Shocklet-like Structures Upstream of Mercury: New Insights into Hermean Foreshock Dynamics

Shocklets are nonlinear compressive magnetosonic structures generated by the steepening of ultra-low-frequency (ULF) waves and dispersive effects in collisionless foreshocks. At Earth, they are commonly associated with diffuse suprathermal ion populations, sharp magnetic compressions, and whistler precursors, providing key diagnostics of wave–particle interactions and energy transfer upstream of planetary bow shocks. While shocklets have been extensively studied at Earth, their occurrence and properties at Mercury remain largely unexplored.

Here we investigate shocklet-like structures in Mercury’s foreshock using 20 Hz magnetic field observations from the MESSENGER mission. The analysis surveys upstream intervals containing ULF wave activity, including both low-frequency (<0.03 Hz) and higher-frequency (~1–2 Hz) fluctuations. The former are regarded as Hermean analogs of the wave populations known to evolve into shocklets in Earth’s foreshock, while the latter may correspond to whistler-like precursor activity.

More than 200 candidate events were identified and classified according to waveform morphology and polarization properties. One category consists of Earth-like shocklets exhibiting steepened leading edges, clear magnetic compression, linear or elliptical polarization, and frequent whistler precursors. A second, more abundant population is composed of ULF magnetosonic waves with superposed higher-frequency fluctuations, displaying weaker steepening and less clearly defined polarization signatures. These observations suggest that nonlinear wave steepening at Mercury occurs under a broader range of wave conditions than typically observed at Earth.

The observed diversity of shocklet-like structures further indicates that Mercury’s foreshock environment may differ fundamentally from the terrestrial case. Mercury’s weaker bow shock and likely reduced levels of reflected-ion-driven turbulence could favor the coexistence of multiscale wave activity and more coherent shocklet-like structures. These results provide new insight into how planetary-scale conditions regulate nonlinear wave evolution in collisionless plasmas and establish an observational framework for interpreting upstream wave phenomena during the orbital phase of BepiColombo.