Estimation of the exposure for the air shower detection mode of EUSO-SPB1

EUSO-SPB1 was a balloon-borne pathfinder mission of the JEM-EUSO (Joint Experiment Missions for the Extreme Universe Space Observatory) program. A 12-day long balloon flight started from New Zealand on April 25th, 2017 on the NASA's Super Pressure Balloon. With capability of detecting EeV energy air showers, the data acquisition was performed using a 1 m^2 two-Fresnel-lens UV-sensitive telescope with fast readout electronics in the air shower detection mode over ~30 hours at ~16--30 km above South Pacific. Using a variety of approaches, we searched for air shower events. Up to now, no air shower events have been identified. The effective exposure, regarding the role of the clouds in particular, was estimated based on the air shower and detector simulations together with a numerical weather forecast model. Compared with the case assuming the fully clear atmosphere conditions, more than ~60% of showers are detectable regardless the presence of the clouds. The studies in the present work will be applied in the follow-up pathfinders and in the future full-scale missions in the JEM-EUSO program.