Quasi-static magnetohydrodynamic turbulence at high Reynolds number

We analyse the anisotropy of homogeneous turbulence in an electrically conducting fluid submitted to a uniform magnetic field, for low magnetic Reynolds number, in the quasi- static approximation. We interpret disagreeing previous predictions between linearized theory and simulations: in the linear limit, the kinetic energy of transverse velocity components, normal to the magnetic field, decays faster than the kinetic energy of the axial component, along the magnetic field (Moffatt (1967)); whereas many numerical studies predict a final state characterised by dominant energy of transverse velocity components. We investigate the corresponding nonlinear phenomenon using Direct Numerical Simulations of freely-decaying turbulence, and a two-point statistical spectral closure based on the Eddy Damped Quasi-Normal Markovian model. The transition from the three-dimensional turbulent flow to a "two-and-a-half-dimensional" flow (Montgomery & Turner (1982)) is a result of the combined effects of short-time linear Joule dissipation and longer time nonlinear creation of polarisation anisotropy. It is this combination of linear and nonlinear effects which explains the disagreement between predictions from linearized theory and results from numerical simulations. The transition is characterized by the elongation of turbulent structures along the applied magnetic field, and by the strong anisotropy of directional two-point correlation spectra, in agreement with experimental evidence. Inertial equatorial transfers in both DNS and the model are presented to describe in detail the most important equilibrium dynamics. Spectral scalings are maintained in high Reynolds number turbulence attainable only with the EDQNM model, which also provides simplified modelling of the asymptotic state of quasi-static MHD turbulence.Comment: Journal of Fluid Mechanics, 201

Interactions Between Solar Neutrinos and Solar Magnetic Fields

We attempt to correlate all of the available solar-neutrino capture-rate data with the strong magnetic fields these neutrinos encounter in the solar interior along their Earth-bound path. We approximate these fields using the (photospheric, magnetograph-measured) surface magnetic flux from central latitude bands, time delayed to proxy the solar interior. Our strongest evidence for anticorrelation is for magnetic fields within the central $\pm 5^{0}$ solar-latitude band that have been delayed by 0.34 years. Assuming a neutrino-magnetic interaction, this might indicate that interior fields travel to the solar surface in this period of time. As more solar-neutrino flux information is gathered, the question of whether such evidence is physical or statistical in nature should be resolved, providing that new data span enough solar cycles and that correlation studies focus on these isolated fields.Comment: 17 pages, (+ 5 ps figs + aasms4.sty attached, uuencoded) Submitted to AP

The quest for the solar g modes

Solar gravity modes (or g modes) -- oscillations of the solar interior for which buoyancy acts as the restoring force -- have the potential to provide unprecedented inference on the structure and dynamics of the solar core, inference that is not possible with the well observed acoustic modes (or p modes). The high amplitude of the g-mode eigenfunctions in the core and the evanesence of the modes in the convection zone make the modes particularly sensitive to the physical and dynamical conditions in the core. Owing to the existence of the convection zone, the g modes have very low amplitudes at photospheric levels, which makes the modes extremely hard to detect. In this paper, we review the current state of play regarding attempts to detect g modes. We review the theory of g modes, including theoretical estimation of the g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the techniques that have been used to try to detect g modes. We review results in the literature, and finish by looking to the future, and the potential advances that can be made -- from both data and data-analysis perspectives -- to give unambiguous detections of individual g modes. The review ends by concluding that, at the time of writing, there is indeed a consensus amongst the authors that there is currently no undisputed detection of solar g modes.Comment: 71 pages, 18 figures, accepted by Astronomy and Astrophysics Revie

Perspectives in Global Helioseismology, and the Road Ahead

We review the impact of global helioseismology on key questions concerning the internal structure and dynamics of the Sun, and consider the exciting challenges the field faces as it enters a fourth decade of science exploitation. We do so with an eye on the past, looking at the perspectives global helioseismology offered in its earlier phases, in particular the mid-to-late 1970s and the 1980s. We look at how modern, higher-quality, longer datasets coupled with new developments in analysis, have altered, refined, and changed some of those perspectives, and opened others that were not previously available for study. We finish by discussing outstanding challenges and questions for the field.Comment: Invited review; to appear in Solar Physics (24 pages, 6 figures

Développement d'un modèle pré-clinique à l'infection du Chikungunya

National audienceContexte, objectifs: La compréhension de la maladie à Chikungunya qui se caractérise par l'induction de douleur articulaire et musculaire chronique à long terme. Le laboratoire a développé un modèle d'infection expérimentale par le virus du Chikungunya chez le macaque cynomolgus afin d'explorer celle-ci à moyen et long terme. Résultats: Ce modèle reproduit lors de l'infection aigüe les signes cliniques et un profil d'évolution des paramètres virologiques et immunologiques proches de l'infection humaine. Nous avons identifié les sites de réplication virale et mis en évidence la persitance du virus dans les tissus ce qui permet pour la première fois de donner un début d'explication à la pathologie à long terme. Nous avons utilisé ce modèle pour évaluer une méthode prophylactique proposé lors de l'épidémie de la Réunion en 2006. Conclusion: Associé aux essais sur des modèles murins représentatifs de l'infection en phase aiguë, l'utilisation des primates non-humain est indispensable pour valider en préclinique des stratégies vaccinales ou prophylactiques s'appliquant à la période chronique de l'infection