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

Abstract 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.