A Magma Ocean at the base of Earth's mantle (BMO), if stirred by sufficiently vigorous convection, may have a sufficient electrical conductivity to sustain a magnetic field.
However, this possibility rests on several results that are based on knowledge obtained mostly from numerical simulations of Earth-core dynamos, which arguably operate in a different regime -- both in terms of geometry (a thick spherical shell) and dynamical balance.
With the help of dedicated numerical simulations in a thin spherical shell geometry, we study how such magnetic fields would look like (intensity, geometry) and what are the required conditions for an Earth-like magnetic field to be produced by a BMO.
How to cite: Schaeffer, N., Aurnou, J., and Labrosse, S.: Magnetic field generation by a Basal Magma Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14023, https://doi.org/10.5194/egusphere-egu25-14023, 2025.
While the Earth's magnetic field has existed for 4Gyr or more, its inner core is much younger - recent estimates for the age of the inner core go back no further than 1.5Gyr. Consequently, Earth’s dynamo has been running in a full sphere for much of its life, in contrast to the present day dynamo operating in a spherical shell. However, despite their geophysical relevance, full shpere dynamos remain rare in literature.
Here, we present results from a first parameter study on rapidly rotating dynamos in a full sphere geometry, representative of the Earth's dynamo before the nucleation of the inner core. Since we cannot rely on the buoyancy release of the inner core, our dynamos are driven by internal heat sources and fixed flux boundary conditions take account of the secular cooling of the planet. We show that - depending on the input paramters (Ekman, Rayleigh and magnetic Prandtl number) - such dynamos can produce a variety of different solutions, including dipolar and multipolar dynamos as well as stable and chaotically-reversing dynamos.
How to cite: Burmann, F., Luo, J., Marti, P., and Jackson, A.: Early-Earth dynamos in a full-sphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1349, https://doi.org/10.5194/egusphere-egu25-1349, 2025.
The geomagnetic field is generated and self-sustained by dynamo action in the Earth's liquid outer core. The dynamo is driven by thermo-chemical convection that derives energy from the secular cooling and inner core growth. In addition, the geodynamo is controlled by thermally inhomogeneous core-mantle boundary (CMB). The CMB controls the heat transfer from the core to the mantle. Such heterogeneous CMB heat flux affects the flow and magnetic field patterns generated by the dynamo. The present study investigates the back reaction of magnetic fields on the onset of convective instability inside the inner core tangent cylinder by incorporating various laterally varying thermal structures at the top plate of a plane layer convection model. Different orientations of imposed magnetic fields of various strengths have been implemented at various rotation rates. Consequently, localised convective flow clusters have been developed in the regions of heat flow higher than the mean heat flux as a consequence of imposed laterally heterogeneous thermal structures. Additionally, convective clusters have developed with both odd and even orders of thermal heterogeneity, resulting in laterally asymmetric and symmetric structures respectively. As a result of rapid rotation, small-scale columnar rolls are formed in a weak magnetic field, regardless of the magnetic field orientation. However, under a strong magnetic field with a horizontally imposed magnetic field, large-scale convection rolls are developed.
How to cite: Barman, T., Haldar, T., and Sahoo, S.: Back reaction of magnetic field on rotating convection in presence of thermal heterogeneity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16896, https://doi.org/10.5194/egusphere-egu25-16896, 2025.
In the present study the combined convection in a rapidly rotating plane layer under the conditions that are characteristic of the near-polar regions in the planetary interiors is investigated. The combined thermal and compositional convection in a slowly rotating plane layer was previously considered for oceans, where convection is supported by thermal effects and is suppressed by compositional effects. The present work analyses the occurrence of convection by both of these effects with a predominant compositional effect in the Earth’s outer core and with various effects in the deep interiors of the known planets and moons. The self-consistent estimates of typical physical quantities give similarity coefficients for the small ratio dissipation/convection generation (s coincides with inverse Rayleigh number) and the ratio thermal convection/compositional convection (r). The third small coefficient (δ linked to the Ekman number) is the ratio of the characteristic size normal to the axis of rotation to the layer thickness. The effect of the important parameters δ and s on the stability of the combined thermal and compositional convection in a rapidly rotating plane layer is proposed in the literature by Starchenko (2017). To investigate the linear stability of this problem here, the normal mode method is employed. The critical values of s and A (the critical wave number) observed to be depend on r for different values of δ and both Prandtl numbers that could imitate Solar System’s planets and moons at different ages. The obtained results coincide with those obtained by pioneers in the literature. The weakly nonlinear behaviour near to the primary instability threshold has been investigated using the spatiotemporal Landau-Ginzburg (LG) equation with cubic nonlinearity. Using the multiple scale analysis, the LG equation obtained and it is similar to those in the literature having different relaxation time, nonlinear coefficient, and coherence lengths. The heat transfer rate is studied using these coefficients. This equation is used to determine the domain for Eckhaus and zigzag as secondary instabilities.
