Linear Polarization Signals Research Articles

The polarization angle in the wings of Ca I 4227: A new observable for diagnosing unresolved photospheric magnetic fields

Context. When observed in quiet regions close to the solar limb, many strong resonance lines show conspicuous linear polarization signals, produced by scattering processes (i.e., scattering polarization), with extended wing lobes. Recent studies indicate that, contrary to what was previously believed, the wing lobes are sensitive to the presence of relatively weak longitudinal magnetic fields through magneto-optical (MO) effects. Aims. We theoretically investigate the sensitivity of the scattering polarization wings of the Ca I 4227 Å line to the MO effects, and we explore its diagnostic potential for inferring information on the longitudinal component of the photospheric magnetic field. Methods. We calculate the intensity and polarization profiles of the Ca I 4227 Å line by numerically solving the problem of the generation and transfer of polarized radiation under non-local thermodynamic equilibrium conditions in one-dimensional semi-empirical models of the solar atmosphere, taking into account the joint action of the Hanle, Zeeman, and MO effects. We consider volume-filling magnetic fields as well as magnetic fields occupying a fraction of the resolution element. Results. In contrast to the circular polarization signals produced by the Zeeman effect, we find that the linear polarization angle in the scattering polarization wings of Ca I 4227 presents a clear sensitivity, through MO effects, not only to the flux of the photospheric magnetic field, but also to the fraction of the resolution element that the magnetic field occupies. Conclusions. We identify the linear polarization angle in the wings of strong resonance lines as a valuable observable for diagnosing unresolved magnetic fields. Used in combination with observables that encode information on the magnetic flux and other properties of the observed atmospheric region (e.g., temperature and density), it can provide constraints on the filling factor of the magnetic field.

Polarization of the Lyα Lines of H i and He ii as a Tool for Exploring the Solar Corona

Abstract The near-Earth space weather is driven by the quick release of magnetic free energy in the solar corona. Probing this extremely hot and rarified region of the extended solar atmosphere requires modeling the polarization of forbidden and permitted coronal lines. To this end, it is important to develop efficient codes to calculate the Stokes profiles that emerge from given three-dimensional (3D) coronal models and this should be done taking into account the symmetry breaking produced by the presence of magnetic fields and non-radial solar wind velocities. We have developed such a tool with the aim of theoretically predicting and interpreting spectropolarimetric observations of the solar corona in permitted and forbidden lines. In this paper, we show the results of a theoretical investigation of the linear polarization signals produced by scattering processes in the H i Lyα line at 1216 Å and in the He ii Lyα line at 304 Å using 3D coronal models by Predictive Science Inc. These spectral lines have very different critical magnetic fields for the onset of the Hanle effect (53 G and 850 G, respectively), as well as different sensitivities to the Doppler effect caused by the solar wind velocities. We study under which circumstances simultaneous observations of the scattering polarization in these Lyα lines can facilitate the determination of magnetic fields and macroscopic velocities in the solar corona.

Linear Polarization of the 21 cm Line from the Epoch of Reionization

Abstract The 21 cm linear polarization due to Thomson scattering off free electrons can probe the distribution of neutral hydrogen in the intergalactic medium during the epoch of reionization, complementary to the 21 cm temperature fluctuations. A previous study estimated the strength of polarization with a toy model and claimed that it can be detected with 1 month observation of the Square Kilometre Array (SKA). Here we revisit this investigation with account of nonlinear terms due to inhomogeneous reionization, using semi-numerical reionization simulations to provide the realistic estimation of the 21 cm TE and EE angular power spectra ( C ℓ TE and C ℓ EE ). We find that (1) both power spectra are enhanced on sub-bubble scales but suppressed on super-bubble scales, compared with previous results; (2) C ℓ TE displays a zero crossing at ℓ < 100, and its angular scale is sensitive to the scale dependence of H i bias on large scales; (3) the ratios of the power spectrum to its maximum value during reionization at a given ℓ, i.e., C ℓ TE / C ℓ , max TE and C ℓ EE / C ℓ , max EE , show robust correlations with the global ionized fraction. However, measurement of this signal will be very challenging not only because the overall strength is weaker than the sensitivity of SKA, but also because of the polarized foregrounds from diffuse synchrotron emission, and Faraday rotation, which modifies the observed polarization. Nevertheless, the 21 cm linear polarization signal may still likely be detectable through other approaches, e.g., its cross-correlation with other probes.

