Sub-Alfvénic solar wind interaction with a magnetosphere: 3D hybrid simulation

Sub-Alfvénic flows have been observed within magnetic clouds of interplanetary Coronal Mass Ejections (ICMEs), where enhanced magnetic field strength coincides with low plasma density. These conditions can significantly change the planetary space environment by enabling direct interaction between sub-Alfvénic solar wind and a planet’s magnetosphere. To investigate this regime, we perform a 3D global simulation using the hybrid code MENURA, modeling a plasma flow that transitions from super-Alfvénic to sub-Alfvénic as it encounters the magnetosphere. The upstream Alfvén speed is varied using a smoothed, step-like analytical function under pressure balance. The interplanetary magnetic field is oriented perpendicular to the Sun–planet direction, representative of local magnetic cloud conditions within ICMEs at 1AU. Our results reveal significant magnetospheric changes under sub-Alfvénic solar wind conditions: The bow shock rapidly weakens and dissipates while expanding to distances well beyond its original subsolar position, with pronounced expansion along the flanks. These findings provide new insight into magnetospheric dynamics under varying solar wind regimes and improve our understanding of planetary plasma environments. Furthermore, they offer valuable context for interpreting past observations from the MESSENGER mission and ongoing measurements from ESA’s BepiColombo mission.