Explicit IMF By-dependence in geomagnetic activity

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 shown that IMF 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). For NH summer the dependence on the B_y sign is reversed. 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 for a given value of solar wind coupling function, geomagnetic activity is about 40% stronger for B_y > 0 than for B_y < 0 in NH winter. We also discuss recent advances in the physical understanding of the B_y-effect. Our results highlight the importance of the IMF B_y-component for space weather and must be taken into account in future space weather modeling.