Saturn’s wonky magnetic field

Earth is not the only planet in the solar system that features the colourful polar light shows we know as the aurora borealis and aurora australis, or northern and southern lights. Jupiter and Saturn have similar auroral displays, which arise from an interaction between planetary magnetic fields and the constant stream of electrons and protons that make up the solar wind.

Saturn, viewed from the Earth-orbiting Hubble Space Telescope, showing auroral light around the planet's north and south poles

Some years ago the Cassini spacecraft provided us with stunning images of Saturn’s aurorae. Now we have observations from the Hubble Space Telescope with Saturn’s rings edge-on, showing at this near-equinox point in its orbit the giant planet’s northern and southern lights. The Hubble website features a number of still images and animations of varying resolution.

Think of a planetary magnetic field and you will likely imagine a symmetric bar magnet arrangement, with north and south poles, and onion-like lines of magnetic force joining them. In reality the situation is a little more complex, with a magnetic field – whether it be that of Earth, or of a gas giant such as Jupiter and Saturn – generated by complex motions of electrically conducting fluid in a planet’s core. The resulting field departs from the idealised model of a simple bar magnet, and there can be significant differences in the magnetic field between the north and south poles.

Differences in field strength and shape will affect the way in which the charged particles that give rise to aurorae are confined by the field lines. In the case of Saturn, the northern so-called auroral oval is slightly smaller and more intense than the one around the south pole, which indicates that the auroral particles in the north are accelerated to higher energies as they spiral around and down the magnetic field lines to the planet’s upper atmosphere. We already knew this from Cassini observations, but the Hubble results confirm the bigger picture.

The Hubble telescope and Cassini spacecraft may have provided us with impressive scientific insight into Saturn’s auroral light shows, but there is surely nothing to beat watching Earth’s aurorae from the ground on a crisp winter night. That is something I greatly miss now that I am no longer a research geophysicist with regular fieldwork duties in Arctic Scandinavia.