Communications 2002 2003

Langlais B., and M. Purucker, A Polar Magnetic Paleopole on Mars?Polar Lithospheric Field From Multiple Satellite Observations, AGU Fall Meeting, San Francisco, USA, 2003.
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Hulot, G., and Langlais B., Investigating the Nature of the Current Decrease of the Earth's Magnetic Dipole Moment, Invited Contribution, AGU Fall Meeting, San Francisco, USA, 2003.
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Langlais B., M. Purucker, and S. Vennerstrøm, Polar Lithospheric Field From Multiple Satellite Observations, 2nd CHAMP Science Meeting, Potsdam, Germany, 2003.
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Langlais B., Magnetic Field Secular Variation: The Satellite Perspective, IUGG XXIII assembly, Sapporo, Japan, 2003.
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Langlais, B., M. Purucker, and M. Mandea, The crustal magnetic field of Mars: its implications on the lithosphere properties, IUGG XXIII assembly, Sapporo, Japan, 2003.
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Langlais B., M. Purucker, and S. Vennerstrøm, Polar Lithospheric Field From Multiple Satellite Observations, IUGG XXIII assembly, Sapporo, Japan, 2003.
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Chambodut, A., M. Mandea, and B. Langlais, Geomagnetic field models for epochs 1995 and 2000, IUGG XXIII assembly, Sapporo, Japan, 2003.
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Langlais, B., M. Purucker, and M. Mandea, The crustal magnetic field of Mars: its correlation with some impact craters, EGS/AGU/EUG Joint meeting, Nice, April 2003.
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Mandea, M., F Primdhal, M. H. Acuna, G. Chanteur, Y. Cohen, J. E. P. Connerney, E. Friis-Christensen, G. Hulot, B. Langlais, M. Menvielle, M. Purucker, N. Olsen, K. Schwingenschuh, P. Tarits, S. Vennerstrom, and M. Wieczorek, External and internal magnetic fields at Mars: New results and upcoming missions, EGS/AGU/EUG Joint meeting, Nice, April 2003.
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Hulot G., C. Eymin, B. Langlais, Mioara Mandea and N. Olsen, Small-scale structure of the Geodynamo inferred from Ørsted and Magsat satellite data, Orsted International Science Team Meeting, Copenhagen, September 22-27, 2002.
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Chambodut A., J. Schwarte, B. Langlais, H. Lühr and M. Mandea, The selection of data in field modeling, 4th Ørsted International Science Team Conference , Copenhagen, September 23-27, 2002.
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Langlais B., M. Purucker, and M. Mandea, The orientation of the magnetization in the martian lithosphere, AGU spring meeting, Washington, May 28-31, 2002.
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Langlais B., Use of multi-satellite datasets to model the lithospheric magnetic field in polar areas, AGU spring meeting, Washington, May 28-31, 2002.
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Langlais B., M. Mandea, and M. Purucker, Sur l'aimantation de la lithosphère martienne, Réunion des Sciences de la Terre, Nantes, April 9-12, 2002 (in French).
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Hulot G., C. Eymin, B. Langlais, Mioara Mandea and N. Olsen, Small-scale structure of the Geodynamo inferred from Ørsted and Magsat satellite data, 1st CHAMP Science Meeting, Potsdam, January 22 - 25, 2002.
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Purucker M., N. Olsen, D. Ravat, P. Schwintzer, and B. Langlais, 1st CHAMP Science Meeting, Improving global crustal temperature, geologic variation, and lithospheric strength models utilizing the lithospheric magnetic field derived from CHAMP, Potsdam, January 22 - 25, 2002.
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Langlais B., and M. Purucker, A Polar Magnetic Paleopole on Mars?Polar Lithospheric Field From Multiple Satellite Observations, AGU Fall Meeting, San Francisco, USA, 2003.

