Azimuthal Direction Research Articles

Cosmic-Ray Ionization Rate in Protoplanetary Disks with Sheared Magnetic Fields

We investigate the effects of magnetic-field configurations on the ionization rate by cosmic rays in protoplanetary disks. First, we consider cosmic-ray propagation from the interstellar medium (ISM) to the protoplanetary disks and showed that the cosmic-ray density around the disk should be 2 times lower than the ISM value. Then, we compute the attenuation of cosmic rays in protoplanetary disks. The magnetic fields in the disk are stretched to the azimuthal directions, and cosmic rays need to detour while propagating to the midplane. Our results show that the detouring effectively enhances the column density by about two orders of magnitude. We employ a typical ionization rate by cosmic rays in diffuse ISM, which is considered too high to be consistent with observations of protoplanetary disks, and find that the cosmic rays are significantly shielded at the midplane. In the case of the disk around IM Lup, the midplane ionization rate is very low for r ≲ 100 au, while the value is as large as a diffuse ISM in the outer radii. Our results are consistent with the recent Atacama Large Millimeter/submillimeter Array observation that indicates the radial gradient in the cosmic-ray ionization rate. The high ionization rate in the outer radii of disks may activate the magnetorotational instability that was thought to be suppressed due to ambipolar diffusion. These results will have a strong influence on the dynamical and chemical evolutions of protoplanetary disks.

Effect of Strong Azimuthal Swirl on Ignition and Light-Around in an Annular Combustor

Abstract In this paper, we investigate the effect of strong azimuthal swirl on ignition dynamics in a laboratory-scale annular combustor. Bulk azimuthal swirl was produced by a novel angled injector configuration, producing swirling jet flames oriented downwards toward the combustor backplane and in the azimuthal direction, replicating a simplified version of the SAFRAN spinning combustor concept. To provide more realistic flow conditions, the design included rich-quench-lean (RQL) staging via a circumferential distribution of dilution ports and an effusion cooled combustor backplane. High-speed imaging and an azimuthal array of photomultipliers to measure OH* chemiluminescence were used to characterize the ignition dynamics for different injector velocities and global equivalence ratios. The mass flows through the injectors, dilution ports, and effusion cooled backplane were independently metered so that the injector equivalence ratio and global equivalence ratio could be separately controlled. The light-around times were found to have no clear correlation with the injector velocity since the rich injector equivalence ratio meant the flame burned in a nonpremixed mode even though the global equivalence ratio was lean due to the RQL staging. However, it was found that lower injector velocities extended the lean ignition limit based on the global equivalence ratio. The ignition sequence during light-around (order in which the injectors are ignited) was found to be highly repeatable, igniting each consecutive injector in the anticlockwise direction (the direction of bulk swirl). In rare cases, the ignition sequence was observed to branch in both directions. Finally, in an effort to extend the lean ignition limit, the effect of azimuthal staging was investigated. Two configurations were tested. In the first configuration, the injectors on one half of the annulus were operated at a fixed equivalence ratio whereas the other half of the annulus was operated at a different equivalence ratio. In the second configuration, every second injector had the same equivalence ratio. Both configurations extended the lean extinction limit but the first configuration was the most effective.

Detection of Ships Cruising in the Azimuth Direction Using Spotlight SAR Images with a Deep Learning Method

Spotlight synthetic aperture radar (SAR) achieves a high azimuth resolution with long integration times. Meanwhile, the long integration times also cause defocused and smeared images of moving objects such as cruising ships This is a typical imaging mechanism for moving objects in Spotlight SAR images. Conversely, ships can be classified as stationary or moving from the amount of smearing, and this classification method is, in general, based on manual observation. This paper proposes an automatic method for detecting cruising ships using deep learning known as the “You Only Look Once (YOLO) v5 model”, which is one of the frameworks of the YOLO family. In this study, ALOS-2/PALSAR-2 L-band Spotlight SAR images over the waters around the Miura Peninsula, Japan, were analyzed using the YOLO v5 model with a total of 53 ships’ images and compared with Automatic Identification System (AIS) data. The results showed a precision of approximately 0.85 and a recall rate of approximately 0.89 with an F-measure of 0.87. Thus, sufficiently high values were achieved in the automatic detection of moving ships using the deep learning method with the YOLO v5 model. As for false detections, images of breakwaters were classified as ships cruising in the azimuth direction. Further, range moving ships were found to be difficult to detect. From the present preliminary study, it was found that the YOLO v5 model is limited to ships cruising predominantly in the azimuth direction.

