Abstract

The most important parameter in the coupling between solar wind and geomagnetic activity is the B_z‐component of the interplanetary magnetic field (IMF). However, recent studies have shown that IMF B_y is an additional, independent driver of geomagnetic activity. We use here local geomagnetic indices from a large network of magnetic stations to study how IMF B_y affects geomagnetic activity at different latitudes for all solar wind and, separately, during coronal mass ejections. We show that geomagnetic activity, for all solar wind, is 20% stronger for B_y > 0 than for B_y < 0 at subauroral latitudes of about 60° corrected geomagnetic latitude. During coronal mass ejections, the B_y‐effect is larger, about 40%, at slightly lower latitudes of about 57° (corrected geomagnetic) latitude. These results highlight the importance of the IMF B_y‐component for space weather at different latitudes and must be taken into account in space weather modeling.