The atypical generation mechanism of Titan's Schumann resonance

International audienceThe observation of a presumed Schumann resonance (SR) during the landing of the Huygens Probe in Titan's atmosphere on January 2005 was subsequently reported and interpreted by Béghin et al. (2007, 2009, 2010, 2012) as being generated through the interaction of Saturn's magnetosphere with Titan's ionosphere rather than from the lightning activity that prevails on Earth. Beyond this atypical behavior, the existence of a single mode instead of the usual multimodal structure of terrestrial SRs and a comprehensive analysis of the physical generation mechanism remained to be investigated. The purpose of the present work is to draw up the baselines of a global model reconciling the Huygens data and the proposed generation mechanism. Based upon relevant observations obtained after several tenths of Titan's flybys by the Cassini orbiter, the modeling involves macro plasma physics processes as well as a global analysis of the mechanisms at the moon's scale. The clue to the SR's generation mechanism is shown to be the low-frequency modulation of the Pedersen current sheets that are induced in the ionopause region by the corotating Saturn's magnetosphere. The modulation principle involves a wave coupling between the ion-acoustic instabilities driven by the longitudinal current sheets and the electromagnetic quasi-transverse whistler mode. The spectral distribution of the sole second eigenmode seen by the Huygens Probe is found to comply with the ionopause plasma parameters measured by Cassini during the bipolar configuration of Titan-Saturn interaction that is thought to have occurred during the probe descent in the atmosphere

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

Lightning detection in planetary atmospheres

Lightning in planetary atmospheres is now a well-established concept. Here we discuss the available detection techniques for, and observations of, planetary lightning by spacecraft, planetary landers and, increasingly, sophisticated terrestrial radio telescopes. Future space missions carrying lightning-related instrumentation are also summarised, specifically the European ExoMars mission and Japanese Akatsuki mission to Venus, which could both yield lightning observations in 2016.Comment: Accepted for publication in Weather as part of a special issue on Advances in Lightning Detectio

Evaluation of iron status in European adolescents through biochemical iron indicators: the HELENA Study

BACKGROUND/OBJECTIVES: To assess the iron status among European adolescents through selected biochemical parameters in a cross-sectional study performed in 10 European cities. SUBJECTS/METHODS: Iron status was defined utilising biochemical indicators. Iron depletion was defined as low serum ferritin (SF8.5 mg/l) plus iron depletion. Iron deficiency anaemia (IDA) was defined as ID with haemoglobin (Hb) below the WHO cutoff for age and sex: 12.0 g/dl for girls and for boys aged 12.5-14.99 years and 13.0 g/dl for boys aged ≥15 years. Enzyme linked immunosorbent assay was used as analytical method for SF, sTfR and C-reactive protein (CRP). Subjects with indication of inflammation (CRP >5 mg/l) were excluded from the analyses. A total of 940 adolescents aged 12.5-17.49 years (438 boys and 502 girls) were involved. RESULTS: The percentage of iron depletion was 17.6%, significantly higher in girls (21.0%) compared with boys (13.8%). The overall percentage of ID and IDA was 4.7 and 1.3%, respectively, with no significant differences between boys and girls. A correlation was observed between log (SF) and Hb (r = 0.36, P < 0.01), and between log (sTfR) and mean corpuscular haemoglobin (r = -0.30, P < 0.01). Iron body stores were estimated on the basis of log (sTfR/SF). A higher percentage of negative values of body iron was recorded in girls (16.5%) with respect to boys (8.3%), and body iron values tended to increase with age in boys, whereas the values remained stable in girls. CONCLUSIONS: To ensure adequate iron stores, specific attention should be given to girls at European level to ensure that their dietary intake of iron is adequate.status: publishe

Rapport national sur les travaux français exécutés de 1975 à 1978 dans la discipline "ionosphère-magnétosphère" : chapitre III de la Section IV du rapport national présenté à l'A.G. de l'UGGI - Canberra 1979

La finalisé de cette discipline est de comprendre les phénomènes et les lois qui régissent la stabilité et la dynamique de tout l'environnement ionisé qui entoure la Terre depuis les basses couches de l'ionosphère jusqu'à l'onde de choc créée par notre planète dans le vent solaire. Les équipes françaises ont réalisé que ces objectifs ne seraient atteints que dans le cadre d'une étroite coopération nationale d'abord, puis internationale. On remarque d'autre part, surtout ces dernières années, un effort de coordination, de synthèse et d'analyse théorique qui a été notamment suscité par une Action Thématique Programmée, placée sous l'égide du CNRS dans le cadre de l'IMS (International Magnetospheric Study), effort qui devrait s'accroître encore dans les années à venir. On a, dans ce rapport, classé les travaux effectués tout d'abord en moyens et méthodes mis en oeuvre, puis description plutôt morphologique du milieu et étude des processus macroscopiques et phénomènes particuliers, tous travaux concourant de manière indispensable à la partie centrale de nos études : les phénomènes globaux fonda- mentaux (circulation générale des basses couches, circuits électro-moteurs et interaction planète-vent solaire)

Series expansion of electrostatic potential radiated by a point source in isotropic Maxwellian plasma

