Outer Core Structure Research Articles

Estimating lateral structure in the Earth's outer core

Lateral density structure could exist in the Earth's fluid outer core, owing to the gravitational perturbations induced by the asphericity of the mantle. We take a realistic tomographic image of lateral heterogeneities in the topography of the core—mantle boundary (CMB), as an upper bound for the amplitude of lateral structure in the surface of constant gravitational potential, at the same average depth. With this boundary condition, we apply the theory of Wahr and de Vries [1989], to estimate quantitatively the corresponding lateral density anomalies in the outer core structure; we find that their value, relative to PREM, must be everywhere smaller than ∼0.1%. The resulting, theoretical maps of the outer core are dominated by long‐wavelength structure, with a pattern closely resembling that of the assumed CMB topography; they do not correlate with any of the existing tomographic images of the core.

Tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases (PKP, Pdiff)

The mapping of variations in P wave speed in the deep mantle is restricted by the uneven sampling of P waves, in particular beneath the Southern Hemisphere. To enhance data coverage, we augmented the ∼1.6 million summary rays of P, pP, and pwP that we used in previous studies with differential travel times of diffracted and refracted core phases. For the core‐refracted differential travel time residuals (PKPAB‐PKPDF and PKPAB‐PKPBC) we used 1383 cross‐correlated digital waveforms as well as ∼27,000 routinely processed bulletin data. We used the waveform data to define quality criteria for the selection and reduction of the bulletin PKP data. For PKPDF‐Pdiff we only considered 543 records derived from waveform cross correlation. No PcP data were used in this study. We used optical ray theory to calculate the ray paths associated with the P, pP, pwP, and PKP data, which are measured at 1 Hz. However, to account for the large Fresnel zones of the low‐frequency (∼50 mHz) PKPDF‐Pdiff data we estimated the three‐dimensional shape of the Fréchet sensitivity kernels from kernels calculated by normal mode summation. The use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high‐frequency data to constrain small‐scale structure. The differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers, and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary (CMB). However, images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle. Here, we do not make a priori assumptions about the mantle source of anomalous time differentials. From test inversions we conclude that (both upper and lower) mantle structures that are poorly resolved by P data can be mapped into the core along PKP paths but that the effect of outer core structures, if any, on the mantle model is small. Compared to the inversion of the P, pP, and pwP alone, the inclusion of the PKPAB‐PKPDF and PKPAB‐PKPBC and PKPDF‐Pdiff data improves the resolution of structure beneath 2200 km depth. In particular, the joint inversion puts better constraints on the long‐wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the CMB that is in agreement with but still smaller than inferences from shear wave studies. Resolution tests indicate that in some regions the enhanced definition of structure is significant, but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle.

A Modification of the Inner and Outer Core for Reactor Pressure Vessel Lifetime Extension

The feasibility of nuclear power plant lifetime extension was examined by reducing the fast neutron fluence at the reactor pressure vessel (RPV) and relieving irradiation embrittlement of materials, and thus ensuring enough structural integrity beyond the design lifetime. Two fluence reduction options, peripheral assembly replacement and additional shield installation in the outer core structures, were applied to the Kori Unit-1 reactor, and the fluence reduction effect was carefully analyzed. For an accurate estimate of the neutron fluence at the RPV and a reasonable description of the modified peripheral assemblies, a full-scope explicit modeling of a Monte Carlo simulation was employed in all calculations throughout this study. The Kori Unit-1 cycle-16 core was modeled on a three-dimensional representation by using the MCNP4B code, and the fluence distribution was estimated at the inner wall beltline around the circumferential weld of the RPV. On the basis of fracture toughness requirements of the RPV, the two modified cases were predicted to have an additional life of 7 to 10 effective full-power years. Throughout the core nuclear characteristics analyses, it was confirmed that the critical peaking factors for safe reactor operation were satisfied with the design limits.

