability[END_REF]).

In addition to this question of flow saturation, a better knowledge of the statistics of the excited turbulence is necessary to understand the energy repartition between the various time and length scales in planetary applications: is FIG. 3. Volume rendering of the enstrophy in the bulk of a librating ellipsoid, unstable to the libration driven elliptical instability. From left to right: inertial modes excited during the initial exponential growth of the instability; first saturation of the flow; and quasi-steady saturated turbulent state. For this numerical simulation, the libration frequency is equal to four times the spin frequency and the Ekman number E num,lib = 10 -4 . Reprinted from [START_REF] Favier | Generation and maintenance of bulk turbulence by libration-driven elliptical instability[END_REF] with the permission of AIP Publishing.

this mechanically forced turbulence of rotating turbulence type, of Kolmogorov turbulence type, or of wave turbulence type? Only the latest experimental investigations with quantitative flow measurements [START_REF] Am Grannan | Experimental study of global-scale turbulence in a librating ellipsoid[END_REF][START_REF] Am Grannan | The turbulent response of planetary interior fluid layers to tidal and librational forcings[END_REF] and the latest numerical simulations reaching low enough Ekman number [START_REF] Am Grannan | The turbulent response of planetary interior fluid layers to tidal and librational forcings[END_REF][START_REF] Favier | Generation and maintenance of bulk turbulence by libration-driven elliptical instability[END_REF] have begun to quantitatively investigate the asymptotic time and space spectra of the tide and libration driven turbulence. Strong signatures of triadic resonance cascade have been exhibited, as well as some possible universal behaviors coincident with rotating turbulence, with a spatial and temporal energy spectra scaling approaching k -3 and ω -3 , respectively (see e.g. figure 4). Nevertheless, present investigations are inherently limited for planetary applications by the relative large values of the accessible elliptical deformations and Ekman numbers. Indeed, for studying the threshold and initial growth of the instability, it was sufficient to reproduce in experiments and simulations the ratio between elliptical forcing and dissipation: the trick then consisted in artificially increasing the forcing to compensate for the overestimated dissipation. But the study of non-linear effects a priori necessitates tackling the planetary relevant limit of small ellipticity, hence of very small Ekman number. "Local" numerical approaches then offer a nice way to reach these limits, considering only a small rectangular domain within the rotating ellipsoidal body, with periodic boundary conditions [START_REF] Barker | Non-linear evolution of the tidal elliptical instability in gaseous planets and stars[END_REF]. Such models allow high and reasonably fast resolution of the full non-linear dynamics without solving the complex boundary layers, the ellipsoidal flow being imposed as a background. In their first study, Barker and Lithwick [START_REF] Barker | Non-linear evolution of the tidal elliptical instability in gaseous planets and stars[END_REF] showed that the nonlinear outcome of the resonant flow leads to the formation of long-lived geostrophic vortices, shutting off the elliptical instability. But questions remain regarding the realistic attenuation of these vortices, ending up being the size of the considered box following an inverse cascade mechanism. For instance, adding Joule dissipation from a weak initial magnetic field prevents those large-scale vortices from forming, promoting a quasi-steady state of dissipation [START_REF] Barker | Non-linear evolution of the elliptical instability in the presence of weak magnetic fields[END_REF].

Clearly, the non-linear fate of the elliptical instability in a planetary context deserves additional investigations in the near future, combining not only experiments and numerical tools, but also theoretical analyses.

