Correlation of Earth’s magnetic field with lower mantle thermal and seismic structure

Numerical models of the geodynamo are usually classified in two categories: those denominated dipolar modes, observed when the inertial term is small enough, and multipolar fluctuating dynamos, for stronger forcing. We show that a third dynamo branch corresponding to a dominant force balance between the Coriolis force and the Lorentz force can be produced numerically. This force balance is usually referred to as the strong-field limit. This solution co-exists with the often described viscous branch. Direct numerical simulations exhibit a transition from a weak-field dynamo branch, in which viscous effects set the dominant length scale, and the strong-field branch in which viscous and inertial effects are largely negligible. These results indicate that a distinguished limit needs to be sought to produce numerical models relevant to the geodynamo and that the usual approach of minimizing the magnetic Prandtl number (ratio of the fluid kinematic viscosity to its magnetic diffusivity) at a given Ekman number is misleading.

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“…Olson et al 2011;Gubbins et al 2007), though without pointing to the existence of a weak-and a strong-field branch.…”

Section: Physical Interpretationmentioning

confidence: 99%

Strong-field spherical dynamos

Dormy

2016

J. Fluid Mech.

101

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“…For the Earth, the most popular prescribed heat flux patterns are either proportional to seismic velocity anomalies obtained from tomography models of the lowermost mantle to mimic realistic conditions, or spherical harmonic degree and order 2 which is the dominant term in these tomography models (Aubert et al 2007(Aubert et al , 2008Bloxham 2002;Christensen and Olson 2003;Davies et al 2008; *Correspondence: Hagay.Amit@univ-nantes.fr 1 CNRS UMR 6112, Université de Nantes, Laboratoire de Planétologie et de Géodynamique, 2 rue de la Houssinière, Nantes, F-44000, France Full list of author information is available at the end of the article Glatzmaier et al 1999;Gubbins et al 2007Gubbins et al , 2011Heimpel and Evans 2013;Kutzner and Christensen 2004;Olson and Amit 2014;Olson and Christensen 2002;Olson et al 2010;Sreenivasan 2009;Sreenivasan and Gubbins 2011;Takahashi et al 2008). For other planets, various single harmonics are often used, for example degree-1 for Mars (Amit et al 2011a;Dietrich and Wicht 2013;Stanley et al 2008) and degree-2 for Mercury and Saturn (Cao et al 2014;Stanley 2010, respectively), reflecting the low-resolution information on planetary deep interiors.…”

Section: Generalmentioning

confidence: 99%

Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

Amit

Choblet

Olson

et al. 2015

Prog. in Earth and Planet. Sci.

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Mantle control on planetary dynamos is often studied by imposing heterogeneous core-mantle boundary (CMB) heat flux patterns on the outer boundary of numerical dynamo simulations. These patterns typically enter two main categories: Either they are proportional to seismic tomography models of Earth's lowermost mantle to simulate realistic conditions, or they are represented by single spherical harmonics for fundamental physical understanding. However, in reality the dynamics in the lower mantle is much more complicated and these CMB heat flux models are most likely oversimplified. Here we term alternative any CMB heat flux pattern imposed on numerical dynamos that does not fall into these two categories, and instead attempts to account for additional complexity in the lower mantle. We review papers that attempted to explain various dynamo-related observations by imposing alternative CMB heat flux patterns on their dynamo models. For present-day Earth, the alternative patterns reflect non-thermal contributions to seismic anomalies or sharp features not resolved by global tomography models. Time-dependent mantle convection is invoked for capturing past conditions on Earth's CMB. For Mars, alternative patterns account for localized heating by a giant impact or a mantle plume. Recovered geodynamo-related observations include persistent morphological features of present-day core convection and the geomagnetic field as well as the variability in the geomagnetic reversal frequency over the past several hundred Myr. On Mars the models aim at explaining the demise of the paleodynamo or the hemispheric crustal magnetic dichotomy. We report the main results of these studies, discuss their geophysical implications, and speculate on some future prospects.

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“…κ also appears in the Rayleigh number, which measures the convective vigor of a system. Thus, the thermal conductivity of the lower mantle affects the structure, thickness, and dynamics of the CMB (1, 2), the style and structure of mantle convection (3)(4)(5), and the amount of heat conducted from the core to the mantle (6) that in turn influences the generation of the Earth's magnetic field (7).…”

mentioning

confidence: 99%

Lattice thermal conductivity of MgO at conditions of Earth’s interior

Tang

Dong

2010

Proc. Natl. Acad. Sci. U.S.A.

150

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Thermal conductivity of the Earth's lower mantle greatly impacts the mantle convection style and affects the heat conduction from the core to the mantle. Direct laboratory measurement of thermal conductivity of mantle minerals remains a technical challenge at the pressure-temperature (P-T) conditions relevant to the lower mantle, and previously estimated values are extrapolated from low P-T data based on simple empirical thermal transport models. By using a numerical technique that combines first-principles electronic structure theory and Peierls-Boltzmann transport theory, we predict the lattice thermal conductivity of MgO, previously used to estimate the thermal conductivity in the Earth, at conditions from ambient to the core-mantle boundary (CMB). We show that our first-principles technique provides a realistic model for the P-T dependence of lattice thermal conductivity of MgO at conditions from ambient to the CMB, and we propose thermal conductivity profiles of MgO in the lower mantle based on geotherm models. The calculated conductivity increases from 15 -20 W∕K-m at the 670 km seismic discontinuity to 40 -50 W∕K-m at the CMB. This large depth variation in calculated thermal conductivity should be included in models of mantle convection, which has been traditionally studied based on the assumption of constant conductivity.first-principles | phonon transport theory | phonon lifetime | high pressure | Lower Mantle T hermal conductivity (κ) is one of the most important mineral properties in determining the heat budget of the Earth. Heat in the Earth's interior is transferred by convection in the mantle and core and regulated by conduction at thermal boundary layers. As defined by Fourier's law of heat conduction J Q ¼ −κ · ∇T, determines the conducting heat flow density (J Q ) in the presence of a temperature gradient ∇T. κ also appears in the Rayleigh number, which measures the convective vigor of a system. Thus, the thermal conductivity of the lower mantle affects the structure, thickness, and dynamics of the CMB (1, 2), the style and structure of mantle convection (3-5), and the amount of heat conducted from the core to the mantle (6) that in turn influences the generation of the Earth's magnetic field (7).Despite its importance, thermal conductivity remains as one of the least constrained physical properties of minerals, especially at lower-mantle pressures (P) and temperatures (T) and approximately 1,900-4,000 K (9-12). Experimental data at deep mantle conditions are scarce due to the technical difficulty of measuring thermal conductivity at these extremes. Thermal conductivity of lower-mantle minerals is often estimated either by extrapolating data from lower P-T conditions and/or employing theoretical models with parameters fitted with lower P-T data (1, 13). However, direct extrapolation to deep mantle conditions can be unreliable beyond the P-Trange of the measurements, and empirical models are often based on untested assumptions. For example, the sound velocities are used to approximate phonon vel...

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Correlation of Earth’s magnetic field with lower mantle thermal and seismic structure

Cited by 107 publications

References 23 publications

Strong-field spherical dynamos

Strong-field spherical dynamos

Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

Lattice thermal conductivity of MgO at conditions of Earth’s interior

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