The saturation properties of EMIC waves and the associated heating of cold ions

Electromagnetic ion cyclotron (EMIC) waves in the Earth’s inner magnetosphere are driven by ring current ions and play an essential role in electron and ion dynamics. Since their frequency ranges (0.1 ~ 10 Hz) are close to the cyclotron frequencies of protons (H⁺), singly ionized helium and oxygen ions (He⁺, O⁺), they can efficiently heat the cold ions via cyclotron resonance. In this study, we run a hybrid simulation in a homogeneous plasma including hot H⁺ (~100 keV) cold H⁺, He⁺, and O⁺, to investigate the EMIC wave properties after saturation and the associated heating of cold ions. We find that the spectrum of EMIC waves evolves towards smaller frequency as the waves saturate, resulting from the relaxation of the temperature anisotropy of hot H⁺. Accompanying with the frequency evolution, the efficient scattering on the ions shifts to heavier ions; that is,  the cold H⁺, He⁺, and O⁺ are heated sequentially. The H⁺ and He⁺ are mainly heated perpendicularly with respect to the background magnetic field line, while the O⁺ are mainly heated near the field-aligned direction. Our study can advance the understanding of EMIC wave properties and their coupling with cold ions in the magnetosphere.