Numerical simulations of the magnetorotational instability (MRI) with zero initial net flux in a non-stratified isothermal cubic domain are used to demonstrate the importance of magnetic boundary ...conditions. In fully periodic systems, the level of turbulence generated by the MRI strongly decreases as the magnetic Prandtl number (Pm), which is the ratio of kinematic viscosity and magnetic diffusion, is decreased. No MRI or dynamo action below Pm = 1 is found, agreeing with earlier investigations. Using vertical field conditions, which allow the generation of a net toroidal flux and magnetic helicity fluxes out of the system, the MRI is found to be excited in the range 0.1 ≤ Pm ≤ 10, and the saturation level is independent of Pm. In the vertical field runs, strong mean-field dynamo develops and helps to sustain the MRI.
Aims. To study the existence of large-scale convective dynamos under the influence of shear and rotation. Methods. Three-dimensional numerical simulations of penetrative compressible convection with ...uniform horizontal shear are used to study dynamo action and the generation of large-scale magnetic fields. We consider cases where the magnetic Reynolds number is either marginal or moderately supercritical with respect to small-scale dynamo action in the absence of shear and rotation. Our magnetic Reynolds number is based on the wavenumber of the depth of the convectively unstable layer. The effects of magnetic helicity fluxes are studied by comparing results for the magnetic field with open and closed boundaries. Results. Without shear no large-scale dynamos are found even if the ingredients necessary for the α-effect (rotation and stratification) are present in the system. When uniform horizontal shear is added, a large-scale magnetic field develops, provided the boundaries are open. In this case the mean magnetic field contains a significant fraction of the total field. For those runs where the magnetic Reynolds number is between 60 and 250, an additional small-scale dynamo is expected to be excited, but the field distribution is found to be similar to cases with smaller magnetic Reynolds number where the small-scale dynamo is not excited. In the case of closed (perfectly conducting) boundaries, magnetic helicity fluxes are suppressed and no large-scale fields are found. Similarly, poor large-scale field development is seen when vertical shear is used in combination with periodic boundary conditions in the horizontal directions. If, however, open (normal-field) boundary conditions are used in the x-direction, a large-scale field develops. These results support the notion that shear not only helps to generate the field, but it also plays a crucial role in driving magnetic helicity fluxes out of the system along the isocontours of shear, thereby allowing efficient dynamo action.
Summary
Background
Human rhinovirus (HRV) infections are a major cause of exacerbations in chronic respiratory conditions such as asthma and chronic obstructive pulmonary disease, but HRV‐induced ...immune responses of the lower airway are poorly understood. Earlier work examining cytokine release following HRV infection has focused on epithelial cells because they serve as the principal site of viral replication, and internalization and replication of viral RNA appear necessary for epithelial cell mediator release. However, during HRV infection, only a small proportion of epithelial cells become infected. As HRV‐induced cytokine levels in vivo are markedly elevated, this observation suggests that other mechanisms independent of direct viral infection may induce epithelial cell cytokine release.
Objective
Our aim was to test for the importance of interactions between human bronchial epithelial cells (HBECs) and monocytic cells in the control of mediator release during HRV exposure.
Methods
In vitro models of HRV serotype‐16 (HRV16) infection of primary HBECs and human monocytic cells, in mono or co‐culture, were used. We assessed HRV16‐induced CXCL10 and CCL2 protein release via ELISA.
Results
Co‐culture of human monocytic and bronchial epithelial cells promoted a synergistic augmentation of CXCL10 and CCL2 protein release following HRV16 challenge. Transfer of conditioned media from HRV16‐treated monocytic cells to epithelial cultures induced a robust release of CXCL10 by the epithelial cells. This effect was greatly attenuated by type I IFN receptor blocking antibodies, and could be recapitulated by IFN‐α addition.
Conclusions
Our data indicate that epithelial CXCL10 release during HRV infection is augmented by a monocytic cell‐dependent mechanism involving type I IFN(s). Our findings support a key role for monocytic cells in the amplification of epithelial cell chemokine production during HRV infection, and help to explain how an inflammatory milieu is created in the lower airways even in the absence of extensive viral replication and epithelial infection.
Cite this as: N. L. Korpi‐Steiner, S. M. Valkenaar, M. E. Bates, M. D. Evans, J. E. Gern and P. J. Bertics, Clinical & Experimental Allergy, 2010 (40) 1203–1213.
