On the role of oceans in the geomagnetic induction by Sq along the 210◦ magnetic meridian region

Abstract

We have utilized solar quiet daily variation (Sq) data recorded at a network of temporary and permanent magnetic observatories, operated along the western Pacific coast, in the 210 ◦ Magnetic Meridian (MM) region, spanning both hemispheres. The selected data sets correspond to solar-quiet year 1996. We have determined the ionospheric source current systems for all three Lloyd’s seasons prior to calculating the Sq-induction response. As the selected data sets encompass the whole range of the western Pacific, we studied the role of self-induction effect (SIE) of the ocean at 24-hr period of Sq on the induction response, following the theory developed by Rikitake (1960). Our calculations show a considerable influence of the SIE on the induction response at 24-hr period, implying that ignoring the SIE in induction studies in oceanic regions leads to erroneous interpretation of the results. We have observed a large regional bias in the derived induction response estimates along the 210◦MM region. As the ocean effect also depends on the regional subsurface structure and its conductivity, we believe that in addition to the SIE, the bias in the estimated response functions could be due to the strong influence of the coupling of the electric currents induced in the ocean and the highly heterogeneous upper mantle that has resulted from the active lithospheric convergence between the Pacific and Philippine Sea plates all along the western Pacific. We interpret the derived induction responses in the light of the tectonic significance of the 210◦MM region. We also discuss the well-defined seasonal differences in ionospheric source current systems by comparing them with those reported for the East Asian sector.