Core Dynamics, Core-mantle Interactions and Earth’s Rotation Variation Weijia Kuang

1. Topographic torque Time Magnetic torque Time Core Dynamics, Core-mantle Interactions and Earth’s Rotation Variation Weijia Kuang Space geodetic and geomagnetic observations demonstrated that Earth’s rotation rate, i.e. length of day (LOD) variation on decadal time scales results from exchange of angular momentum between the solid mantle and the liquid core. What core-mantle interaction mechanisms could produce a coupling torque sufficient for the observed decadal LOD variation? In the past decades, two coupling mechanisms, electromagnetic and topographic core mantle couplings, have been extensively studied and the coupling torques are estimated from geomagnetic observations together with assumptions on the properties of lower mantle and magnetic field and core flow near the core-mantle boundary (CMB). The electromagnetic coupling arises from magnetic force on an electrically conducting D”-layer. The topographic coupling arises from core pressure acting on non axi-symmetric CMB. However, the techniques (approximations) used successfully for axial core angular momentum decouple core dynamical processes from torque evaluation, and therefore could lead to erroneous (kinematic) coupling torques. We use our MoSST core dynamics model on two potential problems: the effect of the core flow evaluated from observed geomagnetic secular variation and a spherical CMB on topographic coupling (Figure 1) , and the effect of the toroidal field diffused to the D”-layer on electromagnetic coupling (Figure 2). We find that dynamically consistent torques (with our model) differ completely from those kinematic torques. Our results demonstrate the importance of dynamical consistencies on (1) core-mantle coupling torques, and (2) lower mantle properties to explain Earth’s rotation variation. Figure 1: The (non-dimensional) topographic torques from our MoSST model.. The kinematic torque (solid line) is more than an order of magnitude stronger than the dynamically consistent torque (dotted line), suggesting that inconsistencies between core flow and the CMB topography significantly over-estimate the coupling torque. Figure 2: The (non-dimensional) magnetic torques from our MoSST model.. The solid line is the difference between the kinematic torque and the dynamically consistent torque (dash-doted line). This difference is comparable to the full torque itself, suggesting that the toroidal field diffused into the D”-layer is important in the torque evaluation. .