Five Years of Mini-EUSO Observations from the ISS: Summary of Key Results

TL;DR

Five years of Mini-EUSO observations establish space-based UV detection from the ISS as a viable path toward future UHECR observatories, with an energy threshold above $10^{21}$ eV and three data-time scales $D1=2.5\,\mu$s, $D2=320\,\mu$s, and $D3=40.96$ ms. The work presents end-to-end calibration using a ground-based UV flasher, generates global UV Earth maps, and delivers a space-based meteor survey (>$24{,}000$ events) alongside elves observations up to 800 km and multiple SLTs, enabling discrimination between UHECR-like signals and atmospheric transients. These results validate detector concepts, inform background estimation for missions like POEMMA and JEM-EUSO, and contribute to atmospheric, planetary, and transient UV science. The dataset and methodologies enhance planning for next-generation space-based UHECR observatories and support robust separation of genuine cosmic-ray events from terrestrial and atmospheric backgrounds.

Abstract

Mini-EUSO is the first space-borne detector of the JEM-EUSO (Joint Exploratory Missions for Extreme Universe Space Observatory) program, operating on the International Space Station (ISS) since October 2019. Designed to search for Ultra-High Energy Cosmic Rays (UHECRs) above 10$^{21}~$eV and capable of placing a stringent upper limit on their flux at these extreme energies, paving the way to future space-based UHECR observatories, Mini-EUSO has completed more than 150 observation sessions over five years, accumulating approximately 750 hours of data. The mission has produced the first global UV emission maps of Earth and provided valuable insights into lightning phenomena and Transient Luminous Events (TLEs), such as elves, as well as artificial light sources and meteors. Notably, Mini-EUSO has conducted the first systematic space-based meteor survey, detecting over 22,000 meteors and identifying three interstellar candidates. Among the observed TLEs, the most interesting class of phenomena are elves, which appear as expanding ring-shaped structures occurring at $\sim$90 km altitude. Mini-EUSO has detected elves with varying structures and different numbers of concentric rings, from single-ring up to five rings. Thanks to its imaging capabilities, fast time resolution, and favorable observational geometry, Mini-EUSO is uniquely suited to studying this kind of lightning phenomena, providing unprecedented insight into their dynamics. Additionally, the instrument has demonstrated the capability of a space-based detector to identify short light transients resembling extensive air shower signals while distinguishing them from those produced by UHECRs. This contribution presents a comprehensive summary of the Mini-EUSO mission, its status, and main results.

Figures (9)

• Figure 1: Left: Mini-EUSO installed on the UV-transparent window during a data-taking session. Middle: Schematic view of the instrument. Right: The focal plane, made of an array of MAPMTs, for a total of 2304 pixels arranged in a $48\times48$ matrix.
• Figure 2: Left: Picture of the flasher used for the end-to-end calibration. Right: Foreseen FoV of the Mini-EUSO telescope superimposed to the geographical map for the passage over Sant’Antimo Abbey, on October 30, 2022. The background picture is taken from the Black Marble 2016 dataset available at https://visibleearth.nasa.gov/images/144898/earth-at-night-black-marble-2016-color-map
• Figure 3: Efficiency of the Mini-EUSO focal surface. Left: Map of the efficiency. Right: histogram of the efficiency values. The color scale (in percent) is common to the two panels. The average value is 7.3%. More than 50% of the pixels have an efficiency higher than 6% and lower than 11%.
• Figure 4: Counts observed in moonless conditions over Europe and the Mediterranean area. Sparsely populated areas, like the Sahara desert and the Carpathian and Apennine mountains, appear darker than the surroundings.
• Figure 5: Percentage of time that a space-based UHECR detector will spend below a given background rate, taking into account the day and Moon cycle, as well as the presence of cities and other anthropogenic sources.