Detection of inner core solidification from observations of antipodal P K I I K P

P K I I K P waves, reflected from the underside of the inner core boundary, are very sensitive to the S velocity in the uppermost 80 km of the inner core at antipodal distances, undergoing a phase change and a factor of 4 amplification as the distance approaches 180°. Modeled P K I I K P waveforms are consistent with a near‐zero shear modulus in the uppermost inner core in a 20–40 km thick patch beneath the eastern equatorial hemisphere. This bright spot of P K I I K P reflection correlates with a thin zone of low P velocity inferred from the complexity of P K I K P waveforms sampling this patch. Estimates of grain sizes from seismic attenuation, the absence of backscattered P K i K P coda in this region, and a prediction for enhanced heat flow through this patch suggest that it is a region of solidification rather than melting.