Bright Structures Research Articles

Analysis of Network Security Measures for Protecting Organizational Data and Implement Digital Ideas

Objectives: The goal of this study is to proposed analysis of Efficacious Intellectual Framework for Host based IDS improves the quality of the Association Rule Mining. Methods: For these 15 examiners were appropriated. To create great reactions, a solitary survey is utilized to quantify numerous hypothetical builds of the review. The poll is separated into three fundamental segments, where, the main area manages general inquiries. In the subsequent segment, the respondents are mentioned to pick one explicit elective that presents the situation with given secuity practice in their organization. The Herberos convention was utilized in this review. Results: Brute-Force method is used to reduce the total running time of the framework when the proposed framework with suggested IRG method is used for unknown rule generation and single keyword pattern matching. The proposed framework consumes less memory and less execution time. Conclusions: This approach is taken advantage of to increase the expectation precision. In this commitment, a creative Bright Characterization Structure is wanted to uplift the exactness of the grouping in the IDS. The Proposed PFSM technique prompted seize the unimportant component in the IDS dataset for the characterization of the organization. From the result achieved, it is validated that the proposed approach achieved its uniqueness over the normal element determination method executed in the Interruption Discovery Framework. Additionally, it likewise progresses the forecast exactness and reduces the mistake rates. The minimization of blunder rates is the result of incredible order precision.

A Tale of NGC 3785: The formation of an ultra-diffuse galaxy at the end of the longest tidal tail

Aims. We present the discovery of an extended and faint tail observed in the isolated environment associated with galaxy NGC 3785. This study additionally provides observational evidence supporting the formation of ultra-diffuse galaxies (UDGs) at the end of the tail. Methods. We utilized the Gnuastro software to detect and analyze the low surface brightness structures in the optical g- and r-bands using data from the Dark Energy Camera Legacy Survey. We created a detection map to identify the faint tail and measured its length using cubic spline fitting. Additionally, we found 84 star-forming clumps along the tail and performed photometric analysis on the tail portion after applying a significance threshold on the signal-to-noise ratio. Results. We have measured the projected length of the tail, which is ∼390 kpc. We propose that this tail arises from the interaction of the NGC 3785 with a gas-rich galaxy, which ends up as a UDG at the end of the tail.

Recovery of coronal dimmings

Context. Coronal dimmings are regions of reduced emission in the lower corona observed in the wake of coronal mass ejections (CMEs), representing their footprints. Studying the lifetime evolution of coronal dimmings helps us to better understand the recovery and replenishment of the corona after large-scale eruptions. Aims. We study the recovery of dimmings on different spatial scales to enhance our understanding of the replenishment and dynamics of the corona after CMEs. Methods. In order to investigate the long-term evolution of coronal dimming and its recovery, we propose two approaches that focus on both the global and the local evolution of dimming regions: the fixed mask approach and the pixel boxes approach. We present four case studies (September 6, 2011; March 7, 2012; June 14, 2012; and March 8, 2019) in which a coronal dimming is associated with a flare/CME eruption. We analyzed each event with the same methodology, using extreme-ultraviolet filtergrams from the Solar Dynamics Observatory’s Atmospheric Imaging Assembly (SDO/AIA) and Solar TErrestrial RElations Observatory’s Extreme UltraViolet Imager (STEREO/EUVI) instruments. We identified the dimming region by image segmentation, then restricted the analysis to a specific portion of the dimming and tracked the time evolution of the dimming brightness and area. In addition, we study the behavior of small subregions inside the dimming area, of about 3 × 3 pixels, to compare the recovery in different regions of the dimming. Results. Three out of the four cases show a complete recovery 24 hours after the flare/CME eruption. The primary recovery mechanism identified in the observations is the expansion of coronal loops into the dimming region. The recovery of the brightness follows a two-step trend, with a steeper and quicker segment followed by a slower one. In addition, some parts of the dimming, which may be core dimmings, are still present at the end of the analysis time and do not recover within 3 days, whereas the peripheral regions (secondary dimmings) show a full recovery. Conclusions. The high temporal and spatial resolution of SDO/AIA observations combined with multi-view data of the STEREO/EUV instrument reveal high-situated coronal loops expanding after CME eruptions, which cover dimming regions and gradually increase their intensity. Our developed approaches enable the analysis of dimmings alongside these bright structures, revealing different timescales of recovery for core and secondary or twin dimming regions. Combined with magnetic field modeling, these methods lay the foundation for further systematic analysis of dimming recovery and enhance the knowledge gained from already-analyzed events.

