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Using statistically significant data at 1 AU, it has recently been shown (Bale et al.) that in the solar wind, when the Knudsen number K {sub T} (the ratio between the electron mean free path and the electron temperature scale height) drops below about 0.3, the electron heat flux q intensity rapidly approaches the classical collisional Spitzer-Harm limit. Using a fully kinetic model including the effect of Coulomb collisions and the expansion of the solar wind with heliocentric distance, we observe that the heat flux strength does indeed approach the collisional value for Knudsen numbers smaller than about 0.3 in very good agreement with the observations. However, closer inspection of the heat flux properties, such as its variation with the heliocentric distance and its dependence on the plasma parameters, shows that for Knudsen numbers between 0.02 and 0.3 the heat flux is not conveniently described by the Spitzer-Harm formula. We conclude that even though observations at 1 AU seem to indicate that the electron heat flux intensity approaches the collisional limit when the Knudsen drops below ∼0.3, the collisional limit is not a generally valid closure for a Knudsen larger than 0.01. Moreover, the good agreement between the heat flux frommore » our model and the heat flux from solar wind measurements in the high-Knudsen number regime seems to indicate that the heat flux at 1 AU is not constrained by electromagnetic instabilities as both wave-particle and wave-wave interactions are neglected in our calculations.« less

Magnetic reconnection in laboratory, space and astrophysical plasmas is often invoked to explain explosive energy release and particle acceleration. However, the timescales involved in classical models within the macroscopic MHD regime are far too slow to match the observations. Here we revisit the tearing instability by performing visco-resistive two-dimensional numerical simulations of the evolution of thin current sheets, for a variety of initial configurations and of values of the Lunquist number $S$, up to $10^7$. Results confirm that when the critical aspect ratio of $S^{1/3}$ is reached in the reconnecting current sheets, the instability proceeds on ideal (Alfv\'enic) macroscopic timescales, as required to explain observations. Moreover, the same scaling is seen to apply also to the local, secondary reconnection events triggered during the nonlinear phase of the tearing instability, thus accelerating the cascading process to increasingly smaller spatial and temporal scales. The process appears to be robust, as the predicted scaling is measured both in inviscid simulations and when using a Prandtl number $P=1$ in the viscous regime., Comment: Accepted for publication in Plasma Physics and Controlled Fusion

We present a one dimensional kinetic simulation model which can be used to simulate the stationary state of a semicollisional plasma. Results of a simple simulation are presented and compared to Fokker-Planck calculations. The model is particularly well suited for the diluted solar atmosphere.

We have addressed the problem of combined magnetic and velocity shear driven instabilities in the context of the heliospheric plasma. New high-order numerical methods have been used to analyze the instability dynamics of the heliospheric current-sheet interacting with the structure determined by the slow component of the solar wind on the solar equatorial plane above the helmet streamers. Preliminary results are presented.

The solar wind is a weakly collisional non homogeneous plasma; gradients associated to density, velocity shears and current sheets are often observed. In situ observations also show that the solar wind plasma is far from thermal equilibrium and particle distribution functions are not isotropic. The presence of a temperature anisotropy can be the source of free energy for kinetic instabilities and their unstable fluctuations may grow and propagate in the plasma. However, how these fluctuations evolve in a non homogeneous medium and how they interact and influence local coherent structures, is still an open question. We report preliminary numerical simulations that describe the evolution of current sheets in a non thermal plasma, focusing on the interaction between kinetic effects driven by a proton temperature anisotropy and magnetic reconnection processes.

We investigate the role of kinetic instabilities driven by a proton anisotropy on the onset of magnetic reconnection by means of 2-D hybrid simulations. The collisionless tearing of a current sheet is studied in the presence of a proton temperature anisotropy in the surrounding plasma. Our results confirm that anisotropic protons within the current sheet region can significantly enhance/stabilize the tearing instability of the current. Moreover, fluctuations associated to linear instabilities excited by large proton temperature anisotropies can significantly influence the stability of the plasma and perturb the current sheets, triggering the tearing instability. We find that such a complex coupling leads to a faster tearing evolution in a regime with larger perpendicular temperature when an ion-cyclotron instability is generated by the anisotropic proton distribution functions. On the contrary, in the presence of the opposite anisotropy, fire hose fluctuations excited by the unstable background protons with larger parallel temperature are not able to efficiently destabilize the current sheets, which remain stable for a long time after fire hose saturation. We discuss possible influences of this novel coupling on the solar wind and heliospheric plasma dynamics., 6 pages, 6 figures, Accepted for publication in the Astrophysical Journal

