'These processes are a universal mechanism for aurora
generation.'
The way in which the radiant displays of colors in the sky
known as auroras form on Earth may be how these lights arise throughout the
solar system, according to new findings from Mercury.
Artist's illustration of ESA/JAXA BepColombo mission flying through precipitating electrons that can trigger X-ray auroras on the surface of Mercury (Credit: Thibaut Roger/Europlanet (CC BY-SA 4.0)) |
Mercury,
the smallest and least massive of the solar system's planets, also possesses a
magnetosphere. However, its magnetosphere is typically only about 5% the size
of Earth's. This is because the magnetic field of Mercury is less than 1% as
strong as our planet's, said Sae Aizawa, (a planetary scientist and space
plasma physicist at the Institute for Astrophysics and Planetology Research in
Toulouse, France).
The magnetosphere of a planet often acts as a barrier to
solar wind flow, in the same way as a boulder in a stream serves as an obstacle
to flowing water. However, previous research found that, depending on how Earth
and Mercury are oriented with respect to the interplanetary magnetic field
from the
sun, the magnetospheres of both those planets regularly become open to the
solar wind.
BepiColombo observations during its first Mercury flyby (Credit: Sae Aizawa) |
On Earth, the entry of the solar wind into the magnetosphere
drives the motions of electrons and other electrically charged particles. At
the poles, these electric currents flow into the planet's atmosphere, race down
Earth's magnetic field lines and crash into molecules in the upper atmosphere,
generating auroral light.
Previous research detected very high-energy electrons and
X-rays from Mercury. However, those studies lacked the tools to monitor the
relatively low-energy electrons that usually power auroras.
Now Aizawa and her colleagues have discovered evidence that
auroras on Mercury have the same origins as they do on Earth. The results
suggest that "these processes are a universal mechanism for aurora
generation," Aizawa said.
In the new study, the scientists analyzed data from
the BepiColombo mission,
a joint effort from the European Space Agency and the Japan Aerospace
Exploration Agency to study Mercury. Launched in 2018, BepiColombo made its
first of six flybys of Mercury in 2021 and is planned to orbit Mercury starting
in 2025, making it the first mission to another planet to consist of two
satellites.
BepiColombo found that relatively low-energy electrons in
Mercury's magnetosphere can accelerate in the planet's dawn areas and then get
pumped into magnetic field lines on the world's nightside. Since Mercury has an
incredibly tenuous atmosphere compared to Earth's, these electrons do not
strike the air, but instead slam onto Mercury's surface, generating X-ray
auroras.
All in all, although the planets of the solar system differ
in many respects from each other, such as the strength of their magnetic fields
and the composition of their atmospheres, the way in which electrons can get
accelerated at planets to generate auroras appears universal across these
worlds, Aizawa said.
The scientists detailed their findings in the journal
Nature Communications.
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