A global map of the Martian magnetic field is computed using MGS magnetic measurements and an equivalent source approach. Sources are located arbitrarily (i.e. with a constant spacing) near the surface of Mars. The magnetic field and the magnetization (from the equivalent source solution) present similar features: magnetic anomalies are mostly located South of the crustal dichotomy, the largest craters as well as the largest volcanoes of the North hemisphere do not seem to present any significant magnetic anomaly. In one case at least, a volcanic edifice, Apollinaris Patera, seems to be associated with a magnetic anomaly. This volcano is located at (9.3 deg. S, 174.4 deg. E), North of Terra Cimmeria and Terra Sirenum, where the largest magnetic anomalies were detected. It rises about 5 km above the surrounding terranes. Its shape is a 200-km wide dome, with a 75-km wide caldera on its submit. This is one of the largest volcanoes in the Southern Hemisphere. It is associated with a relatively high gravity anomaly. Crater counts and stratigraphic relations suggest a middle Hesperian age for its younger flow, but it presumably had a long history. We present the modeling of the magnetic anomaly located above this volcano. We use a single equivalent source, located below the volcano. The direction of the magnetization of the source leads to a polar position (70 degrees latitude) for the paleopole. Assuming that the last eruptive stages occurred while the Martian dipolar dynamo was still active, then this paleopole position would indicate that the Martian magnetic field was almost aligned on the present day rotation axis of the planet.

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2003

Hulot, G., and Langlais B., Investigating the Nature of the Current Decrease of the Earth's Magnetic Dipole Moment, Invited Contribution, AGU Fall Meeting, San Francisco, USA, 2003.

The Earth's dipole moment is currently decreasing fast, at a much faster pace than the one it would experience were this pace due to a free diffusive dipole decay mode. This suggests that an active process is going on that is responsible for this current decay. In this paper we will briefly review various aspects of the present field behavior, which could be related to this fast decay of the dipole and which could thus provide us with some information about the processes that could be responsible for it. Of particular interest is the fact that both the field and its secular variation display a strong hemispheric Pacific/Atlantic asymmetry. Also of interest is the amount of diffusion involved in the process. All those issues will be discussed in view of recent satellite data.

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2003

Langlais B., M. Purucker, and S. Vennerstrøm, Polar Lithospheric Field From Multiple Satellite Observations, 2nd CHAMP Science Meeting, Potsdam, Germany, 2003.

Mapping the magnetic field of lithospheric origin is a key to better understanding the properties of the oceanic and/or continental lithosphere. Models are usually based on local, airborne surveys, or on satellite datasets. The first kind of data doesn't allow large scale anomalies to be well modeled. The second one usually relied on the scalar measurements of the POGO series, or on the short time mission MAGSAT. The launches of Ørsted (Feb. 1999), CHAMP (Jul. 2000) and SAC-C (Nov. 2000) gave a new opportunity for studying the magnetic field of the Earth from the satellite perspective. This mini-constellation, which can be considered as a small satellite observatory network, provides for the first time multiple, simultaneous field measurements at satellite altitude. Of particular interest is the data density above the North and South pole areas of the Earth. In this study, we use the equivalent dipole source method to model the lithospheric field over polar (+/-60 degrees) areas. We use a multiple step approach. First, data are selected globally, with respect to the local time and external magnetic perturbations. Spherical harmonic (SH) models are built, using either short-time data subsets, or pluri-annual measurements. The core contribut ions are then removed from the measurements. The computed residuals are carefully selected, in order to remove as much external perturbations as possible. Because only the total intensity enters the SH model, it is possible to use the average of the component perpendicular to the main field direction to determine the quiet intervals. The sorted residuals are then used to infer the susceptibility of field-aligned dipoles located in the lithosphere. The modeled dipoles are finally used to predict the magnetic field of lithospheric origin at a constant altitude. A comparison with other models is shown. A discussion with respect to the season (winter or summer) is presented, as well as a comparison between the North and South pole areas.

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2003

Langlais B., Magnetic Field Secular Variation: The Satellite Perspective, IUGG XXIII assembly, Sapporo, Japan, 2003.

