First, the size of Jupiter means that it has a larger core than the other planets.
A larger core means more core particles are interacting to generate a stronger electrical current. The other factor is the speed of Jupiter's rotation. Jupiter's strong magnetic field is generated in Jupiter's thick layers of metallic hydrogen.
Its magnetic field creates a magnetosphere to surround the planet and shield it from solar winds. It traps more charged particles than Earth's. They create belts of intense radiation around Jupiter.
The strong magnetic field is produced by the rotation of the planet and convection in a metallic core. Because Jupiter is so large and spins so fast, its magnetic field is very strong. It is over 20,000 times stronger than Earth's.
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