Satellite Magnetic Field Data Research Articles

Satellite Magnetics Suggest a Complex Geothermal Heat Flux Pattern beneath the Greenland Ice Sheet

Geothermal heat flow is key to unraveling several large-scale geophysical systems, including the inner workings of the Greenlandic ice sheet, and by extension, the possibility of understanding the past and prior global climate. Similarly, it could provide insight into the paleo-trace of the Icelandic mantle plume, which in turn is integral in answering long-standing questions on the origin of mountains in western and eastern Greenland and in Norway. This study documents the results from an intra-scientific field approach, which combines geological, petrophysical, and satellite magnetic field data in a nonlinear probabilistic inversion. These results include Curie depths with associated uncertainties and Geothermal Heat Flux estimates. While baselines remain challenging to evaluate due to the strong nonlinearity of the problem posed, stress testing reveals a high robustness of the predicted spatial variations, which largely disagree with the classic straightforward northwest–southeast or east–west plume trace across Greenland. Instead, our results indicate a complex heat flux pattern, including a localized region with anomalously heightened heat flux near the origin of the Northeast Greenland Ice Stream.

Occurrence Rates of Electromagnetic Ion Cyclotron (EMIC) Waves With Rising Tones in the Van Allen Probes Data Set.

In Fourier time-frequency power spectrograms of satellite magnetic field data, electromagnetic ion cyclotron (EMIC) waves may feature discrete, rising tone structures that rapidly increase in frequency. Using data from the Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) fluxgate magnetometer, we conducted a statistical study of EMIC waves from September 2012 through June 2016. We compared the occurrence rates and spatial distributions for all EMIC waves with those for rising tone EMIC waves as a function of magnetic local time (MLT) and L shell, as well as a function of R XY and Z in solar-magnetic (SM) coordinates. Overall, EMIC waves occurred during 2.4% of the time period considered, but rising tone EMIC waves were only found during 0.2% of the time period considered. About 7%-8% of the minutes of orbital coverage with H+ or He+ band EMIC waves had rising tones. The regions of peak occurrence rates for H+ and He+ band waves, as well as waves with rising tones, were found in the noon and dusk sectors for 4<L<6. The preferred regions for H+ waves as a function of R XY and Z SM suggest an association with magnetospheric compressions near noon and interactions between plumes and the ring current near dusk. Peak occurrence rates for O+ band waves were found between 2<L<4 at all MLT, and over a wide range of L shells near dusk. No rising tones were found in the O+ band.

Analysis of Swarm Satellite Magnetic Field Data for the 2015 Mw 7.8 Nepal Earthquake Based on Nonnegative Tensor Decomposition

A non-negative tensor decomposition (NTD) approach has been developed to analyze the ionospheric magnetic field data of the Swarm Alpha and Charlie satellites for the Mw7.8 2015 Nepal earthquake. All available satellite data were analyzed regardless of geomagnetic activity. We used the amplitude time-frequency spectra of the two-satellite data to build third-order tensors and decomposed them into three components. One of these components seems to be more affected by seismicity. In particular, the cumulative number of anomalous tracks of this component displays accelerated growth that conforms to a Sigmoid fit from 60 to 40 days before the mainshock. Subsequently, until 10 days before the earthquake, it shows a weak accelerating trend that obeys a power-law behavior and then resumes linear growth after the mainshock. Moreover, the cumulative anomaly was indicated not to be caused by geomagnetic activity, solar activity or other non-seismic factors. An investigation of the foreshocks around the epicenter reveals that the cumulative Benioff strain also exhibited two accelerated growths before the mainshock, which is consistent with the cumulative result of ionospheric anomalies. In the first acceleration stage, seismicity appeared in the region surrounding the epicenter, and most of the ionospheric anomalies were offset away from the epicenter. During the second acceleration stage, some foreshocks occurred closer to or on the mainshock fault, and ionospheric anomalies also appeared near two faults around the epicenter. Furthermore, the correspondence between the ionospheric anomalies and the anomalies in different geo-layers can be explained by the lithosphere&#x2013;atmosphere&#x2013;ionosphere coupling model.

