Night one — late February

Arrival

The bus dropped me at the fjord road an hour after the sun gave up. By the time I had the stove lit, the sky to the north held a pale green arc, steady as a bridge, and I stood in the doorway with my gloves in my hand, not wearing them.

What I was looking at began, days ago, on the sun. The solar wind — a thin, endless exhalation of charged particles — streams past Earth and drapes our magnetic field back into a long tail on the night side. Electrons accelerated in that tail pour down along the field lines toward the poles, and a hundred kilometres over my head they strike the upper air.

The green: atomic oxygen, struck by those electrons, emits light at a wavelength of 557.7 nanometres — the signature colour of the aurora, brightest roughly 100 to 250 kilometres up. It is green partly because that emission is strong, and partly because our eyes are most sensitive right there.

Night four

The colours, sorted by altitude

Tonight the arc grew a deep red crown, so faint I kept checking it against the camera. The red sits above the green, always. This is not weather; it is chemistry arranged by altitude.

High up, past 250 kilometres, the air is so thin that an excited oxygen atom can wait out its full temper — nearly two minutes — before releasing a photon of red light at 630 nanometres. Any collision in that time would quietly steal the energy instead. Lower down, collisions come fast, so only the quicker green transition survives. And at the very bottom edge, below about 100 kilometres, molecular nitrogen takes the hits and fringes the curtain with pink and violet.

Reading the sky vertically: red above 250 km (oxygen, patient), green 100–250 km (oxygen, quick), pink-violet at the lower hem (nitrogen). The aurora is a bar chart of the atmosphere.

Night seven

Substorm

For three hours the arc did nothing, and I nearly went in. Then the whole sky tore. The single band split into curtains, the curtains rippled like something breathing, and for maybe eight minutes the snow in front of the hut was green enough to read by. Then it all faded to a smudge, as if embarrassed.

That is a substorm, and it has a mechanism: the stretched magnetotail stores energy the way a bent branch does, and when the field lines snap back — reconnection — the stored energy accelerates electrons earthward in a burst. The explosive phase really does last minutes, not hours. Watchers on the ground have logged this rhythm for over a century: quiet arc, sudden breakup, slow recovery.

Kp: geomagnetic activity is summarised in the planetary K-index, 0 to 9, in three-hour blocks. The auroral oval — a ring of light centred on the geomagnetic pole, not the geographic one — swells toward the equator as Kp climbs. At my latitude, Kp 2 is a good night. Kp 5 is a storm.

Night nine

Why curtains

Everyone who sees a big display asks the same question I did: why does it hang in folds, like fabric? The sky has no loom.

It does have a magnetic field. Up here the field lines run nearly vertical, and the incoming electrons spiral tightly along them — they cannot drift sideways. So the light is drawn in tall rays along the field, and where the electron rain falls in sheets, the sheets glow: drapery, seen edge-on and face-on at once. The sharp lower hem marks the altitude where the atmosphere finally grows dense enough to stop the electrons almost all at once.

The scale is humbling: a curtain's rays can stretch hundreds of kilometres tall, yet the sheet itself may be only a few hundred metres thick — a wall of light the height of a country and the width of a village street.

Night eleven — leaving

What the sun sends

Last night was the best of the trip, and I owed it to a hole. Coronal holes — dark, open-field regions of the sun's outer atmosphere — let the solar wind escape fast, and when such a stream sweeps past Earth the nights that follow run bright. The great storms come instead from coronal mass ejections: a billion tonnes of magnetised plasma thrown off the sun, arriving a day or three later. Both trace the sun's eleven-year cycle of temper.

I packed the hut by headlamp this morning. People ask if the aurora makes a sound. It doesn't, not at my feet — the light is born in air a hundred kilometres up, far too thin to carry a whisper down. The loudest thing on the fjord road was my own breath. That, and the bus, exactly on time, headlights small and yellow and human.

For the record: the narrator and the hut are invented. Every mechanism in these pages — the solar wind, reconnection, the emission altitudes and wavelengths, the oval, the Kp scale — is real, and described as physicists understand it.