Transition Region Research Articles

Ultra-miniaturized Bloch mode metasplitters for one-dimensional grating waveguides.

We present, for the first time, to our knowledge, power splitters with multiple channel configurations in one-dimensional grating waveguides (1DGWs) that maintain crystal lattice-sensitive Bloch mode profiles without perturbation across all output channels, all within an ultra-miniaturized footprint of just 2.1 × 2.2 μm2. This novel capability reduces the need for transition regions, simplifies multi-channel configurations of 1DGWs, and maximizes the effective use of chip area. The pixelated metamaterial approach, integrated with a time-domain heuristic algorithm, is utilized to concurrently achieve broadband operation, optimized dispersion control, and minimal loss. We experimentally demonstrate that the 1 × 2 and 1 × 3 metasplitters achieve average minimum losses per channel of 3.80 dB and 5.36 dB, respectively, which are just 0.80 dB and 0.59 dB above ideal splitting. The measurements for both designs demonstrate a 1 dB bandwidth of 15 nm, with excellent uniformity across all output channels. These versatile metasplitter designs can serve as fundamental building blocks for ultrahigh-bandwidth, densely integrated photonic circuits and in scenarios where slow light is essential.

A data-driven framework for developing a unified density-modulus relationship for the human lumbar vertebral body.

Despite the broad agreement that bone stiffness is heavily dependent on the underlying bone density, there is no consensus on a unified relationship that applies to both cancellous and cortical compartments. Bone from the two compartments is generally assessed separately, and few mechanical test data are available for samples from the transitional regions between them. In this study, we present a data-driven framework integrating experimental testing and numerical modeling of the human lumbar vertebra through an energy balance criterion, to develop a unified density-modulus relationship across the entire vertebral body, without the necessity of differentiation between trabecular and cortical regions. A dataset of 25 spinal segments harvested from fresh-frozen human spines consisting of L1 vertebrae with adjacent intervertebral disks and neighboring T12 and L2 endplates was examined through a systematic process. Each specimen was subjected to axial compression using a custom-designed radiolucent device, and the deformation at multiple points during the ramp was quantified using digital volume correlation applied to the time-lapse series of microcomputed tomography images acquired during loading. A finite element model of each specimen was constructed from quantitative computed tomography images, with the experimental displacement fields imposed to replicate the observed deformation. The optimal density-modulus relationship, both in exponential and polynomial forms, was then determined by using data-driven techniques to match the numerical strain energy with the experimental external work. The resulting relationships effectively recovered bone tissue modulus at the microscale. Subsequently, the unified relationships were applied to investigate the vertebral structure-property correlations at the macroscale: as expected, compressive stiffness exhibited a moderate correlation with bone mineral density, whereas bending stiffness was revealed to correlate strongly with bone mineral content. These findings support the accuracy of the developed density-modulus relationships for the vertebral body and indicate the potential of the proposed framework to extend to other properties of interest such as vertebral strength and toughness.

Surface Circular Photogalvanic Effect in Tl-Pb Monolayer Alloys on Si(111) with Giant Rashba Splitting.

We have found that surface superstructures made of "monolayer alloys" of Tl and Pb on Si(111), having giant Rashba effect, produce nonreciprocal spin-polarized photocurrent via circular photogalvanic effect (CPGE) by obliquely shining circularly polarized near-infrared (IR) light. CPGE is here caused by the injection of in-plane spin into spin-split surface-state bands, which is observed only on Tl-Pb alloy layers but not on single-element Tl nor Pb layers. In the Tl-Pb monolayer alloys, despite their monatomic thickness, the magnitude of CPGE is comparable to or even larger than the cases of many other spin-split thin-film materials. A model analysis has provided the relative permittivity ε* of the monolayer alloys to be ∼1.0, which is because the monolayer exists at a transition region between vacuum and the substrate. The present result opens the possibility that we can optically manipulate the spins of electrons even on monolayer materials.

