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We experimentally and numerically study the temporal dynamics of light scattered by large clouds of cold atoms after the exciting laser is switched off, in the low intensity (linear-optics) regime. Radiation trapping due to multiple scattering as well as subradiance lead to decay much slower than the single atom fluorescence decay. These two effects have already been observed separately, but the interplay between them remained to be understood. Here, we show that with well chosen parameters of the driving field, the two effects can occur at the same time, but follow different scaling behaviors. The subradiant decay is observed at late time and its rate is independent of the detuning, while the radiation trapping decay is observed at intermediate time and depends on the detuning through the optical depth of the sample. Numerical simulations based on random walk process and coupled-dipole equations support our interpretations. Our study clarifies the different interpretations and physical mechanisms at the origin of slow temporal dynamics of light in cold atoms.

Clouds regulate the Greenland Ice Sheet’s surface energy balance through the competing effects of shortwave radiation shading and longwave radiation trapping. However, the relative importance of these effects within Greenland’s narrow ablation zone, where nearly all meltwater runoff is produced, remains poorly quantified. Here we use machine learning to merge MODIS, CloudSat, and CALIPSO satellite observations to produce a high-resolution cloud radiative effect product. For the period 2003–2020, we find that a 1% change in cloudiness has little effect (±0.16 W m −2 ) on summer net radiative fluxes in the ablation zone because the warming and cooling effects of clouds compensate. However, by 2100 (SSP5-8.5 scenario), radiative fluxes in the ablation zone will become more than twice as sensitive (±0.39 W m −2 ) to changes in cloudiness due to reduced surface albedo. Accurate representation of clouds will therefore become increasingly important for forecasting the Greenland Ice Sheet’s contribution to global sea-level rise. Here the authors use remote sensing observations and machine learning to show that clouds will become increasingly important for determining the Greenland Ice Sheet’s contribution to global sea levels due to decreasing albedo in the ablation zone.

The radiation trapping effect (RTE) of electrons in the interaction of an ultra-intense laser and a near-critical-density plasma-filled gold cone is numerically investigated by using the particle-in-cell code EPOCH. It is found that, by using the cone, the threshold laser intensity for electron trapping can be significantly decreased. The trapped electrons located behind the laser front and confined near the laser axis oscillate significantly in the transverse direction and emit high-energy photons in the forward direction. With parameters optimized, a narrow photon angular distribution and a high-energy conversion efficiency from the laser to the photons can be obtained. The proposed scheme may offer possibilities to demonstrate the RTE of electrons in experiments at approachable laser intensities and serve as a novel table-top ray source.

Urban heat intensification and its implications for environmental health due to solar radiation absorption is a key driver of the Urban Heat Island (UHI) effect, particularly in Mediterranean climates. In dense urban fabrics, shortwave radiation is repeatedly reflected within street canyons, affecting pedestrian-level radiative exposure. This study introduces the Urban Canyon Shortwave Trapping Index ( STI c ), a dimensionless parameter quantifying geometry-induced shortwave accumulation near the ground. Shortwave fluxes were simulated using the SOLWEIG model over the centre of Nicosia, Cyprus, under typical summer conditions. Wall albedo was systematically varied (0.2–0.8) to represent a range of common building materials and cooling strategies, including Nature-Based Solution (NBS). Results show that STI c increases with both wall reflectivity and canyon depth, reaching values above 2 in high-albedo, high aspect ratio configurations. Maximum thermal trapping occurs between 11:00 and 14:00 local time, coinciding with peak in solar altitude. Spatial variability of STI c also increases with wall albedo, indicating greater sensitivity to morphological differences under more reflective conditions. These findings support the integration of both morphological and material-based strategies in UHI mitigation and thermal comfort improvement and sustainable urban development.

A numerical model of a pulsed self-sustained discharge in mixtures of H 2 and He is created to describe spontaneous emission in Lyman and Werner bands. The model considers processes involving the electronic and vibrational states of H 2 , the electronic states of helium and hydrogen atoms, positive and negative ions, and spontaneous emission in Lyman and Werner bands with allowance for its trapping. Equations of electron–vibrational kinetics are solved along with the Boltzmann equation for the electron energy distribution function, equations of the external electrical circuit, and equations describing preionization by a beam of fast electrons. A simplified model with effective VV exchange constants is used for the vibrational kinetics of hydrogen molecules, allowing the number of necessary calculations to be reduced. Processes of electron attachment and detachment during collisions with vibrationally excited H 2 molecules are considered. The model is verified by a detailed comparison with experimental data on discharge characteristics and radiation energy available in the literature. It is shown that in a pulsed discharge with preionization, the efficiency of radiation in Lyman and Werner bands can be as high as 16% of the energy deposited in the discharge.