International audienceA new algebraic approach is proposed to calculate the electrostatic potential distributed around a point source in isotropic Maxwellian plasma. The method derives a power series expansion of the radial distance from the source with frequency-dependent coefficients. Distance and frequency are normalized to the Debye length and to the plasma frequency, respectively, so that the expression keeps its entire generality whatever the experimental conditions might be. The proposed method is based upon the Mittag-Lefler expansion of the inverse of the plasma dispersion function for the infinite series of Landau poles. After mathematical clarification of the validity of this expansion, a significant correction of the previous works leads to a self-consistent interpretation of the true contribution of the higher-order poles at large distance from the source. The power series expansion is compared to the classical so-called ''Landau wave approximation'' which is proved to include in reality the contribution of higher-order poles independently from the plasma temperature. For practical use the power expansion is needed to obtain a precise result at distances from the source shorter than about 15 Debye lengths, while the Landau wave approximation gives correct results at larger distances. This work provides all necessary baselines for precise three-dimensional modeling of mutual impedance devices to be used in space plasma experiments where the Debye length is comparable to the spacecraft size

Self-consistent modeling of induced magnetic field in Titan’s atmosphere accounting for the generation of Schumann resonance

International audienceThis model is worked out in the frame of physical mechanisms proposed in previous studies accounting for the generation and the observation of an atypical Schumann Resonance (SR) during the descent of the Huygens Probe in the Titan’s atmosphere on 14 January 2005. While Titan is staying inside the subsonic co-rotating magnetosphere of Saturn, a secondary magnetic field carrying an Extremely Low Frequency (ELF) modulation is shown to be generated through ion-acoustic instabilities of the Pedersen current sheets induced at the interface region between the impacting magnetospheric plasma and Titan’s ionosphere. The stronger induced magnetic field components are focused within field-aligned arcs-like structures hanging down the current sheets, with minimum amplitude of about 0.3 nT throughout the ramside hemisphere from the ionopause down to the Moon surface, including the icy crust and its interface with a conductive water ocean. The deep penetration of the modulated magnetic field in the atmosphere is thought to be allowed thanks to the force balance between the average temporal variations of thermal and magnetic pressures within the field-aligned arcs. However, there is a first cause of diffusion of the ELF magnetic components, probably due to feeding one, or eventually several SR eigenmodes. A second leakage source is ascribed to a system of eddy-Foucault currents assumed to be induced through the buried water ocean. The amplitude spectrum distribution of the induced ELF magnetic field components inside the SR cavity is found fully consistent with the measurements of the Huygens wave-field strength. Waiting for expected future in-situ exploration of Titan’s lower atmosphere and the surface, the Huygens data are the only experimental means available to date for constraining the proposed model

Instability problem of the electric field antennas on the Polar spacecraft

International audienceThis paper is an application of the surface charge distribution method to the modeling of the electric antennas installed on board the Polar spacecraft in order to identify the plasma conditions leading to the instability of the sphere-preamplifier-stubguard system. First we present an analytic approach, which allows us to understand the physical mechanism and to define the conditions of the instability. We then show the results of the numerical modeling for the more common types of instabilities observed in flight. The latter, which we call type-1 oscillations, are observed solely in the low-L plasmasphere region. The modeling predicts that the oscillations can occur in a weakly magnetized Maxwellian plasma in the upper hybrid range (fp < f < f•) when the Debye length lies between well-defined limits. The frequency modulation of the oscillations in this range versus the spin angle of the antennas with respect to the Earth's magnetic field is well explained by the model. The type-2 oscillations are observed occasionally and occur always in the exterior cusp, at large L values. They are most likely to be associated with high-density clouds of solar wind streaming plasma entering into that region. Our analytic modeling indeed predicts that the instability conditions can be satisfied when the antennas are crossing a 10 eV electron flow, with a density of-100 cm-3 and a bulk velocity of the order of 200 km s-•

Titan's native ocean revealed beneath some 45 km of ice by a Schumann-like resonance

International audienceAfter five years of thorough analysis of data from the Huygens Probe that descended into Titan's atmosphere in January 2005, we report major findings inferred from measurements of low frequency waves and atmospheric conductivity. The data account for the observation of a Schumann-like resonance trapped within Titan's atmospheric cavity. On Earth, this phenomenon is triggered by lightning and was anticipated to be observed on Titan, as it provides a tool to reveal the presence of a ground conductive boundary to sustain the resonance of the cavity. The Huygens observations show that the major electric field component of the signal is horizontal, which is inconsistent with lightning sources. We interpret, however, the observed signal as a second spherical harmonic of Titan's cavity, triggered and sustained by strong electric currents induced in the ionosphere by Saturn's magnetospheric plasma flow. The present study describes the characteristics of such trapped modes that allow us to constrain the parameters of the cavity and to infer the presence of a conductive layer at 45 km (±15 km) below the surface. By comparison with the presence of subsurface conductive ocean in the Galilean icy satellites, we conclude that Titan should have pursued similar processes of internal dynamics. To date, this result represents the only evidence for a buried ocean in Titan

Space Plasma Physics: Phenomena Induced by Charged Particle Beams

International audienceThe effects of electron beam emissions from Spacelab were recorded with onboard diagnostic instruments. The variation of the Spacelab-shuttle potential with respect to the ambient plasma near the scientific air lock was investigated. Data on the waves and instabilities triggered by the electron beams are discussed. Within the electron gyrofrequency and electron plasma frequency range, strong signals were detected by both electric and magnetic antennas during the beam emissions. The frequencies of the emitted waves were compared to the characteristic plasma frequencies to enable mode identification