63 research outputs found
Entanglement of helicity and energy in kinetic Alfven wave/whistler turbulence
The role of magnetic helicity is investigated in kinetic Alfv\'en wave and
oblique whistler turbulence in presence of a relatively intense external
magnetic field . In this situation, turbulence is
strongly anisotropic and the fluid equations describing both regimes are the
reduced electron magnetohydrodynamics (REMHD) whose derivation, originally made
from the gyrokinetic theory, is also obtained here from compressible Hall MHD.
We use the asymptotic equations derived by Galtier \& Bhattacharjee (2003) to
study the REMHD dynamics in the weak turbulence regime. The analysis is focused
on the magnetic helicity equation for which we obtain the exact solutions: they
correspond to the entanglement relation, , where and
are the power law indices of the perpendicular (to ) wave
number magnetic energy and helicity spectra respectively. Therefore, the
spectra derived in the past from the energy equation only, namely and
, are not the unique solutions to this problem but rather
characterize the direct energy cascade. The solution is a limit
imposed by the locality condition; it is also the constant helicity flux
solution obtained heuristically. The results obtained offer a new paradigm to
understand solar wind turbulence at sub-ion scales where it is often observed
that .Comment: 26 pages, submitted to the special issue of JPP "Present achievements
and new frontiers in space plasmas
A Universal Law for Solar-Wind Turbulence at Electron Scales
The interplanetary magnetic fluctuation spectrum obeys a Kolmogorovian power
law at scales above the proton inertial length and gyroradius which is well
regarded as an inertial range. Below these scales a power law index around
is often measured and associated to nonlinear dispersive processes.
Recent observations reveal a third region at scales below the electron inertial
length. This region is characterized by a steeper spectrum that some refer to
it as the dissipation range. We investigate this range of scales in the
electron magnetohydrodynamic approximation and derive an exact and universal
law for a third-order structure function. This law can predict a magnetic
fluctuation spectrum with an index of which is in agreement with the
observed spectrum at the smallest scales. We conclude on the possible existence
of a third turbulence regime in the solar wind instead of a dissipation range
as recently postulated.Comment: 11 pages, will appear in Astrophys.
[Plasma 2020 Decadal] Disentangling the Spatiotemporal Structure of Turbulence Using Multi-Spacecraft Data
This white paper submitted for 2020 Decadal Assessment of Plasma Science concerns the importance of multi-spacecraft missions to address fundamental questions concerning plasma turbulence. Plasma turbulence is ubiquitous in the universe, and it is responsible for the transport of mass, momentum, and energy in such diverse systems as the solar corona and wind, accretion discs, planet formation, and laboratory fusion devices. Turbulence is an inherently multi-scale and multi-process phenomenon, coupling the largest scales of a system to sub-electron scales via a cascade of energy, while simultaneously generating reconnecting current layers, shocks, and a myriad of instabilities and waves. The solar wind is humankind's best resource for studying the naturally occurring turbulent plasmas that permeate the universe. Since launching our first major scientific spacecraft mission, Explorer 1, in 1958, we have made significant progress characterizing solar wind turbulence. Yet, due to the severe limitations imposed by single point measurements, we are unable to characterize sufficiently the spatial and temporal properties of the solar wind, leaving many fundamental questions about plasma turbulence unanswered. Therefore, the time has now come wherein making significant additional progress to determine the dynamical nature of solar wind turbulence requires multi-spacecraft missions spanning a wide range of scales simultaneously. A dedicated multi-spacecraft mission concurrently covering a wide range of scales in the solar wind would not only allow us to directly determine the spatial and temporal structure of plasma turbulence, but it would also mitigate the limitations that current multi-spacecraft missions face, such as non-ideal orbits for observing solar wind turbulence. Some of the fundamentally important questions that can only be addressed by in situ multipoint measurements are discussed
Analyse des signaux pour un dispositif de mesure et de stimulation du systĂšme nerveux central
