The Earth's solid inner core (IC) remains one of the most enigmatic parts of our planet, despite numerous studies in the fields of seismology, geodynamics, mineral physics, and materials science. Making up only 1% of the Earth's total volume with a temperature of over 5,000°C, understanding the structure, current dynamics, and evolution of the IC is vital to understanding the Earth's thermal history (see Tkalčić [2017] for an in-depth discussion). In particular, this provides insights into the geodynamo, without which life would not exist as we know it today. Since its discovery, new models for the structure of the IC are being proposed with increasing complexity in terms of both seismic velocity and attenuation. Discussions on IC anisotropy arose following a study by Poupinet et al. (1983), who observed that a laterally homogeneous velocity model does not fit the global PKP differential travel time data. Later, Morelli et al. (1986) noticed that absolute PKIKP phases travel up to 3 s faster parallel to the Earth's rotational axis. Anisotropy, rather than isotropic heterogeneity was confirmed in the same year by Woodhouse et al. (1986), who used the hypothesis of IC anisotropy to explain the previously enigmatic observation of the IC sensitive normal modes (e.g., 10 S 2). The seismic body waves sensitive to the IC structure and the corresponding travel time data used predominantly in IC research are illustrated in Figure 1.

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