Centre d'Etudes Supérieures de la Renaissance

Magnetohydrodynamic (MHD) turbulence in the solar wind is observed to show the spectral behavior of classical Kolmogorov fluid turbulence over an inertial subrange and departures from this at short wavelengths, where energy should be dissipated. Here we present the first measurements of the electric field fluctuation spectrum over the inertial and dissipative wave number ranges in a Beta > or approximately = 1 plasma. The k(-5/3) inertial subrange is observed and agrees strikingly with the magnetic fluctuation spectrum; the wave phase speed in this regime is shown to be consistent with the Alfvén speed. At smaller wavelengths krho(i) > or = 1 the electric spectrum is enhanced and is consistent with the expected dispersion relation of short-wavelength kinetic Alfvén waves. Kinetic Alfvén waves damp on the solar wind ions and electrons and may act to isotropize them. This effect may explain the fluidlike nature of the solar wind.

Cassini's successful orbit insertion has provided the first examination of Saturn's magnetosphere in 23 years, revealing a dynamic plasma and magnetic environment on short and long time scales. There has been no noticeable change in the internal magnetic field, either in its strength or its near-alignment with the rotation axis. However, the external magnetic field is different compared with past spacecraft observations. The current sheet within the magnetosphere is thinner and more extended, and we observed small diamagnetic cavities and ion cyclotron waves of types that were not reported before.

Abstract. Using four-point magnetic field measurements by the Cluster spacecraft, we statistically analyze the magnetic field and electric current configurations during rapid crossings of the current sheet observed in July-October 2001 at geocentric distances of 19 RE. The database includes 78 crossings, specially selected to apply multi-point data analysis techniques to calculate vector derivatives. Observed bipolar variations of jz, often with | jz |>jy, indicate that the electric currents follow kinks of the current sheet. The current density varies between 5-25nA/m2. The half-thickness of the current sheet during flapping varies over a wide range, from 1 to 20 ion thermal gyroradii (Lcp), calculated from average temperature and lobe magnetic field for each crossing). We found no relationship between the tilt angle of the current sheet normal and the half-thickness. In 68 cases the magnetic field curvature vector has a positive (earthward) X-component. Ten cases with a negative (tailward) curvature, associated with reconnection, were detected within 0<YGSM<7 RE. The minimum curvature radii vary mainly between 1 and 10 Lcp, and the adiabaticity parameter κ≤1 for 73% of the events. The electric current density during flapping is often off-central, i.e. the main current density is shifted from the neutral sheet (| Bx |<5nT) to the Northern or Southern Hemisphere. This is most likely a temporal effect related to the flapping. The analysis shows that the flapping motion of the current sheet is associated with kink-like waves on the sheet surface. The kink fronts, tilted in the Y-Z plane, moved toward dawn in the morning half and toward dusk in the evening half of the magnetotail.

H i mass. We show that it is plausible that the FIR excess is due to cold atomic gas that is optically thick in the 21 cm line, while the contribution by a pure H 2 phase with no CO emission remains a possible explanation.

Abstract. The Whisper instrument yields two data sets: (i) the electron density determined via the relaxation sounder, and (ii) the spectrum of natural plasma emissions in the frequency band 2–80 kHz. Both data sets allow for the three-dimensional exploration of the magnetosphere by the Cluster mission. The total electron density can be derived unambiguously by the sounder in most magnetospheric regions, provided it is in the range of 0.25 to 80 cm-3 . The natural emissions already observed by earlier spacecraft are fairly well measured by the Whisper instrument, thanks to the digital technology which largely overcomes the limited telemetry allocation. The natural emissions are usually related to the plasma frequency, as identified by the sounder, and the combination of an active sounding operation and a passive survey operation provides a time resolution for the total density determination of 2.2 s in normal telemetry mode and 0.3 s in burst mode telemetry, respectively. Recorded on board the four spacecraft, the Whisper density data set forms a reference for other techniques measuring the electron population. We give examples of Whisper density data used to derive the vector gradient, and estimate the drift velocity of density structures. Wave observations are also of crucial interest for studying small-scale structures, as demonstrated in an example in the fore-shock region. Early results from the Whisper instrument are very encouraging, and demonstrate that the four-point Cluster measurements indeed bring a unique and completely novel view of the regions explored.Key words. Space plasma physics (instruments and techniques; discontinuities, general or miscellaneous)

