Power spectra of geomagnetic field variations at synchronous orbit were calculated for the month of August 1974. Two segments of approximately 9 hours each, centered at local noon and local midnight, were used for the analysis. Each spectrum was parameterized as a straight line in the log-log regime. The power spectral density at 1 mHz was higher for local night (particularly for the D component) than for local day and increased with geomagnetic activity as indicated by &Sgr;Kp for the half day. The slopes of the straight line fits to the spectra ranged mainly from --1.5 to --3.0 and showed no clear difference night to day (except for the H component) and no clear relationship to geomagnetic activity. The day-night difference in power can be explained mainly by broadband noise caused by substorms. The average power spectrum at synchronous orbit, as represented by the ATS 6 observations, was compared with reported observations from the interplanetary medium, the magnetosheath, the region inside the magnetosphere near the magnetopause, and the ground.