Signal and Noise Separation From Satellite Magnetic Field Data Through Independent Component Analysis: Prospect of Magnetic Measurements Without Boom and Noise Source Information

AbstractWe propose an application of the independent component analysis (ICA) to separate satellite‐induced time‐varying stray fields from magnetic field data obtained using onboard multiple magnetometers. The ICA is a method for estimating source signals at multiple sites so that the estimated source signals can become statistically independent of each other. Since stray field variations are statistically independent of external natural field variations, the ICA method is expected to separate the natural variations from stray fields. Thus, we applied the ICA to magnetic field data from the first Quasi‐Zenith Satellite, which has two triaxial fluxgate magnetometers, without using an extendable boom. First, we removed the long‐period trend from the original data to create detrended data. Then, we applied the FastICA algorithm to the detrended data and obtained six independent components (ICs). The stray fields were successfully separated into three ICs (noise ICs), and the natural signals were represented by the other three ICs (signal ICs). Finally, we restored the observed signals from the signal ICs, and confirmed that the natural phenomena variations were not altered by the processing step. We also proposed a selection method of the noise ICs using the C coefficient, which is the coefficient of the variance of the mixing vectors. There was a large difference in C between the ICs whose C coefficients are the largest third and fourth ones. Overall, these results demonstrate the possibility that the ICA method can support for boom‐less magnetic observations in future satellite missions.

Analysis of Swarm Satellite Magnetic Field Data Before the 2016 Ecuador (Mw = 7.8) Earthquake Based on Non-negative Matrix Factorization

In this paper, based on non-negative matrix factorization (NMF), we analyzed the ionosphere magnetic field data of the Swarm Alpha satellite before the 2016 (Mw = 7. 8) Ecuador earthquake (April 16, 0.35°N, 79.93°W), including the whole data collected under quiet and disturbed geomagnetic conditions. The data from each track were decomposed into basis features and their corresponding weights. We found that the energy and entropy of one of the weight components were more concentrated inside the earthquake-sensitive area, which meant that this weight component was more likely to reflect the activity inside the earthquake-sensitive area. We focused on this weight component and used five times the root mean square (RMS) to extract the anomalies. We found that for this weight component, the cumulative number of tracks, which had anomalies inside the earthquake-sensitive area, showed accelerated growth before the Ecuador earthquake and recovered to linear growth after the earthquake. To verify that the accelerated cumulative anomaly was possibly associated with the earthquake, we excluded the influence of the geomagnetic activity and plasma bubble. Through the random earthquake study and low-seismicity period study, we found that the accelerated cumulative anomaly was not obtained by chance. Moreover, we observed that the cumulative Benioff strain S, which reflected the lithosphere activity, had acceleration behavior similar to the accelerated cumulative anomaly of the ionosphere magnetic field, which suggested that the anomaly that we obtained was possibly associated with the Ecuador earthquake and could be described by one of the Lithosphere–Atmosphere–Ionosphere Coupling (LAIC) models.

Swarm Satellite Magnetic Field Data Analysis Prior to 2019 Mw = 7.1 Ridgecrest (California, USA) Earthquake

This work presents an analysis of the ESA Swarm satellite magnetic data preceding the Mw = 7.1 California Ridgecrest earthquake that occurred on 6 July 2019. In detail, we show the main results of a procedure that investigates the track-by-track residual of the magnetic field data acquired by the Swarm constellation from 1000 days before the event and inside the Dobrovolsky’s area. To exclude global geomagnetic perturbations, we select the data considering only quiet geomagnetic field time, defined by thresholds on Dst and ap geomagnetic indices, and we repeat the same analysis in two comparison areas at the same geomagnetic latitude of the Ridgecrest earthquake epicentre not affected by significant seismicity and in the same period here investigated. As the main result, we find some increases of the anomalies in the Y (East) component of the magnetic field starting from about 500 days before the earthquake. Comparing such anomalies with those in the validation areas, it seems that the geomagnetic activity over California from 222 to 168 days before the mainshock could be produced by the preparation phase of the seismic event. This anticipation time is compatible with the Rikitake empirical law, recently confirmed from Swarm satellite data. Furthermore, the Swarm Bravo satellite, i.e., that one at highest orbit, passed above the epicentral area 15 min before the earthquake and detected an anomaly mainly in the Y component. These analyses applied to the Ridgecrest earthquake not only intend to better understand the physical processes behind the preparation phase of the medium-large earthquakes in the world, but also demonstrate the usefulness of a satellite constellation to monitor the ionospheric activity and, in the future, to possibly make reliable earthquake forecasting.

