ALBERT

Description: On the timescales of mantle convection (~100 Myrs), downwelling mantle flows due to buoyancy-driven flows depress the core-mantle boundary (CMB) into the outer core (OC) creating a dynamic topography. Seismological studies have suggested that the peak-to-peak amplitudes are less than 10 km. If the solid grain boundaries of mantle silicates and oxides are sufficiently wetted, percolation of liquid iron alloy into the matrix can create an interconnected network to form a metal-silicate mush at CMB topography lows. Since timescales of large-scale flows at the top of the OC are much smaller than viscous compaction timescales of the silicate matrix in the mushy layer, hydrodynamic and magnetic pressure gradients in the OC can drive Darcy flow inside the porous region. This will not only create an electrically conducting region at the solid side of the CMB that distorts the magnetic field, but also produce stronger mechanical coupling within the mushy layer via viscous stresses between the matrix and fluid. These combined effects can result in torques at the CMB that transfers angular momentum between the mantle and OC, thus affecting variations in the length of day. Lastly, flows in the mushy layer facilitate chemical and isotopic exchange between the liquid iron and silicate matrix which has implications on the chemical evolution of the Earth. We will present a study that analyses the pressure torques at the CMB due to the presence of a porous layer, and how surface core flows can be affected by permeability and porosity of the layer.