Superdiffusive and Subdiffusive Transport of Energetic Particles in Solar Wind Anisotropic Magnetic Turbulence

The transport of energetic particles in a mean magnetic field and the presence of anisotropic magnetic turbulence are studied numerically, for parameter values relevant to the solar wind. A numerical realization of magnetic turbulence is set up in which we can vary the type of anisotropy by changing the correlation lengths lx, ly, lz. We find that for lx, ly lz, transport can be non-Gaussian, with superdiffusion along the average magnetic field and subdiffusion perpendicular to it. Decreasing the lx/lz ratio down to 0.3, Gaussian diffusion is obtained, showing that the transport regime depends on the turbulence anisotropy. Implications for energetic particle propagation in the solar wind and for diffusive shock acceleration are discussed

A Monte Carlo simulation of magnetic field line tracing in the solar wind

International audienceIt is well known that the structure of magnetic field lines in solar wind can be influenced by the presence of the magnetohydrodynamic turbulence. We have developed a Monte Carlo simulation which traces the magnetic field lines in the heliosphere, including the effects of magnetic turbulence. These effects are modelled by random operators which are proportional to the square root of the magnetic field line diffusion coefficient. The modelling of the random terms is explained, in detail, in the case of numerical integration by discrete steps. Furthermore, a proper evaluation of the diffusion coefficient is obtained by a numerical simulation of transport in anisotropic magnetic turbulence. The scaling of the fluctuation level and of the correlation lengths with the distance from the Sun are also taken into account. As a consequence, plasma transport across the average magnetic field direction is obtained. An application to the propagation of energetic particles from corotating interacting regions to high heliographic latitudes is considered

Dynamique urbaine à Prague de la fin de la période communiste à nos jours : l’importation du modèle occidental du commerce de détail

Suite à la chute du mur de Berlin et des régimes communistes en 1989, les pays d’Europe Centrale et Orientale instaurèrent une économie de transition devant faciliter le passage d’une économie dite de commandement à une véritable économie de marché. Des processus de restitution, de privatisation et de libéralisation furent amorcés à des vitesses plus ou moins rapides selon les pays. Dans ce nouveau contexte de transition, les villes d’Europe Centrale et Orientale connaissent des transformations qui touchent autant les paysages urbains que les organisations spatiales. À Prague, si le centre-ville est conforté dans sa place de premier centre commercial et commerçant de la ville, l’évolution actuelle se caractérise par la construction planifiée de véritables centres commerciaux près des zones résidentielles construites sous le régime communiste.Since the fall of the Berlin Wall and the communist regimes in 1989, central and eastern European countries established an economy of transition to facilitate the change from a planned economy to a truly market economy. Processes of restitution, privatization and liberalization were engaged with varying rapidity according to the country. Under this new political and economic climate, the cities of Central and Eastern Europe are bringing change to urban landscapes as well as their spatial organisation. In Prague, if the center is confirmed in its role of the first central place of the city, the current development is the planned construction of new commercial centres near the residential areas built under the communist regime.Nach dem Fall der Mauer in Berlin und der kommunistischen Regime 1989 richteten die Länder Mittel- und Osteuropas eine Übergangswirtschaft ein, um den Wechsel von der Planwirtschaft zu einer echten Marktwirtschaft zu erleichtern. Die Prozesse der Rückgabe, Privatisierung und Liberalisierung wurden je nach Land mehr oder weniger rasch eingeleitet. In diesem neuen Kontext des Übergangs erfuhren die Städte Mittel- und Osteuropas Veränderungen, die sowohl die Städtischen Landschaften wie die Raumorganisationen berührten. Wenn in Prag das Statzentrum verstärkt den Platz des ersten Handels- und Geschäftszentrums der Stadt einnimmt, wird die aktuelle Entwicklung charakterisiert durch die planmässige Errichtung echter Geschäftszentren nahe der unter dem kommunistischen Regime erbauten Wohnzonen