How to cite: Rani, H., Nayak, K., Starchenko, S. V., and Rameshwar, Y.: Linear and Weakly Nonlinear Stability of Combined Convection in a Rapidly Rotating Plane Layer in Planetary Convection Models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1151, https://doi.org/10.5194/egusphere-egu25-1151, 2025.
Seismic studies, mineral physics, thermal evolution models and geomagnetic observations offer conflicting evidence about the presence of a stably stratified layer at the top of the Earth's fluid outer core. Such a convectively stable layer could have a strong influence on the Earth's hydromagnetic waves, propagating underneath the core-mantle boundary (CMB) that are used to probe the outermost region of the core. Here we present numerical solutions for the eigenmodes in a neutrally stratified sphere permeated by a magnetic field with and without a top stable layer, allowing for fluid exchanges between the stable layer and the neutrally stratified bulk of the core and angular momentum exchanges across the CMB through viscous- and electromagnetic coupling. On interannual time-scales, we find torsional Alfvén waves that are only marginally affected by weak to moderate stratification strength in the outer layer. At decadal time-scales similarly weak stable layers promote the appearance of waves, that propagate primarily within the stable layer itself and resemble Magneto-Archimedes-Coriolis (MAC) waves, even though they interact with the adiabatic fluid core below.
How to cite: Seuren, F., Triana, S., Rekier, J., Dehant, V., and Van Hoolst, T.: The influence of a stably stratified layer on the Earth's outer core waves., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19659, https://doi.org/10.5194/egusphere-egu25-19659, 2025.
It is widely accepted that the convection of the liquid metallic outer core is the driver of the dynamo-produced magnetic field in terrestrial bodies, yet the core composition and the processes which occur within the core are difficult to study due to the extreme temperatures (T) and pressures (P). By examining the phase changes that occur with varying P, T, and composition (X), phase diagrams may be constructed for expected core mimetic compositions. The constructed phase diagrams of Fe-Si alloys along with known or modeled P,T conditions of solid/liquid phases within the cores of interest can then be used to determine likely compositions of these cores.
Experiments were conducted in a 1000-ton cubic anvil press at P in the range 2-5 GPa and T into the liquid state. A central 5-hole BN cylinder held 5 different Fe-Si sample compositions simultaneously with a thermocouple located at the base of the BN cylinder, and was surrounded by a graphite furnace within a pyrophyllite cubic pressure cell. Fe-Si samples were prepared from pure Fe up to 33 wt% Si using mixtures of powders with known compositions. Following quenching of each experiment, the samples underwent electron microprobe analysis and along with textural analyses, these are used to map the T-X phase diagram at constant P. These phase diagrams will then be applied to the cores of small terrestrial bodies, such as the Moon, Mercury, and Vesta, to identify potential core compositions that are consistent with observational data and models that employ prescribed interior compositions and phases.
How to cite: Kalman, B., Yong, W., and Secco, R.: Phase Transitions of Solid and Liquid Fe-Si Alloys with Applications to Planetary Core Composition and Dynamo Processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2585, https://doi.org/10.5194/egusphere-egu25-2585, 2025.
The Earth’s inner core presents interesting properties such as seismic velocity anisotropy and a complex internal structure that is still under investigation. Establishing the phase diagram of the relevant iron alloys and, first, of pure iron itself is necessary to improve our understanding of planetary cores.
