19 research outputs found

    The Source Regions of Whistlers.

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    We present a new method for identifying the source regions of lightning‐generated whistlers observed at a fixed location. In addition to the spatial distribution of causative lightning discharges, we calculate the ratio of lightning discharges transmitted into ground detectable whistlers as a function of location. Our method relies on the time of the whistlers and the time and source location of spherics from a global lightning database. We apply this method to whistlers recorded at 15 ground‐based stations in the Automatic Whistler Detector and Analyzer Network operating between 2007 and 2018 and to located lightning strokes from the World Wide Lightning Location Network database. We present the obtained maps of causative lightning and transmission rates. Our results show that the source region of whistlers corresponding to each ground station is around the magnetic conjugate point of the respective station. The size of the source region is typically less than 2,000 km in radius with a small fraction of sources extending to up to 3,500 km. The transmission ratio is maximal at the conjugate point and decreases with increasing distance from it. This conforms to the theory that whistlers detected on the ground propagated in a ducted mode through the plasmasphere, and thus, the lightning strokes of their causative spherics must cluster around the footprint of the ducts in the other hemisphere. Our method applied resolves the whistler excitation region mystery that resulted from correlation‐based analysis methods, concerning the source region of whistlers detected in Dunedin, New Zealand

    Etude de la magnétosphère terrestre par l'analyse multipoint des données de la mission CLUSTER. Contributions à la caractérisation des frontières et de la magnétosphère interne

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    CLUSTER is the first space mission dedicated to the three-dimensional study of the terrestrial magnetosphere. Its polar orbit and four spacecraft tetrahedron formation allow it to make in situ measurements in various regions of the magnetosphere, in particular in the plasmasphere. This PhD thesis brings together several studies on plasma structures encountered by the CLUSTER spacecraft along their orbit, during the time period 2001-2004. The physical quantity analysed here is mainly the electronic density, but also the DC magnetic field. In a first part of the thesis, two multipoint analysis tools, the spatial gradient and the time delay, are described, tested - using synthetic datasets then real data - and discussed for typical situations. When conditions are favourable, these analyses reveal the boundary motion and orientation, as well as the dimensions of the structures encountered, impossible to estimate with a single satellite mission. Another part of the thesis deals more globally with the plasmasphere, focusing particularly on the topology and dynamics of small- and large-scale density structures. This study is based on the multipoint analyses previously introduced. It also presents and discusses datasets from other CLUSTER instruments, global images of the plasmasphere from the IMAGE mission, and results from statistical studies and numerical simulations. Finally, the thesis describes a statistical study based on another dataset: the power spectral densities of electromagnetic waves measured at low altitude by the Viking spacecraft in 1986.CLUSTER est la première mission spatiale permettant l'étude à trois dimensions de la magnétosphère terrestre. Son orbite polaire et sa configuration de quatre satellites en tétraèdre lui permettent d'effectuer des mesures in situ dans plusieurs régions de la magnétosphère, notamment dans la plasmasphère. Cette thèse rassemble une série de travaux sur les structures de plasma rencontrées sur l'orbite de CLUSTER durant la période 2001-2004. La quantité physique étudiée est essentiellement la densité électronique, mais aussi le champ magnétique continu. Dans une première partie, deux outils d'analyse multipoints, le gradient spatial et la méthode des délais, sont décrits, testés et discutés sur des jeux de données synthétiques, puis sur des données réelles, pour diverses situations typiques. Dans le cadre de conditions favorables, ces analyses révèlent le mouvement, l'orientation de frontières, ainsi que les dimensions des structures rencontrées, impossibles à estimer par une mission composée d'un seul satellite. Une seconde partie s'intéresse plus globalement à la plasmasphère, notamment à la topologie et à la dynamique de ses structures de densité à petite et grande échelle. Cette étude s'appuie sur les analyses multipoints vues précédemment, mais elle utilise également des données d'autres instruments de CLUSTER, des images globales de la plasmasphère fournies par la mission IMAGE, ainsi que des résultats d'études statistiques et de simulations numériques. Enfin, la thèse décrit une analyse statistique menée sur un autre jeu de données: les densités de puissance spectrales des ondes électromagnétiques mesurées à basse altitude par le satellite Viking durant l'année 1986

    Improving Predictions of the 3D Dynamic Model of the Plasmasphere

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    In this perspective paper, we review and discuss different ways that can be used to improve the predictions of the models of the plasmaspheric region. The density of the background cold plasma and the plasmapause position are very important to determine the formation and propagation of waves and interactions with the other regions of the magnetosphere. Improvement of predictions includes refinement of the forecast of the geomagnetic indices that influence the density and the temperature of the particles in some models. Progress is also necessary for the understanding of the physical processes that affect the position of the plasmapause and its thickness since this boundary is not always very sharp, especially during low geomagnetic activity. These processes include the refilling after geomagnetic storms and substorms, the links with the ionosphere, and the expanding plasmaspheric wind during prolonged quiet periods. Using observations from in situ satellites like Van Allen Probes (EMFISIS and HOPE instruments), empirical relations can be determined to improve the dependence of the density and the temperature as a function of the radial distance, the latitude, and the magnetic local time, inside and outside the plasmasphere. This will be the first step for the improvement of our 3D dynamic SWIFF plasmaspheric model (SPM)

    Etude de la magnétosphère terrestre par l'analyse multipoint des données de la mission CLUSTER (contributions à la caractérisation des frontières et de la magnétosphère interne)

