London: Unravelling a long-standing puzzle in
astronomy, an international team of scientists have discovered
that Saturn`s aurora, an ethereal ultraviolet glow which
illuminates the planets upper atmosphere near the poles,
pulses roughly once per Saturnian day.
The length of a Saturnian day has been under much
discussion since it was discovered that the traditional
`clock` used to measure the rotation period of Saturn, a gas
giant planet with no solid surface for reference, apparently
does not keep good time.
Saturn, like all magnetised planets, emits radio waves
into space from the polar regions.
These radio emissions pulse with a period near to 11 h,
and the timing of the pulses was originally, during the
Voyager era, thought to represent the rotation of the planet.
However, over the years the period of the pulsing of the
radio emissions has varied, and since the rotation of a planet
cannot be easily sped up or slowed down, the hunt for the
source of the varying radio period has become one of the most
perplexing puzzles in planetary science.
Now, in a paper to be published in Geophysical Research
Letters (August 6), Nichols et al. use images from the
NASA/ESA Hubble Space Telescope of Saturn`s auroras obtained
between 2005-2009 to show that, not only do the radio
emissions pulse, but the auroras beat in tandem with the
"This is an important discovery for two reasons. First,
it provides a long-suspected but hitherto missing link between
the radio and auroral emissions, and second, it adds a
critical tool in diagnosing the cause of Saturn`s irregular
"Auroras, more commonly known as the `northern lights` on
Earth, are caused when charged particles in space are
funnelled along a planet`s magnetic field into the planet`s
upper atmosphere near the poles, whereupon they impact the
atmospheric particles and cause them to glow.
"This happens when a planet`s magnetic field is stressed
by, for example, the buffeting from the stream of particles
emitted by the Sun, or when moons such as Enceladus or Io
expel material into the near-planet space," the team led by Dr
Jonathan Nichols of the University of Leicester said.
Saturn`s radio waves were long suspected to be emitted by
the charged particles as they hurtle toward the poles, but no
radio-like pulsing had been observed in Saturn`s aurora, an
enigmatic disconnect between the two supposedly-related
However, Nichols et al. found that by using the clock of
the radio pulsing to organise the auroral data, and stacking
the results from all the Hubble Saturn auroral images obtained
from 2005-2009 on top of each other, the auroral pulsing
finally revealed itself.
"This confirms that the auroras and the radio emissions
are indeed physically associated, as suspected. This link is
important, since it implies that the pulsing of the radio
emissions is being imparted by the processes driving Saturn`s
aurora, which in turn can be studied by the NASA/ESA
spacecraft Cassini, presently in orbit around Saturn.
"It thus takes us a significant step toward solving the
mystery of the variable radio period," Dr Nichols added.