How long is a day? On Earth and other rocky planets, it is fairly easy to measure – take some sort of a topographic feature and watch it spin round the planet. On the gas giants, it is more difficulty. With there being no ground spinning round, other things have to be taken into account. For Jupiter, we can look at the varied cloud bands, see how the winds move in different directions and muddle something together. For the Sun, another big ball of gas, we can look at magnetic features such as sun spots anchored on the surface and see how they rotate. For Saturn, which lacks much cloud detail and has no visible magnetic features, there’s more of a quandary.
One idea was to listen to radio emissions. Saturn Kilometric Radiation (SKR) is a form of radiowaves emitted by charged particles accelerating down the magnetic field lines. Should the field lines be spinning in coordination with the planet, then there’s your clock. However, SKR, first timed by the Voyager probes, has been remeasured over the years and surprisingly has slowed down far faster than a planet’s rotation might be expected to.
Now, while the rest of us were missing out on the aurora over the UK due to cloud, research has been published that shows the aurorae on Saturn have bright spots corresponding to radio loud SKR – whenever there’s a strong radio signal, there’s a strong auroral signal. Although this may seem obvious – both aurorae and radio emission being due to the motion of charged particles, both those trapped on magnetic field lines and those heading down into the atmosphere to precipitate – it does suggest that whatever mechanism is behind the slowing of the SKR rotation rate also plays a part in the auroral acceleration mechanism.
The observations behind the research were made possible by using the Hubble Space Telescope at a time when Saturn was able to be viewed ring on. At such a time, both poles are visible enough for their aurorae to be seen at once and conjugate auroral events – things happening at both auroral regions at once – could be seen. It was these relative brightenings and dimmings that were tracked to SKR volume differences.