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    CLUSTER est la première mission spatiale permettant l étude à trois dimensions de la magnétosphère terrestre. Son orbite polaire et sa configuration de quatre satellites en tétraèdre lui permettent d effectuer des mesures in situ dans plusieurs régions de la magnétosphère, notamment dans la plasmasphère. Cette thèse rassemble une série de travaux sur les structures de plasma rencontrées sur l orbite de CLUSTER durant la période 2001-2004. La quantité physique étudiée est essentiellement la densité électronique, mais aussi le champ magnétique continu. Dans une première partie, deux outils d analyse multipoints, le gradient spatial et la méthode des délais, sont décrits, testés et discutés sur des jeux de données synthétiques, puis sur des données réelles, pour diverses situations typiques. Dans le cadre de conditions favorables, ces analyses révèlent le mouvement, l orientation de frontières, ainsi que les dimensions des structures rencontrées, impossibles à estimer par une mission composée d un seul satellite. Une seconde partie s intéresse plus globalement à la plasmasphère, notamment à la topologie et à la dynamique de ses structures de densité, à petite et grande échelle. Cette étude s appuie sur les analyses multipoints vues précédemment, mais elle utilise également des données d autres instruments de CLUSTER, des images globales de la plasmasphère fournies par la mission IMAGE, ainsi que des résultats d études statistiques et de simulations numériques. Enfin, la thèse décrit une analyse statistique menée sur un autre jeu de données: les densités de puissance spectrales des ondes électromagnétiques mesurées à basse altitude par le satellite Viking durant l année 1986.ORLEANS-BU Sciences (452342104) / SudocSudocFranceF

    Statistical analysis of plasmaspheric plumes with Cluster/WHISPER observations

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    International audiencePlasmaspheric plumes have been routinely observed by the four Cluster spacecraft. This paper presents a statistical analysis of plumes observed during five years (from 1 February 2001 to 1 February 2006) based on four-point measurements of the plasmasphere (outside 4 Earth radii) as it is sampled by the spacecraft in a narrow local time sector before and after perigee. Plasmaspheric plumes can be identified from electron density profiles derived from the electron plasma frequency determined by the WHISPER wave sounder onboard Cluster. As the WHISPER instrument has a limited frequency range (2 - 80 kHz) only plumes with densities below 80 cm(-3) can be identified in this way. Their occurrence is studied as a function of several geomagnetic indices (K-p, am and D-st). Their transverse equatorial size, magnetic local time distribution, L position and density variation are discussed. Plasmaspheric plumes are observed mostly for moderate K-p and never for small D-st. They are found mainly in the afternoon and pre-midnight MLT sectors. Comparisons are also made between the density profiles of the plumes as they are crossed on the in- and outbound legs of the orbit, before and after perigee crossing, respectively

    Links between the plasmapause and the radiation belts boundaries as observed by Cluster instruments

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    International audienceThe Cluster mission offers the exceptional opportunity to study the relations between the position of the plasmapause and the position of the radiation belt boundaries with identical sensors on multiple spacecraft. We compare the positions of the radiation belt edges deduced from CIS observations (electrons with energy larger than 2 MeV) with the positions of the plasmapause derived from WHISPER data (electron plasma frequency). In addition, we compare those results with the boundary positions determined from RAPID observations (electrons with energy between 244.1 and 406.5 keV). The period of 1 April 2007 to 31 March 2009 has been chosen for the analysis because at that time Cluster's perigee was located at lower radial distances than during the earlier part of the mission (as close as 2 Re, deep inside the plasmasphere and the radiation belts). This time period corresponds to a long solar activity minimum. Differences are observed between the radiation belt boundary positions obtained from the two different instruments: The radiation belt positions are related to the energy bands. The plasmapause position is more variable than the radiation belt boundary positions, especially during small geomagnetic activity enhancements. A correspondence is observed between the plasmapause position determined by WHISPER and the outer edge of the outer radiation belt of energetic electrons (> 2 MeV) observed by CIS. There may be an apparent contradiction with previous studies that indicated a correlation between the inner edge of the outer radiation belt and the plasmapause. Radiation belt losses through plasmaspheric waves, however, occur on a longer time scale and are longitudinally averaged because of energetic particle drift motion, so the radiation belt boundaries should not necessarily reflect the instantaneous plasmapause position at one particular local time. Moreover, during higher geomagnetic activity time periods, the plasmapause is located closer to the inner boundary of the outer radiation belt

    Plasmasphere observations with Cluster data supplemented with data from the Dynamics Explorer-1 and Van Allen Probes missions

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    International audienceSince 2000 the four Cluster spacecraft have crossed the Earth's plasmasphere along a polar orbit every 2.5 days, with various perigee altitudes (from 1.5 to 4 RE), different configurations (string of pearls, tetrahedron) and changing separations (from 10 to 100 000 km). The resulting dataset allows different types of inner magnetosphere studies and provides insight in plasmasphere dynamics, including changes in plasmapause position. Plasmaspheric plumes can also be studied on a case-by-case basis, in a statistical manner and in relation with wave activity (EMIC, electromagnetic rising tone, whistler waves).Moreover, data from an old mission, Dynamics Explorer-1, have recently become available. In particular, densities and temperatures for many ions (H+, He+, He++, O+, and O++) have been derived from the RIMS (Retarding Ion Mass Spectrometer) instrument and are available from October 1981 to January 1985. Such composition data, not available from the Cluster satellites, allow in particular to analyze the distributions of those ions in the plasmasphere boundary layer, as a function of magnetic local time and geomagnetic activity.Finally, since 2012, the two Van Allen Probes satellites are orbiting the inner magnetosphere in the magnetic equatorial plane and with a low perigee, allowing a crossing of the plasmasphere every 9 hours. The EMFISIS (Electric and Magnetic Field Instrument Suite and Integrated Science) instrument onboard both spacecraft can determine the electron density in a very large density range (up to 3000 cm-3) using several methods. This gives a different opportunity to analyze the plasmapause and plasmaspheric plumes from a different perspective
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