The Graduate University for Advanced Studies

National Institute of Polar Research

Graduate School of Science, The University of Tokyo

Graduate School of Science, Osaka University

Nagoya University

Japan Atomic Energy Agency

Overview of Aurora Borealis

Auroras are phenomena in which the atmosphere emits light when electrons falling into the atmosphere from outer space around the Earth collide with atoms and molecules in the polar atmosphere. The light in auroras is mainly produced by electrons with energies of a few keV that can reach an altitude of around 100 km, but sometimes electrons with energies higher than a few hundred keV that can penetrate deeper into the atmosphere arrive. Such high-energy electrons are thought to change the composition of the atmosphere through collisions with the atmosphere, and are thought to be a factor in the destruction of stratospheric ozone. Therefore, clarifying when and how the energetic electrons arrive is an important clue to understanding how the Earth's atmosphere is affected through its interaction with the universe.

Previous studies have shown that when active auroral activity is observed, such as auroral explosions and pulsating auroras that appear after auroral explosions, large amounts of high-energy electrons are known to fall into the atmosphere, and they are thought to be the main source of ionization at relatively low altitudes (50 to 80 km) in the atmosphere. On the other hand, atmospheric ionization associated with quiet auroral explosions before they occur has received little attention.

On July 24-25, 2018, observations by the large atmospheric radar PANSY radar (Figure) at Showa Station, Antarctica, revealed that atmospheric ionization was occurring at an altitude as low as 68 km about ten minutes before the auroral explosion. At the same time, the high-energy electrons were observed by the Arase satellite, which was located in space just beyond the magnetic field lines from Showa Station. Using this observation data, the radiation behavior analysis code PHITS estimated atmospheric ionization when electrons entered the atmosphere. The results are consistent not only with the ionization height observed by the PANSY radar, but also with the ionization strength observed by the RIOMETER at Showa Station.

In addition, using the global simulation "REPPU," which can reproduce the magnetospheric variations that lead to the auroral explosion, we estimated the extent of the electron fallout region, which was not captured by observations, and found that it could extend as far as 4000 km in the east-west direction and 120 degrees in longitude. Taking into account the duration of ionization estimated from observations, this suggests that the impact of atmospheric ionization due to high-energy electron fallout before an auroral explosion can be several tens of percent of that during active auroral activity.

Thus, the study, which combined state-of-the-art simulations, large facilities in Antarctica, and cutting-edge observations by satellites, revealed that there is a non-negligible impact of atmospheric ionization even before the auroral explosion.

The results of this research have been published in the Journal of Space Weather and Space Climate.

Figure: Antenna group of the PANSY radar, a large atmospheric radar at Showa Station, Antarctica.

Professor Kaoru Sato, Associate Professor Satoshi Kasahara, and Assistant Professor Kunihiro Katsuraka of the Department of Earth and Planetary Science participated in this research.

For details, please visit the website of the Graduate University for Advanced Studies.