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| Whistler-mode wave magnetic field (blue arrows with spiral) propagating along the magnetic field (purple) interacting with electrons (red) passing through it. Credit: University of Tokyo |
A team from Nagoya University in Japan has observed, for the first time, the
energy transferring from resonant electrons to whistler-mode waves in space.
Their findings offer direct evidence of previously theorized efficient
growth, as predicted by the non-linear growth theory of waves. This should
improve our understanding of not only space plasma physics but also space
weather, a phenomenon that affects satellites.
When people imagine outer space, they often envision it as a perfect vacuum.
In fact, this impression is wrong because the vacuum is filled with charged
particles. In the depths of space, the density of charged particles becomes
so low that they rarely collide with each other.
Instead of collisions, the forces related to the electric and magnetic
fields filling space, control the motion of charged particles. This lack of
collisions occurs throughout space, except for very near to celestial
objects, such as stars, moons, or planets. In these cases, the charged
particles are no longer traveling through the vacuum of space but instead
through a medium where they can strike other particles.
Around the Earth, these charged-particle interactions generate waves,
including electromagnetic whistler-mode waves, which 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. Since electromagnetic fields are so important in space
weather, studying these interactions should help scientists predict
variations in the intensity of highly energetic particles. This might help
protect astronauts and satellites from the most severe effects of space
weather.
A team comprising Designated Assistant Professor Naritoshi Kitamura and
Professor Yoshizumi Miyoshi of the Institute for Space and Earth Science
(ISEE) at Nagoya University, together with researchers from the University
of Tokyo, Kyoto University, Tohoku University, Osaka University, and Japan
Aerospace Exploration Agency (JAXA), and several international
collaborators, mainly used data obtained using low-energy electron
spectrometers, called Fast Plasma Investigation-Dual Electron Spectrometers,
on board 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 of using a wave
particle interaction analyzer, they succeeded in directly detecting the
ongoing energy transfer from resonant electrons to whistler-mode waves at
the location of the spacecraft 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 anybody has directly observed the efficient growth of
waves in space for the wave-particle interaction between electrons and
whistler-mode waves," explains Kitamura.
"We expect that the results will contribute to research on various
wave-particle interactions and to also 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 electrons to
high energies in the radiation belt, which are sometimes called 'killer
electrons' because they inflict damage on satellites, as well as the loss of
high-energy electrons in the atmosphere, which form diffuse auroras."
Reference:
N. Kitamura et al, Direct observations of energy transfer from resonant
electrons to whistler-mode waves in magnetosheath of Earth, Nature
Communications (2022).
DOI: 10.1038/s41467-022-33604-2
Tags:
Physics