The iron phase diagram at high pressure and temperature is still discussed despite numerous experimental and simulation studies. Indeed, discrepancies still exist on the melting curve and the existence of a high pressure and high temperature cubic phase is debated. New techniques must be developed to address those issues.
The European X-ray Free Electron Laser (EuXFEL) offers a high brilliance pulsed X-ray beam. The pulses duration is below 50 fs and can be synchronized with the DiPOLE 100-X laser, enabling X-ray diffraction experiments during dynamic compression. This type of experiments was first tested at the EuXFEL in 2023 by an international consortium that was followed by a second experiment in 2024.
Those experiments allow the collection of high-quality in situ X-ray diffraction data and visar measurements of shocked and off-Hugoniot Iron. After establishing procedures for the processing of such data, comparison between results from the two experiments will ensure repeatability. In this presentation, we will show first results including solid phases and melt diffraction patterns collected during those experiments.
This work is the result of experiments performed under the EuXFEL 2740 and 6659 community proposals led by M. McMahon, K. Appel, J. Eggert and G. Morard.
How to cite: Ginestet, H. and Merkel, S. and the EuXFEL 2740 and 6659 community proposals: Laser driven shock compression of Iron at the EuXFEL, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9786, https://doi.org/10.5194/egusphere-egu25-9786, 2025.
The growth of Earth's solid inner core powers the geodynamo in the liquid outer core, creating a global magnetic field that helps to shield the planet from harmful solar radiation. However, the origins of the inner core are still not fully understood. Traditional models of core evolution overlook the necessity for liquids to be supercooled below their melting point before freezing. Recent estimates of the required supercooling for the inner core's homogeneous nucleation are unrealistically high and conflict with the expected current thermal structure of the core. Through molecular dynamics simulations, we show that nucleation from an Fe1-xCx liquid, with x=0.1-0.15, reduces the supercooling requirement to 250-400 K, broadly compatible with expected current thermal profiles of the core. Though these compositions are not a complete description of core chemistry, which requires at least ternary systems, they are consistent with a number of constraints derived from seismology, mineral physics, and geochemistry. Crucially, our results demonstrate that whilst some potential compositions of the core cannot explain the presence of the inner core, others can. The nucleation process of the inner core can therefore provide a new and strong constraint on core composition.
How to cite: Wilson, A., Davies, C., Andrew, W., and Alfè, D.: Constraining the Composition of Earth’s Core: Insights from Nucleation in FeC Liquids, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11647, https://doi.org/10.5194/egusphere-egu25-11647, 2025.
Dynamo action in liquid Fe planetary cores varies according to alloyed light elements such as S and Si. This study experimentally constrains the thermal conductivity of Fe-S-Si alloys at planetary core conditions, which may be used in combination with thermal evolution models to find the total thermal convective force in the core. A sample of Fe alloy with 16wt%S and 2wt%Si was chosen as a predicted composition of the core of Asteroid 4 Vesta, based on studies of HED meteorites [1-2]. This is near the miscibility limit of S and Si in liquid Fe [3].
Experiments were performed at 2-5 GPa in a 1000-ton cubic anvil press and at up to 9 GPa in a 3000-ton multi-anvil press. Temperatures as high as 2100 K into the melt of Fe-S-Si. The electrical resistivity of the liquid Fe-S-Si alloy was measured in situ; to find the electronic component of the thermal conductivity, the Wiedemann-Franz Law was used. To confirm the sample composition and homogenization, electron microprobe analysis was performed on samples recovered from various stages of melting, yielding compositional maps of Fe, S, and Si across each sample.
The individual effects of S and Si on the electrical resistivity of liquid Fe are seen in the results for the conditions of small planetary cores. Fe-16wt%S-2wt%Si has an electrical resistivity of 300-450 µΩ·cm at the complete melt in the pressure range of 2-7 GPa. Pure Fe at the same pressures is at most half this value [4], meaning that a moderate amount of S greatly decreases thermal conductivity in the liquid core. These results may be used to find the adiabatic heat flux at the top of the core of a given planetary body, with direct application to the formation of a magnetic dynamo in the liquid cores of objects such as Vesta, Ganymede, and Mars.
