198,658 research outputs found
Spontaneous current-layer fragmentation and cascading reconnection in solar flares: II. Relation to observations
In the paper by B\'arta et al. (arXive:astro-ph:/1011.4035, 2010) the authors
addressed some open questions of the CSHKP scenario of solar flares by means of
high-resolution MHD simulations. They focused, in particular, on the problem of
energy transfer from large to small scales in decaying flare current sheet
(CS). Their calculations suggest, that magnetic flux-ropes (plasmoids) are
formed in full range of scales by a cascade of tearing and coalescence
processes. Consequently, the initially thick current layer becomes highly
fragmented. Thus, the tearing and coalescence cascade can cause an effective
energy transfer across the scales. In the current paper we investigate whether
this mechanism actually applies in solar flares. We extend the MHD simulation
by deriving model-specific features that can be looked for in observations. The
results of the underlying MHD model showed that the plasmoid cascade creates a
specific hierarchical distribution of non-ideal/acceleration regions embedded
in the CS. We therefore focus on the features associated with the fluxes of
energetic particles, in particular on the structure and dynamics of emission
regions in flare ribbons. We assume that the structure and dynamics of
diffusion regions embedded in the CS imprint themselves into structure and
dynamics of flare-ribbon kernels by means of magnetic-field mapping. Using the
results of the underlying MHD simulation we derive the expected structure of
ribbon emission and we extract selected statistical properties of the modelled
bright kernels. Comparing the predicted emission and its properties with the
observed ones we obtain a good agreement of the two.Comment: 7 pages, 5 figure
Graphene-Based Nanostructures in Electrocatalytic Oxygen Reduction
Application of graphene-type materials in electrocatalysis is a topic of
growing scientific and technological interest. A tremendous amount of research
has been carried out in the field of oxygen electroreduction, particularly with
respect to potential applications in the fuel cell research also with use of
graphene-type catalytic components. This work addresses fundamental aspects and
potential applications of graphene structures in the oxygen reduction
electrocatalysis. Special attention will be paid to creation of catalytically
active sites by using non-metallic heteroatoms as dopants, formation of
hierarchical nanostructured electrocatalysts, their long-term stability, and
application as supports for dispersed metals (activating interactions)
Problem-orientable numerical algorithm for modelling multi-dimensional radiative MHD flows in astrophysics -- the hierarchical solution scenario
We present a hierarchical approach for enhancing the robustness of numerical
solvers for modelling radiative MHD flows in multi-dimensions. This approach is
based on clustering the entries of the global Jacobian in a hierarchical manner
that enables employing a variety of solution procedures ranging from a purely
explicit time-stepping up to fully implicit schemes. A gradual coupling of the
radiative MHD equation with the radiative transfer equation in higher
dimensions is possible. Using this approach, it is possible to follow the
evolution of strongly time-dependent flows with low/high accuracies and with
efficiency comparable to explicit methods, as well as searching
quasi-stationary solutions for highly viscous flows. In particular, it is shown
that the hierarchical approach is capable of modelling the formation of jets in
active galactic nuclei and reproduce the corresponding spectral energy
distribution with a reasonable accuracy.Comment: 28 pages, 9 figure
A Taxonomy of Data Grids for Distributed Data Sharing, Management and Processing
Data Grids have been adopted as the platform for scientific communities that
need to share, access, transport, process and manage large data collections
distributed worldwide. They combine high-end computing technologies with
high-performance networking and wide-area storage management techniques. In
this paper, we discuss the key concepts behind Data Grids and compare them with
other data sharing and distribution paradigms such as content delivery
networks, peer-to-peer networks and distributed databases. We then provide
comprehensive taxonomies that cover various aspects of architecture, data
transportation, data replication and resource allocation and scheduling.
Finally, we map the proposed taxonomy to various Data Grid systems not only to
validate the taxonomy but also to identify areas for future exploration.
Through this taxonomy, we aim to categorise existing systems to better
understand their goals and their methodology. This would help evaluate their
applicability for solving similar problems. This taxonomy also provides a "gap
analysis" of this area through which researchers can potentially identify new
issues for investigation. Finally, we hope that the proposed taxonomy and
mapping also helps to provide an easy way for new practitioners to understand
this complex area of research.Comment: 46 pages, 16 figures, Technical Repor
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