The researchers observed unexpected, rapid “electron precipitation” from low-Earth orbit using the ELFIN mission, a pair of tiny satellites built and operated on the UCLA campus by undergraduate and graduate students guided by a small team of staff mentors.
By combining the ELFIN data with more distant
observations from NASA’s THEMIS spacecraft, the scientists determined that the
sudden downpour was caused by whistler waves, a type of electromagnetic wave
that ripples through plasma in space and affects electrons in the Earth’s
magnetosphere, causing them to “spill over” into the atmosphere.
Their findings, published March 25 in the journal Nature
Communications, demonstrate that whistler waves are responsible for
far more electron rain than current theories and space weather models predict.
“ELFIN is the first satellite to measure these
super-fast electrons,” said Xiaojia Zhang, lead author and a researcher in
UCLA’s department of Earth, planetary and space sciences. “The mission is
yielding new insights due to its unique vantage point in the chain of events
that produces them.”
Central to that chain of events is the near-Earth
space environment, which is filled with charged particles orbiting in giant
rings around the planet, called Van Allen radiation belts. Electrons in these
belts travel in Slinky-like spirals that literally bounce between the Earth’s
north and south poles. Under certain conditions, whistler waves are generated
within the radiation belts, energizing and speeding up the electrons. This
effectively stretches out the electrons’ travel path so much that they fall out
of the belts and precipitate into the atmosphere, creating the electron rain.
One can imagine the Van Allen belts as a large
reservoir filled with water — or, in this case, electrons, said Vassilis
Angelopolous, a UCLA professor of space physics and ELFIN’s principal
investigator. As the reservoir fills, water periodically spirals down into a
relief drain to keep the basin from overflowing. But when large waves occur in
the reservoir, the sloshing water spills over the edge, faster and in greater
volume than the relief drainage. ELFIN, which is downstream of both flows, is
able to properly measure the contributions from each.
The low-altitude electron rain measurements by ELFIN,
combined with the THEMIS observations of whistler waves in space and
sophisticated computer modeling, allowed the team to understand in detail the
process by which the waves cause rapid torrents of electrons to flow into the
atmosphere.
The findings are particularly important because
current theories and space weather models, while accounting for other sources
of electrons entering the atmosphere, do not predict this extra whistler
wave-induced electron flow, which can affect Earth’s atmospheric chemistry,
pose risks to spacecraft and damage low-orbiting satellites.
The researchers further showed that this type of
radiation-belt electron loss to the atmosphere can increase significantly
during geomagnetic storms, disturbances caused by enhanced solar activity that
can affect near-Earth space and Earth’s magnetic environment.
“Although space is commonly thought to be separate
from our upper atmosphere, the two are inextricably linked,” Angelopoulos said.
“Understanding how they’re linked can benefit satellites and astronauts passing
through the region, which are increasingly important for commerce,
telecommunications and space tourism.”
Since its inception in 2013, more than 300 UCLA
students have worked on ELFIN (Electron Losses and Fields investigation), which
is funded by NASA and the National Science Foundation. The two microsatellites,
each about the size of a loaf of bread and weighing roughly 8 pounds, were
launched into orbit in 2018, and since then have been observing the activity of
energetic electrons and helping scientists to better understand the effect of
magnetic storms in near-Earth space. The satellites are operated from the UCLA
Mission Operations Center on campus.
Source: https://newsroom.ucla.edu/releases/researchers-discover-source-of-superfast-electron-rain
Journal article: https://www.nature.com/articles/s41467-022-29291-8
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