References:
[1] Steenstra, E.S., Dankers, D., Berndt, J., Klemme, S., Matveev, S., van Westrenen, W., 2019. Icarus, v. 317, p. 669-681.
[2] Pringle, E.A., Savage, P.S., Badro, J., Barrat, J.-A., Moynier, F., 2013. Earth Planet. Sci. Lett., v. 373, p. 75-82.
[3] Chabot, N.L., Wollack, E.A., Klima, R.L., Minitti, M.E., 2014. Earth Planet. Sci. Lett., v. 390, p. 199-208.
[4] Yong, W., Secco, R.A., Littleton, J.A.H., Silber, R.E., 2019. Geophys. Res. Lett., v. 46, p. 11065-11070.
How to cite: Lenhart, E., Yong, W., and Secco, R.: The heat flux through the cores of small terrestrial planetary bodies from electrical resistivity measurements of liquid Fe-S-Si at high pressures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2878, https://doi.org/10.5194/egusphere-egu25-2878, 2025.
The instabilities occurring in a horizontal layer of ferromagnetic fluid which is heated from below and kept rotating about vertical axis in the presence of vertical magnetic fluid is analysed using weakly nonlinear analysis. It is observed that either stationary convection or oscillatory convection occurred as the first instability depending on the control parameters. The amplitude equations are derived in the vicinity of the onset of stationary convection and oscillatory convection by assuming the fluid as anisotropic. At the onset of stationary convection the conditions for generalized Eckhuas instability and near the Liptz point the conditions for zig zag instability are obtained. Localized convection is studied from cubic-quintic amplitude equation. Heat transfer rate in terms of the Nusselt number is computed from the cubic amplitude equation. From the cubic-quintic amplitude equation, the subcritical fluid behavior near the onset of oscillatory convection is analysed.
How to cite: Rameshwar, Y., Jozef, B., Anil Kumar, O., and Rani, H. P.: Weakly Nonlinear Analysis of Rotating Anisotropic Ferromagnetic Rayleigh-Bénard convection, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14312, https://doi.org/10.5194/egusphere-egu25-14312, 2025.
The Fe-Ni alloy is believed to be the main component of Earth's core. Yet, Ni’s effects on the inner core’s structure and formation process are often disregarded due to its similarity to Fe. Using ab initio simulations, we find that Ni can stabilize bcc structures and accelerate Fe’s crystallization at high temperatures and inner core pressures. We computed the Gibbs free energy and phase diagram for liquid and solid solutions of Fe-Ni alloys under inner core conditions, providing new insights into the possible structure of the inner core. The results offer new constraints for the study of the core’s composition and formation.
How to cite: Sun, Y., Wei, L., Ho, K.-M., and Wentzcovitch, R.: The effect of Ni on the formation and structure of Earth’s inner core, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20372, https://doi.org/10.5194/egusphere-egu25-20372, 2025.
An upper bound of heat transfer has been published recently [1] for the first time in the case of compressible convection. This concerns only the anelastic liquid approximation, but the best place where such an approximation is valid might well be the core of terrestrial planets. In this work, I will apply this result to the specific case and geometry of the outer core. A big difference is also the fact that not only viscous dissipation but also Joule heating are sources of entropy production.
For a given forcing (Rayleigh number), we will see that there is a trade-off between a maximal heat flux and a maximal Joule dissipation. An upper bound can be obtained for both quantities, but they cannot both reach that bound.
We shall also consider the case of terrestrial planets of larger radii than the Earth. A number of exoplanets are suspected to be in that case. We will investigate the consequences of larger compressibility on their internal structure [2] and obtain upper bounds of heat flux and Joule dissipation.
[1] T Alboussière, Y Ricard, S Labrosse, "Upper bound of heat flux in an anelastic model for Rayleigh–Bénard convection", JFM 999, 2024
[2] Y Ricard, T Alboussière, "Compressible convection in super-Earths", PEPI 341, 2023
How to cite: Alboussiere, T.: Bounds on heat transfer and dissipation in the core, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13013, https://doi.org/10.5194/egusphere-egu25-13013, 2025.
