How are atmospheric 'green ghosts' formed? We might finally have an answer

The atmospheric drama of a red sprite is often accompanied by a green ghostly glow if you only look close enough to spot it.

Colloquially dubbed as "mesospheric ghosts," this phenomenon have been observed in last few years. But, till date, it had puzzled the scientific community. However, now, a study has an explanation that may tell us how these 'green ghosts' are formed.

Sprites and ghosts

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As per Astronomy, high up in the atmosphere, near the edge of space, sometimes, before a thunderstorm evaporates away, a dazzling flash of red appears briefly. These are called sprites and have been properly observed in the last few years.

Since 2019, scientists discovered that one in 100 sprites, which fall under the umbrella of transient luminous events (TLEs) is accompanied by an elusive, mysterious 'green ghost'.

Writing in Nature Communications, study author María Passas-Varo said: "TLEs are generated as a result of the intense electrical activity associated with lightning discharges."

These happen in the mesosphere, Earth's layer of atmosphere that extends from about 31 to 53 miles (50 to 85 kilometres) above ground and is located above the stratosphere.

Passas-Varo, a space scientist at the Institute of Astrophysics of Andalusia in Spain, explains that "the specific type of TLE that forms depends on factors such as the altitude, the type of lightning discharge, and the characteristics of the atmospheric layers involved."

What they found

The scientist and her team spent more than four yearsgathering data of around 2,000 sprites to determine which elements or atmosphere are responsible for the unique hue of these 'green ghosts'.

Using an instrument called the Granada Sprite Spectrograph and Polarimeter, or GRASSP, they examined the light from sprites.

"To record these rare phenomena, you have to aim the spectrograph … at the altitude where the ghost is likely to appear, which becomes a matter of luck combined with expertise," says Passas-Varo.

"In almost four years of recording, from more than 2,000 spectra, we have just 42 spectra from the top of a sprite," she said, adding that out of those, only one, which occurred Sept 21, 2019, was a strong enough signal to study.

What they found was a surprise. As per Astronomy, the researchers found that metals like iron and nickel are located high up in the atmosphere, much higher than they expected to find them.

This study lays the groundwork for future atmospheric research by providing a rare, yet valuable glimpse into the mesosphere. There's "still a lot of work to do," says María Passas-Varo.

(With inputs from agencies)