Ultraviolet
and infrared images from NASA's Cassini spacecraft and Hubble Space
Telescope show active and quiet auroras at Saturn's north and south
poles. Full caption
The
dark region seen on the face of the sun at the end of March 2013 is a
coronal hole (just above and to the right of the middle of the picture),
which is a source of fast solar wind leaving the sun. Image Credit: NASA/SDO/AIA. Full image and caption
While
the curtain-like auroras we see at Earth are green at the bottom and
red at the top, NASA's Cassini spacecraft has shown us similar
curtain-like auroras at Saturn that are red at the bottom and purple at
the top. Image Credit: NASA/JPL-Caltech/SSI. Full image and caption
NASA
trained several pairs of eyes on Saturn as the planet put on a dancing
light show at its poles. While NASA's Hubble Space Telescope, orbiting
around Earth, was able to observe the northern auroras in ultraviolet
wavelengths, NASA's Cassini spacecraft, orbiting around Saturn, got
complementary close-up views in infrared, visible-light and ultraviolet
wavelengths. Cassini could also see northern and southern parts of
Saturn that don't face Earth.
The result is a kind of step-by-step choreography detailing how the
auroras move, showing the complexity of these auroras and how scientists
can connect an outburst from the sun and its effect on the magnetic
environment at Saturn.
"Saturn's auroras can be fickle -- you may see fireworks, you may see
nothing," said Jonathan Nichols of the University of Leicester in
England, who led the work on the Hubble images. "In 2013, we were
treated to a veritable smorgasbord of dancing auroras, from steadily
shining rings to super-fast bursts of light shooting across the pole."
The Hubble and Cassini images were focused on April and May of 2013.
Images from Cassini's ultraviolet imaging spectrometer (UVIS), obtained
from an unusually close range of about six Saturn radii, provided a look
at the changing patterns of faint emissions on scales of a few hundred
miles (kilometers) and tied the changes in the auroras to the
fluctuating wind of charged particles blowing off the sun and flowing
past Saturn.
"This is our best look yet at the rapidly changing patterns of
auroral emission," said Wayne Pryor, a Cassini co-investigator at
Central Arizona College in Coolidge, Ariz. "Some bright spots come and
go from image to image. Other bright features persist and rotate around
the pole, but at a rate slower than Saturn's rotation."
The UVIS images, which are also being analyzed by team associate
Aikaterini Radioti at the University of Liege, Belgium, also suggest
that one way the bright auroral storms may be produced is by the
formation of new connections between magnetic field lines. That process
causes storms in the magnetic bubble around Earth. The movie also shows
one persistent bright patch of the aurora rotating in lockstep with the
orbital position of Saturn's moon Mimas. While previous UVIS images had
shown an intermittent auroral bright spot magnetically linked to the
moon Enceladus, the new movie suggests another Saturn moon can influence
the light show as well.
The new data also give scientists clues to a long-standing mystery about the atmospheres of giant outer planets.
"Scientists have wondered why the high atmospheres of Saturn and
other gas giants are heated far beyond what might normally be expected
by their distance from the sun," said Sarah Badman, a Cassini visual and
infrared mapping spectrometer team associate at Lancaster University,
England. "By looking at these long sequences of images taken by
different instruments, we can discover where the aurora heats the
atmosphere as the particles dive into it and how long the cooking
occurs."
The visible-light data have helped scientists figure out the colors
of Saturn's auroras. While the curtain-like auroras we see at Earth are
green at the bottom and red at the top, Cassini's imaging cameras have
shown us similar curtain-like auroras at Saturn that are red at the
bottom and purple at the top, said Ulyana Dyudina, an imaging team
associate at the California Institute of Technology, Pasadena, Calif.
The color difference occurs because Earth's auroras are dominated by
excited nitrogen and oxygen molecules, and Saturn's auroras are
dominated by excited hydrogen molecules.
"While we expected to see some red in Saturn's aurora because
hydrogen emits some red light when it gets excited, we also knew there
could be color variations depending on the energies of the charged
particles bombarding the atmosphere and the density of the atmosphere,"
Dyudina said. "We were thrilled to learn about this colorful display
that no one had seen before."
Scientists hope additional Cassini work will illuminate how clouds of
charged particles move around the planet as it spins and receives
blasts of solar material from the sun.
"The auroras at Saturn are some of the planet's most glamorous
features – and there was no escaping NASA's paparazzi-like attention”,
said Marcia Burton, a Cassini fields and particles scientist at NASA's
Jet Propulsion Laboratory, Pasadena, Calif., who is helping to
coordinate these observations. "As we move into the part of the 11-year
solar cycle where the sun is sending out more blobs of plasma, we hope
to sort out the differences between the effects of solar activity and
the internal dynamics of the Saturn system."
There is still more work to do. A group of scientists led by Tom
Stallard at the University of Leicester is busy analyzing complementary
data taken during the same time window by two ground-based telescopes in
Hawaii -- the W.M. Keck Observatory and NASA's Infrared Telescope
Facility. The results will help them understand how particles are
ionized in Saturn's upper atmosphere and will help them put a decade of
ground-based telescope observations of Saturn in perspective, because
they can see what disturbance in the data comes from Earth's atmosphere.
Jia-Rui Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook@jpl.nasa.gov
