Anisotropic viscous dissipation in transient reconnecting plasmas

Aims. We examine the global energy losses associated with reconnecting coronal plasmas. Methods. Using planar magnetic reconnection simulations we compute resistive and bulk viscous losses in transient coronal plasmas. Resistive scalings are computed for the case of incompressible reconnection powered by large scale vortical flows. These results are contrasted with an example of magnetic merging driven by the coalescence instability. Results. We demonstrate that the large scale advective flows, required to sustain resistive current sheets, may be associated with viscous losses approaching flare-like rates of 10²⁹ erg s⁻¹ . More generally, bulk viscous dissipation appears likely to dominate resistive dissipation for a wide variety of magnetic merging models. We emphasize that these results have potentially important implications for understanding the flare energy budget

Gaussian decomposition of HI surveys. V. Search for very cold clouds

In the previous papers of this series, we have decomposed into Gaussian components all the HI 21-cm line profiles of the Leiden-Argentina-Bonn (LAB) database, and studied statistical distributions of the obtained Gaussians. Now we are interested in separation from the general database of the components the "clouds" of closely spaced similar Gaussians. In this paper we describe the new cloud-finding algorithm. To separate the clouds of similar Gaussians, we start with the single-link hierarchical clustering procedure in five-dimensional (longitude, latitude, velocity, Gaussian width and height) space, but make some modifications to accommodate it to the large number of components. We also use the requirement that each cloud may be represented at any observed sky position by only one Gaussian and take into account the similarity of global properties of the merging clouds. As a test, we apply the algorithm for finding the clouds of the narrowest HI 21-cm line components. Using the full sky search for cold clouds, we easily detect the coldest known HI clouds and demonstrate that actually they are a part of a long narrow ribbon of cold clouds. We model these clouds as a part of a planar gas ring, deduce their spatial placement, and discuss their relation to supernova shells in the solar neighborhood. We conclude that the proposed algorithm satisfactorily solves the posed task. We guess that the study of the narrowest HI 21-cm line components may be a useful tool for finding the structure of neutral gas in solar neighborhood.Comment: 11 pages, 6 figures, short version will be published in "Astron. Astrophys", the version with full-resolution figures at http://www.aai.ee/~urmas/ast/Kits.pd

Path-tracing Monte Carlo Library for 3D Radiative Transfer in Highly Resolved Cloudy Atmospheres

Interactions between clouds and radiation are at the root of many difficulties in numerically predicting future weather and climate and in retrieving the state of the atmosphere from remote sensing observations. The large range of issues related to these interactions, and in particular to three-dimensional interactions, motivated the development of accurate radiative tools able to compute all types of radiative metrics, from monochromatic, local and directional observables, to integrated energetic quantities. In the continuity of this community effort, we propose here an open-source library for general use in Monte Carlo algorithms. This library is devoted to the acceleration of path-tracing in complex data, typically high-resolution large-domain grounds and clouds. The main algorithmic advances embedded in the library are those related to the construction and traversal of hierarchical grids accelerating the tracing of paths through heterogeneous fields in null-collision (maximum cross-section) algorithms. We show that with these hierarchical grids, the computing time is only weakly sensitivive to the refinement of the volumetric data. The library is tested with a rendering algorithm that produces synthetic images of cloud radiances. Two other examples are given as illustrations, that are respectively used to analyse the transmission of solar radiation under a cloud together with its sensitivity to an optical parameter, and to assess a parametrization of 3D radiative effects of clouds.Comment: Submitted to JAMES, revised and submitted again (this is v2

Dynamic planar magnetic reconnection solutions for incompressible plasmas

The planar magnetic reconnection problem for viscous, resistive plasmas is addressed. We show that solutions can be developed by superposing transient nonlinear disturbances onto quiescent “background” fields. The disturbance fields are unrestricted in form, but the spatial part of the background field must satisfy ∇2K= -λK. This decomposition allows previous analytic reconnection solutions, based on one-dimensional disturbance fields of “plane wave” form, to be recovered as special cases. However, we point out that planar disturbance fields must be fully two-dimensional to avoid the pressure problem associated with analytic merging models, that is, to avoid unbounded current sheet pressures in the limit of small plasma resistivities. The details of the reconnection problem are then illustrated using cellular background field simulations in doubly periodic geometries. The flux pile-up rate is shown to saturate when the pressure of the current sheet exceeds the hydromagnetic pressure of the background field. Although the presaturation regime is well described by one-dimensional current sheet theory, the nonlinear postsaturation regime remains poorly understood. Preliminary evidence suggests that, although after saturation the early evolution of the field can be described by slow Sweet-Parker scalings, the first implosion no longer provides the bulk of the energy release

Transport on river networks: A dynamical approach

This study is motivated by problems related to environmental transport on river networks. We establish statistical properties of a flow along a directed branching network and suggest its compact parameterization. The downstream network transport is treated as a particular case of nearest-neighbor hierarchical aggregation with respect to the metric induced by the branching structure of the river network. We describe the static geometric structure of a drainage network by a tree, referred to as the static tree, and introduce an associated dynamic tree that describes the transport along the static tree. It is well known that the static branching structure of river networks can be described by self-similar trees (SSTs); we demonstrate that the corresponding dynamic trees are also self-similar. We report an unexpected phase transition in the dynamics of three river networks, one from California and two from Italy, demonstrate the universal features of this transition, and seek to interpret it in hydrological terms.Comment: 38 pages, 15 figure

Magnetic annihilation and reconnection in two dimensions

The problems of incompressible, planar, magnetic annihilation and reconnection are discussed. We first emphasize that steady-state reconnection solutions can be constructed from annihilation models involving harmonic velocity fields. We show, however, that the only harmonic velocity profile capable of supporting inviscid magnetic annihilation is the traditional stagnation point flow φ= −αxy. The implication is that further steady-state planar reconnection models derived from annihilation solutions are impossible. We go on to show that certain classes of nonharmonic stream functions allow reconnection solutions to be developed, once the constraint of time independence is relaxed. In particular, we construct an exact reconnection model based on cellular inflows into a periodic assemblage of magnetic X-points