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SUMMARY: Paleomagnetic records provide us with information about the extreme geomagnetic events known as excursions and reversals, but the sparsity of available data limits detailed knowledge of the process and timing. To date there are no agreed on criteria for categorizing such events in terms of severity or longevity. In an analogy to categorizing storms in weather systems, we invoke the magnitude of the global (modified) paleosecular variation index $P_{i_D}$ to define the severity of the magnetic field state, ranging in level from 0 to 3, and defined by instantaneous values of $P_{i_D}$ with level 0 being normal ($P_{i_D}\lt 0.5$) to extreme ($P_{i_D}\ge 15$). We denote the time of entry to an excursional (or reversal) event by when $P_{i_D}$ first exceeds 0.5, and evaluate its duration by the time at which $P_{i_D}$ first returns below its median value, termed the end of event threshold. We categorize each excursional event according to the peak level of $P_{i_D}$ during the entire event, with a range from Category-1 (Cat-1) to Cat-3. We explore an extended numerical dynamo simulation containing more than 1200 events and find that Cat-1 events are the most frequent (72 per cent), but only rarely lead to actual field reversals where the axial dipole, $g_1^0$, has reversed sign at the end of the event. Cat-2 account for about 20 per cent of events, with 34 per cent of those leading to actual reversals, while Cat-3 events arise about 8 per cent of the time but are more likely to produce reversals (43 per cent). Higher category events take as much as 10 times longer than Cat-1 events. Two paleomagnetic field models separately cover the Laschamp excursion and Matuyama–Brunhes (M-B) reversal which are Cat-2 events with respective durations of 3.6 and 27.4 kyr. It seems likely that Cat-2 may be an underestimate for M-B due to limitations in the paleomagnetic records. Our overall results suggest no distinction between excursions and reversals other than a reversal having the ending polarity state opposite to that at the start.

Paleomagnetic records provide us with information about the extreme geomagnetic events known as excursions and reversals, but the sparsity of available data limits detailed knowledge of the process and timing. To date there are no agreed on criteria for categorizing such events in terms of severity or longevity. In an analogy to categorizing storms in weather systems, we invoke the magnitude of the global (modified) paleosecular variation index |$P_{i_D}$| to define the severity of the magnetic field state, ranging in level from 0 to 3, and defined by instantaneous values of |$P_{i_D}$| with level 0 being normal (⁠|$P_{i_D}\lt 0.5$|⁠) to extreme (⁠|$P_{i_D}\ge 15$|⁠). We denote the time of entry to an excursional (or reversal) event by when |$P_{i_D}$| first exceeds 0.5, and evaluate its duration by the time at which |$P_{i_D}$| first returns below its median value, termed the end of event threshold. We categorize each excursional event according to the peak level of |$P_{i_D}$| during the entire event, with a range from Category-1 (Cat-1) to Cat-3. We explore an extended numerical dynamo simulation containing more than 1200 events and find that Cat-1 events are the most frequent (72 per cent), but only rarely lead to actual field reversals where the axial dipole, |$g_1^0$|⁠ , has reversed sign at the end of the event. Cat-2 account for about 20 per cent of events, with 34 per cent of those leading to actual reversals, while Cat-3 events arise about 8 per cent of the time but are more likely to produce reversals (43 per cent). Higher category events take as much as 10 times longer than Cat-1 events. Two paleomagnetic field models separately cover the Laschamp excursion and Matuyama–Brunhes (M-B) reversal which are Cat-2 events with respective durations of 3.6 and 27.4 kyr. It seems likely that Cat-2 may be an underestimate for M-B due to limitations in the paleomagnetic records. Our overall results suggest no distinction between excursions and reversals other than a reversal having the ending polarity state opposite to that at the start. [ABSTRACT FROM AUTHOR]

