Uranus’s magnetic field toggles on and off every day


The strange geometry of Uranus’s magnetic field means it opens and closes each time the planet spins, writes Andrew Masterson.


A composite image showing an extremely bright aurora on Uranus.
A composite image showing an extremely bright aurora on Uranus. The auroras are produced by periodic fluctuations in the magnetic field interacting with the solar wind.
ESA/Hubble & NASA, L. Lamy / Observatoire de Paris

Uranus’s magnetosphere – the region surrounding the planet in which the magnetic field dominates – gets switched on and off every day, according to researchers at the Georgia Institute of Technology in the US.

In a paper published in the Journal of Geophysical Research: Space Physics, the scientists report that the magnetosphere flips between “open” and “closed” modes once every rotation of the planet.

When the system is in open mode, solar wind flows in, circulating across the planet. In closed mode, the magnetosphere forms a protective barrier, deflecting the wind back out into space.

To make their findings, the researchers, Xin Cao and Carol Paty, used data gathered by NASA’s Voyager 2 probes as it passed Uranus in 1986.

The information revealed a magnetosphere markedly different to that which surrounds the Earth.

On our planet, it aligns quite closely with the spin axis, which means it whirls around like a top. Because of that, the magnetosphere is always facing towards the sun. It flips between open and closed modes quite often, but irregularly, always in response to strong solar winds.

This mode switching, by the way, is one of the causes for the colourful auroras sometimes seen at extreme northerly and southern latitudes.

Uranus’s magnetosphere, in contrast, exhibits precise regularity in its mode changes. This, say the researchers, is because it lies at an angle of roughly 60 degrees to the planet’s spin axis, causing its interaction with incoming solar winds to vary dramatically during the 17 hours it takes for a full rotation.

“Uranus is a geometric nightmare,” says Paty.

“The magnetic field tumbles very fast, like a child cartwheeling down a hill head over heels. When the magnetised solar wind meets this tumbling field in the right way, it can reconnect and Uranus’s magnetosphere goes from open to closed to open on a daily basis.”

The researchers suggest the robust and regular changes to the magnetosphere may mean that the icy planet has spectacular auroras across its breadth every day.

However, given that the planet is 3,218,688,000 kilometres from Earth, even our best eye in the sky, the Hubble Space Telescope, is too far away to see any but the very brightest of them.

  1. http://onlinelibrary.wiley.com/doi/10.1002/2017JA024063/full
  2. http://onlinelibrary.wiley.com/doi/10.1002/2017JA024063/full
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