Scientists Unravel Mystery of “Green Ghosts” in Upper Atmosphere

Ghostly green lights have long been observed dancing in Earth’s upper atmosphere alongside unusual lightning strikes. Now, scientists have captured the first spectroscopic measurements of these “green ghosts,” finally unraveling the origin of their eerie glow.

Mysterious Ghosts Observed for Decades

Transient luminous events like sprites, elves, and blue jets have been documented in the upper atmosphere for over 30 years, appearing above intense thunderstorms. Among these are the even more puzzling “green ghosts” – faint glowing green patches that seem to pulsate and dance alongside the lightning.

First glimpsed in 2017, green ghosts remained an atmospheric enigma – their cause and composition unknown. Sightings were rare and unpredictable, making study difficult. But culminating analysis of past observations has finally allowed physicists to capture spectroscopic recordings of the apparitions.

Key Findings From Spectroscopic Measurements

Analysis of these new spectral signatures published Dec 12 in AGU Advances reveals:

• Green ghosts emit light concentrated around 557 nanometers, corresponding to a yellow-green hue.
• This glow comes from oxygen atoms in long-lasting excited state.
• Excited oxygen likely comes from dissociation of ozone in mesosphere by lightning-generated electrons.

“Our key finding is that the green emission comes from atomic oxygen, and the way that atomic oxygen is produced points to a mechanism involving lightning-generated electrons breaking ozone molecules apart,” summarized first author Dr. William Daffer of the United States Air Force Academy.

Link to Lightning-Triggered Electron Avalanche

Researchers propose the following mechanism for green ghost formation:

1. Intense lightning sends high energy electrons racing upwards.
2. Electrons collide with and break apart ozone molecules around 56 km altitude.
3. This produces long-lived excited oxygen atoms, causing faint green glow.
4. Glow persists for up to 2 seconds unlike short-lived sprites.

This electron avalanche theory is supported by ghost locations slightly above sprite halos, the altitude at which streamers often diverge.

“The locations of the green ghosts and the spectrum that we measured are consistent with the proposed mechanism involving lightning-generated electrons,” said Daffer.

New Insights Compared to Past Hypotheses

Prior to spectral analysis, hypotheses for the green glow included:

• Excited nitrogen emissions: Ruled out by lack of expected spectral lines
• Meteoric metals like iron or magnesium: Not enough concentration at high altitude

“There were a lot of ideas floating around out there, but measuring the spectrum allowed us to narrow the possibilities down,” noted Daffer. This demonstrates the value of spectroscopic evidence in illuminating atmospheric chemistry.

Implications for Atmospheric Science

Unraveling the green ghost mystery provides wider insights into electrical and chemical processes in Earth’s atmosphere.

• Ghosts reveal lightning and thunderstorm impacts on upper atmospheric composition – triggering chemical changes at high altitude through electron collisions.

• Findings improve understanding of sprite halos and diverse transient luminous events.

• Research techniques can be applied to study other atmospheric phenomena like meteors.

“We’re still trying to understand the impact lightning has high up in the atmosphere,” said Daffer. “Learning more about these glows gives insight into lightning-driven chemistry.”

Outlook: Monitoring Continues

With the ghostly glow’s origin now revealed, researchers emphasize this is just the beginning of unlocking its secrets. More sightings are needed to investigate fine-scale formation mechanisms and variability.

Specialized cameras and spectrographs remain trained on thundercloud hotspots to potentially capture the next apparitions. Global monitoring networks also watch for activity, while satellite instruments probe atmospheric composition.

“We got very lucky to catch the emission signature of green ghosts associated with only a handful of lightning flashes back in 2017,” said Dr. Daffer. “We’re excited to see what continued observations might reveal about these mysterious luminous events and their chemiluminescent glow.”

Key Events in Green Ghost Discovery

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