The science

How the aurora works.

A 150-million-kilometre chain reaction that starts on the Sun and ends as light 100 to 300 km above your head.

Solar wind (gold) flowing past Earth's magnetic field. Not to scale.

8.3 min

for sunlight to reach Earth

1–4 days

for solar wind and eruptions to arrive

100–300 km

height of most aurora

557.7 nm

the green light of oxygen

01 · The chain of eventsFrom the Sun to your sky

The Sun constantly blows a thin, electrically charged gas into space called the solar wind. It carries the Sun's magnetic field with it. Every so often the Sun also throws off a much bigger cloud, a coronal mass ejection, which can reach Earth in as little as a day.

Earth's magnetic field deflects almost all of it. The aurora is what happens to the small share of energy that gets in.

Leaves the Sun
Solar wind streams out all the time. Eruptions add bursts of faster, denser plasma.
300–800 km/s
Crosses space
The wind takes days to cover 150 million km. A fast eruption can do it in under a day.
~18 h to 4 days
Passes the monitor
Spacecraft at the L1 point, 1.5 million km sunward, measure it before it arrives.
15–60 min warning
Joins the field
When the wind's field points south, it links up with Earth's field and energy pours in.
Bz negative
Loads the tail
Energy builds in Earth's stretched magnetic tail on the night side, then snaps back.
Substorms
Lights the air
Electrons race down field lines and strike oxygen and nitrogen, which glow.
1–10 keV electrons

Sunlight

8.3 min

Fastest eruptions

~18 hours

Typical eruption

2–3 days

Solar wind at 400 km/s

4.3 days

Fig. 1 · Travel time from the Sun to Earth. Bars use a logarithmic scale.

02 · The magnetosphereEarth's magnetic shield

Earth's magnetic field carves a bubble in the solar wind called the magnetosphere. On the day side the wind squashes it to about 10 Earth radii. On the night side it stretches into a tail hundreds of Earth radii long.

The key is the direction of the magnetic field the wind carries, called Bz. When it points north, the two fields repel and the wind slides past. When it points south, the fields join in a process called magnetic reconnection, energy floods into the tail, and the aurora brightens. Try both below.

Fig. 2 · Side view with the Sun to the left. Gold dots are solar wind particles; green streaks are particles that make it in and reach the auroral zones. Not to scale.

03 · ColorsWhy green, why red

Incoming electrons knock oxygen and nitrogen into excited states. Each gas releases that energy as light of particular colors, and height decides which color wins.

Oxygen's green light comes from a state that lasts about 0.7 seconds. Its red light comes from a state that lasts nearly two minutes. Lower down, the air is so dense that an excited atom usually bumps into a neighbour and loses its energy before it can glow red. So red survives only high up, where the air is thin, and green dominates below.

Red: high oxygenTakes nearly two minutes to emit, so it only shows where collisions are rare. Dominant in big storms, when it can reach far south.

Green: oxygenThe common color. Our eyes are most sensitive near this wavelength, which is part of why it stands out.

Blue and violet: nitrogen ionsFaint to the eye, strongest at the tops of curtains lit by sunlight and in fast, energetic bursts.

Pink edge: nitrogenA crisp pink or magenta fringe along the bottom of bright, fast-moving curtains, where the most energetic electrons reach lowest.

Fig. 3 · Where each aurora color forms. Heights are typical; real displays vary with storm strength.

04 · The spectrumThe light, line by line

Run aurora light through a prism and it does not spread into a smooth rainbow. It breaks into a few sharp lines, each the fingerprint of one atom or molecule. Astronomers call these emission lines.

That is why the aurora's colors look so pure. The famous green is a single wavelength, 557.7 nanometres, from oxygen.

Fig. 4 · The main visible lines of the aurora. Line heights show typical relative brightness and are illustrative; they change from display to display.

05 · Where it happensThe auroral oval

Aurora does not form over the geographic pole. It forms in a ring around the geomagnetic pole, which sits over the Canadian Arctic. That is why Alaska and northern Canada see more aurora than Siberia at the same latitude.

The ring is called the auroral oval. It is wider and reaches farther south on the night side. As geomagnetic activity rises, the whole oval swells toward the equator. Drag the slider to see how far it spreads.

Kp index3

Aurora on the horizon from

Fig. 5 · View from above the North Pole. The oval is drawn from the standard Kp boundary and today's Sun position; the shaded half is night right now. Cities light up when aurora would reach their horizon.

06 · Measuring stormsThe Kp scale

Magnetometers around the world measure how much Earth's magnetic field wobbles in each three-hour window. Combined, they give the planetary K-index, Kp, from 0 (quiet) to 9 (extreme).

NOAA turns the top of the scale into storm levels, G1 to G5, like hurricane categories. The rarer the level, the farther toward the equator the aurora reaches.

