The northern lights start as a wind off the Sun and end as glowing air 100 km over your head. Follow the energy the whole way.
Data: NOAA SWPC Real-Time Solar Wind and planetary K-index
An aurora is air glowing. Electrons coming down from space hit oxygen and nitrogen 100 to 300 km up and hand over some of their energy. Each atom holds it for a moment, then lets it go as light of one exact colour. A neon sign works the same way; here the power supply is Earth's magnetic field.
The Sun's outer atmosphere is so hot that it boils off into space as the solar wind: roughly a million tonnes of charged gas every second, moving at 300 to 800 km/s. It carries a stretched-out piece of the Sun's magnetic field with it. Large eruptions called coronal mass ejections throw denser, faster clouds that arrive anywhere from under a day to about four days later.
Not all wind is equal. What reaches Earth on a given night is usually one of three things, and they make very different aurora.
| Background wind | Fast stream | CME | |
|---|---|---|---|
| Comes from | The Sun’s quiet regions, all the time | A coronal hole, a patch where the Sun’s field opens into space | An eruption, often above a sunspot group |
| Speed | 300 to 450 km/s | 500 to 800 km/s | 500 to over 2,000 km/s |
| Density | 3 to 10 per cm³ | Thin in the stream, piled up at its front edge | Dense in the sheath behind its shock |
| Bt | 3 to 7 nT | 10 to 20 nT at the front edge, then 5 to 10 | 15 to 50 nT, rarely 70 or more |
| Bz | Drifts north and south | Flips every few minutes | Turns slowly and can stay south for many hours |
| Aurora | Quiet arcs, Kp 0 to 3 | Repeated substorms, often G1 to G2, sometimes G3 | Can reach G3 to G5 if Bz stays south; little if it points north |
| Timing | Always there | Lasts 2 to 5 days and comes back every 27 days | Arrives 1 to 4 days after the eruption; the main storm lasts about a day |
Fast streams run into the slower wind ahead of them and squeeze it, so a stream’s front edge carries a burst of dense, strongly magnetised gas before the fast wind itself arrives. They dominate the years when the solar cycle is winding down; CMEs are most common near solar maximum, and nearly every extreme storm on record has been a CME.
Speed is how fast the wind is moving. Faster wind delivers more energy per second and hits the magnetosphere harder.
Density is how many particles are in each cubic centimetre. Together with speed it sets the wind’s pressure, which squeezes the magnetosphere smaller. On its own, a dense wind with a northward field makes little aurora.
Bt is the total strength of the magnetic field the wind carries, in nanotesla. It sets the ceiling, because the north–south part can never be bigger than the whole.
Bz is the north–south part, measured in Earth’s frame. Negative means south, the direction that links up with Earth’s field. It is the single most important number and the hardest to forecast: spacecraft such as SOLAR-1, IMAP, DSCOVR and ACE measure it about 1.5 million km upstream, which gives roughly 15 to 60 minutes of warning. A rough rule is that fast wind with Bz below −10 nT for several hours means a strong storm.
In this model, Kp comes from the Newell coupling function, v4/3 Bt2/3 sin8/3(θ/2), where θ is the field’s clock angle and whatever part of Bt isn’t Bz is treated as east–west. It is averaged over a few seconds of model time, because the magnetosphere takes a while to respond, while the aurora’s brightness follows Bz from moment to moment. The magnetopause distance uses the Shue et al. (1998) model. The time trace is sped up so a few seconds stand for many minutes.
Charged particles can't easily cross magnetic field lines, so Earth's field works as a shield. The wind is pushed around a bubble called the magnetosphere, squashed to about 10 Earth radii on the side facing the Sun and drawn out into a long tail on the night side.