The geomagnetic fields are generated by dynamo action driven by thermo-chemical convection in the deep interior. The convective instability occurs due to the density gradient of fluid, which depends on the temperature (thermal) and light element concentration (chemical composition), and they diffuse at different rates. We investigate rotating double-diffusive convection (RDDC) in the two-dimensional plane layer. We focus on classical convection, a diffusive regime with unstable thermal and chemical composition gradients. A systematic investigation on the impact of various thermal and compositional boundary conditions, such as fixed temperature, fixed composition, fixed heat flux, and fixed compositional flux, and their combinations, on the onset of convection by fixing mechanical boundary condition as no-slip is carried out in the present study. In particular, we choose a compositionally dominated regime by fixing the Rayleigh ratio (ratio of thermal to compositional Rayleigh number) equal to 0.5 for both non-rotating and rotating cases. With varying compositional Rayleigh numbers, the critical thermo-chemical Rayleigh number is estimated at the onset. The onset Rayleigh number, with fixed temperature and compositional boundary condition at both the upper and bottom boundary, is higher than fixed flux conditions for both the non-rotating and rotating cases, and this trend persists with increase (decrease) in compositional (thermal) Prandtl number at the low diffusivity ratio regime. However, at the high diffusivity ratio regime, the trends substantially change with changing diffusivity.
How to cite: Singh, S., Barman, T., and Sahoo, S.: Impact of boundary condition on the onset of thermo-chemical convection at the Earth’s core, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9161, https://doi.org/10.5194/egusphere-egu25-9161, 2025.
The Earth's liquid outer core is mainly composed of iron and nickel. The secular cooling of the inner core releases latent heat and light elements, driving convection in the liquid outer core and promoting the upward transport of these lighter elements, thereby forming thermal and compositional driven convection. However, due to the uncertainty of the temperature distribution within the liquid outer core, two types of convection may occur: top-heavy and salt-fingers double-diffusive convection, the latter characterized by a thermal stable stratification where the thermal gradient is stabilizing. Most dynamo models, however, do not account for such complex driving mechanisms. Instead, they simplify the system by assuming no distinction between thermal and compositional convection, which is termed the co-density model. In our study, we compared the top-heavy double-diffusive model with the co-density model within the strong field regime, where the Lorentz force plays a significant role. Our results suggest that, under strong fields and similar magnetic Reynolds numbers, different types of buoyancy do not show significant differences in driving the dynamo process. Furthermore, we investigate the effects of varying the strength of thermal stratification on the dynamo. Our analysis indicates that when the thermal stratification becomes sufficiently strong, it can suppress convection entirely, ultimately halting the dynamo process.
How to cite: Fan, W. and Lin, Y.: Double-diffusive convection driven dynamos in the strong-field regime, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8115, https://doi.org/10.5194/egusphere-egu25-8115, 2025.
Posters on site: Mon, 28 Apr, 10:45–12:30 | Hall X2
The Earth’s inner core is made of a solid iron alloy. Seismic observations suggest a structure and an anisotropy which leads to variations in both the velocity and the attenuation of the seismic waves. Attenuation is the loss of energy during the propagation of the seismic waves. Whether this attenuation arises from intrinsic properties of the iron alloys or extrinsic origins remains an open question. In this context, studying attenuation in metallic alloys could help improving our knowledge about the physical properties and the geodynamic of the inner core.
Different sources of attenuation exist in the core: extrinsic and intrinsic sources. The first one is linked to external environment that impact the wave propagation, such as scattering or heterogeneities. Intrinsic sources are related to the properties of the material itself. This work focuses on the latter and particularly on the anelastic relaxation, which is one of the source of internal friction.
In this work, we are re-investigated these problems to understand attenuation mechanisms in metals at high temperature. The experiments are conducted on a dynamic mechanical analysis (DMA) instrument with control of temperature and oxygen fugacity albeit at ambient pressure. Thus, we use an analogous material which presents similar crystallographic structure and is expected to behave like the inner core.
Here, we will present the first tests realized with variable frequency and constant temperature and discuss the future steps of the project.
How to cite: Carin, L., Chantel, J., Hilairet, N., and Merkel, S.: Preliminary work for experiments on inner core attenuation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3763, https://doi.org/10.5194/egusphere-egu25-3763, 2025.
