The interaction of the solar wind with the Earth's magnetic field produces geomagnetic activity, which is critically dependent on the orientation of the interplanetary magnetic field (IMF). Most solar wind coupling functions quantify this dependence on the IMF orientation with the so-called IMF clock angle in a way, which is symmetric with respect to the sign of the B y component. However, recent studies have suggested that the sign of B y is an additional, independent driver of high-latitude geomagnetic activity, leading to higher (weaker) geomagnetic activity in Northern Hemisphere (NH) winter for B y > 0 (B y < 0). In this paper we quantify the size of this explicit B y effect with respect to the solar wind coupling function, both for northern and southern high-latitude geomagnetic activity. We show that high-latitude geomagnetic activity is significantly (by about 40%-50%) suppressed for B y < 0 in NH winter and for B y > 0 in Southern Hemisphere winter. When averaged over all months, high-latitude geomagnetic activity in NH is about 12% weaker for B y < 0 than for B y > 0. The B y effect affects the westward electrojet strongly, but hardly at all the eastward electrojet. We also show that the suppression of the westward electrojet in NH during B y < 0 maximizes when the Earth's dipole axis points toward the night sector, that is, when the auroral region is maximally in darkness.While the polarity of IMF B y does not have any independent role in the solar wind-magnetosphere coupling functions, it plays a significant role in modulating the IMF B z component observed in the GSM coordinate system via the Russell-McPherron (RMP) effect (Russell & McPherron, 1973). The RMP effect arises due to the

The most important driver of geomagnetic activity is the north–south ( Bz) component of the interplanetary magnetic field (IMF), which dominates the solar wind‐magnetosphere coupling and all solar wind coupling functions. While the east–west ( By) IMF component is also included in most coupling functions, its effect is always assumed to be symmetric with respect of its sign. However, recent studies have shown that, for a fixed value of any coupling function, geomagnetic activity is stronger for By>0 than for By<0 in Northern Hemisphere winter. In Southern Hemisphere winter, the dependence on the By sign is reversed. In this paper, we use measurements of National Oceanic and Atmospheric Administration Polar‐Orbiting Operational Environmental Satellites to show that the flux of magnetospheric electrons precipitating into the atmosphere also exhibits an explicit By dependence. This By dependence is strong in the midnight and dawn sectors where it causes a related By effect in the westward electrojet and geomagnetic activity by modulating ionospheric conductivity.