L’évolution des comportements d’achat à Prague : du communisme au fun shopping

L’organisation spatiale du commerce de détail à Prague évolue très rapidement. Depuis 1997, les centres commerciaux modernes se sont multipliés, et la dernière génération d’entre eux introduit le concept de fun shopping. Pour les professionnels du marketing, il s’agit de rendre l’activité d’achat plus agréable en l’associant à la récréation et la socialisation. Les habitants de Prague modifient leurs comportements d’achat en empruntant le mode de vie à l’occidentale, ou du moins l’image qu’en ont les Pragois, et qui s’identifie par la société de consommation. Au courant de l’année 2004, une enquête a été réalisée auprès de 400 habitants de Prague. Le but de cette étude était l’identification de divers types de comportement spatial par l’analyse des déplacements à but d’achat et de mesurer le rôle des centres commerciaux comme lieu de socialisation. Les premiers résultats sont présentés dans cet article.The retailing spatial organization of Prague evolves with rapidity. Since the opening of the first modern shopping centre in the outer-suburbs of the Czech capital city, shopping centres and hypermarkets spread, and the last generation introduces, in the city, the concept o fun shopping. For professionals, the shopping activity must be combined with leisure and recreation. Today, modern shopping malls provide food courts, multiplex cinemas, fitness centres. Inhabitants of Prague are changing their shopping habits and are adopting a western way of life that is identified with the consumption society. In 2004, a sample of 400 inhabitants, located within Prague, has been interviewed. The goal of the survey was the definition of different kinds of shopping behaviours and the definition of the role of shopping centres as new location for socialization. The first results of this survey are presented in this paper

Dynamique urbaine à Prague de la fin de la période communiste à nos jours : l’importation du modèle occidental du commerce de détail

Suite à la chute du mur de Berlin et des régimes communistes en 1989, les pays d’Europe Centrale et Orientale instaurèrent une économie de transition devant faciliter le passage d’une économie dite de commandement à une véritable économie de marché. Des processus de restitution, de privatisation et de libéralisation furent amorcés à des vitesses plus ou moins rapides selon les pays. Dans ce nouveau contexte de transition, les villes d’Europe Centrale et Orientale connaissent des transformations qui touchent autant les paysages urbains que les organisations spatiales. À Prague, si le centre-ville est conforté dans sa place de premier centre commercial et commerçant de la ville, l’évolution actuelle se caractérise par la construction planifiée de véritables centres commerciaux près des zones résidentielles construites sous le régime communiste.Since the fall of the Berlin Wall and the communist regimes in 1989, central and eastern European countries established an economy of transition to facilitate the change from a planned economy to a truly market economy. Processes of restitution, privatization and liberalization were engaged with varying rapidity according to the country. Under this new political and economic climate, the cities of Central and Eastern Europe are bringing change to urban landscapes as well as their spatial organisation. In Prague, if the center is confirmed in its role of the first central place of the city, the current development is the planned construction of new commercial centres near the residential areas built under the communist regime.Nach dem Fall der Mauer in Berlin und der kommunistischen Regime 1989 richteten die Länder Mittel- und Osteuropas eine Übergangswirtschaft ein, um den Wechsel von der Planwirtschaft zu einer echten Marktwirtschaft zu erleichtern. Die Prozesse der Rückgabe, Privatisierung und Liberalisierung wurden je nach Land mehr oder weniger rasch eingeleitet. In diesem neuen Kontext des Übergangs erfuhren die Städte Mittel- und Osteuropas Veränderungen, die sowohl die Städtischen Landschaften wie die Raumorganisationen berührten. Wenn in Prag das Statzentrum verstärkt den Platz des ersten Handels- und Geschäftszentrums der Stadt einnimmt, wird die aktuelle Entwicklung charakterisiert durch die planmässige Errichtung echter Geschäftszentren nahe der unter dem kommunistischen Regime erbauten Wohnzonen

