Quiet Ionospheric Currents and Earth Conductivity Profile Computed from Quiet-time Geomagnetic Field Changes in the Region of Australia

Abstract

Equivalent ionospheric source currents representing the quiet-day geomagnetic field variations were established for a half-sector. of the Earth that included Australia. The analysis used a spherical harmonic separation of the external and internal fields for the extremely quiet conditions existing in 1965. Month-by-month behaviour of the current system indicated a clockwise vortex source with a maximum of 12.8xl04 A in January and a minimum of 4.4xl04 A in June. The focus location shifted from about -32·5° geomagnetic latitude in summer to about - 30.0° in winter. The separated external and internal field coefficients were used to estimate the Earth's upper mantle electrical conductivity (J" at a depth d from about 250 to 350 km as (J" =0·00067 exp(O· 012 d) S m - I and from about 350 to 550 km as (J" = 0·0014exp(0·0088d) Sm- I .