Earth's core internal dynamics 1840–2010 imaged by inverse geodynamo modelling

International audienceInverse geodynamo modelling seeks to estimate the dynamic state of Earth's core from geo-magnetic data and the statistical information brought by a prior self-consistent, 3-D numerical model of the geodynamo. The method rests on the use of least-squares inversions under constraints and estimates hidden quantities by taking advantage of linear relations and long-range statistical correlations with the magnetic observations. The data, together with their error statistics, are provided by geomagnetic field models COV-OBS, gufm-sat-Q3 and CM4, covering epochs 1840–2010. The prior numerical model is the recently published coupled Earth dynamo, the output of which presents a high degree of morphological semblance to the geomagnetic field while reproducing the main features of its secular variation. An analysis of the inversion misfits to the data shows that the prior model generally accounts well for the main field and secular variation data within their specified errors, throughout the investigated time period. Inverted core flows are confirmed to be mainly organized in columns parallel to the Earth's rotation axis. A previously observed giant eccentric columnar gyre of westward-drifting flow is found to be present from epoch 1870 onwards, and its structure is shown to slowly rotate westwards at a rate up to 0.1 • yr −1 , confirming an earlier prediction based on direct numerical modelling. Temporal variations in the axisymmetric part of the gyre accurately account for observed variations of the length of the day in recent epochs. Inverted magnetic structures support a mechanism of azimuthal flux expulsion by convective columns to explain the origin of low-latitude magnetic flux patches existing beneath the Atlantic. The 1840–2010 time average of the inverted density anomaly field has a longitudinal hemispheric structure, with most of the buoyancy in the Eastern hemisphere, consistent with rapid surges of convective columns imaged in this hemisphere, with earlier proposals of a faster inner core freezing there, and with a possible east-to-west convective translation of the solid inner core. The typical timescales of flow variation observed in the inversions are two to three times shorter than those naturally produced by a direct simulation of the prior model, underlining its limits in fully rendering the Earth's core short timescale dynamics

The predictability of advection-dominated flux-transport solar dynamo models

Space weather is a matter of practical importance in our modern society. Predictions of forecoming solar cycles mean amplitude and duration are currently being made based on flux-transport numerical models of the solar dynamo. Interested in the forecast horizon of such studies, we quantify the predictability window of a representative, advection-dominated, flux-transport dynamo model by investigating its sensitivity to initial conditions and control parameters through a perturbation analysis. We measure the rate associated with the exponential growth of an initial perturbation of the model trajectory, which yields a characteristic time scale known as the e-folding time $\tau_e$. The e-folding time is shown to decrease with the strength of the $\alpha$-effect, and to increase with the magnitude of the imposed meridional circulation. Comparing the e-folding time with the solar cycle periodicity, we obtain an average estimate for $\tau_e$ equal to 2.76 solar cycle durations. From a practical point of view, the perturbations analysed in this work can be interpreted as uncertainties affecting either the observations or the physical model itself. After reviewing these, we discuss their implications for solar cycle prediction.Comment: 33 pages, 12 figure

A mean-field Babcock-Leighton solar dynamo model with long-term variability

Dynamo models relying on the Babcock-Leighton mechanism are successful in reproducing most of the solar magnetic field dynamical characteristics. However, considering that such models operate only above a lower magnetic field threshold, they do not provide an appropriate magnetic field regeneration process characterizing a self-sustainable dynamo. In this work we consider the existence of an additional \alpha-effect to the Babcock-Leighton scenario in a mean-field axisymmetric kinematic numerical model. Both poloidal field regeneration mechanisms are treated with two different strength-limiting factors. Apart from the solar anti-symmetric parity behavior, the main solar features are reproduced: cyclic polarity reversals, mid-latitudinal equatorward migration of strong toroidal field, poleward migration of polar surface radial fields, and the quadrature phase shift between both. Long-term variability of the solutions exhibits lengthy periods of minimum activity followed by posterior recovery, akin to the observed Maunder Minimum. Based on the analysis of the residual activity during periods of minimum activity, we suggest that these are caused by a predominance of the \alpha-effect over the Babcock-Leighton mechanism in regenerating the poloidal field.Comment: 21 pages, 6 figure

