High up in the atmosphere, near the boundary of space, a dazzling, fleeting flash of red sometimes briefly appears above a thunderstorm before evaporating away.
These events, which occur far above when lightning strikes in the lower atmosphere, are called sprites. They fall under the umbrella of transient luminous events (TLEs) and only in the past few decades have we been able to observe them. And only since 2019 have we discovered that one out of 100 sprites producers an even more elusive, even more mysterious phenomenon: a greenish glow called a ghost. A new study published today in Nature Communications hints at when and why sprites might form these ghosts.
Sprites and ghosts
“TLEs are generated as a result of the intense electrical activity associated with lightning discharges,” says study lead author María Passas-Varo of the Consejo Superior de Investigaciones Científicas Instituto de Astrofísica de Andalucía in Granada, Spain. “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.”
Not all TLEs are sprites. There are a cornucopia of these events, all a little bit different, and typically named after mythological creatures like elves, gnomes, pixies, and trolls. It’s an area of atmospheric research that’s still relatively new, and always in need of more data.
Since June 2019, Passas-Varo and her team have been observing lightning storms with an instrument called the Granada Sprite Spectrograph and Polarimeter, or GRASSP. GRASSP examines the light from sprites to determine what elements or molecules in the atmosphere are responsible for the emission. With it, they have recorded at least 2,000 sprites, generating one of the biggest databases of these events in their efforts to look specifically for the ghosts they sometimes create.
“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,”
Passas-Varo says. “In almost four years of recording, from more than 2,000 spectra, we have just 42 spectra from the top of a sprite.” And from those, she says, only one, which occurred Sept. 21, 2019, was a strong enough signal to study.
Metals in the mesosphere
Sprites and ghosts occur in the mesosphere, the layer of Earth’s atmosphere that sits above the stratosphere and extends from about 31 to 53 miles (50 to 85 kilometers) above the ground. The mesosphere is extremely thin, with only about 1/100,000 the atmospheric pressure at sea level. And one of the biggest mysteries surrounding ghosts is what, chemically, causes them. Certain elements likely become charged in the atmosphere during these events, and the researchers wanted to know which ones.
The answer was quite surprising: It’s metals, like iron and nickel, and they’re located higher in the atmosphere than the study authors expected to find them.
That’s because while the atmospheric layers above the mesosphere are known to have bits of iron and nickel, likely deposited by meteors as they streak through, these metals had never before been detected in the mesosphere. Passas-Varo says it’s likely that gravity waves (think ocean-like waves but through the air, not to be confused with gravitational waves) could push the microscopic bits of iron and nickel down to these lower altitudes, where they generate ghosts under the right circumstances.
Now that some ghosts have been caught, Passas-Varo and her colleagues are hatching ideas on how to capture more of them.
The first is to add a green filter to some of their field cameras used to detect lightning. This could help them cross-reference the events with data from the Mesospheric Airglow/Aerosol Tomography and Spectroscopy satellite, which looks for gravity waves in Earth’s atmosphere. In doing so, they can see whether gravity waves play a role in whether a sprite produces a ghost.
They’re also considering building a new, GRAASP-like instrument with a different type of lens and slit at the front to take readings. A cylindrical lens funneling light into a vertical slit, Passas-Varo says, will make it easier to image ghosts for further study.
There’s “still a lot of work to do,” she says. But this study lays the groundwork for future atmospheric ghostbusting — and might even unearth some new atmospheric “spirits” along the way.