Infrared Stealth Coating with Tunable Structural Color Based on ZnO Spheres.

It is important to develop low infrared (IR) emissive coating with tunable structure color to improve the infrared-visible stealth performance of military equipment. In this work, uniform ZnO spheres are used as building units to construct photonic structures with both bright adjustable structure color and low IR emissivity due to the relatively high refractive index and low IR emissivity of ZnO. The vivid tunable structural colors are provided by the photonic bandgap of ZnO photonic crystals (PCs) or the quasi-bandgap of amorphous photonic crystals (APCs), respectively. Both ZnO PCs and APCs exhibited low IR emissivity in 3-5µm. The IR emissivity of 255nm ZnO PCis 0.483 and the IR emissivity of 255nm ZnO APC is 0.492 at 25°C. With the increase of temperature, the IR emissivity offurther decreased to 0.295 and 0.312 at 300°C. These structures can be applied to a variety of surfaces, and all these structures have good thermal and light stability as well. This work may open a simple and effective way to fabricate materials with good infrared-visible stealth performance, expanding the application of ZnO PCs and APCs coatings in the camouflage area.

Formation mechanism of bright and dark concentric-ring pattern in dielectric barrier discharge

In this work, a bright and dark concentric-ring pattern is reported in a dielectric barrier discharge for the first time. The spatiotemporal dynamics of the bright and dark concentric-ring pattern are investigated with an intensified charge-coupled device and photomultiplier tubes. The results indicate that the bright and dark concentric-ring pattern is composed of three concentric-ring sublattices. These are bright concentric-ring structures, dark concentric-ring structures and wider concentric-ring structures, respectively. The bright concentric-ring structures and dark concentric-ring structures are alternately distributed. The bright concentric-ring structures are located at the centre of the wider concentric-ring structures. The wider concentric-ring structures first form from the outer edge and gradually develop to the centre. The essence of all three concentric-ring structures is the individual discharge filaments. The optical emission spectra of different sublattices are acquired and analysed. It is found that the plasma parameters of the three concentric-ring sublattices are different. Finally, the formation mechanism of the bright and dark concentric-ring pattern is discussed.

Saturation and absorption effects on the transmission properties of Kerr-type nonlinear layers

The effects of absorption and saturation on soliton states in Kerr-type nonlinear layers are theoretically investigated. In addition to the usual gray and bright soliton structures observed in nonlinear slabs, a flat soliton, i.e., a particular soliton excitation with electric field amplitude independent of the position within the layer, is researched in cases of self-defocusing and self-focusing nonlinearities. Effects caused by the combination of absorption and saturation, such as the shift and extinction of the flat soliton peak, the decrease in the amplitude of the electric field within the nonlinear layer, and the suppression of the multistable behavior of the transmission coefficient in the vicinity of the soliton peaks, are discussed. The present theoretical results are compared and found in good quantitative agreement with previous experimental measurements.Graphical

Numerous bidirectionally propagating plasma blobs near the reconnection site of a solar eruption

A current sheet is a common structure involved in solar eruptions. However, it is observed in a minority of the events, and the physical properties of its fine structures during a solar eruption are rarely investigated. Here, we report an on-disk observation that displays 108 compact, circular, or elliptic bright structures, presumably plasma blobs, propagating bidirectionally along a flare current sheet during a period of ∼24 min. Using extreme ultraviolet images, we investigated the temporal variation of the blob number around the flare’s peak time. The current sheet connects the flare loops and the erupting filament. The width, duration, projected velocity, temperature, and density of these blobs are ∼1.7 ± 0.5 Mm, ∼79 ± 57 s, ∼191 ± 81 km s−1, ∼106.4 ± 0.1 K, and ∼1010.1 ± 0.3 cm−3, respectively. The reconnection site rises with a velocity of ≤69 km s−1. The observational results suggest that plasmoid instability plays an important role in the energy-release process of solar eruptions.

