Interplanetary sleuthing by a tag team of spacecraft has revealed a new link between the glowing ultraviolet auroras at Saturn's poles and the planet's mysterious radio emissions.
As the Saturn kilometric radiation (so known because the radio emissions' wavelengths are measured in kilometers), or SKR, emanates from the gas-giant planet, its intensity oscillates every 10.5 hours or so, nearly in concert with the planet's rotation. In fact, the length of the radio cycle was once taken to represent the rotational period of the planet. (On gas giants such as Saturn, where there is no surface to speak of, determining rotational speed is no trivial task.) But the duration of an SKR cycle later proved to vary by several minutes over the years, whereas a planet's rotational speed should remain nearly constant on such short timescales.
Using radio measurements taken by the Cassini spacecraft, which has been exploring the Saturnian system since 2004, along with ultraviolet auroral observations from the Earth-orbiting Hubble Space Telescope, Jonathan Nichols and his colleagues located a periodic pulsing in the auroral power that matches the ebb and flow of the so-called SKR. Nichols, a research associate in the Radio and Space Plasma Physics Group at the University of Leicester in England, and his colleagues reported their finding August 6 in Geophysical Research Letters. Understanding the two phenomena, which are both produced by interactions between charged particles and the planet's magnetic field, may help to explain the variations in the timing of the radio pulses and to shed light on Saturn's geomagnetic workings. The full complexity of the interaction between Saturn's magnetosphere, its planetary rotation, and charged particles, including those of the solar wind, is not well understood.
Auroras arise from the impact of streams of charged particles into a planet's atmosphere; the phenomena occur at the poles because magnetic field lines converge there, effectively steering charged particles into the atmosphere. On Earth, the auroras (known colloquially as the northern and southern lights) are primarily driven by the solar wind, but there appear to be other factors in play at Saturn, says study co-author John Clarke, an astrophysicist at Boston University. "The next question is what produces the charged particles," Clarke says. "That is still fairly uncertain in the case of Saturn."
The strong linkage between Saturnian auroras and radiation is not entirely surprising, notes William Kurth, a research scientist in the physics and astronomy department at the University of Iowa, who did not contribute to the new study. "I like to think of these radio emissions as being simply a long-wavelength emission, much longer than optical wavelengths, that are tied up with the same processes that produce the auroras," Kurth says. The kilometric radiation, he adds, is thought to result from electrons accelerated by electric fields in the planet's magnetosphere, and those electric fields also contribute to the generation of the polar auroras.
After all, similar connections have been found between Earth's auroras and terrestrial radio emissions. "We've believed from studies here at Earth that there should be a good correlation between these kilometric radio emissions from the auroras and the auroras themselves," Kurth says. "The reason that this hasn't been established [for Saturn] prior to this paper is that data hasn't been readily available."
A number of studies had shown that the SKR emissions and the planet's auroras emanate from the same latitudes and appear generally correlated in intensity. But establishing a regular, periodic behavior of the auroras akin to that of the radio waves required years of data, Clarke says. "The new piece here is that the UV aurora has not been observed to pulse in the way the radio emissions do," Clarke says.
Getting a solid body of data using Hubble can be a tricky business; securing observing time on the orbiting telescope is a highly competitive endeavor. But in a series of Hubble campaigns between 2005 and 2009, Nichols and his colleagues compiled enough images to show that the auroral activity at the planet's poles tends to peak in synchrony with the intensity of the SKR. "Jon [Nichols] had to look at years of data to pull that result out of it," Clarke says.