The downward continuation of magsat crustal anomaly field over Southeast Asia

Achache, J.; Abtout, Abdeslam; Mouël, Jean‐Louis Le

doi:10.1029/jb092ib11p11584

Cited by 34 publications

(10 citation statements)

References 49 publications

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“…In practice one, therefore, has to employ special downward continuation (e.g. Achache et al 1987) or use a joint inversion (Ravat et al 2002b) technique in order to evaluate the field close to the Earth's surface. In summary, from the point of view of these various applications, especially the high-degree coefficients (=small-scale features) of our new models have to be as accurate and noise free as possible in order to be suitable for the outlined purposes.…”

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confidence: 99%

Earth's lithospheric magnetic field determined to spherical harmonic degree 90 from CHAMP satellite measurements

Maus

Rother

Hemant

et al. 2006

Geophysical Journal International

103

107

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S U M M A R YThe CHAMP magnetic field mission is providing highly reliable measurements from which the global lithospheric magnetic field can be determined in unprecedented resolution and accuracy. Using almost 5 yr of data, we derive our fourth generation lithospheric field model termed MF4, which is expanded to spherical harmonic degree and order 90. After subtracting from the full magnetic field observations predicted fields from an internal field model up to degree 15, an external field model up to degree two, and the predicted magnetic field signatures for the eight dominant ocean tidal constituents, we fit and remove remaining external fields and polar electrojet signatures in a track-by-track scheme. From a subset of least disturbed tracks, we estimate the MF4 model by least squares, damping ill-determined coefficients by regularization. The resulting MF4 model provides a good representation of the lithospheric field down to an altitude of about 50 km at lower latitudes, with reduced accuracy in the polar regions. Crustal features come out significantly sharper than in previous models. In particular, bands of magnetic anomalies along subduction zones become visible by satellite for the first time.

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confidence: 99%

Earth's lithospheric magnetic field determined to spherical harmonic degree 90 from CHAMP satellite measurements

Maus

Rother

Hemant

et al. 2006

Geophysical Journal International

103

107

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“…These authors have attributed this effect to the equatorial electro jet, an electric current system flowing along the equator in the E layer of the ionosphere, below the satellite. This effect was found to have opposite signs at dawn and at dusk Achache et al [1987]. pointed to this as evidence for the existence of a counter electro jet current at dawn[Gouin and Mayaud, 1967].…”

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confidence: 99%

New global vector magnetic anomaly maps derived from Magsat data

Cohen¹,

Achache²

1990

J. Geophys. Res.

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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...

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“…It can be seen on Figure 3 that the dawn and dusk maps obtained are very similar for both the vertical component (Z) and the north component (X) of the anomaly field indicating that time-dependent external field effects have been satisfactorily reduced. In particular, the long-wavelength signal associated with the equatorial electrojet [Yanagisawa and Kono, 1985;Achache et al, 1987] is not present in these maps. Figure 4 shows the vertical component (Z) computed separately from dawn and dusk maps of Figure 3 averaged over all longitudes.…”

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confidence: 86%

Long‐wavelength magnetic anomalies in the Caribbean: Plate boundaries and allochthonous continental blocks

Counil¹,

Achache²,

Galdéano³

1989

J. Geophys. Res.

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Vector magnetic anomaly maps are derived at satellite altitude from Magsat data over the Caribbean and Middle America. To minimize smoothing of the anomaly field caused by averaging the data over the whole altitude range of Magsat measurements, two maps are derived in two nonoverlapping altitude ranges: 350–420 and 420–550 km. A composite map of the vertical component of the anomaly field is then computed at ground level. This map displays large positive anomalies associated with fast subduction zones. These anomalies define discontinuities in the downgoing slab associated with seismic gaps. Positive anomalies are also consistently observed above allochthonous blocks of continental origin in the Caribbean area and suggest that the average magnetization is larger in the continental lithosphere. Elongated magnetic lows are observed above transform boundaries of the Caribbean plate. The Cayman Trough anomaly is consistent with magnetization of the order of 1 A/m in the upper mantle. Demagnetization of the lithosphere along the Bocono‐El Pilar fault system which bounds the Caribbean plate to the south may be accounted for by frictional heating along shear zones. The pattern of anomalies along the southern margin of the United States is related to its accretionary tectonic history. Various estimates of the vertically integrated magnetization of both oceanic and continental lithospheres are computed using these anomalies and provide constraints on the vertical distribution of lithospheric sources of Magsat anomalies.

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The downward continuation of magsat crustal anomaly field over Southeast Asia

Cited by 34 publications

References 49 publications

Earth's lithospheric magnetic field determined to spherical harmonic degree 90 from CHAMP satellite measurements

Earth's lithospheric magnetic field determined to spherical harmonic degree 90 from CHAMP satellite measurements

New global vector magnetic anomaly maps derived from Magsat data

Long‐wavelength magnetic anomalies in the Caribbean: Plate boundaries and allochthonous continental blocks

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