Paleosecular variation analysis is a primary tool for characterizing ancient geomagnetic behavior and its evolution through time. This study presents a new high‐quality directional data set, paleosecular variation of the Paleogene (PSVP), with and without correction for serial correlation, compiled from 1,667 sites from 45 different localities from the Paleogene and late Cretaceous (84–23 Ma). The data set is used to study the variability, structure, and latitude dependence of the geomagnetic field during that period by varying selection criteria and PSV models. Modeled values for the equatorial virtual geomagnetic pole (VGP) dispersion have over‐lapping uncertainty intervals within their uncertainty bounds between 8.3° and 18.6° for the past 250 Ma. We investigate the suitability of two descriptive models of PSV, Model G‐style quadratic fits and covariant Giant Gaussian Process models, and find that both styles of model fail to satisfactorily reproduce the latitude dependent morphology of PSV, but suggest that estimates of the equatorial VGP dispersion may still robustly characterize aspects of Earth's long‐term field morphology. During this time where the PSV behavior has not changed substantially, the reversal frequency has varied widely. The lack of a clear relationship between PSV behavior and reversal frequency is not trivially explained in the context of published findings regarding numerical geodynamo simulations. Plain Language Summary: The geomagnetic field is saved in volcanic rocks, which can be used to study the direction and intensity of this field millions of years later. The magnetic field is formed in the geodynamo in the Earth's outer core. The study of geomagnetic field changes through time gives us information on the changes in the Earth's outer core, lower mantle and inner core. In this study, all the data from the Paleogene and Late Cretaceous were gathered (84–23 million years ago) and studied to see how much the geomagnetic field changed through that time. With this study we now have information on the variability of the field for the entirety of the past 320 million years. We see that during the last 250 million years, the directional variability of the field seems to have varied little, even though the frequency of reversals varied massively. Some numerical simulations of the dynamo process occurring in Earth's core had previously suggested that both of these two things were directly correlated to the magnitude of heat flowing from the outer core to the mantle. Our new results suggest a more complex, nuanced picture. Key Points: A new data set, paleosecular variation of the Paleogene, comprising 1,667 paleomagnetic directions from 45 different localities of volcanics aged between 84 and 23 MaMedian equatorial dispersions of virtual geomagnetic poles have overlapping uncertainty bounds since 250 MaLarge fluctuations in the reversal frequency are not associated with large fluctuations in the inferred median axial dipole dominance [ABSTRACT FROM AUTHOR]

Abstract Paleosecular variation analysis is a primary tool for characterizing ancient geomagnetic behavior and its evolution through time. This study presents a new high‐quality directional data set, paleosecular variation of the Paleogene (PSVP), with and without correction for serial correlation, compiled from 1,667 sites from 45 different localities from the Paleogene and late Cretaceous (84–23 Ma). The data set is used to study the variability, structure, and latitude dependence of the geomagnetic field during that period by varying selection criteria and PSV models. Modeled values for the equatorial virtual geomagnetic pole (VGP) dispersion have over‐lapping uncertainty intervals within their uncertainty bounds between 8.3° and 18.6° for the past 250 Ma. We investigate the suitability of two descriptive models of PSV, Model G‐style quadratic fits and covariant Giant Gaussian Process models, and find that both styles of model fail to satisfactorily reproduce the latitude dependent morphology of PSV, but suggest that estimates of the equatorial VGP dispersion may still robustly characterize aspects of Earth's long‐term field morphology. During this time where the PSV behavior has not changed substantially, the reversal frequency has varied widely. The lack of a clear relationship between PSV behavior and reversal frequency is not trivially explained in the context of published findings regarding numerical geodynamo simulations.

In this study, the characteristics of azimuthally asymmetric equivalent potential temperature (θe) distributions in the outer core of tropical cyclones (TCs) encountering weak and strong vertical wind shear are examined using a Lagrangian trajectory method. Evaporatively forced downdrafts in the outer rainbands can transport low-entropy air downward, resulting in the lowest θe in the downshear-left boundary layer. Quantitative estimations of θe recovery indicate that air parcels, especially those originating from the downshear-left outer core, can gradually revive from a low entropy state through surface enthalpy fluxes as the parcels move cyclonically. As a result, the maximum θe is observed in the downshear-right quadrant of a highly sheared TC. The trajectory analyses also indicate that parcels that move upward in the outer rainbands and those that travel through the inner core due to shear make a dominant contribution to the midlevel enhancement of θe in the downshear-left outer core. In particular, the former plays a leading role in such θe enhancements, while the latter plays a secondary role. As a result, moist potential stability occurs in the middle-to-lower troposphere in the downshear-left outer core. [ABSTRACT FROM AUTHOR]