Aims. We study turbulent transport coefficients that describe the evolution of large-scale magnetic fields in turbulent convection. Methods. We use the test field method, together with ...three-dimensional numerical simulations of turbulent convection with shear and rotation, to compute turbulent transport coefficients describing the evolution of large-scale magnetic fields in mean-field theory in the kinematic regime. We employ one-dimensional mean-field models with the derived turbulent transport coefficients to examine whether they give results that are compatible with direct simulations. Results. The results for the α-effect as a function of rotation rate are consistent with earlier numerical studies, i.e. increasing magnitude as rotation increases and approximately $\cos \theta$ latitude profile for moderate rotation. Turbulent diffusivity, $\eta_{\rm t}$, is proportional to the square of the turbulent vertical velocity in all cases. Whereas $\eta_{\rm t}$ decreases approximately inversely proportional to the wavenumber of the field, the α-effect and turbulent pumping show a more complex behaviour with partial or full sign changes and the magnitude staying roughly constant. In the presence of shear and no rotation, a weak α-effect is induced which does not seem to show any consistent trend as a function of shear rate. Provided that the shear is large enough, this small α-effect is able to excite a dynamo in the mean-field model. The coefficient responsible for driving the shear-current effect shows several sign changes as a function of depth but is also able to contribute to dynamo action in the mean-field model. The growth rates in these cases are, however, well below those in direct simulations, suggesting that an incoherent α-shear dynamo may also act in the simulations. If both rotation and shear are present, the α-effect is more pronounced. At the same time, the combination of the shear-current and ${\Omega}\times{ J}$-effects is also stronger than in the case of shear alone, but subdominant to the α-shear dynamo. The results of direct simulations are consistent with mean-field models where all of these effects are taken into account without the need to invoke incoherent effects.
Aims. The alpha- and gamma-effects, which are responsible for the generation and turbulent pumping of large scale magnetic fields, respectively, due to passive advection by convection are determined ...in the rapid rotation regime corresponding to the deep layers of the solar convection zone. Methods. A 3D rectangular local model is used for solving the full set of MHD equations in order to compute the electromotive force (emf), vec{\mathcal = \overline{\vec{u} \times \vec{b}}, generated by the interaction of imposed weak gradient-free magnetic fields and turbulent convection with varying rotational influence and latitude. By expanding the emf in terms of the mean magnetic field, mathcal{E}_i = a_{ij} \overline{B}_j, all nine components of a sub( )ijare computed. The diagonal elements of a sub( )ijdescribe the alpha-effect, whereas the off-diagonals represent magnetic pumping. The latter is essentially the advection of magnetic fields by means other than the underlying large-scale velocity field. Comparisons are made to analytical expressions of the coefficients derived under the first-order smoothing approximation (FOSA). Results. In the rapid rotation regime the latitudinal dependence of the alpha-components responsible for the generation of the azimuthal and radial fields does not exhibit a peak at the poles, as is the case for slow rotation, but at a latitude of about 30degr. The magnetic pumping is predominantly radially down- and latitudinally equatorward as in earlier studies. The numerical results compare surprisingly well with analytical expressions derived under first-order smoothing, although the present calculations are expected to lie near the limits of the validity range of FOSA.
We study stellar convection using a local three-dimensional MHD model, with which we investigate the influence of rotation and large-scale magnetic fields on the turbulent momentum and heat transport ...and their role in generating large-scale flows in stellar convection zones. The former is studied by computing the turbulent velocity correlations, known as Reynolds stresses, the latter by calculating the correlation of velocity and temperature fluctuations, both as functions of rotation and latitude. We find that the horizontal correlation, Q sub( theta phi ), capable of generating horizontal differential rotation, attains significant values and is mostly negative in the southern hemisphere for Coriolis numbers exceeding unity, corresponding to equatorward flux of angular momentum. This result is also in accordance with solar observations. The radial component Q sub(r phi ) is negative for slow and intermediate rotation indicating inward transport of angular momentum, while for rapid rotation, the transport occurs outwards. Parametrisation in terms of the mean-field Lambda -effect shows qualitative agreement with the turbulence model of Kichatinov & Ruediger (1993) for the horizontal part H proportional to Q sub( theta phi )/cos theta , whereas for the vertical Lambda -effect, V proportional to Q sub(r phi )/sin theta , agreement only for intermediate rotation exists. The Lambda -coefficients become suppressed in the limit of rapid rotation, this rotational quenching being stronger and occurring with slower rotation for the V component than for H. We have also studied the behaviour of the Reynolds stresses under the influence of a large-scale azimuthal magnetic field of varying strength. We find that the stresses are enhanced by the presence of the magnetic field for field strengths up to and above the equipartition value, without significant quenching. Concerning the turbulent heat transport, our calculations show that the transport in the radial direction is most efficient at the equatorial regions, obtains a minimum at midlatitudes, and shows a slight increase towards the poles. The latitudinal heat transport does not show a systematic trend as a function of latitude or rotation.
