Magnetic local time and latitude distribution of ionospheric large-scale EMIC wave events: Swarm observations

In the present work, the magnetic local time and latitude (MLT and MLat) distributions of ionospheric large-scale (> 20° MLat) electromagnetic ion cyclotron waves were investigated using high-resolution 50-Hz geomagnetic field data from Swarm A and B satellites. Both longitudinal and transverse waves were studied in a comparative manner for different geomagnetic activities and seasons. Frequent occurrences of large-scale waves in the South Atlantic Anomaly and North America, where the longitudinal waves propagate over the longest distance, were observed. Waves appear mostly in the 02–10 MLT sector wherein the pre-noon longitudinal waves propagate farthest in latitudinal range. With the enhancement of geomagnetic activity, both transverse and longitudinal waves increase in occurrence. The dayside occurrence rate is higher during weak geomagnetic activity, whereas the situation is reversed on the nightside and duskside. The dayside waves are located outside of the mid-latitude trough, and the nightside waves are located near (inside) the equatorward boundary of the mid-latitude trough. Large-scale waves tend to occur at the equinox when the absolute value of the dipole tilt angle is less than 10°, while the long-distance transmission in the waveguide occurs in the pre-noon in summer. Longitudinal waves propagate in the region where the electron density is higher than that of the transverse waves. This study reveals potential factors that contribute to the occurrence of ionospheric waveguide events.