The temporal changes of the Earth's magnetic field are usually modeled using observatory data. Indeed the secular variation models rely on long-term series of magnetic measurements. Up to now, only the observatories were able to provide such measurements. The first half of the Decade of Geopotential Research has seen the launch of three satellites, Ørsted, CHAMP and SAC-C. These three satellites are dedicated to monitoring the space and temporal changes of the Earth's magnetic field. In this study we present the combined use the vector and scalar measurements made on-board these satellites to model the secular variation. First, data are selected with respect to their local time and to the external activity indices. These data are sorted so that their geographical distribution are as equiangular as possible. Then, secular variation models are derived using two approaches. In the first one, we solve for the main field and the secular variation Gauss coefficients together. In the second, we first derive short-time main field models, from which the secular variation is derived by subtracting the coefficients from consecutive models. A comparison of the methods is presented, in terms of rms behavior and field spectra. We also discuss the core properties inferred from these models, including the core radius estimation, and the diffusive times of the magnetic field.

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2003

Langlais, B., M. Purucker, and M. Mandea, The crustal magnetic field of Mars: its implications on the lithosphere properties, IUGG XXIII assembly, Sapporo, Japan, 2003.

While several landers are planned for the next decade, the only instruments which will provide unequivocal insight into the magnetic properties of the lithosphere are the magnetometers on Netlander and the Mossbauer spectrometer on the Mars Exploration Rover. Thus, magnetic studies of the Martian lithosphere still highly rely on measurements made on board satellites such as Mars Global Surveyor (MGS) and the yet to come Nozomi. Models built from such datasets are the only way to probe the properties of the Martian lithosphere. In this paper, we present the recent advances made in the modeling of the Martian magnetic anomalies, based on the 80-430 km altitude MGS measurements. Using an equivalent source dipole approach, we produce a constant altitude map of the Martian magnetic field. The dipole network consists of an icosahedral (spherical triangles) distribution, with a mean horizontal resolution set to 173 km (2.92 degrees at the equator). For practical purposes, a constant 40-km thick layer is considered, its depth being adjusted to offer a better fit to the data. On the contrary to Earth-like studies, magnetization directions are solved as part of the solution. This map first confirms the hemispheric distribution of the high intensity, short scale magnetic anomalies. Second, this method gives an unique opportunity to access the magnetization contrasts of the Martian lithosphere. The magnetization ranges +/- 12 A/m for a 40-km thick layer. Correlations of the magnetization contrasts with some relatively small impact craters (300 km and up) are shown, that possibly indicat e a thinner magnetic lithosphere (20 km). Implications for the magnetic field at the surface of Mars are discussed, as well as the magnetization range and the look for a Martian magnetic paleopole.

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2003

Langlais B., M. Purucker, and S. Vennerstrøm, Polar Lithospheric Field From Multiple Satellite Observations, IUGG XXIII assembly, Sapporo, Japan, 2003.

Mapping the magnetic field of lithospheric origin is a key to better understanding the properties of the oceanic and/or continental lithosphere. Models are usually based on local, airborne surveys, or on satellite datasets. The first kind of data doesn't allow large scale anomalies to be well modeled. The second one usually relied on the scalar measurements of the POGO series, or on the short time mission MAGSAT. The launches of Ørsted (Feb. 1999), CHAMP (Jul. 2000) and SAC-C (Nov. 2000) gave a new opportunity for studying the magnetic field of the Earth from the satellite perspective. This mini-constellation, which can be considered as a small satellite observatory network, provides for the first time multiple, simultaneous field measurements at satellite altitude. Of particular interest is the data density above the North and South pole areas of the Earth. In this study, we use the equivalent dipole source method to model the lithospheric field over polar (+/- 60 degrees) areas. We use a multiple step approach. First, data are selected globally, with respect to the local time and external magnetic perturbations. Spherical harmonic (SH) models are built, using short-time data subsets. The core contributions are then removed from the measurements. The computed residuals are carefully selected, in order to remove as much external perturbations as possible. Because only the total intensity enters the SH model, it is possible to use the average of the component perpendicular to the main field direction to determine the quiet intervals. The sorted residuals are then used to infer the susceptibility of field-aligned dipoles located in the lithosphere. The modeled dipoles are finally used to predict the magnetic field of lithospheric origin at a constant altitude. A comparison with other studies is shown. A discussion with respect to the season (winter or summer) is presented, as well as a comparison between the North and South pole areas.

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2003

Chambodut, A., M. Mandea, and B. Langlais, Geomagnetic field models for epochs 1995 and 2000, IUGG XXIII assembly, Sapporo, Japan, 2003.

