超伝導量子干渉素子（SQUID）磁力計を用いた地磁気観測

―極低周波自然電磁波の高感度検出を目指して―

抄録

We developed the Low-Tc Superconducting Quantum Interference Device（LTS SQUID） magnetometer system to observe geomagnetic fields with high sensitivity. The system is comprised of a three-axis SQUID magnetometer housed in a glass fiber reinforced plastic（GFRP） cryostat, low-drift readout electronics with flux locked loop（FLL）, and a 27-bit data logger with a global positioning system（GPS） and telemetry function. All components operate with batteries. The SQUID magnetometers were electromagnetically shielded in liquid helium, using a cylindrical, copper shield. The system noise was approximately 25 fT/ √Hz in the 0.5 – 100 Hz frequency range, and the temperature drift of the system was ~1 pT/°C. The liquid helium boil-off rate of the system was 1.2 L/d, and the system could operate for 4 weeks. The system could detect weak geomagnetic signals, such as the Schumann resonance with eighth harmonics, and the Ionospheric Alfvén Resonance appearing at night. Installing two SQUID magnetometer systems at different places around 150 km apart, we performed comparative measurements of the geomagnetic fields successfully. We confirmed that the SQUID magnetometer system was useful for detecting the natural electromagnetic signals with high sensitivity in the extremely low frequency （ELF） range.