Search

International audience; In July 2017, the MMS constellation was evolving in the magnetotail with an apogee of 25 Earth radii and an average inter-satellite distance of 10 km (i.e. at electron scales). On 23rd of July around 16:19 UT, MMS was located at the edge of the current sheet which was in a quasi-static state. Then, MMS suddenly entered in the central plasma sheet and detected the local onset of a small substorm as indicated by the AE index (∼400 nT). Fast earthward plasma flows were measured during about 1 hour starting with a period of quasi-steady flow and followed by a saw-tooth like series of plasma jets. This plasma transport sequence ended up by a flow reversal still occurring close to the magnetic equator. Thanks to the unprecedented MMS measurement capability, these different phases are analyzed in terms of wave activity, current signatures, particle acceleration and heating. The origin of these two phases of plasma transport is discussed.

International audience; Magnetic reconnection is a fundamental process occurring in thin current sheets where a change in the magnetic field topology leads to fast magnetic energy conversion into energy of charged particles. A key yet poorly understood aspect is how the reconnection electric field is sustained in the diffusion region by the different terms in the generalized Ohm's law. In particular, the role of the pressure and inertia terms is not yet fully understood as well as the importance of the anomalous resistivity term and its source. Simulations have provided some estimations of the different terms; however direct observations have been scarce so far. The four-spacecraft Magnetospheric Multiscale Mission (NASA/MMS) allows, for the first time, the full evaluation of the generalized Ohm's law in the diffusion region. Here we present MMS observations at a few subsolar diffusion region crossings on October,3 rd 2015 where MMS spacecraft were separated by 25 km. We compare the measured electric field with the electric field due to both kinetic effects (electron pressure tensor, electron inertia terms) and to anomalous resistivity associated to different wave modes. The electric field is balanced by the Hall term at ion scales as expected. At smaller scales, preliminary results indicate that the electric field is mainly balanced by the divergence of the electron pressure tensor, although the contribution of anomalous resistivity is not negligible.

International audience; Reciprocal Vectors of the tetrahedron have been used since the beginning of the CLUSTER mission for estimating gradients of physical fiels, either scalar (density) or vector (magnetic field). An introduction to the method was given in chapter 14 of the ISSI book SR001 published in 1998 ''Analysis Methods for Multi-Spacecraft Data'', and an updated review of the method has been presented in the second ISSI book SR008 ''Multi-Spacecraft Analysis Methods Revisited'' in 2008. This method encompasses the curlometer tool based on Ampère's theorem, moreover it allows a detailed analysis of errors affecting the estimated gradients and it handles symmetrically the four spacecraft. The quality of the estimated gradient depends upon the proximity of the real tetrahedron to the regular tetrahedron : geometrical errors due to uncertainties in spacecraft positions grow rapidly when the tetrahedron is flat or elongated. A new approach has been designed to remedy this caveat to some extent. By contrast to the original method Generalized Reciprocal Vectors (GRV's)result from the search of an optimal weighting of the data provided by the four spacecraft. We will present applications to MMS cases during crossings of current sheets.

International audience; We analyze two ion scale dipolarization fronts associated with field-aligned currents detected by the Magnetospheric Multiscale mission during a large substorm on 10 August 2016. The first event corresponds to a fast dawnward flow with an antiparallel current and could be generated by the wake of a previous fast earthward flow. It is associated with intense lower hybrid drift waves detected at the front and propagating dawnward with a perpendicular phase speed close to the electric drift and the ion thermal velocity. The second event corresponds to a flow reversal: from southwward/dawnward to northward/duskward associated with a parallel current consistent with a brief expansion of the plasma sheet before the front crossing and with a smaller lower hybrid drift wave activity. Electromagnetic electron phase-space holes are detected near these low-frequency drift waves during both events. The drift waves could accelerate electrons parallel to the magnetic field and produce the parallel electron drift needed to generate the electron holes. Yet we cannot rule out the possibility that the drift waves are produced by the antiparallel current associated with the fast flows, leaving the source for the electron holes unexplained.

