The interaction of the solar wind with Earth’s bow shock gives rise to several distinct ion populations upstream and downstream of the shock. Using numerical computation of trajectories in the de ...Hoffmann‐Teller frame of reference and previous analytical results, we investigate whether membership of these populations is determined by a particle’s incident velocity in the solar wind rest frame. The shock is assumed to be a planar surface acting as an electrostatic potential jump. Ions with insufficient energy to overcome this potential are assumed to reflect specularly. We find that ions escaping upstream come from the wings of the incident distribution, assumed to be Maxwellian, and have incoming velocities in the plasma rest frame that point upstream close to the magnetic field direction. Ions escaping after several reflections have more field‐aligned incident velocities in the upstream frame on average than those escaping after one reflection. Different ion populations occupy separate regions of the incident distribution.
Plain Language Summary
The Earth’s bow shock decelerates the approaching supersonic solar wind. Unlike at ordinary hydrodynamic shock waves, the conversion of kinetic into thermal energy is accomplished in a collisionless way. An important contribution to this heating process is made by solar wind ions that reflect at the shock. After gyration in the upstream magnetic field, most of these ions return to the shock and pass downstream but some reflect again or even escape without further shock interactions. We use a mixture of previous analytical results and computation of ion trajectories in a model planar shock to try to establish whether these various groups of ions originate from different parts of the incident solar wind velocity distribution. We find that a particle’s individual upstream velocity in the solar wind rest frame does determine its belonging to one or other of the above ion populations. In particular, ions that escape the shock after reflection have high speeds in the incident distribution and velocities that are oriented along the upstream magnetic field.
Key Points
Different ion populations near low Mach number collisionless shocks arise from separate portions of the incident velocity distribution
Incoming plasma frame velocities for escaping ions point upstream close to the magnetic field and are from the wings of the distribution
Most escaping ions have further reflections and incident velocities more field aligned than the more energetic single‐reflection ions
A major goal in Earth Science has been to understand how geochemical characteristics of lavas at the Earth's surface relate to the location and formation history of specific regions in the Earth's ...interior. For example, some of the strongest evidence for the preservation of primitive material comes from low 4He/3He ratios in ocean island basalts, but the location of the primitive helium reservoir(s) remains unknown. Here we combine whole‐mantle seismic tomography, simulations of mantle flow, and a global compilation of new and existing measurements of the 4He/3He ratios in ocean island basalts to constrain the source location of primitive 4He/3He material. Our geodynamic simulations predict the present‐day surface expression of plumes to be laterally offset from their lower mantle source locations. When this lateral offset is accounted for, a strong relationship emerges between minimum 4He/3He ratios in oceanic basalts and seismically slow regions, which are generally located within the two large low shear‐wave velocity provinces (LLSVPs). Conversely, no significant relationship is observed between maximum 208Pb*/206Pb* ratios and seismically slow regions in the lowermost mantle. These results indicate that primitive materials are geographically restricted to LLSVPs, while recycled materials are more broadly distributed across the lower mantle. The primitive nature of the LLSVPs indicates these regions are not composed entirely of recycled slabs, while complementary xenon and tungsten isotopic anomalies require the primitive portion of the LLSVPs to have formed during Earth's accretion, survived the Moon‐forming giant impact, and remained relatively unmixed during the subsequent 4.5 billion years of mantle convection.
Plain Language Summary
Geochemical variations in volcanic rocks erupted at Earth's surface indicate differences in mantle composition, but our understanding of the location, formation, and history of compositionally distinct mantle domains remains incomplete. In particular, some hotspot lavas contain signatures of primitive regions within the mantle that have remained relatively isolated and unprocessed throughout Earth's history. Here we use models of mantle flow to predict the locations within the mantle that are sampled by the mantle plumes associated with hotspot volcanism. Combining these models of mantle flow with state‐of‐the‐art seismic images and a comprehensive catalog of hotspot lava geochemistry, we find that hotspots with a more primitive geochemical signature (as indicated by the isotopes of helium) sample the two large low shear‐velocity provinces in the lowermost mantle. Complementary constraints from xenon and tungsten isotope ratios associated with primitive materials then require these continent‐sized provinces in Earth's deep interior formed early in Earth's history, survived the violent Moon‐forming giant impact, and remained relatively unmixed with the rest of the solid Earth over the past 4.5 billion years.
