The effect of thermal boundary conditions on dynamos driven by internal heating

The early dynamos of Mars and Earth probably operated without an inner core being present. They were thus exclusively driven by secular cooling and radiogenic heating which can both be modeled by homogeneously distributed heat sources. Some previous dynamo simulations that explored this driving mode found dipole dominated magnetic fields, while other reported multipolar configurations. Since these models differed both in the employed outer thermal boundary conditions and in the size of the inner core, which was still retained for practical reasons, the cause for the variation in field geometry remained unclear. Here we investigate this issue and find that strong dipole dominated fields are preferred for fixed heat flux conditions whereas weaker multipolar fields are typical for fixed temperature conditions. The size of the inner core, on the other hand, proved to be of minor influence. The stronger dipolar fields for fixed heat flux conditions promote larger convective structures. Since the mantle of the terrestrial planets controls the heat flux rather than the temperature at the core-mantle boundary, our results suggest that the early dynamos of Mars and Earth would have produced dipole dominated magnetic fields.