Abstract
The amplitude of the annual variation in the horizontal component of Sq is larger in the southern hemisphere than the northern hemisphere. This asymmetry may be represented as the superposition of a world-wide annual variation with its maximum at or near perihelion and summer-winter annual variations of equal amplitudes in the two hemispheres and maxima at the local summer solstice. The amplitude of the world-wide annual variation, as a fraction of the yearly mean value of Sq, is about 0.07 ± 0.02. The variation in ionospheric conductivity associated with the earth's orbital eccentricity is inadequate alone to account for this annual variation in Sq. If joule heating by the Sq currents is appreciable, its annual and semi-annual variations could account for the similar variations in the temperature of the thermosphere.
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