Night time measurement of the UV background by EUSO-Balloon

International audienceA precise characterization of the Earth’s UV background during night is essential for the observation of ultra-high-energy cosmic ray induced extensive air showers (EAS) from space. We have analyzed data from the flight of the EUSO-Balloon pathfinder mission that took place near Timmins (Canada) in the moonless night from 24 to 25 August 2014. The EUSO-Balloon telescope imaged the UV background in the wavelength range of 290 – 430 nm from an altitude of ∼ 38 km with an 1 m2 refractor telescope with ∼ 11◦ field-of-view pointed in nadir direction. The UV data were complemented by the data of the Infrared camera onboard EUSO-Balloon, which was operating in the wavelength ranges of 10.37 – 11.22 µm and 11.57 – 12.42 µm. We have combined the UV and IR images to study the upward UV radiance from the Earth’s surface and Earth’s atmosphere. This allowed us to estimate the level of UV background in clear atmosphere conditions without man-made light and also to investigate the influence of clouds on the UV background. Comparison of the UV and IR images reveals an evident dependence of the upward UV radiance on the atmospheric conditions. This opens a possibility to use the UV albedo measurement for characterization of the cloud coverage. The efficiency of detection of EAS from space by EUSO like detectors varies depending on the level of the UV background. Using the available EUSOBalloon data, we discuss the role of UV background variations due to clouds and man-made light on the exposure and effective area of EAS observations