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Refractive indices of photochemical haze analogs for Solar System and exoplanet applications: A cross-laboratory comparative study between the PAMPRE and COSmIC experimental setups
No full textInternational audiencePrevious observations of Titan, Pluto, and Solar System gas giants, along with recent observations of exoplanet atmospheres with the James Webb Space Telescope, have taught us that photochemical hazes are ubiquitous and form in a variety of temperature, gas composition, and irradiation environments. Despite their crucial role in understanding their impact on observations and on the planetary radiative budget, the refractive indices of these haze particles remain unknown and are strongly influenced by changes in gas-phase chemistry. In this study, we performed a cross-laboratory investigation to assess the effect of the experimental setup and gas composition on the refractive indices of Titan, Pluto, and exoplanet haze analogs. We report new data in a broad spectral range from UV to far-IR (up to 200 µm) for future use in climate models and retrieval frameworks. We compare the refractive indices of laboratory haze analogs produced from six different gas compositions, in which we varied the relative abundances N 2 /CH 4 and CH 4 /CO in the initial gas mixture, using the PAMPRE (LATMOS, France) and COSmIC (NASA Ames Research Center, USA) experimental setups. We observed strong variations in the k values in the spectral range from UV to near-IR between the different analogs, which are caused by both the experimental setup and changes in the gas N 2 /CH 4 ratio. We find that the gas N 2 /CH 4 ratio has a stronger influence on the haze refractive indices in the entire spectral range compared to the gas CH 4 /CO ratio. The experimental setup is the primary factor affecting the refractive indices, confirming that the gas residence time, irradiation, pressure, and gas temperature are important parameters influencing the composition of the solid analog. The higher n and k values in the UV-visible range, along with the stronger amine, alkene, aromatic, and/or hetero-aromatic signatures in the mid-IR for the COSmIC analogs, are consistent with a greater incorporation of nitrogen into the COSmIC solid analogs compared to the PAMPRE analogs, even at similar nitrogen abundances in the gas phase. Haze analogs produced in gas mixtures without nitrogen, similar to the stratospheres of Solar System gas giants and the H 2 -dominated atmospheres of sub-Neptunes, are generally more transparent with lower n values across the entire spectral range from UV to mid-IR and should therefore be carefully considered in climate and observational applications. The variations in IR absorption features between hazes produced with and without nitrogen could help constrain the presence of N 2 in exoplanet atmospheres
Sampling from multi-modal distributions on Riemannian manifolds with training-free stochastic interpolants
No full textIn this paper, we propose a general methodology for sampling from un-normalized densities defined on Riemannian manifolds, with a particular focus on multi-modal targets that remain challenging for existing sampling methods. Inspired by the framework of diffusion models developed for generative modeling, we introduce a sampling algorithm based on the simulation of a non-equilibrium deterministic dynamics that transports an easy-to-sample noise distribution toward the target. At the marginal level, the induced density path follows a prescribed stochastic interpolant between the noise and target distributions, specifically constructed to respect the underlying Riemannian geometry. In contrast to related generative modeling approaches that rely on machine learning, our method is entirely training-free. It instead builds on iterative posterior sampling procedures using only standard Monte Carlo techniques, thereby extending recent diffusion-based sampling methodologies beyond the Euclidean setting. We complement our approach with a rigorous theoretical analysis and demonstrate its effectiveness on a range of multi-modal sampling problems, including high-dimensional and heavy-tailed examples
Selective Disruption of Plasmodium falciparum mitochondrial DNA via G-Quadruplex-Binding Ligand RHPS4 Provides a Novel Antimalarial Strategy
No full textInternational audienceABSTRACT Malaria caused by Plasmodium falciparum remains a major health threat, killing over 600,000 people annually. The spread of resistance to all major antimalarials, including artemisinins, highlights the urgent need for new drugs with distinct mechanisms of action. Here we show that the G-quadruplex ligand RHPS4, an acridine derivative, displays strong antiplasmodial activity against both drug-sensitive and -resistant P. falciparum strains and clinical isolates. RHPS4 primarily targets the trophozoite stage and induces major mitochondrial alterations, including reduction of mitochondrial DNA (mtDNA) and transcriptional dysfunctions. Bioinformatic analyses identified at least eight putative G4-forming sequences within the parasite’s mtDNA. Biophysical studies confirmed G4 folding of at least one sequence and its interaction with RHPS4. These findings indicate that RHPS4 disrupts P. falciparum mitochondrial metabolism through G4 stabilization, leading to parasite death, and establish mtDNA G4 structures as novel therapeutic targets for antimalarial development