- Un des enjeux actuels en Neurosciences est de pouvoir enregistrer simultanĂ©ment les activitĂ©s d'un grand nombre de cellules au sein de grands rĂ©seaux de neurones, et de pouvoir stimuler de maniĂšre dynamique ces rĂ©seaux afin d'en contrĂŽler les activitĂ©s. Le but du projet Neurocom est de rĂ©aliser un systĂšme multiĂ©lectrode haute densitĂ© intĂ©grĂ© sur silicium, permettant d'enregistrer et de stimuler de grands rĂ©seaux de neurones in vitro. Ce dispositif sera constituĂ© d'une microstructure d'Ă©lectrodes stĂ©rilisable hybridĂ©e sur un circuit analogique intĂ©grĂ© (prĂ©amplification, filtrage, multiplexage, stimulation), lui-mĂȘme interfacĂ© via une carte numĂ©rique de commande et acquisition reliĂ©e Ă un PC. Afin de pouvoir mieux apprĂ©hender les phĂ©nomĂšnes bioĂ©lectriques et Ă©lectrochimiques Ă l'interface capteur et donc mieux spĂ©cifier le cahier des charges et l'architecture du systĂšme, la maquette de test NEUROCOM1 a Ă©tĂ© conçue en Ă©lectronique discrĂšte et est actuellement utilisĂ©e pour conduire diffĂ©rents tests
Role of mprF1 and mprF2 in the Pathogenicity of Enterococcus faecalis
Aujourd hui, Enterococcus faecalis est considĂ©rĂ© comme l un des plus importants agents pathogĂšnes causant des maladies nosocomiales. En raison de sa rĂ©sistance innĂ©e et acquise aux antibiotiques, l identification de nouvelles cibles pour le traitement de cette bactĂ©rie est une grande prioritĂ©. Le facteur Multiple Peptide RĂ©sistance (MprF), qui a Ă©tĂ© dĂ©crit en premier chez Staphylococcus aureus, modifie le phosphatidylglycĂ©rol avec de la lysine et rĂ©duit ainsi la charge nĂ©gative de l enveloppe cellulaire. Ceci a comme consĂ©quence d augmenter la rĂ©sistance aux peptides antimicrobiens cationiques (PAC). Deux gĂšnes paralogues putatifs (mprF1 et mprF2) ont Ă©tĂ© identifiĂ©s chez E. faecalis par recherche BLAST en utilisant le gĂšne dĂ©crit chez S. aureus. Une caractĂ©risation de ces deux gĂšnes d E. faecalis ainsi que des mĂ©canismes conduisant Ă une rĂ©sistance aux PAC, pourrait aider Ă dĂ©velopper des nouvelles stratĂ©gies thĂ©rapeutiques contre ce pathogĂšne. Deux mutants de dĂ©lĂ©tion et un double mutant ont Ă©tĂ© construits par recombinaison homologue chez E. faecalis. L analyse des phospholipides des membranes cytoplasmiques des deux mutants mprF1 et mprF2 par chromatographie sur couche mince a montrĂ© que seule l inactivation de mprF2 inhibe la synthĂšse de trois amino-phosphatidlyglycĂ©rol distincts (comme la Lysine-PG, l Alanine-PG et l Arginine-PG). De plus, le mutant mprF2 est Ă©galement plus sensible aux PAC que la souche sauvage. La capacitĂ© de formation d un biofilm est gĂ©nĂ©ralement considĂ©rĂ©e comme un facteur important de virulence, ce qui est Ă©galement le cas pour les entĂ©rocoques. Le mutant mprF2 montre une capacitĂ© accrue dans ce phĂ©nomĂšne. Ceci semble ĂȘtre du Ă une augmentation de la concentration d ADN extracellulaire dans le biofilm formĂ© par ce mutant. Curieusement, cette augmentation est indĂ©pendante d une autolyse. Le mutant mprF2 est Ă©galement plus rĂ©sistant Ă l opsonophagocytose. Cependant, le gĂšne mprF2 ne joue aucun rĂŽle dans les bactĂ©riĂ©mies de souris et les endocardites de rats.En revanche, aucun phĂ©notype n a Ă©tĂ© trouvĂ© pour un mutant mprF1 jusqu Ă prĂ©sent. Cette mutation ne modifie ni la synthĂšse de l aminoacyl-PG en condition de laboratoire ni la rĂ©sistance aux PAC et Ă l opsonophagocytose. Par consĂ©quent, il semble que mprF2 soit le seul gĂšne mprF fonctionnel chez E. faecalis. NĂ©anmoins, contrairement Ă d autres bactĂ©ries, mprF2 ne semble pas ĂȘtre un facteur de virulence majeur pour cette espĂšce.Enterococcus faecalis is regarded nowadays as one of the most important nosocomial pathogens. Due to its innate and acquired resistance to antibiotics, identification of new targets for antimicrobial treatment of E. faecalis is a high priority. The multiple peptides resistance factor (MprF), which was first described in Staphylococcus aureus, modifies phosphatidylglycerol with lysine and reduces the negative charge of the membrane, thus increasing resistance to cationic antimicrobial peptides (CAMPs). Two putative mprF paralogs (mprF1 and mprF2) were identified in E. faecalis by Blast search using the well-described S. aureus gene as a lead. A better understanding of these two genes and mechanisms leads to enterococcal resistance to CAMPs might help designing therapeutic strategies against this bacteria. Two single deletion mutants and double mutant in E. faecalis were created by homologues recombination. Analysis of cell membrane phospholipids from both mutants by thin-layer chromatography showed that inactivation of mprF2 abolished the synthesis of three distinct amino-phosphatidylglycerol (mostly likely Lysin-PG, Alanine-PG and Argine-PG). The CAMPs testing