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Pulsating aurora from electron scattering by chorus waves

Abstract

Auroral substorms, dynamic phenomena that occur in the upper atmosphere at night, are caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy1,2. These storms are characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn1,3. Pulsating aurorae, which are quasiperiodic, blinking patches of light tens to hundreds of kilometres across, appear at altitudes of about 100 kilometres in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. This auroral pulsation, with periods of several to tens of seconds, is generated by the intermittent precipitation of energetic electrons (several to tens of kiloelectronvolts) arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere4,5,6,7. A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves8,9,10,11. However, no direct observational evidence of this interaction has been obtained so far12. Here we report that energetic electrons are scattered by chorus waves, resulting in their precipitation. Our observations were made in March 2017 with a magnetospheric spacecraft equipped with a high-angular-resolution electron sensor and electromagnetic field instruments. The measured13,14 quasiperiodic precipitating electron flux was sufficiently intense to generate a pulsating aurora, which was indeed simultaneously observed by a ground auroral imager.

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Figure 1: Schematic of electron scattering by chorus waves, resulting in a pulsating aurora.
Figure 2: Auroral snapshots around the footprint of ERG.
Figure 3: In situ observations by ERG.
Figure 4: Correlations between electron flux and wave power spectral density.

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Acknowledgements

The observations presented here were obtained with the help of Mitsubishi Heavy Industries, Ltd, Meisei Electric Co., Ltd, Hamamatsu Photonics Co. Ltd, YS DESIGN Co., Ltd, NIPPI Co. Ltd, Sumitomo Heavy Industries, Ltd and TIERRA TECNICA Co. Ltd. We acknowledge the work of the members of the ERG project team over several years. Y.M. is supported by JSPS Kakenhi (15H05747, 15H05815 and 16H06286). Y. Kasahara is supported by JSPS Kakenhi (16H04056 and 16H01172). H.U.F. is supported by grant AGS-1004736 from the National Science Foundation (NSF) of the USA. I.S. is supported by JSPS Kakenhi (17H06140). We thank NASA for contract NAS5-02099, S. Mende and E. Donovan for use of the ASI data, the Canadian Space Agency for logistical support in fielding and data retrieval from the ground-based observatory stations, and the NSF for support of the Ground-based Imager and Magnetometer Network for Auroral Studies programme through grant AGS-1004736. The ERG (Arase) satellite science data is available from the ERG Science Centre operated by the Institute of Space and Astronautical Science of the Japan Aerospace eXploration Agency and the Institute for Space–Earth Environmental Research of Nagoya University (https://ergsc.isee.nagoya-u.ac.jp/index.shtml.en). We are grateful to J. Hohl for assistance in editing the manuscript. We also thank N. Umemura for assistance in source data archiving. S. Kasahara thanks T. Mukai and M. Fujimoto for discussions.

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Authors

Contributions

S. Kasahara developed the MEP-e instrument used in this study with S.Y. and T.M., identified the event, analysed the combined dataset and wrote the paper. Y.M. oversaw the production of the combined dataset and discussed its interpretation. Y. Kasahara, S.M. and A.K. provided Plasma Wave Experiment data and discussed the interpretation. A.M. provided MaGnetic Field experiment data. Y. Kazama assisted in the evaluation of MEP-e data through comparison with the Low-Energy Particle experiments – electron analyser. H.U.F. and V.A. provided ASI/THEMIS data and discussed the event and presentation of the results. S. Kurita evaluated the spacecraft footprint with Y.M. and discussed the event. K.K. and K.S. discussed the event and presentation. I.S. oversaw the ERG project and discussed the interpretation of the event. All authors reviewed the manuscript.

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Correspondence to S. Kasahara.

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Extended data figures and tables

Extended Data Figure 1 In situ observations by ERG with an additional dataset.

a, b, Frequency–time spectrograms of the power spectral densities of the electric (a) and magnetic field (b), showing chorus waves. The magenta and white lines indicate 0.5fce and 1.0fce, respectively, based on local magnetic field observations. c–e, Energy–time spectrograms for differential fluxes of loss-cone electrons parallel (pitch angles PA < 2°) (c) and anti-parallel (PA > 178°) (d) to the magnetic field and electrons perpendicular to the magnetic field (PA = 80°–100°) (e). Quasiparallel (PA = 20°–40°) and quasiantiparallel (PA = 140°–160°) electrons show essentially the same trend as that of the perpendicular flux. f, Flux(PA < 2°)/flux(PA = 20°–40°); g, flux(PA > 178°)/flux(PA = 140°–160°). The graphs in b and c are the same as those in Fig. 3a and b, respectively (but replotted here for comparison with a and d). The faint signature of an upper hybrid resonance wave at 12–16 kHz in a at about 11:10 ut is consistent with the assumed density of approximately 3 cm−3.

