CHAMP Satellite Data Research Articles

Distribution of Magnetic Anomalies in China and Adjacent Areas Derived from CHAMP Satellite Data

AbstractBased on the new and higher degree geomagnetic model POMME‐4.2S from CHAMP satellite data, we have calculated satellite magnetic anomalies and their vertical gradients at altitude 400 km in China and adjacent areas. The distributions of seven magnetic components are shown. In addition, we also compared the impact of the truncation level on the magnetic anomalies and analyzed the relationship between the anomalies and the lithospheric structure. The results indicate that the ∆X(∆Y) magnetic anomaly foci appear on the south (west) and north (east) sides of that of ∆Zin the main magnetic anomaly areas of the Sichuan, Tarim and Songliao basins. The configuration of the magnetic anomaly distribution is determined when the spherical harmonic model is up to 90 degrees, and only a minor modification to the magnetic anomaly is required for higher degrees. The anomaly and its vertical gradient are varying synchronously in the strong magnetic anomaly areas. The satellite magnetic anomaly has no direct corresponding relations with the topographic change, distribution of faults, depth of the Moho and the thickness of the lithosphere. However, this anomaly is connected with the depth of the Curie isotherm interface and the thickness of the magnetic layer, which is positive (negative) in the areas of deeper (shallower) Curie isotherm surface and thicker (thinner) magnetic layer.

Electrical conductivity in the lower mantle: Constraints from CHAMP satellite data by time-domain EM induction modelling

Time-domain EM induction is used to obtain 1-D conductivity structure in the Earth using seven years of vector magnetometer measurements by the CHAMP satellite. An optimal model is obtained by means of quasi-Newton minimization with regularization. Sensitivity of data to deep Earth conductivity is checked by limited grid search of the model space in the vicinity of the optimal model. While regularization plays a considerable role in constraining the conductivity model, data misfit poses upper limits on the lower mantle and D ″ conductivity at 2 S/m, and 10 S/m, respectively.

Characteristics of the Secular Variation and Secular Acceleration Distributions of the Main Geomagnetic Field from the CHAMP Satellite

AbstractUsing the new geomagnetic model POMME‐4.2S derived from the CHAMP satellite data, we have analyzed the characteristics of the distributions of seven secular variation components and three secular acceleration components , Ÿ , in China and the world. We also compared the differences of the distributions between the secular variation models POMME‐4.2S and CGRF‐SV (Chinese Geomagnetic Reference Field‐Secular Variation). The results suggest that the quadrupole plays the main role in the geomagnetic secular variation, and the octupole occupies the largest portion in the contribution to the secular acceleration. The decrease of global secular variation is much larger than its increase in 2004.0 A.D. in the background of a decreasing trend of the geomagnetic field. The secular acceleration changes more at middle and low latitudes than that at higher latitudes. Moreover, the negative and positive magnetic anomalies of Ÿ and components form several alternating areas along longitudes, and these two kinds of anomalies of component are nearly symmetrical to the equator. An evident relationship is present for the magnetic anomaly distributions of the three secular acceleration components , Ÿ , . That is, the foci of magnetic anomalies exist in the south and north, and Ÿ in the east and west sides of the foci of magnetic anomalies. The amounts of change of the secular variation and acceleration in China are small and their distributions are relatively uniform.

Radio vision of the vertical structure of the layers and a study of radio-wave propagation conditions in the atmosphere using high-stability satellite signals

From an analysis of the CHAMP (Germany) and FORMOSAT-3 (Taiwan–USA) satellite data it follows that the second-order time derivative of the eikonal (eikonal acceleration) and the Doppler frequency shift are two most important parameters indispensable for the radio vision of layers in the atmosphere and the ionosphere. Measurements of the temporal evolution of the Doppler shift permit one to study the vertical structure of the atmosphere under the condition of its spherical symmetry. Analysis of the amplitude and phase of interrelated variations in the eikonal acceleration and radio-wave intensity permits one to detect the layers in the atmosphere and the ionosphere. Eikonal variations are converted into refraction attenuation variations, which allows the integral absorption to be determined with the refraction effect on the radio-wave intensity cancelled out. This is necessary for measurements of the water-vapor density and gas minorities during multifrequency radio-occultation sounding along the satellite-to-satellite paths. The obtained results can be of common value for other remote-sounding paths, as well.

