Written by Hsu Tsu-yueh. Picture by ERG team.

With self-made high-precision space science measuring instrument, National Cheng Kung University and Academia Sinica take part in the ERG satellite mission by Japan Aerospace Exploration Agency (JAXA). The research team found auroral electrons were accelerated at an altitude of 30,000 kilometers. This unexpected finding revealed that what scientists have believed about auroras for fifty years could be incorrect. By extending the study, the research team hopes to contribute to understanding about electron acceleration mechanisms seen in other solar system planets or celestial body magnetospheres.

During nighttime of January 18^th in Japan, the study was published in Scientific Reports, which is under the international journal Nature. Aurora is the light emitted when accelerated electrons in space hit the Earth’s atmosphere, the research team explained. In the past, it was believed the electrons were accelerated several thousand kilometers above ground, but the exact altitude for acceleration was unclear. In this study, the Low-Energy Particle Experiments-Electron Analyzer (LEP-e), with its precision and high angular resolution, allowed the team to make the major breakthrough in auroral research.

 “By coordinating the plasma data sent by LEP-e onboard ERG satellite and data from ground-based THEMIS all-sky cameras, we identified the auroral electron acceleration region.” The study result showed that the auroral electron acceleration region can be located 30,000 kilometers above ground, overturning previous belief that auroral electrons were accelerated below the altitude of 20,000 kilometers.

Arase satellite and THEMIS cameras on the ground showed that the aurora acceleration region extended to the upper side of the satellite, and that electrons accelerated from ultra-high altitude were pouring into the aurora brightening region.

The Japan-Taiwan collaboration ERG satellite was launched in 2016, carrying a full set of fine scientific instruments in order to measure plasma particles with different energy and electromagnetic field in space. LEP-e, made by Taiwanese ERG team and on board the satellite, possess high angular resolution and is capable of measuring electrons with the potential to cause auroras.

The Taiwanese ERG team expressed that this is the first time Taiwan has manufactured such fine and high-resolution instrument for space measuring. The Taiwanese team was in charge of designing, manufacturing, calibration, and data formatting of the instrument. Ever since the satellite was launched, the instrument has been in good condition, continuously conducting measurements at 200~32,000 kilometers above ground. It is the farthest any Taiwan-made instrument has traveled away from Earth, and it is also the first time that a Taiwan‐made particle instrument is to cruise in the magnetosphere exploring the regions of intense ionizing radiation.

Aside from Earth, electron acceleration mechanisms can also be seen in other solar system planets or celestial body magnetospheres such as auroras at Jupiter or Saturn and pulsar magnetospheres. The research team expects that future studies will advance the understanding of electron acceleration mechanisms in different types of celestial bodies.

JAXA ERG satellite bears the mission of exploring Earth’s radiation belts, understanding the particle acceleration mechanisms, and looking for clues about complex wave-particle interactions. In 2014, the Taiwanese team was invited to join JAXA ERG satellite mission and was in charge of building the LEP-e, the crucial instrument that enables the measuring of low energy electrons in space.

NCKU Institute of Space and Plasma Sciences Professor Sunny Wing-Yee Tam and Assistant Professor Tzu-Fang Chang are both participating in the international collaborative space mission. The successful space mission demonstrates the nation’s strength in space technology, and the scientific breakthrough along its way is even more precious, Professor Tam said. The ERG mission is a model cross-nation collaboration for Taiwan, which is currently in the third phase of its Long-Term National Space Technology Development Program. It is important for Taiwan to carefully choose research topics and instrument development plans for space science in order to ensure funding is well-spent on research projects with good potential and international prospects.

The Exploration of energization and Radiation in Geospace (ERG) satellite mission is operated by JAXA Institute of Space and Astronautical Science (ISAS). The instruments deployed on the satellite were jointly developed and manufactured by JAXA ISAS, official institutions in Japan, Tokai National Higher Education and Research System, Nagoya University, Kanazawa University, Tohoku University, Kyoto University, and Kyushu Institute of Technology. The Taiwanese ERG team, consisting of Academia Sinica Institute of Astronomy and Astrophysics and NCKU Institute of Space and Plasma Sciences, is the only instrument team outside of Japan to participate in the ERG satellite mission. 

More information:

1. Paper:
   Active auroral arc powered by accelerated electrons from very high altitudes, published in Scientific Reports on 18^th January 2021.
   Authors:
   Shun Imajo (Nagoya University JP), Yoshizumi Miyoshi (Nagoya University JP), Yoichi Kazama, (Institute of Astronomy and Astrophysics, Academia Sinica TW), Kazushi Asamura (Institute of Space and Astronautical Science JP), Iku Shinohara (Institute of Space and Astronautical Science JP), Kazuo Shiokawa (Nagoya University JP), Yoshiya Kasahara (Kanazawa University JP), Yasumasa Kasaba (Tohoku University JP), Ayako Matsuoka (Kyoto University JP), Shiang-Yu Wang (Institute of Astronomy and Astrophysics, Academia Sinica TW), Sunny Tam (National Cheng Kung University TW), Tzu-Fang Chang (National Cheng Kung University TW), Bo-Jhou Wang, (Institute of Astronomy and Astrophysics, Academia Sinica TW), Vassilis Angelopoulos (University of California, Los Angeles US), Chae-Woo Jun (Nagoya University JP), Masafumi Shoji (Nagoya University JP), Satoko Nakamura (Nagoya University JP), Masahiro Kitahara (Nagoya University JP), Mariko Teramoto (Kyushu Institute of Technology JP), Kurita Satoshi (Kyoto University JP) and Tomoaki Hori (Nagoya University JP)
2. 《オーロラ粒子の加速領域は超高高度まで広がっていた－オーロラ粒子の加速の定説を覆す発見 》：https://reurl.cc/dVVzeD
3. New Finding on auroral electron acceleration: The acceleration region extends to unexpectedly high altitudes: https://reurl.cc/MZZ1XK
4.  《重大新發現：極光電子加速區域竟達3萬公里高空》：https://reurl.cc/V33p85
5.  Taiwan ERG Data Center: https://reurl.cc/0DDgEl
6.  Japan ERG Science Center: https://reurl.cc/3NNrO0