186,892 research outputs found
Vpliv socialno-kulturnega okolja na razvoj glasbene nadarjenosti otrok v 2. triletju osnovne šole
No full textModelling of asymmetric nanojets in coronal loops
No full textContext. Observations of reconnection jets in the solar corona are emerging as a possible diagnostic for studying highly elusive coronal heating. Such jets, and in particular those termed nanojets, can be observed in coronal loops and have been linked to nanoflares. However, while models successfully describe the bilateral post-reconnection magnetic slingshot effect that leads to the jets, observations reveal that nanojets are unidirectional or highly asymmetric, with only the jet travelling inward with respect to the coronal loop's curvature being clearly observed.
Aims. The aim of this work is to address the role of the curvature of the coronal loop in the generation and evolution of asymmetric reconnection jets. Methods. We first use a simplified analytical model in which we estimate the post-reconnection tension forces based on the local intersection angle between the pre-reconnection magnetic field lines and their post-reconnection retracting length towards new equilibria. Second, we use a simplified numerical magnetohydrodynamic (MHD) model to study how two opposite propagating jets evolve in curved magnetic field lines.
Results. Through our analytical model, we demonstrate that in the post-reconnection reorganised magnetic field, the inward directed magnetic tension is inherently stronger (by up to three orders of magnitude) than the outward directed one and that, with a large enough retracting length, a regime exists where the outward directed tension disappears, leading to no outward jet at large, observable scales. Our MHD numerical model provides support for these results, proving also that in the subsequent time evolution the inward jets are consistently more energetic. The degree of asymmetry is also found to increase for small-Angle reconnection and for more localised reconnection regions.
Conclusions. This work shows that the curvature of the coronal loops can play a major role in the asymmetry of the reconnection jets and that inward directed jets are more likely to occur and are more energetic than the corresponding outward directed ones
An isogeometric method for linear nearly-incompressible elasticity with local stress projection
No full textIn this paper, we propose an isogeometric method for solving the linear nearly-incompressible elasticity problem. The method is similar to the (B) over bar formulation where the volumetric strain is projected on a lower degree spline space in order to prevent volumetric locking. In our method, we adopt a local projection on a coarser mesh, chosen in order to guarantee optimal convergence. Moreover the locality of the projector allows to maintain the sparsity of the stiffness matrix, that is, the efficiency of the method. The analysis of the method is based on the inf-sup stability of the associated mixed formulation via a macro-element technique for spline functions. The numerical tests confirm the theory of the method. (C) 2016 Elsevier B.V. All rights reserved.MN
Reconnection nanojets in the solar corona
No full textThe solar corona is shaped and mysteriously heated to millions of degrees by the Sun’s magnetic field. It has long been hypothesized that the heating results from a myriad of tiny magnetic energy outbursts called nanoflares, driven by the fundamental process of magnetic reconnection. Misaligned magnetic field lines can break and reconnect, producing nanoflares in avalanche-like processes. However, no direct and unique observations of such nanoflares exist to date, and the lack of a smoking gun has cast doubt on the possibility of solving the coronal heating problem. From coordinated multi-band high-resolution observations, we report on the discovery of very fast and bursty nanojets, the telltale signature of reconnection-based nanoflares resulting in coronal heating. Using state-of-the-art numerical simulations, we demonstrate that the nanojet is a consequence of the slingshot effect from the magnetically tensed, curved magnetic field lines reconnecting at small angles. Nanojets are therefore the key signature of reconnection-based coronal heating in action
G. Antolin, O. S. Α., Catalogo de los codices latinos de la real biblioteca del Escorial.
No full textSalaville Sévérien. G. Antolin, O. S. Α., Catalogo de los codices latinos de la real biblioteca del Escorial. . In: Échos d'Orient, tome 15, n°93, 1912. p. 184