The Long-term Evolution of the Solar Transition Region

The Long-term Evolution of the Solar Transition Region

A Scenario for Origin of Global 4 mHz Oscillations in Solar Corona

We establish a spherically symmetric model of solar atmosphere, which consists of the whole chromosphere and low corona below the 1.25 solar radius. It is a hydrodynamic model with heating in the chromosphere through an artificial energy flux. We performed a series of simulations with our model and found oscillations with a peak frequency of ∼4 mHz in the power spectrum. We confirmed that this resulted from the p-mode excited in the transition region and amplified in a resonant cavity situated in the height range ∼4×103–2×104 km. This result is consistent with global observations of Alfvénic waves in corona and can naturally explain the observational ubiquity of 4mHz without the difficulty of the p-mode passing through the acoustic-damping chromosphere. We also confirmed that acoustic shock waves alone cannot heat the corona to the observed temperature, and found mass upflows in the height range ∼7×103–7×104 km in our model, which pumped the dense and cool plasma into the corona and might be the mass supplier for solar prominences.

Dynamical properties of hydrogen fluid at high pressures.

The properties of the hydrogen fluid at high pressures are still of interest to the scientific community. The experimentally unreachable dynamical properties could provide new insights into this field. In 2020 [Cheng et al., Nature 585, 217-220 (2020)], the machine-learned approach allows the calculation of the self-diffusion coefficient in the warm dense hydrogen with higher precision. After that, the work [van de Bund et al., Phys. Rev. Lett. 126(22), 225701 (2021)] reports the abinitio treatment of isotopic effects on diffusion in H2/D2 and a significant increase in its value in the region of the phase transition. Both works indicate the anomalous growth of diffusion, but the reasons for this phenomenon are unclear. In the present work, we reveal the plasma-like behavior of the diffusion growth. We apply the classical molecular dynamics method using a machine learning potential developed on the abinitio modeling for the prediction of diffusion and shear viscosity coefficients. We consider dependencies of the vibrational spectrum, molecule lifetime, diffusion, and shear viscosity coefficients on density along the isotherms in the temperature range from 600 to 1100K.

Observations of umbral flashes in the resonant sunspot chromosphere

Context. In sunspot umbrae, the core of some chromospheric lines exhibits periodic brightness enhancements known as umbral flashes. The consensus is that they are produced by the upward propagation of shock waves. This view has recently been challenged by the detection of downflowing umbral flashes and the confirmation of a resonant cavity above sunspots. Aims. We aim to determine the propagating or standing nature of the waves in the low umbral chromosphere and confirm or refute the existence of downflowing umbral flashes. Methods. Spectroscopic temporal series of Ca II 8542 Å, Ca II H, and Hα in a sunspot were acquired with the Swedish Solar Telescope. The Hα velocity was inferred using bisectors. Simultaneous inversions of the Ca II 8542 Å line and the Ca II H core were performed using the code NICOLE. The nature of the oscillations were determined and insights into the resonant oscillatory pattern were gained by analyzing the phase shift between the velocity signals and examining the temporal evolution. Results. Propagating waves in the low chromosphere are more common in regions with frequent umbral flashes, where the transition region is shifted upward, making resonant cavity signatures less noticeable. In contrast, areas with fewer umbral flashes show velocity fluctuations that align with standing oscillations. Evidence suggests dynamic changes in the location of velocity-resonant nodes due to variations in the transition region height. Downflowing profiles appear at the onset of some umbral flashes, but upflowing motion dominates during most of the flash. These downflowing flashes are more common in standing umbral flashes. Conclusions. We confirm the existence of a chromospheric resonant cavity above sunspot umbrae. It is produced by wave reflections at the transition region. The oscillatory pattern depends on the transition region height, which exhibits spatial and temporal variations due to the impact of the waves.

Vector substrate design for grain boundary engineering: boosting oxygen evolution reaction performance in LaNiO3.

The realization and subsequent control of emerging structural and electronic phases in solid materials has significantly enhanced their functionalities, thereby benefiting both fundamental research and practical applications. The grain boundary (GB), as a transitional region within the crystal lattice, exhibits atomic shifts and distinct energy profiles. These unique characteristics offer a promising avenue for the discovery of advanced active catalytic phases for carbon, oxygen, hydrogen, and nitrogen evolution/reduction reactions. However, the challenge lies in isolating and controlling the quantity of grain boundaries in conventional catalysts, which hinders the identification of their functional attributes. In this study, we successfully engineered the (001)/(110), (001)/(111), and (110)/(111) GBs in LaNiO3 (LNO) using a vector substrate design approach. Subsequent evaluation of these GBs in the oxygen evolution reaction (OER) revealed that LNO (110)/(111) exhibited the fastest surface reconstruction into Ni oxyhydroxide and the most superior OER performance, achieving 2.36 mA cm-2 at η = 400 mV. This outstanding performance is attributed to its strongest Ni-O covalency and the proximity of the O 2p-band center to the Fermi level. This research aims to address the challenges associated with isolating and controlling GBs for optimized OER performance, while also providing comprehensive insights into the relationship between GBs and surface reconstruction behaviors.

