The dancing sky: 6 years of night-sky observations at Cerro Paranal

Aims. This work provides the results of the first six years of operation by the systematic night-sky monitoring at ESO-Paranal (Chile). Methods. The UBVRI night-sky brightness was estimated on about 10 000 VLT-FORS1 archival images, obtained on more than 650 separate nights, distributed over 6 years, and covering the descent from maximum to minimum of sunspot cycle n. 23. Additionally, a set of about 1000 low-resolution, optical, night-sky spectra were extracted and analysed. Results. The unprecedented database discussed in this paper has led to detecting a clear seasonal variation of the broad-band night-sky brightness in the VRI passbands, similar to the well-known semi-annual oscillation of the Na I D doublet. The spectroscopic data demonstrate that this seasonality is common to all spectral features, with the remarkable exception of the OH rotational-vibrational bands. A clear dependency on the solar activity is detected in all passbands and is particularly pronounced in the U band, where the sky brightness decreased by similar to 0.6 mag arcsec(-2) from maximum to minimum of solar cycle n. 23. No correlation is found between solar activity and the intensity of the Na I D doublet and the OH bands. A strong correlation between the intensity of N I 5200 angstrom and [OI] 6300, 6364 angstrom is reported here for the first time. The paper also addresses the determination of the correlation time-scales with solar activity and the possible connection with the flux of charged particles emitted by the Sun.