Anomalous splitting of free oscillations: A reevaluation of possible interpretations

The splitting of normal modes sensitive to structure in the inner core is larger than can be explained by three‐dimensional heterogeneity confined to the earth's mantle. A preferred interpretation of this anomalous splitting involves inner core anisotropy, providing a unified explanation of these data and of observed trends in the travel times of PKP waves that sample the inner core. We reexamine this interpretation, as well as a previously suggested alternative one in terms of outer core structure. Our motivation comes from recent results which indicate that simple, smooth models of inner core anisotropy are in disagreement with some PKP observations. We invert a recently assembled high quality dataset, comprising modes sensitive to mantle and outermost core structure, as well as modes sensitive to mantle, outer core and inner core structure. We compare models parametrized to include either inner core anisotropy or heterogeneity in the outer core. We show the following: (1) Outer core models, with fewer parameters, provide good overall fits to most modes with weak or strong sensitivity in the inner core, except for mode 3S2, the inner core mode with the strongest splitting. (2) Vs and ρ profiles obtained for models with outer core structure using mantle and core modes are in better agreement with those obtained using data sensitive to mantle structure alone, whereas the Vp structure in D′ shows a stronger c20 component when inner core anisotropy is considered. (3) Lateral heterogeneity restricted to the mantle, in particular strong heterogeneity in D′, fails to account consistently for the splitting of all modes. Outer core models thus deserve further investigation and should be weighted against more complex models of inner core anisotropy to explain anomalous splitting of normal modes and PKP travel times. The outer core structure obtained, while arguably unrealistic, is consistent with concentration of light elements, either confined to the Taylor cylinder tangent to the inner core or in polar caps at the top and bottom of the outer core. A small amount of shear within such polar caps might account for the excess splitting of some core modes.

A Fast Neutron Fluence Reduction Strategy for Pressure Vessel Lifetime Extension

The feasibility of nuclear power plant lifetime extension was examined by the evaluation of fast neutron (>1 MeV) fluence at the reactor pressure vessel (RPV). A fluence reduction option, additional shields installation in outer core structures, was applied to the Kori Unit 1 reactor, and its fluence reduction effect was carefully analyzed. Full-scope explicit modeling of Monte Carlo simulation with MCNP4A code was employed in the fast neutron fluence calculations. An optimized design option was found from various choices in geometry and material for shield structure. It is expected that the magnitude of fast neutron fluence would be reduced by 39% at the circumferential weld of the RPV. On the basis of the criterion of pressurized thermal shock requirement of the RPV, it is predicted to obtain the additional life of 4.4 effective full power years. It was also investigated that the nuclear characteristics and thermal hydraulic factors at the internal core were negligibly influenced by the installation of additional shield structure.

Conformations and lipophilicity profiles of some cyclic β-(1→3)- and β-(1→6)-linked oligogalactofuranosides

The conformational features of cyclooligosaccharides composed of β-(1→3)- and β-(1→6)-linked galactofuranose units, i.e., cyclo[ d-Gal fβ-(1→3)] n with n=4 ( 1) and 5 ( 2), and cyclo[ d-Gal f β-(1→6)] n with n=3 ( 3) and 4 ( 4), were investigated by means of Monte Carlo simulations. The flexibility of the macrocyclic backbone strongly favors bent and asymmetrical conformations over round geometries. Generation of the molecular surfaces of the global minimum-energy structures reveals disk-type shapes for 1– 4 without through-going central cavities, yet distinct indentations close to the O-2/O-3 groups, respectively. The molecular lipophilicity patterns prove these surface dents to be hydrophobic for the β-(1→6)-linked cyclogalactans 3 and 4, whereas their β-(1→3)-linked counterparts display an inverse situation with a hydrophobic outer core structure.

