Sanguora

Sanguora, a tall oracle on a high spire, trails a slow red veil of light far above a thin band of green aurora.

The Physics

Atomic oxygen’s red auroral doublet at 630.0 nm and 636.4 nm is the forbidden ¹D₂ → ³P transition (to ³P₂ for 630.0 nm and ³P₁ for 636.4 nm), mainly magnetic dipole. The ¹D₂ level lives about 110 s, around 150 times longer than the green line’s upper level. Collisions therefore quench it everywhere below roughly 200 km, and the red aurora shines only from the thinnest high air. ¹D₂ is also the lower level of the 557.7 nm green line, so each green photon leaves an atom ready to emit red.

τ(¹D₂) ≈ 110 s

I₆₃₀ ∝ P(¹D₂) · A₆₃₀ / (A_tot + k_q[M])

• τ: radiative lifetime of ¹D₂ (s)
• I₆₃₀: 630.0 nm emission rate per unit volume (photons m⁻³ s⁻¹)
• P(¹D₂): production rate of ¹D₂ atoms (m⁻³ s⁻¹)
• A₆₃₀: Einstein coefficient of the 630.0 nm branch (s⁻¹)
• A_tot: total radiative decay rate of ¹D₂, = 1/τ (s⁻¹)
• k_q[M]: collisional quenching rate per atom (s⁻¹)

Sources: Astron. & Geophys. 47, 1.11 (2006), doi:10.1111/j.1468-4004.2006.47111.x; Stoffregen & Derblom, “Life-time of the atomic oxygen 6300 Å line”, Nature 185, 28 (1960), doi:10.1038/185028a0.

Honest caveat: The green cascade is not the only source of ¹D₂: slow electrons and chemical reactions populate it directly. “Above about 200 km” is approximate and varies with solar activity.

The glow is the approximate line colour: a pure spectral colour lies outside the sRGB range a print can show.

In character

Down there the air is loud. Up here I have a hundred and ten seconds of silence, and I spend every one of them on red.

On the hoodie

Front: a red veil curl over a ¹D→³P ladder. Back: Sanguora in full pixel art with “FORBIDDEN. OBSERVED ANYWAY.” and “110 SECONDS, OR NOTHING.” Gildan 18500 Heavy Blend hoodie, Black, S to 5XL.

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