Key Points
Geodynamic models predict the lateral offset of plume source locations in the lower mantle from their present‐day surface expressions
These models predict a strong relationship between primitive (low) 4He/3He ratios and seismically slow regions of the lower mantle
These models predict no significant relationship between recycled, high 208Pb*/206Pb* ratios, and seismically slow regions of the lower mantle
Depth-dependent geoacoustic properties are inferred from wide-angle frequency-domain reflection-coefficient data at two sites with different mud-layer thicknesses on the New England Mud Patch. A ...trans-dimensional Bayesian inversion is employed to estimate geoacoustic properties and uncertainties from these data using the viscous grain shearing sediment model and spherical wave reflection-coefficient predictions. Results near the thick-mud (SWAMI) site show a nearly uniform sound velocity over the upper approximately 9.2m, followed by a transition layer with velocity increasing nonlinearly by ~280 m/s over 1.8 m. At the thin-sediment (VC31-2) site, the velocity profile exhibits a similar transition layer. Estimates of intrinsic velocity and attenuation dispersion are also obtained. Over the measurement band of about 400-1300 Hz, the velocity in the fine-grained sediments (mud) at both sites varies by only a few meters per second, i.e., velocity is nearly independent of frequency. The attenuation of the fine-grained sediments at both sites follows a nearly linear frequency dependence. The geoacoustic inferences compare reasonably closely with independent measurements including core measurements, chirp-reflection data, and angle of intromission data.
Objective: Estimating the elasticity distribution in the cornea is important because corneal elasticity is usually influenced by corneal pathologies and surgical treatments, especially for early ...corneal sclerosis. Because the thickness of the cornea is typically less than 1 mm, high-resolution ultrasound elastography as well as the Lamb wave model is required for viscoelastic property estimation. In the present study, an array high-frequency ultrasound (HFUS) elastography method based on ultrafast ultrasound imaging was proposed for estimating the viscoelastic properties of porcine cornea. Methods: The elastic wave was generated by an external vibrator, after which the wave propagation image was obtained using a 40-MHz array transducer. Viscoelasticity estimation was performed by fitting the phase velocity curve using the Lamb wave model. The performance of the proposed HFUS elastography system was verified using 2-mm-thick thin-layer gelatin phantoms with gelatin concentrations of 7% and 12%. Ex vivo experiments were carried out using fresh porcine cornea with artificial sclerosing. Results: Experimental results showed that the estimated elasticity was close to the standard value obtained in the phantom study when the Lamb wave model was used for elasticity measurement. However, the error between the standard elasticity values and the elasticity values estimated using group shear wave velocity was large. In the ex vivo eyeball experiments, the estimated elasticities and viscosities were respectively 9.1 ± 1.3 kPa and 0.5 ± 0.10 Pa·s for a healthy cornea and respectively 15.9 ± 2.1 kPa and 1.1 ± 0.12 Pa·s for a cornea with artificial sclerosis. A 3D HFUS elastography was also obtained for distinguishing the region of sclerosis in the cornea. Conclusion: The experimental results demonstrated that the proposed HFUS elastography method has high potential for the clinical diagnosis of corneal diseases compared with other HFUS single-element transducer elastography systems.
Purpose
Compared to conventional arterial spin labeling (ASL) methods, velocity‐selective ASL (VSASL) is more sensitive to artifacts from eddy currents, diffusion attenuation, and motion. Background ...suppression is typically suboptimal in VSASL, especially of CSF. As a result, the temporal SNR and quantification accuracy of VSASL are compromised, hindering its application despite its advantage of being delay‐insensitive.
Methods
A novel dual‐module VSASL (dm‐VSASL) strategy is developed to improve the SNR efficiency and the temporal SNR with a more balanced gradient configuration in the label/control image acquisition. This strategy applies for both VS saturation (VSS) and VS inversion (VSI) labeling. The dm‐VSASL schemes were compared with single‐module labeling and a previously developed multi‐module schemes for the SNR performance, background suppression efficacy, and sensitivity to artifacts in simulation and in vivo experiments, using pulsed ASL as the reference.