International audience; Multi-spacecraft missions like the ESA mission CLUSTER and the NASA mission MMS are essential to improve our understanding of physical processes in space plasmas. Several methods were designed in the 90's during the preparation phase of the CLUSTER mission to estimate gradients of physical fields from simultaneous multi-points measurements [1, 2]. Both CLUSTER and MMS involve four spacecraft with identical full scientific payloads including various sensors of electromagnetic fields and different type of particle detectors. In the standard methods described in [1, 2], which are presently in use, data from the four spacecraft have identical weights and the estimated gradients are most reliable when the tetrahedron formed by the four spacecraft is regular. There are three types of errors affecting the estimated gradients (see chapter 14 in [1]) : i) truncature errors are due to local non-linearity of spatial variations, ii) physical errors are due to instruments, and iii) geometrical errors are due to uncertainties on the positions of the spacecraft. An assessment of truncature errors for a given observation requires a theoretical model of the measured field. Instrumental errors can easily be taken into account for a given geometry of the cluster but are usually less than the geometrical errors which diverge quite fast when the tetrahedron flattens, a circumstance occurring twice per orbit of the cluster. Hence reliable gradients can be estimated only on part of the orbit. Reciprocal vectors of the tetrahedron were presented in chapter 4 of [1], they have the advantage over other methods to treat the four spacecraft symmetrically and to allow a theoretical analysis of the errors (see chapters 4 of [1] and 4 of [2]). We will present Generalized Reciprocal Vectors for weighted data and an optimization procedure to improve the reliability of the estimated gradients when the tetrahedron is not regular. A brief example using CLUSTER or MMS data will be given. This approach also operates for any number of spacecraft. References [1] Analysis Methods for Multi-Spacecraft Data, ISSI Scientific Report, SR-001, Eds. G. Paschmann and P.W. Daly, ISSI, Bern, Switzerland, 1998. [2] Multi-Spacecraft Analysis Methods Revisited, ISSI Scientific Report, SR-008, Eds. G. Paschmann and P.W. Daly, ISSI, Bern, Switzerland, 2008.

International audience; Magnetic reconnection is a fundamental process occurring in thin current sheets where a change in the magnetic field topology leads to fast magnetic energy conversion into energy of charged particles. A key yet poorly understood aspect is how the reconnection electric field is sustained in the diffusion region by the different terms in the generalized Ohm's law. In particular, the role of the pressure and inertia terms is not yet fully understood as well as the importance of the anomalous resistivity term and its source. Simulations have provided some estimations of the different terms; however direct observations have been scarce so far. The four-spacecraft Magnetospheric Multiscale Mission (NASA/MMS) allows, for the first time, the full evaluation of the generalized Ohm's law in the diffusion region. Here we present MMS observations at a few subsolar diffusion region crossings on October,3rd 2015 where MMS spacecraft were separated by 25 km. We compare the measured electric field with the electric field due to both kinetic effects (electron pressure tensor, electron inertia terms) and to anomalous resistivity associated to different wave modes. The electric field is balanced by the Hall term at ion scales as expected. At smaller scales, preliminary results indicate that the electric field is mainly balanced by the divergence of the electron pressure tensor, although the contribution of anomalous resistivity is not negligible.

International audience; Les Search Coil Magnetometers (SCM) de chacun des 4 satellites MMS sont en fonctionnement depuis le lancement de la mission en mars 2015 acquérant chaque jour autour d’un gigaoctet de données par satellite (en mode survey, burst et high burst). Il est important pour le LPP de se doter d'un outil de visualisation rapide de ces données afin de surveiller le fonctionnement de l’instrument et de détecter d’éventuelles anomalies via notamment l’analyse des calibrations en vol qui sont effectuées au moins une fois par orbite. Cet outil, est actuellement en développement au LPP. Il permettra également aux scientifiques de sélectionner les données en identifiant des évènements intéressants, puis de télécharger directement le fichier de données via la base miroir locale, ainsi que les aperçus associés (position de la constellation, quicklook fields). Le volume imposant des données (> 10To/an) rend en effet pertinentes la construction et la gestion d’une base miroir locale incluant les données SCM mais également les données calibrées (L2) des autres instruments afin de permettre aux scientifiques de diminuer les temps de transfert de ces données très volumineuses et de réaliser des études statistiques sur des grands ensembles de données. C’est cette base qui, par exemple, devrait alimenter les projets de machine-learning en cours de développement au LPP.

