Determination of the 1-D distribution of electrical conductivity in Earth’s mantle from Swarm satellite data
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 17 December 2012
Accepted: 22 July 2013
Published: 22 November 2013
We present an inversion scheme to recover the (1-D) depth profile of mantle conductivity from satellite magnetic data, which takes into account 3-D effects arising from the distribution of oceans and continents. The scheme is based on an iterative inversion of C-responses, which are estimated from time series of the dominating external (inducing) and internal (induced) spherical harmonic coefficients of the magnetic potential due to a magnetospheric source. These time series will be available as a Swarm Level-2 data product. We verify our approach by using synthetic, but realistic time series obtained by simulating induction due to a realistic magnetospheric source in a 3-D “target” conductivity model of the Earth. This model contains not only a laterally heterogeneous layer representing oceans and continents, but also 3-D inhomogeneities in the mantle. The inversion for mantle conductivity is initiated with a uniform conductivity model. Convergence is reached within a few iterations. The recovered model agrees well with the laterally averaged target model, although the latter comprises large jumps in conductivity. Our 1-D inversion scheme is therefore ready to process Swarm data.
Temporal variations of Earth’s magnetic field have long been used to infer global 1-D conductivity profiles of Earth’s mantle, mostly from continental geomagnetic observatories (e.g. Schmucker, 1985; Schultz and Larsen, 1987; Olsen, 1998). In contrast to data from observatories, which are sparse and irregularly distributed (with only few in oceanic regions), data of uniform quality with a good spatial coverage can be obtained from low-Earth-orbit (LEO) platforms. This enables the derivation of a globally-averaged conductivity profile that is not biased towards continental regions.
However, analysis of satellite data is more challenging compared to analysis of observatory data for two reasons: First, LEO satellites move typically with a speed of 7–8 km/s and thus measure a mixture of temporal and spatial changes of the magnetic field. Second, satellites pass over both continents and oceans, and therefore the magnetic satellite data are affected by induction in the oceans (cf. Tarits and Grammatica, 2000; Everett et al., 2003; Kuvshinov and Olsen, 2005) in a complicated way. In spite of these difficulties, a number of attempts has been made to probe mantle conductivity from space (e.g. Didwall, 1984; Olsen, 1999; Olsen et al., 2002; Constable and Constable, 2004; Velímský et al., 2006). Most recently, Kuvshinov and Olsen (2006) derived a global model of mantle conductivity from five years of CHAMP, Ørsted and SAC-C magnetic data and demonstrated the necessity of taking into account the contributions of a laterally heterogeneous surface shell representing the distribution of oceans and continents. The authors accounted for the ocean effect by correcting the magnetic field at orbit altitudes.
As in the paper by Kuvshinov and Olsen (2006), we present in this paper a methodology to determine the 1-D conductivity profile of Earth’s mantle by inverting global C-responses. These are estimated from time series of the dominating external (inducing) and internal (induced) spherical harmonic expansion (SHE) coefficients of the magnetic potential that describes the signals of magnetospheric origin. Both time series will be available as Swarm Level-2 data product MMA SHA 2, determined by the Comprehensive Inversion chain (CI, Sabaka et al., 2013) of the Swarm Level-2 Processing Facility SCARF (Olsen et al., 2013). The CI aims to separate magnetic contributions from various sources (originating in the core, lithosphere, ionosphere and magnetosphere) in the form of corresponding SHE coefficients. As Kuvshinov and Olsen (2006), we also correct for the ocean effect, but our correction scheme differs from that presented by the authors in two aspects. First, our scheme is iterative, i.e. a correction for the ocean effect is applied multiple times. Second, we do not make the detour of predicting the magnetic field at orbit altitudes, but directly correct the estimated C-responses.
2. Inversion Algorithm
2.1 Estimation of global C-responses
2.2 Correction of estimated C-responses
Calculation of BX-D,unit(r = a, ϑ, φ,ω), i.e. the magnetic field at Earth’s surface due to a unit amplitude magnetospheric ring current source at a set of frequencies ω. Note that the calculated magnetic field only varies in longitude φ if the model contains 3-D heterogeneities.
Transformation of to CX-D(ω) using Eq. (3).
To compute the magnetic field in a 3-D conductivity model (item 1 of the above list), we use a 3-D contracting integral equation solver, extensively described in Kuvshinov and Semenov (2012).
2.3 Derivation of the 1-D conductivity model
We derive the 1-D conductivity model from the corrected C -responses Ccorr(ω) by using the quasi-Newton algorithm of Byrd et al. (1995). The inversion is stabilized by minimizing the first derivative of log(conductivity) with respect to log(depth). Inversion is performed for several regular-ization parameters (i.e. several degrees of smoothing). The solution is picked from a trade-off curve (L-curve), which relates data misfit and model complexity (Hansen, 1992).
3. Model Study
In order to test the performance of our inversion scheme, we generate synthetic data (i.e. time series of induced coefficients) in a test 3-D conductivity model, hereinafter referred to as “target model”, and afterwards recover the 1-D conductivity structure of the target model (i.e. its laterally averaged conductivity) from the data. We introduce the target conductivity model (Subsection 3.1), describe how we generate the test data (Subsection 3.2) and finally present the results of our model study (Subsection 3.3).
3.1 Target conductivity model
3.2 Generation of the test data set
Hourly mean time series of external SHE coefficients in a geomagnetic dipole coordinate system (up to degree 3 and order 1) have been derived by analysis of 4.5 years of observatory data (July 1998-December 2002), details of the derivation are given in Olsen et al. (2005). Time series of internal SHE coefficients are synthesized by simulating induction in the target conductivity model (Subsection 3.1), cf. companion paper by Puthe and Kuvshinov (2013).
We use the time series of external and internal coefficients to predict the magnetospheric field at orbit altitudes (with a sampling frequency of 1 Hz). Adding the contributions due to different sources (core, lithosphere and ionosphere) yields the magnetic field at orbit altitudes, which is then analyzed by the CI. The external and internal SHE coefficients of the magnetic potential due to magnetospheric sources recovered by the CI constitute a realistic test data set for our inversion scheme. A more detailed description of the generation of the test data set is provided in Olsen et al. (2013).
3.3 Inversion results
We have presented a scheme to invert satellite magnetic data for a global depth profile of mantle conductivity. The scheme is based on the inversion of C-responses and comprises a correction for the ocean effect. In spite of the iterative architecture and a number of processing steps, final results can be obtained within a few hours. The repeated calculation of the magnetic field in a 3-D model is the most expensive step in terms of computational cost.
The algorithm has been tested by simulating induction due to a realistic magnetospheric source in a realistic 3-D (target) conductivity model and recovering the 1-D conductivity structure of this model (i.e. its laterally averaged conductivity) from the synthetic data. In spite of large conductivity jumps and a number of 3-D heterogeneities in the target model, an excellent recovery of the 1-D conductivity of Earth’s mantle has been achieved from crustal depths to the core-mantle boundary. The algorithm has thus proved to be workable and ready to digest Swarm data. Moreover, the inversion results provide a useful initial guess for 3-D mantle conductivity studies with Swarm data (cf. companion papers by Puthe and Kuvshinov, 2013; Velímský, 2013).
The authors thank Nils Olsen for a careful and constructive review of the manuscript. This work has been supported by the European Space Agency through ESTEC contract No. 4000102140/10/NL/JA, by the Swiss National Science Foundation under grant No. 2000021-140711/1, and in part by the Russian Foundation for Basic Research under grant No. 12-05-00817-a.
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