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Nonlinear cascades in two-dimensional turbulent magnetoconvection

The dynamics of spectral transport in two-dimensional turbulent convection of electrically conducting fluids is studied by means of direct numerical simulations (DNS) in the frame of the magnetohydrodynamic (MHD) Boussinesq approximation. The system performs quasi-oscillations between two different regimes of small-scale turbulence: one dominated by nonlinear MHD interactions, the other governed by buoyancy forces. The self-excited change of turbulent states is reported here for the first time. The process is controlled by the ideal invariant cross-helicity, $H^\mathrm{C}=\int_S \mathrm{d}S \mathbf{v}\cdot\mathbf{b}$. The observations are explained by the interplay of convective driving with the nonlinear spectral transfer of total MHD energy and cross-helicity.