Evolution of the storm magnetic field disturbance around Earth's surface and the associated ring current as deduced from multiple ground observatories

Based on the continuous magnetic measurements of the multiple ground observatories, the structure of the geomagnetic field disturbance and its temporal variations have been investigated, and the evolution of the storm ring current is deduced accordingly. Assuming that the geomagnetic disturbance around the Earth's surface is approximately linearly varying in space, the spatial gradient of the corrected H (geomagnetic horizontal) component can be obtained from the multiple ground observations. It is found that the maximum difference of the corrected H component around the Earth's surface, as calculated by the gradient of the corrected H component multiplied by the Earth diameter, represents well the asymmetric index Asy-H. The antidirection of the gradient of the corrected H component can be regarded as pointing to the position of the maximum partial ring current. It is shown that for ordinary magnetic storms with Dst>-200nT, the maximum partial ring current is located around the duskside during the main phase and the earlier stage of the recovery phase. At the later stage of the recovery phase, the position of the maximum partial ring current can possibly keep rotating eastward, indicating that the energetic electrons may play a significant role and the main ring current carriers may be the electrons at this stage. For the super storms (Dst<-200nT), the position of the maximum partial ring current is not as regular. Based on physical considerations, this investigation also provides new formulas of the definitions to the symmetric index Sym-H and asymmetric index Asy-H.