MESSENGER Observations of a Possible Alfvén Wing at Mercury Driven by a Low Alfvénic Mach Number Interplanetary Coronal Mass Ejection

We investigate Mercury's response to rare, low Alfvénic Mach number $\left(M_A\right)$ solar wind conditions using observations from the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission. This study provides compelling evidence of Mercury's altered magnetospheric state under these extreme conditions, including the first observational confirmation of Alfvén wing formation at the planet. Our analysis estimates that the upstream conditions during the interplanetary coronal mass ejection (ICME) were sub-to trans-Alfvénic ($M_A\le$ 1.5). These unusually low $M_A$ solar wind conditions were driven by large interplanetary magnetic fields (IMF) associated with an ICME impact observed by MESSENGER on 30 December 2011. During this $\sim$17 hr event, MESSENGER completed one orbital pass through Mercury's magnetosphere, capturing magnetic field and plasma observations of its altered state. We compare these observations to a three-dimensional magnetohydrodynamic simulation of the event and to MESSENGER observations under typical $M_A$ conditions ($M_A\approx$ 5.0). Compared with its nominal state, the dayside magnetosphere during the ICME exhibited a weaker, more expanded bow shock and significantly lower plasma density within the magnetosheath. On the nightside, MESSENGER observed a highly inclined magnetic field relative to the typical magnetospheric magnetic field, populated with high-density plasma consistent with the formation of an Alfvén wing– a characteristic feature of sub-Alfvénic magnetospheric interactions. This investigation of Mercury under extreme $M_A$ conditions provides insights into the nominal, sub-Alfvénic interactions between many outer planet moons and their host planet's magnetosphere and also informs our understanding of the many exoplanetary-stellar wind interactions occurring in low-$M_A$ environments.