In the blink of an eye, an enormous bright red light flashes above a thundercloud, spreading energetic branches that extend five times taller than Mount Everest and look like jellyfish tendrils and angel’s wings.
These mysterious phenomena are known as Transient Luminous Events (TLEs), and are usually invisible to the naked eye because they happen on millisecond timescales, too fast to be seen. They occur between 50 to 100 kilometers above the ground, a long-ignored area of the atmosphere that is too high for aircraft but too low for satellites to investigate. There, the thin air interacts with strong electrical fields to ionize molecules and create arcing plasmas.
These spectacles are relatively new to science. Pilots had reported enigmatic bright flashes throughout the 20th century, but their anecdotal evidence didn’t amount to proof. The first image of a TLE was captured accidentally in 1989 when a University of Minnesota professor aimed a low-light TV camera at the sky to film a rocket launch. Replaying the tape later on, Professor John R. Winckler saw brilliant columns of light extending from the tops of storm clouds. Hearing of the finding, NASA officials immediately ordered a review of video tapes taken from the space shuttle that looked at lightning events on Earth. They found dozens more examples of TLEs, and later scientists have been recording them ever since.
“One of the neatest things about TLEs is that first image in 1989 was just a serendipitous capture,” said amateur radio astronomer Thomas Ashcraft, who has been photographing the events for several years.
Using a relatively simple camera and radio dish, Ashcraft has seen a whole bestiary of odd TLE phenomena. The most common are sprites, tall and highly structured bursts of light that appear above thunderstorms. They ionize the nitrogen in our atmosphere, causing a red glow. Often, they happen in conjunction with “Emissions of Light and Very Low Frequency Perturbations due to Electromagnetic Pulse Sources,” also known as ELVES, which are enormous halos of light that shoot outward to cover up to 500 kilometers in a millisecond. Though they are too short-lived to see, ELVES can produce bright afterglows that some people have mistaken for UFOs. Other TLEs have names like blue jets and trolls.
To deliver great TLE shots, Ashcraft first checks radar maps of the local area around his observatory in Santa Fe, New Mexico. Red spots on such maps indicate strong lightning cells, which increases the probability of sprite activity. Because the phenomena are mostly visible in near infrared wavelengths, he uses a modified off-the-shelf DLSR camera from which he removed the clear glass filter covering the CCD that blocks infrared light.
By taking continuous three-second exposures, Ashcraft records thousands of pictures each night. He then goes through the catalog looking for a sprite to appear. If he spots something, he can check a video camera that he has running during the night to see if captured more detail there. He shares his most interesting findings with other sprite observers, who may chime in with their own pictures from other positions.
From Santa Fe, Ashcraft says he can usually catch sprites up to 1,000 kilometers away. “I can see big storms out over the Great Plains, usually beyond Oklahoma City and into Nebraska,” he said. “After that, the curvature of the Earth gets in the way.”