POGO Satellite Research Articles

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.

A comprehensive model of the quiet-time, near-Earth magnetic field: phase 3

SUMMARY The near-Earth magnetic field is caused by sources in the Earth’s core, ionosphere, magnetosphere, lithosphere and from coupling currents between the ionosphere and the magnetosphere, and between hemispheres. Traditionally, the main field (low degree internal field) and magnetospheric field have been modelled simultaneously, with fields from other sources being modelled separately. Such a scheme, however, can introduce spurious features, especially when the spatial and temporal scales of the fields overlap. A new model, designated CM3 (Comprehensive Model: phase 3), is the third in a series of efforts to coestimate fields from all of these sources. This model has been derived from quiet-time Magsat and POGO satellite and observatory hourly means measurements for the period 1960‐1985. It represents a significant advance in the treatment of the aforementioned field sources over previous attempts, and includes an accounting for main field influences on the magnetosphere, main field and solar activity influences on the ionosphere, seasonal influences on the coupling currents, a priori characterization of the influence of the ionosphere and the magnetosphere on Earth-induced fields, and an explicit parametrization and estimation of the lithospheric field. The result is a model that describes well the 591 432 data with 16 594 parameters, implying a data-to-parameter ratio of 36, which is larger than several popular field models.

Role of the ambient magnetic field B in the longitudinal variation of equatorial electrojet

A review of studies of longitudinal variations of equatorial electrojet (EEJ) intensity confirms evidence of the longitudinal variation from POGO satellites data. This shows major peaks of EEJ intensity at 100oE and 190oE with a subsidiary peak at 190oE. The connection between EEJ intensity and the ambient magnetic field B has been probed with regression and power law analyses. These reveal no statistically significant relationship between the intensity of EEJ and B, and their fittings of observations are poor. Comparison of the longitudinal profile of B along the dip equator with average longitudinal profile of the zonal electric field Ey at the dip equator proves that on the average Ey is not independent of longitude as was assumed by earlier workers. It has been suggested that Joule heating of the atmosphere which depends on the square of B produces perturbations of ionospheric drift velocity, and that the perturbations contribute to the longitudinal variation of EEJ intensity. Keywords: ambient magnetic field, equatorial electroject magnetic signature, longitudinal variation of equatorial electroject current. (Global Journal of Pure and Applied Sciences: 2002 8(4): 517-528)

Error control coding schemes in digital systems

The rare opportunity given by the unprecedented spatial and temporal coverage of POGO satellites data is exploited to present all possible cross sections of equatorial electrojet (EEJ) current in the group of papers. Here the all-longitude means at 7 hours centred on local noon and the daytime means at 36 longitudes, of the vertical distribution parameters of EEJ current density are provided. The altitude–local time cross section of EEJ current density has one contour cell peaking at (12 h, 106 km) while the altitude-longitude cross section of EEJ current density has three contour cells peaking at (1000E, 106 km), (1900E, 106 km) and (2900E, 106 km). In both cross sections the contour of a given current density extends farther from the altitude of peak current density when the peak current density at the centre of EEJ is larger. This accounts for the wavy nature of the altitude-longitude cross section. Attention is drawn to two rocket measurements that support this phenomenon. The following conclusions are also reached. (a) The continuous distribution of current density model reproduces the altitude distribution parameters of EEJ current density very well, (b) the altitude distribution parameters of EEJ current density in India and Peru are not significantly different and (c) The altitude distribution parameters of EEJ current density from rockets data and from POGO Satellites data are not significantly different and that (d) It is essential to take into account the lack of symmetry between the vertical distributions of EEJ current density below and above the altitude of EEJ peak current density. (Global Journal of Pure and Applied Sciences: 2002 8(3): 381-388)

Spatial and temporal distribution of ionospheric currents-4: altitude – local time and altitude – longitude cross sections of Equatoriall Electrojet current density

The rare opportunity given by the unprecedented spatial and temporal coverage of POGO satellites data is exploited to present all possible cross sections of equatorial electrojet (EEJ) current in the group of papers. Here the all-longitude means at 7 hours centred on local noon and the daytime means at 36 longitudes, of the vertical distribution parameters of EEJ current density are provided. The altitude–local time cross section of EEJ current density has one contour cell peaking at (12 h, 106 km) while the altitude-longitude cross section of EEJ current density has three contour cells peaking at (1000E, 106 km), (1900E, 106 km) and (2900E, 106 km). In both cross sections the contour of a given current density extends farther from the altitude of peak current density when the peak current density at the centre of EEJ is larger. This accounts for the wavy nature of the altitude-longitude cross section. Attention is drawn to two rocket measurements that support this phenomenon. The following conclusions are also reached. (a) The continuous distribution of current density model reproduces the altitude distribution parameters of EEJ current density very well, (b) the altitude distribution parameters of EEJ current density in India and Peru are not significantly different and (c) The altitude distribution parameters of EEJ current density from rockets data and from POGO Satellites data are not significantly different and that (d) It is essential to take into account the lack of symmetry between the vertical distributions of EEJ current density below and above the altitude of EEJ peak current density. (Global Journal of Pure and Applied Sciences: 2002 8(3): 389-398)

