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Active auroral arc powered by accelerated electrons from very high altitudes

Imajo, Shun Miyoshi, Yoshizumi Kazama, Yoichi Asamura, Kazushi Shinohara, Iku Shiokawa, Kazuo Kasahara, Yoshiya Kasaba, Yasumasa Matsuoka, Ayako Wang, Shiang-Yu Tam, Sunny W. Y. Chang, Tzu‑Fang Wang, Bo‑Jhou Angelopoulos, Vassilis Jun, Chae-Woo Shoji, Masafumi Nakamura, Satoko Kitahara, Masahiro Teramoto, Mariko Kurita, Satoshi Hori, Tomoaki 京都大学 DOI:10.1038/s41598-020-79665-5

2021.01.18

概要

Bright, discrete, thin auroral arcs are a typical form of auroras in nightside polar regions. Their light is produced by magnetospheric electrons, accelerated downward to obtain energies of several kilo electron volts by a quasi-static electric field. These electrons collide with and excite thermosphere atoms to higher energy states at altitude of ~ 100 km; relaxation from these states produces the auroral light. The electric potential accelerating the aurora-producing electrons has been reported to lie immediately above the ionosphere, at a few altitudes of thousand kilometres1. However, the highest altitude at which the precipitating electron is accelerated by the parallel potential drop is still unclear. Here, we show that active auroral arcs are powered by electrons accelerated at altitudes reaching greater than 30, 000 km. We employ high-angular resolution electron observations achieved by the Arase satellite in the magnetosphere and optical observations of the aurora from a ground-based all-sky imager. Our observations of electron properties and dynamics resemble those of electron potential acceleration reported from low-altitude satellites except that the acceleration region is much higher than previously assumed. This shows that the dominant auroral acceleration region can extend far above a few thousand kilometres, well within the magnetospheric plasma proper, suggesting formation of the acceleration region by some unknown magnetospheric mechanisms.

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Acknowledgements

The present study was supported by JSPS KAKENHI 15H05747, 16H06286, 17H00728, 20H01959, and 19K03949.

This study was supported by JSPS Bilateral Open Partnership Joint Research Projects. The LEP-e development

is supported by the Academia Sinica, the National Cheng Kung University, and also the Ministry of Science

and Technology of Taiwan under Contract No. MOST 105-311z1-Y-001-042 and MOST 106-2111-M-001-011.

T.-F. Chang acknowledges support from Taiwan grant MOST 109-2635-M-006-001. The authors would also like

to extend sincere thanks to all members of the ERG project, the ERG Science Center, the THEMIS team, and

contributors to SPEDAS and IDL GEOPACK software.

Author contributions

S.I. identified the event, analyzed the combined dataset, and wrote the paper. Y.M. oversaw the production of the

combined dataset and discussed its interpretation. I.S. oversaw the ERG project and discussed the interpretation

of the event. K.S. discussed the interpretation of the event and drafting the paper. Y. Kazama, S-Y.W., S.W.Y.T.,

T-F.C., B-J.W., and C-W.J. provided the low-energy particle experiments-electron analyser (LEP-e) data. K.A.

provided the low-energy particle experiments-ion analyser (LEP-i) data and discussed the interpretation of the

event. Y. Kasahara, Y. Kasaba, M.S., S.N., and M.K. provided plasma wave experiment/electric field detector

(PWE/EFD) data. A.M., M.T., and S.K., provided MaGnetic Field experiment (MGF) data. V.A. provided ASI/

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THEMIS data and discussed drafting the paper. T.H. developed the pitch angle sorting tool for LEP-i data and

discussed the interpretation of the event. All authors reviewed the manuscript.

Competing interests The authors declare no competing interests.

Additional information

Supplementary Information The online version contains supplementary material available at https​://doi.

org/10.1038/s4159​8-020-79665​-5.

Correspondence and requests for materials should be addressed to S.I.

Reprints and permissions information is available at www.nature.com/reprints.

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