How does the aurora form? The Sun blows a stream of charged particles, the solar wind, past Earth, and our magnetic field steers them. Here you watch it happen: particles spiral along the field lines, and the ones aimed almost straight down a line reach the upper atmosphere near both poles and light it up in the real colours of the northern and southern lights. The rest bounce back and forth, trapped in the Van Allen belts. Turn up the solar wind for a storm, or strengthen the field and watch the spirals tighten. It's the full-screen version of the aurora mode in our charged particle lab.
Charged particles from the solar wind follow Earth's magnetic field. The ones aimed along a field line spiral down into the atmosphere and make it glow.
Reaching the atmosphere0 /s
Trapped in radiation belts0
Spiral radius (r = mv/qB)1.00×
Particles moving almost along the field (inside the loss cone) reach the atmosphere.
Particles with steeper pitch angles mirror and bounce between the hemispheres, trapped in the Van Allen belts.
Green: oxygen at ~100–250 km (557.7 nm) Red: oxygen above ~250 km (630 nm) Purple: nitrogen at the lower edge
Not to scale: the aurora is only ~100–400 km high, and its height is exaggerated here so it shows up.
The aurora is drawn only where particles actually land. Nothing here is painted on: each glowing ray marks a spot where a simulated particle reached the atmosphere. That puts the light on a ring around each magnetic pole, at about 60–65° latitude, which is exactly where the real auroral ovals sit, and never on the pole itself. Want to know whether you'll see one tonight? Check the live aurora forecast.
How the Aurora Forms
From the Sun to a glowing curtain, in three steps you can see in the simulation.
1
The solar wind arrives
The Sun streams out electrons and protons at hundreds of kilometres per second. Earth's magnetic field deflects most of that solar wind around the planet (the streaks fading at the left edge), but some particles are captured into the magnetosphere, mostly from the long tail on the night side. That's why the right-hand (night-side) ovals glow brighter. The solar wind slider sets how many arrive, from a calm day to a geomagnetic storm.
2
The magnetic field steers them
A magnetic force always acts sideways to a particle's motion, so it can't speed it up, only bend its path into a spiral around a field line. Near the poles the field lines crowd together, and the stronger field reflects most particles back (the magnetic mirror), so they bounce between hemispheres, trapped in the Van Allen belts (orange). Only particles moving almost along the field, inside the loss cone, make it all the way down (cyan).
3
Collisions make the light
Those particles slam into the upper atmosphere 100–400 km up. They excite oxygen and nitrogen, which give the energy back as light: green from oxygen around 100–250 km, red from oxygen higher up, and purple from nitrogen along the lower edge. It happens in both hemispheres at once, as the northern lights (aurora borealis) and the southern lights (aurora australis).
How It Works
Real force law, real trajectories, and an aurora that comes out of the physics instead of being drawn in.
1
The Lorentz force and a Boris pusher
Each particle feels F = q·v×B from an approximate slice of Earth's dipole field. We advance it with a Boris pusher, the standard scheme for charged particles, which rotates the velocity by the exact gyro-angle each step. Speed is conserved to machine precision, so spirals never drift outward. It is the same code that drives our charged particle lab.
2
Loss cone versus trapped
Particles are released at the equator of the outer field lines (4.2–6.2 Earth radii). About two-thirds move almost along the field and follow it down into the atmosphere; the rest have steep pitch angles and mirror before they get there. The readouts count both. The field strength slider multiplies the planet's field: the spiral radius r = mv/qB shrinks in proportion, so tripling the field makes spirals a third as wide.
3
Where the light goes
Every time a particle reaches the atmosphere, the spot where it lands brightens and then fades over a couple of seconds. Those spots add up to the curtains, so the ovals, their brightness and the day–night difference all come straight from the simulated trajectories. Curtain heights are exaggerated so they're visible at this scale.
How does the aurora form?
The Sun constantly blows out a stream of charged particles called the solar wind. Earth's magnetic field deflects most of it, but some particles are captured into the magnetosphere, mostly on the night side, and guided along the magnetic field lines toward the polar regions. Particles moving almost along a field line reach the upper atmosphere, roughly 100 to 400 km up, where they collide with oxygen and nitrogen. The collisions excite those atoms and molecules, and as they settle back down they give off light: the aurora.
