Upstream ultra‐low frequency waves in Mercury's foreshock region: MESSENGER magnetic field observations

Abstract

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. The availability of MESSENGER orbiter data enables us for the first time to conduct an in‐depth study of upstream waves in Mercury's foreshock. This paper reports first results of an observational study of upstream ULF waves in Mercury's foreshock using high‐time resolution magnetic field data from the MESSENGER spacecraft to understand the general morphology of these waves. We find that the most common wave phenomenon in Mercury's foreshock has frequencies ~ 2 Hz, with properties similar to the 1 Hz whistler waves in the Earth's foreshock. Their generation appears to be generic to the shock and not affected by the weak strength and small size of Mercury's bow shock. On the other hand, the most common wave phenomenon in the Earth's foreshock is the large‐amplitude 30 second waves, identified as fast magnetosonic waves generated by backstreaming ions. Similar waves at Mercury have wave frequencies at ~ 0.3 Hz, but occur only sporadically. The general lack of strong “30 second” 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.