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°. 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° 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°. 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. Plain Language Summary The magnetic field of the Earth is tilted with respect to the Sun; as the planet orbits and rotates, the angle of this tilt changes. When the interplanetary magnetic field—which is carried by the solar wind—meets the Earth's magnetic field, a cavity is formed in the solar wind. Depending on the tilt angle, some mass and energy is able to penetrate into this cavity, affecting conditions in near‐Earth space, in the upper atmosphere, and on the ground. To better understand how the tilt angle controls this, we perform computer simulations of the interaction between the solar wind and the Earth's magnetic field. We show that when the tilt angle is larger, less mass and energy can penetrate