Frequency characteristics of high-sensitivity fluxgate magnetometers at Kakioka, Kanoya, and Memambetsu Magnetic Observatories
Akira YAMAZAKI and Toshiaki MISHIMA
Abstract
Beginning in 2012, geomagnetic 0.1 s sampling interval values at the Kakioka Magnetic Observatory and its branches at Memambetsu and Kanoya have been produced by direct A/D conversion of the output signal from high-resolution fluxgate magnetometers. Before their use, an analog high-pass filter, which passed signals with periods shorter than 150 s, had been used for the output signal of the magnetometers; therefore the frequency characteristics of the 0.1 s values have changed markedly. Three new high-resolution fluxgate magnetometers were manufactured to replace the original magnetometers, and we have examined their frequency characteristics. A calibrating system of three axial large square coils in the absolute observation house at Kakioka was used. Although the result was close to the specification of the magnetometer, which is a 5-Hz lowpass filter, the characteristics of the Z component were somewhat different from those of the H and D components. We also examined the frequency characteristics measured by the calibration coil inside the fluxgate sensors. In a comparison of the results from the large coil system with those from the internal coil, the frequency characteristics of the three components were slightly different. We investigated the cause of the differences and found that induction currents inside the aluminum parts around the fluxgate sensor, such as the sensor bottom disk, sensor cover, and sensor-mounting stand, influenced the frequency characteristics. Since the observed geomagnetic field includes the effects of the induction currents in the interior of these aluminum parts, we should adopt the calibration data as measured by the large coil system.
Received 3 August 2012; accepted 12 November 2012
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