Spatial Distribution of Pc1/EMIC Waves Relative to the Nightside Ionospheric Footprint of the Plasmapause

Pc1 pulsations cover the 0.2–5 Hz frequency range with electromagnetic ion cyclotron (EMIC) waves of magnetospheric origin being generally accepted as their most important source. In the ionosphere, the initially transverse EMIC waves can couple to the compressional mode and propagate long distances in the ionospheric waveguide. By studying Pc1 waves in the topside ionosphere, we can obtain information on the spatial distribution of both the transverse (incident EMIC) and compressional waves. In the present paper, we make use of a new Swarm L2 product developed for characterizing Pc1 waves to explore the spatial distribution of these waves relative to the midlatitude ionospheric trough (MIT), which corresponds to the ionospheric footprint of the plasmapause (PP) at night. It is shown that the vast majority of Pc1 events are located inside the plasmasphere and that the spatial distributions clearly follow changes in the MIT/PP position at all levels of geomagnetic activity. In the topside ionosphere, the number of transverse Pc1 (incident EMIC) waves rapidly decreases outside the PP, while their occurrence peak is located considerably equatorward (|ΔMlat| = −5° to −15°) of the PP footprint, that is, inside the plasmasphere. On the other hand, the compressional Pc1 waves can propagate in the ionosphere outside the PP toward the poles, while in the equatorial direction there is a secondary maximum in their spatial distribution at low magnetic latitudes. Our results suggest that mode conversion taking place in the inductive ionosphere plays a crucial role in the formation of the presented spatial distributions.