The asymmetric distribution of convective available potential energy (CAPE) in the outer core of sheared tropical cyclones (TCs) is examined using the National Centers for Environmental Prediction Final operational global analysis data. Larger (smaller) CAPE tends to appear in the downshear (upshear) semicircle. This downshear-upshear contrast in CAPE magnitude becomes much more statistically significant in moderate-to-strong shear. The azimuthally asymmetric CAPE is closely associated with the near-surface equivalent potential temperature (θe). Larger surface winds occur in the upshear semicircle in strongly sheared TCs, contributing to larger surface latent heat fluxes in those quadrants. More low-level air well fueled by the larger surface latent heat fluxes in the upshear quadrants is cyclonically advected into the downstream quadrants. As a result, larger near-surface θe and CAPE are found in the outer core in the downshear quadrants. [ABSTRACT FROM AUTHOR]

Abstract Fowl cholera caused by Pasteurella multocida exerts a massive economic burden on the poultry industry. Lipopolysaccharide (LPS) is essential for the growth of P. multocida genotype L1 strains in chickens and specific truncations to the full length LPS structure can attenuate bacterial virulence. Here we further dissected the roles of the outer core transferase genes pcgD and hptE in bacterial resistance to duck serum, outer membrane permeability and virulence in ducks. Two P. multocida mutants, ΔpcgD and ΔhptE, were constructed, and silver staining confirmed that they all produced truncated LPS profiles. Inactivation of pcgD or hptE did not affect bacterial susceptibility to duck serum and outer membrane permeability but resulted in attenuated virulence in ducks to some extent. After high-dose inoculation, ΔpcgD showed remarkably reduced colonization levels in the blood and spleen but not in the lung and liver and caused decreased injuries in the spleen and liver compared with the wild-type strain. In contrast, the ΔhptE loads declined only in the blood, and ΔhptE infection caused decreased splenic lesions but also induced severe hepatic lesions. Furthermore, compared with the wild-type strain, ΔpcgD was significantly attenuated upon oral or intramuscular challenge, whereas ΔhptE exhibited reduced virulence only upon oral infection. Therefore, the pcgD deletion caused greater virulence attenuation in ducks, indicating the critical role of pcgD in P. multocida infection establishment and survival.

The characteristics of convective-scale downdrafts in the outer core of tropical cyclones in the lower- and upper-layer vertical wind shear (VWS) are investigated based on two high-resolution idealized numerical experiments. Four types of outer-core downdrafts, originating from the lower troposphere, the midtroposphere, the upper level, and the tropopause, respectively, are found. The downdrafts originating from the lower and midtroposphere can penetrate down near the surface, and those originating from the tropopause in upper-layer VWS tend to penetrate more downward than in lower-layer VWS. Downdrafts tend to be located in the more upwind portion of the downshear-right quadrant in lower-layer VWS than in upper-layer VWS. The frequency of downdrafts outside and upwind of the parent updraft increases with the increasing downdraft top height. Vertical momentum budgets indicate that downward-oriented buoyancy due to the evaporational cooling of rainwater and precipitation drag mainly contribute to the occurrence of low-level downdrafts, and the midlevel and upper-level downdrafts originate due to precipitation drag and are strengthened by the downward-oriented, buoyancy-induced perturbation pressure gradient. The processes governing the downdrafts from the tropopause are different between the two experiments. More icy-type particles are produced and transported outward at upper levels in the lower-layer shear experiment, resulting in larger downward-oriented buoyancy due to the sublimational cooling of icy-type particles and contributing to the development/maintenance of the downdraft from the tropopause in that experiment. However, the downward-oriented perturbation pressure gradient leads to the development/maintenance of the downdraft from the tropopause in the upper-layer shear experiment.