On the (Mis)Interpretation of the Scattering Polarization Signatures in the Ca ii 8542 Å Line through Spectral Line Inversions

Abstract Scattering polarization tends to dominate the linear polarization signals of the Ca ii 8542 Å line in weakly magnetized areas (B ≲ 100 G), especially when the observing geometry is close to the limb. In this paper, we evaluate the degree of applicability of existing non-LTE spectral line inversion codes (which assume that the spectral line polarization is due to the Zeeman effect only) at inferring the magnetic field vector and, particularly, its transverse component. To this end, we use the inversion code STiC to extract the strength and orientation of the magnetic field from synthetic spectropolarimetric data generated with the Hanle-RT code. The latter accounts for the generation of polarization through scattering processes as well as the joint actions of the Hanle and the Zeeman effects. We find that, when the transverse component of the field is stronger than ∼80 G, the inversion code is able to retrieve accurate estimates of the transverse field strength as well as its azimuth in the plane of the sky. Below this threshold, the scattering polarization signatures become the major contributors to the linear polarization signals and often mislead the inversion code into severely over- or underestimating the field strength. Since the line-of-sight component of the field is derived from the circular polarization signal, which is not affected by atomic alignment, the corresponding inferences are always good.

Solving the Paradox of the Solar Sodium D_{1} Line Polarization.

Twenty-five years ago, enigmatic linear polarization signals were discovered in the core of the sodium D_{1} line. The only explanation that could be found implied that the solar chromosphere is practically unmagnetized, in contradiction with other evidences. This opened a paradox that has challenged physicists for many years. Here we present its solution, demonstrating that these polarization signals can be properly explained in the presence of magnetic fields in the gauss range. This result opens a novel diagnostic window for exploring the elusive magnetism of the solar chromosphere.

The Effects of Three-dimensional Radiative Transfer on the Resonance Polarization of the Ca i 4227 Å Line

Abstract The sizable linear polarization signals produced by the scattering of anisotropic radiation in the core of the Ca i 4227 Å line constitute an important observable for probing the inhomogeneous and dynamic plasma of the lower solar chromosphere. Here we show the results of a three-dimensional (3D) radiative transfer complete frequency redistribution investigation of the line’s scattering polarization in a magnetohydrodynamical 3D model of the solar atmosphere. We take into account not only the Hanle effect produced by the model’s magnetic field but also the symmetry breaking caused by the horizontal inhomogeneities and macroscopic velocity gradients. The spatial gradients of the horizontal components of the macroscopic velocities produce very significant forward scattering polarization signals without the need of magnetic fields, while the Hanle effect tends to depolarize them at the locations where the model’s magnetic field is stronger than about 5 G. The standard 1.5D approximation is found to be unsuitable for understanding the line’s scattering polarization, but we introduce a novel improvement to this approximation that produces results in qualitative agreement with the full 3D results. The instrumental degradation of the calculated polarization signals is also investigated, showing what we can expect to observe with the Visible Spectro-Polarimeter at the upcoming Daniel K. Inouye Solar Telescope.

