On the collapse of Mercury's dayside magnetosphere

Mercury's interaction with the solar wind is unique in our solar system due to its weak global magnetic field and the planet's proximity to the Sun. The magnetosphere of Mercury is therefore highly dynamic, especially during times of coronal mass ejections (CMEs), which travel at high speeds compared to the solar wind and carry mass and magnetic field from the Sun. It has been long hypothesized and modeled, but never observed, that the magnetic flux on Mercury's dayside may be completely eroded or compressed below the surface during extreme conditions. Such an event allows Mercury to temporarily act as an unmagnetized airless body, with the CME plasma interacting directly with Mercury's surface, weathering the regolith and sputtering particles into the exosphere. Here, using MESSENGER data, we present the first observations of a CME compressing Mercury's dayside magnetosphere below the surface, accompanied by a ∼40% increase in the peak magnetospheric field and a ∼300% increase in the magnetotail field. These phenomena have not been observed at any planet until now. BepiColombo, arriving at Mercury during the next solar maximum, will be well positioned to observe such magnetospheric collapse events in more detail, and we make predictions for these observations of the magnetosphere/CME interaction. The collapse of Mercury's dayside magnetosphere has important implications for exoplanets in the so-called "habitable zone" of magnetically active M dwarf stars, as such events are likely to be much more common in these systems. Loss of planetary magnetospheres could significantly contribute to planetary atmospheric loss, affecting habitability and the potential for emergence of life.