Implementation of an unshielded SQUID as a geomagnetic sensor

The fluxgate magnetometer has long been the standard instrument of magnetic observatories due to its ease of use and sensitivity in the nanotesla range. Recently more sensitive magnetic sensors have become a requirement to study in particular the interaction between earthquakes and the ionosphere. The Superconducting QUantum Interference Device (SQUID) is capable of detecting magnetic flux in the femtotesla range and is well suited for detecting these interactions. Traditionally however, these devices have not been used to study the ionosphere due to shielding requirements. The Laboratoire Souterrain a Bas Bruit (LSBB) in France employs a low critical temperature (Low-Tc) SQUID for geomagnetic research, but it is placed in a unique low noise environment, 500 meters underground, that makes it impractical for other observatories to replicate. In this work, we implemented a completely unshielded high-Tc SQUID system at a magnetic observatory to complement fluxgate measurements. Here we discuss the implementation of the 3-axis SQUID magnetometer from an engineering perspective, including hut and rig design, placement, data acquisition, noise measurements, and possible future developments.