Occurrence and controls of ~ 1 Hz waves in Mercury’s magnetosphere: insights from MESSENGER observations

<title>Abstract</title> Mercury’s magnetosphere is highly dynamic, with processes occurring on much shorter spatial and temporal scales than at Earth due to its weak magnetic field and proximity to the Sun. Wave-driven transport of energy and momentum is a key manifestation of these dynamics. We present a comprehensive statistical survey of ~ 1 Hz waves using the full MESSENGER magnetometer dataset. These waves occur on average 10–20% of the time within Mercury’s magnetosphere, predominantly on closed field lines. Their occurrence is enhanced under conditions that favour an expanded closed-field region, such as a northward interplanetary magnetic field or low internal magnetospheric disturbance. Wave power peaks in the post-midnight sector, mirroring asymmetries seen in other plasma parameters and suggesting a generation mechanism linked to particle injections along closed field lines near the magnetic equator. While the precise wave mode is not determined here, their frequency range, polarization, and occurrence context are broadly consistent with electromagnetic ion cyclotron-like waves. These results provide the first global characterization of ~ 1 Hz waves and their dependence on upstream and internal conditions, establishing a foundation for interpreting future BepiColombo observations and for understanding wave-driven dynamics in Mercury’s magnetosphere.