Results
Dm‐VSASL enabled more robust labeling and efficient backgroud suppre across brain tissues, especially of CSF, resulting in significantly reduced artifacts and improved temporal SNR. Compared to single‐module labeling, dm‐VSASL significantly improved the temporal SNR in gray (by 90.8% and 94.9% for dm‐VSS and dm‐VSI, respectively; P < 0.001) and white (by 41.5% and 55.1% for dm‐VSS and dm‐VSI, respectively; P < 0.002) matter. Dm‐VSI also improved the SNR of VSI by 5.4% (P = 0.018).
Conclusion
Dm‐VSASL can significantly improve the robustness of VS labeling, reduce artifacts, and allow efficient background suppression. When implemented with VSI, it provides the highest SNR efficiency among VSASL methods. Dm‐VSASL is a powerful ASL method for robust, accurate, and delay‐insensitive perfusion mapping.
Entropy theory applied to hydrometric measurements enables establishing a relationship between the maximum and the mean velocities of flow passing a river section. In many developing countries, river ...discharge is still estimated by the velocity‐area method following the procedure established based on the point‐velocities measured at 0.6D depth from the water surface of many verticals of flow section. This study explores the establishment of a relationship between the maximum point‐velocity estimated at 0.6D depth of the vertical located at or nearer to the deepest flow depth, and the maximum point‐velocity occurring somewhere along the same vertical which in turn can be linked to the sectional mean flow velocity based on the established entropy theory. The appropriateness of the proposed two‐steps based approach of discharge estimation is first verified on the Tiber River and the Po River of Italy. The study finds that the values of the maximum point‐velocity estimated using the proposed approach closely matches with the recorded maximum flow velocity values along the same flow‐depths. After this verification, the proposed method was applied for discharge estimation at two Indian river gauging stations, where only the point‐velocity measurements at 0.6D depth locations of the verticals are recorded. The study finds that the discharges estimated by both the proposed approach and the velocity‐area method closely match with each other with the estimated Nash‐Sutcliffe Efficiency (NSE) values greater than 0.99. Thus the proposed two‐steps approach involving the entropy concept based relationship for discharge estimation enables to replace the tedious and time‐consuming velocity‐area approach.
Key Points
Proposes a new method to estimate discharge using u0.6D,max velocity measurement. Estimation of state equilibrium constant using the historical records of the velocity‐area method
It is establishing a relationship between u0.6D,max and umax
The potential of the approach is demonstrated through uncertainty analysis for the Indian Rivers
Aortic Dissection (AD) is a complex pathology that affects the aorta. Diagnosis, management and treatment remain a challenge as it is a highly patient-specific pathology and there is still a limited ...understanding of the fluid-mechanics phenomena underlying clinical outcomes. Although in vitro models can allow the accurate study of AD flow fields in physical phantoms, they are currently scarce and almost exclusively rely on over simplifying assumptions. In this work, we present the first experimental study of a patient-specific case of AD. An anatomically correct phantom was produced and combined with a state-of-the-art in vitro platform, informed by clinical data, employed to accurately reproduce personalised conditions. The complex AD haemodynamics reproduced by the platform was characterised by flow rate and pressure acquisitions as well as Particle Image Velocimetry (PIV) derived velocity fields. Clinically relevant haemodynamic indices, that can be correlated with AD prognosis – such as velocity, shear rate, turbulent kinetic energy distributions – were extracted in two regions of interest in the aortic domain. The acquired data highlighted the complex nature of the flow (e.g. recirculation regions, low shear rate in the false lumen) and was in very good agreement with the available clinical data and the CFD results of a study conducted alongside, demonstrating the accuracy of the findings. These results demonstrate that the described platform constitutes a powerful, unique tool to reproduce in vitro personalised haemodynamic conditions, which can be used to support the evaluation of surgical procedures, medical devices testing and to validate state-of-the-art numerical models.
Using logging data and samples collected by the four expeditions of the International Ocean Discovery Program NanTroSEIZE drilling campaign that occupied Site C0002 in the Nankai margin located ...southwest of Japan, we determined effective porosity and sonic velocity profiles down to 3 km below the seafloor in the accretionary prism, above the seismogenic plate interface. We measured cation exchange capacity to differentiate clay‐bound water content from effective porosity, which is representative of compaction. The decrease in effective porosity with depth is fitted with a single exponential decay, which suggests hydrostatic conditions for most of the borehole. The Erickson‐Jarrard template was adapted to account for the actual clay‐bearing nature of the sediments. We used this improved P‐wave velocity–effective porosity relationship to fill shipboard data gaps and predict P‐ and S‐wave velocity profiles at Site C0002.