The Origin of Observed Magnetic Variability for a Sol on Mars From InSight

AbstractDay‐night variations in the magnetic field at Mars have been previously observed at satellite altitudes. The InSight Fluxgate Magnetometer (IFG) has provided the first evidence for diurnal magnetic field variations at the martian surface. IFG data show diurnal variations with typical peak amplitudes of 20–40nT in the early morning to midmorning; the amplitude of the magnetic field varies over the first 389 sols of the mission and peaks between sols 50 and 100. Temperature variations, solar array currents, and lander activities all generate magnetic fields. Particularly, the first two of these also produce signals with clear diurnal variations. We first assess the IFG data calibration and conclude that temperature and solar array currents have only minimal effects on the variability we observe in the final calibrated magnetic field data. We use satellite magnetic field data and a Mars global circulation model to make predictions for the temporal evolution of wind‐driven fields in the ionosphere. Such fields vary due to seasonal changes in the ionization profile and the winds, and in the altitude range of the dynamo region, that is, the region in which electric currents can be produced. We find that the amplitude and seasonal variability of the surface magnetic fields are generally consistent with those predicted from wind‐driven currents. Moreover, a regional dust storm in the vicinity of the InSight landing site, which started around sol 45, might be responsible for the higher magnetic field amplitudes observed in the IFG data in the early part of the mission.

Constraints on the resistivity of the oceanic lithosphere and asthenosphere from seafloor ocean tidal electromagnetic measurements

SUMMARY The electromagnetic (EM) field generated by ocean tidal flow is readily detectable in both satellite magnetic field data, and in ocean-bottom measurements of electric and magnetic fields. The availability of accurate charts of tidal currents, constrained by assimilation of modern satellite altimetry data, opens the possibility of using tidal EM fields as a source to image mantle electrical resistivity beneath the ocean basins, as highlighted by the recent success in defining the globally averaged lithosphere–asthenosphere boundary (LAB) with satellite data. In fact, seafloor EM data would be expected to provide better constraints on the structure of resistive oceanic lithosphere, since the toroidal magnetic mode, which can constrain resistive features, is a significant component of the tidal EM field within the ocean, but is absent above the surface (in particular in satellite data). Here we consider this issue in more detail, using a combination of simplified theoretical analysis and 1-D and 3-D numerical modelling to provide a thorough discussion of the sensitivity of satellite and seafloor data to subsurface electrical structure. As part of this effort, and as a step toward 3-D inversion of seafloor tidal data, we have developed a new flexible 3-D spherical-coordinate finite difference scheme for both global and regional scale modelling, with higher resolution models nested in larger scale solutions. We use the new 3-D model, together with Monte Carlo simulations of errors in tidal current estimates, to provide a quantitative assessment of errors in the computed tidal EM signal caused by uncertainty in the tidal source. Over the open ocean this component of error is below 0.01 nT in Bz at satellite height and 0.05 nT in Bx on the seafloor, well below typical signal levels. However, as coastlines are approached error levels can increase substantially. Both analytical and 3-D modelling demonstrate that the seafloor magnetic field is most sensitive to the lithospheric resistance (the product of resistivity and thickness), and is more weakly influenced (primarily in the phase) by resistivity of the underlying asthenosphere. Satellite data, which contain only the poloidal magnetic mode, are more sensitive to the conductive asthenosphere, but have little sensitivity to lithospheric resistance. For both seafloor and satellite data’s changes due to plausible variations in Earth parameters are well above error levels associated with source uncertainty, at least in the ocean interior. Although the 3-D modelling results are qualitatively consistent with theoretical analysis, the presence of coastlines and bathymetric variations generates a complex response, confirming that quantitative interpretation of ocean tidal EM fields will require a 3-D treatment. As an illustration of the nested 3-D scheme, seafloor data at five magnetic and seven electric stations in the northeastern Pacific (41○N, 165○W) are fit with trial-and-error forward modelling of a local domain. The simulation results indicate that the lithospheric resistance is roughly 7 × 108 Ωm2. The phase of the seafloor data in this region are inconsistent with a sharp transition between the resistive lithosphere and conductive asthenosphere.