Many dynamo studies extrapolate numerical model results to planetary conditions by empirically constructing scaling laws. The seminal work of Christensen and Aubert (2006) proposed a set of scaling laws that have been used throughout the geoscience community. These scalings make use of specially-constructed parameters that are independent of fluid diffusivities, anticipating that large-scale turbulent processes will dominate the physics in planetary dynamo settings. With these 'diffusion-free' parameterizations, the results of current numerical dynamo models extrapolate directly to fully-turbulent planetary core systems; the effects of realistic fluid properties merit no further investigation. In this study, we test the validity of diffusion-free heat transfer scaling arguments and their applicability to planetary conditions. We do so by constructing synthetic heat transfer datasets and examining their scaling properties alongside those proposed by Christensen and Aubert (2006). We find that the diffusion-free parameters compress and stretch the heat transfer data, eliminating information and creating an artificial alignment of the data. Most significantly, diffusion-free heat transfer scalings are found to be unrelated to bulk turbulence and are instead controlled by the onset of non-magnetic rotating convection, itself determined by the viscous diffusivity of the working fluid. Ultimately, our results, in conjunction with those of Stelzer and Jackson (2013) and King and Buffett (2013), show that diffusion-free scalings are not validated by current-day numerical dynamo datasets and cannot yet be extrapolated to planetary conditions.

Pseudomonas aeruginosa is a highly prevalent gram-negative bacterium that is becoming more difficult to treat because of increasing antibiotic resistance. As chemotherapeutic treatment options diminish, there is an increased need for vaccines. However, the creation of an effective P. aeruginosa vaccine has been elusive despite intensive efforts. Thus, new paradigms for vaccine antigens should be explored to develop effective vaccines. In these studies, we have focused on the synthesis of two L-rhamnose–bearing epitopes common to glycoforms I and II of the outer core domain of Pseudomonas aeruginosa lipopolysaccharide, α-L-Rha-(1→6)-α-D-Glc-(1→4)-α-D-GalN-(Ala)-α-aminooxy (3) and α-L-Rha-(1→3)-β-D-Glc-(1→3)-α-D-GalN-(Ala)-α-aminooxy (4), respectively. The target trisaccharides were both prepared starting from a suitably protected galactosamine glycoside, followed by successive deprotection and glycosylation with suitably protected D-glucose and L-rhamnose thioglycosides. Global deprotection resulted in the formation of targets 3 and 4 in 22 and 35% yield each. Care was required to modify basic reaction conditions to avoid early deprotection of the N-oxysuccinamido group. In summary, trisaccharides related to the L-rhamnose–bearing epitopes common to glycoforms I and II of the outer core domain of Pseudomonas aeruginosa lipopolysaccharide have been prepared as their aminooxy glycosides. The latter are expected to be useful in chemoselective oxime-based bioconjugation reactions to form Pseudomonas aeruginosa vaccines.

The transport properties of liquid iron alloys at high pressure (P) and temperature (T) are essential for understanding the formation, composition, and evolution of planetary cores. Light alloying elements (e.g., Si, O, S, C, N, H) in liquid iron are particularly relevant due to the density deficit of Earth's core, yet high P-T experimental diffusion studies involving such alloys remain scarce, with large uncertainties on the P and T dependence required for extrapolation to core conditions. In this study, we measured the chemical diffusion of carbon in liquid iron over a P-T range of 3-15 GPa and 1700-2450 K using a multi-anvil apparatus. Diffusion couples consisting of pure Fe and Fe-2.5 wt%C cylinders were placed end-to-end in an MgO capsule in a vertical orientation. Carbon concentration profiles were measured by electron microprobe and modeled numerically to correct for non-isothermal diffusion that occurred prior to reaching the peak temperature. Carbon diffusion coefficients range from 6 × 10-9 m2s-1 to 2 × 10-8 m2s-1, with global Arrhenian fit parameters D0 = 1.4 ± 0.5 × 10-7 m2s-1, AE = 43 ± 6 kJ/mol, and ∆V = -0.06 ± 0.19 cm3mol-1. A negligible P effect is consistent with previous studies of oxygen diffusion in liquid iron and high-T calculations but differs from larger ∆V values previously reported from carbon self-diffusion experiments for liquid Fe3C and simulations for an Fe-Ni-C alloy. Carbon diffusion coefficients determined here are approximately three times faster than those reported from Fe-Ni-C liquid simulations, which highlights the potential significance of compositional effects on mass transport properties of liquid iron alloys and the need for expanding the P-T experimental diffusion data set currently available in the literature to more complex and geologically relevant compositions. [ABSTRACT FROM AUTHOR]

