Polar Cap ULF Pulsations: Coordinated Radar-Magnetometer Observations

This study uses multi-instrument geomagnetic and ionospheric observations in Antarctica to examine the causative mechanisms of the ultra-low-frequency (ULF) variations of the electrodynamics of the polar cap—the least explored part of the coupled solar wind-magnetosphere-ionosphere system. The ionospheric oscillations corresponding to Pc5–6 pulsation band (quasi-periods about 3–15 min) were detected by SuperDARN Mac-Murdo radar. The ground geomagnetic response B was examined using magnetometers at polar latitudes in Antarctica. We have selected events with quasi-periodic variations of the ionospheric Doppler velocity V and simultaneously observed by ground magnetometers. Supposedly, this class of events is produced by magnetospheric MHD waves. We have determined the effective wave impedance, that is V/B ratio, which enabled us to identify the physical nature of specific polar cap pulsations. This type of polar Pc5–6 pulsations has been associated with Alfven mode, though the occurrence of periodic disturbances in the region with open or strongly extended field lines seems puzzling.