Tuning in to Saturn's radio broadcast
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 23, 2008

Observations from NASA's Cassini spacecraft have been used to build the first three dimensional picture of the sources of intense radio emissions in Saturn's magnetic field, known as the Saturn Kilometric Radiation (SKR).

Discovered by NASA's Voyager spacecraft in 1980, SKR is the most intense component of radio emissions from Saturn, with frequencies between about 10 kilohertz and 1.2 megahertz corresponding to the Long Wave and Medium Wave broadcasting bands. These emissions produce eerie sounds and are generated by high-energy electrons spiralling around magnetic field lines that are threaded through Saturn's aurora. Previous Cassini observations have shown that the SKR is closely correlated with the intensity of Saturn's UV aurora and the pressure of the solar wind in the vicinity of the Solar System giant.

Seen from space, an aurora appears as a ring of light circling a planet’s polar region, typically where magnetic poles reside. This image shows a series of Hubble photos showing the evolution of an auroral display on Saturn. Image: NASA/ESA/J. Clarke (Boston University).

Unlike the Earth’s aurora which is visible to the naked eye at polar latitudes, Saturn's aurora is only seen in ultraviolet light. Local brightening in Saturn’s aurora often follows the rotation of the planet and exhibits rapid variations on time scales of minutes. These variations and regularities indicate that the aurora is primarily shaped and powered by a continual tug-of-war between Saturn's magnetic field and the flow of charged particles from the Sun.

Speaking at the European Planetary Science Congress held in Munster, Germany this week, Dr Baptist Cecconi of the Observatoire de Paris describes the results of the Cassini flyby of 25-26 September 2006, made using the Radio and Plasma Wave Science (RPWS) instrument. This flyby maximised the science return since Cassini approached from the southern hemisphere and swooped out from the northern hemisphere, allowing the instruments to take measurements from about 30 degrees below to about 30 degrees above the equatorial plane.

“The radio sources are clustered around curving magnetic field lines,” he says. “Because the radio signals are beamed out from the source in a cone-shape, we can only detect the sources as Cassini flies through the cone. When Cassini flies at high altitudes over the ring planes, we see the sources clearly clustered around one or two field lines. However, at low latitudes we get more refraction and so the sources appear to be scattered.”

Click for animation showing the sources of the radio emission in Saturn's magnetic field and the corresponding active magnetic field line footprints. Image: Cecconi, Lamy and Zarka.

Cecconi’s model shows that the active magnetic field lines can be traced back to near-polar latitudes in both the northern and southern hemisphere, matching well with the location of Saturn's UV aurora.

"For the purposes of the model, we've imagined a screen that cuts through the middle of Saturn, set up at right-angles to the line between Cassini and the centre of the planet,” he explains. “We've mapped the footprints of the radio sources projected onto the screen, which tilts as Cassini moves along its orbital path, and its orientation with respect to Saturn changes. We've also traced the footprints of the magnetic field lines back to the cloud tops of Saturn.”

The observations also revealed some minor differences between emissions in the northern and southern hemispheres, but the strongest emission was seen in the western part of Saturn's sunlit hemisphere, corresponding to a region of Saturn's magnetopause where electrons are thought to be accelerated by the interaction of the solar wind and Saturn's magnetic field.