AURORA is a fictional glass-roof lodge at 69°N. Its sky is one full-screen fragment shader — layered noise curtains in magnetospheric green and violet — and the hero's Kp-index slider is wired straight into the physics, because aurora tourism genuinely lives and dies by that number.
Every northern-lights lodge shows the same longexposure photograph. This one computes the phenomenon instead, and hands the guest the real control variable: the planetary K-index. At Kp 1 the curtains hug the horizon; at Kp 9 they reach the zenith and shift violet — which is what actually happens when the solar wind leans in.
The two aurora colors are the real emission lines — atomic oxygen's green at 557.7 nm, molecular nitrogen's violet — and the shader hands over from green to violet with altitude, as the sky does.
Josefin Sans — display, in its airy 300 with 600 for the charged words; Scandinavian geometry without the cliché.
| Element | How it works |
|---|---|
| Curtain shader | Three layered curtains, each an fBm-displaced horizontal band: one noise field waves the curtain's centreline, a second modulates its thickness, and a high-frequency field carves the vertical rays. Intensity falls off per layer for depth. |
| Kp coupling | One uniform scales curtain extent, width, brightness, and the green→violet handover. The slider eases toward its target so storms build rather than switch. |
| Physical color | Colour ramps run altitude-wise from oxygen green to violet; snow on the ridge picks up a fraction of the live aurora light. |
| Scenery in-shader | Stars (hashed cells with twinkle clocks), an fBm mountain ridge, snow grain, and vignette — the entire scene is one draw call. |
| Accessibility | Reduced motion freezes shader time (a still sky, still responsive to Kp). The slider is a native range input announcing 0–9. |
| You do | The page does |
|---|---|
| Drag Kp toward 9 | Curtains climb to the zenith, brighten, and turn violet; the snow catches more light |
| Drag Kp to 0 | A quiet green whisper on the horizon — the honest average night |
| Wait | The curtains drift and re-fold; no loop, ever |