Session EM8: Global Studies EM8-1

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1 Session EM8: Global Studies EM8-1 Induction from Magnetic Satellite Measurements and Implications for Mantle Conductivity Steven Constable (Scripps Institution of Oceanography, Catherine Constable (Scripps Institution of Oceanography) Monika Korte (Scripps Institution of Oceanography) 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. We have isolated induction by harmonic Dst in satellite data 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) CMP3 comprehensive model. The Dst signal is then clearly evident in the mid-latitude satellite passes lower than 50 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 Dst signals for the dawn passes are half those of the dusk passes, but the ratio of internal to external fields, and thus the impedance, is the same for both series. The sum of equatorial external and internal components of the field averaged over dawn and dusk passes provides an excellent estimate of the Dst index. The dawn and dusk timeseries can be separately processed to provide globallyaveraged, frequency dependent impedances. When combined with longer period impedances derived from geomagnetic observatory data, Earth s radial electrical conductivity structure can be estimated, 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 atwo orders of magnitude jump in conductivity. A global map of the internal (induced) component of the magnetic field provides a qualitative estimate of 3D 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.

2 EM8-2 ONE-DIMENSIONAL CONDUCTIVITY STRUCTURE IN THE MID-MANTLE BENEATH THE NORTH PACIFIC Takao Koyama (IFREE, JAMSTEC, JAPAN; Hisayoshi Shimizu (Earthquake Research Institute, Univ. of Tokyo, JAPAN) Hisashi Utada (Earthquake Research Institute, Univ. of Tokyo, JAPAN) Alan D. Chave (Woods Hole Oceanographic Institution, USA) In this paper, we present a model of one-dimensional electrical conductivity structure in the mid-mantle beneath the North Pacific by a semi-global analysis of MT and GDS responses. We used the voltage data of eight submarine cables and the magnetic field data at eight geomagnetic observatories/stations as well as C responses published by Fujii and Schultz [2002]. In the data analysis, the global source field were approximated by P1 0 distribution at the period over 1 day. The ocean-continental contrast of the electrical conductivity was fully considered at grid points 2 degrees interval both in longitude and latitude. A 3-D global induction code by the modified IDM was used for forward solver but 1-D conductivity structures were inversely determined. We first obtained a set of optimum 1-D conductivities that minimizes the misfit between observations and calculations with a smooth constraints. We then carried out inversion with additional mathematical constraint that allows discontinuity in the conductivity. From the inversion with smooth constraint, we obtained a model in which conductivity increases monotonically in the mantle transition zone. Discontinuities were assumed at 400 and 650 km depth corresponding to phase transition, resulting a large (1.5 orders of magnitude) increase at 400 km depth and a small (a factor of three) jump at 650 km depth. It is interesting to find a similarity between the latter result and a model suggested by the laboratory measurement of the electrical conductivity of Mg 1.8 Fe 0.2 SiO 4 by Xu et al. [1998].

