LESIA - Observatoire de Paris

VP/Medoc - service de calcul de vent solaire : état actuel et développements en cours

Filippo Pantellini — 2013-06-24T08:09:56Z

VP est un modèle fluide de vent solaire incluant la transition chromosphérique, allant de la surface à une fraction d'unité astronomique. Dès lors qu'on s'intéresse à la dynamique du plasma de la couronne et du vent solaire, on peut avoir un jour envie de comparer ses idées/son modèle aux prédictions d'un "autre" modèle. Sur le site "VP", on donne un aperçu raisonnable de la physique du modèle et on propose à l'utilisateur de regarder des résultats publiés en utilisant à distance des routines IDL, de façon à pouvoir commodément comparer avec son propre modèle. Dans un second temps, on propose du calcul à la demande : nous lançons les programmes en suivant les desiderata de l'utilisateur. Dans un troisième temps, on travaille sur la physique pour essayer de faire progresser le modèle et donc l'outil de recherche ainsi que le service. On détaillera nos tentatives actuelles pour retrouver des lois empiriques connues dans le cadre d'un modèle intégrant ce que nous croyons savoir de la dissipation turbulente.

Kinetic multi-component simulations for the solar wind small-scale turbulence: ion temperature anisotropy

Filippo Pantellini — 2013-05-31T09:21:30Z

A natural laboratory to study plasma physics is represented by the solar wind. The solar wind is a multi-component and weakly collisional system, and is generally observed from spacecraft measurements to be in a fully turbulent regime. Therefore, the nonlinear dynamics of a collisionless plasma is well described by the self-consistent Vlasov theory, taking into account proton, alpha particle and electron dynamics. We present Vlasov numerical simulations of a turbulent multi-ion plasma, using a low-noise hybrid Vlasov-Maxwell code in a five-dimensional phase space configuration (two dimensions in physical space and three dimensions in velocity space). Ions are treated as kinetic particles, so the Vlasov equation is solved for proton and alpha particle distribution functions, while electrons are considered as a fluid. The ion dynamics at short spatial scales display several interesting aspects, mainly consisting in the departure of the distribution functions from the typical Maxwellian configuration, under the effect of the turbulence. During the nonlinear evolution, coherent structures appear, such as vortices and current sheets and, in between magnetic islands, reconnection events occur. In regions of high magnetic stress, temperature anisotropy is found to be higher. Preferential perpendicular heating is observed for both ion species, although alpha particles display a more significant anisotropy. Moreover, according with the solar wind observations, the results show that the temperature anisotropy of alpha particles is correlated to the proton temperature anisotropy and to the alpha particle drift speed with respect to protons. This study helps understanding some of the complex features commonly observed in the turbulent solar wind.

Improving the accuracy of the Cepheid method to determine the Hubble constant : A precision distance to the LMC

Filippo Pantellini — 2013-05-17T12:52:48Z

I will briefly discuss the basic uncertainties affecting the Cepheid method to set up the cosmic distance scale, and derive the Hubble constant. I will then report on the work of our group to improve on one crucial aspect of the Cepheid method, which is to determine an accurate distance to the Large Magellanic Cloud as the best-suited fiducial galaxy to measure Cepheid distances to more distant galaxies, using a unique sample of late-type eclipsing binary systems in the LMC.

I will also report on an independent determination of the LMC distance from an application of the infrared surface brightness technique on LMC Cepheids, which yields a check on the distance result from the eclipsing binaries.

SÉMINAIRE ANNULÉ : Effect of shear flow on dynamo action in a rotating layer

Filippo Pantellini — 2013-05-17T12:48:23Z

SÉMINAIRE ANNULÉ

FIRST, imageur haute dynamique Perspectives du masquage/réarrangement de pupille

Filippo Pantellini — 2013-03-27T11:05:54Z

FIRST est un interféromètre fibré pour télescope monolithique fonctionnant dans le visible. Son principe combine la technique du réarrangement de pupille avec le filtrage spatial par fibres monomodes, permettant l'étalonnage de la fonction de transfert du télescope. De premiers résultats sur ciel ont été obtenus sur des étoiles binaires, à la limite de diffraction du télescope Shane de 3m de l'Observatoire Lick. Ces premiers résultats sont prometteurs pour des développements futurs en masquage / réarrangement de pupille, et offrent des perspectives intéressantes pour la détection de systèmes exoplanétaires.

