Results 1 - 10 of 203
Results 1 - 10 of 203. Search took: 0.017 seconds
|Sort by: date | relevance|
[en] In addition to the weak-dipolar state and to the fluctuating-multipolar state, widely discussed in the literature, a third regime has been identified in (Dormy 2016 J. Fluid. Mech. 789 500–13). It corresponds to a strong-dipolar branch which appears to approach, in a numerically affordable regime, the magnetostrophic limit relevant to the dynamics of the Earth’s core. We discuss the transitions between these states and point to the relevance of this strong-dipolar state to Geodynamo modelling. (paper)
[en] The paper 'Direct observations of the viscosity of Earth's outer core and extrapolation of measurements of the viscosity of liquid iron' by D.E. Smylie, V.V. Brazhkin, and A. Palmer [Phys. Usp. 52 (1) 79 (2009)] is subject to critique for its proposed approach to estimating the viscosity of the Earth's outer core. (methodological notes)
[en] Complete text of publication follows. From geomagnetic observations recorded above the Earth's surface, down ward continuation of a potential magnetic field gives r.m.s. radial field values of the order 0.3 mT at the core-mantle boundary (CMB). Further assumptions are needed to infer the field sustaining the geodynamo inside the conducting Earth's core, which strength cannot be directly probed. A few mT is found by applying scaling laws derived from either force balance or power dissipation arguments in numerical simulations. Nutation data and theory indicate 7 mT at the inner core boundary (ICB) to account for the dissipation in the Earth's core. Recently, the magnetostrophic balance applied to quasi-geostrophic core flow models indicated as well a few mT inside the core. However, a 60-year signal found in the length-of-day variation (lod) has been associated with torsional waves carried by a much weaker internal field, of amplitude similar to that obtained at the CMB. Instead, we find in the present study a 6-year period torsional oscillation that predicts well both the phase and the amplitude of the 6-year lod signal detected over the second half of the twentieth century. Triggered every 6 years at the ICB, the waves propagate outwards through the Earth's core in a few years time. It suggests a large internal magnetic field strength that reconciles with previous estimates from geodynamo simulations, and supports the idea of a gravitational coupling between the inner core and the mantle.
[en] Complete text of publication follows. Among the main problems in geomagnetism are the reversals, particularly by their long term irregularity, and the existence of superchrons. In this work we show evidence supporting the idea that geomagnetic reversals, both in frequency and morphology, are controlled by anomalies at the base of the mantle, in particular by large topographic structures. This observation comes from the comparison between observed topography at the base of the mantle, and from rate of oceanic plate spreading (which may be the ultimate cause of topographic anomalies at the base of the mantle) and frequency of reversals. To explain these facts we use the argument that topographic magnetostrophic waves can generate the alpha effect of dynamo theory. The amplitude of the alpha effect is proportional to the amplitude of the topographic feature. As it has been long known, frequency of reversals depends on the amplitude of alpha. Then we come to the conclusion that periods with high frequency of reversals are related to epochs when the core mantle boundary is irregular, and periods with low frequency of reversals are associated with a smooth core mantle boundary. Since alpha effect is a source of poloidal field its concentration may cause the observed clustering of VGPs close high amplitude topographic regions. Finally we show that these long term variations of alpha effect due to variations in the amplitude of topographic features can generate a distribution of polarity intervals similar to the observed one by using a nonstationary stochastic dynamo model.
[en] Complete text of publication follows. When deriving a core field model from satellite and observatory data, some smoothing constraints are usually applied to the model solutions. Such constraints lead to core field model temporal behaviors that are not compatible with the frozen-flux approximation. We derived the GRIMM-2 core field model from eight years of CHAMP satellite and observatory vector data. The data selection technique used is the same as in the previous GRIMM version. The field model is co-estimated together with a model of the flow at the top of the liquid outer core. The smoothing constraints are then applied exclusively on the flow model such that GRIMM-2 is compatible with the frozen-flux approximation. Further hypothesis have been considered as tangential geostrophy or possible horizontal diffusion. The small scale structures of the obtained field model, its secular variation and acceleration, are all reliably resolved. We find that the observed geomagnetic jerks can be well described and that the core field acceleration has to vary rapidly in time in order to obtain an acceptable fit to the data.
[en] Complete text of publication follows. We have recently used an ensemble technique to invert core flows from models of the geomagnetic SV. We have generated random realisations of the small scale unknown magnetic field bk at the CMB that we have added to models of the known large-scale magnetic field B. The resulting magnetic field enters the matrix that connects the flow to a SV model. Each realization of b, bk corresponds to a flow solution uk. The common part of all the flow solutions is obtained by taking the average of the ensemble of flow solutions. We have investigated how well magnetic observatory records are predicted from our flow solutions. We find that each member of the ensemble of solutions (uk) interacting with its associated (B+ bk) accounts well for the observatory records. We find also that the ensemble average interacting with B accounts for most of the records of the main geomagnetic jerks. We are thus in the position to discuss what flow changes are responsible for jerks and other rapid changes of the core field. We investigate whether it is possible to discriminate between jerks and other sudden variations from this standpoint. We are now developing a fully self-consistent model of the rapid Secular Variation to complement this kinematic study. We will argue that our recent inference of rather strong magnetic fields in the core interior, about 10 times as intense as at the core surface, is consistent with the observation of rapid changes of the geomagnetic SV.
[en] Complete text of publication follows. Taylor states are magnetic field morphologies in a sphere or shell that obey Taylor's constraint, namely the vanishing of the azimuthal component of the Lorentz torque on every cylinder coaxial with the rotation axis of the Earth. We report on simple models of the internal magnetic field structure in a sphere and a spherical shell that are compatible with morphologies of the magnetic field at the core surface.