Analysis of above-the-limb cosmic rays for EUSO-SPB2

The Extreme Universe Space Observatory on a Super Pressure Balloon 2 (EUSO-SPB2) experiment is a pathfinder mission for future space-based instruments targeting the fluxes of Ultra-High Energy Cosmic Rays (UHECR), with energies exceeding 1EeV and very high energy diffuse and transient neutrinos, with energies exceeding 1PeV. Using two telescope designs: the Fluorescence Telescope (FT) and the Cherenkov Telescope (CT), EUSO-SPB2 made novel observations of the backgrounds relevant for space-based detection. EUSO-SPB2 will launch from Wanaka, NZ in Spring of 2023, for a long duration (up to 100d) flight at a nominal float altitude of 33km. In this contribution, we will focus on the CT’s capability to measure cosmic rays from above Earth’s limb via the Cherenkov emission produced by the resultant Extensive Air Showers (EAS). Using the EASCherSim optical Cherenkov generation code, we provide an updated estimate of the event rate of above-the-limb cosmic rays for the CT, taking into account updated values for the trigger efficiency as determined during the field testing of the instrument. We take particular care to consider the longitudinal development of EAS in rarefied atmosphere, accounting for the energy dependent elongation rate. In addition, we consider improvements to the magnetic field modeling present in EASCherSim and illustrate their impact on the observed events and detection thresholds. Finally, we compare these simulations to preliminary flight data from EUSO-SPB2.