Dipole Tilt Effect on Magnetopause Reconnection and the Steady-State Magnetosphere-Ionosphere System: Global MHD Simulations

The Earth's dipole tilt angle changes both diurnally and seasonally and introduces numerous variabilities in the coupled magnetosphere-ionosphere system. By altering the location and intensity of magnetic reconnection, the dipole tilt influences convection on a global scale. However, due to the nonlinear nature of the system, various other effects like dipole rotation, varying interplanetary magnetic field (IMF) orientation, and nonuniform ionospheric conductance can smear tilt effects arising purely from changes in coupling with the solar wind. To elucidate the underlying tilt angle dependence, we perform magnetohydrodynamic (MHD) simulations of the steady-state magnetosphere-ionosphere system under purely southward IMF conditions for tilt angles from 0-90 degrees. We identify the location of the magnetic separator in each case and find that an increasing tilt angle shifts the 3-D X line southward on the magnetopause due to changes in magnetic shear angle. The separator is highly unsteady above 50 degrees tilt angle, characteristic of regular flux transfer event (FTE) generation on the magnetopause. The reconnection rate drops as the tilt angle becomes large, but remains continuous across the dayside such that the magnetosphere is open even for 90 degrees. These trends map down to the ionosphere, with the polar cap contracting as the tilt angle increases, and region I field-aligned current (FAC) migrating to higher latitudes with changing morphology. The tilt introduces a north-south asymmetry in magnetospheric convection, thus driving more FAC in the Northern (sunward facing) hemisphere for large tilt angles than in the Southern independent of conductance. These results highlight the strong sensitivity to onset time in the potential impact of a severe space weather event.