Natural NSB during solar minimum.

Since 2018, Solar Cycle 24 has entered into a solar minimum. During this period, 11M of zenithal night sky brightness (NSB) data have been collected at different dark sites around the planet, including astronomical observatories and natural protected areas, with identical broadband TESS photometers (based on Unihedron SQM TLS237 sensor). A detailed observational review of the multiple effects that contribute to the NSB measurement has been conducted with optimal filters designed to avoid brightening effects by the Sun, the Moon, clouds and astronomical sources (the Galaxy and zodiacal light). The natural NSB has been calculated from the percentiles for 44 different photometers by applying these new filters. The pristine night sky was measured to change with an amplitude of 0.1 mag/arcsec$^2$ in all the photometers, which, it is suggested, is due to NSB variations on scales of up to months and is compatible with semi-annual oscillations. We report the systematic observation of short time variations in the NSB on the vast majority of the nights and find these to be related to airglow events forming above the mesosphere.