Posted on behalf of Mark Zastrow.
In the predawn hours of 4 March 2002, as the United States and its allies battled Al Qaeda in the mountains of Afghanistan, a US army helicopter was sent to drop reinforcements on Takur Ghar, a mountain peak blanketed by snow — and enemy fire. Attempts to warn the chopper off by satellite radio failed. At the landing zone, it was hit by a rocket-propelled grenade and crash-landed, stranding its force in a fierce firefight that killed four US soldiers.
Now, research suggests that space weather — in the form of enormous bubbles of plasma high above Earth’s atmosphere — disrupted the chopper’s satellite communications.
Plasma populates the upper layers of Earth’s ionosphere during the day, when sunlight breaks atmospheric particles into their charged constituents. At sunset, turbulence can develop as the plasma recombines, forming buoyant regions of lower density than their surroundings. These bubbles typically form near the magnetic equator, which snakes around the planet at low latitudes. During the night, they can grow to be tens of kilometres wide and extend towards the poles for thousands of kilometres. Smaller-scale turbulence inside these writhing tubes distorts radio waves that pass through it the way heat roiling above hot tarmac sets distant images dancing.
Typically, this distortion — called scintillation — is forecast by measuring the loss of signal along the line of sight from ground stations to communications satellites, or directly by satellites that fly through the bubbles. But in work published online this month in Space Weather, scientists analysed ultraviolet images from NASA’s TIMED satellite, which passed over the Afghanistan theatre at the time of the battle.
Their work indicates that a plasma bubble lay roughly 500 kilometres over the battlefield, directly between a pair of communications satellites overhead. The bubble’s clearly defined perimeter suggested the presence of radio-disruptive turbulence within.
The team notes that the initial disruption from space bubbles was probably small, but interference from radio echoes off the surrounding peaks could have greatly amplified the signal breakup. Lead author Michael Kelly, of the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, also notes that the battle occurred in the absence of a solar storm; such storms carry the potential for even greater impacts on military or emergency response communications. “It’s kind of this esoteric topic and yet it can affect society in very compelling ways,” he says.
Keith Groves of Boston College in Massachusetts says that it would be unusual for plasma bubbles to form at the latitude of Afghanistan, but not implausible. Groves led development of the ground-based system the US Air Force currently uses to measure radio wave scintillation. He’s sceptical that ultraviolet images alone can indicate the small-scale turbulence that is the culprit, but thinks they could be a powerful tool when used in tandem with current techniques.
Correction: This post has been changed to indicate that the ground-based system for measuring radio wave scintillation developed by Keith Groves is deployed by the US Air Force.