G. Antolin, O. S. Α., Catalogo de los codices latinos de la real biblioteca del Escorial.
No full textSalaville Sévérien. G. Antolin, O. S. Α., Catalogo de los codices latinos de la real biblioteca del Escorial. . In: Échos d'Orient, tome 15, n°93, 1912. p. 184
In Situ Generation of Transverse Magnetohydrodynamic Waves from Colliding Flows in the Solar Corona
No full textTransverse magnetohydrodynamic (MHD) waves permeate the solar atmosphere and are a candidate for coronal heating. However, the origin of these waves is still unclear. In this Letter, we analyze coordinated observations from Hinode/Solar Optical Telescope (SOT) and Interface Region Imaging Spectrograph ( IRIS) of a prominence/coronal rain loop-like structure at the limb of the Sun. Cool and dense downflows and upflows are observed along the structure. A collision between a downward and an upward flow with an estimated energy flux of 107-108 erg cm-2 s-1 is observed to generate oscillatory transverse perturbations of the strands with an estimated ≈40 km s-1 total amplitude, and a short-lived brightening event with the plasma temperature increasing to at least 105 K. We interpret this response as sausage and kink transverse MHD waves based on 2D MHD simulations of plasma flow collision. The lengths, density, and velocity differences between the colliding clumps and the strength of the magnetic field are major parameters defining the response to the collision. The presence of asymmetry between the clumps (angle of impact surface and/or offset of flowing axis) is crucial for generating a kink mode. Using the observed values, we successfully reproduce the observed transverse perturbations and brightening, and show adiabatic heating to coronal temperatures. The numerical modeling indicates that the plasma β in this loop-like structure is confined between 0.09 and 0.36. These results suggest that such collisions from counter-streaming flows can be a source of in situ transverse MHD waves, and that for cool and dense prominence conditions such waves could have significant amplitudes
A simple and effective method based on strain projections to alleviate locking in isogeometric solid shells
No full textIn this work, we focus on the family of shell formulations referred to as "solid shells", where the simulation of shell-type structures is performed by means of a mesh of 3D solid elements, with typically only one element through the thickness. We propose a novel approach for alleviating the various locking phenomena, which typically appear in thin structures, based on the projection of strains onto discontinuous coarser polynomial spaces defined at element level. In particular, we present and investigate two different formulations based on this approach. Several numerical experiments prove the very good performance of both formulations. The main advantages of the presented approach compared to existing solid shell formulations are its simplicity and numerical efficiency.MNS12 figure
Fast and accurate elastic analysis of laminated composite plates via isogeometric collocation and an equilibrium-based stress recovery approach
No full textA novel approach which combines isogeometric collocation and an equilibrium-based stress recovery technique is applied to analyze laminated composite plates. Isogeometric collocation is an appealing strong form alternative to standard Galerkin approaches, able to achieve high order convergence rates coupled with a significantly reduced computational cost. Laminated composite plates are herein conveniently modeled considering only one element through the thickness with homogenized material properties. This guarantees accurate results in terms of displacements and in-plane stress components. To recover an accurate out-of-plane stress state, equilibrium is imposed in strong form as a post-processing correction step, which requires the shape functions to be highly continuous. This continuity demand is fully granted by isogeometric analysis properties, and excellent results are obtained using a minimal number of collocation points per direction, particularly for increasing values of length-to-thickness plate ratio and number of layers
MHD simulations of the in situ generation of kink and sausage waves in the solar corona by collision of dense plasma clumps
No full textContext. Magnetohydrodynamic (MHD) waves are ubiquitous in the solar corona where the highly structured magnetic fields provide efficient wave guides for their propagation. While MHD waves have been observed originating from lower layers of the solar atmosphere, recent studies have shown that some can be generated in situ by the collision of dense counter-propagating flows. Aims. In this theoretical study, we analyse the mechanism that triggers the propagation of kink and sausage modes in the solar corona following the collision of counter-propagating flows, and how the properties of the flows affect the properties of the generated waves. Methods. To study in detail this mechanism we ran a series of ideal 2D and 3D MHD simulations where we varied the properties of the counter-propagating flows; by means of a simple technique to estimate the amplitudes of the kink and sausage modes, we investigated their role in the generation and propagation of the MHD waves. Results. We find that the amplitude of the waves is largely dependent on the kinetic energy of the flows, and that the onset of kink or sausage modes depends on the asymmetries between the colliding blobs. Moreover, the initial wavelength of the MHD waves is associated with the magnetic configuration resulting from the collision of the flows. We also find that genuine 3D systems respond with smaller wave amplitudes. Conclusions. In this study, we present a parameter space description of the mechanism that leads to the generation of MHD waves from the collision of flows in the corona. Future observations of these waves can be used to understand the properties of the plasma and magnetic field of the solar corona
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