ULF geomagnetic power nearL= 4 5. cross-power spectral studies of geomagnetic variations 2-27 mHz in conjugate areas

By using cross-power spectral analysis techniques the amplitudes and polarization characteristics of geomagnetic variations in the three frquency bands of 2–5 mHz, 10–15 mHz, and 15–27 mHz are studied nonstatistically as functions of latitude and local time for 22 days. It is found that during the local day the diurnal changes in the polarization characteristics of the magnetic field variations in the highest- and lowest-frequency bands are very similar to the polarization characteristics arising from the analyses of discrete well-defined Alfvén wave events in the same frequency ranges. This important result lends very strong additional evidence to the hypothesis that most local day magnetic fluctuations are controlled largely by a source external to the magnetosphere, probably the solar wind. During local night the results are consistent with a source propagation direction predominantly from local morning toward local evening. The results for the 10- to 15-mHz band are less clear and suggest that the geomagnetic variations are affected to a significant extent by plasma gradients (probably at the plasmapause) generally occurring within the latitude range of the present set of stations.