It starts on the Sun
Every aurora begins as a wind — not of air, but of electrically charged particles blowing off the Sun in every direction, all the time.
The Sun's outer atmosphere, the corona, is heated to more than a million degrees. It is so hot that the Sun's gravity cannot hold on to it, so it streams away into space as the solar wind: a thin gas of electrons and protons (a plasma) that carries a piece of the Sun's magnetic field with it.
Near Earth the wind is astonishingly thin — around 5 particles per cubic centimetre — but it moves fast:
- Slow wind: roughly 300–500 km/s.
- Fast wind from coronal holes (open-field regions on the Sun): about 600–800 km/s. Coronal holes can survive for several solar rotations, so their storms often repeat every ~27 days.
- Coronal mass ejections (CMEs): huge clouds of magnetised plasma, often around a billion tonnes, blasted out at up to ~3,000 km/s. CMEs cause the biggest storms.
Spacecraft parked at the L1 point, 1.5 million km sunward of Earth, sample the wind before it arrives. That is where the 15–60 minute "aurora alerts" come from.
Drag the speed slider and watch the travel time change — the numbers are calculated from the real Sun–Earth distance (149.6 million km). Sizes and distances in the drawing are not to scale.
Earth's invisible shield — and its back door
Earth is a giant magnet. Its field, made by churning liquid iron in the outer core, pushes the solar wind aside and carves out a protected bubble: the magnetosphere.
On the day side the wind squeezes the bubble to about 10 Earth radii (~64,000 km). On the night side it stretches it into a long magnetotail that reaches far beyond the Moon's orbit. Most of the wind simply flows around.
The back door opens when the solar wind's own magnetic field points south (scientists write Bz < 0). Then it is opposite to Earth's field at the front of the bubble and the two reconnect — they break and rejoin. Solar-wind field lines become tied to Earth, get dragged over the poles into the tail, reconnect again there and snap back toward Earth. That snap flings particles down along the field lines toward the poles.
This loop is the Dungey cycle, proposed by James Dungey in 1961. When the wind's field points north, reconnection moves to high latitudes behind the poles, much less energy gets in and the aurora is usually faint.
That is why aurora forecasters obsess over a single number: Bz. Fast wind plus a strong southward Bz that lasts for hours is the recipe for a great night.
Why green? Why red?
A few thousand kilometres above the poles, electric fields accelerate electrons to energies of roughly 1–10 keV. They spiral down the field lines at tens of thousands of km per second and crash into the thin upper air.
Each collision can kick an oxygen atom or a nitrogen molecule into an excited state. When it relaxes, it releases the extra energy as light of one exact colour — a fingerprint of that atom. The trick is how long each atom must wait before it can glow:
A "forbidden" transition: the atom must sit undisturbed for about 0.7 seconds. That works from roughly 100 km upward. Below that, collisions steal the energy first.
Even more patient: the excited state lives about 110 seconds. Only very high up, where air is so thin that collisions are rare, can it survive long enough to glow. Lower down it is "quenched".
Emitted almost instantly — in well under a millionth of a second. Mixed with the green it gives many curtains their faint blue tint.
During strong storms, very energetic electrons reach deeper into denser air, where nitrogen's red bands plus the blue give the curtains a pink hem.
So the colour of an aurora is really a map of altitude and energy: soft (low-energy) electrons stop high up and make red; harder ones reach down to where green, blue and pink are made.
Arcs, rays, coronas — and the substorm
Aurora is rarely steady. It often follows a rhythm called a substorm, which can repeat several times in one night. Step through it below, or press play.
The auroral oval
The aurora isn't everywhere at once. It forms a glowing ring around each magnetic pole. The ring stays fixed facing away from the Sun while Earth turns beneath it — so any place under the ring gets its show at night.
Why the ring is off-centre
Earth's magnetic axis is tilted about 10° from its spin axis, and the magnetic pole sits over the Canadian Arctic, near northwest Greenland. The oval is centred there — not on the geographic North Pole. That pulls it south over North America and Iceland and north over Siberia.
What matters is your magnetic latitude. On a quiet night the oval's night-side edge lies near 66° magnetic latitude, and it moves about 2° toward the equator for every step of Kp — down to roughly 48° at Kp 9 (NOAA SWPC).
How the map works
Magnetic latitudes and magnetic local time are computed from the AACGM-v2 model used by researchers. The oval's shape follows the Kp rule above, with its day-side edge several degrees further poleward. Real ovals are lumpier and change minute by minute, so treat it as a guide, not a forecast.
"Low in sky" means the oval is within about 3° (≈330 km) of your magnetic latitude; "camera only" means within about 7°, where a long exposure may catch a glow on the horizon.
Dark skies, the right hour, the right season
You need three things at once: darkness, clear sky, and activity. Pick a date to see what the Sun and Moon are doing over Iceland — calculated for that exact night.
The best hours
Activity peaks around magnetic midnight, when you are on the part of the oval that faces straight away from the Sun, where substorms usually erupt. Iceland keeps UTC all year with no daylight saving, and its magnetic midnight falls at roughly 00:30 on the clock. So 22:00–02:00 is the classic window, though strong storms can light up the sky as soon as it gets dark.
The best season
From about late April to mid-August the Sun never sinks 12° below Reykjavík's horizon, so the sky stays too bright. From late August to mid-April the nights are dark enough. The weeks around the equinoxes (March and September) are extra good: geomagnetic storms are statistically more common, largely because of how Earth's tilted magnetic field lines up with the solar wind's field — the Russell–McPherron effect (1973).