3 EM8-3 THREE-DIMENSIONAL CONDUCTIVITY STRUCTURE IN THE MID-MANTLE BENEATH THE NORTH PACIFIC Takao Koyama (IFREE, JAMSTEC, JAPAN; Hisayoshi Shimizu (Earthquake Research Institute, Univ. of Tokyo, JAPAN) Hisashi Utada (Earthquake Research Institute, Univ. of Tokyo, JAPAN) In this paper, we present a model of three-dimensional electrical conductivity structure in the mid-mantle beneath the North Pacific by a semi-global three-dimensional inversion of EM data. We used data of voltage variations obtained by eight submarine cables and magnetic field variations at eight geomagnetic observatories/stations as well as C responses published by Fujii and Schultz [2002]. We developed a new three-dimensional inversion algorithm by combining the steepest descent and the quasi-newton methods. An integral equation solver using the modified IDM (e.g. Singer, 1995) was used as a forward solver. We assumed that 3-D heterogeneous conductivity distribution can be separated into radially symmetric (1-D) structure and 3-D perturbation. The 1-D reference model is presented by our companion paper in this workshop (Koyama et al., 2002). 3-D perturbation was inversely estimated so that the observation-calculation misfit is minimized with an additional mathematical constraint (perturbation is smooth in space). The 1-D reference model consists of 17 layers with 50 km thickness. Perturbation is assumed to exist in the depth range from 350 km to 850 km in 1/4 of the Earth centered by the North Pacific, that is, in the horizontal range from 90 to 270 degrees of longitude and from 0 to 90 degrees of co-latitude. This region of perturbation is divided into grids of 10 degrees horizontally and 50 km in the depth with unknown conductivity parameters, while the grid size for calculation is 2 degrees horizontally and 50 km in the depth. We have made a checkerboard test and found the spatial resolution depends on the distribution of observation sites (cables and magnetic stations). Considering this, following features can be pointed out that can be comparable to results of seismic tomography (e.g. Fukao et al., 2001; Mégnin and Romanowicz, 2000): 1) There is a conducting zone in the transition zone beneath Hawaii with a conductivity perturbation of factor of three. 2) The uppermost lower mantle beneath Philippines is less conductive by a half. 3) The transition zone beneath Mariana is more conductive by three times. Assuming the heterogeneities purely of thermal origin, we found that they correspond to temperature anomalies of degrees.

4 EM8-4 NUCLEAR EXPLOSIONS AND LIGHTNING LOCALIZATION: NEW USES FOR GLOBAL EM Jeffrey Ridgway (ISL Inc., San Diego, CA, Steven Constable and Catherine de Groot-Hedlin (Scripps Institution of Oceanography, La Jolla, CA), and Martin Fuellekrug, (Institut fur Meteorologie und Geophysik, U. of Frankfurt/Main, Germany) The fusion of seismic and magnetotelluric data holds promise in discriminating between underground nuclear blasts and earthquakes. We have recently initiated a project to explore the usage of regional arrays of orthogonal magnetometers to detect EM effects of nuclear blasts. MT/EM measurements can sense several different effects of such explosions: the acoustic wave which travels slowly away from the blast center, the upward propagating wave which generates Alfv n waves, and the direct electromagnetic pulse (EMP) which propagates through the earth s atmosphere waveguide after traveling from a buried location to the surface. These signals are mixed in with noise signals which mask their presence: plane wave ionospheric resonances, and worldwide lightning which can mimic EMPs from explosions. We have experience in detection and localization of worldwide lightning pulses from triangulation of synchronized global magnetic observatories (e.g. Fuellekrug and Constable, 2000). These procedures yield scientific information that is useful in its own right. One method involves calculating the azimuth of propagation from the Poynting vector at 3 globally distributed stations, and determining the triangle of uncertainty of the source, by the intersection of the 3 great circles. Another method relies on precise timing of arrival of the pulses at widely spaced observatories. These methods have already proven successful. An animated movie of lightning discharge distribution will be presented, to demonstrate the effectiveness of this technique. In addition to relying on lighting triangulation to distinguish these signals from explosion-caused EMPs, the proposed work will explore the usage of regional sensor arrays (~100 km between sensors) to identify and remove coherent plane-wave noise from the ionosphere. We will draw on our experience with multivariate MT reduction algorithms from Egbert (1997) and others, for coherent noise mitigation. A demonstration experiment, planned to synchronize its timing with regional mining explosions, will validate the concept, using an array of magnetic sensors in California to derive directional EMP location information, and to identify and mitigate plane-wave signals which constitute noise. The ultimate goal is a global array of synchronized, multi-sensor regional arrays, which will be useful for nuclear test monitoring, atmospheric EM studies, worldwide lightning storm activity, and other global electromagnetic phenomena. Egbert, G. D., Robust multiple-station magnetotelluric data processing, Geophys. J. Int., vol. 130, pp Fuellekrug, M. and S. Constable, Global triangulation of intense lightning discharges, Geophysical Res. Let., 27,

