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

Like supersonically fast fighter jets creating sonic shocks in the air, planet Earth typically moves in the magnetized solar wind at super-Alfvénic speeds and generates a bow shock. Here we report a rare regime of Earth's magnetosphere interaction with a sub-Alfvénic solar wind brought by an erupted magnetic flux rope from the Sun, called a coronal mass ejection (CME). The terrestrial bow shock disappears, leaving the magnetosphere exposed directly to the cold CME plasma and the strong magnetic field from the Sun's corona. Our global magnetohydrodynamic (MHD) simulations show that the magnetosphere transforms from its typical windsock-like configuration to having wings that magnetically connect our planet to the Sun. NASA's Magnetospheric Multiscale (MMS) mission further observes cold CME ions simultaneously with energized counter-streaming electrons, indicating dual-wing reconnection between the CME and Alfvén-wing field lines. The observations show dynamic generation of new wing filaments, and dual-wing reconnection converting the open flux in the wings to closed flux, forming dayside magnetic flux circulation. The picture revealed by the MMS measurements differs from the typical cartoons of Alfvén wings, and goes beyond the global MHD simulation which does not produce dual-wing reconnection. How much the dayside flux circulation modifies the typical Dungey cycle is among the open questions brought by the unusual cosmic event. Our work creates new possibilities to understand how sub-Alfvénic plasma wind may impact astrophysical bodies in our solar and other stellar systems.