The effect of a vertical uniform magnetic field on Rayleigh-Bénard convection is investigated experimentally. We confirm that the threshold of convection is in agreement with linear stability theory up to a Chandrasekhar number $Q\simeq 4\times {10}^6,$ higher than in previous experiments. We characterize two convective regimes influenced by MHD effects. In the first one, the Nusselt number Nu proportional to the Rayleigh number Ra, which can be interpreted as a condition of marginal stability for the thermal boundary layer. For higher Ra, a second regime ${\mathrm{Nu}\mathrm{\sim }\mathrm{Ra}}^{0.43}$ is obtained.

• Received 3 July 2000

DOI:https://doi.org/10.1103/PhysRevE.62.R4520