Spatial distribution pattern and long-term trend of atmospheric methane in the Atlantic-Mediterranean transition region based on TROPOMI and GOSAT measurements.

Methane (CH4) spatial distribution and its trends in the Atlantic-Mediterranean transition region of southwestern Europe were studied using TROPOMI and GOSAT observations. TROPOMI XCH4 provided insights into the distribution across the entire Iberian Peninsula, highlighting the high XCH4 mixing ratios in the two main valleys and the southern sub-plateau, characterized by agricultural and livestock activities. The marine-continental region in the southwestern Iberian Peninsula was identified as a major CH4 hot spot. Focusing on this region, the seasonal distribution of CH4 and its trends were studied. The lowest XCH4 levels were recorded in a semi-natural landscape in a mountain range with an average mixing ratio of 1871.9±18.5nmolmol-1, while the highest trend was recorded at 10.9±0.7nmolmol-1year-1 for the period 2019-2023. Conversely, the highest XCH4 levels were obtained over an inland wetland, averaging 1888.7±15.8nmolmol-1; while recording the lowest regional trend, 6.4±0.9nmolmol-1year-1. Aiming to identify potential regional changes in the XCH4 trend, GOSAT observations were used, revealing a long-term trend of 8.7±0.3nmolmol-1year-1 from 2009 to 2023. Focusing on the first and last five years of the series, trends of 6.5±0.9nmolmol-1year-1 and 14.9±1.2nmolmol-1year-1 were detected, indicating a trend acceleration ratio of 2.3. An analogous pattern was found in the Atlantic Ocean and central Mediterranean at the same latitude, with ratios of 2.7 and 2.4 respectively. Furthermore, these outcomes were corroborated by ground-based observations from the Azores and Lampedusa stations, which belong to the NOAA network. Despite CH4 increasing globally and its trend accelerating, this work highlights the complexity of its dynamics, with regional variations in its mixing ratios, horizontal distribution, and temporal trends.

Canopy elastic turbulence: Insights and analogies to canopy inertial turbulence.

Canopy flows occur when a moving fluid encounters a matrix of free-standing obstacles and are found in diverse systems, from forests and marine ecology to urban landscapes and biology (e.g. cilia arrays). In large-scale systems, involving Newtonian fluids (like water or air), canopy flows typically exhibit inertial turbulence due to high Reynolds numbers (Re). However, in small-scale systems like cilia, where Re is low, but the fluid can be viscoelastic (like mucus), the relevant control parameter is the Weissenberg number (Wi), quantifying elastic stresses in the flow. Here, we investigate the flow of a viscoelastic polymer solution over a microscopic canopy within a microfluidic device. As the Weissenberg number increases, the flow undergoes distinct transitions, eventually becoming unstable beyond a critical Wi. At high Wi, we observe the emergence of elastic turbulence (ET), a chaotic flow regime that, despite differing underlying mechanisms, exhibits striking similarities to large-scale canopy inertial turbulence. Similar to canopy inertial turbulence, ET within the canopy can be spatially divided into distinct regions: a porous layer within the canopy, a mixing layer at the canopy tips, a transitional region just above the canopy, and a Poiseuille-like flow further up. The separation of the flow into different regions reveals a new analogy between inertial turbulence and ET, providing a fresh insight into ET flows and expanding their potential for innovative microfluidic designs and real-world applications.

