A sudden change in the solar-wind dynamic pressure (P_SW) considerably amplifies convection. This process is ident in the variations in the PC index, which depends on the transpolar ionospheric current and is frequently used to estimate the interplanetary electric field (E_M) penetrating into the magnetosphere from the solar wind. Owing to the effect of P_SW pulses, the linear dependence between the value of E_M and the PC index is violated during magnetic storms. The ionospheric electric field reaches extremely large values. In addition, a sudden change in P_SW causes a fast reconfiguration of the convection system. Results of the numerical simulation of the electric potential distribution in the polar-cap ionosphere before, during, and after the passage of the high P_SW front are presented.