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Abstract

Multipath remains one of the major challenges in high-precision GNSS positioning. The multipath hemispherical map (MHM) based on satellites’ location repeatability in space is a popular method to mitigate GNSS multipath effects, but its performance depends on the availability of sufficient satellite orbital tracks in the skyplot. For instance, for BDS-3 medium Earth orbiters and Galileo satellites with 7-day and 10-day orbital repeat times, respectively, the skyplot of their orbital tracks will be too sparse to cover the shifting orbital tracks on the succeeding days, if only a few days of observations are used to construct MHMs. In this study, we establish an interoperable MHM using the overlap frequency signals of GPS, Galileo and BDS-3 (i.e., GPS L1/L5, Galileo E1/E5a and BDS-3 B1C/B2a). We compared the performance of GPS/Galileo/BDS-3 MHM (i.e., MP_GEC) and single-constellation MHMs (i.e., MP_G, MP_E and MP_C). The mean reduction rates of the L1/E1/B1C and L5/E5a/B2a carrier-phase residuals for the MP_GEC applied to GPS, Galileo and BDS-3 are 36% and 48%, respectively, which are 10–30% points larger compared to the MP_G, MP_E and MP_C. The MP_GEC constructed using 4 days of observations reduced the Galileo RMS positioning errors by 26%, 31% and 29% for the east, north, and up components, respectively, showing improvements of about 16, 18 and 17% points compared to the MP_E, and even approaching the RMS errors of the MP_E constructed using 10 days of observations. The results show that the interoperable GPS/Galileo/BDS-3 MHM is able to improve the spatial resolution, modeling efficiency and correction performance in mitigating multipath effects for high-precision GNSS positioning.

Abstract

The national Beidou Navigation Satellite System (BDS) ground-based augmentation network (BGAN) of China is constructed with the existing GNSS observation resources of industrial sectors and local governments, based on the concept of joint building and sharing with sustainable development. This study provides a detailed introduction to the design, construction and operation of a meteorological application system based on BGAN, and validation of its water vapor products. BDS and GPS real-time observation of atmospheric water vapor is achieved nationwide in China and multi-GNSS applications. Through the application of multi-GNSS data and validation of the water vapor products from 2018 to 2020, the accuracy of precipitable water vapor (PWV) derived from BDS only is equivalent to that from GPS only. The root mean square error (RMSE) between them is about 2 mm with high correlation coefficient. Based on radiosonde data, the validation is conducted with the products of BDS-PWV, GPS-PWV, and Combined-PWV derived with multi-GNSS of BDS and GPS. The error characteristics of the three products show a consistent trend over the months. The bias is relatively small. The RMSE of the three products is in the range of 2.18–2.73 mm. The BDS-PWV has the largest RMSE, followed by GPS-PWV, and Combined-PWV has the smallest RMSE.

SummaryIn this work, the propagation and attenuation of vertically polarized surface waves when interacting with terraced slopes is studied experimentally and numerically. To validate the devised simulation, a laboratory-scale physical model is tested in order to examine the attenuation properties of this well-known artificial landform. The experiment involves formation of a terraced slope, in a laboratory setup, via use of an unconsolidated granular medium made of silica microbeads. This granular medium exhibits a gravity-induced power-law stiffness profile, resulting in a depth-dependent velocity profile. A piezoelectric actuator was used to excite vertically polarized surface acoustic modes localized near the surface of the medium. The three components of the particle velocity field of these modes were measured by means of a three-dimensional laser Doppler vibrometer. In accordance with the terraced slope, a simple inclined plane was further tested to investigate and highlight the differences in terms of wave propagation along these two different ground formations. The results of this research provide significant experimental evidence that the terraced slopes form mechanisms which attenuate low frequency surface waves, thus acting as metasurfaces. This work suggests the use of laboratory-scale physical models to investigate the wave propagation in different landforms, which extend beyond typical horizontal ground morphologies, and which could be linked to atypical wave propagation properties, possibly even influencing propagation of seismic waves.

