CORRELATIONS OF Dst INDEX WITH THE INTERPLANETARY ELECTRIC FIELD

Correlations of the Dst index with various interplanetary parameters are studied by using the solar wind and interplanetary magnetic field data from the ACE spacecraft. Among all parameters investigated, E_y, 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, E_y suddenly begins to increase and that the increase of E_y, 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 E_y is much easier to be identified than those corresponding curves of V、V²B_z、VB₂ and ε. Weak disturbances in the magnetospheric background field, fluctuations of both the IMF Bz and the interplanetary E_y 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.