International audience; Spatial and high-time-resolution properties of the velocities, magnetic field, and 3-D electric field within plasma turbulence are examined observationally using data from the Magnetospheric Multiscale mission. Observations from a Kelvin-Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a first look at the properties of turbulence in the KHI. For the first time direct observations of both the high-frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra exhibit power law behavior with changes in slope near the ion gyrofrequency and lower hybrid frequency. The work provides the first observational evidence for turbulent intermittency and anisotropy consistent with quasi two-dimensional turbulence in association with the KHI. The behavior of kinetic-scale intermittency is found to have differences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics in the KHI.

International audience; Dipolarization fronts (DFs), embedded in bursty bulk flows, play a crucial role in Earth's plasma sheet dynamics because the energy input from the solar wind is partly dissipated in their vicinity. This dissipation is in the form of strong low-frequency waves that can heat and accelerate energetic electrons up to the high-latitude plasma sheet. However, the dynamics of DF propagation and associated low-frequency waves in the magnetotail are still under debate due to instrumental limitations and spacecraft separation distances. In May 2015 the Magnetospheric Multiscale (MMS) mission was in a string-of-pearls configuration with an average intersatellite distance of 160 km, which allows us to study in detail the microphysics of DFs. Thus, in this letter we employ MMS data to investigate the properties of dipolarization fronts propagating earthward and associated whistler mode wave emissions. We show that the spatial dynamics of DFs are below the ion gyroradius scale in this region (˜500 km), which can modify the dynamics of ions in the vicinity of the DF (e.g., making their motion nonadiabatic). We also show that whistler wave dynamics have a temporal scale of the order of the ion gyroperiod (a few seconds), indicating that the perpendicular temperature anisotropy can vary on such time scales.

International audience; The instrument suite of MMS mission includes one search coil and two fluxgate magnetometers on each observatory and can therefore provide the possibility to study events that cross the sensitive frequency range of both instrument types. It is therefore desirable to have a common merged data product that combines the best parts of both instruments. Extensive ground based test measurements have been performed to identify the properties of the instruments. New models for compensating time and frequency properties were created and the difference to existing calibrations is analyzed. Considerations for application as well as signal processing constraints are discussed and the resulting method is applied on data from the mission. Dipolarization events can serve as suitable example, since they contain a variety of disturbances with different characteristic scales: transient filamentary properties, sharp fronts, reconfiguration of the current sheet, wave signatures on kinetic as well as fluid scales, thus requiring highest sensitivity data in a wide frequency range. We show these different signatures relevant to the dipolarization events based on the analysis of these newly produced data.

International audience; Plasma jets aka bursty bulk flows play a crucial role in Earth's magnetosphere dynamics, in particular during substorms where they can penetrate down to the geosynchronous orbit. The energy input from the solar wind is partly dissipated in jet fronts (also called dipolarization fronts) in the form of strong whistler waves that can heat and accelerate energetic electrons. The ratio of the energy transported during jets to the substorm energy consumption can reach one third or more due to kinetic-scale phenomena, that are still under debate due to instrumental limitations. The recently-launched Magnetospheric Multiscale (MMS) mission has already detected numerous plasma jet events, and evolves in a tetrahedral configuration (with an average inter- satellite distance of 160 km and unprecedent resolutions up to 16,000 samples/s) that allows to study in detail the microphysics of these phenomena. Thus in this study we employ MMS data to investigate the energy dissipated in jet fronts related to the generation of whistler waves, and the interaction of such waves with energetic electrons in the vicinity of the flow/jet braking region near the equatorial boundary between tail and inner magnetosphere. We also make use of ray tracing simulations to evaluate their propagation properties, as well as their impact on particles in the off-equatorial magnetosphere.

