Bimodal nature of solar wind–magnetosphere–ionosphere–thermosphere coupling

The solar wind–magnetosphere–ionosphere–thermosphere system has two ways to transfer the force that the solar wind exerts at the magnetopause to the Earth: (1) a dipole interaction mode and (2) a thermospheric drag mode. This paper uses MHD simulations to discuss both modes comparatively. The dipole interaction mode is relatively well known and so is used as a model against which to contrast the less well known thermospheric drag mode. The thermospheric drag mode itself consists of two distinct mechanisms, a direct mechanism in which the solar wind pulls on the thermosphere and (here discussed for the first time) an indirect mechanism involving an interaction between the region 1 current system and the geomagnetic dipole which increases the drag on the thermosphere nearly an order of magnitude over the direct drag mechanism. This additional force appears to be necessary to account for observed high-latitude thermospheric winds that correlate with IMF Bz.