Exploring geodynamo's scaling laws is of great importance when a tremendous gap still lies between realistic physics and estimated model parameters. Existing scaling laws need to be tested by numerical simulations. To boost these, taking the outer core viscosity ν, we studied its impacts on weak and strong field geodynamo outputs by varying ν in two orders of magnitudes. In the weak field mode, the fluid velocity u varies with ν by a scaling law of u∼ν0.52. While in the strong field mode, u varies very slowly by a scaling law of u∼ν0.10. The magnetic field B does not change much with ν when the driving force is not too strong (Ra=12,000) but decreases with ν by a scaling law of B∼ν−0.20 when geodynamo operates in a very vigorous mode (Ra=50,000). The reason that u increases with ν is essential that increasing ν breaks the Taylor‐Proudman constraint and drops the critical Rayleigh number, but this kind of increase of u does not give a stronger B. Furthermore, we conducted a local force balance analysis and demonstrated that the balance shifts under different ν. In a quasi‐MAC regime, the Lorentz force instead of the inertial force enters into the first‐order agostrophic force balance, though the viscous force still plays a role. Comparing with other scaling laws previous wisdom holds, we propose that the effect of viscosity diffusion is non‐negligible for both existing weak and strong dynamo regimes. Still, appropriate assumptions can be made to take this variation into account. Key Points: The reason why velocity increases with increasing viscosity is givenA clever numerical method used to study the influence of viscosity diffusion on justifying weak or strong field geodynamo processThe direct scaling law between a certain physical parameter, viscosity, with velocity and magnetic field of the outer core [ABSTRACT FROM AUTHOR]

In this paper, the authors present the statistical characteristics of the buoyancy of outer-core convective-scale updrafts in numerically simulated sheared tropical cyclones (TCs). The total buoyancy is predominantly positive in weak-to-strong ambient vertical shears, whereas much of the total buoyancy under an extreme shear environment becomes negative. Thermal buoyancy positively contributes to the total buoyancy value. For weakly and moderately sheared TCs, the updraft buoyancy is statistically significantly stronger downshear but smaller upshear. Such a downshear preference of strong buoyancy becomes less evident as the shear magnitude increases. The total buoyancy of updrafts shows a decreasing tendency with radius. Both thermal and dynamic buoyancy do not significantly correlate with vertically averaged vertical mass fluxes. This also leads to no significant correlation between the total buoyancy and vertical mass fluxes of outer-core updrafts.

Fowl cholera caused by Pasteurella multocida exerts a massive economic burden on the poultry industry. Lipopolysaccharide (LPS) is essential for the growth of P. multocida genotype L1 strains in chickens and specific truncations to the full length LPS structure can attenuate bacterial virulence. Here we further dissected the roles of the outer core transferase genes pcgD and hptE in bacterial resistance to duck serum, outer membrane permeability and virulence in ducks. Two P. multocida mutants, ΔpcgD and ΔhptE, were constructed, and silver staining confirmed that they all produced truncated LPS profiles. Inactivation of pcgD or hptE did not affect bacterial susceptibility to duck serum and outer membrane permeability but resulted in attenuated virulence in ducks to some extent. After high-dose inoculation, ΔpcgD showed remarkably reduced colonization levels in the blood and spleen but not in the lung and liver and caused decreased injuries in the spleen and liver compared with the wild-type strain. In contrast, the ΔhptE loads declined only in the blood, and ΔhptE infection caused decreased splenic lesions but also induced severe hepatic lesions. Furthermore, compared with the wild-type strain, ΔpcgD was significantly attenuated upon oral or intramuscular challenge, whereas ΔhptE exhibited reduced virulence only upon oral infection. Therefore, the pcgD deletion caused greater virulence attenuation in ducks, indicating the critical role of pcgD in P. multocida infection establishment and survival. [ABSTRACT FROM AUTHOR]

Abstract—The structure of seismic wavefield at distances close to antipodal is analyzed using two numerical methods—DSM and AXISEM—for calculating full waveforms. The obtained empirical estimates of the amplitude ratio of PKIIKP and PKIKP phases indicate that the S-wave velocity in the upper 40 km of the inner core may not be higher than 3.1 km/s. The properties of the PKPc-dif waves suggest a lower P-wave velocity in the lower 100 km of the outer core. [ABSTRACT FROM AUTHOR]

