scholarly journals Three-lobed near-infrared Stokes V profiles in the quiet Sun

2018 ◽  
Vol 616 ◽  
pp. A109 ◽  
Author(s):  
Christoph Kiess ◽  
Juan Manuel Borrero ◽  
Wolgang Schmidt
Keyword(s):  
Magnetic Properties ◽  
Circular Polarization ◽  
Optical Depth ◽  
Near Infrared ◽  
Solar Spectrum ◽  
Line Of Sight ◽  
Physical Parameters ◽  
Solar Photosphere ◽  
Complex Signals ◽  
Resolution Element

Context. The 1.5-m GREGOR solar telescope can resolve structures as small as 0.4′′ at near-infrared wavelengths on the Sun. At this spatial resolution the polarized solar spectrum shows complex patterns, such as large horizontal and/or vertical variations of the physical parameters in the solar photosphere. Aims. We investigate a region of the quiet solar photosphere exhibiting three-lobed Stokes V profiles in the Fe I spectral line at 15 648 Å. The data were acquired with the GRIS spectropolarimeter attached to the GREGOR telescope. We aim at investigating the thermal, kinematic and magnetic properties of the atmosphere responsible for these measured complex signals. Methods. The SIR inversion code is employed to retrieve the physical parameters of the lower solar photosphere from the observed polarization signals. We follow two different approaches. On the one hand, we consider that the multi-lobe circular polarization signals are only produced by the line-of-sight variation of the physical parameters. We therefore invert the data assuming a single atmospheric component that occupies the entire resolution element in the horizontal plane and where the physical parameters vary with optical depth τ (i.e., line-of-sight). On the other hand, we consider that the multi-lobe circular polarization signals are produced not by the optical depth variations of the physical parameters but instead by their horizontal variations. Here we invert the data assuming that the resolution element is occupied by two different atmospheric components where the kinematic and magnetic properties are constant along the line-of-sight. Results. Both approaches reveal some common features about the topology responsible for the observed three-lobed Stokes V signals: both a strong (>1000 Gauss) and a very weak (<10 Gauss) magnetic field with opposite polarities and harboring flows directed in opposite directions must co-exist (either vertically or horizontally interlaced) within the resolution element. Conclusions.

scholarly journals Ground Reflectance Retrieval on Horizontal and Inclined Terrains Using the Software Package REFLECT

2018 ◽  
Vol 10 (10) ◽  
pp. 1638
Author(s):  
Yacine Bouroubi ◽  
Wided Batita ◽  
François Cavayas ◽  
Nicolas Tremblay
Keyword(s):  
Optical Properties ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Software Package ◽  
Near Infrared ◽  
Solar Spectrum ◽  
Sensor Calibration ◽  
Topographic Effects ◽  
Atmospheric Conditions ◽  
Spectral Bands

This paper presents the software package REFLECT for the retrieval of ground reflectance from high and very-high resolution multispectral satellite images. The computation of atmospheric parameters is based on the 6S (Second Simulation of the Satellite Signal in the Solar Spectrum) routines. Aerosol optical properties are calculated using the OPAC (Optical Properties of Aerosols and Clouds) model, while aerosol optical depth is estimated using the dark target method. A new approach is proposed for adjacency effect correction. Topographic effects were also taken into account, and a new model was developed for forest canopies. Validation has shown that ground reflectance estimation with REFLECT is performed with an accuracy of approximately ±0.01 in reflectance units (for the visible, near-infrared, and mid-infrared spectral bands), even for surfaces with varying topography. The validation of the software was performed through many tests. These tests involve the correction of the effects that are associated with sensor calibration, irradiance, and viewing conditions, atmospheric conditions (aerosol optical depth AOD and water vapour), adjacency, and topographic conditions.

