Observations of Oblique Plasmaspheric Hiss from Van Allen Probes

Abstract:

New insight into plasmaspheric hiss wave properties is provided through statistical analysis of Van Allen Probes EMFISIS observations. Results indicate that the wave normal angle of plasmaspheric hiss is predominantly field aligned at larger L shells. In contrast, at lower L shells, it is found that two populations of plasmaspheric hiss exist; a primary approximately field aligned population, and a secondary more oblique population. Detailed investigation of this oblique secondary population reveals that it is most prevalent inside L = 3, at frequencies with f/f_ce > 0.01 (or f > 700 Hz), during low geomagnetic activity levels, and is generally confined between 19 and 9 in MLT. Apart from the different L shell ranges, the structure of this oblique plasmaspheric hiss population is similar to that observed for obliquely propagating chorus waves in the equatorial region, perhaps suggesting a causal link between the two wave modes. The plausibility of this link is confirmed using the HOTRAY ray tracing code, with the evolution of ray trajectories indicating that chorus waves generated at oblique angles can become field aligned at high latitudes before entering the plasmasphere to be observed as oblique plasmaspheric hiss at low L shells. The observed variation in the oblique plasmaspheric hiss population with geomagnetic activity is also compatible with oblique chorus as the source. That is, the higher fluxes of Landau resonant electrons associated with elevated substorm activity cause an increase in the damping experienced by oblique chorus waves outside of the plasmasphere, stunting the amount of chorus wave power that can access the plasmasphere and evolve into oblique plasmaspheric hiss. These new results highlight the variable distribution in both power spectral density and wave normal angle of two discrete plasmaspheric hiss wave modes. These factors must be considered in future studies if we are to gain a more complete understanding, as well as an accurate predictive capability, of particle interactions with plasmaspheric hiss.