The Polarization of Ambient Noise on Mars

AbstractSeismic noise recorded at the surface of Mars has been monitored since February 2019, using the InSight seismometers. This noise can reach −200 dB. It is 500 times lower than on Earth at night and it increases of 30 dB during the day. We analyze its polarization as a function of time and frequency in the band 0.03–1 Hz. We use the degree of polarization to extract signals with stable polarization independent of their amplitude and type of polarization. We detect polarized signals at all frequencies and all times. Glitches correspond to linear polarized signals which are more abundant during the night. For signals with elliptical polarization, the ellipse is in the horizontal plane below 0.3 Hz. In the 0.3‐1Hz high frequency band (HF) and except in the evening, the ellipse is in the vertical plane and the major axis is tilted. While polarization azimuths are different in the two frequency bands, they both vary as a function of local hour and season. They are also correlated with wind direction, particularly during the daytime. We investigate possible aseismic and seismic origins of the polarized signals. Lander or tether noise can be discarded. Pressure fluctuations transported by wind may explain part of the HF polarization but not the tilt of the ellipse. This tilt can be obtained if the source is an acoustic emission coming from high altitude at critical angle. Finally, in the evening when the wind is low, the measured polarized signals may correspond to the seismic wavefield of the Mars background noise.

Interaction of Magnetic Fields with a Vortex Tube at Solar Subgranular Scale

Abstract Using high-resolution spectropolarimetric data recorded with the Swedish 1 m Solar Telescope, we have identified several instances of granular lanes traveling into granules. These are believed to be the observational signature of underlying tubes of vortical flow with their axis oriented parallel to the solar surface. Associated with these horizontal vortex tubes, we detect in some cases a significant signal in linear polarization, located at the trailing dark edge of the granular lane. The linear polarization appears at a later stage of the granular lane development, and is flanked by patches of circular polarization. Stokes inversions show that the elongated patch of linear polarization signal arises from the horizontal magnetic field aligned with the granular lane. We analyze snapshots of a magnetohydrodynamic numerical simulation and find cases in which the horizontal vortex tube of the granular lane redistributes and transports the magnetic field to the solar surface causing a polarimetric signature similar to what is observed. We thus witness a mechanism capable of transporting magnetic flux to the solar surface within granules. This mechanism is probably an important component of the small-scale dynamo supposedly acting at the solar surface and generating the quiet-Sun magnetic field.

The Magnetic Field of the Active Planet-hosting M Dwarf AU Mic

Abstract AU Mic is a young, very active M dwarf star with a debris disk and at least one transiting Neptune-size planet. Here we present a detailed analysis of the magnetic field of AU Mic based on previously unpublished high-resolution optical and near-infrared spectropolarimetric observations. We report a systematic detection of circular and linear polarization signatures in the stellar photospheric lines. Tentative Zeeman Doppler imaging modeling of the former data suggests a nonaxisymmetric global field with a surface-averaged strength of about 90 G. At the same time, linear polarization observations indicate the presence of a much stronger ≈2 kG axisymmetric dipolar field, which contributes no circular polarization signal due to the equator-on orientation of AU Mic. A separate Zeeman broadening and intensification analysis allowed us to determine a mean field modulus of 2.3 and 2.1 kG from the Y- and K-band atomic lines, respectively. These magnetic field measurements are essential for understanding environmental conditions within the AU Mic planetary system.

Observational indications of magneto-optical effects in the scattering polarization wings of the Ca I 4227 Å line

Context. Several strong resonance lines, such as H I Ly-α, Mg II k, Ca II K, and Ca I 4227 Å, are characterized by deep and broad absorption profiles in the solar intensity spectrum. These resonance lines show conspicuous linear scattering polarization signals when observed in quiet regions close to the solar limb. Such signals show a characteristic triplet-peak structure with a sharp peak in the line core and extended wing lobes. The line core peak is sensitive to the presence of magnetic fields through the Hanle effect, which however is known not to operate in the line wings. Recent theoretical studies indicate that, contrary to what was previously believed, the wing linear polarization signals are also sensitive to the magnetic field through magneto-optical (MO) effects. Aims. We search for observational indications of this recently discovered physical mechanism in the scattering polarization wings of the Ca I 4227 Å line. Methods. We performed a series of spectropolarimetric observations of this line using the Zurich IMaging POLarimeter camera at the Gregory-Coudé telescope at Istituto Ricerche Solari Locarno in Switzerland and at the GREGOR telescope in Tenerife (Spain). Results. Spatial variations of the total linear polarization degree and linear polarization angle are clearly appreciable in the wings of the observed line. We provide a detailed discussion of our observational results, showing that the detected variations always take place in regions in which longitudinal magnetic fields are present, thus supporting the theoretical prediction that they are produced by MO effects.

