Core Anisotropy Research Articles

Impact behavior of periodic, stochastic, and anisotropic minimal surface-lattice sandwich structures

Recent advancements in 3D printing technologies have made it possible to fabricate intricate lattice architectures with high precision. These lattices can now be utilized to design lightweight sandwich structures that serve multiple functions. To enhance the impact loading performance of these structures, it is crucial to understand how the lattice's topological properties, particularly those with minimal surface attributes like periodic or stochastic Primitive and Gyroid triply periodic minimal surfaces (TPMS) and spinodal-like stochastic cellular materials, associate with the mechanical properties of sandwich structures while keeping the skin thickness fixed. Thus, this paper explores the low-velocity impact behavior of various sheet/shell-based minimal surface-latticed cores of sandwich structures with woven composite skins. The elasto-plastic-damage numerical simulations consider lattice core periodicity, randomness, and anisotropy while keeping the relative density constant. Core lattice randomness and anisotropy are designed using the Gaussian Random Field (GRF) method for spinodal-based stochastic cellular materials and stochastic TPMS. The simulation results showed that the periodic Primitive-lattice core exhibits high out-of-plane shearing strength, enabling the sandwich structure to demonstrate the highest perforation limit. GRF spinodal-based core achieved the highest peak load due to its anisotropic mechanical properties. However, the post-yielding bending of the lattice sheet limited its ability to resist perforation, and absorb and dissipated energy. Interestingly, the stochastic Gyroid TPMS topology, with its inherent densely-distributed microstructure, showed high sensitivity to loading rate, resulting in enhanced energy absorption and dissipation of the sandwich structure. These findings offer valuable insights for optimizing multifunctional sandwich structures with superior impact performance and their design for additive manufacturing.

Studying inner core and lower mantle structure with a combination of PKP and converted SKP and PKS waves

SUMMARYStructure of the inner core is often measured through traveltime differences between waves that enter the inner core (PKPdf) and waves that travel through the outer core only (PKPab and PKPbc). Here we extend the method to converted waves PKSdf and SKPdf and compare results to PKP wave measurements. PKSdf and SKPdf have a very similar path to PKPdf and if velocity variations are present in the inner core, all three wave types should experience them equally. Since traveltime deviations can be due to velocity changes (either isotropic or anisotropy) as well as wave path deviations born from heterogeneity, we simultaneously investigate wave path directions and traveltimes of PKP, SKP and PKS waves for several source-array combinations. One of the path geometries is the anomalous polar corridor from South Sandwich to Alaska, which has strong traveltimes anomalies for PKPdf relative to more normal equatorial path geometries. Here we use array methods and determine slowness, traveltime and backazimuth deviations and compare them to synthetic data. We find that path deviations from theoretical values are present in all wave types and paths, but also that large scatter exists. Although some of the path deviations can be explained by mislocation vectors and crustal variations, our measurements argue that mantle structure has to be considered when investigating inner core anisotropy. Our polar path data show similar traveltime residuals as previously published, but we also find slowness residuals for this path. Interestingly, SKPdf and PKSdf for the South Sandwich to Alaska path show traveltime residuals that are partly opposite to those for PKPdf, possibly due to an interaction with a localized ultra-low velocity zone where waves enter the core.

Spatial magnetization profile in spherical nanomagnets with surface anisotropy: Green's function approach

We consider a single spherical nanomagnet and investigate the spatial magnetization profile in the continuum approach, using the Green's function formalism. The energy of the (many-spin) nanomagnet comprises an isotropic exchange interaction, a uniaxial anisotropy in the core and Néel's surface anisotropy, and an external magnetic field. We derive a semi-analytical expression for the magnetization vector field for an arbitrary position r within and on the boundary of the nanomagnet, as a solution of a homogeneous Helmholtz equation with inhomogeneous Neumann boundary conditions. In the absence of core anisotropy, we use the solution of this boundary problem and infer approximate analytical expressions for the components m α , α = x, y, z, as a function of the radial distance r and the direction solid angle. Then, we study the effects of the nanomagnet's size and surface anisotropy on the spatial behavior of the net magnetic moment. In the presence of a core anisotropy, an approximate analytical solution is only available for a position r located on the surface,i.e. r = R n, where R is the radius of the nanomagnet and n the verse of the normal to the surface. This solution yields the maximum spin deviation as a result of the competition between the uniaxial core anisotropy and Néel's surface anisotropy. Along with these (semi-)analytical calculations, we have solved a system of coupled Landau–Lifshitz equations written for the atomic spins, and compared the results with the Green's function approach. For a plausible comparison with experiments, e.g. using the technique of small-angle magnetic neutron scattering, we have averaged over the direction solid angle and derived the spatial profile in terms of the distance r. We believe that the predictions of the present study could help to characterize and understand the effects of size and surface anisotropy on the magnetization configurations in nanomagnet assemblies such as arrays of well-spaced platelets.

