Limits on lateral density and velocity variations in the Earth's outer core

Abstract

An approximate analysis is given for the likely fractional lateral density variations (δρ/ρ) in the outer core, caused by large scale-length fluid dynamical processes. It is first shown that fractional density and fractional seismic velocity variations are probably comparable, so that fluid dynamic arguments have relevance to seismic data. In regions of nearly neutral stability in the outer core, an analysis of convective vigour indicates an upper bound of δρ/ρ≲10^(−8). If the outer core possesses one or more layers of strong static stability then stationary contributions to δρ can be larger, if they are associated with axisymmetric (m = 0) harmonics, because of stabilizing zonal winds. Baroclinic instabilities nevertheless limit δρ/ρ≲ 10^(−6) but may not exist if the static stability is sufficiently large. Shear instabilities always limit δρ/ρ≲ 10^(−4). Magnetic field effects suggest comparable or more stringent upper bounds. It is concluded that scientists undertaking analysis of the Earth's geoid or seismic travel times or normal modes can safely assume that there are negligible lateral variations in the outer core.