The recent call for revision of the IGRF/DGRF models for 1995 and 2000 epochs provides an excellent opportunity for studying the global andregional structure of the main geomagnetic field and of its secular variation. Available datasets consist of new, high-quality, satellite measurements, and of time series of the global magnetic observatory network. A 2000.0 main-field model (up to degree/order 13) and secular-variation one (up to degree/order 8) are derived using Ørsted vector and scalar data spanning twelve months centered on the considered epoch. In order to minimize the diurnal disturbances linked to the Sun, only satellite measurements made in the shadow region were kept. Then data were selected with respect to the geomagnetic indices: Kp(h) <= 1+; Kp(h +/- 3) <= 2- ; |Dst| <= 5 nT and |dDst/dt| <= 3nT/h. A series of secular-variation models from 1995 to 2000 is also computed using observatory data, weighted by a scheme based on their geographical distribution. These resulting models are presented, as well as their comparison with other models for the same epochs.

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2003

Langlais, B., M. Purucker, and M. Mandea, The crustal magnetic field of Mars: its correlation with some impact craters, EGS/AGU/EUG Joint meeting, Nice, April 2003.

The Martian magnetic field is characterized by its short spatial length scales and locally high intensity anomalies. The magnetic experiment on-board the Mars Global Surveyor satellite provided a dual altitude coverage, one near 200±100 km (AB and SPO phases), and another one near 400±30 km (MO phase). These datasets are used to model the magnetic field at a constant altitude. Because there is no global, Earth-like, inducing magnetic field, the magnetization directions are solved for as part of the solution. Many a priori parameters can influence the model, among which are the dipole mesh resolution, and the depth to the sources. We present modeling results, showing the evolution of statistics with respect to these a priori parameters. The final model consists of a mesh of 40-km thick dipoles, located 20 km below the surface, on a 170-km mean horizontal resolution grid. A discussion with respect to the residuals is presented. Comparisons with previously published models show that our model is at least equivalent or even better. Nonetheless, the magnetization model gives us an unique opportunity to explore the properties of the Martian lithosphere. Given an assumed 40-km thick magnetized layer, magnetization ranges 12 A/m. We also present some strong correlations between relatively small impact craters ( > 300 km diameter) and magnetization contrasts. Finally, we discuss the implications of the directional information and calculated paleopoles.

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2003

Mandea, M., F Primdhal, M. H. Acuna, G. Chanteur, Y. Cohen, J. E. P. Connerney, E. Friis-Christensen, G. Hulot, B. Langlais, M. Menvielle, M. Purucker, N. Olsen, K. Schwingenschuh, P. Tarits, S. Vennerstrom, and M. Wieczorek, External and internal magnetic fields at Mars: New results and upcoming missions, EGS/AGU/EUG Joint meeting, Nice, April 2003.

Separation of internal and external magnetic field sources from MGS satellite data alone could be improved using magnetic proxies. We discuss two approaches. The solar wind monitoring observations from ACE (Vennerstrom et al, 2002) can be extrapolated to Mars and can be compared with MGS observations over the northern hemisphere at times when ACE and Mars are both on the same Parker spiral field line. It is also possible to directly compare magnetic conditions at Earth with those at Mars when both are on the same field line.
Magnetic measurements onboard orbiters may allow achieving separation between internal and external sources. In addition to the already mentioned global variations, time-varying magnetospheric fields are probably also associated with the spatially- restricted mini-magnetospheres, especially in the Cimmeria region in the south; such a separation would for sure help in understanding the dynamics of these structures. The highest frequency signals are likely to be associated with spherics (Grimm, 2002), perhaps associated with dust storms.
There are several time-varying magnetic fields, which could be used as probes of the Martian subsurface from magnetic measurements onboard orbiters. Olsen (2002, and in preparation) and Tarits (personal communication) have shown evidence for global ionospheric fields, some of which most certainly vary on a daily basis. Finally, external field variations over the course of a solar cycle would allow probing into the Martian lower mantle. A prerequisite for the recognition of such signals would be a mission in the latter part of the decade. We hope that the MagNet experiment on NetLander, in conjunction with a simultaneously recording orbital magnetometer (as it was proposed in MEMOIRE project), will give us a means of separating temporal and spatial variations of the magnetic field.

