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For
the first time, scientists have captured images of auroras above the
giant ice planet Uranus, finding further evidence of just how peculiar a
world that distant planet is. Detected by means of carefully scheduled
observations from the Hubble Space Telescope, the newly witnessed
Uranian light show consisted of short-lived, faint, glowing dots—a world
of difference from the colorful curtains of light that often ring
Earth’s poles.
In
the new observations, which are the first to glimpse the Uranian aurora
with an Earth-based telescope, the researchers detected the luminous
spots twice on the dayside of Uranus—the side that’s visible from
Hubble. Previously, the distant aurora had only been measured using
instruments on a passing spacecraft. Unlike auroras on Earth, which can
turn the sky greens and purples for hours, the newly detected auroras on
Uranus appeared to only last a couple minutes.
In
general, auroras are a feature of the magnetosphere, the area
surrounding a planet that is controlled by its magnetic field and shaped
by the solar wind, a steady flow of charged particles emanating from
the sun. Auroras are produced in the atmosphere as charged solar wind
particles accelerate in the magnetosphere and are guided by the magnetic
field close to the magnetic poles—that’s why the Earthly auroras are
found around high latitudes.
But
contrary to the Earth—or even Jupiter and Saturn—“the magnetosphere of
Uranus is very poorly known,” said Laurent Lamy, with the Observatoire
de Paris in Meudon, France, who led the new research.
The
results from his team, which includes researchers from France, the
United Kingdom, and the United States, will be published Saturday in Geophysical Research Letters, a journal of the American Geophysical Union.
Auroras
on Uranus are fainter than they are on Earth, and the planet is more
than 4 billion km (2.5 billion miles) away. Previous Earth-bound
attempts to detect the faint auroras were inconclusive. Astronomers got
their last good look at Uranian auroras 25 years ago when the Voyager 2
spacecraft whizzed past the planet and recorded spectra from of the
radiant display.
“This
planet was only investigated in detail once, during the Voyager flyby,
dating from 1986. Since then, we’ve had no opportunities to get new
observations of this very unusual magnetosphere,” Lamy noted.
Planetary
scientists know that Uranus is an oddball among the solar system’s
planets when it comes to the orientation of its rotation axis. Whereas
the other planets resemble spinning tops, circulating around the Sun,
Uranus is like a top that was knocked on its side—but still keeps
spinning.
The
researchers suspect that the unfamiliar appearance of the newly
observed auroras is due to Uranus’ rotational weirdness and peculiar
traits of its magnetic axis. The magnetic axis is both offset from the
center of the planet and lists at an angle of 60 degrees from the
rotational axis—an extreme tilt compared to the 11 degree difference on
Earth. Scientists theorize that Uranus’s magnetic field is generated by a
salty ocean within the planet, resulting in the off-center magnetic
axis.
The
2011 auroras differ not only from Earth’s auroras but also from the
Uranian ones previously detected by Voyager 2. When that spacecraft made
its flyby decades ago, Uranus was near its solstice—its rotational axis
was pointed toward the Sun. In that configuration, the magnetic axis
stayed at a large angle from the solar wind flow, producing a
magnetosphere similar to the Earth’s magnetosphere, although more
dynamic. Under those 1986 solstice conditions, the auroras lasted longer
than the recently witnessed ones and were mainly seen on the nightside
of the planet, similar to what’s observed on Earth, Lamy said. Hubble
can’t see the far side of the planet, however, so researchers don’t know
what types of auroras, if any, were generated there.
The
new set of observations, however, is from when the planet was near
equinox, when neither end of the Uranian rotational axis aims at the
Sun, and the axis aligns almost perpendicular to the solar wind flow.
Because the planet’s magnetic axis is tilted, the daily rotation of
Uranus during the period around the equinox causes each of its magnetic
poles to point once a day toward the Sun, likely responsible for a very
different type of aurora than the one that was seen at solstice, Lamy
explained.
“This
configuration is unique in the solar system,” added Lamy, who noted
that the two transient, illuminated spots observed in 2011 were close to
the latitude of Uranus’s northern magnetic pole.
Capturing
the images of Uranus’s auroras resulted from a combination of good luck
and careful planning. In 2011, Earth, Jupiter and Uranus were lined up
so that the solar wind could flow from the Sun, past Earth and Jupiter,
and then toward Uranus. When the Sun produced several large bursts of
charged particles in mid-September 2011, the researchers used
Earth-orbiting satellites to monitor the solar wind’s local arrival two
to three days later. Two weeks after that, the solar wind sped past
Jupiter at 500 km per second (310 miles per second). Calculating that
the charged particles would reach Uranus in mid-November, the team
scrambled to scheduled time on the Hubble Space Telescope.
Ever
since the Voyager 2 flyby demonstrated that Uranus was a “strange
beast,” said Fran Bagenal, a planetary scientist with the University of
Colorado in Boulder, “we’ve been really keen to get a better view. This
was a very clever way of looking at that.”
A
better understanding of Uranus’ magnetosphere could help scientists
test their theories of how Earth’s magnetosphere functions, she added.
“We have ideas of how things work on Earth and places like Jupiter and
Saturn, but I don’t believe you really know how things work until you
test them on a very different system.”
Earth-based detection of Uranus’ aurorae
Source: American Geophysical Union