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Uranus auroras glimpsed from Earth

Charles Black, Founder and CEO of Sen
Apr 16, 2012, 7:00 UTC

Sen—The aurorae on Uranus have been spotted for the first time since 1986, providing astronomers with new insight into the magnetosphere of the planet.

The aurorae discovered are very different from the stunning light shows we can see on Earth. On Uranus, the aurorae appeared as short-lived, faint glowing dots.

The recent aurorae, which occurred in November 2011, were imaged by the Hubble Space Telescope.

The previous detection of aurorae in 1986 came from a Voyager 2 flyby, which recorded spectra from the light show.

The Uranian aurorae are very faint, as the planet is over four billion kilometres away, which is why many previous attempts to catch the aurorae in action failed. 

“This planet was only investigated in detail once, during the Voyager flyby, dating from 1986. Since then, we’ve had no opportunities to get new observations of this very unusual magnetosphere,” said research team leader Laurent Lamy from the Observatoire de Paris in Meudon, France.

It wasn’t a coincidence that Hubble was trained on Uranus at the time of the aurorae; astronomers had predicted that such a display might occur. In mid-September 2011, the Sun belched out a mass of charged particles, which arrived at the Earth several days afterwards. Two weeks later, the charged particles whizzed past Jupiter at 500 kilometres per second, allowing astronomers to calculate when they would reach Uranus. 

Unlike the sheets of coloured light that dance for hours in the Earth’s atmosphere, these Uranian aurorae were short lived and only persisted for a few minutes. They were seen as luminous spots, and were detected twice near the northern magnetic pole.

While Voyager 2 mainly saw aurorae on Uranus’ nightside, researchers detected the aurorae on the dayside of the planet - the only side visible from Hubble.

The recent aurorae do not resemble the 1986 aurorae, which lasted much longer than the 2011 display. This is due to different positions of the planet’s rotational axis at different points in its orbit. During the Voyager 2 flyby, Uranus’ rotational axis was pointed at the Sun, meaning the magnetic axis, which is offset from the rotational axis by 60 degrees, was at a large distance from the flow of the solar wind. In 2011, a different alignment occurred, resulting in each of the magnetic poles were pointing towards the Sun once a day and causing a different type of aurora.

Uranus is the seventh planet from the Sun and was discovered in 1781 by William Herschel. 

Uranus has a diameter of 50,700 kilometres (31,500 miles) - about four times that of Earth. The ice giant orbits the Sun at an average distance of almost 2.9 billion kilometres (1.8 billion miles) and effectively lies on its back, tilted over by 97.8 degrees on its axis. 

Whilst a day on Uranus is shorter than an Earth day (at 17.2 Earth hours), its takes Uranus 84 years to make a single orbit of the Sun.

Learn more about Uranus in our feature by Sen columnist Mark Thompson.