Geomagnetic field fluctuations at synchronous orbit 1. Power spectra

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 ∑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.