We present numerical simulations of the solar wind using a fully kinetic model which takes into account the effects of particle's binary collisions in a quasi-neutral plasma in spherical expansion. Starting from an isotropic Maxwellian velocity distribution function for the electrons, we show that the combined effect of expansion and Coulomb collisions leads to the formation of two populations: a collision-dominated cold and dense population almost isotropic in velocity space and a weakly collisional, tenuous field-aligned and antisunward drifting population generated by mirror force focusing in the radially decreasing magnetic field. The relative weights and drift velocities for the two populations observed in our simulations are in excellent agreement with the relative weights and drift velocities for both core and strahl populations observed in the real solar wind. The radial evolution of the main moments of the electron velocity distribution function is in the range observed in the solar wind. The electron temperature anisotropy with respect to the magnetic field direction is found to be related to the ratio between the collisional time and the solar wind expansion time. Even though collisions are found to shape the electron velocity distributions and regulate the properties of the strahl, it is found that the heat flux is conveniently described by a collisionless model where a fraction of the electron thermal energy is advected at the solar wind speed. This reinforces the currently largely admitted fact that collisions in the solar wind are clearly insufficient to force the electron heat flux obey the classical Spitzer-Harm expression where heat flux and temperature gradient are proportional to each other. The presented results show that the electron dynamics in the solar wind cannot be understood without considering the role of collisions.

Introduction: This study describes a pivotal clinical trial of a new minimally invasive mesotherapy technique for facial rejuvenation. Methods: The authors utilized two formulations: formulation A with hyaluronic acid, vitamins, amino acids, minerals, coenzymes, and antioxidant substances; formulation B with hyaluronic acid and idebenone. Fifty participants were enrolled in the study and divided in two groups. Group 1 (50-65 years) treated with formulation A. Group 2 (35-50 years) treated with formulation B. The groups underwent four sessions of mesotherapy involving multiple injections. Treatment was conducted at 15 day intervals. All participants had pre- and posttreatment photographs. Punch biopsies were taken from randomly selected participants, baseline and after 6 weeks, and stained for interleukin (IL)-6, IL- 1b, matrix metalloproteinase (MMP)-1, and collagen 1. Clinical evaluation was based on the Global Aesthetic Scale (GAIS) and on the Wrinkle Severity Rating Scale (WSRS). Results: The results produced were statistically analyzed and resulted in a significant and long-lasting effect on facial rejuvenation. Evaluation of photographs at 0, 1, and 2 months revealed significant clinical improvement: brightness, texture, and firmness of the skin. The analysis of the GAIS and WSRS scores in the two groups demonstrated statistically significant results after 2 months. The biopsies taken from randomly selected participants at baseline and after 3 months showed a decrease in IL- 1b, IL-6, and MMP1, and an increase in collagen 1. Conclusion: The new minimally invasive mesotherapy technique described can improve the clinical appearance of the skin in different age groups. © The Author(s) 2013.

Nano and micrometre sized dust particles travelling through the heliosphere at several hundreds of km?s?1 have been repeatedly detected by interplanetary spacecraft. When such fast moving dust particles hit a solid target in space, an expanding plasma cloud is formed through the vaporization and ionization of the dust particles itself and part of the target material at and near the impact point. Immediately after the impact the small and dense cloud is dominated by collisions and the expansion can be described by fluid equations. However, once the cloud has reached ?m dimensions, the plasma may turn collisionless and a kinetic description is required to describe the subsequent expansion. In this paper we explore the late and possibly collisionless spherically symmetric unconstrained expansion of a single ionized ion?electron plasma using N-body simulations. Given the strong uncertainties concerning the early hydrodynamic expansion, we assume that at the time of the transition to the collisionless regime the cloud density and temperature are spatially uniform. We also neglect the role of the ambient plasma. This is a reasonable assumption as long as the cloud density is substantially higher than the ambient plasma density. In the case of clouds generated by fast interplanetary dust grains hitting a solid target, some 107 electrons and ions are liberated and the in vacuum approximation is acceptable up to meter order cloud dimensions. As such a cloud can be estimated to become collisionless when its radius has reached ?m order dimensions, both the collisionless approximation and the in vacuum approximation are expected to hold during a long lasting phase as the cloud grows by a factor 106. With these assumptions, we find that the transition from the collisional to the collisionless regime could occur when the electron Debye length ?D within the cloud is much smaller than the cloud radius R0, i.e. ?????D/R0???1. This implies a quasi-neutral expansion regime where the radial electron and ion density profiles are equal through most of the cloud except at the cloud?vacuum interface. The consequence of ? being much smaller than unity implies that the electrostatic fields within a cloud generated by a dust impact on a neutral target is ?100 times weaker than in the case of grains hitting a spacecraft, where the positive potential of the target is strong enough to strip-off all the electrons from the expanding cloud leading to a ?Coulomb explosion? like regime (e.g. Peano et al 2007 Phys. Plasmas \bf 14 056704).