Disordered Photonic Crystal Slabs for Thin-Film Photovoltaics

2018 ◽  
Author(s):  
Chelsea Carlson
Keyword(s):  
Thin Film ◽  
Photonic Crystal ◽  
Near Infrared ◽  
Crystalline Silicon ◽  
Solar Spectrum ◽  
Silicon Layer ◽  
Physical Parameters ◽  
Photonic Crystal Slabs ◽  
Optical Applications ◽  
Conversion Efficiencies

Photonic crystal nanostructures are the foundation for many optical applications such as nanochip waveguides, optical fibres, and high-Q nanocavities. Recently, researchers have begun to explore the use of photonic crystal slabs to increase the overall absorption of sunlight in thin-film solar photovoltaic (PV) cells. Currently, amorphous silicon (a-Si:H) thin-film technologies can only achieve efficiencies of up to 16% in laboratories and less than 10% in manufactured commercial products. The difficulty in improving these efficiencies arises from the inherent band gap properties of the crystalline silicon layer: the natural photonic bandgap in the near infrared (IR) region of light prohibits almost a third of the entire available solar spectrum from being absorbed. Some of this loss can be salvaged by increasing the thickness of the silicon layer, but this drives the price of the cell up and has very limited potential. However, using photonic crystal nanostructures in the active layer of the cell can decrease the reflection of light at the surface and increase the photon path within the film, enhancing the collection and conversion efficiencies over a broad spectrum. The absorption can be further increased by introducing pseudo-disorder within the structures. The purpose of this study was to explore the physical parameters of this disorder and quantitatively optimize absorption.

scholarly journals Physical parameters of molecular clumps in the S254—S258 star formation region

2021 ◽  
Author(s):  
E. A. Popova ◽  
◽  
D. A. Ladeyschikov ◽  
M. S. Kirsanova ◽  
A. M. Sobolev ◽  
...  
Keyword(s):  
Star Formation ◽  
Optical Depth ◽  
Near Infrared ◽  
Continuum Emission ◽  
Physical Parameters ◽  
Star Formation Region ◽  
Formation Region ◽  
Molecular Lines ◽  
The Difference ◽  
Infrared Extinction

This research is focusing on studying of physical parameters in the S254—S258 star-formation complex. The following parameters were derived: column densities, clump masses, and highdensity tracers optical depth, including CS and HCO+ molecules. Different gas and dust tracers maps were used, including CO molecular lines, Bolocam 1.1 mm continuum emission and near-infrared extinction map. We also used the available CS and HCO+ data, which cover part of the S254-S258 region to make conclusions regarding the distribution of highdensity gas. The resulted values of physical parameters obtained through different tracers are slightly different. The discussion about the possible reason for the difference is provided.

scholarly journals A hundred new eclipsing binary system candidates studied in a near-infrared window in the VVV survey

2020 ◽  
Vol 37 ◽  
Author(s):  
L. V. Gramajo ◽  
T. Palma ◽  
D. Minniti ◽  
R. K. Saito ◽  
J. J. Clariá ◽  
...  
Keyword(s):  
Near Infrared ◽  
Galactic Disk ◽  
Extensive Study ◽  
Eclipsing Binaries ◽  
Physical Parameters ◽  
Eclipsing Binary ◽  
Mean Values ◽  
Median Period ◽  
First Results ◽  
Contact Binaries

Abstract We present the first results obtained from an extensive study of eclipsing binary (EB) system candidates recently detected in the VISTA Variables in the Vía Láctea (VVV) near-infrared (NIR) Survey. We analyse the VVV tile d040 in the southern part of the Galactic disc wherein the interstellar reddening is comparatively low, which makes it possible to detect hundreds of new EB candidates. We present here the light curves and the determination of the geometric and physical parameters of the best candidates found in this ‘NIR window’, including 37 contact, 50 detached, and 13 semi-detached EB systems. We infer that the studied systems have an average of the $K_s$ amplitudes of $0.8$ mag and a median period of 1.22 days where, in general, contact binaries have shorter periods. Using the ‘Physics Of Eclipsing Binaries’ (PHOEBE) interactive interface, which is based on the Wilson and Devinney code, we find that the studied systems have low eccentricities. The studied EBs present mean values of about 5 700 and 4 900 K for the $T_1$ and $T_2$ components, respectively. The mean mass ratio (q) for the contact EB stars is $\sim$ 0.44. This new galactic disk sample is a first look at the massive study of NIR EB systems.

