Mutual Impedance Probe in Collisionless Unmagnetized Plasmas With Suprathermal Electrons—Application to BepiColombo

Context: Mutual impedance experiments are active electric probes providing in-situ space plasma measurements. Such active experiments consist of a set of electric antennas used as transmitter(s) and receivers(s) through which various dielectric properties of the plasma can be probed, giving therefore access to key plasma parameters such as, for instance, the electron density or the electron temperature. Since the beginning of the space exploration, such active probes have been launched and operated in Earth's ionospheric and magnetospheric plasmas. More recently and in the coming years, mutual impedance probes have been and will be operated onboard exploratory planetary missions, such as Rosetta, BepiColombo and JUICE, to probe the cometary plasma of 67P/Churyumov-Gerasimenko, the Hermean and the Jovian magnetospheres, respectively.Aims: Some analytic modeling is necessary to calibrate and analyse mutual impedance observations in order to access to macroscopic bulk plasma quantities. In situ particle observations from various space missions have confirmed that space plasmas are out of local thermodynamic equilibrium. This means that particle velocity distributions can be far from a Maxwellian distribution, exhibiting for instance temperature anisotropies, beams or a suprathermal population. The goal of this paper is to characterize the effect of suprathermal electrons on the instrumental response in order to assess the robustness of plasma diagnostics based on mutual impedance measurements in plasmas characterized by a significant amount of suprathermal particles.Methods: The instrumental response directly depends on the electron velocity distribution function (evdf). In this work, we choose to model suprathermal electrons by considering different approaches using: (i) a kappa evdf, (ii) a double-Maxwellian evdf or (iii) a mix of a Maxwellian evdf and a kappa evdf. For each case, we compute the spatial distribution of the electrostatic potential induced by the transmitters, discretized and modeled here as an ensemble of pulsating point charges.Results: We apply our modeling by building synthetic mutual impedance spectra of the PWI/AM2P probe, lauched in October 2018 onboard the Mercury Magnetospheric Orbiter (MIO/MMO) spacecraft of the BepiColombo exploratory space mission, in order to calibrate and analyse the future electron observations in the plasma environment of Mercury

Bedload transport and obstacles to sediment continuity in Morvan Rivers

En France, les récentes évolutions réglementaires (DCE, 2000 ; LEMA, 2006) obligent à assurer une certaine continuité sédimentaire au droit des barrages. L’exigence se pose singulièrement dans le massif du Morvan, où une dizaine de barrages se sont installés depuis le milieu du XIXe siècle. Les questionnements y sont complexes : plusieurs des cours d’eau sur lesquels les ouvrages ont été édifiés ont été assez peu étudiés (lit plat de moyenne énergie à substrat grossier mixte). En outre, les systèmes fluviaux étudiés – l’Yonne supérieure, la Cure et le Chalaux – sont marqués par une anthropisation ancienne, caractérisée notamment par des usages multiples antérieurs aux barrages (moulins, flottage du bois). L’énergie de ces rivières, en partie régie par les barrages, est un facteur important mais insuffisant pour comprendre le charriage actuel. Celui-ci s’avère particulièrement influencé par certains paramètres morphologiques (substrat très grossier, structures sédimentaires, pente). Or les conditions morphologiques actuelles paraissent largement résulter de plusieurs décennies de modification des apports liquides et solides par les barrages. Leurs effets sur les processus fluviaux doivent donc être appréhendés à plusieurs échelles temporelles. Les travaux indiquent qu’ils dépendent également des influences héritées des anciens usages hydrauliques. La trajectoire hydromorphologique passée de la rivière semble aussi jouer sur les impacts morpho-sédimentaires qui ont pu être observés à la suite d’un démantèlement de barrage. Plutôt qu’une table rase ou un retour en arrière, les barrages ou leur démantèlement représentent ainsi davantage une nouvelle évolution de la trajectoire fluviale, faite de ruptures, de continuités et de mutations.In France, recent regulatory developments (WFD, 2000; LEMA, 2006) require sediment continuity across the dams to be ensured. This requirement is particularly acute in the Morvan massif, where a dozen dams have been installed since the middle of the 19th century. The questions are complex: several of the rivers on which the structures have been built have only been poorly studied (medium energy plane bed with mixed coarse substrate). In addition, the fluvial systems - the Upper Yonne, the Cure and the Chalaux - are marked by a long anthropogenic influence, characterized in particular by multiple uses prior to the dams (mills, log driving). Those rivers’ energy, partly governed by the dams, is an important factor but insufficient to understand current bedload transport. The latter is indeed significantly influenced by certain morphological parameters (very coarse substrate, sedimentary structures, slope). However, current morphological conditions appear to be largely the result of several decades of disturbance in liquid and solid inputs by dams. Their effects on river processes must therefore be understood on several time scales. The study indicates that they also depend on the influences inherited from previous hydraulic uses. The past hydromorphic trajectory of the river also seems to influence the morpho-sedimentary impacts that were observed following a dam removal. Rather than a clean slate or a step backwards, dams or their removal thus represent a new evolution of the river trajectory, made up of ruptures, continuities and mutations

An adaptive mapping framework for the management of peat soils: A new Irish peat soils map

