Theoretical understanding of chromospheric inhomogeneities

Detailed theoretical studies of chromospheric inhomogeneities consider dynamics as well as radiative transfer of mass flow as a consequence of energy deposition. It is shown that pressure is exerted by the heating waves, especially in inhomogeneous structures, where they can be defracted. A dynamical model is formulated that depicts the inhomogeneous structure of the chromosphere-corona transition region through mass flow regimes

Resonance scattering at lyman-alpha by an atomic hydrogen cell

Hydrogen cell and ion chamber for obtaining photoelectric data on resonance scattering at lyman alpha lin

A numerical parametric study of the mechanical action of pulsatile blood flow onto axisymmetric stenosed arteries

International audienceIn the present paper, a fluid-structure interaction model is developed, questioning how the mechanical action of the blood onto an atheromatous plaque is affected by the length and the severity of the stenosis. An axisymmetric model is considered. The fluid is assumed Newtonian. The plaque is modelled as a heterogeneous hyperelastic anisotropic solid composed of the arterial wall, the lipid core and the fibrous cap. Transient velocity and pressure conditions of actual pulsatile blood flow are prescribed. The simulation is achieved using the Arbitrary Lagrangian Eulerian scheme in the COMSOL commercial Finite Element package. The results reveal different types of behavior in function of the length (denoted L) and severity (denoted S) of the stenosis. Whereas large plaques (L > 10 mm) are mostly deformed under the action of the blood pressure, it appears that shorter plaques (L < 10 mm) are significantly affected by the shear stresses. The shear stresses tend to deform the plaque by pinching it. This effect is called: "the pinching effect". It has an essential influence on the mechanical response of the plaque. For two plaques having the same radius severity S = 45%, the maximum stress in the fibrous cap is 50% larger for the short plaque (L = 5 mm) than for a larger plaque (L = 10 mm), and the maximum wall shear stress is increased by 100%. Provided that they are confirmed by experimental investigations, these results may offer some new perspectives for understanding the vulnerability of short plaques

On the two-dimensionalization of quasistatic magnetohydrodynamic turbulence

We analyze the anisotropy of turbulence in an electrically conducting fluid in the presence of a uniform magnetic field, for low magnetic Reynolds number, using the quasi-static approximation. In the linear limit, the kinetic energy of velocity components normal to the magnetic field decays faster than the kinetic energy of component along the magnetic field [Moffatt, JFM 28, 1967]. However, numerous numerical studies predict a different behaviour, wherein the final state is characterized by dominant horizontal energy. We investigate the corresponding nonlinear phenomenon using Direct Numerical Simulations. The initial temporal evolution of the decaying flow indicates that the turbulence is very similar to the so-called "two-and-a-half-dimensional" flow [Montgomery & Turner, Phys. Fluids 25(2), 1982] and we offer an explanation for the dominance of horizontal kinetic energy.Comment: 17 pages, 8 figure

Update on g-mode research

Since the beginning of this century we have attended a blooming of the gravity-mode research thanks to the unprecedented quality of the data available, either from space with SoHO, or from the ground-based networks as BiSON or GONG. From the first upper limit of the gravity-mode amplitudes fixed at 10 mm/s at 200 microHz given by Appourchaux et al. (2000), on one hand, a peak was supposed to be a component of the l=1, n=1 mixed mode (Garcia et al. 2001a, b; Gabriel et al. 2002) and, on the other hand, a couple of patterns --multiplets-- were attributed to gravity modes (Turck-Chieze et al. 2004; Mathur et al. 2007). One of these patterns, found around 220 microHz, could be labeled as the l=2, n =-3 g mode, which is expected to be the one with the highest surface amplitude (Cox and Guzik 2004). Finally, in 2007, Garcia et al. were able to measure the fingertips of the dipole gravity modes looking for their asymptotic properties. In the present paper we present an update of the recent developments on this subject with special attention to the 220 microHz region, the dipole asymptotic properties and the impact of the incoming g-mode observations on the knowledge of the solar structure and rotation profile.Comment: Paper accepted for publication in Astronomische Nachrichten. 9 pages, 8 figure

