Most of us think of Alaska or Iceland when we imagine aurorae. Maybe you’ve seen them at lower latitudes, like in Minnesota or New York, or (rarely) even Arizona. But aurorae light up other places, too. I don’t mean places like Egypt; I mean places like Jupiter and Saturn.
On these planets (like on Earth) glowing rings encircle both poles. But Saturn’s main auroral oval has puzzled astronomers for a long time. Unlike for aurorae on Earth and Jupiter, researchers can’t determine which of three processes creates it.
We’re not ready to decide that debate yet. But scientists from the UK and the United States describe in the June 18th Nature something even more interesting: a second auroral oval on Saturn that clearly does not work the same way as Earth’s.
Aurorae arise when charged particles stream in along a planet’s magnetic-field lines and crash into its upper atmosphere. On Earth, these particles come from the solar wind. They enter Earth’s magnetosphere through “open” field lines at the poles that connect to the field embedded in the solar wind itself, providing a direct path from the Sun to the top of our atmosphere.
On Jupiter, however, the charged particles come from the volcanic moon Io. Thin, ionized gas from Io is caught by Jupiter’s magnetic field, which rotates rapidly with the planet. The ions can’t keep up with Jupiter’s fast rotation at the equator, though; they stop co-rotating with the planet and slide along the magnetic field lines to Jupiter’s polar regions. The aurorae that result appear at latitudes specifically tied to this “co-rotation breakdown.”
On Saturn, the newly discovered second auroral oval glows at Saturn’s co-rotation breakdown latitude, too.
The new oval is only a quarter as bright as the main oval, though. Researchers have not yet completed ultraviolet studies to corroborate the infrared data that led to the discovery.
Scientists also don’t know yet where the ions feeding into Saturn’s magnetic field come from. “Until relatively recently, it was thought that sputtering off the surface of the icy moons and rings would be the dominant source for Saturn’s plasma,” writes principal investigator Tom Stallard (University of Leicester). He notes that the moon Enceladus and its ice-geyser plume likely provide Saturn’s magnetosphere with about one tenth the material that Io injects into Jupiter’s.
Nevertheless, given their results the team concludes that modeling Saturn’s aurorae as a “hybrid” of Earth and Jupiter’s lights is now unreasonable.