Exploring the Heavens with Laser Light
Imagine a glowing green pencil that reaches so far into the night sky it seems to pierce the Big Dipper. Such is the sight on a hillside above the Chatanika River valley, where scientists at Poker Flat Research Range aim lasers skyward. With lasers, they hope to learn more about the upper tiers of Earth's atmosphere.
Besides looking really neat as they cut through winter air, lasers allow scientists to gather information from miles above without leaving the ground. Laser light is the primary tool of Richard Collins, who allowed me to tag along with him to Poker Flat. Collins is a researcher at the Geophysical Institute of the University of Alaska Fairbanks. Unlike a standard light bulb that emits light in all directions, a laser's energy is focused in one direction. Because lasers retain their narrow beam characteristics for exceptional distances, Collins is able to send pulses of laser light high enough to reach the part of the atmosphere he studies.
The laser gives Collins a view of the mesosphere, a region from thirty to fifty miles above sea level, just below where the bottom of the aurora forms. Collins is funded to study the mesosphere because scientists think this area will cool as Earth's surface warms, and they want to find out why. Because the mesosphere is a tough place to study--balloons carrying sensors pop before they get that high, and satellites can't orbit that low--scientists know little about the region when compared to their knowledge of the atmosphere above and below.
The mesosphere is the home of shooting stars, where meteors flame out as they hurdle toward Earth at speeds as fast as 30 miles per second. Meteors, pebble-size fragments left over from the birth of the solar system, glow with the heat of friction as they collide with gas molecules in the mesosphere.
When a meteor burns, it leaves a trail of smoke and atoms of metal. Because one of these metals, sodium, reflects light extremely well, Collins uses this cosmic exhaust to check the winds in the mesosphere. He aims a laser the color of street lamps (which glow with the action of excited sodium atoms) straight up. The laser light travels to the mesosphere and bounces off the sodium atoms; the returning light is captured by a telescope next to the laser. A computer calculates the distance a pulse of laser light traveled before being reflected back. The sodium atoms act like ink in water; their movement allows Collins to track the winds of the mesosphere.
The laser also allows Collins to see noctilucent, or "luminous night" clouds. People at high latitudes can see noctilucent clouds with the naked eye in late summer. The rare clouds show themselves only after sunset or before sunrise, which gives a hint of their elevation, about 50 miles above the ground (most clouds form at much lower elevations in the troposphere; the tallest thunderclouds flatten like the top of an anvil at an altitude of about seven miles).
Oddly, temperatures in the mesosphere are coldest when it is warmest on the ground. This leads to the formation of noctilucent clouds above Alaska in August. Because the clouds have only been reported since the 1870s, scientists wonder if perhaps human activity causes or intensifies the clouds, which may be the result of pollution and a fingerprint of global change. Measurements taken throughout the year, through the waxing and waning of the seasons, are important in understanding how the entire atmosphere might evolve over the long haul. Collins gathers information from the mesosphere with an incredibly simple tool--a column of colored light that reaches where more complicated machines fail.
