Aurora
Green curtains with red tops and a violet hem ripple over a still lake, and every so often the lowest one flares and a surge races west across the sky. Beside it, solar-wind particles pushed through Earth's magnetic field show where the light comes from: about 99 in 100 are turned away, and the rest spiral down into two rings round the poles. Turn up the solar wind and the rings slide toward the equator, until even Boca Raton gets a faint red glow.
In the first years of the twentieth century the Norwegian physicist Kristian Birkeland hung a magnetised brass ball, his terrella, inside a vacuum chamber and fired a beam of electrons at it. The ball lit up in two glowing rings, one round each magnetic pole. He argued that the aurora was the same thing at planetary scale: charged particles from the Sun, caught by Earth's field and steered to the polar regions. Most physicists of the day thought he was wrong. Satellites proved him right in the 1960s.
His colleague Carl Størmer spent decades working out by hand how a single charged particle moves in a magnetic dipole, and found something odd. A particle coming in from far away cannot reach the ground just anywhere: below a cutoff latitude set by its momentum, the field turns it back every time. Only the polar caps are open. That is the small panel here: a stream of particles from the Sun, each pushed through a dipole field with the Boris method, most of them deflected, and the few that land landing only in the green caps.
The big panel is what those landings look like from underneath. The colours are atoms. Oxygen gives the green at 557.7 nanometres, from about 100 to 200 km up. Oxygen also gives the red at 630.0 nm, but only higher up, because the red state takes nearly two minutes to radiate and in denser air a collision steals the energy first. Ionised nitrogen gives the violet fringe at the hem, where the most energetic electrons reach deepest. Those three heights are why a curtain is green at the bottom, red at the top and purple along the lower edge.
On 10 and 11 May 2024 the strongest geomagnetic storm in two decades pushed the auroral oval so far south that people in Florida photographed it. The geometry is set here: from Boca Raton the green base of a Kp 9 curtain is below the horizon, but its red top, 400 km up, clears it, so what Florida got was a low red glow, brighter in phone cameras than to the eye. Pick Boca Raton, push the solar wind to the top, and see for yourself. Then try the naked-eye setting: the eye is nearly blind to faint red, which is why so many people only saw that night in their photos.
Fifteen pendulums that never touch each other snake, split into groups, dissolve into chaos and snap back into line every minute — and you can hear it.