Earth's Alfvén wings driven by the April 2023 Coronal Mass Ejection

We report Magnetospheric Multiscale observations of the dayside magnetosphere when the solar wind becomes sub-Alfvénic, causing the windsock-like magnetosphere to transform into Alfvén wings. The event occurred in the magnetic cloud of a Coronal Mass Ejection (CME) on April 24, 2023. We highlight the following outstanding features: (1) a layer of accelerated cold CME flow directly adjacent to the wing and to the magnetopause, presenting a rare regime of magnetosphere interaction with unshocked CME plasma; (2) wing filaments created by magnetic reconnection, representing new channels of magnetic connection between the magnetosphere and foot points of the Sun's erupted flux rope; (3) cold CME ion deceleration with little heating across the magnetopause. The reported measurements advance our knowledge of CME interaction with planetary magnetospheres, and open new opportunities to understand how sub-Alfvénic plasma flows impact astrophysical bodies such as Mercury, moons of Jupiter, and exoplanets close to their host stars.