A marriage between satellite observations and modelling has shown that acceleration of electrons in the magnetosphere can be explained by scattering of these particles by plasma oscillations known as chorus waves. See Letter p.411
Both dramatic dropout1 and increase2,3 in the flux of high-energy electrons trapped in Earth's dipole-like magnetic field have been reported along with the formation of a storage ring4 — a third 'radiation belt' nested between the usual inner and outer zones of million-electronvolt ionized particles (plasma) that surround Earth. This ring lies in a region called the slot, which is formed by scattering of electrons by plasma waves and their loss to the atmosphere5,6. On page 411 of this issue, Thorne et al.7 present convincing evidence for local acceleration of electrons by the same type of plasma wave or oscillation that causes electron loss — called a whistler wave because of its occurrence in the audio frequency range. This mode of oscillation occurs in two types depending on the density of the cold plasma in the inner region of the magnetosphere: 'hiss' and 'chorus'. These colourful names go back to the early days of listening to these modes with audio-range radio receivers8,9.
Figure 1 shows a classic picture of Earth's magnetosphere and the wave modes originally sketched by Thorne and Kennel10. These have subsequently been augmented with other plasma-wave types11,12 that affect the radiation-belt electrons, including ultra-low frequency and electromagnetic ion cyclotron (EMIC) waves. Electrons and ions transported from the tail of the magnetosphere excite chorus and EMIC waves at the dawn and dusk sides of the magnetosphere (right and left parts of the figure), respectively.
In their study, Thorne and colleagues report observations taken by the Van Allen Probes launched last year13. The satellites recorded a rapid electron-acceleration event that occurred during a disturbance of Earth's magnetosphere, known as a geomagnetic storm, on 8–9 October 2012. The authors modelled these observations and showed that chorus waves can explain this acceleration event. What allows these waves to interact so effectively with electrons is the right-hand rotation of the waves' electric field, which occurs in the same direction as electron gyration, allowing electrons to be accelerated. Such polarization of the electric field allows the waves to resonate with the electron gyration, causing either electron loss to the atmosphere or rapid acceleration, depending on the energy of electrons and their relative velocity parallel and perpendicular to the magnetic field. The energy of electrons interacting with the chorus depends on the density of the cold plasma, which co-rotates with Earth.
A dramatic loss of the entire outer-zone million-electronvolt electrons had been observed on 8 October 2012 (ref. 4). Thorne and colleagues' analysis now reveals a rapid rebuilding and enhancement of the outer zone on 9 October in six-dimensional (phase-) space, extended to include the velocity dimension (see Extended Data Fig. 4, where the enhancement is illustrated as a peak on the dawn side of the magnetosphere). The authors compared this enhancement with a phase-space density distribution that peaks at a greater distance from Earth than the observed enhancement does. Observations from the Van Allen Probes have shown3 the rapid development of this peak now modelled by the authors, but the peak had been difficult to explain with radial diffusion in phase-space density from greater distance to where the peak is located — long the paradigm for replenishment of the outer-zone electrons14. The authors' modelling now demonstrates that this effect can cause the peak.
Thorne and colleagues' study represents a breakthrough in understanding the complex interplay of radiation-belt electrons with plasma waves, which affects the electrons' acceleration and loss. More investigations in this field should combine the type of analysis performed by Thorne et al. with models of radial transport that include measured electric and magnetic-field amplitudes in the ultra-low-frequency wave regime15, which oscillate with the longitudinal-drift period of electrons14. Future work should also incorporate the effects of large-amplitude coherent whistler waves16, which have been observed in improved high-resolution measurements from the Van Allen Probes17.
Turner, D. L., Shprits, Y., Hartinger, M. & Angelopoulos, V. Nature Phys. 8, 208–212 (2012).
Horne, R. B. et al. Nature 437, 227–230 (2005).
Reeves, G. D. et al. Science 341, 991–994 (2013).
Baker, D. N. et al. Science 340, 186–190 (2013).
Kennel, C. F. & Petschek, H. E. J. Geophys. Res. 71, 1–28 (1966).
Lyons, L. R. & Thorne, R. M. J. Geophys. Res. 78, 2142–2149 (1973).
Thorne, R. M. et al. Nature 504, 411–414 (2013).
Morgan, M. G. & Allcock, G. McK. Nature 177, 30–31 (1956).
Helliwell, R. A. Whistlers and Related Ionospheric Phenomena (Stanford Univ. Press, 1965).
Thorne, R. M. & Kennel, C. F. J. Geophys. Res. 76, 4446–4453 (1971).
Summers, D., Thorne, R. M. & Xiao, F. J. Geophys. Res. 103, 20487–20500 (1998).
Shprits, Y. Y., Li, W. & Thorne, R. M. J. Geophys. Res. 111, A12206 (2006).
Kessel, R. L., Fox, N. J. & Weiss, M. Space Sci. Rev. 179, 531–543 (2012).
Fälthammar, C.-G. J. Geophys. Res. 70, 2503–2516 (1965).
Loto'aniu, T. M. et al. J. Geophys. Res. 115, A12245 (2010).
Cattell, C. et al. Geophys. Res. Lett. 35, L01105 (2008).
Wygant, J. R. et al. Space Sci. Rev. 179, 183–220 (2013).
About this article
Hybrid fluid‐particle simulation of whistler‐mode waves in a compressed dipole magnetic field: Implications for dayside high‐latitude chorus
Journal of Geophysical Research: Space Physics (2017)
Annales Geophysicae (2015)