Parametric instabilities have often been invoked to explain some of the features of Alfvenic turbulence as observed in the (fast) solar wind plasma, namely the gradual reduction of cross helicity and its final saturation with heliocentric distance. Moreover, this instability could also be responsible for direct plasma heating and creation of transverse small-scale Alfvenic fluctuations, as required by recent models for coronal heating and solar wind acceleration. Here we discuss numerical simulations of the long-term nonlinear evolution of Alfven waves, both in the MHD and hybrid regimes, performed by the plasma theory group in Florence in the last decade and we show, for the first time, 2-D MHD simulations of the parametric decay of oblique arc-polarized waves.

The properties of spontaneous reconnection of a current sheet analyzed via direct three-dimensional simulations are presented. In particular the non-linear dynamics of resistive instabilities has been studied in absence or in presence of velocity shears. It is shown that full three-dimensional simulations allow the inclusion of a rich variety of (ideal) secondary instabilities which, depending on the initial equilibrium magnetic field configuration, determine the final fate of the system in the fully non linear regime. In particular in presence of a guide-field the dynamic is similar to what observed in two-dimensional simulations with energy driven toward both smaller and larger scales and energy spectra anisotropy. For different magnetic field configurations, the final state is characterized by the disruption of the coalesced structure created during the resistive phase and the system is characterized by a more chaotic state. A discussion on the importance of high-order numerical techniques in numerical simulations of magnetic reconnection is also present.

The parametric instability of a monochromatic shear Alfven wave in oblique propagation with respect the am- bient magnetic field is investigated in a kinetic regime, per- forming one-dimensional (1-D) and two-dimensional (2-D) hybrid simulations. The parallel component of the mother wave is found to be subject to a parametric decay which excites an ion-acoustic wave along the magnetic field and a backward propagating daughter shear Alfven wave, as in the instability for a purely parallel mother wave. At the same time, the acoustic wave generation supports the acceleration of a velocity beam in the ion distribution function, due to the non-linear trapping of protons. Moreover, the instabil- ity leads to the generation of broad band oblique spectra of coupled Alfvenic and compressive modes with variable per- pendicular wavevectors, and, as a consequence, the magnetic field after saturation is characterized by a strong transverse modulation. A 30 0.21 0.33 0.12 0.007 0.008 B 45 0.21 0.33 0.12 0.006 0.008 C 60 0.21 0.33 0.12 0.004 0.008

[1] Using numerical simulations in a hybrid regime, we studied the evolution of large-amplitude Alfven waves subject to modulational and decay instabilities, including the effects of ion kinetics. We considered both a monochromatic and incoherent spectrum of waves, different wave polarizations and amplitudes, and different plasma regimes, ranging from β 1. We found in all cases that ion dynamics affects the instability evolution and saturation; as a feedback, wave-particle interactions provide a nonlinear trapping of resonant particles that importantly change the properties of the ion velocity distribution functions. In particular, we observed a proton acceleration along the magnetic field and in some cases the formation of a parallel velocity beam traveling faster than the rest of the distribution. For the range of parameters used in our simulations, the fundamental ingredient in generating an ion beam is observed to be the parallel electric field carried by the density fluctuations driven by the ion-acoustic modes generated by the parametric instabilities.