Mélange irréversible et aspect énergétique de la turbulence stratifié

L'efficacité du mélange turbulent dans les fluides stratifiés reste un problème largement ouvert, aux nombreuses applications géophysiques et industrielles. Ce problème peut être simplement formulé: pour une certaine quantité d'énergie injecté dans un écoulement, quelle partie est utilisée pour mélanger le champ de densité de manière irréversible? Dans ce contexte, nous avons réalisé un ensemble de simulations numériques directes (DNS) d'un fluide stratifié turbulent 3D en résolvant l'équation de Navier-Stokes avec approximation de Boussinesq. Une méthode standard pseudo-spectrale (Fourier) est utilisée avec 1024^3 points. Une zone poreuse de pénalisation est introduite pour prendre en compte les conditions aux limites de non flux et de vitesse nulle, en haut et en bas de la boîte alors que le plan horizontal est périodique. Un champ de vitesse turbulente est imposé initialement, il perturbe le champ de flottabilité initialement stable : l'écoulement stratifié est dans un état de turbulence décroissante. La simulation est effectuée sur ~ 48 temps de retournement initial t = L / U (où L et U sont respectivement l'échelle de longueur intégrale et la vitesse moyenne des fluctuations turbulentes initiales). Les paramètres de la simulation sont choisis pour fixer le nombre de Reynolds à Re = 1000 et nous avons fait varier le nombre de Richardson sur une large gamme de valeurs : Ri = 1,4,64,256,1024,4096. Selon Winters et al. 1995, il est possible de distinguer : - l'énergie cinétique Ec - l'énergie potentielle Ep associée au champ plein de flottabilité - l'énergie potentielle de fond Eb associée au champ de flottabilité trié verticalement - l'énergie potentielle disponible Ea = Ep - Eb    Nous avons pu obtenir un bilan d'énergie impliquant ces quantités ainsi que leur transfert au cours du temps. Le transfert de l'énergie cinétique Ex vers l'énergie potentielle Ep sature au bout d'un certains de temps de retournement lié au nombre de Richardson. Cette saturation est associée à un maximum d'énergie disponible Ea, alors que l'énergie potentielle de fond Eb croit de manière monotone mais plus rapidement lorque le nombre de Richardson est élevé (??).  Nous avons ensuite calculé l'efficacité de mélange, définie comme le rapport entre la quantité d'énergie de fond (Eb) dissipée au cours de la simulation et la quantité totale d'énergie injecté. Récemment, une approche issue de la mécanique statistique a été proposée afin de modéliser les variations de cette efficacité de mélange en fonction du nombre de Richardson (Venaille et al. 2016). Nous obtenons une très bonne concordance entre les résultats de DNS et les prédictions théoriques

Structuration de la convection mixte en milieu poreux confiné latéralement et chauffé par le bas : effets d'inertie

Nous étudions la naissance de la convection dans un milieu poreux chauffé par le bas en présence d'un écoulement horizontal, et plus particulièrement l'influence de l'inertie poreuse et du rapport de forme transversal a du milieu. Nous montrons que l'état de conduction est déstabilisé au profit de rouleaux longitudinaux fixes (R.L) si a est entier et au profit de rouleaux propagatifs purement transversaux (R.T) si a est inférieur à une valeur limite ac <1. Pour a>ac et non entier, la convection naît sous la forme de structures tridimensionnelles (3D) oscillatoires pour a>1 ou sous la forme de R.T pour acReK*

Solar cycle variation in solar f-mode frequencies and radius

Using data from the Global Oscillation Network Group (GONG) covering the period from 1995 to 1998, we study the change with solar activity in solar f-mode frequencies. The results are compared with similar changes detected from the Michelson Doppler Imager (MDI) data. We find variations in f-mode frequencies which are correlated with solar activity indices. If these changes are due to variation in solar radius then the implications are that the solar radius decreases by about 5 km from minimum to maximum activity.Comment: To appear in Solar Physic

Antiretroviral Treatment Start-Time during Primary SIVmac Infection in Macaques Exerts a Different Impact on Early Viral Replication and Dissemination

BACKGROUND: The time of infection is rarely known in human cases; thus, the effects of delaying the initiation of antiretroviral therapy (ART) on the peripheral viral load and the establishment of viral reservoirs are poorly understood. METHODOLOGY/PRINCIPAL FINDINGS: Six groups of macaques, infected intravenously with SIV(mac251), were given placebo or antiretroviral therapy to explore reservoir establishment; macaques were treated for 2 weeks, with treatment starting 4 hours, 7 or 14 days after infection. Viral replication and dissemination were measured in the gut (rectum), in the lung and in blood and lymphoid tissues (peripheral lymph nodes), by quantifying viral RNA, DNA and 2LTR circles. We used immunohistochemistry (CD4 and CD68) to assess the impact of these treatments on the relative amount of virus target cells in tissue. Treatment that was started 4 hours post-infection (pi) decreased viral replication and dissemination in blood and tissue samples, which were assessed on day 14 (RNA/DNA/2LTR circles). The virus remained detectable and lymphoid tissues were activated in LN and the gut in both placebo- and ART-treated animals. Viral RNA in plasma continued to be lower in macaques treated seven days after infection; however, this was not the case for viral DNA in peripheral blood mononuclear cells. There was a small but significant difference in RNA and DNA levels in tissues between placebo- and ART-treated animals on day 21. When started 14 days after infection, treatment resulted in a limited decrease in the plasma viral load. CONCLUSIONS: Treatment that was started 4 hours after infection significantly reduced viral replication and dissemination. When started 7 days after infection, it was of slight virological benefit in peripheral blood and in tissues, and treatment was even less effective if started 14 days pi. These data favor starting ART no longer than one week after intravenous SIV(mac251) exposure