Viridael

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

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 s

Line emitted when A ≳ k_q[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)
• k_q: 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. Sources: McLennan & Shrum, Proc. R. Soc. A 108, 501 (1925), doi:10.1098/rspa.1925.0088; Astron. & Geophys. 47, 1.11 (2006); Whiter et al., Ann. Geophys. 41, 1 (2023).

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 character

Hold 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.

On the hoodie

Front: a green droplet in a ¹S→¹D ladder glyph. Back: Viridael in full pixel art with “FORBIDDEN. OBSERVED ANYWAY.” and “HOLD IT 0.74 SECONDS.” Gildan 18500 Heavy Blend hoodie, Black, S to 5XL.

Part of the Forbidden Transitions set with Nebulia, Coronix, Flipwarden, Sanguora, Sulphra. Browse all physics hoodies, more astrophysics & cosmology designs, or the Deep Cuts.