SummaryShear wave velocity (Vs) is a fundamental property of elastic media whose estimation from PS converted waves is challenging and requires modeling the boundary where P to S conversion occurs. This paper presents a PS tomography where seismic wave conversion/reflection points correspond to reflectors modelled with the level-set function set to zero (φ(x, z) = 0). The proposed method aims for stable Vs inversion in a seismic acquisition setting using multicomponent receivers. Synthetic models simulating true Vs, Vp and the location of the geological reflector are used in the study. The inversion starts by locating a flat reflector, φ(x, z) = 0, which defines the zone Ω1 between the surface and the reflector, where the initial Vs and Vp fields are also set. To calculate the traveltimes of incident PT (P wave that propagates in Ω1 from source to the reflector) , converted PS, and reflected PP waves, for both observed and modelled data (forward problem), the methodology proposed by Rawlinson and Sambridge is adopted. This method uses the arrival times of the P-waves, Tpt, from the seismic source at each reflector point as secondary sources generating the times Tps and Tpp. These times are calculated as a solution to the Eikonal equation by using the Fast Marching method. The PS and PP residual times are minimized by updating Vs, Vp, and φ(x, z) = 0 through adjoint variables designed from a formulation using Lagrange Multipliers in a variational context. The performance of the algorithm is evaluated for models with synclinal, sinusoidal and monoclinal reflector geometries using numerical tests considering the inversion of: 1) φ, given the true values of Vs and Vp; 2) φ and Vs, given the true value of Vp; 3) φ and Vp, given the true value of Vs; and 4) the three parameters φ, Vs, and Vp, simultaneously. Good results are obtained by inverting Vs and φ, given the true value of Vp. The simultaneous inversion of the three parameters exhibits promising results, despite the illumination problems caused by the different distribution of the PS, PP, and PT time gradients due to the geometry of the reflectors and the acquisition setting (sources-receivers in the same plane). The proposed tomography estimates Vs and reflector positions which could help in statics corrections and improve the lithological characterization of near surface.

SummaryConstitutive laws are a necessary ingredient in calculations of glacial isostatic adjustment (GIA) or other surface loading problems (e.g., loading by ocean tides). An idealized constitutive law governed by the Maxwell viscoelastic model is widely used but increasing attention is being directed towards more intricate constitutive laws that, in particular, include transient rheology. In this context, transient rheology collectively refers to dissipative mechanisms activated in addition to creep modeled by the Maxwell viscoelastic model. Consideration of such viscoelastic models in GIA is in its infancy and to encourage their wider use, we present constitutive laws for several experimentally derived transient rheologies and outline a flexible method in which to incorporate them into geophysical problems, such as the viscoelastic deformation of the Earth induced by surface loading. To further motivate this need, we demonstrate, via the Love number collocation method, how predictions of crustal displacement depart significantly between Earth models that adopt only Maxwell viscoelasticity and those with transient rheology. Throughout this paper, we highlight the differences in terminology and emphases between the rock mechanics, seismology, and GIA communities, which have perhaps contributed towards the relative scarcity in integrating this broader and more realistic class of constitutive laws within GIA. We focus on transient rheology since the associated deformation has been demonstrated to operate on timescales that range from hours to decades. With ice mass loss enhanced at similar timescales as a consequence of anthropogenically caused climate change, the ability to model GIA with more accurate constitutive laws is an important tool to investigate such problems.

Abstract

The Tianwen-1 mission, marking China’s inaugural venture into Mars exploration, successfully deployed the Zhurong rover on Utopia Planitia. This review primarily focuses on the insights provided by the Mars Rover Penetrating Radar (RoPeR), a pivotal component of the mission’s scientific payload. The article synthesizes the RoPeR findings with an emphasis on the geological evolution and potential habitability of the Zhurong rover landing site. The study meticulously investigates the genesis and spatial distribution of water ice within Utopia Planitia, establishing correlations with the Martian climatic and hydrological history, the formation of typical landforms and mineral evidence associated with water ice/liquid water of this region. It further discusses the potential habitability of Mars’ subsurface in the current environmental context. This review also expatiates the techniques used in the analysis of RoPeR data, including the methodology for processing polarimetric radar data and the inversion of the dielectric properties of the Martian subsurface. Through this comprehensive review, we aim to present a cohesive picture of the Tianwen-1 mission’s findings, particularly the Zhurong rover results, and their implications for understanding Mars’ geological past, water ice, and assessing its habitability.

