We examined the effect of magnetic disturbances in two polar caps on the generation of magnetospheric substorms. For this purpose we investigated the correlation between the AL index (showing substorm activity in the Northern hemisphere) and two geomagnetic activity indices, the Polar Cap (PC) index and Polar Magnetic (PM) index showing the magnetic disturbances in the Northern and Southern polar caps. For the analysis we used the data for four years when geomagnetic activity indices were available in both hemispheres. We obtained an unexpected yet important result: while in northern winter the correlation between AL index and northern PC/PM indices is very good, in northern summer the AL index correlates much better with southern PC/PM indices. Thus, substorm activity in summer months correlates much better with geomagnetic activity not in the nearby polar cap but in the opposite polar cap. This effect may be caused by the interhemispheric field-aligned currents flowing from the summer high-latitude ionosphere and closing through the ionosphere in the opposite auroral zone. An interesting feature of these interhemispheric currents is that they are directed opposite to the substorm field-aligned currents in the summer hemisphere but along the substorm field-aligned currents in the winter hemisphere. This leads to decreasing the total field-aligned currents and their contribution to magnetic disturbances in the summer hemisphere but increasing these currents and related magnetic disturbances in winter hemisphere, which explains the experimental results obtained in our study.