Spatial and temporal distribution of ionospheric currents-3: latitude- local time and latitude- Longitude across section of Equatorial Electrojet current density and intensity

A comprehensive analysis of the POGO satellites data of 1967 –1969 has enabled us to present the daytime means, based on 09 hr to 15 hr, at each of 36 longitudes, and the all-longitude means at each of 7 local time hours, for each of 9 parameters of the equatorial electrojet (EEJ) necessary for the cross sections. The latitude-local time cross sections of EEJ current density and intensity each displays three contour cells. The forward current contour cell peaks at (12h, 00.) and the return current contour cells peak at (12h, 5.130N) and (12h, 5.130S). The latitude-longitude cross sections of EEJ current density and intensity each displays nine contour cells. The three forward current contour cells peat at (00, 1000E), (00, 1900E) and (00, 2900E). The three return current contour cells to the north peak at (5.350N, 1000E), (5,050N, 1900E) and (5.300, 2900E), while the three to peak at (5.350S, 1000), (5.050S, 1900E), while the three to the south peak at (5.350S, 1000E), (5.050S, 1900E) and (5,300S, 290E). The return current spreads thinly over an area of about four times the area of the forward current. Consequently, the ratio of the peak return current density or intensity to the peak forward current density or intensity is only about one quarter. On the average the percentage ratio is –23.25 0.41. Arising from the decrease of EEJ with latitude while at the same time it increases everywhere as the peak current at the magnetic dip equator increases, the cross sections show that the contours extend farther in latitude when the peak current at the dip equator is larger. Key Words: latitude –local time, latitude. longitude cross sections, current density, and intensity inonospheric currents. (Global Journal of Pure and Applied Sciences: 2002 8(3): 361-370)

A study of local time and longitudinal variability of the amplitude of the equatorial electrojet observed in POGO satellite data

The local time and longitudinal variations of the amplitude of the equatorial electrojet (EE) are investigated using the POGO series satellite data. An iterative normalization scheme is developed to separate the variations of the amplitude of the EE with respect to longitude and local time. The resulting local time variation of the EE compares favorably with observatory based studies; however, the longitudinal variation is not well resolved. The altitude of the satellite is another factor that affects the amplitude of the EE. We use a simple linear scaling of amplitude of the EE with the inverse of the satellite altitude. An experiment including only the satellite passes below 450 km altitude indicates that there may be problems in correcting the data from higher altitudes. The reason behind the discrepancy between the results from the data limited to below 450 km and the full data set is not understood and requires further investigation.

Induction studies with satellite data

The natural variations of the Earth's magnetic field of periods spanning from milliseconds to decades can be used to infer the conductivity-depth profile of the Earth's interior. Satellites provide a good spatial coverage of magnetic measurements, and forthcoming missions will probably allow for observations lasting several years, which helps to reduce the statistical error of the estimated response functions. Two methods are used to study the electrical conductivity of the Earth's mantle in the period range from hours to months. In the first, known as the potential method, a spherical harmonic analysis of the geomagnetic field is performed, and the Q-response, which is the transfer function between the internal (induced) and the external (inducing) expansion coefficients is determined for a specific frequency. In the second approach, known as the geomagnetic depth sounding method, the C-response, which is the transfer function between the magnetic vertical component and the horizontal derivative of the horizontal components, is determined. If one of these transfer functions is known for several frequencies, models of the electrical conductivity in the Earth's interior can be constructed. This paper reviews and discusses the possibilities for induction studies using high-precision magnetic measurements from low-altitude satellites. The different methods and various transfer functions are presented, with special emphasis on the differences in analysing data from ground stations and from satellites. The results of several induction studies with scalar satellite data (from the POGO satellites) and with vector data (from the Magsat mission) demonstrate the ability to probe the Earth's conductivity from space. However, compared to the results obtained with ground data the satellite results are much noisier, which presumably is due to the shorter time series of the satellite studies. The results of a new analysis of data from the Magsat satellite indicate higher resistivity in oceanic areas than in continental areas. However, since this holds for the whole range of periods between 2 and 20 days, this difference probably is not caused purely by differences in mantle conductivity (for which one would expect less difference for the longer periods). Further studies with data from recently launched and future satellites are needed.

