Sky Polarization Research Articles

Spectral properties of identified polarized-light sensitive interneurons in the brain of the desert locustSchistocerca gregaria

Many migrating animals employ a celestial compass mechanism for spatial navigation. Behavioral experiments in bees and ants have shown that sun compass navigation may rely on the spectral gradient in the sky as well as on the pattern of sky polarization. While polarized-light sensitive interneurons (POL neurons) have been identified in the brain of several insect species, there are at present no data on the neural basis of coding the spectral gradient of the sky. In the present study we have analyzed the chromatic properties of two identified POL neurons in the brain of the desert locust. Both neurons, termed TuTu1 and LoTu1, arborize in the anterior optic tubercle and respond to unpolarized light as well as to polarized light. We show here that the polarized-light response of both types of neuron relies on blue-sensitive photoreceptors. Responses to unpolarized light depended on stimulus position and wavelength. Dorsal unpolarized blue light inhibited the neurons, while stimulation from the ipsilateral side resulted in opponent responses to UV light and green light. While LoTu1 was inhibited by UV light and was excited by green light, one subtype of TuTu1 was excited by UV and inhibited by green light. In LoTu1 the sensitivity to polarized light was at least 2 log units higher than the response to unpolarized light stimuli. Taken together, the spatial and chromatic properties of the neurons may be suited to signal azimuthal directions based on a combination of the spectral gradient and the polarization pattern of the sky.

A multifrequency angular power spectrum analysis of the Leiden polarization surveys

The Galactic synchrotron emission is expected to be the most relevant source of astrophysical contamination in cosmic microwave background polarization measurements, at least at frequencies 30'. We present a multifrequency analysis of the Leiden surveys, linear polarization surveys covering essentially the Northern Celestial Hemisphere at five frequencies between 408 MHz and 1411 MHz. By implementing specific interpolation methods to deal with these irregularly sampled data, we produced maps of the polarized diffuse Galactic radio emission with pixel size of 0.92 deg. We derived the angular power spectrum (APS) (PI, E, and B modes) of the synchrotron dominated radio emission as function of the multipole, l. We considered the whole covered region and some patches at different Galactic latitudes. By fitting the APS in terms of power laws (C_l = k l^a), we found spectral indices that steepen with increasing frequency: from a = -(1-1.5) at 408 MHz to a = -(2-3) at 1411 MHz for 10 < l < 100 and from a = -0.7 to a = -1.5 for lower multipoles (the exact values depending on the considered sky region and polarization mode). The bulk of this steepening can be interpreted in terms of Faraday depolarization effects. We then considered the APS at various fixed multipoles and its frequency dependence. Using the APSs of the Leiden surveys at 820 MHz and 1411 MHz, we determined possible ranges for the rotation measure, RM, in the simple case of an interstellar medium slab model. Taking also into account the polarization degree at 1.4 GHz, we could break the degeneracy between the identified RM intervals. The most reasonable of them turned out to be RM = 9-17 rad/m^2.

Dual-field imaging polarimeter using liquid crystal variable retarders

An imaging Stokes-vector polarimeter using liquid crystal variable retarders (LCVRs) has been built and calibrated. Operating in five bands from 450 to 700 nm, the polarimeter can be changed quickly between narrow (12 degrees ) and wide (approximately 160 degrees) fields of view. The instrument is designed for studying the effects of differing sky polarization upon the measured polarization of ground-based objects. LCVRs exhibit variations in retardance with ray incidence angle and ray position in the aperture. Therefore LCVR-based Stokes polarimeters exhibit unique calibration challenges not found in other systems. Careful design and calibration of the instrument has achieved errors within +/-1.5%. Clear-sky measurements agree well with previously published data and cloudy data provide opportunities to explore spatial and spectral variations in sky polarization.

Fine structure of the compound eye of the fungus beetle Neotriplax lewisi (Coleoptera, Cucujiformia, Erotylidae)

Abstract. In many ways, the apposition eye of the erotylid fungus beetle Neotriplax lewisi resembles that of chrysomelids: its 400–500 mostly hexagonal ommatidia are of the acone type and possess “open rhabdoms,” a tapetum is not present, and axons penetrate the basement membrane in distinct bundles of eight. The eye also shows some unusual features that, at present, defy clear functional interpretation. Firstly, the cuticle of the interfacetal areas stains differently from that of the corneal lenses and, secondly, the two rhabdom systems in each ommatidium (central and peripheral) both possess microvilli that are oriented in such a way as to permit e‐vector discrimination. On the basis of comparisons with other open rhabdom eyes, it is postulated that vision in N. lewisi involves neither high resolving power nor superior absolute sensitivity. However, this beetle can distinguish illuminated from shaded areas, and seems specialized to make use of sky polarization (probably in the UV and green regions of the spectrum) and/or the position of the sun as a course‐stabilizing function during flights.

