2023-12-13 11:15:27
In the mesosphere, a region of the atmosphere that was believed to be devoid of electrical activity, gigantic and brief luminous flashes occur whose formal scientific discovery, just three decades ago, surprised the scientific community. Given their elusive nature, they received names taken from ‘A Midsummer Night’s Dream’ (Shakespeare), such as elves or elves. Related to storm lightning, but located tens of kilometers above the clouds, among them are ghosts (or Ghosts, from English GreenisH Optical emission from Sprite Tops), greenish flashes whose origin was attributed to oxygen.
Now, the first spectroscopic study of these events associates them with metals, such as iron or nickel, which had never been included in optical models of mesospheric flashes.
The study was carried out by a team led by the Institute of Astrophysics of Andalusia (IAA), dependent on the Higher Council for Scientific Research (CSIC) in Spain.
The ghosts do not appear in isolation, but are observed in the upper region of the sprites (goblins), a spectacular luminous event of just hundredths of a second that shows a diffuse upper part and a lower region populated by tentacles (filaments of ionized air between ten and one hundred meters thick). Goblins can extend from forty to almost one hundred kilometers above the ground and sometimes present a greenish ghost on the diffuse upper part, which lasts hundreds of milliseconds after the disappearance of the goblin that generated it.
“Since the first Transient Light Events (or TLEs) were documented in 1989, which is how we know this family of events, people around the world have sought to capture them during electrical storms. And, in fact, it was an amateur scientist who first photographed a ghost, the last discovered member of the TLE family,” says María Passas Varo, a researcher at the Institute of Astrophysics of Andalusia who heads the team of authors of the work.
Until now, the main hypothesis to explain this greenish flash that appears on some more intense elves pointed to the interaction of charged particles (ions) with the atomic oxygen present in the atmosphere, a phenomenon already identified in the greenish color of the auroras. To corroborate this, the scientific team that developed this work began a systematic observation campaign in June 2019 to obtain spectra of the upper region of the elves (a spectrum allows us to know parameters such as the temperature or composition of a celestial object).
The red structure whose shape is somewhat reminiscent of a jellyfish is an elf. Just below, a storm can be seen. The image was captured from space. (Photo: NASA)
“One in every hundred very intense goblins generates a ghost. We have analyzed more than two thousand ghosts, and only forty-two corresponded to the upper region of the elf, where ghosts usually appear. It takes a lot of luck and a lot of skill to point the instrument at the right height, because the observation slit is very narrow and you have to predict where the goblin is going to appear. Of the forty-two spectra, only one had a sufficiently intense signal-to-noise ratio,” details Passas Varo.
That spectrum revealed, for the first time, what produces ghosts, a process in which oxygen contributes very little. The team found, instead, that the greenish flash is mainly due to metals, iron and nickel, elements that had never been taken into account when developing optical models for TLEs.
“Just that day we observed gravity waves, atmospheric disturbances produced by intense vertical movements in the air, such as those generated by storms,” adds María Passas Varo. “We knew that in the atmosphere there are layers of metals, which come from the entry of interstellar dust into the atmosphere, and everything indicates that these fluctuations in the density of the air make the altitude of these layers of metals variable: thus, this “Variability would be responsible for the fact that ghosts do not always appear.”
The discovery of these metal atoms implies an update of the models for transient luminous events, the understanding of which is in turn fundamental to understanding how the planet’s global electrical circuit works.
The study is titled “Spectroscopy of a mesospheric ghost reveals iron emissions.” And it has been published in the academic journal Nature Communications. (Source: Silbia López de Lacalle / IAA / CSIC)
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