Magsat Satellite Data Research Articles

An overlapping additive Schwarz preconditioner for the Laplace-Beltrami equation using spherical splines

We present an overlapping domain decomposition technique for solving the Laplace---Beltrami equation on the sphere with spherical splines. We prove that the condition number of the additive Schwarz operator is bounded by $O\left(H^2/h^2\right)$ , where H and h are the sizes of the coarse and fine meshes, respectively. In the case that the degree of the splines is even, a better bound $O\left(\max_{1\leq k \leq J}\left(1+H_k/\delta_k\right)\right)$ is proved. Here J is the number of subdomains, H k is the size of the kth subdomain, and ? k is the size of the overlap of the kth subdomain. The method is illustrated by numerical experiments on large point sets taken from magsat satellite data.

A domain decomposition method for solving the hypersingular integral equation on the sphere with spherical splines

We present an overlapping domain decomposition technique for solving the hypersingular integral equation on the sphere with spherical splines. We prove that the condition number of the additive Schwarz operator is bounded by O(H/δ), where H is the size of the coarse mesh and δ is the overlap size, which is chosen to be proportional to the size of the fine mesh. In the case that the degree of the splines is even, a better bound O(1 + log2(H/δ)) is proved. The method is illustrated by numerical experiments on different point sets including those taken from magsat satellite data.

An overlapping additive Schwarz preconditioner for interpolation on the unit sphere with spherical radial basis functions

The problem of interpolation of scattered data on the unit sphere has many applications in geodesy and Earth science in which the sphere is taken as a model for the Earth. Spherical radial basis functions provide a convenient tool for constructing the interpolant. However, the underlying linear systems tend to be ill-conditioned. In this paper, we present an additive Schwarz preconditioner for accelerating the solution process. An estimate for the condition number of the preconditioned system will be discussed. Numerical experiments using MAGSAT satellite data will be presented.

Long-term trends in paleointensity: The contribution of DSDP/ODP submarine basaltic glass collections

The Deep Sea Drilling Project and the Ocean Drilling Program have been collecting fresh appearing submarine basaltic glass from the world’s oceans for over three decades. This glass has proved nearly ideal for estimating paleointensity variations of the Earth’s magnetic field. We compile here data for 726 paleointensity experiments from six publications on paleointensity using DSDP/ODP glass. We also include new data for an additional 225 specimens. These were obtained through the so-called “IZZI” paleointensity experiment of [Tauxe, L., Staudigel, H., 2004. Strength of the geomagnetic field in the cretaceous normal superchron: new data from submarine basaltic glass of the troodos ophiolite. Geochem. Geophys. Geosyst. 5 (2), Q02H06, doi:10.1029/2003GC000635] whereby infield-zerofield steps are alternated with the zerofield-infield steps to enhance quality assessment of the resulting data. The entire collection of data from 951 experiments was prepared for uploading to the MagIC data base ( http://earthref.org), including original measurements, interpretations, and useful metadata. Excellent results were obtained throughout the depth ( > 1400 mbsf) and age (0–160 Ma) range sampled. DSDP/ODP glass data are compared with published paleointensity data meeting minimal acceptance criteria from the time interval 1–160 Ma. Paleolatitudes were estimated for all cooling units in a self-consistent manner for use in calculating virtual axial dipole moments. We conclude: (1) There is about a 20% difference in mean values between the SBG and the lava flow data ( 48 ± 36 and 57 ± 29 ZAm 2 respectively). The difference is caused by the fact that there are more higher values in the lava flow data than in the SBG data set rather than a difference in the minimum values. Lava flows cooling over a periods of days to months can account for the discrepancy. (2) The positive relationship between polarity interval length and average paleofield intensity first hypothesized by [Cox, A.V., 1968. Lengths of geomagnetic polarity intervals. J. Geophys. Res. 73, 3247–3260] is supported by data compiled here. The Brunhes data (for which we have only a minimum estimate for polarity interval length) are consistent with a long polarity interval, suggesting that instead of racing toward reversal [Hulot, G., Eymin, C., Langlais, B., Mandea, M., Olsen, N., 2002. Small-scale structure of the geodynamo inferred from oersted and magsat satellite data. Nature 416, 620–623], we could instead be in the midst of a long stable polarity interval. (3) Because the average value appears to be a function of polarity interval length, it is probably not useful to calculate a mean value. Nonetheless, it appears that most of the time (apart from the Brunhes and the longest polarity intervals), the average dipole moment is substantially less than the present day value as suggested by [Juarez, T., Tauxe, L., Gee, J.S., Pick, T., 1998. The intensity of the earth’s magnetic field over the past 160 million years. Nature 394, 878–881].