Nature Geoscience, Published online: 22 April 2024; doi:10.1038/s41561-024-01397-2

In a part of the Apennines, where the Earth’s crust is thin and heat flow is high, production of CO2 from deep below the mountains dominates over near-surface weathering processes that consume this greenhouse gas. Ultimately, the magnitude of deep CO2 release tips the balance towards a landscape that is a net carbon emitter.

Nature Geoscience, Published online: 22 April 2024; doi:10.1038/s41561-024-01421-5

A global gauge-corrected monthly river flow and storage dataset suggests that residence time is a key driver of water storage and variability and indicates substantial freshwater discharge to the ocean from the Maritime Continent.

Abstract

The ionosphere crucially impacts on Global Navigation Satellite System (GNSS) positioning accuracy and integrity. Recently some network-based methods have shown the potential to construct a regional/global vertical total electron content (VTEC) or grid ionospheric vertical delay (GIVD) map for accuracy augmentation purposes. However, how to use these advanced methods for integrity augmentation has not been adequately investigated. The authors have investigated a regional ionospheric integrity monitoring strategy based on the radial basis function neural network (RBF-NN), using GNSS TEC observations. Similar to the SBAS approach, the GIVD map is constructed so as to enhance positioning accuracy, and the corresponding grid ionospheric vertical error (GIVE) map is constructed for protection level calculation to enhance positioning integrity. To reduce the GIVD residuals and the GIVE values, the local ionospheric spatial activity index (LISAI) is proposed as an indicator of local ionospheric spatial activity level. The RBF-NN structure parameters are able to be adaptively determined via hierarchical clustering. Modeling results in the China region have verified that the proposed GIVD modeling method is slightly better than the classical WAAS-Kriging method. The proposed GIVE modeling method significantly outperforms WAAS-Kriging, achieving an improvement of around 46% and 25% during the ionospheric calm and active periods, respectively.

Publication date: Available online 8 April 2024

Source: Advances in Space Research

Author(s): Houyin Xi, Bin Chen, Tianwen Chen, Xiaodong Zhang, Min Luo

Abstract

During geomagnetic storms relativistic outer radiation belt electron flux exhibits large variations on rapid time scales of minutes to days. Many competing acceleration and loss processes contribute to the dynamic variability of the radiation belts; however, distinguishing the relative contribution of each mechanism remains a major challenge as they often occur simultaneously and over a wide range of spatiotemporal scales. In this study, we develop a new comprehensive model for storm-time radiation belt dynamics by incorporating electron wave-particle interactions with parallel propagating whistler mode waves into our global test-particle model of the outer belt. Electron trajectories are evolved through the electromagnetic fields generated from the Multiscale Atmosphere-Geospace Environment (MAGE) global geospace model. Pitch angle scattering and energization of the test particles are derived from analytical expressions for quasi-linear diffusion coefficients that depend directly on the magnetic field and density from the magnetosphere simulation. Using a study of the 17 March 2013 geomagnetic storm, we demonstrate that resonance with lower band chorus waves can produce rapid relativistic flux enhancements during the main phase of the storm. While electron loss from the outer radiation belt is dominated by loss through the magnetopause, wave-particle interactions drive significant atmospheric precipitation. We also show that the storm-time magnetic field and cold plasma density evolution produces strong, local variations of the magnitude and energy of the wave-particle interactions and is critical to fully capturing the dynamic variability of the radiation belts caused by wave-particle interactions.

