CORRELATIONS OF Dst INDEX WITH THE INTERPLANETARY ELECTRIC FIELD

Correlations of the Dst index with various interplanetary parameters are stud ied by using the solar wind and interplanetary magnetic field data from the ACE spacecraft. Among all parameters investigated, Ey, the down-dusk component of the interplanetary electric field is found to be best correlated with the Dst index. During strong and moderate geomagnetic storms, it is seen that 2 to 3 hours before the storm main phase starts, Ey suddenly begins to increase and that the increase of Ey, is almost synchronous with the decrease of the Dst, with a time a head about 2 to 3 hours. Such an abruptly changing characteristic curve of Ey is much easier to be identified than those corresponding curves of V , V2Bz .. VB2 and e. Weak disturbances in the magnetospheric background field, fluctuations of both the IMF Bz and the interplanetary Ey all strongly influence the development of storms, causing multiple-step main phase decrease and enhancement of the storm intensity. The physical mechanism responsible for the close correlation of Dst with the interplanetary electric field is discussed. A viewpoint that the present study of multiple-step main phase storms needs to be improved is proposed.