When southern Alaska’s sleepy Mt. Augustine erupted in 2006—billowing ash and halting all air traffic—atmospheric researchers at the University of Alaska Fairbanks bolted to attention. UAF’s resident expert on the middle layers of Earth’s atmosphere, Richard L. Collins, used specialized lasers to detect clouds of volcanic aerosols aloft over Alaska. His lasers were able to track the clouds even in places where the aerosols were too disperse to be located by remote sensing techniques.

Despite the laser’s ash-tracking power, this recent research barely gets mentioned by the fast-talking Collins as he unapologetically blows a puff of second-hand smoke into a green laser beam pointed straight into the sky. The cigarette smoke makes the thin beam easy to see in this dark room. The Irish-born, slightly irreverent Collins is explaining how his team uses this and other powerful lasers to study the layers 10 to 75 miles (16 to 120 km) above the planet’s surface.  This region that includes the stratosphere, mesosphere and lower thermosphere is called the middle atmosphere.

The green-beam laser currently taking center stage is just one tool in Collins’ Lidar (Light Detection And Ranging) Research Laboratory (LRL) located at the Poker Flat Research Range, 30 miles north of Fairbanks near Chatanika, Alaska. Here, researchers are using different types of lidars to study different things in the middle atmosphere including noctilucent clouds, atmospheric metal content, thermal structure, the aurora and more. Such phenomena are tricky to measure because they occur at altitudes that are too high for planes and weather balloons to fly and too low for most orbiting satellite-borne instruments. But they are just right for lidar.

Lidar instruments are optical radar systems that measure scattered light to study distant objects. A laser is used to transmit light into the sky. A telescope collects the light echoes that are reflected back by the atoms, molecules and particles in the air. A computer keeps track of the time it takes for the light to travel out and back and so determines the height from which the echoes came. The light echoes that return to the telescope first provide measurements of the atmosphere closest to the ground, while the echoes that arrive in later time intervals provide measurements from greater heights. In the first millionth of a second after the laser pulse is transmitted, the telescope detects echoes from the ground to 160 yards away. In the second millionth of a second, the telescope detects echoes from 160 yards to 320 yards away, and so on. Thus, the lidar makes a measurement of the vertical profile of the atmosphere slice by slice from the ground upward.

Lidars come in many different varieties. Collins’ team uses mainly Rayleigh and resonance lidar systems. Rayleigh lidars send out laser light that gets reflected back from all the particles in the path. The Rayleigh lidar at the LRL uses a Neodynium;YAG laser that produces pulses of bright green light. The transmitted light is so intense that the lidar system measures the echo from the air itself. Resonance lidars use light of a specific wavelength (or with a specific color) to cause particular atmospheric atoms and molecules to fluoresce. The color is created by a dye and used in a dye laser that is precisely tuned in wavelength for a specific target species.

Collins and his team of graduate students are collaborating with researchers from across the United States and the world in their middle atmosphere research. He is widely known to students as a challenging teacher and a passionate researcher, but one who takes the time to make sure students learn tough topics. Each of his graduate students is involved in experimental, hands-on lidar work at LRL. Following are two snapshots of current work.

The vortex at the top of the world

Over each of Earth’s poles sits a vortex—a constantly moving, elongated cyclone stretching through the lower and middle layers of the atmosphere. Scientists know that each vortex forms in the winter months, when temperature gradients between the equator and pole are steep. When spring comes and temperatures warm, the vortices disappear.

Brentha Thurairajah, a graduate student working with Collins, is using the Rayleigh lidar to take high-resolution density and temperature measurements—specifically in the stratosphere and mesosphere—to better understand how the general circulation of the middle atmosphere impacts the Arctic vortex.

By mapping these temperature changes over time, Thurairajah hopes to learn more about how the different layers of the atmosphere impact, or are coupled to, each other. In particular, she is interested in any feedback mechanisms that may exist—for example, events in the stratosphere or mesosphere that trigger events in the troposphere or vice versa. Such information could eventually improve weather-forecasting skills worldwide.

In addition, such information about the workings of the arctic vortex could help explain its role in ozone depletion. It is inside the cold centers of these vortices that the antarctic and arctic ozone holes form. Chemicals build up in the vortices during the winter. When spring comes, the sunlight catalyzes reactions between these built-up chemicals and Earth’s protective ozone, rapidly breaking the ozone apart. As part of the International Polar Year, Thurairajah cooperates with other lidar researchers in Norway, Greenland and Nunavut to better understand middle atmosphere circulation and the arctic vortex.