Extending comprehensive models of the Earth's magnetic field with Ørsted and CHAMP data

SUMMARY A new model of the quiet-time, near-Earth magnetic field has been derived using a comprehensive approach, which includes not only POGO and Magsat satellite data, but also data from the Orsted and CHAMP satellites. The resulting model shows great improvement over its predecessors in terms of completeness of sources, time span and noise reduction in parameters. With its well separated fields and extended time domain of 1960 to mid-2002, the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997. Because all sources are coestimated in a comprehensive approach, intriguing north–south features typically filtered out with other methods are being discovered in the lithospheric representation of the model, such as the S Atlantic spreading ridge and Andean subduction zone lineations. In addition, this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection. The F-region currents, through which the satellites pass, are now treated as lying within meridional planes, as opposed to being purely radial. Results are consistent with those found previously for Magsat, but an analysis at Orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite. Besides the model, a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset. This has proven very useful in diagnosing the cause of peculiar artefacts in the Magsat vector data, which seem to suggest the presence of a small misalignment bias in the vector magnetometer.

Small-scale structure of the geodynamo inferred from Oersted and Magsat satellite data.

The 'geodynamo' in the Earth's liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the Earth's surface. Here we use data from the currently operating Danish Oersted satellite, and from the US Magsat satellite that operated in 1979/80, to identify and interpret variations in the magnetic field over the past 20 years, down to length scales previously inaccessible. Projected down to the surface of the Earth's core, we found these variations to be small below the Pacific Ocean, and large at polar latitudes and in a region centred below southern Africa. The flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs (and retrograde vortices with lows) in the historical (400-year average) magnetic field. This pattern is analogous to those seen in a large class of numerical dynamo simulations, except for its longitudinal asymmetry. If this asymmetric state was reached often in the past, it might account for several persistent patterns observed in the palaeomagnetic field. We postulate that it might also be a state in which the geodynamo operates before reversing.

A comparison of surface magnetic data and MAGSAT data over Italy and its surroundings

A new composite map of magnetic surface (MAGSURF) anomaly for Italy and its adjacent region has been derived from sea and ground surveys on the basis of an improved definition of the Italian Magnetic Reference Field and its temporal change. Spectral analysis of the MAGSURF anomaly field shows a regional scale energy in the wavelength range 145–500 km, due to the presence of various crustal sources. The regional MAGSURF anomaly map shows poor correlation with the corresponding MAGSAT scalar anomaly map, whose power spectrum reveals wavelength components in the range 300–700 km. The MAGSURF data are upward-continued to an elevation of 100 km for comparison with MAGSAT satellite data, downward-continued to the same altitude of 100 km and low-pass filtered for wavelengths larger than 500 km. Upward-continued surface data and downward continued satellite data show good morphological similarity for wavelengths in the range 300–500 km at an altitude of 100 km. Satellite data permit the characterization of magnetic signatures due to sources located in the middle-lower crust of Sardinia, Lombardia, Molise and Dalmatia, while further surface regional anomalies are connected with upper crust bodies of Mid Tyrrhene and East Sicily.

Magnetic Anomaly Fields Determined from Gradient Measurements at Stratospheric Altitudes and from Magsat Satellite Data.

The magnetic anomaly fields (MAF) obtained from the data of gradient magnetic measurements at stratospheric altitudes along transcontinental routes, passing over the Russian territory, and from the Magsat satellite data are investigated. The stratospheric gradient measurements are shown to be more suitable for separating and interpreting regional magnetic anomalies of crustal sources as compared to the data obtained at aircraft and satellite altitudes. It is also shown that, at stratospheric altitudes, one should use the device for magnetic field measurements that contains three sensors spread uniformly along the vertical line within four kilometers. This allows us to obtain the rate of MAF change in the stratospheric layer, which makes it possible to study in more detail the geometry of sources and to estimate with high precision (about 1 km) the locations of positive and negative zones of the first (and second) vertical derivatives of MAF. In general, the stratospheric gradient surveys successfully supplement satellite ones.

