SONIC RESONANCE. “Electromagnetic ion cyclotron (EMIC) waves in turn are excited near the duskside plasmapause as a result of cyclotron resonance with anisotropic ring current ions, and also interact with the van Allen belt electron population. Again, it is likely that more than one process is active in the inner magnetosphere during any given time.”
https://link.springer.com/article/10.12942/lrsp-2007-1