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2003

Hulot G., C. Eymin, B. Langlais, Mioara Mandea and N. Olsen, Small-scale structure of the Geodynamo inferred from Ørsted and Magsat satellite data, Ørsted International Science Team-4, Copenhagen, September 23 - 27, 2002.

The main source of the Earth's magnetic field lies within the liquid core where the "Geodynamo" operates. The "Main Field" (MF) it produces dominates the large and medium scales of the field observed at the Earth's surface. We took advantage of the present Danish Ørsted and 1979/1980 US Magsat satellites, to identify and interpret MF variations over 20 years, going down to length scales previously inaccessible. At the core surface, we found these changes to be weak below the Pacific Ocean and strong at polar latitudes and in a region centred below South Africa. Core surface flows accounting for these changes are characterised by a westward flow remarkably concentrated in retrograde polar vortices and by an asymmetric ring within which prograde (retrograde) vortices are grouped and correlated with well-known highs (lows) in the averaged (400 years) historical MF. This pattern is analogous to the one seen in a large class of dynamo simulations, except for its azimuthal asymmetry. This asymmetric state may have often been reached in the past, which would account for several claimed characteristics of the paleomagnetic field. It could also be a state through which the Geodynamo goes before reversing.

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2002

Chambodut A., J. Schwarte, B. Langlais, H. Lühr and M. Mandea, The selection of data in field modeling, 4th Ørsted International Science Team Conference , Copenhagen, September 23 - 27, 2002.

The launch of the Ørsted and CHAMP high-precision geomagnetic field satellites has opened new horizons for a better understanding of the Earth's magnetic field. Geomagnetic field models based on spherical harmonic analysis (SHA) of geomagnetically quiet, night-time, vector and scalar measurements acquired on board the Ørsted and CHAMP spacecrafts were estimated by different groups of researchers. Because of the different parametrization factors in applying the SHA (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 discrepencies, we computed different field models based on different subsets (obtained using various selection criteria). We first evaluate the influence of the used geomagnetic indices on the external field. Second, we present different methods to select the night-side dataset. Finally, we compare our models with some previously published models (Langlais et al., 2002; Holme et al., 2002).

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2002

Langlais B., M. Purucker, and M. Mandea, The orientation of the magnetization in the martian lithosphere, AGU spring meeting, Washington, May 28-31, 2002.

Eos. Trans. AGU, 83(19), Spring Meet. Suppl., Abstract GP42A-05, 2002.

Using a previously described conjugate gradient technique to compute the magnetization of a equivalent source dipole mesh, we develop a new model of the Martian lithosphere magnetization using Mars Global Surveyor magnetic data acquired during the AeroBraking, Science Phase Orbit and Mapping Orbit phases of the mission. A special attention was carried out on the out-of-limit data screening. Our new model is compared to previously published ones in terms of fitting the observations. The major improvements with respect to the previous models comes from the use of the raw AB1/2 data instead of the AB-binned one. By comparing solutions derived on partial datasets we discuss about the orientation of the magnetization, which appears to be mostly vertical: assuming a pure vertical magnetization, predicted and observed field components are correlated up to 0.99, 0.98 and 0.96 for Br, Bt and Bp, respectively. Using our magnetization model we compute altitude-normalized maps for altitudes ranging between 200 and 400 km. The magnetic field is compared to other physical properties, such as the crater density, the gravity and the topography.

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2002

Langlais B., Use of multi-satellite datasets to model the lithospheric magnetic field in polar areas, AGU spring meeting, Washington, May 28-31, 2002.

Eos. Trans. AGU, 83(19), Spring Meet. Suppl., Abstract GP21A-02, 2002.

The launches of Ørsted (Feb. 1999), CHAMP (Jul. 2000) and SAC-C (Nov. 2000) provide a new opportunity for studying the magnetic field of the Earth. This mini-constellation provides for the first time multiple, simultaneous field measurements at satellite altitude, which can be considered as a small satellite observatory network. Among the many studies that can be done using the data of this network, I chose to analyse the lithospheric field behavior in polar areas, where the data geographical distribution is denser. Considering the data sample available for this special virtual session, I removed from the measurements the main field of nuclear origin. The residuals are used to determine what would be the magnetization of an equivalent source dipole distribution over the North and South pole areas and then to represent the field at a constant altitude. I present the results of these computations and discuss about the amplitude of the non-modeled field with respect to local time.

