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Strength of Mercury's magnetic field MESSENGER measured the strength of Mercury's magnetic field during its two 2008 flybys. Despite profound differences in the geometry of the solar wind magnetic field during the two encounters, and despite the fact that each flyby passed over a different hemisphere, the strength of the magnetic field matched to within two percent for the two different flybys. This suggests that Mercury's field is generated within its core, and that it is tightly aligned with the rotational axis. NASA / JHUAPL / CIW