Secular variations of the Earth's apparent resistivity

Monitoring carried out by the generalized magnetovariation (GMV) and geomagnetic depth sounding (GDS) methods has shown a valuable variability of yearly apparent resistivities from the yr 1911 to 2010 for the selected European geomagnetic observatories. These deep soundings were carried out by the two methods mentioned above, correspondingly for shorter and longer periods in the range from 8h to a month. The possible changes in the source signals through an uncorrelated part of the observed field were mainly suppressed by robust, coherence-based statistical data processing. The detected resistivity variations have periods of more than several yr, while inequality of induction arrows obtained by the GMV method may be seen already during a couple of yr. The 11-yr variability of the apparent resistivity correlates with Wolf numbers with some phase lags. The variations with shorter periods have been found to change synchronously with the number of the earthquakes at a region including some seismically active areas in Europe. The peculiarities and possible origins of the phenomenon are discussed. ► Secular variations of the Earth's apparent resistivity have been detected. ► Relations between the Earth's resistivities and Wolf numbers have been found. ► Significant changes of the imaginary induction arrow have been detected.