Electromagnetic ion cyclotron (EMIC) waves, driven by ring current ion temperature anisotropy in the Earth’s magnetosphere, play a key role in accelerating and precipitating relativistic electrons in the radiation belts. Their excitation and saturation are significantly affected by the surrounding cold plasma. Previous studies have shown that background cold helium ions can influence the growth and saturation of EMIC waves, yet the role of cold oxygen ions in wave saturation remains less understood. In this paper, we use linear theory and nonlinear hybrid simulation to investigate the effect of cold oxygen ions in the EMIC wave growth and saturation in a homogeneous plasma containing hot and cold protons, cold helium and cold oxygen ions. Our findings reveal that increasing the cold oxygen ion concentration decreases the EMIC wave growth rate and broadens the spectral width of stop bands near the helium and oxygen gyrofrequencies. Furthermore, an increasing oxygen ion concentration notably reduces the saturation amplitude of EMIC waves in cases where the helium band is dominant, while cases with a dominant hydrogen band remain unaffected. Cold ions are heated during wave excitation, with cold helium ions heating more significantly than cold protons and oxygen ions. However, an increasing cold oxygen ion concentration reduces the heating efficiency for cold helium ions. These results offer insights into how cold plasma modifies the spectral properties and amplitudes of EMIC waves, shedding light on energy transfer from hot protons to cold plasma through EMIC waves.

Magnetosonic waves are electromagnetic emissions from a few to 100 Hz primarily confined near the magnetic equator both inside and outside the plasmasphere. Previous studies proved that MS waves can transport equatorially mirroring electrons from an equatorial pitch angle of 90$^\circ$ down to lower values by bounce resonance. But the dependence of bounce resonance effect on wave or background plasma parameters is still unclear. Here we applied a test particle simulation to investigate electron transport coefficients, including diffusion and advection coefficients in energy and pitch angle, due to bounce resonance with MS waves. We investigate five wave field parameters, including wave frequency width, wave center frequency, latitudinal distribution width, wave normal angle and root-mean-square of wave magnetic amplitude, and two background parameters, $L$-shell value and plasma density. We find different transport coefficients peaks resulted by different bounce resonance harmonics. Higher order harmonic resonances exist, but the effect of fundamental resonance is much stronger. As the wave center frequency increases, higher order harmonics start to dominate. With wave frequency width increasing, the energy range of effective bounce resonance broadens, but the effect itself weakens. The bounce resonance effect will increase when we decrease the wave normal angle, or increase the wave amplitude, latitudinal distribution width, L-shell value, and plasma density. The parametric study will advance our understanding of the favorable conditions of bounce resonance.