Noctilucent clouds and solar activity
Since in 1885 for the first time in 3 scientific journals was reported about striking evening phenomena in the sky, this "noctilucent clouds" (NLC) called phenomenon is observed quite often. The spectacular view is caused by very small ice particles (approx. 50nm radius) in the altitude area between 80 and 90 km. NLC can be observed by bare eye only immediately after sunset, or before sunrise, if the sunlight still illuminates the high altitudes, on the other hand, however, the perturbing daylight has already strongly decreased. The formation of ice particles strongly depends on the temperature and to a certain extent also on the water vapor. Only during the summer the necessary low temperatures fall short in high latitudes by less than 150 K. Already very small changes of temperature or water vapor strongly affect altitude and brightness of the NLC. Thus NLC become an important indicator for changes in the atmosphere. Variations of frequency, altitude and brightness of the clouds possibly admit conclusions on climatic changes. However, in addition extensive time series are necessary. The longest measurements up to now are visual observations of the NLC frequency and enclose approx. 40 years. From satellites measurements of the frequency and brightness are carried out for approx. 28 years. Both series of measurements show variations with periods of about 11 years, with a clear anti-correlation to the solar activity. Model calculations show that this should be caused by variations of temperature and water vapor which are generated by the solar radiation cycle. However, herewith this explains neither the observed amplitudes (e.g., in the brightness variation) nor the phase shift observed between NLC parameters and solar activity.
The IAP builds since 1997 a continuous NLC time series with the RMR-lidar at ALOMAR in North Norway which now covers a complete solar cycle. Presently all together more than 1250 hours of NLC measurements are available, which is the largest NLC data set acquired by lidar. From these data the frequency, altitude as well as the brightness of the clouds are determined precisely.