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The Forefront of Space Science

Nature's light show: planetary aurora
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Different components of the auroral emission can be seen in a single auroral image, and we can learn a lot from their analysis. Firstly it is obvious that the main oval is not really oval-shaped at all. Rather it is shaped like a kidney bean due to an anomaly in the magnetic field in this region, which makes it asymmetric around the pole. Outside of the main oval are the footprints of the large Galilean moons: Io, Europa and Ganymede. These appear as bright spots of light, with tails that fade further from the moons. Inside the main oval are the elusive polar emissions. These are the most variable component of Jupiter's aurora, continuously changing their location and brightness. Even after years of study, no-one is certain what is causing these emissions, although many theories suggest there should be an Earth-like solar wind interaction. This is one of the major aims of upcoming missions to Jupiter: to work out the processes which drive these intriguing emissions.

Figure 2
Figure 2. Jupiter's northern ultraviolet aurora imaged by the Hubble Space Telescope. Credit: NASA / J.T. Clarke.

The grandfather of the solar system

Moving further outward in the solar system we reach Saturn, the sixth planet from the Sun and the second largest. Saturn orbits at a distance nine times further than the Earth, meaning that one Saturn year lasts nearly thirty Earth years. Saturn is a gas giant like Jupiter and its outstanding feature is its incredible rings, made up of lumps of ice some as small as specks of dust and some as large as cars. Saturn also has more than 60 moons, made of rock and ice. Because of this, one might expect Saturn's aurora to be formed by the same process as Jupiter's, if rather scaled down from Io's extreme volcanism. However, careful analyses of auroral images taken by the Hubble Space Telescope and the Cassini orbiter at Saturn have revealed that Saturn's main auroral oval is instead controlled by the solar wind, in a similar way to that at the Earth. This has been determined by the fact that the aurora brighten and change their shape and location when the solar wind blows harder. The Cassini spacecraft, now in the sixth year of its tour of Saturn, has allowed many detailed studies into Saturn's magnetic field and the particles present both close to the planet, and in the distant equatorial plane. These studies also suggest there may be additional processes occurring which could lead to fainter 'Jupiter-like' emission outside the main oval, and an auroral spot from the moon Enceladus.

Figure 3
Figure 3. Three images of Saturn's aurora taken by the Hubble Space Telescope, showing the intensifications that occur in times of strong solar wind. The aurora are imaged in the ultraviolet and shown here in blue superposed on a visible image of the planet. Credit: NASA / J.T. Clarke.

There is still much to learn from each of the unique bodies in our solar system, and studying the aurora provides useful, not to mention beautiful, insights into the complex physical processes governing the dynamics of the different worlds. So, while scientists and engineers continue to design spacecraft and instruments to study more and more distant planets orbiting other stars, it is important not to forget about our nearest neighbours, our local environment, and how much more we can discover close to home.

Sarah V. Badman

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