Aurora Borealis
Solar particles → Earth’s field → aurora. Builds on Lorentz Force.
You’re in northern Norway watching a spectacular aurora borealis (Northern Lights) — curtains of green and red light dancing across the sky. Your friend says it’s just sunlight reflecting off ice crystals in the upper atmosphere.
What actually causes the aurora?
The aurora is NOT reflected sunlight — it’s atmosphere glowing! Charged particles streaming from the Sun (the solar wind) encounter Earth’s magnetic field. The field deflects most particles, but some get funneled along field lines toward the magnetic poles. When these high-energy particles collide with oxygen and nitrogen molecules 100–300 km up, the molecules get excited and emit light: green from oxygen at ~100 km, red from oxygen at ~200 km, and blue-purple from nitrogen.
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Watch charged particles from the solar wind encounter Earth’s magnetic field. See how they spiral along field lines and converge at the poles. Where they hit the atmosphere, the aurora glows — green at lower altitudes, red higher up.
F = qv × B
The cross product means the force is perpendicular to both velocity and field — so the particle spirals along the field line. Near the poles, the converging field lines funnel particles into the atmosphere.
Without Earth’s magnetic field, the solar wind would strip away our atmosphere. Mars lost its magnetic field billions of years ago, and most of its atmosphere was blown away by the solar wind.
During solar storms, intense particle bombardment can damage satellite electronics, disrupt GPS signals, and even knock out power grids on Earth — the same particles that make auroras beautiful can be destructive.
Jupiter, Saturn, and even Uranus and Neptune have auroras. Jupiter’s aurora is powered partly by its moon Io’s volcanic particles getting caught in Jupiter’s enormous magnetic field.
Scientists monitor the Sun for coronal mass ejections (CMEs) that can trigger geomagnetic storms. A CME can make auroras visible as far south as Florida or Texas — and can threaten power infrastructure.
“Solar particles ride Earth’s magnetic highway to the poles, where they paint the sky in green, red, and purple. The aurora is the Lorentz force made beautiful.”
Aurora Lab
Explore how solar wind particles interact with Earth’s magnetic field to create the aurora. Adjust solar wind speed and density, tilt Earth’s magnetic axis, and see how the auroral oval changes. Explore the altitude-dependent colors.
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Start with moderate solar wind and watch particles spiral toward the poles. Increase solar wind density — the aurora brightens and the oval expands toward the equator. Toggle field lines to see the magnetic funnel. Use the altitude slider to explore colors: green dominates at 100 km, red at 200 km, blue-purple from nitrogen at the edges. Try tilting Earth’s axis to see how it shifts the aurora.
The aurora is the Lorentz force in action at planetary scale. Charged solar wind particles spiral along Earth’s magnetic field lines and are funneled toward the poles. The colors come from specific atmospheric gases excited at specific altitudes: green oxygen at ~100 km, red oxygen at ~200 km, blue-purple nitrogen.