Seeing purple in nighttime aurora

The different colors in the aurora, or northern lights, are evidence that different types of atoms and molecules are excited in the night sky. When particles coming off the sun, known as the solar wind, collide with atoms and molecules in the Earth’s upper atmosphere, they become excited. This extra energy is eventually released as a photon of light. The wavelength, or color, of the released light depends on just how much extra energy the atom or molecule gained and lost when it was excited.
The aurora is created when Earth’s magnetic field traps the solar wind particles close to the planet and allows them to interact with Earth’s atmosphere and become excited. Different colors appear because each element emits a characteristic wavelength of light. Excited oxygen appears as red or green light; excited nitrogen appears purple.

Graduate student Agatha Light is working with Collins to detect aurorally excited nitrogen molecules using the LRL’s dye laser. Such nitrogen measurements have never been done before, and much of Light’s challenge is finding the best dye to detect and measure the molecules of interest. Once she identifies the dye and calibrates the laser so that it can measure excited nitrogen reliably, she must wait for a purple, nitrogen-rich aurora on which to test the new method. If all goes well, Light intends to analyze at least one purple aurora during the 2008–2009 winter observation season.

Back in the dark room of the LRL, team leader Collins warns his visitors that “the beam can detach the retina from your eye instantly” as he wafts more cigarette smoke into the green laser beam. Collins is careful to highlight the dangers of working with lidar instruments. His talk is half science, half instruction.
If nothing else, the warnings get the attention of his audience in a hurry and prompt young visitors to start asking questions about the instruments and the research. When Collins asks the room full of visiting college students for questions, multiple arms shoot into the air. Clearly, he has captured their attention.

Further reading

Lidar Research Laboratory
http://www.gi.alaska.edu/splidar/

International Polar Year Middle Atmosphere Circulation Research
http://research.iarc.uaf.edu/IPY-CTSM/overview.php

A Message in the Sky?

Noctilucent clouds are seen in the skies of Fairbanks in the early-morning hours of August 2007. These clouds only form during summer months when heat in the lower atmosphere rises and cools. Some researchers believe this type of cloud is the result of global climate change since they were not documented until after the Industrial Revolution.

Every August in the arctic latitudes, mysterious sets of high-flying (50–53 mile, 80–85 km), thin, icy clouds are illuminated by the setting sun. These sporadic clouds were first documented by scientists in 1885 and are known as noctilucent, or “night luminous” clouds (NLC). For just a few weeks in late summer, Collins and his team turn their attention—and their instruments—to measure and monitor these NLCs with the hopes of understanding more about what causes them.

Currently, a scientific debate is brewing about the cause and trend of NLCs. The clouds were not documented until 1885, so many scientists speculate that NLCs are a byproduct of the Industrial Revolution and global climate change. Because of how the clouds form, warmer temperatures should mean an increase in NLC observations. And according to some, says Collins, the NLCs have indeed become more prevalent over time.
 
NLCs form only in the summer when air in the lower atmosphere rises and cools, creating an icy layer of NLCs in the mesosphere. Counter to common logic, the arctic mesosphere is colder in the summer than in the winter because the warm air rising through the mesosphere at the summer pole expands and cools.  The rising air cools more rapidly than it can be heated by the sun and so winter in the mesoshere arrives in our midsummer.

NLC studies conducted over the past century do appear to show an increase in cloud formation in the Arctic. But skeptics of the theory that NLC formation is a canary for global climate change suggest that current NLC trends are too short to detect long-term changes in cloud formation and that the annual variability in NLC formation could be expected in a naturally varying system. It seems further NLC research is needed, but that is easier said than done.

In 2008, UAF lidar observations of NLCs were hampered by uncooperative weather. One night, a regular, low-flying cloud blocked the lidar’s view of the NLCs. On another night, the clouds were visible on the northern horizon to humans but not detected overhead by the lidar lasers. The NLCs lingered in the sky to the northeast of the lab, out of reach of the laser beam, and teasing the eager reseachers below. On that night, no solid NLC measurements were obtained, according to graduate student Brentha Thurairajah.

This fleeting and finicky observation window is one reason the NLCs remain such a mystery to scientists. Conditions must be perfect to get good lidar measurements: the clouds must be directly overhead, the sky must be clear, and the sun must set enough for the clouds to be visible to the human eye. If NLCs are indeed a symptom of a warming world, it will take more work to diagnose the extent of the disease.