Quiet-day ionospheric currents and their application to upper mantle conductivity in Australia
© 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. 1998
Received: 10 December 1996
Accepted: 10 December 1997
Published: 6 June 2014
This study concerns the use of selected geomagnetic field records to establish the 1990 quiet-day current system (Sq) for Australia and to use the ionospheric current source of Sq for a determination of the Earth’s deep electrical conductivity. The primary data set came from a chain of eight, three-component magnetometer stations that was operated along a north-south line in central Australia. Additional records, necessary for boundary conditions, were added to the data set. A regional spherical harmonic analysis (SHA) allowed the separation of the internal and external field contributions to the Sq variations. Mapping of the equivalent ionospheric current from the external field showed that the Sq contour focus passed near the —30° geomagnetic latitude of central Australia with a 5° latitude variation between winter and summer and a corresponding change from about 80 to 200 kA in strength. A special transfer function allowed the computation of an equivalent conductivity-depth profile of central Australia from the paired external and internal coefficients of the SHA. A regression line through the conductivity estimates gives a profile that starts at 0.025 S/m for a depth of 130 km, rising gradually to about 0.045 S/m at 250 km, then steepens to 0.11 S/m at 360 km and rises moderately to 0.13 S/m at 470 km near the base of the upper mantle. No data were obtained through the mantle transition zone. Computations gave 0.18 S/m in the region of 800 km depth. Previous conductivity models for the upper mantle beneath central Australia, although less specific in values, are consistent with our profile. At depths greater than 500 km, the regression profile is in agreement with the conductivity distribution beneath the Tasman Sea determined from seafloor magnetotellurics, although both measurements lack high resolution at such depths.