* Astronomy

A new study has suggested that a new type of aurora may exist on Saturn, creating a faint ring around one the planet's poles.
According to a report in New Scientist, the planet has oval-like auroras that periodically brighten its poles. Its main auroras are thought to be caused by the solar wind.
This type of aurora is thought to be much like the Earth's, which is caused by the solar wind, a stream of charged particles emanating from the Sun.

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This is an artist's concept of the Saturnian plasma sheet based on data from Cassini magnetospheric imaging instrument. It shows Saturn's embedded "ring current," an invisible ring of energetic ions trapped in the planet's magnetic field.


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Credit: NASA/JPL/Space Science Institute

Saturn is at the centre, with the red "donut" representing the distribution of dense neutral gas outside Saturn's icy rings. Beyond this region, energetic ions populate the plasma sheet to the dayside magnetopause filling the faintly sketched magnetic flux tubes to higher latitudes and contributing to the ring current. The plasma sheet thins gradually toward the nightside. The view is from above Saturn's equatorial plane, which is represented by grid lines. The moon Titan's location is shown for scale. The location of the bow shock is marked, as is the flow of the deflected solar wind in the magnetosheath.

The auroral radio transmissions of giant planets are normally used to estimate their internal rotations. But in the case of Saturn, these emissions have variations on a monthly scale, which cannot be put down to rotation. A team of astronomers led by a researcher of CNRS of the Space Studies Laboratory and Astrophysics Instrumentation (UMR CNRS, Observatory of Paris), showed that these variations were controlled  by the solar wind.
Visually, the measurement of the rotation period of giant planets is vague because it gives only the combination between internal rotation and speed of the clouds, both of which are unknown factors. As the auroral radio transmissions of planets are produced by electrons moving in the planetary magnetic field, they are in theory dependent on the  planetary motion, and this is why are used they to measure the internal rotation of giant planets. For Jupiter, one can measure the rotation with a precision better than a one millionth part. In the case of Saturn, radio measurements had provided a rotational period  of 10h 39m 24s ±7s. But of later measurements by Ulysses and Cassini showed that this value could vary ±6 min (thus ±1%) on  yearly or monthly time  scale. The same variations  also seemed  to affect the magnetic field measured by Cassini.

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 Title: Modulation of Saturn's radio clock by solar wind speed
Author: Philippe Zarka, Laurent Lamy, Baptiste Cecconi, Renée Prangé & Helmut O. Rucker

The internal rotation rates of the giant planets can be estimated by cloud motions, but such an approach is not very precise because absolute wind speeds are not known a priori and depend on latitude: periodicities in the radio emissions, thought to be tied to the internal planetary magnetic field, are used instead. Saturn, despite an apparently axisymmetric magnetic field, emits kilometre-wavelength (radio) photons from auroral sources. This emission is modulated at a period initially identified as 10 h 39 min 24 ± 7 s, and this has been adopted as Saturn's rotation period. Subsequent observations, however, revealed that this period varies by plusminus6 min on a timescale of several months to years. Here we report that the kilometric radiation period varies systematically by plusminus1% with a characteristic timescale of 2030 days. Here we show that these fluctuations are correlated with solar wind speed at Saturn, meaning that Saturn's radio clock is controlled, at least in part, by conditions external to the planet's magnetosphere. No correlation is found with the solar wind density, dynamic pressure or magnetic field; the solar wind speed therefore has a special function. We also show that the long-term fluctuations are simply an average of the short-term ones, and therefore the long-term variations are probably also driven by changes in the solar wind.

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