ULF geomagnetic power near L = 4. III. Statistical study of power spectra in conjugate areas during December solstice

Results are reported of statistical studies of 2-hour power spectra of nonoverlapping H component magnetic record segments. The records were obtained in January 1972 at an antarctic magnetometer station (Siple) near L=4 and at three stations (L ∼4.4, ∼4.0, ∼3.2) at approximately the same magnetic longitude in the northern conjugate area. The K index measured near the longitude of the stations was ≤5 during the interval studied. (The explicit dependence of magnetic power on geomagnetic activity is examined in part four of this series (Lanzerotti and Surkan, 1974).) It is found in the present paper that during more disturbed periods, higher power levels are accompanied by steeper spectral slopes; this finding implies a concentration of energy at the lower frequencies at these times. Relatively higher power levels are observed during the local afternoon and late evening than during other periods of the day. On the basis of power levels and spectral slopes, Siple (L ∼ 4) is more nearly conjugate to a northern station at L ∼4.4 than to a station at L ∼4 during the 1971–1972 December solstice period. This finding may arise from hemispherical differences in ionospheric conductivities and, as such, may be dependent upon season.