Here we determined the P wave velocity of liquid Fe84Si16 up to 56 GPa based on inelastic X‐ray scattering measurements in a laser‐heated diamond‐anvil cell. We found that silicon significantly increases the P wave velocity of liquid iron under high pressure. The equation of state (EoS) of liquid Fe‐Si is obtained from the present sound velocity measurements. When extrapolating the present data to core pressures with the EoS, the silicon concentration must be limited to less than 1.9−0.5+1.2 wt.% in the outer core to explain its P wave velocity. In contrast, if silicon is the sole light element, 4.0−0.6+1.0 to 4.8−1.4+0.9 wt.% is necessary to account for the outer core density deficit. Thus, a liquid Fe‐Si cannot explain both the density and sound velocity simultaneously, suggesting that silicon is not the predominant light element in the core. Recent core formation models have predicted that the initial core contained 2 to 9 wt.% Si, which makes the P wave velocity much faster than that observed in the present‐day outer core. The core may have changed its composition by crystallizing SiO2 or (Mg,Fe)SiO3. It is possible that the initial core with <4 wt.% Si and ~5 wt.% O segregated from silicate under moderately oxidizing conditions and its composition evolved into a silicon‐poor one (<1.9 wt.% Si), which is compatible with the seismological observations. Such moderate Si content in the original core could also account for the high MgO/SiO2 and 30Si/28Si ratios in Earth's mantle relative to chondritic values. Key Points: Sound velocity of liquid Fe‐Si alloy was measured at high pressures to 56 GPaSilicon content in the present‐day outer core should be limited to 1.9 +1.2−0.5 wt.%Core may have formed under moderately oxidizing conditions, and its composition may have evolved into a Si‐poor one [ABSTRACT FROM AUTHOR]

Stably stratified layers are present in stellar interiors (radiative zones) as well as planetary interiors – recent observations and theoretical studies of the Earth's magnetic field seem to indicate the presence of a thin, stably stratified layer at the top of the liquid outer core. We present direct numerical simulations of this region, which is modelled as an axisymmetric spherical Couette flow for a stably stratified fluid embedded in a dipolar magnetic field. For strong magnetic fields, a super-rotating shear layer, rotating nearly 30% faster than the imposed rotation rate difference between the inner convective dynamo region and the outer boundary, is generated in the stably stratified region. In the Earth context, and contrary to what was previously believed, we show that this super-rotation may extend towards the Earth magnetostrophic regime if the density stratification is sufficiently large. The corresponding differential rotation triggers magnetohydrodynamic instabilities and waves in the stratified region, which feature growth rates comparable to the observed timescale for geomagnetic secular variations and jerks. In the stellar context, we perform a linear analysis which shows that similar instabilities are likely to arise, and we argue that it may play a role in explaining the observed magnetic dichotomy among intermediate-mass stars. [ABSTRACT FROM AUTHOR]

Vigorous convection in Earth's outer core led to the suggestion that it is chemically homogeneous. However, there is increasing seismic evidence for structural complexities close to the outer core's upper and lower boundaries. Both body waves and normal mode data have been used to estimate a P wave velocity, Vp, at the top of the outer core (the E′ layer), which is lower than that in the Preliminary Reference Earth Model. However, these low Vp models do not agree on the form of this velocity anomaly. One reason for this is the difficulty in retrieving and measuring SmKS arrival times. To address this issue, we propose a novel approach using data from seismic arrays to iteratively measure SmKS‐ SKKS differential travel times. This approach extracts individual SmKS signal from mixed waveforms of the SmKS series, allowing us to reliably measure differential travel times. We successfully use this method to measure SmKS time delays from earthquakes in the Fiji‐Tonga and Vanuatu subduction zones. SmKS time delays are measured by waveform cross correlation between SmKS and SKKS, and the cross‐correlation coefficient allows us to access measurement quality. We also apply this iterative scheme to synthetic SmKS seismograms to investigate the 3‐D mantle structure's effects. The mantle structure corrections are not negligible for our data, and neglecting them could bias the Vp estimation of uppermost outer core. After mantle structure corrections, we can still see substantial time delays of S3KS, S4KS, and S5KS, supporting a low Vp at the top of Earth's outer core. Key Points: We develop an array‐based iterative method to measure SmKS‐ SKKS (m=3–5) differential travel timesThree‐dimensional mantle structure effects must be considered in studies of SmKS differential travel timesOur measurements support a low Vp at the top of outer core [ABSTRACT FROM AUTHOR]

