Mercury's magnetic field is approximately a magnetic dipole (meaning the field has only two magnetic poles)[8] apparently global,[9] on planet Mercury.[10] 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.[11] The origin of the magnetic field can be explained by dynamo theory.[12] The magnetic field is strong enough near the bow shock to slow the solar wind, which induces a magnetosphere.[13]