The thermal conductivity of Earth’s core is a key parameter to investigate thermal evolution of the Earth, as well as the characteristics of the dynamo which drives Earth’s magnetic field, however it has been the subject of intense controversy. At the heart of this controversy are the persistent discrepancies between direct measurements of iron thermal conductivity, ab initio calculations of thermal and electrical conductivity and experimental electrical conductivity measurements. Here we present new data on the thermal conductivity of hcp-Fe up to 135 GPa and 3000 K, combined with direct X-ray based methods for the in situ measurement of sample geometry – the largest source of uncertainty in thermal conductivity measurements. Our results reaffirm a ‘low’ thermal conductivity for iron at the conditions of Earth’s core-mantle boundary, but revise this value upwards to between 60 W/m/K and 80 W/m/K which can be reconciled with the lower end of values reported using ab initio theory and electrical experiments.
How to cite: Edmund, E., Dobrosavljevic, V. V., Geballe, Z. M., and Goncharov, A. F.: Revisiting the Thermal Conductivity of Iron at Earth’s Core-Mantle Boundary, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6140, https://doi.org/10.5194/egusphere-egu25-6140, 2025.
We combined recent experimental data with analytical models to investigate the evolution of the Grüneisen parameter (γ) for iron under conditions relevant to the cores of rocky planets ranging from 1 to 5 Earth masses. γ relates thermal and elastic properties of materials and is a critical factor for understanding the dynamic behavior of planetary interiors. Previous sound speed measurements of the iron γ at Earth's core conditions, combined with seismic velocity data, significantly enhanced our understanding of the planet's interior [1]. Extending these studies to extreme conditions of larger planets can thus offer new insights into their internal properties. Recent laser-driven shock experiments measured γ for both liquid and solid iron at pressures of up to 3 TPa [2, 3]. By fitting this expanded dataset with the Altshuler and Anderson formalisms [4], we derived updated γ values that allowed us to assess temperature profiles for a range of planetary core sizes. These preliminary findings enabled us to assess the efficiency of thermal convection in super-Earth cores, providing valuable insights into their dynamic behavior.
References:
[1] Antonangeli & Ohtani. Progress in Earth and Planetary Science 2 (2015): 1-11.
[2] Huff et al. (2024) Phys. Rev. B, 109.18,184311.
[3] Smith et al. (2018) Nat. Astr., 2.6, 452.
[4] Clesi & Deguen (2024) GJI, 237 (3), 1275.
How to cite: Suer, T.-A., Clesi, V., Huff, M., and Marshall, M.: The Grüneisen parameter of iron under extreme conditions and its influence on thermal convection in super-earth cores , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14927, https://doi.org/10.5194/egusphere-egu25-14927, 2025.
Iron is the main constituent of the Earth's and terrestrial planetary cores. It is in the body-centered-cubic (bcc) structure under ambient conditions and transforms into the face-centered-cubic structure (fcc) upon heating at ambient pressure and into the hexagonal-closed-packed (hcp) structures at ~15 GPa at ambient temperature. Reaching Earth's inner core conditions in experiments is not trivial, and most reports of experiments approaching those pressures and temperature refer to the hcp structure for pure iron. First principles calculations, however, show that the energy difference between hcp and cubic phases of Fe is small at inner core conditions and some have argued for stable cubic Fe phases in the Earth's inner core.
In this work, we explore the phase diagram of Fe up to over 200 GPa and up to melting through a different thermodynamical pathway from conventional laser-heated diamond anvil cell experiments. The experiments rely on new facilities at the European X-Ray Free-Electron Laser, which provides extremely intense X-ray flashes repeated up to every 220 ns. The facility, coupled with the High Energy Density (HED) instrument, allows heating, melting, and crystallizing iron samples repeatedly and probe for its crystal structure as the sample cools from its previous state.
The experiments show a complex phase diagram for iron, and the observations of different crystal structures for iron as samples are moved through different thermodynamic states. Here, I will present these new experiments and preliminary results that can be obtained on Fe, along with our work on experimental metrology, which are actively pursuing at present.
Presentation on behalf of the EuXFEL 3063 community proposal, led by S. Merkel and G. Morard (doi: 10.22003/XFEL.EU-DATA-003063-00), and the EuXFEL 5700 community proposal, led by A. Dewaele and S. Merkel (doi: 10.22003/XFEL.EU-DATA-005700-00).