Improving situation awareness of a single human operator interacting with multiple unmanned vehicles: first results

In the context of the supervision of one or several unmanned vehicles by a human operator, the design of an adapted user interface is a major challenge. Therefore, in the context of an existing experimental set up composed of a ground station and heterogeneous unmanned ground and air vehicles we aim at redesigning the human-robot interactions to improve the operator's situation awareness. We base our new design on a classical user centered approach

Modelling the palaeo-evolution of the geodynamo

International audienceAlthough it is known that the geodynamo has been operating for at least 3.2 Ga, it remains difficult to infer the intensity, dipolarity and stability (occurrence of reversals) of the Precam-brian magnetic field of the Earth. In order to assist the interpretation of palaeomagnetic data, we produce models for the long-term evolution of the geodynamo by combining core ther-modynamics with a systematic scaling analysis of numerical dynamo simulations. We update earlier dynamo scaling results by exploring a parameter space, which has been extended in order to account for core aspect ratios and buoyancy source distributions relevant to Earth in the Precambrian. Our analysis highlights the central role of the convective power, which is an output of core thermodynamics and the main input of our updated scalings. As the thermal evolution of the Earth's core is not well known, two end-member models of heat flow evolution at the core–mantle boundary (CMB) are used, respectively, terminating at present heat flows of 11 TW (high-power scenario) and 3 TW (low power scenario). The resulting models predict that until the appearance of the inner core, a thermal dynamo driven only by secular cooling, and without any need for radioactive heating, can produce a dipole moment of strength comparable to that of the present field, thus precluding an interpretation of the oldest palaeomagnetic records as evidence of the inner core presence. The observed lack of strong long-term trends in palaeointensity data throughout the Earth's history can be rationalized by the weakness of palaeointensity variations predicted by our models relatively to the data scatter. Specifically, the most significant internal magnetic field increase which we predict is associated to the sudden power increase resulting from inner core nucleation, but the dynamo becomes deeper-seated in the core, thus largely cancelling the increase at the core and Earth surface, and diminishing the prospect of observing this event in palaeointensity data. Our models additionally suggest that the geodynamo has lied close to the transition to polarity reversals throughout its history. In the Precambrian, we predict a dynamo with similar dipolarity and less frequent reversals than at present times, due to conditions of generally lower convective forcing. Quantifying the typical CMB heat flow variation needed for the geodynamo to cross the transition from a reversing to a non-reversing state, we find that it is unlikely that such a variation may have caused superchrons in the last 0.5 Ga without shutting down dynamo action altogether

Review of contemporary sound installation practices in Québec

Continuing a trend of publications investigating sound art within a specific geographical context, this paper proposes an original view of the sound installation practice in Que´bec. This study is part of a research project aiming at building new theoretical and practical tools for the documentation of such artworks. In this paper we present the outcomes of the first phase and its connection with the bigger picture of the project, which is the questioning of the relevance of spatial audio recordings with six degrees of freedom (6DoF) for mediating the capture of knowledge relating to the sensory experience of a work. During the first phase, we developed a conceptual descriptive framework based on a mixed-methods approach, top-down and bottom-up, consisting in a systematic review of literature paralleled with a categorization of contemporary sound art production in Que´bec based on publicly available documentation. This process led to a formal and quantitative depiction of the Que´bec scene, which aims to guide both the selection of case studies for the next phases but also to be part of the conceptual tools for investigating the sensory experience of these works. This quantitative depiction of the scene will thus foster a qualitative investigation of the sensory experience of sound art installations and the knowledge that may be lost in standard written documentation practice with an original methodological framework