Publications

B. Langlais, B., M. Purucker, and M. Mandea, The crustal magnetic field of Mars, J. Geophys. Res., 10.1029/2003JE002048, 2003, in press.
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Macmillan, S., S. Maus, T. Bondar, A. Chambodut, V. Golovkov, R. Holme, B. Langlais, V. Lesur, F. Lowes, H. Luhr,¨ W. Mai, M. Mandea, N. Olsen, M. Rother, T. Sabaka, A. Thomson and I. Wardinski, The 9th-Generation International Geomagnetic Reference Field International, Geophys. J. Int., 155, pp. 10511056, 2003.
Also in EOS, Trans. Amer. Geophys. Union, 84, 46, 2003.
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Chambodut, A., J,. Schwarte, B. Langlais, H. Lühr, and M. Mandea, The selection of data in main field modeling, in: OIST-4 Proceedings , P. Stauning et al (eds.), Copenhagen, 2003.
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B. Langlais, B., M. Mandea, and P. Ultré-Guérard, High-resolution magnetic field modeling: application to MAGSAT and Ørsted data, Phys. Earth Plan. Int., 135, pp. 77-91, 2003.
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Purucker M. E., B. Langlais, N. Olsen, G. Hulot, and M. Mandea, The southern edge of cratonic North America: evidence from new satellite magnetometer observations, Geophys. Res. Lett., 10.1029/2001GL013645, May 2002.
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Mandea M., and B. Langlais, Observatory crustal magnetic biases during MAGSAT and Ørsted satellite missions, Geophys. Res. Lett., 10.1029/2001GL013693, August 2002.
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Hulot G., C. Eymin, B. Langlais, M. Mandea, and N. Olsen, Small-scale structure of the geodynamo inferred from Ørsted and MAGSAT satellite data, Nature, 416, pp. 620-623, 2002.
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Langlais B., and M. Mandea, An IGRF candidate main geomagnetic field model for epoch 2000 and a secular variation model for 2000-2005, Earth Planets Space , 52, pp. 1137-1148, 2000.
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Mandea M., and B. Langlais, Use of Ørsted scalar data in evaluating the pre-Ørsted main field candidate models for the IGRF 2000, Earth Planets Space, 52, pp. 1167-1170, 2000.
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Lowes F.J., T. Bondar, V.P. Golovkov, B. Langlais, S. Macmillan and M. Mandea, Evaluation of the candidate Main Field model for IGRF 2000 derived from preliminary Ørsted data, Earth Planets Space, 52, pp. 1183-1186, 2000.
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Olsen N., R. Holme, G. Hulot, T. Sabaka, T. Neubert, L. Tøffner-Clausen, F. Primdahl, J. Jørgensen, J.M. Léger, D. Barraclough, J. Bloxham, J. Cain, C. Constable, V. Golovkov, A. Jackson, P. Kotzé, B. Langlais, S. Macmillan, M. Mandea, J. Merayo, L. Newitt, M. Purucker, T. Risbo, M. Stampe, A. Thompson and C. Voorhies, Ørsted Initial Field Model, Geophys. Res. Lett., 27, pp. 3607-3610, 2000.
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Mandea M., S. Macmillan, T. Bondar, V. Golovkov, B. Langlais, F. Lowes, N. Olsen, J. Quinn, and T. Sabaka, International geomagnetic reference field - 2000, Phys. Earth Plan. Int., 120, pp. 39-42, 2000.
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B. Langlais, B., M. Purucker, and M. Mandea, The crustal magnetic field of Mars, J. Geophys. Res., 10.1029/2003JE002048, 2003, in press.

The equivalent source dipole technique is used to model the three components of the Martian lithospheric magnetic field. We use magnetic field measurements made on board the Mars Global Surveyor spacecraft. Different input dipole meshes are presented and evaluated. Because there is no global, Earth-like, inducing magnetic field, the magnetization directions are solved for together with the magnetization intensity. A first class of models is computed using either low-altitude or high-altitude measurements, giving some statistical information about the depth of the dipoles. Then, a second class of models is derived, based on measurements made between 80 and 430 km altitude. 4840 dipoles are placed 20 km below the surface, with a mean spacing of 2.92 degreees (173 km). Residual rms values between observations and predictions are as low as 15 nT for the total field, with associated correlation coefficient equal to 0.97. The resulting model is used to predict the magnetic field at 200-km constant altitude. We present the maps of the magnetic field and of the magnetization. Downward continuation of a spherical harmonic model derived from our equivalent source solution suggests intermediate scale lithospheric fields at the surface probably exceed 5000 nT. Given an assumed 40-km thick magnetized layer, with a mean volume per dipole equal to 3.6.10^6 km^3, the magnetization components range between +/-12 A/m. We also present apparent correlations between some impact craters (>= 300 km diameter) and magnetization contrasts. Finally, we discuss the implications of the directional information and possible magnetic carriers. Top

