Short Large-Amplitude Magnetic Structures (SLAMS) at Mercury observed by MESSENGER

Abstract. We present the first observations of Short Large-Amplitude Magnetic Structures (SLAMS) at Mercury. We have investigated approximately four years of MESSENGER data to identify SLAMS in the Mercury foreshock. Defining SLAMS as magnetic field compressional structures, with an increase in magnetic field strength of at least twice the background magnetic field strength, when MESSENGER is located in the solar wind, we find 435 SLAMS. The SLAMS are found either in regions of a general ultra-low frequency (ULF) wave field, at the boundary of such a ULF wave field, or in a few cases isolated from the wave field. We present statistics on several properties of the SLAMS, such as temporal scale size, amplitude, and the presence of whistler-like wave emissions. We find that SLAMS are mostly found during periods of low interplanetary magnetic field strength, indicating that they are more common for higher solar wind Alfvénic Mach number (M_A). We use the Tao solar wind model to estimate solar wind parameters to verify that M_A is indeed larger during SLAMS observations than otherwise. Finally, we also investigate how SLAMS observations are related to foreshock geometry.