La captation audiovisuelle enmathématiques
No full textWe describe an experimental activity involving the recording of mathematical work sessions, aimed at documenting the work of mathematicians and contributing to an archive of the daily aspects of research. The feedback from the filmed images provides a new investigative tool and a source of reflection on our practices.Nous décrivons une activité expérimentale de captation de séances de travail mathématiques, qui a pour but de documenter le travail du mathématicien, en contribuant à nourrir un fonds d’archive sur les aspects quotidiens de l’activité de recherche. Le retour d’expérience de l’image filmée offre un nouvel outil d’investigation et une source de réflexions sur nos pratiques.</div
Estimation of discrete distributions in relative entropy, and the deviations of the missing mass
No full textInternational audienceWe study the problem of estimating a distribution over a finite alphabet from an i.i.d. sample, with accuracy measured in relative entropy (Kullback–Leibler divergence). While optimal bounds on the expected risk are known, high-probability guarantees remain less well-understood. First, we analyze the classical Laplace (add-one) estimator, obtaining matching upper and lower bounds on its performance and establishing its optimality among confidence-independent estimators. We then characterize the minimax-optimal high-probability risk and show that it is achieved by a simple confidence-dependent smoothing technique. Notably, the optimal non-asymptotic risk incurs an additional logarithmic factor compared to the ideal asymptotic rate. Next, motivated by regimes in which the alphabet size exceeds the sample size, we investigate methods that adapt to the sparsity of the underlying distribution. We introduce an estimator using data-dependent smoothing, for which we establish a high-probability risk bound depending on two effective sparsity parameters. As part of our analysis, we also derive a sharp high-probability upper bound on the missing mass
RHITA: a web tool for real-time detection of extreme weather events
No full textExtreme weather hazards are increasing and stakeholders need rapid, transparent information during unfolding events. We present RHITA (Real-time Hazard Identification and Tracking Algorithm), an open-source framework and web tool for near real-time detection and tracking of weather-related hazards over Europe. RHITA identifies grid cells exceeding local quantile thresholds, groups them into spatial clusters, and links clusters through time to reconstruct three-dimensional events in longitude, latitude, and time. For each event, RHITA provides intensity, extent and duration metrics and estimates rarity through return periods derived from a long historical record.RHITA is operated with ECMWF open forecasts for daily monitoring and ERA5 reanalysis for a consistent historical archive from 1950 to 2024. We target four hazards: heatwaves, cold spells, heavy precipitation and strong winds. Key spatial and temporal parameters are optimized against EM-DAT disaster records (2000 to 2023). Applying RHITA to ERA5 yields a European climatology of hazard events and reveals robust increases in heatwave frequency, intensity and affected area, a decline in cold spell frequency, and more heterogeneous signals for heavy precipitation and strong winds at the continental scale. RHITA provides open access data and an interactive interface to support rapid hazard characterization, event contextualization and downstream risk analysis
Consumer Choice between Recommendation Algorithms - Experimental Evidence
No full textInternational audienceRegulators are increasingly concerned with the power of large online platforms to bias consumer recommendations. In light of these concerns, I study experimentally whether giving consumers the choice between several recommendation algorithms can improve consumer welfare. I develop a novel experimental setting in which I first elicit the subjects' preferences and then expose them to choice tasks intermediated by personalized recommendation algorithms. When faced with a costly choice between two algorithms, subjects have a positive willingness-to-pay for better recommendations. However, subjects underestimate the potential gain from the better algorithm: a \$1 increase in estimated gain raises willingness-to-pay by only \$0.15 on average. These findings suggest that giving consumers power over recommendation algorithms to curtail potential abuse is not straightforward. However, it may be a viable business for platforms to offer improved recommendation algorithms to consumers for a fee