Solar Flare Ribbon Fronts. II. Evolution of Heating Rates in Individual Flare Footpoints

Solar flare ribbon fronts appear ahead of the bright structures that normally characterize solar flares, and can persist for an extended period of time in spatially localized patches before transitioning to “regular” bright ribbons. They likely represent the initial onset of flare energy deposition into the chromosphere. Chromospheric spectra (e.g., He i 10830 Å and the Mg ii near-UV lines) from ribbon fronts exhibit properties rather different to typical flare behavior. In prior numerical modeling efforts we were unable to reproduce the long lifetime of ribbon fronts. Here we present a series of numerical experiments that are rather simple but which have important implications. We inject a very low flux of nonthermal electrons (F = 5 × 108 erg s−1 cm−2) into the chromosphere for 100 s before ramping up to standard flare energy fluxes (F = 1010−11 erg s−1 cm−2). Synthetic spectra not only sustained their ribbon-front-like properties for significantly longer: in the case of harder nonthermal electron spectra, the ribbon front behavior persisted for the entirety of this weak-heating phase. Lengthening or shortening the duration of the weak-heating phase commensurately lengthened or shortened the ribbon front lifetimes. Ribbon fronts transitioned to regular bright ribbons when the upper chromosphere became sufficiently hot and dense, which happened faster for softer nonthermal electron spectra. Thus, the lifetime of flare ribbon fronts are a direct measure of the duration over which a relatively low flux of high-energy electrons precipitates to the chromosphere prior to the bombardment of a much larger energy flux.

Vessel-targeted compensation of deformable motion in interventional cone-beam CT

The present standard of care for unresectable liver cancer is transarterial chemoembolization (TACE), which involves using chemotherapeutic particles to selectively embolize the arteries supplying hepatic tumors. Accurate volumetric identification of intricate fine vascularity is crucial for selective embolization. Three-dimensional imaging, particularly cone-beam CT (CBCT), aids in visualization and targeting of small vessels in such highly variable anatomy, but long image acquisition time results in intra-scan patient motion, which distorts vascular structures and tissue boundaries. To improve clarity of vascular anatomy and intra-procedural utility, this work proposes a targeted motion estimation and compensation framework that removes the need for any prior information or external tracking and for user interaction. Motion estimation is performed in two stages: (i) a target identification stage that segments arteries and catheters in the projection domain using a multi-view convolutional neural network to construct a coarse 3D vascular mask; and (ii) a targeted motion estimation stage that iteratively solves for the time-varying motion field via optimization of a vessel-enhancing objective function computed over the target vascular mask. The vessel-enhancing objective is derived through eigenvalues of the local image Hessian to emphasize bright tubular structures. Motion compensation is achieved via spatial transformer operators that apply time-dependent deformations to partial angle reconstructions, allowing efficient minimization via gradient backpropagation. The framework was trained and evaluated in anatomically realistic simulated motion-corrupted CBCTs mimicking TACE of hepatic tumors, at intermediate (3.0 mm) and large (6.0 mm) motion magnitudes. Motion compensation substantially improved median vascular DICE score (from 0.30 to 0.59 for large motion), image SSIM (from 0.77 to 0.93 for large motion), and vessel sharpness (0.189 mm−1 to 0.233 mm−1 for large motion) in simulated cases. Motion compensation also demonstrated increased vessel sharpness (0.188 mm−1 before to 0.205 mm−1 after) and reconstructed vessel length (median increased from 37.37 to 41.00 mm) on a clinical interventional CBCT. The proposed anatomy-aware motion compensation framework presented a promising approach for improving the utility of CBCT for intra-procedural vascular imaging, facilitating selective embolization procedures.

