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The first steps in Earth core weather forecasting

Why has the Earth's magnetic field intensity decreased by 10% between 1840 and the present day? Will the South-Atlantic anomaly in this field, above which satellite equipment suffers the most damage, become even more pronounced in the coming century? When will the next magnetic pole inversion occur? Operational geodynamic modelling aims to answer these questions, and is now providing some initial insights in two articles published in Nature Communications and Geophysical Journal International.

The first steps in Earth core weather forecasting

Publication date: 27/01/2016

Press, Research

Related teams :
Geological Fluid Dynamics

The Earth’s magnetic field interacts strongly with human technological activities. Among the areas where knowledge of the spatial and temporal variations in this field is important are the operation of low-altitude satellites, the determination of course in navigation applications such as those embedded in smartphones, exploration geophysics, and the protection of large-scale electrical infrastructures. To give a concrete example, we know, for example, that the rate of occurrence of failures on satellites increases sharply when they are in a zone of low magnetic intensity known as the South Atlantic Anomaly. This is to be expected, since the Earth’s magnetic field is less effective in protecting our planet from the charged particles of the solar wind in this zone.

How can variations in the Earth’s magnetic field be explained and predicted? The source of this field is the slow cooling of our planet’s interior, which creates convection movements in the core, a ball of liquid iron 2900 kilometres below our feet. In such an electrically conductive fluid, the movements generate a dynamo effect, which converts a fraction of the energy released by the cooling into electromagnetic energy. Computer simulations of this geodynamic effect have developed considerably over the last twenty years or so, providing a better understanding of the fundamentals of this mechanism. These simulations are now sufficiently advanced to reproduce most of the large-scale characteristics and centennial variations of the magnetic field.

Based on this state of the art, the question of prediction can be addressed by drawing on data assimilation techniques, which have mainly been developed in meteorology and oceanography over the last few decades. The principle of data assimilation is to guide the evolution of the system simulated by computer calculations by injecting observations. For several years (see A. Fournier et al., 2010, reference below), researchers at the Institut de Physique du Globe de Paris and the Institut des Sciences de la Terre (Grenoble) have been working on the development of geomagnetic data assimilation, with the support of their organisations, CNRS/INSU, CNES, GENCI and the Agence Nationale de la Recherche. In collaboration with a researcher from the Danish National Space Institute (DTU Space, Copenhagen), representatives of this group are currently delivering operational results from this geodynamic modelling. This work is based on data from the European Space Agency’s Swarm mission, a constellation of three geomagnetic measurement satellites launched at the end of 2013, in which CEA-Leti (Grenoble), the IPGP and the CNES played an active part in the design for France.

Left: visualisation of the internal dynamic structure of the core, estimated from surface observations and the behaviour of a geodynamic computer model. The vortex structures of the flow are shown in red and blue. The internal magnetic field is represented in orange and violet, as well as by field lines in the volume that are twisted by the flow. Right: map (centred on the Atlantic) of the large-scale flow at the surface of the core, which is organised like a large conveyor belt causing intense westward drift in the Atlantic hemisphere.

One of the strengths of data assimilation is that it enables the internal dynamics of the core to be estimated from surface observations, using the statistics (provided by the computer model) that characterise these dynamics. This approach reveals the presence of a large vortex at the surface of the core, which, like a conveyor belt, constantly transports the magnetic field from the poles to the equator in Asia, and from the equator to the poles in America. If the magnetic field were not so asymmetric between the eastern and western hemispheres, its intensity would remain stable over time. However, the presence of the weak anomaly in the South Atlantic unbalances this mechanism, so that there is a lack of magnetic field returning to the poles, and therefore a decrease in the dipole, which makes up the bulk of the field visible at the surface. This is why its intensity has been decreasing since the first absolute measurements were made by K. F. Gauss in 1840.

The question of the decay of the Earth’s magnetic field is therefore intimately linked to the respective positions of the eddy and the anomaly in the South Atlantic. Data assimilation simulations predict that they will remain linked over the coming century. As a result of the strong lateral drift that the gyre creates in the equatorial regions of the Atlantic, the anomaly and the gyre itself should also be pushed around 3,000 kilometres westwards (at the Earth’s surface) over the next 100 years. However, this shift will not change the mechanism described above, and the magnetic dipole should therefore continue to decline at the same rate over the next century. The South Atlantic anomaly will also deepen significantly, widening the problem area for satellite flight.

Animation of magnetic field intensity curves at the Earth’s surface (micro-teslas) from 2015 to 2115 as part of a geomagnetic data assimilation prediction. The South Atlantic anomaly is represented by the dark blue lines. The assimilation predicts a significant deepening of this anomaly, as well as a westward drift of its centre.

Evolution of the geomagnetic dipole intensity (micro-teslas) over the last 175 years, and prediction for the coming century, with its margin of uncertainty.

Can we predict the behaviour of the Earth’s magnetic field over the longer term, and in particular predict the next pole reversal? Such reversals occur at a chaotic rate, averaging around four events per million years. The last reversal occurred 780,000 years ago. We are therefore living through a relatively long period of stable polarity, which has nevertheless seen multiple phases of growth and decline in the magnetic field.

To continue the atmospheric analogy, while the evolution of the South Atlantic anomaly is a matter for meteorology, the prediction of inversions is a matter for climatology, in other words very long-term variations. These variations are unfortunately unpredictable, due to the famous ‘butterfly effect’ which describes the sensitivity of a chaotic system to initial conditions. Any error in the initial determination of the state of the system leads to an error in the prediction, which increases exponentially as time passes. The doubling time of this error, which for the atmosphere is of the order of a few days, is around thirty years for the Earth’s core (see Hulot et al. 2010, reference below). Predicting a phenomenon that could occur in the next hundred thousand years or so is therefore quite simply impossible, just as we cannot know with any certainty what the weather will be like this time next year.

Geodynamics also includes very short-term phenomena such as geomagnetic tremors, or sudden changes in the rate of variation of the magnetic field. Space measurement missions such as Swarm have revealed several such jolts over the last 15 years, but numerical geodynamic models are still unable to reproduce them operationally, due to limitations in the computing power available. An approach that combines high-quality geomagnetic measurements with advances in simulations should enable us to discover the origins of these as yet unexplained phenomena in the future.

Ref:

  • C. Finlay, J. Aubert, N. Gillet: Gyre-driven decay of the geomagnetic dipole, Nature Communication, doi: 10.1038/NCOMMS10422
  • J. Aubert: Geomagnetic forecasts driven by thermal wind dynamics in the Earth’s core, Geophysical Journal International 203, 1738-1751, 2015.

For more perspective, the following resources can also be consulted:

  • A. Fournier et al : An Introduction to Data Assimilation and Predictability in Geomagnetism , Space Science Reviews, 155, 247-291, 2010
  • G. Hulot, F. Lhuillier, J. Aubert: Earth’s dynamo limit of predictability, Geophysical Research Letters 37, 2010.
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