Viridael: Forbidden [O I] 557.7 nm Green Aurora Line

Cosmology & the Sky arc, page 151 of 193. Back to The Pixelated Physics Universe

Who they are

Viridael, a hooded keeper with closed eyes, holds a single droplet of green light beneath auroral curtains that stop at a haze line.

Viridael is a character of the Cosmology & the Sky arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.

The physics

The aurora’s dominant green line at 557.7 nm, first examined spectroscopically by Anders Ångström in 1867, was identified in 1925 by J. C. McLennan and G. M. Shrum as the forbidden ¹S₀ → ¹D₂ transition of atomic oxygen, both levels inside the 2p⁴ ground configuration (an electric-quadrupole transition). The ¹S₀ level is metastable, lifetime about 0.74 s. Below roughly 100 to 120 km, collisions de-excite it before it can radiate, so the green curtain has a lower edge.τ(¹S₀) ≈ 0.74 sLine emitted when A ≳ kq[M] (quenching rate per atom below radiative rate) τ: radiative lifetime of the upper level (s) A: Einstein spontaneous-emission coefficient, = 1/τ for a single decay channel (s⁻¹) kq: collisional quenching rate coefficient (m³ s⁻¹) [M]: number density of colliding air molecules (m⁻³), which rises steeply with falling altitude Statistically the green emission peaks near 114 km.

No single equation is shown for this character; the physics above is stated in words because it is an observation or a piece of history, and we do not invent formulas.

Honest caveat: Published lifetimes for ¹S₀ range from about 0.7 to 0.8 s; we use 0.74 s. The quench altitude is a range, not a sharp line.The glow is the approximate line colour: a pure spectral colour lies outside the sRGB range a print can show.In characterHold still. I can keep this light for less than a second, and only above the noise of the crowded air. That is enough to paint the sky.

Crossovers

Viridael meets Coronix (Forbidden Fe XIV Coronal Line). Coronix and Viridael both glow green with forbidden lines. Coronix's comes from highly ionised iron in the million-kelvin corona; Viridael's comes from neutral oxygen in Earth's upper atmosphere.

Viridael meets Sanguora (Forbidden [O I] 630.0 nm Red Aurora Line). Viridael's green 557.7 nm line and Sanguora's red 630.0 nm line come from the same oxygen atoms. The red state lives much longer, so it only survives high up, where collisions are rare.

Wear the lore

Concept keywords: Forbidden [O I] 557.7 nm Green Aurora Line, Viridael, Cosmology & the Sky, physics lore, Pixelated Physics