INTEGRATION OF HIGH-TEMPERATURE SURFACE HEAT EXCHANGE

Despite the discrete nature of the interaction of droplets of sprayed liquid with a high-temperature surface, the inevitable formation of a liquid film leads to the fact that the main qualitative laws of the heat exchange that occurs in this case are characteristic of the known process of heat exchange during boiling. At the same time, the presence of extensive theoretical and experimental studies of “large-volume boiling” and the process of steam generation in channels does not allow us to establish the conditions of heat exchange during the thermal interaction of a dispersed liquid - water with different concentrations of surfactants with a high-temperature surface. We did not find any materials about this process in the scientific literature. With a number of features common to the above two cases of heat exchange (the presence of boiling crises, film and bubble modes, etc.), cooling a high-temperature surface with a droplet medium containing various concentrations of surfactants has significant distinctive features due to the hydrodynamics of the process, which is the subject of further study. To fully explore the above task, the following must be done: Develop a methodology for experimental study of local conditions of non-stationary heat exchange of sprayed water with different concentrations of surfactants. Develop and manufacture an experimental setup on which to conduct research on the effects of irrigation density, surface temperature, degree of liquid underheating, its speed and angle of impingement on the surface. Develop a mathematical model for calculations of heat flows, heat transfer coefficients, dynamics of hydraulic methods of liquid dispersion – water with different concentrations of surfactants, critical heat flows and surface temperatures in the transition region from film to bubble boiling as a function of determining factors. Establish the independent influence of the degree of non-stationarity of the process on the heat exchange conditions.

Thermodynamical topology with multiple defect curves for dyonic AdS black holes

Dyonic black holes with quasitopological electromagnetism exhibit an intriguing phase diagram with two separated first-order coexistence curves. In this paper, we aim to uncover its influence on the black hole thermodynamical topology. At first, we investigate the phase transition and phase diagram of the dyonic black holes. Comparing with previous study that there is no black hole phase transition region for a middle pressure, we find this region can narrow or disappear by fine tuning the coupling parameter. Instead, two first-order phase transitions can be observed. Importantly, we uncover that such novel phase diagram shall lead to a multiple defect curve phenomenon in black hole topology where each dyonic black hole is treated as one defect in the thermodynamical parameter space. By examining the topology, it is shown that there could be one, three, or five black hole states for given pressure and temperature. For each case, the topological number is calculated. Our results show that the topological number always takes value of +1, keeping unchanged even when the multiple defect curves appear. Therefore, our study provides an important ingredient on understanding the black hole thermodynamical topology.

Spectra of standing kink waves in loops and the effects of the lower solar atmosphere

Understanding the effects of the lower solar atmosphere on the spectrum of standing kink oscillations of coronal loops, in both the decaying and decayless regime, is essential for developing more advanced tools for coronal seismology. We aim to reveal the effects of the chromosphere on the spatial profiles and frequencies of the standing kink modes, create synthetic emission maps to compare with observations, and study the results using spatial and temporal coronal seismology techniques. We excited transverse oscillations in a 3D straight flux tube using (a) a broadband footpoint driver, (b) a sinusoidal velocity pulse, and (c) an off-centre Gaussian velocity pulse, using the PLUTO code. The flux tube is gravitationally stratified, with footpoints embedded in chromospheric plasma. Using the FoMo code, we created synthetic observations of our data in the Fe IX 17.1 nm line and calculated the spectra with the Automatic Northumbria University Wave Tracking code. We also numerically solved the generalised eigenvalue system for the 1D wave equation to determine the effects of the stratification on the kink modes of our system. The synthetic observations of the loops perturbed by the velocity pulses show a single dominant mode that our 1D analysis reveals to be the third harmonic of the system. For the broadband driver, the synthetic emission shows multiple frequency bands, associated with both the loop and the driver. Finally, using seismological techniques, we highlight the possibility of misidentifying the observed third, sixth, and ninth harmonics with the first, second, and third harmonics of the coronal part of the loop. Unless more advanced techniques of spatial seismology are used with many data points from observations along the entire loop length, this misidentification can result in overestimating the mean magnetic field by a factor equal to the period ratio of the fundamental over the third harmonic. For longer coronal loops it is easy to misidentify the detected standing kink modes for lower-order modes of the system, which can have important seismological implications. To prevent these errors and properly constrain the value of the estimated mean magnetic field, additional observations of the loops footpoints using transition region and chromospheric lines are necessary.

