Interplanetary magnetic field B_y controlled Alfvén wings at Earth during encounter of a coronal mass ejection

In the vicinity of Earth's orbit, the typical solar wind Alfvén Mach number exceeds 5, and the super-Alfvénic solar wind drives a conventional magnetosphere configuration. However, at the ejecta phase of an interplanetary coronal mass ejection (ICME) event, the Alfvén Mach number may experience a significant reduction due to the intensified interplanetary magnetic field (IMF) strength and decreased density. On 24 April 2023, an ICME reached Earth's orbit. The solar wind density dropped to as low as 0.3 amu/cc while the IMF strength is about 25 nT. As a result, the solar wind flow transitions to a sub-Alfvénic state with an Alfvén Mach number of 0.4, providing opportunities to investigate the interaction of planetary magnetospheres with low Mach number solar wind. We carry out global simulations to investigate the responses of Earth's magnetosphere to the sub-Alfvénic ICME ejecta. The global magnetohydrodynamic (MHD) simulation 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 IMF By component, causing the Alfvén wings to extend towards the dawn and dusk sides. In this paper, we present the structures of the magnetic field, plasma flow, and current system around the Alfvén wings. The global magnetospheric convection under the sub-Alfvénic solar wind condition is discussed in depth. Our results achieve a new level of understanding about the interaction between a magnetized body and sub-Alfvénic upstream conditions, and provide guidance for future observations.