Magnetospheres
Magnetic Fields, Solar Wind & Auroras
Solar wind streams in from the left. The bow shock decelerates it, the magnetopause deflects it, the magnetotail stretches downwind. Particles that leak through the polar cusps spiral down field lines and excite the auroral ovals.
A planet generates a global magnetic field when its interior hosts a convecting electrically conductive fluid — Earth's liquid Fe-Ni outer core, Jupiter's metallic hydrogen mantle, or the icy ionic fluid in Uranus and Neptune.
The fluid's motion plus the planet's rotation stretches and twists existing field lines into a self-sustaining dipole. Stop the convection (Mars) or stop the rotation (Venus) and the dynamo dies.
Without a magnetosphere, atmospheric retention drops by 100–1000×.
Solar-wind particles that breach the polar cusps spiral down magnetic field lines and slam into the upper atmosphere near the magnetic poles, exciting atmospheric atoms into glowing rings — the auroral ovals.
Colour comes from atomic species: O at high altitude → red, O at lower altitude → green, N₂ → pink/violet. The ED engine renders aurora-like effects on certain bodies in scenic systems.
Jupiter's auroras are 100× brighter than Earth's and continuously powered by Io's volcanic plume — not just the solar wind.
| Body | Strength | Source | Retention | Note |
|---|---|---|---|---|
| Mercury | 1% of Earth | Liquid Fe core (weak dynamo) | Lost most volatiles | Compressed magnetosphere — no atmosphere shield |
| Venus | Negligible | Induced by solar wind only | CO₂ retained by mass | No internal dynamo — high atmospheric loss |
| Earth | 1.0× (reference) | Liquid Fe outer core | Strong protection | Aurora borealis/australis from cusp particles |
| Mars | Crustal patches only | Lost dynamo ~4 Gyr ago | Atmosphere stripped | Magnetic anomalies in southern crust |
| Jupiter | 20,000× Earth | Liquid metallic-H mantle | Massive — extends past Saturn | Most powerful in the system; deadly radiation |
| Saturn | 600× Earth | Metallic H, smaller core | Strong | Aurora visible at radio + UV wavelengths |
| Uranus | 50× Earth | Ionic water mantle (offset) | Strong | Magnetic axis tilted 60° from rotation axis |
| Neptune | 30× Earth | Ionic water mantle (offset) | Strong | Magnetic axis offset from planet centre |
| Ganymede | 1% of Earth | Internal dynamo (only moon!) | — | Embedded inside Jupiter's magnetosphere |