Environmental magnetic noises change with time influence biomagnetic measurements. There are two types: artificial magnetic noises, and those caused by natural fields For example, the former are caused by the movement of objects composed of magnetic materials, such as cars or trains, elevators, and fan-pumps of air conditioners, on the other hand, the latter are caused by natural fields, such as volcanoes or solar wind. The magnitude of geomagnetism is about 10 -4 T, and that of movements of cars or trains, elevators, and fan-pumps of air conditioners are 10-6 ∼ 10 -10 T. By contrast, the magnitude of magnetoencephalogram or magnetocardiogram is 10-12 ∼ 10 -14 T. Although they are, very weak, environmental magnetic noises cannot be ignored we measured the temporal vertical component of geomagnetic changes with a SQUID (Superconducting Quantum Interference Device) magnetometer in the vicinity of the crater of the Mount Sakurajima volcano for about 40 days. taken, By comparing our data taken by SQUID with the data of by the magnetic observatory of the Japan Meteorological Agency, we confirmed that our data are convincing. The influence of magnetic fluctuation caused by volcanoes on biomagnetic measurement is also discussed.