A method for transforming 2D imaging features with the AFSS‐loaded rotating Luneberg lens reflector

The micro-motion target can perform micro-Doppler modulation on the electromagnetic wave due to motion components in different directions, leading to the defocus effect of the target imaging feature along the azimuth direction. This phenomenon has been widely concerned and investigated in the field of target recognition and anti-recognition. However, the micro-motion target fails to transform image features along the distance direction according to its slow mechanical modulation speed. In comparison, electromagnetic metasurfaces can flexibly control the characteristics of electromagnetic waves with faster modulation speed through electronic control. It can realize the image feature transformation along the range direction. Inspired by this idea, a joint modulation method of rotational micro-motion and electronic control through an active frequency selective surface (AFSS)-loaded rotating Luneberg lens reflector is proposed. The signal models of micro-motion modulation and non-periodic AFSS modulation are established separately. On this basis, the imaging properties of the joint modulation are further analysed and the two-dimensional (2D) defocusing phenomenon of the image is discovered. Moreover, the simulation of the measured synthetic aperture radar (SAR) data with different modulation methods is conducted to demonstrate the effectiveness of the proposed method.

Relating atmospheric aerosol amounts to blue to red ratio and grayscale contrast fluctuations using digitalization of routine webcam photographs taken in the urban environment of Vienna

Visibility and visual contrast depend on several factors such as aerosol concentration, fog attenuation and humidity, viewing direction relative to the sun as well as trace gas amounts. Usually, visibility is determined by observers or by measurement devices such as visiometers. Only few studies reported on the relationship between visibility and aerosol concentration using digital cameras. In the present study, we develop and test new algorithms for the determination of the amount of aerosols by using digital photography. Two years of routine webcam recordings are used for this study. The digital images are taken at the meteorological measurement platform, which is situated on the roof of a university campus building and overlooking the city of Vienna. The photographs are automatically taken as part of Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements, which are continuously performed during daytime at different azimuthal and elevation angles. Two new algorithms for estimating aerosol amounts by quantifying digital images taken on cloud-free days are introduced within the scope of the present study: (1) the ratio of the blue to red (B/R) RGB channels within a vertical pixel line and (2) the grayscale contrast fluctuations (GCF) within a horizontal pixel line. Vertical B/R and horizontal GCF are compared against ground-based aerosol optical depth (AOD) and in-situ particulate matter (PM) < 10 μm (PM10) measurements. Interestingly, the comparisons show promising results for specific azimuthal directions and during certain daytime periods, e.g. for constellations when the sun is to the side or directly behind the camera. In terms of coefficients of determination (R2), B/R correlates best with AOD during summer, while GCF shows better performance in winter. R2 values of up to 0.86 and 0.73 are found between horizontal line GCF and in-situ PM10 and between vertical line B/R and AOD, respectively. For a better interpretation of the correlations, simultaneous measurements of relative humidity (RH) are included in the analyses.

Three-dimensional large-scale and very-large-scale coherent structures in a turbulent axisymmetric jet

The direct numerical simulation database from Shin et al. (J. Fluid Mech., vol. 823, 2017, pp. 1–25) is used to study three-dimensional vortical and very-large-scale coherent structures in a turbulent round jet at a Reynolds number of $7300$ . In particular, horseshoe vortices and their role in the formation of very-large-scale coherent structures in the jet near and intermediate fields are assessed. The swirling strength criterion together with conditional averaging are used to visualize volumetric vortical structures. It is shown that, similar to wall-bounded turbulent flows, the turbulent jet is populated with symmetric and asymmetric horseshoe-like vortices, which induce high-momentum and low-momentum regions in the flow. However, unlike what is found for wall-bounded flows, inverse horseshoe-like vortices are common in the turbulent jet. They prevail in the shear region around the potential core in the jet near field and contribute to the mixing of the potential core in the jet. In the jet near field, groups of axially aligned horseshoe structures induce long streaky structures, which are periodic in the azimuthal and streamwise directions. In the jet intermediate field, very-large-scale motions (VLSMs) consisting of high-momentum regions, flanked on either side by low-momentum regions, are found to be associated with groups of horseshoe vortices. Instantaneous three-dimensional flow fields suggest that horseshoe vortices tend to concatenate and form organized spiral as well as axially aligned coherent VLSMs. A detection scheme is introduced to identify and average over these VLSMs. This conditional averaging reveals that spiral VLSMs and axially aligned VLSMs constitute $72\,\%$ and $28\,\%$ of the total VLSMs, respectively.

