Green ‘Ghosts’ in Earth’s Upper Atmosphere Involves Large-Scale Electric Discharges [Study]

The green ghosts observed in the Earth's upper atmosphere are a rare phenomenon called mesospheric ghosts. They are infrequent, and photographers have been chasing and sharing images of them.

Green Ghost in Earth's Upper Atmosphere Explained

Green ghosts, also known as mesospheric ghosts, are infrequent and faint greenish glow 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 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.

"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 studies with the Solar System Department at the Andalusian Institute of Astrophysics in Spain, explained. "They are exceptionally rare, with only one in every 100 sprites developing a green ghost."

The green ghosts were initially spotted in 2019 by Hank Schyma, a citizen scientist. When Schyma was recording a sprite storm over Oklahoma in May of that year, he noticed a greenish light that appeared on top of some energetic sprites and lasted for several milliseconds. Although several skywatchers had inadvertently captured the green ghosts earlier, Schyma was the first to recognize the phenomena properly.

The most common explanation for the surfaced green atmospheric phenomenon was that excited oxygen atoms released light in the green wavelength. The term "ghost" originates from an acronym for Sprite Tops' GreenisH Optical emission. Following Schyma's YouTube video presentation of his findings, the TLE community began to pay attention.

Varo and her colleagues saw Schyma's video. From there, they began their research to understand the nature of the green ghost. They used a spectrograph. The green hue was supposed to be caused by excited oxygen atoms, but spectroscopic data were lacking; therefore, the materials involved in the process remained unknown.

"From imaging campaigns, it is known that one in every 100 energetic jellyfish sprites develops a mesospheric ghost," Varo said. However, according to her, sprites only last for 20 milliseconds, "so the long-lasting event has to be a ghost."

The researchers' data analysis confirmed the earlier theory, which showed faint but distinct signs of excited oxygen atoms. However, scientists also discovered that additional elements present in the environment, such as atoms of iron, nickel, and nitrogen, were responsible for the green hue. In the future, the researchers want to find more similar strange occurrences.

Green, Purple Lights Mistaken As Auroras

In related news, people would easily conclude that the purple and green lights in the night skies are aurora borealis or northern lights. However, depending on its position, the lights could be STEVE (Strong Thermal Emission Velocity Enhancement), not auroras.

STEVE is as colorful as auroras but closer to the equator. These two phenomena - STEVE and aurora - are unrelated but share the same mechanism. When charged particles heat up high in the atmosphere, further south than usual auroras, STEVE's mauve streaks occur.

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