Annual and semiannual changes of the quiet daily variations (Sq) in the geomagnetic field at North American locations

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Abstract
An analytic representation of the quiet daily variation of the geomagnetic field (Sq) was obtained for 1965 in the North American region, with a mean error of less than 10% of the daily variation range. The month‐by‐month 24‐, 12‐, 8‐, and 6‐hour Fourier components of the field in H, D, and Z had significant annual and semiannual changes. At the equator the Sq(H) semiannual change was dominant for all Fourier components. From 15° to 50° latitude the 24‐hour component annual change of Sq(D) was the principal seasonal variation in this region of the ionospheric current vortex. Great enhancements of the 24‐hour component of the daily field occurred at high latitude, with strong summertime maxima in the annual variation of H, D,, and Z above 70° latitude. The results, in general, agreed with the expectation for ionospheric current sources in all regions.