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Space magnetometry and the Swarm mission

Hulot, G., and 11 colleagues 2015. Swarm's absolute magnetometer experimental vector mode, an innovative capability for space magnetometry. Geophysical Research Letters 42, 1352-1359.

This very successful mission aims at investigating all sources of the Earth’s magnetic, as well as the ionospheric environment (see Alken et al., GRL, 2015; Thébault et al., EPS, 2016; Chulliat et al., and further early results in the two special issues edited by Hulot et al., GRL, 2015, and Olsen et al., EPS, 2016). The Geomagnetism team is also specifically in charge of the new generation absolute magnetometers on board these satellites (ASM instruments, PI, G. Hulot), the nominal role of which is to provide 1 Hz scalar measurements of the intensity of the field for both science investigations and calibration of the vector relative magnetometers also on board the satellites (see Toffner-Clausen et al. EPS, 2016). These instruments, however, can also be run in two additional experimental modes: a “Burst mode”, providing 250 Hz scalar data, and a “Vector mode”, providing 1 Hz vector and scalar data (see Léger et al., EPS, 2015). The burst mode has been run only over a few days, which led, among other results to the demonstration that the unique sensitivity of these instruments could provide exciting information about yet poorly investigated extremely low frequency lightning signals propagating through the ionosphere (results soon to be published). The vector mode was also shown to be extremely valuable, to the extent that it eventually became the nominal mode of the ASM instruments on the mission. It routinely provides the 1 Hz scalar data nominally required for the mission and 1 Hz experimental vector data. As discussed in details in Léger et al., EPS, (2015) and in Hulot et al. GRL (2015), these data were shown to be of such good quality that it could be used to derive core and lithospheric field models, only limited by slight distortions of the mechanical link between the ASM instrument and the set of star cameras (providing the attitude restitution also needed for such modelling purposes). As a matter of fact, this also made it possible to build a main field model that could be proposed as a candidate model (Vigneron et al., EPS, 2015) contributing to the already widely used 2015 International Geomagnetic Reference Field (IGRF) model (Thébault et al., EPS, 2015). These results show that such ASM instruments can be used as stand-alone self-calibrated magnetometers, leading to the possibility of considerably simplifying and miniaturizing magnetometry payloads on future satellite missions and prompting the Geomagnetic team to propose the NanoMagSat nanosatellite mission



Space weather

Love, J. J., Coïsson, P., Pulkkinen, A. 2016. Global statistical maps of extreme-event magnetic observatory 1 min first differences in horizontal intensity. Geophysical Research Letters 43, 4126-4135.

Space weather is studied by the geomagnetism team from the point of view of its effect on the Earth, as the planet’s magnetic field is strongly perturbed during solar-driven events. The current solar cycle reached a lower maximum than previous cycles, only leading to few major storms. Much stronger storms, however, could occur during the next cycle, raising concerns and prompting the civil society to ask scientists to quantify the possible magnitude of such extreme events, in order to be able to assess the risks associated with it. Magnetic storms can have severe impacts over modern technological systems like power grids or satellite services, including satellite navigation. Specifically, power grids are vulnerable to induced currents, the most relevant example being the magnetic storm of March 1989, when the Hydro-Québec power grids collapsed during many hours.

Data from magnetic observatories contain information on the local response to the forcing from the Sun, both during quiet and disturbed conditions. One of the parameters of interest to assess the induction hazard is the rapid variation of the horizontal component of the magnetic field, since it is directly correlated with the occurrence of induced currents, both in the Earth lithosphere and in the electric power lines. A specific study has been conducted (Love et al., 2016) to analyse and estimate the strongest rapid variations that can be expected on the ground. The longest available time-series of 1-minute data from 34 ground geomagnetic observatories were used, covering almost four solar cycles between 1974 and 2014. The largest magnitudes of once per year, once per decade and once per century events were estimated by extrapolation, providing representative values for the geomagnetic latitude of each analyzed observatory. Maximum variations were found to be around 55° magnetic latitude, where they can exceed 1000 nT/min in the European sector. These statistical data were finally used to produce a global model of extreme variations that can now be used for estimating the hazard linked to geomagnetic induced currents, when coupled with measurements of ground conductivity.



