This story about noctilucent clouds on NASA’s Science website made me think about a few things.
What I wonder is this: could noctilucent clouds be a proxy for cosmic ray interactions? While there are a lot of high energy galactic cosmic rays (GCR’s), there are GCR”s that are coming in at low energies as well. The lower the energy, the higher in the atmosphere would be their primary target area. The lower the energy, the more the earth’s magnetic field is deflecting them, redirecting then towards the magnetic poles like the protons of the solar wind (which have much lower energies).
Noctilucent clouds are not yet well understood, and given the size of the NLC ice crystals, cosmic ray interaction could be a possible trigger for their formation. As many of you know, cosmic rays leave tiny nucleation tracks, in the atmosphere. much like they do when entering a cloud chamber in a nuclear physics lab. I built one once as high school student and watched cosmic rays and other background radiation zip through.
Tracking particles and cosmic rays in a cloud chamber
As occurs in a supercooled cloud chamber (dry ice is involved) it would seem to me that cosmic ray interaction with very rarefied supercooled water vapor could be occurring in the 60-90 km altitude range. While the mechasism of a cloud chamber relies on supersaturation to leave a visbile trail, an interaction that forms a small ice crystal in the nanometer range may not need supersaturation. And, given that NLC’s form mostly at high latitudes, as aurora borealis does, there may be an interactive component of some sorts with earth’s magnetic field.
I’m also thinking NLC’s may very well be the equivalent of “dark matter in the universe” for our atmospheric interface with space and incoming solar radiation. As mentioned in the article “There is a substantial population of invisible noctilucent clouds, a population of much smaller ice crystals (< 30 nm) that don’t scatter much sunlight.” They may be small, but may have an albedo effect of some sort that is undiscovered. This is all just conjecture on my part, but I thought it would make for interesting discussion. If nothing else, NLC’s illustrate that we still don’t know how all aspects of the atmosphere work, and the portion that is closest to space is the one that is the most difficult to measure. I welcome discussion. – Anthony
UPDATE: Here is an excellent powerpoint presentation on NLC’s:
h/t to Fernando Mafili from comments
August 25, 2008: When in space, keep an eye on the window. You never know what you might see.
Last month, astronauts on board the International Space Station (ISS) witnessed a beautiful display of noctilucent or “night-shining” clouds. The station was located about 340 km over western Mongolia on July 22nd when the crew snapped this picture:
Above: Noctilucent clouds photographed by the crew of the ISS: more.
Atmospheric scientist Gary Thomas of the University of Colorado has seen thousands of noctilucent cloud (NLC) photos, and he ranks this one among the best. “It’s lovely,” he says. “And it shows just how high these clouds really are–at the very edge of space.”
He estimates the electric-blue band was 83 km above Earth’s surface, higher than 99.999% of our planet’s atmosphere. The sky at that altitude is space-black. It is the realm of meteors, high-energy auroras and decaying satellites.
What are clouds doing up there? “That’s what we’re trying to find out,” says Thomas.
People first noticed NLCs at the end of the 19th century after the 1883 eruption of Krakatoa. The Indonesian supervolcano hurled plumes of ash more than 50 km high in Earth’s atmosphere. This produced spectacular sunsets and, for a while, turned twilight sky watching into a worldwide pastime. One evening in July 1885, Robert Leslie of Southampton, England, saw wispy blue filaments in the darkening sky. He published his observations in the journal Nature and is now credited with the discovery of noctilucent clouds.
Scientists of the 19th century figured the clouds were some curious manifestation of volcanic ash. Yet long after Krakatoa’s ash settled, NLCs remained.
“It’s a puzzle,” says Thomas. “Noctilucent clouds have not only persisted, but also spread.” In the beginning, the clouds were confined to latitudes above 50o; you had to go to places like Scandinavia, Siberia and Scotland to see them. In recent years, however, they have been sighted from mid-latitudes such as Washington, Oregon, Turkey and Iran
Above: Noctilucent clouds over Mt. Sabalan, a 15,784 ft extinct volcano in northwestern Iran. Photo credit: Siamak Sabet. [more]
“This year’s apparition over Iran (pictured above) was splendid,” says Thomas. The Persian clouds appeared on July 19th, just a few days before the ISS display, and were photographed from latitude 38o N. “That’s pretty far south,” he says.
The genesis and spread of these clouds is an ongoing mystery. Could they be signs of climate change? “The first sightings do coincide with the Industrial Revolution,” notes Thomas. “But the connection is controversial.”
NASA is investigating. The AIM satellite, launched in April 2007, is now in polar orbit where it can monitor the size, shape and icy make-up of NLCs. The mission is still in its early stages, but already some things have been learned. Thomas, an AIM co-Investigator, offers these highlights:
1. Noctilucent clouds appear throughout the polar summer, are widespread, and are highly variable on hourly to daily time scales. A movie made from daily AIM snapshots shows the 2007 NLC season unfolding over the north pole: watch it.
Right: A daily snapshot of noctilucent cloud activity over the North Pole in 2007. Click on the image to set the scene in motion. Credit: AIM/Goddard Space Flight Center Scientific Visualization Studio.
2. There is a substantial population of invisible noctilucent clouds. Thomas explains: “NLCs are made of tiny ice crystals 40 to 100 nanometers wide—just the right size to scatter blue wavelengths of sunlight. This was known before AIM. The spacecraft has detected another population of much smaller ice crystals (< 30 nm) that don’t scatter much sunlight.” Clouds made of these smaller crystals are stealthy and hard to see, but a key part of the overall picture.
3. Some of the shapes in noctilucent clouds, resolved for the first time by AIM’s cameras, resemble shapes in tropospheric clouds near Earth’s surface. AIM science team members have described the similarities as “startling.” The dynamics of weather at the edge of space may not be as unEarthly as previously supposed.
These findings are new and important, but they don’t yet unravel the central mysteries:
- Why did NLCs first appear in the 19th century?
- Why are they spreading?
- What is ice doing in a rarefied layer of Earth’s upper atmosphere that is one hundred million times dryer than air from the Sahara desert?
AIM has just received a 3-year extension (from 2009 to 2012) to continue its studies. “We believe that more time in orbit and more data are going to help us answer these questions,” says Thomas.
Meanwhile, it’s a beautiful mystery. Just ask anyone at the edge of space.
h/t to Jack Simmons for the NASA story link.