Variability of Millennial-Scale Trends in the Geomagnetic Axial Dipole

The historical trend in the axial dipole is sufficient to reverse the field in less than 2 kyr. Assessing the prospect of an imminent polarity reversal depends on the probability of sustaining the historical trend for long enough to produce a reversal. We use a stochastic model to predict the variability of trends for arbitrary time windows. Our predictions agree well with the trends computed from paleomagnetic models. Applying these predictions to the historical record shows that the current trend is likely due to natural variability. Furthermore, an extrapolation of the current trend for the next 1 to 2 kyr is highly unlikely. Instead, we compute the trend and time window needed to reverse the field with a specified probability. We find that the dipole could reverse in the next 20 kyr with a probability of 2%

Visco-magnetic torque at the core mantle boundary

A magneto-hydrodynamic model of boundary layers at the Core-Mantle Boundary (CMB) is derived and used to compute the viscous and electromagnetic torques generated by the Earth's nutation forcing. The predicted electromagnetic torque alone cannot account for the dissipation estimated from the observations of the free core nutation. The presence of a viscous boundary layer in the electromagnetic skin layer at the CMB, with its additional dissipative torques, may explain the geodetic data. An apparent Ekman number at the top of the core between 3 and $5 10^{-11}$ is inferred depending on the electrical conductivity of the mantle

Oceanografia sísmica. Una nova eina per entendre els oceans

L'oceanografia sísmica s'està convertint en una eina pràctica per estudiar la circulació oceànica a gran escala, els processos de mescla a mesoescala i la seva dinàmica. A més, s'ha demostrat la seva utilitat per quantificar paràmetres com ara la temperatura i la salinitat. Des de 2003, s'ha emprès la recerca en la millora i l'adaptació de la sísmica de reflexió, una eina robusta ben acceptada en el món acadèmic i la indústria dels hidrocarburs per visualitzar l'escorça profunda i els marges de les plaques tectòniques, i per localitzar possibles reservoris de petroli, respectivament. La necessitat urgent d'identificar amb precisió els mecanismes responsables del canvi climàtic fa que l'oceanografia sísmica sigui encara de més interès per als oceanògrafs físics. Atesa la gran contribució dels oceans al transport de calor (més o menys equivalent a l'atmosfera encara que amb molt menys gruix), és necessari entendre els processos oceànics i les imatges detallades de les estructures oceàniques, com ara els remolins, fronts i escales termohalines. L'oceanografia sísmica proporciona aquesta imatge detallada, així com la quantificació de les propietats intrínseques oceàniques. La sobreabundància d'arxius amb dades sísmiques marines, molts d'ells amb registres de reflexions relativament febles de l'oceà, ofereix un conjunt de dades a escala mundial pràcticament il·limitat amb el qual es pot estudiar la circulació oceànica. L'oceanografia sísmica no només ofereix l'oportunitat de representar espacialment l'estructura termohalina, sinó que l'accés a bases de dades històriques pot donar informació sobre el comportament temporal de la circulació, i això és especialment important de cara a la comprensió del canvi climàtic global

Constructing Stochastic Models for Dipole Fluctuations from Paleomagnetic Observations

Records of relative paleointensity are subject to several sources of error. Temporal averaging due to gradual acquisition of magnetization removes high-frequency fluctuations, whereas random errors introduce fluctuations at high frequency. Both sources of error limit our ability to construct stochastic models from paleomagnetic observations. We partially circumvent these difficulties by recognizing that the largest affects occur at high frequency. To illustrate we construct a stochastic model from two recent inversions of paleomagnetic observations for the axial dipole moment. An estimate of the noise term in the stochastic model is recovered from a high-resolution inversion (CALS10k.2), while the drift term is estimated from the low-frequency part of the power spectrum for a long, but lower-resolution inversion (PADM2M). Realizations of the resulting stochastic model yield a composite, broadband power spectrum that agrees well with the spectra from both PADM2M and CALS10k.2. A simple generalization of the stochastic model permits predictions for the mean rate of magnetic reversals. We show that the reversal rate depends on the time-averaged dipole moment, the variance of the dipole moment and a slow timescale that characterizes the adjustment of the dipole toward the time-averaged value. Predictions of the stochastic model give a mean rate of 4.2 Myr^(−1), which is in good agreement with observations from marine magnetic anomalies

