テレスコープアレイ実験サイトにおける バイスタティックライダーによる大気透明度評価

抄録

One of the main missions of the Telescope Array (TA) project is to characterize Ultra-High Energy Cosmic Rays (UHECR) based on observations of atmospheric fluorescence, which is primarily emitted from nitrogen molecules excited by secondary particles of UHECR. The TA system is distributed over an area of 700 km 2 in the wilderness. The atmospheric fluorescence is detected at three measurement stations at the site using telescopes called Fluorescence Detectors (FDs). Ultraviolet (UV) lights from nitrogen fluorescence are absorbed by other atmospheric molecules and aerosols, affecting the measurement with FDs. Therefore, the transparency of the atmosphere for UV lights has been measured on site using a well-characterized Mie-LIDAR system. The main issue was that the Mie-LIDAR could provide an atmospheric transparency at one measurement station, which might not be the same as the one measured at the other stations, since they are apart from each other by several tens of kilometers. To address this issue, we installed a new UV laser system at the location equidistant from the three stations in order to measure atmospheric transparency at all stations at the same time. A parallel UV laser beam was emitted vertically from the ground to the sky, and the scattered light was measured with FDs at each station. After four years of measurements, we concluded that the atmospheric transparency at the three stations was equivalent with a 95% correlation