The problem of three-dimensional combined magnetic and velocity shear driven instabilities of a compressible magnetized jet modeled with a plane neutral/current double vortex sheet in the framework of the resistive magnetohydrodynamics is addressed. The resulting dynamics given by the stream+current sheet interaction is analyzed and the effects of a variable geometry of the basic fields are considered. Depending on the basic asymptotic magnetic field configuration, a selection rule of the linear instability modes can be obtained. Hence, the system follows a two-stage path developing either through a fully three-dimensional dynamics with a rapid evolution of kink modes leading to a final turbulent state, or rather through a driving two-dimensional instability pattern that develops on parallel planes on which a reconnection+coalescence process takes place., Comment: 33 pages, 15 figures, accepted for publication in Physics of Plasmas

We present one-dimensional molecular dynamics simulations of a two-species, initially uniform, freely evolving granular system. Colliding particles swap their relative position with a 50% probability allowing for the initial spatial ordering of the particles to evolve in time and frictional forces to operate. Unlike one-dimensional systems of identical particles, two-species one-dimensional systems of quasi-elastic particles are ergodic and the particles' velocity distributions tend to evolve towards Maxwell-Boltzmann distributions. Under such conditions, standard fluid equations with merely an additional sink term in the energy equation, reflecting the non-elasticity of the interparticle collisions, provide an excellent means to investigate the system's evolution. According to the predictions of fluid theory we find that the clustering instability is dominated by a non-propagating mode at a wavelength of the order 10 pi L/N epsilon , where N is the total number of particles, L the spatial extent of the system and epsilon the inelasticity coefficient. The typical fluid velocities at the time of inelastic collapse are seen to be supersonic, unless N epsilonor= 10 pi . Species segregation, driven by the frictional force occurs as a result of the strong temperature gradients within clusters which pushes the light particles towards the clusters' edges and the heavy particles towards the center. Segregation within clusters is complete at the time of inelastic collapse.

Resumen en español: "No obstante que numerosos estudios científicos han demostrado el impacto positivo de los árboles de sombra de los cafetales sobre la biodiversidad, el mantenimiento del ciclaje de nutrimentos y el control del microclima del agroecosistema, ante la inestabilidad de los precios del café en los últimos diez años, muchos productores han decidido tumbar la sombra. Poco se sabe sobre el impacto que esto pueda tener sobre los microorganismos edáficos y en particular de los grupos funcionales de nemátodos en estos agroecosistemas. Los nemátodos son buenos indicadores de las condiciones del suelo, algunas especies tienen un papel importante en la degradación de la materia orgánica y otras pueden causar daño a la planta de café. El objetivo de esta investigación, utilizando un análisis estadístico descriptivo y geoestadístico, fue estudiar el impacto de la ausencia, presencia y heterogeneidad de los árboles de sombra sobre los grupos funcionales de nemátodos. Se encontró que la luz y los nemátodos tienen una distribución espacialmente dependiente y se distribuyen de manera heterogénea. La distribución de los grupos funcionales de nemátodos y su escala de autocorrelación, no cambiaron aunque cambió el manejo de la sombra. Sin embargo, en el cafetal cultivado bajo sol, se encontraron correlaciones entre la luz y los índices: fitoparásitos/nemátodos de vida libre (Fit/Libre) (r = 0.44, P

[1] We report an analysis of the proton temperature anisotropy evolution from 0.3 to 2.5 AU based on the Helios and Ulysses observations. With increasing distance the fast wind data show a path in the parameter space (β∥p, T⊥p/T∥p). The first part of the trajectory is well described by an anticorrelation between the temperature anisotropy T⊥p/T∥p and the proton parallel beta, while after 1 AU the evolution with distance in the parameter space changes and the data result in agreement with the constraints derived by a fire hose instability. The slow wind data show a more irregular behavior, and in general it is not possible to recover a single evolution path. However, on small temporal scale we find that different slow streams populate different regions of the parameter space, and this suggests that when considering single streams also the slow wind follows some possible evolution path.