Identifying 3‐D Vortex Structures At/Around the Magnetopause Using a Tetrahedral Satellite Configuration

AbstractIdentifying vortices is the key to understanding the turbulence in plasma shear layers. This paper aims to provide general guidelines for identifying 3‐D vortex structures. Currently, no single precise definition of a vortex is universally accepted, despite the significance of vortices in fluid and plasma dynamics. Recently, various vortex identification methods using Galilean invariance have been proposed by numerous researchers. These methods are general for different fluid and plasma visualization applications. In the present paper, we describe how we have identified 105 vortex structures by applying these methods to Cluster data near the duskside of the magnetopause. Four sets of Cluster satellite magnetic field data are used to linearly approximate the magnetic field. We identify the 3‐D magnetic vortex structures by using various vortex identification criteria as follows: (i) the first criterion is Q‐criterion that defines vortices as regions in which the vorticity energy prevails over other energies; (ii) the second criterion is the λ2‐criterion that is related to the minus eigenvalue of the Hessian matrix of the pressure terms; and (iii) the third criterion called the geometrical line‐type method requires the existence of Galilean‐invariant vortex core inside the four Cluster tetrahedral regions. In reality, both Q‐ and λ2‐criteria are also related to Galilean invariance. The present analysis evidences that the geometrical line‐type method is more precise than the other two using Cluster satellite magnetic field data.

Structure of ULF Pc3 waves at low altitudes

Recent low‐orbiting observations at satellites with high‐accuracy magnetometers onboard (Oersted, CHAMP, and ST5) have provided a detailed picture of the Pc3 wave structure in the topside ionosphere. Pc3 waves were detected very clearly in the compressional component of the satellite magnetic field data, whereas on the ground their signature was found in the H component. The occurrence of a significant compressional component in Pc3 pulsations in the topside ionosphere was also evidenced by radio‐sounding measurements of ionospheric plasma oscillations. The following possibilities of ULF compressional disturbance excitation are considered: (1) an incident Alfvén wave generates an evanescent fast mode as a result of its interaction with the anisotropically conducting ionosphere; (2) transport of ULF wave energy from a distant source toward the ionosphere predominantly occurs by a fast mode. We estimate quantitatively the expected relationships between the Pc3 wave magnetic components above the ionosphere and on the ground produced by these different mechanisms and have derived simple analytical relationships between the compressional and ground signals for both mechanisms. Numerical modeling with the use of exact formulas has shown that these approximations work well over a wide range of wave scales. This model has been applied to the interpretation of Pc 3 waves observed by CHAMP in the upper ionosphere and by ground stations at midlatitudes. In general, the observed ratio between the compressional component in space and the ground signal corresponds better to the scenario of direct fast mode transmission to the ground.

Field-aligned currents onboard the Intercosmos Bulgaria-1300 satellite in comparison with modeled FAC

The statistical field-aligned current (FAC) distribution has been demonstrated by [Iijima, T., Potemra, T.A., 1976. The amplitude distribution of field-aligned currents at northern high latitudes observed by Triad. Journal of Geophysical Research 81(13), 2165–2174] and many other authors. The large-scale (LS) FACs have been described by different empirical/statistical models [Feldstein, Ya. I., Levitin, A.E., 1986. Solar wind control of electric fields and currents in the ionosphere. Journal of Geomagnetism and Geoelectricity 38, 1143; Papitashvili, V.O., Rich, F.J., Heinemann, M.A., Hairston, M.R., 1999. Parameterization of the Defense Meteorological Satellite Program ionospheric electrostatic potentials by the interplanetary magnetic field strength and direction. Journal of Geophysical Research 104, 177–184; Papitashvili, V.O., Christiansen, F., Neubert, T., 2002. A new model of field-aligned currents derived from high-precision satellite magnetic field data. Geophysical Research Letters, 29(14), 1683, doi:10.1029/2001GL014207; Tsyganenko, N.A., 2001. A model of the near magnetosphere with a dawn–dusk asymetry (I. Mathematical structure). Journal of Geophysical Research 107(A8), doi:10.1029/2001JA000219; Weimer, D.R., 1996a. A new model for prediction of ionospheric electric potentials as a function of the IMF. In: Snowmass’96 Online Poster Session; Weimer, D.R., 1996b. Substorm influence on the ionospheric convection patterns. In: Snowmass’96 Online Poster Session; Weimer, D.R., 2001. Maps of ionospheric field-aligned currents as a function of the interplanetary magnetic field derived from Dynamic Explorer 2 data. Journal of Geophysical Research 106, 12,889–12,902; Weimer, D.R., 2005. Improved ionospheric electrodynamic models and application to calculating Joule heating rates. Journal of Geophysical Research 110, A05306, doi:10.1029/2004JA010884]. In the present work, we compare two cases of LS FAC obtained from magnetic field measurements onboard the Intercosmos Bulgaria-1300 satellite with three models: two empirical [Tsyganenko, N.A., 2001. A model of the near magnetosphere with a down-dusk asymetry (I. Mathematical structure). Journal of Geophysical Research 107(A8), doi:10.1029/2001JA000219; Weimer, D.R., 2005. Improved ionospheric electrodynamic models and application to calculating Joule heating rates. Journal of Geophysical Research 110, A05306, doi:10.1029/2004JA010884] and one computer-based MHD-simulation in “The Community Coordinated Modeling Center” (CCMC) [Toth, G., et al., 2005. Space weather modeling framework: a new tool for the space science community. Journal of Geophysical Research 110, A12226, doi:10.1029/2005JA011126]. We found that the position of the measured FAC is close to the positions predicted by the models, but the measured density can be greater than the model FAC densities. We discuss the possible reasons for the observed discrepancy between the measured and modeled FACs.

