Propagation of Very Oblique Chorus Waves Near a Plasmaspheric Plume Boundary

In a case study using Van Allen Probe B, we investigate chorus wave observations near the western edge of a plasmaspheric plume characterized by steep density gradients. Initially, wave vectors are oriented anti-Earthward, but they become very oblique and eastward as the probe approaches the plume boundary. Treating the plume boundary as an azimuthal density gradient, ray tracing can reproduce the observed wave vector directions. Ray tracing shows that the azimuthal density gradient strongly inclines the wave vectors eastward. Consequently, waves are reflected upon reaching the Gendrin angle and cannot enter the plume. We establish an analytical criterion for the azimuthal density enhancement, determining the condition for chorus waves to enter plumes near the equatorial region. Our results partly explain the oblique chorus near plumes observed by Hartley, Chen et al. (2022, https://doi.org/10.1029/2022GL098710), offering insight into wave-particle interactions by chorus waves with the influence of azimuthal density structures.