assay demonstrated that the deletion mutant of mprF2 was more susceptible to CAMPs than the wild type. Biofilm formation is usually regarded as a virulence factor which provides an important way for enterococci to cause infections. Inactivation of mprF2 led to increase the biofilm formation which we showed that it was due to the accumulation of eDNA in the biofilm, but the release of eDNA is independent from autolysis. The mprF2 mutant was resistance to killing by opsonophagocytosis more than wild type. However, the mprF2 gene plays no role in bacteremia in mice and rat endocarditis. Our results showed that non polar effect mprF1 mutant does not affect in the synthesis of aminoacyl-PG in the laboratory condition. It also has no effect on susceptible to CAMPs, opsonic killing and autolysis. Therefore, it seems that mprF2 is the only functional mprF gene in E. faecalis in the laboratory condition. Unlike mprF found in other bacteria, mprF does not seem to be a major virulence factor in enterococci.CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF
Contribution of plasmid-encoded peptidase S8 (PrtP) to adhesion and transit in the gut of Lactococcus lactis IBB477 strain
Fluidization of collisionless plasma turbulence
In a collisionless, magnetized plasma, particles may stream freely along magnetic field lines, leading to âphase mixingâ of their distribution function and consequently, to smoothing out of any âcompressiveâ fluctuations (of density, pressure, etc.). This rapid mixing underlies Landau damping of these fluctuations in a quiescent plasmaâone of the most fundamental physical phenomena that makes plasma different from a conventional fluid. Nevertheless, broad power law spectra of compressive fluctuations are observed in turbulent astrophysical plasmas (most vividly, in the solar wind) under conditions conducive to strong Landau damping. Elsewhere in nature, such spectra are normally associated with fluid turbulence, where energy cannot be dissipated in the inertial-scale range and is, therefore, cascaded from large scales to small. By direct numerical simulations and theoretical arguments, it is shown here that turbulence of compressive fluctuations in collisionless plasmas strongly resembles one in a collisional fluid and does have broad power law spectra. This âfluidizationâ of collisionless plasmas occurs, because phase mixing is strongly suppressed on average by âstochastic echoes,â arising due to nonlinear advection of the particle distribution by turbulent motions. Other than resolving the long-standing puzzle of observed compressive fluctuations in the solar wind, our results suggest a conceptual shift for understanding kinetic plasma turbulence generally: rather than being a system where Landau damping plays the role of dissipation, a collisionless plasma is effectively dissipationless, except at very small scales. The universality of âfluidâ turbulence physics is thus reaffirmed even for a kinetic, collisionless system
On the violation of the zeroth law of turbulence in space plasmas
The zeroth law of turbulence states that, for fixed energy input into large-scale motions, the statistical steady state of a turbulent system is independent of microphysical dissipation properties. This behaviour, which is fundamental to nearly all fluid-like systems from industrial processes to galaxies, occurs because nonlinear processes generate smaller and smaller scales in the flow, until the dissipation â no matter how small â can thermalise the energy input. Using direct numerical simulations and theoretical arguments, we show that in strongly magnetised plasma turbulence such as that recently observed by the Parker Solar Probe spacecraft, the zeroth law is routinely violated. Namely, when such turbulence is âimbalancedâ â when the large-scale energy input is dominated by AlfvĂ©nic perturbations propagating in one direction (the most common situation in space plasmas) â nonlinear conservation laws imply the existence of a âbarrierâ at scales near the ion gyroradius. This causes energy to build up over time at large scales. The resulting magnetic-energy spectra bear a strong resemblance to those observed in situ, exhibiting a sharp, steep kinetic transition range above and around the ion-Larmor scale, with flattening at yet smaller scales. The effect thus offers a possible solution to the decade-long puzzle of the position and variability of ion-kinetic spectral breaks in plasma turbulence. The existence of the âbarrierâ also suggests that, how a plasma is forced at large scales (the imbalance) may have a crucial influence on thermodynamic properties such as the ion-to-electron heating ratio
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