Light scattering of large rough particles application to cometary grains

While the electromagnetic field scattered by a spherical particle is classically obtained by the Helmholtz equation, the general case of an arbitrary particle may be investigated in the general framework of the interaction of a wave with a scattering potential. A wave function then satisfies the Schroedinger equation. The general solution of the Schroedinger equation is given. The main disadvantage of this approach are its restriction to large particles and its scalar nature preventing the calculation of the polarization. However, Perrin and Lamy have shown how to avoid the second limitation and retrieve a vectorial description. They proved that in the case of large spheres when the ad hoc assumptions are satisfied, the expression of the scattering amplitude may be approximated by an expansion series in partial waves, i.e., on a discrete basis. The analogy may be generalized, and the ratio of the two components for a rough particle obtained by taking the ratio of the reflectivities for the two directions of polarization. These reflectivities involve the simple and double reflections calculated following the method developed by Wolff for rough surfaces. The theory is further detailed

Periodic behaviour of coronal mass ejections, eruptive events, and solar activity proxies during solar cycles 23 and 24

We report on the parallel analysis of the periodic behaviour of coronal mass ejections (CMEs) based on 21 years [1996 -- 2016] of observations with the SOHO/LASCO--C2 coronagraph, solar flares, prominences, and several proxies of solar activity. We consider values of the rates globally and whenever possible, distinguish solar hemispheres and solar cycles 23 and 24. Periodicities are investigated using both frequency (periodogram) and time-frequency (wavelet) analysis. We find that these different processes, in addition to following the $\approx$11-year Solar Cycle, exhibit diverse statistically significant oscillations with properties common to all solar, coronal, and heliospheric processes: variable periodicity, intermittence, asymmetric development in the northern and southern solar hemispheres, and largest amplitudes during the maximum phase of solar cycles, being more pronounced during solar cycle 23 than the weaker cycle 24. However, our analysis reveals an extremely complex and diverse situation. For instance, there exists very limited commonality for periods of less than one year. The few exceptions are the periods of 3.1--3.2 months found in the global occurrence rates of CMEs and in the sunspot area (SSA) and those of 5.9--6.1 months found in the northern hemisphere. Mid-range periods of $\approx$1 and $\approx$2 years are more wide spread among the studied processes, but exhibit a very distinct behaviour with the first one being present only in the northern hemisphere and the second one only in the southern hemisphere. These periodic behaviours likely results from the complexity of the underlying physical processes, prominently the emergence of magnetic flux.Comment: 33 pages, 15 figures, 2 table

Un problème mal posé ? La formation scientifique et technique des bibliothécaires

Soulignée de façon récurrente par des études échelonnées dans le temps, la sous-représentation des scientifiques en bibliothèque doit-elle aboutir à une critique de la formation ou, plus largement, à l\u27évolution d’une conception « latine » de la diffusion culturelle

The solar wind as seen by SOHO/SWAN since 1996: comparison with SOHO/LASCO C2 coronal densities

We update the SOHO/SWAN H Lyman-alpha brightness analysis to cover the 1996-2008 time interval. A forward model applied to the intensity maps provides the latitude and time dependence of the interstellar Hydrogen ionisation rate over more than a full solar cycle. The hydrogen ionisation, being almost entirely due to charge-exchange with solar wind ions, reflects closely the solar wind flux. Our results show that the solar wind latitudinal structure during the present solar minimum is strikingly different from the previous minimum, with a much wider slow solar wind equatorial belt which persists until at least the end of 2008. We compute absolute values of the in-ecliptic H ionisation rates using OMNI solar wind data and use them to calibrate our ionisation rates at all heliographic latitudes. We then compare the resulting fluxes with the synoptic LASCO/C2 electron densities at 6 solar radii. The two time-latitude patterns are strikingly similar over all the cycle. This comparison shows that densities at 6 solar radii can be used to infer the solar wind type close to its source, with high (resp. low) densities tracing the slow (resp. fast) solar wind, simply because the density reflects at which altitude occurs the acceleration. The comparison between the two minima suggests that the fast polar wind acceleration occurs at larger distance during the current minimum compared to the previous one. This difference, potentially linked to the magnetic field decrease or(and) the coronal temperature decrease should be reproduced by solar wind expansion models.Comment: Proceedings of the SOHO-23 conference, Sept 21-25, 200

Prix versus quantités : Les politiques environnementales d’incitation au développement des énergies renouvelables

La confirmation du risque de changement climatique et la volonté affichée des Etats européens de poursuivre des objectifs ambitieux de production d’électricité renouvelable pour y répondre invitent à s’interroger sur l’efficacité comparée des instruments d’incitation utilisés. Ceux-ci sont clairement des instruments homologues de ceux utilisés par les politiques environnementales. Le choix entre les instruments jouant sur les quantités - fixation d’objectifs nationaux et enchères concurrentielles, ou bien imposition de quotas et échanges de certificats verts -, et les instruments jouant sur les prix - tarifs d’achats administrés - relève du même type de débat que pour ces politiques. Toutefois l’objet de l’intervention publique est particulier : il est de stimuler le changement technique et d’accélérer les apprentissages technologiques en vue d’amener les techniques renouvelables vers la zone de compétitivité avec les techniques classiques après internalisation des coûts environnementaux. La comparaison des instruments doit donc être posée en relation avec les caractéristiques des processus d’innovation et des conditions d’adoption - incertitudes sur les courbes de coût, dynamiques d’apprentissage - qui conduisent à se référer aussi à des critères d’efficience dynamique.Pour ce faire on examine l’efficacité des différents cadres incitatifs au développement des énergies renouvelables, à la fois d’un point de vue théorique en opposant les approches par les prix à celles par les quantités, et d’un point de vue pratique en se référant aux expériences concrètes de mise en œuvre de ces différents instruments. Le papier conclut à l’efficacité plus grande du système de prix administrés comparé au dispositif d’enchères, mais met en évidence les qualités théoriques de l’approche par les certificats verts qui devront être confirmées par la pratique, compte tenu de l’influence des règles et des structures de marché sur les performances de ce type d’approche

Influence of homocysteine on the physical structure and molecular mobility of elastin network in cultured arteries

The thermal and dielectric properties of the elastin network were investigated in arteries cultured with physiological and pathological concentrations of homocysteine, an aminoacid responsible of histological impairments in human arteries. The glass transition of this amorphous protein was investigated by Differential Scanning Calorimetry (DSC). To explore the molecular dynamics of the elastin network in the nanometer range, we used Thermally Stimulated Currents (TSC), a dielectric technique running at low frequency and measuring the dipolar reorientations in proteins subjected to a static electrical field. Combining TSC and DSC experiments with determination of the activation parameters of relaxation times reveals the molecular mobility of the proteins. The major differences in the relaxation behavior of elastin between arteries cultured with physiological and pathological concentrations of homocysteine are discussed