Revealing asymmetry on mid-plane of protoplanetary disc through modelling of axisymmetric emission: methodology

ABSTRACT This study proposes an analytical framework for deriving the surface brightness profile and geometry of a geometrically thin axisymmetric disc from interferometric observation of continuum emission. Such precise modelling facilitates the exploration of faint non-axisymmetric structures, such as spirals and circumplanetary discs. As a demonstration, we simulate interferometric observations of geometrically thin axisymmetric discs. The proposed method can reasonably recover the injected axisymmetric structures, whereas Gaussian fitting of the same data yielded larger errors in disc orientation estimation. To further test the applicability of the method, it was applied to the mock data for $m=1,2$ spirals and a point source, which are embedded in a bright axisymmetric structure. The injected non-axisymmetric structures were reasonably recovered except for the innermost parts, and the disc geometric parameter estimations were better than Gasussian fitting. The method was then applied to the real data of Elias 20 and AS 209, and it adequately subtracted the axisymmetric component, notably in Elias 20, where substantial residuals remained without our method. We also applied our method to continuum data of PDS 70 to demonstrate the effectiveness of the method. We successfully recovered emission from PDS 70 c consistently with previous studies, and also tentatively discovered new substructures. The current formulation can be applied to any data for disc continuum emission, and aids in the search of spirals and circumplanetary discs, whose detection is still limited.

Tidal features and disc thicknesses of edge-on galaxies in the SDSS Stripe 82

ABSTRACT We examine deep optical images of edge-on galaxies selected from the Sloan Digital Sky Survey (SDSS) Stripe 82. The entire sample consists of over 800 genuine edge-on galaxies with spectroscopic redshifts out to $z\sim 0.2$. To discern the faintest details around the galaxies, we use three different data sources with a photometric depth of down to 30 mag arcsec$^{-2}$ in the r band: SDSS Stripe 82, Hyper Suprime-Cam Strategic Program, and DESI Legacy Imaging Surveys. Our analysis of the deep images reveals a variety of low surface brightness features. 49 galaxies exhibit prominent tidal structures, including tidal tails, stellar streams, bridges, and diffuse shells. Additionally, 56 galaxies demonstrate peculiar structural features such as lopsided discs, faint warps, and dim polar rings. Overall, we detect low surface brightness structures in 94 galaxies out of 838, accounting for 11 per cent of the sample. Notably, the fraction of tidal structures is only 5.8 per cent, which is significantly lower than that obtained in modern cosmological simulations and observations. Previous studies have shown that strongly interacting galaxies have stellar discs about 1.5–2 times thicker than those without apparent interactions. In an analysis where tidal features are carefully masked for precise disc axis ratio measurements, we show that discs of galaxies with tidal features are 1.33 times thicker, on average, than control galaxies that do not have visible tidal features. Furthermore, we find that edge-on galaxies with tidal structures tend to have a higher fraction of oval and boxy discs than galaxies without tidal features.

Relationship between TIGRE solar S-index and USET Ca ii K full disk images

Full disk observations of the solar chromosphere in the Ca ii K line represent a valuable dataset for studies of solar magnetic activity. The well known S-index is widely used to investigate the magnetic activity of stars, however, its connection to the coverage of stellar magnetic structure is still poorly understood. We use the archives of full disk Ca ii K images taken by the Royal Observatory of Belgium with the Uccle Solar Equatorial Table (USET) to derive the area fraction of the brightest chromospheric structures over the last decade. These data have allowed us to study the end of the solar cycle 24 and the beginning of the solar cycle 25. Our aim is to compare this dataset with the solar S-index from the Telescopio Internacional de Guanajuato Robotico Espectroscopico (TIGRE) lunar spectroscopy to analyze the relationship between a disk coverage index and an integrated spectrum. We also searched for periodic modulations in our two datasets to detect the solar rotation period. We used more than 2700 days of observations since the beginning of the Ca ii K observations with USET in July 2012. We performed a calibration of the images (re-centering and center-to-limb variation correction). The brightest regions of the solar surface (plages and enhanced network) were then segmented using an algorithm based on an intensity threshold. We computed the area fraction over the solar disk and compared it with the S-index from TIGRE. For the detection of periodic modulations, we applied a discrete Fourier power spectrum method to both datasets. A tight linear relationship was found between the USET area fraction and the TIGRE S-index, with an improved correlation obtained in the low-activity regime by considering the enhanced network. In both time series, we detected the modulation caused by the rotation of bright structures on the solar disk. However, this detection is constrained in the case of TIGRE due to its observation strategy. We studied the correlation between the disk coverage with chromospheric structures and the variability of the S-index on an overlapping period of ten years. We concluded that the disk coverage index is a good proxy for the S-index and will be useful in future studies of the magnetic activity of solar-type stars. The USET area fraction dataset is most appropriate for evaluating the solar rotation period and will be used in future works to analyze the impact of the inclination of the stellar rotation axis on the detectability of such periodic modulations in solar-type stars.

