Saturn is a bit of an enigma so far as auroral physics goes. Unlike Uranus and Neptune, it is close enough for instruments and probes to make a significant series of observations of the auroral region. Unlike Mars and Venus, the aurora is intrinsically bright. The source of the enigma is the question of whether Saturn has an internally driven aurora like Jupiter or an externally driven one like the Earth.
The Earth’s aurora
I have described in earlier posts the causes of Earth’s aurora – the solar wind (particles of hot gas expanding away from the Sun) interacts with the Earth’s magnetosphere (the region where the Earth’s magnetic field is stronger than the solar wind’s) by connecting the two together as it hits the front and disconnecting as it passes beyond the back. As it’s doing this, electrons and protons it feeds into Earth’s polar regions cause a current system that then accelerates stuff from the magnetosphere into the atmosphere. The movie below shows this happening, with a simulation of measurements of solar wind particle density, direction and auroral activity.
The important thing to remember about this is that the aurorae of the Earth are controlled by external forces in general. They react to the changing solar wind, which in turn reacts to solar activity, though there are internal factors that help this along. The Maunder minimum of solar activity in the 16th century saw the Sun become unusually quiet and the aurora vanished as a result.
That’s all fine and well, but can a planet have a strong internal auroral source? Well, the answer’s yes. The strongest aurora in the solar system is the Jovian aurora and that is entirely internal. The volcanic moon Io spews out sulphur and hydrogen and all sorts of other things into a big donut shape around Jupiter. Sunlight then ionises this and those electrons and ions are spun up as electrons and ions have to follow the speed at which the magnetic field spins to stop currents forming. At a certain distance from the planet, this means they have to orbit too fast and so they stop co-rotating with the field and spin at a slower rate. Having two sets of electric charges moving at different speeds causes currents, and these currents link to the atmosphere through magnetic field lines. This causes the main auroral oval on Jupiter.
You’ll see on the image above that there are two other auroral signals, one called ‘polar emission’ and one called ‘satellite footprints’. The first one is generated the same way Earth gets its aurorae – through the Dungey cycle mentioned above, the second ones are due to ionisation of gases on the satellites themselves – the satellites orbit at different speeds than the magnetosphere rotates and as a result, there is a current that generates these.
So is Saturn Jupiter-like, Earth-like or a unique case of its own? Well it seems to be something like the latter, the evidence for either case can be picked up and for this, you just need to look in the two wavelength regions aurorae are studied in. Saturn’s aurora can’t be studied easily in the visible due to the massive amounts of reflected sunlight overwhelming auroral signals. They can be studied in the ultraviolet or the infrared however (and they have radio signals too). In the ultraviolet, studies with the Hubble Space Telescope have shown a tremendous terrestrial style aurora, in that it reacts to the solar wind. The solar wind at Saturn isn’t a gentle, constant breeze, but an ever-changing thing that saves up its energy for three days, blasts all at once, does nothing for another three days, then blasts again and so on. The result is the aurora turn on and off as the image below dramatically demonstrates.
But note how in the description above, I mention the density or power of the aurora changes, which causes this, whereas in the Dungey cycle and the movie above, magnetic field direction has the strongest effect on Earth. The image above is very reminiscent of an image taken by Kristian Birkeland a century earlier when firing electrons at a magnetised sphere called a terella, when trying to explain Earth’s aurora. Although this failed to explain Earth’s auroral distribution, the spiral has a familiar look to the one above.
Further hits to the idea of an Earth-like aurora came with the arrival of Cassini to the Kronian system. Cassini looked at Saturn in infrared light. Spectroscopic studies of infrared Doppler shifts in the auroral region had already suggested a quiet oval with a Jupiter like structure – plasma close to the planet corotating with the magnetic field, plasma further away rotating more slowly. When Cassini first arrived, it snapped such a large, quiescent oval.
But it still had a look of a spiral. Maybe it was the same as in the ultraviolet after all? Well, they kept snapping away as Cassini got closer, they even tried taking the UV aurora with the HST at the same time as Cassini and it soon became clear that the IR aurora was different and more Jovian like. Although there are clearly at least two sets of processes going on – one similar to the UV aurora and one similar to a low energy Jovian aurora – there is generally a constant oval and intermittent polar emission, which is what we have at Jupiter, just rebalanced a little. A closer Cassini image is below.
Now the question is where do the two processes come from? We’ve explained the first away by thinking of Saturn as Earth, but with a bigger magnetosphere and a pulsed solar wind shaking material out of the magnetotail – the UV told us that much. The infrared on the other hand has this constant, though broken, oval. It all starts in the right place, is a bit bitty, but looks like a Jupiter style thing. But Jupiter has a satellite gushing out all this volcanic material – what does Saturn have? Well, for the answer to that, lets look at another Cassini image.
The image shows the rings of Saturn, not just the ones we see but even more lit up by the Sunlight passing through dust. The Sun peaks through a little on the bottom left of the planet and a small blue pixel can be seen on the upper left, just away from the main rings – the Earth. The material for Saturn’s auroral oval comes from these vast swathes of dust and ice. The rings of Saturn rotate at the orbital velocities of the particles they’re made of, smaller faster fragments are further from the planet, so the rings generally rotate at the same angular velocity all the way out, but this is a different angular velocity to the magnetic field. The Sun ionises the rings just as it ionises Io’s volcanic dust cloud. Electromagnetic effects such as the Spokes (dark dust on the rings such as the image below) are seen as a result, and maybe, just maybe, this is enough to power a current system that generates the infrared oval. And maybe once the solar wind causes material to be dumped closer to this current system, the extra plasma fires it harder and helps to ignite the Dawn flares. Time and lots of further study will tell.