Modèle statistique de chauffage de la couronne solaire calme

ORLEANS-BU Sciences (452342104) / SudocSudocFranceF

Wave characteristics analysis in the vicinity of shocks in space plasmas (cLUSTER satellite observations, numerical simulation and interpretation)

Cette thèse est consacrée à l étude des processus d ondes au voisinage des chocs dans les plasmas spatiaux. La propagation des ondes de Langmuir dans un plasma présentant des inhomogéne ités aléatoires de densité a été modélisée numériquement; les résultats obtenus ont été comparés aux données des instruments WHISPER et WBD à bord des satellites CLUSTER. Les résultats de modélisation et l étude statistique portant sur l intensité des ondes de Langmuir observées dans le préchoc terrestre et le vent solaire ont montré que le théorème central limite n est pas applicable aux statistiques sur l intensité, du fait du nombre insuffisant d inhomogénéités. Il en résulte que la fonction de distribution de probabilité pour le logarithme des énergies d ondes n atteint pas la distribution normale. D autre part la détection à distance de la zone quasi-perpendiculaire du front de choc terrestre a pu être effectuée en analysant la modulation des ondes de Langmuir et celle des ondes électrostatiques avec fréquence décalée à proximité de la limite du pré-choc. Il a été montré que la probabilité d observation de la non-stationnarité du front de choc augmente avec le nombre de Mach du choc. Enfin le rayonnement de transition des électrons relativistes au front de choc quasi-perpendiculaire a été calculé pour expliquer le mécanisme de l émission électromagnétique observée par les satellites près du front de choc interplanétaire le 22 janvier 2004. Les paramètres du calcul correspondent aux véritables paramètres de l évènement. Le spectre du rayonnement de transition établi théoriquement a son maximum dans le même domaine de fréquence que pour les mesures.The doctoral thesis is devoted to the investigation of wave processes in the vicinity of space plasma shocks. The numerical modelling of the Langmuir wave propagation in a plasma with random density inhomogeneities was performed and its results were compared with experimental data obtained by WHISPER and WBD instruments of the CLUSTER spacecraft project. The analysis results showed that the Central Limit Theorem is not applicable to the Langmuir wave intensity statistics in the Earth s foreshock and the solar wind, because of insufficient number of inhomogeneities affecting the amplitude of the waves. Consequently the normal distribution of the probability distribution function for logarithm of wave energies is not achieved. The remote sensing of quasi-perpendicular part of the Earth s bow shock front was performed using the analysis of the modulation of Langmuir, upshifted and downshifted wave intensities close to the foreshock boundary. The probability of the shock front nonstationarity observation was found to grow with shock Mach number. The transition radiation of the relativistic electrons at the interplanetary quasi-perpendicular shock front was calculated to explain the mechanism of electromagnetic emission observed by satellites near the shock front on January, 22, 2004. The parameters of the calculation corresponded to the actual parameters of the event. The theoretically predicted spectrum of transition radiation has its maximum in the same frequency region as in the measurements.ORLEANS-SCD-Bib. electronique (452349901) / SudocSudocFranceF

Injection and acceleration of ions at collisionless shocks: Kinetic simulations

International audienceKinetic simulations of collisionless shocks have provided a wealth of information on injection and acceleration of thermal ions into a diffusive acceleration process. At quasi-parallel shocks upstream diffuse ions and the induced upstream turbulence are an integral part of the collisionless shock structure. Before injected into a diffusive acceleration process thermal ions are trapped near the shock and are accelerated to higher energies. The injection and acceleration process for thermal ions at quasi-perpendicular shocks depends on the possibility of these ions to recross the shock many times. A viable mechanism for injection is cross-field diffusion of the specularly reflected ions after they have crossed the shock into the downstream region. Determination of the cross-field diffusion coefficient in strong turbulence suggests that specularly reflected ions can recross the quasi-perpendicular shock and can get further accelerated. At more oblique shocks the same injection process as at quasi-parallel shocks can work: particles gain high enough velocities during their first shock encounter so that they can escape the shock along the magnetic field in the upstream direction. Because of the form of the pickup ion distribution in velocity space there seems to be no problem for accelerating these ions at either quasi-parallel, quasi-perpendicular, or perpendicular shocks

