The geomagnetic variation anomaly in the northern Pyrenees: study of the temporal variation

Abstract : The existence and the spatial properties of an important geomagnetic anomaly in the north of the eastern part of the Pyrenees have been described in a previous paper of this journal (Babour et al.). The temporal dependance of the anomalous magnetic field and its relation with the normal field is now studied. Due to its remarkable property — separation of spatial and temporal variations — only one component of the anomalous field at one station needs to be considered. A high coherency between the variations of the anomalous field and those of the normal field projected in the direction 40°E of north is found. Transfer functions are obtained in the frequency domain and also directly in the time domain using two different methods. The various estimates of the transfer function are in good agreement for the period range 20min–2hr. As the anomaly is due to a concentration of currents induced elsewhere, the obtained transfer function is interpreted in terms of a non-local relation associated with the area where the induction takes place which is identified with oceanic regions. Using reasonable assumptions, the transfer function is shown to be connected to the ratio of the electric to magnetic field over oceanic areas. The existence of a highly conducting medium at 50 km depth is proposed. The mechanism responsible for the concentration of currents is qualitatively discussed in relation with the geometry of continental margins.