An experimental foundation for electromagnetic shower formation in the geomagnetic field

At very high energies the Earth magnetic field, although in many other connections considered rather weak, acts as a strong field on photons and leptons. This paper discusses the intimate connection between this effect and the corresponding ‘strong field effects’ observed at accessible accelerator energies in aligned single crystals. As such, these effects constitute an experimental verification of the theoretical basis used for simulations of the development of electromagnetic showers in magnetic fields, in particular the geomagnetic field. A short discussion of more general aspects of the shower development in the fields present at different distance scales is included.