scholarly journals Sparse interferometric Stokes imaging under the polarization constraint (Polarized SARA)

2018 ◽  
Vol 478 (4) ◽  
pp. 4442-4463 ◽  
Author(s):  
Jasleen Birdi ◽  
Audrey Repetti ◽  
Yves Wiaux
Keyword(s):  
Stokes Parameter ◽  
Stokes Parameters ◽  
Data Sets ◽  
Radio Interferometry ◽  
Matlab Code ◽  
Polarimetric Imaging ◽  
Primal Dual ◽  
Projection Techniques ◽  
Superior Image Quality ◽  
Novel Algorithm

ABSTRACT We develop a novel algorithm for sparse imaging of Stokes parameters in radio interferometry under the polarization constraint. The latter is a physical non-linear relation between the Stokes parameters, imposing the polarization intensity as a lower bound on the total intensity. To solve the joint inverse Stokes imaging problem including this bound, we leverage epigraphical projection techniques in convex optimization and we design a primal–dual method offering a highly flexible and parallelizable structure. In addition, we propose to regularize each Stokes parameter map through an average sparsity prior in the context of a reweighted analysis approach (SARA). The resulting method is dubbed Polarized SARA. Using simulated observations of M87 with the Event Horizon Telescope, we demonstrate that imposing the polarization constraint leads to superior image quality. For the considered data sets, the results also indicate better performance of the average sparsity prior in comparison with the widely used Cotton–Schwab clean algorithm and other total variation based priors for polarimetric imaging. Our matlab code is available online on GitHub.

scholarly journals Modelling of the Magnetic Configuration of CP Stars from Polarimetric Observations

1998 ◽  
Vol 11 (2) ◽  
pp. 679-681
Author(s):  
M. Landolfi
Keyword(s):  
Magnetic Field ◽  
Stokes Parameter ◽  
Quadratic Field ◽  
Mean Field ◽  
Longitudinal Component ◽  
Stokes Parameters ◽  
Longitudinal Field ◽  
The Mean ◽  
The Magnetic Field ◽  
Polarized Radiation

The observational quantities commonly used to study the magnetic field of CP stars – the mean field modulus and the mean longitudinal field, as well as the ‘mean asymmetry of the longitudinal field’ and the ‘mean quadratic field’ recently introduced by Mathys (1995a,b) – are based either on the Stokes parameter / or on the Stokes parameter V. However, a complete description of polarized radiation requires the knowledge of the full Stokes vector: in other words, we should expect that useful information is also contained in linear polarization (the Stokes parameters Q and U); or rather we should expect the information contained in (Q, U) and in V to be complementary, since linear and circular polarization are basically related to the transverse and the longitudinal component of the magnetic field, respectively.

Discovering Frequent Embedded Subtree Patterns from Large Databases of Unordered Labeled Trees

2008 ◽  
pp. 3235-3251
Author(s):  
Yongqiao Xiao ◽  
Jenq-Foung Yao ◽  
Guizhen Yang
Keyword(s):  
Knowledge Discovery ◽  
Web Sites ◽  
Synthetic Data ◽  
Research Interest ◽  
Data Sets ◽  
Labeled Trees ◽  
Computational Overhead ◽  
Large Databases ◽  
Frequency Counts ◽  
Novel Algorithm

Recent years have witnessed a surge of research interest in knowledge discovery from data domains with complex structures, such as trees and graphs. In this paper, we address the problem of mining maximal frequent embedded subtrees which is motivated by such important applications as mining “hot” spots of Web sites from Web usage logs and discovering significant “deep” structures from tree-like bioinformatic data. One major challenge arises due to the fact that embedded subtrees are no longer ordinary subtrees, but preserve only part of the ancestor-descendant relationships in the original trees. To solve the embedded subtree mining problem, in this article we propose a novel algorithm, called TreeGrow, which is optimized in two important respects. First, it obtains frequency counts of root-to-leaf paths through efficient compression of trees, thereby being able to quickly grow an embedded subtree pattern path by path instead of node by node. Second, candidate subtree generation is highly localized so as to avoid unnecessary computational overhead. Experimental results on benchmark synthetic data sets have shown that our algorithm can outperform unoptimized methods by up to 20 times.

scholarly journals Multilabel Classification with Principal Label Space Transformation

2012 ◽  
Vol 24 (9) ◽  
pp. 2508-2542 ◽  
Author(s):  
Farbound Tai ◽  
Hsuan-Tien Lin
Keyword(s):  
Singular Value ◽  
Data Sets ◽  
Classification Problems ◽  
Real World Data ◽  
Multilabel Classification ◽  
Binary Relevance ◽  
Space Transformation ◽  
Empirical Performance ◽  
Value Decomposition ◽  
Novel Algorithm

We consider a hypercube view to perceive the label space of multilabel classification problems geometrically. The view allows us not only to unify many existing multilabel classification approaches but also design a novel algorithm, principal label space transformation (PLST), that captures key correlations between labels before learning. The simple and efficient PLST relies on only singular value decomposition as the key step. We derive the theoretical guarantee of PLST and evaluate its empirical performance using real-world data sets. Experimental results demonstrate that PLST is faster than the traditional binary relevance approach and is superior to the modern compressive sensing approach in terms of both accuracy and efficiency.

scholarly journals The V-Stokes Parameter as a Manifestation of Envelope Activity for Cool, Bright, Evolved Stars