Macmillan, S., S. Maus, T. Bondar, A. Chambodut, V. Golovkov, R. Holme, B. Langlais, V. Lesur, F. Lowes, H. Luhr,¨ W. Mai, M. Mandea, N. Olsen, M. Rother, T. Sabaka, A. Thomson and I. Wardinski, The 9th-Generation International Geomagnetic Reference Field International, Geophys. J. Int., 155, pp. 10511056, 2003.

The International Association of Geomagnetism and Aeronomy has recently released the 9th- Generation International Geomagnetic Reference Field--the latest version of a standard math- ematical description of the Earth's main magnetic field used widely in studies of the Earth's deep interior, its crust and its ionosphere and magnetosphere. The coefficients were recently fi- nalized at the XXIII General Assembly of the International Union of Geophysics and Geodesy, held at Sapporo in Japan in 2003 July. The IGRF is the product of a huge collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observatories and surveys around the world. Top

Chambodut, A., J,. Schwarte, B. Langlais, H. Lühr, and M. Mandea, The selection of data in main field modeling, in: OIST-4 Proceedings , P. Stauning et al (eds.), Copenhagen, 2003.

The launch of the Ørsted and CHAMP high-precision geomagnetic field satellites has opened new horizons for a better understanding the Earth's magnetic field. Geomagnetic field models from Oersted and CHAMP spacecraft missions based on spherical harmonic analysis of geomagnetically quit, night-time, vector and scalar measurements were estimated by different groups of researchers. Because of different parametrisation factors in applying the spherical harmonic analysis (degree/order of internal/external parts) and different data selection criteria (geomagnetic indices: Kp, Dst, etc... ; night-side data ; only scalar data at high geomagnetic latitudes), the obtained models are different. In order to estimate these differences, we computed main field models from several data sets, which have been obtained by using modified selection criteria. Firstly, the influence of the external field by the choice of the geomagnetic indices is evaluated. Then, several methods in getting night-side data are applied. Finally, we compare our models with some already published models (Langlais et al., 2003; Holme et al., 2002, Sabaka et al., 2002). Top

B. Langlais, B., M. Mandea, and P. Ultré-Guérard, High-resolution magnetic field modeling: application to MAGSAT and Ørsted data, Phys. Earth Plan. Int., 135, pp. 77-91, 2003.

Launched on 23rd February 1999, the Ørsted satellite opened the decade of geopotential field research. This is the first satellite to measure the three components of the Earth's magnetic field since MAGSAT (19791980). Ørsted orbital parameters are very similar to those of MAGSAT, allowing a first-order comparison of the 1979 and 2000 magnetic fields. Using the available vector and scalar data over the first 14 months of the Ørsted mission and applying classical selection criteria (local time, external magnetic activity), we compute a 29-degree/order main-field model and a 13-degree/order secular-variation model for the period 19992000. The applied method and the accuracy of the derived model are discussed. We compare the resulting main-field model to a similar one derived from MAGSAT data. Results of this comparison are presented, such as (i) morphology and energy spectrum of the secular variation and (ii) morphology of the crustal magnetic field at MAGSAT and Ørsted epochs.

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Purucker M. E., B. Langlais, N. Olsen, G. Hulot, and M. Mandea, The southern edge of cratonic North America: evidence from new satellite magnetometer observations, Geophys. Res. Lett., 10.1029/2001GL013645, 2002.

A global model is developed for both induced and remanent magnetizations in the terrestrial lithosphere. The model is compared with, and well-described by, Ørsted satellite observations. Interpretation of the observations over North America suggests that the large total field anomalies, associated with spherical harmonic degrees 15-26 and centered over Kentucky and the south-central United States, are the manifestations of the magnetic edges of the southern boundaries of cratonic North America. The techniques and models developed here may be of use in defining other cratonic boundaries.

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Mandea M., and B. Langlais, Observatory crustal magnetic biases during MAGSAT and Ørsted satellite missions, Geophys. Res. Lett., 10.1029/2001GL013693, 2002.

Computing main-field models derived from MAGSAT and Ørsted satellite magnetic measurements, the crustal influence on satellite data can be treated as random noise. Satellite data can thus be used in conjunction with magnetic observatory measurements to isolate the non-core field at the observatory locations. Crustal biases for the horizontal northward X, eastward Y and vertical downward Z components are computed for all magnetic observatories where data are available for MAGSAT (1979-1980) and Ørsted (1999-2000) epochs, to study their correlation over twenty years. For a set of observatories installed after 1979 new crustal biases are computed.

