Air shower

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Simulation of an air shower generated by a proton with an energy of 1TeV that hits the atmosphere at a height of 20 km.

An air shower is a shower of particles in the earth's atmosphere that is generated by a high-energy photon or a particle of cosmic rays . It is an “avalanche” of elementary particles and electromagnetic radiation that extends for many kilometers . The original ( primary ) particle first meets an atom in the air with which it interacts. As a result of this interaction, further particles ( secondary particles ) are created, which in turn react with the air atoms and generate further particles. The formation of this cascade excites the nitrogen atoms, which then emit the excitation energy as fluorescent light .

Origin and properties of the air shower

Exemplary reaction scheme of an air shower triggered by a proton.

The primary particle can be a proton , electron , atomic nucleus , photon or, less commonly, a positron . The particle cascade consists predominantly of electrons, positrons and photons. The electrons and positrons generated by deflection in the Coulomb field of the atomic nuclei and electrons shell bremsstrahlung . The high-energy photon generates an electron-positron pair through pair production .

As long as the energy of the electrons and positrons is greater than the critical energy (approx. 80 MeV ), the number of particles in the air shower increases. If the mean energy of the electrons and positrons falls below the critical energy, the electrons and positrons mainly lose energy through ionization of the atoms, whereby no more high-energy photons are generated: the shower maximum is reached and the particle cascade dies out.

Depending on the type and energy of the primary particle, the shape of the shower, the number of particles produced and the location of the shower maximum vary. For a particle with an energy of 10 7 GeV falling perpendicular to the earth's surface, hadrons , muons and electrons can be detected at sea ​​level . They form a shower front only a few meters thick, which has a radius of about 100 m lateral to the original direction of incidence of the primary particle. Air showers from primary particles with energies of less than 100 GeV, on the other hand, are no longer directly detectable at sea level.

Detection methods

Air showers can be detected using various methods:

  • Air shower fields: Here the charged particles and photons are detected. By measuring the relative delay at different points on the shower front, the direction of incidence of the original particle can be reconstructed. By measuring the number of particles, one can deduce the original energy of the primary particle. Experiments using this technique (exemplary): KASCADE-Grande , IceTop, the surface detector from IceCube , Pierre Auger Observatory , Tunka Grande
  • Air Cherenkov detectors: The charged particles in the shower front move at a speed that is almost the speed of light in a vacuum. Since the speed of propagation of light in the earth's atmosphere is about 1/1000 slower than in a vacuum, some particles move at a speed that is greater than that of light in air. This leads to a coherent polarization of the medium along the flight path, which can be detected as Cherenkov radiation . The light appears as a bluish flash with a duration of a few billionths of a second ( nanosecond ). This flash cannot be seen with the human eye, but air Cherenkov light can be detected with a correspondingly fast photon detector. Experiments using this technique (exemplary): HESS , Tunka-133 and Tunka-HiSCORE
  • Fluorescence light telescopes: Air showers stimulate the nitrogen molecules in the air to fluoresce weakly, so that the profile of the air shower can be observed from the side on dark and clear nights. This requires special telescopes that are very light-sensitive and have a high time resolution. The method is therefore comparatively complex, but also comparatively precise. Experiments using this technique (exemplary): Pierre Auger Observatory
  • Radio antennas: The electrons and positrons in the air shower are slightly deflected in the earth's magnetic field, so that radio radiation is emitted. This effect is coherently reinforced by the high density of electrons in the shower front. The resulting radio signal is detectable and can be used to observe air showers. The main advantage over the air Cherenkov and fluorescence light methods is that radio measurements can be carried out not only on clear nights, but around the clock. Experiments using this technique (exemplary): LOPES , LOFAR , Pierre Auger Observatory , TREND , Tunka-Rex

Part of the cosmic radiation is responsible for the aurora borealis (without cascades, as the energy is lower).

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