A critical ingredient for auroras exists much higher in space than previously
thought, according to new research in the journal Scientific Reports. The
dazzling light displays in the polar night skies require an electric
accelerator to propel charged particles down through the atmosphere.
Scientists at Nagoya University and colleagues in Japan, Taiwan and the U.S.
have found that it exists beyond 30,000 kilometers above the Earth's
surface—offering insight not just about Earth, but other planets as well.
The story of aurora formation begins with supersonic plasma propelled from
the sun into space as high-speed, charged particles. When these charged
particles get close to Earth, they are deflected and funneled in streams
along the planet's magnetic field lines, eventually flowing towards the
poles.
"Most electrons in the magnetosphere don't reach the part of the upper
atmosphere called the ionosphere, because they are repelled by the Earth's
magnetic field," explains Shun Imajo of Nagoya University's Institute for
Space-Earth Environmental Research, the study's first author.
But some particles receive a boost of energy, accelerating them into Earth's
upper atmosphere where they collide with and excite oxygen and nitrogen
atoms at an altitude of about 100 kilometers. When these atoms relax from
their state of excitation, they emit the auroral lights. Still, many details
about this process remain a mystery.
"We don't know all the details of how the electric field that accelerates
electrons into the ionosphere is generated or even how high above Earth it
is," Imajo says.
Scientists had assumed electron acceleration happened at altitudes between
1,000 and 20,000 kilometers above Earth. This new research revealed the
acceleration region extends beyond 30,000 kilometers.
"Our study shows that the electric field that accelerates auroral particles
can exist at any height along a magnetic field line and is not limited to
the transition region between the ionosphere and magnetosphere at several
thousand kilometers," says Imajo. "This suggests that unknown magnetospheric
mechanisms are at play."
The team reached this finding by examining data from ground-based imagers in
the US and Canada and from the electron detector on Arase, a Japanese
satellite studying a radiation belt in Earth's inner magnetosphere. The data
was taken from 15 September 2017 when Arase was at about 30,000 kilometers
altitude and located within a thin active auroral arc for several minutes.
The team was able to measure upward and downward movements of electrons and
protons, ultimately finding the acceleration region of electrons began above
the satellite and extended below it.
To further investigate this so-called very high-altitude acceleration
region, the team next aims to analyze data from multiple aurora events,
compare high-altitude and low-altitude observations, and conduct numerical
simulations of electric potential.
"Understanding how this electric field forms will fill in gaps for
understanding aurora emission and electron transport on Earth and other
planets, including Jupiter and Saturn," Imajo says.
Reference:
Shun Imajo et al. Active auroral arc powered by accelerated electrons from
very high altitudes, Scientific Reports (2021). DOI:
10.1038/s41598-020-79665-5