Tide-induced magnetic signals and their errors derived from CHAMP and Swarm satellite magnetometer observations

Satellite-measured tidal magnetic signals are of growing importance. These fields are mainly used to infer Earth’s mantle conductivity, but also to derive changes in the oceanic heat content. We present a new Kalman filter-based method to derive tidal magnetic fields from satellite magnetometers: KALMAG. The method’s advantage is that it allows to study a precisely estimated posterior error covariance matrix. We present the results of a simultaneous estimation of the magnetic signals of 8 major tides from 17 years of Swarm and CHAMP data. For the first time, robustly derived posterior error distributions are reported along with the reported tidal magnetic fields. The results are compared to other estimates that are either based on numerical forward models or on satellite inversions of the same data. For all comparisons, maximal differences and the corresponding globally averaged RMSE are reported. We found that the inter-product differences are comparable with the KALMAG-based errors only in a global mean sense. Here, all approaches give values of the same order, e.g., 0.09 nT-0.14 nT for M2. Locally, the KALMAG posterior errors are up to one order smaller than the inter-product differences, e.g., 0.12 nT vs. 0.96 nT for M2. Graphical Abstract

Inversion of the satellite observations of the tidally induced magnetic field in terms of 3-D upper-mantle electrical conductivity: Method and synthetic tests

<p><span><span>We have developed and tested a new frequency-domain, spherical harmonic-finite element approach to the inverse problem of global electromagnetic (EM) induction. It is based on the quasi-Newton minimization of data misfit and regularization, and uses the adjoint approach for fast calculation of misfit gradients in the model space. Thus, it allows for an effective inversion of satellite-observed magnetic field induced by tidally driven flows in the Earth's oceans in terms of 3-D structure of the electrical conductivity in the upper mantle.</span></span><span><span> Before proceeding to the inversion of Swarm-derived models of tidal magnetic signatures, we have performed a series of </span></span><span><span>parametric studies</span></span><span><span>, using a 3-D conductivity model WINTERC-e as a testbed.</span></span></p><p><span>The WINTERC-e model has been derived using state-of-the-art laboratory conductivity measurements of mantle minerals, and thermal and compositional model of the lithosphere and upper mantle WINTERC-grav. The latter model is based on the inversion of global surface waveforms, satellite gravity and gradiometry measurements, surface elevation, and heat flow data </span><span><span>in a thermodynamically self-consistent framework. </span></span><span><span>Therefore, the WINTERC-e model, independent of any EM data, represents an ideal target for synthetic tests of the 3-D EM inversion.</span></span><span> </span></p><p><span><span>We tested the impact of </span></span><span><span>the </span></span><span><span>satellite </span></span><span><span>altitude</span></span><span><span>, </span></span><span><span>the truncation degree of the </span></span><span><span>spherical-harmonic </span></span><span><span>expansion of the tidal signals, the random</span></span><span><span> noise in data</span></span><span><span>,</span></span><span> </span><span><span>and </span></span><span><span>of the </span></span><span><span>sub-</span></span><span><span>continental conductivity</span></span><span> </span><span><span>on the </span></span><span><span>ability to recover the sub-oceanic upper-mantle conductivity structure.</span></span><span><span> We </span></span><span><span>demonstrate </span></span><span><span>that </span></span><span><span>with </span></span><span><span>suitable regularization </span></span><span><span>we</span></span><span> </span><span><span>can successfully reconstruct the 3D upper-mantle conductivity below world oceans.</span></span></p>

Crustal Characterization of Portugal's mainland based on Magnetotelluric measurements.

<p>In the last decades, the phenomena of Geomagnetic Induced Currents (GICs) have received special attention as one of the main hazards of Space Weather and has been widely investigated. In the high and mid-latitudes, these large GICs can flow in power systems and become problematic and even severe enough to cause a complete system shutdown. Two major factors determine GICs: (1) the strength and orientation of the electric field in the power system, which depends on the ionospheric and magnetospheric currents as well as on the crust and mantle conductivity; and (2) the electric power network characteristics. The Earth's conductivity can be obtained based on geophysical measurements that give the distribution of the conductivity in-depth and laterally. A realistic model of conductivity can be built based on the interpretation of Magnetotelluric (MT) soundings. The power of this geophysical method resides in the fact that it uses a natural source of energy, which allows estimating the conductivity distribution from a dozen of meters to some kilometres in depth.</p><p>We present a 3D resistivity model of the entire Portugal mainland based on more than 40 broadband MT soundings spaced 50x50km. The present study aims to contribute to a better understanding of Portugal's crust and its main geological structures. As a more practical application, knowledge of the presence of resistivity/conductivity bodies is important to obtain more precise GICs estimations. </p><p> </p>

Lateral variations of electrical conductivity in the lower mantle constrained by Swarm and CryoSat-2 missions

The electrical conductivity is an important geophysical parameter connected to the thermal, chemical, and mineralogical state of the Earth’s mantle. In this paper, we apply the previously developed methodology of forward and inverse EM induction modeling to the latest version of satellite-derived spherical harmonic coefficients of external and internal magnetic field, and obtain the first 3-D mantle conductivity models with contributions from Swarm and CryoSat-2 satellite data. We recover degree 3 conductivity structures which partially overlap with the shape of the large low-shear velocity provinces in the lower mantle.