Plain Language Summary
Effective porosity reflects the network of pores that affects the transport properties and the history of sediment compaction. However, evaluating this property is not straightforward given that standard methods provide total porosity, which also includes clay‐bound water. We can estimate the effective porosity using cation exchange capacity of the rocks. This method was used at Site C0002, the deepest hole in history of the International Ocean Discovery Program, drilled down to the heart of the Nankai accretionary prism, located southwest of Japan. We show that the effective porosity follows a simple exponential decay law with depth. We also demonstrate that there is a single relationship between effective porosity and P‐wave velocity. We use this relationship to fill the data gaps in the P‐ and S‐wave velocity profiles at Site C0002. Conversely, this provides a direct method to estimate effective porosity, and hence, the compaction state, from seismic profiles acquired across the Nankai accretionary prism.
Key Points
Effective porosity was derived from cation exchange capacity analysis on samples from Site C0002, the deepest International Ocean Discovery Program borehole ever drilled
Total and effective porosities decay exponentially all along Site C0002, down to the heart of the Nankai accretionary prism (3,000 mbsf)
Modifying the parameters of the Erickson‐Jarrard model using effective porosity and clay content improves the P‐wave velocity prediction
Purpose
Background suppression (BS) is recommended in arterial spin labeling (ASL) for improved SNR but is difficult to optimize in existing velocity‐selective ASL (VSASL) methods. Dual‐module VSASL ...(dm‐VSASL) enables delay‐insensitive, robust, and SNR‐efficient perfusion imaging, while allowing efficient BS, but its optimization has yet to be thoroughly investigated.
Methods
The inversion effects of the velocity‐selective labeling pulses, such as velocity‐selective inversion (VSI), can be used for BS, and were modeled for optimizing BS in dm‐VSASL. In vivo experiments using dual‐module VSI (dm‐VSI) were performed to compare two BS strategies: a conventional one with additional BS pulses and a new one without any BS pulse. Their BS performance, temporal noise, and temporal SNR were examined and compared, with pulsed and pseudo‐continuous ASL (PASL and PCASL) as the reference.
Results
The in vivo experiments validated the BS modeling. Strong positive linear correlations (r > 0.82, p < 0.0001) between the temporal noise and the tissue signal were found in PASL/PCASL and dm‐VSI. Optimal BS can be achieved with and without additional BS pulses in dm‐VSI; the latter improved the ASL signals by 8.5% in gray matter (p = 0.006) and 12.2% in white matter (p = 0.014) and tended to provide better temporal SNR. The dm‐VSI measured significantly higher ASL signal (p < 0.016) and temporal SNR (p < 0.018) than PASL and PCASL. Complex reconstruction was found necessary with aggressive BS.
Conclusion
Guided by modeling, optimal BS can be achieved without any BS pulse in dm‐VSASL, further improving the ASL signal and the SNR performance.
Earth's cratonic mantle lithosphere is distinguished by high seismic wave velocities that extend to depths greater than 200 km, but recent studies disagree on the magnitude and depth extent of the ...velocity gradient at their lower boundary. Here we analyze and model the frequency dependence of Sp waves to constrain the lithosphere‐asthenosphere velocity gradient at long‐lived stations on cratons in North America, Africa, Australia, and Eurasia. Beneath 33 of 44 stations, negative velocity gradients at depths greater than 150 km are less than a 2–3% velocity drop distributed over more than 80 km. In these regions the base of the typical cratonic lithosphere is gradual enough to be explained by a thermal transition. Vertically sharper lithosphere‐asthenosphere transitions are permitted beneath 11 stations, but these zones are spatially intermittent. These results demonstrate that lithosphere‐asthenosphere viscosity contrasts and coupling fundamentally differ between cratons and younger continents.
Key Points
Quantitative constraints are placed on the lithosphere‐asthenosphere gradient beneath cratons in North America, Africa, Australia, and Eurasia
The lithosphere‐asthenosphere transition beneath cratons is typically consistent with thermal transition
Rarer observations of strong lithosphere‐asthenosphere boundary phases are possibly caused by localized enrichment of volatiles and melt
PDF
