A study of annual variations in the geomagnetic total intensity with special attention to detecting volcanomagnetic signals

This paper investigates the cause of annual variations in the geomagnetic t otal intensity that are often seen especially in the volcanic areas. As a h ypothesis of the cause, a model was proposed, in which such a change is pro duced by changes in the inhomogeneous magnetization of near-surface rocks d ue to temporal changes of the atmospheric temperature. This hypothesis was tested by field and laboratory experiments. First, amplitude and phase diff erence of annual variations in the total intensity and ground temperature d ata were determined by time series analyses. Considering thermal diffusion from the surface into the ground, the phase difference between the total in tensity and temperature was converted to a characteristic depth, and then t he amplitude of annual temperature variation at the depth was estimated. Fi nally, the observed total intensity variations were compared with the expec ted change on the basis of the temperature dependence of rock's magnetizati on obtained by a laboratory experiment and the local magnetic anomaly obtai ned by a magnetic survey at each magnetometer site. A good agreement betwee n the observed and expected changes was obtained, which strongly suggests t hat the hypothesis is correct. It was also shown that a correction of annua l variations by using temperature data will enable us more accurate detecti on of volcanomagnetic signals.