Оценка электронного содержания плазмосферы и высоты перехода O⁺/H⁺ во время геомагнитной бури в феврале 2022 г. по данным Иркутского радара НР

We study the topside ionosphere above the NmF2 ionization maximum and the transition region between the ionosphere and the plasmasphere. We analyze the interaction between the topside ionosphere and the plasmasphere during a strong geomagnetic storm in February 2022, using data from the Irkutsk Incoherent Scatter Radar (IISR) and total electron content data from global navigation satellite systems. To determine the ionosphere and plasmasphere electron contents, an original technique is employed to calculate the integral content of ion density from IISR data, which takes into account the two-component composition of ionospheric plasma. We compare different functions of approximation of the topside ionosphere. The original technique was adjusted for use with IISR Ne data fitted based on the β-Chapman profile. We compare the plasmasphere electron content during quiet and magnetically disturbed days, as well as the dynamics of the O⁺/H⁺ transition height, which is the upper boundary of the ionosphere and the lower boundary of the plasmasphere.

Estimated plasmasphere electron content and O⁺/H⁺ transition height during the February 2022 geomagnetic storm from Irkutsk IS radar data

We study the topside ionosphere above the NmF2 ionization maximum and the transition region between the ionosphere and the plasmasphere. We analyze the interaction between the topside ionosphere and the plasmasphere during a strong geomagnetic storm in February 2022, using data from the Irkutsk Incoherent Scatter Radar (IISR) and total electron content data from global navigation satellite systems. To determine the ionosphere and plasmasphere electron contents, an original technique is employed to calculate the integral content of ion density from IISR data, which takes into account the two-component composition of ionospheric plasma. We compare different functions of approximation of the topside ionosphere. The original technique was adjusted for use with IISR Ne data fitted based on the β-Chapman profile. We compare the plasmasphere electron content during quiet and magnetically disturbed days, as well as the dynamics of the O⁺/H⁺ transition height, which is the upper boundary of the ionosphere and the lower boundary of the plasmasphere.

Are rootzone soil moisture dynamics and thresholds associated with surface layer?

Abstract The identification of evapotranspiration regimes, primarily the water-limited and energy-limited regimes, separated by the critical soil moisture (CSM) threshold, is fundamental to analyzing land–atmosphere interactions. To better understand the soil moisture (SM) dynamics happening synchronously in the soil column, we aim to estimate the rootzone (0–28 cm and 0–100 cm) CSM thresholds and associated regimes at a global scale, which was not previously attempted. We propose the use of the covariability of soil diurnal temperature amplitude (derived from the GLDAS) and SM (ERA5) to estimate the CSM, which overcomes the data uncertainty and multivariate dependencies of traditional methods. We find that transitional climatic regions, encompassing the western USA, Brazilian savanna, Sahelian grassland, South African savanna, peninsular India, and Mediterranean region, are global hotspots of frequent rootzone regime shifting with significant seasonality—the wet regime prevails in the fall season, while the dry regime takes over at other times of the year. The CSM values of 0–28 cm and 0–100 cm layers are mostly in the 0.2–0.35 and 0.25–0.4 m3m−3 range, respectively. We find that landscape aridity and bioclimatic characteristics primarily determine the spatial distribution of CSM and associated regimes. Furthermore, we investigate the hydrological link between the surface and rootzone layers. We note that the rootzone and surface CSM and regimes are strongly correlated, although the 0–28 cm layer indicates a relatively stronger connection compared to the 0–100 cm layer. The shallower (deeper) rootzone layer shows regimes similar to those on the surface for more than 80% (65%–80%) of the time. We observe that the strength of association between surface and rootzone regimes increases from arid (herbaceous vegetated regions) to humid (woody) regions and during wet to dry seasons. Overall, a strong association in regime dynamics between surface and subsurface layers suggests the potential applicability of remotely sensed surface SM as a surrogate to study rootzone regime responsiveness to soil–plant–atmosphere interactions.

