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The Forefront of Space Science

Nature's light show: planetary aurora
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Extra-terrestrial aurora

Other planets in our solar system have aurora too, and studying them tells us about the extra-terrestrial environments. Jupiter, the largest planet in the solar system, has such a strong magnetic field that if you could see its extent in the sky from Earth, it would appear the same size as the Sun, even though it is five times further away. Jupiter's strong magnetic field is one characteristic of its space environment, as well as its super-fast rotation; one day on Jupiter lasts less than ten hours. Add into this fast, energetic environment the volcanic moon Io and you can expect to see some fantastic interactions taking place. It is these characteristics of Jupiter that lead to the formation of its main aurora, rather than an Earth-like interaction with the solar wind.

Io is slightly larger than Earth's moon and orbits close to Jupiter. It is the most volcanically active body in the solar system, releasing around 1 tonne every second of oxygen, sulphur and other materials into its local space. It is this material, and the way that energy is transferred from the planet to make it rotate around the planet, that generates Jupiter's aurora. In this way the moon Io, located deep inside Jupiter's magnetic environment, is the origin of Jupiter's main auroral oval encircling its magnetic pole, rather than the solar wind blowing outside.

The Hubble Space Telescope

Jupiter is a gas giant planet - it has no solid surface and its upper atmosphere is made mainly of hydrogen gas. This means that the brightest auroral emissions (more than ten times brighter than the Earth's aurora) are emitted from hydrogen at ultraviolet wavelengths. The human eye can only see a small portion of the full light spectrum (we see 'visible' light in colours red to violet) so telescopes are used to observe at other wavelengths like the UV. One useful and well-known telescope is the Hubble Space Telescope, which is in orbit around the Earth at a height of less than 600 km. Hubble carries several different cameras and instruments and has been used many times to observe aurora on Jupiter.

Figure 1
Figure 1. The Hubble Space Telescope in orbit above the Earth. Credit: NASA.

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