Inner core anisotropy and heterogeneity level

An analysis of core phase travel times is performed in order to estimate the relative contributions of heterogeneities and anisotropy and their distribution inside the inner core. A stochastic analysis performed on the equatorial paths, which are poorly sensitive to anisotropy, indicates that the inner core exhibits a heterogeneity level smaller than 0.3 % at all depths on scale lengths larger than 200 km. This result, extended to all path orientations, suggests a predominant effect of the anisotropy on travel times anomalies. A simultaneous inversion of absolute and differential core phase travel times for any path orientation favours a simple inner core model in which a large, homogeneous structure with 3% uniform anisotropy is asymmetrically surrounded by a homogeneous isotropic layer with a thickness of 100-200 km on the western hemisphere, increasing to 400 km over a large part of the eastern hemisphere.