The Microwave Emission Model of Layered Snowpacks (MEMLS) was originally developed for microwave emissions of snowpacks in the frequency range 5–100 GHz. It is based on six-flux theory to describe radiative transfer in snow including absorption, multiple volume scattering, radiation trapping due to internal reflection and a combination of coherent and incoherent superposition of reflections between horizontal layer interfaces. Here we introduce MEMLS3&a, an extension of MEMLS, which includes a backscatter model for active microwave remote sensing of snow. The reflectivity is decomposed into diffuse and specular components. Slight undulations of the snow surface are taken into account. The treatment of like- and cross-polarization is accomplished by an empirical splitting parameter q. MEMLS3&a (as well as MEMLS) is set up in a way that snow input parameters can be derived by objective measurement methods which avoid fitting procedures of the scattering efficiency of snow, required by several other models. For the validation of the model we have used a combination of active and passive measurements from the NoSREx (Nordic Snow Radar Experiment) campaign in Sodankylä, Finland. We find a reasonable agreement between the measurements and simulations, subject to uncertainties in hitherto unmeasured input parameters of the backscatter model. The model is written in Matlab and the code is publicly available for download through the following website: http://www.iapmw.unibe.ch/research/projects/snowtools/memls.html.

Urban heat island effects, which are the phenomenon wherein higher ambient air temperatures are measured in cities than in rural areas, have worsened urban thermal environments over the past decades. This study aims to analyze the effects of urban physical environments on the duration of high air temperature, using climate data collected from 217 Automatic Weather Stations in Seoul, Korea. In order to specify radiation trapping effects, interaction effects between sky view factors (SVF) and albedo values were analyzed using multiple regression analysis. The results indicate that increases in commercial and traffic areas lead to longer durations of high air temperature and that high urban porosity shortens the duration of high air temperature by improving urban ventilation. This study also indicates that the duration of high air temperature has a negative association with SVF; however, an analysis of interaction effects indicates that high-albedo materials diminish the positive effects of high SVF largely because of radiation trapping effects. These findings suggest that urban ventilation paths, high SVF, and materials with an appropriate albedo value play important roles in improving thermal comfort conditions, such as the duration of high air temperature.

Raman interactions in alkali vapours are used in applications such as atomic clocks, optical signal processing, generation of squeezed light and Raman quantum memories for temporal multiplexing. To achieve a strong interaction the alkali ensemble needs both a large optical depth and a high level of spin-polarisation. We implement a technique known as quenching using a molecular buffer gas which allows near-perfect spin-polarisation of over 99.5 % in caesium vapour at high optical depths of up to ∼ 2 × 10 5 ; a factor of 4 higher than can be achieved without quenching. We use this system to explore efficient light storage with high gain in a GHz bandwidth Raman memory.

The increase of the Urban Heat Island (UHI) is a continuing phenomenon in dense cities. Technically, the building of heat gain generated from modified solar radiation in tropical urban spaces requires the scientific solution. The investigation on the behavior of urban microclimate, especially solar radiation in the urban canyon, was comprehensively investigated by early urban energy studies. However, the solar radiation trapping effect as the behavior of solar radiation in different type of urban configurations is still unclear. This study, therefore, aims to investigate the impact of urban configurations on the solar radiation performance in Kuala Lumpur tropical context. By using ENVI-met V3.1, this study simulates four urban configurations situated in two scenarios of canyon directions; East-West and South-North. The results recorded that the short wave and long wave solar radiation varies in a different type of urban configurations. It concludes that besides the Sky View Factor (SVF) and Height to Width (H/W) aspect ratio, the direction of urban canyon towards the exposure of the source of solar radiation significantly influences the solar radiation trapping effect. This finding will be a significant reference to strategies for the urban configuration to improve the microclimate in the urban spaces and mitigating climate change.

Optical emission spectroscopy has been widely used in low-temperature argon plasma diagnostics. A coronal model is usually used to analyze the measured line ratios for diagnostics with a single temperature and density. However, many plasma processing conditions deviate from single temperature and density, optically thin conditions, or even coronal plasma conditions due to cascades from high-lying states. In this paper, we present a collisional-radiative model to investigate the validity of coronal approximations over a range of plasma conditions of Te = 1–4 eV and Ne = 108–1013 cm−3. The commonly used line ratios are found to change from a coronal limit where they are independent of Ne to a collisional-radiative regime where they are not. The effects of multiple-temperature plasma, radiation trapping, wall neutralization, and quenching on the line ratios are investigated to identify the plasma conditions under which these effects are significant. This study demonstrates the importance of the completeness of atomic datasets in applying a collisional-radiative model to low-temperature plasma diagnostics.