Interplanetary Magnetic Field <i>B</i> _<i>y</i> Controlled Alfvén Wings at Earth During an Encounter of a Coronal Mass Ejection

Abstract On 24 April 2023, an ICME reached Earth's orbit. The solar wind density dropped to 0.3 amu/cc while the IMF strength was about 25 nT. As a result, the solar wind flow transitions to a sub‐Alfvénic state with an Alfvén Mach number of 0.4. We carry out global magnetohydrodynamic simulations to investigate the responses of Earth's magnetosphere to the ICME ejecta. The results show the formation of Alfvén wings as the solar wind becomes sub‐Alfvénic. Furthermore, the sub‐Alfvénic period was characterized by the dominance of the IMF component, causing the Alfvén wings to extend toward the dawn and dusk flanks. We investigate the global magnetospheric convection of this sub‐ Alfvénic case and find that the overall convection is mediated by the Alfvén wings, while the magnetic field convection in inner magnetosphere is similar to the super‐Alfvénic case.