A major structural intersection in the basement of the Okavango Basin, NE of Tsumeb, Namibia, indicated by satellite magnetometry and other data

The Global Magnetic Anomaly Map, based on POGO satellite measurements (Regan et al., 1975), which proved to be very useful in the study of deep controls of mineralization in the western United States (Kutina, 1980, 1991) and central Africa (Kutina 1993), has been used to examine the region enclosing the large basins in the southern and southwestern parts of Africa. A narrow, N-S trending ridge of magnetic highs has been found to extend along the main axis of the Okavango Basin and is interpreted to reflect an important north-south trending structure in the basement. This ridge is interrupted in the area of its intersection with the projected course of a NE-trending structural lineament of the Damara Belt, along which a number of metallic deposits of Namibia, including Tsumeb, are located. The structural setting of this intersection indicates that it may be very favorable for concentration of metals, but its actual economic value depends on whether processes of reactivation have led to concentration or remobilization of metals from the basement into shallower depths of the Okavango Basin.

The Geomagnetic Field 1900-1995, Including the Large-Scale Field from Magnetospheric Sources, and the NASA Candidate Models for the 1995 Revision of the IGRF

This paper reports on a continuous representation of the main geomagnetic field of degree 13 for the 1900-1995 time period, including a degree 1 representation of the field of external origin, designated GSFC(S95). The model employs a cubic B-spline basis with equi-spaced knots for the temporal variation in the secular variation of the internal field. Hence, the temporal variation of the spherical harmonic coefficients is represented by integrals of cubic B-splines. In the derivation, a suite of different forms is utilized for representation of the external field: (1) GSFC(S95-a), in which the external terms are proportional to the annual averages of the as index, (2) GSFC(S95-s), in which the external terms are represented by unconstrained cubic B-splines, (3) GSFC(S95-sc), in which the cubic B-spline representation of the external field is constrained to be near the GSFC(S95-a) model for years prior to about 1940, and, (4) GSFC(S95-nx), in which there is no external field representation. The NASA candidate models for the 1995 revision of the IGRF are extracted from GSFC(S95-sc). Data sources include the Magsat and POGO satellites, observatory annual means, decimated land survey, marine total-field, aeromagnetic, and repeat data. Random data uncertainties are assigned by statistical binning procedures, while systematic error is accounted for via the correlated weight matrix procedure of Langel et al. (1989). The data are not sufficient to resolve all model parameters, and thus, regularization via quadratic penalty functions is employed. For the internal field this included minimizing the average of the square of the radial field secular variation and acceleration over the core-mantle boundary and through time. Comparison of the GSFC(S95-sc) model with the ufm1 model of Bloxham and Jackson (1992) for their common time span shows good general agreement, especially with respect to secular variation coefficient signatures and overall data statistics. The major differences are manifested in a better fitting of Magsat and POGO data by GSFC(S95-sc), but better fitting of early survey data by ufm1. This is expected and reflects the relative influence of the data and penalty function in the particular model. The external field of GSFC(S95-sc) exhibits a very prominent solar-cycle variation in the q10 coefficient, though with about a 2-year time lag. The internal field exhibits a well behaved Rn spectrum throughout the time span indicating sufficient constraints being applied to the poorly observed, high-degree secular variation. Finally, the westward drift synthesized for degrees 2 to 5 shows a 0.76 linear correlation with the length-of-day (lod) variations of Jordi et al. (1994) when the latter leads the former by 11 years.

The near-Earth magnetic field from magnetospheric and quiet-day ionospheric sources and how it is modeled

Early attempts to model fields from sources in the magnetosphere during magnetically quiet times were unsuccessful. This is attributed to the presence of fields from lithospheric sources and to deficiencies in the data distribution. The availability of data from the Magsat satellite allowed definitive determination of the first-degree field from magnetospheric sources. Temporal variation of those fields on time scales of hours to a few years was implemented by use of a proxy function thought to mimic that variation. The Dst index was chosen as the proxy function. Extension of determination of the first-degree magnetospheric field to earlier times, namely to 1900, is accomplished first by use of the annual average of the aa index as a proxy function and then by use of cubic B-splines as temporal basis functions. The model first-degree field is well correlated with the annual sunspot number, both in its general level changes and in its detailed year-to-year variation. The correlation coefficient of the year-to-year variation of the first-degree, zero order, external term with the annual sunspot number is 0.77 when the external term lags the sunspot number by 2 years. Another extension of the results made possible by the Magsat data is simultaneous determination of a model of the fields from the magnetosphere, the ionosphere, and internal to the Earth. In this case, data from the Magsat and POGO satellites and magnetic observatory hourly mean and annual mean data were analyzed together. Though still in an experimental stage, these models are shown to fit all the data closely and to be able to readily isolate the fields according as the source is in the magnetosphere, ionosphere, or Earth.