When the wind's magnetic field points south, opposite to Earth's at the nose of the bubble, the two fields can join. This is magnetic reconnection. Field lines peel open, get dragged over the poles and pile up in the tail. The tail stretches until it reconnects too, snapping back toward Earth. Electrons already stored in the tail get pushed down the field lines and sped up to a few thousand electronvolts, much of that in the last few thousand kilometres above the aurora. A burst of this is called a substorm, the moment when quiet arcs suddenly brighten and start to dance.
So the solar wind supplies the energy, but few of the electrons that make the light came from the Sun. Most were already inside Earth's magnetosphere.
The field lines that reach into the tail come down in a band around each magnetic pole, usually between 65° and 75° magnetic latitude: the auroral oval. The oval stays lined up with the Sun while Earth turns underneath it, so towns in the far north slide under it every night. The southern hemisphere has a matching oval, the aurora australis. In a storm the ovals swell toward the equator, which is why big events are seen far from the Arctic.
Green, at 557.7 nm, comes from oxygen at about 100 to 240 km. The atom holds that energy for under a second before it glows.
Red, at 630 nm, also comes from oxygen, but from a state that lasts close to two minutes. Lower down, a collision with another molecule steals the energy before the light comes out, so red only survives above about 200 km where the air is very thin. Strong storms pour more energy in up there, which is why big displays turn crimson.
Molecular nitrogen adds blue and violet. When the electrons arrive with extra energy they reach down to about 90 km, where nitrogen's deep red mixes with the green into a pink hem along the lower edge.
Near the poles the field lines plunge almost straight down, and the electrons follow them, so the glow stretches into vertical rays. Long sheets of falling electrons make the curtains, and ripples in those sheets make the folds that seem to sway.
The ground view puts you at a real place and draws the oval where the current solar wind would put it on Earth's night side. If the oval passes over you, the aurora fills the sky and its rays seem to meet overhead. If it lies to the north, you see it side-on from hundreds of kilometres away. Earth's curve hides the bottoms of the curtains first, so from farther south only the tall red tops clear the horizon. That is why aurora seen far from the Arctic is so often a red glow low in the north.
In big storms the oval can slide south of places like Tromsø, and the aurora moves into the southern sky there. In the southern hemisphere everything is mirrored: the aurora australis sits toward the south.
Cameras and eyes disagree. A long exposure collects faint colour, especially the deep red at 630 nm, which night vision barely registers. Eye shows roughly what you would notice without a camera: faint aurora looks grey-white, and only bright displays show clear green.
Kp is a 0 to 9 index of how disturbed Earth’s magnetic field is, reported in thirds (5−, 5, 5+) every three hours; G1 to G5 is the storm scale used in space weather forecasts. The Kp here is an estimate from the wind you set, and the oval latitudes and places are a rough guide for each Kp. Latitudes are magnetic (AACGM), which can differ from geographic latitude by 10° or more: Edmonton is farther north magnetically than Stockholm. The oval's night-side edge moves from about 67° at Kp 0 to 47° at Kp 9, tuned so that the places listed roughly match NOAA's storm-scale descriptions. A place counts as seeing the aurora when its tops reach 10° above the horizon. Real storms vary. The oval’s height above the ground is exaggerated in the space view, and nothing else is drawn to scale.
Live in the Solar wind panel reads the real-time solar wind from NOAA’s Space Weather Prediction Center (swpc.noaa.gov, data at services.swpc.noaa.gov/json/rtsw). It is measured at the L1 point by SOLAR-1, IMAP, DSCOVR and ACE, using whichever craft NOAA marks as active. Each reading is shifted by its travel time to Earth, the coupling is averaged over the hour before, and NOAA’s own estimated Kp is shown alongside the model’s. It refreshes every minute. Pages opened as Claude artifacts can’t connect to other websites, so Live needs a self-hosted copy of this page.
1 to 5 steps, W solar wind panel, B H C L background, fast stream, CME or live data, [ ] push Bz south or north, V space or ground, G field lines, E eye or camera, R reset the view, / hide the interface. Drag to look around; pinch or scroll to zoom.