Initial Weakening Research Articles

Early emergence and determinants of human-induced Walker circulation weakening

The Walker circulation is projected to slow down in response to greenhouse gas warming. However, detecting the impact of human activities on changes in the Walker circulation is challenging due to the significant influence of internal variability. Here, based on ensembles of multiple climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we show evidence that the emergence of the human-induced weakening of Walker circulation tends to occur earlier in the middle-upper troposphere than at the surface. This earlier emergence is attributed to a more pronounced initial weakening response of the middle-upper tropospheric Walker circulation to atmospheric CO2 radiative forcing. We further reveal that the emergence time of a weaker Walker circulation varies across models. This intermodel spread is governed by an ocean thermostat that operates by modulating the zonal sea surface temperature gradient over the tropical Indo-Pacific region. Our findings address the key question of whether and how to detect human-induced large-scale atmospheric circulation changes and provide valuable insights for assessing the associated risks.

Electrochemical Weakening Material Strength for Accelerated Thinning of Silicon Carbide Substrates

This paper presents a novel approach to expedite the thinning of silicon carbide (SiC) substrates, a crucial step in enhancing material performance and extending chip longevity by reducing volume resistivity. The traditional methods of SiC substrate thinning, such as diamond wheel grinding and plasma etching, have inherent limitations, including the risk of wafer breakage and high processing costs. In response to these challenges, we propose an innovative technique involving the electrochemical etching attenuation of SiC strength, offering a cost-effective and efficient solution.In this study, we employed an electrochemical method to initially weaken the SiC substrate from the surface to a defined depth, followed by fine-tuning its thickness using a conventional diamond grinding wheel. Remarkably, our results demonstrate that SiC substrates subjected to electrochemical treatment exhibit well-structured lattices free from residual stresses. This novel approach not only reduces the risk of wafer breakage but also enhances the material's integrity (Figure 1).To gain deeper insights into the effects of electrochemistry on SiC material strength, advanced characterization techniques, including scanning electron microscopy, were employed. Our findings reveal that the electrochemically treated SiC surface exhibits a sponge-like morphology, with a cross-sectional profile showcasing prominent columnar structures (Figure 2). This sponge-like structure is attributed to the removal of silicon atoms during anodization, affirming the effectiveness of electrochemical methods in rapidly attenuating the material strength of SiC substrates, thereby facilitating their efficient thinning through grinding without compromising substrate integrity.In summary, this research highlights the potential of electrochemical methodologies for the initial weakening of SiC substrates, leading to accelerated and cost-effective thinning processes. This breakthrough not only expedites SiC substrate thinning but also contributes significantly to the advancement of SiC substrate thinning and grinding techniques, promising substantial benefits for future semiconductor device fabrication. Figure 1

Brittle initiation of dissolution–precipitation creep in plagioclase-rich rocks: insights from the Bergen arcs, Norway

The initiation of ductile shear zones commonly occurs spatially associated with fluid-rock reactions along brittle precursors. In many cases the relative timing of fracturing, fluid infiltration, reaction, and recrystallisation is unclear, making it difficult to disentangle mechanisms of shear zone initiation from subsequent deformation and recrystallisation. Here we present the study of the transition from a dry plagioclase-diopside-garnet-scapolite host granulite-facies lithology to (1) a low strain amphibolite-facies rock, and (2) a transition from low strain to high strain amphibolite-facies lithologies. Hydration of the granulite-facies precursor at amphibolite-facies conditions produces an assemblage comprised dominantly of plagioclase-amphibole-zoisite-clinozoisite-kyanite-scapolite-quartz. Detailed study of plagioclase chemistry and microstructures across these two transitions using Electron Backscatter Diffraction (EBSD) and Wavelength Dispersive Spectrometry (WDS) allows us to assess the degree of coupling between deformation and fluid-rock reaction across the outcrop. Plagioclase behaves dominantly in a brittle manner at the hydration interface and so the initial weakening of the rock is attributed to grain size reduction caused by fracture damage and fluid infiltration at amphibolite-facies conditions. Extensive fracturing-induced grain size reduction locally increases permeability and allows for continuing plagioclase and secondary mineral growth during shear. Based on plagioclase microstructures, such as, an inherited but dispersed crystallographic preferred orientation (CPO), truncation of chemical zoning, and the dominance of fine (5–150 µm), slightly elongate, polygonal grains we conclude that deformation is dominantly facilitated by dissolution–precipitation creep assisted by grain boundary sliding in the shear zone.

