Ash detector: laser device could protect aircraft in flight.

Analysis of a volcanic plume that wafted over central Alaska suggests that polarized laser beams can detect airborne ash, which in large concentrations can be a significant threat to aircraft. Instruments that fire such beams and interpret their reflections could be ground based, mounted on satellites, or even installed on airplanes, the scientists say.

On Jan. 11, 2006, the Augustine volcano on an uninhabited island about 275 kilometers southwest of Anchorage began a monthlong series of eruptions. Plumes from the eruptions occasionally reached heights of 12 km and interrupted flights in and out of Anchorage International Airport, says Ken Dean, a volcanologist at the University of Alaska in Fairbanks. Thick clouds of volcanic ash can clog an aircraft's engines, sandblast its cockpit windows, and cause millions of dollars' worth of damage (SN: 9/13/03, p. 168).

At first, winds blew Augustine's plume of ash and steam toward the east, away from densely populated areas. Then, late in January, weather patterns shifted. Parts of the plume then headed northeast toward Fairbanks, where the university operates various ground-based instruments that probe the atmosphere.

One of those devices analyzes the reflected signals from a high-powered laser that fires polarized beams of infrared light. This radar instrument "is not routinely turned on," says Dean, but when computer models suggested that the plume from Augustine would be carried over the area at altitudes below 6 km, "we looked at [the plume] as a target of opportunity."

On the afternoon of Feb. 2, the plume wasn't thick enough to be visible and caused no problems for flights in and out of Fairbanks, says Dean. However, a redder-than-normal glow at sunset hinted that the atmosphere over Fairbanks, about 700 km from the volcano, contained a tenuous veil of particles. Observations with the laser confirmed the presence of airborne ash, Dean and his colleagues report in the April 28 Geophysical Research Letters.

When polarized light reflects off spherical particles, such as water droplets, it remains polarized, Dean explains. However, light reflected from the plume aerosols was partially depolarized, a sign that some of the airborne particles were irregularly shaped. The largest effect came from particles suspended at altitudes between 1.7 and 3.8 km above sea level, a range that roughly matched the computer-predicted dispersal of the ash.

Satellites monitor Earth in various wavelengths that can detect airborne ash. However, space-based observations didn't reveal signs of volcanic ash over Fairbanks in early February, probably because the plume had largely dissipated before it reached the region, says Tom Murray of the Alaska Volcano Observatory in Anchorage.

The new findings indicate that ground-based laser radar can detect volcanic ash in atmospheric concentrations far below those that would cause damage to aircraft, says Dean. Such instruments, as well as devices installed on aircraft, could provide an important complement to the system that now warns pilots about threats from airborne ash.