Are collisions with electrons important for modeling the polarization of the lines of the C2 solar molecule?

Observations of the second solar spectrum (SSS) revealed the existence of prominent linear polarization signals due to lines of the C2 molecule. Interpretation of the SSS is the only tool to obtain the weak and turbulent magnetic field which is widespread in the Quiet Sun. However, this interpretation is conditioned by the determination of accurate collisional data. In this context, we present a formulation of the problem of the calculation of the polarization transfer rates by collisions of polarized C2 states with electrons. The obtained formulae are applied to determine, for the first time, the polarization transfer rates between the C2 states of the Swan band electronic system (a3Πu – d3Πg) and electrons for temperatures going up from 1000 to 10 000 K. However, due to the closeness of the electronic states of the C2 molecule, the two electronic d3Πg and a3Πu cannot be disconnected from the other electronic levels and, thus, a model based on only two states is not sufficient to describe the formation of the lines in the Swan band. Consequently, we also calculated the collisional polarization transfer rates in the case where the first eight electronic states of C2 are taken into account. All rates are given as functions of the temperature by power laws. Our results should be useful for future solar applications.

A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating

ABSTRACT The Galactic centre black hole candidate Sgr A* is the best target for studies of low-luminosity accretion physics, including with near-infrared (NIR) and submillimetre wavelength long baseline interferometry experiments. Here, we compare images and spectra generated from a parameter survey of general relativistic MHD simulations to a set of radio to NIR observations of Sgr A*. Our models span the limits of weak and strong magnetization and use a range of sub-grid prescriptions for electron heating. We find two classes of scenarios can explain the broad shape of the submillimetre spectral peak and the highly variable NIR flaring emission. Weakly magnetized ‘disc-jet’ models where most of the emission is produced near the jet wall, consistent with past work, as well as strongly magnetized (magnetically arrested disc) models where hot electrons are present everywhere. Disc-jet models are strongly depolarized at submillimetre wavelengths as a result of strong Faraday rotation, inconsistent with observations of Sgr A*. We instead favour the strongly magnetized models, which provide a good description of the median and highly variable linear polarization signal. The same models can also explain the observed mean Faraday rotation measure and potentially the polarization signals seen recently in Sgr A* NIR flares.

Properties of M Dwarf Flares at Millimeter Wavelengths

Abstract We report on two millimeter flares detected by the Atacama Large Millimeter/submillimeter Array at 220 GHz from AU Mic, a nearby M dwarf. The larger flare had a duration of only ∼35 s, with peak L R = 2 × 1015 erg s−1 Hz−1, and lower limit on linear polarization of . We examine the characteristics common to these new AU Mic events and those from Proxima Cen previously reported in MacGregor et al.—namely short durations, negative spectral indices, and significant linear polarization—to provide new diagnostics of conditions in outer stellar atmospheres and details of stellar flare particle acceleration. The event rates (∼20 and 4 events days−1 for AU Mic and Proxima Cen, respectively) suggest that millimeter flares occur commonly but have been undetected until now. Analysis of the flare observing frequency and consideration of possible incoherent emission mechanisms confirms the presence of MeV electrons in the stellar atmosphere occurring as part of the flare process. The spectral indices point to a hard distribution of electrons. The short durations and lack of pronounced exponential decay in the light curve are consistent with formation in a simple magnetic loop, with radio emission predominating from directly precipitating electrons. We consider the possibility of both synchrotron and gyrosynchrotron emission mechanisms, although synchrotron is favored given the linear polarization signal. This would imply that the emission must be occurring in a low density environment of only modest magnetic field strength. A deeper understanding of this newly discovered and apparently common stellar flare mechanism awaits more observations with better-studied flare components at other wavelengths.

