Caltech Scientist Proposes Explanation for Puzzling Property of Night-Shining Clouds at the Edge of Space
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For Immediate Release
September 25, 2008
Caltech Scientist Proposes Explanation for Puzzling Property of
Night-Shining Clouds at the Edge of Space
PASADENA, Calif.-- An explanation for a strange property of
noctilucent clouds--thin, wispy clouds hovering at the edge of space
at 85 km altitude--has been proposed by an experimental plasma
physicist at the California Institute of Technology (Caltech),
possibly laying to rest a decades-long mystery.
Noctilucent clouds, also known as night-shining clouds, were first
described in 1885, two years after the massive eruption of Krakatoa,
a volcanic island in Indonesia, sent up a plume of ash and debris up
to 80 km into Earth's atmosphere. The eruption affected global
climate and weather for years and may have produced the first
noctilucent clouds.
The effects of Krakatoa eventually faded, but the unusual electric
blue clouds remain, nestled into a thin layer of Earth's mesosphere,
the upper atmosphere region where pressure is 10,000 times less than
at sea level. The clouds, which are visible during the deep twilight,
are most often observed during the summer months at latitudes from 50
to 70 degrees north and south--although in recent years they have
been seen as far south as Utah and Colorado. Noctilucent clouds are a
summertime phenomenon because, curiously, the atmosphere at 85 km
altitude is coldest in summer, promoting the formation of the ice
grains that make up the clouds.
"The incidence of noctilucent clouds seems to be increasing, perhaps
because of global warming," says Paul M. Bellan, a professor of
applied physics at Caltech.
Twenty-five years ago, researchers at Poker Flat, Alaska, discovered
that the clouds were highly reflective to radar. This unusual
property has long puzzled scientists. Bellan, reporting in the August
issue of the Journal of Geophysical Research-Atmospheres, now has an
explanation: the ice grains in noctilucent clouds are coated with a
thin film of metal, made of sodium and iron. The metal film causes
radar waves to reflect off ripples in the cloud in a manner analogous
to how X-rays reflect from a crystal lattice.
Sodium and iron atoms collect in the upper atmosphere after being
blasted off incoming micrometeors. These metal atoms settle into a
thin layer of vapor that sits just above the altitude at which
noctilucent clouds occur. Astronomers recently have been using the
sodium layer to create laser-illuminated artificial guide stars for
adaptive optics telescopes that remove the distorting affects of
atmospheric turbulence to produce clearer celestial images.
Measurements of the density of sodium and iron atomic vapor layers
show that the metal vapor is depleted by over 80 percent when
noctilucent clouds are present. "Noctilucent clouds have been shown
to act very much like a flycatcher for sodium and iron atoms," Bellan
says. Indeed, in laboratory experiments, other researchers have found
that at the frigid temperatures (-123 degrees Celsius) within
noctilucent clouds, atoms in sodium vapor quickly become deposited on
the surface of ice to form a metallic film.
"If you have metal-coated ice grains in noctilucent clouds, the radar
reflectivity can become enormous" he says. "This reflectivity is not
the sum of reflections from individual ice grains, which would not
produce a very large reflection. Instead, what happens is that
ripples in the cloud of metal-coated ice grains reflect in unison and
reinforce each other, somewhat like an army marching in step across a
bridge causes the bridge to vibrate."
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Contact: Kathy Svitil
(626) 395-8022
ksv...@caltech.edu