Numerical simulation of tropical cyclones by an axisymmetric nonhydrostatic model

As an approach to study the mesoscale processes within a typhoon, an axisymmetric nonhydrostatic numerical model is developed without the use of convective parameterization. Many simulated characteristics are consistent with radar and aircraft observations, such as the maximum of vertical and tangential wind, the inflow concentrated near the surface, the outward slope of the eyewall updraft, etc. The model duplicates not only the outward propagation of mesoscale convective systems, but also the inward movement of convective rings, the rate of which coincides with the observation. Besides, the model gives good simulations of the life cycle of convective rings, and indicates that the convective rings far from the eyewall play important roles in the fluctuation of typhoon intensity. Numerical results also exhibit the existence of coupling between outer and inner core structure. Analyses of the simulations show that convective momentum transport generates local maximum absolute angular momentum in the middle and upper troposphere. The momentum anomaly results in symmetric instability, which provides the environment to form convective rings. While the momentum anomaly moves outward with the outflow in the middle and upper troposphere, it initiates a series of convective rings with aid of other direct factors, which explains the outward propagation of convective systems. The simulations exhibit the life cycle of a typical convective ring in terms of three stages, or the developing, mature and dissipating stage. Analysis shows that the symmetric instability and the convective instability promote each other, and their cooperation makes the life of convective rings longer.

Constraining outermost core velocity with SmKS waves

SmKS waves (m=2,3,4), seismic waves that travel as S‐waves in the mantle, P‐waves in the core, and reflect (m‐1) times on the underside of the core‐mantle boundary, are well‐suited for constraining outermost core VP structure. This is due to closeness of the mantle paths and also the shallow depth range these waves travel in the outermost core. High quality digitized WWSSN and Canadian network recordings from a deep focus Java Sea event which sample the outer core beneath the northern Pacific, the Artic, and northwestern North America (roughly ⅛th of the globe), are utilized as an example to show the strength of SmKS waves to resolve outermost core structure. S3KS‐S2KS and S4KS‐S3KS differential travel times were measured and compared to those from reflectivity synthetics created from core models of past studies. Constraints and uncertainties in outer core structure using SmKS are discussed. For the event studied, the PREM core model, with possibly a small VP decrease in the outermost 50 km of the core, provides a good fit to the data.

Travel times of S and SKS: Implications for three‐dimensional lower mantle structure beneath the central Pacific

The travel times of S and SKS phases from deep‐focus Fiji‐Tonga events recorded in North America by Canadian Station Network and World Wide Seismographic Station Network stations exhibit a strong azimuthal dependence. The differential times of S‐SKS exhibit relatively little scatter and are used to investigate three‐dimensional mantle models for the central Pacific region. S‐SKS times are corrected for Earth's ellipticity, and residuals are computed with respect to the iasp91 reference model. Anomalously large S‐SKS times, ranging up to 9 s larger than iasp91 predictions for Δ > 100°, suggest a slow lower mantle beneath the central Pacific. These results are compared to the predictions of three tomographic models: MDLSH of Tanimoto (1990), model SH12_WM13 of Su et al. (1992), and model SH. 10c. 17 of Masters et al. (1992). The three‐dimensional whole mantle models have been parameterized into 11 spherical shells. Qualitative agreement between the tomographic model predictions and observations is encouraging, varying from fair to good. However, inconsistencies are present and suggest anomalies in the lower mantle of scale length smaller than the present 2000 + km scale resolution of tomographic models. Laterally varying outer core structure is not necessary to explain the anomalies in this data set, although such a scenario cannot be resolved with this data.

Structural differences in the outer core region of lipopolysaccharides derived from members of the genus Salmonella

Spontaneous and P22-resistant rough mutants, respectively, selected from Salmonella IV (18: z36, z38:-) and S. djakarta (48: z4, z24:-), appeared to lack the epitope recognized by the T6 monoclonal antibody which had been previously shown to correspond to the terminal alpha-1,2-linked N-acetyl-D-glucosamine residue of the Salmonella lipopolysaccharide (LPS) Ra core. LPSs and core oligosaccharides were therefore prepared from these two rough mutants and analysed by chemical and serological methods. Sugar analyses as well as methylation and 13C-NMR studies indicated that rough mutants derived from these two serotypes indeed possessed outer core structures differing from those of the well-characterized Salmonella Ra core. Serological data corroborated the chemical findings. Proposed structures of the outer core regions of these two R-types are presented and the significance of the findings is discussed.

