Rare 'Ghosts' Seen in Earth's Upper Atmosphere Explained by Physicists

Researchers have uncovered new details about a rarely seen phenomenon involving green "ghosts" in the Earth's upper atmosphere, according to a study.

The findings, published in the journal Nature Communications, help to explain the physical processes that produce these events—namely, why these "ghosts" glow green.

The phenomena in question, known as mesospheric ghosts, are infrequent and faint greenish glows that can remain for hundreds of milliseconds when special forms of lightning known as sprites occur.

Sprites are spectacular, large-scale electric discharges that appear as luminous red-orange flashes. They are a type of short-lived lightning—occurring above the altitudes of normal lightning—known as transient luminous events (TLEs).

The most common TLEs include sprites and other atmospheric phenomena such as halos, elves, blue jets and gigantic jets. Some sprites are known as jellyfish sprites, based on their appearance, which resembles the animal.

TLEs were serendipitously observed for the first time in 1989. Since then, sky photographers all around the globe have been spending nights chasing and sharing images of these breathtaking flashes of light.

The rare mesospheric ghosts, which occur at the top of certain energetic sprites, are also classed as TLEs.

"Like all members of the family of TLEs, ghosts develop through complex interactions between thunderstorms and the Earth's atmosphere," María Passas Varo, an author of the study who is with the Solar System Department at the Andalusian Institute of Astrophysics in Spain, told Newsweek.

"They are exceptionally rare, with only one in every 100 sprites developing a green ghost," she said.

The green ghosts were first discovered in 2019 by a citizen scientist, Hank Schyma. In May of that year, Schyma was documenting a sprite storm over Oklahoma when he observed a greenish glow appearing on top of certain energetic sprites that lasted for several milliseconds. Some skywatchers had unwittingly recorded the green ghosts before this, but Schyma was the first to identify the phenomenon.

"I have photos predating discovery, but I did not appreciate their significance until Hank pointed them out in video," another citizen scientist and TLE observer, Thomas Ashcraft, who was not involved in the study, told Newsweek.

After Schyma's observations, the main hypothesis that emerged to explain the green atmospheric phenomenon was that it was the result of oxygen atoms being excited and giving off light in the green spectrum. This is where the "ghost" name comes from—an acronym for GreenisH Optical emission from Sprite Tops. After Schyma shared his observations in a YouTube video, the TLE community started to pay attention to these rare greenish emissions.

"Our group decided to study the green features on top of sprites after watching this YouTube video," Varo said.

She went on: "Hank Schyma realized that something green happened on top of certain huge sprites, and the main hypothesis to explain this greenish glow was the emission of atomic oxygen. Then, sky photographers all around the world realized that green ghosts were not due to sensor artifacts and they were a new phenomenon."

After seeing the video, Varo and her colleagues began a research project to understand the nature of the green ghosts. To do this, they used a spectrograph, a device that detects light and records its component colors, which can provide scientists with data about what is producing it.

Although excited oxygen atoms were thought to be responsible for the green color, the materials involved in the process had yet to be identified because of a lack of spectroscopic observations.

In June 2019, Varo and her colleagues began a campaign of spectroscopic observations in Castellgalí, located near Spain's northeastern coast in the Barcelona province. They took spectroscopic measurements within the wavelengths that humans can see, primarily in the range that includes green light.

During this campaign, the scientists captured more than 2,000 images and spectra recordings of TLEs. But observing the green ghosts was "extremely difficult" with the device that the team was using.

"From imaging campaigns, it is known that one in every 100 energetic jellyfish sprites develops a mesospheric ghost," Varo said.

To record the rare ghosts, the spectrograph has to be aimed at the altitude where they are likely to appear.

"You have to predict where the sprite is going to appear and to aim to the top of that sprite. It is a matter of luck combined with expertise to capture ghost spectra," Varo said.

In almost four years of recording, the team members managed to successfully record only 42 spectra from the tops of sprites. And they observed a green ghost in only one of these.

The successful observation came on September 21, 2019, when the team spotted a jellyfish sprite during a thunderstorm above the Mediterranean. The spectra data they collected from the sprite indicated the presence of a green ghost on its upper side.

While they did not capture any color images of the ghost, the researchers knew they had recorded one because the light produced by the phenomenon was primarily observed in the green spectrum. In addition, the event lasted over 500 milliseconds.

"Sprites last only 20 milliseconds approximately, so this long-lasting event has to be a ghost," Varo said.

Analyzing the data, the researchers detected weak but certain traces of excited oxygen atoms, confirming the previous hypothesis. But they also found that other elements in the atmosphere—including iron, nickel and nitrogen atoms—were contributing to the green color. The researchers now hope that more of these unusual phenomena will be detected in the future.

"Lots of work still to do. This is the first ghost spectrum but not the last," Varo said.

Update 12/12/23, 12:28 p.m. ET: This story was updated with additional comments from Thomas Ashcraft.