Saturation of the ionospheric electric field during substorm

Using magnetic measurements from a worldwide stations network that was used in the international CDAW-9C program, on the basis of the magnetogram inversion technique we have investigated the dependence of the ionospheric and magnetospheric electric fields during strong substorms on the AE indices. It is shown that a traditional method of estimating the EMF in the polar ionosphere as the difference between extreme values of the electric potential throughout the latitude range Phi >60 degrees leads to a loss of important information. The method used to determine the potential difference separately for the dayside and nightside sectors revealed substantially differing dependencies of the dayside and nightside EMF on the AE indices. The dayside EMF saturates prior to the "expansion phase", and decreases in the course of it. It is shown that this decrease can be caused by an abrupt drop of the magnitude of the interplanetary electric field, which decrease preceded the main "expansion onset" of substorm. Unlike the dayside EMF, the nightside EMF, together with the AE index, is enhanced in the course of the "expansion onset" as a consequence of the reconnection that produces an accelerated convection in the near-Earth plasma sheet.