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—Paleomagnetic studies of several Late Cretaceous volcanic sections of the Okhotsk–Chukotka volcanic belt have been carried out in the Bilibino region of the Chukotka Autonomous Okrug and along the Pevek–Egvekenot road. Extensive collections have been acquired and analyzed. The laboratory experiments isolated the ancient characteristic magnetization component reflecting the direction of the geomagnetic field at the time of formation of the studied rocks (~85 Ma ago). The primary character of the revealed characteristic magnetization component is supported by the positive regional fold test and by the coincidence of the paleomagnetic pole calculated from this component with that previously obtained for Chukotka from the rocks of similar age (Stone et al., 2009). The paleomagnetic pole calculated from the combination of the previous and our newly obtained data (Plat = 69.3°, Plong = 180.7°, N = 99, A95 = 5.1°) indicates that the sampled rocks were formed in the immediate vicinity of the geographic pole. The reliability of the existing Late Cretaceous paleomagnetic poles for Eurasia and North America is analyzed, and the refined poles are calculated for these plates for the time of ~85 Ma. The reconstruction of the Chukotka–Kolyma–Omolon block’s position relative to Eurasia and North America allowing for the paleomagnetic poles calculated for that time is proposed. The reconstruction implies that from the formation time of the studied rocks up to the present, the Chukotka–Kolyma–Omolon block has undergone relatively small (tens to first hundreds of km) southward movements relative to the North American plate and has been noticeably shifted (by a few hundred km) relative to the Eurasian plate. Our reconstruction is close to that proposed in (Otofuji et al., 2015) but, in contrast to the latter, it does not require a collision between the Chukotka–Kolyma–Omolon block and Eurasia after 80 Ma ago.

International audience; Despite progress in the knowledge of secular variation during the first millennium BCE in Europe, data coverage remains poor at the earliest periods, especially in some regions as in the Central Mediterranean area. This study presents three new directional and six new intensity data between the 13th and the 4th centuries BCE on archaeological kilns, pottery and baked clay fragments from South Italy and France. Archaeodirections were determined after thermal demagnetizations and archaeointensities using the Thellier-Thellier protocol with corrections for the anisotropy and cooling rate effects. The new data confirm the large deviation of the direction from a Geocentric Axial Dipole field, the high geomagnetic field strength and the fast secular variation observed in Europe during the earliest half of the first millennium BCE. Another characteristic of this period is a difference of ~25° between the longitudes of the virtual geomagnetic poles inferred from European and Middle East data. This unusual behaviour can be mainly related to the Levantine Iron Age anomaly (LIAA) and its expansion from the Middle East to Europe. However, the review of the global directional database shows that almost all virtual geomagnetic poles, 96% of them coming from Europe, the Middle East, East Asia, North America and Hawaii, are 10–25° away from the rotation axis towards North Russia between 1000 and 600 BCE. The calculation of a mean global VGP curve suggests that the North geomagnetic pole followed a clockwise motion during this period with a dipole tilt up to around 14°. This study shows that a dipole axis tilt may have played an important role in the rapid secular variation in western Eurasia, although part of this variation may also be related to non-dipole fields associated with the LIAA.

The analysis of the statistical characteristics of the geomagnetic field generated in the numerical geodynamo models has shown that the distribution of the spherical harmonic coefficients in some cases is not Gaussian but, instead, has much in common with the Laplace distribution. The shape of the corresponding histograms depends on the time scale, which allows interpreting the obtained data in terms of a mixture of Gaussian distributions. The similar effects associated with the intermittence were observed in the experiments in a turbulent fluid flow. Hence, the behavior of secular variations in the magnetic field of the Earth should perhaps be described in terms of a mixture of several Gaussian stationary processes corresponding to switching between the different regimes of geodynamo generation.

International audience; From 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.

