The Battle of Takur Ghar, fought at 10,200 feet near a snowcapped mountain peak, started in the early morning hours of March 4, 2002. It was part of Operation Anaconda, a plan to block all escape routes from the Shah-i-Kot valley and destroy al-Qaeda and Taliban fighters who had massed in the frigid hills of eastern Afghanistan. Things did not go as planned.
During the operation, U.S. Navy SEALs were pinned down on the ridge dividing the upper and lower parts of the valley. A Chinook helicopter carrying U.S. Rangers was deployed to Takur Ghar to rescue the SEALs. After the helo was en route, military officers repeatedly and desperately radioed the rescue team of 21 men not to land on the mountaintop because enemies armed with rocket-propelled grenades and heavy machine guns had taken the territory. The Chinook never received the message. It crash-landed under intense fire.
How could communications fail so completely at such a critical moment? Atmospheric scientist Michael Kelly and space scientist Larry Paxton, both of the Johns Hopkins University Applied Physics Laboratory, have an idea. According to the researchers, military radio operators there may have run afoul of a little-known source of radio interference: plasma bubbles.
"When we first started looking into this, we were very skeptical—we weren't sure we would find anything at all," Kelly says. "But as the work unfolded, it does seem like these bubbles would have an impact."
Like Bubbles Rising Through Water
Plasma is made up of electrically charged particles such as protons and electrons. It dominates the ionosphere, the layer of the atmosphere that begins about 50 miles above Earth's surface. Giant bubbles around 60 miles wide can form in the ionosphere; these pockets, which are low in electron density, are known to bend and disperse radio waves, interfering with communications.
Extreme ultraviolet radiation from the sun reacts with air molecules in the atmosphere to form this plasma. And while sunlight keeps the plasma stable during the day, at night the electrically charged particles recombine to form electrically neutral atoms and molecules. This recombination happens more quickly at lower altitudes, making the plasma there less dense. As a result, it bubbles up through the denser plasma above like air bubbles rising through water.
Spring is peak season for plasma bubbles above Afghanistan, Kelly and Paxton say. Given the location and timing of the Battle of Takur Ghar, the scientists thought a plasma bubble could have been present then.
Tendrils of low-density, charged particles are called plasma bubbles, and turbulence at their edges can skew radio frequency waves passing through them. APL researchers provide evidence that plasma bubbles may have contributed to the communications outages during a 2002 Afghanistan battle. (Photo Credit: NASA)
To see whether they were right, the scientists investigated data from the Global Ultraviolet Imager (GUVI) instrument aboard NASA's Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) mission, which launched in 2001 to study the composition and dynamics of the upper atmosphere.
Study co-author Joseph Comberiate, a space physicist at the Johns Hopkins University Applied Physics Laboratory, developed a technique to convert the 2-D images from TIMED into 3-D representations of plasma bubbles. "This work was really a synthesis of a lot of new technological capabilities and cutting-edge research," Comberiate says.
Indeed, they found, on the day of the Battle of Takur Ghar there was a plasma bubble directly between the doomed helicopter and the communications satellite that military officials used to try the reach the Chinook. This plasma bubble, in addition to radio interference caused by the complex terrain in the area, may help explain the communications outage. The scientists detailed their findings in the journal Space Weather.
The model these researchers devised could help to detect and to predict the movements of plasma bubbles for several hours after they form, they say. This knowledge could provide soldiers with an early warning system as to when and where they might experience radio interference, allowing them to adapt by using different radio frequencies. "This could be an important tool for helping to mitigate future impacts," Comberiate says. "This scenario could happen again."
Space physicist Jonathan Makela at the University of Illinois at Urbana-Champaign, who did not take part in this research, agrees.
"This is a good real-world example of the impacts that irregularities in the ionosphere can have," he says. "You can imagine a weather-service-type constellation of satellites that come up with real-time maps of where you might experience communication or navigation outages."