Multi-mode wave observations at Mercury’s Foreshock

The solar wind interaction with Mercury’s magnetic field generates a bow shock in front of the planet. As at Earth, the region upstream of the shock that is magnetically connected to it, and known as foreshock, is permeated by a variety of waves. The characteristic frequencies and wave properties so far reported are (i) high frequency 2 Hz whistler waves (similar to the 1 Hz waves at Earth), (ii) intermediate frequency of 0.8 Hz, whose properties and formation mechanism remains unknown and (iii) lower frequency compressive waves in the 0.3 Hz range (corresponding to the large amplitude 30-s waves observed at Earth’s foreshock). The existence of ultra-low frequency waves indicates that backstreaming ions are able to drive instabilities as in the terrestrial case. However, simultaneous occurrence of different modes at Earth is not often observed.  In this work we use Messenger magnetic field data to study some examples of extended regions at Mercury’s foreshock where multiple wave modes at frequencies 2, 0.8 and 0.3 Hz co-exist. The waves can maintain coherence over long intervals of time which may be related to the fact that the shock is weaker with Mach numbers in the range 2-5, and so that less backstreaming ions and density gradients are expected. Future work using plasma data from the BepiColombo mission are needed to understand in more detail wave generation and evolution in Mercury’s environment.