Three curtains of light, each an exponential falloff around a noise-warped line. It behaves like an aurora because it is built like one.
See the itineraries →exp(−distance × k) gives the soft vertical bleed that a gradient stop cannot. k controls how tight each curtain is.
An fbm sample displaces the curtain line, so it drifts rather than sliding — the difference between weather and animation.
Each curtain has its own frequency and phase, so they cross without ever syncing into a single band.