International audience; The nature of extra-nuclear ultraluminous X-ray sources (ULX) in nearby galaxies continues to be an enigma, since their adopted isotropic high-energy output would surpass the Eddington limit of even the most massive stellar black holes. Many ultraluminous X-ray sources are surrounded by emission nebulae that show indications of both shock ionization and X-ray ionization. Relatively compact X-ray ionized nebulae can be used to independently infer the luminosities, and thus to exclude possible beaming effects into our line of sight. Larger bubble-like nebulae reach several hundred parsec diameters and provide important information on the formation and/or mass loss history of ULX. We point out the close relationship to microquasars and the previously unique SS 433 system with its radio nebula W 50.

Despite much observational and theorical effort little is presently known about the nature of the luminous non-nuclear X-ray sources which appear to largely surpass the Eddington limit of a few solar masses. The first part of this work shows the multiwavelentgh study of a sample of these objects with ROSAT and XMM-Newton observations and through an optical survey done at OHP, ESO and CFHT. These objects have been studied for decades and for the first time several nebulæ are seen near the X-ray source. One of these nebulæ seem to be X-ray photoionised, meaning that the X-ray emission of this object surpasses the Eddington limit of more than several tenths solar masses ! The second part of this thesis is more dedicated to a technical task which consisted in writing a software used in the XMM-Newton Processing Pipeline., Malgré de nombreux efforts tant sur le plan observationnel que théorique, nous ne connaissons que très peu de choses sur la nature des sources X très lumineuses qui n'appartiennent pas au noyau de la galaxie hôte et qui semblent dépasser (ou dépassent) très largement la limite d'Eddington d'un objet de quelques masses solaires. Ce travail présente dans une première partie l'étude multi-longueurs d'onde d'un échantillon de certains de ces objets à travers les observations X des satellites ROSAT et XMM-Newton et les résultats d'observations optiques menées à l'Observatoire de Haute-Provence, à l'ESO et au CFHT. Ces objets ont été étudiés depuis des décennies sans qu'ils aient livré aucun de leurs secrets, et pour la première fois de nombreuses nébuleuses en émission ont été découvertes à proximité de la source X. Et plus encore, une de ces nébuleuses semble être photoionisée par les rayons X ce qui tend à prouver par la même occasion que l'émission X de l'objet dépasse largement la limite d'Eddington d'un objet de quelques dizaines de masses solaires ! Une deuxième partie de ce travail est consacrée à une tâche plus technique qui fut l'écriture d'un programme intégré à une chaîne de traitement automatique des données du satellite européen XMM-Newton.

International audience; Un certain nombre de galaxies proches abritent, en dehors de leur noyau, des sources X extrêmement lumineuses qui, apparemment dépassent la limite d'Eddington d'un objet compact de quelques masses solaires. Nous avons réalisé une étude photométrique et spectroscopique d'un échantillon de ces sources X brillantes en utilisant pour cela le télescope de 193 cm de l'OHP, le CFHT et le NTT de l'ESO. Ceci nous a permis de mettre en évidence l'association d'une source X avec une nébuleuse hautement ionisée, il s'agit de la première identification optique d'une source X super-Eddington. cette découverte est un argument en faveur de l'existence de trous noirs de plusieurs dizaines de masses solaires dans les systèmes binaires. Nous avons également découvert des nébuleuses annulaires de quelques centaines de parsecs apparemment excitées par les sources X très lumineuses.

Cluster Spatio-Temporal Analysis of Field Fluctuations (STAFF) high resolution data that are available at Cluster Active Archive (CAA) comprise two main parts, corresponding to data issued from the two onboard data analysers. The STAFF waveform analyser (STAFF-SC) provides data in the frequency range 0.1–10 Hz or 0.1–180 Hz, depending on the spacecraft telemetry rate. The Spectrum Analyser (STAFF-SA) calculates the complete spectral matrix elements for five wave components, the three magnetic components from the STAFF experiment and the two electric components from the Electric Field and Wave (EFW) experiment, in the frequency range 8–4,000 Hz. From STAFF-SC the CAA data comprise waveform data in telemetry units, complex spectra in physical units, dynamic spectra plots, and possibly in the future waveform data in physical units. The CAA products from STAFF-SA are the spectral matrix data in physical units. In the future, STAFF-SA value added products containing wave polarisation and propagation characteristics will be delivered.