Gross thermodynamics of two-component core convection

We model the inner core by an alloy of iron and 8 per cent sulphur or silicon and the outer core by the same mix with an additional 8 per cent oxygen. This composition matches the densities of seismic model, Preliminary Reference Earth Model (PR-EM). When the liquid core freezes S and Si remain with the Fe to form the solid and excess 0 is ejected into the liquid. Properties of Fe, diffusion constants for S, Si, 0 and chemical potentials are calculated by first-principles methods under the assumption that S, 0, and Si react with the Fe and themselves, however, not with each other. This gives the parameters required to calculate the power supply to the geodynamo as the Earth's core cools. Compositional convection, driven by light O released at the inner-core boundary on freezing, accounts for half the entropy balance and 15 per cent of the heat balance. This means the same magnetic field can be generated with approximately half the heat throughput needed if the geodynamo were driven by heat alone. Chemical effects are significant: heat absorbed by disassociation of Fe and 0 almost nullify the effect of latent heat of freezing in driving the dynamo. Cooling rates below 69 K Gyr(-1) are too low to maintain thermal convection everywhere; when the cooling rate lies between 35 and 69 K Gyr(-1) convection at the top of the core is maintained compositionally against a stabilizing temperature gradient; below 35 K Gyr(-1) the dynamo fails completely. All cooling rates freeze the inner core in less than 1.2 Gyr, in agreement with other recent calculations. The presence of radioactive heating will extend the life of the inner core, however, it requires a high heat flux across the core-mantle boundary. Heating is dominated by radioactivity when the inner core age is 3.5 Gyr. We, also, give calculations for larger concentrations of O in the outer core suggested by a recent estimation of the density jump at the inner-core boundary, which is larger than that of PREM. Compositional convection is enhanced for the higher density jumps and overall heat flux is reduced for the same dynamo dissipation, however, not by enough to alter the qualitative conclusions based on PREM. Our preferred model has the core convecting near the limit of thermal stability, an inner-core age of 3.5 Gyr and a core heat flux of 9 TW or 20 per cent of the Earth's surface heat flux, 80 per cent of which originates from radioactive heating

Implication of the lopsided growth for the viscosity of Earth's inner core

Two main seismic features characterize the Earth's inner core: a North-South polar anisotropy and an East-West asymmetry of P-wave velocity and attenuation. Anisotropy is expected if shear deformation is induced by convective motions. Translation has recently been put forward as an important mode of convection of the inner core. Combined with a simple diffusive grain growth model, this mechanism is able to explain the observed seismic asymmetry, but not the bulk anisotropy. The source of anisotropy has therefore to be sought in the shear motions caused by higher modes of convection. Using a hybrid finite-difference spherical harmonics Navier-Stokes solver, we investigate the interplay between translation and convection in a 3D spherical model with permeable boundary conditions at the inner core boundary. Three parameters act independently: viscosity, internal heating and convection velocity in the outer core. Our numerical simulations show the dominance of pure translation for viscosities of the inner core higher than $10^{20}$ Pas. Translation is almost completely hampered by convective motions for viscosities lower than $10^{18}$ Pas. Between these values, translation and convection develop, but convective downwellings are restricted to the coldest hemisphere where crystallization occurs. On the opposite side, shear is almost absent, thereby allowing grain growth. We propose that the coexistence of translation and convection observed in our numerical models leads to a seismic asymmetry but localizes deformation only in one hemisphere.Comment: Submitted to PNAS on Februray 10th 2012, rejected on March 19th 2012. Submitted to Earth Planet. Sci. Lett. on March 20th, 2012. (10 pages, 8 figures

Physical Properties of Iron in the Inner Core

The Earth's inner core plays a vital role in the dynamics of our planet and is itself strongly exposed to dynamic processes as evidenced by a complex pattern of elastic structure. To gain deeper insight into the nature of these processes we rely on a characterization of the physical properties of the inner core which are governed by the material physics of its main constituent, iron. Here we review recent research on structure and dynamics of the inner core, focusing on advances in mineral physics. We will discuss results on core composition, crystalline structure, temperature,and various aspects of elasticity. Based on recent computational results, we will show that aggregate seismic properties of the inner core can be explained by temperature and compression effects on the elasticity of pure iron, and use single crystal anisotropy to develop a speculative textural model of the inner core that can explain major aspects of inner core anisotropy.Comment: 23 pages, 16 figures. To appear in AGU Geodynamics Series book on "Core structure, dynamics, and rotation", V. Dehant et al. (eds.

