In June 2019, scientists in Spain went searching for ghosts haunting the skies above the Mediterranean Sea. These green-hued wisps, dancing above pink-red, extremely high-altitude lightning during thunderstorms, had been discovered only in May that year. What were they? The only way to know was to capture one.
But that would prove to be a troublesome task. These ghosts are aptly named: they are difficult to see with the naked eye and appear for just a heartbeat dozens of miles above ground.
“Seeing a ghost is really difficult,” said María Passas-Varo, a researcher at the Institute of Astrophysics of Andalusia in Spain.
But on Sept. 21, 2019, they finally caught one with a specialized camera: a green spirit flickering at the crown of a jellyfish-shaped maelstrom of fuchsia lightning 50 miles above the sea. And after painstakingly disentangling the various wavelengths of light emitted by the ghost, the scientists unveiled its elemental makeup.
In a study published Tuesday in the journal Nature Communications, Dr. Passas-Varo and her colleagues revealed that the ghost’s pale emerald complexion came, in part, from excited oxygen, similar to the green glow of auroras; nitrogen plays a role, too.
But the main contributor was another element: iron. That was a surprise because the metal was ultimately being delivered from space.
Better understanding ghosts and other ephemeral lightning like entities can help scientists interpret the difficult-to-parse chemistry and physics of Earth’s upper atmosphere.
“There are layers of metals that dance” in and above thunderstorms, Dr. Passas-Varo said.
Ghosts are a type of transient luminous event, or T.L.E., which were first described by scientists in 1989. T.L.E.s can include blue jets, which fire upward from thunderstorm clouds, as well as crimson-tinged upper atmospheric lightning that can come in many shapes, like carrots and jellyfish, and is known as a sprite.
T.L.E.’s “are like fireworks,” Dr. Passas-Varo said. And little is definitively known about them — especially ghosts, the first of which was observed atop a sprite storm over Oklahoma in May 2019.
To capture their own ghost, her team pointed a spectrographic camera — one that can use light to ascertain chemistry — at the upper atmosphere from an observation post in Castellgalí, Spain. All they could do was wait for sprite thunderstorms to appear, cross their fingers and hope that at least one sprite would be briefly decorated with a ghost, and that their camera was pointed at the right place.
Eventually, they found one flitting about on a jellyfish sprite.
“It was a matter of luck,” Dr. Passas-Varo said.
This one was largely powered by extraterrestrial iron, not atmospheric oxygen. The camera also revealed the presence of nickel, sodium and silicon. The complex chemical soup responsible for this ghost even added a yellow-orange tinge to its green glow.
All of those elements often come from micrometeoroids and deep-space dust particles that are nearly constantly plunging into the upper atmosphere. That means that ghosts could effectively be seen as interplanetary visitors.
Still, some researchers said not too many conclusions should be drawn from the new paper’s findings.
“The metallic traces are interesting, but I’ll caution that this was only a single event,” said Chris Vagasky, a lightning researcher at the University of Wisconsin-Madison who was not involved in the new work. To see if all ghosts are iron-fueled spooks, he added, “it would be nice to see the results from multiple ghosts.”
He has no doubt that the search for ghosts, and other T.L.E.s, will continue — largely because these phantoms are inherently beguiling.
“It’s really incredible to think that there is so much more occurring during a thunderstorm than what you can see or hear,” Dr. Vagasky said.