Twilight

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Amit Bihari Lall - One of the best experts on this subject based on the ideXlab platform.

Adriaan M. Dokter - One of the best experts on this subject based on the ideXlab platform.

  • flocking behaviour in the Twilight ascents of common swifts apus apus
    Ibis, 2019
    Co-Authors: Adriaan M. Dokter, Cecilia Nilsson, Johan Bäckman
    Abstract:

    Among the many unique flight behaviours of Common Swifts Apus apus, the most puzzling may be their ascents to high altitudes during both dusk and dawn. Twilight ascents have been hypothesized to be functionally related to information acquisition, including integration of celestial orientation cues, high-altitude visual landmarks and sampling of weather conditions. However, their exact purpose remains unknown. We tracked Common Swifts with tracking radar at their breeding grounds in southern Sweden, and present evidence that during the dusk ascent and dawn descent they often occur in flocks, whereas during the dusk descent and dawn ascent phase they do not. This flocking behaviour suggests that swifts may benefit from conspecific interactions during Twilight ascents and descents, possibly through more robust cue acquisition and information exchange in groups, or extending social behaviour also seen in screaming parties before dusk. (Less)

  • Twilight ascents by common swifts apus apus at dawn and dusk acquisition of orientation cues
    Animal Behaviour, 2013
    Co-Authors: Adriaan M. Dokter, Susanne Akesson, Hans Beekhuis, W Bouten, Luit Buurma, Hans Van Gasteren, Iwan Holleman
    Abstract:

    Common swifts are specialist flyers spending most of their life aloft, including night-time periods when this species roosts on the wing. Nocturnal roosting is preceded by a vertical ascent in Twilight conditions towards altitudes of up to 2.5 km, behaviour previously explained as flight altitude selection for sleeping. We examined the nocturnal flight behaviour of swifts, as uniquely identified by a Doppler weather radar in central Netherlands using continuous measurements during two consecutive breeding seasons. Common swifts performed Twilight ascents not only at dusk but also at dawn, which casts new light on the purpose of these ascents. Dusk and dawn ascents were mirror images of each other when timereferenced to the moment of sunset and sunrise, suggesting that the acquisition of Twilight-specific light-based cues plays an important role in the progression of the ascents. Ascent height was well explained by the altitude of the 280 K isotherm, and was not significantly related to wind, cloud base height, humidity or the presence of nocturnal insects. We hypothesize that swifts profile the state of the atmospheric boundary layer during Twilight ascents and/or attempt to maximize their perceptual range for visual access to distant horizontal landmarks, including surrounding weather. We compare Twilight profiling by swifts with vertical Twilight movements observed in other taxa, proposed to be related to orientation and navigation.

Iwan Holleman - One of the best experts on this subject based on the ideXlab platform.

  • Twilight ascents by common swifts apus apus at dawn and dusk acquisition of orientation cues
    Animal Behaviour, 2013
    Co-Authors: Adriaan M. Dokter, Susanne Akesson, Hans Beekhuis, W Bouten, Luit Buurma, Hans Van Gasteren, Iwan Holleman
    Abstract:

    Common swifts are specialist flyers spending most of their life aloft, including night-time periods when this species roosts on the wing. Nocturnal roosting is preceded by a vertical ascent in Twilight conditions towards altitudes of up to 2.5 km, behaviour previously explained as flight altitude selection for sleeping. We examined the nocturnal flight behaviour of swifts, as uniquely identified by a Doppler weather radar in central Netherlands using continuous measurements during two consecutive breeding seasons. Common swifts performed Twilight ascents not only at dusk but also at dawn, which casts new light on the purpose of these ascents. Dusk and dawn ascents were mirror images of each other when timereferenced to the moment of sunset and sunrise, suggesting that the acquisition of Twilight-specific light-based cues plays an important role in the progression of the ascents. Ascent height was well explained by the altitude of the 280 K isotherm, and was not significantly related to wind, cloud base height, humidity or the presence of nocturnal insects. We hypothesize that swifts profile the state of the atmospheric boundary layer during Twilight ascents and/or attempt to maximize their perceptual range for visual access to distant horizontal landmarks, including surrounding weather. We compare Twilight profiling by swifts with vertical Twilight movements observed in other taxa, proposed to be related to orientation and navigation.

Oleg S Ugolnikov - One of the best experts on this subject based on the ideXlab platform.

  • detection of molecular scattering field by polarization analysis of sky background during the transitive Twilight and temperature measurements near the stratopause
    arXiv: Atmospheric and Oceanic Physics, 2017
    Co-Authors: Oleg S Ugolnikov, Igor A Maslov, B. V. Kozelov
    Abstract:

    The simplest version of the method of detection of single molecular scattering basing on the polarization measurements of the Twilight sky background by all-sky cameras is considered. The method can be used during the transitive Twilight (solar zenith angles 94-98 degrees), when effective single scattering takes place in upper stratosphere and lower mesosphere. The application of the method to the multi-year measurements in the vicinity of Moscow and Apatity allows determination of temperature of these atmospheric layers and estimation of contribution and properties of multiple scattering during the transitive Twilight period.

