Swirls in the solar corona

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement No. 695075).Context. Vortex flows have been found in the photosphere, chromosphere, and low corona in observations and simulations. It has been suggested that vortices play an important role in channeling energy and plasma into the corona. However, the impact of vortex flows on the corona has not been studied directly in a realistic setup. Aims. We investigate the role vortices play for coronal heating using high-resolution simulations of coronal loops. The vortices are not artificially driven and they arise, instead, self-consistently from magnetoconvection. Methods. We performed 3D resistive (magnetohydrodynamic) MHD simulations with the MURaM code. Studying an isolated coronal loop in a Cartesian geometry allows us to resolve the structure of the loop interior. We conducted a statistical analysis to determine vortex properties as a function of height from the chromosphere into the corona. Results. We find that the energy injected into the loop is generated by internal coherent motions within strong magnetic elements. A significant part of the resulting Poynting flux is channeled through the chromosphere in vortex tubes forming a magnetic connection between the photosphere and corona. Vortices can form contiguous structures that reach up to coronal heights, but in the corona itself, the vortex tubes get deformed and eventually lose their identity with increasing height. Vortices show increased upward directed Poynting flux and heating rate in both the chromosphere and corona, but their effect becomes less pronounced with increasing height. Conclusions. While vortices play an important role for the energy transport and structuring in the chromosphere and low corona, their importance higher up in the atmosphere is less clear since the swirls are less distinguishable from their environment. Vortex tubes reaching the corona reveal a complex relationship with the coronal emission.Publisher PDFPeer reviewe

Low Ply Drawings of Trees

We consider the recently introduced model of \emph{low ply graph drawing}, in which the ply-disks of the vertices do not have many common overlaps, which results in a good distribution of the vertices in the plane. The \emph{ply-disk} of a vertex in a straight-line drawing is the disk centered at it whose radius is half the length of its longest incident edge. The largest number of ply-disks having a common overlap is called the \emph{ply-number} of the drawing. We focus on trees. We first consider drawings of trees with constant ply-number, proving that they may require exponential area, even for stars, and that they may not even exist for bounded-degree trees. Then, we turn our attention to drawings with logarithmic ply-number and show that trees with maximum degree $6$ always admit such drawings in polynomial area.Comment: This is a complete access version of a paper that will appear in the proceedings of GD201

Electrostatic attraction of nanoobjects - a versatile strategy towards mesostructured transition metal compounds

This highlight summarizes current challenges of mesostructuring and focuses on the scope and the potential of the ELAN – (electrostatic attraction of nanoobjects) strategy in mesostructuring of transition metal compounds. It discusses the limitations of this concept and highlights prominent examples. ELAN exploits the Coulomb attraction between inorganic precursors and polymeric templates in order to prevent macrophase separation. Essential requirements for ELAN are tailor-made, mesoscopic polyelectrolytic templates and charged molecular oligo-ions or stable colloids carrying a surface charge. The ELAN-strategy is highly reliable and opens the way to crystalline, mesoporous transition metal compounds with predefined polymorphism. It also provides the possibility to adjust wall chemistry and reactivity as well as the flexibility to synthesise different mesostructures (spheres, non-woven arrays or hexagonally ordered phases)

Mesostructured ZnO/Au nanoparticle composites with enhanced photocatalytic activity

Ease of catalyst separation from reaction mixtures represents a significant advantage in heterogeneous photocatalytic wastewater treatment. However, the activity of the catalyst strongly depends on its surface-to-volume ratio. Here, we present an approach based on cylindrical polybutadiene-block-poly(2-vinylpyridine) polymer brushes as template, which can be simultaneously loaded with zinc oxide (ZnO) and gold (Au) nanoparticles. Pyrolytic template removal of the polymer yields in mesostructured ZnO/Au composites, showing higher efficiencies in the photocatalytic degradation of ciprofloxacin and levofloxacin (generic antibiotics present in clinical wastewater) as compared to neat mesostructured ZnO. Upscaling of the presented catalyst is straightforward promising high technical relevance

Segregation by thermal diffusion of an intruder in a moderately dense granular fluid

A solution of the inelastic Enskog equation that goes beyond the weak dissipation limit and applies for moderate densities is used to determine the thermal diffusion factor of an intruder immersed in a dense granular gas under gravity. This factor provides a segregation criterion that shows the transition between the Brazil-nut effect (BNE) and the reverse Brazil-nut effect (RBNE) by varying the parameters of the system (masses, sizes, density and coefficients of restitution). The form of the phase-diagrams for the BNE/RBNE transition depends sensitively on the value of gravity relative to the thermal gradient, so that it is possible to switch between both states for given values of the parameters of the system. Two specific limits are considered with detail: (i) absence of gravity, and (ii) homogeneous temperature. In the latter case, after some approximations, our results are consistent with previous theoretical results derived from the Enskog equation. Our results also indicate that the influence of dissipation on thermal diffusion is more important in the absence of gravity than in the opposite limit. The present analysis extends previous theoretical results derived in the dilute limit case [V. Garz\'o, Europhys. Lett. {\bf 75}, 521 (2006)] and is consistent with the findings of some recent experimental results.Comment: 10 figure