The distinguished limit for energetically robust large scale dynamos driven by rapidly rotating convection*

Natural dynamos, such as the Earth's outer core, generate global scale magnetic field despite the inferred presence of small scale convectively-driven turbulence. Helicity, which is a measure of the correlation between the velocity and vorticity fields, is thought to be an important ingredient for the generation of these global scale magnetic fields. Previous simulations show that as the buoyancy forcing is increased the relative helicity decreases and the small scales of the magnetic field become energetically dominant. However, asymptotic theory predicts that energetically robust large scale dynamos, defined as large scale dynamos in which the largest scale of the magnetic field is energetically dominant relative to the small scale magnetic field, are preferred when a particular, or distinguished, limit is taken that is characterized by rapid rotation and an induction equation balance in which stretching and diffusion are comparable on the small scales. Here we use a suite of direct numerical simulations of rotating convection-driven dynamos in the asymptotic regime that confirms this theory, demonstrating that energetically robust large scale dynamos can be generated for strongly forced turbulent convection despite small relative helicity.