The origin and nature of Ultra-High Energy Cosmic Rays (UHECRs) remain unsolved in contemporary astroparticle physics. To give an answer to these questions is rather challenging because of the extremely low flux of a few per km 2 per century at extreme energies (i.e. E > 5×10 19 eV). The central objective of the JEM-EUSO program, Joint Experiment Missions for Extreme Universe Space Observatory, is the realisation of an ambitious space-based mission devoted to UHECR science. A super-wide-field telescope will look down from space onto the night sky to detect UV photons emitted from air showers generated by UHECRs in the atmosphere. The JEM-EUSO program includes several missions from ground (EUSO-TA), from stratospheric balloons (EUSO-Balloon, EUSO-SPB1, EUSO-SPB2), and from space (TUS, Mini-EUSO) employing fluorescence detectors to demonstrate the UHECR observation from space and prepare the large size missions K-EUSO and POEMMA. We review the scientifical objectives associated with the developing projects of the JEM-EUSO program and the technological achievements allowing them.
Super Pressure Balloon (EUSO-SPB1) is a pathfinder of the JEM-EUSO program, which aims to observe Ultra High Energy Cosmic Rays (UHECRs) from near-space. It was launched from Wanaka (New Zealand) on April 25, 2017 UTC and was terminated after twelve days of flight in the South Pacific Ocean. A good knowledge of the atmospheric conditions and cloud properties, such as the Cloud Top Height (CTH), is fundamental to correctly reconstruct the energy and geometry of air showers produced by cosmic rays passing through the atmosphere. One of the methods used to retrieve the CTH is based on Numerical Weather Prediction models. In this work, we consider in particular the Weather Research and Forecasting (WRF) model. A first model test is made on the WRF parametrizations for elementary processes, applying a top-bottom directed algorithm based on two quantities: cloud fraction and optical depth. The validated procedure is then applied to the SPB1 trajectory, retrieving the CTH every ten minutes for the days of the flight. A comparison is made with the analyzed data taken from MODerate resolution Imaging Spectroradiometer (MODIS) satellite images, once per day, to understand the reliability of the method. Another way to retrieve the CTH is the so-called radiative method, that allows to calculate the Cloud Top Temperature (CTT). A vertical temperature profile is needed to transform the CTT into CTH. When radiosoundings are not available, WRF can provide vertical temperature profiles. The conversion from the CTT to the CTH is then made.
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