Field Strength at Mercury's Core-Mantle Boundary from Dipole Extrapolation and the Northward Offset Asymmetry

Mercury's magnetic field is generated at the top of a core that lies only about four hundred kilometres below the surface, closer in relative terms than in any other planet. This paper extrapolates the published dipole field inward to the core-mantle boundary with the inverse-cube law, and computes the north-south polar field asymmetry produced by the published northward offset of the dipole by a fifth of a planetary radius. The headline results are: dipole_extrapolation_factor = 1.762; equatorial_field_at_cmb_nT = 335; polar_field_at_cmb_nT = 669. The field at the core-mantle boundary is only about five hundred nanotesla at the pole, remarkably weak for a dynamo-sustaining region, and the offset dipole makes the northern polar surface field more than three times the southern, an asymmetry that maps directly onto hemispheric differences in solar-wind access to the surface.