Borok-Munich Absolute Palaeointensity Database

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Earth's dynamo limit of predictability controlled by magnetic dissipation

International audienceTo constrain the forecast horizon of geomagnetic data assimilation, it is of interest to quantify the range of predictability of the geodynamo. Following earlier work in the field of dynamic meteorology, we investigate the sensitivity of numerical dynamos to various perturbations applied to the magnetic, velocity and temperature fields. These perturbations result in some errors, which affect all fields in the same relative way, and grow at the same exponential rate λ = τ −1 e , independent of the type and the amplitude of perturbation. Errors produced by the limited resolution of numerical dynamos are also shown to produce a similar amplification, with the same exponential rate. Exploring various possible scaling laws, we demonstrate that the growth rate is mainly proportional to an advection timescale. To better understand the mechanism responsible for the error amplification, we next compare these growth rates with two other dynamo outputs which display a similar dependence on advection: the inverse τ −1 SV of the secular-variation timescale, characterizing the secular variation of the observable field produced by these dynamos; and the inverse (τ mag diss) −1 of the magnetic dissipation time, characterizing the rate at which magnetic energy is produced to compensate for Ohmic dissipation in these dynamos. The possible role of viscous dissipation is also discussed via the inverse (τ kin diss) −1 of the analogous viscous dissipation time, characterizing the rate at which kinetic energy is produced to compensate for viscous dissipation. We conclude that τ e tends to equate τ mag diss for dynamos operating in a turbulent regime with low enough Ekman number, and such that τ mag diss < τ kin diss. As these conditions are met in the Earth's outer core, we suggest that τ e is controlled by magnetic dissipation, leading to a value τ e = τ mag diss ≈ 30 yr. We finally discuss the consequences of our results for the practical limit of predictability of the geodynamo

Statistical properties of reversals and chrons in numerical dynamos and implications for the geodynamo

International audienceWe analyse a series of very long runs (equivalent to up to 50 Myr) produced by chemically-driven dynamos. All runs assume homogeneous boundary conditions, an electrically conducting inner-core (except for one run) and only differ by the choice of the Rayleigh number Ra★. Introducing dynamo-based definitions of reversals, chrons and related concepts, such as "failed reversals" and "segments" (bounded by reversals or failed reversals), we investigate the distributions of chron and segment lengths, those of reversal and failed reversal durations, the way dipole field behaves through reversals and failed reversals, and the possible links between the axial dipole intensity and chron or segment lengths. We show that chron and segment lengths are very well described in terms of a Poisson process (with no occurrence of superchrons), while distributions of reversal and failed reversal durations are better fitted by log-normal distributions. We found that reversal rates generally increase in proportion to Rm-Rmc,Rm being the magnetic Reynolds number and Rmc a critical value. In contrast, reversal and failed reversal durations appear to be mainly controlled by the core's magnetic diffusion timescale. More generally, we show that much of the reversing behaviour of these dynamos can be understood by examining their signature in a (g10,g11,h11) phase-space plot. This reveals that the run with an insulating inner-core is very different and has only two distinct modes of opposite polarity, which we argue is the reason it displays less reversals and failed reversals, and has a clear tendency to produce an intensity "overshoot" and some systematic pattern in the dipole pole behaviour through reversals and failed reversals. This contrasts with conducting inner-core runs, which display an additional central unstable mode, the importance of which increases with Rm, and which is responsible for the more complex reversing behaviour of these dynamos. Available paleomagnetic data suggest that the current geodynamo could have such a (small) central mode, which would thus imply a strong sensitivity of the frequency and complexity of reversals and of the likelihood of failed reversals, to changes in the geodynamo's driving parameters through geological times

The geomagnetic secular-variation timescale in observations and numerical dynamo models

International audienceThe knowledge of the spatial power spectra of the main geomagnetic field and of its secular variation makes it possible to define typical timescales t n for each spherical harmonic degree n. Investigating both observations and numerical dynamos, we show that a one‐parameter law of the form t n = t SV/n is satisfied for the non‐dipole field, given the statistical way the observed t n are expected to fluctuate. Consequently, we determine the corresponding secular‐ variation timescale t SV from either instantaneous or time‐ averaged spectra, leading to a value of 415 ± 45 55 yr for recent satellite field models. In the broader context of geomagnetic data assimilation, t SV could provide a sensible and convenient means to rescale the time axis of dynamo simulations

