Pulsating Aurora Caused by the “Propagation Path” of Electromagnetic Wave

May. 13, 2025 | GATEWAY to Academic Articles

ITO Yuri / National Institute of Polar Research and
Graduate University for Advanced Studies, SOKENDAI

In this study, we uncovered a mechanism whereby auroras are generated within a “pathway” formed by electromagnetic waves from space to the Earth through joint observations with a satellite and ground-based observation networks. The result is expected to contribute to understanding the space environment near the Earth.
Chorus waves^*1, which are one of the electromagnetic waves naturally occurring in space near the Earth (i.e., geospace), scatter high energetic electrons. The energy of these scattered electrons is typically ~10 kilo-electron volts and the electrons are mainly scattered near the magnetic equator. The scattered electrons precipitate into the Earth’s atmosphere and generate blinking auroras known as “pulsating auroras” ^*2. During pulsating auroras, it is suspected that electrons with the energies higher than the pulsating auroral electrons themselves, tens of kilo-electron volts or more, simultaneously precipitate into the atmosphere. The presence of these higher energy electrons can damage the environment in geospace, making the pulsating aurora--as a gateway to these energetic interlopers--an important phenomenon to study. Previous studies have not fully clarified how electrons trapped in the magnetosphere, with energies ranging from tens of kilo-electron volts to a few mega-electron volts, are distributed spatiotemporally, or under what conditions these energetic electrons precipitate into the atmosphere. In this study, high-latitude propagation of chorus waves, the patchy structure of pulsating auroras, and precipitation of tens to 100 kilo-electron volts electrons were simultaneously observed by conjugate observations by satellite, ground-based optical instruments, and an atmospheric radar. Based on the results, it was found that the tube-like “propagation path” of electromagnetic waves helps high-latitude propagation of chorus waves and precipitation of relativistic (high energy) electrons and creates the patchy structure of the pulsating auroral shape, reflecting the cross-section of the “propagation path”. This result indicates that the spatial distribution of the energetic electrons can be visualized through the “propagation path” of electromagnetic waves by monitoring the shape of pulsating auroras from the ground, and it is expected to contribute to improvements of the accuracy of space weather forecasts^*3.

Research Summary

Auroras are caused via collision with particles such as nitrogen and oxygen in the Earth’s atmosphere and precipitating electrons from the magnetosphere^*4 formed in space near the Earth (i.e., geospace). Electrons with the energy of about 10 kilo-electron volts (keV) are scattered by whistler-mode chorus waves, which are naturally generated near the magnetospheric equator, and precipitate into the Earth’s atmosphere. These precipitating electrons cause emission of “pulsating auroras”.

Recent studies with the Arase satellite^*5 and other instruments have suggested that, during pulsating auroras, electrons with tens of kilo- to several mega-electron volts, which are higher than typical energy of pulsating auroral electrons, simultaneously precipitate into the polar atmosphere, corresponding to the shape of pulsating aurora. In addition, the highly energetic electrons cause ozone depletion in the mesosphere and upper stratosphere. Therefore, it is an important issue to understand the characteristics of pulsating auroras and related environmental changes in geospace.

Chorus waves generated near the magnetic equator usually decay away from the magnetic field lines as they propagate in the north-south direction, and it is considered that they cannot propagate to high magnetic latitudes above 20º, far from the magnetic equator. On the other hand, it has been suggested that for highly energetic electrons to precipitate into the atmosphere, it is necessary for chorus waves to propagate to higher latitudes along magnetic fields, where they scatter electrons. This means that there is a physical mechanism, which drives high-latitude propagation of chorus waves and a relationship between the shape of pulsating aurora and the energy of pulsating auroral electrons. However, there have been no actual cases observed by magnetospheric satellites, ground-based optical instruments, and atmospheric radar that satisfy sufficient conditions to clarify the physical mechanism controlling these relationships. Thus, we focused on a single event of observations with the Arase satellite, which has a unique orbit inclination of about 31º and can observe chorus waves propagating to higher latitudes and energetic particles (Figure 1).

In this study, we analyzed a single event of satellite–ground joint observations by Japanese satellite “Arase”, all-sky high-speed imagers installed in Tromsø, Norway, and the “European Incoherent SCATter: EISCAT” radar^*6. In this event, simultaneous observations were established for more than two hours, and a series of changes in the magnetosphere, the pulsating auroral shape, and the energy of precipitating electrons were successfully observed. This event, in which simultaneous observations with the three instruments were conducted for long duration, was a very valuable case in which all of the following three conditions were satisfied: 1) The orbit of the Arase satellite passed over the observation site between midnight and morning, when pulsating auroras frequently occur; 2) The weather of observation site was clear in order to determine what kind of aurora was being observed in the sky above; and 3) The satellite's footprint, which can deviate from the prediction due to the effects of changes in the magnetospheric environment, was within the field of view of the EISCAT radar and optical instruments.

