The northern lights

The oval is NOAA's OVATION model, run on the solar wind measured a million and a half kilometres sunwards of us, half an hour to an hour before it arrives. Each point is the chance of aurora straight overhead. You do not have to stand under it: a curtain of light is tall, and from a long way south you can see its top over the horizon. So the question for Eindhoven is how close the edge comes, and how high that puts the top of the curtain in our northern sky.

Kp is how hard the Earth's magnetic field is being shaken, from 0 (calm) to 9, worked out every three hours from magnetometers at a dozen observatories; the figure here is NOAA's minute-by-minute estimate, and the verdict also looks at NOAA's forecast for the next twelve hours. On a quiet night it sits around 1 or 2. NOAA calls Kp 5 a minor storm; at Kp 7 the aurora is typically seen down to 50° geomagnetic latitude, at Kp 6 to 55°. Eindhoven is at about 52½°, so a Kp of 6 to 7 is roughly what we need, and a clear, dark sky to the north.

If the verdict says go: wait until it is properly dark, find a spot with no lamps between you and the northern horizon, give your eyes a few minutes, and take a photo in night mode. The phone often shows it first.

The colours are layers. Electrons from the solar wind, steered down along the magnetic field, hit the thin air a hundred kilometres up and more. Oxygen glows green between about 100 and 250 kilometres; higher up, where the air is so thin that an excited oxygen atom can wait up to two minutes for its turn to shine without bumping into anything, it glows red. Nitrogen at the bottom edge adds violet and a pink fringe. From far south, the Earth hides the green and only the red top clears the horizon. And in the big storms that bring it this far, more of the incoming electrons are slow ones, which give up their energy high up, where the red is made. Between the two, the aurora over the Netherlands is usually red.

And why phones see it better than people: a camera collects light for a second or more, and its sensor sees colour at any brightness. Our eyes at night see with rods, which are sensitive but colour-blind, so a faint aurora looks like a grey-white glow, a cloud that does not move with the wind. Press your eyes to see the difference. On a strong night, like May 2024, the colour shows to the eye too.