Source field problems in northern parts of the Baltic Shield

Theoretical calculations using a stationary ionospheric electrojet model are presented for a resistivity model of interest in studies of the Baltic Shield. The Earth model includes the undistorted East European Platform resistivity-versus-depth model, and crustal and asthenospheric conducting layers. The results show that close to the source region, for periods > 15 min, the magnetic field depends only weakly on the Earth's conductivity. Accordingly, the resolution of apparent resistivity curves obtained by the horizontal spatial gradient (HSG) technique becomes poor at longer periods close to the source region. A comparison with experimental HSG apparent resistivity from Kiruna and Kevo in northern Scandinavia supports the existence of high conductivity in the lower crust. The sharp decrease of apparent resistivity at periods > 1 h may—within the limits of experimental error—be a result of the non-uniform source rather than an indication of a good conductor in the upper mantle.