The Effects of IMF BY on the Twisting of Cusp

The cusp region plays a crucial role in the interaction between the solar wind and the Earth's magnetosphere, where solar wind particles can enter the magnetosphere directly. This study first reports that the cusp boundary exhibits a twisting structure, which intensifies with increasing altitude, as demonstrated by global magnetohydrodynamic (MHD) simulations. It is further revealed that the dawn-dusk component of the interplanetary magnetic field (IMF) significantly influences the degree of cusp twisting. This effect can be attributed to the tilt of the magnetic reconnection X-line and the subsequent tilt of the plasma flow directions, modulated by the IMF B_Y. Moreover, the relationship between the cusp twisting deformations at different altitudes and the magnitude of the IMF B_Y is quantitatively analyzed across the entire cusp region based on systematic MHD simulation runs. A remarkable enhancement in the twisting angle is indicated with increasing IMF B_Y and altitude, varying from 0° to 7.6°. The orientation of the cusp twisting follows that of the magnetotail and current sheet dynamics reported in previous studies, implying that the cusp twisting reported here is an essential part of the global effect of non-zero IMF B_Y on the dayside magnetopause.