Lateral Variations of Electrical Conductivity in the Lower Mantle Constrained by Swarm and CryoSat-2 Missions

The electrical conductivity is an important geophysical parameter connected to the thermal, chemical, and mineralogical state of the Earth's mantle. In this paper we apply the previously developed methodology of forward and inverse EM induction modelling to the latest version of satellite-derived spherical harmonic coefficients of external and internal magnetic field, and obtain the first 3-D mantle conductivity models with contributions from Swarm and CryoSat-2 satellite data. We recover degree 3 conductivity structures which partially overlap with the shape of the large low-shear velocity provinces in the lower mantle.

Global-to-Cartesian 3-D EM modeling using a nested IE approach with application to long-period responses from island geomagnetic observatories

<p>There is a significant interest in constraining the mantle conductivity beneath oceans. One of the main sources of data that can be used to reveal the conductivity distribution in the oceanic mantle are time-varying magnetic fields measured at island geomagnetic observatories. From these data local electromagnetic (EM) responses are estimated and then inverted in terms of conductivity. The challenge here is that island responses are strongly distorted by the ocean induction effect (OIE) originating from the lateral conductivity contrasts between the conductive ocean and resistive land. OIE is generally modeled by global simulations using relatively coarse grids (down to 0.25 degree resolution) to represent the bathymetry. Insufficiently accurate accounting for the OIE may lead to the wrong interpretation of the observed responses. We study whether the small-scale bathymetry features influence the island responses. To address this question we developed a global-to-Cartesian 3-D EM modeling framework based on a nested integral equation approach, which allows to efficiently account for the effects of high-resolution bathymetry. Two geomagnetic observatories, located in Indian (Cocos Island) and Pacific (Oahu Island) Oceans, are chosen to study the OIE in long-period responses. Numerical tests demonstrate that accounting of the very local bathymetry (down to 1 km resolution) dramatically change modeling results. Remarkably, the anomalous behavior of the imaginary parts of the responses at Cocos Island, namely, the change of sign at short periods, is reproduced by using highly detailed bathymetry.</p>

Probing upper mantle electrical conductivity with daily magnetic variations using global-to-local transfer functions

Summary We present new transfer functions (TFs) that can handle external electromagnetic (EM) sources of complex geometry. These TFs relate global expansion coefficients describing the source with a locally measured EM field. In this study, the new TFs concept was applied to the daily magnetic variations measured at the ground. The parameterisation of the source in terms of spherical harmonics was adopted. We used nearly 20 years of data from 125 mid-latitude observatories and explored how the results are affected by (I) solar activity conditions, (II) the choice of the prior conductivity model used for the source coefficient estimation, and (III) the presence of ocean tidal magnetic signals. We found that choosing magnetically quiet periods is beneficial due to simpler source morphology, and the choice of prior conductivity model may significantly affect the source coefficients and TFs at short periods. We further observed significant contributions by ocean tidal magnetic signals at coastal and island observatories and corrected for them. Finally, the estimated TFs were inverted for the mantle conductivity at several locations representing different geological settings.

Lowermost mantle thermal conductivity constrained from experimental data and tomographic models

SUMMARY Heat transfer through Earth's mantle is sensitive to mantle thermal conductivity and its variations. Thermal conductivities of lower mantle minerals, bridgmanite (Bm) and ferropericlase (Fp), depend on pressure, temperature, and composition. Because temperature and composition are expected to strongly vary in the deep mantle, thermal conductivity may also vary laterally. Here, we compile self-consistent data on lattice thermal conductivities of Bm and Fp at high pressure to model lower mantle thermal conductivity and map its possible lateral variations. Importantly, our data set allows us to quantify the influence of iron content on mantle conductivity. At the bottom of the mantle, the thermal conductivity for a pyrolitic mantle calculated along an adiabat with potential temperature 2000 K is equal 8.6 W m–1 K–1. Using 3-D thermochemical models from probabilistic tomography, which include variations in temperature, iron content, and bridgmanite fraction, we then calculate possible maps of conductivity anomalies at the bottom of the mantle. In regions known as low shear-wave velocity provinces, thermal conductivity is reduced by up to 26 per cent compared to average mantle, which may impact mantle dynamics in these regions. A simple analysis of threshold and saturation effects related to the iron content shows that our estimates of thermal conductivity may be considered as upper bounds. Quantifying these effects more precisely however requires additional mineral physics measurements. Finally, we estimate variations in core–mantle boundary heat flux, and find that that these variations are dominated by lateral temperature anomalies and are only partly affected by changes in thermal conductivity.