Interplay between Anisotropy- and Skewness-driven Whistler Instabilities in the Solar Wind under the Core–Strahlo Model

Temperature anisotropy and field-aligned skewness are commonly observed nonthermal features in electron velocity distributions in the solar wind. These characteristics can act as a source of free energy to destabilize different electromagnetic wave modes, which may alter the plasma state through wave–particle interactions. Previous theoretical studies have mainly focused on analyzing these nonthermal features and self-generated instabilities individually. However, to obtain a more accurate and realistic understanding of the kinetic processes in the solar wind, it is necessary to examine the interplay between these two energy sources. By means of linear kinetic theory, in this paper we investigate the excitation of the parallel propagating whistler mode, when it is destabilized by electron populations exhibiting both temperature anisotropy and field-aligned strahl or skewness. To describe the solar wind electrons, we adopt the core–strahlo model as an alternative approach. This model offers the advantage of representing the suprathermal features of halo and strahl electrons, using a single skew–kappa distribution already known as the strahlo population. Our findings show that when the electron strahlo exhibits an intrinsic temperature anisotropy, this suprathermal population becomes a stronger and more efficient source of free energy for destabilizing the whistler mode. This suggests the greater involvement of the anisotropic strahlo in processes conditioned by wave–particle interactions. The present results also suggest that the contribution of core anisotropy can be safely disregarded when assessing the importance of instabilities driven by the suprathermal population. This allows for a focused study, particularly regarding the regulation of the electron heat flux in the solar wind.

Revealing the Band-Edge Exciton Fine Structure of Single InP Nanocrystals.

We investigate the fundamental optical properties of single zinc-blende InP/ZnSe/ZnS nanocrystals (NCs) using frequency- and time-resolved magneto-photoluminescence spectroscopy. At liquid helium temperature, highly resolved spectral fingerprints are obtained and identified as the recombination lines of the three lowest states of the band-edge exciton fine structure. The evolutions of the photoluminescence spectra and decays under magnetic fields show evidence for a ground dark exciton level 0L with zero angular momentum projection along the NC main elongation axis. It lies 300 to 600 μeV below the ±1L bright exciton doublet, which is finely split by the NC shape anisotropy. These spectroscopic findings are well reproduced with a model of exciton fine structure accounting for shape anisotropy of the InP core. Our spectral fingerprints are extremely sensitive to the NC morphologies and unveil highly uniform shapes with prolate deviations of less than 3% from perfect sphericity.

Crystallographic texture formation in Fe-9wt%Si alloy during deformation and phase transition at high pressure

SUMMARY The seismic anisotropy of the Earth's solid inner core has been the topic of much research. It could be explained by the crystallographic preferred orientation (CPO) developing during convection. The likely phase is hexagonal close-packed iron (hcp), alloyed with nickel and some lighter elements. Here we use high energy synchrotron X-rays to study CPO in Fe-9wt%Si, uniaxially compressed in a diamond anvil cell in radial geometry. The experiments reveal that strong preferred orientation forms in the low-pressure body-centred cubic (bcc) phase that appears to be softer than pure iron. CPO is attributed to dominant {110}<111> slip. The onset of the bcc→hcp transition occurs at a pressure of ≈15 GPa, and the alloy remains in a two phase bcc + hcp state up to 40 GPa. The hcp phase forms first with a distinct {11$\bar{2}$0} maximum perpendicular to compression. Modelling shows that this is a transformation texture, which can be described by Burgers orientation relationship with variant selection. Experimental results suggest that bcc grains oriented with <100> parallel to compression transform into hcp first. The CPO of the hcp changes only slowly during further pressure and deviatoric stress increase at ambient temperature. After heating to 1600 K, a change in the hcp CPO is observed with alignment of (0001) planes perpendicular to compression that can be interpreted as dominant (0001)<11$\bar{2}$0> slip, combined with {10$\bar{1}$2}<$\bar{1}$011> mechanical twinning, which is similar to the deformation modes suggested previously for pure hcp iron at inner core conditions.

