Instrumental Polarization Research Articles

Analysis of interference fringes in a long-wave infrared full Stokes polarimeter based on rotating waveplates

In the long-wave infrared (LWIR, 8–15 µm) band, the interference effect of polarization elements becomes an issue in polarimetry due to defects in the anti-reflective coatings. The paper describes an analysis and optimization method for the rotating-waveplates-based Stokes polarimeter, to eliminate interference fringes and improve polarization measurement accuracy in LWIR. An interference model was established based on the theory of polarized light and thin-film optics. Different modulation schemes were simulated and analyzed to obtain an optimized Stokes polarimeter, reducing the instrumental polarization to less than 1E-3. Furthermore, experimental validation was conducted by the Accurate Infrared Magnetic Field Measurements of the Sun (AIMS) telescope. The result shows that the instrumental polarization was less than 2E-3, consistent with the simulation.

A MeerKAT Polarization Survey of Southern Calibration Sources

We report on full-Stokes L-band observations of 98 MeerKAT calibration sources. Linear polarization is detected in 71 objects above a fractional level of 0.2%. We identify ten sources with strong fractional linear polarization and low Faraday rotation measure that could be suitable for wide-band absolute polarization calibration. We detect significant circular polarization from 24% of the sample down to a detection level of 0.07%. Circularly polarized emission is seen only for flat spectrum sources α > −0.5. We compare our polarized intensities and Faraday synthesis results to data from the NVSS at 1400 MHz and the ATCA SPASS survey at 2300 MHz. NVSS data exist for 54 of our sources and SPASS data for 20 sources. The percent polarization and rotation measures from both surveys agree well with our results. The residual instrumental linear polarization for these observations is measured at 0.16%, and the residual instrumental circular polarization is measured at 0.05%. These levels may reflect either instabilities in the relative bandpass between the two polarization channels with either time or antenna orientation, or atmospheric/ionospheric variations with pointing direction. Tracking of the hourly gain solutions on J0408-6545 after transfer of the primary gain solutions suggests a deterioration of the gain stability by a factor of several starting about 2 hr after sunrise. This suggests that observing during the nighttime could dramatically improve the precision of polarization calibration.

Effect of instrumental polarization with a half-wave plate on the B-mode signal: prediction and correction

We study the effect of incident unpolarized signal converted to polarized light produced by a realistic half-wave plate (HWP) and evaluate the impact of the effect in the measurement of Cosmic Microwave Background (CMB) B-mode polarization signal targeting to probe the tensor-to-scalar ratio r. The HWP is modeled with the Mueller formalism, and coefficients are decomposed for any incident angle into harmonics of the HWP rotation frequency due to azimuthal angle dependence. Although we use a general formalism, band-averaged matrix coefficients are calculated as an example for a 9-layer sapphire HWP using EM propagation simulations. We perform simulations of multi-detector observations in a band centered at 140 GHz using LiteBIRD instrumental configuration. We show both theoretically and with the simulations that most of the artefacts on Stokes parameter maps are produced by the dipole leakage on B-modes induced by the fourth harmonics MQI (4f) and MUI (4f). The resulting effect is strongly linked to the spin-2 focal plane scanning cross linking parameters. We develop a maximum likelihood-based method to correct the IP leakage by joint fitting of the Mueller matrix coefficients as well as the Stokes parameter maps. We show that the residual leakage after correction leads to an additional noise limited uncertainty on r of the order of 10-7, independently of the value of the Mueller matrix coefficients. We discuss the impact of the monopole signal and the potential coupling with other systematic effects such as gain variations and detector nonlinearities.