Why is the aurora green, and what makes it red or purple?
Each colour comes from a particular gas at a particular height. The common green glow is atomic oxygen emitting at 557.7 nm, at around 100 to 250 km. Above about 250 km, oxygen emits a deep red at 630 nm; that transition is slow, so it only survives high up where collisions are rare. Nitrogen gives the purple, blue and pink tints, often along the lower edge of a curtain.
Why do auroras happen near the poles?
The field lines that reach far out into space, where solar-wind particles are captured, come back down to Earth at high latitudes. So the particles land on a ring around each magnetic pole called the auroral oval, typically around 60 to 75 degrees magnetic latitude, rather than on the pole itself. The same thing happens in both hemispheres at once: the northern lights (aurora borealis) and the southern lights (aurora australis).
What is the loss cone, and why are some particles trapped?
As a charged particle spirals toward a pole, the field lines crowd together and the stronger field turns its forward motion into spinning, until it is reflected back: the magnetic-mirror effect. Whether it mirrors above the atmosphere depends on its pitch angle, the angle between its velocity and the field. Particles moving within a narrow cone around the field direction, the loss cone, reach the atmosphere and make aurora. The rest bounce between the hemispheres and stay trapped, which is how the Van Allen radiation belts hold their particles.
Is the simulator to scale?
No — the simulator is not drawn to scale: the aurora is only about 100 to 400 km high on a planet 6,371 km in radius, so the curtains are drawn several times taller to be visible. Particles move far more slowly than real ones so you can follow them, and the field is an idealised dipole in a single slice, whereas the real magnetosphere is squashed on the day side and stretched into a long tail on the night side. The physics of the motion (spiralling, mirroring and where particles land) is computed from the Lorentz force.
A ready-to-assign lab using the simulation above. Free to use — nothing to install or sign up for, and no student data leaves the browser.
High school, College10–20 minInteractive simulation
Learning objectives
Explain how the solar wind and Earth’s magnetic field produce the aurora.
Distinguish particles that reach the atmosphere (the loss cone) from particles trapped in the radiation belts.
Relate the aurora’s colours to the gases and heights that emit them.
Variables to change
Solar wind strength
Magnetic field strength
Procedure
Start at Moderate solar wind and field strength ×1.0, and note where on Earth the aurora appears.
Record the “Reaching the atmosphere” and “Trapped in radiation belts” readouts.
Raise the solar wind to Storm and record both readouts again.
Return to Moderate, raise the field strength to ×3.0, and compare the spirals and the spiral-radius readout.
Observations
Record where the aurora forms, how the two particle counts change with the solar wind, and how the spiral size changes with the field strength.
Questions
Where does the aurora appear, and why not exactly at the poles?
What happened to the rate of particles reaching the atmosphere when the solar wind increased?
Why do some particles bounce between the hemispheres instead of reaching the atmosphere?
Using r = mv/(qB), by what factor does tripling the field change the spiral radius?
Why is most aurora green, and what produces red and purple?
Explanation
Charged particles from the solar wind are captured into Earth’s magnetic field and spiral along the field lines. Those moving nearly along the field (inside the loss cone) reach the upper atmosphere on the auroral ovals, where collisions make oxygen and nitrogen glow; the rest are reflected by the magnetic mirror and stay trapped in the Van Allen belts.
Answer key (for instructors)
1. On a ring at about 60–65° latitude around each magnetic pole — the auroral oval — because the field lines that reach far into space, where particles are captured, meet the Earth there.
2. It rose — more incoming particles means more reach the atmosphere, so the aurora brightens, as in a geomagnetic storm.
3. Their pitch angle is too steep: as the field strengthens toward a pole, the magnetic mirror reflects them before they reach the air, trapping them in the radiation belts. Only particles in the loss cone get through.
4. It becomes one third (0.33×) — the radius is inversely proportional to B.
5. Green is oxygen emitting at 557.7 nm around 100–250 km; red is oxygen at 630 nm above about 250 km; purple and blue come from nitrogen lower down.
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