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A UK team of researchers have discovered a secondary aurora sparkling on Saturn that is much more like Jupiter’s in origin than it is the Earth’s. Planetary aurorae are produced when charged particles stream along the magnetic field of a planet and into its atmosphere from the surrounding space environment. On Earth these charged particles come from the solar wind – a stream of particles that emanate from the Sun. On Jupiter, however, the dominant source of aurora inducing particles is its own moons, particularly Io, which spews out roughly one tonne of volcanic material every second. Some of this becomes ionised and is pulled in by Jupiter’s magnetic field, co-rotating in a plasma sheet around the planet. But, as the particles spread out, the magnetic field weakens and breaks down, causing the particles to crash into Jupiter’s atmosphere and sparking the aurora.
Seen from space, an aurora appears as a ring of light circling a planet’s polar region, typically where magnetic poles reside. This image shows a series of Hubble photos showing the evolution of an auroral display on Saturn. Image: NASA/ESA/J. Clarke (Boston University). Previously, only one main auroral oval had been observed on Saturn, and the primary source of the particles was unclear. Measurements had already shown that Saturn’s main auroral oval is located significantly poleward of the latitude at which ions no longer co-rotate with the planet, meaning that it cannot be Jupiter-like in formation. But now a secondary oval 25 percent as bright as the main oval has been spotted near the equator, rotating with the planet’s magnetic field. It is a weak equivalent of Jupiter’s main oval, its relative dimness being due to the lack of a large source of ions such as from Io. “At Saturn, scientists were unsure whether the aurora was caused by the solar wind or by particles from its own system,” says Tom Stallard of the University of Leicester. “When we discovered the second zone of aurorae on Saturn we realised this aurora, unlike the one already seen on Saturn, was behaving in the same way as Jupiter’s, largely unaffected by the solar wind, dominated by the rotation of the planet.” Modelling the aurorae on Jupiter and Saturn shows that both exhibit aurora in the positions where the co-rotation between the planet and its plasma sheet breaks down. “We can now say that some of Saturn’s aurorae are like Jupiter’s and they have a common formation process,” says Stan Cowley of the University of Leicester. “Further, our discovery of the secondary aurora on Saturn suggests that we shall also find one on Jupiter within its polar region.” Although the morphology of the aurorae of both Jupiter and Saturn are significantly different, and have much still to tell us about the interaction of the planets with the solar wind, the processes by which they are formed seem to be similar. Summing up the new research in a paper published in Science this week, the authors conclude “it is no longer reasonable to consider Saturn's aurorae a 'hybrid' of those of the Earth and Jupiter, but rather that the aurorae of Jupiter and Saturn are variants of the same formation processes.”
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