Accurate mapping for effective management of peat soils is necessary to help reduce GHG emissions and improve environmental quality. However, mapping remains a major challenge as definitions of peat soils vary substantially between jurisdictions and organizations, while field data are sparse and difficult to produce, and remote sensing of limited use for converted peatlands. Using an Adaptive Mapping Framework, this paper compiles disparate data sources to update the Derived Irish Peat Map to better facilitate its application for environmental issues. This includes incorporation of areas considered to be underlain by shallow peat soils (≥ 10 cm and ≥ 8.6 % Organic Matter content), and augmentation of the overall test dataset with an additional ∼ 20,000 points.The workflow for map generation employed 20 Decision Tree Output Maps (DTOMs), aggregated into 33 Map Combinations (MCs). These facilitated the addition of new areas and removal of areas where the presence of peat could not be confidently identified. The MC selected for the update had the highest accuracy metrics (≥ 74 %), consisting of DTOMs with a user accuracy ≥ 66 % and assessed over a minimum number of test points ≥ 50. The resulting map reveals peat to underlie 1.66 M ha of Ireland (∼ 23.3 % of the country), identified with values of 83 % for overall accuracy and F1 score for peat areas. This extent is 13.2 % larger than that delineated in previous versions and at least 23.2 % larger than areas presented in other previous maps. The methodology also allows transparency from which data sources can be distinguished to define different key peat thickness ranges (≥ 10 cm, ≥ 30–40 cm), better facilitating assessment of environmental impacts arising from land use change.This approach has potential relevance for peat mapping globally, notably in areas containing disparate datasets (e.g., land cover, soil map, etc.), or employing different production methods. The accuracy metrics generated also suggest that the approach can be used as a basis for implementing or updating national and international regulations concerning carbon-rich soils in comparable settings to those encountered in Ireland

Hydro-morphometric parameters controlling travel distance of pebbles and cobbles in three gravel bed streams

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Multi time scale influence of dams on bedload transport

Dams are ones of the main sources of anthropogenic disturbance to the ecology and geomorphology of rivers. The aim of the present study is to understand the mechanisms underlying their influence on downstream bedload transport in three gravel-bed rivers in the Morvan massif, France. The hydrological disturbance caused by four dams is examined at a short (2.4–2.7 years) and longer (21–28 year) time scale. At the short time scale, bedload displacement was monitored (RFID) at 8 study sites around the dams. Morpho-sedimentary characterization of the bed substrate was performed at the study sites and combined with analyses of the long profile evolution and the current cross profile. The flood regime has been to a varying extent durably reduced by the dams depending on their size and purpose: the mean annual maximum flood was reduced by 9 to 40% and the number of flood events by 27 to 73.5% over the 21–28 years period. Sediment availability and loose structures were found above the dams and below medium-sized dams (15 m) influencing strongly or moderately the flood regime (configuration II). These morpho-sedimentary features significantly affect the current bedload dynamics, creating conditions more or less favorable for the mobility of the present and incoming bedload. The cumulative mean bedload distances of RFID tracers in configuration II are significantly lower (6.8–45 m) than in configuration I (78–315 m). The current flow management of the dams has only a moderate effect on the bedload distances recorded, as shown by the mean virtual bedload velocities, which confirm the different dynamics (0.42–0.91 m/d, and 0.62–6.44 m/d, for II and I respectively). Our results demonstrate how modifications of dams on flow and bedload discharge altered the downstream morphology, but also that this inherited morphology may now be the main controlling factor of bedload transport. These findings invite further discussion about the most appropriate ways to restore rivers downstream of dams when dealing with multi-decadal inherited morphological features

Morpho-sedimentary dynamics associated to dam removal. The Pierre Glissotte dam (central France)

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Automatic detection of the electron density from the WHISPER instrument onboard CLUSTER II

International audienceThe Waves of HIgh frequency and Sounder for Probing Electron density by Relaxation(WHISPER) instrument, is part of the Wave Experiment Consortium (WEC) of the CLUSTER IImission. The instrument consists of a receiver, a transmitter, and a wave spectrumanalyzer. It delivers active (when in sounding mode) and natural electric field spectra. Thecharacteristic signature of waves indicates the nature of the ambient plasma regime and, combinedwith the spacecraft position, reveals the different magnetosphere boundaries and regions. Thethermal electron density can be deduced from the characteristics of natural waves in natural modeand from the resonances triggered in sounding mode, giving access to a key parameter of scientificinterest and major driver for the calibration of particles instrument.Until recently, the electron density derivation required a manual time/frequency domaininitialization of the search algorithms, based upon visual inspection of WHISPER active and naturalspectrograms and other datasets from different instruments onboard CLUSTER.To automate this process, knowledge of the region (plasma regime) is highly desirable. A Multi-Layer Perceptron model has been implemented for this purpose. For each detected region, a GRU,recurrent network model combined with an ad-hoc algorithm is then used to determine the electrondensity from WHISPER active spectra. These models have been trained using the electron densitypreviously derived from various semi-automatic algorithms and manually validated, resulting in anaccuracy up to 98% in some plasma regions. A production pipeline based on these models has beenimplemented to routinely derive electron density, reducing human intervention up to 10 times. Workis currently ongoing to create some models to process natural measurements where the data volumeis much higher and the validation process more complex. These models of electron densityautomated determination will be useful for future other space missions