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Convection in the liquid outer core of the Earth is driven by thermal andchemical perturbations. The main purpose of this study is to examine the impactof double-diffusive convection on magnetic field generation by means of 3Dglobal geodynamo models, in the so-called "top-heavy" regime ofdouble-diffusive convection, when both thermal and compositional backgroundgradients are destabilizing. Using a linear eigensolver, we begin by confirmingthat, compared to the standard single-diffusive configuration, the onset ofconvection is facilitated by the addition of a second buoyancy source. We nextcarry out a systematic parameter survey by performing $79$ numerical dynamosimulations. We show that a good agreement between simulated magnetic fieldsand the geomagnetic field can be attained for any partitioning of theconvective input power between its thermal and chemical components. On thecontrary, the transition between dipole-dominated and multipolar dynamos isfound to strongly depend on the nature of the buoyancy forcing. Classicalparameters expected to govern this transition, such as the local Rossby numberor the degree of equatorial symmetry of the flow, fail to capture the dipolebreakdown. A scale-dependent analysis of the force balance instead reveals thatthe transition occurs when the ratio of inertial to Lorentz forces at thedominant length scale reaches $0.5$, regardless of the partitioning of thebuoyancy power. The ratio of integrated kinetic to magnetic energy $E_k/E_m$provides a reasonable proxy of this force ratio. Given that $E_k/E_m \ll 1$ inthe Earth's core, the geodynamo is expected to operate far from thedipole-multipole transition. It hence appears that the occurrence ofgeomagnetic reversals is unlikely related to dramatic and punctual changes ofthe amplitude of inertial forces in the Earth's core, and that anothermechanism must be sought.

Geomagnetic semblance and dipolar-multipolar transition in top-heavy double-diffusive geodynamo models