Secrets of Volcano Lightning Probed During Alaska Eruption
W4KRL at arrl.net
Mon Jun 4 10:39:16 CDT 2012
The searing hot plumes of ash rising above explosive volcanic blasts often
burst with lightning storms, the largest of which rival the most powerful
thunderstorms known on Earth. Now the most complete and detailed
measurements of volcanic lightning are yielding insights into the roots of
these electrical discharges, and could potentially help avert threats to
global air traffic.
Lightning is often seen crackling in the plumes of explosive volcanic
eruptions, such as that of the Icelandic volcano Eyjafjallajökull. However,
active volcanoes are often quite remote from civilization and their
outbursts are sudden and unpredictable, which is why scientists have rarely
investigated the origins of this lightning in detail. Even when scientists
can get a close view of eruptions, the intensely murky nature of volcanic
clouds hides most of their lightning from view.
Fortunately, due to seismic activity two months in advance of the explosive
eruption of Alaska's Mount Redoubt in 2009, scientists were able to install
four portable lightning mapping stations around the volcano to monitor
lightning during the entire sequence of an eruption for the first time.
The instruments were deployed in wintry conditions in late January and early
February 2009, along the west coast of the Kenai Peninsula, 50 miles (80
kilometers) east of Mount Redoubt. The 10,200-foot-tall (3,100 meters)
volcano, located at the northeastern end of the Aleutian volcanic arc,
erupted in March and April 2009.
"Setting up equipment in subzero temperatures, pounding stakes through
frozen ground, was tough," recalled researcher Sonja Behnke, an atmospheric
physicist at the New Mexico Institute of Mining and Technology at Socorro.
Lightning releases a succession of bursts of very-high-frequency radio
emissions. The array of stations the researchers set up imaged the lightning
in the murky plume by monitoring these outbursts.
"One of the most surprising things about our results from Redoubt was just
the amount of lightning we saw during each explosive eruption," Behnke said.
During the course of Redoubt's eruption, more than 30 separate explosions
were spotted, with the largest explosions triggering intense lightning
storms that lasted up to 70 minutes and generated up to 7,000 lightning
flashes. Smaller explosions produced fewer than 10 discernible lightning
flashes over spans of less than 10 minutes, and some triggered no detectable
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