scholarly journals Broadband Linear Polarization in Ap Stars

1993 ◽  
Vol 138 ◽  
pp. 305-309
Author(s):  
Marco Landolfi ◽  
Egidio Landi Degl’Innocenti ◽  
Maurizio Landi Degl’Innocenti ◽  
Jean-Louis Leroy ◽  
Stefano Bagnulo
Keyword(s):  
Magnetic Field ◽  
Circular Polarization ◽  
Linear Polarization ◽  
Rotation Axis ◽  
Spectral Lines ◽  
Line Of Sight ◽  
Long Time ◽  
Rotating Star ◽  
Ap Stars ◽  
The Magnetic Field

AbstractBroadband linear polarization in the spectra of Ap stars is believed to be due to differential saturation between σ and π Zeeman components in spectral lines. This mechanism has been known for a long time to be the main agent of a similar phenomenon observed in sunspots. Since this phenomenon has been carefully calibrated in the solar case, it can be confidently used to deduce the magnetic field of Ap stars.Given the magnetic configuration of a rotating star, it is possible to deduce the broadband polarization at any phase. Calculations performed for the oblique dipole model show that the resulting polarization diagrams are very sensitive to the values of i (the angle between the rotation axis and the line of sight) and β (the angle between the rotation and magnetic axes). The dependence on i and β is such that the four-fold ambiguity typical of the circular polarization observations ((i,β), (β,i), (π-i,π-β), (π-β,π-i)) can be removed.

scholarly journals Spectral Derivatives of Optical Depth for Partitioning Aerosol Type and Loading

2021 ◽  
Vol 13 (8) ◽  
pp. 1544
Author(s):  
Tang-Huang Lin ◽  
Si-Chee Tsay ◽  
Wei-Hung Lien ◽  
Neng-Huei Lin ◽  
Ta-Chih Hsiao
Keyword(s):  
Optical Depth ◽  
Complex Refractive Index ◽  
Solar Spectrum ◽  
Collective Models ◽  
Anthropogenic Pollutants ◽  
Satellite Signal ◽  
Aerosol Compositions ◽  
Theoretical Simulations ◽  
Derivatives Of ◽  
Aerosol Index

Quantifying aerosol compositions (e.g., type, loading) from remotely sensed measurements by spaceborne, suborbital and ground-based platforms is a challenging task. In this study, the first and second-order spectral derivatives of aerosol optical depth (AOD) with respect to wavelength are explored to determine the partitions of the major components of aerosols based on the spectral dependence of their particle optical size and complex refractive index. With theoretical simulations from the Second Simulation of a Satellite Signal in the Solar Spectrum (6S) model, AOD spectral derivatives are characterized for collective models of aerosol types, such as mineral dust (DS) particles, biomass-burning (BB) aerosols and anthropogenic pollutants (AP), as well as stretching out to the mixtures among them. Based on the intrinsic values from normalized spectral derivatives, referenced as the Normalized Derivative Aerosol Index (NDAI), a unique pattern is clearly exhibited for bounding the major aerosol components; in turn, fractions of the total AOD (fAOD) for major aerosol components can be extracted. The subtlety of this NDAI method is examined by using measurements of typical aerosol cases identified carefully by the ground-based Aerosol Robotic Network (AERONET) sun–sky spectroradiometer. The results may be highly practicable for quantifying fAOD among mixed-type aerosols by means of the normalized AOD spectral derivatives.

scholarly journals Exploring the inner parsecs of active galactic nuclei using near-infrared high resolution polarimetric simulations with MontAGN

2018 ◽  
Vol 612 ◽  
pp. A69 ◽  
Author(s):  
L. Grosset ◽  
D. Rouan ◽  
D. Gratadour ◽  
D. Pelat ◽  
J. Orkisz ◽  
...  
Keyword(s):  
Active Galactic Nuclei ◽  
Optical Depth ◽  
Near Infrared ◽  
Galactic Nuclei ◽  
Constant Density ◽  
Characteristic Model ◽  
Infrared Observations ◽  
Current Estimation ◽  
Direct Light ◽  
Ngc 1068

