Mini–EUSO (Multiwavelength Imaging New Instrument for the Extreme Universe Space Ob-
servatory, known as UV atmosphere in the Russian Space Program) is the first detector of the
JEM-EUSO program to observe the Earth from the International Space Station (ISS) and to vali-
date from there the observational principle of a space-based detector for UHECR measurements.
Mini-EUSO is a telescope operating in the near UV range, mainly between 290 – 430 nm, with a
square Focal Surface (FS) corresponding to a Field of View (FoV) of ∼ 44 ◦ . Its spatial resolution
at ground level is ∼ 6.3 km. Mini-EUSO was launched with the uncrewed Soyuz MS-14. The
first observations, from the nadir-facing UV transparent window in the Russian Zvezda module,
took place on October 7, 2019. The detector size (37 × 37 × 62 cm 3 ) was mainly constrained by
the window size. The detector is usually installed during onboard night-time a couple of times per
month, approximately at 18:30 UTC with operations lasting about 12 hours. So far, 139 sessions
of data acquisition have been performed. Data are stored locally on USB solid state disks. After
each data-taking session ∼10% of stored data, usually corresponding to the beginning and the
end of each session, are copied and transmitted to ground to verify the correct functioning of the
instrument. Till now, the data of the first full 44 sessions returned to Earth and is being analysed.
The Mini–EUSO FS, or Photon Detector Module (PDM), consists of a square matrix of 36 Multi-
Anode Photomultiplier Tubes (MAPMTs). Each MAPMT consists of 8 × 8 pixels. A group of
2 × 2 MAPMTs forms an Elementary Cell (EC). In total there are 2304 channels. Each EC has an
independent high voltage power supply (HVPS) and board connecting four MAPMTs. The HVPS
system, based on a Cockroft-Walton circuit, has an internal safety mechanism which operates
either reducing the collection efficiency or the gain of the MAPMTs when particularly bright
signals occurr. The optics are based on two 25 cm diameter Fresnel lenses with a point spread
function of 1.2 pixels. UV bandpass filters are glued in front of the MAPMTs.
The system has a single photon-counting capability with a double pulse resolution of ∼ 6 ns.
Photon counts are summed in Gate Time Units (GTUs) of 2.5 𝜇s. The PDM Data Processor stores
the 2.5 𝜇s GTU data stream (D1) in a running buffer on which runs the trigger code. Sums of
128 frames (320 µs, D2) are continuously calculated and stored in another buffer where a trigger
algorithm, at this time scale, is running. Similarly, sums on 128 D2 frames (40.96 ms, D3) are
calculated in real time and continuously stored. Every 5.24 s, 128 packets of D3 data, up to 4 D2
packets and up to 4 D1 packets (if triggers were present) are sent to the storage (SSDs).
Mini-EUSO measured the terrestrial UV background with unparalleled precision. The fraction of
time in which the atmospheric UV light intensity allows the UHECR observation from space is
∼18%, compatible with the expectations from simulations conducted at the time of the JEM-EUSO
studies. The trigger rate on spurious events remains within the requirements in nominal background
conditions. It proved effective in detecting Short Light Transients (SLT), demonstrating indirectly
that the JEM-EUSO technology can detect UHECRs from space as they show similarities in terms
of light profile, intensity, duration and pixel pattern on the FS of Mini-EUSO, even though all
these characteristics do not match at the same time for a single event and are not mistaken for
real EAS-induced signals. The ability of Mini-EUSO to detect and study atmospheric phenomena
like ELVES, and the ones linked to SLTs is unique and beyond the capabilities of any other
atmospheric detector. That makes a space-based detector for UHECRs a unique instrument for
the atmospheric science field. Mini-EUSO demonstrated to be the first experiment from space to
perform a systematic study of meteor light curves and flux in a wide range of magnitudes.