A statistical analysis of a Galactic all sky survey at 1.4 GHz

AbstractRadio surveys at frequencies of ∼ 1 GHz allow to map the synchrotron emission (SE) in a frequency range where (except for very low Galactic latitudes or towards localized regions) it dominates over the other radio components. New all sky total intensity and polarization data at 1.4 GHz have been recently collected.We focus on the Galactic radio emission correlation properties described in terms of angular power spectrum (APS).We present for the first time the APS, in both total intensity and polarization modes, for some representative Galactic cuts and suitable APS power law (PL) parametrizations. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

On-board calibration system for millimeter-wave radiometers based on reference-polarized signal injection

This paper presents the architecture of a novel on-board calibration system for millimeter-wave radiometers aimed at measuring the Q,U Stokes parameters of the sky emission. This technique requires the injection of differently polarized reference signals, which is carried out by means of a specific component mounted in the antenna-feed system. The presented technique was developed by Cortiglioni et al. for the calibration of the Balloon-borne Radiometers for Sky Polarization Observations radiometers, specifically designed to measure the polarized sky emission by a correlation technique. A procedure for the accurate evaluation of the polarization state of the calibration signals is also reported. Numerical and experimental results are presented for the Ka-band calibrator in the operational conditions

Deep 1.4-GHz observations of diffuse polarized emission

Polarized diffuse emission observations at 1.4 GHz in a high Galactic latitude area of the Northern celestial hemisphere are presented. The 3.2 x 3.2 deg 2 field, centred at RA = 10 h 58 m , Dec. = +42°18' (B1950), has Galactic coordinates l ∼ 172°, b ∼ +63° and is located in the region selected as northern target of the Balloon-borne Radiometers for Sky Polarization Observations experiment. Observations have been performed with the Effelsberg 100-m telescope. We find that the angular power spectra of the E and B modes have slopes of β E = -1.79 ± 0.13 and β B = -1.74 ± 0.12, respectively. Because of the very high Galactic latitude and the smooth emission, a weak Faraday rotation action is expected, which allows both a fair extrapolation to cosmic microwave background polarization (CMBP) frequencies and an estimate of the contamination by the Galactic synchrotron emission. We extrapolate the E-mode spectrum up to 32 GHz and confirm the possibility to safely detect the CMBP E-mode signal in the Ka band found in another low-emission region. Extrapolated up to 90 GHz, the Galactic synchrotron B mode looks to compete with the cosmic signal only for models with a tensor-to-scalar perturbation power ratio T/S < 0.001, which is even lower than the T/S value of 0.01 found to be accessible in the only other high Galactic latitude area investigated to date. This suggests that values as low as T/S = 0.01 might be accessed at high Galactic latitudes. Such low-emission values can allow a significant redshift of the best frequency to detect the CMBP B mode, also reducing the contamination by Galactic dust, and opening interesting perspectives to investigate inflation models.

BaR-SPOrt: the instrument to be accommodated at Dome C

The BaR-SPOrt (Balloon-Borne Radiometers for Sky Polarization Observations) experiment, a program of the Agenzia Spaziale Italiana (ASI) co-funded by PNRA (Progetto Nazionale di Ricerca in Antartide) was originally designed as a payload for long duration balloons flights. The changing scenario, both scientific and strategic, has led us to propose it for the starting winter campaign of at the Concordia Base. Here the instrument and the features making it suitable to operate at Dome-C are described. After the initial setup, BaR-SPOrt should not require any kind of routine intervention by a dedicated base staff. The experiment will just need electrical power (less than 2 kW) and a suitable accommodation on the field. It can be fully monitored and controlled, including the data acquisition, through its own telemetry/telecommand link using IRIDIUM modems. Both the receiver and the critical electronics are housed inside a temperature-controlled vacuum chamber, providing the properly stabilized environment. The cold part of the radiometer employs a closed loop mechanical cryo-cooler that provides temperatures <70 ± 0.1 K with low power consumption (<200 W).