Solar dynamo

Hung, C.P., Jouve, L., Brun, A.S., Fournier, A., Talagrand, O. 2015. Estimating the Deep Solar Meridional Circulation Using Magnetic Observations and a Dynamo Model: A Variational Approach. The Astrophysical Journal 814, 151.

Thanks to the synergistic possibilities offered by the USPC IdEx and the UnivEarthS LabEx, we have joined forces with astrophysicists of AIM/CEA in Saclay to try and apply the data assimilation skills that we gained in the geomagnetic context to study another magnetic body of primary importance for us Earthlings: The Sun. This paper presents proof-of-concept experiments, which demonstrate that it is possible to estimate the large-scale meridional circulation inside the Sun by combining a mean-field model of its dynamo with observations of the magnetic field at its surface. We cast the problem in a variational framework and estimate the meridional circulation that can best account for the fluctuations of the solar magnetic field. It is indeed generally accepted that the large-scale meridional circulation is the clock that regulates solar activity. Over the past few years, our group has developed, together with the Paleomagnetism group, a growing interest for the yearly to millenial fluctuations of solar activity, the latter being inferred from cosmogenic nuclides records. By combining models of the geomagnetic field over the archeological period (Licht et al., 2013) with such records, we have highlighted the fact that solar activity can be adequately described by transitions over a limited set of modes (Usoskin et al., 2014). In addition, we have used a similar approach to estimate the impact of geomagnetic spikes on the production rate of those cosmogenic nuclides in the Earth's atmosphere. We conclude that a corroborative evidence for the existence of such spikes (inferred initially from stratigraphic data) in the cosmogenic record is likely to remain elusive (Fournier et al., 2015).



Deformations and rotation of the Earth at geological time-scale.

Greff-Lefftz, M., Besse, J. 2012. Paleo movement of continents since 300 Ma, mantle dynamics and large wander of the rotational pole. Earth and Planetary Science Letters 345, 151-158.

Wander of the Earth rotational axis (or dipolar axis) is known to be mainly linked to mass distribution changes in the mantle and at the Earth’s surface. In collaboration with J. Besse (Paleomagnetism team), we performed sensitivity experiments based on the position of subductions and superplumes, deriving models for the temporal evolution of 3D mass anomalies in the mantle and computing associated inertia perturbations and polar wander (Greff and Besse, 2014). In particular, in the highlight paper, we tried to estimate the exact amount of subduction, based on the study of subduction related volcanism in the past and plate motion history: the peri-Pacific subductions seem to be a quasi permanent feature of the Earth's history at least since the Paleozoic, while the "Tethyan" subductions have a complex history with successive collisions of continental blocs and episodical rebirth of E-W subduction trending zones. Assuming that subducted slabs sink vertically into the mantle and taking into account two large-scale upwellings at the bottom of the mantle, constrained by intra-plate volcanism in the past, we computed the temporal evolution of the rotational axis since 280 Ma and compared our results to the Apparent Polar Wander (APW) observed by paleomagnetism. We found that a major trend of both paleomagnetic and computed polar wander are successive oscillatory clockwise or counter-clockwise motions, with tracks separated by abrupt cusps. These cusps result from earlier major geodynamic events: the 230 Ma cusp is related to the end of active subduction due to the closure of the Rheic Ocean basin after the Hercynian continental collision (340-300 Ma) and to renewed subduction zone West of Laurentia, whereas the 190 Ma cusp results from the Kimmerian collision (270-230 Ma) and the subsequent end of the Neo-Tethys ocean subduction. A similar approach allowed us to also explain, through inertial interchanges, an ultra-rapid episode of Neoproterozoic TPW (Robert et al. 2017).

This simple geodynamical model finally allowed us to fit present-day geoid and gravity anomalies with good accuracy at a global scale (Panet el al., 2014; Greff et al., 2016) and to investigate dynamical topography and lithospheric stresses in the geological past (Greff et al., 2017).



Advances in dynamo simulation.