The amount of literature on both melting and thermal conductivity of iron at Earth's core conditions is overwhelming and the discrepancies are very large. There is a broad range of experimental techniques each of which is flawed to a certain degree, which may explain the discrepancy. In this report, we present new data using a different method for determining the phase behavior and resistivity of iron in the laser-heated diamond cell by measuring the electrical resistance of both solid and liquid iron wires. The experiment avoids some of the major flaws of previous experiments, the most important of which is the detection of the onset of melting. These measurements confirm a shallow melting curve found earlier and the resistivity data imply a trend towards low thermal conductivity in the liquid outer core. Image 1 • Thermal conductivity and melting of iron close to outer core condition was investigated. • New measurements in a laser-heated diamond cell is presented. • Low thermal conductivity and shallow melting curve was observed. • Low thermal conductivity supports the models of a slowly cooling core and an old core. [ABSTRACT FROM AUTHOR]

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We examined the phase relation of Fe2S to 306 GPa and 3,000 K and found that Fe2S forms an orthorhombic structure with space group Pnma above 190 GPa. Reexamination of previous X‐ray diffraction data demonstrated that hexagonal close‐packed Fe coexists with Pnma Fe2S at >2,700 K and ~290 GPa, while it concurs with CsCl (B2)‐type stoichiometric FeS at lower temperatures. Our results indicate that Fe2S is the most Fe‐rich iron sulfide above ~250 GPa where Fe3S is not stable. It is most likely that eutectic melting occurs between Fe and Fe2S at 330 GPa corresponding to inner core boundary conditions, instead of between Fe and Fe3S below 250 GPa. The compression curve of Fe2S obtained to 294 GPa shows that it is not dense enough to account for the inner core density, suggesting that the Fe‐Fe2S eutectic liquid gives the maximum sulfur concentration in the outer core. Plain Language Summary: The chemistry of the Earth's core has long remained enigmatic; the observed density and velocity profiles indicate that the core includes some light alloying elements. The present study focuses on sulfur, one of the major candidates, and experimentally investigates the phase relations in the Fe‐FeS system under high pressures relevant to the core. The main finding of this work is that 1) Fe2S adopts the orthorhombic Pnma structure above 190 GPa and 2) it is the most iron‐rich sulfide at >250 GPa where Fe3S decomposes. Our data also show that Fe2S is much lighter than the observed inner core density, suggesting that sulfur concentration in the liquid core is not high enough to crystallize Fe2S. Key Points: Fe2S undergoes a phase transition to the orthorhombic Pnma phase above 190 GPaFe and Fe2S form a eutectic system above ~250 GPa, where Fe3S is not stablePnma Fe2S is not dense enough to account for the inner core density [ABSTRACT FROM AUTHOR]

The weak intrinsic magnetic field of Mercury is intimately tied to the structure and cooling history of its metallic core. Recent constraints about the planet's internal structure suggest the presence of a FeS layer overlying a silicon-bearing core. We performed 4-electrode resistivity experiments on core analogues up to 10 GPa and over wide temperature ranges in order to investigate the insulating properties of core materials. Our results show that the FeS layer is liquid and insulating, and that the electrical resistivity of a miscible Fe-Si(-S) core is comparable to the one of an immiscible Fe-S, Fe-Si core. The difference in electrical resistivity between the FeS-rich layer and the underlying Fe-Si(-S) core is at least 1 log unit at pressure and temperature conditions relevant to Mercury's interior. Estimates of the lower bound of thermal conductivity for FeS and Fe-Si(-S) materials are calculated using the Wiedemann-Franz law. A thick (>40 km) FeS-rich shell is expected to maintain high temperatures across the core, and if temperature in this layer departs from an adiabat, then this might affect the core cooling rate. The presence of a liquid and insulating shell is not inconsistent with a thermally stratified core in Mercury and is likely to impact the generation and sustainability of a magnetic field. • Electrical resistivity measured up to 10 GPa on Fe-Si(-S) and CaS samples. • Increasing the Si content increases resistivity and Fe-Si alloys are less resistive than FeS. • The FeS-rich layer at the core-mantle interface of Mercury is liquid and insulating. • Lower bounds of thermal resistivity are calculated for Fe alloys. • A solid inner core might not be present in present-day Mercury core. [ABSTRACT FROM AUTHOR]

