H+, He+, He++, O++, N+ EMIC Wave Occurrence and Its Dependence on Geomagnetic Conditions: Results From 7 Years of Van Allen Probes Observations

Abstract

Electromagnetic ion cyclotron (EMIC) waves are believed to play an important role in the dynamics of the inner magnetosphere, including the ring current, the radiation belts and potentially, the cold plasma. In this work, we investigate their occurrence in the magnetosphere and the geomagnetic and solar wind conditions which lead to their excitation. We use an automated detection algorithm of EMIC waves observed by Van Allen Probes over the entire mission duration between 2012 and 2019. Consistent with earlier studies, we find that the H⁺ band occurrence maximizes in the dayside magnetosphere during enhancements of solar wind dynamic pressure. Both the H⁺ and He⁺ band are also generated along the duskside magnetosphere during disturbed geomagnetic conditions. In addition, to H⁺ and He⁺ bands commonly surveyed, we investigate the occurrence of H⁺ waves above and below 0.5 H⁺ gyrofrequency, as well as wave occurrence in the N⁺ and O⁺⁺ bands. Most H⁺ waves are observed in the band below 0.5f_H+. We find several events in the N⁺ band, indicative of their very low occurrence. The O⁺⁺ band is observed during disturbed geomagnetic conditions and high solar wind dynamic pressure at low L-shells. Its radial localization coincides with the O⁺⁺ torus. This study provides a comprehensive picture of EMIC wave distribution and insight into ion composition in the inner magnetosphere during variable geomagnetic conditions.