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BACKGROUND AND MOTIVATION

This topic is a subject of active research in geophysical community and was exploited in a recent science-fiction motion picture The Core (although the scientific facts in the movie were misrepresented to enhance entertainment). See a recent video by the American Geophysical Union on Inge Lehmann, the discoverer of the Earth's inner nucleus here.

The inner core’s surface seems to be rough and mushy at places, and its interior deformed by internal stresses. It might contain heterogeneity at various scales, and host another smaller shell more elusive to our seismological probes. Despite its small volume (less than 1% of the Earth's volume), the Earth's inner core contains about 10% of the total magnetic field energy. It plays a crucial role in outer core liquid motions and the geodynamo, which generates the Earth's magnetic field. Without the magnetic field, life on Earth would be impossible. Embedded in the liquid outer core with a strong magnetic field and exposed to large gravitational pulling from the surrounding mantle, it spins faster than the mantle, and, at times, it slows down. But how do we know all this?

As seismic waves produced by earthquakes, explosions, and other natural phenomena reverberate through the solid Earth, they are reflected or scattered from discontinuities within and between the crust, mantle, and inner and outer core. Changes in the composition and temperature of Earth's minerals cause the waves to change their speed, bend, and even reverse their paths, all of which is manifested in recorded seismograms. The waves that traverse the inner core are thus the only direct probe available, and they become a subject of study.

Despite the fact that seismology leads the observational efforts, major advances in our understanding of the Earth's inner core would have not been possible without combining seismological observations with the results from geodynamics, magnetohydrodynamic modelling, mineral physics, and mathematical geophysics. The geophysical values derived from seismological analyses, e.g., the inner-outer core density ratio, the strength of anisotropy in the inner core, or the amount of differential rotation with respect to the mantle, are all being used in geodynamical modelling.

Understanding inner core structure and dynamics, including energy exchange across the liquid core boundaries, helps Earth and Planetary scientists to better understand planetary formation, the workings of the Earth's magnetic field, and the age of the inner core, the time capsule to understanding Earth's past, present, and future. During the past several decades, our understanding of the inner core's internal structure and dynamics has completely changed owing to modern observational seismology and the expansion of worldwide seismographic stations.

We have recently discovered that the differential rotation of the inner core with respect to the mantle is variable in time, which reconciled the old discrepancy in the results from the earthquake doublets and normal modes studies. We found that the inner core accelerated and decelerated more in recent years, but more data are needed to confirm this observation. We have also recently discovered a way to detect shear waves in the inner core, and owing to our new method, we confirmed its solidity and improved constraints on its shear properties. A lot remains to be done; both to improve our observational capaticty, and to provide new constraints on the mineralogical state of iron at high-pressure–high-temperature conditions.

Ongoing research on the Earth's inner core is focusing on the following topics:

• the inner core isotropic and anisotropic velocity and attenuation structures using direct waveform modelling, travel times of PKP waves, and seismic tomography,
• shear properties of the inner core using observations and waveform modelling of seismic and correlation wavefield,
• character and radial dependence of anisotropy within the inner core; the innermost inner core,
• structure and dynamics near the inner-core boundary,
• differential rotation of the inner core with respect to the rest of the mantle.  

The following approaches can be pursued to achieve a better spatial sampling and imaging resolution of the inner core:

1. Deployment of seismic stations at extreme geographic latitudes and ocean floor. The Australian deployments of the seismographic stations across Australia and in the surrounding regions, including Antarctica and Southern Ocean help in achieving this objective;
2. Development and application of new seismological and mathematical geophysics techniques (e.g. array signal processing, search for exotic inner core phases, correlation methods, Bayesian inversion using rigorous treatment of uncertainty, and machine learning applications for data collection), which will allow us to collect and use large amounts of data that were previously not available.

NATURE OF PROJECT(S): computational (e.g. running software on Terrawulf or National Computational Infrastructure computers), numerical, observational, interpretational (e.g. see the Inner Core book and a couple of recent papers on this topic here and here).

ESSENTIAL BACKGROUND: PHYS3070 (Physics of the Earth)

POSSIBLE PROJECT(S): Any of the approaches mentioned above could become student projects. Highly motivated students with backgrounds in geophysics, physics, astronomy, computer sciences, engineering or mathematics are invited to join me on this exciting journey to the centre of our planet.