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

Abstract

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 (By) of the Interplanetary Magnetic Field (IMF). In this paper we present observations strongly suggesting an explicit IMF By effect on the solar wind-magnetosphere coupling. When the Earth's dipole is tilted in the direction corresponding to northern winter, positive IMF By is found to on average lead to a larger polar cap than when IMF By is negative during otherwise similar conditions. This explicit IMF By 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 By to likely be a result of differences in the dayside reconnection rate.