On Monday, Nasa's Juno spacecraft is due to make its plunge into Jupiter's atmosphere. The arrival marks the start of the mission's unparalleled study of the gas giant and has been five years in the making.
As part of the mission, Juno will enter an orbit flying above Jupiter's poles to study the Jovian magnetic field and aurorae from this viewpoint for the first time. Scientists want to find out what is driving the aurorae on Jupiter because it differs from the features of aurorae seen on Earth.
Learning about aurorae can reveal a wealth of information about our neighbouring planets and, where we have both images and solar wind data from spacecraft, will help us look to "exoplanets' beyond our solar system.
In particular, discovering which radio waves are given off by an aurora can tell us about a planet's magnetic field and this could prove especially useful in the hunt for alien life.
On Earth, tourists flock to see the aurorae in ordinary, visible wavelengths of light. The most intense aurorae are caused by solar storms – when charged particles rain down on the upper atmosphere, excite gases, and cause them to glow red, green and purple – Jupiter has an additional source for its aurorae.
Light is also given off in other wavelengths, like ultraviolet and radio waves, and because exoplanets are so far away – and orbiting around very bright stars – it is often difficult to see a visible aurora on these planets. By comparison, radio waves give a strong signal of the aurora that is not so easily obscured by the parent star.
Jupiter's aurorae were first discovered by the Voyager 1 spacecraft in 1979. A thin ring of light on Jupiter's nightside looked like a stretched-out version of our own aurorae on Earth. Only later was it discovered that the aurorae were best visible in the ultraviolet.
The Hubble Space Telescope recently captured a selection of Jupiter's aurorae and not only are they huge, they are also hundreds of times more energetic than aurorae on Earth. Unlike those on Earth, Jupiter's displays don't end.
Dr Jonathan Nichols and Professor Steve Milan from the University of Leicester are studying radio emissions from brown dwarfs – the only objects outside the solar system where we have detected aurora. "If we can understand the reasons why the brown dwarf is emitting in the radio then we can scale that up to exoplanets,” explained Nichols.
Brown dwarfs are a cross between planets and stars and powerful emitters of radio waves. Measuring radio emissions tells us about the magnetic field of the brown dwarf – and magnetic fields are critical. Life on Earth depends on our magnetic field; it provides protection from being bombarded by dangerous space particles, such as the electrons and protons in the "solar wind" streaming from the Sun. Consequently, if you're looking for habitable planets, you need to be looking for a good magnetic field.
Nichols and Milan have created a new model of auroral radio emissions from Earth-like planets. The model reveals the power of the radio waves that we would expect to see from planets like ours so could will help identify the planets most likely to support life.
Previously, researchers have used a scaling law based on the solar system, meaning they plotted radio power output against a solar wind input for all planets, then extrapolated from this. The problem with this approach is that not all aurorae are generated in the same way.
Earthly aurorae occur because of the impact of the solar wind with the Earth's magnetic field, whereas we would still see aurorae on Jupiter even without the solar wind. Jupiter's aurorae happen because sulphur from volcanic eruptions on its moon, Io, gets caught up in the magnetic field of the rapidly rotating planet. This generates electric currents that accelerate electrons to the surface and cause auroral displays.
The solar wind may add to this, affecting the brightness of the aurorae seen and because of the differences in the generation of aurorae, scaling laws are not an accurate way of determining radio emission from exoplanets.
To combat this problem, Nichols and Milan's model describes the actual physical processes that generate aurorae on Earth and can thereby calculate how the radio power output changes based on changes in the solar wind and planet's situation - its magnetic field and orbital distance from its star. Specifically, auroral processes on Earth follow what is known as the Dungey Cycle.
As the solar wind hits the Earth's magnetic field it breaks, reconnects and opens up field lines on the Sun-side of the planet, dragging them up and over the Earth, laying the magnetic field lines down behind in a stretched-out 'tail'. As more and more is laid down, this pushes magnetic field lines into contact in the tail. These eventually break and reconnect, catapulting particles down towards our atmosphere.
This forms very vivid aurorae on the night-side of the planet. After reconnection, the newly closed field lines push back towards the day side, drifting sideways round the Earth. This whole process is the Dungey cycle, and it was modelled by Nichols and Milan by considering how particles and currents flow in the upper atmosphere.
The new model indicates radio emissions to be expected from exoplanets under different stellar wind conditions and at varying distances from the parent star. It reveals the power of radio waves we are looking for is too low to be detected by our current telescopes. Plus, in the future, more sensitive receivers like the Square Kilometre Array "should be able to detect plenty of exoplanets," continued Nichols.
"If you could measure the magnetic field of exoplanets it would tell you so much more than the spectral or atmospheric measurements can. It has the potential to be transformative for exoplanet science."
This is yet further evidence the aurora is so much more than just a beautiful tourist attraction and could lead to discovering if we are alone in the universe.
Dr Melanie Windridge is author of Aurora: In search of the Northern Lights, out now