Rapid local acceleration of relativistic radiation-belt electrons by magnetospheric chorus

Abstract

Recent analysis of satellite data obtained during the 9 October 2012 geomagnetic storm identified the development of peaks in electron phase space density1, which are compelling evidence for local electron acceleration in the heart of the outer radiation belt2,3, but are inconsistent with acceleration by inward radial diffusive transport4,5. However, the precise physical mechanism responsible for the acceleration on 9 October was not identified. Previous modelling has indicated that a magnetospheric electromagnetic emission known as chorus could be a potential candidate for local electron acceleration6,7,8,9,10, but a definitive resolution of the importance of chorus for radiation-belt acceleration was not possible because of limitations in the energy range and resolution of previous electron observations and the lack of a dynamic global wave model. Here we report high-resolution electron observations11 obtained during the 9 October storm and demonstrate, using a two-dimensional simulation performed with a recently developed time-varying data-driven model12, that chorus scattering explains the temporal evolution of both the energy and angular distribution of the observed relativistic electron flux increase. Our detailed modelling demonstrates the remarkable efficiency of wave acceleration in the Earth’s outer radiation belt, and the results presented have potential application to Jupiter, Saturn and other magnetized astrophysical objects.

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Figure 1: Conditions leading to rapid relativistic electron acceleration on 9 October 2012.
Figure 2: Modelling the global distribution of chorus and electron scattering rates.
Figure 3: Comparison of Fokker–Planck simulation results with observations.

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Acknowledgements

This work was supported by JHU/APL contracts 967399 and 921647 under NASA’s prime contract NAS5-01072. The analysis at UCLA was supported by the EMFISIS sub-award 1001057397:01 and by the ECT sub-award 13-041. We thank OMNIweb for providing geomagnetic indices and solar wind parameters used in this study and the NOAA POES team for providing POES electron data.

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Authors

Contributions

R.M.T. developed the project, directed the theoretical analysis, and was mainly responsible for writing the paper. W.L. conducted the data analysis on electrons and chorus waves and helped construct the global model of chorus. B.N. developed the method to infer wave intensity using POES electron data, performed the wave modelling, and evaluated the diffusion coefficients. Q.M. performed the Fokker–Planck simulations. J.B. and L.C. assisted with the global wave modelling. D.N.B. and S.G.K. provided the REPT data. H.E.S. and G.D.R. led the RBSP-ECT energetic particle team and G.D.R. and M.G.H. provided the phase space density. M.G.H. was responsible for the electron pitch angle data. C.A.K., W.S.K. and G.B.H. provided the EMFISIS wave data. J.B.B., J.F.F. and S.G.C. provided the MagEIS data.

Corresponding author

Correspondence to R. M. Thorne.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Bz , SYM-H and evolution of ratios (30–100 keV) in various MLT sectors.

a, The z component of the interplanetary magnetic field in the GSM coordinate. b, SYM-H index. ch, The ratio (colour coded) of precipitated and trapped electron fluxes (30–100 keV) measured by multiple POES satellites in six distinct MLT sectors. The grey shaded region represents the time interval in which the simulation was performed.

Extended Data Figure 2 Electron diffusion rates due to chorus scattering at L = 5.

Drift and bounce averaged electron pitch angle diffusion coefficients (colour coded) (, ad), momentum diffusion coefficients (, eh), and mixed (pitch angle, momentum) diffusion coefficients (, il) for four ut intervals during the observed electron acceleration event, based on the parameters shown in Extended Data Table 1.

Extended Data Figure 3 Spline fitting of the observed electron PSD data.

The red solid line represents the spline fitting result, and the ‘plus sign’ data points represent the observed electron phase space density at 90° pitch angles from both MagEIS and REPT two hours before 20:00 ut on 8 October 2012, when Van Allen probe B travelled through L ≈ 5.

Extended Data Figure 4 Schematic illustration of local electron acceleration by chorus.

The top panel shows electron fluxes before (left) and after (right) the storm. The injection of low-energy plasma sheet electrons into the inner magnetosphere (1) causes chorus wave excitation in the low-density region outside the cold plasmasphere (2). Local energy diffusion associated with wave scattering leads to the development of strongly enhanced phase space density just outside the plasmapause (3). Subsequently, radial diffusion can redistribute the accelerated electrons inwards or outwards from the developing peak (4).

Extended Data Table 1 Adopted model parameters

Supplementary information

Supplementary Information

This file contains: Supplementary Section SI, a more detailed description of how the global wave amplitudes of chorus emissions is constructed from the ratio of precipitated to trapped (30–100 keV) electron flux obtained from POES spacecraft data; Supplementary Section S2, a detailed description of how electron diffusion coefficients using available data on the global wave characteristics of chorus and the global distribution of plasma density were constructed and Supplementary Section S3, additional information on the boundary conditions and the initial electron distribution used in the 2D simulation. (PDF 350 kb)

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Thorne, R., Li, W., Ni, B. et al. Rapid local acceleration of relativistic radiation-belt electrons by magnetospheric chorus. Nature 504, 411–414 (2013). https://doi.org/10.1038/nature12889

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