  • noctilucent cloud polarimetry Twilight measurements in a wide range of scattering angles
    Planetary and Space Science, 2016
    Co-Authors: Oleg S Ugolnikov, Igor A Maslov, B. V. Kozelov, Janna M Dlugach
    Abstract:

    Abstract Wide-field polarization measurements of the Twilight sky background during several nights with bright and extended noctilucent clouds in central and northern Russia in 2014 and 2015 are used to build the phase dependence of the degree of polarization of sunlight scattered by cloud particles in a wide range of scattering angles (from 40° to 130°). This range covers the linear polarization maximum near 90° and large-angle slope of the curve. The polarization in this angle range is most sensitive to the particle size. The method of separation of scattering on cloud particles from the Twilight background is presented. Results are compared with T-matrix simulations for different sizes and shapes of ice particles; the best-fit model radius of particles (0.06 μm) and maximum radius (about 0.1 μm) are estimated.

  • ubvri Twilight sky brightness at eso paranal
    Astronomy and Astrophysics, 2006
    Co-Authors: F Patat, Oleg S Ugolnikov, O V Postylyakov
    Abstract:

    Context. Twilight studies have proved to be important tools to analyze the atmospheric structure with interesting consequences on the characterization of astronomical sites. Active discussio ns on this topic have been recently restarted in connection with the evaluation of Dome C, Antarctica as a potential astronomical site and several s ite-testing experiments, including Twilight brightness m easurements, are being prepared. Aims. The present work provides for the first time absolute photome tric measurements of Twilight sky brightness for ESO-Paranal (Chile), which are meant both as a contribution to the site monitoring and as reference values in the analysis of other sites, inclu ding Dome C. Methods. The U BVRI Twilight sky brightness was estimated on more than 2000 FORS1 archival images, which include both flats and standard stars observations taken in Twilight, covering a Sun zenith distance range 94 ◦ -112 ◦ . Results. The comparison with a low altitude site shows that Paranal V Twilight sky brightness is about 30% lower, implying that some fraction of multiple scattering has to take place at an altitude of a fe w km above the sea level.

  • ubvri Twilight sky brightness at eso paranal
    arXiv: Astrophysics, 2006
    Co-Authors: F Patat, Oleg S Ugolnikov, O Postylyako
    Abstract:

    Twilight studies have proved to be important tools to analyze the atmospheric structure with interesting consequences on the characterization of astronomical sites. Active discussions on this topic have been recently restarted in connection with the evaluation of Dome C, Antarctica as a potential astronomical site and several site-testing experiments, including Twilight brightness measurements, are being prepared. The present work provides for the first time absolute photometric measurements of Twilight sky brightness for ESO-Paranal (Chile), which are meant both as a contribution to the site monitoring and as reference values in the analysis of other sites, including Dome C. The UBVRI Twilight sky brightness was estimated on more than 2000 FORS1 archival images, which include both flats and standard stars observations taken in Twilight, covering a Sun zenith distance range 94-112 deg. The comparison with a low altitude site shows that Paranal V Twilight sky brightness is about 30% lower, implying that some fraction of multiple scattering has to take place at an altitude of a few km above the sea level.

  • effects of multiple scattering and atmospheric aerosol on the polarization of the Twilight sky
    Journal of Quantitative Spectroscopy & Radiative Transfer, 2004
    Co-Authors: Oleg S Ugolnikov, O V Postylyakov, Igor A Maslov
    Abstract:

    The paper presents a review of a number of wide-angle polarization CCD-measurements of the Twilight sky in V and R color bands with effective wavelengths 550 and 700 nm. The basic factors affecting (usually decreasing) the polarization of the Twilight sky are the atmospheric aerosol scattering and multiple scattering. These effects were distinguished from each other, and a method of multiple-scattering separation is discussed. The results are compared with the data of numerical simulation of radiative transfer in the atmosphere for different aerosol models. The whole Twilight period is divided into different stages with different mechanisms forming the Twilight-sky polarization properties.

Cecilia Nilsson - One of the best experts on this subject based on the ideXlab platform.

  • flocking behaviour in the Twilight ascents of common swifts apus apus
    Ibis, 2019
    Co-Authors: Adriaan M. Dokter, Cecilia Nilsson, Johan Bäckman
    Abstract:

    Among the many unique flight behaviours of Common Swifts Apus apus, the most puzzling may be their ascents to high altitudes during both dusk and dawn. Twilight ascents have been hypothesized to be functionally related to information acquisition, including integration of celestial orientation cues, high-altitude visual landmarks and sampling of weather conditions. However, their exact purpose remains unknown. We tracked Common Swifts with tracking radar at their breeding grounds in southern Sweden, and present evidence that during the dusk ascent and dawn descent they often occur in flocks, whereas during the dusk descent and dawn ascent phase they do not. This flocking behaviour suggests that swifts may benefit from conspecific interactions during Twilight ascents and descents, possibly through more robust cue acquisition and information exchange in groups, or extending social behaviour also seen in screaming parties before dusk. (Less)