We report in situ observations from the Cluster and FAST spacecraft showing the deposition of energy into the auroral ionosphere from broadband ULF waves in the cusp and low‐latitude boundary layer. A comparison of the wave Poynting flux with particle energy and flux at both satellites indicates that energy transfer from the broadband waves to the plasma occurs through field‐aligned electron acceleration, transverse ion acceleration, and Joule heating. These processes are shown to result in precipitating electron fluxes sufficient to drive bright aurora and cause outflows of energized electrons and O + ions from the ionosphere into the low‐latitude boundary layer. By solving an eigenmode equation for Alfvén waves in the observed plasma environment, it is shown that the broadband waves observed at Cluster and FAST are dispersive Alfvén waves. It is demonstrated that these waves have wavelengths perpendicular to the geomagnetic field extending from significant fractions of an L shell down to ion gyroradii and electron inertial lengths and wave frequencies in the plasma frame from 1 mHz up to 50 mHz. These waves are shown to have wavelengths along the geomagnetic field of the order of the field line length between the ionosphere and the equatorial plane and become field line resonances (FLRs) when on closed field lines. It is shown that the inclusion of nonlinear and/or nonlocal kinetic effects is required in the description of these waves to account for accelerated particles observed. On the basis of the wave polarization and spectral properties observed from Cluster and FAST it is speculated that these waves are generated through the mode conversion of surface Alfvén waves driven by tailward flows in the low‐latitude boundary layer.

We identify drift-kinetic Alfvén waves in the vicinity of a reconnection X line on the Earth's magnetopause. The dispersive properties of these waves have been determined using wavelet interferometric techniques applied to multipoint observations from the Cluster spacecraft. Comparison of the observed wave dispersion with that expected for drift-kinetic Alfvén waves shows close agreement. The waves propagate outwards from the X line suggesting that reconnection is a kinetic Alfvén wave source. Energetic O+ ions observed in these waves indicate that reconnection is a driver of auroral ion outflow.

Les figures les plus complexes de l'histoire de la musique sont souvent celles que l'on éprouve bien de la peine à situer dans une catégorie esthético-historique. Johannes Ciconia est incontestablement au nombre de celles-ci. Il est fin des uns et commencement des autres, souvent enrobé d'un mystère que confortent les incertitudes quant à sa carrière, comme si le personnage, l'homme, avait cherché à disparaître derrière des œuvres dont on ne peut parfois pas lui attribuer la paternité. En un peu plus d'une vingtaine d'années, un portrait plus précis de Ciconia a émergé. De nouveaux éléments biographiques sont apparus : son œuvre, musicale et théorique, a fait l'objet d'une édition critique ; des analyses révèlent l'ampleur de son invention ; des interprétations, la beauté de son inspiration.<br />Ce volume se veut une relecture critique de tous ces apports, mais aussi un élargissement et un approfondissement des problématiques.<br />(Contributions de Jane Alden, Margaret Bent, Galliano Ciliberti, David Fallows, Jan Herlinger, Annette Kreutziger-Herr, Pedro Memelsdorff, Stefano Mengozzi, Yolanda Plumley, Anne Stone, et Philippe Vendrix)

We demonstrate from observations that kinetic Alfvén waves may play an important role in facilitating magnetic reconnection. These waves radiate outwards from the diffusion region oblique to the magnetic field in a conelike pattern delimited by the X line separatrices with outward energy fluxes equivalent to that contained in the outstreaming ions. It is shown that the wave vectors reverse across the X and symmetry lines and have a large out of plane component. We estimate that these waves drive significant transport through the diffusion region.

Observations at the Earth's magnetopause identify mode conversion from surface to kinetic Alfvén waves at the Alfvén resonance. Kinetic Alfvén waves radiate into the magnetosphere from the resonance with parallel scales up to the order of the geomagnetic field-line length and spectral energy densities obeying a k(perpendicular)(-2.4) power law. Amplitudes at the Alfvén resonance are sufficient to both demagnetize ions across the magnetopause and provide field-aligned electron bursts. These waves provide diffusive transport across the magnetopause sufficient for boundary layer formation.

