Abstract: We built an inversion model for separating internal and external equivalent current system S_q by the spherical harmonic analysis method, which was used to inverse daily for S_q 10 days around two minimum shift dates of April 24 and May 9, 2008 before the Wenchuan 8.0 earthquake. Combining with the index of geomagnetic storms (Dst), we analyzed the relationship between minimum point shift of the geomagnetic vertical component in diurnal variation and current system. We found that, on the days of minimum point shift of the geomagnetic vertical component in diurnal variation, the strength on the center of vortex has a tendency to decrease in internal and external equivalent current system for both the north and the south hemisphere, and the size of the internal field decreased bigger. Further, the center of vortex shifted on the days of minimum point shift of vertical component geomagnetic in diurnal variation.

Key words: Minimum point shift of the geomagnetic vertical component in diurnal variation, Spherical harmonic analysis, Current system S_q, The 2008 Wenchuan earthquake