Geomagnetic field spectra from eight standard observations at geomagnetic latitudes below 30?? were studied to determine the field characteristics unique to the equatorial region. Emphasis was placed upon those variations having periods between 5 min and 4 hr for a selection of magnetically quiet, average, and active days in 1965. The power spectral density at the equator was about ten times that the near 30?? latitude. The initial manifestation of the equatorial electrojet as evidenced by the east-west alignment of the horizontal field or the change in vertical amplitudes occurred below about 20?? latitude. Induced current effects upon the vertical component from which the Earth conductivity might be inferred could best be obtained at times and latitudes unaffected by the electrojet current. Values of about 1.6 ?? 103 mhos/m for an effective skin depth of 500-600 km were determined. The spectral amplitudes increased linearly with geomagnetic activity index, Ap. The spectral slope had a similar behavior at all latitudes. The slope changed systematically with Ap-index and showed a diurnal variation, centered on local noon, that changed form with geomagnetic activity.
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Spectral characteristics of geomagnetic field variations at low and equatorial latitudes