Observations of noctilucent clouds from Lithuania

Abstract

We present an analysis of systematic visual and photographic observations of noctilucent clouds seen from Lithuania in the years 1973–2009. The main trends in the noctilucent cloud occurrence frequency and the mean brightness are derived from statistical and correlation analysis. A clear signature of the solar activity cycle is imprinted on the noctilucent cloud occurrence frequency and mean brightness, both showing distinct anti-correlation with the sunspot numbers; however, no statistically significant increase of either noctilucent cloud occurrence frequency or brightness has been detected at least over past 19 yr (1991–2009). The only statistically significant positive trend is established for the numbers of very bright noctilucent cloud displays in the years 1973–2009. The most recent noctilucent cloud observations are linked to variations of local mesospheric temperatures, measured by the Aura satellite.