This picture is from a model of how the earth's magnetic field is generated.
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Glaztmeir and Roberts of LANL
The Generation of Neptune's Magnetosphere
Magnetospheres are generated with 1.) magnetic materials and 2.) with movement of the magnetic material.
The Earth-like planets generate magnetospheres within the iron cores at their centers. However, Neptune has almost no iron core.
Neptune's magnetic field is created by movement in its icy shell.
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