Search for: All records

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.

Observations of magnetospheric chorus being triggered by lightning‐induced whistlers are rare but provide a unique opportunity to remotely diagnose wave‐particle interactions in the Earth's radiation belts. The observations presented herein are unique in that whistlers, originating from lightning, are seen to trigger upper band chorus repeatedly over the course of 2 hr. Each whistler exhibits a distinct upper frequency cutoff that is used to estimate the anisotropy of the hot plasma distribution. Resulting anisotropy estimates are in good agreement with previous in situ measurements. While the anisotropy determines wave growth in the linear regime, access to the nonlinear regime requires the in situ wave amplitude to exceed the threshold for phase trapping of energetic electrons. The results suggest that while upper band chorus is less favorable to be spontaneously generated, the conditions in this band are more conducive for triggering of the chorus instability by an external input wave.