Same mechanism as the 3D curtain view, drawn as a ground-level 2D scene plus live physics readouts instead of a decorative ribbon. Solar-wind protons and electrons are funnelled by Earth's magnetic field lines into the upper atmosphere, where they collide with oxygen and nitrogen atoms and knock electrons into excited states. The colour you see is the wavelength released when each electron relaxes back down:
O (excited) -> O + photon @ 557.7 nm (green, ~0.7-1 s decay, needs O2/N2 collisions rare above ~250 km)
O (excited) -> O + photon @ 630.0 nm (red, ~110 s decay, only survives at low density / high altitude)
N2+ (excited) -> N2+ + photon @ 427.8 nm (blue-violet, fast decay, only reached by the most energetic particles, low altitude)
- Precipitation altitude selects which transition dominates: green around 100-200 km, red above ~200 km where low density lets the slow 630 nm decay survive, blue-violet below ~100 km where only the fastest particles penetrate.
- Wind speed & density set the ram pressure and hence the total kinetic energy dumped into the ionosphere — brighter curtains and lower penetration altitude at high speed/density.
- Kp index expands the auroral oval equatorward (colat. ≈ 15° + 3.9°×Kp) and raises particle energy, pushing colour from green toward red-capped, then blue-fringed at the most extreme storms.
- Gas mix reweights the oxygen (green/red) vs nitrogen (blue/violet) contribution, as if simulating a different planet's upper atmosphere.