We study the linear and nonlinear evolution of the tearing instability on thin current sheets by means of two-dimensional numerical simulations, within the framework of compressible, resistive magnetohydrodynamics. In particular we analyze the behavior of current sheets whose inverse aspect ratio scales with the Lundquist number S as S 1=3 . This scaling has been recently recognized to yield the threshold separating fast, ideal reconnection, with an evolution and growth which are independent of S provided this is high enough, as it should be natural having the ideal case as a limit for S! 1. Our simulations conrm that the tearing instability growth rate can be as fast as 0:6 A 1 , where A is the ideal Alfv enic time set by the macroscopic scales, for our least diusive case with S = 10 7 . The expected instability dispersion relation and eigenmodes are also retrieved in the linear regime, for the values of S explored here. Moreover, in the nonlinear stage of the simulations we observe secondary events obeying the same critical scaling with S, here calculated on the local, much smaller lengths, leading to increasingly faster reconnection. These ndings strongly support the idea that in a fully dynamic regime, as soon as current sheets develop, thin and reach this critical threshold in their aspect ratio, the tearing mode is able to trigger plasmoid formation and reconnection on the local (ideal) Alfv enic timescales, as required to explain the explosive aring activity often observed in solar and astrophysical plasmas. Subject headings: plasmas { MHD { methods: numerical.

The measure of the third-order structure function, Y, is employed in the solar wind to compute the cascade rate of turbulence. In the absence of a mean field B0=0, Y is expected to be isotropic (radial) and independent of the direction of increments, so its measure yields directly the cascade rate. For turbulence with mean field, as in the solar wind, Y is expected to become more two dimensional (2D), that is, to have larger perpendicular components, loosing the above simple symmetry. To get the cascade rate one should compute the flux of Y, which is not feasible with single-spacecraft data, thus measurements rely upon assumptions about the unknown symmetry. We use direct numerical simulations (DNS) of magneto-hydrodynamic (MHD) turbulence to characterize the anisotropy of Y. We find that for strong guide field B0=5 the degree of two-dimensionalization depends on the relative importance of shear and pseudo polarizations (the two components of an Alfv\'en mode in incompressible MHD). The anisotropy also shows up in the inertial range. The more Y is 2D, the more the inertial range extent differs along parallel and perpendicular directions. We finally test the two methods employed in observations and find that the so-obtained cascade rate may depend on the angle between B0 and the direction of increments. Both methods yield a vanishing cascade rate along the parallel direction, contrary to observations, suggesting a weaker anisotropy of solar wind turbulence compared to our DNS. This could be due to a weaker mean field and/or to solar wind expansion., Comment: Some text editing and typos corrected, 13 pages, 6 figures, to be published in ApJ

Magnetic field polarity inversions embedded in the predominantly unipolar fast solar wind have been observed by the Ulysses spacecraft at high latitudes. Such reversals have the nature of folded back field lines which we suggest are generated by the interaction of standard large amplitude, low frequency, Alfvenic turbulence with velocity shears in the fast solar wind. We present 2D magnetohydrodynamic simulations of a very low frequency and high amplitude Alfven wave propagating away from the sun embedded in a velocity shear structure such as a microstream and show how reversals in the magnetic field lines are generated naturally on a time-scale consistent with their observation at Ulysses. The generated magnetic field and plasma signals are similar to those observed. We discuss the role turbulence-stream shear interactions might play in limiting differential velocities in the asymptotic high speed solar wind. Citation: Landi, S., P. Hellinger, and M. Velli (2006), Heliospheric magnetic field polarity inversions driven by radial velocity field structures, Geophys. Res. Lett., 33, L14101, doi:10.1029/2006GL026308.

[1] Oblique fire hose instability is investigated using hybrid simulations for proton betas of the order of one and for proton parallel temperatures sufficiently greater than the perpendicular ones. The simulations confirm previous simulation results showing that this instability has self-destructing properties and efficiently reduces the proton temperature anisotropy. A parametric study using one-dimensional standard hybrid simulations shows that stronger changes in the temperature anisotropy and stronger wave emissions appear for larger initial temperature anisotropies. An ideal, slow plasma expansion, modeled by a two-dimensional hybrid expanding box simulation, leads to a generation of proton temperature anisotropy. The anisotropy leads first to destabilization of the dominant parallel fire hose, which interacts mainly with minor supra-Alfvenic protons, whereas the evolution of core protons is determined by expansion. Consequently, the effective anisotropy is only slightly reduced and the system eventually becomes unstable with respect to the oblique fire hose instability. The oblique fire hose strongly scatters the protons and removes the anisotropy disrupting the parallel fire hose. An important portion of the fluctuating wave energy is dissipated to protons, and only long wavelength waves remain in the system. The system with low wave activity then develops again larger temperature anisotropies, and the evolution repeats itself. It is concluded that both parallel and oblique proton fire hose instabilities constrain the proton temperature anisotropy in the expanding solar wind, with the latter one constituting a final frontier for the anisotropy. These results give a possible explanation of some apparent discrepancies between observations and linear predictions. In addition, a simple bounded anisotropy model is developed to include some of the kinetic effects of the fire hose instabilities in fluid models.