1988 ◽  
Vol 108 ◽  
pp. 189-190
Author(s):  
B. D. Holenstein ◽  
R. H. Koch ◽  
R. J. Pfeiffer
Keyword(s):  
Radiation Field ◽  
Stokes Parameter ◽  
Stokes Parameters ◽  
Close Binaries ◽  
Evolved Stars

At Pennsylvania’s Flower and Cook Observatory, instrumentation has been developed to measure simultaneously the four Stokes parameters of the filtered radiation field from a celestial source. The instrumental Q/U/V-parameters have been found to be very small and well-behaved. Thus far, the program has concentrated on cool bright giants and supergiants and on hot, evolving close binaries. A single season’s investigation of Alp Ori has already been reported (Holenstein 1987) and the present paper is a summary of current results for the cool, evolved program stars.For Psi1 Aur, V CVn, 6 Gem, 72 Leo and 119 Tau no V-signal at the level of 3σhas been detected from data from the 1986-1987 season. At the level of 0.0n%, unambiguous and variable V-signals have been detected for VV Cep, Mu Cep, Alp Her, Alp Ori, Bet Peg, and Alp Sco.

A Novel Deduction of Stokes Parameters Based on 2-ordered Basis Matrices

2014 ◽  
Vol 556-562 ◽  
pp. 3598-3601
Author(s):  
Jin Zu Ji
Keyword(s):  
Electromagnetic Wave ◽  
Complex Number ◽  
Coordinate Transformation ◽  
Stokes Parameter ◽  
The Other ◽  
Stokes Parameters ◽  
Algebra Structure ◽  
Wave Polarization ◽  
Matrix Operation ◽  
Matrix Problems

Four two-ordered basis matrices were introduced to deduct Stokes parameter representing electromagnetic wave polarization. Two of them have algebra features similar to unity and imaginary of complex number, so an exponential function was defined which can easily express some matrix operation such as coordination rotation in 2-dimension. The other 2 basis matrices were also incorporated to deduce the Stokes parameters. The four matrices have some fatigue algebra structure which was incorporated in deduction. The coordinate transformation was converted to some matrix problems and we get a more concise result.

scholarly journals A New Multiphase Soft Segmentation with Adaptive Variants

2013 ◽  
Vol 2013 ◽  
pp. 1-9
Author(s):  
Hongyuan Wang ◽  
Fuhua Chen ◽  
Yunmei Chen
Keyword(s):  
Gaussian Distribution ◽  
Random Variables ◽  
Bias Field ◽  
Projection Methods ◽  
Membership Functions ◽  
Dual Theory ◽  
Piecewise Constant ◽  
Primal Dual ◽  
The Mean ◽  
Novel Algorithm

Soft segmentation is more flexible than hard segmentation. But the membership functions are usually sensitive to noise. In this paper, we propose a multiphase soft segmentation model for nearly piecewise constant images based on stochastic principle, where pixel intensities are modeled as random variables with mixed Gaussian distribution. The novelty of this paper lies in three aspects. First, unlike some existing models where the mean of each phase is modeled as a constant and the variances for different phases are assumed to be the same, the mean for each phase in the Gaussian distribution in this paper is modeled as a product of a constant and a bias field, and different phases are assumed to have different variances, which makes the model more flexible. Second, we develop a bidirection projected primal dual hybrid gradient (PDHG) algorithm for iterations of membership functions. Third, we also develop a novel algorithm for explicitly computing the projection fromRKto simplexΔK-1for any dimensionKusing dual theory, which is more efficient in both coding and implementation than existing projection methods.

scholarly journals RETRIEVAL OF AEROSOL PHASE FUNCTION AND POLARIZED PHASE FUNCTION FROM POLARIZATION OF SKYLIGHT FOR DIFFERENT OBSERVATION GEOMETRIES

2018 ◽  
Vol XLII-3 ◽  
pp. 857-863
Author(s):  
L. Li ◽  
L. L. Qie ◽  
H. Xu ◽  
Z. Q. Li
Keyword(s):  
Zenith Angle ◽  
Phase Function ◽  
Solar Zenith Angle ◽  
Stokes Parameter ◽  
Principal Plane ◽  
Stokes Parameters ◽  
Retrieval Algorithm ◽  
Optical Parameters ◽  
Skylight Polarization ◽  
Observation Geometry

The phase function and polarized phase function are important optical parameters, which describe scattering properties of atmospheric aerosol particles. Polarization of skylight induced by the scattering processes is sensitive to the scattering properties of aerosols. The Stokes parameters <i>I, Q, U</i> and the polarized radiance <i>L<sub>p</sub></i> of skylight measured by the CIMEL dual-polar sun-sky radiometer CE318- DP can be use to retrieve the phase function and polarized phase function, respectively. Two different observation geometries (i.e., the principal plane and almucantar) are preformed by the CE318-DP to detect skylight polarization. Polarization of skylight depends on the illumination and observation geometries. For the same solar zenith angle, retrievals of the phase function and the polarized phase function are still affected by the observation geometry. The performance of the retrieval algorithm for the principal plane and almucantar observation geometries was assessed by the numerical experiments at two typical high and low sun’s positions (i.e. solar zenith angles are equal to 45&amp;deg; and 65&amp;deg;). Comparing the results for the principal plane and almucantar geometries, it is recommended to utilize the principal plane observations to retrieve the phase function when the solar zenith angle is small. The Stokes parameter <i>U</i> and the polarized radiance <i>L<sub>p</sub></i> from the almucantar observations are suggested to retrieve the polarized phase function, especially for short wavelength channels (e.g., 440 and 500&amp;thinsp;nm).

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