Complex Whistler-Mode Wave Features Created by a High Density Plasma Duct in the Magnetosphere

A Van Allen Probes observation of a high-density duct alongside whistler-mode wave activity shows several distinctive characteristics: (a)-within the duct, the wave normal angles (WNA) are close to zero and the waves have relatively large amplitudes, this is expected from the classic conceptualization of ducts. (b)-at L-shells higher than the duct's location a large "shadow" is present over an extended region that is larger than the duct itself, and (c)-the WNA on the earthward edge of the duct is considerably higher than expected. Using ray-tracing simulations it is shown that rays fall into three categories: (a) ducted (trapped and amplified), (b) reflected (scattered to resonance cone and damped), and (c) free (non-ducted). The combined macroscopic effect of all these ray trajectories reproduce the aforementioned features in the spacecraft observation. The near-earth space environment is a complex system that is dominated by the interaction of radio waves with charged particles. A class of radio waves known as "whistler-mode" waves, have a drastic impact on energetic particles; understanding properties of whistler waves is thus of considerable interest. It is well-known that whistler-mode waves can be trapped within "ducts," which are perturbations in plasma density. Ducts act like pipes that guide waves over thousands of kilometers and routinely connect one hemisphere to the other. Although ducting has been studied for over 75 years, prior research has primarily focused on this guiding effect. Instead, we use spacecraft observations alongside computer simulations to show that ducts have a dramatic effect on waves that are never directly confined to the duct. It is found that a "shadow" region extends far outside the duct caused by trapped waves that leave behind a gap in power. Furthermore, external waves also collide with the duct and ride along the edge at an unexpectedly oblique angle. The results of the work indicate that wave propagation in the vicinity of ducts is more complex than previously thought and must be carefully examined for an accurate understanding of the space environment. A Van Allen Probes duct observation shows that whistler waves are field aligned inside, absent on one side, and oblique on the other Raytracing shows that rays are either trapped in the duct, reflected toward the resonance cone, or free (non-ducted) Power density and wave normal angle distribution constructed from 10,000 rays reproduces the observed wave properties