Explicit IMF B y -Dependence of Energetic Protons and the Ring Current.

The most important parameter driving the solar wind-magnetosphere interaction is the southward ( B z ) component of the interplanetary magnetic field (IMF). While the dawn-dusk ( B y ) component of the IMF is also known to play an important role, its effects are usually assumed to be independent of its sign. Here we demonstrate for the first time a seasonally varying, explicit IMF B y -dependence of the ring current and Dst index. Using satellite observations and a global magnetohydrodynamic model coupled with a ring current model, we show that for a fixed level of solar wind driving the flux of energetic magnetospheric protons and the growth-rate of the ring current are greater for B y  < 0 ( B y  > 0) than for B y  > 0 ( B y  < 0) in Northern Hemisphere summer (winter). While the physical mechanism of this explicit B y -effect is not yet fully understood, our results suggest that IMF B y modulates magnetospheric convection and plasma transport in the inner magnetosphere.