Aurora Borealis
Charged solar wind particles spiral down magnetic field lines and collide with atmospheric atoms, exciting them to emit light. Adjust the solar wind, magnetic field tilt, and particle energy to sculpt the curtains.
The Physics of Aurora
Auroras arise when the Sun ejects a stream of charged particles — the solar wind — which is deflected by Earth's magnetic field. Along field lines near the magnetic poles, particles funnel into the upper atmosphere (90–300 km altitude) and collide with oxygen and nitrogen atoms.
These collisions excite electrons to higher energy levels. As they fall back, photons are emitted at characteristic wavelengths. Oxygen at ~110 km glows green (557.7 nm), oxygen at higher altitudes glows red (630 nm), and nitrogen produces blue/purple hues. The colors depend on altitude and particle energy.
Particles spiral along field lines via the Lorentz force: F = q(v × B). The gyro-radius shrinks as they descend into denser magnetic field regions (mirror force), but sufficiently energetic particles penetrate to the emission altitude. The spiral pitch angle — the ratio of perpendicular to parallel velocity — determines how deep they reach.
The Kp index (0–9) measures global geomagnetic disturbance. During a major geomagnetic storm (Kp ≥ 7), the auroral oval expands equatorward and aurora can be seen at mid-latitudes. The curtain structure arises from field-aligned current sheets driven by magnetospheric dynamics.