Statistical Properties of Quasi-Periodic Electromagnetic Ion Cyclotron Waves: ULF Modulation Effects

Electromagnetic ion cyclotron (EMIC) waves effectively scatter relativistic electrons in Earth's radiation belts and energetic ions in the ring current. Empirical models parameterizing the EMIC wave characteristics are important elements of inner magnetosphere simulations. Two main EMIC wave populations included in such simulations are the population generated by plasma sheet injections and another population generated by magnetospheric compression due to the solar wind. In this study, we investigate a third class of EMIC waves, generated by hot plasma sheet ions modulated by compressional ultra-low frequency (ULF) waves. Such ULF-modulated EMIC waves are mostly observed on the dayside, between magnetopause and the outer radiation belt edge. We show that ULF-modulated EMIC waves are weakly oblique (with a wave normal angle $\approx 20^{◦}\pm 10^{◦}$) and narrow-banded (with a spectral width of $\sim 1/3$ of the mean frequency). We construct an empirical model of the EMIC wave characteristics as a function of $L$-shell and MLT. The low ratio of electron plasma frequency to electron gyrofrequency $\left(f_{pe}/f_{ce}\sim 5-10\right)$ around the EMIC wave generation region does not allow these waves to scatter energetic electrons. However, these waves provide very effective (comparable to strong diffusion) quasi-periodic precipitation of plasma sheet protons.