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Langlais B., and M. Mandea, An IGRF candidate main geomagnetic field model for epoch 2000 and a secular variation model for 2000-2005, Earth Planets Space , 52, pp. 1137-1148, 2000.

A candidate main geomagnetic field model for epoch 2000, and a secular variation model for the period 2000-2005, are proposed. The main field model is to degree and order 10, the secular variation one to degree and order 8. These models are derived using the method of least squares. A 1997.5 main field model was derived from annual mean values provided by geomagnetic observatories for the 1997.5 epoch, repeat station measurements made in 1997 and reduced to 1997.5, and scalar data since 1995 adjusted to 1997.5. A weighting scheme based on both geographical distribution and data quality was applied. This model was then extrapolated to the 2000.0 epoch, using previously derived secular variation models. To derive these secular variation models, twenty six main field models were firstly computed for epochs 1975.5 through 2000.5, using annual mean values of the X, Y , Z components of the magnetic field from observatories, with the same geographical distribution every year. When missing, annual mean values for 1998, 1999 and 2000 were estimated from extrapolated monthly means, using exponential smoothing and taking account of the seasonal variation. From these twenty six models, twenty five annual secular variation models were extracted, by taking the differences between consecutive main field models. Finally, to produce the IGRF candidate secular variation model, each Gauss coefficient of this set of secular variation models was extrapolated to give values for each year to 2005, using exponential smoothing. So, a mean secular variation model was obtained for the period 2000 2005 and this is proposed for adoption.

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Mandea M., and B. Langlais, Use of Ørsted scalar data in evaluating the pre-Ørsted main field candidate models for the IGRF 2000, Earth Planets Space, 52, pp. 1167-1170, 2000.

For describing the main field model at the 2000.0 epoch and the secular variation over the 2000 2005 time-span, three candidate models for the International Geomagnetic Reference Field (IGRF 2000) were proposed at the beginning of 1999, called in alphabetical order IPGP00 (proposed by IPGP), IZMI00 (proposed by IZMIRAN) and USUK00 (proposed by USGS/BGS). A fourth model, IGRF95 (the updated IGRF 1995), was suggested by the Working Group chairman. The modelling methods and the data used are presented by each team elsewhere in this special issue. This study is an attempt to test these models using the total field intensity provided by the Ørsted satellite, the only data available from that satellite at the time when the two tests describing here were done. The first test consists of evaluating the differences between the real and the synthetic data computed from the candidate models. The second test compares the capability of the candidate models to reduce the Backus effect, using a predictive dip-equator position and Ørsted data. Both tests show that the quality of the candidate models is far from being acceptable, and, therefore, a new candidate model for the main field, using vectorial Ørsted data, is required.

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Lowes F.J., T. Bondar, V.P. Golovkov, B. Langlais, S. Macmillan and M. Mandea, Evaluation of the candidate Main Field model for IGRF 2000 derived from preliminary Ørsted data, Earth Planets Space, 52, pp. 1183-1186, 2000.

On this occasion the selection of the IGRF for 2000 was left to a small Task Force. Before it was accepted by the Task Force as IGRF 2000, the final candidate model (a truncated version of Ørsted(10c/99)) was compared with a comprehensive set of independent surface and satellite data. The method, data selection, and results of this comparison are described.

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Olsen N., R. Holme, G. Hulot, T. Sabaka, T. Neubert, L. Tøffner-Clausen, F. Primdahl, J. Jørgensen, J.M. Léger, D. Barraclough, J. Bloxham, J. Cain, C. Constable, V. Golovkov, A. Jackson, P. Kotzé, B. Langlais, S. Macmillan, M. Mandea, J. Merayo, L. Newitt, M. Purucker, T. Risbo, M. Stampe, A. Thompson and C. Voorhies, Ørsted Initial Field Model, Geophys. Res. Lett., 27, pp. 3607-3610, 2000.

Magnetic measurements taken by the Ørsted satellite during geomagnetic quiet conditions around January 1, 2000 have been used to derive a spherical harmonic model of the Earth's magnetic field for epoch 2000.0. The maximum degree and order of the model is 19 for internal, and 2 for external, source fields; however, coefficients above degree 14 may not be robust. Such a detailed model exists for only one previous epoch, 1980. Achieved rms misfit is < 2 nT for the scalar intensity and < 3 nT for one of the vector components perpendicular to the magnetic field. For scientific purposes related to the Ørsted mission, this model supercedes IGRF 2000.

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