The Geomagnetic Spectrum For 1980 and Core-Crustal Separation

Abstract

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 new model for the (geo)magnetic power spectrum, with application to planetary dynamo radii

“…Using a field model based on MAGSAT measurements, Langel and Estes (1982) found a core radius of 3311 km for n = 2-12. A more elaborated SH model led to a depth of 80 km below the CMB, again omitting the n = 1 term (Cain et al, 1989).…”

Terrestrial Magnetism: Historical Perspectives and Future Prospects

“…The break at harmonic number (spherical harmonic degree) n c ≈ 15 has increased, and will probably continue to increase, as data improves, indicating that some of the intermediate wavelength signal is noise. From Cain et al (1989) the long wavelength part of the field, for mapping purposes say, as is the case for the IGRF, but scientific studies of the core field require something better.…”

Unusual behaviour of the IGRF during the 1945–1955 period

“…(3) makes it converge more slowly than does the series in (3). Besides, some other different factors proceeding the sum square of the spherical harmonic coefficients and different exponents of (a/b) are used (Cain et al, 1989). Displays of log M n (Joules) vs. degree n at the ground surface (r = a = 6371 km) and the CMB (r = b = 3480 km) for different epochs of 1900-2000 are given in Fig.…”