Explicit IMF B_y-Dependence in Geomagnetic Activity: Quantifying Ionospheric Electrodynamics

摘要

Geomagnetic activity is mainly driven by the southward (B_z) component of the interplanetary magnetic field (IMF), which dominates all solar wind coupling functions. Coupling functions also depend on the absolute value of the dawn-dusk (B_y) component of the IMF, but not on its sign. However, recent studies have shown that for a fixed level of solar wind driving, auroral electrojets in the Northern Hemisphere (NH) are stronger for B_y > 0 than for B_y < 0 during NH winter. In NH summer,the dependence on the B_y sign is reversed. While this B_y sign dependence, also called the explicit B_ydependence,is very strong in the winter hemisphere, it is weak in the summer hemisphere. Moreover,the explicit B_y-dependence is much stronger in the westward electrojet than in the eastward electrojet. In this study, we study how the explicit IMF B_y-dependence is coupled with large-scale field-aligned currents (FACs) by using FAC measurements from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and an empirical ionospheric conductance model. We model the complete ionospheric electrodynamics by solving the current continuity equation, and show that during periods of elevated solar wind driving (B_z < 0), the IMF B_y component modulates Regions 1 and 2 FACs in the dawn sector of the winter hemisphere. This leads to an explicit B_y-dependence in ionospheric conductance and the westward electrojet. We also show that the B_y-dependence of FACs and conductance is weak in the dusk sector, which explains the earlier observation of the weak B_y-dependence of the eastward electrojet.