This work describes the month‐by‐month behavior of the equivalent ionospheric current systems derived from spherical harmonic analyses of the quiet time geomagnetic field daily variations in 1965 at selected observatories representing the three southern global half‐sector regions separately encompassing South America, Africa, and Australia. These external Sq current patterns were vortices having mid‐latitude foci with midday summertime amplitudes reaching 16.2×104 A above the midnight level. The wintertime amplitudes were about 10×104 A smaller. At low latitudes there was a large intrusion of the opposite hemisphere external Sq current system into the wintertime hemisphere at prenoon hours, displacing the primary current vortex to later postnoon hours. The behavior of the southern hemisphere external currents were found to be seasonally similar to those of the northern hemisphere for the same year. The quiet year behavior was compared to the results for the 1958 active year determined earlier by Matsushita. The winter‐to‐summertime increase in focus current was found to be similar in amplitude for the 2 years. The active year summertime and equinoctial current focus amplitudes were about 2.3 times the amplitudes of corresponding months in the quiet year.

A special selection of extremely quiet 1965 geomagnetic daily variation records from North America, Europe, central Asia, and east Asia was used to determine monthly spherical harmonic analysis coefficients representing the Sq fields from these regions. The coefficients permitted a separation of the external and internal contributions to the daily variations. Month‐by‐month behavior of the equivalent external current system and contours of the daily range of this current showed regional differences in amplitude and location of the equivalent ionospheric currents with seasonal change. Current foci locations reached 35° to 42° geomagnetic latitude in summer and 18° to 35° in winter with a regional dependence. The largest daily range of external current was 14.6×104 A in North America, 15.3×104 A in Europe, 13.7×104 A in central Asia, and 12.3×104 A in east Asia. Continental differences in the behavior of the current seemed to be explained by the relative position of the geographic and geomagnetic poles with respect to these regions.

The Sq current systems for quiet days of 1976 and 1977 were studied for observatories established near 76° E. longitude in the northern hemisphere. We used a special spherical harmonic analysis technique to separate the internal and external contributions of the quiet field variations and to determine the equivalent external current contours responsible for the Sq field variations on the sixth and twenty‐first of each month. The average ratio of the external to internal spectral power was 6.7 in 1976 and 7.4 in 1977. Focus positions for the India‐Siberia region external Sq current vortex, near 22° to 29°, were found to be at lower geomagnetic latitudes than for Europe and North America. The usual mid‐latitude vortex current, reaching 11.0 to 13.2 × 104 A in summertime, disappeared completely during winter months in both the years. The Sq current position was affected clearly by the off‐spin axis position of the north geomagnetic pole. Throughout the 2 years, we also found small current vorticies near 70° to 80° geomagnetic latitude, which we ascribed to auroral region activity on quiet days.

Spherical harmonic analysis coefficients of the external and internal parts of the quiet-day geomagnetic field variations (Sq) separated for the seven continental regions of the observatories have been used to determine conductivity profiles to depths of about 600km by the Schmucker equivalent substitute conductor method. The profiles give evidence of increases in conductivity between about 150 and 350 km depth, then a general increase in conductivity thereafter. For South America we found a high conductivity at shallow depths. The European profile showed a highly conducting layer near 125km. At the greater depths, Europe, Australia and South America had the lowest values of conductivity. North America and east Asia had intermediate values whereas the African and central Asian profiles both showed the conductivities rising rapidly beyond 450km depth. The regional differences indicate that there may be considerable lateral heterogeneity of electrical conductivity in the Earth's upper mantle.

The electrical conductivity profile ofthe Earth at depths of about 50 to 500 km was determined using the quiet ionospheric current variations observed at a line of stations near 75° East longitude. We found conductivity values ofabout 0.06 S/m from 50 to approximately 350 km depth with slight relative maxima near 125 and 275 kin, interpersed by relative minima near 210 and 330 km. Thereafter, the conductivity increased sharply toward a value of about 0.18 S/m at 500 km with no indication of leveling off. A comparison with regional seismic wave-velocity models shows good correspondence between high conductivity and low-velocity zones. The conduction by hydrogen-saturated pyroxene is envisaged as a possible mechanism for the high conductivity and its variation in the upper mantle.