Astronomers have for the first time captured images of auroras above the giant planet Uranus.
Uranus auroras, as seen by the Hubble Space Telescope in 2011 (Laurent Lamy)
The images, taken by the NASA/ESO Hubble Space Telescope, reveal auroras on Uranus consisted of short-lived, faint, glowing dots. Unlike auroras on Earth, which can turn the sky greens and purples for hours, the newly detected auroras on Uranus appeared to only last a couple minutes.
In the new observations, which are the first to glimpse the Uranian aurora with an Earth-based telescope, the astronomers detected the luminous spots twice on the dayside of Uranus – the side that’s visible from Hubble.
Previously, the distant aurora had only been measured using instruments on a passing spacecraft.
“Contrary to the Earth – or even Jupiter and Saturn – the magnetosphere of Uranus is very poorly known,” said Dr. Laurent Lamy of the Observatoire de Paris in Meudon, France, who led the study published in Geophysical Research Letters.
Auroras on Uranus are fainter than they are on Earth, and the planet is more than 4 billion kilometers (2.5 billion miles) away. Previous Earth-bound attempts to detect the faint auroras were inconclusive. Astronomers got their last good look at Uranian auroras 25 years ago when the Voyager 2 spacecraft whizzed past the planet and recorded spectra from of the radiant display.
“This planet was only investigated in detail once, during the Voyager flyby, dating from 1986,” Dr. Lamy said. “Since then, we’ve had no opportunities to get new observations of this very unusual magnetosphere.”
The new set of observations is from when the planet was near equinox, when neither end of the Uranian rotational axis aims at the Sun, and the axis aligns almost perpendicular to the solar wind flow.
“Because the planet’s magnetic axis is tilted, the daily rotation of Uranus during the period around the equinox causes each of its magnetic poles to point once a day toward the Sun, likely responsible for a very different type of aurora than the one that was seen at solstice,” Dr. Lamy explained. “This configuration is unique in the Solar system.”
Capturing the images of Uranus’s auroras resulted from a combination of good luck and careful planning.
“Ever since the Voyager 2 flyby demonstrated that Uranus was a strange beast,” said Dr. Fran Bagenal, a planetary scientist at the University of Colorado in Boulder. “We’ve been really keen to get a better view. This was a very clever way of looking at that.”
“A better understanding of Uranus’ magnetosphere could help scientists test their theories of how Earth’s magnetosphere functions,” the scientists added. “We have ideas of how things work on Earth and places like Jupiter and Saturn, but I don’t believe you really know how things work until you test them on a very different system.”