The Sky Polarisation Observatory (SPOrt) Program

The goal of the Sky Polarisation Observatory (SPOrt) Program is the measurement of the sky linearly polarised emission in the 22-90 GHz frequency range from the International Space Station (2003-2004). The instrument configuration together with most relevant ground support activities are presented. In particular, the development of hardware solutions for high sensitive polarimetric measurements has been addressed by the SPOrt team to match the experiment requirements.

Polarization singularities in the clear sky

Ideas from singularity theory provide a simple account of the pattern of polarization directions in daylight. The singularities (two near the Sun and two near the anti-Sun) are points in the sky where the polarization line pattern has index +1/2 and the intensity of polarization is zero. The singularities are caused by multiple scattering that splits into two each of the unstable index +1 singularities at the Sun and anti-Sun, which occur in the single-dipole scattering (Rayleigh) theory. The polarization lines are contours of an elliptic integral. For the intensity of polarization (unnormalized degree), it is necessary to incorporate the strong depolarizing effect of multiple scattering near the horizon. Singularity theory is compared with new digital images of sky polarization, and gives an excellent description of the pattern of polarization directions. For the intensity of polarization, the theory can reproduce not only the zeros but also subtle variations in the polarization maxima.It is not one of the least wonders of terrestrial physics, that the blue atmosphere which overhangs us, exhibits in the light which it polarises phenomena somewhat analogous to those of crystals with two axes of double refractionBrewster D 1863 Trans. R. Soc. Ed. 23 205–10

Measuring CMB polarization from ISS: the SPOrt experiment

The SPOrt (Sky Polarization Observatory) experiment aims to measure CMBP (cosmic microwave background polarization) on about 80% of the sky from space. Selected by ESA to fly on board the ISS in 2006, it is funded by the Italian Space Agency (ASI). As shown also by the recent WMAP release, CMBP data, besides of removing various degeneracies among cosmological parameters, provided new and important information on the cosmic opacity τ and, therefore, on very early cosmic objects which reionized the world at z ∼ 15. Most such information is obtained from low-l spectral components, that SPOrt, with its HPBW resolution of 7∘ will explore with a high level of sensitivity. The 4 polarimeters of SPOrt work at 22, 32 and (2x) 90 GHz. At lower frequencies they will provide a (nearly) all-sky survey of Galactic synchrotron polarized emission, while data at the higher frequency will measure the CMBP signal. Correlating SPOrt with anisotropy data, by other experiments, shall therefore provide significant cosmological information. We performed a number of simulations of SPOrt performance, aimed to determine how far τ and/or other parameter(s) concerning reionization are constrained by the expected data. We also considered a possible interplay between reionization histories and Dark Energy nature. Besides of information on technological developments for systematics reduction, long term stability and observing time efficiency, we report here recent outputs on the expected SPOrt performance in constraining cosmological models.

How well does the Rayleigh model describe the E-vector distribution of skylight in clear and cloudy conditions? A full-sky polarimetric study.

We present the first high-resolution maps of Rayleigh behavior in clear and cloudy sky conditions measured by full-sky imaging polarimetry at the wavelengths of 650 nm (red), 550 nm (green), and 450 nm (blue) versus the solar elevation angle thetas. Our maps display those celestial areas at which the deviation deltaalpha = /alphameas - alphaRyleigh/ is below the threshold alphathres = 5 degrees, where alphameas is the angle of polarization of skylight measured by full-sky imaging polarimetry, and alphaRayleigh is the celestial angle of polarization calculated on the basis of the single-scattering Rayleigh model. From these maps we derived the proportion r of the full sky for which the single-scattering Rayleigh model describes well (with an accuracy of deltaalpha = 5 degrees) the E-vector alignment of skylight. Depending on thetas, r is high for clear skies, especially for low solar elevations (40% < r < 70% for thetas < or = 13 degrees). Depending on the cloud cover and the solar illumination, r decreases more or less under cloudy conditions, but sometimes its value remains remarkably high, especially at low solar elevations (rmax = 69% for thetas = 0 degrees). The proportion r of the sky that follows the Rayleigh model is usually higher for shorter wavelengths under clear as well as cloudy sky conditions. This partly explains why the shorter wavelengths are generally preferred by animals navigating by means of the celestial polarization. We found that the celestial E-vector pattern generally follows the Rayleigh pattern well, which is a fundamental hypothesis in the studies of animal orientation and human navigation (e.g., in aircraft flying near the geomagnetic poles and using a polarization sky compass) with the use of the celestial alpha pattern.

