Daytime auroras may not occur on Earth, but on Saturn, the spectacular light shows illuminate the ringed planet's polar skies even at high noon.
Saturn's daytime auroras have surprised researchers, who previously hypothesized that auroras only occur when charged particles from the sun hit the edge of the planet's magnetic field on the nightside.
Using data gathered from Cassini's 13-year mission on Saturn, they discovered that even though the laws of physics are at work in other planets, different environments drive different processes that lead to different outcomes.
How Do Auroras Occur?
When magnetic fields collide in a process called magnetic reconnection, the magnetic lines break and release huge amounts of energy.
For example, the sun's solar flares shoot out plasma that carry electrically charged particles, which are channeled to the Earth's magnetic poles via its magnetic field. When these electrically charged particles, which naturally have their own magnetic field, come in contact with the Earth's magnetosphere, the magnetic lines break, change direction, and are then converted into heat and kinetic energy.
In other words, it is the earth's magnetic field colliding with charged particles streaming from the sun that create the breathtaking auroras that light up the north and south poles. It is the same process that scientists have seen in other planets that have auroras, including Venus, Mars, Jupiter, Saturn, and Uranus.
However, new research has uncovered that a slightly different process occurs on Saturn, which accounts for the appearance of daytime auroras over the Saturnian poles.
Saturn's Daytime Aurora
On Earth, magnetic collisions only happen on the side facing away from the sun. This happens because electrically charged particles coming from the sun cannot penetrate the Earth's magnetic field at daytime.
The reconnection only happens at a point called the magnetopause, which is found right at the edge of the magnetic field. On Saturn, however, magnetic collisions occur right inside the magnetic field.
"We were surprised to find that on Saturn, magnetic reconnection can happen not only on the Sun-facing side, but well inside the magnetosphere," says Andrew Coates of the Mullard Space Science Laboratory at the University College London. "This suggests there's a difference process at play."
The researchers believe that Saturn's fast rotation plays a role in generating noontime auroras. Although Saturn is 760 times larger than the Earth, its days are just 10 Earth Hours long.
Its fast spin may cause the dayside magnetodisc, a ring of plasma near the equator, to thin out and allow magnetic fields to collide during the day. It was previously thought that solar winds on the dayside of the magnetodisc compressed the plasma ring to prevent reconnections from happening.
Differences in Saturn's magnetosphere could also contribute to the anomaly. For instance, the magnetic field may also be affected by Saturn's rings and its multiple moons. The water vapor and ice particles unleashed by volcanoes on the moon Enceladus may also have an effect.
Researchers believe this could also help astronomers find unknown auroras in other planets. It could also point them to better leads in explaining mysterious X-ray pulses found coming from Jupiter.
Details of the study are published in the journal Nature Astronomy.