There are some systematic differences between the 1-min Polar Cap (PC) indices of magnetic activity derived from data from the southern hemispheric station, Vostok, and the northern hemispheric station, Thule. First of all, differences in sign of the PC index are observed when BZ is northward: negative values of the PC index are common in the summer polar cap. Secondly, predominance of larger PC index values is seen in winter hemisphere for extremely high levels of magnetic activity (PC > 12) when the ionospheric electric field in summer and winter polar caps tends to approach an asymptotic value of E = 40--45 mV/m. It is suggested that ionospheric conductivity in the winter polar cap can be significantly increased owing to bombardment of the ionosphere by solar protons (E = 1--10 MeV) in association with polar cap absorption events.