Magnetosphere of Saturn - Dynamics - Aurorae

Aurorae

Saturn has bright polar aurorae, which have been observed in the ultraviolet, visible and near infrared light. The aurorae usually look like bright continuous circles (ovals) surrounding the poles of the planet. The latitude of auroral ovals varies in the range of 70–80°; the average position is 75 ± 1° for the southern aurora, while the northern aurora is closer to the pole by about 1.5°. From time to time either aurorae can assume a spiral shape instead of oval. In this case it begins near midnight at a latitude of around 80°, then its latitude decreases to as low as 70° as it continues into the dawn and day sectors (counterclockwise). In the dusk sector the auroral latitude increases again, although when it returns to the night sector it still has a relatively low latitude and does not connect to the brighter dawn part.

Unlike Jupiter's, the Saturn's main auroral ovals are not related to the breakdown of the co–rotation of the plasma in the outer parts of the planet's magnetosphere. The aurorae on Saturn are thought to be connected to the reconnection of the magnetic field under the influence of the Solar wind (Dungey cycle), which drives an upward current (about 10 million amperes) from the ionosphere and leads to the acceleration and precipitation of energetic (1–10 kev) electrons into the polar thermosphere of Saturn. The saturnian aurorae are more similar to those of the Earth, where they are also Solar wind driven. The ovals themselves correspond to the boundaries between open and closed magnetic field lines—so called polar caps, which are thought to reside at the distance of 10–15° from the poles.

The aurorae of Saturn are highly variable. Their location and brightness strongly depends on the Solar wind pressure: the aurorae become brighter and move closer to the poles when the Solar wind pressure increases. The bright auroral features are observed to rotate with the angular speed of 60–75% that of Saturn. From time to time bright features appear in the dawn sector of the main oval or inside it. The average total power emitted by the aurorae is about 50 GW in the far ultraviolet (80–170 nm) and 150–300 GW in the near-infrared (3–4 μm—H₃+ emissions) parts of the spectrum.