Plasmon-enhanced rotational dynamics of anisotropic core-shell polymeric-metallic microparticles

The development of efficient and cost-effective micromachines is a challenge for applied and fundamental science, given their wide fields of usage. Light is a suitable tool to move small objects in a noncontact way, given its capabilities in exerting forces and torques. However, when complex manipulation is required, micro-objects with proper architecture could play a specific role. Here we report on the rotational dynamics of core-shell particles, with a polymeric nematic core of ellipsoidal shape capped by Au nanoparticles. They undergo a peculiar synchronous spinning and orbital motion when irradiated by a simple Gaussian beam, which originates from the coupling of the metallic nanoparticles’ optical response and the core anisotropies. The rotation capabilities are strongly enhanced when the trapping wavelength lies in the plasmonic resonance region: indeed, the spin kinetic energy reaches values two orders of magnitude larger than the one of bare microparticles. The proposed strategy brings important insights into optimizing the design of light controlled micro-objects and might benefit applications in microfluidics, microrheology, and micromachining involving rotational dynamics.

Early Cambrian renewal of the geodynamo and the origin of inner core structure

Paleomagnetism can elucidate the origin of inner core structure by establishing when crystallization started. The salient signal is an ultralow field strength, associated with waning thermal energy to power the geodynamo from core-mantle heat flux, followed by a sharp intensity increase as new thermal and compositional sources of buoyancy become available once inner core nucleation (ICN) commences. Ultralow fields have been reported from Ediacaran (~565 Ma) rocks, but the transition to stronger strengths has been unclear. Herein, we present single crystal paleointensity results from early Cambrian (~532 Ma) anorthosites of Oklahoma. These yield a time-averaged dipole moment 5 times greater than that of the Ediacaran Period. This rapid renewal of the field, together with data defining ultralow strengths, constrains ICN to ~550 Ma. Thermal modeling using this onset age suggests the inner core had grown to 50% of its current radius, where seismic anisotropy changes, by ~450 Ma. We propose the seismic anisotropy of the outermost inner core reflects development of a global spherical harmonic degree-2 deep mantle structure at this time that has persisted to the present day. The imprint of an older degree-1 pattern is preserved in the innermost inner core.

Magnetic properties and Tb diffusion of sintered Nd–Fe–B magnets by suspension plasma spraying

For the first time, a TbF3 coating as a diffusion source was prepared on the surface of sintered Nd–Fe–B magnets by suspension plasma spraying. A sintered Nd–Fe–B magnet with a high coercivity and low remanence loss was obtained. The uniform and dense coating favored grain-boundary diffusion of heavy rare-earth elements, which improved the coercivity of Nd–Fe–B magnets. The effect of grain-boundary diffusion and TbF3 coating amount on the microstructure and magnetic properties of the magnet were studied. Optimal magnetic properties were obtained for a heat treatment of 900 °C for 10 h, when the TbF3 coating amount was 0.2132 mg/mm2. The coercivity reached 23.72 kOe with an increase of 60.16% compared with the original magnet, whereas the remanence did not decrease significantly. Microstructure analysis showed that a Tb-layer core–shell structure formed around the Nd2Fe14B grains after diffusion. The high magnetocrystalline anisotropy of the Tb core–shell structure formation improved the Nd–Fe–B magnet coercivity significantly. Therefore, the method of plasma spray grain-boundary diffusion is efficient and feasible.

Two-step nucleation of the Earth’s inner core

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.

The anisotropy of hcp iron under inner core conditions: the effect of light elements

In recent decades, global seismic observations have identified increasingly complex anisotropy of the Earth’s inner core. Numerous seismic studies have confirmed hemispherical variations in the inner core’s anisotropy. Here, based on <i>ab initio</i> molecular dynamics calculations, we report how the anisotropy of hexagonal close-packed (hcp)-iron, under inner core conditions, could be altered when alloyed with light elements. We find that light elements in binary allows with iron — hcp-Fe-X (X = C, O, Si, and S) — could have significant effects on density, sound velocities, and anisotropy, compared with the behavior of pure hcp-iron; the anisotropy of these binary alloys depends on combined effects of temperature and the particular alloying light element. Furthermore, the change in anisotropy strength with increasing temperature can be charted for each alloy. Alloying pure iron with some light elements such as C or O actually does not increase but decreases core anisotropy at high temperatures. But the light element S can significantly enhance the elastic anisotropy strength of hcp-Fe.

