Saturn's aurorae images 'unique to science'

Scientists from the University of Leicester have led an international study to capture space images that are unique to science.

Researchers using the NASA/ESA Hubble Space Telescope (HST) recently took advantage of the rare opportunity to record Saturn when its rings are edge-on, resulting in a unique movie featuring both of the giant planet's poles.

Saturn is only in this position every 15 years and this prime orientation allowed a sustained study of both of its beautiful and dynamic aurorae that decorate its poles much like the northern and southern lights on our own planet.

The study is a result of two Hubble Space Telescope programmes led by Jonathan Nichols at the University of Leicester.

Dr. Nichols said: “Hubble has proved to be one of mankind's most important scientific tools, and this is the first time that a group in the UK has led a HST programme to observe the aurorae on another world.

“However, scientists at the University of Leicester, including Prof. Stan Cowley and Dr. Emma Bunce in the Radio & Space Plasma Physics Group, did not just observe using HST, they are also actively involved in the Cassini mission which is observing many different aspects of Saturn's aurore and magnetic field, and which was recently extended to 2017 by NASA. The HST and Cassini observations combine to form a significant scientific force.”

It takes Saturn almost thirty years to orbit our Sun so chances to image both of its poles simultaneously are rare. Since 1994, Hubble has been snapping pictures of the planet at a good angle, but 2009 brought the unique opportunity for Hubble to image Saturn with rings edge-on. In addition, the ringed planet was approaching its equinox when both poles are equally illuminated by the Sun's rays [1].

These recent observations go well beyond just a still image and allowed researchers to monitor the behaviour of both Saturn's poles in the same shot over a sustained period of time and create a movie. Over several days during January and March 2009, Hubble collected data from the ringed planet that aided astronomers studying both its northern and southern swirling aurorae. Given the rarity of such an event, this new footage will likely be the last and best equinox movie that Hubble captures of our planetary neighbour.

Dr. Nichols added: “It is particularly exciting to know that these images are unique to science. They have not, and will never again, be obtained using Hubble. This is because HST pictured Saturn at a very special vantage point, near its equatorial plane. Due to Saturn's long orbit, HST will not see this view again in its lifetime. This sustained series images of simultaneous north-south aurora are important scientifically, since they cannot be obtained at any other planet, including Earth. They tell us a great deal about the nature of the planet's magnetic field and the processes which generate aurore in a way not possible at Earth. It's a great example of how planetary science can fully complement the study of the Earth.”

Despite its great distance, the Sun is still Saturn's parent star and a parents' influence is far reaching. The hot Sun constantly emits particles that reach all of the planets of the solar system in the form of solar wind. When this electrically charged stream gets close to a planet, the planet's magnetic field traps the particles, bouncing them back and forth between its two poles. The magnetic field thus focuses the particles on the polar regions, where they interact with atoms in the upper layers of the atmosphere creating aurorae, the familiar glow that the inhabitants of the Earth's polar regions know as the northern and southern lights.

The light show of Saturn's aurorae appears symmetric at the two poles. [2] However, analysing the new data in greater detail, astronomers discovered some subtle differences between the northern and southern aurorae, which reveal important information about Saturn's magnetic field. The northern auroral oval is slightly smaller and more intense than the southern one, implying that Saturn's magnetic field is not equally distributed across the orb of the planet; it is slightly uneven and is stronger in the north than the south. As a result, the electrically charged particles in the north are accelerated to higher energies as they are fired toward the atmosphere than those in the south. This confirms a previous result obtained by the space probe Cassini, in orbit around the ringed planet since 2004.

• Dr Nichols will be discussing this subject as part of the Leicester Physics Centre Public Lecture entitled 'Lots in Space'. It takes place on Tuesday 16 March at 18:30 in Lecture Theatre 3 of the Ken Edwards building, University of Leicester.