Why Iceland is built for aurora
The whole country sits between about 63° (Vík, on the south coast) and 67° (the northern Westfjords) magnetic latitude — right under the edge of the quiet-time oval. By the NOAA rule that means aurora can reach overhead at a Kp of just 0–2, depending on where you stand, so on most clear, dark nights there is something to see.
Check two forecasts
The Icelandic Met Office aurora forecast shows activity and a cloud map. Clouds matter most: when the south is overcast, the north or east is often clear. For live solar wind and Bz, see NOAA SWPC.
Get out of the light
Even 15 minutes outside town makes a huge difference. Give your eyes about 20 minutes to adapt, and use a red light, not a white phone screen. A bright Moon washes out faint aurora but lights the landscape beautifully.
Camera settings to start with
Tripod · widest lens · f/1.8–2.8
ISO 1600–3200 · 2–10 s exposure
Manual focus on a bright star
Use shorter exposures when the aurora is moving fast, or the rays blur. Phone night modes work surprisingly well.
Stay safe
Never stop on the road to watch — pull fully into a proper parking area. Check road and weather conditions before driving at night in winter, dress for wind, and tell someone where you're going.
A thousand years of looking up
- c. 1250
The Norwegian Konungs skuggsjá ("The King's Mirror") describes the norðrljós seen from Greenland and weighs up theories about what causes them.
- 1619
Galileo Galilei is usually credited with the name aurora borealis: Aurora, Roman goddess of dawn, and Boreas, Greek god of the north wind.
- 1716
Edmond Halley suggests the aurora is linked to Earth's magnetism and follows its field lines.
- 1741
In Uppsala, Olof Hiorter and Anders Celsius notice compass needles twitching whenever the aurora is active — proof it is magnetic.
- 1859
Richard Carrington sees a white-light solar flare. About 17.6 hours later the biggest storm on record hits: aurora is seen from the Caribbean, and telegraph lines spark.
- 1896–1913
Kristian Birkeland fires electron beams at a magnetised sphere (his "terrella") and argues that particles from the Sun cause aurora and drive huge electric currents. Satellites confirmed his currents decades later.
- 1910s–40s
Carl Størmer photographs aurora from two places at once and triangulates the heights: most light comes from about 90–150 km, and some rays reach far higher.
- 1961
James Dungey proposes that magnetic reconnection lets the solar wind drive the magnetosphere — the Dungey cycle.
- 1964
Syun-Ichi Akasofu describes the auroral substorm from all-sky camera films.
- 1973
Christopher Russell and Robert McPherron explain why storms cluster around the equinoxes.
- 2018
STEVE is described in Science Advances, discovered with the help of citizen-science aurora photographers.
- 2024
On 10–11 May the strongest geomagnetic storm since 2003 (G5) sends aurora to unusually low latitudes worldwide. In October, NASA and NOAA announce that Solar Cycle 25 has reached its maximum.
Myths and facts
"The aurora is sunlight reflecting off polar ice." No — it's light emitted by gas atoms 100–400 km up, and it happens over the open ocean too.
"Vikings thought the lights were Valkyries' armour." Popular online, but it doesn't appear in the medieval Norse sources. The oldest Norse description, in Konungs skuggsjá, is a sober discussion of possible natural causes.
"You need extreme cold." Cold has nothing to do with it. Winter just brings long, dark nights, and clear winter nights happen to be cold.
"It only happens in winter." The aurora happens all year round. In the Arctic summer the sky is simply too bright to see it.
The southern lights. The aurora australis happens at the same time as the northern lights, around the south magnetic pole. From land, Tasmania, southern New Zealand and Antarctica are the best places to see it.
In Icelandic it's norðurljós, "northern lights". Finns call it revontulet, "fox fires", after a folk tale about an arctic fox sweeping snow into the sky with its tail.
Could you explain it to a friend?
Where the science comes from
- NOAA Space Weather Prediction Center — Aurora Tutorial and Tips on Viewing the Aurora (Kp–latitude rule, viewing hours, emission heights).
- Icelandic Meteorological Office — Aurora forecast and Aurora and the Earth's magnetic field.
- "The altitude of green OI 557.7 nm and blue N₂⁺ 427.8 nm aurora", Annales Geophysicae 41, 1–12 (2023) — mean peak heights of ~115 km.
- Gillies et al. (2017), "Identifying the 630 nm auroral arc emission height", JGR Space Physics.
- Dungey, J. W. (1961), "Interplanetary magnetic field and the auroral zones", Physical Review Letters 6, 47.
- Akasofu, S.-I. (1964), "The development of the auroral substorm", Planetary and Space Science 12, 273.
- Russell, C. T. & McPherron, R. L. (1973), "Semiannual variation of geomagnetic activity", JGR 78, 92.
- MacDonald, E. A. et al. (2018), "New science in plain sight: citizen scientists lead to the discovery of optical structure in the upper atmosphere", Science Advances 4, eaaq0030.
- SIDC (Royal Observatory of Belgium), Solar Cycle 25 reached its maximum in October 2024.
- Shepherd, S. G. (2014), "Altitude-adjusted corrected geomagnetic coordinates: definition and functional approximations", JGR Space Physics 119 — computed with the
aacgmv2package for the globe and map. - Leirvogur Magnetic Observatory (University of Iceland) — Reykjavík-area corrected geomagnetic latitude ≈ 64.5°.
- Sun and Moon positions use standard low-precision astronomical formulas (accurate to a fraction of a degree), computed in your browser.
The sky simulations are physically proportioned — real altitudes, Earth's curvature, field-line tilt and emission heights — but they are artistic renderings, not photographs. Electron stopping heights in Chapter 3 are approximate values from standard energy-deposition models.