Spatial distribution and solar cycle variability of EMIC waves observed by THEMIS

Electromagnetic ion cyclotron (EMIC) waves are one of the most frequently observed plasma wave modes in Earth’s magnetosphere and play an important role in particle precipitation and magnetospheric energy redistribution. In this study, we present a comprehensive statistical analysis of H⁺-band and He⁺-band EMIC waves observed by the THEMIS mission from January 2012 to June 2025. The spatial distribution and wave properties, including occurrence rate, wave amplitude, ellipticity, normal angle, mean frequency, and frequency bandwidth, are systematically examined. Consistent with earlier studies, our results show that H⁺-band EMIC waves predominantly occur in the dawn and afternoon sectors of the outer magnetosphere, while He⁺-band EMIC waves are mainly concentrated in the afternoon sector. In the dawn sector, both H⁺- and He⁺-band EMIC waves exhibit more oblique normal angles and predominantly linear polarization. In contrast, EMIC waves in the afternoon sector tend to have more parallel normal angles and left-hand polarization. In addition, both EMIC wave occurrence and wave properties display clear solar-cycle-dependence. Both H⁺- and He⁺-band EMIC waves have higher occurrence rate during solar minimum than solar maximum. H⁺-band EMIC waves tend to be left-hand polarized during solar minimum and more linearly polarized during solar maximum, whereas He⁺-band EMIC waves exhibit the opposite polarization behavior. These results provide new statistical evidence for the modulation of EMIC wave generation and propagation by the solar cycle.