Statistical Maps of Foreshock Waves Utilising 23 Years of Cluster Data

Ultra-low frequency (ULF) magnetosonic waves arise from the backstreaming ion population in the quasi-parallel foreshock region, participating in several key foreshock processes such as particle acceleration and shock reformation both directly and by steepening into transient structures such as SLAMS (short, large-amplitude magnetic structures). To better understand the effects of upstream solar wind conditions on these multi-scale processes, we use the 23-year Cluster dataset to study ULF waves under a range of solar wind conditions, combining Cluster data with the upstream OMNI product to produce Geocentric Interplanetary Medium (GIPM) coordinate mappings of foreshock wave properties. This method enables us to compare foreshock observations across changing solar wind conditions, by accounting for the changes in foreshock location and scale with varying IMF direction and dynamic pressure. We present the first quantitative maps of compressive and transverse foreshock wave power as a function of cone angle and Mach number, and study the ULF wave power dependence on Mach number, solar wind speed, density and background magnetic field strength, finding a slight increase in normalised foreshock wave power with increasing Mach number. We find the magnetic field strength to be the strongest determinant of foreshock wave power: wave power increases with decreasing field strength. The solar wind speed and density play more minor roles. We find that the relative changes in ULF-band power in the pristine solar wind are larger than in the foreshock under changing solar wind conditions. In the magnetosheath, we find higher ULF-band wave power on the quasi-parallel side, compared to quasi-perpendicular. These results set the context for future missions investigating waves in the solar wind, foreshock, and the magnetosheath, such as HelioSWARM and Plasma Observatory.