A research group led by Designated Professor Lynn Kistler (Also a Professor at the University of New Hampshire, USA), Professor Yoshizumi Miyoshi and Designated Associate Professor Tomoaki Hori of the Institute for Space-Earth Environmental Research at Nagoya University, in collaboration with the Japan Aerospace Exploration Agency (JAXA), the University of Tokyo, Osaka University, the University of New Hampshire, Lockheed Martin Advanced Technology Center and the Harvard Smithsonian Astronomical Observatory, has shown that the primary cause of space storms, previously believed to be created by plasma of solar origin, is actually of terrestrial origin. Data from four scientific satellites of Japan, NASA of the U.S.A. and the European Space Agency(ESA)were analyzed. It was found that the plasma in the Earth's magnetosphere changed from solar origin to terrestrial origin during space storms. This achievement is expected to lead to the prediction of space storms and was published in the October 30 issue of the international journal Nature Communications.
The area that the Earth's magnetic field exerts its influence (magnetosphere) and charged particles carrying electricity (plasma) exist in the space surrounding the Earth (geospace). One of the origins of this is solar plasma (solar wind), which enters the region called the plasma sheet which lies beyond the inner magnetosphere on the night side of the Earth, before being transported to the inner magnetosphere near the Earth. The other is plasma in the Earth's upper atmosphere (ionosphere), which is believed to be composed of hydrogen and oxygen ions leaking into space.
Expulsions of particles form the sun causes an increase in the density and velocity of the solar wind, resulting in a space storm in geospace. This increase in charged particles causes severe auroral activity to be observed on the earth surface, as well as satellite malfunctions and power outages on the ground.
The development of space storms has been thought to occur due to a significant increase in hydrogen ions from the solar wind within the inner magnetosphere. On the other hand, since the origin of these ions could not be identified, it was unclear to what extent plasma of terrestrial origin contribute to the development of space storms.
To distinguish between ions carried by the solar wind and those of terrestrial origin, the research group focused on α-particles (divalent-charged helium ions), which are found only in solar wind. They investigated the fluctuations in the size of space storms in the solar wind and geospace and examined the densities of hydrogen ions, oxygen ions and α-particles within the inner magnetosphere. Furthermore, they explored the origins of hydrogen ions in the magnetosphere by comparing the ratios of hydrogen ions to α-particles captured by various satellites.
They analyzed the space storms that occurred on September 7−10, 2017, for JAXA's geospace exploration satellite Arase, NASA's scientific satellite MMS and solar observing satellite Wind, and ESA's scientific satellite Cluster. At that time, Wind was located near the Sun, Cluster near the Earth, Arase near the night side of the Earth and MMS near the night side of the Earth in the plasma sheet region farther from Arase. An analysis was conducted using the ideal locations of satellites that measured α-particles and other particles. If the ratio of hydrogen ions to α-particles observed by Arase is close to that observed by Wind, the hydrogen ions observed by Arase are of solar origin, and if the ratios are significantly different, they must be of terrestrial origin.
The results of the analysis showed that the hydrogen ions observed by Arase were of solar origin until 20:00 on the 7th; however, as a space storm developed after 21:00, the percentages observed by Wind and Arase began to shift significantly. This suggested that the hydrogen ions observed by Arase were of terrestrial origin.
It was also found that as the space storm progressed, the amount of oxygen ions of terrestrial origin increased and exceeded the amount of hydrogen ions, contributing to the development of the space storm. Space storms have various impacts on the Earth and predicting them is considered a major challenge; however, this study found that the effects of ions of terrestrial origin should be considered.
Kistler commented, "This is just one example. We found that hydrogen and oxygen ions from the Earth's atmosphere play a significant role in the occurrence of space storms; however, space storms occur at a considerable frequency. Because we successfully demonstrated the applicability of our analytical method, we believe that by applying this method to other space storms, we can verify the universality of our findings."
Miyoshi added, "The Sun exhibits extreme diversity in its activity. In 2017, it was at the end of an 11-year solar cycle in terms of solar activity. In 2020, it entered a quiet state, but over the past year and this year, solar activity has been increasingly vigorous. It is expected to reach its maximum solar activity in 2025. Arase satellite is currently conducting observations smoothly, and we hope to reveal the universality and diversity of this knowledge by observing various space storms in the future."
Publication: Nature Communications
Title: The variable source of the plasma sheet during a geomagnetic storm
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.