The magnetic field of Mercury as measured by the MESSENGER spacecraft
Séminaires Planétologie et Sciences Spatiales
Salle 727 - Lamarck A -
Studying planetary magnetic fields may bring clues for understanding the physical processes of a planetary interior. Mercury is the only terrestrial planet (except the Earth) that has a core dynamo that generates a global magnetic field. Hermean magnetic field measurements acquired over the northern hemisphere by the MESSENGER spacecraft provide crucial information on the magnetic field of the planet. The Time-Dependent Equivalent Source Dipole method is developed here to model magnetic field and possible secular variation with measurements distributed over a limited spatial region. Tests with synthetic data distributed on regular grids as well as at spacecraft positions confirm the validity of the method. The method is applied to almost three terrestrial years of MESSENGER measurements. Ignoring the secular variation terms, sidereal and solar day Hermean field models are computed and interpreted. A dominantly zonal field with small-scale non axisymmetric features co-rotating with the Sun in Mercury Body Fixed is found in the sidereal day models. When modeling the field during one complete solar day, these small-scale features decrease and the field becomes even more axisymmetric. However, comparing different solar day models reveal a strong variability. The lack of any coherent non-axisymmetric feature recovered by our method, which was designed to allow for such small-scale structures, provides strong evidence for the large-scale and close-to-axisymmetry structure of the internal magnetic field of Mercury.