An assessment of long-wavelength magnetic anomalies over Canada

The reliability of the long-wavelength portion (> 300 km) of the magnetic field over Canada, as represented by the national aeromagnetic anomaly database compiled by the Geological Survey of Canada (GSC), is assessed by comparison with two independent data sets: a high-altitude country-wide survey carried out by the former Earth Physics Branch (EPB) and data from the MAGSAT and POGO satellite missions. The different altitudes at which each data set was measured (300 m, ~4 km, and ~400 km), and their different resolution and time span of observations allow a determination of the integrity of selected wavelength bands in each data set. The (upward-continued) EPB and MAGSAT–POGO fields compare well for wavelengths of 300–2500 km. The GSC data show significant differences to the former, indicating that the levelling and merging of several hundred individual surveys has degraded the longer wavelength components of the magnetic field. Replacing the GSC wavelength components >300 km with those from the EPB field produces a magnetic data set containing more dependable information within the largest possible waveband.

Global Variations of the POGO electrojet parameters during the solstices.

A method of analysis has been used to obtain electrojet parameters from the POGO Satellites' data during the solstitial seasons. This yields enormous values about 432 each, of electrojet's halfwidth w, peak eastward intensity J0, and total eastward current I+, -covering the entire 360° longitudes round the earth, and within 6 hours of local day time in the December and June solstitial months of the years 1968 and 1969 respectively. The daytime all-longitude averages of these parameters for the combined solstices are: (226±11)km for w, (207±28) Akm-1 for J0 and (51±7)×103 A for I+. This second coverage of the earth, with a new set of data, shows that J0 and I+ vary considerably with longitude, unlike w, which remains fairly constant throughout the globe. Interestingly, we find that, apart from the expected maximum in J0 and I+ around longitude 280°E, subsidiary maxima also occur at about 100°E, and 190°E, in confirmation of the equinoctial results of ONWUMECHILI and AGU (1981). It is suggested that the longitudinal variations in J0 and I+, may arise from the longitudinal differences in conductivities, wind systems and possibly changes in electric field at electrojet altitudes.

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.

Estimation of lower crust magnetization from satellite derived anomaly field

Various lines of evidence point to the lower crust as the source of the long-wavelength magnetic anomaly field measured by the POGO and Magsat satellites. Using seismically determined lower crust thicknesses and equivalent source inversion of the satellite anomaly data, magnetization for the lower crust for much of the United States has been calculated. The average magnetization for two hundred sixty-six 150 × 150 km areas is 3.5 A/m with a standard deviation of 1.1 A/m. These values are consistent with laboratory measurements of mafic-ultramafic rocks expected in the lower crust, and in agreement with previous estimates of lower crust magnetization based on long-wavelength aeromagnetic data. Average lower crust thickness for the same areas is 18.2 km ( σ = 6.4 km). Thus, over large regions, it appears that variation in magnetization and variation in magnetic layer thickness contribute almost equally in causing the anomaly field variation at satellite altitude.

Crustal geomagnetic field: two-dimensional intermediate-wavelength spatial power spectra

Two-dimensional Fourier spatial power spectra of equivalent magnetization values are presented for a region that includes a large portion of the western United States. The magnetization values were determined by inversion of POGO satellite data, assuming a magnetic crust 40 km thick, and were located on an 11 × 10 array with 300 km grid spacing. The spectra appear to be in good agreement with values of the crustal geomagnetic field spatial power spectra given by McLeod and Coleman (1980) and with the crustal field model given by Serson and Hannaford (1957). The spectra show evidence of noise at low frequencies in the direction along the satellite orbital track (N-S), indicating that for this particular data set additional filtering would probably be desirable. These findings illustrate the value of two-dimensional spatial power spectra both for describing the geomagnetic field statistically and as a guide for diagnosing possible noise sources.