Wide Bandwidth Observations of Pulsars C, D, and J in 47 Tucanae

Abstract We report the first wide-band observations of pulsars C, D, and J in the globular cluster 47 Tucanae (NGC 104) using the Ultra-Wideband Low (UWL) receiver system recently installed on the Parkes 64 m radio telescope. The wide frequency range of the UWL receiver (704–4032 MHz), along with the well-calibrated system, allowed us to obtain flux density measurements and polarization pulse profiles. The mean pulse profiles have significant linear and circular polarization, allowing for determination of the Faraday rotation measure for each pulsar. Precise measurements of the dispersion measures show a significant deviation in the value for pulsar D compared to earlier results. Searches for new pulsars in the cluster are ongoing and we have determined optimal bands for such searches using the Parkes UWL receiver system.

Simulating the Solar Corona in the Forbidden and Permitted Lines with Forward Modeling. I. Saturated and Unsaturated Hanle Regimes

Abstract The magnetic field in the corona is important for understanding solar activity. Linear polarization measurements in forbidden lines in the visible/IR provide information about coronal magnetic direction and topology. However, these measurements do not provide a constraint on coronal magnetic field strength. The unsaturated, or critical regime of the magnetic Hanle effect is potentially observable in permitted lines for example in the UV, and would provide an important new constraint on the coronal magnetic field. In this paper we present the first side-by-side comparison of forbidden versus permitted linear polarization signatures, examining the transition from the unsaturated to the saturated regime. In addition, we use an analytic 3D flux rope model to demonstrate the Hanle effect for the line-of-sight versus plane-of-sky (POS) components of the magnetic field. As expected, the linear polarization in the unsaturated regime will vary monotonically with increasing magnetic field strength for regions where the magnetic field is along the observer’s line of sight. The POS component of the field produces a linear polarization signature that varies with both the field strength and direction in the unsaturated regime. Once the magnetic field is strong enough that the effect is saturated, the resulting linear polarization signal is essentially the same for the forbidden and permitted lines. We consider how such observations might be used together in the future to diagnose the coronal magnetic field.

The diurnal variation of polarization characteristics of the Earth treated as an exoplanet

The diurnal polarization variation of the Earth, treated as an exoplanet with an unresolved disk but resolvable from its host star, is presented in three wavelength bands centered at 490 nm, 670 nm and 865 nm respectively according to French satellite-borne PARASOL data. We aim to estimate disk-integrated polarization of the Earth with a phase angle of 55°. It is shown that: (1) the linear polarization signal and its variation are ascribed to the combination of surface feature distribution and atmospheric conditions acting as a variable polarimetric modulator; and (2) the polarimetric wavelength dependence is strong due to the atmospheric wavelength sensitivity. During the period when the PARASOL data were acquired, the cloud coverage ranged from 44.0% to 57.7%, and the polarimetric diurnal variation amplitude was within 1.8% in the 490 nm band, 1.3% in the 670 nm band and 1.5% in the 865 nm band.

Few-mode ring-core quantum dots-doped optical fiber amplifier

Abstract Optical fiber amplifier is a vital device in optical communication system, which can directly amplify optical signal. We theoretically propose a type of few-mode quantum dots-doped fiber amplifier (FM-QDFA) in this paper, capable of providing amplification for a number of linear polarization (LP) signal mode groups. The main part of the amplifier is a ring-core fiber doped with PbS quantum dots. And refractive index difference between the annular area and cladding is set to 0.035, which weakens the modal coupling between different LP mode groups. The simulation results show that the ring-core optical fiber not only supports transmission for LP(L=0∼6,1) signal mode group, but also provides relatively uniform gain for LP(L=1∼3,1) signal mode group across the wavelength band 1500 nm–1630 nm that covers the C + L band. For these three signal modes, we explore the differential modal gain (DMG) under 11 pump mode groups (LP(L=0∼10,1)). Noticing that the highest gain for LP(1,1) signal mode is approximately 31 dB obtained at the wavelength of 1530 nm and the DMG is less than 0.5 dB with a LP (7,1) pumping mode.