Comprehensive study of ULF upstream waves observed in the topside ionosphere by CHAMP and on the ground

Abstract. Based on magnetic field measurements from the satellite CHAMP, a detailed picture could be obtained of the upstream wave (UW) distribution in the topside ionosphere. The low, near-polar orbit of CHAMP, covering all local times, allows the global distribution of this type of pulsation to be revealed. The observations from space are compared to recordings of the ground-based MM100 meridional array covering the latitude range 66° to 42° in magnetic coordinates. UWs show up very clearly in the compressional component of the satellite magnetic field data, whereas on the ground, their signature is found in the H component, but it is mixed with oscillations from field line resonant pulsations. Here we first introduce a procedure for an automated detection of UW signatures, both in ground and space data. Then a statistical analysis is presented of UW pulsations recorded during a 132-day period, centred on the autumn 2001 equinox. Observations in the top-side ionosphere reveal a clear latitudinal distribution of the amplitudes. Largest signals are observed at the equator. Minima show up at about 40° latitude. The coherence between ground and satellite wave signatures is high over wide latitude and longitude ranges. We make suggestions about the entry mechanism of UWs from the foreshock region into the magnetosphere. The clear UW signature in satellite recordings between −60° and 60° latitude allows for detailed investigations of the dependence on solar wind conditions. We test the control of solar wind speed, interplanetary magnetic field strength and cone angle on UWs. For the first time, it is possible to derive details of the Doppler-shift effect by modifying the UW frequency from direct observations. The results reconcile foreshock wave generation predictions with near-Earth observations.

Wavelet modelling of the spherical inverse source problem with application to geomagnetism

This paper is concerned with the modelling of ionospheric current systems from induced magnetic fields measured by satellites in a multiscale framework. Scaling functions and wavelets are used to realize a multiscale analysis of the function spaces under consideration and to establish a multiscale regularization procedure for the inversion of the considered vectorial operator equation. Based on the knowledge of the singular system a regularization technique in terms of certain product kernels and corresponding convolutions can be formed. In order to reconstruct ionospheric current systems from satellite magnetic field data, an inversion of the Biot-Savart law in terms of multiscale regularization is derived. The corresponding operator is formulated and the singular values are calculated. The method is tested on real magnetic field data of the satellite CHAMP and simulated data of the proposed satellite mission SWARM.

A new model of field‐aligned currents derived from high‐precision satellite magnetic field data

A model of field‐aligned currents is derived from high‐precision magnetic field measurements from the Ørsted and Magsat satellites, being parameterized by the interplanetary magnetic field strength and direction for summer, winter and equinox. The high‐precision data allow the model to be determined directly by a simple 2‐D curl technique combined with fitting of spherical harmonic functions. New elements of the model are: (a) the FAC patterns are determined separately for both polar regions, resolving the seasonal dependence of interhemispheric asymmetries, (b) the IMF ∼ 0, ground‐state patterns are also resolved; these elements are obtained for the first time. From the model, the total upward/downward currents have been determined for various IMF conditions. The ratio of the summer/winter currents is ∼1.35 and the equinox currents ∼1. The model allows FAC mapping for IMF |B| ≤ 12 nT, except during magnetic storms and substorms.

