Semi-annual variation of the geomagnetic field
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. 1999
Received: 29 September 1998
Accepted: 21 April 1999
Published: 6 June 2014
Nighttime hourly mean values of D, H and Z (or X, Y and Z in a few cases) from 113 observatories for the interval 1964.0 to 1966.0 have been analyzed to determine the semi-annual variation. Results from the 84 observatories with dip latitudes between ±60° have been subjected to spherical harmonic analysis to determine the coefficients of the internal and external parts. Only those coefficients that are found to be significantly different from zero at the 5 per cent level have been included.
One of the main objectives is to obtain a reliable estimate, with confidence limits, of the internal/external ratio at a very low frequency for constraining estimates of the deep conductivity of the mantle. It is shown that a model that includes only the principal P 1 0 term can lead to a seriously misleading internal/external ratio.