Lack of the α-1,2-linked N-acetyl-D-glucosamine epitope in the outer core structures of lipopolysaccharides from certain O serogroups and subspecies of Salmonella enterica

A total of 176 strains of Salmonella enterica representing 116 serotypes were tested for the presence of the T6 epitope of the α-1,2-linked N-acetyl-D-glucosamine residue by reaction with a murine monoclonal antibody T6 specific for this structure in the Salmonella Ra core lipopolysaccharide (LPS). All 20 serotypes (70 strains) belonging to serogroups A to E were positive for the T6 epitope while 29% of the 96 serotypes (106 strains) belonging to O serogroups F to 67 were negative; 12 serotypes (12 strains) of subspecies IIIb Salmonella were positive for the T6 epitope, but 10 serotypes (11 strains) of subspecies IIIa Salmonella were found to lack this epitope. In T6-positive strains, the epitope was accessible to antibody binding in both the unsubstituted free rough core LPS and in the rough core LPS substituted with a few repeating units of O side chains. The presence or absence of the T6 epitope in Salmonella strains was not affected by culture conditions, the source of the isolate, the age of the culture or the presence of fimbriae antigens.

A Murine Monoclonal Antibody that Recognizes a Genus-Specific Epitope in the Salmonella Lipopolysaccharide Outer Core

A murine monoclonal antibody 105 made from spleen cells of a mouse immunized with a mixture of common Salmonella serotypes reacted specifically with salmonellae from the most frequently encountered O serogroups of A (O:2) to E (O:3), and with strains from the less common O serogroups that represent the subspecies I, II, IIIb, IV, V and VI. Specificity for Salmonella was demonstrated by the lack of reactivity of monoclonal antibody 105 with any of the 30 other different species of Gram-positive and Gram-negative bacteria tested including 16 species in the family of Enterobacteriaceae. Studies to elucidate its binding epitope have shown that it reacts with the three distal sugar residues joined through specific anomeric linkages as present only in the Salmonella lipopolysaccharide outer core, which explains its specificity for the Salmonella. The failure of monoclonal antibody 105 to react with a subspecies IIIa Salmonella suggested a different outer core structure in this strain of Salmonella and also that monoclonal antibodies to the outer core of Salmonella lipopolysaccharide should be useful in the molecular analysis of their diversity.

Smooth lipopolysaccharide of salmonella adelaide has an atypical salmonella ra core

Two wild isolates as well as two laboratory strains of Salmonella adelaide obtained from different geographical areas failed to react with a monoclonal antibody directed against the terminal α-1,2-linked N-acetylglucosamine residue of the outer core of Salmonella lipopolysaccharide (LPS). This finding was confirmed by the lack of reactivities of Salmonella Ra LPS with S. adelaide antiserum or of S. adelaide R oligossacharide with Salmonella Ra serum. Furthermore, S. adelaide proved to be resistant to lysis by phage FO1, which binds to a receptor also believed to involve the terminal α-1,2-linked N-acetylglucosamine of the Salmonella R oligosaccharide. These results suggest that the outer core structure of S. adelaide LPS may be different from that of S. typhimurium and other Salmonella strains.

A murine monoclonal antibody specific for the outer core oligosaccharide of Salmonella lipopolysaccharide

Immunoglobulin G3 murine monoclonal antibody T6 specific for the lipopolysaccharide of Salmonella O serogroups A to E was established. By using R mutants of Salmonella spp., Escherichia coli, and Shigella spp., the major reactive epitope with T6 was tentatively identified as the terminal disaccharide, N-acetylglucosamine 1.2----alpha glucose, of the core oligosaccharide. T6 was reactive with 10 clinical isolates of each of the Salmonella O serogroups A to E but not with 58 isolates of other gram-negative bacteria. Its selective reactivity against Salmonella spp. renders T6 a potentially more useful reagent than the conventional polyvalent serum for the identification of Salmonella spp. It may also serve as a useful molecular tool for the study of the outer core structure of all Salmonella and related species.

