Uranus's weird auroras captured by Hubble

Strange auroras that are very different from Earth's northern lights have been observed on Uranus using the Hubble Space Telescope.
Uranus's auroras appeared briefly as a faint spot close to the planet's northern magnetic pole. (Laurent Lamy)

Strange auroras that are very different from Earth's northern lights have been observed on Uranus using the Hubble Space Telescope.

A faint, glowing dot appeared close to the northern magnetic pole of the ice giant planet for just a couple of minutes on two days in November 2011, reported a news release from the American Geophysical Union describing a study of the phenomenon published Saturday in the organization's journal Geophysical Research Letters.

On Earth, auroras usually take the form of green, red and purplish curtains of light in the night sky (the side that faces away from the sun) near the poles that can dance and ripple for hours.

Auroras are produced when the solar wind, a flow of charged particles from the sun, interacts with a planet's magnetic field. In the case of Earth, the particles are drawn to the magnetic poles, which are close to the geographic North and South poles.

They interact with oxygen and nitrogen molecules in the atmosphere, producing the northern lights and the southern lights.

In the case of Uranus, located roughly four billion kilometres away from Earth,  a research team led by Laurent Lamy of the Observatoire de Paris in Meudon, France, managed to get images of its auroras by predicting exactly when a powerful solar wind unleashed by the sun in September 2011 would arrive at Uranus.

They pointed Hubble in the right direction at that time, which was about two months later, witnessing the aurora's brief appearance on the "day" side of the planet, the side facing the sun.

The researchers think the strange appearance of Uranus's aurora has to do with its unusual magnetic field. While the Earth's magnetic and geographic poles (on either side of its axis of rotation) are near each other, Uranus's magnetic poles are offset from its geographic poles — by 60 degrees. Also, the way the planet is currently rotating causes each of its magnetic poles to point toward the sun once a day.

"This configuration is unique in the solar system," said Lamy in a statement.

The auroras on Uranus have been detected once before, by the spacecraft Voyager during a flyby in 1986. At that time, the spacecraft detected longer auroras on its night side, the side facing away from the sun, which Hubble cannot see.

On that occasion, the magnetic poles were pointed at a large angle to the sun, as the Earth's magnetic poles are, rather than directly at the sun as they were at the time of the newly detected aurora.