Saturn's high-altitude winds generate an extraordinary aurorae, study
finds

Date:
February 8, 2022
Source:
University of Leicester
Summary:
Space scientists have discovered a never-before-seen mechanism
fueling huge planetary aurorae at Saturn.



FULL STORY ========================================================================== Leicester space scientists have discovered a never-before-seen mechanism fuelling huge planetary aurorae at Saturn.


========================================================================== Saturn is unique among planets observed to date in that some of its
aurorae are generated by swirling winds within its own atmosphere,
and not just from the planet's surrounding magnetosphere.

At all other observed planets, including Earth, aurorae are only formed
by powerful currents that flow into the planet's atmosphere from the surrounding magnetosphere. These are driven by either interaction with
charged particles from the Sun (as at the Earth) or volcanic material
erupted from a moon orbiting the planet (as at Jupiter and Saturn).

This discovery changes scientists' understanding of planetary aurorae
and answers one of the first mysteries raised by NASA's Cassini probe,
which reached Saturn in 2004: why can't we easily measure the length of
a day on the Ringed Planet? When it first arrived at Saturn, Cassini
tried to measure the bulk rotation rate of the planet, that determines
the length of its day, by tracking radio emission 'pulses' from Saturn's atmosphere. To the great surprise of those making the measurements,
they found that the rate appeared to have changed over the two decades
since the last spacecraft to have flown past the planet - - Voyager 2,
also operated by NASA -- in 1981.

Leicester PhD researcher Nahid Chowdhury is a member of the Planetary
Science Group within the School of Physics and Astronomy and corresponding author for the study, published in Geophysical Research Letters. He said: "Saturn's internal rotation rate has to be constant, but for decades researchers have shown that numerous periodic properties related to the
planet -- the very measurements we've used at other planets to understand
the internal rotation rate, such as the radio emission -- tend to change
with time. What's more, there are also independent periodic features seen
in the northern and southern hemispheres which themselves vary over the
course of a season on the planet.



==========================================================================
"Our understanding of the physics of planetary interiors tells us the
true rotation rate of the planet can't change this quickly, so something
unique and strange must be happening at Saturn. Several theories have been touted since the advent of the NASA Cassini mission trying to explain the mechanism/s behind these observed periodicities. This study represents
the first detection of the fundamental driver, situated in the upper
atmosphere of the planet, which goes on to generate both the observed
planetary periodicities and aurorae.

"It's absolutely thrilling to be able to provide an answer to one of the longest standing questions in our field. This is likely to initiate some rethinking about how local atmospheric weather effects on a planet impact
the creation of aurorae, not just in our own Solar System but farther
afield too." Astronomers and planetary scientists based at the University
of Leicester led a study alongside colleagues from NASA's Jet Propulsion Laboratory (JPL), the Japan Aerospace Exploration Agency (JAXA), and the Universities of Wisconsin- Madison, Boston, and Lancaster, plus Imperial
and University Colleges, London, to resolve the decades-old question.

They measured infrared emissions from the gas giant's upper atmosphere
using the Keck Observatory in Hawai'i and mapped the varying flows of
Saturn's ionosphere, far below the magnetosphere, over the course of a
month in 2017.

This map, when fixed against the known pulse of Saturn's radio aurorae,
showed that a significant proportion of the planet's aurorae are generated
by the swirling pattern of weather in its atmosphere and are responsible
for the planet's observed variable rate of rotation.



========================================================================== Researchers believe the system is driven by energy from Saturn's
thermosphere, with winds in the ionosphere observed between 0.3 and 3.0 kilometres per second.

Dr Tom Stallard, Associate Professor in Planetary Astronomy at the
University of Leicester, added: "The University of Leicester has long been involved in measuring the effects of this new discovery -- we've observed
how the pulsing aurorae and the wobbling magnetic field lines stretching
out into space highlight an apparently changing rotation rate. For two
decades our researchers, along with the wider scientific community,
have speculated about what might be driving these strange periodicities.

"Over the years, scientific meetings have had late-night discussions about whether the volcanic moon Enceladus might be the cause, or interactions
with the thick atmosphere of the moon Titan, or perhaps interactions
with Saturn's bright rings. But recently, many researchers have focused
on the possibility that it is Saturn's upper atmosphere that causes
this variability.

"This search for a new type of aurora harks back to some of the earliest theories about Earth's aurora. We now know that aurorae on Earth are
powered by interactions with the stream of charged particles driven
from the Sun. But I love that the name Aurora Borealis originates
from the 'the Dawn of the Northern Wind'. These observations have
revealed that Saturn has a true Aurora Borealis -- the first ever aurora
driven by the winds in the atmosphere of a planet." Dr Kevin Baines,
a JPL-Caltech-based co-author of the study and a member of the Cassini
Science Team, added: "Our study, by conclusively determining the origin
of the mysterious variability in radio pulses, eliminates much of the
confusion into Saturn's bulk rotation rate and the length of the day
on Saturn." Because of the variable rotation rates observed at Saturn, scientists have been prevented from using the regular pulse of radio
emission to calculate the bulk internal rotation rate. Fortunately, a
novel method was developed by Cassini scientists using gravity-induced perturbations in Saturn's complex ring system, which now seems to be the
most accurate means of measuring the planet's bulk rotational period,
which was determined in 2019 to be 10 hours, 33 minutes and 38 seconds.

Planetary research at the University of Leicester spans the breadth of
our Solar System -- and beyond.

Leicester researchers are members of the Juno mission made up of a global
team of astronomers observing Saturn's massive neighbour, Jupiter,
and are leading observations of the Solar System's outer planets from
the recently-launched James Webb Space Telescope. Leicester also plays a leading role in science and instrumentation on the European Space Agency (ESA)'s Jupiter Icy Moons Explorer (JUICE), due for launch later in 2022.

'This research was supported by a NASA Keck PI Data Award, administered
by the NASA Exoplanet Science Institute.

Video: https://youtu.be/jRtjGTJWu-E ========================================================================== Story Source: Materials provided by University_of_Leicester. Note:
Content may be edited for style and length.


========================================================================== Related Multimedia:
* Saturn's_extraordinary_aurorae ========================================================================== Journal Reference:
1. M. N. Chowdhury, T. S. Stallard, K. H. Baines, G. Provan, H. Melin,
G. J.

Hunt, L. Moore, J. O'Donoghue, E. M. Thomas, R. Wang, S. Miller,
S. V.

Badman. Saturn's Weather‐Driven Aurorae Modulate Oscillations
in the Magnetic Field and Radio Emissions. Geophysical Research
Letters, 2022; 49 (3) DOI: 10.1029/2021GL096492 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220208085022.htm
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