Auxiliary antenna design for anti‐interference of satellite communication earth station

AbstractIn this study, a novel elliptical widebeam auxiliary antenna is proposed for anti‐interference of satellite communication earth station. The wide beam width of about 180° in horizontal plane (H‐plane) is achieved by increasing the width of the substrate, reducing the size of the ground plane and adding a V‐shaped aluminum frame. And the narrow beam width of about 30° in elevational plane (E‐plane) is achieved by designing a binary element array. Only two auxiliary antennas are needed for sampling to cover interference from any azimuth direction in H‐plane. The antenna is fabricated and measured, and the results are in good agreement with simulation. The corresponding −10 dB impedance bandwidth is about 17% with maximum gain of 6.5 dBi. The 3 dB beam widths are respectively 33° and 178° in E‐plane and H‐plane. Furthermore, simulation and experiment of the proposed auxiliary antenna in interference cancelation system verify its anti‐interference performance. According to sidelobe canceler, an average interference cancelation ratio of about 27 dB is achieved.

Low-Profile VHF Antenna Based on Quarter-Mode Substrate-Integrated Waveguide Structure

A traditional whip antenna is suitable for terrestrial wireless communication due to its omnidirectional radiation pattern which is capable of transmitting and receiving signals from all directions. However, the length of a whip antenna is long for the very high frequency (VHF) band applications; for example, the ideal length is 80 cm at 90 MHz, which restrains the mobility when mounted on a vehicle. Many studies have been conducted to develop a low-profile antenna that can substitute a whip antenna but most of them are for relatively high frequency (GHz) applications. In this paper, a planar low-profile antenna operating in the VHF band is proposed. The height of the antenna is only 9 cm by utilizing a quarter-mode substrate-integrated waveguide (QMSIW) structure with shorting pins at the edges. The latter promotes omnidirectional radiations despite its short height. The antenna design was optimized by 3D full-wave simulation software. Subsequently, an antenna prototype was fabricated and tested. The results demonstrated that the antenna resonated at 86 MHz with a maximum gain of 1.7 dB along the azimuth direction.

Target-Oriented High-Resolution and Wide-Swath Imaging with an Adaptive Receiving–Processing–Decision Feedback Framework

High-resolution and wide-swath (HRWS) synthetic aperture radar (SAR) is a promising technique for applications such as maritime surveillance. In the maritime environment, normally only a few targets such as ships are interested. However, before detecting them, considerable receiving resources and computation time are required to receive the echoes of the whole scene and process them to obtain imaging results. This is a heavy burden for online monitoring on platforms with limited resources. To address these issues, different from the concept of whole-scene-oriented imaging, we propose a target-oriented imaging concept, which is implemented by an adaptive receiving–processing–decision feedback framework with feedback connection. (1) To reduce receiving resource consumption, we propose a two-dimensional adaptive receiving module. It receives sub-band echoes of targets only through dechirping and subaperture decomposition in the range and azimuth directions, respectively. (2) To reduce computation time, we propose a target-oriented processing module. It processes subarea echoes of targets only through parallelly conducting inverse fast Fourier transform (IFFT) and back projection (BP) in the range and azimuth directions, respectively. (3) To allocate resources reasonably, we propose a decision module. It decides the necessary receiving window, bandwidth, and image resolution through constant false alarm rate (CFAR) detection. (4) To allocate resources adaptively, we connect three modules with a closed loop to enable feedback. This enables progressive target imaging and detection from rough to fine. Experimental results verify the feasibility of the proposed framework. Compared with the current one, for a typical scenario, at least 30% of the system’s resources and 99% of computation time are saved.

Sparse SAR Imaging Method for Ground Moving Target via GMTSI-Net

Ground moving targets (GMT), due to the existence of velocity in range and azimuth direction, will lead to the deviation from their true position and defocus in the azimuth direction during the synthetic aperture radar (SAR) imaging process. To address this problem and compress the amount of echo data, a sparse SAR imaging method for ground moving targets is proposed. Specifically, we first constructed a two-dimensional sparse observation model of the GMT based on matched filter operators. Then, the observation model was solved by a deep network, GMT sparse imaging network (GMTSI-Net), which was mainly obtained by unfolding an iterative soft threshold algorithm (ISTA)-based iterative solution. Furthermore, we designed an adaptive unfolding module in the imaging network to improve the adaptability of the network to the input of echo data with different sampling ratios. The proposed imaging network can achieve faster and more accurate SAR images of ground moving targets under a low sampling ratio and signal-to-noise ratio (SNR). Simulated and measured data experiments were conducted to demonstrate the performance of imaging quality of the proposed method.

