The spatio-temporal characteristics of geomagnetic induction vectors in North China

More than five years' variations of geomagnetic induction vectors for 13 geomagnetic stations in North China were obtained by using the robust estimation method, which has been verified and compared with the ordinary least square and the weighted least square methods. We proposed the following three principles of selecting a specified period of the results from the robust estimation method: (a) the relatively higher coherency between horizontal and vertical component at this period, (b) the much more stable results of the robust estimation at this period, and (c) the skin depth within the crust for the period. After investigating the stability of coherency resulting from different time window (one day and ten days), we chose the results with 640 second period for all stations finally, to be analyzed forward. Significance annual background variations were revealed from the time series results of the geomagnetic induction vectors with 640 second period, and were extracted the long-term background (more than six months) from them using the Fourier harmonic analysis. A similar phase variation was obtained for most station excepting DAL, BJI and JIH stations, possibly related to different underground conductive structures. The further statistical analyses of the geomagnetic vectors of BJI, JIH, LYH, WHN, HUH and TAY stations indicated that a relatively higher conductivity layer might exist around these stations, while the results of QIX, TSY, LZH, TAA and MAL stations showed some complexity of the electrical conductivity structures in the crust. After removing the long-term background from the time series, we found that there exists a rough monthly variation of the geomagnetic induction vectors. Such period variation is consistent with the geomagnetic activity index, and may relate to the period of the solar rotation.