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

Abstract

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.