We determine mass transport and structural properties of binary liquid iron alloys over a wide density (5.055–11.735 g·cm−3) and temperature range (2,500–6,500 K) using first‐principles molecular dynamics. Compositions consist of 96 at% Fe and 4 at% ϕ, where ϕ = H, C, N, O, Mg, Si, S, or Ni. Self‐diffusion coefficients (D) of Fe and ϕ range from 3.5·10−9 to 1.9·10−7 m2·s−1. Results show a relation between mean atomic radius and diffusivity ratio for the alloying element and iron: Si and Ni are "iron‐like" with similar atomic radii and D compared with those of Fe; H, C, N, O, and S are "small non‐iron‐like" with smaller atomic radii and larger D; and Mg transitions from "large non‐iron‐like" with a larger atomic radius and smaller D at low density to iron‐like under conditions of the Earth's core. The effect of pressure on D for C, N, and O is negligible for densities below ~8 g·cm−3, accompanied by an increase in average coordination numbers to ~6, and an increase in mean atomic radii. For densities above ~8 g·cm−3, diffusivities and atomic radii of these elements decrease monotonically with pressure, which is typical for the iron‐like alloying elements as well as for H, Mg, and S over the whole compression range. While atomic radius ratios move toward unity with compression, diffusivity ratios for the alloying element relative to iron tend to increase for the "non‐iron‐like" elements with density. Plain Language Summary: The Earth's core is mostly iron with substantial amounts of other elements that dissolved into core‐forming metallic melt during the planet's accretion. Although the exact composition of the Earth's core remains unknown, we can obtain important insight into its chemistry and ongoing physical processes by studying the behavior of alloying elements dissolved in liquid iron, such as their transport rates and incorporation mechanisms. Here we investigate the behavior of eight elements (hydrogen, carbon, nitrogen, oxygen, sulfur, silicon, magnesium, and nickel) that are relevant candidates for the Earth's core composition using molecular dynamic simulations, which allow extensive exploration of the effects of pressure and temperature beyond conditions accessible in the laboratory. Our results show a distinct relationship between an alloying element's size and its diffusivity, which can be used to predict these properties for other elements not studied here. Differences in diffusivity mean that certain elements equilibrated in core‐forming liquids faster than others. Differences in atomic size mean that certain elements fit more easily into the liquid iron structure than others, which affects their solubility. Understanding the trends by which diffusion rates and atomic radii change with pressure and temperature allows accurate modeling of the Earth's core under extreme conditions. Key Points: Molecular dynamic simulations show relationship between mass transport and structural properties of alloying elements in liquid FeRadius ratio of alloying atoms and Fe approaches unity at high pressure, while diffusivity ratio of small alloying atoms and Fe increasesBinary alloy viscosity is strongly reduced by H, weakly reduced by C, N, O, or S at high pressure, and unaffected by Si, Mg, or Ni [ABSTRACT FROM AUTHOR]

The convective state of the top of Earth's outer core is still under debate. Conflicting evidence from seismology and geomagnetism provides arguments for and against a thick stably stratified layer below the core-mantle boundary. Mineral physics and cooling scenarios of the core favor a stratified layer. However, a non-zero secular variation of the total geomagnetic energy on the core-mantle boundary is evidence for the presence of radial motions extending to the top of the core. We compare the secular variation of the total geomagnetic energy with the secular variation of the geomagnetic dipole intensity and tilt. We demonstrate that both the level of cancellations of the sources and sinks of the dipole intensity secular variation, as well as the level of cancellations of the sources and sinks of the dipole tilt secular variation, are either larger than or comparable to the level of cancellations of the sources and sinks of the total geomagnetic energy secular variation on the core-mantle boundary, indicating that the latter is numerically significant hence upwelling/downwelling reach the top of the core. Radial motions below the core-mantle boundary are either evidence for no stratified layer or to its penetration by various dynamical mechanisms, most notably lateral heterogeneity of core-mantle boundary heat flux.