Magnetospheric Conditions Controlling ~1 Hz Waves at Mercury

The magnetosphere of Mercury is highly dynamic, a consequence of its small size, weak intrinsic magnetic field, and proximity to the Sun. One intriguing phenomenon is the presence of magnetic field fluctuations around 1 Hz. Here, we present a comprehensive statistical survey of these waves using the full span of the MESSENGER magnetometer measurements from 2011 to 2015. We find that ~1 Hz waves are observed during 10-20 % of the time that the spacecraft spent on closed field lines in Mercury’s magnetosphere, as determined from the KT17 magnetic field model. Wave occurrence is increased under magnetospheric conditions that favour an expanded closed field line region. We present the first global characterisation of the ~1 Hz waves at Mercury and demonstrate their dependence on both external drivers, such as upstream IMF conditions, and internal magnetospheric activity, such as the occurrence of identified dipolarization events. These results are discussed in the context of the BepiColombo mission, which will provide new opportunities to identify the nature of these waves and to assess their role in Mercury’s highly dynamic plasma environment.