Effect of Grain-Size and Textural Weakening in Polyphase Crustal and Mantle Lithospheric Shear Zones

Strain localization to form narrow plate boundary shear zones in Earth’s lithosphere requires a significant amount of strain weakening. Here we investigate the relative contributions of grain-size-sensitive weakening versus textural weakening in polyphase shear zones in granitoid and peridotitic rocks through two-dimensional centimetre-scale bulk simple shear numerical models. The models deploy both constant grain size (only textural weakening) and dynamic grain-size evolution based on the paleowattmeter. Our results reveal that for granitoid rocks, textural weakening dominates, especially at temperatures around 550 °C, while grain-size-sensitive weakening plays a secondary yet significant role. For peridotitic rocks, intense weakening is evident below temperatures of ~1000 °C due to grain-size reduction, while textural weakening has a minor effect on weakening for experiments above 1000 °C. Two-dimensional experiments are compared to one-dimensional, single-phase models to reveal the effect of geometrical complexities in stress and grain-size evolution. These results are discussed in the context of natural lithospheric shear zones and are compared with established piezometers for individual mineral phases. Our findings underscore the vital role of grain-size-sensitive rheologies, particularly in the mantle lithosphere, for the initial weakening of ductile shear zones. These insights offer quantitative constraints that advance our understanding of the long-term strength of lithospheric plate boundaries.

Tree Storey Structure and Condition Dynamics in Middle-Taiga Native Spruce Forests of the Northern Ural Foothills

The paper focuses on the dynamic processes occurring in indigenous spruce phytocenoses developing on the territory of one of the large reserves of spontaneous dark coniferous taiga in the European North – the Pechoro-Ilychsky Reserve. The aim of the study was to assess the size structure dynamics of the forest stands and the undergrowth, as well as changes in their vital status in naturally developing spruce stands of the green moss and long-stem moss groups of forest types in the foothills of the Northern Urals (the basin of the upper reaches of the Pechora River). The spruce stands, mixed in composition and complex in structure, form a cyclic uneven-aged type of structure. For the middle taiga indigenous spruce forests, based on the assessment of inequality in the woody plants’ sizes distribution, the results of the long-term structure monitoring of the forest stands and the undergrowth on four permanent sample plots were analysed. In forest stands of different types, a similar dynamics of tree differentiation by trunk volume and undergrowth by trunk height was observed. There is a significant accumulation of small woody plants individuals, with a smooth, evenly fading distribution of their numbers in the direction of size increase. Under the canopy of forest stands there is a continuous regrowth process. According to the data of stationary observations of the vitality and damage degree of trees and undergrowth of spruce forests, based on a visual assessment of a woody plant state according to the characteristics of the crown, a slightly weakened condition of both the upper tree layers of the phytocenosis and the lower – undergrowth was revealed. Spruce stands are at the stage of initial weakening or are already weakened, the undergrowth is predominantly healthy.

