A geophysicist is calibrating a highly sensitive fluxgate magnetometer inside a custom-built non-ferrous wooden research cabin. The cabin contains no electrical wiring, permanent magnets, or metallic components. When the instrument is powered on, the display shows a steady, non-zero magnetic flux density reading of 47.0 μ 47.0\ \mu\,47.0 μT.
State the source of the magnetic field being detected by the magnetometer.