Magnetovariational sounding of the Earth using observatory and MAGSAT satellite data

Deep magnetovariational soundings have been carried out using data from the geomagnetic observatory net and the MAGSAT satellite. Complex apparent resistivities have been estimated from satellite data by two independent methods. The joint results have been interpreted assuming spherical symmetry of the Earth's layers. The global estimation of the electrical structure of the Earth's mantle has confirmed the existence of a layer with high conductance at a depth of approximately 600–700 km. The nature and possible causes of this layer are discussed.

Laboratory simulation of the large‐scale Birkeland current system in the polar region with northward interplanetary magnetic field

The large scale Birkeland current system in the polar region has been studied by using the UCR‐T1 space simulation facility (diameter 1.3 m, length 11 m). Intense plasma flow simulating the solar wind interacts with the terrella magnetic field which simulates the planetary dipole field. The northward interplanetary magnetic field (BIMF) is simulated by an externally applied uniform magnetic field throughout the interaction chamber. Magnetic field measurements above the polar cap show that for the northward interplanetary magnetic field, a stable well‐developed W pattern in the transverse component of magnetic field, prevails for the entire period of the interaction. These results have a remarkable similarity with the space observations obtained from the Magsat satellite data. The W pattern can be interpreted as an evidence for a large‐scale Birkeland current system which may be associated with the field line reconnection in the magnetotail.

Canadian geomagnetic reference field 1985.

The new technique of spherical cap harmonic analysis has enabled a model to be constructed of both the main geomagnetic field and its secular variation, that applies to Canada and its adjacent areas over a time range of 25 years. The technique permits the optimum utilization of data over only a portion of the earth's surface, while ensuring that the curl and divergence of the total magnetic field vector are identically zero. With this technique, proper consideration can be taken of altitude variation, a particularly important factor when dealing with satellite data. The effect of crustal anomalies on the determination of the secular variation (SV) model was eliminated by fitting SV independently of main field, and then integrating the SV model. Main field data at epoch 1985.0 were derived by updating Canadian aeromagnetic and Magsat satellite data by means of that integrated SV model. A spherical cap harmonic residual model was constructed from the updated main field data after subtraction of the 1980 International Geomagnetic Reference Field (IGRF 1980) extrapolated to 1985.0. Merging this residual model for 1985.0 and the integrated SV model provided a model with maximum spatial index of 16 for the main field terms at epoch (temporal degree 0) and of 4 for the integrated secular variation terms (temporal degrees 1 to 3), over a spherical cap of half angle 30°. The vector sum of the residual model and the IGRF 1980 at 1985.0 is called the Canadian Geomagnetic Reference Field 1985, and applies from 1960 to 1985 with extrapolation possible to 1990. This reference field has been used to produce the Magnetic Charts of Canada for epoch 1985.0.

Acquisition of long wavelength magnetic anomalies pre-dates continental drift

Using the low altitude MAGSAT satellite data, NASA recently published a new global magnetic map of anomalies. Using this new accurate map and paleomagnetic reconstructions of Gondwanaland, we point out that long wavelength anomalies were continuous in continents that are presently separated. Thus, continental drift would not significantly affect the geometry of these deep-source anomalies. Accordingly, the magnetization of the rocks responsible for these anomalies should mainly be induced.

Pogo and Pangaea

Long wavelength magnetic anomalies of crustal origin derived from the POGO and MAGSAT satellite data often display a strong continuity across the now-rifted continental margins when the continents are reassembled into Pangaea. These anomalies predate the breakup of the supercontinent and represent major blocks whose crustal properties are broadly similar even though those blocks are no longer contiguous.

Magnetic anomaly maps from 40°N to 83°N derived from MAGSAT satellite data

Stringent selection criteria using Kp indices, local magnetic activity estimates, and visual intercomparisons, followed by a statistical screening procedure have enabled the production of preliminary scalar and vector component magnetic anomaly maps derived from Magsat data at northern high latitudes, in spite of frequent external field disturbances. Comparisons between the maps and with other magnetic maps and geological information confirm their general validity.