A study of the magnetic effects of the Equatorial Electrojet (EEJ) along the East Asian and West African sectors
No full textInternational audienceThis paper presents a comparative analysis of the Equatorial Electrojet (EEJ) effects along the East Asian (140° E) and West African (5° W) sectors using geomagnetic field data recorded in 1993. The data were obtained within the frameworks of the International Equatorial Electrojet Year (IEEY) campaign for West Africa and the Solar-Terrestrial Energy Program (STEP) project for the Asian sector. The analysis was conducted under quiet magnetic conditions (Am < 20 nT) to facilitate the investigation of regular daily variations () induced by the EEJ. Key electrodynamic parameters of the EEJ such as ribbon width, center position, and maximum current intensity were estimated using latitudinal profiles of the H and Z components of the geomagnetic field. The results reveal significant latitudinal and longitudinal variations characterized by a wider EEJ ribbon in Asia (∼714 km) compared to West Africa (∼601 km), and a higher peak current intensity in West Africa (214.3 ± 30.9 A/km) relative to Asia (142.4 ± 31.9 A/km). Also, in 1993, the EEJ centre was located north of the magnetic equator in West Africa and south of it in Asia, indicating a pronounced hemispheric asymmetry. Seasonal variations show higher EEJ intensities during the equinoxes than during the solstices in both longitude sectors. Throughout 1993, the EEJ peak intensity over West Africa consistently surpassed that over East Asia, irrespective of the magnetic season. Furthermore, the frequency of occurrence of the counter-electrojet (CEJ) was higher in West Africa, especially during the morning hours. This study shows that EEJ morphology and strength are modulated by longitude-dependent factors, including ionospheric conductivity, neutral winds, atmospheric tides, and the geomagnetic field
Stress scenarios of cyber loss processes with dependencies
No full textCyber risk has become one of the most critical threats to organizations, characterized by its contagious nature and its dependence on vulnerability disclosures. These features make it particularly challenging for insurers to quantify systemic exposures and design stress tests that assess portfolio resilience under extreme scenarios. In this paper, we develop a model that explicitly incorporates the role of vulnerabilities in cyber attack dynamics and captures contagion and clustering features. Using the general framework of Multivariate Self-Exciting Processes with Dependencies (MSPD), we provide closed-form expressions for the expectation and the variance of a cyber loss process under an externally excited Hawkes model, and we derive closed-form formulas for the surplus under two stress scenarios: (i) an excess of claims in the portfolio and (ii) a massive disclosure of critical vulnerabilities. This novel approach enables trajectory-based stress tests by evaluating the loss process under deterministic scenarios of claims or vulnerability disclosures. By quantifying the impact of these scenarios, our framework offers insurers practical tools for cyber risk stress testing, portfolio management, and regulatory compliance
One- and Two-Photon Polymerization of Solvent- and Filler-Free Aromatic Organic Precursors: Toward 3D-Printed Semiconducting Microstructures
No full textInternational audienceOver the past decades, a great deal of attention has been dedicated toward developing semiconductive polymers (SP), which are now considered as a critical class of photoactive and electroactive materials. However, the processing of many SPs involves the use of solvents, leading to the fabrication of samples with limited shapes, mostly flat 2D thin films. To develop 3D-shaped materials with potential semiconducting properties, this work tackles the development of solvent-and filler-free resins for the fabrication via additive manufacturing of microscale organic semiconductors. Two molecular liquids based on electron-rich carbazole and triarylamine units are used to formulate photoresists. The successful reactivity of the formulations was investigated, both at the macroscopic scale (one-photon polymerization through UV curing) and at the microscopic scale (two-photon polymerization (2PP) via direct laser writing), with Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. Subsequently, the thermal properties of the macroscopic samples were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Optimization of the printing parameters for 2PP led to the elaboration of 3D micrometer-scale samples whose morphology was assessed by scanning electron microscopy (SEM). Finally, electrical measurements revealed a semiconducting behavior as the samples were found to conduct current after p-doping with iodine