Recorded 27 January 2025. Alban Pothérat of Coventry University presents "Magnetohydrodynamic (MHD) approximations in geophysical rotating convection" at IPAM's Rotating Turbulence: Interplay and Separability of Bulk and Boundary Dynamics Workshop. Abstract: The framework for understanding the interplay between magnetic fields and planetary interiors is dominated by the dynamo problem: what mechanism in the fluid mechanics of planetary interiors produces the planetary magnetic fields that we observe? Fully answering this question requires to take into account the two-way coupling between electromagnetic and mechanical quantities. The full magnetohydrodynamic (MHD) equations are the golden standard for this purpose, and for that reason, they are an expected ingredient of most numerical simulations aiming for planetary realism. They are, however, numerically costly, and since they incorporate all MHD effects, diffusive, propagative, advective, for both magnetic and mechanical quantities, they make it sometimes difficult to identify the exact magnetohydrodynamic mechanism at play. For these reasons, the full MHD equations are not always best suited to understand all aspects of planetary cores. Instead, depending on their specific timescales and length scales, convection, waves and even slowly evolving global flow patterns can be captured by more restrictive approximations, such as the low-Rm approximation or the quasi-static MHD approximation, whose simpler form offers greater physical insight. A major benefit of this approach is to guide the interpretation of laboratory experiments in planetary context. Examples of such experiments will be presented, and in particular, recent experiments showing how the Earth's magnetic field may control the flow into the Tangent Cylinder. Learn more online at: https://www.ipam.ucla.edu/programs/workshops/rotating-turbulence-interplay-and-separability-of-bulk-and-boundary-dynamics/