151,995 research outputs found
Magneto-acoustic waves in sunspots from observations and numerical simulations
We study the propagation of waves from the photosphere to the chromosphere of
sunspots. From time series of cospatial Ca II H (including its line blends)
intensity spectra and polarimetric spectra of Si I 1082.7 nm and He I 1083.0 nm
we retrieve the line-of-sight velocity at several heights. The analysis of the
phase difference and amplification spectra shows standing waves for frequencies
below 4 mHz and propagating waves for higher frequencies, and allows us to
infer the temperature and height where the lines are formed. Using these
observational data, we have constructed a model of sunspot, and we have
introduced the velocity measured with the photospheric Si I 1082.7 nm line as a
driver. The numerically propagated wave pattern fits reasonably well with the
observed using the lines formed at higher layers, and the simulations reproduce
many of the observed features. The observed waves are slow MHD waves
propagating longitudinally along field lines.Comment: proceedings of GONG 2010/SOHO 24 meeting, June 27 - July 2, 2010,
Aix-en-Provence, Franc
Local tropical linear spaces
In this paper we study general tropical linear spaces locally: For any basis
B of the matroid underlying a tropical linear space L, we define the local
tropical linear space L_B to be the subcomplex of L consisting of all vectors v
that make B a basis of maximal v-weight. The tropical linear space L can then
be expressed as the union of all its local tropical linear spaces, which we
prove are homeomorphic to Euclidean space. Local tropical linear spaces have a
simple description in terms of polyhedral matroid subdivisions, and we prove
that they are dual to mixed subdivisions of Minkowski sums of simplices. Using
this duality we produce tight upper bounds for their f-vectors. We also study a
certain class of tropical linear spaces that we call conical tropical linear
spaces, and we give a simple proof that they satisfy Speyer's f-vector
conjecture.Comment: 13 pages, 1 figure. Some results are stated in a bit more generality.
Minor corrections were also mad
Improving Sparsity in Kernel Adaptive Filters Using a Unit-Norm Dictionary
Kernel adaptive filters, a class of adaptive nonlinear time-series models,
are known by their ability to learn expressive autoregressive patterns from
sequential data. However, for trivial monotonic signals, they struggle to
perform accurate predictions and at the same time keep computational complexity
within desired boundaries. This is because new observations are incorporated to
the dictionary when they are far from what the algorithm has seen in the past.
We propose a novel approach to kernel adaptive filtering that compares new
observations against dictionary samples in terms of their unit-norm
(normalised) versions, meaning that new observations that look like previous
samples but have a different magnitude are not added to the dictionary. We
achieve this by proposing the unit-norm Gaussian kernel and define a
sparsification criterion for this novel kernel. This new methodology is
validated on two real-world datasets against standard KAF in terms of the
normalised mean square error and the dictionary size.Comment: Accepted at the IEEE Digital Signal Processing conference 201
Taxonomic note on the type species of Centris (Melanocentris) (Hymenoptera: Apidae)
Centris (Melanocentris) Friese is one of many subgenera that have been proposed throughout the taxonomic history of the bee genus Centris Fabricius. The lack of critical study of the type specimens of its type species, Centris atra Friese, resulted in the synonymy of Melanocentris with subgenus Ptilotopus Klug. Subsequently, Melacentris Moure was described as a new subgenus to group the large number of species identified as Melanocentris before the synonymy was proposed. The study of the syntypes of C. atra and the designation of a lectotype (herein) leads to the revalidation of Melanocentris as a subgenus distinct from Ptilotopus, and necessitates the new synonymy of Melacentris with Melanocentris
About Twistor Spinors with Zero in Lorentzian Geometry
We describe the local conformal geometry of a Lorentzian spin manifold
admitting a twistor spinor with zero. Moreover, we describe the
shape of the zero set of . If has isolated zeros then the metric
is locally conformally equivalent to a static monopole. In the other case
the zero set consists of null geodesic(s) and is locally conformally
equivalent to a Brinkmann metric. Our arguments utilise tractor calculus in an
essential way. The Dirac current of , which is a conformal Killing vector
field, plays an important role for our discussion as well
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