Differential geomagnetic field measurements at the edge of the Denver Basin

Abstract

Two self-calibrating rubidium vapor magnetometers were used to investigate the time variations of the difference in total field between two sites in Colorado. Magnetometer 1 was located at the Boulder Magnetic Observatory, just to the east of the Rocky Mountain Front Range, and 2 was 12 km magnetically east of 1 at Gunbarrel Hill. The range of the measured difference field ΔF during four quiet time nights was 0.4 γ. During three moderately disturbed days there were several 6-hour periods in each of which the range was less than 0.1 γ. During 11 days of data there was no apparent long-term drift. Analysis of four types of geomagnetic events showed that for 11 frequencies (100-s to 24-hour periods) the variation in the difference field was never more than 8% of the total field variation. For six ‘bay’ events, the 1-hour coherence between the ‘noise’ ΔF and the declination D was 0.95. The amplitude of the transfer function from D to ΔF was found to be in rough agreement with that calculated from previous models of earth conductivity in this region, but the phase disagreed by about 1 rad. The magnetometers were the first field-usable ones of this design to be built, and their stability was checked by operating them at separations of up to 100 m. The relative stability over a number of days was found to be 0.05 γ rms, most of the variation being diurnal and of unknown origin.