A team from Nagoya University in Japan has observed, for the first time, energy transfer from resonant electrons to whistler-mode waves in space. Their findings provide direct evidence for the previously theorized efficient growth, as predicted by nonlinear wave growth theory. This should improve our understanding not only of the physics of space plasma, but also of space weather, a phenomenon that affects satellites.
When people imagine outer space, they often see it as a perfect void. In fact, this impression is incorrect because the vacuum is full of charged particles. In deep space, the density of charged particles becomes so low that they rarely collide with each other. Instead of collisions, forces related to the electric and magnetic fields that fill space control the movement of charged particles. This lack of collisions occurs throughout space, except for very close celestial objects such as stars, moons, or planets. In these cases, the charged particles no longer travel through the vacuum of space but through a medium where they can hit other particles.
Around Earth, these charged particle interactions generate waves, including whistler-mode electromagnetic waves, that scatter and accelerate some of the charged particles. When diffuse auroras appear around the poles of planets, observers are seeing the results of an interaction between waves and electrons. Because electromagnetic fields are so important in spacetime, studying these interactions should help scientists predict variations in the intensity of high-energy particles. This could help protect astronauts and satellites from the more severe effects of space weather.
A team consisting of Designated Assistant Professor Naritoshi Kitamura and Professor Yoshizumi Miyoshi from Nagoya University’s Institute of Space and Earth Sciences (ISEE), along with researchers from the University of Tokyo, Kyoto University , Tohoku University, Osaka University, and the Japan Aerospace Exploration Agency. (JAXA), and several international collaborators, primarily used data obtained using low-energy electron spectrometers, called Fast Plasma Investigation-Dual Electron Spectrometers, aboard NASA’s Magnetospheric Multiscale spacecraft. They analyzed interactions between electrons and whistler-mode waves, which were also measured by the spacecraft. By applying a method using a wave-particle interaction analyzer, they were able to directly detect the ongoing energy transfer from resonant electrons to whistler-mode waves at the location of the spaceship in space. From this, they derived the growth rate of the wave. The researchers published their results in Nature Communications.
The most important finding was that the observed results were consistent with the hypothesis that non-linear growth occurs in this interaction. “This is the first time anyone has directly observed the efficient growth of waves in space for the wave-particle interaction between electrons and whistler-mode waves,” explains Kitamura. “We hope that the results will contribute to research on various wave-particle interactions and also to improve our understanding of the progress of plasma physics research. As more specific phenomena, the results will contribute to our understanding of the acceleration of high-energy electrons in the radiation belt, which are sometimes called “killer electrons” because they cause damage to satellites, as well as the loss of high-energy electrons in the atmosphere, which form diffuse auroras” .
//Funding//
This work was supported by Grant-in-Aid for Scientific Research (17H06140, 18H03727, 21K13979) from the Japan Society for the Promotion of Science.
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