Polarimetric differential imaging with VLT/NACO

Context. The observed diversity of exoplanets can possibly be traced back to the planet formation processes. Planet-disk interactions induce sub-structures in the circumstellar disk that can be revealed via scattered light observations. However, a high-contrast imaging technique such as polarimetric differential imaging (PDI) must first be applied to suppress the stellar diffraction halo. Aims. In this work we present the PDI PiPelIne for NACO data (PIPPIN), which reduces the archival polarimetric observations made with the NACO instrument at the Very Large Telescope. Prior to this work, such a comprehensive pipeline to reduce polarimetric NACO data did not exist. We identify a total of 243 datasets of 57 potentially young stellar objects observed before NACO’s decommissioning. Methods. The PIPPIN pipeline applies various levels of instrumental polarisation correction and is capable of reducing multiple observing setups, including half-wave plate or de-rotator usage and wire-grid observations. A novel template-matching method is applied to assess the detection significance of polarised signals in the reduced data. Results. In 22 of the 57 observed targets, we detect polarised light resulting from a scattering of circumstellar dust. The detections exhibit a collection of known sub-structures, including rings, gaps, spirals, shadows, and in- or outflows of material. Since NACO was equipped with a near-infrared wavefront sensor, it made unique polarimetric observations of a number of embedded protostars. This is the first time detections of the Class I objects Elia 2-21 and YLW 16A have been published. Alongside the outlined PIPPIN pipeline, we publish an archive of the reduced data products, thereby improving the accessibility of these data for future studies.

Zero-polarization candidate regions for the calibration of wide-field optical polarimeters

Context. The calibration of optical polarimeters relies on the use of stars with negligible polarization (i.e., unpolarized standard stars) for determining the instrumental polarization zero point. For wide-field polarimeters, calibration is often done by imaging the same star over multiple positions in the field of view (FoV), which is a time-consuming process. A more effective technique is to target fields containing multiple standard stars. While this method has been used for fields with highly polarized stars, there are no such sky regions with well measured unpolarized standard stars. Aims. We aim to identify sky regions with tens of stars exhibiting negligible polarization that are suitable for a zero-point calibration of wide-field polarimeters. Methods. We selected stars in regions with extremely low reddening, located at high Galactic latitudes. We targeted four ~40′ × 40′ fields in the northern and eight in the southern equatorial hemispheres. Observations were carried out at the Skinakas Observatory and the South African Astronomical Observatory. Results. We found two fields in the north and seven in the south characterized by a mean polarization lower than p < 0.1%. Conclusions. At least 9 out of the 12 fields can be used for a zero-point calibration of wide-field polarimeters.

Characterization of Instrumental Polarization in the MOPTOP Instrument

MOPTOP is a polarimeter currently installed on the fully robotic 2 m Liverpool Telescope, designed to allow accurate measurements of the variability of the polarization of astronomical objects like gamma-ray bursts and blazars. In this work, we present the characterization of the field position dependent instrumental polarization added by MOPTOP to the acquired images. This characterization is done by the acquisition of images of low polarized standard stars, for different positions over the CCD frame, for the filters BVRIL. These images were reduced using the MOPTOP pipeline, providing us with the q and u Stokes parameters. These values, in turn, were subtracted from the q and u values of the respective star, found in literature, resulting in the spatial distribution of the residual polarization added by the instrument. A plane was fitted to the obtained result, allowing us to calculate the q and u values added by MOPTOP.

Characterization of High-polarization Stars and Blazars with DIPOL-1 at Sierra Nevada Observatory

We report here the performance and first results of the new multiband optical polarimeter DIPOL-1, installed at the Sierra Nevada Observatory 90 cm T90 telescope (SNO, Granada, Spain). DIPOL-1 is equipped with a plane parallel calcite plate and λ/2 retarder for modulating the intensity of two perpendicularly polarized beams, and a high readout speed CMOS camera that allows for fast, time-dense coverage. We characterize the performance of this instrument through a series of tests on zero- and high-polarization standard stars. The instrumental polarization in the Nasmyth focus was well determined, with a very stable contribution of 4.0806% ± 0.0014% in the optical R band. For bright high-polarization standards (m R < 8) we reach precisions <0.02% in polarization degree and 0.°1 in polarization angle for exposures of 2–4 min. The polarization properties of these stars have been constrained, providing more recent results also about possible variability for future studies of some of the most used calibrators. Moreover, we have tested the capability of observing much fainter objects, in particular through blazar observations, where we reach a precision of <0.5%−0.6% and <0.°5 for faint targets (m R ∼ 16.5) with exposures of ∼1 hr. For brighter targets (m R ∼ 14.5−15), we can aim for time-dense observations with errors <0.2%−0.4% and <1°−1.°5 in 5–20 min. We have successfully performed a first campaign with DIPOL-1, detecting significant polarized emission of several blazars, with special attention to the highest ever polarization degree measured from blazar 3C 345 at ∼32%.