Observed Near-Inertial Waves Trapped in a Propagating Anticyclonic Eddy

Abstract Near-inertial waves (NIWs) trapped in a propagating anticyclonic eddy (AE) are investigated along the eddy path at three areas spanning 660 km by using two mooring arrays and a cruise transect. In the upstream area, the reconstructed three-dimensional structure reveals that NIWs are concentrated within the eddy core with wave current amplitudes exceeding 0.2 m s−1; vertically, due to the critical layer effect caused by eddy baroclinicity, NIWs are trapped at depths around 200 and 315 m with frequencies estimated to be ω1 ≈ 0.918f and ω2 ≈ 0.985f, respectively. After the AE propagates southwestward for hundreds of kilometers, the NIWs of frequency ω1 are still detectable inside the AE, while NIWs of frequency ω2 are absent because of the equatorward migration of the AE on a beta plane. Meanwhile, the wave kinetic energy downstream is trapped closer to the eddy center in radial direction, with the wave amplitude decaying roughly in a Gaussian form along the eddy radius, and becomes more homogeneous in the azimuthal direction, showing a more regular trapping form in the three-dimensional view. Investigation on wind shows that trapped NIWs are likely to be generated by a typhoon but less affected by the wind during the eddy passage time. By an energy analysis, we find that enhanced wave dissipation near the critical layer is roughly balanced by the energy transfer from mean flows, and therefore the trapped wave kinetic energy is largely conserved during the long-distance migration.

Contact Resistance Dependent Transient Loss of REBCO No-Insulation Magnet UsingT-AFormula During Ramping Process

The transient loss during the ramping process is an important evaluation metric for commercial large-scale magnet systems to meet refrigeration requirements; this loss is considerably influenced by contact resistivity. Due to the intrinsic bypass current of the no-insulation pancake, two parts of the transient loss were considered in this study: turn-to-turn loss generated by the radial direction current and the magnetization loss produced by the azimuthal direction current. A turn-distributed model was used to calculate the non-uniform current distribution, where the pancake coil is radially subdivided into each turn. The radial current was used to calculate the turn-to-turn loss. The turn azimuthal current was applied to the boundary condition of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T-A</i> formula to calculate the magnetization loss in a cylindrical coordinate system. The superconductor index value model was used for the simulations, where the critical current density is the conductor engineering current density estimated by a neural network fitting model. The contact resistance dependent turn-to-turn loss and magnetization loss were presented. This approach is expected to be used in magnet-level NMR/ MRI systems.

INCREASING THE SIGNAL-TO-NOISE RATIO IN EDDY CURRENT NDT BY CREATING OF AN INHOMOGENEOUS ELECTROMAGNETIC FIELD

In this article, using the example of multistrand rope inspection, the ways of increasing the signal-to-noise ratio in eddy current control are considered. Based on the results of numerical simulation, a comparative analysis of encircling addycurrent probes with different lengths of the exciting coil is carried out. A probe with a narrow exciting coil has a pronounced dependence of the ratio of signals from defects in different layers on the position of the measuring coils. The use of measuring coils located at different distances from the center of the exciting coil makes it possible to increase the signal-to-noise ratio due to the separation in the radial direction of the parts of the object that mainly form signals and noise in different measuring coils. The use of a system of surface measuring coils instead of encircling measuring coils makes it possible to increase the signal-to-noise ratio due to the separation in the azimuthal direction of the parts of the object that mainly form signals and noise in each of the surface measuring coils.

Detecting slow-moving landslides using InSAR phase-gradient stacking and deep-learning network

