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The origin and nature of extreme energy cosmic rays (EECRs), which have energies above the 5 ⋅ 10 19 eV —the Greisen-Zatsepin-Kuzmin (GZK) energy limit, is one of the most interesting and complicated problems in modern cosmic-ray physics. Existing ground-based detectors have helped to obtain remarkable results in studying cosmic rays before and after the GZK limit, but have also produced some contradictions in our understanding of cosmic ray mass composition. Moreover, each of these detectors covers only a part of the celestial sphere, which poses problems for studying the arrival directions of EECRs and identifying their sources. As a new generation of EECR space detectors, TUS (Tracking Ultraviolet Set-up), KLYPVE and JEM-EUSO, are intended to study the most energetic cosmic-ray particles, providing larger, uniform exposures of the entire celestial sphere. The TUS detector, launched on board the Lomonosov satellite on April 28, 2016 from Vostochny Cosmodrome in Russia, is the first of these. It employs a single-mirror optical system and a photomultiplier tube matrix as a photo-detector and will test the fluorescent method of measuring EECRs from space. Utilizing the Earth’s atmosphere as a huge calorimeter, it is expected to detect EECRs with energies above 10 20 eV . It will also be able to register slower atmospheric transient events: atmospheric fluorescence in electrical discharges of various types including precipitating electrons escaping the magnetosphere and from the radiation of meteors passing through the atmosphere. We describe the design of the TUS detector and present results of different ground-based tests and simulations.

EUSO-Balloon is a pathfinder for JEM-EUSO , the mission concept of a spaceborne observatory which is designed to observe Ultra-High Energy Cosmic Ray (UHECR)-induced Extensive Air Showers (EAS) by detecting their UltraViolet (UV) light tracks “from above.” On August 25, 2014, EUSO-Balloon was launched from Timmins Stratospheric Balloon Base (Ontario, Canada) by the balloon division of the French Space Agency CNES. After reaching a floating altitude of 38 km, EUSO-Balloon imaged the UV light in the wavelength range ∼290–500 nm for more than 5 hours using the key technologies of JEM-EUSO . The flight allowed a good understanding of the performance of the detector to be developed, giving insights into possible improvements to be applied to future missions. A detailed measurement of the photoelectron counts in different atmospheric and ground conditions was achieved. By means of the simulation of the instrument response and by assuming atmospheric models, the absolute intensity of diffuse light was estimated. The instrument detected hundreds of laser tracks with similar characteristics to EASs shot by a helicopter flying underneath. These are the first recorded laser tracks measured from a fluorescence detector looking down on the atmosphere. The reconstruction of the direction of the laser tracks was performed. In this work, a review of the main results obtained by EUSO-Balloon is presented as well as implications for future space-based observations of UHECRs.

The current status of the KLYPVE orbital detector of ultrahigh energy cosmic rays, which is scheduled to be deployed on board the Russian module of the International Space Station, is discussed. The main focus is on describing possible optical systems for the instrument.

The TUS detector was a highly sensitive orbiting telescope. Due to the spacecraft’s polar orbit, the detector was able to observe the UV emission of the atmosphere above the polar auroral oval. Events with vintensity variations characteristic of pulsating auroras were detected along the equatorial boundary of the auroral oval. These variations occurred during prolonged geomagnetic disturbances. When compared to data from charged particle detectors, they revealed an increased flux of precipitating high-energy electrons with energies of more than 100 keV along with UV pulsations.

The authors describe a EUSO-SPB2 balloon experiment to study ultra-high energy and extreme energy cosmic rays along with high-energy astrophysical neutrinos. The main characteristics of the fluorescent and Cherenkov telescopes are given. A multi-channel photodetectoris calibrated as part of the pre-flight preparation.

The main goal of the TUS experiment was to search for and study ultra-high energy cosmic rays with energies E > 70 EeV. The TUS detector registered a number of unusual events, the origin of which is unclear. Events that are unique and not similar to extensive air shower (EAS) are the subject of the study presented in this paper. Events such as gamma-ray bursts (GRBs), out-of-aperture upward going EASs accompanied by lightning flashes, as well as terrestrial gamma-ray flashes (TGFs) are considered as their possible sources.

In Russia several space missions are now planned to study transient luminous events in the atmosphere and high‐energy charged particles at satellite altitudes. The experimental goal is to investigate the origin of the high‐energy electrons and gamma ray quanta for specific transient luminous events (TLEs) and their role in the ionosphere‐magnetosphere system. Simultaneous measurements of electrons at the orbit of the satellite and TLE atmospheric radiation in many wavelength bands will be performed in two missions, Tatiana‐2 and RELEC. In the TUS mission UV transient event detection will be accompanied by measurements of the weak UV emission from the “seed” electrons of extensive air showers of extremely high‐primary energies.

In a new Tatiana‐2 mission the measurement of transient luminous events (TLE) in the Earth atmosphere in nadir direction are planned. Near UV temporal images of TLE in millisecond scale will be measured together with temporal profiles in 8 channels of wide spectrum of TLE emission. Simultaneously temporal variation of electron flux at the satellite orbit will be measured. Aims of these measurements are to continue research of bright UV flashes, started in the Tatiana‐1 mission (Universitetsky–Tatiana satellite), their global distribution, their rate over oceans and continents, and their possible correlation with lunar phase. Special attention will be paid to search for correlation between UV flashes from the atmosphere and variations of electron flux in the atmosphere‐magnetosphere system.

In the autumn of 2021, a multichannel imaging photometer of the Pulsating Aurora Imaging Photometer System was installed at the Verkhnetulomsky Observatory. During the first season of operation (2021/2022), measurements were made over the course of 163 nights in three modes of temporal resolution: 2.5 μs, 320 μs, and 41 ms. The high temporal resolution makes it possible to investigate the fine temporal structure of the emission, which are short (less than 1 s) bursts of UV radiation, so-called “microbursts” that can be single or follow in series. The long-term series of microbursts registered on November 27–29, 2021, were analyzed. It is shown that the series of bursts have a complex temporal structure, individual bursts have several peaks with intervals of 100–400 ms, the intervals between bursts are of the order of 1 s, and they appear in packs lasting from several seconds to minutes. The series appear both in quiet geomagnetic conditions and during substorms; the frequency and amplitude of bursts in the second case are significantly larger.

KLYPVE (K-EUSO) is a planned space experiment aimed at studying ultra-high-energy cosmic rays by detecting fluorescent and Cherenkov radiation from extensive air showers in the Earth’s night atmosphere from near-Earth orbit. The observatory is being developed as a part of the JEM-EUSO program. The threshold of registration will be around 50 EeV, and the annual statistics of events will be more than 50. The KLYPVE mission can significantly supplement the data of ground-based experiments.