Geostationary Environment Monitoring Spectrometer (GEMS) polarization characteristics and correction algorithm

Abstract. The Geostationary Environment Monitoring Spectrometer (GEMS) is the first geostationary earth orbit (GEO) environmental instrument, onboard the Geostationary Korea Multi-Purpose Satellite–2B (GEO-KOMPSAT-2B) launched on 19 February 2020, and is measuring reflected radiance from the earth's surface and atmosphere system in the range of 300–500 nm in the ultraviolet–visible (UV–Vis) region. The radiometric response of a satellite sensor that measures the UV–Vis wavelength region can depend on the polarization states of the incoming light. To reduce the sensitivity due to polarization, many current low earth orbit (LEO) satellites are equipped with a scrambler to depolarize the signals or a polarization measurement device (PMD) that simultaneously measures the polarization state of the atmosphere, then utilizes it for a polarization correction. However, a novel polarization correction algorithm is required since GEMS does not have a scrambler or a PMD. Therefore, this study aims to improve the radiometric accuracy of GEMS by developing a polarization correction algorithm optimized for GEMS that simultaneously considers the atmosphere's polarization state and the instrument's polarization sensitivity characteristics. The polarization factor and axis were derived by the preflight test on the ground as a function of wavelengths, showing a polarization sensitivity of more than 2 % at some specific wavelengths. The polarization states of the atmosphere are configured as a look-up table (LUT) using the Vector Linearized Discrete Ordinate Radiative-Transfer model (VLIDORT). Depending on the observation geometry and atmospheric conditions, the observed radiance spectrum can include a polarization error of 2 %. The performance of the proposed GEMS polarization algorithm was assessed using synthetic data, and the errors due to polarization were found to be larger in clear regions than in cloudy regions. After the polarization correction, polarization errors were reduced close to zero for almost all wavelengths, including the wavelength regions with high peaks and curvatures in the GEMS polarization factor, which sufficiently demonstrates the effectiveness of the proposed polarization correction algorithm. From the actual observation data after the launch of GEMS, the diurnal variation for the spatial distribution of polarization error was confirmed to be minimum at noon and maximum at sunrise/sunset. This can be used to improve the quality of GEMS measurements, the first geostationary environmental satellite, and then contribute to the retrieved accuracy of various Level-2 products, such as trace gases and aerosols in the atmosphere.

Early high-resolution millimeter-wave maps from ToITEC

TolTEC is a polarization-sensitive camera at millimeter wavelengths with unprecedented sensitivity and 5-10 arcsecond resolution in three photometric bands. TolTEC achieved first light on the 50-meter Large Millimeter Telescope in July 2022, just prior to a planned summer telescope maintenance shutdown, and began commissioning observations when the telescope resumed observations in December 2022. The commissioning program consisted of observations of targeted nebulae, molecular clouds, polarized quasars, galaxies, and clusters of galaxies. The goals of these observations include demonstrating the science capabilities of the camera and characterizing the performance and instrumental properties, including noise, beams, and polarization sensitivity in its three bands centered at 1.1, 1.4, and 2.0 mm |with angular resolutions of 5, 6, and 10 arcseconds, respectively. We present early results from the commissioning observations of the Crab Nebula. Upcoming observations will include selected proposals from the broader astronomy community in Mexico and the United States, as well as legacy surveys focused on mapping galactic molecular clouds, cold dust emission in local galaxies, polarized dust emission in filaments around star-forming regions, massive galaxy clusters, and distant obscured star-forming galaxies.

