June 11, 2013 - NORTH POLE - Every summer, something strange and wonderful happens high above the
north pole. Ice crystals begin to cling to the smoky remains of
meteors, forming electric-blue clouds with tendrils that ripple
hypnotically against the sunset sky. Noctilucent clouds—a.k.a.
"NLCs"--are a delight for high-latitude sky watchers, and around the
Arctic Circle their season of visibility is always eagerly anticipated.
 |
| Satellite images of noctilucent clouds
(NLCs), which hover over Earth's poles at
the edge of space. The data come from NASA's
AIM
spacecraft. The north polar "daisy"
pictured below is a composite of near-realtime
images from AIM assembled by researchers at
the University of Colorado's Laboratory for
Atmospheric and Space Physics (LASP). |
News flash: This year, NLCs are getting an early start. NASA's AIM
spacecraft, which is orbiting Earth on a mission to study noctilucent
clouds, started seeing them on May 13th.
"The 2013 season is remarkable because it started in the northern
hemisphere a week earlier than any other season that AIM has observed,"
reports Cora Randall of the Laboratory for Atmospheric and Space Physics
at the University of Colorado. "This is quite possibly earlier than
ever before."
The early start is extra-puzzling because of the solar cycle.
Researchers have long known that NLCs tend to peak during solar minimum
and bottom-out during solar maximum—a fairly strong anti-correlation.
"If anything, we would have expected a later start this year because the
solar cycle is near its maximum," Randall says. "So much for
expectations."
For sky watchers, this means it's time to pay attention to the sunset
sky, where NLCs are most often seen. An early start could herald
brighter clouds and wider visibility than ever before.
 |
| On Sunday, June 9th, Alan Dyer of Gleichen, Alberta, Canada, went outside to see the colors of the sunset. He got more than he bargained for. Stacked atop the rosy glow of twilight were dual bands of electric-blue noctilucent clouds and green auroras. "At times the auroral curtains appeared superimposed on the noctilucent clouds," says Dyer. "It isn't often we see the two phenomena together." |
Noctilucent clouds were first noticed in the mid-19th century after the
eruption of super-volcano Krakatoa. Volcanic ash spread through the
atmosphere, painting vivid sunsets that mesmerized observers all around
the world. That was when the NLCs appeared. At first people thought
they must be some side-effect of the volcano, but long after Krakatoa's
ash settled the noctilucent clouds remained.
"They've been with us ever since," says Randall. "Not only that, they are spreading."
When AIM was launched in 2007, the underlying cause of NLCs was still
unknown. Researchers knew they formed 83 km above Earth's surface where
the atmosphere meets the vacuum of space--but that's about all. AIM
quickly filled in the gaps.
 |
| Astronomers in the Netherlands have discovered a
world where the sun is square. It is Earth. On June
6th Jan Koeman was watching the sunset from Lauwersoog,
and this is what he saw. "The sunset was a very weird
one," says Koeman. "Inversions in the
atmosphere gave it some very odd
shapes." |
"It turns out that meteoroids play an important role in the formation of
NLCs," explains Hampton University Professor James Russell, the
principal investigator of AIM. "Specks of debris from disintegrating
meteors act as nucleating points where water molecules can gather and
crystallize."
Early NLCs (geometry, med) NLCs appear during summer because that is when water molecules are wafted up from the lower atmosphere to mix with
the "meteor smoke." That is also the time when the upper atmosphere is
ironically coldest.
Back in the 19th century, NLCs were confined to high latitudes. You had
to go to Alaska or Scandinavia to see them. In recent years, however,
they have been sighted as far south as Utah, Colorado, and Nebraska.
Some researchers believe that the spread of NLCs is a sign of climate
change.
 |
| Glowing electric-blue at the edge of space, noctilucent clouds have
surprised researchers by appearing early this year. The unexpected
apparition hints at a change in the "teleconnections" of Earth's
atmosphere. Credit: Science@NASA
|
One of the greenhouse gases that has become more abundant in Earth's
atmosphere since the 19th century is methane. "When methane makes its
way into the upper atmosphere, it is oxidized by a complex series of
reactions to form water vapor," says Russell. "This extra water vapor is
then available to grow ice crystals for NLCs."
The early start of the 2013 season appears to be caused by a change in atmospheric “teleconnections.”
WATCH: NASA's ScienceCasts - An Early Start for Noctilucent Clouds.
“Half-a-world away from where the northern NLCs are forming, strong
winds in the southern stratosphere are altering global circulation
patterns,” explains Randall. "This year more water vapor is being pushed
into the high atmosphere where NLCs love to form, and the air there is
getting colder."
"All of this has come as an interesting surprise for us," notes Russell.
"When we launched AIM, our interest was in the clouds themselves. But
now NLCs are teaching us about connections between different layers of
the atmosphere that operate over great distances. Our ability to study
these connections will surely lead to new understanding about how our
atmosphere works."
For more information about NASA’s AIM mission, visit:
www.nasa.gov/aim. -
NASA.