Natural Night Sky Brightness during Solar Minimum

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 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 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/arcsec2 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.

SG-WAS: A New Wireless Autonomous Night Sky Brightness Sensor

“…Following Table 1, it takes approximately 4.5 s to take one measurement at magnitude 22, 1.7 s at magnitude 21, and so on. In total, ten continuous measurements at magnitude 22 are taken in about 45 s. As the possible variations in sky brightness produced by airglow are not measurable on this time scale [10], the standard deviation of these measurements corresponds only to the instrumental error, which is provided along with the NSB every 5 min in the SG-WAS.…”

“…Figure 9 shows the zenithal brightness curve on 14 May 2021 for five SG-WAS and two TESS-W placed together. In the inner plot, the dark and clear period (as defined by [10]) is shown enlarged. Note that the SG-WAS curves are smoother than those of TESS-W, which is a result of the averaging process.…”

“…The SG-WAS curves are very similar to each other, with their difference being less than one-hundredth of a magnitude. [10] enlarged. The SG-WAS curves are smoother than those of TESS-W as a consequence of the 10measurement averaging process.…”

“…The emissions of ALAN from cities can be monitored using instruments on Earth-orbiting satellites [7,8]. However, evaluating the effects of these emissions on night sky brightness (NSB) in very dark places (normally natural protected areas or astronomical observatories) requires extensive ground-based observations carried out by photometer networks [9,10]. These networks are also necessary for testing light pollution models that predict NSB as a function of the location and distribution of artificial sources and the scattering of light in the atmosphere [11].…”

“…The vertical lines indicate the end (left) and beginning (right) of astronomical twilight (black), the leaving of the zodiacal light (blue) and the entrance of the Galaxy (green) in the FOV of the device. The inner graph shows the dark period[10] enlarged. The SG-WAS curves are smoother than those of TESS-W as a consequence of the 10-measurement averaging process.…”