A time-averaged regional model of the Hermean magnetic field

•First quasi-hemispheric magnetic field model based on the entire MESSENGER’s mission lifetime.•First regional model in the Northern hemisphere of Mercury to the spatial resolution of about 900km downward continued to the core-mantle boundary.•First order separation of internal and external fields at Mercury over the Northern hemisphere.•A new estimate for the internal axial quadrupole to axial dipole ratio of 0.27 was derived. This paper presents the first regional model of the magnetic field of Mercury developed with mathematical continuous functions. The model has a horizontal spatial resolution of about 830km at the surface of the planet, and it is derived without any a priori information about the geometry of the internal and external fields or regularization. It relies on an extensive dataset of the MESSENGER’s measurements selected over its entire orbital lifetime between 2011 and 2015. A first order separation between the internal and the external fields over the Northern hemisphere is achieved under the assumption that the magnetic field measurements are acquired in a source free region within the magnetospheric cavity. When downward continued to the core-mantle boundary, the model confirms some of the general structures observed in previous studies such as the dominance of zonal field, the location of the North magnetic pole, and the global absence of significant small scale structures. The transformation of the regional model into a global spherical harmonic one provides an estimate for the axial quadrupole to axial dipole ratio of about g20/g10=0.27. This is much lower than previous estimates of about 0.40. We note that it is possible to obtain a similar ratio provided that more weight is put on the location of the magnetic equator and less elsewhere.