Gastine, T., Wicht, J., Aubert, J. 2016. Scaling regimes in spherical shell rotating convection. Journal of Fluid Mechanics 808, 690-732.

Rayleigh-Bénard convection in rotating spherical shells can be considered as a simplified analogue of many astrophysical and geophysical fluid flows. In this paper, we used three-dimensional direct numerical simulations to study this physical process. We constructed a dataset of more than 200 numerical models that cover a broad parameter range. We have investigated the scaling behaviours of both local (length scales, boundary layers) and global (Nusselt and Reynolds numbers) properties across various physical regimes, from onset of rotating convection to weakly-rotating convection. In a narrow parameter range, we have established that our numerical data approaches a physical regime that becomes almost independent of the diffusivities of the system. Using a decomposition of the viscous dissipation rate into bulk and boundary layer contributions, we have derived a new theoretical scaling of the flow velocity that accurately describes the numerical data. This scaling law can then be extrapolated to estimate the flow velocity of the convective interiors of rapidly-rotating planets.

Several dedicated diagnostic tools have also been developed to complete this study. Among them, we have implemented in the open-source dynamo code MagIC (https://github.com/magic-sph/magic), a measure of the forces that control the dynamics of the system. This force balance diagnostic has since been employed to precisely determine the forces that control the numerical geodynamo models (Yadav et al. 2016, Aubert et al. 2017). In those studies, we have demonstrated that a triple force balance between Coriolis force, buoyancy and Lorentz force controls the dynamics of the turbulent dynamo simulations that are relevant to model the geodynamo.



Data assimilation for the secular variation of the geomagnetic field

Fournier, A., Aubert, J., Thébault, E. 2015. A candidate secular variation model for IGRF-12 based on Swarm data and inverse geodynamo modelling. Earth, Planets, and Space 67, 81.

This article presents the properties of the candidate secular variation model that we proposed for the construction of the 12th generation of the International Geomagnetic Reference Field, or IGRF (Thebault et al., 2015). It describes the average time rate-of-change of the main geomagnetic field expected between 2015.0 and 2020.0, up to spherical harmonic degree 8, as prescribed by the IGRF task force. The main novelty of this model is that its prediction is based on the integration of a numerical model of the geodynamo, which spontaneously generates a dynamic magnetic field whose variability resembles that of the Earth (Aubert et al., 2013). To define a suitable initial condition, we relied on data collected by the Swarm constellation between November 2013 and September 2014. This IGRF candidate model is one of the achievements of the ANR-funded program AVSGeomag (2011-2016), whose goal was precisely to design a set of methods that could combine physics and observations for geomagnetic analysis (e.g. Fournier et al., 2013; Sanchez et al., 2016). Our efforts in this line of research are not circumscribed to the historical and modern era: for instance, Morzfeld et al., (2017) resort to low-dimensional models of the geomagnetic field to analyse geomagnetic reversals.



Regional modelling and the lithospheric field

Lesur, V., Hamoudi, M., Choi, Y., Dyment, J., Thébault, E. 2016. Building the second version of the World Digital Magnetic Anomaly Map (WDMAM). Earth, Planets, and Space 68, 27.

The part of the crust above the Curie temperature often has a magnetisation that generates an observable magnetic field that the Earth surface. This magnetisation carries information on the crust age, structure, temperature and chemistry. It is therefore a signal worth studying at all wavelengths, from few meters to global –i.e. thousand of kilometres, scales. It was the aim of the World Digital Magnetic Anomaly Map (WDMAM, Lesur at al., 2016) to gather all available information on the observed crustal magnetic field. From the smallest wavelengths to few hundred kilometres, the information was provided by aeromagnetic and marine surveys. At the largest wavelengths the core magnetic field dominates the signal (Vigneron et al., 2015; Thébault et al., 2015) and it is a challenge for scientists to decipher the crustal field at these scales. In between, for wavelengths ranging from 3000km to 200km, the magnetic field had to be extracted from satellite data (Thebault et al., 2016). The ESA Swarm satellite mission has been specifically designed for the objective of revealing the lithospheric field at the few hundred kilometre scales.