UCLA

The magnetic fields of planets and other bodies are created and sustained due to the turbulent motions of an internal fluid layer, a process known as dynamo action. Forward models are required to characterize the dynamics of rotating convective turbulence driving dynamo action due to the inability to obtain direct measurements of the internal fluid layers of planetary bodies. The characteristic flow velocities and length scales of dynamo systems remain poorly constrained due to the difficulty of modeling realistic planetary core conditions. Thus, the goal of this dissertation is to explore these key properties of core-style convection. To do so, I have conducted novel experiments aimed to better quantify the features of quasi- geostrophic turbulence using the UCLA large-scale rotating convection device, ‘NoMag’.

I have completed a systematic study to simultaneously measure the heat transfer and bulk velocities of different rotating convective regimes at some of the most extreme laboratory conditions possible to date. The study of heat transfer is employed in most forward models of core-style convection. In laboratory experiments in particular, due to the relative difficulty of collecting velocity measurements, those of heat transfer alone are assessed, the dynamics of which are assumed to describe the the bulk velocity dynamics of the system. On the contrary, I utilize laser doppler velocimetry to obtain measurements of bulk velocities concurrently with the collection of temperature measurements for the characterization of system heat transfer. I find that heat transfer behavior is consistent with the results of past studies and is largely controlled by boundary layer dynamics. I further find that velocity behaviors do not directly coincide with heat transfer behaviors in the parameter space studied. Instead, I show that a dynamical flow regime of quasi-geostrophic turbulence relevant to core flows is robustly reached, suggesting that it is possible to access realistic bulk dynamics in models that remain far from planetary core conditions.

Using the results of this study, I estimate the characteristic length scales of the flows of each experiment. These estimates from my data are compared with length scale estimates of numerous numerical models of planetary core convection. I conclude from this meta-analysis of forwards models that all evidence to date suggests that the theorized characteristic length scales of planetary dynamo systems co-scale with one another and are thus non-separable.

In two other studies that comprise the remainder of this dissertation, I further examine the applicability of laboratory models towards planetary settings. An experimental study on the influence of centrifugal buoyancy on rotating convection in water and in liquid metal was completed, where results agree with the recent numerical work of Horn and Aurnou (2018). It is found that the transition from Coriolis to centrifugally dominated convection depends on the strength of the centrifugal buoyancy relative to the gravitational buoyancy and the geometry of the cylinder in which experiments are conducted. These results are useful to ensure that the regime of rotating convection explored in a given experiment is relevant to planetary core flows, i.e. not centrifugally dominated. Separately, I conducted a series of spin up experiments with well-established theory to calibrate the NoMag apparatus and its measuring components. Further, the results from spin up experiments conducted with rough boundaries might have geophysical implications for the possible viscous coupling at Earth’s core mantle boundary, as well as turbulent mixing in the global ocean.

The results of the studies presented in this dissertation clarify the relevance of long theorized and poorly tested dynamic length and velocity scalings of planetary core flows. Flows that are quasi-geostrophically turbulent are robustly observed in the laboratory data collected in this dissertation. The need for next generation models of planetary core flows is motivated by the results of the work herein. In particular, studies in which the characteristic length scales of core-style flows are directly quantified will undoubtedly enhance our

understanding of the multi-scale turbulent physics driving planetary dynamo systems.