Dynamics of shielding the polar cap electric field

The polar cap (PC) is parted into old cap observed before substorm, and new one which forms during substorm, frames the old cap and expands PC total area. The old PC and the new one are the basis of old and new magnetosphere tail respectively. We examined the weak substorm just at the beginning of 20.11.2000 superstorm. The old PC partly shielded from penetration of electric field of convection resulting during the substorm, dominated in the total PC area. The shielding coefficient is determined as the ratio β=Ψ2/Ψ02, where Ψ02 and Ψ2 are open magnetic fluxes through the old PC before the substorm and during its development, respectively. In the course of substorm under consideration, the coefficient β has been varying from 1 to 0.4 at the maximum of the expansion phase. After that, β restored to starting value. The decrease of β at the loading phase can be interpreted as a result of involving a part of magnetic flux of the old PC into the process of Poynting vector e' transfer from the solar wind to the magnetosphere. Under such approach, weakening of this process at the unloading phase causes β increase. As estimated, the decrease/increase in β during the substorm leads to the several-fold increase/decrease in e'.