Diffusion and dispersion of passive tracers: Navier-Stokes versus MHD turbulence

A comparison of turbulent diffusion and pair-dispersion in homogeneous, macroscopically isotropic Navier-Stokes (NS) and nonhelical magnetohydrodynamic (MHD) turbulence based on high-resolution direct numerical simulations is presented. Significant differences between MHD and NS systems are observed in the pair-dispersion properties, in particular a strong reduction of the separation velocity in MHD turbulence as compared to the NS case. It is shown that in MHD turbulence the average pair-dispersion is slowed down for $\tau_\mathrm{d}\lesssim t\lesssim 10 \tau_\mathrm{d}$, $\tau_\mathrm{d}$ being the Kolmogorov time, due to the alignment of the relative Lagrangian tracer velocity with the local magnetic field. Significant differences in turbulent single-particle diffusion in NS and MHD turbulence are not detected. The fluid particle trajectories in the vicinity of the smallest dissipative structures are found to be characterisically different although these comparably rare events have a negligible influence on the statistics investigated in this work.Comment: Europhysics Letters, in prin

Subdiffusive transport in intergranular lanes on the Sun. The Leighton model revisited

In this paper we consider a random motion of magnetic bright points (MBP) associated with magnetic fields at the solar photosphere. The MBP transport in the short time range [0-20 minutes] has a subdiffusive character as the magnetic flux tends to accumulate at sinks of the flow field. Such a behavior can be rigorously described in the framework of a continuous time random walk leading to the fractional Fokker-Planck dynamics. This formalism, applied for the analysis of the solar subdiffusion of magnetic fields, generalizes the Leighton's model.Comment: 7 page

The role of oxygen ions in the formation of a bifurcated current sheet in the magnetotail

Cluster observations in the near-Earth magnetotail have shown that sometimes the current sheet is bifurcated, i.e. it is divided in two layers. The influence of magnetic turbulence on ion motion in this region is investigated by numerical simulation, taking into account the presence of both protons and oxygen ions. The magnetotail current sheet is modeled as a magnetic field reversal with a normal magnetic field component $B_n$, plus a three-dimensional spectrum of magnetic fluctuations $\delta {\bf B}$, which represents the observed magnetic turbulence. The dawn-dusk electric field E$_y$ is also included. A test particle simulation is performed using different values of $\delta {\bf B}$, E$_y$ and injecting two different species of particles, O$^+$ ions and protons. O$^+$ ions can support the formation of a double current layer both in the absence and for large values of magnetic fluctuations ($\delta B/B_0 = 0.0$ and $\delta B/B_0 \geq 0.4$, where B$_0$ is the constant magnetic field in the magnetospheric lobes).Comment: 8 pages, 8 figures. J. Geophys. Res., in pres

Simulating Heliospheric and Solar Particle Diffusion using the Parker Spiral Geometry

Cosmic Ray transport in curved background magnetic fields is investigated using numerical Monte-Carlo simulation techniques. Special emphasis is laid on the Solar system, where the curvature of the magnetic field can be described in terms of the Parker spiral. Using such geometries, parallel and perpendicular diffusion coefficients have to be re-defined using the arc length of the field lines as the parallel displacement and the distance between field lines as the perpendicular displacement. Furthermore, the turbulent magnetic field is incorporated using a WKB approach for the field strength. Using a test-particle simulation, the diffusion coefficients are then calculated by averaging over a large number of particles starting at the same radial distance from the Sun and over a large number of turbulence realizations, thus enabling one to infer the effects due to the curvature of the magnetic fields and associated drift motions.Comment: accepted for publication at Journal of Geophysical Research - Space Physic

La consommation à Prague : impacts sur l’espace urbain: Etudes du CEFRES n° 2, septembre 2004

Editeur : CEFRES39p.Depuis la fin des régimes communistes, les villes des pays est-européens connaissent des mutations profondes. Prague, capitale de la Tchécoslovaquie, puis de la République tchèque à partir de 1993, constitue un des exemples d’espaces urbains qui change rapidement de visage sous l’effet de la réintroduction d’une économie fonctionnant selon les principes du marché. Nous avons choisi d’aborder cette transformation par l’analyse de la géographie du commerce de détail dans l’espace urbain pragois et des habitudes d’achats des habitants de la ville