Thanks to the Hubble Space Telescope, astronomers have caught a rare display of auroras on Uranus, which ranks among the solar system's oddest planets.
Unlike the beautiful, rippling curtains of greenish light we've been seeing in earthly skies over the past few months, the Uranian auroras are short-lived bright spots sitting on top of the ice giant's bluish cloud tops. But they're caused by a similar mechanism, involving the interaction of electrically charged particles with atoms and ions in the planet's upper atmosphere.
NASA's Voyager 2 probe picked up the first evidence of Uranus' auroras in 1986. "Since then, we've had no opportunities to get new observations of this very unusual magnetosphere," Laurent Lamy, an astronomer at the Observatoire de Paris, said today in a news release. There have been a fewhints of auroral observations, but Hubble's views from last November rank as the best views yet. Lamy and his colleagues provide the details in a paper published by Geophysical Research Letters.
The team took advantage of a lucky break and a favorable planetary alignment: Last year, Earth, Jupiter and Uranus were lined up so that energetic solar emissions could flow past each planet in turn. When the sun produced several outbursts in September, the astronomers timed the flow of the particle storm past Earth a couple of days later, and then detected the flow past Jupiter two weeks after that. On the basis of those readings, they calculated that the outburst would reach Uranus in mid-November, and scrambled to schedule observing time on the Hubble Space Telescope.
Uranus is an oddity because it basically rotates on its side as it orbits the sun. The orientation of its magnetosphere is tilted 60 degrees with respect to its rotational axis. As a result, during the current season, each of the planet's magnetic poles turns to face the sun in the course of a Uranian day. "This configuration is unique in the solar system," Lamy said.
Hubble was well-placed to catch the auroral flashes on the sunlit side, near Uranus' north magnetic pole. Each flash appeared to last only a couple of minutes, the astronomers said.
These new findings solidify Uranus' place on the list of planets flashing with auroral lights. Jupiter and Saturn are also on the list. Mars is thought to be capable of localized auroral effects, even though it doesn't have a global magnetic field. (In fact, some observers suspect we saw evidence of those effects last month.) Earlier this month, astronomers reported seeing auroral-type activity on Venus as well.
Lights on Earth
And then there's Earth. Last October, a solar outburst sparked northern lights that could be seen as far south as the state of Mississippi, and over the past month, higher-latitude residents have been treated to almost as many fireworks displays as Disneyland tourists typically get to see. Although the approach of summer is starting to cut down on the opportunities to see auroras in the Northern Hemisphere, some folks got great views as recently as last night. Here are a few of the highlights:
'Where in the Cosmos'
Today's picture of auroral displays on Uranus served as this week's "Where in the Cosmos" picture puzzle on the Cosmic Log Facebook page. It took only a couple of minutes for Shirley Beningo to blurt out which celestial body was shown in the picture, and what the bright spots were. To reward her for her quick cosmic vision, I'm sending her a pair of cardboard 3-D glasses, wrapped up in a 3-D picture of yours truly. Ashley Nicole and Gerry Marien came in as the runners-up, and are eligible for 3-D glasses as well. Be sure to click the "like" button on the Cosmic Log Facebook page so you're ready for next Friday's "Where in the Cosmos" contest.
Earlier stories of auroral glories:
- Farewell to the northern lights
- Northern lights make for must-see TV
- Southern exposure for auroral lights
- Sky lights go wild, north and south
- Solar storm lights up northern skies
- Slideshow: The best of the northern lights
- Cosmic Log's auroral archive
In addition to Lamy, the authors of "Earth-Based Detection of Uranus' Aurorae" include R. Prange, K.C. Hansen, J.T. Clarke, P. Zarka, B. Cecconi, J. Aboudarham, N. Andre, G. Branduardi-Raymont, R. Gladstone, M. Barthelemy, N. Achilleos, P. Guio, M.K. Dougherty, H. Melin, S.W.H. Cowley, T.S. Stallard, J.D. Nichols and G. Ballester.
Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto,"my book about the controversial dwarf planet and the search for other worlds.