Experimental study on infrared thermal response characteristics of water-bearing concrete under drop hammer impact

The dynamic impact damage of concrete will lead to serious safety accidents, and the water environment will aggravate the degree of damage. In this paper, an infrared monitoring experimental system for drop hammer impact damage of water-bearing concrete was designed and established. The mechanical properties, infrared radiation temperature (IRT) and infrared thermography of concrete impact damage with different soak time were tested and analyzed. The influence of water on the infrared generation mechanism of concrete was discussed. The results show that water has an initial weakening effect on concrete, which weakens the cementation between concrete particles and aggregate strength, resulting in a decrease in concrete strength. Compared with dry concrete, the infrared temperature of water-bearing concrete increased more during impact damage. With the increase of soak time, the average infrared temperature (AIRT) and the maximum infrared temperature (MIRT) of water-bearing concrete decreased. When dry concrete was damaged, tensile cracks were mainly produced and accompanied by heat absorption. When water-bearing concrete was damaged, high temperature shear cracks appeared. The high temperature generated at the moment of impact failure of water-bearing concrete may be caused by the cavitation phenomenon of water in the microscopic pores of concrete during the drop hammer impact and the combined effect of concrete microstructure damage. The research results are of great significance for monitoring and evaluating the stability of the dynamic evolution process of concrete structures.

The Effect of Initial Vortex Asymmetric Structure on Tropical Cyclone Intensity Change in Response to an Imposed Environmental Vertical Wind Shear

AbstractPrevious studies have investigated how the environmental vertical wind shear (VWS) may trigger the asymmetric structure in an initially axisymmetric tropical cyclone (TC) vortex and how TC intensity changes in response. In this study, the possible effect of the initial vortex asymmetric structure on the TC intensity change in response to an imposed environmental VWS is investigated based on idealized full‐physics model simulations. Results show that the effect of the asymmetric structure in the initial TC vortex can either enhance or suppress the initial weakening of the TC in response to the imposed environmental VWS. When the initial asymmetric structure is in phase of the VWS‐induced asymmetric structure, the TC weakening will be enhanced and vice versa. Our finding calls for realistic representation of initial TC asymmetric structure in numerical weather prediction models and observations to better resolve the asymmetric structure in TCs.

Evidence for an Extreme Cooling Event Prior to the Laschamp Geomagnetic Excursion in Eifel Maar Sediments

We present a timeseries of flood and slumping phases in central Europe for the past 65,000 years from event layers in sediment cores from infilled Eifel maar basins (Germany). Palynological, petrographic and organic carbon (chlorins) records are used to understand the precise timing of these events. Periods of increased flood activity seem to coincide with Heinrich stadials in marine sediment records, which are associated with cold and more arid climate conditions, indicating a vegetation response within the maars’ catchment areas. This multi-proxy correlation reveals prominent slumps at different maar sites during Greenland Stadial (GS) 12. The stratigraphy is based on sediment records from the Auel infilled maar and we thus call this event Auel Cold Event (ACE). Frozen and fractured sediment packages within the slump suggest deep frost or permafrost conditions for the region during the stadial. The results agree well with sediment archives and archeological sites across Europe that report severely cold and arid conditions for the stadial. This supports the assumption that GS12 was indeed one of the coldest periods of the last glacial cycle rather than the Heinrich stadials. Based on our age model, the ACE occurred at 43,500 yr b2k (years before the year 2000), which coincides with the initial weakening of Earth’s magnetic field strength prior to the Laschamp geomagnetic excursion.

The Time-Dependent Response of a Two-Basin Ocean to a Sudden Surface Temperature Change

Abstract Building on previous work using single-basin models, we here explore the time-dependent response of the Atlantic meridional overturning circulation (AMOC) to a sudden global temperature change in a two-basin ocean–ice model. We find that the previously identified mechanisms remain qualitatively useful to explain the transient and the long-term time-mean responses of the AMOC in our simulations. Specifically, we find an initial weakening of the AMOC in response to warming (and vice versa for cooling), controlled by the mid-depth meridional temperature contrast across the Atlantic basin. The long-term mean response instead is controlled primarily by changes in the abyssal stratification within the basin. In contrast to previous studies we find that for small-amplitude surface temperature changes, the equilibrium AMOC is almost unchanged, as the abyssal stratification remains similar due to a substantial compensation between the effects of salinity and temperature changes. The temperature-driven stratification change results from the differential warming/cooling between North Atlantic Deep Water and Antarctic Bottom Water, while the salinity change is driven by changes in Antarctic sea ice formation. Another distinct feature of our simulations is the emergence of AMOC variability in the much colder and much warmer climates. We discuss how this variability is related to variations in deep-ocean heat content, surface salinity, and sea ice in the deep convective regions, both in the North Atlantic and in the Southern Ocean, and its potential relevance to past and future climates.

