rygoid

Rygoid is a term used in atmospheric science to describe a rare, transient luminous cloud formation that appears in the upper troposphere and lower stratosphere, typically between 12 and 18 kilometers above sea level. The phenomenon is characterized by a rosette‑shaped glow that can range from a few hundred meters to several kilometers in diameter. The name derives from the Greek word rigo, meaning “arrow,” reflecting the commonly observed directional movement of the cloud along prevailing wind vectors.

Observation of rygoids began in the 1970s during high‑altitude photographer expeditions in the Pacific. Since then, satellite data from the GOES‑16 and the Himawari‑8 series have documented dozens of occurrences, largely over oceanic regions where upper‑level jet streams are strongest. The visual signature is often accompanied by weak, diffuse auroral activity at lower altitudes, suggesting a connection to upper atmospheric electric fields.

Physical explanations for the rygoid involve ionization fronts generated by energetic charged particles precipitating from the magnetosphere. Researchers propose that localized electric potentials create a filamentary glow that propagates along atmospheric conductivity gradients. In situ measurements by research aircraft have found sodium and potassium ion densities elevated within the bright core of the cloud, implying that chemiluminescence contributes to the observed light emission.

The width of the luminous ring can vary seasonally, with the most frequent sightings recorded during late spring to early summer months. Traditional weather forecasting models do not explicitly simulate rygoids, though their occurrence has been noted to influence short‑term variations in upper‑level wind shear.

Because of their rarity and the difficulty of capturing high‑resolution data, rygoids remain a subject of ongoing research within the communities of upper‑atmospheric physics and remote sensing optical diagnostics. Future studies aim to quantify their frequency, energy budgets, and potential impacts on aerosol chemistry and upper‑level fog formation.