Undisturbed Mesosphere Optical Properties from Wide-Angle Frequent Twilight Sky Polarimetry

The paper describes the first results of all-sky polarization measurements of twilight background started in central Russia in the very beginning of summer 2011. Time-frequent data of sky intensity and polarization over the wide range of sky point zenith distances is used to separate the multiple scattering and to build the altitude dependency of scattering coefficient and polarization in the mesosphere (altitudes from 60 to 90 km) at different angles. Undisturbed structure of the mesosphere without the noticeable aerosol stratification during the observation days allows to estimate the atmosphere temperature at these altitudes.

💡 Research Summary

The authors present the first results of a wide‑angle twilight sky polarimetry campaign conducted near Moscow (55.2° N, 37.5° E) during the early summer of 2011. Using a fish‑eye lens system equipped with a rotating polarization filter, a Sony DSI‑Pro CCD, and a 540 nm bandpass filter, they recorded the intensity and linear polarization of the twilight sky over a field of about 140°. The observations spanned from before sunset through the entire night to after sunrise, with exposure times ranging from 0.001 s in bright twilight to 15 s in dark twilight, and a temporal cadence of roughly 2 s (bright) to 18 s (dark). Two consecutive clear nights (31 May and 1 June) were selected; the latter featured a partial solar eclipse that uniformly reduced sky brightness but did not affect the derived mesosphere parameters because the eclipse occurred during the multiple‑scattering‑dominated dark twilight stage.

The methodology relies on the well‑known geometry of single scattering during twilight: when the solar zenith angle (SZA) exceeds 90°, lower atmospheric layers are in Earth’s shadow, and effective scattering occurs at an altitude H≈18 km for the chosen wavelength. By examining pairs of sky points symmetric with respect to the solar vertical (zenith distances +z and –z), the authors separate the contributions of single scattering (J), multiple scattering (j), and the night‑sky background (n). In the SZA range > 100°, where single scattering is negligible, the intensity ratio I(+z)/I(–z) follows a straight line, allowing the determination of coefficients A and B that describe the linear relationship between the two components. These coefficients are then used in a set of algebraic expressions (Eqs. 5 and 9 in the paper) to retrieve the intensity and polarization of the single‑scattered component for SZA values where it is still present (approximately 97–100° depending on z).

The analysis shows that the polarization of the single‑scattered light in the 60–90 km altitude range is very close to the Rayleigh‑theory value (≈0.06) and exhibits only a slight decrease with altitude, indicating an essentially aerosol‑free mesosphere. The derived single‑scattering coefficients, combined with Rayleigh scattering theory, enable an estimate of the atmospheric temperature in this layer; the authors obtain values around 180 K, consistent with the known summer cooling of the mid‑latitude mesosphere.

The study demonstrates that wide‑field, high‑cadence twilight polarimetry can effectively disentangle multiple scattering and provide reliable measurements of mesosphere optical properties and temperature without the need for lidar or satellite instruments. Limitations include potential systematic errors from the fish‑eye lens’s radial flat‑field non‑uniformity and uncertainties in the night‑sky background polarization, especially when local light pollution contributes. The authors suggest that extending the technique to multiple wavelengths, longer observation periods, and improved calibration would allow monitoring of mesospheric aerosol variations (e.g., during meteor showers) and long‑term climate trends.