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.