Figure 1
![Figure 1](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1880-5981-66-98/MediaObjects/40623_2014_Article_88_Fig1_HTML.jpg)
Trend in the seasonal numbers of Moscow NLC nights. (a) The number of NLC nights according to the Moscow data for 1990 to 2013 without a weather correction factor. (b) Relative (weather-corrected) numbers of NLC nights. The script in the upper left corner shows the 2-D linear regression model versus time and solar Ly α flux. The contribution of both arguments is given by the blue line, the contribution of the time-trend is shown by the black thick line, and the thin red curves denote 95% confidence limits.