Aims. In this paper we aim to constrain the properties of dust structures in the central first parsecs of active galactic nuclei (AGN). Our goal is to study the required optical depth and composition of different dusty and ionised structures. Methods. We developed a radiative transfer code called Monte Carlo for Active Galactic Nuclei (MontAGN), which is optimised for polarimetric observations in the infrared. With both this code and STOKES, designed to be relevant from the hard X-ray band to near-infrared wavelengths, we investigate the polarisation emerging from a characteristic model of the AGN environment. For this purpose, we compare predictions of our models with previous infrared observations of NGC 1068, and try to reproduce several key polarisation patterns revealed by polarisation mapping. Results. We constrain the required dust structures and their densities. More precisely, we find that the electron density inside the ionisation cone is about 2.0 × 109 m−3. With structures constituted of spherical grains of constant density, we also highlight that the torus should be thicker than 20 in term of K-band optical depth to block direct light from the centre. It should also have a stratification in density: a less dense outer rim with an optical depth at 2.2 μm typically between 0.8 and 4 for observing the double scattering effect previously proposed. Conclusions. We bring constraints on the dust structures in the inner parsecs of an AGN model supposed to describe NGC 1068. When compared to observations, this leads to an optical depth of at least 20 in the Ks band for the torus of NGC 1068, corresponding to τV ≈ 170, which is within the range of current estimation based on observations. In the future, we will improve our study by including non-uniform dust structures and aligned elongated grains to constrain other possible interpretations of the observations.

scholarly journals Sea Ice Albedo from MISR and MODIS: Production, Validation, and Trend Analysis

2018 ◽  
Vol 11 (1) ◽  
pp. 9 ◽  
Author(s):  
Said Kharbouche ◽  
Jan-Peter Muller
Keyword(s):  
Sea Ice ◽  
Near Infrared ◽  
Time Window ◽  
Solar Spectrum ◽  
High Accuracy ◽  
Dynamic Surface ◽  
Spectral Bands ◽  
Langley Research Center ◽  
Terra Satellite ◽  
Cloud Mask

The Multi-angle Imaging SpectroRadiometer (MISR) sensor onboard the Terra satellite provides high accuracy albedo products. MISR deploys nine cameras each at different view angles, which allow a near-simultaneous angular sampling of the surface anisotropy. This is particularly important to measure the near-instantaneous albedo of dynamic surface features such as clouds or sea ice. However, MISR’s cloud mask over snow or sea ice is not yet sufficiently robust because MISR’s spectral bands are only located in the visible and the near infrared. To overcome this obstacle, we performed data fusion using a specially processed MISR sea ice albedo product (that was generated at Langley Research Center using Rayleigh correction) combining this with a cloud mask of a sea ice mask product, MOD29, which is derived from the MODerate Resolution Imaging Spectroradiometer (MODIS), which is also, like MISR, onboard the Terra satellite. The accuracy of the MOD29 cloud mask has been assessed as >90% due to the fact that MODIS has a much larger number of spectral bands and covers a much wider range of the solar spectrum. Four daily sea ice products have been created, each with a different averaging time window (24 h, 7 days, 15 days, 31 days). For each time window, the number of samples, mean and standard deviation of MISR cloud-free sea ice albedo is calculated. These products are publicly available on a predefined polar stereographic grid at three spatial resolutions (1 km, 5 km, 25 km). The time span of the generated sea ice albedo covers the months between March and September of each year from 2000 to 2016 inclusive. In addition to data production, an evaluation of the accuracy of sea ice albedo was performed through a comparison with a dataset generated from a tower based albedometer from NOAA/ESRL/GMD/GRAD. This comparison confirms the high accuracy and stability of MISR’s sea ice albedo since its launch in February 2000. We also performed an evaluation of the day-of-year trend of sea ice albedo between 2000 and 2016, which confirm the reduction of sea ice shortwave albedo with an order of 0.4–1%, depending on the day of year and the length of observed time window.