The second challenging prospect in my opinion concerns the magnetohydrodynamics of flows driven by tides and libration, which is still largely unknown. In particular, while dynamo action from elliptical instability has been assumed in several planetary applications [START_REF] Andrault | The deep earth may not be cooling down[END_REF][START_REF] Bars | An impact-driven dynamo for the early moon[END_REF], it has up to now effectively been realized only twice, in numerical simulations of limited dynamical regimes. The reason for this stands in the huge numerical challenge in solving flows in fully three-dimensional geometries with the addition of the induction equations (see e.g. [START_REF] Cébron | Magnetohydrodynamic simulations of the elliptical instability in triaxial ellipsoids[END_REF]). The first numerical realization of a libration driven dynamo dates back to 2013 [START_REF] Wu | On a dynamo driven topographically by longitudinal libration[END_REF], considering the kinematic dynamo of an unstable flow at E = 3 × 10 -3 for a simplified geometry, where the planetary core is spheroidal and librating about an axis perpendicular to its symmetry axis. The first tidal dynamo was shown the following year by [START_REF] Cébron | Tidally driven dynamos in a rotating sphere[END_REF], considering a FIG. 4. Power-spectrum of the saturated turbulent flow in (a) tidal experimental and numerical studies and (b) libration experimental and numerical studies. In all cases, the forcing frequency is equal to four times the spin frequency. Ekman numbers are respectively equal to E exp,tide = 1.5 × 10 -5 , E num,tide = 5 × 10 -5 , E exp,lib = 2.7 × 10 -5 , E num,lib = 10 -4 . Reproduced from [START_REF] Am Grannan | The turbulent response of planetary interior fluid layers to tidal and librational forcings[END_REF].

Grannan et al. Tides and Libration

spherical geometry where a "virtual" body force locally transforms circular streamlines into ellipsoidal streamlines. A magnetic field with a dominant dipolar component was obtained from a self-consistent dynamo calculation at E = 5 × 10 -3 , as shown in figure 5. In addition to these studies, [START_REF] Barker | Non-linear evolution of the elliptical instability in the presence of weak magnetic fields[END_REF] also exhibited dynamo action in their local numerical model, for a much more turbulent tidal flow; but again the extension of local computations towards global conclusions still deserves investigations. In short, those three seminal studies have opened the way for a systematic exploration of the parameter space and for a systematic characterization of the obtained magnetic field, similarly to what has been done since the first convective dynamo realization in 1995 [START_REF] Gary | A three-dimensional convective dynamo solution with rotating and finitely conducting inner core and mantle[END_REF]. Among the exciting questions to be answered for planetary applications, one can mention the typical shape of the generated magnetic field (i.e. dipolar or not, with an amplitude fixed by energy balances [START_REF] Pa Davidson | Scaling laws for planetary dynamos[END_REF]), as well as the existence of magnetic field reversals. Here also significant progress is expected in the next years, through a thoughtful use of the various numerical tools and analytical approaches, and even laboratory experiments.

IV. BEYOND TIDE AND LIBRATION DRIVEN ELLIPTICAL INSTABILITIES

In conclusion, research in core fluid dynamics lives in a very exciting time today, in the context of an intense activity in planetary exploration, involving past, current and future space missions: see for instance the paleomagnetic reanalyses of Moon's Apollo samples (e.g. [START_REF] Garrick-Bethell | Early lunar magnetism[END_REF]); the ongoing Dawn mission around asteroid Vesta and dwarf planet Ceres (e.g. [START_REF] Ct Russell | Dawn at vesta: Testing the protoplanetary paradigm[END_REF]); the forthcoming Juice mission, which will spend at least three years making detailed observations of the giant gaseous planet Jupiter and three of its largest moons, Ganymede, Callisto and Europa; and the ongoing NASA and ESA extrasolar systems explorations. The analysis and interpretation of available and forthcoming data necessitate innovative fundamental models, providing alternative mechanisms to the standard convective models in explaining the variety of behaviors and magnetic fields observed in planets, both in our Solar System and in extrasolar ones. For the specific case of tide and libration driven elliptical instabilities, combined theoretical, numerical, and experimental approaches have led to significant progress. Yet many open questions remain, regarding especially the non-linear saturation and turbulent state of the flows, as well as the shape and intensity of the corresponding dynamo. Beyond tide and libration driven elliptical instabilities, other routes towards core turbulence and dynamos have also been recently explored. For instance, the nonlinear self-interaction of an excited inertial mode, directly forced by harmonic forcing, drives an intense and localized axisymmetric jet that becomes unstable at low Ekman number because of a shear instability [START_REF] Sauret | Tide-driven shear instability in planetary liquid cores[END_REF]; the characteristics of the excited turbulence and the dynamo capability of this flow remain to be studied. Also, the precession driven flow has recently been reinvestigated, showing the prevalence of a shear-driven parametric instability [START_REF] Lin | Shear-driven parametric instability in a precessing sphere[END_REF]; an inverse cascade then sets in, leading to the formation of large scale cyclones capable of dynamo action [START_REF] Lin | Precession-driven dynamos in a full sphere and the role of large scale cyclonic vortices[END_REF]. Besides, other types of mechanical forcing, like nutation and latitudinal libration [START_REF] Vantieghem | Latitudinal libration driven flows in triaxial ellipsoids[END_REF], are present at the planetary scale, but their study is still in its infancy. Finally, it is worth mentioning that while

