4,550 research outputs found
Signal detection for spectroscopy and polarimetry
The analysis of high spectral resolution spectroscopic and
spectropolarimetric observations constitute a very powerful way of inferring
the dynamical, thermodynamical, and magnetic properties of distant objects.
However, these techniques are photon-starving, making it difficult to use them
for all purposes. One of the problems commonly found is just detecting the
presence of a signal that is buried on the noise at the wavelength of some
interesting spectral feature. This is specially relevant for
spectropolarimetric observations because typically, only a small fraction of
the received light is polarized. We present in this note a Bayesian technique
for the detection of spectropolarimetric signals. The technique is based on the
application of the non-parametric relevance vector machine to the observations,
which allows us to compute the evidence for the presence of the signal and
compute the more probable signal. The method would be suited for analyzing data
from experimental instruments onboard space missions and rockets aiming at
detecting spectropolarimetric signals in unexplored regions of the spectrum
such as the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) sounding
rocket experiment.Comment: 10 pages, 5 figures, accepted for publication in A&
Estimating the magnetic field strength from magnetograms
A properly calibrated longitudinal magnetograph is an instrument that
measures circular polarization and gives an estimation of the magnetic flux
density in each observed resolution element. This usually constitutes a lower
bound of the field strength in the resolution element, given that it can be
made arbitrarily large as long as it occupies a proportionally smaller area of
the resolution element and/or becomes more transversal to the observer and
still produce the same magnetic signal. Yet, we know that arbitrarily stronger
fields are less likely --hG fields are more probable than kG fields, with
fields above several kG virtually absent-- and we may even have partial
information about its angular distribution. Based on a set of sensible
considerations, we derive simple formulae based on a Bayesian analysis to give
an improved estimation of the magnetic field strength for magnetographs.Comment: 8 pages, 7 figures, accepted for publication in A&
Cartografía de severidad de incendios forestales a partir de la combinación del modelo de mezclas espectrales y la clasificación basada en objetos
This study shows an accurate and fast methodology in order to evaluate fire severity classes of large forest fires. A single Landsat Enhanced Thematic Mapper multispectral image was utilized in this study with the aim of mapping fire severity classes (high, moderate and low) using a combined-approach based in an spectral mixing model and object-based image analysis. A large wildfire in the Northwest of Spain is used to test the model. Fraction images obtained by Landsat unmixing were used as input data in the object-based image analysis. A multilevel segmentation and a classification were carried out by using membership functions. This method was compared with other simplest ones in order to evaluate the suitability to distinguish between the three fire severity classes above mentioned. McNemar’s test was used to evaluate the statistical significance of the difference between approaches tested in this study. The combined approach achieved the highest accuracy reaching 97.32% and kappa index of agreement of 95.96% and improving accuracy of individual classes.Este estudio presenta una metodología rápida y precisa para la evaluación de los niveles de severidad que afectan a grandes incendios forestales. El trabajo combina un modelo de mezclas espectrales y un análisis de imágenes basado en objetos con el objetivo de cartografiar distintos niveles de severidad (alto, moderado y bajo) empleando una imagen multiespectral Landsat Enhanced Thematic Mapper. Este modelo es testado en un gran incendio forestal ocurrido en el noroeste de España. Las imágenes fracción obtenidas tras aplicar el modelo de mezclas a la imagen Landsat fueron utilizadas como datos de entrada en el análisis basado en objetos. En este se llevó a cabo una segmentación multinivel y una posterior clasificación usando funciones de pertenencia. Esta metodología fue comparada con otras más simples con el fin de evaluar su conveniencia a al hora de distinguir entre los tres niveles de severidad anteriormente mencionados. El test de McNemar fue empleado para evaluar la significancia estadística de la diferencia entre los métodos testados en el estudio. El método combinado alcanzó la más alta precisión con un 97,32% y un índice Kappa del 95,96%, además de mejorar la precisión de los niveles individualmente
Scattering polarization in the CaII Infrared Triplet with Velocity Gradients
Magnetic field topology, thermal structure and plasma motions are the three
main factors affecting the polarization signals used to understand our star. In
this theoretical investigation, we focus on the effect that gradients in the
macroscopic vertical velocity field have on the non-magnetic scattering
polarization signals, establishing the basis for general cases. We demonstrate
that the solar plasma velocity gradients have a significant effect on the
linear polarization produced by scattering in chromospheric spectral lines. In
particular, we show the impact of velocity gradients on the anisotropy of the
radiation field and on the ensuing fractional alignment of the CaII levels, and
how they can lead to an enhancement of the zero-field linear polarization
signals. This investigation remarks the importance of knowing the dynamical
state of the solar atmosphere in order to correctly interpret
spectropolarimetric measurements, which is important, among other things, for
establishing a suitable zero field reference case to infer magnetic fields via
the Hanle effect.Comment: 14 pages, 10 figures, 3 appendixes, accepted for publication in Ap
Near-IR internetwork spectro-polarimetry at different heliocentric angles
The analysis of near infrared spectropolarimetric data at the internetwork at
different regions on the solar surface could offer constraints to reject
current modeling of these quiet areas.
We present spectro-polarimetric observations of very quiet regions for
different values of the heliocentric angle for the Fe I lines at 1.56 micron,
from disc centre to positions close to the limb. The spatial resolution of the
data is 0.7-1". We analyze direct observable properties of the Stokes profiles
as the amplitude of circular and linear polarization as well as the total
degree of polarization. Also the area and amplitude asymmetries are studied.
We do not find any significant variation of the properties of the
polarimetric signals with the heliocentric angle. This means that the magnetism
of the solar internetwork remains the same regardless of the position on the
solar disc. This observational fact discards the possibility of modeling the
internetwork as a Network-like scenario. The magnetic elements of internetwork
areas seem to be isotropically distributed when observed at our spatial
resolution.Comment: Sorry, this is the version with the correct bibliography. Some
figures had to be compressed. Accepted for publication in A&
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