ALBERT

Description: The inner core boundary marks the phase transition between the solid inner core and the fluid outer core. As the site of inner core solidification, the boundary provides insight into the processes generating the seismic structures of the inner core. In particular, it may hold the key to understanding the previously observed hemispherical asymmetry in inner core seismic velocity, anisotropy and attenuation. Here, we use a large PKiKP-PcP amplitude ratio and travel time residual dataset to investigate velocity and density contrast properties near the inner core boundary. Although hemispherical structure at the boundary has been proposed by previous inner core studies, we find no evidence for hemispheres in the amplitude ratios or travel time residuals. In addition, we find that the amplitude ratios are much larger than can be explained by variations in density contrast at the inner core boundary or core-mantle boundary. This indicates that PKiKP is primarily observed when it is anomalously large, due to focussing along its ray path. Using data in which PKiKP is not detected above the noise level, we calculate an upper estimate for the ICB density contrast of 1.2 kg m −3 . The travel time residuals display large regional variations, which differ on long and short length scales. These regions may be explained by large scale velocity variations in the F-layer just above the inner core boundary, and/or small scale topography of varying magnitude on the ICB, which also causes the large amplitudes. Such differences could arise from localised freezing and melting of the inner core.