A Statistical Study on the Upstream Ultra-low Frequency Waves in Mercury's Foreshock seen by MESSENGER

In this work, we present the first statistical analysis of the main properties of ultra-low frequency waves in Mercury's foreshock, making use of high-time resolution MESSENGER magnetic field measurements. Specifically, we analyze the frequency, polarization and occurrence rate of different wave modes in the Hermean foreshock to improve the current understanding on the wave-particle interaction processes taking place in this region. In agreement with Le et al., (2013), we find that waves with a narrow power spectral density peak in the 0.2-0.35 Hz range propagate quasi-parallel to the background magnetic field (~10 degrees), and most of them present a left-handed polarization in the spacecraft frame. These waves have similar properties to the '30 second waves' observed in the Earth's foreshock, previously associated with fast magnetosonic waves generated by backstreaming protons. In sharp contrast with the terrestrial foreshock, the occurrence rate of waves with frequencies in the 0.2-0.35 Hz range is relatively low in the Hermean foreshock, suggesting significant lower fluxes of backstreaming protons due to the relatively weaker bow shock strength and the smaller foreshock size. In addition, we observe that the occurrence rate of these waves is larger for relatively small Interplanetary Magnetic Field (IMF) cone angles. This variability is likely related to the relatively higher linear wave growth rates and the larger foreshock size associated with this IMF cone angle range. Additionally, we find that the occurrence rate of waves with frequencies near 2 Hz in the spacecraft frame is significantly larger compared to the 0.2-0.35 Hz waves and have similar properties to the previously reported 1 Hz whistler waves in the Earth's foreshock. Finally, we also present an analysis on the dependence of the amplitude of both low frequency wave modes on MESSENGER's location in the Hermean foreshock.