The Hemispherical Magnetic Field of Ancient Mars

The measurements of the Martian crustal magnetization by the Mars Global Surveyor spacecraft revealed a characteristic equatorial dichotomy. Strong magnetic anomalies are found exclusively in the southern hemisphere, whereas the north is nearly devoid of any magnetization. The strong magnetization might attest the presence of an ancient Martian core dynamo, but to explain the hemispherical distribution is rather challenging. Here we study, whether it is possible to modify a core dynamo such that the magnetic field at the surface matches the crustal magnetization in amplitude and equatorial dichotomy. Taking the thermal state of the ancient Martian interior into account, we enforce hemispherical dynamos by applying a sinusoidal anomaly to the core mantle boundary heat flux. We show in detail, how convection and magnetic induction are affected by the boundary forcing. Furthermore, we investigate the dependence on model parameters and boundary conditions. The core mantle boundary heat flux anomaly consistently triggers a thermal wind dominated dynamo mode that induces a magnetic field confined to one hemisphere. Interestingly, all dynamo solutions that match the crustal hemisphericity show fast and regular oscillations including polarity inversions. We analyze if and how such a hemispherical magnetic field can be translated into a crustal magnetization pattern and conclude that the fast magnetic cycles are incompatible with the slowly acquired crustal magnetization. The special symmetries of flow and magnetic field in the boundary forced hemispherical dynamos allow us to compare the cycle periods to Parker waves, where we find a surprisingly good agreement.