Van Allen Probes Observations of Chorus Wave Vector Orientations: Implications for the Chorus-to-Hiss Mechanism

Using observations from the Van Allen Probes EMFISIS instrument, coupled with ray tracing simulations, we determine the fraction of chorus wave power with the conditions required to access the plasmasphere and evolve into plasmaspheric hiss. It is found that only an extremely small fraction of chorus occurs with the required wave vector orientation, carrying only a small fraction of the total chorus wave power. The exception is on the edge of plasmaspheric plumes, where strong azimuthal density gradients are present. In these cases, up to 94% of chorus wave power exists with the conditions required to access the plasmasphere. As such, we conclude that strong azimuthal density gradients are actually a requirement if a significant fraction of chorus wave power is to enter the plasmasphere and be a source of plasmaspheric hiss. This result suggests it is unlikely that chorus directly contributes a significant fraction of plasmaspheric hiss wave power. Plain Language Summary Plasmaspheric hiss waves are typically observed inside a high-density region of geospace known as the plasmasphere. Chorus waves are typically observed at higher altitudes, beyond the plasmasphere region, where the density is substantially lower. Despite the differences between these two wave types, it has been proposed that chorus waves may propagate in such a way that they enter the plasmasphere, where they become a source of plasmaspheric hiss. However, this mechanism can only occur if chorus waves have a specific set of initial conditions. In this study, we find that chorus waves are rarely observed with these required conditions. Only in a spatially limited region close to the edge of plasmaspheric plume structures, where chorus wave power is typically weaker, do we observe a significant fraction of chorus waves that exist with the conditions required to propagate into the plasmasphere. This result qualitatively indicates that chorus waves may not be a substantial source of plasmaspheric hiss.