Abstract
Annual geomagnetic variations with a maximum amplitude of 5 nT, and in phase with ground temperature variations at a depth of 1–2 m, were observed in the baseline values of fluxgate magnetometers installed at three JMA magnetic observatories. A possible origin of the annual variations is a change in magnetization of the soil due to changes in ground temperature. In order to examine the effect of temperature changes on soil magnetization, we measured the magnetic properties of soil samples collected at the JMA observatories. Magnetization of soil samples in a magnetic field of 0.05 mT ranged within 0.05 × 10−3−1.6 × 10−3 A m2/kg and the temperature dependence of magnetization ranged within 0.3 × 10−6−14 × 10−6 A m2/kg °C, except for a sample having an extraordinarily strong magnetization. Based on the measured magnetization, and their temperature dependence, of samples from Memambetsu, which shows the largest values among the samples from the three observatories, we determined the distribution of the geomagnetic field and its annual variation produced by soil magnetization. The maximum amplitude of annual variation in the geomagnetic field is 7 nT, which is consistent with the observed annual variation of the baseline value of the magnetometers.
Highlights
Geomagnetic observatories record variations of the geomagnetic field with variometers and determine the absolute values of the records with a separate procedure called absolute measurements
We measured the magnetic properties of soil samples collected from the three Japan Meteorological Agency (JMA) observatories, and we have examined the effect of temperature change on the soil magnetization to the geomagnetic annual variations
It is consistent with the amplitude of the annual variations, in the baseline values of the fluxgate magnetometers, which was largest at MMB and smallest at KNY
Summary
Geomagnetic observatories record variations of the geomagnetic field with variometers and determine the absolute values of the records with a separate procedure called absolute measurements. Absolute measurements of the total force, inclination and declination of the geomagnetic field are carried out with proton magnetometers and magnetic theodolites. The baseline values for the fluxgate magnetometers (96FM, 90FM and 95FM) at Memambetsu (MMB), Kakioka (KAK), and Kanoya (KNY), magnetic observatories (Fig. 2) show different annual variations. At KNY, linearly decreasing, and increasing, trends at rates of 1–2 nT/yr are notable in the H and Z components, and an additional annual variation of amplitude 1 nT is notable in the H , Z and D components Observing conditions, such as a change in temperature and the tilt of the magnetometers, are the most plausible causes of annual variations, but cannot fully explain the observed annual variations.
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