Curie isotherm map of Indian subcontinent from satellite and aeromagnetic data

Abstract Estimating depth to the bottom of the magnetic crust (synonymously called the Curie isotherm) on a regional scale from long wavelength magnetic anomalies requires that large areas of survey data be used for the calculations. There is still no consensus on the minimum survey area required to arrive at a reliable estimate of the Curie isotherm depth. From the available aeromagnetic data over India, depth to the Curie isotherm is estimated using spectral techniques. We found that the significant spectral maxima required to calculate the depth to the Curie isotherm existed only for 4° × 4° blocks in southern peninsular India up to 18 °N latitude and for 5° × 5° blocks for the rest of the region in Central India. As aeromagnetic data coverage over India is incomplete, we also calculate the magnetic crustal thickness from the lithospheric model of the CHAMP satellite data for the whole country, using an iterative forward modeling approach. The calculated Curie isotherm is shallow in the mobile belts and deeper in the cratons in both the derivations. As the Curie isotherms calculated from the two data sets collected at very different altitudes using totally different techniques, match reasonably well, it lends credence to the methodology adopted. The derived Curie isotherm depth map is in accordance with the basic structural trend of the major tectonic units within the Indian subcontinent. A comparison is made of the calculated Curie isotherm depth with Moho depths along available DSS profiles over India and it is found that the Curie depth is generally shallower than the Moho depth implying that it possibly represents a thermal boundary rather than a compositional change. Further, we find that high magnitude earthquakes are associated with high gradients in Curie depth.

GRIMM: the GFZ Reference Internal Magnetic Model based on vector satellite and observatory data

SUMMARY In this paper the new GFZ Reference Internal Magnetic Model (GRIMM) is presented. The model has been derived from nearly 6 yr of CHAMP satellite data and 5 yr of observatory hourly means. At high latitudes, full vector satellite data are used at all local times which allows a separation between, on one hand, the fields generated by ionosphere and field aligned currents, and, on the other hand, the fields generated in the Earth's core and lithosphere. This selection technique leads to a data set without gaps during the polar summers resulting in a core field model that has an unprecedented time resolution. The modelled static core field, secular variation and lithospheric field are all in good agreement with previously published magnetic field models. Order five B-splines are used to model the variation in time of the core field. The energy in the secular acceleration has, therefore, a smooth behaviour in time and increases continuously from 2003.5. Mapping the field acceleration from 2001.5 to 2005.5 reveals its rapid and complex evolution over this time period at the Earth's surface. Due to the applied regularization technique, the acceleration energy in spherical harmonics 6–11 is significantly larger than for other models and we show that such a spectrum is acceptable.

An improved geomagnetic data selection algorithm for global geomagnetic field modelling

A spherical harmonic degrees 60, global internal field model is described (called BGS/G/L/0706). This model includes a degree 15 core and piecewise-linear secular variation model and is derived from quiet-time Ørsted and Champ satellite data sampled between 2001.0 and 2005.0. For the satellite data selection, a wide range of geomagnetic index and other data selection filters have been used to best isolate suitably quiet magnetospheric and ionospheric conditions. Only a relatively simple, degree one spherical harmonic, external field model is then required. It is found that a new ‘Vector Magnetic Disturbance’ index (VMD), the existing longitude sector A indices, the auroral zone index IE, and the polar cap index PC are better than Kp and Dst at rejecting rapidly varying external field signals at low, middle, auroral and polar latitudes. The model quality is further enhanced by filling spatial and temporal gaps in the quiet data selection with a second selection containing slightly more disturbed data. It is shown that VMD provides a better parametrization than Dst of the large-scale, rapidly changing, external field. The lithospheric field model between degrees 16 and 50 is robust and displays good coherence with other recently published models for this epoch. BGS/G/L/0706 also shows crustal anomalies consistent with other studies, although agreement is poorer in the southern polar cap. Intermodel coherency reduces above about degree 40, most likely due to incompletely filtered signals from polar ionospheric currents and auroral field aligned currents. The absence of the PC index for the southern hemisphere for 2003 onwards is a particular concern.

