Possible effects of lateral heterogeneity in the D″ layer on electromagnetic variations of core origin

This paper examines the consistency of a model for typical spatial features of rapid geomagnetic secular variations in which electromagnetic (EM) scattering due to the lateral heterogeneity of mantle conductivity plays an important role. When geomagnetic field variations of a core origin were separated into poloidal and toroidal modes, previous studies suggested that the scattering effect on the poloidal mode is probably weak. However, the toroidal field originating in the core may extend to the (very) deep mantle and can be converted into a poloidal field of observable intensity by scattering. In this paper, scattering of the EM field due to the D″ layer, where the lateral heterogeneity is supposed to be most significant in the mantle, is studied using three-dimensional numerical calculations. The results show that the spatial features of 60-year variations at the surface can be explained by scattering, although it is possible to interpret that they are directly reflecting those of poloidal field variations originating in the core. The same explanation may be applicable to geomagnetic jerks, assuming that they are represented by a shorter time-scale (1 year) variation. Although a magnetic field observation itself does not enable us to distinguish whether or not observed variations originated from poloidal field variations in the core or by a scattering of the toroidal field in the D″ layer, our calculation results indicate that detection of electric field and LOD variations of corresponding periods provide strong constraints on the mechanisms of these phenomena, especially the spatial pattern of decadal variations.