Downward field continuation in combining satellite and ground-based internal magnetic field data

When ground-based and satellite internal magnetic field data are available they are referred to different concentric spheres outside a sphere containing all the sources. Suppose that ground-based and satellite simultaneous data sets are treated independently. For both independent and simultaneous Spherical Harmonic Analysis (SHA) field models are calculated by the method of least squares via the set of Gauss coefficients { g n m ; h n m } where a definite truncation index N is used. Furthermore, there is supposed that the Gauss coefficients are correctly determined under these conditions so that no error discussion has to be included. By both of the models—the ground-based and the satellite SHA model—for all points outside the sphere containing all the sources the values of the internal field can be determined, but both models will give different results because for the same truncation index N there is a different approximation quality of the infinite series expansion SHA for the ground in comparison to satellite altitude. The mathematical reason is that the convergence quality of the infinite SHA expansion depends on the distance of the reference sphere from the source region to which the expansion is referred. The differences can be evaluated when the relevant functional systems for both SHA models are investigated for which the different set of Gauss coefficients are determined. If ground-based and satellite internal magnetic field data are combined it is necessary to pay attention to such differences. From the physical point of view it is clear that the physical content of the data depends on the distance of the reference sphere from the source region. Shorter wavelengths of the field are strongly attenuated at satellite altitudes. Therefore, it proves to be advantageous to include a downward field continuation when ground-based and satellite internal magnetic field data are combined although this is an ill-posed inverse problem from the mathematical point of view ( Anger, G., 1990. Inverse Problems in Differential Equations. Akademie, Berlin: Plenum Press, London; Huestis, S.P. Parker, R.L., 1979. Upward and downward continuation as inverse problems. Geophys. J. R. Astr. Soc. 57, 171–188). From the convergence quality of simultaneous SHA field models referred to related concentric reference spheres a regularizing criterion for a downward continuation of the Gauss coefficients had been derived which uses inherent mathematical characteristics of the SHA procedure [ Webers, W., 1998. On Combining Ground-based and Satellite Magnetic Field Data (Scientific Technical Report STR 98/21). Geo Forschungs Zentrum Potsdam (pp. 384–401); Webers, W., 1999. On determining reference fields from ground-based and satellite field data. Phys. Chem. Earth (A) 24, 943–947]. Applying this procedure to downward field continuation for the International Geomagnetic Reference Field DGRF 1990 demonstrates the different physical content of simultaneous field models at concentric spheres with their different structures. Moreover, mathematical upward and downward continuation in connection with simultaneous SHA field models based on simultaneous independent data sets at satellite altitude and at the ground will open new insights how internal and external constituents contribute to magnetic field records at different altitudes.

Automatic identification of large‐scale field‐aligned current structures

The study of large‐scale field‐aligned currents (LSFACs) has depended on visual inspection of data plots, which is often subjective and time‐consuming, and therefore limits the efficiency of analyses. The present paper reports a new procedure to automatically identify the spatial structure of LSFACs from satellite magnetic field measurements. The procedure is based on the concept of the first‐order B spline fitting with variable node positions, which may be envisioned as fitting line segments to a line plot. The fitting is made for the maximum variance component of magnetic variations in the plane perpendicular to the background magnetic field. If the distribution of LSFACs can be approximated as an infinite sheet, each slope of the plot corresponds to the crossing of a FAC sheet. The number of node points, which determines the number of FAC sheets, is one of the fitting parameters and is optimized for each orbit so that the Akaike information criterion (AIC) is minimized. Whereas other methods, such as a spherical harmonic fitting, seek to obtain two‐dimensional distributions of FACs from assembled data, the present method is basically the automation of the way we visually examine a plot of satellite magnetic field data, and it can be applied even to a single satellite pass. Therefore the procedure can be implemented to real‐time data processing and now‐casting. The procedure should also provide a powerful tool for data mining. Magnetic field data from the Defense Meteorological Satellite Program‐F7 (DMSP‐F7) were used for demonstration, and the present paper reports the initial results.