scholarly journals Combining visible and infrared radiometry and lidar data to test ice clouds optical properties

2010 ◽  
Vol 10 (3) ◽  
pp. 7215-7264
Author(s):  
A. Bozzo ◽  
T. Maestri ◽  
R. Rizzi
Keyword(s):  
High Resolution ◽  
Optical Depth ◽  
Near Infrared ◽  
Natural Variability ◽  
Ice Clouds ◽  
Cloud Optical Depth ◽  
Cloud Parameters ◽  
Modeling Methodology ◽  
Clear Cases ◽  
Spectral Data Analysis

Abstract. Measurements taken during the 2003 Pacific THORPEX Observing System Test (P-TOST) by the MODIS Airborne Simulator (MAS), the Scanning High-resolution Interferometer Sounder (S-HIS) and the Cloud Physics Lidar (CPL) are compared to simulations performed with a line-by-line and multiple scattering modeling methodology (LBLMS). Formerly used for infrared hyper-spectral data analysis, LBLMS has been extended to the visible and near infrared with the inclusion of surface bi-directional reflectance properties. A number of scenes are evaluated: two clear scenes, one with nadir geometry and one cross-track encompassing sun glint, and three cloudy scenes, all with nadir geometry. CPL data is used to estimate the particulate optical depth at 532 nm for the clear and cloudy scenes. Cloud optical depth is also retrieved from S-HIS infrared window radiances, and it agrees with CPL values, to within natural variability. MAS data are simulated convolving high resolution radiances. The paper discusses the results of the comparisons for the clear cases and for the three cloudy cases. LBLMS clear simulations agree with MAS data to within 20% in the shortwave (SW) and near infrared (NIR) spectrum and within 2 K in the infrared (IR) range. It is shown that cloudy sky simulations using cloud parameters retrieved from IR radiances systematically underestimate the measured radiance in the SW and NIR by nearly 50%, although the IR retrieved optical thickness agree with same measured by CPL. MODIS radiances measured from Terra are also compared to LBLMS simulations in cloudy conditions using retrieved cloud optical depth and effective radius from MODIS, to understand the origin for the observed discrepancies. It is shown that the simulations agree, to within natural variability, with measurements in selected MODIS SW bands. The paper dwells on a possible explanation of these contraddictory results, involving the phase function of ice particles in the shortwave.

scholarly journals The VMC survey

2018 ◽  
Vol 613 ◽  
pp. L8 ◽  
Author(s):  
F. Niederhofer ◽  
M.-R. L. Cioni ◽  
S. Rubele ◽  
T. Schmidt ◽  
K. Bekki ◽  
...  
Keyword(s):  
Near Infrared ◽  
Two Dimensions ◽  
Magellanic Clouds ◽  
Line Of Sight ◽  
Main Body ◽  
Proper Motions ◽  
Velocity Pattern ◽  
Spatially Resolved ◽  
Magnitude Diagram ◽  
Over Time

We present the first spatially resolved map of stellar proper motions within the central (~3.1 × 2.4 kpc) regions of the Small Magellanic Cloud (SMC). The data used for this study encompasses four tiles from the ongoing near-infrared VISTA survey of the Magellanic Clouds system and covers a total contiguous area on the sky of ~6.81 deg2. Proper motions have been calculated independently in two dimensions from the spatial offsets in the Ks filter over time baselines between 22 and 27 months. The reflex motions of approximately 33 000 background galaxies are used to calibrate the stellar motions to an absolute scale. The resulting catalog is composed of more than 690 000 stars which have been selected based on their position in the (J − Ks, Ks) color-magnitude diagram. For the median absolute proper motion of the SMC, we find (μαcos(δ), μδ) = (1.087 ± 0.192 (sys.) ± 0.003 (stat.), −1.187 ± 0.008 (sys.) ± 0.003 (stat.)) mas yr−1, consistent with previous studies. Mapping the proper motions as a function of position within the SMC reveals a nonuniform velocity pattern indicative of a tidal feature behind the main body of the SMC and a flow of stars in the south-east moving predominantly along the line-of-sight.

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