Landslides are a major geohazard that endangers human lives and properties. Recently, efforts have been made to use Synthetic Aperture Radar Interferometry (InSAR) for landslide monitoring. However, it is still difficult to effectively and automatically identify slow-moving landslides distributed over a large area due to phase unwrapping errors, decorrelation, troposphere turbulence and computational requirements. In this study, we develop a new approach combining phase-gradient stacking and a deep-learning network based on YOLOv3 to automatically detect slow-moving landslides from large-scale interferograms. Using Sentinel-1 SAR images acquired from 2014 to 2020, we developed a burst-based, phase-gradient stacking algorithm to sum up phase gradients in short-temporal-baseline interferograms along the azimuth and range directions. The stacked phase gradients clearly reveal the characteristics of localized surface deformation that is mainly caused by slow-moving landslides and avoids the errors due to phase unwrapping in partially decorrelated areas and atmospheric effects. Then, we trained the improved Attention-YOLOv3 network with stacked phase-gradient maps of manually labeled landslides to achieve quick and automatic detection. We applied our method in an ∼180,000 km2 area of southwestern China and identified 3,366 slow-moving landslides. By comparing the results with optical imagery and previously published landslides in this region, the proposed method can achieve automatic detection over a large area precisely and efficiently. From the derived landslide density map, we determined that most landslides are distributed along the three large rivers and their branches. In addition to some counties with known high-density landslides, approximately 10 more counties with high landslide density were exposed, which should attract more attention to their risks for geohazards. This application demonstrates the potential value of our newly developed method for slow-moving landslide detection over a nation-wide area, which can be employed before applying more time-consuming time-series InSAR analysis.

Ultrasonic and seismic constraints on crystallographic preferred orientations of the Priestley Glacier shear margin, Antarctica

Abstract. Crystallographic preferred orientations (CPOs) are particularly important in controlling the mechanical properties of glacial shear margins. Logistical and safety considerations often make direct sampling of shear margins difficult, and geophysical measurements are commonly used to constrain the CPOs. We present here the first direct comparison of seismic and ultrasonic data with measured CPOs in a polar shear margin. The measured CPO from ice samples from a 58 m deep borehole in the left lateral shear margin of the Priestley Glacier, Antarctica, is dominated by horizontal c axes aligned sub-perpendicularly to flow. A vertical-seismic-profile experiment with hammer shots up to 50 m away from the borehole, in four different azimuthal directions, shows velocity anisotropy of both P waves and S waves. Matching P-wave data to the anisotropy corresponding to CPO models defined by horizontally aligned c axes gives two possible solutions for the c-axis azimuth, one of which matches the c-axis measurements. If both P-wave and S-wave data are used, there is one best fit for the azimuth and intensity of c-axis alignment that matches the measurements well. Azimuthal P-wave and S-wave ultrasonic data recorded in the laboratory on the ice core show clear anisotropy of P-wave and S-wave velocities in the horizontal plane that match that predicted from the CPO of the samples. With quality data, azimuthal increments of 30∘ or less will constrain well the orientation and intensity of c-axis alignment. Our experiments provide a good framework for planning seismic surveys aimed at constraining the anisotropy of shear margins.

Azimuthal Motion of Poleward Moving Auroral Forms

AbstractPoleward moving auroral forms (PMAFs), the most common dayside auroral structure, images the process of energy and particle transport from the solar wind to the magnetosphere via magnetic reconnection. This form of aurora moves not only poleward, but also in the azimuthal (dawn‐dusk) direction. However, the underlying mechanism responsible for the azimuthal motion of PMAFs are not thoroughly investigated. Here based on all‐sky imager observations in the southern hemisphere, we statistically determine that the azimuthal motion of PMAFs is highly dependent on the dawn‐dusk component of the interplanetary magnetic field (IMF By). Using three‐dimensional (3‐D) global hybrid simulations, we demonstrate that IMF By controls the azimuthal motion of PMAFs through plasma flow derived from magnetic tension of magnetopause flux ropes. Our study, by focusing on the azimuthal motion of PMAFs, allows a better understanding of space weather, especially from a 3‐D perspective.

Three-dimensional photoelectron holography with trichromatic polarization-tailored laser pulses

We present a three-dimensional (3D) photoelectron wave packet holography scheme based on polarization-tailored trichromatic femtosecond laser pulses for the determination of quantum phases in atomic multiphoton ionization (MPI). Experimentally, we combine supercontinuum polarization pulse shaping with photoelectron tomography for the reconstruction of the 3D photoelectron momentum distribution (PMD). To demonstrate the 3D photoelectron holography scheme, we superimpose a sculptured wave packet encoding a relative continuum phase with a reference wave packet. In particular, we create a sculptured angular momentum superposition wave packet by (2 + 1) resonance-enhanced MPI of potassium atoms using a counter-rotating circularly polarized bichromatic pulse sequence. The sculptured wave packet, consisting of states with different orbital angular momentum quantum numbers, interferes with the reference wave packet generated by direct three-photon ionization with a circularly polarized pulse of the third color. Depending on the circularity of the reference pulse, interference of both wave packets gives rise to 3D photoelectron holograms with c 2 or c 4 rotational symmetry in the laser polarization plane, i.e., in the azimuthal direction. In the polar direction, the azimuthal interference pattern undergoes a phase-shift revealing the relative quantum phase between the p- and f-type continuum partial waves in the sculptured wave packet. We determine the relative continuum phase by fitting the parameters of an analytical model of the hologram to the measured 3D PMD and confirm the result by direct extraction of the continuum phase difference from the polar-angle-dependent azimuthal phase-shift of the photoelectron angular distribution.

