★★☆ Moderate

🌌 Aurora Borealis

Watch electrons from the solar wind spiral along Earth's magnetic field lines, crash into oxygen and nitrogen atoms at 80–300 km altitude, and excite them into their characteristic aurora colours. Adjust solar wind intensity, particle energy, and geomagnetic latitude.

Activity:

Solar wind intensity (Kp) Kp 3

Particle energy (keV) 10 keV

Geomag. latitude (°) 65°

Simulation speed 1.0×

Kp index: 3 Active particles: 0 Altitude range: 80–300 km Dominant colour: Green (O 557.7 nm)

Green — O(¹S) at 100–150 km

Red — O(¹D) at 200–300 km

Purple/Blue — N₂ at 80–100 km

The Physics of Auroras

Solar-wind electrons (and some protons) travel along open field lines into the polar cusps. They spiral around field lines due to the Lorentz force F = qv × B (gyro-radius r = mv⊥/qB). Colliding with O and N₂ in the thermosphere, they excite electrons to higher orbitals. De-excitation emits photons: green (O at 557.7 nm, 100 km), red (O at 630 nm, >200 km), and blue/purple (N₂, <100 km). High Kp index = stronger solar wind = aurora visible at lower latitudes.

Aurora Colours Explained

Green (557.7 nm) — Most common. Oxygen atom excited at 100–150 km altitude by electron impact. The ¹S → ¹D transition emits green light with a lifetime of ~0.7 s.

Red (630 nm) — High-altitude aurora (>200 km). Oxygen ¹D → ³P transition, but very slow (110 s lifetime) — only occurs where the atmosphere is sparse enough to avoid collisional quenching.

Purple/Blue — Low-altitude (<100 km). Molecular nitrogen N₂ excited electronic states. Very energetic particles are needed to reach this depth.

Kp index — Planetary geomagnetic disturbance index (0–9). Kp ≥ 5 is a geomagnetic storm. During extreme events (Kp 9, like the 2024 May storm) aurora is visible down to 40° latitude.