Recently we have measured the electrical conductivity of (Mg0.9Fe0.1)SiO3 perovskite and post-perovskite at high pressure and temperature in a laser-heated diamond-anvil cell (DAC). The results demonstrate that the conductivity of post-perovskite is > 102 S/m, higher by three orders of magnitude than that of perovskite at similar pressure range, and does not vary greatly with temperature. The highly conductive post-perovskite layer above the core-mantle boundary would, by electromagnetic coupling, enhance the exchange of angular momentum between the fluid core and solid mantle, which can explain the observed changes in length of a day on decadal timescales. Heterogeneity in the conductivity of the lowermost mantle is likely to depend on changes in chemistry of the boundary region, not fluctuations in temperature.
