QUIET-DAY IONOSPHERIC CURRENTS AND THEIR APPLICATION TO UPPER-MANTLE CONDUCTIVITY IN AUSTRALIA

This study concerns the use of selected geomagnetic field records to e stablish the 1990 quiet-day current system (Sq) for Australia and to u se the ionospheric current source of Sq for a determination of the Ear th's deep electrical conductivity. The primary data set came from a ch ain of eight, three-component magnetometer stations that was operated along a north-south line in central Australia, Additional records, nec essary for boundary conditions, were added to the data set. A regional spherical harmonic analysis (SHA) allowed the separation of the inter nal 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 degrees geomagnetic latitude of central Australia with a 5 degrees latitude variation between wint er and summer and a corresponding change from about 80 to 200 kA in st rength. A special transfer function allowed the computation of an equi valent conductivity-depth profile of central Australia from the paired external and internal coefficients of the SHA. A regression line thro ugh 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 th rough the mantle transition zone. Computations gave 0.18 S/m in the re gion of 800 km depth. Previous conductivity models for the upper mantl e beneath central Australia, although less specific in values, are con sistent with our profile. At depths greater than 500 km, the regressio n profile is in agreement with the conductivity distribution beneath t he Tasman Sea determined from seafloor magnetotellurics, although both measurements lack high resolution at such depths.