On the use of tricoherent analysis to detect non-linear wave-wave interactions

International audienceZur Beschreibung nichtlinearer Effekte in dispersiven Medien kann man den Ansatz der Drei- und Vier-Wellen-Wechselwirkungen verwenden. Man benötigt eine Analyse der Spektren höherer Ordnung, um sich gegenseitig beeinflussende Wellen in Zufallssignalen zu identifizieren. Das Bikohärenzspektrum, d.h. das normierte Kumulantenspektrum dritter Ordnung, ist in zahlreichen Untersuchungen zur Analyse von Drei-Wellen-Wechselwirkungen in numerischen, labormäβigen und natürlichen Experimenten verwendet worden. Hier entwickeln wir das Trikohärenzspektrum zur Detektion von Vier-Wellen-Wechselwirkungen. Wir berechnen statistische Eigenschaften des Trispektrums und von Trikohärenzschätzungen sowie das Vertrauensintervall der letzteren. Wir legen Ergebnisse numerischer Schätzungen der Trikohärenz zu synthetischen Signalen vor.For the description of non-linear effects in dispersive media, the approach of three- and four-wave interactions may be used. Higher-order spectral analysis is required for identifying interacting waves in random signals. The bicoherence spectrum, which is the normalized cumulant spectrum of the third order, has been applied to numerous studies for analysing three-wave interactions in numerical, laboratory and natural experiments. Here, we develop the Incoherence spectrum to detect four-wave interactions, calculate statistical properties of the trispectrum and tricoherence estimators as well as the confidence interval for the latter, and present results of numerical estimations of the tricoherence using synthetic signals.Afin de décrire les effets non-linéaires dans un milieu dissipatif, il est possible d'utiliser l'approche par interaction de trois ou quatre ondes. L'analyse spectrale d'ordre supérieur est nécessaire pour identifier les ondes en interaction dans les signaux stochastiques. Le spectre de bicohérence, qui est le spectre cumulant normalisé du troisième ordre, a été utilisé dans de nombreuses études pour analyser l'interaction de trois ondes dans des expériences numériques, de laboratoire ou naturelles. Ici, nous développons le spectre de tricohérence pour détecter les interactions de quatre ondes, calculons les propriétés statistiques du trispectre et des estimateurs de tricohérence, de même que l'intervalle de confiance pour ce dernier, et présentons le résultat d'estimations numériques de la tricohérence utilisant des signaux synthétiques

Measurements of the magnetic field fluctuations in the Solar Orbiter project

International audienceWe estimated the characteristic amplitudes and frequencies of different kinds of waves in the Solar wind on the characteristic distance of the order of 0.2 AU that corresponds to the orbit of the Solar Orbiter. These estimates are based on the measurements of Helios I and 2 satellites. On this basis we have evaluated the experimental requirements for the magnetic field sensors onboard of the Solar Orbiter. We present the characteristics of the search coil magnetometers that are developed in the LPCE. In addition to traditional search coils we present the new magnetic loop antenna characteristics that can be used for the high frequency wide band magnetic field fluctuations measurements in the range 10 kHz-20 MHz

The Radio & Plasmas Waves instrument on the Solar Orbiter mission : science objectives and capabilities

International audienceWe will review the science objectives of the Radio & Plasmas Waves (RPW) instrument on the Solar Orbiter mission. Among those the study of the connectivity between the solar corona and the inner Heliosphere as close as from 0.3 AU and the kinetic behavior of the Solar Wind are of prime importance. We present then the RPW technical capabilities which will allow in-situ and remote sensing measurements of both electrostatic and electromagnetic fields and waves in a broad frequency range, typically from a fraction of Hertz to a few tens of MHz

Morphology of Langmuir wave active sites in the Earth's foreshock observed from the four points CLUSTER constellation : analysis of a case event at small separation.

International audienceHigh frequency fluctuating electric field upstream from the Earth bowshock, at frequencies near the local electron plasma frequency Fpe, but varying within a considerable range (~ 0.1 to 1.1 Fpe), have been observed in situ by several plasma wave instruments. Referred to as `Langmuir waves' when found near the plasma frequency, as `upshifted'or `downshifted' oscillations when found above or below Fpe, they are associated with the flux of electrons which move upstream from the bowshock. The Cluster constellation offers a unique opportunity to perform a spatio-temporal analysis of the active sites of such electrostatic emissions when at small (~ 100 km) average separation. We present a case event taken in this context, over a few minutes time interval. The presence of so called 30 s (period) ULF waves in DC magnetic field and plasma density indicates a position in the ion foreshock. The HF electrostatic fluctuations are measured in the 2 - 80 kHz band with a good time resolution: the instrument performs signal acquisitions in snapshots of 6.3 ms duration at a recurrence of 13.3 ms, transmitting to ground a survey of large band signal variations at resolution 13.3 ms, and, at 107 ms recurrence, the accumulation of frequency spectra (at 160 Hz resolution) derived on board for all the snapshots. The HF fluctuations, as observed from the accumulated spectra, can be sorted in several classes: (a) thermal noise above the plasma frequency, the position of which is precisely measured by the sounder at regular intervals,(b)Langmuir waves in the close vicinity of Fpe, sometimes associated with emissions at 2 Fpe, (c) downshifted waves, (c)broadband noise at the low frequency end of the Whisper range.In many instances, more than one class (excluding thermal noise, always present) are associated in the same accumulated spectrum. Although the emissions are quite sporadic in term of amplitude variations, the global characteristics, in term of frequency signatures, can persist over a few seconds, the changes being seen on all spacecraft at nearby time boundaries. We can thus define regions, or active sites, of a given class (simple or composite) of frequency signatures. We shall present and discuss an analysis of a) the size and velocity of given regions (from simple boundary time analysis), b) occurrence of a given site as compared to global plasma parameters, such as DC magnetic field configuration, solar wind velocity, or global features in the ion and electron populations, c) directivity properties (in the antenna spin plane) of the spectral signatures, compared to DC magnetic field and solar wind velocity orientation