13,475 research outputs found
High-frequency Oscillations in Small Magnetic Elements Observed with Sunrise/SuFI
We characterize waves in small magnetic elements and investigate their
propagation in the lower solar atmosphere from observations at high spatial and
temporal resolution. We use the wavelet transform to analyze oscillations of
both horizontal displacement and intensity in magnetic bright points found in
the 300 nm and the Ca II H 396.8 nm passbands of the filter imager on board the
Sunrise balloon-borne solar observatory. Phase differences between the
oscillations at the two atmospheric layers corresponding to the two passbands
reveal upward propagating waves at high frequencies (up to 30 mHz). Weak
signatures of standing as well as downward propagating waves are also obtained.
Both compressible and incompressible (kink) waves are found in the small-scale
magnetic features. The two types of waves have different, though overlapping,
period distributions. Two independent estimates give a height difference of
approximately 450+-100 km between the two atmospheric layers sampled by the
employed spectral bands. This value, together with the determined short travel
times of the transverse and longitudinal waves provide us with phase speeds of
29+-2 km/s and 31+-2 km/s, respectively. We speculate that these phase speeds
may not reflect the true propagation speeds of the waves. Thus, effects such as
the refraction of fast longitudinal waves may contribute to an overestimate of
the phase speed.Comment: 14 pages, 7 figure
The effect of magnetic dipolar interactions on the interchain spin wave dispersion in CsNiF_3
Inelastic neutron scattering measurements were performed on the ferromagnetic
chain system CsNiF_3 in the collinear antiferromagnetic ordered state below T_N
= 2.67K. The measured spin wave dispersion was found to be in good agreement
with linear spin wave theory including dipolar interactions. The additional
dipole tensor in the Hamiltonian was essential to explain some striking
phenomena in the measured spin wave spectrum: a peculiar feature of the
dispersion relation is a jump at the zone center, caused by strong dipolar
interactions in this system. The interchain exchange coupling constant and the
planar anisotropy energy were determined within the present model to be J'/k_B
= -0.0247(12)K and A/k_B = 3.3(1)K. This gives a ratio J/J' \approx 500, using
the previously determined intrachain coupling constant J/k_B = 11.8$. The small
exchange energy J' is of the same order as the dipolar energy, which implies a
strong competition between the both interactions.Comment: 18 pages, TeX type, 7 Postscript figures included. To be published in
Phys. Rev.
A Probabilistic Analysis of Kademlia Networks
Kademlia is currently the most widely used searching algorithm in P2P
(peer-to-peer) networks. This work studies an essential question about Kademlia
from a mathematical perspective: how long does it take to locate a node in the
network? To answer it, we introduce a random graph K and study how many steps
are needed to locate a given vertex in K using Kademlia's algorithm, which we
call the routing time. Two slightly different versions of K are studied. In the
first one, vertices of K are labelled with fixed IDs. In the second one,
vertices are assumed to have randomly selected IDs. In both cases, we show that
the routing time is about c*log(n), where n is the number of nodes in the
network and c is an explicitly described constant.Comment: ISAAC 201
Categorification of persistent homology
We redevelop persistent homology (topological persistence) from a categorical
point of view. The main objects of study are diagrams, indexed by the poset of
real numbers, in some target category. The set of such diagrams has an
interleaving distance, which we show generalizes the previously-studied
bottleneck distance. To illustrate the utility of this approach, we greatly
generalize previous stability results for persistence, extended persistence,
and kernel, image and cokernel persistence. We give a natural construction of a
category of interleavings of these diagrams, and show that if the target
category is abelian, so is this category of interleavings.Comment: 27 pages, v3: minor changes, to appear in Discrete & Computational
Geometr
Highly efficient, tunable single photon source based on single molecules
The authors studied spatially isolated terrylene molecules immobilized in a quasiplanar optical λ/2-microresonator using confocal microscopy and spectroscopy at variable temperatures. At T = 1.8 K, they observed individual molecules relaxing into microresonator-allowed vibronic levels of their electronic ground state by emission of single fluorescence photons. Coupling the purely electronic transition of embedded molecules to the longitudinal photonic mode of the microresonator resulted in an ultimate spectral narrowing and an increased collection efficiency of the emitted single photon wave trains
On The Evolution of Magnetic White Dwarfs
We present the first radiation magnetohydrodynamics simulations of the
atmosphere of white dwarf stars. We demonstrate that convective energy transfer
is seriously impeded by magnetic fields when the plasma-beta parameter, the
thermal to magnetic pressure ratio, becomes smaller than unity. The critical
field strength that inhibits convection in the photosphere of white dwarfs is
in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG
field strengths observed in magnetic white dwarfs, implying that these objects
have radiative atmospheres. We have then employed evolutionary models to study
the cooling process of high-field magnetic white dwarfs, where convection is
entirely suppressed during the full evolution (B > 10 MG). We find that the
inhibition of convection has no effect on cooling rates until the effective
temperature (Teff) reaches a value of around 5500 K. In this regime, the
standard convective sequences start to deviate from the ones without convection
owing to the convective coupling between the outer layers and the degenerate
reservoir of thermal energy. Since no magnetic white dwarfs are currently known
at the low temperatures where this coupling significantly changes the
evolution, effects of magnetism on cooling rates are not expected to be
observed. This result contrasts with a recent suggestion that magnetic white
dwarfs with Teff < 10,000 K cool significantly slower than non-magnetic
degenerates.Comment: 11 pages, 12 figures, accepted for publication in the Astrophysical
Journa
Plant Disease Resistance Inducing Activity of 7-Oxo- and 7-Hydroxysterols
The 7-oxosterols 1–2 and the 7-hydroxysterols 3–6 induce resistance toward the fungal pathogens Puccinia striiformis West, and Puccinia hordei Otth in barley and wheat. Primary leaves of the plants were sprayed with solutions of the compounds (10-4 mol/l in 1% aqu. ethanol) followed, 2 days later, by challenge inoculation with the fungal pathogens. The results indicate that 7a- and 7β-hydroxylated epimers of β-sitosterol and cholesterol show the highest value of induced resistance (39-49% reduction of infection sites). No enhanced resistance toward the fungi Erysiphe graminis DC f. sp. tritici and hordei and Cochliobolus sativus Ito & Kuribayashi was observed. © 1995 Verlag der Zeitschrift für Naturforschung. All rights reserved
Assessing Agreement between Two Measurement Systems: An Alternative to the Limits of Agreement Approach
The comparison of two measurement systems is important in medical and other contexts. A common goal is to decide if a new measurement system agrees suitably with an existing one, and hence whether the two can be used interchangeably. Various methods for assessing interchangeability are available, the most popular being the limits of agreement approach due to Bland and Altman. In this article, we review the challenges of this technique and propose a model-based framework for comparing measurement systems that overcomes those challenges. The proposal is based on a simple metric, the probability of agreement, and a corresponding plot which can be used to summarize the agreement between two measurement systems. We also make recommendations for a study design that facilitates accurate and precise estimation of the probability of agreement
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