Hemispheric asymmetry of ionospheric electromagnetic ion cyclotron waves and the association with the South Atlantic Anomaly

This study investigates ionospheric electromagnetic ion cyclotron (EMIC) waves on the basis of Swarm satellite observations from 2015 to 2018, focusing on hemispheric asymmetries and their modulation by the South Atlantic Anomaly (SAA). Both the power spectral density (PSD) and occurrence rate of EMIC waves are significantly enhanced in the SAA relative to other regions at similar latitudes. Waves observed in the SAA region generally exhibit lower frequencies and originate from slightly higher L shells than their northern counterparts. He-band waves show stronger hemispheric contrasts in PSD, while O-band waves display larger differences in frequencies and occurrence rates. The Southern Hemisphere shows higher PSD and occurrence rates than the Northern Hemisphere during 00–12 local time, which becomes particularly pronounced near equinoxes. Although PSD and occurrence rate enhancements in the SAA region are more prominent during geomagnetically quiet periods, hemispheric differences in frequency and L shell intensify under disturbed conditions. EMIC wave occurrence also increases with solar wind dynamic pressure; PSD asymmetry is greater during low-pressure intervals, whereas frequency differences become more pronounced during high-pressure intervals. These findings deepen our understanding of the hemispheric asymmetry of EMIC waves and the mechanisms by which they are modulated by the SAA.