High latitude, dayside ULF signals observed from ground in Greenland

Abstract. Ultra-low frequency (ULF) waves propagate through the cusp and generate distinct summertime signatures in high latitude ground-based magnetometer measurements. In this study, we apply four years of data from the high time resolution West Greenland magnetometer chain and perform a statistical analysis of ULF signal distribution as a function of season, magnetic latitude, magnetic local time, and interplanetary magnetic field parameters. We find that ULF signals at the highest latitudes, in the cusp and beyond, are sensitive to seasonal change, indicating that the ionospheric currents that generate the signal depend on solar illumination to obtain sufficient conductivities. This effect, in concert with dipole tilt, is investigated, and a clear cusp-related ULF signal population during summer was found. In winter, this population merges with other ULF signals associated with Alfvénic interhemispheric bouncing further south and thus disappears. Earlier studies, which have mainly been performed during winter conditions, failed to unambiguously identify cusp ULF signals. Furthermore, we discuss other aspects of our statistical analysis and briefly address implications for other known cusp phenomena.