Simulation of High-contrast Polarimetric Observations of Debris Disks with the Roman Coronagraph Instrument

The Nancy Grace Roman Space Telescope Coronagraph Instrument will enable the polarimetric imaging of debris disks and inner dust belts in the optical and near-infrared wavelengths, in addition to the high-contrast polarimetric imaging and spectroscopy of exoplanets. The Coronagraph uses two Wollaston prisms to produce four orthogonally polarized images and is expected to measure the polarization fraction with measurement errors <3% per spatial resolution element. To simulate the polarization observations through the Hybrid Lyot Coronagraph (HLC) and Shaped Pupil Coronagraph (SPC), we model disk scattering, the coronagraphic point-response function, detector noise, speckles, jitter, and instrumental polarization and calculate the Stokes parameters. To illustrate the potential for discovery and a better understanding of known systems with both the HLC and SPC modes, we model the debris disks around Epsilon Eridani and HR 4796A, respectively. For Epsilon Eridani, using astrosilicates with 0.37 ± 0.01 as the peak input polarization fraction in one resolution element, we recover the peak disk polarization fraction of 0.33 ± 0.01. Similarly, for HR 4796A, for a peak input polarization fraction of 0.92 ± 0.01, we obtain the peak output polarization fraction as 0.80 ± 0.03. The Coronagraph design meets the required precision, and forward modeling is needed to accurately estimate the polarization fraction.

Three-dimensional instrument polarization analysis and optimization of liquid-crystal-based Stokes polarimeter.

The Stokes polarimeter based on liquid crystal variable retarders (LCVRs) is a space polarization measurement technology widely used. However, due to the tilt of the optic axis of the LCVR with the driving voltage in the direction of light propagation and the interference in LCVR, the LCVRs-based Stokes polarimeter produces a large instrument polarization, which affects the accurate polarization measurement. In this paper, we combine polarization ray tracing with multi-beam interference, and establish a general three-dimensional polarization analysis model of the LCVRs-based Stokes polarimeter. The simulation results of adjusting the LCVR voltage to reduce the instrument polarization are analyzed, and the variation of polarization measurement accuracy with the field of view before and after optimization of the LCVRs-based Stokes polarimeter is simulated and analyzed. A LCVR structure with additional films for matching the refractive index is proposed. According to the simulation results, this structure can significantly reduce the interference effects and reduce the impact of variations in liquid crystal layer thickness on the interference effects.

Calibrating VLBI Polarization Data Using GPCAL. I. Frequency-dependent Calibration

In this series of papers, we present new methods of frequency- and time-dependent instrumental polarization calibration for very long baseline interferometry (VLBI). In most existing calibration tools and pipelines, it has been assumed that instrumental polarization is constant over frequency within the instrument bandwidth and over time. The assumption is not always true and may prevent an accurate calibration, which can result in degradation of the quality of linear polarization images. In this paper, we present a method of frequency-dependent instrumental polarization calibration that is implemented in GPCAL, a recently developed polarization calibration pipeline. The method is tested using simulated data sets generated from real Very Long Baseline Array (VLBA) data. We present the results of applying the method to real VLBA data sets observed at 15 and 43 GHz. We were able to eliminate significant variability in cross-hand visibilities over frequency that is caused by frequency-dependent instrumental polarization. As a result of the calibration, linear polarization images were slightly to modestly improved as compared to those obtained without frequency-dependent instrumental polarization calibration. We discuss the reason for the minor impact of frequency-dependent instrumental polarization calibration on existing VLBA data sets and prospects for applying the method to future VLBI data sets, which are expected to provide very large bandwidths.

