43,564 research outputs found
Three-dimensional non-LTE radiative transfer computation of the Ca 8542 infrared line from a radiation-MHD simulation
Interpretation of imagery of the solar chromosphere in the widely used
\CaIIIR infrared line is hampered by its complex, three-dimensional and non-LTE
formation. Forward modelling is required to aid understanding. We use a 3D
non-LTE radiative transfer code to compute synthetic \CaIIIR images from a
radiation-MHD simulation of the solar atmosphere spanning from the convection
zone to the corona. We compare the simulation with observations obtained with
the CRISP filter at the Swedish 1--m Solar Telescope. We find that the
simulation reproduces dark patches in the blue line wing caused by Doppler
shifts, brightenings in the line core caused by upward-propagating shocks and
thin dark elongated structures in the line core that form the interface between
upward and downward gas motion in the chromosphere. The synthetic line core is
narrower than the observed one, indicating that the sun exhibits both more
vigorous large-scale dynamics as well as small scale motions that are not
resolved within the simulation, presumably owing to a lack of spatial
resolution.Comment: accepted as ApJ lette
On the Efficiency of Optimising Shallow Backtracking in Prolog
The cost of backtracking has been identified as one of the bottlenecks in
achieving peak performance in compiled Prolog programs. Much of the backtracking in
Prolog programs is shallow, i.e. is caused by unification failures in the head of a
clause when there are more alternatives for the same procedure, and so special treatment
of this form of backtracking has been proposed as a significant optimisation. This
paper describes a modified WAM which optimises shallow backtracking. Four different
implementation approaches are compared. A number of benchmark results are presented,
measuring the relative tradeoffs between compilation time, code size, and run time. The
results show that the speedup gained by this optimisation can be significant
Completed representation ring spectra of nilpotent groups
In this paper, we examine the `derived completion' of the representation ring
of a pro-p group G_p^ with respect to an augmentation ideal. This completion is
no longer a ring: it is a spectrum with the structure of a module spectrum over
the Eilenberg-MacLane spectrum HZ, and can have higher homotopy information. In
order to explain the origin of some of these higher homotopy classes, we define
a deformation representation ring functor R[-] from groups to ring spectra, and
show that the map R[G_p^] --> R[G] becomes an equivalence after completion when
G is finitely generated nilpotent. As an application, we compute the derived
completion of the representation ring of the simplest nontrivial case, the
p-adic Heisenberg group.Comment: This is the version published by Algebraic & Geometric Topology on 26
February 200
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