Natural Night Sky Brightness during Solar Minimum

TL;DR

This study analyzes natural night sky brightness during Solar Cycle 24's minimum, using extensive measurements from dark sites worldwide to understand variations caused by atmospheric and celestial factors.

Contribution

It provides a comprehensive observational review of NSB variations during solar minimum, utilizing a large dataset with optimized filters to isolate natural brightness changes.

Findings

Natural NSB varies by about 0.1 mag/arcsec$^2$ over months.

Short-term NSB variations are linked to airglow events.

NSB exhibits semiannual oscillations.

Abstract

In 2018, Solar Cycle 24 entered into a solar minimum phase. During this period, 11 million zenithal night sky brightness (NSB) data were collected at different dark sites around the planet, including astronomical observatories and natural protected areas, with identical broadband Telescope Encoder and Sky Sensor photometers (based on the Unihedron Sky Quality Meter TSL237 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 other 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 is suggested to be due to NSB variations on scales of up to months and to be compatible with semiannual oscillations. We report the systematic observation of short-time variations in NSB on the vast majority of the nights and find these to be related to airglow events forming above the mesosphere.