On the morning of Monday, 4 March 2002, sometime just before the sun came up, an MH-47E Chinook helicopter carrying a group of U.S. Army Rangers flew low across a rugged Afghan landscape. Their destination, 33°20′34″N 69°12′49″E, was a snowcapped mountain called Takur Ghar. It was a rescue mission; hours earlier a team of Navy SEALS had been shot down by al-Qaida forces at the mountain’s summit and needed extraction. But the Rangers had been given the wrong coordinates and were headed right into the same al-Qaida forces that shot the SEALS down. Back at the U.S. command post, radio operators tried desperately to warn the Chinook, but the message was never received, and the helicopter was downed by another al-Qaida rocket-propelled grenade. The Rangers’ rescue mission turned into a 17-hour firefight—one of the deadliest engagements of the war for U.S. forces, costing seven lives.
The jagged peaks of Afghanistan have caused plenty of communications difficulties for U.S. forces, but researchers suspect that the doomed rescue mission may have fallen victim to a less visible source of interference: plasma bubbles. Their research, published online this month in Space Weather, suggests that turbulent pockets of ionized gas may have deflected the military satellite radio signals enough to cause temporary communications blackouts in the region.
“I wasn’t really expecting to find that there was a space weather impact,” says Michael Kelly, the study’s lead author and a space scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “But everything we found was kind of consistent with that possibility so we kept digging.”
Plasma bubbles form between 85 km and 595 km above Earth’s surface in a layer of the atmosphere called the ionosphere. In this layer, ultraviolet radiation from the sun strips electrons from gas molecules and turns them into charged ions.
Charged gases in the ionosphere, also referred to as plasmas, require an enormous amount of solar energy to sustain. The amount of plasma in the ionosphere drops off after the sun sets. Ions recombine with the neutral atmosphere—a reaction that occurs preferentially at lower altitudes because the neutral atmosphere is denser than the ionosphere. As the number of ions dwindles, the plasma at these lower altitudes loses density faster. The result is predictable: Like layers of vinegar and oil in a salad dressing, the plasma rearranges according to density.
“The heavy fluid wants to go down and the light fluid wants to go up,” explains Jonathan Makela, a computer engineer at University of Illinois, Urbana-Champaign, who was not involved in the study. The plasma bubbles aren’t slow either. Despite their potential to grow to be more than a thousand kilometers in size, they rocket upward at hundreds of meters per second through the ionosphere.
As the plasma bubbles grow, they cause electromagnetic turbulence that can distort the passage of radio waves. Kelly equates the interference to the way a beam of light bends as it passes from air into water: The radio signal refracts as it enters and exits the plasma bubble.
He and his team suspected that the bubbles could have been responsible for the communications blackout that ultimately doomed the Army Rangers. Furthermore, like tornadoes or hurricanes, plasma bubbles have their own season when atmospheric conditions are most favorable for their formation. The Takur Ghar battle occurred during Afghanistan’s “bubble season.” It was a reasonable hypothesis, but proving it might’ve been impossible if it weren’t for a lucky coincidence.
At just about the time when things were becoming completely FUBAR atop Takur Ghar, NASA’s Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite had just completed a pass over the area. By using data from TIMED’s Global Ultraviolet Imager, scientists were able to confirm the presence of a plasma bubble between the battlefield and the satellites trying to relay the urgent radio message from the base. The analysis relied on a technique in which plasma bubbles can be forecast by applying a tomographic reconstruction technique to the ultraviolet data.
Although it’s impossible to conclude that the plasma bubble definitely caused the radio blackout, Kelly’s research has confirmed that the bubble was there at just the right time, Makela says. “I think this paper does a good job of showing that [plasma bubbles] can have impacts on operations.”
What had begun as a rescue mission for the Rangers quickly devolved into a struggle for survival. If the TIMED satellite was overhead, why weren’t U.S. commanders aware of the potential for a communications blackout? The answer speaks to the importance of the research, Kelly says: The techniques used in the paper hadn’t been developed yet. “The science has made tremendous strides in the last decade and, quite frankly, this technique didn’t exist back then. So nobody could’ve provided this data to them when the mission took place,” he says. “This incident is consistent with a space weather impact … but we’re not saying that somebody should’ve done something differently.”