Abstract

The most important driver of geomagnetic activity is the north–south (\(B_z\)) component of the interplanetary magnetic field (IMF), which dominates the solar wind-magnetosphere coupling and all solar wind coupling functions. While the east–west (\(B_y\)) 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 \(B_y > 0\) than for \(B_y < 0\) in Northern Hemisphere winter. In Southern Hemisphere winter, the dependence on the \(B_y\) 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 \(B_y\) dependence. This \(B_y\) dependence is strong in the midnight and dawn sectors where it causes a related \(B_y\) effect in the westward electrojet and geomagnetic activity by modulating ionospheric conductivity.