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Implications of Mini-EUSO measurements for a space-based observation of UHECRs

Mario Bertaina, Matteo Battisti, JEM-EUSO collaboration

TL;DR

The study investigates how Mini-EUSO measurements can inform the feasibility and design of a space-based UHECR observatory. By combining in-orbit UV background measurements, end-to-end calibration, and ESAF-based simulations, it projects M-EUSO's exposure, duty cycle, and performance in both nadir and limb observation modes. Key findings indicate a duty cycle of roughly 9–19% under favorable backgrounds, and potential annual exposures of about $3×10^{4}$ km$^2$ sr yr in nadir mode and ~$1×10^{5}$ km$^2$ sr yr in tilted mode above $3×10^{20}$ eV, with energy resolutions around $20$–$30 ext{%}$ and angular resolutions of a few degrees. These results support the viability of a space-based UHECR mission with substantial exposure and enable robust anisotropy studies, while highlighting the need for cloud-aware exposure determination. The work sets concrete expectations for M-EUSO performance and guides mission planning, calibration, and background handling strategies.

Abstract

Mini--EUSO (Multiwavelength Imaging New Instrument for the Extreme Universe Space Observatory, known as \emph{UV atmosphere} in the Russian Space Program) is the first mission of the JEM-EUSO program on board the International Space Station. It was launched in August 2019 and it is operating since October 2019 being located in the Russian section (Zvezda module) of the station and viewing our planet from a nadir-facing UV-transparent window. The instrument is based on the concept of the original JEM-EUSO mission and consists of an optical system employing two Fresnel lenses of 25 cm each and a focal surface composed of 36 Multi-Anode Photomultiplier tubes, 64 channels each, for a total of 2304 channels with single photon counting sensitivity and an overall field of view of 44$^\circ \times $44$^\circ$. Mini-EUSO can map the night-time Earth in the near UV range (predominantly between 290 nm and 430 nm), with a spatial resolution of about 6~km and different temporal resolutions of 2.5~$μ$s, 320~$μ$s and 41 ms. Mini-EUSO observations are extremely important to better assess the potential of a space-based detector in studying Ultra-High Energy Cosmic Rays (UHECRs) such as K-EUSO and POEMMA. In this contribution we focus the attention on the results of the UV measurements and we place them in the context of UHECR observations from space, namely the estimation of exposure for the planned M-EUSO (Multi-messenger Extreme Universe Space Observatory) mission.

Implications of Mini-EUSO measurements for a space-based observation of UHECRs

TL;DR

The study investigates how Mini-EUSO measurements can inform the feasibility and design of a space-based UHECR observatory. By combining in-orbit UV background measurements, end-to-end calibration, and ESAF-based simulations, it projects M-EUSO's exposure, duty cycle, and performance in both nadir and limb observation modes. Key findings indicate a duty cycle of roughly 9–19% under favorable backgrounds, and potential annual exposures of about km sr yr in nadir mode and ~ km sr yr in tilted mode above eV, with energy resolutions around – and angular resolutions of a few degrees. These results support the viability of a space-based UHECR mission with substantial exposure and enable robust anisotropy studies, while highlighting the need for cloud-aware exposure determination. The work sets concrete expectations for M-EUSO performance and guides mission planning, calibration, and background handling strategies.

Abstract

Mini--EUSO (Multiwavelength Imaging New Instrument for the Extreme Universe Space Observatory, known as \emph{UV atmosphere} in the Russian Space Program) is the first mission of the JEM-EUSO program on board the International Space Station. It was launched in August 2019 and it is operating since October 2019 being located in the Russian section (Zvezda module) of the station and viewing our planet from a nadir-facing UV-transparent window. The instrument is based on the concept of the original JEM-EUSO mission and consists of an optical system employing two Fresnel lenses of 25 cm each and a focal surface composed of 36 Multi-Anode Photomultiplier tubes, 64 channels each, for a total of 2304 channels with single photon counting sensitivity and an overall field of view of 4444. Mini-EUSO can map the night-time Earth in the near UV range (predominantly between 290 nm and 430 nm), with a spatial resolution of about 6~km and different temporal resolutions of 2.5~s, 320~s and 41 ms. Mini-EUSO observations are extremely important to better assess the potential of a space-based detector in studying Ultra-High Energy Cosmic Rays (UHECRs) such as K-EUSO and POEMMA. In this contribution we focus the attention on the results of the UV measurements and we place them in the context of UHECR observations from space, namely the estimation of exposure for the planned M-EUSO (Multi-messenger Extreme Universe Space Observatory) mission.

Paper Structure

This paper contains 6 sections, 3 figures.

Table of Contents

  1. Introduction
  2. The Mini-EUSO instrument and operation
  3. Typical background variability during an orbit of Mini-EUSO
  4. Mini-EUSO duty cycle and exposure
  5. Preliminary expected performance of M-EUSO based on Mini-EUSO results
  6. Acknowledgements

Figures (3)

  • Figure 1: a) The figure shows the trigger efficiency of Mini-EUSO as a function of energy for different background conditions (see Sect. \ref{['sec:exposure']} for details). b) Image (top) and light profile (down) of a Short Light Transients detected by Mini-EUSO near Sri Lanka. The event has duration and light intensity similar to what is expected from an EAS signal even though their kinematics do not match (see discussion on Sect. \ref{['sec:exposure']}).
  • Figure 2: The figure shows a portion of an orbit of Mini-EUSO on Session 30 (January 9$^{th}$ 2021). Image a) shows the UV map of Mini-EUSO as a function of longitude and latitude. The counts/pix/GTU are color-coded in image b). They have been taken in D3 mode but normalized to the D1 GTU. Plot c) displays the light curve along the orbit of one pixel of Mini-EUSO. Plot d) presents the map of artificial lights taken from falchi (with a scale in fraction above natural light intensity), while e) the atmospheric conditions during the passage over ocean by means of a GFS post-processing analysis of the weather forecasts. The orbit of Mini-EUSO is highlighted by the three lines indicating the center and edges of the FoV. See text for details.
  • Figure 3: Fraction of time (FoT) a space-based detector would spend below a given background level (BL), accounting for the diurnal and lunar cycles, as well as anthropogenic light sources. The values assume a Mini-EUSO detector efficiency of 8.7%, consistent with that used in ESAF simulations. BLs below 0.4 cts/pix/GTU are never observed, while the last bin integrates the FoT in which it exceeded 30 cts/pix/GTU.