A bonding evolution theory study of the reaction between methylidyne radical, CH(X2Π), and cyclopentadiene, C5H6

AbstractIn the present work, bonding evolution theory (BET) is applied to gain insight about the complex reaction between methylidyne radical, CH (X2Π) and cyclopentadiene, C5H6. The novelty of this work is that all reaction pathways take place in the doublet electronic state and an unpaired electron is always present. Therefore, taking the aforementioned reaction as explicative example, we have shown how to apply the BET tool to these kinds of open‐shell systems, by splitting the wavefunctions into the corresponding alpha and beta parts. As an added value, we have included a point‐by‐point description of the algorithm we use to make it available for the readers. Hence, a complete analysis of bond breaking/forming and charge redistribution along the multi‐channels connecting reactants to products via the transition states and intermediates is presented. We show how the BET brings about the representation of the electronic flow in complex molecular rearrangements like the one herein studied, yielding a transparent rationalization based on the electron density redistribution. The present study allows us to conclude that along the different processes giving rise to the benzene product, the breaking of a CC sigma bond initiates the electronic rearrangement in two cases, but not in the third one. The last step in these processes can be described as an initial weakening of the CH bond with a quasi‐hydride formation and a final retro‐transfer of electrons from the quasi‐hydride to the CH bond. On the other hand, in the way to the fulvene product, the breaking of the CC sigma bond takes place after previous electronic redistribution. Neither the last step of the fulvene formation process nor the interesting H transfer described in the second one, can be explained without the wavefunction splitting technique herein detailed and exemplified.

Impact of injection rate ramp-up on nucleation and arrest of dynamic fault slip

Fluid injection into underground formations reactivates preexisting geological discontinuities such as faults or fractures. In this work, we investigate the impact of injection rate ramp-up present in many standard injection protocols on the nucleation and potential arrest of dynamic slip along a planar pressurized fault. We assume a linear increasing function of injection rate with time, up to a given time t_c after which a maximum value Q_m is achieved. Under the assumption of negligible shear-induced dilatancy and impermeable host medium, we solve numerically the coupled hydro-mechanical model and explore the different slip regimes identified via scaling analysis. We show that in the limit when fluid diffusion time scale t_w is much larger than the ramp-up time scale t_c, slip on an ultimately stable fault is essentially driven by pressurization at constant rate. Vice versa, in the limit when t_c/t_w gg 1, the pressurization rate, quantified by the dimensionless ratio dfrac{Q_m t_w}{t_c Q^*} with Q^* being a characteristic injection rate scale, does impact both nucleation time and arrest distance of dynamic slip. Indeed, for a given initial fault loading condition and frictional weakening property, lower pressurization rates delay the nucleation of a finite-sized dynamic event and increase the corresponding run-out distance approximately proportional to propto left( dfrac{Q_m t_w}{t_c Q^*}right) ^{-0.472}. On critically stressed faults, instead, the ramp-up of injection rate activates quasi-static slip which quickly turn into a run-away dynamic rupture. Its nucleation time decreases non-linearly with increasing value of dfrac{Q_m t_w}{t_c Q^*} and it may precede (or not) the one associated with fault pressurization at constant rate only.

