The relation between Ultralow Frequency (ULF) waves and Short Large Amplitude Magnetic Structures (SLAMS) in the Mercury foreshock

Ultralow frequency (ULF) waves in the Hermean foreshock are believed to be driven by the interaction of solar wind ions reflected from the bow shock with the original solar wind beam. The direction of the interplanetary magnetic field (IMF) determines what regions are accessible to the reflected ions, and where therefore ULF waves are growing to an observable amplitude. Short Large-Amplitude Magnetic Structures (SLAMS) have been suggested to form when the growth of the ULF waves enter a non-linear, explosive stage. We use observations of foreshock ULF waves and SLAMS based on MESSENGER magnetic field data to investigate the relation between the two types of phenomena. We study the spatial extent of both SLAMS and ULF waves for different IMF directions, and relate them to the angle q_Bn between the IMF and the bow shock normal. At Earth, a majority of SLAMS have an elliptical polarization in the opposite sense to the ULF waves. We investigate whether this is the case also at Mercury, and also check if there is a continuous change of distribution of polarization organized by amplitude of SLAMS and structures with an amplitude intermediate between ULF waves and SLAMS, sometimes known as ‘shocklets’. The results are discussed in terms of possible similarities between terrestrial and Hermean SLAMS formation mechanisms, with a particular focus on possible extensions of these studies to be performed by the upcoming BepiColombo mission.