The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind–magnetosphere-ionosphere coupling

Observations have shown that the cross-polar cap potential tends to maximize near the equinoxes, when on averageμ= 0, with smaller values observed near the solstices. This is similar to the well-known semiannual variation in geomagnetic activity.

We use numerical model simulations to investigate the role of two possible mechanisms that may be responsible for the influence ofμon the magnetosphere-ionosphere system: variations in the coupling efficiency between the solar wind and the magnetosphere and variations in the ionospheric conductance over the polar caps. Under southward interplanetary magnetic field (IMF) conditions, variations in ionospheric conductance at high magnetic latitudes are responsible for 10–30% of the variations in the cross-polar cap potential associated withμ, but variations in solar wind–magnetosphere coupling are more important and responsible for 70–90%.