The concept of energy provided a unifying link for mechanical and thermal phenomena (remember our microscopic description of temperature as the motion of atoms). The concept of field brought electricity, magnetism and light into a common framework. A field can be considered a type of energy in space, or energy with position.

The easiest field to understand is the magnetic field from a bar magnet or electric coil. An electric current is simply the motion of electron or ions. Their motion produces a magnetic interaction that we call a magnetic field.

The concept of a field is a mathematical construct to help visualize the effects of magnetic forces. The lines of the field indicate the direction a particle will move (be forced) if dropped into the field at that point.

Note also that the strength of the field determines how hard the particle will be forced. And the more bunched the field lines, the stronger the magnetic force. For Jupiter, this means the magnetic forces direct charged particles towards the poles (north for positive charges, south for negative charges). And the magnetic field is strongest at the poles