We have discovered the nature of atmospheric ghosts

In the highest layers of the atmosphere live ghosts, elves, trolls, halos, fairies, giants(giant blue jets) and blue jets (blue jets). All these are extravagant, bright and ephemeral atmospheric phenomena, the nature of which is gradually revealed.

Using GRASSP (GRANada sprite spectrograph and polarimeter), we study the properties of the light they emit and have recorded more than 2000 transient light events (TLE) since 2019. Of all of them, we only have a ghostly spectrum (they are actually elusive), but it has allowed us to identify the metals responsible for its fleeting green glow.

Scientific observations

These elusive optical phenomena occur in the mesosphere and observations of them are already described in the scientific literature of the late 19th and early 20th centuries:

“…continuous flashes of light… rose to a considerable height, resembling rockets rather than lightning…” (Mackenzie and Toynbee, 1886); “…the luminous trail rose about 15 degrees at the speed of a rocket or even faster…” (Everett, 1903); “…scattered fan-shaped flashes of greenish color extending upward in a clear sky…” (Wilson, 1956).

In 1989 they were immortalized for the first time and randomly. A team of researchers from the University of Minnesota (USA) was conducting an experimental campaign pointing to the horizon to calibrate some polarimetric equipment when, unwittingly, they detected a very short and strong light emission rising from the top of a distant storm towards the ionosphere.

Elf name

Because of their brevity and elusive behavior, in 1993 University of Alaska Fairbanks professor Dave Sentman proposed giving them the name elves (spritesin English) after attending a Shakespeare performance. Summer Night Dreamwhere a goblin named Pook appears and disappears as if by magic.

Since the general acceptance of this mysterious name for elves, and as the family of transient luminous phenomena (TLPs) has acquired members based on new discoveries, they have been christened with fantastic names: elves, fairies, giants (giant blue jets)…and also ghosts, of course.

Storm chaser

And from the red goblin to the green ghost. In May 2019, storm chaser Hank Shima uploaded a video to YouTube in which the first atmospheric phenomenon described by Nobel Prize winner Charles Thomas Wilson was recorded: a very diffuse green glow, lasting less than a second, occurring at an altitude of about 80 km above a jellyfish-type sprite.

The video is shocking.

Shima dubbed this new TLE a ghost, from the English. ghostas an abbreviation for Green emissions from excited oxygen on top of sprites.

Because at that time the main hypothesis explaining its diffuse greenness was the mechanisms of de-excitation of atomic oxygen in the mesosphere, similar to what happens in auroras.

More than 2,000 ghosts and only one ghost have been recorded.

From the Atmospheric Electricity Group of the Institute of Astrophysics of Andalusia, we decided to use our spectrograph GRASSP (GRANada Sprite Spectrograph and Polarimeter) to analyze the light of the ghosts and check whether oxygen is indeed responsible for their green color.

We have recorded over 2000 images and spectra of transient light phenomena (short-term light events in English TLEs) in the spectral range from 500 to 600 nanometers since June 2019.

And out of all of them, we only have the ghostly ghost. Because catching the ghost of a ghost is not easy.

We know from TLE image records that only one in a hundred jellyfish sprites wears a ghost as a crown. Well, to record the spectrum, we need to point to an area of ​​the sky where we think the ghost will appear, but we only have information about the location of the parent storm, which must be over 200 km away. be able to see a ghost beyond the horizon.

We point to the sky above the storm and wait for the ghost to appear in the projection of the very narrow slit of the spectrograph.

If a ghost appears but is not in the projection of the slit, we miss that spectrum, so hunting a ghost requires a lot of experience and a lot of luck because not only are they very rare, but they must also appear in the field of view of our spectrograph.

After nearly four years of recording and over 2,000 goblin ghosts, we only have 42 of the top ones left, from the region where the ghosts originate. And of those 42, only one showed a strong enough signal-to-noise ratio to extract valuable information.

Iron, Nickel and Sodium

We detected a weak spectral line of atomic oxygen at 557.7 nm, but to our surprise, we also detected spectral lines of atomic iron at 583.46 nm and 586.72 nm, much more intense than oxygen, and a spectral line of molecular nitrogen at 560.80 nm. .

These emissions have a long radiative lifetime and, in combination with atomic oxygen excited at a wavelength of 557.73 nm, may explain the persistence of the greenish glow. We also found traces of other metals in the ghost’s spectrum: not only iron, but also nickel and sodium. The presence of metals is not new. In fact, metal layers exist near the mesopause due to meteorite ablation of interplanetary dust particles that enter the Earth’s atmosphere at high speed.

All of these species emit wavelengths between 500 and 600 nm, which corresponds to the spectral range of the green channel of most commercial photosensors, covering not only green (500–565 nm), but also yellow (565–590 nm) and some orange (590 nm) colors . – 600 nm). So our ghost was indeed greenish, bordering on yellow, although in a commercial camera it would have looked green.

Shrouded in gravitational waves

That night we also observed a pattern of concentric stripes in the background of the images, which could indicate that we were in a gravitational wave scenario at that moment, which would not be nonsense, since updrafts Storms Electrical storms cause gravitational waves, and this the thunderstorm was very active.

Gravitational waves can influence the density of the nighttime lower boundary of the mesosphere’s metallic layers, and this may be one explanation for why ghosts are so rare.

We have uncovered the nature not of ghosts and spirits from the other world, but of a luminous natural phenomenon, the spectrum of which has never before been recorded. Since it is unique, we cannot rely on this result to draw any conclusion at this time. We can only assure that the emissions of this particular ghost were due to metals and, of course, oxygen and nitrogen.

We need to continue observing the skies to hunt for new ghosts, and thus have more data to create current theoretical models that will provide a physical explanation for the atmospheric mysteries that remain to be solved.