New global vector magnetic anomaly maps derived from Magsat data

Abstract

New global maps of the vector magnetic anomaly field of Earth are derived from Magsat data. hnprovement on earlier published vector and scalar maps is achieved with the following procedure: (1) The noise level of vector measurements associated with the discontinuities of the in-flight sensor attitude determination is reduced by selecting the data. (2) A monthly model of the main field is computed to account for secular variation, using only quiet day data. (3) The time dependence of the ring current effect is taken into account using hourly values of the Dst index based on ground observations. (4) Sectorial indices based on ground observations are introduced to select profiles corresponding to low magnetic activity. These indices provide a better discrimination of quiet data than the global Kp index usually used for that purpose. (5) A time-and longitude-dependent dynamic model of the equatorial electrojet is derived and used to correct magnetic measurements at low latitudes. Anomaly maps of X and Z components are obtained using selected measurements within the reduced altitude range: 350425 km. These maps are compared with earlier Magsat anomaly maps, and their respective resolutions are discussed. These maps evidence large anomalies of lithospheric origin within the oceanic domain. In particular, the Z map displays a characteristic pattern of long-wavelength anomalies in these areas. It is first shown that some of these anomalies, particularly in the North Atlantic and Indian oceans, may be correlated with the age of the lithosphere. Then, a significant similarity between magnetic and geoid anomalies above ocean basins is evidenced. This suggests that long-wavelength oceanic magnetic anomalies may also be attributed to variations of the thickness of the magnetized layer associated with ocean floor topography. result, these maps have hardly been used to investigate the distribution of magnetic sources in the lithosphere or even to derive local surface magnetization models.Most previous studies of the anomaly field observed by Magsat (see, e.g., Magsat special sections of Geophysical Research Letters, 9, 1982, and Journal of Geophysical Research, 90, 1985) were based on scalar maps and focused on the interpretation and modeling of local short-wavelength anomalies. Indeed, since the main and surface anomaly fields are of comparable magnitude at wavelengths near 3000 km [Langel and Estes, 1982a], the anomaly field of lithospheric origin is poorly represented at these wavelengths by total field anomaly maps (i.e., maps of the projection of the vector anomaly field onto the direction of the main field).Yet the solution to many problems regarding the global distribution of magnetization in the lithosphere and its relations with other geophysical parameters would benefit from accurate global vector anomaly maps: Is the thickness of the magnetic layer related to the age (or the temperature) of the lithosphere or does the magnetization depend significantly on the composition of the crust? Is remanent magnetization a sign...

Equatorial electrojet at African longitudes: first results from magnetic measurements

“…The signature of this current was not at all observed at dawn. Cohen and Achache (1990) used measurements of the X and Z components to show that ionospheric currents were systematically observed to¯ow eastward at dusk, but westward at dawn, suggesting that a CEJ eect was, most of the time, measured at dawn (80% of the days). Langel et al (1993) pointed out the fact that the EEJ description obtained from MAGSAT data is biased by the necessary use of a main ®eld model, and concluded that the CEJ eect was not really observed in the MAGSAT dawn data.…”

“…Ionospheric soundings have allowed studies on the associated ionospheric electric ®elds as well as on electron densities and on ionospheric plasma instabilities (Balsley, 1973). Magnetic measurements onboard low-altitude satellites have also been used to study the EEJ on a global scale (Cain and Sweeney, 1973;Onwumechilli andAgu, 1980, 1981a), and to isolate its magnetic contribution from core and lithospheric ®elds (Cohen and Achache, 1990;Langel et al, 1993).…”

Error Analysis of Total Field Models Derived from POGS Data

“…By providing a dense and global coverage of the Earth's surface with vector measurements of the Earth's magnetic field from November 1979 until March 1980, the Magsat mission has enabled the derivation of main field models with high accuracy (e.g., Langel et al, 1982;Gubbins and Bloxham, 1985;Lowes and Martin, 1987;Bloxham and Jackson, 1992) as well as the compilation of accurate global high degree anomaly field models (e.g., Arkani-Hamed and Strangway, 1986;Cain et al, 1989;Schmitz et al, 1989;Cohen and Achache, 1990;Hamoudi et al, 1995). However, global field models based on scalar data only are plagued with a systematic error of significant amplitude known as the Backus effect (e.g., Backus, 1970;Stern and Bredekamp, 1975;Stern et al, 1980).…”

MAGSAT Vector and Scalar Anomalous Magnetic Fields over Russian-Indian Area.

“…To check up the reliability of obtained results we fulfilled the comparison our magnetic maps with the magnetic maps published before (Langel et al, 1982b;Agarual et al, 1986;Cohen and Achache, 1990;Singh and Rajaram, 1990). Such comparison showed that there is a general agreement with the global anomaly maps and with the regional maps given by Indian scientists.…”

“…At present the MAGSAT data are used extensively for the preparation and interpretation of the anomalous magnetic field maps. These investigations realized in two directions: the first direction is connected with the preparation of the global maps of the anomalous magnetic field (Langel et al, 1982b;Cain et al, 1984;Yanagisawa and Kono, 1985;Arkani-Hamed and Strangway, 1986;Cohen and Achache, 1990) and the second direction is connected with the preparation of the analogous maps for the separate regions. As to the first direction it is necessary to pay attention to the fact that global magnetic maps are depicted usual either in color or black-white versions without numerical values.…”