How to cite: Merkel, S. and Ginestet, H. and the EuXFEL 3063 and 5700 community proposals: Study of iron phases at planetary core conditions using static experiments at the European XFEL, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17768, https://doi.org/10.5194/egusphere-egu25-17768, 2025.
Sudden changes in the secular variation of the geomagnetic field, the geomagnetic jerks, provide information about the dynamics of the core at short timescales. How this dynamics may be coupled to changes in the core-mantle boundary (CMB) topography is not fully understood, due to the difficulty of obtaining direct observations on this region. Yet, it could be a key factor in explaining rapid changes in the geomagnetic field. Here, we use satellite measurements on the Earth’s gravity field variations in order to constrain potential mass redistributions at the CMB. We conduct an analysis of GRACE satellite and Satellite Laser Ranging (SLR) measurements of the Earth's gravity field from 2003 to 2015. The combination of second-order spatial derivatives of the gravity potential with a multi-scale temporal analysis allows for an enhanced separation of superimposed signals in the gravity field, based on their different spatial patterns and timescales. This way, we identify a significant transient north-south gravity anomaly at the boundary between the Atlantic Ocean and the African continent with maximum intensity in January 2007, with a timescale of 2-3 years. This signal cannot be fully explained by variations in surface water mass sources, suggesting an origin within the solid Earth. We show that the observed anomaly may be associated with mass redistributions during perovskite-to-post-perovskite phase transition triggered by moving thermal anomalies in the African Large Low Shear Velocity Province (LLSVP). This dynamic process results in decimetric variations in the CMB topography over months to years. We study how core flows in a stratified layer at the top of the core are impacted by this topography change, and discuss the corresponding signals in the geomagnetic field.
How to cite: Gaugne, C., Panet, I., Mandea, M., Greff, M., and Rosat, S.: GRACE observations of rapid mass variations at the core-mantle boundary during deep mantle phase transitions in interaction with core flow, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8808, https://doi.org/10.5194/egusphere-egu25-8808, 2025.
We investigate torsional Alfvén eigenmodes in the Earth's outer core. These eigenmodes exhibit energy equally distributed between magnetic and kinetic components, with their motion predominantly columnar. This columnar nature has previously enabled the development of approximate inviscid one-dimensional models. In contrast, our study employs a three-dimensional numerical model that incorporates viscosity, an electrically conductive inner core, and a thin, conductive layer at the bottom of the mantle. This configuration allows angular momentum exchange between the outer core, the solid inner core, and the mantle. Using this model, we systematically examine the key properties of these modes, particularly their columnarity, torques, and decay rates. We study how these properties vary with the magnetic diffusivity and viscosity of the outer core, as well as with the electrical conductance of the mantle's bottommost layer.
How to cite: Triana, S., Rekier, J., Barik, A., Gerick, F., Seuren, F., and Dehant, V.: A numerical model of torsional Alfven eigenmodes in the Earth's core, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18135, https://doi.org/10.5194/egusphere-egu25-18135, 2025.
Internal fluid layers can contribute to energy dissipation within planets, thereby influencing the planet’s rotational parameters. Traditionally, dissipation and angular momentum transfer in such fluid layers have been analysed assuming smooth surfaces. Here, we account for the effects of topographical irregularities, particularly the wave drag caused by inertial waves.
In rapidly rotating fluids, topography can excite inertial waves that propagate deep into the fluid interior. These waves contribute to the fluid drag exerted at the topography. We present a theoretical model for the drag caused by topographically excited inertial waves, validated through a two-step approach.
In the first step, we validate our model for the simplest case: steady flow over a monochromatic topography in a periodic Cartesian box. Numerical simulations are conducted using the computational fluid dynamics solver Nek5000, showing that the drag scales with the square of the topography height (h^2) for low-slope topographies. For steeper slopes exceeding unity, the drag becomes wavelength-dependent.
In the second step, we examine a more complex case involving the differential rotation of the fluid and the monochromatic topography in a cylinder. We demonstrate experimentally and numerically that the torque from inertial wave drag can be predicted from our previous results, with the resulting torque exhibiting the same scaling properties as the drag in the periodic box.
