  This paper presents time series observations and analysis of broadband night
sky airglow intensity 4 September 2018 through 30 April 2020. Data were
obtained at 5 sites spanning more than 8500 km during the historically deep
minimum of Solar Cycle 24 into the beginning of Solar Cycle 25. New time series
observations indicate previously unrecognized significant sources of broadband
night sky brightness variations, not involving corresponding changes in the
Sun's 10.7cm solar flux, occur during deep solar minimum. Even during a deep
solar minimum the natural night sky is rarely, if ever, constant in brightness.
Changes with time scales of minutes, hours, days, and months are observed.
Semiannual night sky brightness variations are coincident with changes in the
orientation of Earth's magnetic field relative to the interplanetary magnetic
field. Solar wind plasma streams from solar coronal holes arriving at Earth's
bow shock nose are coincident with major night sky brightness increase events.
Sites more than 8500 km along the Earth's surface experience nights in common
with either very bright or very faint night sky airglow emissions. The reason
for this observational fact remains an open question. It is plausible,
terrestrial night airglow and geomagnetic indices have similar responses to the
solar energy input into Earth's magnetosphere. Our empirical results contribute
to a quantitative basis for understanding and predicting broadband night sky
brightness variations. They are applicable in astronomical, planetary science,
space weather, light pollution, biological, and recreational studies.
