An Explicit IMF B-gamma Dependence on Solar Wind-Magnetosphere Coupling

Presently, all empirical coupling functions quantifying the solar wind-magnetosphere energy-or magnetic flux conversion assume that the coupling is independent of the sign of the dawn-dusk component (B-gamma) of the Interplanetary Magnetic Field (IMF). In this paper we present observations strongly suggesting an explicit IMF B-gamma effect on the solar wind-magnetosphere coupling. When the Earth's dipole is tilted in the direction corresponding to northern winter, positive IMF B-gamma is found to on average lead to a larger polar cap than when IMF B-gamma is negative during otherwise similar conditions. This explicit IMF B-gamma effect is found to reverse when the Earth's dipole is inclined in the opposite direction (northern summer) and is consistently observed from both hemispheres. We interpret the different responses of the polar cap size due to the sign of IMF B-gamma to likely be a result of differences in the dayside reconnection rate.