This two-step approach provides the foundation for understanding angular momentum transfer in planetary interiors. It sets the stage for calculating the resulting torque over a full spherical shell.
How to cite: Giraud, V., Noir, J., Cébron, D., Monville, R., and Burman, F.: Investigating the drag force due to inertial waves generated by topography, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7333, https://doi.org/10.5194/egusphere-egu25-7333, 2025.
We investigate experimentally the flow structure in a fluid layer heated from below and cooled from above, where, additionnally, a horizontal temperature gradient is imposed on the top plate. This is a model system for the dynamics in subglacial lakes where such a competition between Rayleigh-Bénard Convection (RBC) and Horizontal Convection (HC) is thought to happen, and an experimental realisation of the numerical work of Couston et al. (2022). We evidence a hysteretic transition from a RBC flow structure to a HC flow structure when the ratio of the horizontal heat flux to the vertical heat flux, Λ, is 4e-4 when Λ is decreasing, and 7e-4 when Λ is increasing. These values are lower than the threshold value found in the two-dimensional Direct Numerical Simulation (DNS), of order 1e-2, which has an impact on the flow structure prediction for several subglacial lakes. Additionnally, for larger values of Λ, we observe that the warmest part of the top plate becomes warmer than the bottom plate, and a stable temperature gradient settles below the warm side of the top plate. Thermal plumes are no longer visible in this region, and seem to be replaced by internal gravity waves.
How to cite: Bret, C., Bu, Y., Rabaux, V., Chanut, V., Couston, L., Chilla, F., and Salort, J.: Competition between Rayleigh-Bénard and Horizontal Convection: an experimental model for subglacial lakes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10923, https://doi.org/10.5194/egusphere-egu25-10923, 2025.
In rotating magnetoconvection (RMC) models, the turbulent state of the Earth's fluid core is parameterized by the isotropic (i) and anisotropic (a) diffusive coefficients, specifically, the viscosity (ν), thermal diffusivity (κ), and magnetic diffusivity (η). It can be used as the basic state, which is useful for the study of stability analysis as each physical state. The linear stability analysis is performed on RMC model of the horizontal fluid planar layer heated from below and cooled from above, rotating about its vertical axis and permeated by a horizontal homogeneous magnetic field. The normal mode method in the form of horizontal rolls is applied on the RMC model. A comparison is made between the results based on the fastest growing (F) modes with the highest growth rate and the marginal (M) modes. The F modes are studied for four different i and a combinations of diffusivities (νκη) = (aaa, aai, iai, iii) as (f, p, h, i) cases. Both the anisotropic and isotropic parameters have a significant impact on the instability caused by a large Rayleigh number, R, in all occurrences of F modes. The F modes are strongly and differently influenced by the f, p, h and i, cases. In all the investigated cases the wave number and maximum growth rate based on the R and the anisotropic parameter, α (ratio of horizontal to vertical diffusivities), are independent of Ekman number, Ez, Elsasser number, Λz and are the same. The effect of all anisotropy cases is more significant for the F modes than the M modes on the occurrence of convection modes. The F modes show much better results than the M modes related to the parameters, R, Ez, Λz, inverted magnetic Prandtl number, pz , and Roberts number, qz that are typical for the Earth’s outer core. The present RMC approach allows to easily deal with very huge R, very small Ez and huge wave numbers, particularly in F modes which the geodynamo simulations are unable to do. In M as well as in F modes, the inequality α > 1 (α < 1) inhibits (facilitates) the convection, at all anisotropy cases. The QG balance of forces could prevail in α << 1 conditions in the Earth's outer core and the MAC balance could be in the uppermost layer of the core with α >> 1.
Keywords: Rotating magnetoconvection; fastest growing modes; marginal modes; anisotropic diffusivities; molecular and turbulent diffusivities; Earth’s core conditions.
How to cite: Rani, H. P., Brestenský, J., and Nayak, K.: Rotating Magnetoconvection with Diffusivities Parameterized by Turbulent State of the Earth's Core, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20527, https://doi.org/10.5194/egusphere-egu25-20527, 2025.