Observations of anomalous splitting and their interpretation in terms of aspherical structure

The problem of inverting for the aspherical structure of the earth is complicated by the nonlinear dependence of low‐frequency seismic waveforms on aspherical structure. In an attempt to overcome this obstable, we report on the application of two complementary techniques. The first, a data space technique called singlet stripping, which linearly recombines seismic recordings to estimate singlet resonance functions, has been applied to 190 International Deployment of Accelerometers and Global Digital Seismographic Network recordings from five large events. More than 290 singlets from 34 low harmonic degree multiplets appear to have been resolved. A subset of these measurements has been compared with those produced from the second technique, a nonlinear regression, which iteratively estimates coefficients which are linear functionals of aspherical structure. Both techniques agree that most multiplets are normally split, with singlet frequency distributions insignificantly different from those predicted for a rotating, hydrostatic (RH) earth model. The main result of this paper is that both techniques also agree that approximately a third of the multiplets are anomalously split, some of which span frequency bands up to 2.5 times greater than predicted for an RH model. All of the anomalously split multiplets are SKS, PKP, or PKIKP equivalent. The observation of anomalously widely split multiplets is highly robust and provides compelling evidence for the existence of deep large‐scale, nonhydrostatic aspherical structure. The inverse problem for the axisymmetric part of aspherical structure has been performed in the hope of illuminating anomalous splitting. Unless a large amount of structure in the core is included, we are unable to construct a smooth axisymmetric model which accurately predicts the splitting characteristics for the anomalous multiplets while simultaneously fitting the normally split multiplets. The location and nature of this core heterogeneity are unclear, but we find that a simple outer core structure is sufficient to give a reasonable fit to the data. There are good theoretical reasons for believing that such nonhydrostatic outer core structure is geophysically unreasonable, yet differential travel time data sensitive to core structure apparently require similar large scale heterogeneity in the core. Although this dilemma remains unresolved, spectral fitting techniques like the nonlinear regression can be applied to many more multiplets than considered here and it is not unreasonable to predict that reliable large scale aspherical models of the deep earth soon will become available.

Monoclonal antibodies to enterobacterial common antigen and to Escherichia coli lipopolysaccharide outer core: demonstration of an antigenic determinant shared by enterobacterial common antigen and E. coli K5 capsular polysaccharide

We established hybridoma cell lines producing monoclonal antibodies against enterobacterial common antigen (ECA) and a substructure of the outer core of different Escherichia coli lipopolysaccharides (LPSs). Anti-ECA antibodies 865 and 898 reacted with ECA in extracts of heated E. coli and with ECA-bound R1 and R4 core-containing LPS preparations, as well as with a purified sample of ECA from Salmonella montevideo. Antibody 865, but not antibody 898, cross-reacted with K5 capsular polysaccharide, suggesting that 4-linked alpha-N-acetylglucosamine is part of an antigenic determinant shared by both K5 polysaccharide and ECA. Anti-LPS antibody 786 recognized an outer core structure common to E. coli K-12, B, R2, and R4 core type LPS, but not to R1 and R3 core type LPS. Its most probable target is the trisaccharide sequence Hexp(1----2)-alpha-D -Glcp(1----3) alpha-D-Glcp----(Hepp) (where Hex is hexose, p is phosphate, Glc is glucose, and Hep is heptose), the first glucose being the immunodominant moiety. These monoclonal antibodies may be used not only for the detection of ECA, K5, and LPS core structures but also for analysis of the molecular forms resolved on polyacrylamide gels (banding patterns) of both ECA and LPS, independently of one another.