Nonlinear Wave Growth Analysis of Chorus Emissions modulated by field line resonance and mirror-mode ULF waves

Previous studies have found that chorus waves can be generated in the troughs of the compressional ULF waves. Here, we report for the first time the periodic excitation of chorus waves near ULF wave crests, which is attributed to different modes of the observed ULF waves. We demonstrate that latitudinal profile of the ULF waves can play important roles in excitation of chorus waves on the basis of nonlinear generation theory of chorus emissions. Field line resonance (FLR) mode results in chorus wave excitation near ULF wave troughs, while the mirror mode causes chorus near the wave crest. Chorus wave occurrence near the ULF wave crests is attributed to the antisymmetric field profiles of mirror-mode ULF waves, which periodically modulate the threshold amplitude by modifying the second-order derivative of the background dipole field. FLR ULF wave with the symmetric profile of magnetic field with respect to the equator fosters chorus wave excitation near ULF wave trough. The good agreement between the theory and the observations highlights the effects of ULF wave field configuration in modulating chorus waves.