Excitation of vertical breathing motion in disc galaxies by tidally-induced spirals in fly-by interactions

ABSTRACT It is now clear that the stars in the Solar neighbourhood display large-scale coherent vertical breathing motions. At the same time, Milky Way-like galaxies experience tidal interactions with satellites/companions during their evolution. While these tidal interactions can excite vertical oscillations, it is still not clear whether vertical breathing motions are excited directly by the tidal encounters or are driven by the tidally-induced spirals. We test whether excitation of breathing motions are directly linked to tidal interactions by constructing a set of N-body models (with mass ratio 5:1) of unbound single fly-by interactions with varying orbital configurations. We first reproduce the well-known result that such fly-by interactions can excite strong transient spirals (lasting for ${\sim}2.9{-}4.2\,{\rm Gyr}$) in the outer disc of the host galaxy. The generation and strength of the spirals are shown to vary with the orbital parameters (the angle of interaction, and the orbital spin vector). Furthermore, we demonstrate that our fly-by models exhibit coherent breathing motions whose amplitude increases with height. The amplitudes of breathing motions show characteristic modulation along the azimuthal direction with compressing breathing motions coinciding with the peaks of the spirals and expanding breathing motions falling in the inter-arm regions – a signature of a spiral-driven breathing motion. These breathing motions in our models end when the strong tidally-induced spiral arms fade away. Thus, it is the tidally-induced spirals which drive the large-scale breathing motions in our fly-by models, and the dynamical role of the tidal interaction in this context is indirect.

Adverse impact of macro-textured superhydrophobicity on contact time reduction at high Weber numbers

Macro textured superhydrophobic surfaces are reported to be highly effective in contact time reduction and thus a potential candidate for anti-icing applications. However, we find that the macro textured superhydrophobic cylindrical surfaces have higher contact time compared to its counterpart without any macro texturing at high impact Weber numbers (We)for the various range of curvature ratios of the cylinder and aspect ratios of the macro texturing. The asymmetric spreading is aided by the preferential jetting and flow redirection results in a stable lamella in the azimuthal direction resulting in the contact time enhancement for intermediate We number regimes. The retraction velocity is also adversely influenced by the presence of ribs in these We number regimes thus resulting in enhanced residence time. However, the dynamics at the high We regimes are governed by the hole nucleation and film rupturing. The presence of ribs is found to reduce the nucleation inception and nucleation density resulting in higher contact time compared to the non-ribbed superhydrophobic surfaces. An outcome based map was developed based on the systematic experimental observations over a wide spectrum of parametric variation. Detailed analysis is presented to explain the counter-intuitive observations.

Probing Magnetic Fields in Protoplanetary Disk Atmospheres through Polarized Near-IR Light Scattered by Aligned Grains

Magnetic fields play essential roles in protoplanetary disks. Magnetic fields in the disk atmosphere are of particular interest, as they are connected to the wind-launching mechanism. In this work, we study the polarization of the light scattered off of magnetically aligned grains in the disk atmosphere, focusing on the deviation of the polarization orientation from the canonical azimuthal direction, which may be detectable in near-IR polarimetry with instruments such as VLT/SPHERE. We show with a simple disk model that the polarization can even be oriented along the radial (rather than azimuthal) direction, especially in highly inclined disks with toroidally dominated magnetic fields. This polarization reversal is caused by the anisotropy in the polarizability of aligned grains and is thus a telltale sign of such grains. We show that the near-IR light is scattered mostly by μm-sized grains or smaller at the τ = 1 surface and such grains can be magnetically aligned if they contain superparamagnetic inclusions. For comparison with observations, we generate synthetic maps of the ratios of U ϕ /I and Q ϕ /I, which can be used to infer the existence of (magnetically) aligned grains through a negative Q ϕ (polarization reversal) and/or a significant level of U ϕ /I. We show that two features observed in the existing data, an asymmetric distribution of U ϕ with respect to the disk minor axis and a spatial distribution of U ϕ that is predominantly positive or negative, are incompatible with scattering by spherical grains in an axisymmetric disk. They provide indirect evidence for scattering by aligned nonspherical grains.