Transmission of soliton for a coupled Radhakrishnan–Kundu–Lakshmanan equation in an optical fiber using the Jacobi elliptical sn function method

We examine a coupled Radhakrishnan–Kundu–Lakshmanan (cRKL) equation that includes self-phase modulation, third order dispersion, self-steepening, and cross-phase modulation effects. A propagating ultrafast pulse in nonlinear systems is described by the given equation. Utilizing the coupled Radhakrishnan–Kundu–Lakshmanan (cRKL) model, we explored the dynamics of various innovative optical solitons, such as wing-shaped, W-shaped, and brilliant solitons. We used mathematical methods like the Jacobi elliptical sn function method to get the exact analytical solution. Our results demonstrate that the self-phase modulation (SPM), self-steepening (SS), cross-phase modulation (XPM), and third order dispersion (TOD) can be effectively varied to influence the structure of the solitons. It is demonstrated that the precise balance between the self-steepening effect, the self-phase modulation, and the group velocity dispersion forms these novel W-shaped chirp-free dark solitons and dark and bright solitonic structures. By choosing appropriate settings for the physical variables, it is possible to create a graphic representation of the properties of optical solitons. The study and application of nonlinear materials with third order dispersion, cross-phase modulation, self-phase modulation effect, and self-steepening nonlinearity may take new paths as a result of our findings.

On optical soliton solutions of the higher-order Lakshmanan-Porsezian-Daniel model having the cubic-quintic-septic law in the presence of spatio-temporal and chromatic dispersion

The higher-order Lakshmanan-Porsezian-Daniel equation (LPDE) with the cubic-quintic-septic (CQS) law having spatiotemporal and chromatic dispersion terms (STD-CD) is examined to derive new optical soliton solutions. To accomplish this aim, we operated on a simple version of the new extended auxiliary equation method (SAEM26). The optical soliton solutions of the LPDE with CQS as well as STD-CD are constructed in detail. Moreover, 3D-surface, contour, and 2D plots are presented for the bright and periodic singular soliton solutions. Additionally, the effects of diverse model parameters on the bright soliton structure are surveyed, and these effects are displayed with 2D graphics. The findings established in this work can positively contribute to research in nonlinear optics, while the SAEM26 can be effectively applied to similar nonlinear models.

On some novel solitonic structures for the Zhiber–Shabat model in modern physics

Abstract In this article, the modified Kudryashov and extended simple equation methods are employed to obtain analytical solutions for the Zhiber–Shabat problem. The outcomes of this study clearly indicate that the provided methodologies are appropriate techniques for generating some new exact solutions for nonlinear evolution equations. Furthermore, the nature of the solutions would be presented in three dimensions for various parameters applying the most advanced scientific instruments. The physical behavior of the solutions are graphically displayed, and it is established that the acquired solutions are newly constructed in the form of bright, dark, optical, singular, and bell-shaped periodic soliton wave structures. The properties of the nonlinear model have been illustrated using 3D, 2D, and contour plots by selecting an appropriate set of parameters, which is demonstrated to visualize the physical structures more productively. Finally, it is concluded that similar strategies can also be implemented to study many contemporary models. To the best of our knowledge, the current work presents a novel case study that has not been previously studied in order to generate several new solutions to the governing model appearing in diverse disciplines. The results show that the strategies that have been employed are more effective and capable than the traditional methods found in previous research.

