The evolution characteristics of geomagnetic disturbances during geomagnetic storms

With the data of the geomagnetic meridian observatories in China, 25 geomagnetic storms in the period of 1997 to 1999 have been analyzed by using three kinds of methods which are Natural Orthogonal Components (NOC), Correlation Analysis and Fourier Analysis. Three steps have been done to decompose the storm-time variation (Dst) correlating with the universal time (UT), the solar daily variation (S-q) and the disturbance daily variation (S-D) depending on the local solar time (LT). The results show that the spacial distribution and the temporal evolution characteristic of equatorial symmetric ning current are reflected clearly by Dst variation. The amplitude of S,, reached maximum or minimum during most main phases in MZL, BMT and QGZ observatories. It reflects the variation of the geomagnetic field from the motion of the S. focus with latitude. During main phases, the amplitude of SD was the largest. It decreased slowly during the recovery phase. Correlation Analysis and Fourier Analyses offer a useful way to extract the S-q and S-D variations.