Spatial Distribution and Wave Property of Dual-Frequency EMIC Waves: Swarm Observations

The spatial distributions of single-frequency and dual-frequency Electromagnetic ion cyclotron (EMIC) waves in the subauroral ionosphere are investigated under varying geomagnetic activities, using high-resolution magnetic field data from dual Swarm satellites spanning from 2015 to 2017. Single-frequency EMIC waves predominantly occur in the dawn sector, whereas dual-frequency waves exhibit peaks around both dawn and dusk. The occurrence rate of dual-frequency waves shows a more pronounced increase with increasing geomagnetic activity. As magnetic storms evolve, both types of EMIC waves shift from dusk to dawn. The South Atlantic Anomaly (SAA) emerges as a high-incidence region for ionospheric EMIC waves. Dual-frequency EMIC waves display lower frequencies compared to other regions. Additionally, the low-frequency components of dual-frequency waves observed at higher latitudes demonstrate greater power density and longer durations than their high-frequency counterparts. This suggests that higher frequency waves experience more significant damping during propagation. Most dual-frequency EMIC waves observed in the ionosphere belong to the O-band and He-band waves, indicating that magnetospheric bands below the cyclotron frequency of H+ are more likely to propagate into the ionosphere.