  Mini-EUSO (Multiwavelength Imaging New Instrument for the Extreme Universe
Space Observatory) is a telescope observing the Earth from the International
Space Station since 2019. The instrument employs a Fresnel-lens optical system
and a focal surface composed of 36 multi-anode photomultiplier tubes, 64
channels each, for a total of 2304 channels with single photon counting
sensitivity. Mini-EUSO also contains two ancillary cameras to complement
measurements in the near infrared and visible ranges. The scientific objectives
of the mission range from the search for extensive air showers generated by
Ultra-High Energy Cosmic Rays (UHECRs) with energies above 10$^{21}$ eV, the
search for nuclearites and Strange Quark Matter (SQM), up to the study of
atmospheric phenomena such as Transient Luminous Events (TLEs), meteors and
meteoroids. Mini-EUSO can map the night-time Earth in the near UV range
(between 290-430 nm) with a spatial resolution of about 6.3 km (full field of
view of 44{\deg}) and a maximum temporal resolution of 2.5 $\mu$s, observing
our planet through a nadir-facing UV-transparent window in the Russian Zvezda
module. The detector saves triggered transient phenomena with a sampling rate
of 2.5 $\mu$s and 320 $\mu$s, as well as continuous acquisition at 40.96 ms
scale. In this paper we discuss the detector response and the flat-fielding and
calibration procedures. Using the 40.96 ms data, we present $\simeq$6.3 km
resolution night-time Earth maps in the UV band, and report on various
emissions of anthropogenic and natural origin. We measure ionospheric airglow
emissions of dark moonless nights over the sea and ground, studying the effect
of clouds, moonlight, and artificial (towns, boats) lights. In addition to
paving the way forward for the study of long-term variations of natural and
artificial light, we also estimate the observation live-time of future UHECR
detectors.