L'injection des particules énergétiques solaires de la couronne au milieu interplanétaire

Filippo Pantellini — 2013-03-01T16:15:23Z

Lors des éruptions solaires, des particules sont accélérées à haute énergie et peuvent ensuite se propager dans le milieu interplanétaire jusqu'à la Terre, perturbant ainsi l'activité technologique humaine. Avant de se propager dans le milieu interplanétaire, ces particules énergétiques doivent s'être échapper du site d'accélération dans la couronne au milieu interplanétaire. Cette étape, habituellement considérée comme évidente lors de l'étude des événement à particules, repose sur la dynamique du champ magnétique lors de l'éruption. Pour que les particules énergétiques accèdent au milieu interplanétaire, un couplage entre le champ magnétique fermé de l'éruption et le champ ouvert vers l'espace interplanétaire doit s'opérer. A partir de simulations MHD de configurations magnétiques, représentant des régions actives solaires types, j'ai élaboré deux modèles permettant d'expliquer comment les particules, accélérées dans un région active fermée, sont injectées dans le champ ouvert du milieu interplanétaire. Par ailleurs, la dynamique de la reconnexion magnétique, lors de ces éruptions modélisées, présente des propriétés permettant d'expliquer les sources d'émissions radio des particules énergétiques observées dans la couronne ainsi que les mesures multi-points in-situ des particules énergétiques solaires.

Jupiter/Saturn H3+ Auroral Emission Model for Electron Energy Estimation

Filippo Pantellini — 2013-02-25T11:23:52Z

Auroral electron energy is a key parameter as reflecting the magnetospheric activities and controlling upper atmospheric heating and conductance. We investigate the feasibility of characterizing the Jovian auroral electron energy and flux via H3+ infrared (IR) emission line analysis instead of traditional method via ultraviolet (UV) emission. Ground based telescopes can monitor Jovian infrared auroral activities continuously for an extended time interval compared to the more restricted temporal coverage of ultraviolet observations. Since the departure from local thermodynamic equilibrium (LTE) varies with vibrational levels and altitude, measurements of the relative emission line intensities reveal the altitude of emission and hence the electron energy. The combination of three H3+ line-intensity ratios is required to determine the electron energy and the background temperature. The feasibility issue is evaluated by studying how the observational error propagates into the error of the estimated electron energy. We also test this method to multi-line analysis and much better accuracy is expected. Since saturnian H3+ emissions vary far more substantially according to temperature variations, the method described here is not applicable to observations of Saturn. In this seminar, I would like to discuss several predictions for the magnetosphere-ionosphere coupling system from our model.

Rapidly rotating late B-type stars : spots or pulsations ?

Filippo Pantellini — 2013-02-25T11:16:23Z

Recent space based photometry (CoRoT & MOST) delivered a few late B-type stars, where the variability shows intriguing similarity with the light curves of cool, differentially rotating spotted stars. A pilot study (Degroote et al., 2009) showed that it is possible to explain both the light curve variability and the spectral line profiles with a differentially rotating spot model. However, white light photometry alone is not sufficient to disentangle variability due to spots or pulsations. Much effort is devoted to building models that can explain all of the observations simultaneously in one consistent model, a method which we have generalised also to binary and multiple stars, and other types of observations. After the pilot study, more targets were looked for and found. The multicolour spaced based photometry and ground based spectroscopy available for some of these targets allow for a follow-up study to explain the observations and understand these objects.

