Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow

The magnetorotational (MRI) dynamo has long been considered one of the possible drivers of turbulent angular momentum transport in astrophysical accretion disks. However, various numerical results suggest that this dynamo may be difficult to excite in the astrophysically relevant regime of magnetic Prandtl number (Pm) significantly smaller than unity, for reasons currently not well understood. The aim of this article is to present the first results of an ongoing numerical investigation of the role of both linear and nonlinear dissipative effects in this problem. Combining a parametric exploration and an energy analysis of incompressible nonlinear MRI dynamo cycles representative of the transitional dynamics in large aspect ratio shearing boxes, we find that turbulent magnetic diffusion makes the excitation and sustainment of this dynamo at moderate magnetic Reynolds number (Rm) increasingly difficult for decreasing Pm. This results in an increase in the critical Rm of the dynamo for increasing kinematic Reynolds number (Re), in agreement with earlier numerical results. Given its very generic nature, we argue that turbulent magnetic diffusion could be an important determinant of MRI dynamo excitation in disks, and may also limit the efficiency of angular momentum transport by MRI turbulence in low Pm regimes.Comment: 7 pages, 6 figure

Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?

In Keplerian accretion disks, turbulence and magnetic fields may be jointly excited through a subcritical dynamo process involving the magnetorotational instability (MRI). High-resolution simulations exhibit a tendency towards statistical self-organization of MRI dynamo turbulence into large-scale cyclic dynamics. Understanding the physical origin of these structures, and whether they can be sustained and transport angular momentum efficiently in astrophysical conditions, represents a significant theoretical challenge. The discovery of simple periodic nonlinear MRI dynamo solutions has recently proven useful in this respect, and has notably served to highlight the role of turbulent magnetic diffusion in the seeming decay of the dynamics at low magnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), a common regime in accretion disks. The connection between these simple structures and the statistical organization reported in turbulent simulations remained elusive, though. Here, we report the numerical discovery in moderate aspect ratio Keplerian shearing boxes of new periodic, incompressible, three-dimensional nonlinear MRI dynamo solutions with a larger dynamical complexity reminiscent of such simulations. These "chimera" cycles are characterized by multiple MRI-unstable dynamical stages, but their basic physical principles of self-sustainment are nevertheless identical to those of simpler cycles found in azimuthally elongated boxes. In particular, we find that they are not sustained at low Pm either due to subcritical turbulent magnetic diffusion. These solutions offer a new perspective into the transition from laminar to turbulent instability-driven dynamos, and may prove useful to devise improved statistical models of turbulent accretion disk dynamos.Comment: 12 pages, 8 figures, submitted to A&

Stability analysis of secondary modes, driven by the phase space island

We present a new theoretical approach, based on the Hamiltonian formalism, to investigate the stability of islands in phase space, generated by trapping of energetic particles (EPs) in plasma waves in a tokamak. This approach is relevant to MHD modes driven by EPs (EP-MHD) such as toroidal Alfvén eigenmodes (TAEs), EP-driven geodesic acoustic modes (EGAMs) or fishbones. A generic problem of a single isolated EP-MHD mode is equivalent to and hence can be replaced by a 2D Hamiltonian dynamics in the vicinity of the phase space island. The conventional Langmuir wave/bump-on-tail problem is then used as a representative reduced model to describe the dynamics of the initial EP-MHD. Solving the Fokker-Planck equation in the presence of pitch angle scattering, velocity space diffusion and drag and retaining plasma drifts in a model, we find a 'perturbed' equilibrium, associated with these phase space islands. Its stability is then explored by addressing the Vlasov/Fokker-Planck-Poisson system. The Lagrangian of this system provides the dispersion relation of the secondary modes and allows an estimate of the mode onset. The secondary instabilities have been confirmed to be possible but under certain conditions on the primary island width and in a certain range of mode numbers. The threshold island width, below which the mode stability is reached, is calculated. The secondary mode growth rate is found to be maximum when the associated resonant velocity approaches the boundary of the primary island. This, in turn, leads to a conclusion that the onset of the secondary mode can be prevented provided the primary wave number is the lowest available

Effects of agonists of peroxisome proliferator-activated receptor γ on proteoglycan degradation and matrix metalloproteinase production in rat cartilage in vitro

