What are electric and magnetic fields? A field is an influence or force that exists throughout space, that one body exerts on another. Gravity is one such field. All matter responds to gravity, but if matter is electrically charged or contains currents, it also responds to electric and magnetic forces. Electric charges and changing magnetic fields produce electric fields. Here on Earth, we have learned how to produce electricity by using motors, called generators, which cause magnetic fields to change. These changing magnetic fields create the electric current we use in our homes. Electric charges in motion (an electric current) and changing electric fields produce magnetic fields. For instance, flows of molten metals deep inside Earth's core cause currents which sustain a huge magnetic field that extends far into space. As seen in the diagram above, magnetic fields are represented by lines of force. When the lines are close together, the force of the field is great; when they are far apart, the force is weak. Earth's magnetic field lines spread out from the south polar region and come together in the north polar region. The field is strong at the poles and weak at Earth’s equator. Charged particles are guided by Earth's magnetic field (see Protective Shield for information about this fascinating interaction). Understanding how plasmas interact with electric and magnetic fields gives us a better idea of what is happening between the Sun and Earth and elsewhere in the universe, but many mysteries remain.