Attention-based deep learning framework for automatic fundus image processing to aid in diabetic retinopathy grading

Background and objectiveEarly detection and grading of Diabetic Retinopathy (DR) is essential to determine an adequate treatment and prevent severe vision loss. However, the manual analysis of fundus images is time consuming and DR screening programs are challenged by the availability of human graders. Current automatic approaches for DR grading attempt the joint detection of all signs at the same time. However, the classification can be optimized if red lesions and bright lesions are independently processed since the task gets divided and simplified. Furthermore, clinicians would greatly benefit from explainable artificial intelligence (XAI) to support the automatic model predictions, especially when the type of lesion is specified. As a novelty, we propose an end-to-end deep learning framework for automatic DR grading (5 severity degrees) based on separating the attention of the dark structures from the bright structures of the retina. As the main contribution, this approach allowed us to generate independent interpretable attention maps for red lesions, such as microaneurysms and hemorrhages, and bright lesions, such as hard exudates, while using image-level labels only. MethodsOur approach is based on a novel attention mechanism which focuses separately on the dark and the bright structures of the retina by performing a previous image decomposition. This mechanism can be seen as a XAI approach which generates independent attention maps for red lesions and bright lesions. The framework includes an image quality assessment stage and deep learning-related techniques, such as data augmentation, transfer learning and fine-tuning. We used the architecture Xception as a feature extractor and the focal loss function to deal with data imbalance. ResultsThe Kaggle DR detection dataset was used for method development and validation. The proposed approach achieved 83.7 % accuracy and a Quadratic Weighted Kappa of 0.78 to classify DR among 5 severity degrees, which outperforms several state-of-the-art approaches. Nevertheless, the main result of this work is the generated attention maps, which reveal the pathological regions on the image distinguishing the red lesions and the bright lesions. These maps provide explainability to the model predictions. ConclusionsOur results suggest that our framework is effective to automatically grade DR. The separate attention approach has proven useful for optimizing the classification. On top of that, the obtained attention maps facilitate visual interpretation for clinicians. Therefore, the proposed method could be a diagnostic aid for the early detection and grading of DR.

Comprehensive Analysis of a Filament-embedding Solar Active Region at Different Stages of Evolution

Active regions are the brightest structures seen in the solar corona, so their physical properties hold important clues to the physical mechanisms underlying coronal heating. In this work, we present a comprehensive study for a filament-embedding active region as determined from observations from multiple facilities including the Chinese Hα Solar Explorer. We find three types of dynamic features that correspond to different thermal and magnetic properties, i.e., the overlying loops—1 MK cool loops, the moss region—2–3 MK hot loops’ footprints, and the sigmoidal filament. The overlying cool loops, which have a potential field, always show Doppler blueshifts at the east footprint and Doppler redshifts at the west, indicating a pattern of “siphon flow.” The moss-brightening regions, which sustain the hot loops that have a moderate sheared field, always show downward Doppler redshifts at the chromosphere, which could be a signature of plasma condensing into the inner region adjacent to the filament. The sigmoidal filament, which has strongly sheared field lines along the polarity inversion line, however, shows a different Doppler velocity pattern in its middle part, i.e., an upward Doppler blueshift at the double-J-shaped stage indicating tether-cutting reconnection during the filament channel formation and then a downward redshift showing the plasma condensation for the sigmoidal filament formation. The present work shows overall properties of the filament-embedding active region, constraining the heating mechanisms of different parts of the active region and providing hints regarding the mass loading of the embedded filament.

Study of the Intensity Distribution of the Green Line in the Inner Corona

The corona is a key region of solar atmospheric activity, and the source of solar-terrestrial space weather. Limited by observation, research on the plasma structure and magnetic field state of the lower coronal atmosphere is still very lacking, and there are few international studies on the brightness stratification of the lower coronal atmosphere in the visible light band. In this paper, the coronal green line (FeXIV5303 Å) observation data of Lijiang coronagraph YOGIS (Yunnan Observatories Green-line Imaging System) is used to analyze the bright structure and mid-coronal loops in the inner coronal region (1.03R⊙–1.25R⊙, where R⊙ represents the solar radius) effectively. By fitting the exponential decay of the intensity of the bright structures at the radial height of the sun and comparing these fitting results, it is found that the decay index of the static inner coronal loops obtained is around a fixed value. Then, the more obvious coronal loops are extracted, and by performing the same exponential fitting on the intensities of different heights, the obtained decay index is also relatively similar to that of the bright structure, which provides a reference for further study of the evolution of physical parameters in the corona.

