  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 (CMEs). 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 $60^{\circ}$ corrected geomagnetic (CGM)
latitude. During CMEs, the $B_y$-effect is larger, about 40%, at slightly lower
latitudes of about $57^{\circ}$ (CGM) 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.