Level	Kp	How often, per 11-year cycle	Aurora seen as far as
G5Extreme	9	4 storms · 4 days	Florida and southern Texas, about 40° magnetic latitude
G4Severe	8	100 storms · 60 days	Alabama and northern California, about 45°
G3Strong	7	200 storms · 130 days	Illinois and Oregon, about 50°
G2Moderate	6	600 storms · 360 days	New York and Idaho, about 55°
G1Minor	5	1,700 storms · 900 days	High latitudes, such as northern Michigan and Maine
Kp 0–4Quiet to active	0–4	Most of the time	Alaska, northern Canada, Iceland, northern Scandinavia

Fig. 6 · NOAA Space Weather Prediction Center geomagnetic storm scale. Frequencies are NOAA's averages per solar cycle.

07 · The aurora's heartbeatAnatomy of a substorm

A night of aurora is rarely steady. It usually builds through cycles called substorms, as the magnetic tail stores energy and then releases it.

First a calm arc drifts slowly toward the equator. Then, often near local magnetic midnight, it suddenly breaks up: it brightens many times over, folds into curtains, and races across the sky. Afterwards it fades into soft, pulsing patches. The cycle can repeat several times a night.

GrowthQuiet arc · 30–60 min

BreakupBright, fast curtains · 10–30 min

RecoveryPulsing patches · 1–2 h

Fig. 7 · A substorm seen looking toward the magnetic pole, compressed from about two hours into 24 seconds.

08 · The long rhythmThe 11-year solar cycle

The Sun's activity rises and falls over roughly 11 years, tracked by counting sunspots. More sunspots mean more eruptions and more storms.

Cycle 25 peaked in late 2024 and is now declining. That does not mean the show is over: some of the biggest storms on record came in a cycle's declining years, such as the Halloween storms of 2003.

Fig. 8 · Monthly sunspot number (faint) with its 13-month average (bright), 1950 to today, and NOAA's forecast to 2030 with its range. Data: NOAA Space Weather Prediction Center.

09 · SeasonsWhy the equinoxes run hot

Geomagnetic storms are more common around the March and September equinoxes than in midwinter or midsummer. Around the equinoxes, Earth's tilt lines its magnetic field up so that a southward-pointing solar wind field connects more easily. Scientists call this the Russell–McPherron effect.

Fig. 9 · Schematic, not measured data. Relative storm activity through the year.

10 · Both polesNorth and south at once

The northern and southern lights are twins. The same magnetic field lines that end over Alaska curve through space and land over Antarctica, so displays in the two hemispheres often happen at the same time and look like rough mirror images. Scientists call these conjugate aurora.

Fig. 10 · A single magnetic field line links the two auroral zones. Not to scale.

11 · Odd and wonderfulStranger than it looks

A few things people often ask about, and a few they do not know to ask.

STEVE

A narrow purple ribbon, sometimes with a green "picket fence" below it, that aurora photographers kept catching. Studied seriously from 2016 with help from those photographers, it turned out not to be an aurora at all.

It comes from a fast river of very hot gas in the upper atmosphere rather than particles raining in. The name stands for Strong Thermal Emission Velocity Enhancement.

Can you hear it?

Some observers report faint crackling or clapping during strong displays. For a long time scientists doubted it, since the aurora is far too high for sound to carry.

Recordings in Finland suggest some sounds may come from electric charges discharging in a layer of air about 70 m above the ground during geomagnetic activity. It is still debated.

Other worlds

Any planet with an atmosphere and a magnetic field can have aurora. Jupiter's are hundreds of times more energetic than Earth's, fed partly by material from its volcanic moon Io. Saturn, Uranus and Neptune have them too.

Mars has no global magnetic field, but spacecraft have still seen patchy aurora there.

Power and GPS

The same storms that paint the sky push electric currents through long power lines and pipelines. In March 1989 a severe storm collapsed the Hydro-Québec grid, leaving millions without power for about nine hours.

Storms also disturb GPS accuracy and high-frequency radio, which is why NOAA issues space weather alerts.

Cameras see more

In faint displays, your eyes switch to their night vision, which sees little color. A camera on a long exposure keeps adding light, so it often shows vivid green and red when you only see a pale gray glow.

That is why a phone in night mode is a good first test on a doubtful night.

The great storms

The Carrington Event of September 1859 sent aurora toward the tropics and sparked telegraph lines. The storm of May 2024 was the strongest in about two decades, reaching G5 with aurora photographed far outside the usual zones.

Records like these are why space weather is now forecast like ordinary weather.

Sources: NOAA SWPC, Aurora · NOAA Space Weather Scales · NOAA Solar Cycle Progression · NASA, Auroras