Thermal state observed by declination magnetometers on Aso volcano

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A declination manetometer was developed with categories in sensitive and power saving. Two sets were installed near the active crater of Aso volcano, site C1 and W1, in March 1997. The purpose of this study is a detection of geomagnetic change associated with volcanic activity. We can estimate a thermal state beneath the crater through the magnetic change.Fluxgate magnetometer for declination was developed in 1996 FY.The stability is necessary to monitor the active volcano, Measurement of EY component of the geomagnetic field is the best. The device were buried in 2m depth to keep constant temperature. The specifications of the device ; sensitivity ; lOOnT/volt, resolution ; 1pT, stability ; 1nT/degree, power ; l2volts 12OmA.The volcanomagnetic change observed with proton magnetometer as follows ; the volcanic activity of Aso in 1997 was calm and kept cooling phase, but the phase was changed in heating on March 1998. The observed decrination data in this period gives us some new ivolcanomagnetic effects.