Neurobiology of polarization vision in the locust Schistocerca gregaria.

The polarization pattern of the blue sky serves as an important reference for spatial orientation in insects. To understand the neural mechanisms involved in sky compass orientation we have analyzed the polarization vision system in the locust Schistocerca gregaria. As in other insects, photoreceptors adapted for the detection of sky polarization are concentrated in a dorsal rim area (DRA) of the compound eye. Stationary flying locusts show polarotactic yaw-torque responses when illuminated through a rotating polarizer from above. This response is abolished after painting the DRAs. Central stages of the polarization vision system, revealed through tracing studies, include dorsal areas in the lamina and medulla, the anterior lobe of the lobula, the anterior optic tubercle, the lateral accessory lobe and the central complex. Physiological analysis of polarization-sensitive (POL) neurons has focussed on the optic tubercle and on the central complex. Each POL neuron was maximally excited at a certain e-vector (phimax) and was maximally inhibited at an e-vector perpendicular to phimax. The neurons had large visual fields, and many neurons received input from both eyes. The neuronal organization of the central complex suggests a role as a spatial compass within the locust brain.

The Sky Polarization Observatory

The Sky Polarization Observatory (SPOrt) is an ASI-funded experiment specifically designed to measure the sky polarization at 22, 32 and 90 GHz, which was selected in 1997 by ESA to be flown on the International Space Station. Starting in 2006 and for at least 18 months, it will be taking direct and simultaneous measurements of the Stokes parameters Q and U at 660 sky pixels, with FWHM=7°. Due to development efforts over the past few years, the design specifications have been significantly improved with respect to the first proposal. Here we present an up-to-date description of the instrument, which now warrants a pixel sensitivity of 1.7 μK for the polarization of the cosmic background radiation, assuming two years of observations. We discuss SPOrt scientific goals in the light of WMAP results, in particular in connection with the emerging double-reionization cosmological scenario.

NEAR-EARTH OBSERVATIONS BY WIDELY SEPARATED TELESCOPES

We suggest the idea of a total-sky survey at the International Space Station by four small wide-angle telescopes with polarization filters and charge-coupled device arrays separated several metres from one another. The video information processing will be carried out by a real-time multiprocessor system on board the station. This experiment would allow us to observe the sunlit space debris and meteoroids of centrimetre size with estimations of their distances and velocities at distances up to 20 km from the station and to investigate the interplanetary and interstellar medium by making polarization sky maps and detecting the weak-contrast features on them.

Systematic effects in the measurement of polarization by the PLANCK telescope

We present estimates of the response to polarized signals by the PLANCK telescope. These estimates are based on a set of simulations, using a physical optics code (GRASP8), for linearly polarized detectors at different frequencies and located in different positions of an early design of the PLANCK focal plane. We show how the optical aberration introduced by the studied focal plane configuration affects absolute and relative orientation of the sky signals polarization planes. In addition, we compute the spurious signal introduced by the telescope optics into a PLANCK-type measurement of the sky polarization. Our analysis shows that the spurious polarization expected in a PLANCK-like experiment is typically of the order of 0.2% of the unpolarized intensity.

Ultrastructure and orientation of ommatidia in the dorsal rim area of the locust compound eye

In many insect species, a dorsal rim area (DRA) in the compound eye is adapted to analyze the sky polarization pattern for compass orientation. In the desert locust Schistocerca gregaria, these specializations are particularly striking. The DRA of the locust consists of about 400 ommatidia. The facets have an irregular shape, and pore canals are often present in the corneae. Screening pigment is missing in the region of the dioptric apparatus suggesting large receptive fields. The rhabdoms are shorter, but about four times larger in cross-section than the rhabdoms of ordinary ommatida. Eight retinula cells contribute to the rhabdom. The microvilli of retinula cell 7 and of cells 1, 2, 5, 6, 8 are highly aligned throughout the rhabdom and form two blocks of orthogonal orientation. The microvilli in the minute rhabdomeres of retinula cells 3 and 4, in contrast, show no particular alignment. As in other insect species, microvillar orientations are arranged in a fan-like pattern across the DRA. Photoreceptor axons project to distinct areas in the dorsal lamina and medulla. The morphological specializations in the DRA of the locust eye most likely maximize the polarization sensitivity and suggest that the locust uses this eye region for analysis of the sky polarization pattern.

