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The SPHERE-2 balloon-borne detector designed for extensive air shower (EAS) observations using EAS optical Vavilov-Cherenkov radiation (``Cherenkov light), reflected from the snow-covered surface of Lake Baikal is described. We briefly discuss the concept behind the reflected Cherenkov light method, characterize the conditions at the experimental site and overview the construction of the tethered balloon used to lift the SPHERE-2 telescope above the surface. This paper is mainly dedicated to a detailed technical description of the detector, including its optical system, sensitive elements, electronics, and data acquisition system (DAQ). The results of some laboratory and field tests of the optical system are presented.
In this paper we review the main issues that are relevant for the detection of Extensive Air Showers (EAS) from space. EAS are produced by the interaction of Ultra-High Energy Cosmic Particles (UHECP) with the atmosphere and can be observed from an orbiting telescope by detecting air fluorescence UV light. We define the requirements and provide the main formulas and plots needed to design and optimize a suitable telescope. We finally estimate its expected performances in ideal conditions.
We report on a measurement of thermal neutrons, generated by the hadronic component of extensive air showers (EAS), by means of a small array of EN-detectors developed for the PRISMA project (PRImary Spectrum Measurement Array), novel devices based on a compound alloy of ZnS(Ag) and $^{6}$LiF. This array has been operated within the ARGO-YBJ experiment at the high altitude Cosmic Ray Observatory in Yangbajing (Tibet, 4300 m a.s.l.). Due to the tight correlation between the air shower hadrons and thermal neutrons, this technique can be envisaged as a simple way to estimate the number of high energy hadrons in EAS. Coincident events generated by primary cosmic rays of energies greater than 100 TeV have been selected and analyzed. The EN-detectors have been used to record simultaneously thermal neutrons and the air shower electromagnetic component. The density distributions of both components and the total number of thermal neutrons have been measured. The correlation of these data with the measurements carried out by ARGO-YBJ confirms the excellent performance of the EN-detector.
With the Auger Engineering Radio Array (AERA) of the Pierre Auger Observatory, we have observed the radio emission from 561 extensive air showers with zenith angles between 60$^circ$ and 84$^circ$. In contrast to air showers with more vertical incidence, these inclined air showers illuminate large ground areas of several km$^2$ with radio signals detectable in the 30 to 80,MHz band. A comparison of the measured radio-signal amplitudes with Monte Carlo simulations of a subset of 50 events for which we reconstruct the energy using the Auger surface detector shows agreement within the uncertainties of the current analysis. As expected for forward-beamed radio emission undergoing no significant absorption or scattering in the atmosphere, the area illuminated by radio signals grows with the zenith angle of the air shower. Inclined air showers with EeV energies are thus measurable with sparse radio-antenna arrays with grid sizes of a km or more. This is particularly attractive as radio detection provides direct access to the energy in the electromagnetic cascade of an air shower, which in case of inclined air showers is not accessible by arrays of particle detectors on the ground.
As of 2023, the Square Kilometre Array will constitute the worlds largest radio telescope, offering unprecedented capabilities for a diverse science programme in radio astronomy. At the same time, the SKA will be ideally suited to detect extensive air showers initiated by cosmic rays in the Earths atmosphere via their radio emission. With its very dense and uniform antenna spacing in a fiducial area of one km$^2$ and its large bandwidth of 50-350 MHz, the low-frequency part of the SKA will provide very precise measurements of individual cosmic ray air showers. These precision measurements will allow detailed studies of the mass composition of cosmic rays in the energy region of transition from a Galactic to an extragalactic origin. Also, the SKA will facilitate three-dimensional tomography of the electromagnetic cascades of air showers, allowing the study of particle interactions at energies beyond the reach of the LHC. Finally, studies of possible connections between air showers and lightning initiation can be taken to a new level with the SKA. We discuss the science potential of air shower detection with the SKA and report on the technical requirements and project status.
A search for tau neutrino induced showers with the MAGIC telescopes is presented. The MAGIC telescopes located at an altitude of 2200 m a.s.l. in the Canary Island of La Palma, can point towards the horizon or a few degrees below across an azimuthal range of about 80 degrees. This provides a possibility to search for air showers induced by tau leptons arising from interactions of tau neutrinos in the Earth crust or the surrounding ocean. In this paper we show how such air showers can be discriminated from the background of very inclined hadronic showers by using Monte Carlo simulations. Taking into account the orography of the site, the point source acceptance and the event rates expected have been calculated for a sample of generic neutrino fluxes from photo-hadronic interactions in AGNs. The analysis of about 30 hours of data taken towards the sealeads to a 90% C.L. point source limit for tau neutrinos in the energy range from $1.0 times 10^{15}$ eV to $3.0 times 10^{18}$ eV of about $E_{ u_{tau}}^{2}times phi (E_{ u_{tau}}) < 2.0 times 10^{-4}$ GeV cm$^{-2}$ s$^{-1}$ for an assumed power-law neutrino spectrum with spectral index $gamma$=-2. However, with 300 hours and in case of an optimistic neutrino flare model, limits of the level down to $E_{ u_{tau}}^{2}times phi (E_{ u_{tau}}) < 8.4 times 10^{-6}$ GeV cm$^{-2}$ s$^{-1}$ can be expected.