The Arase satellite observed chorus waves at magnetic latitude above 20º, i.e., chorus waves propagating to higher latitudes along field lines and highly energetic electrons. Ground-based observations simultaneously observed patchy structure of pulsating aurora and the ionization caused by precipitation of electrons with energy of tens to 100 kilo-electron volts into the atmosphere (Figure 2). The results mean that some “factor” makes chorus waves propagate to higher latitudes and occur wave-particle interactions with highly energetic electrons. Based on this result, we have proposed a physical mechanism that the “propagation path” of electromagnetic waves drives the long-distance propagation of chorus waves and makes precipitation of high energetic electrons and pulsating auroral patches reflecting cross-section of the path (Figure 3). The “propagation path” of electromagnetic waves is a tube-like region (duct) where the electron density is lower/higher than the surrounding area, which allows waves to propagate farther, and sometimes the waves travel to the ground along the path [Matsuda et al., 2021, Geophysical Research Letters, 48].

We also tried to prove the proposed physical mechanism by comparing changes in the magnetospheric electron density and pulsating auroral emission. Since the Arase satellite does not directly observe the electron density, it is necessary to derive it from the observed specific waves using a physical equation or by reconstructing it from the potential when the satellite is charged reflecting the electron density and the electron temperature of the surrounding environment. However, the derivation of the electron density is not easy because the required waves sometimes hide in other waves and cannot be detected, or it may be difficult to separate the contribution of the electron density from that of the electron temperature. In this study, the electron density was reconstructed by comparing the satellite potential, for which data was always present during the observation, to a specific wave that were read for ~30 minutes. As a result, spatial variation in the electron density corresponding to the patchy structure of pulsating aurora, which was detected by the Arase satellite traversing the “propagation path”, were confirmed. Thus, correspondence between the existence of the “propagation path” of electromagnetic waves, the shape of pulsating aurora, and the energy of precipitating electrons were clarified.

This finding means that the energy of precipitating electrons can be estimated by monitoring the shape of pulsating auroras on the ground. In addition, it is expected to possibly visualize the distribution of electrons with energy of tens of kilo- to a few mega-electron volts in space by tracing the “propagation path” from pulsating auroral patches.

Terminologies

• ^*1 Chorus wave: The electromagnetic waves naturally occurring in space near the Earth. As it propagates, it causes the energetic electrons moving on the same magnetic field lines precipitate into the Earth's atmosphere. When these electrons collide with atmospheric particles, pulsating auroras emit.
• ^*2 Pulsating aurora: One of auroras characterized by quasi-periodic emission like heart pulsation in seconds to tens of seconds and often occur from midnight to the daytime side.
• ^*3 Space weather forecast: A technique for predicting disturbances in the near-Earth space environment caused by solar activity.
• ^*4 Magnetosphere: It is the region affected by the Earth's unique magnetic field. The magnetosphere is stretched in the anti-sunward direction by the “solar wind,” a high-speed flow of electrically charged particles (plasma) ejected from the Sun into planetary space.
• ^*5 Arase satellite: A Japanese scientific satellite launched in 2016. The magnetosphere has a region called the “radiation belt” where highly energetic (e.g., tens of kilo- to a few mega-electron volts) electrons exist in large quantities and are repeatedly created and lost. The purpose of this project is to clarify the processes by which it is created and lost, and how geospace storms caused by solar wind disturbances develop.
• ^*6 EISCAT (European Incoherent SCATter) radar: A large-sized atmospheric radar installed in Tromsø, Norway. It is operated by the EISCAT scientific association of six countries: Japan, Norway, Sweden, Finland, the United Kingdom, and China. It uses a large parabolic antenna to emit powerful radio waves, detects weak radio waves scattered back from the atmosphere and observes the upper atmosphere.

Information

Author

Yuri Ito
April 2018–March 2022 The University of Electro-Communications
April 2022–March 2024 Department of Computer and Network Engineering, Graduate School of Informatics and Engineering, The University of Electro-Communications
April 2024–Present Ph.D. Student, The Graduate University for Advanced Studies, SOKENDAI
April 2024–Present Project Researcher, National Institute of Polar Research

A word from the co-author at ISAS (Professor SHINOHARA Iku)

Ms. Ito Yuri is a graduate student in the Polar Science Program at SOKENDAI. Her ambitious project aims to extract spatial information that cannot be obtained solely from satellite observations by integrating satellite data with various ground-based observations. Arase (ERG) has revealed that the electromagnetic waves traveling through space scatter electrons across a wide energy range within the inner magnetosphere and radiation belt. This scattering causes electrons to precipitate into the Earth's upper atmosphere and be lost. However, the spatial distribution of this phenomenon remains unclear. Her result is an epoch-making achievement that has used the unique opportunity of simultaneous observations from multiple sources: ground-based aurora and radar observations, along with the Arase observation. This research clarifies that the patchy structure of pulsating auroras reflects the path of the electromagnetic waves that cause high-energy electron precipitation. In the future, we hope her work will further clarify the spatial structure of the electromagnetic wave path in space by utilizing the coordinated observations between Arase and ground-based observations.

Professor SHINOHARA Iku
Ph.D., Department of Earth and Planetary Science, The University of Tokyo
Head of Arase Operations Team
Head of Science Satellite Operation and Data Archive Unit
Professor, Department of Solar System Sciences