Fifty miles above your head, sunlight is stripping electrons off the air. In the same instant, other electrons are finding their way back onto atoms. Neither of those processes is the ionosphere. The running difference between them is.
A strange thing to have bounced a century of radio off. Like a candle flame (No. 1) it is a shape held up by relentless throughput; unlike a flame it has no container, no boundary and no material of its own — just ordinary upper air, roughly 80 to 600 kilometres up, with a small fraction of its electrons knocked loose. That fraction is settled by a race.
Solar extreme ultraviolet and X-rays ionise whatever they hit; free electrons then find ions and recombine. Where ions stay put, the standing density is the square root of production over the recombination coefficient — and that square root is why a century of radio could be bet on it. Loss goes as density squared, because two particles must find each other; production goes as density once. The layer punishes its own success. Quadruple the sunlight and you get double the electrons — which is how it shrugs off a solar output swinging by nearly a factor of ten over the eleven-year cycle.
Its height is settled the same way. Production peaks where the air has swallowed most of the incoming light: below, more gas but no light left; above, light and nothing to ionise. The peak sits at the crossover — whose altitude depends on the scale height of the air and the Sun’s angle, not on how bright the Sun is. Turn the Sun up tenfold and the layer gets denser and stays put. Brightness sets how much; geometry sets where.
Now the number that makes this a standing wave, not a stock. In the E region electrons go by dissociative recombination of NO⁺ and O₂⁺: coefficient near 10⁻⁷ cm³ per second, against a few hundred thousand electrons per cubic centimetre. Divide: an E-region electron at midday can expect about thirty seconds. The mirror a noon signal bounces off is replaced end to end roughly a hundred times an hour, holding its height and density throughout.
Which tells you what sunset does. Production stops; loss does not, because recombination is a collision and collisions do not need light. The E layer, with its half-minute memory, goes almost at once, and F1 merges away. What survives is F2 — highest and oddest, down by roughly a factor of ten and still there at dawn, because at 300 kilometres an oxygen ion can only die by first meeting a nitrogen molecule, and there are hardly any left. The killer is scarce, not slow. And F2 has no business existing: its peak is not a peak in production. Production falls with height, loss falls faster — rate-limited by that same thin nitrogen — so the quotient climbs until diffusion stops it. The layer carrying the world’s night-time radio is not a pile of anything. It is a ratio that runs out of room, a shape which, as one standard treatment puts it, “just resembles a Chapman layer by accident.”
The lowest layer does better than vanish. The D region, 60 to 90 kilometres up, is built almost entirely out of a trace gas: its dominant source is one solar line, Lyman-alpha at 121.5 nanometres, ionising nitric oxide — the only constituent up there whose ionisation potential, 9.25 electronvolts, is low enough to be reached by it. Oxygen needs 10.08 and is spared. And Lyman-alpha only gets that low through a coincidental gap in oxygen’s own absorption spectrum.
It does not reflect; it absorbs. It is dense enough that an electron shaken by a passing wave hits a molecule before it can hand the energy back, so the wave becomes heat — worst at low frequencies, which is why this is a broadcast-band problem, not a shortwave one. At sunset the absorber closes, and some of that is storage rather than destruction: electrons there are also lost by attaching to oxygen molecules, day and night, while the step that frees them again needs sunlight. In the dark a share of them sit parked on O₂ as negative ions, deaf to radio. Sunrise does not build them all from nothing; it shakes them loose.
That is the midnight skip entire. By day a distant station is absorbed low down and you hear only its ground wave; after dark the absorber stands aside, the signal reaches the F layer and lands hundreds of miles off, sounding like it was never far.
Set it beside No. 29, the ozone layer: same sky, same Sun, the same trick of a stock held by a reaction rather than stored. But ozone is destroyed by absorbing ultraviolet — itself a photochemical step — so sunset closes both halves of ozone’s ledger at once and the layer waits out the night with its books open. The ionosphere’s destruction is a collision, and collisions carry on in the dark. Sunset closes only the income.
That asymmetry is the difference between a shield you can count on around the clock and a mirror re-earned every morning. Marconi’s signal crossed the Atlantic on 12 December 1901; Heaviside and Kennelly explained it the following year, correctly, on no evidence at all. In the hundred and twenty-five years since, the mirror has never once failed to be there.
≈
One loop I’m watching
Next: a lawn kept by the mouths eating it. On the African savanna, patches grazed hard and repeatedly do not die back — they hold themselves short, young and unusually rich in nitrogen, which is exactly what draws the herds back to graze them again. The ecologist Sam McNaughton named them grazing lawns in 1984. A plant community maintained by the consumption that ought to destroy it, and a standing pattern in a food web: the lawn keeps its shape while every blade and every grazer in it is replaced. Next time.