Analysis of the Transition of an Explosive Cyclone to a Mediterranean Tropical-like Cyclone

This study investigates the dynamics of the development phases of a Mediterranean tropical-like cyclone (medicane) in the southern Ionian Sea, on 28 September 2018 that caused high impact phenomena in the central and eastern Mediterranean, focusing on the transition from explosive cyclone to medicane. The symmetry and the warm core structure of the system have been demonstrated via phase space diagrams determining three phases of the system development that are then supported on a dynamical basis. During the first phase of the system, baroclinic instability triggered the formation of the explosive cyclone, when strong upper-level PV anomalies at the dynamic tropopause level moved towards a pre-existed area of enhanced low-level baroclinicity over the coastal areas of Libya along with positive SST anomalies. The surface frontal structure was enhanced under the influence of the upper-level dynamic processes. During the second phase when the medicane formed, low-level diabatic processes determined the evolution of the surface cyclone, without any significant support from baroclinic processes in the upper troposphere. The distortion of the low-level baroclinicity and the frontal structure began after the initial weakening of the upper-level dynamics. During the third phase, the system remained barotropic, being affected by similar mechanisms as in the second phase but with lower intensity. The transition mechanism is not only the result of the seclusion of warm air in the cyclone core but, mainly, the continuation of an explosive cyclone or an intense cyclone when the occlusion began to form.

Characterizing the Distribution of Temperature and Normal Stress on Flash Heated Granite Surfaces at Seismic Slip Rates

AbstractAt seismic slip rates, flash‐weakening can significantly reduce the coefficient of friction, and the magnitude of weakening increases with surface temperature. To quantify the distribution of flash temperature, high‐speed double‐direct shear experiments were conducted on Westerly granite blocks using velocity steps from 1 mm/s to 900 mm/s at 9 MPa normal stress. We employed a high‐speed infrared camera to measure surface temperatures on the moving block during sliding, and utilized a novel sliding‐surface geometry to control the mm‐scale contact history. Following the initial weakening upon the velocity step, the blocks slide at a constant coefficient of friction. Surface temperatures are inhomogeneously distributed across the sliding surface, and increase with displacement. To determine the local normal stress distribution at the mm‐scale, we combine a one‐dimensional thermal model with conventional flash‐weakening models that incorporate a surface temperature‐dependence informed by the controlled, mm‐scale contact history. Early contacts experience local normal stress exceeding 40 times the applied normal stress. As sliding progresses, the local normal stress at the hottest contacts decreases as contact area increases, leading to local normal stresses ranging from 2 to 6 times the applied normal stress on most contacts by 30 mm of slip. Increases in surface temperature, which would decrease the coefficient of friction, are buffered by wear processes that increase contact area and decrease the local normal stress. Treatments of flash heating are advanced by incorporating improved characterization of the state of the sliding surface at the mm and larger scales during sliding.

Thermal Weakening Friction During Seismic Slip: Experiments and Models With Heat Sources and Sinks

AbstractExperiments that systematically explore rock friction under crustal earthquake conditions reveal that faults undergo abrupt dynamic weakening. Processes related to heating and weakening of fault surfaces have been invoked to explain pronounced velocity weakening. Both contact asperity temperature Ta and background temperature T of the slip zone evolve significantly during high‐velocity slip due to heat sources (frictional work), heat sinks (e.g., latent heat of decomposition processes), and diffusion. Using carefully calibrated High‐Velocity Rotary Friction experiments, we test the compatibility of thermal weakening models: (1) a model of friction based only on T in an extremely simplified, Arrhenius‐like thermal dependence; (2) a flash heating model which accounts for the evolution of both V and T; (3) same but including heat sinks in the thermal balance; and (4) same but including the thermal dependence of diffusivity and heat capacity. All models reflect the experimental results but model (1) results in unrealistically low temperatures and model (2) reproduces the restrengthening phase only by modifying the parameters for each experimental condition. The presence of dissipative heat sinks in stage (3) significantly affects T and reflects on the friction, allowing a better joint fit of the initial weakening and final strength recovery across a range of experiments. Temperature is significantly altered by thermal dependence of (4). However, similar results can be obtained by (3) and (4) by adjusting the energy sinks. To compute temperature in this type of problem, we compare the efficiency of three different numerical approximations (finite difference, wavenumber summation, and discrete integral).