Space-time modeling of the main magnetic field by combined methods of spherical harmonic analysis and natural orthogonal components

A simple method for obtaining a space-time model of the main magnetic field from the high-precision satellite survey data is described. At the first stage, the CHAMP satellite data for one-day interval are expanded into the spherical harmonics with constant coefficients. This yields a set of daily mean spherical harmonic models (DMSHM) over the survey interval of a few years. At the second stage, the coefficients of this set are used as source data for expansion into the natural orthogonal components (NOC). It is shown that the terms of the NOC series decrease rapidly, and the accuracy of the space-time model of the main geomagnetic field over the time interval under discussion is not worse than the accuracy of the models obtained by traditional methods.

Investigation of a secular variation impulse using satellite data: The 2003 geomagnetic jerk

Observatory monthly means provide an excellent opportunity to study the temporal changes of the magnetic field at a given location. Unfortunately, the uneven distribution of the present observatory network makes it difficult to determine the global field change pattern. Recently, we have developed an approach to extract satellite monthly means at a regular network of “virtual observatories” at 400 km altitude, based on CHAMP magnetic measurements. Using monthly means for 2001–2005 from those “virtual observatories” we investigate the space–time structure of the short-period variation of the Earth's magnetic field by means of a Spherical Harmonic Expansion, followed by a separation into external (magnetospheric) and internal part. This allows, for the first time, to study the secular variation globally and directly from satellite magnetic data. Analyzing the time series of the magnetic field at the “virtual observatories” as well as those of the spherical harmonic expansion coefficients, we detect a secular variation impulse (an abrupt jump in the second time derivative of the magnetic field) in the CHAMP satellite data during the first months of the year 2003. The jerk occurred simultaneously in the northern and southern hemispheres in a rather limited area near 90° E, with maximum jerk strength at about ± 30° latitude, a region also characterized by a strong secular acceleration (second time derivative of the magnetic field). We show that the 2003 geomagnetic jerk is not worldwide in occurrence and that there is an evidence for this event in the length-of-day variation.

A global lithospheric magnetic field model with reduced noise level in the Polar Regions

Due to persistent ionospheric current systems at high latitudes, satellite magnetic measurements have high noise levels in the Polar regions. Consequently, small‐scale lithospheric features are much more difficult to resolve there than at lower latitudes. Using linear combinations of spherical harmonics (SH), we define localized functions which are band‐limited to a given maximum degree. Thus, a model's SH resolution can be varied over the globe. We produce a lithospheric field model from CHAMP satellite data which resolves features to SH degree 90 at low latitudes, but only to SH degree 60 in the Polar regions. The new model can be downward continued from satellite altitude to the Earth's surface without generating spurious high‐latitude anomalies.

Modeling of the geomagnetic field secular variation based on the satellite surveys

The accuracy of the geomagnetic field secular variation (SV), obtained using different methods of analysis based on different data sets, has been estimated. A comparison of different SV models (developed by different groups of researchers) as candidates for compiling the IGRF 10th generation is the example of such an estimation. These models were obtained based on the Oersted and CHAMP satellite data. In the present work these models are compared in order to estimate the modeling algorithms and the effect of initial data selection on modeling results. To solve this problem, several SV models have been constructed based on the independent algorithms and data sets. The differences between the candidate models using different data proved to be of the same order. A comparison of the spatial power spectra indicated that all models constructed using data subjected to the frequency selection considerably differ from the models the data of which were not selected.

CHAOS-a model of the Earth's magnetic field derived from CHAMP, Ørsted, and SAC-C magnetic satellite data

SUMMARY We have derived a model of the near-Earth magnetic field (up to spherical harmonic degree n = 50 for the static field, and up to n = 18 for the first time derivative) using more than 6.5 yr of high-precision geomagnetic measurements from the three satellites Orsted, CHAMP and SAC-C taken between 1999 March and 2005 December. Our modelling approach goes in several aspects beyond that used for recent models: (i) we use different data selection criteria and allow for higher geomagnetic activity (index Kp ≤ 2o), thus we include more data than previous models; (ii) we describe the temporal variation of the core field by splines (for n ≤ 14); (iii) we take magnetometer vector data in the instrument frame and co-estimate the Euler angles that describe the transformation from the magnetometer frame to the star imager frame, avoiding the inconsistency of using vector data that have been aligned using a different (pre-existing) field model; (iv) we account for the bending of the CHAMP optical bench connecting magnetometer and star imager by estimating Euler angles in 10 day segments and (v) we co-estimate degree-1 external fields separately for every 12 hr interval. The model provides a reliable representation of the static (core and crustal) field up to spherical harmonic degree n = 40, and of the first time derivative up to n = 15.