Abstract

The aurora often appears as an approximately oval shape surrounding the magnetic poles, and is a visible manifestation of the intricate coupling between the Earth's upper atmosphere and the near-Earth space environment. While the average size of the auroral oval increases with geomagnetic activity, the instantaneous shape and size of the aurora is highly dynamic. The identification of auroral boundaries holds significant value in space physics, as it serves to define and differentiate regions within the magnetosphere connected to the aurora by magnetic field lines. In this work, we demonstrate a new method to estimate the spatiotemporal variations of the poleward and equatorward boundaries in global UV images. We apply our method, which is robust against outliers and occasional bad data, to 2.5 years of UV imagery from the Imager for Magnetopause-to-Aurora Global Exploration satellite. The resulting data set is compared to recently published boundaries based on the same images (Chisham et al., 2022, https://doi.org/10.1029/2022JA030622), and shown to give consistent results on average. Our data set reveals a root mean square boundary normal velocity of 149 m/s for the poleward boundary and 96 m/s for the equatorward boundary and the velocities are shown to be stronger on the nightside than on the dayside. Interestingly, our findings demonstrate an absence of correlation between the amount of open magnetic flux and the amount of flux enclosed within the auroral oval.

Abstract

We present a new geodetic strain rate and rotation rate model for Greece that has been derived using a highly dense GPS velocity field. The spatial distribution and the resolved rates of the various velocity gradient tensor quantities provided updated constraints on the present-day upper crustal deformation in the region and revealed new details not reported previously. The spatial distribution of the second invariant demonstrated that the overall magnitude of strain rates is highest across two well-defined provinces. The first follows the North Anatolian Fault and its two branches within the north Aegean, crosses central Greece and through the Gulf of Corinth it terminates in western Greece, while the second encompasses the extensional province of western Turkey and the eastern Aegean Sea islands. Our estimates revealed that shearing affects some of the fault-bounded grabens of central Greece that lie to the SW of the North Aegean Basin implying considerable oblique extension. We identified a narrow region of counterclockwise rotation whose location and kinematics have been induced by the net effect across the intersection of the clockwise rotating domains of western and central Greece. The Aegean microplate and the Anatolian plate are separated by a wide transition zone which accommodates the curved stretching of the entire plate system. In both edges of the Hellenic forearc the dominant mode of crustal strain is E-W extension. We found that earthquakes of M ≥ 5.6 are spatially well-correlated with high-strain areas, indicating that strain rate mapping could be used to inform future probabilistic seismic hazard analyses.

Abstract

The CsCl-type (B2) phase of FeSi (B2-FeSi) has been proposed as a candidate phase in the ultralow-velocity zones (ULVZs) at the base of the lower mantle and in the Earth's inner core. However, the elastic properties of B2-FeSi under relevant conditions remain unclear. Here we determine the density, elastic constants, and velocities of B2-FeSi at high pressures (90–390 GPa) and temperatures (3,000–6,000 K) relevant to the Earth's lower most mantle and the inner core, using first-principles molecular dynamics simulations. At the base of the lower mantle, B2-FeSi shows significantly lower velocities and a higher density than those of the ambient mantle. Mechanical mixing models suggest the presence of ∼27–39 vol% B2-FeSi in the silicate mantle is consistent with the reduced velocities and the elevated density of ULVZs observed seismically. On the other hand, the hcp-Fe and B2-FeSi mixture exhibits higher bulk sound velocity compared to the PREM under inner core conditions. Adding superionic H in the interstitial sites of B2-FeSi lowers its density but has little effect on the bulk sound velocity of B2-FeSi, precluding H-bearing B2-FeSi as a major component in the Earth's inner core.

Abstract

Fault zones exhibit 3D variable thickness, a feature that remains inadequately explored, particularly with regard to the impact on fluid flow. Upon analyzing an analytic solution, we examine 3D thermal-hydraulic (TH) dynamical models through a benchmark experiment, which incorporates a fault zone with thickness variations corresponding to realistic orders of magnitude. The findings emphasize an area of interest where vigorous convection drives fluid flow, resulting in a temperature increase to 150°C at a shallow depth of 2.7 km in the thickest sections of the fault zone. Moreover, by considering various tectonic regimes (compressional, extensional, and strike-slip) within 3D thermal-hydraulic-mechanical (THM) models and comparing them to the benchmark experiment, we observe variations in fluid pressure induced by poroelastic forces acting on fluid flow within the area of interest. These tectonic-induced pressure changes influence the thermal distribution of the region and the intensity of temperature anomalies. Outcomes of this study emphasize the impact of poroelasticity-driven forces on transfer processes and highlight the importance of addressing fault geometry as a crucial parameter in future investigations of fluid flow in fractured systems. Such research has relevant applications in geothermal energy, CO2 storage, and mineral deposits.

