Physics of auroral phenomena : proceedings of the 40th annual seminar, Apatity, 13-17 March, 2017 / [ed. board: N. V. Semenova, A. G. Yahnin]. - Апатиты : Издательство Кольского научного центра РАН, 2017. - 143 с. : ил., табл.

size distribution with width 0 = 1 .4 [ Von Savigny and Burrows, 2007], than mean particle radius will be close to a half of monodisperse value, being equal to 30 nm (asterisks in Fig. 3). Stratospheric aerosol layer [Junge et al., 1961] appears at the altitudes about 20 km and consists of sulfur acid droplets [Rosen, 1971]. The number and size of particles sufficiently increase after volcanic eruptions, when large amount of sulfur dioxide is emitted to the stratosphere. However, aerosol is also found in the stratosphere during volcanically-quiet period. Stratospheric aerosol particles are larger than the ones on NLC, they have weaker wavelength dependence of scattering. This leads to red excess of light scattered by these particles on the background of Rayleigh and multiple scattering. Effect gets stronger in dusk area, since aerosol particles scatter most part of radiation in a forward direction. О.S. Ugolnikov et al. Figure 4. Twilight sky brightness ratio in symmetric points of solar vertical (evening twilight ofMarch, 27, 2016). Arrows show the effect of aerosol scattering in stratosphere. 90 92 94 96 98 100 Solar zenith angle z, degrees 1.0 В (463 nm) Scattering altitude for R band, h(0,z), km [10 15 20 30 40 50 Figure 5. Retrieved characteristics of particle log-normal size distribution: solid line and gray areas (single, double and triple error) - twilight analysis (the same date as in Fig. 4), 21.4 km, dashed line - [Bourassa et al., 2008]. Median radius, r, microns During spring and summer 2016, all-sky RGB-photometry was conducted near Moscow (55.2°N, 37.5°E). Fig. 4 shows the behavior of brightness ratio in symmetrical solar vertical points (zenith angle 45°) during the evening twilight o f March, 27, 2016. This value was described in [Ugolniko-v, 1999]. It increases from the sunset to the deep stage of twilight (solar zenith angle 97°) due to increasing difference o f effective single scattering altitudes in these solar vertical points. During the even darker stage brightness asymmetry disappears, as the single scattering fades on the background o f multiple scattering. Effects noted above are observed for all spectral bands. However, additional dusk excess of brightness is seen for R channel at solar zenith angle about 93° (arrow in Fig. 4), it corresponds to effective scattering in the Junge layer. Analysis of sky color behavior in a solar vertical during the twilight performed in [Ugolnikov and Maslov, 2017] allowed studying the size distribution of sulfur acid droplets. The diagram analogous to NLC in Fig. 3 is shown in Fig. 5 in comparison with space limb measurements [Bourassa et al., 2008]. The range o f possible parameters is a thin line, that is typical for such particles. Fig. 6 shows the vertical profile o f mean radius with assumed size 118