We propose a hybrid semi-Lagrangian scheme for the Vlasov–Poisson equation that combines the Numerical Flow Iteration (NuFI) method with the Characteristic Mapping Method (CMM). Both approaches exploit the semi-group property of the underlying diffeomorphic flow, enabling the reconstruction of solutions through flow maps that trace characteristics back to their initial positions. NuFI builds this flow map iteratively, preserving symplectic structure and conserving invariants, but its computational cost scales quadratically with time. Its advantage lies in a compact, low-dimensional representation depending only on the electric field. In contrast, CMM achieves low computational costs when remapping by composing the global flow map from explicitly stored submaps. The proposed hybrid method merges these strengths: NuFi is employed for accurate and conservative local time stepping, while CMM efficiently propagates the solution through submap composition. This approach reduces storage requirements, maintains accuracy, and improves structural properties. Numerical experiments demonstrate the effectiveness of the scheme and highlight the trade-offs between memory usage and computational cost. We benchmark against a semi-Lagrangian predictor-corrector scheme used in modern gyrokinetic codes, evaluating accuracy and conservation properties.
Recent evidence from Parker Solar Probe on the suprathermal electrons with Kappa-type velocity distributions in the outer corona has revived interest in the kinetic-based macro-modeling of the solar (SW), aiming to explain its properties. Invoked in kinetic modeling of nonequilibrium plasmas, standard Kappa distributions (SKDs) have been adjusted to the regularized Kappa distributions (RKDs) to fix the inconsistencies of SKD and develop consistent fluid modeling of space plasmas. We propose a new analysis of these properties at large heliocentric distances based on the existence of RKD electrons at the exobase. This new semi-analytic formalism is inspired by the methodology proposed initially by Meyer-Vernet and Issautier (1998), https://doi.org/10.1029/98ja02853. Compared to SKDs, the results for RKDs have extended applicability, since all moments can be defined and calculated consistently for all values of the x parameter, even lower than the critical ones (e.g., kappa(c) = 3/2 imposed to the second-order moment) of SKDs. However, the excess energy of the more energetic suprathermal electrons associated with low values of kappa less than or similar to 3/2, is regulated by the RKD-specific cutoff parameter alpha < 1. The estimates for, for example, the temperature and bulk velocity of the SW, remain at realistic values even for small 3/2 < kappa less than or similar to 2, which would otherwise exceed specific observations. One can thus model a higher abundance of suprathermal electrons at the exobase (e.g., kappa <= 3/2), which is plausible for the sources of energetic events (flares and coronal mass ejections), and also in the astrospheres of stars with coronas hotter than the Sun's. Plain Language Summary The solar wind is a flow of charged particles, electrons and ions/protons, emerging from the outer solar corona and reaching speeds between 300 and 800 km/s in interplanetary space. Kinetic-exospheric models show that the solar wind can be driven by light but energetic, suprathermal electrons that escape solar gravity and create an electric field that pulls protons away from the Sun. We provide an improved semi-analytic model of the long-distance solar wind (r > 0.3 AU) that uses a regularized Kappa distribution to realistically capture the effects of suprathermal electrons at the exobase of a few solar radii. This distribution resolves the inconsistencies and limitations of standard Kappa distributions, offering a consistent extension of such macro-models, even in the presence of enhanced suprathermal electrons at the exobase, characteristic of more energetic coronal outflows.