Homing behaviour in the sleepy lizard (Tiliqua rugosa): the role of visual cues and the parietal eye

To investigate how visual cues are integrated into a navigational strategy for homing in the Australian sleepy lizard (Tiliqua rugosa), lizards were displaced beyond their home range, either with full access to visual cues or with no access to visual cues during the displacement. Homeward orientation was significantly worse when lizards were denied visual cues during the displacement than when they were not. However when lizards were displaced with their field of view restricted to the sky, their homeward orientation was equally as good as that of lizards displaced with no visual restriction. These experiments suggest that sleepy lizards use celestial cues to determine the compass bearing of the outward journey, and reverse this bearing to orient in the homeward direction (course reversal). In a subsequent experiment, lizards oriented randomly with respect to home when the parietal eye was entirely covered with a patch during the displacement and return, while control lizards fitted with a sham parietal eye patch were well oriented towards home. In both groups, the lateral eyes were unobstructed and had complete access to visual cues including celestial cues and landmarks. These results suggest that the parietal eye plays a highly significant role in sleepy lizard homing, perhaps mediating a sky polarization compass sense.

How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation.

One of the biologically most important parameters of the cloudy sky is the proportion P of the celestial polarization pattern available for use in animal navigation. We evaluated this parameter by measuring the polarization patterns of clear and cloudy skies using 180 degrees (full-sky) imaging polarimetry in the red (650 nm), green (550 nm) and blue (450 nm) ranges of the spectrum under clear and partly cloudy conditions. The resulting data were compared with the corresponding celestial polarization patterns calculated using the single-scattering Rayleigh model. We show convincingly that the pattern of the angle of polarization (e-vectors) in a clear sky continues underneath clouds if regions of the clouds and parts of the airspace between the clouds and the earth surface (being shady at the position of the observer) are directly lit by the sun. The scattering and polarization of direct sunlight on the cloud particles and in the air columns underneath the clouds result in the same e-vector pattern as that present in clear sky. This phenomenon can be exploited for animal navigation if the degree of polarization is higher than the perceptual threshold of the visual system, because the angle rather than the degree of polarization is the most important optical cue used in the polarization compass. Hence, the clouds reduce the extent of sky polarization pattern that is useful for animal orientation much less than has hitherto been assumed. We further demonstrate quantitatively that the shorter the wavelength, the greater the proportion of celestial polarization that can be used by animals under cloudy-sky conditions. As has already been suggested by others, this phenomenon may solve the ultraviolet paradox of polarization vision in insects such as hymenopterans and dipterans. The present study extends previous findings by using the technique of 180 degrees imaging polarimetry to measure and analyse celestial polarization patterns.

Polarization patterns of the summer sky and its neutral points measured by full–sky imaging polarimetry in Finnish Lapland north of the Arctic Circle

Using 180° field–of–view (full–sky) imaging polarimetry, the patterns of the degree and angle of polarization of the entire summer sky were measured on 25 June 1999 at a location north of the Arctic Circle in Finnish Lapland as a function of the angular solar zenith distance. A detailed description of the used full–sky imaging polarimeter and its calibration is given. A series of the degree and angle of polarization pattern of the full sky is presented in the form of high–resolution circular maps measured in the blue (450 nm) spectral range as a function of the solar zenith distance. Graphs of the spectral dependence of the degree and angle of polarization of skylight at 90° from the Sun along the antisolar meridian are shown. The celestial regions of negative polarization and the consequence of the existence of this anomalous polarization, the neutral points, are visualized. The measured values of the angular zenith distance of the Arago and Babinet neutral points are presented as a function of the zenith distance of the Sun for the red (650 nm), green (550 nm) and blue (450 nm) ranges of the spectrum. The major aim of this work is to give a clear and comprehensive picture, with the help of full–sky imaging polarimetry, of what is going on in the entire polarized skydome. We demonstrate how variable the degree of polarization of skylight and the position of the neutral points can be within 24 h on a sunny, almost cloudless, visually clear day.