Boreal

Two FFGL plugins for Resolume Arena and Avenue in one download: SW Boreal, a source that's the night sky, and SW Boreal Over, an effect that adds the aurora to your clip as light. A sky mask keeps a skyline in front of it, and an Illumination control lights the scene with the sky's own colour.

The idea underneath it: an auroral arc is a sheet of electric charge drifting in crossed electric and magnetic fields, and a charge sheet drifting at E×B is a vortex sheet. The potential is a stream function and the space charge is its vorticity, so a thin arc follows the same equations as a thin sheet of spinning fluid and goes unstable the same way, by rolling itself up (Hallinan & Davis 1970). The curls, the folds and the westward surge of a substorm are all that one instability at different scales, and Curl Size is a physical length.

Four more models follow on from that, with every constant out in the open. Where the sheet's wound tight, the current along the field is high, and by the Knight relation so is the electrons' energy. That energy goes into a real atmosphere (NRLMSIS) at the heights Fang et al.'s parameterisation gives, and excites each line through its own chemistry. The green line's atoms glow after 0.7 s and the red line's after minutes, and lower down the red ones get knocked out of it by collisions first, so the red lives above 200 km, lags behind a moving curl and hangs about afterwards. A camera on a curved Earth looks up through all of it, with the air dimming and reddening whatever's low.

Things that turn up without anyone putting them there: the pink lower border of a hard display, the corona overhead with its rays meeting at the magnetic zenith, arcs on the horizon shrinking to bands, red tops that rise with solar activity, the airglow band low down in every long exposure, and snow that goes green under a strong display.