Extended Data Figure 2 Correlation coefficients for auroral intensity.

a, The colours (red, yellow, cyan and magenta) show the correlation coefficients between the auroral intensity and the loss-cone electron flux. b, Time series data of the loss-cone electron flux and the auroral intensity at a pixel at which electrons and chorus waves have nearly the highest correlation. The auroral intensity is plotted at the same time resolution as the electrons (about 8 s). c, d, The same as a and b, but for the chorus wave intensity. The wave intensity in d is plotted at the same time resolution as the auroral intensity (3 s). In a and c, the background auroral images are magnified around the centre of the field of view of the Pas station. Highest-correlation pixels are consistently located near the centre of both panels, suggesting the spacecraft footprint. The dashed lines in a and c illustrate magnetic coordinates every 2° in latitude and 5° in longitude. The displacement of the model footprint from the high-correlation pixels is approximately −0.5° and −5° in latitude and longitude, respectively, consistent with typical modelling errors20. Cross-correlations were calculated for the period 10:54:00–10:58:00 ut. In other time periods, high-correlation pixels were not commonly obtained, perhaps because of the fine structures of pulsating patches and the equatorial modulation regions near the spacecraft. For example, if the spacecraft leaves the localized modulation region as a result of magnetospheric configuration change, chorus waves and associated electron precipitation disappear at its location, but pulsating patches can continue to be ‘on’ if the equatorial modulation region still exists. In other words, although the spacecraft’s footprint in the ionosphere can leave an illuminated patch owing to spatial reconfiguration of magnetic field line structures or plasma phenomena, the auroral intensity remains high at some pixels. Other reasons that make the above correlations difficult to identify, such as the contribution of soft electron (<10 keV) precipitation to higher-altitude (>100 km) illumination, may be studied in future work.

Extended Data Figure 3 The angular response of MEP-e in two orthogonal directions.

a, Sensor response as a function of elevation angle with respect to the sensor’s mounting plane. b, Response in the sensor’s azimuthal direction, which is orthogonal to the elevation angle. Blue circles, laboratory data; black line, Gaussian model. The model curves were used to obtain the analysis results shown in Extended Data Fig. 4. Profiles for one detector are shown here; similar profiles were obtained for the other 15 detectors.

Extended Data Figure 4 Results of PAD model taking the sensor’s angular resolution into account.

Because of the finite angular resolution, contamination from outside the loss cone cannot be completely negligible. The blue line indicates the model input PAD, which is isotropic except for the step-function drop at the loss-cone angle of 2.5° (the nominal loss-cone angle in the event presented in this paper, based on a local magnetic field of about 100 nT). The red curve shows how the electron PAD is modulated by the effect of the sensor’s finite angular resolution. The grey dashed line indicates the threshold, 2°, for loss-cone selection (that is, if the angle between the centre of the field of view and the magnetic field is smaller than the threshold, the measured flux is considered to be the flux inside the loss cone). For this calculation, the sensor’s field of view is modelled by a Gaussian cone (full-width at half-maximum, 3.5°), based on the ground calibration. For example, even when the middle of the sensor’s field-of-view is centred along the magnetic field line and the actual electron PAD has an ideally empty loss cone, the instrument can inadvertently record about a few tens per cent of the flux from outside the loss cone. In our observations, however, the electron flux in the loss cone most often exhibits a filling ratio larger than 0.5, sometimes about 1, when the precipitation is ‘on’ (Fig. 4 and Extended Data Fig. 1f and g), too large to be explained by contamination alone. Also, synchronization with chorus waves cannot be produced by this instrumental effect.

Supplementary information

Auroral motions obtained by all-sky imagers

Successive clear sky images from two ground stations (Fort Simpson at the upper left, and The Pas at the lower right) are shown as a video. The red cross indicates the nominal spacecraft footprint. Dashed lines illustrate magnetic coordinates every 10o in latitude and 15o in longitude. The presented time period covers that of Fig. 3. (MPG 27428 kb)

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Kasahara, S., Miyoshi, Y., Yokota, S. et al. Pulsating aurora from electron scattering by chorus waves. Nature 554, 337–340 (2018). https://doi.org/10.1038/nature25505

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