Calibrating VLBI Polarization Data Using GPCAL. II. Time-dependent Calibration

We present a new method of time-dependent instrumental polarization calibration for very long baseline interferometry (VLBI). This method has been implemented in the recently developed polarization calibration pipeline GPCAL. Instrumental polarization, also known as polarimetric leakage, is a direction-dependent effect, and it is not constant across the beam of a telescope. Antenna pointing model accuracy is usually dependent on time, resulting in off-axis polarimetric leakages that can vary with time. The method is designed to correct for the off-axis leakages with large amplitudes that can severely degrade linear polarization images. Using synthetic data generated based on real Very Long Baseline Array (VLBA) data observed at 43 GHz, we evaluate the performance of the method. It was able to reproduce the off-axis leakages assumed in the synthetic data, particularly those with large amplitudes. The method has been applied to two sets of real VLBA data, and the derived off-axis leakages show very similar trends over time for pairs of nearby sources. Furthermore, the amplitudes of the off-axis leakages are strongly correlated with the antenna gain correction factors. The results demonstrate that the method is capable of correcting for the off-axis leakages present in VLBI data. By calibrating time-dependent instrumental polarization, the rms noise levels of the updated linear polarization images have been significantly reduced. The method is expected to substantially enhance the quality of linear polarization images obtained from existing and future VLBI observations.

Polarization Optical Design of 8-meter Chinese Giant Solar Telescope

Instrumental polarization is a vital factor for the accurate measurements of the solar magnetic field. It is necessary to optimize the optical design of the large solar telescope to obtain high accuracy solar magnetic field information. In this paper, an optimal design scheme based on four-mirror polarization compensation optics for the 8-meter giant solar telescope is presented. The polarization effect of pupil and field of view are analyzed by the polarization ray tracing programming, the telescope motion and wavelength properties of the field-effect are investigated detailedly. The result shows that, in the near infrared waveband which includes the magnetic sensitive spectral lines of He I 1.083 μm and Fe I 1.565 μm, the polarization-free field size is 0.91′, and the polarization-free field size in the visible band is 0.5′, in which the instrumental polarization of telescope is smaller than 2×10−4.

BEYONDPLANCK

We constrained the level of polarized anomalous microwave emission (AME) on large angular scales usingPlanckLow-Frequency Instrument (LFI) and WMAP polarization data within a Bayesian cosmic microwave background (CMB) analysis framework. We modeled synchrotron emission with a power-law spectral energy distribution, as well as the sum of AME and thermal dust emission through linear regression with thePlanckHigh-Frequency Instrument (HFI) 353 GHz data. This template-based dust emission model allowed us to constrain the level of polarized AME while making minimal assumptions on its frequency dependence. We neglected CMB fluctuations, but show through simulations that these fluctuations have a minor impact on the results. We find that the resulting AME polarization fraction confidence limit is sensitive to the polarized synchrotron spectral index prior. In addition, for prior meansβs &lt; −3.1 we find an upper limit ofpAMEmax ≲ 0.6% (95% confidence). In contrast, for meansβs = −3.0, we find a nominal detection ofpAME = 2.5 ± 1.0% (95% confidence). These data are thus not strong enough to simultaneously and robustly constrain both polarized synchrotron emission and AME, and our main result is therefore a constraint on the AME polarization fraction explicitly as a function ofβs. Combining the currentPlanckand WMAP observations with measurements from high-sensitivity low-frequency experiments such as C-BASS and QUIJOTE will be critical to improve these limits further.

Thin flexible multi-octave metamaterial absorber for millimeter wavelengths.

