Observations of upstream ultra-low-frequency waves in the Mercury´s foreshock

We report the observational results of upstream ULF waves in the Mercury´s foreshock using high-time resolution magnetic field data, 20 samples per second, from the MESSENGER spacecraft. The Mercury´s bow shock is unique in our solar system as it is produced by low Mach number solar wind blowing over a small magnetized body with a predominately radial interplanetary magnetic field. Our study has showed the existence of at least three types of upstream waves: 1) whistler waves at frequencies near 2 Hz, similar to the 1-Hz waves at the Earth; 2) waves with frequencies ~ 0.3 Hz, similar to the large-amplitude 30-s waves at the Earth; 3) fluctuations with spectral peaks centered at ~ 0.8 Hz. Unlike the Earth´s foreshock where the most prominent upstream wave phenomenon is locally generated large-amplitude 30-s magnetosonic waves, the most common foreshock waves are whistler waves generated at the bow shock, with properties similar to the 1-Hz waves in the Earth´s foreshock. Their occurrence characteristics show that the “1-Hz” wave generation is generic to the bow shock and not affected by the strength and size of the shock at Mercury. On the other hand, the “30-s” magnetosonic waves at Mercury occur only sporadically and with small amplitudes. The general lack of strong “30-s” magnetosonic waves at Mercury can be attributed to the lack of strong backstreaming ions due to a weak bow shock and not enough time for wave growth due to the small foreshock size. Superposed on the “1-Hz” whistler waves, there are short bursts of spectral peaks at ~ 0.8 Hz that are new and have not been reported previously in Mariner 10 data. The source of the ~ 0.8 Hz waves remains to be identified.