Shear‐Wave Anisotropy in the Earth's Inner Core

AbstractEarth's inner core (IC) anisotropy is widely used to infer the deep Earth's evolution and present dynamics. Many compressional‐wave anisotropy models have been proposed based on seismological observations. In contrast, inner‐core shear‐wave (J‐wave) anisotropy—on a par with the compressional‐wave anisotropy—has been elusive. Here, we present a new class of the J‐wave anisotropy observations utilizing earthquake coda‐correlation wavefield. We establish that the coda‐correlation feature I2‐J, sensitive to J‐wave speed, exhibits time and amplitude changes when sampling the IC differently. J‐waves traversing the IC near its center travel faster for the oblique than equatorial angles relative to the Earth's rotation axis by at least ∼5 s. The simplest explanation is the J‐wave cylindrical anisotropy with a minimum strength of ∼0.8%, formed through the lattice‐preferred‐orientation mechanism of iron. Although we cannot uniquely determine its stable iron phase, the new observations rule out one of the body‐centered‐cubic iron models.

The Use of Core & Log Anisotropy Parameters into Seismic Data Processing: A Case Study of Deep-Water Reservoir

The seismic far-offset data plays important role in seismic subsurface imaging and reservoir parameters derivation, however, it is often distorted by the hockey stick effect due to improper correction of the Vertical Transverse Isotropy (VTI) during the seismic velocity analysis. The anisotropy parameter η is needed to properly correct the VTI effect. The anisotropy parameters of ε and δ obtained from log and core measurements, can be used to estimate the η values, however, the upscaling effects due to the different frequencies of the wave sources used in the measurements must be carefully taken into account. The objective is to get better understanding on the proper uses of anisotropy parameters in the the velocity analysis of deepwater seismic gather data. To achieve the objective, the anisotropy parameters from ultrasonic core measurements and dipole sonic log were used to model the seismic CDP gathers. The upscaling effects is reflected by the big difference of measured anisotropy values, in which the core measurement value is about 40 times higher than the log measurement value. The CDP gathers modelling results show that, due to the upscaling effect, the log and core-based models show significant differences of far-offset amplitude and hockey sticks responses. The differences can be minimized by scaling-down the log anisotropy values to core anisotropy values by using equations established from core – log anisotropy values cross-plot. The study emphasizes the importances of integrating anisotropy parameters from core and log data to minimize the upscaling effect to get the best η for the VTI correction in seismic velocity analysis.

Practical Method for Drastic Improvement of Output Offset Stability in Bias-Switched Fundamental Mode Orthogonal Fluxgate

Fundamental mode orthogonal fluxgate (FM-OFG) adopting the bias switching technique is a promising compact and low-noise magnetometer for precise DC and low frequency magnetic field measurement. Yet, its output offset stability is insufficient for measurements in the environment with large temperature variations. In this research we have experimentally investigated the cause of the residual offset and its drift found in the bias-switched FM-OFG. Then we proposed a simple and effective method to drastically improve the sensor temperature characteristics of the FM-OFG in a practical feedback design with analog circuitry. Ideally, the induced voltages arisen by the uniaxial anisotropy of an amorphous wire core become symmetrical under flipped excitation polarities, and thus they are canceled by averaging. However, it was revealed that there was certain imbalance between them due to the asymmetry in wire’s magnetic properties, producing remnant component in the final output. The proposed method achieves equilibration of the imbalance component within the demodulation process of the induced voltages to minimize the residual offset and its drift in the final, averaged output. The method was verified by the experiment, and the offset stability of 0.013 nT/°C within the temperature range from −60°C to +70°C was achieved with good repeatability.