Modeling the lithospheric magnetic field over France by means of revised spherical cap harmonic analysis (R‐SCHA)

We have recently proposed the revised spherical cap harmonic analysis (R‐SCHA) modeling technique. The new mathematical functions represent faithfully the spatial variations of potential fields in a restricted area. In this paper, we tackle the inverse problem and outline the efficiency of the new basis functions with respect to real magnetic data. Processing simultaneously repeat stations, observatory, aeromagnetic, and CHAMP satellite data provides our first vector lithospheric field model over France, which extends from surface to 500 km of altitude. The magnetic field is represented with a minimum horizontal spatial representation of 40 km at the mean Earth radius. The magnetic lithospheric map consistency is confirmed with a comparison to known geological features. The model variation with altitude also suggests that the major French magnetic anomaly, the Paris basin anomaly, is produced by a deep‐rooted geological structure. These results demonstrate the superiority of regional modeling over global modeling for delineating small‐scale details in the lithospheric field. In view of forthcoming satellite missions, like Swarm, the revised spherical cap harmonic analysis method will help to accurately represent the lithospheric field for more detailed geological interpretations.

Global lithospheric magnetic field modelling by successive regional analysis

Abstract Present lithospheric field models, like the MF4 and CM4, are produced by least squares estimation using spherical harmonic basis functions with global support. Accounting for the different properties of magnetic data at low and high latitudes, a method that can take regional differences into account is proposed. Using four years of CHAMP satellite data, a detailed lithospheric magnetic field snapshot is obtained at 400 km altitude over the entire sphere by stitching together a dense coverage of regional models. The individual forward models computed on a quasi-regular grid over the Earth are then transformed to spherical harmonics by direct integration. Despite the stitching procedure, the long wavelength lithospheric features are correctly reproduced and small scale features are well resolved. Without regularization, the resulting model is stable to spherical harmonic degree 56. In addition to accounting for regionally varying noise levels, the proposed technique is also well suited to deal with incomplete data coverage issues when combining satellite with near surface data. The method could therefore make an important contribution to one of the main goals of the Swarm mission: to close the spectral gap between satellite and near-surface magnetic surveys.

Improving and testing the empirical equatorial electrojet model with CHAMP satellite data

Abstract. The longitudinal variation of the Equatorial Electrojet (EEJ) intensity has been revised including data from the equatorial station of Baclieu (Vietnam), where an unexpected enhancement of the EEJ magnetic effects is observed. The features of this longitudinal variation were also obtained with the CHAMP satellite, except in the Pacific and Atlantic Oceans, where no ground level data points were available.The EEJ magnetic signatures recorded on board the CHAMP satellite have been isolated for 325 passes in different longitude sectors around local noon. The results have been compared with the EEJ magnetic effects computed using the Empirical Equatorial Electrojet Model (3EM) proposed by Doumouya et al. (2003). The modeled EEJ magnetic effects are generally in good agreement with CHAMP observed EEJ magnetic signatures.

First scalar magnetic anomaly map from CHAMP satellite data indicates weak lithospheric field

Satellite magnetic anomaly maps derived by different techniques from Magsat/POGO data vary by more than a factor of 2 in the deduced strength of the lithospheric magnetic field. Here, we present a first anomaly map from new CHAMP scalar magnetic field data. After subtracting a recent Ørsted main and external field model, we remove remaining unmodeled large‐scale external contributions from 120° track segments by subtracting a best‐fitting uniform field. In order to preserve N/S trending features, the data are not filtered along‐track. Direct integration of the spherically gridded data yields the final degree 14–65 spherical harmonic expansion of the total intensity anomaly at the mean satellite altitude of 438 km. Apart from enhanced long wavelength features and a smoother general appearance, our initial map is strikingly similar to one of the lower strength Magsat/POGO maps.