Over the course of 290 orbits, the Electron Reflectometer onboard Mars Global Surveyor consistently observed a plasma boundary at a median altitude of 380 km, where electron fluxes at energies greater than ∼100 eV change abruptly by about an order of magnitude. Above the boundary, electron energy spectra are consistent with solar wind electrons that have been shocked and then cooled by impact with exospheric neutrals. Below the boundary, electron energy spectra exhibit a broad feature from 20 to 50 eV, which likely results from a blend of unresolved photoionization peaks that have been predicted by published models of ionospheric photoelectrons at Mars. We attribute a second feature at ∼500 eV to oxygen Auger electrons. The 500‐eV flux level measured below the boundary responds to variations in the solar soft x‐ray flux and is consistent with a balance between photoionization and loss by impact with atmospheric neutral atoms.

Abstract. Two distinct populations of reflected and accelerated ions are known to originate from quasi-perpendicular shocks, gyrating ions and reflected ion beams. Recent observations under such bow shock conditions with Cluster have shown strong evidence that both particle distributions appear to emerge from the same reflection process. In this paper the basic production mechanism of field-aligned beams has been investigated by using CLUSTER multi-spacecraft measurements. We have analyzed several quasi-perpendicular shocks with the Cluster Ion Spectrometry experiment (CIS) and followed the spatial and temporal evolution of the reflected and transmitted ion populations across the shock. These observations show that the field-aligned beams most likely result from effective scattering in pitch angle during reflection in the shock ramp. Investigating a low Mach number shock, leakage of a fraction of the thermalized ion distribution in the downstream region does not appear to be the source as the volume in phase space occupied by beam ions is empty downstream of the shock ramp.

Trottmann Christian. La vision béatifique. Des disputes scolastiques à sa définition par Benoît XII. Rome : Ecole française de Rome, 1995. 916 p. (Bibliothèque des Écoles françaises d'Athènes et de Rome, 289)

In terms of statistical fluctuations, stellar population synthesis models are only asymptotically correct in the limit of a large number of stars, where sampling errors become asymptotically small. When dealing with stellar clusters, starbursts, dwarf galaxies or stellar populations within pixels, sampling errors introduce a large dispersion in the predicted integrated properties of these populations. We present here an approximate but generic statistical formalism which allows a very good estimation of the uncertainties and confidence levels in any integrated property, bypassing extensive Monte Carlo simulations, and including the effects of partial correlations between different observables. Tests of the formalism are presented and compared with proper estimates. We derive the minimum mass of stellar populations which is required to reach a given confidence limit for a given integrated property. As an example of this general formalism, which can be included in any synthesis code, we apply it to the case of young (t &lt; 20 Myr) starburst populations. We show that, in general, the UV continuum is more reliable than other continuum bands for the comparison of models with observed data. We also show that clusters where more than 10^5 Mo have been transformed into stars have a relative dispersion of about 10% in Q(He+) for ages smaller than 3 Myr. During the WR phase the dispersion increases to about 25% for such massive clusters. We further find that the most reliable observable for the determination of the WR population is the ratio of the luminosity of the WR bump over the Hbeta luminosity. A fraction of the observed scatter in the integrated properties of clusters and starbursts can be accounted for by sampling fluctuations.

Le present volume de la revue des Annales de Bretagne et des Pays de l'Ouest constitue la publication des Actes du colloque tenu a Tours 4 au 6 mars 2004 pour commemorer le douzieme centenaire de la mort, le 19 mai 804, d' Alcuin, grande figure de l'histoire carolingienne. L'ouvrage est presente par une rapide introduction que l'on suppose etre redigee par les deux organisateurs de la manifestation, Philippe Depreux et Bruno Judic. Cinq domaines extremement riches, denses et largement novateurs ont ete couverts par les contributions au colloque, publiees dans ce volume. D'une part l'environnement d'Alcuin, ses voyages et son installation a Tours, d'autre part, l'etude du gouvernement de l'abbaye Saint- Martin. Le troisieme domaine traite est celui des relations d'Alcuin avec « l'ecriture » au sens large. Enfin, deux « gros » domaines sont traites dans les parties IV et V du volume. La partie IV propose des contributions sur Alcuin, l'exegese et le pouvoir a l'epoque carolingienne. Le cinquieme et dernier domaine aborde — les reseaux d'Alcuin et la formation d'une culture europeenne — presente de nombreuses et riches contributions.

