Saturn is unique among planets observed to date in that some of its aurorae are generated by swirling winds within its atmosphere, and not just from the planet’s surrounding magnetosphere.
This infrared image, taken by NASA’s Cassini spacecraft, shows an aurora at the southern pole of Saturn. Image credit: NASA / Cassini / VIMS Team / University of Arizona / University of Leicester / JPL / ASI.
An abiding mystery following NASA’s Cassini spacecraft’s extended tour of Saturn is also one of the first questions the orbiter raised about the planet: measurements showed that Saturn’s day appeared to be 6 min longer at 10 hours 45 min than that measured by NASA’s Voyager 1 and 2 spacecraft.
Since it is improbable that the interior of Saturn changed its rotation period over the course of only two decades, it was clear that somehow the magnetic fields above the planet were slipping relative to those generated deep within the interior.
This mystery has remained unresolved despite nearly two decades of Cassini observations at Saturn.
“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,” said Nahid Chowdhury, a Ph.D. student in the School of Physics and Astronomy at the University of Leicester.
“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 NASA’s Cassini mission trying to explain the mechanism(s) behind these observed periodicities.”
“Our 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.”
In the study, Chowdhury and colleagues measured infrared emissions from the Saturnian upper atmosphere using the Keck Observatory in Hawai’i and mapped the varying flows of the gas giant’s ionosphere, far below the magnetosphere, over the course of a month in 2017.
The 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.
The researchers believe the system is driven by energy from Saturn’s thermosphere, with winds in the ionosphere observed between 0.3 and 3.0 km/s.
“The search for a new type of aurora harks back to some of the earliest theories about Earth’s aurora,” said Dr. Tom Stallard, a researcher in the School of Physics and Astronomy at the University of Leicester.
“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 ‘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.”
“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,” added Dr. Kevin Baines, a researcher with NASA’s Jet Propulsion Laboratory and the Space Science and Engineering Center at the University of Wisconsin-Madison.
The team’s findings were published in the journal Geophysical Research Letters.
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M.N. Chowdhury et al. Saturn’s Weather-Driven Aurorae Modulate Oscillations in the Magnetic Field and Radio Emissions. Geophysical Research Letters, published online December 28, 2021; doi: 10.1029/2021GL096492
