Viscosity of Earth's Outer Core

The viscosity of Earth's outer liquid core is a fundamental property of great importance in modelling Earth's magnetic field and in many other branches of geophysics and geochemistry. Accurate measurements of viscosity in the F-layer at the bottom of the outer core are provided by the reduction of rotational splitting of the two equatorial translational modes of the inner core, observed with a network of superconducting gravimeters. Independent values are given by the prograde and retrograde modes which differ by about 10%, with a mean value of 1.243× 1011 Pa· s. At the top of the outer core, the viscosity has been measured by the free decays found, for both the retrograde and prograde Free Core Nutations, in the VLBI nutation series provided by Goddard Space Flight Center and the United States Naval Observatory. The four values range just over a factor of two, with a mean value of 2,448 Pa· s. Recently, viscosities ranging from 1011 Pa· s at the bottom of the liquid outer core to 102 Pa· s at the top have been found by Arrhenius extrapolation of laboratory measurements (Brazhkin, JETP Lett. 68, 502, 1998). We report here the results of a similar extrapolation along the melting temperature curve between our measured boundary values to obtain a viscosity profile for the entire outer liquid core.