Spectropolarimetric analysis of an active region filament

Aims. The determination of the magnetic filed vector in solar filaments is made possible by interpreting the Hanle and Zeeman effects in suitable chromospheric spectral lines like those of the He I multiplet at 10 830 Å. We study the vector magnetic field of an active region filament (NOAA 12087). Methods. Spectropolarimetric data of this active region was acquired with the GRIS instrument at the GREGOR telescope and studied simultaneously in the chromosphere with the He I 10 830 Å multiplet and in the photosphere Si I 10 827 Å line. As has been done in previous studies, only a single-component model was used to infer the magnetic properties of the filament. The results are put into a solar context with the help of the Solar Dynamic Observatory images. Results. Some results clearly point out that a more complex inversion had to be performed. First, the Stokes V map of He I does not show a clear signature of the presence of the filament. Second, the local azimuth map follows the same pattern as Stokes V; it appears that polarity of Stokes V is conditioning the inference to very different magnetic fields even with similar linear polarization signals. This indication suggests that the Stokes V could be dominated from below by the magnetic field coming from the active region, and not from the filament itself. This evidence, and others, will be analyzed in depth and a more complex inversion will be attempted in the second part of this series.

Solar Wind Magnetic Field Turbulence over the Solar Activity Cycle Inferred from Coronal Sounding Experiments with Helios Linear-Polarized Signals

Results of experiments on polarized radio sounding of the outer solar corona using the Helios spacecraft from 1975 to 1984 are presented. The characteristic parameters of the temporal spectra of fluctuations in the Faraday rotation of the plane of polarization for heliocentric distances from 3.5 to 5.5 solar radii are obtained. The absolute level of these fluctuations and, consequently, the level of fluctuations of the magnetic field, is almost independent of the solar activity. It is well known that the global structure of the solar wind varies with the solar cycle such that there is slow solar wind at low latitudes and fast solar wind at high latitudes during solar minima. In contrast, a slow solar wind dominates at all latitudes during solar maxima. One explanation for the invariance of the fluctuations observed by sounding the circumsolar plasma is that the mean magnetohydrodynamic turbulence of the low-latitude, slow solar wind depends weakly on the phase of the solar cycle.

Infrequent Occurrence of Significant Linear Polarization in Hα Solar Flares

Abstract We performed statistical and event studies of linear polarization in the Hα line during solar flares. The statistical study revealed that, among 71 Hα flares analyzed, including 64 GOES flares, only one event shows significant linear polarization signals. Such an infrequent occurrence of significant linear polarization in solar flares is consistent with the result by Bianda et al., who studied 30 flares and found no polarization signals. In the event showing the significant polarization, the maximum degree of linear polarization was 1.16% ± 0.06%, and the average direction of the polarization deviated by −142.°5 ± 6.°0 from the solar north. The observed polarization degrees and the directions are consistent with the preceding reports. These strong linear polarization signals did not appear at major flare ribbons, nor did they correlate with either hard or soft X-ray emissions temporally or spatially. Instead, they appeared at a minor flare kernel, which corresponds to one of the footpoints of a coronal loop. The active region caused coronal dimming after the soft X-ray peak. The observed flare shows no direct evidence that the linear polarization is produced by high-energy particles, which are often considered to generate the polarization. On the other hand, our study suggests the possibility that coronal mass ejections, which have been often observed in flares showing linear polarization signals, play an important role for exciting linear polarization at Hα flare kernels.