Polarization patterns of the summer sky and its neutral points measured byfull-sky imaging polarimetry in Finnish Lapland north of the Arctic Circle

Using 180 degrees field-of-view (full-sky) imaging polarimetry: the pattern s of the degree and angle of polarization of the entire summer sky were mea sured on 25 June 1999 at a location north of tire Arctic Circle in Finnish Lapland as a function of the angular solar zenith distance. A detailed desc ription of the used full-sky imaging polarimeter and its calibration is giv en. A series of the degree and angle of polarization pattern of the full sk y 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. G raphs of the spectral dependence of the degree and angle of polarization of skylight at 90 degrees 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 visualiz ed. The measured values of the angular zenith distance of the Arago and Bab inet neutral points art: 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 th e 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 o n in the entire polarized skydome. We demonstrate how variable the degree o f polarization of skylight and the position of the neutral points can be wi thin 24 h on a sunny, almost cloudless, visually clear day.