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| An illustration comparing the moon's impact on ocean tides (blue) with its impact on plasmasphere tides (orange) (Image credit: Chinese Academy of Sciences) |
Researchers have detected fluctuations in Earth's magnetosphere created by the
same tidal forces that the moon exerts on the oceans.
The moon exerts a previously unknown tidal force on the "plasma ocean"
surrounding Earth's upper atmosphere, creating fluctuations that are similar
to the tides in the oceans, a new study suggests.
In the study, published Jan. 26 in the journal Nature Physics, Chinese
scientists used more than 40 years of data collected by satellites to track
the minute changes in the shape of the plasmasphere, the inner region of
Earth's magnetosphere, which shields our planet from solar storms and other
types of high-energy particles.
The plasmasphere is a roughly doughnut-shaped blob of cool plasma that sits
on top of Earth's magnetic field lines, just above the ionosphere, the
electrically charged part of the upper atmosphere. The plasma, or ionized
gas, in the plasmasphere is denser than the plasma in the outer regions of
the magnetosphere, which causes it to sink to the bottom of the
magnetosphere. The boundary between this dense sunken plasma and the rest of
the magnetosphere is known as the plasmapause.
"Given its cold, dense plasma properties, the plasmasphere can be regarded
as a 'plasma ocean,' and the plasmapause represents the 'surface' of this
ocean," the researchers wrote in the paper. The moon's gravitational pull
can distort this "ocean," causing its surface to rise and fall like the
ocean tides.
The moon is already known to exert tidal forces on Earth's oceans, crust,
near-ground geomagnetic field and the gas within the lower atmosphere.
However, until now, nobody had tested to see if there was a tidal effect on
the plasmasphere.
To investigate this question, the researchers analyzed data from more than
50,000 crossings of the plasmasphere by satellites belonging to 10
scientific missions, including NASA's Time History of Events and Macroscale
Interactions during Substorms (THEMIS) mission. The satellites' sensors are
capable of detecting minute changes in the concentrations of plasma, which
allowed the team to map out the exact boundary of the plasmapause in greater
detail than ever before.
The satellite crossings occurred between 1977 and 2015, and during this
period, there were four complete solar cycles. This information allowed the
team to factor in the role of solar activity on Earth's magnetosphere. Once
the sun's influence was accounted for, it started to become clear that
fluctuations in the shape of the plasmapause followed daily and monthly
patterns that were very similar to the ocean's tides, indicating that the
moon was the most likely cause of the plasma tides.
The researchers are unsure exactly how the moon causes the plasma tides, but
their current best guess is that the moon's gravity causes perturbations in
Earth's electromagnetic field. But further research is needed to tell for
sure.
The team thinks this previously unknown interaction between Earth and the
moon could help researchers understand other parts of the magnetosphere in
greater detail, such as the Van Allen radiation belts, which capture highly
energetic particles from solar wind and trap them in the outer
magnetosphere.
"We suspect that the observed plasma tide may subtly affect the distribution
of energetic radiation belt particles, which are a well-known hazard to
space-based infrastructure and human activities in space," the researchers
wrote. Better understanding the tides could therefore help to improve work
in these areas, they added.
The researchers also want to see if plasma in the magnetospheres of other
planets is influenced by those planets' moons. "These findings may have
implications for tidal interactions in other two-body celestial systems,"
they wrote.
Reference:
Heilig, B. Lunar modulations. Nat. Phys. (2023).
DOI: 10.1038/s41567-022-01913-4
Tags:
Space & Astrophysics