Design and performance of the balloon-borne magnetic spectrometer AESOP-Lite

The Anti-Electron Sub-Orbital Payload Low Energy (AESOP-Lite) is designed to determine the source of the negative spectral index of cosmic-ray electrons below 100 MeV through a series of balloon flights. The entry telescope from the classic LEE (Low Electron Energy) instrument was directly integrated into AESOP-Lite, which utilizes a gas-Cherenkov and magnetic-spectrometer configuration to identify the particle type and measure its energy. Its first flight took place May 15–21, 2018 from Kiruna, Sweden accumulating roughly 130 h of exposure above 130,000 ft altitude before landing on Ellesmere Island, Canada. After recovery, work began to upgrade the instrument for its next flight, from McMurdo Station, Antarctica. In this paper, we report on its updated design, calibration and performance. This includes analyses of ground data taken during integration. The observed muon charge separation from ground runs is discussed and compared to the expected performance of the spectrometer, and the first test results of the new time-of-flight (TOF) system are presented. The energy resolution from track reconstruction algorithms and the energy-dependent geometry factor are tested with Monte Carlo simulations.