5 EM8-5 THE ELECTRICAL STRUCTURE OF MARS? Pascal Tarits (IUEM/UBO, Place Nicolas Copernic, F-29280, Plouzane, France, The Mars Global Surveyor (MGS) Nasa mission has provided and is still providing a complete survey of the martian vector magnetic field. MGS is orbiting the planet at 400 km altitude for more than one year. This is the Mapping Orbit Phase. While the data show no significant planetary field, strong remnant magnetization have been identified on the planet s southern hemisphere. As a result, the Interplanetary Magnetic field (IMF) interacts directly with the planet, the weak atmosphere and, locally, with the strong static anomalies. The time-varying field varies from a few hundred nt on the day side (facing the sun) to several tens of nt on the night side. Despite the structure and organization of the planet/imf interaction is still poorly understood, I show evidences that part of the external field is spatially coherent at the planetary scale on the night side. This observation suggests that night side data can be analysed in terms of potential field and split into external and internal contributions. A preliminary investigation of the MGS data is presented and results about the electrical conductivity of Mars are proposed.

6 EM8-6 RESPONSE OF THE MANTLE CONDUCTIVITY VARIATIONS TO THE TRANSIENT EXTERNAL CURRENTS COMPUTED AT SATELLITE HEIGHTS Jakub Velímský (Dept. of Geophysics, Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic; Good spatial coverage of satellite-born magnetometric data, such as those obtained from the MAGSAT, Ørsted, and CHAMP spacecrafts, offers an unprecedented opportunity to improve our knowledge of electrical conductivity in the Earth s mantle. While the problem of electromagnetic induction in the mantle is traditionally solved in the frequency domain for land-based data, the spatio-temporal distribution of satellite data favors a time-domain approach. A forward solver of the geomagnetic induction problem in a heterogenous sphere in the time-domain has been developed recently. The program is based on a hybrid, sphericalharmonic-finite-element spatial parameterization, and uses a semi-implicit time-integration scheme. It has been extensively tested on a set of conductivity models consisting of eccentrically nested homogeneous spheres. We use this code to study the response of the mantle conductivity variations to the transient signals geomagnetic storms. We approximate the complex structure of a geomagnetic storm by a simplified mathematical model, assuming that the magnetic potential due to the ring current has strictly dipolar spatial characteristics, and that its intensity varies smoothly with time as t e αt. The decay time of the recovery phase 1/α ranges from 1 to 10 days. We study the sensitivity of the magnetic field computed at satellite heights to large midmantle intrusions of conductive material, and to a shallow layer of high conductivity contrast due to the distribution of oceans and continents. Our initial results indicate that the midmantle conductivity variations are well resolved in all three components of the synthetic field. However, the sensitivity deteriorates where the mantle is screened by a highly conductive oceanic layer.

7 EM8-7 DIFFUSION OF THE SECULAR VARIATION THROUGH THE HETERO- GENEOUS MANTLE Peter Weidelt, Jens Stadelmann (Institute of Geophysics and Meteorology, Technical University of Braunschweig, Braunschweig, Germany, Whereas the diffusion of the geomagnetic secular variation through a layered conducting mantle is a well-known process now, the effect of strong lateral conductivity contrasts, supposed to exist in the deep mantle close to the CMB, is still an open problem. It has early been suggested by Runcorn that these heterogeneities are the cause of the strong differences in the secular variation pattern of the Atlantic and Pacific hemisphere, where the weak secular variation of the latter is the effect of a partial shielding due to a deep well-conducting slab. This hypothesis is tested by calculating numerically the scattering of the secular variation by a deep non-uniform thin sheet. Formally this is achieved by casting the problem into an integral equation for the sheet-current density in the sheet. The integral equation has a contracting kernel, which allows an iterative inversion. In general, also Gauss- Seidel iteration performs excellently. The magnetic field resulting from the heterogeneous sheet currents is mapped to the surface. Considered are both harmonically oscillating and drifting sources of the secular variation. The first numerical results show that a significant influence of the secular variation by mantle heterogeneities is possible only for periods up to three years for reasonable conductances of the shell. Moreover, the well-conducting core is strongly damping the induction effects in an anomalous shells, which is close to the CMB.