The Earth’s magnetic field is monitored with magnetometers at Fiby (near Uppsala) and at Abisko. The magnetic field fluctuates rapidly depending on solar activity and slowly depending on variations within the mantle of the Earth. The rapid fluctuations are measured every second by a flux-gate magnetometer and the slow fluctuations twice per month by a proton-precession magnetometer (Table 6, #9.2). Data are archived at World Data Center WDC-C1 in Copenhagen, WDC-C2 in Kyoto, and NGDC in Boulder. The Geological Survey of Sweden (SGU) is responsible for the protonprecession magnetometer measurements.