The development of radiation-absorbent materials and devices for millimeter and submillimeter astronomy instruments is a research area of significant interest that has substantial engineering challenges. Alongside a low-profile structure and ultra-wideband performance in a wide range of angles of incidence, advanced absorbers in cosmic microwave background (CMB) instruments are aimed at reducing optical systematics, notably instrument polarization, far beyond previously achievable specifications. This paper presents a metamaterial-inspired flat conformable absorber design operating in a wide frequency range of 80-400GHz. The structure comprises a combination of subwavelength metal-mesh capacitive and inductive grids and dielectric layers, using the magnetic mirror concept for a large bandwidth. The overall stack thickness is a quarter of the longest operating wavelength and is close to the theoretical limit stipulated by Rozanov's criterion. The test device is designed to operate at a 22.5° incidence. The iterative numerical-experimental design procedure of the new metamaterial absorber is discussed in detail, as well as the practical challenges of its manufacture. A well-established mesh-filter fabrication process has been successfully employed for prototype fabrication, which ensures cryogenic operation of the hot-pressed quasi-optical devices. The final prototype, extensively tested in quasi-optical testbeds using a Fourier transform spectrometer and a vector network analyzer, demonstrated performance closely matching the finite-element analysis simulations; that is, greater than 99% absorbance for both polarizations, with only a 0.2% difference, across the frequency band of 80-400GHz. The angular stability for up to ±10∘ has been confirmed by simulations. To the best of our knowledge, this is the first successful implementation of a low-profile, ultra-wideband metamaterial absorber for this frequency range and operating conditions.

MeerKAT Holography Measurements in the UHF, L, and S Bands

Radio holographic measurements using the MeerKAT telescope are presented for each of its supported observing bands, namely UHF (544–1087 MHz), L (856–1711 MHz), and S (1750–3499 MHz). Because the UHF-band receiver design is a scaled version of that of the L band, the electromagnetic performance in these two bands are expectedly similar to one another. Despite also being linearly polarized, S-band receivers have an entirely different design and distinct performance characteristics from the lower two bands. As introduced in previous work for the L band, evidence of higher order waveguide mode activation also appears in S-band measurements but there are differences in its manifestation. Frequency-dependent pointing (beam squint), beamwidth, beam ellipticity, error beam, instrumental polarization, and cross-polarization power measurements are illustrated for each of MeerKAT’s observational bands in a side-by-side style to facilitate the comparison of features. The derivation of collimation errors and main reflector surface errors from measurements made at these relatively low observation frequencies is also discussed. Results include elevation and ambient temperature effects on collimation, as well as the signatures of collimation degrading over time. The accompanying data release includes a snapshot of full Jones matrix primary beam patterns for all bands and antennas with corresponding derived metrics.

The Rapid ASKAP Continuum Survey III: Spectra and Polarisation In Cutouts of Extragalactic Sources (SPICE-RACS) first data release

AbstractThe Australian SKA Pathfinder (ASKAP) radio telescope has carried out a survey of the entire Southern Sky at 887.5 MHz. The wide area, high angular resolution, and broad bandwidth provided by the low-band Rapid ASKAP Continuum Survey (RACS-low) allow the production of a next-generation rotation measure (RM) grid across the entire Southern Sky. Here we introduce this project as Spectral and Polarisation in Cutouts of Extragalactic sources from RACS (SPICE-RACS). In our first data release, we image 30 RACS-low fields in StokesI,Q,Uat 25$^{\prime\prime}$angular resolution, across 744–1032 MHz with 1 MHz spectral resolution. Using a bespoke, highly parallelised, software pipeline we are able to rapidly process wide-area spectro-polarimetric ASKAP observations. Notably, we use ‘postage stamp’ cutouts to assess the polarisation properties of 105912 radio components detected in total intensity. We find that our StokesQandUimages have anrmsnoise of$\sim$80$\unicode{x03BC}$Jy PSF$^{-1}$, and our correction for instrumental polarisation leakage allows us to characterise components with$\gtrsim$1% polarisation fraction over most of the field of view. We produce a broadband polarised radio component catalogue that contains 5818 RM measurements over an area of$\sim$1300 deg$^{2}$with an average error in RM of$1.6^{+1.1}_{-1.0}$rad m$^{-2}$, and an average linear polarisation fraction$3.4^{+3.0}_{-1.6}$%. We determine this subset of components using the conditions that the polarised signal-to-noise ratio is$&gt;$8, the polarisation fraction is above our estimated polarised leakage, and the StokesIspectrum has a reliable model. Our catalogue provides an areal density of$4\pm2$RMs deg$^{-2}$; an increase of$\sim$4 times over the previous state-of-the-art (Taylor, Stil, Sunstrum 2009, ApJ, 702, 1230). Meaning that, having used just 3% of the RACS-low sky area, we have produced the 3rd largest RM catalogue to date. This catalogue has broad applications for studying astrophysical magnetic fields; notably revealing remarkable structure in the Galactic RM sky. We will explore this Galactic structure in a follow-up paper. We will also apply the techniques described here to produce an all-Southern-sky RM catalogue from RACS observations. Finally, we make our catalogue, spectra, images, and processing pipeline publicly available.