Regional North American gravity and magnetic anomaly correlations

Summary. Correlation of gravity and magnetic anomalies combined with other geological and geophysical data is useful for enhancing the quality of geological interpretation of potential anomaly fields. Maps produced by equivalent point source inversion are used to investigate visual-spatial correlations of surface free-air gravity and POGO satellite magnetic anomalies and regional heat flow and tectonic data for North America and adjacent marine areas. A quantitative analysis of regional potential field anomaly correlations at satellite elevations is also considered utilizing Poisson’s theorem in a moving-window linear regression between derivatives of the anomalous gravity and magnetic fields. An inverse relationship is observed between long-wavelength gravity and magnetic anomalies over continental terrain. Negative gravity and positive magnetic anomalies are areas characterized by relatively thick crust and high magnetization. An example is a prominent magnetic high which corresponds to a trend of gravity minima extending from the Anadarko Basin to the Cincinnati Arch. Negative magnetic and positive gravity anomalies characterize thinner crust and regions of higher heat flow such as the Cordillera of North and Central America and specifically the Yellowstone geothermal region. Although gravity and magnetic anomalies over oceanic areas show poor correlation, the sign of the statistical correlation generally is positive. 1 Crustal correlation considerations The utility of long-wavelength gravity anomalies, covering several degrees of surface area, for geological analysis is well recognized. Magnetic anomalies which have significant energy in wavelengths of several hundred kilometres also have been identified as originating from within the Earth’s crust and perhaps the uppermost mantle. The geological interpretation of these regional anomalies is hindered by the effect of anomaly superposition and source ambiguity which is inherent to the analysis of potential fields. An approach to minimizing these limitations, especially in continental terrains, is to evaluate the correlation between anomalous gravity and magnetic fields. The basis of correlation analysis is the hypothesis, commonly validated in continental areas, that variations in lithology and physical properties of the crystalline crust are reflected

Influence of gravity field uncertainties on the results from Pogo and Magsat geomagnetic surveys

Previous studies have shown several instances where airborne magnetic data, extrapolated to satellite altitude, differ from data obtained by the POGO and MAGSAT satellites. In an effort to explain this difference we have examined errors in the coefficients representing the Earth’s gravitational field which could produce satellite positional variations and therefore would account for the observed differences. Our results indicate that while the magnitude of the orbital errors are in a range that could account for the amplitude of these observed variations, the frequency of the quasi‐periodic orbital error is too large to explain the localized nature of the observed differences. Hence, uncertainties in the coefficients of the gravity model cannot explain the observed phenomenon. In a separate, but related computation, large crustal gravity anomalies have also been examined and shown to be insufficient to produce the desired magnitude of orbital perturbation.

A closer examination of the reduction of satellite magnetometer data for geological studies

Global and regional studies of the data abstracted from the POGO satellites have demonstrated the utility of satellite magnetometer measurements in geological studies. However, detailed examination of these data reveals that there exists a considerable amount of noise resulting from external fields and data reduction techniques. Such noise inhibits more detailed studies with the POGO data and could similarly affect the utility of data from the Magsat mission. Correlation of POGO satellite data with ground observatory records and geomagnetic indices was employed to identify the existence of external field effects and document the data reduction problems. Significant effects of the Sq current system, including the equatorial electrojet, are shown to be still in the data despite the local time screening. Such effects are totally in agreement with theoretical models of this current system and should be considered in Magsat data reduction, as the theoretical model predicts that the Sq current may at times have significant effects even at the dawn/dusk local times of the Magsat orbit. The state of inflation and deflation of the magnetosphere is vividly portrayed in the POGO measurements and is well correlated with the Dst index. The ring current correction applied to the POGO data was seen to be of varying quality depending upon the state of the magnetosphere and the length of satellite track data reduced. These effects combined with the irregular distribution of data from the individual POGO satellites can result in pseudo anomalies as evidenced by a comparison of satellite data reduced in a fashion to minimize them with those previously published over the same area.

Comparison of the POGO satellite and ground measurements of the magnetic field of the equatorial electrojet

The magnitude of the equatorial electrojet signature, S, being a measure of its magnetic field at the POGO satellite and normalized to 400 km altitude, is plotted against its contemporaneous corresponding horizontal magnetic field, X, at a station on or very close to the dip equator under the recording satellite. It is found that they correlate very well; that the ratio k of X to S is 2.28±0.53 and is the same in all regions. Because the vertical component, Z, of the field at Ibadan is largely due to the electrojet, we have also compared it with S, resulting in a good value of 2.03 for k. We also computed a theoretical value of k = 2.58 from Onwumechili's three-dimensional model of the electrojet and this value agrees with and buttresses the observed value of k. It is argued that imperfect removal of the worldwide Sq is most likely to be the major cause of the scatter; that peculiarities of crustal and upper mantle conductivity are not likely to wholly account for the scatter nor for any regional differences in the value of k. However, the previously discordant results have been reconciled and regional differences in the values of k are no longer significant.