The PC index has been put forward with the general aim of funding a key parameter that (1) would estimate the current state of the Earth's magnetic field, and (2) would be readily calculated from routine ground-based measurements at all time. The concept of convection in the near-pole region controlled by the solar wind parameters provides the basis of the method of calculation of the index. The PC index has been derived as a dimensionless quantity parametrized by season, UT, and hemisphere, and calibrated for interplanetary electric field, merging with the magnetosphere. The results of former statistical analyses suggest that the PC index is useful in nowcasting the intensity of the auroral substorms. Twelve strong magnetic disturbances occurring in 2000 have been analyzed to verify this affirmation under specific conditions. Our results show that the PC index, being a measure of the solar wind electric field merging with the magnetosphere, basically follows, with a time delay of about 50 min, variations of the interplanetary electric field observed in the point of libration. Distinctions in details between these quantities, typical of examined events, indicate that characteristics of the solar wind can change on the way from the point L1 to the Earth. Sharp increase of PC index indicates conclusively that auroral substorm will be developed in a matter of minutes. Correlation between the PC and AE indices turned out to be much higher in the absence of D_st variation than in the course of magnetic storm. This regularity suggests that magnetosphere responds in different ways to the same changes in the solar wind parameters, and, therefore, the preceding state of the magnetosphere would be taken into account to foresee its reaction to the solar wind influence.