In the mesosphere, a region of the Atmosphere once thought to lack electrical activity, gigantic and brief flashes of light occur. The discovery of these phenomena, just three decades ago, surprised the scientific community. Given their elusive nature, they were given names inspired by Shakespeare’s “A Midsummer Night’s Dream”, such as sprites or elves. Related to storm lightning but located tens of kilometers above the clouds, among them are the ghosts (from GreenisH Optical emission from Sprite Tops), greenish flashes whose origin was attributed to oxygen. Now, the first spectroscopic study of these events, published in the journal Nature Communications, associates them with metals like iron or nickel, which had never been included in the optical models of mesospheric flashes.
The ghosts do not appear in isolation but are observed in the upper region of sprites, a spectacular luminous event lasting only a few hundredths of a second, displaying a diffuse upper part and a lower region populated with tentacles (filaments of ionized air between ten and a hundred meters thick). Sprites can extend from forty to almost a hundred kilometers above the ground and sometimes present a greenish ghost above the diffuse upper part, which lingers for hundreds of milliseconds after the disappearance of the sprite that generated it.
Since the first documentation of Transient Luminous Events (TLEs) in 1989, which is how we know this family of events, people from around the world have been trying to capture them during electrical storms. In fact, it was an amateur scientist who first photographed a ghost, the latest member discovered in the TLE family, says María Passas Varo, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) leading the study.
Until now, the main hypothesis to explain the greenish flash that appears above some more intense sprites pointed to the interaction of charged particles (ions) with atomic oxygen present in the atmosphere, a phenomenon already identified in the green color of auroras. To corroborate this, the scientific team that conducted this study initiated a systematic observation campaign in June 2019 to obtain spectra of the upper region of sprites (a spectrum allows knowing parameters such as the temperature or composition of a celestial object).
One out of every hundred very intense sprites generates a ghost. We have analyzed more than two thousand spectra, and only forty-two corresponded to the high region of the sprite, where ghosts usually appear. It takes a lot of luck and skill to aim the instrument at the right height because the observation slit is very narrow, and you have to predict where the sprite will appear. Of the forty-two spectra, only one had a signal-to-noise ratio strong enough, details Passas Varo.
That spectrum revealed, for the first time, what produces the ghosts, a process in which oxygen contributes very little. Instead, the team found that the greenish flash is mainly due to metals, iron, and nickel, compounds that had never been considered when developing optical models for TLEs.
On that particular 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 there are layers of metals in the atmosphere, originating from the entry of interstellar dust into the atmosphere, and everything points to the fact that these fluctuations in air density cause the altitude of these metal layers to be variable. Thus, this variability would be responsible for the fact that ghosts do not always appear.
The discovery of these metallic atoms implies an update to the models for transient luminous events, whose understanding is fundamental in understanding how the global electrical circuit of the planet operates.
Sources
Consejo Superior de Investigaciones Científicas (CSIC) | Passas-Varo, M., Van der Velde, O., Gordillo-Vázquez, F.J. et al. Spectroscopy of a mesospheric ghost reveals iron emissions. Nat Commun 14, 7810 (2023). doi.org/10.1038/s41467-023-42892-1
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