Final answer:
Jupiter's magnetic field was discovered through radio emissions, while the magnetic fields of Saturn, Uranus, and Neptune were discovered by spacecraft observations.
Step-by-step explanation:
The magnetic field of Jupiter was discovered by its radio emissions while the magnetic fields of Saturn, Uranus, and Neptune were discovered by spacecraft. The Van Allen radiation belts are similar belts of charged particles that are trapped by Earth's magnetic field. These belts were named after James A. Van Allen, an American astrophysicist who played a crucial role in their discovery. Radio telescopes, such as the Arecibo Radio Telescope and the Parkes Observatory, are used to detect these types of electromagnetic waves from space. Other planets, especially those with strong magnetic fields like Jupiter, have similar belts associated with their magnetic fields.