Advanced Determination of Heat Flow Density on an Example of a West Russian Oil Field

Reliable geothermal data are required for basin and petroleum system modeling. The essential shortcomings of the methods and results of previous geothermal investigations lead to a necessity to reappraise the data on the thermal properties and heat flow. A new, advanced experimental basis was used to provide reliable data on vertical variations in the thermal properties of formation and heat flow for the area surrounding a prospecting borehole drilled through an unconventional hydrocarbon reservoir of the Domanik Formation in the Orenburg region (Russia). Temperature logging was conducted 12.5 months after well drilling. The thermal properties of the rocks were measured with continuous thermal core profiling on all 1699 recovered core samples. Within non-cored intervals, the thermal conductivity of the rocks was determined from well-logging data. The influence of core aging, multiscale heterogeneity and anisotropy, in situ pressure and temperature on the thermal properties of rock was accounted for. The terrestrial heat flow was determined to be 72.6 ± 2.2 mW·m−2—~114% larger than the published average data for the studied area. The experiment presents the first experience of supporting basin modeling in unconventional plays with advanced experimental geothermal investigations.

Quantitation method of loss powers using commercial magnetic nanoparticles based on superparamagnetic behavior influenced by anisotropy for hyperthermia

Local hyperthermia using magnetic nanoparticles has attracted attention as a less invasive cancer therapy. Accurate estimation of heat dissipation of magnetic nanoparticles is important for hyperthermia treatment. In this study, the magnetization properties and heat dissipation of Feraheme®, Resovist®, and Synomag®-D commercial magnetic nanoparticles was measured. The effective core diameter and anisotropy constant were estimated by the Langevin function taking into consideration the effect of anisotropy. The required frequency and intensity of an applied magnetic field with respect to the weight and concentration of magnetic nanoparticles in an assumed spherical tumor were estimated based on measurement and theory of magnetic relaxation. A method to quantitate realistic heat dissipation of magnetic nanoparticles in tumor tissue by measurement of their magnetization properties is introduced. Because the physical rotation of magnetic nanoparticles is inhibited in tumor tissue, the realistic heat dissipation is estimated by solidifying a mass of nanoparticles in an experimental sample. The specific loss power and intrinsic loss power as the heat dissipations of magnetic nanoparticles were estimated from the area of the magnetization curves. The results indicated that only the linear component of magnetization was associated with the loss powers. Our method to quantitate loss powers and the index for applied magnetic field will facilitate the clinical application of hyperthermia treatment using magnetic nanoparticles.

Effects of upper mantle structure beneath Alaska on core-sensitive seismic wave absolute and differential measurements: Implications for estimates of inner core anisotropy

Inner core anisotropy is often studied using differential travel times between the inner core phase, PKPdf, and one of two outer core phases, either PKPbc or PKPab, to eliminate contamination by crust and upper mantle structure. In particular, given the similarity of the two ray paths in the shallow Earth, the PKPbc-df differential travel time is assumed to robustly reflect the structure of the inner core, and not be influenced by mantle structure. Measurements of PKPab-df and PKPbc-df differential times reveal that the inner core is anisotropic: PKPdf rays travel through the inner core ~3% faster along polar paths than along equatorial paths. Even so, measurements of both PKPbc-df and PKPab-df differential travel times on quasi-polar paths between sources in the South Sandwich Islands and stations in Alaska present greater scatter (6 s spread) than other equivalent quasi-polar paths from other parts of the globe (2 s spread). While the South Sandwich Islands to Alaska data help increase spatial sampling of the inner core, including these data in inner core models significantly increases estimates of average global inner core anisotropy strength, by more than 1%. Whether this reflects real spatial variability in the strength of inner core anisotropy or else results from complexity outside of the inner core is uncertain but is crucial for constraining the inner core composition and growth history. Using a regional tomographic model of the Alaskan upper mantle to predict upper mantle effects on PKP travel times, we show that the signature of the Alaska slab is present in trends of observed absolute PKPbc, ab, and df travel times, both as a function of distance and azimuth. Moreover, we demonstrate that the effect of the slab is not fully cancelled by differential measurements. This implies that past models of the inner core are biased towards too strong average anisotropy. In order to better constrain inner core anisotropy in future, differential measurements of core-phase travel times need to be more accurately corrected for upper mantle three-dimensional structure, which in turns requires the construction of higher resolution tomographic models.

