Statistical Properties of Quasi-periodic Electromagnetic Ion Cyclotron
Waves: ULF Modulation Effects
Abstract
Electromagnetic ion cyclotron (EMIC) waves effectively scatter
relativistic electrons in the Earthâ\euro™s radiation belts and
energetic ions in the ring current. Empirical models parameterizing 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 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 wave normal angle
$\approx
20^\circ\pm10^\circ$)
and narrow banded (with spectral width of $\sim 1/3$ of
the mean frequency). We further construct an empirical model of EMIC
wave characteristics as a function of $L$-shell and MLT. The low ratio
of plasma frequency to electron gyrofrequency
($f_{pe}/f_{ce}\sim 5-10$) 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.