Probing lightning-ionosphere coupling with Mini-EUSO: timing and morphology of multi-ring ELVES

Mini-EUSO (Multiwavelength Imaging New Instrument for the Extreme Universe Space Observatory) is a compact ultraviolet telescope operating aboard the International Space Station since 2019, observing the Earth’s atmosphere in the 290-430 nm band from nadir. It is a part of the JEM-EUSO programme, aimed at developing technologies for the observation of ultra-high-energy cosmic rays (UHECRs) from space. The instrument comprises two 25 cm Fresnel lenses and a focal surface of 36 multi-anode photomultiplier tubes (2304 pixels), providing a 44° field of view and a time resolution of 2.5 μs. With an angular pixel size of ∼0.86°, Mini-EUSO has a spatial resolution of ∼6 km at ground level and ∼5 km at ionospheric altitudes, allowing for detailed imaging of fast transient luminous events.

Since the beginning of operations, Mini-EUSO has recorded approximately 50 ELVES. A large fraction of the observed events exhibit complex morphologies, most notably multi-ring structures. Understanding the diversity of ELVES morphologies requires quantitative measurements of their dynamics (ring radius and expansion speed), energetics, and internal ring morphology.

We present results from a dedicated analysis pipeline that reconstructs the spatio-temporal development of ELVES UV emission at microsecond time scales. Mini-EUSO’s fast imaging allows us to measure ring properties such as thickness and brightness variations along the ring, and to follow how these features evolve in time. These measurements help constrain ELVES production mechanisms and the relative role of different EMP propagation paths. In several cases, the reconstructed timing and ring morphology are compatible with, and suggest, a “ground reflection” contribution, where additional ELVES rings may be associated with an EMP component reflected from the Earth’s surface. These observations highlight the capability of compact space-based UV instruments to advance ELVES physics and to probe EMP-ionosphere coupling with unprecedented detail.