They shouldn’t vary at all. But they do. And their behavior in the north is different than in the south. These puzzling behaviors that planetary scientists have struggled to understand are the periodic behavior of Saturn’s magnetic field and its associated phenomena. Solving this mystery is one of the highest priorities in the Cassini Solstice Mission. A team of scientists with NASA's Cassini mission, led by Xianzhe Jia of the University of Michigan, Ann Arbor, has developed new models and simulations that successfully account for these periodic signatures by inserting vortices in Saturn's upper atmosphere or ionosphere (a high layer of the atmosphere where the powerful radiation of the solar wind dissociates atmospheric particles into ions and free electrons). Understanding these signatures gives insight to the workings of Saturn’s magnetosphere, and how it differs from other gas giant planets. It also reveals the diversity of phenomena in our solar system.
The magnetic bubbles around Jupiter and Saturn both generate strong radio emissions. In both cases, the radio waves are generated by electrical currents that flow between a planet’s magnetosphere and its ionosphere. Cloud pattern movements depend on both Jupiter’s spin and atmospheric wind speeds, which vary by latitude, so watching Jupiter’s weather patterns doesn’t allow scientists to measure the length of Jupiter’s day. When a “decametric” (i.e. with a wavelength of tens of meters) radiation was accidentally discovered in the mid-1950s at Jupiter, scientists noted that the intensity of this radiation coming from the planet varied over time, growing stronger and then weaker roughly every 10 hours. Scientists realized they could use the radio-frequency radiation produced by Jupiter to measure its rate of rotation for the first time.
Saturn initially seemed to have similar behavior. The intensity of Saturn’s kilometric radiation ("SKR," with a wavelength measured in kilometers) was observed to vary with a period of about 11 hours, so it was assumed that this was Saturn’s rotation rate. Consequently, the SKR period was adopted by the International Astronomical Union as Saturn’s rotation period, and rotation phase was taken to represent planetary longitude.
However, measurements by Cassini, in orbit around Saturn since 2004, have shown that the periodic signatures in Jupiter and Saturn’s magnetic fields must be driven differently. At Jupiter, the periodic behavior of the magnetic field itself and many other phenomena throughout Jupiter’s magnetosphere are primarily due to the approximately 10-degree tilt of the planet’s magnetic field when compared to Jupiter’s spin axis. This mismatch means that, when Jupiter rotates, its magnetic north and south poles sweep across space rather like how the beam from a lighthouse sweeps across the sea as the light rotates, causing the observed periodic signature. Saturn’s magnetic field, on the other hand, lines up almost exactly with the planet’s spin axis, with less than 0.1 degree between Saturn’s magnetic pole and spin axis. So there is no clear periodic beacon, just as if the lighthouse beam pointed straight up towards the sky rather than towards the ships on the sea looking for guidance. Why, then, do we see periodic behavior in Saturn’s magnetosphere at all?
While the magnetic field generated within Saturn is highly symmetric about its rotation axis, instruments on the Cassini orbiter found that curious periodic signatures are ubiquitous throughout Saturn’s magnetosphere. The signatures have been detected in the magnetic field, plasma properties, plasma waves, and auroral radio and ultraviolet emissions as the planet completes each rotation.
Cassini has also found that the period at which the SKR’s power is modulated drifts over time, changing by as much as roughly 1 percent per year. Though this change seems small, the rotation rate of a body as massive as Saturn could not change that rapidly. This means that the small change of period rules out the possibility of the periodic signature changes being caused by a dynamic change within the planet. It is also not plausible for high order magnetic anomalies to drift fast enough to account for such large shifts. Even more intriguingly, Saturn’s northern and southern periodicities are different, and appear to vary with Saturn’s seasons, presenting scientists with an additional challenge to interpretation of their origin.
Jia’s first model moves the potential source of the changes from Saturn’s interior to the upper atmosphere and ionosphere. It imposes a vortical flow within the ionosphere, coupled to the upper atmosphere. The flow in the ionosphere drives currents into the magnetosphere along magnetic field lines. The rotation of a region of perturbed flow around the spin axis imposes periodicity on the magnetosphere.
Jia created a second model because the phenomena are even more complicated than originally thought. Based on the detection of two separate periods in a number of Cassini observations, a longer period thought to originate in the southern hemisphere and shorter period thought to originate in the north, the researchers have enhanced their model by adding a second vortex at high latitudes in the northern hemisphere.
These ionospheric vortex models are the first detailed quantitative models that reproduce most of the observed periodic magnetospheric features – including periodic signatures in the magnetic field, large-scale electric current flows at Saturn’s equator, periodic density enhancements, periodic "breathing" at the boundaries of Saturn’s magnetosphere and a periodic signature seen in the extended tail of Saturn’s magnetosphere as it gets blown out by the solar wind.
Other scientists, whose papers appear below, have also come up with models suggesting that processes external to the planet, related to the production of plasma by Enceladus, can explain the strange periodic signatures in Saturn's magnetic field, radio emission and other magnetospheric parameters. As Cassini continues to make its way around the Saturn system, scientists will keep an eye on the phenomena and see if they can keep testing their hypotheses.
This Cassini Science League entry is an overview of science papers authored, or co-authored, by at least one Cassini scientist. The information above was derived from or informed by the following publications:
1) " Driving Saturn's Magnetospheric Periodicities from the Upper Atmosphere/Ionosphere: Magnetotail Response to Dual Sources," Xianzhe Jia and Margaret G. Kivelson, Journal of Geophysical Research, Volume 117, A11219, November 2012.
2) “Driving Saturn’s Magnetospheric Periodicities from the Upper Atmosphere/Ionosphere," Xianzhe Jia, Margaret G. Kivelson and Tamas I. Gombosi, Journal of Geophysical Research, Volume 117, A04215, April 2012.
3) "The Variable Rotation Period of the Inner Region of Saturn's Plasma Disk," Donald A. Gurnett, Ann M. Persoon, William S. Kurth, et al., Science Volume 316, Issue 5823, 442-445, April 2007.
4) "Spontaneous Axisymmetry Breaking of the External Magnetic Field at Saturn," Peter Goldreich and Alison J. Farmer, Journal of Geophysical Research-Space Physics, Volume 112, Issue A5, A05225, May 2007.
-- Marcia Burton, Cassini fields and particles scientist, Jo Pitesky, Cassini science communication coordinator and Margaret Kivelson, magnetometer team member, University of Michigan and UCLA.