Concerning the local time asymmetry of Pc5 wave power at the ground and field line resonance widths

A statistical analysis of transverse wave activity in the Pc 5 frequency band using satellite magnetic field data from a 6 month interval shows a symmetric distribution along the geostationary orbit in the magnetosphere, whereas observations on the ground display an asymmetric distribution with regard to the noon sector with maximum wave activity on the morningside. As a possible cause of these different local time variations of the wave activity, the ionospheric screening effect is discussed, which depends on the radial scale lengths or resonance width of the waves as well as on their azimuthal wave number. It is suggested in this study that a local time variation of the resonance width of the waves is caused by a variation of the radial gradient of the plasma density along the geostationary orbit, leading to a slowly varying efficiency of the ionospheric screening effect with local time. Local time variations of the resonance width thus will be of equal importance to explain the observed asymmetry as, for example, local time variations of wave sources causing local time variations of the azimuthal wave number.

The missing dimension in Magsat and POGO anomaly studies

The development of a comprehensive model of the near‐Earth magnetic field in terms of sources in the core, ionosphere, and magnetosphere has allowed the isolation of heretofore unknown north‐south trending anomalies of lithospheric origin. Previously, along‐track filtering of satellite magnetic field data had been necessary in order to remove unmodeled magnetospheric and ionospheric magnetic fields. These fields change between adjacent passes by tens of nanoTeslas and have wavelengths which overlap the crustal spectrum. An example from Australia illustrates the additional information that can be extracted from the Magsat data set when this ‘missing dimension‧ is included. A north‐south trending low covers most of eastern Australia, in agreement with the aeromagnetic map of Australia. This feature is seen in both the dawn and dusk subsets collected from Magsat. The center of this low is roughly coincident with the Eromanga Basin but extends for almost 2000 km in a N‐S direction. The low corresponds to a region of almost featureless magnetic field on the aeromagnetic map. It is also consistent with an elevated geotherm previously proposed from studies of xenoliths in southeastern Australia. Filtering the new map in a north‐south direction reproduces the previous best estimate of the Australian satellite anomaly field. We expect that this comprehensive model will provide a new window into the magnetic field of the earth's lithosphere. The model continues to be enhanced.

Adjustment of UARS, POGS, and DE-1 Satellite Magnetic Field Data for Modeling of Earth's Main Field

In the absence of Magsat quality satellite magnetic field measurements, the use of data of lesser quality is sought. The DE-1 and UARS satellite magnetic field experiments were not designed for the purpose of modeling Earth's main field. The POGS satellite acquired data for modeling of Earth's main field, but at a lower accuracy than Magsat. All of these data were acquired with fluxgate magnetometers, which are subject to calibration drifts. On Magsat the fluxgate magnetometer was calibrated in-flight by comparison with a Cesium-Vapor scalar magnetometer. Such calibration is not possible on the satellites considered here, leaving uncertainty as to the accuracy of the resulting data. A formalism is developed to compare the data from these satellites with a field model derived from all other available data. This model included data from Magsat, so it is highly accurate at 1980. Its accuracy at the epochs of the satellites considered then depends upon the accuracy with which the field at 1980 can be extrapolated to future epochs. Adjustment of DE-1 data requires estimation of only two parameters for data spanning about 10 years. The resulting adjusted data are in good agreement with the model for those 10 years and exhibit residuals generally interpretable in terms of sources in the ionosphere and magnetosphere. Adjustment parameters of the POGS data vary substantially over the three years of available data. The resulting residuals indicate that the adjusted data set is suitable for main field modeling up to about degree 10, as used for the International Geomagnetic Reference Field. However, the resulting residuals are considered to be due mainly to error in measurement and not to geophysical sources. Adjustment of the UARS data is more difficult. The data used are not sufficient to resolve all of the adjustment-model parameters. If a priori information regarding the magnetic field from attitude torquer rods is included, the ambiguity is mostly resolved. Residuals after adjustment are of comparable magnitude and quality to those from POGS.

The search for crustal resources: MAGSAT and beyond

In the decade since global satellite magnetic field data have been available from MAGSAT, notable progress has been made in processing these data for purposes of mapping crustal anomalies. Several regional magnetic anomaly maps compiled using these new techniques (e.g. Kursk region, U.S.S.R.; central Africa; Kiruna, Sweden; and the U.S.A. midcontinent) provide insight into the nature and tectonic evolution of the crust that contribute to conceptual crustal models useful in regional resource exploration. A recent mail survey of geopotential-field specialists involved in resource exploration indicates interest in MAGSAT data and future satellite missions with improved resolution. It is apparent that magnetic anomalies derived from satellite observations can aid in the search for crustal resources.