The patterns of high-degree thermal free convection and its features in a spherical shell

Introduction Mantle convection is thought to be the most effective way of heat transportation in the earth and the source of driving lithospheric plates (Elasser, 1971). The velocity field of plate motion can be split into poloidal and toroidal parts, which are corresponding to the vertical (i.e. radial) and horizontal motions, respectively, in global model. The toroidal component is manifested in the existences of transform faults and the poloidal one in the presences of convergence and diver…

The patterns of high-degree thermal free convection and its features in a spherical shell

A high-degree (degree l = 6 and order m = 0, 1, 2, L, l. High-order model for short) and steady thermal free con-vective motion of an infinite Prandtl number and Boussinesq fluid in a spherical shell is calculated by a Galerkin method. Convection is driven by an imposed temperature drop across top rigid and bottom stress-free isothermal boundaries only heated from below of the shell. In this paper, the scalar poloidal and fluctuating temperature fields are expanded into associated Legendre polynomials with degree l= 6 and order m = 0, 1, 2, L, l. Compared with zero-order model (degree l= 6 and order m= 0), from which 2-D longitudinal (r-q ) profiles can be obtained, high-order model can provide a series of southerly (r-q ), easterly (r-j) and radial (q -j) velocity profiles, which probably reveal more detail features of mass motion in the mantle. It is found that Rayleigh number has great ef-fects on the patterns and velocities of thermal free convection and controls the relative ratio of hot and cold plume in the shell. Probably, the present results mainly reveal the mass motion in the lower mantle, while the striking differences of convection patterns from velocities at different positions have important geodynamical signifi-cances.