International audience; We conducted an absolute palaeointensity (API) survey on Ethiopian volcanics from Early Oligocene (Belessa section) and Middle Miocene (Debre Sina section). After a careful selection based on the reversibility of high-field thermomagnetic curves from samples yielding unambiguous palaeodirections, we restricted our analysis to five (resp. six) cooling units from Belessa (resp. Debre Sina), submitted to both Thellier and Wilson-style experiments. X-ray diffraction measurements and microscopical observations under reflected light gave us good confidence in the reliability of the remanence carriers of the Belessa basalts, with pseudo-brookite exsolutions in titanomagnetite (TM) grains, synonymous of an advanced stage of high-temperature (i.e. deuteric) oxidation. In contrast, the Debre Sina trachybasalts may have been submitted to low-temperature (i.e. post-consolidation) alteration featured by granulation of TM grains and leading to a higher dispersion of API estimates. Compared to the world palaeointensity database', the lower dipole moment found in Ethiopia for the Early Oligocene (30 +/- 8ZAm(2)) than the Middle Miocene (65 +/- 20ZAm(2)) is compatible with a global increase of dipole strength from 30 Ma to present, and also suggestive of a dipole low during the Oligocene. This observation must however be treated with caution because of the over-representation of Plio-Pleistocene determinations in the global database and the partial correlation between dipole strength and number of palaeointensity determinations.

We report palaeomagnetic and Ar-40/Ar-39 dating results from two sequences of basaltic lava flows deposited at the same locality in western China, yet separated in time by similar to 50 Myr: one set lies within the Cretaceous normal superchron at 112-115 Ma and a second at 59-70 Ma spanning the Cretaceous-Palaeogene boundary. We find that magnetic field directions during the superchron exhibit bimodal populations: one with inclinations representative of a dipolar field and another with shallow inclinations that could reflect a more complex, multipolar field. However, the time-dependent variability in field directions was 50% lower during the superchron than after, which implies greater field stability during the superchron. Our results suggest that episodes of less dipolar field behavior occurred within the Cretaceous superchron and raise the question whether a second, more multipolar, field state is more persistent than previously thought.

SUMMARY To 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−1e, 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−1SV of the secular-variation timescale, characterizing the secular variation of the observable field produced by these dynamos; and the inverse (τmagdiss)−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 (τkindiss)−1 of the analogous viscous dissipation time, characterizing the rate at which kinetic energy is produced to compensate for viscous dissipation. We conclude that τe tends to equate τmagdiss for dynamos operating in a turbulent regime with low enough Ekman number, and such that τmagdiss < τkindiss. As these conditions are met in the Earth's outer core, we suggest that τe is controlled by magnetic dissipation, leading to a value τe=τmagdiss≈ 30 yr. We finally discuss the consequences of our results for the practical limit of predictability of the geodynamo.

International audience; Data assimilation in geomagnetism designates the set of inverse methods for geomagnetic data analysis which rely on an underlying prognostic numerical model of core dynamics. Within that framework, the time-dependency of the magnetohydrodynamic state of the core need no longer be parameterized: The model trajectory (and the secular variation it generates at the surface of the Earth) is controlled by the initial condition, and possibly some other static control parameters. The primary goal of geomagnetic data assimilation is then to combine in an optimal fashion the information contained in the database of geomagnetic observations and in the dynamical model, by adjusting the model trajectory in order to provide an adequate fit to the data. The recent developments in that emerging field of research are motivated mostly by the increase in data quality and quantity during the last decade, owing to the ongoing era of magnetic observation of the Earth from space, and by the concurrent progress in the numerical description of core dynamics. In this article we review briefly the current status of our knowledge of core dynamics, and elaborate on the reasons which motivate geomagnetic data assimilation studies, most notably (a) the prospect to propagate the current quality of data backward in time to construct dynamically consistent historical core field and flow models, (b) the possibility to improve the forecast of the secular variation, and (c) on a more fundamental level, the will to identify unambiguously the physical mechanisms governing the secular variation. We then present the fundamentals of data assimilation (in its sequential and variational forms) and summarize the observations at hand for data assimilation practice. We present next two approaches to geomagnetic data assimilation: The first relies on a three-dimensional model of the geodynamo, and the second on a quasi-geostrophic approximation. We also provide an estimate of the limit of the predictability of the geomagnetic secular variation based upon a suite of three-dimensional dynamo models. We finish by discussing possible directions for future research, in particular the assimilation of laboratory observations of liquid metal analogs of Earth's core.

International audience; The 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.

[1] Earth's magnetic field is currently decreasing, reducing the protection it offers against charged particles coming from space and increasing space weather hazards within the near-Earth environment. Modeling the future evolution of the field is thus of considerable interest. But how far in the future this can conceivably be done is still an open question. Here we report on the first systematic investigation of the limit of predictability of fully consistent 3D numerical dynamo simulations, and suggest that the Earth's dynamo is likely unpredictable beyond a century, making decade timescale forecasts of the main magnetic field conceivable, but rendering longer-term predictions, such as the timing of the next reversal, totally unpredictable.