A magnetic profile around the world

An around-the-world profile of magnetic total field intensity has been analyzed. Local anomalies are almost totally absent from Lisbon through the Mediterranean Sea, across the Arabian peninsula, and along the coast of the Arabian Sea to Bombay. Magnetic sources appear to be shallow across the entire Pacific Ocean and in the vicinity of the mid-Atlantic ridge. The major sources under continental areas are much deeper. The Fourier coefficients decrease in amplitude very rapidly up to order 7; beyond that, the envelope of the coefficients slowly decreases, reaching a magnitude of approximately 1 γ at the 2000th harmonic. The harmonics are summed according to fixed wavelength-width intervals. This results in a bimodal distribution indicating crustal sources and core sources. If the single around-the-world profile studied here is typical of all great-circle profiles, it is concluded that spherical harmonic coefficients of order and degree greater than 10 are probably not required to describe the main part of the earth's field to within limits acceptable for world charts.