Distribution of Whistler-Mode Chorus Wave Normal Angles and the Implications for the Chorus-Hiss Connection

The distribution of polar and azimuthal wave vector angles is evaluated for whistler-mode chorus waves using data from the Van Allen Probes EMFISIS instrument. It is found that the majority of wave vectors are azimuthally oriented in the anti-Earthward direction. A bimodal distribution of polar wave angles is observed with a primary population of approximately field-aligned waves and a much smaller secondary population of oblique waves. This result has implications for models of chorus waves which enter the plasmasphere and evolve into plasmaspheric hiss. Ray tracing simulations typically require an initially oblique polar wave vector angle with the azimuthal component of the wave vector oriented Earthwards if a chorus wave is to enter the plasmasphere and be a source of plasmaspheric hiss. The presence of steep azimuthal density gradients in the noon-through-dusk sector due to the plasmaspheric bulge and/or plasmaspheric plumes means that chorus waves with a broader range of both polar wave vector angles and azimuthal wave vector angles can gain access to the plasmasphere. The results of this study suggest that in the absence of azimuthal density gradients, the observed occurrence rates of chorus waves with the initial conditions required to enter the plasmasphere are not substantial. As such, azimuthal density gradients are actually a requirement if a significant population of chorus waves are to enter the plasmasphere and evolve into plasmaspheric hiss.