This lecture has been devoted to describing recent observations that lead to a complex model of the seed:
- Existence or not and amplitude of differential rotation of the seed : inspired by magneto-hydro-dynamic numerical modeling of convection in the outer core in the presence of solid seed (Glazmaier and Roberts, 1995), which predicted such differential rotation, Song and Richards (1996) proposed that the variations, over 30 years of observation, of the PKIKP wave travel times observed on the same polar path, could be explained by a differential rotation of the seed of ~ 1° per year (eastward), exploiting the presence of anisotropy in the seed, with a fast axis of anisotropy inclined to the Earth's axis of rotation. This inspired numerous works in geodynamics and seismology over the following decade. In particular, these challenged Song and Richards' result, concluding that if differential rotation exists, it cannot be greater than ~ 0.15° per year, far less than the original estimate.
- Presence of anisotropy in attenuation anticorrelated with that in compression velocity.
- Presence of topography on the BCI with a lateral wavelength of ~ 10 km and less than 500 m in the radial direction (Cao, Masson and Romanowicz, 2007).
- Variations in seed anisotropy with depth, with the existence of a "seed within a seed" (Ishii and Dziewonski, 2002).
- Hemispherical variations in structure in the superficial part of the seed, recently confirmed by eigenmode splitting studies (Deuss et al., 2010) and possibly due to very small temperature variations at the base of the outer core (~ 10-3°).
