Mercury's magnetic field is approximately a magnetic
dipole (meaning the field has only two magnetic poles) that is significant, and apparently global, on
planet Mercury. Data from
Mariner 10 led to its discovery in 1974; the spacecraft measured the field's strength as 1.1% that of
Earth's magnetic field. The origin of the magnetic field can be explained by
dynamo theory, and because the magnetic field is strong enough near the
bow shock, it slows down the
solar wind, which induces a
magnetosphere.