Investigation of the Ultralow Frequency (ULF) foreshock boundary at Mercury

Ultralow frequency (ULF) waves are found in certain parts of the upstream region of planetary bow shocks. These waves are believed to be driven by the interaction of solar wind ions reflected from the bow shock with the original solar wind beam. The region where ULF waves can possibly be observed is then determined by the regions accessible to the reflected ions within the foreshock (defined as the region magnetically connected to the bow shock). The boundary of the region where ULF waves are observed at Earth is known to also depend on the growth rate of the waves and on the direction of the interplanetary magnetic field (IMF). To identify the ULF foreshock boundary at Mercury, we use MESSENGER observations to investigate the presence or absence of clear ULF wave activity upstream of the bow shock. The boundary of regions where ULF waves are present, as parametrized by the angle θ_Bn between the IMF and the bow shock normal, is identified and the dependence on the IMF is studied. The connection to higher-frequency whistler waves emissions is also investigated. The results are compared to results from other planets, and their connection to other upstream phenomena is discussed. Finally, open questions that can be addressed by the upcoming BepiColombo mission are discussed.