Photograph of the Florida Institute of Technology X-ray detector

X-ray camera points skyward in Florida

Blink and you've missed it. Researchers in the US have captured the world's first X-ray images of lightning, by creating a special camera that can capture radiation at 10 million frames per second. They presented their new findings at the American Geophysical Union (AGU) Fall Meeting in San Francisco and they say that this new view of lightning could help to solve some of the mysteries of this spectacular natural phenomenon.

The research was carried out at the International Center for Lightning Research and Testing, located in Florida. It is one of the few sites in world where lightning is initiated and studied under controlled conditions. By firing rockets with trailing wires into thunder clouds, scientists are able to generate electric fields that are large enough to trigger bolts of lightning, which then propagate back down towards the rocket launch tower.

Joseph Dwyer and colleagues at the Florida Institute of Technology became interested in the fact that lightning emits X-rays as it propagates through the air, a phenomenon that was only noted in the past decade. But given that X-ray sources in lightning travel through the Earth's atmosphere at velocities approaching the speed of light, it is difficult to catch them on camera before they disappear. In addition, they cannot be imaged with standard mirrors and lenses because huge amounts of material are required to prevent X-rays and gamma rays from entering through the sides of a camera.

Tried and true method

Dwyer's team has created a customized camera that has 30 detectors made from a combination of sodium iodide and photomultiplier tubes, each measuring 3 × 3 inch. The device, which is approximately the size of a standard refrigerator, is also equipped with a 3 inch pinhole aperture, and can record X-rays at 10 million frames per second. "This is actually a very old technique for making images, like that seen in a camera obscura," Dwyer says.

We're seeing lightning as Superman would see it with his X-ray vision Joseph Dwyer, Florida Institute of Technology

During July and August this year, Dwyer's team studied four rocket-triggered lightning flashes at the Florida test site. Each flash lasted for approximately two seconds and the resulting sequences of images revealed that X-rays emerged primarily from the vicinity of the lightning tip as it propagated towards the Earth. As the lightning crashed into the control tower it also triggered large bursts of gamma radiation, which were also captured by the camera.

"For the first time we're catching a glimpse of lightning in the X-ray emission," says Dwyer. "We're seeing lightning as Superman would see it with his X-ray vision".

Dwyer hopes that the images can help to explain how bolts of lightning propagate through the air – a process that is still poorly understood. "When lightning propagates it moves in a halting manner called stepping. It will pause, then leap forward, pause, leap forward... We don't know how or why it chooses to do this," he says. "It is difficult to come up with models to explain this motion, since we don't know what the basic picture is, but the images really help. They tell us where the charges are, where the high fields are and where the air is breaking down."