Current Atlantic Meridional Overturning Circulation weakest in last millennium

The Atlantic Meridional Overturning Circulation (AMOC)—one of Earth’s major ocean circulation systems—redistributes heat on our planet and has a major impact on climate. Here, we compare a variety of published proxy records to reconstruct the evolution of the AMOC since about ad 400. A fairly consistent picture of the AMOC emerges: after a long and relatively stable period, there was an initial weakening starting in the nineteenth century, followed by a second, more rapid, decline in the mid-twentieth century, leading to the weakest state of the AMOC occurring in recent decades. The Atlantic Meridional Overturning Circulation (AMOC) is currently distinctly weaker than it has been for the last millennium, according to a synthesis of proxy records derived from a range of techniques.

Bowel adhesion and therapy with the stable gastric pentadecapeptide BPC 157, L-NAME and L-arginine in rats.

BACKGROUNDAfter parietal peritoneum excision with an underlying superficial layer of muscle tissue in rats, there is failed vasculature, and finally, increased adhesion formation. We hypothesized that unlike nitric oxide (NO)-agents, L-NAME and/or L-arginine, the application of the stable gastric pentadecapeptide BPC 157 with its most recent vascular effects (“vascular recruitment”) means attenuated bowel adhesion formation and NO- and malondialdehyde (MDA)-tissue values.AIMTo focused on the bowel adhesion and the therapy with the BPC 157, its most and application of NO-agents.METHODSAlong with defect creation, medication was (1) during surgery, once, at 1 min after defect creation as an abdominal bath (1 mL/rat), BPC 157 (10 µg/kg, 10 ng/kg, 1 mL/rat), an equivolume of saline, L-NAME (5 mg/kg), L-arginine (200 mg/kg) alone and/or combined. Alternatively, medication was (2) intraperitoneally once daily, first application at 30 min after surgery, last application 24 h before assessment at d 7 or d 14. As a postponed therapy to pre-existing adhesion (3), BPC 157 (10 µg/kg, 10 ng/kg intraperitoneally, 1 mL/rat) was given once daily since d 7.RESULTSThe recovery effect of the BPC 157 regimens goes with the presence of abundant vascular vessels in and near the defect, which occurs rapidly. Lastly, also applied as post-treatment, BPC 157 creates attenuated adhesions, minimal or no adhesion. Contrarily, NO-agents have diverse initial and final effects: The initial weakening of blood vessel disappearance and finally, severe worsening of adhesions (L-NAME) vs the initial weakening of blood vessel disappearance and finally, attenuation of adhesions formation (L-arginine), which counteract each other response given together. Importantly, BPC 157 maintains its beneficial effect also when given with NO-agents (L-NAME + BC 157; L-arginine + BPC 157; L-NAME + L-arginine + BPC 157). Finally, with respect to the increased NO- and MDA- values-adhesion tissue formation relation, unlike diverse effect of the NO-agents, the BPC 157 application effect regularly combines decrease on the increased NO- and MDA- values and the beneficial outcome (less adhesion formation).CONCLUSIONBPC 157 therapy can be suited for the realization of the peritoneal defect healing with minimal or no adhesion formation.

Initial weakening in Mixtecan languages

Prosodic boundaries influence patterns of consonantal strengthening and weakening across languages (Fougeron and Keating, 1997; Kakadelis, 2018; Katz and Fricke, 2018; Keating et al., 2003; White et al., 2020). Onset obstruents/nasals in prosodically prominent positions are lengthened and produced with greater contact between active and passive articulators (Fougeron and Keating, 1997; Fletcher, 2010; Keating et al., 2003; Lavoie, 2001). Typically, articulatory strengthening occurs in utterance-initial and word-initial position while weakening/reduction occurs in word-medial position (Katz and Fricke, 2018). We provide phonetic evidence that certain obstruents in word-medial, pre-tonic position are both lengthened and strengthened in Itunyoso Triqui, an indigenous language of Mexico, while word-initial obstruents are often shorter and reduced. We examined 67 min (8933 segments) from a spontaneous speech corpus produced by nine native speakers; and measured duration, voicing lenition, and spirantization. Onset consonants were lengthened slightly in stem-final (stressed) syllables but shortened elsewhere. For sonorant consonants, no utterance-initial lengthening was observed. These findings agree with previous work on a related Mixtecan language—Yoloxochitl Mixtec (DiCanio et al., submitted)—and pose unique challenges for views hypothesizing that consonantal strengthening is either (a) universal or (b) serves to enhance word-level parsing (Katz and Fricke, 2018; White et al., 2020).

