Characterization of magnetostrophic liquid metal convection*

In magnetostrophic convection, the fluid dynamics are dominated by the Elsasser number, $\Lambda$, being of order 1, which describes the ratio of the Lorentz and Coriolis forces. Linear theory predicts magnetostrophic $\Lambda \approx 1$ convection will develop via nearly system-scale modes, leading geophysicists to argue that these modes exist in planetary cores and generate their large-scale dynamo fields. To date, however, magnetostrophic convective modes have yet to be unambiguously identified in laboratory experiments, nor have they been clearly found in liquid metal ($Pm < Pr < 1$) planetary dynamo simulations. Thus, the characterization of magnetostrophic modes is crucial for understanding how dynamo processes operate in deep planetary interiors. To detect and characterize the magnetostrophic regime, we use ultrasonic doppler velocimetry (UDV) and multi-point thermometry to measure the velocities ($Re$) and the global heat transfer efficiency ($Nu$) in a cylindrical rotating magnetoconvection experiment of aspect ratio $\Gamma = D/H=2$ that is filled with liquid gallium ($Pr = 0.027; \ Pm \simeq 10^{-6}$). We will present magnetostrophic experimental results in a range of Rayleigh numbers $10^5 < Ra < 10^7$, and Ekman numbers $5\times 10^{-5} < Ek < 10^{-4}$. We will present vertical and horizontal chord velocity measurements, in which we are seeking to detect the magnetostrophic modes and separate them out from the oscillatory and geostrophic modes that all co-exist in these parameter ranges.