Shocklet-like Structures in Mercury’s Foreshock: New Evidence from MESSENGER

Shocklets are nonlinear compressive magnetosonic structures formed by the steepening of ultra-low-frequency (ULF) waves due to dispersive effects in collisionless foreshocks. At Earth, they are characterized by sharp upstream edges, moderate magnetic compression, and frequent whistler wave precursors.

We investigate shocklet-like structures in Mercury’s foreshock using 20 Hz magnetic field observations from the MESSENGER mission. The analysis targets upstream intervals with broadband ULF activity at frequencies of 2 Hz and below, including both low-frequency (≲0.3 Hz) and higher-frequency (~1–2 Hz) fluctuations analogous to waves known to evolve into shocklets at Earth.

More than 200 candidate events are identified and classified into two main categories based on waveform morphology and polarization. The first consists of Earth-like shocklets, exhibiting sharp leading edges, clear magnetic compression, linear or elliptical polarization, and frequent whistler precursors. The second, more prevalent category comprises ULF magnetosonic waves with superposed higher-frequency fluctuations, displaying weaker steepening and less clear polarization. 

These observations indicate that similar wave-steepening processes operate at Mercury and Earth. However, Mercury’s weaker bow shock and therefore a reduced foreshock turbulence could favor multiscale wave coexistence and a broader diversity of shocklet-like structures.