AbstractObjective To examine the effects of agonists of peroxisome proliferator-activated receptor (PPAR) γ on proteoglycan degradation induced by interleukin (IL)-1β or tumor necrosis factor (TNF)α in cartilage in vitro.Design Proteoglycan degradation was measured as release of radioactivity from rat cartilage explants previously labeled with 35SO2−4. Western blots were used to examine tissue levels of aggrecan neoepitopes NITEGE and VDIPEN, generated by aggrecanases and matrix metalloproteinases (MMP), respectively. Production of MMP-2, -3 and -9 by cultured rat chondrocytes was measured by zymography and by fluorimetric assay.Results IL-1β-induced proteoglycan degradation was likely due to aggrecanase, since it was associated with a strong increase of NITEGE signal. MMP-dependent VDIPEN signal increased only after further incubation with pro-MMP activator APMA. PPAR agonists 15d-PGJ2 and GI262570 (10μM) inhibited IL-1β- and TNFα-induced proteoglycan degradation measured both before and after addition of APMA. The agonists also inhibited cytokine-induced MMP production by isolated chondrocytes.Conclusion This study shows that PPARγ agonists inhibit cytokine-induced proteoglycan degradation mediated by both aggrecanase and MMP. This effect is associated with inhibition of production of MMP-3 and -9. These results support the interest for PPARγ agonists as candidate inhibitors of pathological cartilage degradation. Copyright 2002 OsteoArthritis Research Society International. Published by Elsevier Science Ltd. All rights reserved

The subjective experience of young women with non-metastatic breast cancer: the Young Women with Breast Cancer Inventory

International audienceBackground: The subjective experience of young women with breast cancer has some particular features linked to the impact of the disease and its treatment on their age-related issues (e.g. desire for a child, couple relationship, career management). Despite these specific concerns, no questionnaire currently targets the young breast cancer patient's quality of life, subjective experience or common problems when facing cancer. This study presents the psychometric validation of an inventory that aimed to measure the impact of breast cancer on the quality of life of young women (<45 years of age) with non-metastatic disease. Methods: 546 women aged <45 years when diagnosed with a non-metastatic breast cancer were recruited in 27 French cancer research and treatment centers. They answered a self-reported questionnaire created from verbatim collected by non-directive interviews carried out with 69 patients in a first qualitative study. Exploratory and confirmatory analyses were conducted in order to obtain the final structure of the scale. Internal consistency, test-retest reliability and concurrent validity with quality of life questionnaires currently used (QLQ-C30 and the QLQ-BR23 module) were then assessed. Results: The YW-BCI36 contains 36 items and highlights 8 factors: 1) feeling of couple cohesion, 2) negative affectivity and apprehension about the future, 3) management of child(ren) and of everyday life, 4) sharing with close relatives, 5) body image and sexuality, 6) financial difficulties, 7) deterioration of relationships with close relatives, and 8) career management. Psychometric analyses indicated good internal consistency (Cronbach's alpha values ranging from 0.76 to 0.91) and temporal reliability (Bravais-Pearson correlations ranging from 0.66 to 0.85). As expected, there were quite strong correlations between the YW-BCI36 and the QLQ-C30 and QLQ-BR23 scores (r ranging from 0.20 to -0.66), indicating adequate concurrent validity. Conclusions: The YW-BCI36 was confirmed as a valid scale for evaluating the subjective experience of breast cancer in young women. This instrument could help to identify the problems of these women more precisely, in order to respond to them better by an optimal care management. This scale may improve the medical, psychological and social care of breast cancer patients

A comparison of local simulations and reduced models of MRI-induced turbulence

We run mean-field shearing-box numerical simulations with a temperature-dependent resistivity and compare them to a reduced dynamical model. Our simulations reveal the co-existence of two quasi-steady states, a `quiet' state and an `active' turbulent state, confirming the predictions of the reduced model. The initial conditions determine on which state the simulation ultimately settles. The active state is strongly influenced by the geometry of the computational box and the thermal properties of the gas. Cubic domains support permanent channel flows, bar-shaped domains exhibit eruptive behaviour, and horizontal slabs give rise to infrequent channels. Meanwhile, longer cooling time-scales lead to higher saturation amplitudes.Comment: MNRAS accepted, 9 pages, 11 figure