The angular distribution of chorus waves and the importance of plumes in the chorus-to-hiss mechanism

Van Allen Probes EMFISIS observations in both survey and burst mode are coupled with ray tracing simulations to determine the percentage of chorus wave power that exists with the conditions required to access the plasmasphere and evolve into plasmaspheric hiss. For most chorus source locations, we find only an extremely small fraction of power with the required wave vector orientation. The exception to this is when the chorus source is located on the edge of a plasmaspheric plume. In these cases, strong azimuthal density gradients modify the wave propagation so that large fractions, up to 96%, of chorus wave power can gain access to the plasmasphere. We conclude that this region of the magnetosphere, close to plasmaspheric plume structures, provides an important access region for chorus waves to enter the plasmasphere. However, given that chorus wave power is typically weaker in the region where plumes are frequently observed, this result suggests that it is unlikely that chorus significantly contributes to plasmaspheric hiss. Finally, having identified this crucial region, we propose to directly study the wave properties and propagation characteristics of chorus that occurs in close proximity to plasmaspheric plumes.