Average diurnal characteristics of geomagnetic power spectrums in the period range 4.5 to 1000 seconds

The power spectral density function of geomagnetic rapid variations observed on the earth can in general be considered as the product of three factors in the frequency domain: (1) the spectral distribution of the source or driving function; (2) the coupling or impedance matching to the medium for various propagating modes; and (3) the propagation characteristics of the medium, including the geometrical effects of boundaries. An association of observed spectral components with one or more of these factors requires statistically stable and significant analyses and theoretical studies that can be verified by experiment. It is especially important to determine the variations of the power spectrums with local time, latitude, spatial extent, and Kp index, which has been linked [Snyder et al., 1963] to the bulk velocity of the solar wind. Many authors [Cantwell, 1960; Horton and Hoffman, 1962; Santirocco and Parker, 1963; Smith et al., 1961] have published power spectral density functions of terrestrial measurements, although the primary objective of all these studies was to investigate the conductivity structure of the earth rather than the source mechanisms of the variations. Although the combined results serve to delineate the general shape and level of the spectrum from periods of 1 sec to periods of 10,000 sec, only the work of Santirocco and Parker contains sufficient data from a single location taken with the same instrumentation to indicate the diurnal effects. Even in this study, the choice of an island site to emphasize the conductivity discontinuity and the selection of data intervals containing prominent Pc activity tend to obscure the generality of the results as an indicator of external processes.