Volatile organic compounds (VOCs) and ozone (O3) are key constituents of tropospheric chemistry, affecting both air quality and climate. Forests are major emitters of biogenic VOCs (BVOCs), yet large uncertainties remain regarding the diversity of exchanged compounds, the drivers of their bidirectional fluxes, and their in-canopy chemistry. Long-term and comprehensive in situ datasets remain scarce, limiting our understanding of these complex processes. We conducted a 3-year field campaign (2022–2024) at the Integrated Carbon Observation System mixed temperate forest station of Vielsalm (BE-Vie), combining vertical concentration profile and eddy covariance flux measurements above and below the canopy (concentration dataset: https://doi.org/10.18758/NVFBA74V, Verreyken et al., 2025c; flux dataset: https://doi.org/10.18758/KHV8ZXU2, Dumont et al., 2025a; concentration-turbulence profile dataset: https://doi.org/10.18758/BED4Q2VY, Dumont et al., 2025b). Using a PTR-ToF-MS and an open-source data-processing workflow, we identified 48 significantly exchanged VOCs. The vertical and diurnal gradients of the mixing ratios reflected the interplay between emission, deposition, chemistry, and transport. Combined with a profile of turbulence statistics, these observations offer an opportunity to investigate their behaviour within the canopy. The forest acted as a net VOC source in summer (∼ 1.25 µg m−2 s−1), while deposition dominated in autumn. Many oxygenated VOCs displayed bidirectional exchange. Monoterpenes, isoprene, and methanol were the most abundant flux contributors, but 15–30 (30–43) compounds were needed to account for 90 % of total emissions (depositions), depending on the season. Below-canopy BVOC and O3 fluxes reached ∼ 10 % of above-canopy ones, with proportionally enhanced below-canopy ozone uptake at night. This study provides one of the most detailed long-term datasets of VOC and O3 exchange in a temperate forest and serves as a key reference for improving process-based models of biogenic, physical, and chemical exchange in forest ecosystems.
This study presents the first comprehensive description of the operational GEMS (Geostationary Environment Monitoring Spectrometer) ozone profile retrieval algorithm and evaluates the performance of the reprocessed version 3.0 dataset. The retrieval operates in the 310–330 nm spectral range and yields total degrees of freedom for ozone ranging from 1.5 to 3. Although the vertical sensitivity is limited, GEMS achieves an effective vertical resolution of 5–10 km and is capable of separating tropospheric and stratospheric ozone layers. This work highlights significant algorithmic and calibration improvements in version 3.0. Radiometric offsets in irradiance measurements are corrected using a scaling factor derived from the average ratio to a solar reference, while residual wavelength-dependent biases in the normalized radiance are further mitigated through soft calibration. In addition, shift corrections are applied separately to irradiance and radiance wavelengths. As a result, version 3.0 significantly reduces spectral fitting residuals, lowering them from 0.8 % in version 2.0 to 0.2 % under nominal conditions. This improvement also mitigates altitude-dependent oscillating biases observed in the previous version (+40 DU in the troposphere, −20 DU in the stratosphere). The version 3 ozone profiles show agreements within ±10 DU of ozonesonde observations, with a mean bias of −7.7 % in tropospheric ozone columns and within 5 % in the stratosphere. Furthermore, the retrievals capture day-to-day vertical ozone variability, as demonstrated by comparisons with daily ozonesonde launches in February and March 2024. Integrated ozone columns derived from the profiles also show improved consistency with ground-based total ozone measurements, yielding a mean bias of −3.6 DU and outperforming the GEMS operational total column ozone product.
Comet Interceptor is an ESA science mission with payload contributions from ESA Member States and with an international participation by JAXA. It is the first mission that is being designed, built, and potentially launched before its target is known. This approach will enable the spacecraft to perform the first mission to a Long Period Comet from the Oort Cloud, as these comets have fleeting visits to the inner Solar System lasting only months to years from first discovery, too short for the usual process of mission development to be followed. In this paper we describe a number of factors that need to be considered in selecting a target for the mission, including scientific, orbital, spacecraft and instrument constraints, and discussion of different prioritisation strategies. We find that, in the case where we have a choice of targets, our decisions will mostly be driven by orbital information, which we will have relatively early on, with information on the activity level of the comet an important but secondary consideration. As cometary activity levels are notoriously hard to predict based on early observations alone, this prioritisation / decision approach based more on orbits gives us confidence that a good comet that is compatible with the spacecraft constraints will be selectable with sufficient warning time to allow the mission to intercept it.
