Every planetary body is a source of radio waves, but Saturn seems to be split in two regarding the radio waves controlled by the planet’s rotation. Saturn emits radio waves known as Saturn Kilometric Radiation (SKR), and an instrument on the Cassini spacecraft recently determined that the variations in these radio waves are different in the northern and southern hemispheres. Does this mean that one half of the planet is spinning at a different rate than the other? No, says Cassini scientist Don Gurnett, but “these data just go to show how weird Saturn is. We thought we understood these radio wave patterns at gas giants, since Jupiter was so straightforward. Without Cassini’s long stay, scientists wouldn’t have understood that the radio emissions from Saturn are so different.”
Gurnett is Cassini’s radio and plasma wave science instrument team lead and professor of physics at the University of Iowa, Iowa City.
Not only are the rates different for the different hemispheres, but the northern and southern rotational variations also appear to change with the Saturnian seasons, and the hemispheres have actually swapped rates over the years.
The two radio waves have been converted to the human audio range, can be heard in the video above. Since the radio wave patterns are controlled by the rotation of the planet, the sped-up audio sounds sort of like an a rotating siren.
Scientists don’t think the differences in the radio wave periods has to do with hemispheres actually rotating at different rates, but more likely came from variations in high-altitude winds in the northern and southern hemispheres.
Saturn has been sending scientists mixed signals for decades. Back in the early 1980’s when the Voyager spacecraft flew by the ringed planet, radiation emissions indicated the length of Saturn’s day was about 10.66 hours. But later, data from the Ulysses spacecraft and then Cassini showed radio burst varied by seconds to minutes. A paper in Geophysical Research Letters in 2009 analyzing Cassini data showed that the Saturn Kilometric Radiation was not one wave, but two: a duet, with two singers out of sync. Radio waves emanating from near the north pole had a period of around 10.6 hours; radio waves near the south pole had a period of around 10.8 hours.
The latest data, published in a new paper by Gurnett and his team in the Geophysical Research Letters, show that the southern and northern SKR periods crossed over around March 2010, about seven months after equinox, when the sun shines directly over a planet’s equator. The southern SKR period decreased from about 10.8 hours on Jan. 1, 2008 and crossed with the northern SKR period around March 1, 2010, at around 10.67 hours. The northern period increased from about 10.58 hours to that convergence point.
Seeing this kind of crossover led the Cassini scientists to go back into data from previous Saturnian visits. With a new eye, they saw that NASA’s Voyager data taken in 1980, about a year after Saturn’s 1979 equinox, showed different warbles from Saturn’s northern and southern poles. They also saw a similar kind of effect in the Ulysses radio data between 1993 and 2000. The northern and southern periods detected by Ulysses converged and crossed over around August 1996, about nine months after the previous Saturnian equinox.
Two other papers also seem to confirm this variation and switching. One led by Jon Nichols, University of Leicester, U.K. used data from the Hubble Space Telescope which showed the northern and southern auroras on Saturn wobbled back and forth in latitude in a pattern matching the radio wave variations, from January to March 2009, just before equinox. The radio signal and aurora data are complementary because they are both related to the behavior of the magnetic bubble around Saturn, known as the magnetosphere.
The other paper led by David Andrews, also of University of Leicester, using data from the Cassini magnetometer, showed that from mid-2004 to mid-2009, Saturn’s magnetic field over the two poles wobbled at the same separate periods as the radio waves and the aurora.
“The rain of electrons into the atmosphere that produces the auroras also produces the radio emissions and affects the magnetic field, so scientists think that all these variations we see are related to the sun’s changing influence on the planet,” said Stanley Cowley, a co-author on both papers, co-investigator on Cassini’s magnetometer instrument, and professor at the University of Leicester.
As the sun continues to climb towards the north pole of Saturn, Gurnett’s group has continued to see the crossover trend in radio signals through Jan. 1, 2011. The period of the southern radio signals continued to decrease to about 10.54 hours, while the period of the northern radio signals increased to 10.71 hours.
The teams will continue to monitor the changes in Cassini’s radio signal to better understand the variations.
Gurnett’s paper was published in Geophysical Research Letters.
Source: JPL
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