  The JEM-EUSO (Joint Experiment Missions for Extreme Universe Space
Observatory) program aims at the realization of the ultra-high energy cosmic
ray (UHECR) observation using wide field of view fluorescence detectors in
orbit. Ultra-violet (UV) light emission from the atmosphere such as airglow and
anthropogenic light on the Earth's surface are the main background for the
space-based UHECR observations. The Mini-EUSO mission has been operated on the
International Space Station (ISS) since 2019 which is the first space-based
experiment for the program. The Mini-EUSO instrument consists of a 25 cm
refractive optics and the photo-detector module with the 2304-pixel array of
the multi-anode photomultiplier tubes. On the nadir-looking window of the ISS,
the instrument is capable of continuously monitoring a ~300 km x 300 km area.
In the present work, we report the preliminary result of the measurement of the
UV light in the nighttime Earth using the Mini-EUSO data downlinked to the
ground. We mapped UV light distribution both locally and globally below the ISS
obit. Simulations were also made to characterize the instrument response to
diffuse background light. We discuss the impact of such light on space-based
UHECR observations and the Mini-EUSO science objectives.
