The Red Lightning That Was Photographed by Accident

On the night of July 4, 1989, R.C. Franz and his colleagues at the University of Minnesota were testing a low-light television camera for a sounding-rocket mission and pointed it at the sky almost as an afterthought. The next morning, while reviewing the tape, they found a single frame containing two columns of dim red light reaching upward from a thunderstorm a few hundred kilometers away. They had captured the first photograph of a sprite.

Sprites form between 50 and 90 kilometers up, in the mesosphere, where the air is too thin to support a normal lightning channel. When a particularly strong positive cloud-to-ground stroke fires below, the electric field above the storm momentarily exceeds the breakdown threshold of the rarefied gas overhead. Electrons accelerate, hit nitrogen molecules, and the molecules glow red — the same emission line that makes the upper aurora red. The whole event lasts a few thousandths of a second.

Reports of mysterious red flashes above storms go back to a 1886 letter by C.T.R. Wilson and a separate sighting by Toynbee and Mackenzie. Airline pilots filed observations through the 1960s and 70s. Atmospheric scientists, lacking photographs and unable to reconcile the reports with their physics, mostly filed them under "hallucination" or "reflection" and moved on.

The 1989 image broke the impasse. Within a decade, ground campaigns and shuttle astronauts had recorded thousands. The taxonomy now includes carrot sprites, columnar C-sprites, and the immense jellyfish form that can spread 50 km across the sky. The trigger — why some thunderstorms breed them and others don't — is still not fully understood, which is the part that keeps the chase going.