Au cours des âges, la politique matrimoniale change dans les maisons comtales catalanes : mariage des consanguins au sein du cousinage des années 870-930 ; mariage à une femme étrangère ou bien à un vicomte ou à un châtelain entre 930 et 1080 ; mariage conquérant, rassembleur de royaumes et de comtés au profit de la dynastie de Barcelone, vers 1080-1210. Au service d'une politique, les noces du comte ne coïncident pas toujours avec l'éthique de l'Église, qui préconise le libre choix du conjoint, qui empêche la répudiation arbitraire et qui interdit l'union entre proches parents. Le mariage tient une place centrale dans l'imaginaire aristocratique.

In February 2007, the STEREO‐B spacecraft encountered the magnetosheath, plasma sheet and plasma sheet boundary layer from about 200 R E to 300 R E downtail. This time period was during solar minimum, and there was no storm activity during this month. Using data from the PLASTIC instrument, we find that even during quiet times, O + is a constant feature of the deep magnetotail, with an O + density of about 15% of the O + density in the near‐earth plasma sheet for similar conditions. The tailward flux of the O + is similar to the flux of O + beams that have been observed in the lobe/mantle region of the deep tail. The total outflow rate of the O + down the plasma sheet is 1.1 × 10 24 ions/s, which is 10% of the total outflow rate of 1 × 10 25 ions/s, and of the same order as the estimated loss from dayside transport.

Abstract. This paper presents the first observations with Cluster of a very dense population of thermal ionospheric ions (H+, He+, O+) locally "accelerated" perpendicularly to the local magnetic field in a region adjacent to the magnetopause and on its magnetospheric side. The observation periods follow a long period of very weak magnetic activity. Recurrent motions of the magnetopause are, in the presented cases, unexpectedly associated with the appearance inside closed field lines of recurrent energy structures of ionospheric ions with energies in the 5 eV to ~1000 eV range. The heaviest ions were detected with the highest energies. Here, the ion behaviour is interpreted as resulting from local electric field enhancements/decreases which adiabatically enhance/lower the bulk energy of a local dense thermal ion population. This drift effect, which is directly linked to magnetopause motions caused by pressure changes, allows for the thermal ions to overcome the satellite potential and be detected by the suprathermal CIS Cluster experiment. When fast flowing, i.e. when detectable, the density (~ 1 cm-3) of these ions from a terrestrial origin is (in the cases presented here) largely higher than the local density of ions from magnetospheric/plasma sheet origin which poses again the question of the relative importance of solar and ionospheric sources for the magnetospheric plasma even during very quiet magnetic conditions.Key words. Ionosphere (planetary ionosphere; plasma convection) Magnetospheric physics (magnetopause, cusp and boundary layers)

The interconnection of the interplanetary magnetic field with the geomagnetic field is thought to be the dominant process for mass, energy, and momentum transfer from the magnetosheath into the magnetosphere. Downward precipitating ions from the reconnection site are observed in the cusp by polar orbiting satellites and exhibit sudden changes in their ion‐energy distributions, forming distinctive structures. These structures have been identified as temporal structures, most likely caused by variations of the reconnection rate at the magnetopause, as well as spatial structures caused by spatially separated flux tubes. Comparisons of spatial cusp structures observed by Cluster with simultaneously observed ionospheric convection pattern derived from SuperDARN radar observations showed that spatial cusp structures are linked with separated ionospheric convection cells. It has been suggested that these convection cells and their related spatial cusp structures are driven by multiple reconnection lines at the magnetopause. This study revisits a spatial cusp structure event and shows that the two dispersion events are indeed coming from separated reconnection lines located in different hemispheres.

We study the continuum spectral evolution of 16 gamma-ray bursts detected by the Franco-Soviet SIGNE experiment in 1981-1982 by fitting time resolved (0.5 s) spectra in count space with simple thermal bremsstrahlung and synchrotron models. We find that there is no single characteristic of spectral evolution: we see hard-to-soft, soft-to-hard, luminosity-hardness tracking, and chaotic evolution. We perform correlation studies between instantaneous burst intensity and spectral temperature for seven bursts. While we basically confirm the existence of a correlation between these variables as originally claimed by Golenetskii et al. (1983) we find higher values and a broader range of correlation indices.