104,636 research outputs found
Solar stereoscopy - where are we and what developments do we require to progress?
Observations from the two STEREO-spacecraft give us for the first time the
possibility to use stereoscopic methods to reconstruct the 3D solar corona.
Classical stereoscopy works best for solid objects with clear edges.
Consequently an application of classical stereoscopic methods to the faint
structures visible in the optically thin coronal plasma is by no means straight
forward and several problems have to be treated adequately: 1.)First there is
the problem of identifying one dimensional structures -e.g. active region
coronal loops or polar plumes- from the two individual EUV-images observed with
STEREO/EUVI. 2.) As a next step one has the association problem to find
corresponding structures in both images. 3.) Within the reconstruction problem
stereoscopic methods are used to compute the 3D-geometry of the identified
structures. Without any prior assumptions, e.g., regarding the footpoints of
coronal loops, the reconstruction problem has not one unique solution. 4.) One
has to estimate the reconstruction error or accuracy of the reconstructed
3D-structure, which depends on the accuracy of the identified structures in 2D,
the separation angle between the spacecraft, but also on the location, e.g.,
for east-west directed coronal loops the reconstruction error is highest close
to the loop top. 5.) Eventually we are not only interested in the 3D-geometry
of loops or plumes, but also in physical parameters like density, temperature,
plasma flow, magnetic field strength etc. Helpful for treating some of these
problems are coronal magnetic field models extrapolated from photospheric
measurements, because observed EUV-loops outline the magnetic field. This
feature has been used for a new method dubbed 'magnetic stereoscopy'. As
examples we show recent application to active region loops.Comment: 12 Pages, 9 Figures, a Review articl
Solar Stereoscopy with STEREO/EUVI A and B spacecraft from small (6 deg) to large (170 deg) spacecraft separation angles
We performed for the first time stereoscopic triangulation of coronal loops
in active regions over the entire range of spacecraft separation angles
(, and
). The accuracy of stereoscopic correlation depends mostly on the
viewing angle with respect to the solar surface for each spacecraft, which
affects the stereoscopic correspondence identification of loops in image pairs.
From a simple theoretical model we predict an optimum range of , which is also experimentally confirmed. The best
accuracy is generally obtained when an active region passes the central
meridian (viewed from Earth), which yields a symmetric view for both STEREO
spacecraft and causes minimum horizontal foreshortening. For the extended
angular range of we find a mean 3D
misalignment angle of of stereoscopically
triangulated loops with magnetic potential field models, and for a force-free field model, which is partly caused by
stereoscopic uncertainties . We predict optimum
conditions for solar stereoscopy during the time intervals of 2012--2014,
2016--2017, and 2021--2023.Comment: Solar Physics, (in press), 22 pages, 9 figure
Rapid algorithm for identifying backbones in the two-dimensional percolation model
We present a rapid algorithm for identifying the current-carrying backbone in
the percolation model. It applies to general two-dimensional graphs with open
boundary conditions. Complemented by the modified Hoshen-Kopelman cluster
labeling algorithm, our algorithm identifies dangling parts using their local
properties. For planar graphs, it finds the backbone almost four times as fast
as Tarjan's depth-first-search algorithm, and uses the memory of the same size
as the modified Hoshen-Kopelman algorithm. Comparison with other algorithms for
backbone identification is addressed.Comment: 5 pages with 5 eps figures. RevTeX 3.1. Clarify the origin of the
hull-generating algorith
Multi-Step Processing of Spatial Joins
Spatial joins are one of the most important operations for combining spatial objects of several relations. In this paper, spatial join processing is studied in detail for extended spatial objects in twodimensional data space. We present an approach for spatial join processing that is based on three steps. First, a spatial join is performed on the minimum bounding rectangles of the objects returning a set of candidates. Various approaches for accelerating this step of join processing have been examined at the last year’s conference [BKS 93a]. In this paper, we focus on the problem how to compute the answers from the set of candidates which is handled by
the following two steps. First of all, sophisticated approximations
are used to identify answers as well as to filter out false hits from
the set of candidates. For this purpose, we investigate various types
of conservative and progressive approximations. In the last step, the
exact geometry of the remaining candidates has to be tested against
the join predicate. The time required for computing spatial join
predicates can essentially be reduced when objects are adequately
organized in main memory. In our approach, objects are first decomposed
into simple components which are exclusively organized
by a main-memory resident spatial data structure. Overall, we
present a complete approach of spatial join processing on complex
spatial objects. The performance of the individual steps of our approach
is evaluated with data sets from real cartographic applications.
The results show that our approach reduces the total execution
time of the spatial join by factors
On damping created by heterogeneous yielding in the numerical analysis of nonlinear reinforced concrete frame elements
In the dynamic analysis of structural engineering systems, it is common
practice to introduce damping models to reproduce experimentally observed
features. These models, for instance Rayleigh damping, account for the damping
sources in the system altogether and often lack physical basis. We report on an
alternative path for reproducing damping coming from material nonlinear
response through the consideration of the heterogeneous character of material
mechanical properties. The parameterization of that heterogeneity is performed
through a stochastic model. It is shown that such a variability creates the
patterns in the concrete cyclic response that are classically regarded as
source of damping
Identifying the contributions of Universal Extra Dimensions in the Higgs sector at linear e+ e- colliders
We study the dilepton-dijet signal in the dominant Higgs production channel
at a linear e+ e- collider. We estimate the effects of Universal Extra
Dimension (UED) by a simple analysis of the cross-sections. The heavy
Kaluza-Klein excitations of the Standard Model fields in UED can significantly
alter the decay properties of the Higgs boson to loop-driven final states. We
show that by taking a simple ratio between cross-sections of two different
final states this difference can be very easily highlighted.Comment: Some parts of the text modified. 1 figure added. Version to appear in
IJMP
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