  The nocturnal mesopause region of the Earth's atmosphere radiates
chemiluminescent emission from various roto-vibrational bands of hydroxyl (OH),
which is therefore a good tracer of the chemistry and dynamics at the emission
altitudes. Intensity variations can, e.g., be caused by the general
circulation, gravity waves, tides, planetary waves, and the solar activity.
While the basic OH response to the different dynamical influences has been
studied quite frequently, detailed comparisons of the various individual lines
are still rare. Such studies can improve our understanding of the OH-related
variations as each line shows a different emission profile. We have therefore
used about 90,000 spectra of the X-shooter spectrograph of the Very Large
Telescope at Cerro Paranal in Chile in order to study 10 years of variations of
298 OH lines. The analysis focuses on climatologies of intensity, solar cycle
effect, and residual variability (especially with respect to time scales of
hours and about 2 days) for day of year and local time. For a better
understanding of the resulting variability patterns and the line-specific
differences, we applied decomposition techniques, studied the variability
depending on time scale, and calculated correlations. As a result, the mixing
of thermalized and nonthermalized OH level populations clearly influences the
amplitude of the variations. Moreover, the local times of the variability
features shift depending on the effective line emission height, which can
mainly be explained by the propagation of the migrating diurnal tide. This
behavior also contributes to remarkable differences in the effective solar
cycle effect.