Inferring phase transitions and critical exponents from limited observations with thermodynamic maps

Phase transitions are ubiquitous across life, yet hard to quantify and describe accurately. In this work, we develop an approach for characterizing generic attributes of phase transitions from very limited observations made deep within different phases' domains of stability. Our approach is called thermodynamic maps (TM), which combines statistical mechanics and molecular simulations with score-based generative models. TM enable learning the temperature dependence of arbitrary thermodynamic observables across a wide range of temperatures. We show its usefulness by calculating phase transition attributes such as melting temperature, temperature-dependent heat capacities, and critical exponents. For instance, we demonstrate the ability of TM to infer the ferromagnetic phase transition of the Ising model, including temperature-dependent heat capacity and critical exponents, despite never having seen samples from the transition region. In addition, we efficiently characterize the temperature-dependent conformational ensemble and compute melting curves of the two RNA systems: a GCAA tetraloop and the HIV-TAR RNA, which are notoriously hard to sample due to glassy-like energy landscapes.

Solute Energetics in Aqueous Xanthan Gum Solutions: What Can Be Learned from a Fluorescent Probe?

Xanthan gum (XG) is a well-known anionic polysaccharide that finds broad application in the food and petroleum industries because of its ability to enhance solution viscosity at low concentrations and moderate temperatures. The aim of this work was to use the solvation probe coumarin 153 (C153) to characterize changes in the xanthan gum (XG) solution microstructure as a function of XG concentration and temperature from the perspective of a dissolved solute molecule. We established the utility of C153 fluorescence to track solution changes for XG concentrations that span the transition region from a dilute to a semi-dilute solution, defined by the xanthan gum overlap concentration, C*~0.02 g/dL. The temperature was varied from 293 to 353 K to probe solution conditions wherein XG has been reported to undergo a structural change from helix to random coil conformation, the details of which are still under debate. While C153 fluorescence does not elucidate direct structural information, the emission response is a simple means by which changes in aqueous XG solution can be identified. C153 spectroscopy is observed to correlate with XG conformational changes, as reported in the literature.

The Study of the Molecular Diffusion in Gases and Liquids

The molecular dynamics calculations of the velocity autocorrelation functions and the diffusion coefficients for argon and krypton atoms in argon have been carried out. The data of the molecular dynamics simulations are analyzed to understand the change of the diffusion mechanism with transition from dense gas to liquid. In dense gases, the diffusion mechanism is the same as in rarefied gases, and the temperature dependences of the diffusion coefficient are alike. A new diffusion mechanism appears in liquids. This diffusion mechanism isn’t connected with the jumping motion of the molecules. Probably, it is connected with the motion of the molecules group surrounding a molecule. Then the velocity of the molecule is relaxed with the decreasing of the average velocity of the molecules group. Various structures of the systems within the vapor – liquid phase transition region have been found and investigated. A comparison of the simulation results with the experimental data on diffusion in gaseous and liquid argon yields good agreement.

Investigating numerical stability by scaling heat conduction in a 1D hydrodynamic model of the solar atmosphere

Numerical models of the solar atmosphere are widely used in solar research and provide insights into unsolved problems such as the heating of coronal loops. A prerequisite for such simulations is an initial condition for the plasma temperature and density. Many explicit numerical schemes employ high-order derivatives that require some diffusion, for example isotropic diffusion, for each independent variable to maintain numerical stability. Otherwise, significant numerical inaccuracies and subsequent wiggles will occur and grow at steep temperature gradients in the solar transition region. We tested how to adapt the isotropic heat conduction to the grid resolution so that the model is capable of resolving varying temperature gradients. Our ultimate goal is to construct an atmospheric stratification that can serve as an initial condition for multi-dimensional models. Our temperature stratification spans from the solar interior to the outer corona. From that, we computed the hydrostatic density stratification. Since numerical and analytical derivatives are not identical, the model needs to settle to a numerical equilibrium to fit all model parameters, such as mass diffusion and radiative losses. To compensate for energy losses in the corona, we implemented an artificial heating function that mimics the expected heat input from the 3D field-line braiding mechanism. Our heating function maintains and stabilises the obtained coronal temperature stratification. However, the diffusivity parameters need to be adapted to the grid spacing. Unexpectedly, we find that higher grid resolutions may need larger diffusivities --- contrary to the common understanding that high-resolution models are automatically more realistic and would need less diffusivity. Smaller grid spacing causes larger temperature gradients in the solar transition region and hence a greater potential for numerical problems. We conclude that isotropic heat conduction is an efficient remedy when using explicit schemes with high-order numerical derivatives.