Cooling Mechanisms in a Rotating Brake Disc With a Wire-Woven-Bulk Diamond Cellular Core

Abstract Thermofluidic behaviors governing the enhanced cooling performance of the wire-woven-bulk diamond (WBD) cored brake disc in comparison with the conventional pin-finned brake disc used on heavy vehicles were characterized experimentally. For each type of brake disc, detailed internal thermofluidic data of the two rotating brake discs were obtained using transient thermochromic liquid crystal (TLC) for end-wall heat transfer and particle image velocimetry (PIV) for the inflow field. The results demonstrate that the pin-finned brake disc exhibits a circumferentially periodic curved inline-like passage flow and large dead flow regions, with strong recirculation that reduces its thermal dissipation performance. The cooling advantage of the WBD core is primarily attributed to the combination of enlarged heat transfer surface area (both end-wall and core) and greater utilization of the larger surface due to favorable fluidic behavior developed from the WBD topology. The internal WBD core has approximately three times the surface density of the pin-finned disc which, in combination with the smaller and weaker recirculation zones, leads to more effective usage of the available core surface area for thermal dissipation. The aerodynamic anisotropy of the WBD core induced by its topological anisotropy causes a globally irregular thermofluidic distribution in the brake disc.

New measurements of long-period radial modes using large earthquakes

SUMMARYRadial modes, nS0, are long-period oscillations that describe the radial expansion and contraction of the whole Earth. They are characterized only by their centre frequency and quality factor Q, and provide crucial information about the 1-D structure of the Earth. Radial modes were last measured more than a decade ago using only one or two earthquakes. Here, we measure radial modes using 16 of the strongest and deepest earthquakes of the last two decades. By introducing more earthquake data into our measurements, we improve our knowledge of 1-D attenuation, as we remove potential earthquake bias from our results. For mode 0S0, which is dominated by compressional energy, we measure a Q value of 5982, much higher than previously measured, and requiring less bulk attenuation in the Earth than previously thought. We also show that radial modes cross-couple (resonate) strongly to their nearest spheroidal mode due to ellipticity and inner core cylindrical anisotropy. Cross-coupling improves the fit between data and synthetics, and gives better estimates of the centre frequency and attenuation value of the radial modes. Including cross-coupling in our measurements results in a systematic shift of the centre frequencies of radial modes towards the Preliminary Reference Earth Model. This shift in centre frequencies, has implications for the strength of the radial anisotropy present in the uppermost inner core, with our cross-coupling results agreeing with lower values of anisotropy than the ones inferred from just measuring the modes in self-coupling (isolation). Furthermore, cross-coupling between radial modes and angular-order two modes provides constraints on cylindrical inner core anisotropy, that will help us improve our knowledge of the 3-D structure of the inner core.

Inner core anisotropy measured using new ultra-polar PKIKP paths

SUMMARY We measure the seismic anisotropy of the inner core using PKPbc-PKPdf and PKPab-PKPdf differential traveltimes, as a function of the angle ζ between the Earth’s rotation axis and the ray path in the inner core. Previous research relied heavily on body waves originating in the South Sandwich Islands (SSI) and travelling to seismic stations in Alaska to sample inner core velocities with low ζ (polar paths). These SSI polar paths are problematic because they have anomalous travel time anomalies, there are no ultra-polar SSI paths with ζ &amp;lt; 20° and they only cover a small part of the inner core. Here we improve constraints on inner core anisotropy using recently installed seismic stations at high latitudes, especially in the Antarctic, allowing us to measure ultra-polar paths with ζ ranging from 20°–5°. Our new data show that the SSI’s polar events are fast but still within the range of velocities measured from ray paths originating elsewhere. We further investigate the effect of mantle structure on our data set finding that the SSI data are particularly affected by fast velocities underneath the SSI originating from the subducted South Georgia slab, which is currently located just above the core mantle boundary. This fast velocity region results in mantle structure being misinterpreted as inner core structure and we correct for this using a P-wave tomographic model. We also analyse the effect of velocity changes on the ray paths within the inner core and find that faster velocities significantly change the ray path resulting in the ray travelling deeper into the inner core and spending more time in the inner core. To remove this effect, we propose a simple but effective method to correct each event-station pair for the velocity-dependent ray path changes in the inner core, producing a more reliable fractional traveltime measurement. Combining the new ultra-polar data with mantle and ray path corrections results in a more reliable inner core anisotropy measurement and an overall measured anisotropy of 1.9–2.3 per cent for the whole inner core. This is lower than previous body wave studies (3 per cent anisotropy) and in better agreement with the value of inner core anisotropy measured by normal modes (2 per cent anisotropy). We also identify regional variation of anisotropic structure in the top 500 km of the inner core, which appears to be more complex than simple hemispherical variations. These regional variations are independent of the SSI data and are still present when these data are excluded. We also find a potential innermost inner core with a radius of 690 km and stronger anisotropy.