Generation of Unusually Low Frequency Plasmaspheric Hiss

TitleGeneration of Unusually Low Frequency Plasmaspheric Hiss
Publication TypeJournal Article
Year of Publication2014
AuthorsChen, L, Thorne, RM, Bortnik, J, Li, W, Horne, RB, Reeves, GD, Kletzing, CA, Kurth, WS, Hospodarsky, GB, Spence, HE, Blake, JB, Fennell, JF
JournalGeophysical Research Letters
Start Page5702
Date Published08/2014
KeywordsChorus; Generation; Plasmaspheric Hiss; Ray Tracing; Van Allen Probes
AbstractIt has been reported from Van Allen Probe observations that plasmaspheric hiss intensification in the outer plasmasphere, associated with a substorm injection on Sept 30 2012, occurred with a peak frequency near 100 Hz, well below the typical plasmaspheric hiss frequency range, extending down to ~20 Hz. We examine this event of unusually low frequency plasmaspheric hiss to understand its generation mechanism. Quantitative analysis is performed by simulating wave ray paths via the HOTRAY ray tracing code with measured plasma density and calculating ray path-integrated wave gain evaluated using the measured energetic electron distribution. We demonstrate that the growth rate due to substorm injected electrons is positive but rather weak, leading to small wave gain (~10 dB) during a single equatorial crossing. Propagation characteristics aided by the sharp density gradient associated with the plasmapause, however, can enable these low frequency waves to undergo cyclic ray paths, which return to the unstable region leading to repeated amplification to yield sufficient net wave gain (>40 dB) to allow waves to grow from the thermal noise.
Short TitleGeophys. Res. Lett.

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