The Astrophysical Journal Letters, 789:L25 (5pp), 2014 July 1 Here the Ekman number is E = 5 × 10 -3 and the magnetic Prandtl number (i.e. the ratio of viscosity and magnetic diffusivity) is P m = 5. Reproduced with permission from [START_REF] Cébron | Tidally driven dynamos in a rotating sphere[END_REF] ( c AAS).

the various mechanical forcings should be first studied separately, several of them are present simultaneously in each real planet; non-linear interactions are then to be expected, as highlighted in [START_REF] Cébron | Tilt-over mode in a precessing triaxial ellipsoid[END_REF] and [START_REF] Morize | Experimental determination of zonal winds driven by tides[END_REF]. Mechanical forcings may also superimpose on an existing turbulent convective field, and the subsequent non-trivial couplings deserve in-depth investigation, following [START_REF] Gordon | On the interaction between tides and convection[END_REF]. There is no doubt that the interdisciplinary research area of rotating core fluid dynamics will remain vibrant in the next years, benefiting from the simultaneous advances in fluid metrology, non-standard numerical methods, and planetary exploration.

  FIG.1. Schematic representation of a planet -moon system with an elliptic orbit (top view), illustrating the tidal distortion of the planet, the frozen-in ellipsoidal shape of the moon, and the libration induced by the gravitational torque (see more details in e.g.[START_REF] Noir | Experimental study of libration-driven zonal flows in non-axisymmetric containers[END_REF]). All angles and dimensions have been exaggerated for clarity purpose.

FIG. 2 .

 2 FIG.2. Pictures of (a) the tides set-up at IRPHE, France and (b) the libration set-up at UCLA, USA ( c A. Grannan). Typical radius of both tanks is 10cm, typical spin velocity is 30 -180rpm.

066601- 7

 7 FIG.3. Volume rendering of the enstrophy in the bulk of the ellipsoid (the boundary layers are removed from the visualization) for case A6. We visualize the flow just during the exponential growth of the instability (left, t = 68, see I in Figure2), during the first collapse (middle, t = 75, see II in Figure2), and during the quasi-steady saturated state (right, t = 200). A first movie is available as a supplement showing the collapse of the inertial modes from t ≈ 68 to t ≈ 78 in the librating frame. It can be compared with a similar movie (presented in the frame rotating at constant rate Ω 0 and not in the librating frame) corresponding to case V in Grannan et al.22 A second movie showing the quasi-steady regime (186 < t < 196) is also available. (Multimedia view) [URL: http://dx.doi.org/10.1063/1.4922085.1] [URL: http://dx.doi.org/ 10.1063/1.4922085.2]

35

 35

Figure 8

 8 Figure 8. a) The time-averaged power-spectrum of the saturated turbulent flow in the current tidal studies where f tide " 4 and β " 0.09. The laboratory case, L T 8, where E " 1.5 ˆ10 ´5 is shown in blue. The numerical case, N T 4, where E " 5 ˆ10 ´5 is shown in red. b) The time-averaged power-spectrum of the saturated turbulent flow in the libration studies where lib " 0.8 and f lib " 4. The laboratory case, V from Grannan et al. (2014), where E " 2.7 ˆ10 ´5 is shown in blue. The DNS case, A6 from Favier et al. (2015)

  Figure 4. Spe function of the (A color versio

ACKNOWLEDGMENTS

This paper benefited from numerous fruitful discussions with Benjamin Favier (IRPHE, France) and Jonathan Aurnou (UCLA, USA), as well as from the ongoing Ph.D. work of Alexander Grannan (UCLA, USA) and Thomas Le Reun (IRPHE, France). I acknowledge support from the French Agence Nationale de la Recherche (ANR) under the JCJC-SIMI5 2013 program (proposal LIPSTIC ANR-13-JS05-0004-01), and from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No 681835 -FLUDYCO -ERC-2015-CoG).