Human rhinovirus (HRV)-induced respiratory infections are associated with elevated levels of IFN-gamma-inducible protein 10 (IP-10), which is an enhancer of T lymphocyte chemotaxis and correlates ...with symptom severity and T lymphocyte number. Increased IP-10 expression is exhibited by airway epithelial cells following ex vivo HRV challenge and requires intracellular viral replication; however, there are conflicting reports regarding the necessity of type I IFN receptor ligation for IP-10 expression. Furthermore, the involvement of resident airway immune cells, predominantly bronchoalveolar macrophages, in contributing to HRV-stimulated IP-10 elaboration remains unclear. In this regard, our findings demonstrate that ex vivo exposure of human peripheral blood monocytes and bronchoalveolar macrophages (monocytic cells) to native or replication-defective HRV serotype 16 (HRV16) resulted in similarly robust levels of IP-10 release, which occurred in a time- and dose-dependent manner. Furthermore, HRV16 induced a significant increase in type I IFN (IFN-alpha) release and STAT1 phosphorylation in monocytes. Neutralization of the type I IFN receptor and inhibition of JAK or p38 kinase activity strongly attenuated HRV16-stimulated STAT1 phosphorylation and IP-10 release. Thus, this work supports a model, wherein HRV16-induced IP-10 release by monocytic cells is modulated via autocrine/paracrine action of type I IFNs and subsequent JAK/STAT pathway activity. Our findings demonstrating robust activation of monocytic cells in response to native and/or replication-defective HRV16 challenge represent the first evidence indicating a mechanistic disparity in the activation of macrophages when compared with epithelial cells and suggest that macrophages likely contribute to cytokine elaboration following HRV challenge in vivo.
Context. Earlier work has suggested that large-scale dynamos can reach and maintain equipartition field strengths on a dynamical time scale only if magnetic helicity of the fluctuating field can be ...shed from the domain through open boundaries. Aims. Our aim is to test this scenario in convection-driven dynamos by comparing results for open and closed boundary conditions. Methods. Three-dimensional numerical simulations of turbulent compressible convection with shear and rotation are used to study the effects of boundary conditions on the excitation and saturation of large-scale dynamos. Open (vertical-field) and closed (perfect- conductor) boundary conditions are used for the magnetic field. The shear flow is such that the contours of shear are vertical, crossing the outer surface, and are thus ideally suited for driving a shear-induced magnetic helicity flux. Results. We find that for given shear and rotation rate, the growth rate of the magnetic field is larger if open boundary conditions are used. The growth rate first increases for small magnetic Reynolds number, Rm, but then levels off at an approximately constant value for intermediate values of Rm. For large enough Rm, a small-scale dynamo is excited and the growth rate of the field in this regime increases as Rm1/2. Regarding the nonlinear regime, the saturation level of the energy of the total magnetic field is independent of Rm when open boundaries are used. In the case of perfect-conductor boundaries, the saturation level first increases as a function of Rm, but then decreases proportional to Rm-1 for Rm $\ga$ 30, indicative of catastrophic quenching. These results suggest that the shear-induced magnetic helicity flux is efficient in alleviating catastrophic quenching when open boundaries are used. The horizontally averaged mean field is still weakly decreasing as a function of Rm even for open boundaries.
Using three-dimensional convection simulations, it is shown that a sinusoidal variation of horizontal shear leads to a kinematic α effect with a similar sinusoidal variation. The effect exists even ...for weak stratification and arises owing to the inhomogeneity of turbulence and the presence of impenetrable vertical boundaries. This system produces large-scale magnetic fields that also show a sinusoidal variation in the cross-stream direction. It is argued that earlier investigations overlooked these phenomena partly because of the use of horizontal averaging and also because measurements of α using an imposed field combined with long time averages give erroneous results. It is demonstrated that in such cases the actual horizontally averaged mean field becomes non-uniform. The turbulent magnetic diffusion term resulting from such non-uniform fields can then no longer be neglected and begins to balance the α effect.
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