One of the basic properties of the solar wind, that is the high speed of the fast wind, is still not satisfactorily explained. This is mainly due to the theoretical difficulty of treating weakly collisional plasmas. The fluid approach implies that the medium is collision dominated and that the particle velocity distributions are close to Maxwellians. However the electron velocity distributions observed in the solar wind depart significantly from Maxwellians. Recent kinetic collisionless models (called exospheric) using velocity distributions with a suprathermal tail have been able to reproduce the high speeds of the fast solar wind. In this letter we present new developments of these models by generalizing them over a large range of corona conditions. We also present new results obtained by numerical simulations that include collisions. Both approaches calculate the heat flux self-consistently without any assumption on the energy transport. We show that both approaches - the exospheric and the collisional one - yield a similar variation of the wind speed with the basic parameters of the problem; both produce a fast wind speed if the coronal electron distribution has a suprathermal tail. This suggests that exospheric models contain the necessary ingredients for the powering of a transonic stellar wind, including the fast solar one., Comment: Accepted for publication in The Astrophysical Journal Letters (accepted: 13 May 2005)

We present and discuss a completely self-consistent kinetic simulation of a steady state transonic solar type wind. The equations of motion of an equal number of protons and electrons plunged in a central gravitational field and a self-consistent electric field are integrated numerically. Particles are allowed to make binary collisions with a Coulombian scattering cross-section. The velocity distributions of the particles injected at the boundaries of the simulation domain are taken to be Maxwellian. As anticipated by previous authors we find that the transonic solution implies the existence of a peak in the proton equivalent potential at some distance above the sonic critical point. Collisions appear to be the fundamental ingredient in the process of accelerating the wind to supersonic velocities. For a given temperature at the base of the simulation domain the acceleration efficiency decreases with decreasing density. The reason is that the plasma has to be sufficiently collisional for the heat flux to be converted efficiently into plasma bulk velocity. Concerning the heat flux we find that even when in the vicinity of the sonic point the collisional mean free path of a thermal particle is is significantly smaller than the typical scales of variation of the density or the temperature, the electron heat flux cannot be described conveniently by the classical Spitzer-Harm conduction law; not even in most of the subsonic region. Indeed, in the simulations where a transonic wind forms the heat flux has been found to strongly exceed the Spitzer-Harm flux, in opposition to recently published results from multi-moment models. We emphasize that given the high coronal temperatures we use in our simulations (3 times the typical solar values) we do not expect the results presented in this report to be uncritically transposable to the case of the "real" solar wind. In particular, the quantitative aspects of our results must be handled with some care.

In the solar corona the collisional mean free path for a thermal particle (electrons or protons) is of the order of 10 2 to 10 4 times the typical scale of variation H of macroscopic quantities like the density or the temperature. Despite the relative smallness of the ratio =H, an increasingly large number of authors have become convinced that the heat flux in such a plasma cannot be described satisfactorily by theories which suppose that the local particle velocity distribution functions are close to Maxwellian. We address this question through kinetic simulations of the low solar corona by assuming that non thermal velocity distribution functions are present at the base of the corona. In particular, we show that if one assumes that the electron velocity distribution functions at the base of the corona have suciently strong suprathermal power law tails, the heat flux may flow upwards, i.e. in the direction of increasing temperature. Using kappa velocity distribution functions as prototypes for non thermal velocity distributions, we nd that the heat conduction can be properly described by the classical Spitzer &H ¨ (1953) law provided the kappa index is> 5. This value is much smaller than the value previously found by Dorelli & Scudder (1999). In addition we show that, unless extremely strong power law tails are assumed near the base of the corona (i.e.

