1,312 research outputs found
Fluxtube model atmospheres and Stokes V zero-crossing wavelengths
First results of the inversion of Stokes I and V profiles from plage regions
near disk center are presented. Both low and high spatial resolution spectra of
FeI 6301.5 and FeI 6302.5 A obtained with the Advanced Stokes Polarimeter (ASP)
have been considered for analysis. The thin flux tube approximation,
implemented in an LTE inversion code based on response functions, is used to
describe unresolved magnetic elements. The code allows the simultaneous and
consistent inference of all atmospheric quantities determining the radiative
transfer with the sole assumption of hydrostatic equilibrium. By considering
velocity gradients within the tubes we are able to match the full ASP Stokes
profiles. The magnetic atmospheres derived from the inversion are characterized
by the absence of significant motions in high layers and strong velocity
gradients in deeper layers. These are essential to reproduce the asymmetries of
the observed profiles. Our scenario predicts a shift of the Stokes V
zero-crossing wavelengths which is indeed present in observations made with the
Fourier Transform Spectrometer.Comment: To appear in ApJ Letters (1997) (in press
Effect of Yb concentration on the resistivity and lifetime of CdTe:Ge:Yb codoped crystals
The resistivity and electron lifetime of CdTe:Ge:Yb crystals are reported, demonstrating that the
effect of Yb concentration is crucial for accurate electrical compensation. It is also demonstrated
that the codoping of CdTe with Ge as deep donor and with Yb as rare-earth element could be a
promising way to obtain semiinsulating CdTe crystals with good transport properties. High
resistivity 5 10
9
cm and lifetime 9 s were obtained, thus confirming the beneficial effect of
rare-earth dopingThis work has been partly supported by the projects
CAM SENSORCDT S-0505/MAT/0209, CAM FOTOFLEX S-0505/ENE-123, and EU FP6 PHOLOGIC 017158. E.S.
also acknowledges the Spanish MEC for the fellowship FPU
2003-1388.Peer reviewe
Models and Observations of Sunspot Penumbrae
The mysteries of sunspot penumbrae have been under an intense scrutiny for
the past 10 years. During this time, some models have been proposed and
refuted, while the surviving ones had to be modified, adapted and evolved to
explain the ever-increasing array of observational constraints. In this
contribution I will review two of the present models, emphasizing their
contributions to this field, but also pinpointing some of their inadequacies to
explain a number of recent observations at very high spatial resolution. To
help explaining these new observations I propose some modifications to each of
them. These modifications bring those two seemingly opposite models closer
together into a general picture that agrees well with recent 3D
magneto-hydrodynamic simulations.Comment: 9 pages, 1 color figure. Review talk to appear in the proceedings of
the International Workshop of 2008 Solar Total Eclipse: Solar Magnetism,
Corona and Space Weather--Chinese Space Solar Telescope Scienc
Revealing the Jet Structure of GRB 030329 with High Resolution Multicolor Photometry
We present multicolor optical observations of the nearby (z=0.1685) GRB
030329 obtained with the same instrumentation over a time period of 6 hours for
a total of an unprecedented 475 quasi-simultaneous BVR observations. The
achromatic steepening in the optical, which occurs at t~0.7 days, provides
evidence for a dynamic transition of the source, and can be most readily
explained by models in which the GRB ejecta are collimated into a jet. Since
the current state-of-the-art modeling of GRB jets is still flawed with
uncertainties, we use these data to critically assess some classes of models
that have been proposed in the literature. The data, especially the smooth
decline rate seen in the optical afterglow, are consistent with a model in
which GRB 030329 was a homogeneous, sharp-edged jet, viewed near its edge
interacting with a uniform external medium, or viewed near its symmetry axis
with a stratified wind-like external environment. The lack of short timescale
fluctuations in the optical afterglow flux down to the 0.5 per cent level puts
stringent constraints on possible small scale angular inhomogeneities within
the jet or fluctuations in the external density
Influence of phase-diversity image reconstruction techniques on circular polarization asymmetries
Full Stokes filter-polarimeters are key instruments for investigating the
rapid evolution of magnetic structures on the solar surface. To this end, the
image quality is routinely improved using a-posteriori image reconstruction
methods. We analyze the robustness of circular polarization asymmetries to
phase-diversity image reconstruction techniques. We use snapshots of
magneto-hydrodynamical simulations carried out with different initial
conditions to synthesize spectra of the magnetically sensitive Fe I line at
5250.2 A. We degrade the synthetic profiles spatially and spectrally to
simulate observations with the IMaX full Stokes filter-polarimeter. We also
simulate the focused/defocused pairs of images used by the phase-diversity
algorithm for reconstruction and the polarimetric modulation scheme. We assume
that standard optimization methods are able to infer the projection of the
wavefront on the Zernike polynomials with 10% precision. We also consider the
less favorable case of 25% precision. We obtain reconstructed monochromatic
modulated images that are later demodulated and compared with the original
maps. Although asymmetries are often difficult to define in the quiet Sun due
to the complexity of the Stokes V profiles, we show how asymmetries are
degraded with spatial and spectral smearing. The results indicate that,
although image reconstruction techniques reduce the spatial smearing, they can
modify the asymmetries of the profiles, mainly caused by the appearance of
spatially-correlated noise.Comment: 10 pages, accepted for publication in A&
Glycation does not modify bovine serum albumin (BSA)-induced reduction of rat aortic relaxation: The response to glycated and nonglycated BSA is lost in metabolic syndrome
The effects of nonglycated bovine serum albumin (BSA) and advanced glycosylation end products of BSA (AGE-BSA) on vascular responses of control and metabolic syndrome (MS) rats characterized by hypertriglyceridemia, hypertension, hyperinsulinemia, and insulin resistance were studied. Albumin and in vitro prepared AGE-BSA have vascular effects; however, recent studies indicate that some effects of in vitro prepared AGEs are due to the conditions in which they were generated. We produced AGEs by incubating glucose with BSA for 60 days under sterile conditions in darkness and at 37°C. To develop MS rats, male Wistar animals were given 30% sucrose in drinking water since weanling. Six month old animals were used. Blood pressure, insulin, triglycerides, and serum albumin were increased in MS rats. Contraction of aortic rings elicited with norepinephrine was stronger. There were no effects of nonglycated BSA or AGE-BSA on contractions in control or MS rats; however, both groups responded to L-NAME, an inhibitor of nitric oxide synthesis. Arterial relaxation induced using acetylcholine was smaller in MS rats. Nonglycated BSA and AGE-BSA significantly diminished relaxation in a 35% in the control group but the decrease was similar when using nonglycated BSA and AGE-BSA. This decrease was not present in the MS rats and was not due to increased RAGEs or altered biochemical characteristics of BSA. In conclusion, both BSA and AGE-BSA inhibit vascular relaxation in control artic rings. In MS rats the effect is lost possibly due to alterations in endothelial cells that are a consequence of the illness
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