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G-Cubed: Geochemistry, Geophysics, Geosystems; an electronic journal of the Earth sciences

 

Index Terms

  • Geomagnetism and Paleomagnetism: Geomagnetic induction
  • Mineral Physics: Electrical properties
  • Geomagnetism and Paleomagnetism: Time variations—diurnal to secular
Abstract
Cited By
 

Abstract

Observing geomagnetic induction in magnetic satellite measurements and associated implications for mantle conductivity

Steven Constable

Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, California 92093, USA

Catherine Constable

Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, California 92093, USA

Currents induced in Earth by temporal variations in the external magnetic field have long been used to probe mantle electrical conductivity, but almost exclusively from sparsely distributed land observatories. Satellite-borne magnetometers, such as flown on Magsat, Ørsted, and Champ, offer the prospect of improved spatial coverage. The approach we have taken is to isolate induction by harmonic Dst (“disturbance storm time”) excitation of the magnetospheric ring current in satellite magnetic measurements: this is done by removing the magnetic contributions of the main (core) magnetic field, the crustal magnetic field, and ionospheric fields (cause of the daily variation) using Sabaka et al.'s [2000 , 2002] CMP3 comprehensive model. The Dst signal is then clearly evident in the midlatitude satellite passes lower than 50 degrees geomagnetic latitude. At higher latitudes, auroral and field aligned currents contaminate the data. We fit the internal and external components of the Dst signal for each equatorial pass, exploiting the fact that the geometry for the internal and external components is different for the azimuthal and radial vector components. The resulting timeseries of internal and external field variations shows that the Dst signals for the dawn passes are half those of the dusk passes. The sum of equatorial external and internal components of the field averaged over dawn and dusk passes provides an excellent estimate for the Dst index, and may in fact be superior when used as a proxy for the purposes of removing induced and magnetospheric fields from satellite magnetic data. We call this estimate satellite Dst. Cross spectral analysis of the internal and external timeseries shows both greater power and higher coherence in the dusk data. We processed the transfer function between internal and external dusk timeseries to provide globally-averaged, frequency dependent impedances that agree well with independently derived estimates. We estimate Earth's radial electrical conductivity structure from these impedances using standard regularized inversion techniques. A near-surface conductor is required, of thickness less than 10 km with a conductivity-thickness product almost exactly that of an average Earth ocean. Inversions suggest that an increase in conductivity at 440 km depth, predicted by recent laboratory measurements on high pressure phases of olivine, is not favored by the data, although, as in previous studies, the 670 km discontinuity between the upper and lower mantle is associated with a two orders of magnitude jump in conductivity. A new feature in our inversions is a further increase in lower mantle conductivity at a depth of 1300 km. A global map of the internal (induced) component of the magnetic field provides a qualitative estimate of three-dimensional (3-D) variations in Earth electrical conductivity, demonstrating graphically that the satellite data are responsive to lateral variations in electrical conductivity caused by the continents and oceans.

Received 15 September 2003; accepted 1 December 2003; published 20 January 2004.

Citation: Constable, S., and C. Constable (2004), Observing geomagnetic induction in magnetic satellite measurements and associated implications for mantle conductivity, Geochem. Geophys. Geosyst., 5, Q01006, doi:10.1029/2003GC000634.

Cited By

Adams, Donat J., and Artem R. Oganov (2006), Ab initio molecular dynamics study of CaSiO_{3} perovskite at P-T conditions of Earth’s lower mantle, Phys Rev B, 73(18), 184106, doi:10.1103/PhysRevB.73.184106.

Balasis, Georgios (2004), Local time effects in satellite estimates of electromagnetic induction transfer functions, Geophys Res Lett, 31, L16610, doi:10.1029/2004GL020147.

Balasis, Georgios, and Gary D. Egbert (2006), Empirical orthogonal function analysis of magnetic observatory data: Further evidence for non-axisymmetric magnetospheric sources for satellite induction studies, Geophys Res Lett, 33, L11311, doi:10.1029/2006GL025721.

Kelbert, Anna, Gary D. Egbert, and Adam Schultz (2008), Non-linear conjugate gradient inversion for global EM induction: resolution studies, Geophys J Int, 173(2), 365, doi:10.1111/j.1365-246X.2008.03717.x.

Kuvshinov, Alexei, Hisashi Utada, Dmitry Avdeev, and Takao Koyama (2005), 3-D modelling and analysis of Dst C-responses in the North Pacific Ocean region, revisited, Geophys J Int, 160(2), 505, doi:10.1111/j.1365-246X.2005.02477.x.

Kuvshinov, Alexei, and Nils Olsen (2006), A global model of mantle conductivity derived from 5 years of CHAMP, Ørsted, and SAC-C magnetic data, Geophys Res Lett, 33, L18301, doi:10.1029/2006GL027083.

Martinec, Zdenek, and Heather McCreadie (2004), Electromagnetic induction modelling based on satellite magnetic vector data, Geophys J Int, 157(3), 1045, doi:10.1111/j.1365-246X.2004.02252.x.

Muñoz, Gerard, Antonio Mateus, Jaume Pous, Wiebke Heise, Fernando Monteiro Santos, and Eugenio Almeida (2008), Unraveling middle-crust conductive layers in Paleozoic Orogens through 3D modeling of magnetotelluric data: The Ossa-Morena Zone case study (SW Iberian Variscides), J Geophys Res, 113, B06106, doi:10.1029/2007JB004987.

Pinheiro, K., and A. Jackson (2008), Can a 1-D mantle electrical conductivity model generate magnetic jerk differential time delays?, Geophys J Int, 173(3), 781, doi:10.1111/j.1365-246X.2008.03762.x.

Prieto, G. A., R. L. Parker, D. J. Thomson, F. L. Vernon, and R. L. Graham (2007), Reducing the bias of multitaper spectrum estimates, Geophys J Int, 0(0), 071005051620001, doi:10.1111/j.1365-246X.2007.03592.x.

Velímský, Jakub, Zdeněk Martinec, and Mark E. Everett (2006), Electrical conductivity in the Earth's mantle inferred from CHAMP satellite measurements?I. Data processing and 1-D inversion, Geophys J Int, 166(2), 529, doi:10.1111/j.1365-246X.2006.03013.x.

Whaler, Kathryn A (2007), Geomagnetism in the satellite era, Astron Geophys, 48(2), 2.23, doi:10.1111/j.1468-4004.2007.48223.x.