Oceanografia sísmica. Una nova eina per entendre els oceans

L'oceanografia sísmica s'està convertint en una eina pràctica per estudiar la circulació oceànica a gran escala, els processos de mescla a mesoescala i la seva dinàmica. A més, s'ha demostrat la seva utilitat per quantificar paràmetres com ara la temperatura i la salinitat. Des de 2003, s'ha emprès la recerca en la millora i l'adaptació de la sísmica de reflexió, una eina robusta ben acceptada en el món acadèmic i la indústria dels hidrocarburs per visualitzar l'escorça profunda i els marges de les plaques tectòniques, i per localitzar possibles reservoris de petroli, respectivament. La necessitat urgent d'identificar amb precisió els mecanismes responsables del canvi climàtic fa que l'oceanografia sísmica sigui encara de més interès per als oceanògrafs físics. Atesa la gran contribució dels oceans al transport de calor (més o menys equivalent a l'atmosfera encara que amb molt menys gruix), és necessari entendre els processos oceànics i les imatges detallades de les estructures oceàniques, com ara els remolins, fronts i escales termohalines. L'oceanografia sísmica proporciona aquesta imatge detallada, així com la quantificació de les propietats intrínseques oceàniques. La sobreabundància d'arxius amb dades sísmiques marines, molts d'ells amb registres de reflexions relativament febles de l'oceà, ofereix un conjunt de dades a escala mundial pràcticament il·limitat amb el qual es pot estudiar la circulació oceànica. L'oceanografia sísmica no només ofereix l'oportunitat de representar espacialment l'estructura termohalina, sinó que l'accés a bases de dades històriques pot donar informació sobre el comportament temporal de la circulació, i això és especialment important de cara a la comprensió del canvi climàtic global

On the existence and structure of a mush at the inner core boundary of the Earth

It has been suggested about 20 years ago that the liquid close to the inner core boundary (ICB) is supercooled and that a sizable mushy layer has developed during the growth of the inner core. The morphological instability of the liquid-solid interface which usually results in the formation of a mushy zone has been intensively studied in metallurgy, but the freezing of the inner core occurs in very unusual conditions: the growth rate is very small, and the pressure gradient has a key role, the newly formed solid being hotter than the adjacent liquid. We investigate the linear stability of a solidification front under such conditions, pointing out the destabilizing role of the thermal and solutal fields, and the stabilizing role of the pressure gradient. The main consequence of the very small solidification rate is the importance of advective transport of solute in liquid, which tends to remove light solute from the vicinity of the ICB and to suppress supercooling, thus acting against the destabilization of the solidification front. For plausible phase diagrams of the core mixture, we nevertheless found that the ICB is likely to be morphologically unstable, and that a mushy zone might have developed at the ICB. The thermodynamic thickness of the resulting mushy zone can be significant, from $\sim100$ km to the entire inner core radius, depending on the phase diagram of the core mixture. However, such a thick mushy zone is predicted to collapse under its own weight, on a much smaller length scale ($\lesssim 1$ km). We estimate that the interdendritic spacing is probably smaller than a few tens of meter, and possibly only a few meters

Inversion of torsional oscillations for the structure and dynamics of Earth's core

Oscillations in Earth's liquid core with periods of several decades are inferred from variations in the magnetic field. The observed periods are consistent with a type of hydromagnetic wave known as torsional oscillations. These oscillations represent a set of very-low-frequency normal modes in which the internal magnetic field provides the primary restoring force. By adapting the methods of normal-mode seismology, we construct estimates for the internal structure of the magnetic field and several other key parameters, including the viscosity of the inner core. The structure of the recovered field provides useful insights into the nature of convection. We find evidence of columnar convection in the core, and estimate the strength of the field generated by these flows (≈0.3 mT). We also use the normal modes to recover the excitation source for the oscillations. Much of the excitation appears to originate near the surface of a cylinder that is tangent to the equator of the inner core. Distinct events rise above a background level of excitation, and may be related to instabilities in the geodynam