MAGSAT scalar anomaly map of South America

Ridgway, J. R.; Hinze, William J.

doi:10.1190/1.1442195

Cited by 28 publications

(13 citation statements)

References 16 publications

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“…14(a). The principal features of this map compare well to the MAGSAT scalar anomalies which Ridgway & Hinze (1986) produced at 350 km elevation in Fig. 14(b) by compositing the results of nine separate inversions of smaller, overlapping regions within the study area.…”

Section: Predictions For South Americasupporting

confidence: 61%

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Statistical prediction of satellite magnetic anomalies

Goyal

Frese

Hinze

et al. 1990

Geophysical Journal International

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S U M M A R YThe geological utility of satellite magnetic observations is limited by orbital altitude variations which may be as large as a few hundred kilometres. This study investigates the use of fast and elegant statistical procedures for altitude normalization and gridding of magnetic anomaly data as an alternative to more commonly used equivalent source inversion procedures involving computationally extensive and complex least-squares matrix methods.A standard statistical approach for gridding satellite magnetic anomalies is to recompute numerically averaged values from three-dimensionally distributed observations which are within two standard deviations of an initially determined averaged anomaly estimate. The errors of this procedure for geological analysis are investigated using orbital anomaly simulations of lithospheric sources over a spherical earth. The simulations suggest that numerical averaging errors constitute small and relatively minor contributions to the total error-budget of higher orbital estimates (2400 km), whereas for lower orbital estimates the error of averaging may increase substantially.A more complex statistical procedure involving least-squares collocation in 3-D is found to produce substantially more accurate anomaly estimates as the elevation of prediction is decreased towards the lithospheric sources. Moreover, 3-D collocation is computationally much more efficient and faster to apply than equivalent source inversion methods for altitude-normalizing and gridding magnetic anomaly data. Application of this procedure to MAGSAT magnetic observations of South America demonstrates its utility for producing accurately gridded magnetic anomalies at constant elevation for geological analysis.

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Section: Predictions For South Americasupporting

confidence: 61%

“…The NASA averaging procedure was applied to the orbital profiles of Ridgway & Hinze (1986) to obtain (2") magnetic anomalies at an average elevation of 405 km. This grid of (2") anomalies was then inverted on a point source grid with 4" spacing at 100 km below the earth's surface by least-squares methods.…”

Section: Predictions For South Americamentioning

confidence: 99%

Statistical prediction of satellite magnetic anomalies

Goyal

Frese

Hinze

et al. 1990

Geophysical Journal International

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“…The precise reason for the latter is still under investigation but the prime candidates for this effect are a nonuniform spread of data caused by the satellite having a 5‐day quasi‐synchronism and/or some residual contamination from the ring current. The latter effect should be quite small with along‐track‐filtered residuals because the process emulates the anti ring current filter used by previous researchers notably Ridgway and Hinze [1986].…”

Section: Signal Extraction Methodsmentioning

confidence: 84%

Equatorial electrojet from Ørsted scalar magnetic field observations

et al. 2003

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[1] Previous studies of the longitudinal variation of the local noon electrojet have yielded doubtful results either because of the poor data quality or because the local times of equatorial crossings occurred in the early morning or late afternoon. The recent launch of the Ørsted satellite in a near-circular orbit with slow drift in local time of equatorial crossing has provided the opportunity for researchers to study the electrojet more accurately. Most studies remove the main field using a spherical harmonic model and then search the daytime equatorial passes for the distinctive electrojet trough in total intensity. The present study examines the electrojet for two consecutive 6-month periods and consequently two local time ranges. Pure signal processing is used to remove the main field directly. The residuals are binned separately for night and day passes on a 1°by 1°grid to enhance the signal to noise ratio and are bin centered by a least squares fitted linear model to compensate for the variations in satellite altitude. Thereafter, for each period the compensated night and day binned values are subtracted from each other to produce a difference set. Global plots of the subsequently spatially filtered difference sets reveal an almost constant electrojet 1/e half width of 3°, as seen at satellite altitude apart from a region in the western Pacific. There are four maxima in the electrojet amplitude at 0°-30°E, 90°-120°E, 180°-220°E, and 260°-290°E in each local time range.

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“…Each Magsat profile was then visually examined for spurious activity; only profiles having nearly continuous signals were retained after despiking. The long-wavelength residuals of the ring current correction and very short-wavelength noise were then minimized with a 5"-40" bandpass filter (similar to Ridgway & Hinze 1986). The profiles were then grouped according to months (November to April) and altitude ranges (300-350, 350-400, 400-450, 450-500, 500-550 km), separately for dawn and dusk data sets.…”

Section: Initial Data Processingmentioning

confidence: 99%

Considerations of variations in ionospheric field effects in mapping equatorial lithospheric Magsat magnetic anomalies

Ravat

Hinze

1993

Geophysical Journal International

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SUMMARY Longitudinal, seasonal, and altitude‐dependent variability of magnetic fields is investigated in equatorial latitudes to determine their effect on the isolation of lithospheric Magsat magnetic anomalies. An estimate of ‘ionospheric effect’ was compiled by averaging the total intensity Magsat anomalies as a function of dip latitudes (called ‘dip‐latitude averages’) from dawn and dusk data sets grouped according to longitudes, time (months), and altitudes. Unanticipated seasonal variations were observed in dusk Magsat data over the Indian sector that may contribute to an improved understanding of the equatorial electrojet. The amplitudes of the dawn dip‐latitude averages are small in comparison to the dusk averages and they are of opposite sign as anticipated from the westward and eastward flowing currents at the dip equator at dawn and dusk time, respectively. However, longitudinal variation in the equatorial amplitudes of the dawn diplatitude averages is not entirely consistent with present knowledge of the electrojet field. In the past, dawn Magsat anomalies have been considered to be largely free of ionospheric fields. However, in the preparation of the lithospheric component maps, neglecting to remove the dawn dip‐latitude averages from dawn Magsat data set leaves conspicuous E‐W trending anomaly artifacts in certain regions of the world. Removal of both the dawn and the dusk dip‐latitude averages from their respective data sets substantially improves the agreement between the separately prepared dawn and dusk lithospheric component Magsat maps. Despite the agreement between the resultant lithospheric anomaly maps, dip‐latitude averages do appear to contain some contribution from the lithospheric fields. Thus, processing of the data with the aid of the dip‐latitude averages unavoidably removes a small degree of E‐W oriented lithospheric signal as well.

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MAGSAT scalar anomaly map of South America

Cited by 28 publications

References 16 publications

Statistical prediction of satellite magnetic anomalies

Statistical prediction of satellite magnetic anomalies

Equatorial electrojet from Ørsted scalar magnetic field observations

Considerations of variations in ionospheric field effects in mapping equatorial lithospheric Magsat magnetic anomalies

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