Mercury's Altered Magnetosphere During a Sub-Alfvénic ICME Event: MESSENGER Observations and Inferred Asymmetric Alfvén Wing Formation From Global MHD Simulations

We present observations of a rare configuration of Mercury's magnetosphere in response to sub-Alfvénic upstream conditions, driven by an interplanetary coronal mass ejection (ICME) that impacted the planet on 1 May 2013. Using data from the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft, supported by a global three-dimensional magnetohydrodynamic (MHD) simulation of the event, we demonstrate that Mercury's magnetospheric response during this interval was distinct from the typical super-Alfvénic state. During the sub-Alfvénic upstream conditions, MESSENGER measured a distorted magnetotail with a depleted southern magnetotail lobe. An MHD simulation closely reproduces these observations, providing a plausible global context for the reconfiguration of Mercury's magnetosphere under sub-Alfvénic conditions. The simulation predicts that a pair of Alfvén wings formed during this event, redirecting magnetic flux and plasma within the magnetosphere. The interplanetary magnetic field orientation during this event was primarily sunward/dawnward, generating asymmetric Alfvén wings with respect to the flow direction, in contrast to previously observed north–south wing configurations at the planet. Using Solar Orbiter observations in the inner heliosphere, we estimate that the solar wind is sub-Alfvénic approximately 2.5 times per Earth year near solar maximum, with intervals lasting between 10 s and 12 hr. Studies of these rare, sub-Alfvénic solar wind-magnetospheric interactions provide valuable insights into exoplanet–stellar wind interactions under similarly sub-Alfvénic conditions where in situ observations are not available.