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1. Seismological observation of Earth’s oscillating inner core

   https://doi.org/10.1126/sciadv.abm9916  

   Wang, Wei; Vidale, John E. (June 2022, Science Advances)

   Seismic observations suggest that Earth’s inner core oscillates, rather than superrotates, relative to the mantle and crust. 

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2. Two-step nucleation of the Earth’s inner core

   https://doi.org/10.1073/pnas.2113059119  

   Sun, Yang; Zhang, Feng; Mendelev, Mikhail I.; Wentzcovitch, Renata M.; Ho, Kai-Ming (January 2022, Proceedings of the National Academy of Sciences)

   The Earth's inner core started forming when molten iron cooled below the melting point. However, the nucleation mechanism, which is a necessary step of crystallization, has not been well understood. Recent studies have found that it requires an unrealistic degree of undercooling to nucleate the stable, hexagonal, close-packed (hcp) phase of iron that is unlikely to be reached under core conditions and age. This contradiction is referred to as the inner core nucleation paradox. Using a persistent embryo method and molecular dynamics simulations, we demonstrate that the metastable, body-centered, cubic (bcc) phase of iron has a much higher nucleation rate than does the hcp phase under inner core conditions. Thus, the bcc nucleation is likely to be the first step of inner core formation, instead of direct nucleation of the hcp phase. This mechanism reduces the required undercooling of iron nucleation, which provides a key factor in solving the inner core nucleation paradox. The two-step nucleation scenario of the inner core also opens an avenue for understanding the structure and anisotropy of the present inner core. 

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3. Inner core backtracking by seismic waveform change reversals

   https://doi.org/10.1038/s41586-024-07536-4  

   Wang, Wei; Vidale, John E; Pang, Guanning; Koper, Keith D; Wang, Ruoyan (July 2024, Nature)

   Abstract The solid inner core, suspended within the liquid outer core and anchored by gravity, has been inferred to rotate relative to the surface of Earth or change over years to decades based on changes in seismograms from repeating earthquakes and explosions^1,2. It has a rich inner structure^3–6and influences the pattern of outer core convection and therefore Earth’s magnetic field. Here we compile 143 distinct pairs of repeating earthquakes, many within 16 multiplets, built from 121 earthquakes between 1991 and 2023 in the South Sandwich Islands. We analyse their inner-core-penetrating PKIKP waves recorded on the medium-aperture arrays in northern North America. We document that many multiplets exhibit waveforms that change and then revert at later times to match earlier events. The matching waveforms reveal times at which the inner core re-occupies the same position, relative to the mantle, as it did at some time in the past. The pattern of matches, together with previous studies, demonstrates that the inner core gradually super-rotated from 2003 to 2008, and then from 2008 to 2023 sub-rotated two to three times more slowly back through the same path. These matches enable precise and unambiguous tracking of inner core progression and regression. The resolved different rates of forward and backward motion suggest that new models will be necessary for the dynamics between the inner core, outer core and mantle. 

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4. Support for equatorial anisotropy of Earth’s inner-inner core from seismic interferometry at low latitudes

   https://doi.org/10.1016/j.pepi.2017.03.004  

   Wang, Tao; Song, Xiaodong (March 2018, Physics of the Earth and Planetary Interiors)

   null (Ed.)
5. Origin of temporal changes of inner-core seismic waves

   https://doi.org/10.1016/j.epsl.2020.116267  

   Yang, Yi; Song, Xiaodong (July 2020, Earth and Planetary Science Letters)

   null (Ed.)