Air-shower spectroscopy at horizons

Downward cosmic rays are mostly revealed on the ground by their air-showers diluted and filtered secondary μ+μ−μ+μ− traces and/or by their (Cerenkov - Fluorescent) light because of the high altitude numerous and luminous electromagnetic e+e−,γe+e−,γ shower component. Horizontal and upward air-showers are even more suppressed by deeper atmosphere opacity and by the Earth shadows. In such noise-free horizontal and upward directions rare Ultra High Cosmic rays and rarer neutrino induced air-showers may shine, mostly mediated by resonant PeV νē+e−→W− interactions in air or by higher energy tau air-showers originated by ντντ skimming the Earth. At high altitude (mountains, planes, balloons) the air density is so rarefied that nearly all common air-showers might be observed at their maximal growth at a tuned altitude and direction. The arrival angle samples different distances and the corresponding most probable primary cosmic ray energy. The larger and larger distances (between observer and C.R. interaction) make wider and wider the shower area and it enlarges the probability of being observed (up to three orders of magnitude more than vertical showers); the observation of a maximal electromagnetic shower development may amplify the signal by two–three orders of magnitude (with respect to a suppressed shower at sea level); the peculiar altitude–angle range (ten–twenty km height and ≃80∘–90∘≃80∘–90∘ zenith angle) may disentangle at best the primary cosmic ray energy and composition. Even from existing mountain observatories the up-going air-showers may trace, above the horizons, PeV–EeV high energy cosmic rays and, below the horizons, PeV–EeV neutrino astronomy: their early signals may be captured in already existing gamma telescopes such as Magic at Canarie, while facing the Earth edges during (useless) cloudy nights.