High latitude, dayside rapid geomagnetic variations observed with ground-based magnetometers in Greenland

Abstract. We identify a previously unrecognized population of rapid geomagnetic variations associated with cusp dynamics. These variations produce distinct summertime signatures in the high–latitude ground magnetometer observations near the cusp. In this study, we apply four years of data from the high time resolution West Greenland magnetometer chain and perform a statistical analysis of rapid geomagnetic variations in the ultra low frequency (ULF) regime as a function of season, magnetic latitude, magnetic local time, and interplanetary magnetic field parameters. The magnetic perturbations are characterised using a broadband metric that integrates variability in the 10–600 s period range. This metric captures geomagnetic responses to transient, impulsive, and wave-like magnetospheric phenomena as well as ULF wave activity. We find that rapid geomagnetic variability at the highest latitudes, in the cusp and beyond, is sensitive to seasonal change, indicating that the ionospheric currents generating the observed ground magnetic signatures depend on solar illumination to obtain sufficient conductivities. This effect, in concert with dipole tilt, was investigated, and a clear cusp-related population during summer was found. In winter, this population weakens owing to a lack of sunlight and merges with other signals such as ULF waves associated with Alfvénic interhemispheric bouncing further south, and, thus, becomes indiscernible. Furthermore, we discuss other aspects of our statistical analysis and briefly address relationships between our detected signals and other known cusp phenomena.