Closed (Ganymede) field lines
Open (Jovian-connected) field lines
Auroral ovals
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Ganymede is the only moon known to generate its own internal magnetic field, and that field is strong enough to carve out a genuine miniature magnetosphere entirely embedded inside Jupiter's colossal one. This simulator renders that structure in 3D: closed dipole field lines loop from Ganymede's own core out to a magnetopause standoff distance set by pressure balance against Jupiter's field, while open field lines beyond the polar caps connect straight out into the Jovian magnetosphere. Because Jupiter's magnetic dipole is tilted relative to its spin axis, the background field Ganymede sits in rotates past every ~10 hours, periodically favoring reconnection and shrinking the closed-flux region — the real mechanism behind the observed rocking of Ganymede's auroral ovals. Adjust the dipole strengths and rocking speed and watch the standoff distance, oval colatitude, and open/closed state respond live.