Saturn's Swirling Winds Reveal Spectacular Aurorae Never Seen Before

A new discovery reveals a unique feature when it comes to some of Saturn's aurorae. Observations made with the W. M. Keck Observatory on Maunakea, Hawaii Island, shows that some of the ringed planet's aurorae are generated by swirling winds within its own atmosphere.

This unique observation differs from all other planets, including Earth. Until now, aurorae were thought to only be formed by powerful currents that flow into the planet's atmosphere from the surrounding magnetosphere. Typically these are created by either interactions with charged particles from the Sun, such as at the Earth, or volcanic debris erupting from a moon orbiting the planet, such as with Saturn or Jupiter.

Perhaps the most exciting result of the finding is that it answers one of the questions that was raised when NASA's Cassini probe reached the ringed planet back in 2004. That question involved why it was not an easy task to measure the length of a day on 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," states PhD researcher Nahid Chowdhury, a member of the Planetary Science Group within the University of Leicester's School of Physics and Astronomy and lead author of the study. "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."

Cassini first attempted to measure the bulk rotation rate of the planet, which determines the length of its day, by tracking radio emission pulses from Saturn's atmosphere. Those who were reading the measurements were astonished to find that the rate appeared to have changed over the two decades since the last spacecraft having flown past the planet.

Chowdhury explains, "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 mechanisms behind these observed periodicities." Chowdhury continues, "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."

The team of astronomers and planetary scientists measured infrared emission from the gas giant's upper atmosphere using Keck Observatory's Near-Infrared Spectrograph (NIRSPEC) and mapped the varying flows of Saturn's ionosphere over the course of a month in 2017.

By fixing the infrared emission map against the known pulse of Saturn's radio aurorae, it showed that a large portion of the planet's aurorae are created by the swirling pattern of weather in it's atmosphere and are responsible for the planet's observed variable rate of rotation. This leads researchers to believe that the system is driven by energy from Saturn's thermosphere, with winds in the ionosphere being observed between 0.3 and 3.0 kilometers per second.

"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," says Kevin Baines, a JPL-Caltech-based co-author of the study and a member of the Cassini Science Team.