Abstract

The two phases of El-Niño-Southern Oscillation (ENSO) influence both regional and global terrestrial vegetation productivity on inter-annual scales. However, the major drivers for the regional vegetation productivity and their controlling strengths during different phases of ENSO remain unclear. We herein disentangled the impacts of two phases of ENSO on regional carbon cycle using multiple data sets. We found that soil moisture predominantly accounts for ∼40% of the variability in regional vegetation productivity during ENSO events. Our results showed that the satellite-derived vegetation productivity proxies, gross primary productivity from data-driven models (FLUXCOM) and observation-constrained ecosystem model (Carbon Cycle Data Assimilation System) generally agree in depicting the contribution of soil moisture and air temperature in modulating regional vegetation productivity. However, the ensemble of weakly constrained ecosystem models exhibits non-negligible discrepancies in the roles of vapor pressure deficit and radiation over extra-tropics. This study highlights the significance of water in regulating regional vegetation productivity during ENSO.

Abstract

Despite its importance for the global oxidative capacity, spatially resolved trends and variability of the hydroxyl radical (OH) are poorly constrained. We demonstrate the utility of a tropospheric column OH (TCOH) product, created from machine learning and satellite proxy data, in determining the spatial variability in trends of tropical OH over the oceans during September through November. While OH increases domain-wide by 2.1%/decade from 2005–2019, we find significant spatial heterogeneity in regional trends, with decreases in some areas of 2.5%/decade. Our analysis of the trends in the proxy data indicate anthropogenic-driven changes in emissions of OH drivers as well as increasing temperatures cause these trends. This OH product is potentially a significant advance in constraining OH spatial variability and serves as a useful complement to existing tools in understanding the atmospheric oxidative capacity. Comprehensive observations of TCOH are required to assess the fidelity of this method.

Abstract

Drainage basins are fundamental units of Earth's surface, describing how flows accumulate across landscapes. They are direct expressions of how tectonics and climatic forces alter Earth's surface morphology. Here, we measure the width-to-length ratios (WLRs) of 386,931 drainage basins (average area ∼157 km2), covering all continents except Antarctica and Greenland. Global variations in WLRs are correlated with climatic aridity, whole-basin slope, and local topographic roughness. Basins in arid landscapes tend to be narrower, potentially reflecting a higher prevalence of surface runoff and therefore a stronger slope-parallel component of the transporting flow. Local topographic roughness is associated with wider basins, potentially reflecting greater dispersion of flow directions. Conversely, whole-basin topographic gradients, potentially reflecting gradients in uplift, are associated with narrower basins. However, steeper basins are also often rougher, so revealing the effects of whole-basin slope requires correcting for the confounding effects of roughness variations.

Abstract

The influence of Atlantic Niño on the following El Niño–Southern Oscillation becomes significant since mid-1970s. However, exact mechanisms for this inter-decadal change are still unclear. Here, we perform a set of model pacemaker experiments to probe the relative contributions of the changes in the Atlantic Niño itself and the mean-state under global warming. The results suggest that the warmer background of the tropical Atlantic plays an essential role in enhancing local mean precipitation, inducing stronger divergence and low-level easterlies in the Pacific. Under a favorable condition in the Pacific, even a weak Atlantic Niño-related warming could promote the development of La Niña through cross-basin Walker circulation and the Indian Ocean-relayed Kelvin wave response. In contrast, the Atlantic Niño pattern change itself induces feeble convection anomalies in the western Atlantic, which cannot induce significant atmospheric response in the Pacific. These results imply an important modulation of global warming on the inter-basin connection.