Plasma Waves and Electron Dynamics in the Radiation Belts

Filippo Pantellini — 2013-02-25T11:15:58Z

ELF/VLF waves play a crucial role in the dynamics of radiation belts, and are responsible for the loss and the acceleration of energetic electrons. Modeling of wave-particle interactions requires the best possible knowledge of wave energy and wave-normal directions distribution in L-shells for different magnetic latitudes and magnetic activity conditions. We show that whistler wave normals are directed approximately along the magnetic field (with the mean value about 10-15 degrees) in a vicinity of the geomagnetic equator on the basis of statistical study for ELF/VLF emissions using a whistler frequency range for ten years (2001-2010) of Cluster measurements. The distribution changes with magnetic latitude, the angle for a given frequency tends to the resonance cone and as a result at latitudes about 30 degrees, wave-normals become nearly perpendicular to the magnetic field. Above 20 degrees of latitude the field aligned wave population appears which is explained by Landau damping effects of waves propagation. The obtained results were proved by use of numerical ray tracing simulation Distributions for the diffusion coefficients for day and night sectors and for different geomagnetic activity regimes are obtained. The diffusion coefficients from these distributions are compared with coefficients calculated under assumption of whistler parallel propagation with constant value of variance and wave amplitude along magnetic field line. The analytical validation of diffusion rates was made. The increase of the mean value and the variance of the wave vector distribution with latitude results in significant growth of the pitch-angle diffusion rates due to significant increase of the contribution of higher order cyclotron resonances at large latitudes, which is the most efficient for electrons with small equatorial pitch-angles. The new acceleration mechanism of radiation belts electron based on Landau resonance which explains energy gain up to 1 MeV has been developed.

Three Dimensional Flux Ropes

Filippo Pantellini — 2012-12-18T09:13:25Z

Magnetic flux ropes are due to helical currents and form a dense carpet of arches on the surface of the sun. Occasionally one tears loose as a coronal mass ejection and its rope structure is detected by satellites close to the earth. Current sheets in plasma can tear into filaments and these are nothing other than flux ropes. Ropes are not static, they exert mutual J × B forces causing them to twist about each other and merge. Kink instabilities cause them to violently smash into each other and reconnect at the point of contact. We report on experiments done in in the large plasma device the LAPD (L = 17m, dia = 60cm, 0.3 ≤ B0z ≤ 2.5kG, n ≅ 2 × 10¹² cm −3 ) at UCLA on three dimensional flux ropes. Two or three magnetic flux ropes are generated from initially adjacent pulsed current channels or a series of ropes from the tearing of a current sheet. in a background magnetized plasma. The currents and magnetic fields form exotic shapes with no ignorable direction and no magnetic nulls. Volumetric space-time data show multiple reconnection sites with time-dependent locations. Mach probes measure three dimensional plasma flow which jet out from reconnection regions and spiral around the rope(s) magnetic field. The concept of a quasi-separatrix layer (QSL), a tool to understand and visualize 3D magnetic field line reconnection without null points is introduced. We will explain what it is and how it is derived from our data. Here the QSL is a narrow ribbon-like region(s) that twist between field lines. It (they) will be shown in detail from data sets acquired at up to 50,000 spatial locations. Within the QSL(s) field lines that start close to one another rapidly diverge as they pass through one or more reconnection sites. The magnetic helicity, which is a measure of the linkage of magnetic fields is evaluated from volumetric data in both cases and its rate of change is used to estimate the plasma resistivity. Heating and other co-existing waves (the ropes are Alfvén waves in disguise) will be presented. Flux ropes and QSLs show promise in understanding what 3D reconnection really is!

This work was done at the Basic Plasma Science Facility at UCLA and funded by NSF and DOE

Figure caption: The figure is a rendering of the experimental data. The blue and green tubes are magnetic field lines associated with the flux ropes. (Note several “green” field lines have rotated in space and are smashing into the “blue” ones). The orange field lines are plasma flow. The flow speed is as large as Mach 0.3. The blue surface is a Quasi-Seperatrix layer of value 1000. The flow goes through the QSL at one point and then spirals around the flux ropes. The two axis markers are two meters apart in the z direction (the machine axis). Data was acquired at 50,000 locations over a volume (δx=24 cm, δy = 24 cm, δz = 10m). Many other images and movies will be shown in the talk.