A high degree model of the geomagnetic fielc is derived using an integral technique to extend coefficients beyond the limits allowable with least squares approaches. A previously derived model to n = 29 was first updated with new secular change data for the interval 1979 September-1980 June combined with the previously obtained analysis set of Magsat vector data. Residuals of a new selection of observed vector Magsat data were then analysed by solving for ionospheric-magnetospheric variations and removing their effect. The reduced B, components were then averaged over approximately 3" X 3" blocks of latitude and longitude, and coefficients derived using the Neumann method. These coefficients, when combined with those from the least squares solution, were seen to show significantly greater detail in the structure of the geomagnetic field which appeared to be realistic to n = 50.
The spectrum of a high degree spherical harmonic model of the geomagnetic field is analysed to compute the constants for the core and crustal field contributions. Using a noise estimate of 0.091 nT2 at the mean Magsat radius of 6791 km, the power reduced to the Earth's surface is found to be 9.66 X 10' (0.286)" nT2 for the core, and 19.1 (0.996)" nT2 for the crust. These values show half the crustal power extrapolated to n = 0 compared with a previously published n = 23 model, and a white noise depth of only 14 km below the mean surface. The core spectrum power is 30 per cent less than previously estimated and becomes flat 80 km below the core-mantle boundary. The crustal power level is an eighth of that of an estimate based on one-dimensional analyses of Project MAGNET survey lines. The point where the energy density of the core and crustal components become equal at the Earth's surface is n = 14.2.
A spherical harmonic expansion to degree and order 29 is derived using a selected magnetically quiet sample of Magsat data. Global maps representing the contribution due to terms of the expansion above n = 13 at 400 km altitude are compared with previously published residual anomaly maps and shown to be similar, even in polar regions. An expansion with such a high degree and order displays all but the sharpest features seen by the satellite and gives a more consistent picture of the high‐order field structure at a constant altitude than do component maps derived independently. The appendix is available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B83‐008; $2.50. Payment must accompany order.
Improved techniques for direct least squares analysis of the main geomagnetic field are introduced and tested with simulated data. One improvement involves adding secular change data to the least squares analysis (as well as the usual spatial data). As a result, the secular variation (SV), as observed at fixed magnetic observatories and repeat stations, directly adjusts the SV terms of the spherical harmonic coefficients, while spatial data adjust both the spatial and SV terms. Simulations show that SV errors can be reduced by factors of 2 or 3 by introducing SV as data. The second refinement involved obtaining uncorrelated spherical harmonic coefficient sets through employment of a uniform distribution of data by area or by a compromise of an area‐weighting scheme. The latter was shown to reduce high cross correlations between spherical harmonic coefficients by up to a factor of 3 in simulated tests. This technique is especially useful in analyses of data sets which are unevenly distributed. Justification is given for the area‐weighting scheme and also for the employment of evenly distributed data sets, as may readily be obtained in the case of satellite data. A third refinement, though used previously, that of incorporating declination data weighted by the horizontal intensity and inclination data weighted by total intensity, was shown to have merit on the basis of observed error distributions.
The appendix is available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document B83‐004; $2.50. Payment must accompany order. A combined spatial and secular variation model of the geomagnetic field, labeled M061581, is derived from a selection of Magsat data. Secular variation (SV) data computed from linear fits to midnight hourly values from 19 magnetic observatories were also included in the analysis but were seen to have little effect on the model. The SV patterns from this new model are compared with those from the 1980 IGRF and with those for 1970 computed by the DGRF and with the 1960 patterns computed using the GSFC(12/66) model. Most of the features of the M061581 are identical in location and level with those of the 1980 IGRF. Together they confirm that the reversals in sign of field change seen over Asia and North America between 1965 and 1975 are reverting to the pre‐1965 states. The M061581 model gives −32 nT/yr for the dipole decay rate, larger than the 70% increase already reported since 1965. This abnormally high value is interpreted as being a defect of the model because it appears to result from a much larger (−100 nT/yr) drop in field over the polar regions not indicated by the 1980 IGRF. This north polar decrease is shown to be of external origin as the result of a combination of the seasonal effect of the north polar ionospheric (counterclockwise) afternoon Sqp cell increasing in intensity from the beginning (November 1979) to the end (June 1980) of the Magsat data collection period, coupled with an enhancement of its effect as the orbit lowered from the 350– 550 km initial altitudes to near 200 km just prior to burnup. This experiment indicates that secular variation can be obtained from satellite data for intervals of less than a full year if corrections can be made for seasonal effects and that ‘annual snapshots’ of the field by a satellite would allow easy and accurate models of secular change without the use of any surface data.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.