Analysis of EAS-like events detected by the Mini-EUSO telescope

TL;DR

This paper analyzes EAS-like events detected by Mini-EUSO on the ISS, distinguishing between cosmic-ray induced events and other atmospheric or ground-based phenomena, validating the detection principle for space-based UHECR observation.

Contribution

It demonstrates that Mini-EUSO can differentiate between UHECR-induced events and other transient phenomena, confirming the detection principle for future space-based cosmic-ray observatories.

Findings

Most EAS-like events are ground-based flashes triggered repeatedly.

Some non-repeated events are not of cosmic origin and are classified as SLTs.

The analysis confirms the ability to distinguish UHECR events from other atmospheric phenomena.

Abstract

The Mini-EUSO telescope is the first space-based detector of the JEM-EUSO program. It was launched for the International Space Station on August 22$^{nd}$, 2019 to observe from the ISS orbit ($\sim$420 km altitude) various phenomena occurring in the Earth's atmosphere through a UV-transparent window located in the Russian Zvezda Module. The dimension of the window defines and constrains the dimension of the optics, based on a set of two Fresnel lenses of 25 cm diameter each, almost two orders of magnitude smaller than the system foreseen for a larger space-based detector, like the original JEM-EUSO detector or the future POEMMA. As a consequence, the energy threshold of Mini-EUSO is very high, above $10^{21} $eV. Nevertheless, a series of events that resemble the shape and the time duration of EAS-induced events have been detected in Mini-EUSO data. This contribution presents the most interesting cases, showing that the vast majority of the EAS-like events can be traced back to ground sources repeatedly flashing and triggered many times by Mini-EUSO. Some non-repeated EAS-like events are also present. In these cases, it is possible to exclude their cosmic origin through the comparison with simulated events. Since it is clear that those events can not be originated by a UHECR, we decided to rename them "Short Light Transients" or SLTs. Finally, it was possible to associate some of the SLTs with atmospheric activity. This analysis confirms the validity of the JEM-EUSO detection principle and shows that it is possible for a space-based detector to distinguish between events induced by UHECRs and events with a different origin.