Impact of inner-core size on the dipole field behaviour of numerical dynamo simulations

International audienceFrom a suite of 56 chemically-driven dynamo simulations with aspect ratio (inner to outer core radii) ranging from 0.10 to 0.44, we conduct the first systematic investigation of the impact of inner-core size on the reversing behaviour of dynamos. We show that the growth of the inner core leads to a transition between a "small inner-core" regime ( 0.18), when the field produced is intermediately strong and dipolar, and a "large inner-core" regime (> 0.26), when the field is stronger and more dipolar. During that transition the field is weaker and slightly less dipolar. For aspect ratios 0.20 0.22, reversal frequencies may be more sensitive to changes in the vigour of the convection, allowing high frequencies to be reached much more easily. Although other factors than the size of the inner core likely contribute to controlling the reversal frequency of the Earth's dynamo, we hypothesise that the occurrence of such a transition for the Earth's core between the end of the Precambrian and the end of the Devonian could possibly account for the manifestation of an unusual long-lasting episode of predominantly reversal hyperactivity and complex low intensity fields during that still poorly documented period of time

Caractérisation d'échantillons représentatifs de soudures austénitiques en 316L par spectroscopie de Résonance Ultrasonore (RUS)

Conference : Cofrend 2023 - Les Journées Cofrend/ COFREND DAYS, Marseille, France, , 06 - 08 June 2023Dans la perspective d'améliorer les procédés d'imagerie ultrasonore sur des assemblages soudés, les caractéristiques mécaniques effectives du matériau et leur variabilité au sein de la soudure sont une donnée nécessaire. De nombreuses techniques reposant sur l'utilisation d'ondes élastiques ultrasonores ont déjà été mises en œuvre afin d'obtenir avec précision les constantes élastiques et ainsi d'évaluer l'anisotropie apparente de ces milieux complexes. Dans le cadre de cette étude, la Spectroscopie de Résonance Ultrasonore (RUS) a été mise en œuvre pour caractériser des échantillons prélevés dans un « mur » fabriqué par soudage dans l'hypothèse d'un comportement élastique orthotrope, homogène à l'échelle de l'échantillon. Les fréquences de résonance mesurées ont permis de résoudre le problème inverse à l'aide de méthodes d'optimisation utilisant le gradient et d'obtenir les constantes élastiques pour chacun des échantillons. Les résultats sont comparés à ceux préalablement obtenus par l'utilisation d'une technique de mesure de vitesse de phase en ondes ultrasonores pulsées dans le cadre du projet ANR MUSCAD

Do changes in geomagnetic secular variation, dipole moment and polarity reversal frequency correlate over the past 155 Myr?

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On the reliability of absolute palaeointensity determinations on basaltic rocks bearing a thermochemical remanence - raw data for figures 5-10

We investigated the acquisition of thermochemical remanent magnetization (TCRM) on basaltic rocks from the volcanic island of São Tomé (Gulf of Guinea) and from the southern part of the Red Sea Rift, both containing homogeneous titanomagnetite grains with Curie temperatures of 100-200 °C. The TCRM was created in a rotating thermomagnetometer by cooling the samples from 570 to 200 °C at a rate of 1 °C per hour in the presence of a laboratory magnetic field of 50 μT. The TCRM acquisition occurred at high temperature T > 520 °C through the nucleation of ilmenite lamellae dividing the titanium-magnetite cells. Mutual Fe-Ti diffusion moved the composition of the cells closer to that of magnetite, leading to an increase in the Curie temperature Tс. The TCRM was formed at practically fixed volume of the titanomagnetite cells when Tс exceeded T. The Thellier-style experiments conducted on the samples bearing a laboratory induced TCRM, revealed palaeointensity estimates within 5% to expected value. This conclusion radically differs from previous results obtained in the case of a pure chemical remanent magnetization, and gives hope that a TCRM could be a robust source of palaeomagnetic information, yielding unbiased palaeointensity determinations

Paleomagnetism of Paleocene-Maastrichtian (60–70 Ma) lava flows from Tian Shan (Central Asia): directional analysis and paleointensities

Specimen-level data for paleodirections and absolute paleointensitie