Solar wind, caught on the way down
Charged particles slide along the field lines and light the thin air at the top of the sky.
Green low, violet high
Oxygen glows green around a hundred kilometres up. Higher and thinner, nitrogen adds the violet fringe that hangs above the curtain.
Curtains, not clouds
The rays are the field lines made visible. They sway sideways as the current shifts, and the whole sheet folds slowly along its lower edge.
Some nights nothing, some nights everything
The activity comes in patches. A quiet arc can sit on the horizon for an hour and then, in a minute, fill the sky.