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2002

Langlais B., M. Mandea, and M. Purucker, Sur l'aimantation de la lithosphère martienne, Réunion des Sciences de la Terre, Nantes, April 9-12, 2002.

Réunion des Sciences de la Terre, Nantes, Abstract A-269, 2002.

En utilisant une méthode de gradient conjugué pour déterminer l'aimantation de dipôles équivalents répartis sur une grille uniforme, nous développons un nouveau modèle de l'aimantation de la lithosphère martienne en utilisant les données de la mission Mars Global Surveyor, acquises durant les trois phases d'aérofreinage, de science et de cartographie. L'amélioration principale de notre modèle par rapport aux autres modèles déjà publiés est liée à l'utilisation de données brutes; des phases d'aérofreinage. Nous calculons différents modèles, basés sur des informations partielles. La comparaison de ces modèles et de celui calculé avec toutes les informations nous permet de discuter de l'orientation des dipôles équivalents : ceux-ci apparaissent majoritairement radiaux. En effet, le champ magnétique créé par une telle distribution de dipôles est corrélé avec les observations, avec des coefficients de corrélation atteignant 0.99, 0.98 et 0.96 pour les trois composantes Br, Bt et Bp. Nous utilisons notre modèle de l'aimantation pour calculer le champ magnétique martien à des altitudes comprises entre 200 et 400 km. De telles cartes nous permettent de suivre à la fois l'évolution latérale et verticale du champ magnétique. La morphologie du champ magnétique est mise en parallèle avec d'autres informations, comme la topographie et les anomalies de gravité.

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2002

Hulot G., C. Eymin, B. Langlais, Mioara Mandea and N. Olsen, Small-scale structure of the Geodynamo inferred from Ørsted and Magsat satellite data, 1st CHAMP Science Meeting, Potsdam, January 22 - 25, 2002.

The main source of the Earth's magnetic field lies within the liquid core where the "Geodynamo" operates. The "Main Field" (MF) it produces is partly obscured by sources in the crust, the ionosphere and the magnetosphere. It is possible to try and identify the contribution of each source. Here we take advantage of the present Danish Ørsted and 1979/1980 US Magsat satellites, to identify and interpret MF variations over 20 years, going down to length scales previously inaccessible. At the core surface, we found these changes to be weak below the Pacific Ocean and strong at polar latitudes and in a region centred below South Africa. Core surface flows accounting for these changes are dominated by retrograde polar vortices, and by a remarkable asymmetric ring within which prograde (retrograde) vortices are grouped and correlated with well-known highs (lows) in the averaged (400 years) historical MF. This pattern is analogous to the one seen in a large class of dynamo simulations, except for its azimuthal asymmetry. This asymmetric state may have often been reached in the past, which would account for several claimed characteristics of the paleomagnetic field. It could also be a state through which the Geodynamo goes before reversing.

Top
2002

Purucker M., N. Olsen, D. Ravat, P. Schwintzer, and B. Langlais, Improving global crustal temperature, geologic variation, and lithospheric strength models utilizing the lithospheric magnetic field derived from CHAMP, 1st CHAMP Science Meeting, Potsdam, January 22 - 25, 2002.

Utilizing the magnetically quiet, night-time CHAMP F data from August 2000 to August 2001, we have begun developing improved models of the temperature and nature of the crust in a multi-fold interpretation integrated with the Standard Earth Magnetic Model approach. Our starting point is a model of remanent and induced magnetization from non-satellite observations (ocean floor ages, crustal thickness, and heat flow from tomographic models such as 3SMAC and Crust 5.1). We then compare the predictions from this model with the satellite magnetic field observations and iterate on the model using additional long-wavelength constraints from surface gravity and geology. Finally, we invert for shorter wavelengths using the satellite magnetic field observations. We have applied a preliminary version of this technique over North America (Purucker et al., GRL, in press) with the Oersted data and have shown how the technique can be used to define cratonic boundaries. We are now extending this work to other areas where the lithospheric variation models are not as mature.

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2002

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