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The unexpected origin of plasmaspheric hiss from discrete chorus emissions


Plasmaspheric hiss1 is a type of electromagnetic wave found ubiquitously in the dense plasma region that encircles the Earth, known as the plasmasphere2. This important wave is known to remove3,4,5 the high-energy electrons that are trapped along the Earth’s magnetic field lines6, and therefore helps to reduce the radiation hazards to satellites and humans in space. Numerous theories to explain the origin of hiss have been proposed over the past four decades, but none have been able to account fully for its observed properties. Here we show that a different wave type called chorus7,8, previously thought to be unrelated to hiss, can propagate into the plasmasphere from tens of thousands of kilometres away, and evolve into hiss. Our new model naturally accounts for the observed frequency band of hiss, its incoherent nature, its day–night asymmetry in intensity, its association with solar activity and its spatial distribution. The connection between chorus and hiss is very interesting because chorus is instrumental in the formation of high-energy electrons outside the plasmasphere9, whereas hiss depletes these electrons at lower equatorial altitudes3,4.

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Figure 1: Evolution of chorus into plasmaspheric hiss.
Figure 2: Penetration characteristics of chorus rays.
Figure 3: CRRES satellite data showing chorus and hiss emissions.

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J.B. acknowledges support from the National Science Foundation’s (NSF) Geospace Environment Modeling (GEM) post-doctoral award and NASA, and R.M.T. acknowledges support from an NSF GEM grant. N.P.M. acknowledges support from the Natural Environment Research Council, UK. We thank Roger R. Anderson for provision of the CRRES plasma wave data used in this study.

Author Contributions J.B. performed all the calculations shown in the paper, wrote the manuscript and Supplementary Information section, and composed all the figures. R.M.T. provided consultation on the theoretical aspects of the work and manuscript writing. N.P.M. provided global models of CRRES/LEPA data used in the Landau damping calculations, the dynamic spectrogram shown in Fig. 3, and input into the manuscript writing.

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Correspondence to Jacob Bortnik.

Supplementary information

Supplementary Information

The file contains Supplementary Figures S1-S3 with Legends, Supplementary Discussion sections 1 and 2 and additional references. The Supplementary Figures: illustrate the possible distribution of chorus wave power as a function of wave-normal angle, at the source location (Figure S1), and a single ray path that exhibits the long-lived ‘cyclic trajectory’ behaviour for the first 5 seconds of propagation (Figure S2) and its entire lifetime (Figure S3). The Supplementary Discussion sections 1 & 2: provide a historical context for our work in light of past research on hiss, various suggestions for origin mechanisms, and comments on related work (Section 1). Section 2 provides a discussion on the detailed microphysics of a single ray trajectory from its initiation to its ultimate demise, and gives the reader an intuitive sense for the processes resulting in the entry of certain chorus rays into the plasmasphere and the reason for their dramatically prolonged lifetimes. (PDF 629 kb)

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Bortnik, J., Thorne, R. & Meredith, N. The unexpected origin of plasmaspheric hiss from discrete chorus emissions. Nature 452, 62–66 (2008).

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