Magnetization reversal phenomenon of higher-order lump and mixed interaction structures on periodic background in the (2+1)-dimensional Heisenberg ferromagnet spin equation

This paper mainly researches a useful model that can describe the rapid magnetization reversal process, namely the (2+1)-dimensional Heisenberg ferromagnet spin equation. Based on its Lax pair, we construct the iterative generalized (r,N−r)-fold Darboux transformation (DT), and obtain new position-controllable higher-order lump, periodic wave and rich mixed lump–breather interaction structures on periodic background. In particular, with the increase of N in the iterative generalized (r,N−r)-fold DT, we find that the structure of the lump and periodic wave can flip over with respect to the background. Moreover, we also find a bright lump structure with two peaks and two valleys, which does not flip with the increase of N. We study the magnetization trajectory and magnetization on the Bloch sphere, from which we discover that the magnetization is distributed in an incomplete unit sphere for the flipped case, while the magnetization is distributed in a complete unit sphere for the non-flipped case. These magnetization reversal phenomena are expected to have potential applications in understanding the rapid magnetization reversal process.

Color measurements of the polarized light scattered by the dust in protoplanetary disks

Context. Many reflected light images of protoplanetary disks have been obtained with ground-based high-contrast instruments. Quantitative measurements of the reflected radiation provide strong constraints on the scattering dust which can clarify the dust particle evolution in these disks and the composition of the forming planets. Aims. We derived the wavelength dependence of the polarized reflectivity (Qˆφ/I*)λ or color for a sample of disks to contrain the dust based on these data. Further we searched for systematic differences in the dust properties between subgroups of disks. Methods. We selected from the ESO archive polarized imaging for 11 protoplanetary disks obtained at visible and near-infrared wavelengths with the SPHERE/ZIMPOL and SPHERE/IRDIS high contrast instruments. All disks have bright and well-resolved structures, such as rings or spirals, which allow accurate determinations of the intrinsic polarized reflectivity Qˆφ/I* at multiple wavelengths. For this, we corrected the observations for the smearing effects caused by the point spread functions (PSFs) of the individual dataset with a novel correction procedure applicable to inclined disks. For the 11 disks, we derived a total of 31 Qˆφ/I* values for wavelengths from 0.62 μm (R band) to 2.2 μm (Ks band) and compared our results, if possible, with previous determinations. For each disk, we derived a polarized reflectivity color ηV/IR between a visible band λ < 1 μm and a near-IR band λ > 1 μm and other wavelength combinations. We also consider model calculations for the polarized reflectivity colors η for protoplanetary disks to constrain the scattering properties of the dust. Results. The polarized reflectivities for the investigated disks structures are at a typical level between Qφ/I★ ~ 0.1% to 1.0%. These values depend strongly on the observing conditions and a correction for the PSF smearing effects is essential to obtain the intrinsic values Qˆφ/I*. Corrected values Qˆφ/I* are on average about a factor of 1.6 higher than the observed values. We checked the accuracy of the PSF calibrations procedure with simulations and literature data, and show that the large systematic errors in the observational values are reduced to a relative level ΔQˆφ/Qˆφ ≈ 10% or even less. The high accuracy yields wavelength gradients for the polarized reflectivity (Qˆφ/I*)φ or colors η which are significantly different between different objects. We find in our sample for all disks around Herbig stars (HD 169142, HD 135344B, HD 100453, MWC 758, and HD 142527) a red color ηv/IR > 0.5, while four out of six disks around T Tauri stars (PDS 70, TW Hya, RX J1615, and PDS 66) are gray −0.5 < ηV/IR < 0.5. The red colors support the presence of rather compact dust grains, while the absence of blue colors (except for the near-infrared color of PDS 66) is not compatible with very porous aggregates composed of small monomers. We suspect, that the very red colors ηV/IR ≈ 2 obtained for LkCa 15 and MWC 758 could be the result of an "extra" reddening of the radiation illuminating the disk caused by absorbing hot dust near the star. We discuss the prospects of further improvements for the derivation of dust properties in these disks if the fractional polarization 〈pψ〉 or other parameters of the reflected light are also taken into account in future studies.