Aurelia

Meet Aurelia, a curtain of light painted by charged particles from the Sun.

The Sun streams out a thin wind of electrons and protons. Earth's magnetic field deflects moving charges with the force F = qv × B, which acts sideways to both the motion and the field. Charges therefore spiral along field lines instead of crossing them, and the field lines lead down toward the polar regions. Energised particles striking the upper atmosphere, roughly 100 to 300 km up, excite oxygen and nitrogen, which then glow.

How big is the push? A proton (q = 1.602 × 10⁻¹⁹ C) moving at 400 km/s across a 50 µT field feels F = 1.602 × 10⁻¹⁹ × 4 × 10⁵ × 5 × 10⁻⁵ ≈ 3.2 × 10⁻¹⁸ N, tiny but huge for such a light particle.

Oxygen gives the familiar green and rarer red; nitrogen adds blue and purple. Space-weather forecasters watch solar activity to predict when aurorae will be seen farther from the poles.

A simplification: most auroral particles are accelerated within Earth's magnetosphere, not delivered straight from the solar wind. The same physics also lights aurorae on Jupiter and Saturn.