Active auroral arc powered by accelerated electrons from very high altitudes

Shun Imajo,Tomoaki Hori,Satoko Nakamura,Sunny Wing-Yee Tam ,Chae‐Woo Jun ,A Matsuoka ,Shiang‐Yu Wang ,Bo‐Jhou ‐J Wang ,Y Kazama ,Iku Shinohara ,T F Chang ,Shoji Motomizu ,M Kitahara ,Masaaki Teramoto ,Y Miyoshi ,V Angelopoulos ,Yasumasa Kasaba,Satoshi Kurita,Yoshiya Kasahara,Kazushi Asamura

doi:10.1038/s41598-020-79665-5

Abstract

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.

Highlights

A typical representation of the acceleration region observed by previous low altitude satellites is as follows[6]
Arase was magnetically mapped to a thin active auroral arc; the evolution of the arc was captured by the THEMIS all-sky imager at Rankin Inlet (RANK) (Fig. 1 and Supplementary Video S1)
Ionospheric footprints estimated by several state-of-art magnetic field models[20,21,22,23] show that the satellite was likely in the flux tube of the arc (Supplementary Fig. S1). This arc variation occurred during the recovery phase of an auroral substorm[24], when Arase was located in the plasma sheet boundary layer (PSBL), the transition layer between the plasma sheet and the magnetotail lobe (Supplementary Fig. S2)

Summary

Introduction

A typical representation of the acceleration region observed by previous low altitude satellites is as follows[6]. Since the parallel acceleration makes a pitch angle more field-aligned, in contrast to the mirror force, the electron loss cone width depends on the potential drop below a satellite Such an acceleration region is assumed to lie in the transition region between ionospheric and magnetospheric plasmas, typically at 1000–20,000 km altitudes[1]. THEMIS (Time History of Events and Macroscale Interactions during Substorms) ground-based all-sky imagers[12,13] have sufficiently high temporal and spatial resolutions, which were not to coordinate with past, the previous high-altitude satellite missions This study utilizes this unique opportunity to investigate the auroral acceleration properties at high altitudes using comprehensive particle and field observations (including high-angular resolution electron observations)[11,14,15,16,17,18] with the Arase s atellite[19] and the network of THEMIS ground-based imagers

Methods

Results

Conclusion