Characteristics of geomagnetic power spectra on land and sea in the period range 0.2 to 400 seconds

Abstract

Geomagnetic field fluctuations in the 0.025-5Hz range were measured with a Cesium vapor magnetometer on the sea floor in Monterey Bay, and at a remote land site. Correlation was found between the geomagnetic activity A-index and both the slope and relative magnitude of the power spectra. More active days showed a greater fall-off as frequency increased, and a higher level of the observed power spectra. Fluctuations averaging 0.1nT²/Hz were noted at the low frequencies, dropping to 10⁻⁵nT²/Hz at 1 Hz. Comparisons were made between the land/sea and day/night data to determine the sea and diurnal effects. In addition to swell induced fluctuations, the overall power spectra for sea data is 10-15 dB higher than land data. Diurnal shifts indicate a 20 second period increase in the nighttime Pc 3 fluctuations. Daytime intensity is generally 5 dB above the nighttime data, but with a crossover at 0.05Hz, the nighttime data then running 3 to 5 dB above the day. Further observations were made to determine the applicability of the magnetometer for non-seismic detection of nuclear blast.