Revisiting the Relationship Between Tropical Cyclone Size and Intensity Over the Western North Pacific

AbstractThis study revisits the nonlinear relationship between tropical cyclone (TC) size and intensity over the western North Pacific (WNP). Using best track data from the Japan Meteorological Agency from 2007–2019, we find that the statistical turning point on a fitted size‐intensity curve only exists for TCs with greater lifetime peak intensities (e.g., typhoons). The track and evolution type are two factors determining the statistical size‐intensity relationship. The nonlinear size‐intensity relationship is only observed for recurving typhoons reaching their peak intensities and sizes at different times, due to a nonmonotonic size change during the weakening stage. The slight size decrease in the initial weakening stage can be linked to a TC weakening‐induced shrinkage balanced by a baroclinic expansion when recurving typhoons move into higher latitudes. Finally, different evolution types for WNP typhoons can be determined by the ratio of intensity/size to its theoretical maximum.

Empirical Shear Strength Criterion for Rock Joints Based on Joint Surface Degradation Characteristics During Shearing

Joint surface morphology has essential implications for joint shear behaviors. In this paper, constant normal load (CNL) direct shear tests were performed on 150 artificial rock joint specimens to study the joint surface degradation process under various initial joint roughness coefficient (JRCinitial), joint wall compressive strength (JCS), and normal stress (σn) values. The experimental results revealed that with increases in JRCinitial, JCS, and σn, the joint surface exhibited different levels of increased degradation. With increasing shear displacement, the JRC continued to decline and finally tended to stabilize to a constant value, and we divided the degradation process into an initial weakening phase, a rapid weakening phase, and a constant weakening phase. Based on regression analyses, a JRC degradation model was presented to evaluate the JRC values of rock joints under various displacements, and a new empirical equation was introduced to predict peak shear displacement. Additionally, a piecewise function for the different shear displacement stages was proposed to calculate JRCmobilized by replacing JRC in the JRC–JCS model, and the variation in the friction angle $$\phi$$ at various stages was discussed. Finally, a modified constitutive model was developed to predict shear strength versus displacement curves. The predicted results indicated that the new proposed model performs better than previous models.

Responses of the Thermosphere and Ionosphere System to Concurrent Solar Flares and Geomagnetic Storms

AbstractWe conducted numerical simulations to examine dayside thermosphere and ionosphere responses to concurrent solar flares and a geomagnetic storm during 6–11 September 2017, as well as the interplay of flare and storm effects. We found that E‐region electron density response to the flares was much smaller inside the auroral oval than it was outside the auroral oval due to an extra source of ionization by auroral particle precipitation. During quiet times in this period, flare enhanced ionization, which decreased daytime eastward electric fields, caused an initial weakening of the vertical E × B drifts. This weakening of the vertical E × B drifts was on the order of ~20% for an X8.2 flare in this period. During storm times, flare‐induced initial changes of the vertical plasma drifts varied between being positive and being negative, depending on latitudes, due to storm‐time disturbance dynamo effects. Storm effects also overall weakened flare effects on geopotential height, ionospheric F2 peak height hmF2, and ionospheric F2 peak electron density NmF2. In addition, storm effects changed the latitudinal distributions of flare effects on hmF2, NmF2, vertical plasma drifts, and O/N2. On the other hand, no major X‐class flare occurred shortly before or during the storm. Model simulations indicate that the weaker C‐ and M‐class flares that occurred before and during the storm had a minor effect on the amplitude of the large‐scale traveling atmosphere disturbances generated by the storm.