Secular, annual, and semiannual changes in the baseline level of the earth's magnetic field at North American locations

Secular, annual, and semiannual changes were obtained from the midnight values of field on specially determined days of quiet magnetospheric conditions. From a selection of North American observatories the latitude variation of these changes was evaluated from 20° to 65°. Results for the active and quiet sun years (1958 and 1965) were compared, and changes at Tucson were traced from 1958 through 1974. The year-to-year variance in the determination of a secular change seemed to be decreased by the present analysis method in comparison to a method which uses the annual mean of hourly values. The annual horizontal field variations were largely northward in early summer and southward in early winter; quiet year amplitudes of 2 to 3 γ about doubled in the solar active year. The amplitudes of the semiannual field vectors were about one half that of the annual vectors; the directions were apparently inconsistent from year to year.