Three-dimensional numerical simulations of the tearing instability in the framework of compressible and resistive magnetohydrodynamics are presented. Simulations have been performed with a novel Eulerian conservative high order code, including an explicit resistivity, which uses implicit high order numerical schemes having higher spectral resolution than classical schemes. The linear and non linear evolution of the tearing instability has been followed for force-free and pressure-balanced initial equilibrium configurations. Pressure equilibrium configurations are subject to a secondary instability which drives the system toward a quasi two dimensional structure oriented perpendicularly to the initial configuration. The development of secondary instabilities is suppressed by a guide field allowing the coalescence instability to fully develop in the system. Force-free initial configurations follow an intermediate path with respect the previous cases: Strong coalescence of magnetic islands, due to the non linear evolution of the tearing instability, is observed before the system enters in a phase dominated by 3D modes. The histories of the differing initial current-sheet equilibria have counterparts in the energy spectra that, for all three cases, are observed to be strongly anisotropic.

The industrial production of lactic acid (LA) is mainly based on bacterial fermentation.This process can result in enantiopure or racemic mixture according to the producing organism.Between the enantiomers, L-lactic acid shows superior market value. Recently, we reported a novelanaplerotic pathway called capnophilic lactic fermentation (CLF) that produces a high concentrationof LA by fermentation of sugar in the anaerobic thermophilic bacterium Thermotoga neapolitana.The aim of this work was theidentification of the enantiomeric characterization of the LA producedby T. neapolitana and identification of the lactate dehydrogenase in T. neapolitana (TnLDH) and relatedbacteria of the order Thermotogales. Chemical derivatization and GC/MS analysis were applied todefine the stereochemistry of LA from T. neapolitana. A bioinformatics study on TnLDH was carried out for the characterization of the enzyme. Chemical analysis showed a 95.2% enantiomeric excessof L-LA produced by T. neapolitana. A phylogenetic approach clearly clustered the TnLDH togetherwith the L-LDH from lactic acid bacteria. We report for the first time that T. neapolitana is able toproduce almost enantiopure L-lactic acid. The result was confirmed by bioinformatics analysis onTnLDH, which is a member of the L-LDH sub-family.

Given the average densities in the Solar Wind, the Coulomb mean free paths of both electrons and protons in this medium is of the order of one astronomical unit. Furthermore, and contrary to neutral gases, this mean free path is not constant with the speed of the particles but varies as the square of their energy. The Solar Wind is thus a weakly collisional plasma in which one can expect to observe velocity distribution functions (VDFs) for the particles which are not at thermal/Maxwellian equilibrium. Indeed this is what is observed for the electrons on which we will focus for this PhD Thesis. The electron VDFs actually exhibit three different components: a core, a halo and a strahl. The core electrons are well modeled by a bi-Maxwellian VDF and represent about 95% of the total number density. The non-Maxwellian suprathermal tails consist of two separate parts: the halo, present at all pitch angles, and the strahl, which appears as a beam-like population focused along the ambient magnetic field. What is the origin of these distributions? How do they evolve radially with the Solar Wind expansion? Which precise role do they play in the Solar Wind thermodynamics? These are the main questions to which we want to answer during this project. In addition we expect this PhD thesis to serve as a preparation for the data analysis of the Solar Orbiter and Solar Probe Plus missions which will make measurements in the Solar Wind as close as 60 and 9.5 Solar radii from the Sun respectively., Le libre parcours moyen coulombien des électrons et protons qui constituent le vent solaire est comparable aux hauteurs d’échelle caractéristiques du milieu : de l’ordre de la distance Soleil-Terre. De plus ce libre parcours moyen n’est pas, contrairement au cas d’un gaz neutre, constant avec la vitesse des particules mais varie comme le carré de l’énergie. Le vent solaire est donc un milieu semi-collisionnel ou l’on peut s’attendre à observer des fonctions des vitesses hors équilibre thermodynamique, c’est à dire non-Maxwelliennes, et c’est effectivement ce que l’on observe. Comment et où ces distributions sont-elles crées ? Dans la couronne solaire ou bien au cours de la propagation du vent solaire ? Le but du stage est de commencer à répondre à cette question en partant d’un travail initial sur les « modèles cinétiques dits multi-exobases » (Brandt & Cassinelli, 1966) qu’il faudra modifier afin d’inclure un modèle self-consistant du potentiel électrostatique interplanétaire. Cette thèse s’inscrit dans la préparation des missions Solar Orbiter (ESA) et Solar Probe Plus (NASA) qui seront lancées en 2018-2019 et qui exploreront l’héliosphère interne et la couronne jusqu’à une distance de 9 rayons solaire de la surface de notre étoile.