Portable instrument and current polarization limitations of high sensitivity constant-potential capacitive readout with polymeric ion-selective membranes

In this work, the development of a portable device integrated with electronic circuits, termed PotentioCap, is described for the constant-potential coulometric readout of ion-selective membranes. A range of capacitors (22–220µF) included in the device can be automatically chosen by the control software. The device was evaluated in standard pH solutions and stabilized seawater samples using a hydrogen-selective electrode placed in series with one capacitor. The transient current and integrated charge over time correlate well with that from capacitive readout using a benchtop potentiostat using an external electronic circuit. The capacitor of 47µF is sufficient to amplify the current signal of 0.01 pH unit change with slopes of 2.68µC/decade and 2.49µC/decade in standard pH solutions and stabilized seawater samples, respectively. Unfortunately, however, instrumental control of the measurement protocol, as opposed to the chemical reconditioning reported earlier, results in an order of magnitude larger measurement error (precision of 0.6mpH). To understand this the Nernst-Planck equation is used to describe the effects of constant and exponential decay currents with ion-selective membranes. Numerical simulations show that the passage of 0.2µC of charge across the membrane causes a ∼1 % change in membrane concentration. The resulting phase boundary potential is shown to drift in the same manner as that observed experimentally with current polarized membranes. Furthermore, an improved capacitive model using an RC time constant is proposed by additionally considering the potential change with time. The model results compare favorably with the experimental data. Electric migration is shown not to be a significant contribution to the transient current. We discuss solutions to overcome the observed limitation of potential stability, which currently holds back the realization of ultra-high sensitivity measurements with ion-selective membranes.

Spectropolarimetry as a Means to Address Cloud Composition and Habitability for a Cloudy Exoplanetary Atmosphere in the Habitable Zone

In our solar system, the densely cloud-covered atmosphere of Venus stands out as an example of how polarimetry can be used to gain information on cloud composition and particle mean radius. With current interest running high on discovering and characterizing extrasolar planets in the habitable zone where water exists in the liquid state, making use of spectropolarimetric measurements of directly imaged exoplanets could provide key information unobtainable through other means. In principle, spectropolarimetric measurements can determine if acidity causes water activities in the clouds to be too low for life. To this end, we show that a spectropolarimeter measurement over the range 400–1000 nm would need to resolve linear polarization to a precision of about 1% or better for reflected starlight from an optically thick cloud-enshrouded exoplanet. We assess the likelihood of achieving this goal by simulating measurements from a notional spectropolarimeter as part of a starshade configuration for a large space telescope (a HabEx design, but for a 6 m diameter primary mirror). Our simulations include consideration of noise from a variety of sources. We provide guidance on limits that would need to be levied on instrumental polarization to address the science issues we discuss. For photon-limited noise, integration times would need to be of order 1 hr for a large radius (10 Earth radii) planet to more than 100 hr for smaller exoplanets depending on the star–planet separation, planet radius, phase angle, and desired uncertainty. We discuss implications for surface chemistry and habitability.