A laboratory study of turbulent magnetoconvection: Could thermoelectricity induce asymmetry in geomagnetic secular variation?

It has been proposed that thermoelectric (TE) currents may be important in the vicinity of planetary core boundaries (Stevenson 1987, EPSL; Giampieri & Balogh 2002, P&SS). However, TE-induced core dynamics remain largely unstudied. To address this, we have conducted a series of laboratory experiments of turbulent Rayleigh-Bénard convection with a vertical magnetic field in a cylindrical cell filled with liquid gallium. Thermal measurements are taken at a fixed buoyancy forcing with varying Lorentz force. When buoyant inertia dominates, a large-scale overturning circulation cell develops, which imposes strong lateral temperature gradients onto the tank's top and bottom boundaries. In experiments equipped with electrically conducting boundaries, the large-scale circulation slowly precesses in azimuth when thermoelectrically induced Lorentz forces become comparable to buoyant inertial forces. Moreover, TE introduces an asymmetry in the system: this novel magnetoprecessional mode reverses its traveling direction when the magnetic field polarity is reversed. Extrapolating our results to Earth's core, we estimate the required net Seebeck coefficient to generate TE dynamics at CMB conditions. Furthermore, because TE-driven flows reverse direction as the magnetic field reverses, we hypothesize that thermoelectricity can provide a natural symmetry breaker by driving CMB (or ICB) core flows in opposite directions between normal and reversed geomagnetic field polarities. To test our hypothesis, we need to better constrain the electrical, thermal conductivity, and Seebeck coefficient of the CMB (or ICB), and gather observational evidence of geomagnetic secular variation during field reversals. This study is reported in Xu et al. 2022, JFM